13C and trace-element analysis (e.g., Cd/Ca in calcite) to allow ocean paleochemistry to be used to determine whether during glaciations the site lay under severely nutrient-depleted AAIW or enriched CDW.
The principal shipboard results of Leg 181 drilling are encapsulated by summary plots of stratigraphy, sediments, and average sedimentation rates at each site (Figs. F7, F8, F9). The main scientific target of the leg was the high-resolution study of Neogene sediments in the context of ACC-DWBC evolution. However, drilling in the Southwest Pacific is still in a state of reconnaissance, and, unsurprisingly, Leg 181 sites intersected sediments that range widely in age from Late Cretaceous to Holocene. Together with the results of earlier drilling, which transected sediments of early-middle Eocene and early Oligocene age (Site 277, 278; Fig. F8), a complete deep-marine stratigraphic record between the Late Cretaceous and Holocene is now available for the first time. Detailed study of this data set will undoubtedly lead to significant advances in our understanding of regional Southwest Pacific stratigraphy, micropaleontology, and paleoceanography, as well as helping delineate the origin and evolution of the world's modern thermohaline ocean circulation system.
The principal results from Leg 181 include the following:
Position: 44° 45.332'S, 172° 23.598'E
Start hole: 0345 hr, 23 August 1998
End hole: 1020 hr, 23 August 1998
Time on hole: 6.58 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 406.50
Distance between rig floor and sea level (m): 11.00
Water depth (drill pipe measurement from sea level, m): 395.50
Total depth (from rig floor, mbrf): 412.50
Total penetration (mbsf): 6.01
Coring totals: type: APC; number: 1; cored: 6.00 m; recovered: 100.17%
Formation: lithostratigraphic Unit I: olive-gray silty sand and greenish gray silty clay
Position: 44° 45.332'S, 172° 23.598'E
Start hole: 1015 hr, 23 August 1998
End hole: 0020 hr, 24 August 1998
Time on hole: 14.08 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 407.80
Distance between rig floor and sea level (m): 11.00
Water depth (drill pipe measurement from sea level, m): 396.80
Total depth (from rig floor, mbrf): 563.60
Total penetration (mbsf): 155.54 mbsf
Coring totals: type: APC; number: 17; cored: 155.80 m; recovered: 105.56%
Formation: lithostratigraphic Unit I: 0-92 mbsf; olive-gray silty sand and greenish gray silty clay; lithostratigraphic Unit II: 92-155.8 mbsf; olive-gray silty sand and greenish gray silty clay
Position: 44° 45.332'S, 172° 23.614'E
Start hole: 0020 hr, 24 August 1998
End hole: 1915 hr, 26 August 1998
Time on hole: 66.92 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 407.20
Distance between rig floor and sea level (m): 11.00
Water depth (drill pipe measurement from sea level, m): 396.20
Total depth (from rig floor, mbrf): 902.00
Total penetration (mbsf): 494.80
Coring totals: type: APC; number: 17; cored: 160.30 m; recovered: 108.28%; type: XCB; number: 35; cored: 334.50 m; recovered: 80.32%
Formation: Lithostratigraphic Unit I: 0-92 mbsf; olive-gray silty sand and greenish gray silty clay; lithostratigraphic Unit II: 92-404 mbsf; olive-gray silty sand and greenish gray silty clay; lithostratigraphic Unit III: 404-494.72 mbsf; greenish gray silty clay
Site 1119 is located 96 km east of the eastern shoreline of New Zealand's South Island, offshore from Timaru, within the Canterbury Basin (Fig. F1 in the "Site 1119" chapter). The site was drilled in a water depth of 393 m on the upper slope, 5 km seaward of the edge of the continental shelf. Still further seaward, the slope levels out onto the southern edge of the Chatham Rise platform at 800-1000 m. Clinoform reflectors within the Miocene-Holocene represent earlier positions of the prograding shelf-slope. The reflectors define a shore-parallel zone of Pliocene-Pleistocene sediment drifts, each of which prograded landward and successively accreted to the edge of the shelf.
Site 1119 was drilled to sample the upper slope sediments and underlying sediment drifts. A copious source of sediment is required to build the drifts. Site 1119 samples will allow the provenance of the upper drift sediments to be established, paleocurrent velocities to be inferred, and the late Pliocene-Pleistocene history of the important AAIW water mass to be reconstructed close to its source. The sand-rich intervals encountered in the hole will allow inferences to be made regarding relative sea-level change during this period.
Hole 1119A comprised a single 6.01-m-long core taken for mudline sampling. APC penetration, with essentially full core recovery and little disturbance, was then achieved to a depth of 155.54 mbsf (Hole 1119B) and 160.3 mbsf (Hole 1119C) successively (Table T1, also in ASCII format). Hole 1119C continued to a depth of 494.8 mbsf using the extended core barrel (XCB). Throughout the APC core sections, core voids caused by gas expansion were common in the sands and silts. Core recovery was 105% for Hole 1119B and 89% for Hole 1119C.
The 494.8 m of sediment cored is subdivided into three lithologic units (Fig. F10). The upper 88 m (Unit I) comprises repetitive, sharp-based, silty sand-silty clay couplets. Silty sand beds are usually <2 m thick and have an olive-gray color, coarse texture with shell debris, and small amounts of glauconite. Many microfossils are reworked. The silty clays form thick greenish gray beds (usually >4 m), with mica and scattered macrofossils (including double valves of the subantarctic scallop Zygochlamys delicatula). Nannofossils occur in most samples, radiolarians have sporadic distribution, pyrite is a common accessory mineral, and sponge spicules are prominent. Toward the bottom of lithostratigraphic Unit I, bathyal gastropods appear in the silty muds (e.g., Ellicea), perhaps indicating deepening conditions for the sedimentary couplets with depth in the hole: the sequence as a whole indicates shallowing-upward conditions. Foraminifers and nannoplankton indicate deeper, warmer conditions during deposition of the silty sands (sediment-starved upper slope during interglacial highstands), and deposition of silty-clay intervals during glacial lowstands, in relatively shallower, colder water on a nearshore upper slope. The basic nature of the Unit I sedimentary couplets was apparently controlled by sea-level change, but seismic records show that all but the uppermost parts of Unit I (shallower than 50 m) have a drift geometry. However, the marked sedimentary and environmental cyclicity of Unit I is similar to that already described from Pliocene-Pleistocene sediments nearby on land.
Lithostratigraphic Unit II comprises Subunits IIA from 88.2 to 196 mbsf, IIB from 196 to 285.2 mbsf, and IIC from 285.2 to 401 mbsf. It also contains sedimentary rhythms that differ from the couplets of Unit I. The upper part, Subunit IIA, contains doubly-graded silty sand-silty clay units in which the sand has gradational contacts with the silty clay both above and below. The sands also have a higher calcite content than their counterparts in Unit I and sometimes show incipient cementation. Nannofossils are more frequent than in Unit I. Subunit IIB contains sharp-based sand-silty clay couplets similar to those of Unit I, but in which the sands again have less terrigenous material and include significant amounts of broken shell, benthic foraminifers, and nannofossils. The sands have sharp bases, beneath which are muds displaying sand-filled burrows of Thalassinoides and Planolites. Whole-valve mollusks, conspicuous in the overlying unit, are rare.
The lowest double-graded motif occurs in the bottom part of Core 181-1119C-34X (near the top of Subunit IIC), below which the sharp-based sand-mud couplets continue downhole intermittently to Core 43X (401.0 mbsf, base of Subunit IIC). We interpret the double-graded motif as the deeper water manifestation of the climatic and sea-level changes that produced the sharp-based silty sand-silty clay couplets in Unit I. The sharp-based silty sand-silty clay couplets of Subunit IIB are more difficult to interpret. They may have a similar origin to their counterparts in Unit I, or they may be redeposited (i.e., turbidites). X-radiography may help in making this distinction.
Unit III occurs from 401.1 mbsf to the bottom of the hole at 494.8 mbsf and comprises mostly massive, pale olive-gray silty clay with occasional burrows, which is similar to the inferred glacial silty clays of Units I and II. Unit III contains very thin beds and laminae of light gray sediments that smear-slide analyses suggest are richer in carbonate nannofossils and therefore may be mud turbidites. The trace fossil Zoophycos is scattered throughout, macrofossils and shell fragments are almost absent, and interbeds of olive sand are completely absent. This unit corresponds with the presence of a large sediment drift on the deep seismic line through Site 1119, and we interpret Unit III sediments as deposits on the deeper slopes of the Canterbury drifts. Microfaunas are consistent with deeper water depths for Unit III than for Units I and II.
Preliminary paleontological data indicate that the Unit I/II boundary is ~0.6 Ma (early Castlecliffian), the Unit II/III boundary ~2.2 Ma (early Nukumaruan), and the bottom of hole ~2.8 Ma (late Pliocene, perhaps as old as late Waitotaran) in age. These ages are drawn from cycle counting and faunal evidence. A shipboard paleomagnetic reversal chronology could not be determined unambiguously because of a combination of low intensities and magnetic overprints. Based on the biostratigraphic ages, the average sedimentation rate across the section is ~20 cm/k.y., with lower rates during interglacials (sand) and considerably higher rates during glacials (silt and clay).
Despite the widely varying sedimentation rates and the presence of random core gaps resulting from degassing and possible washing of sand, an excellent spliced multisensor (MST) record for the two APC cores was achieved for Site 1119, based on magnetic susceptibility and natural gamma records. Cyclicity that matches the observed sediment couplets is apparent in both the smoothed magnetic susceptibility and, to a lesser degree, the natural gamma-ray records. Other physical properties data reveal a distinct increase in vane shear strength at ~85 mbsf, which may correlate with an observed seismic discontinuity and indicate a hiatus.
Carbonate concentrations are in the wide range of 0.5% and 75%, indicating strongly variable environmental conditions during sediment deposition (Fig. F10). In contrast, the average concentration of total organic carbon is 0.34% with small variations, which is low for coastal sediments. This may be a result of (1) dilution by either carbonate or terrigenous clastic material, and (2) decomposition, which is manifested in the high methane contents.
Interstitial water profiles show that the sulfate reduction zone occurs at 20 mbsf. There are increases in alkalinity, ammonium, and phosphate concentration across this zone. These increases are controlled by the degradation of sedimentary organic matter in a shallow marine environment. Sulfate concentrations are near zero from 20.15 through 472.3 mbsf, suggesting enhanced diagenetic processes, including the dissolution of magnetic minerals throughout the sequence. The decreases in calcium and magnesium concentrations in the sulfate reduction zone suggest that the precipitation of diagenetic carbonate was promoted by the rapid increase in alkalinity.
Downhole logging at Site 1119 included runs of the following tools: triple combination, Geologic High-Resolution Magnetic Tool (GHMT), and Formation MicroScanner (FMS-sonic). Downhole conditions were irregular below 160.3 mbsf because of damage to the wall during drilling, and some data dropouts are present. After the GHMT tool run, the bottom 20 m of the hole collapsed because of cavings or swelling mud, and, as a result, the data from the two later tool strings do not include this portion of the hole. The poor hole conditions affected the lithodensity and neutron porosity tools and make interpretation of the data difficult. The sonic tool was also affected by the poor hole conditions, but to a lesser degree, and the data obtained from it were used to create integrated travel times for interpretation of seismic survey data at this site. The resistivity, susceptibility, and gamma-ray tools recorded good logs with cyclic signals.
Position: 50° 3.803'S, 173° 22.300'E
Start hole: 0045 hr, 28 August 1998
End hole: 0600 hr, 28 August 1998
Time on hole: 5.25 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 553.90
Distance between rig floor and sea level (m): 11.00
Water depth (drill pipe measurement from sea level, m): 542.90
Total depth (from rig floor, mbrf): 558.50
Total penetration (mbsf): 4.60
Coring totals: type: APC; number: 1; cored: 4.60 m; recovered: 100%
Formation: lithostratigraphic Unit I: 0-4.6 mbsf; foraminifer nannofossil ooze and nannofossil foraminifer ooze
Position: 50° 3.803'S, 173° 22.300'E
Start hole: 0600 hr, 28 August 1998
End hole: 2020 hr, 28 August 1998
Time on hole: 14.33 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 555.20
Distance between rig floor and sea level (m): 11.00
Water depth (drill pipe measurement from sea level, m): 544.20
Total depth (from rig floor, mbrf): 743.20
Total penetration (mbsf): 188.00
Coring totals: type: APC; number: 8; cored: 68.30 m; recovered: 92.55%; type: XCB; number: 13; cored: 119.70 m; recovered: 69.59%
Formation: lithostratigraphic Unit I: 0-4.3 mbsf; foraminifer nannofossil ooze and nannofossil foraminifer ooze; lithostratigraphic Unit II: 4.3-10.2 mbsf; foraminifer nannofossil ooze; lithostratigraphic Unit III: 10.2-54.9 mbsf; foraminifer nannofossil ooze; lithostratigraphic Unit IV: 54.9-188.0 mbsf; nannofossil ooze
Position: 50° 3.815'S, 173° 22.299'E
Start hole: 2020 hr, 28 August 1998
End hole: 0613 hr, 29 August 1998
Time on hole: 33.88 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 556.90
Distance between rig floor and sea level (m): 11.00
Water depth (drill pipe measurement from sea level, m): 545.90
Total depth (from rig floor, mbrf): 601.50
Total penetration (mbsf): 44.60
Coring totals: type: APC; number: 5; cored: 44.60 m; recovered: 100.78%
Formation: lithostratigraphic Unit I: 0-4.6 mbsf; foraminifer nannofossil ooze and nannofossil foraminifer ooze; lithostratigraphic Unit II: 4.6-11.2 mbsf; foraminifer nannofossil ooze; lithostratigraphic Unit III: 11.2-44.6; foraminifer nannofossil ooze
Position: 50° 3.822'S, 173° 22.300'E
Start hole: 0613 hr, 29 August 1998
End hole: 0600 hr, 1 September 1998
Time on hole: 47.78 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 556.90
Distance between rig floor and sea level (m): 11.00
Water depth (drill pipe measurement from sea level, m): 545.90
Total depth (from rig floor, mbrf): 777.60
Total penetration (mbsf): 220.70
Total length of drilled intervals (m): 157.40
Coring totals: type: XCB; number: 9; cored: 63.30 m; recovered: 104.53%
Formation: lithostratigraphic Unit IV: 157.4-220.7 mbsf; nannofossil ooze
Site 1120 is located ~650 km southeast of Stewart Island on the eastern flank of Pukaki Rise, near the middle of the Campbell Plateau, in 543-m deep water (Fig. F1 in the "Site 1120" chapter). The seismic succession is punctuated by seven conspicuous reflectors (R1-R7), several of which may represent unconformities. Few basement highs remained on the Campbell Plateau to shed terrigenous sediment after Late Cretaceous rifting of the eastern plateau margin and subsequent erosion, and the entire succession above reflector R6 (probably Paleocene) is apparently biopelagic carbonate. Times of water-mass change, perhaps accompanied by current activity and sublevation, are indicated by changes in seismic character that take place across younger reflectors D to A, the target of Site 1120 drilling.
Site 1120 was drilled to establish the age of the major unconformities in the Campbell Plateau sequence and to determine the evolution of the shallow circulation across the Plateau. The analysis of seafloor magnetic anomalies suggests that the ACC had its inception in the Oligocene, at ~32 Ma, when Australia separated from Antarctica, thus allowing partial circumpolar flow. At this time, the Campbell Plateau was situated immediately downcurrent from the opening southern ocean and was, therefore, exposed to vigorous current activity. It was anticipated that reflector R2/R3 would be of middle to late Miocene age and correlate with either or both the sharp cooling of Antarctica recorded isotopically at 15-14 Ma, and a phase of known volcanism and minor faulting in eastern South Island, New Zealand, between ~13 and 10 Ma.
Four holes that recovered a sedimentary section spanning the last 23 m.y. were cored with the APC/XCB at Site 1120 to a maximum depth of 220.7 mbsf (Table T2, also in ASCII format). Hole 1120A consists of one single mudline core. Twenty-one cores were taken with the APC/XCB at Hole 1120B from 0 to 188.0 mbsf before operations were put on standby because of excessive heave. After waiting because of weather, five cores were taken with the APC at Hole 1120C to 44.6 mbsf before a broken wireline and deteriorating weather conditions ended operations. Hole 1120D was drilled to 157.4 mbsf and cored with the XCB to 220.7 mbsf when operations again had to be terminated because of the excessive heave.
Site 1120 penetrated a succession of calcareous biogenic oozes that downsection become nannofossil rich, and contain less distinct bedding. Bioturbation ranges from moderate to heavy throughout the core, with identifiable traces of Zoophycos, Palaeophycus, and Planolites. The lithostratigraphy is visually monotonous and featureless for most of the section. However, four units can be characterized, using subtle changes in composition determined from smear slides (Fig. F11). The upper unit (Unit I, 0-4.6 mbsf) comprises glacial/interglacial cycles in alternating beds of foraminifer-oozes and nannofossil-foraminifer oozes. Unit II (4.6-11.2 mbsf) is separated from Unit I by an unconformity and comprises a foraminifer nannofossil ooze with glauconite and rare pyrite concretions. Unit III (11.2-54.9 mbsf) is a foraminifer nannofossil ooze similar to Unit II. The lowermost Unit IV (54.9-220.7 mbsf) is a foraminifer-bearing nannofossil ooze. Foraminifers are less abundant in this unit than in overlying beds, and neither glauconite nor siliceous sponge spicules were observed.
The section is punctuated by a number of significant paraconformities, the first of which lies just below the seafloor at 4.6 mbsf and separates stratified cycles of darker and lighter white ooze of late Pleistocene age from slightly compacted, brownish foraminiferal ooze of middle Pleistocene age below. Another paraconformity may occur at around 13 mbsf, corresponding to a reflection on the site survey 3.5-kHz profile, and a change in microfaunas to latest Miocene forms (5-16 Ma). An expanded section of middle and late Miocene age (13.6-6 Ma) occurs between ~13 and 170 mbsf, apparently without breaks. Another probable unconformity at ~170 mbsf is perhaps marked by reflector R2 on the seismic reflection profile, where nannofossil evidence suggests a biostratigraphic break may occur.
The Pliocene-Pleistocene section at Site 1120 is very thin, and the presence of probable hiatuses throughout the hole indicates that intensified water-mass movements occurred periodically across the Campbell Plateau during the late Neogene. The relatively complete middle to late Miocene section present corresponds to sedimentation rates of 1-2 cm/k.y. over the period ~5.5 to 16 Ma, and indicates quieter, undisturbed biopelagic conditions. The lowest part of the core is of early Miocene age (18-23 Ma) and has a slower sedimentation rate, again consistent with current influence. The occurrence of the usually long-ranging Sphenolithus heteromorphus in only one core (between 167 and 177 mbsf) suggests that up to 2-3 m.y. of late early Miocene and early middle Miocene sediment may be either missing at an unconformity or represented by condensed sedimentation.
Benthic foraminifers indicate that the site has been bathyal throughout the Neogene-Quaternary. Evidence from all planktonic microfossil groups consistently indicates that during the Pleistocene the surface waters and intermediate water masses above the Campbell Plateau were cold. In the Miocene section, changes in the composition of calcareous microfossils reflect cyclic alternations between warmer and colder conditions. The planktonic foraminifers and calcareous nannofossils show a general trend toward colder faunas and floras from the early-middle to the middle-late Miocene.
Physical sediment properties were determined both by high-resolution MST core logging and by index properties measurements. Magnetic susceptibility, gamma-ray attenuation porosity evaluator (GRAPE) density, natural gamma-ray intensity, and digital reflectance data measured with the Minolta Spectrophotometer reveal cyclicities, which were used for stratigraphic correlation in the top 50 mbsf. Detailed hole-to-hole comparisons demonstrated that a nearly complete sedimentary sequence was recovered down to 50 meters composite depth (mcd), with only one small gap at 19 mcd.
A shipboard paleomagnetic reversal chronology could not be determined because of a combination of low intensities and magnetic overprints.
The profiles of interstitial water constituents are controlled by simple diagenetic diffusion processes, which show slightly increasing alkalinity, chloride, calcium, lithium, and silica concentrations, and decreasing potassium and magnesium trends with depth, and no signatures of sulfate reduction. This results from the uniform lithology (carbonate oozes) throughout the hole. The dominant chemical reactions were dissolution of carbonate, silica diagenesis, the possible precipitation of dolomite, and ion-exchange reactions in clay minerals.
Position: 50° 53.876'S, 176° 59.862'E
Start hole: 2225 hr, 1 September 1998
End hole: 1555 hr, 2 September 1998
Time on hole: 17.50 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 4503.10
Distance between rig floor and sea level (m): 11.10
Water depth (drill pipe measurement from sea level, m): 4492.00
Total depth (from rig floor, mbrf): 4511.50
Total penetration (mbsf): 8.40
Coring totals: type: APC; number: 1; cored: 8.40 m; recovered: 99.64%
Formation: lithostratigraphic Subunit IA (0-8.40 mbsf): yellow to yellowish brown silty or sandy clay and grayish brown and light yellowish brown silt, silty sand, and sand beds
Position: 50° 53.876'S, 176° 59.862'E
Start hole: 1555 hr, 2 September 1998
End hole: 1045 hr, 4 September 1998
Time on hole: 42.83 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 4499.00
Distance between rig floor and sea level (m): 11.10
Water depth (drill pipe measurement from sea level, m): 4487.90
Total depth (from rig floor, mbrf): 4638.70
Total penetration (mbsf): 139.70
Coring totals: type: APC; number: 3; cored: 23.00 m; recovered: 129.04%; type: XCB; number: 14; cored: 4.60 m; recovered: 49.85%
Formation: lithostratigraphic Subunit IA (0-15.2 mbsf): yellow to yellowish brown silty or sandy clay and grayish brown and light yellowish brown silt, silty sand, and sand beds; lithostratigraphic Subunit IB (15.2-32.7 mbsf): pale yellow and light yellowish brown clay with fragments of chert layers and nodules; lithostratigraphic Subunit IIA (32.7-132.7 mbsf): white nannofossil ooze with a subordinate content of diatoms, radiolarians, sponge spicules, and silicoflagellates; beds alternate with light greenish gray nannofossil diatom ooze; lithostratigraphic Subunit IIB (132.7-139.7 mbsf): greenish gray nannofossil-bearing clay
Site 1121 is located on the Campbell "drift," which today lies under the DWBC and ACC (Fig. F1 in the "Site 1121" chapter). Drilling at this site showed that, contrary to the pre-Leg 181 view, the sediment accumulation defined by topography and seismic reflection profiles is not a Neogene contourite drift, but rather represents a portion of the original sediment apron that accumulated adjacent to the Campbell Plateau after Late Cretaceous rifting.
The sediment body drilled lies along the foot of the 3000-m-high eastern escarpment of Campbell Plateau and is ~800 km long. The underlying crust dates from the rifting of Campbell Plateau from Marie Byrd Land at ~95 Ma. Up to 1.1 s (~1000 m) of sediment overlies oceanic basement, and probably comprises a basal wedge of nonmarine clastics, followed by latest Cretaceous and Paleogene marine sediments and thin Neogene contourites. In particular, Carter and McCave (1997) surmised that the uppermost sequence of closely spaced reflectors (<160 m thick) was of Pliocene-Pleistocene age. A short gravity core from the site has yielded an abundant assemblage of late Pleistocene and reworked older foraminifers. This sequence, presumed to carry a record of ACC and DWBC history, was the drilling target. The objectives were to obtain well-dated records of isotopic properties of oxygen and carbon that would show possible shifts of the SAF, the onset of and changes in the inflow of Circumpolar Deep Water (CDW) and AABW, and, through grain-size analysis, document changes in the vigor of the deep circulation.
Two holes that recovered a sedimentary section spanning the last 63 m.y. were cored with the APC/XCB at Site 1121 to a maximum depth of 139.7 mbsf (Table T3, also in ASCII format). Hole 1121A consists of one single mudline core. Seventeen cores were taken with the APC/XCB at Hole 1121B from 0 to 139.7 mbsf with 62.9% recovery, when operations were terminated because of bad weather.
The cored section is divided into two units, each subdivided (Fig. F12). Subunit IA (0-15.2 mbsf) is mottled silty and sandy clay with conspicuous manganese nodules. Subunit IB (15.2-32.7 mbsf) is a relatively uniform yellow siliceous clay with cherts as nodules and (broken) layers. This unit is possibly of Neogene age and represents a residual deposit with a very slow net sedimentation rate (~1 m/m.y.). It is the residue of significant erosion by the ACC-DWBC some time after the Paleocene. Unit II includes two subunits dominated by varying mixtures of siliceous and nannofossil-bearing ooze and chalk with up to 50% carbonate. The unit is extensively bioturbated and contains common chert below 71 mbsf. The lithologic features indicate deep marine deposition of pelagic biogenic material close to the CCD at a rate of ~15-30 m/m.y. The inferred erosion in the upper part of the section is corroborated by the ratio of shear strength to overburden pressure which, with values over 0.5 in the upper 20 mbsf, is indicative of overconsolidation. Chert in the lower part of the section precludes similar measurements in the Paleocene Unit II, which is probably also overcompacted.
The uppermost 3 mbsf of the section contains a condensed sequence, with late Pleistocene over early Pleistocene over late Pliocene diatoms in the top meter, underlain by 2 m of early late Pliocene. Foraminifers and coccoliths in the top meter are also of middle to late Pleistocene age. The remainder of the upper 32 mbsf composing Unit I yielded a few late Neogene nannofossils and reworked (?)Paleocene radiolarians. From 32.7 mbsf to the bottom of the hole (Unit II), the sequence provides for the first time in the Southwest Pacific a continuous record of deep marine sediment of late early to late Paleocene age. The rich microfaunas have all major microfossil groups, including nannofossils, diatoms, radiolarians, and benthic foraminifers. Radiolarian Zones RP4, 5, and 6, nannofossil Zones NP4, 5, 6, and 8, benthic foraminiferal Zone CD1, and the diatom Hemiaulus incurvus Zone are present in the record. This has provided tight control on an equally good magnetostratigraphic record in the Paleocene section where magnetochrons C27r, C27n, C26n, and C25r are recognized. The upper condensed section also contains a number of reversals, but, apart from the tentative recognition of the Gauss Chron, the biostratigraphic control is not adequate to identify them. The combined age data indicate sedimentation rates of up to 100 m/m.y. in the Paleocene and ~1 m/m.y. in the Neogene.
The profiles of interstitial water constituents show local fluctuations that are consistent with lithologic control rather than diffusion. The primary chemical reactions were probably silica diagenesis, dissolution of carbonate, and possibly ion-exchange reactions of clay minerals. In particular, relatively high concentrations of dissolved silica in the interval between ~35 and 120 mbsf are related to the dissolution of biosiliceous sediments.
Position: 46° 34.781'S, 177° 23.610'W
Start hole: 2254 hr, 5 September 1998
End hole: 0415 hr, 7 September 1998
Time on hole: 29.35 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 4446.20
Distance between rig floor and sea level (m): 11.20
Water depth (drill pipe measurement from sea level, m): 4435.00
Total depth (from rig floor, mbrf): 4570.10
Total penetration (mbsf): 123.90
Coring totals: type: APC; number: 8; cored: 75.80 m; recovered: 96.97%; type: XCB; number: 5; cored: 48.10 m; recovered: 29.02%
Formation: lithostratigraphic Subunit IA (0-16.6 mbsf): light brownish gray silty clay; lithostratigraphic Subunit IB (16.6-95.0 mbsf): greenish gray silty clay with interbedded dark greenish gray and greenish gray sand and fine sand turbidites; lithostratigraphic Subunit IC (95.0-123.90 mbsf): dark greenish gray silt and very fine sand turbidites, which are intercalated in grayish green and greenish gray silty clay
Position: 46° 34.781'S, 177° 23.610'W
Start hole: 0415 hr, 7 September 1998
End hole: 0625 hr, 7 September 1998
Time on hole: 2.17 hr
Seafloor (drill pipe measurement from rig floor, m mbrf): 4441.00
Distance between rig floor and sea level (m): 11.20
Water depth (drill pipe measurement from sea level, m): 4429.80
Total depth (from rig floor, mbrf): 4450.50
Total penetration (mbsf): 9.50
Coring totals: type: APC; number: 1; cored: 9.50 m; recovered: 103.26%
Formation: lithostratigraphic Subunit IA (0-9.5 mbsf): light brownish gray silty clay above gray silty clay and very fine sand
Position: 46° 34.780'S, 177° 23.622'W
Start hole: 0625 hr, 7 September 1998
End hole: 0800 hr, 12 September 1998
Time on hole: 121.58 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 4443.00
Distance between rig floor and sea level (m): 11.20
Water depth (drill pipe measurement from sea level, m): 4431.80
Total depth (from rig floor, mbrf): 5070.40
Total penetration (mbsf): 627.40
Coring totals: type: APC; number: 13; cored: 103.70; recovered: 103.33%; type: XCB; number: 55, cored: 523.70 m; recovered: 46.65%
Formation: lithostratigraphic Subunit IA (0-22.7 mbsf): light brownish gray silty clay; lithostratigraphic Subunit IB (22.71-109.35 mbsf): greenish gray silty clay with interbedded dark greenish gray and greenish gray sand and fine sand turbidites; lithostratigraphic Subunit IC (109.35-261.7 mbsf): dark greenish gray silt and very fine sand turbidites, which are intercalated in grayish green and greenish gray silty clay; lithostratigraphic Subunit ID (261.7-386.9 mbsf): greenish gray and gray silty clay interbedded with dark greenish gray very fine sand and silt turbidites; lithostratigraphic Subunit IIA (386.9-472.3 mbsf): greenish gray to light greenish gray silty clay with interbeds of dark gray or dark greenish gray fine sand and silt beds; lithostratigraphic Subunit IIB (472.3-494.5 mbsf): dark greenish gray to greenish gray silty clay interspersed with light brownish gray to gray, nannofossil-rich layers; lithostratigraphic Subunit IIC (494.5-550.4 mbsf): dark greenish gray to greenish gray silty clay interspersed with white nannofossil ooze layers; lithostratigraphic Subunit IIIA (550.4-580.62 mbsf): green to greenish gray clayey silt to silty clay with interbeds of greenish to greenish gray fine sand and silt beds and white to gray nannofossil-bearing foraminifer sands; lithostratigraphic Subunit IIIB (580.62-617.85 mbsf): greenish gray fine sand with intraclasts of silty clay and abundant wood fragments (debris flow) and greenish gray to dark greenish gray fine sand-bearing siltstones and light greenish gray foraminifer-bearing nannofossil chalks
Site 1122 is located ~830 km east of South Island, in a water depth of 4430 m on the north (left) bank levee of the abyssal Bounty Fan (Fig. F1 in the "Site 1122" chapter). The fan is located in the most seaward axial deep of the Bounty Trough, a rift basin formed in the Late Cretaceous during the separation of New Zealand and Antarctica across the newly forming mid-Pacific Rise. The Bounty Channel feeds sediment along the axis of the trough and into the path of the Pacific DWBC, which flows north along ~95-m.y.-old oceanic crust of the Southwest Pacific abyssal plain. Subsidence calculations indicate that Site 1122 was situated at depths of ~4700 m in the early Miocene, subsequently shoaling to ~4400 m today because of late Cenozoic sediment deposition. The seismic line through the fan shows that the channel here has over 300 m of incision, with a higher north bank levee, of which the upper parts are composed of a spectacular series of climbing sediment waves, deposited from turbidity current overspill.
Three holes that recovered a sedimentary section spanning the last 16 m.y. were cored with the APC/XCB at Site 1122 to a maximum depth of 627.4 mbsf (Table T4 also in ASCII format). Hole 1122A was cored with the APC to 75.8 mbsf and deepened with the XCB to 123.9 mbsf. One failed mudline core was taken at Hole 1122B. Thirteen cores were taken with the APC at Hole 1122C from 0 to 103.7 mbsf. The hole was deepened with the XCB to 627.4 mbsf. In an attempt to log Hole 1122C, the triple combination was deployed but was unable to pass the bit by more than 12 m. It was decided to recover the logging tool in rapidly deteriorating weather conditions and to terminate operations at the site because of the heavy seas and high winds.
Drilling confirmed that the upper ~300 m of the sediment pile is composed of rhythmic late Pleistocene sand turbidites (Fig. F13), deposited at a high rate of more than 40 cm/k.y. Recovery ranged from good when the turbidite sands were separated by regular muds and cored by APC, to very poor where they were inferred to be dominated by sand and cored by XCB. Between 300 and 450 mbsf, the turbidite sequence passed down into late Pliocene to middle Pleistocene current-worked sands and muds that are inferred to have been deposited under the influence of the DWBC, possibly reinforced by the ACC. A substantial 8-m.y.-long condensed sequence or unconformity exists at ~470-500 mbsf, below which current-influenced sands and muds of late Miocene age (10-18 Ma), with a sedimentation rate of ~5 cm/k.y., were penetrated. Accompanying microfaunas show a shift to less diverse foraminiferal assemblages, suggestive of the incursion of colder waters, an interpretation that is supported by the appearance also of subantarctic diatom floras. The lower part of the Miocene sediments contains abundant coarser grained sand, carbonized wood fragments, and transported shallow-water foraminifers, consistent with the onlap of these sediments onto the angular unconformity observed on the seismic profile. Because of drilling difficulties, the hole terminated just above this unconformity, though poor core recovery and the unfortunate lack of a sonic log makes it difficult to be certain how far above.
Site 1122 yielded an excellent high-resolution record of the input of middle upper Pleistocene sediment into the DWBC-ACC system. The site contained some significant surprises. Preliminary analysis shows that (1) turbidite deposition on the fan levee continued through both glacial and interglacial periods, although the frequency of turbidites was higher in glacials; (2) the major reflecting horizons seen on the seismic profile do not correspond to oxygen isotope stage stratigraphy; and (3) the change from DWBC-influenced sedimentation to New Zealand-derived turbidite/fan levee-influenced sedimentation is unexpectedly young (~0.7 Ma). Seven major tephra horizons were located at Site 1122, and, together with the excellent paleomagnetic record and close micropaleontologic controls, these will provide a tight chronostratigraphic framework for more detailed studies.
A complete magnetostratigraphy was determined in the uppermost 440 mbsf of the APC/XCB section at Site 1122 after AF demagnetization at 20 mT (Fig. F13). All chrons from the Brunhes (C1n) to the onset of C2r.1n (Reunion) at 2.14 Ma could be identified. In the upper Miocene section, below the major unconformity, Chrons C5r.1n and C5r.2n were determined.
Physical sediment properties were determined by high-resolution MST core logging and by index properties measurements. Natural gamma-ray intensity indicates that clay mineral concentration varies strongly between sand and clay layers. Magnetic susceptibility, GRAPE density, and digital reflectance data measured with the Minolta Spectrophotometer reveal cyclicities, which were used for stratigraphic correlation. Detailed hole-to-hole comparisons demonstrated nearly complete recovery of the sedimentary sequence down to 83 mcd with a gap in the continuous record at 73 mcd.
The primary controlling factor on the interstitial water chemistry at Site 1122 is sulfate reduction and methane genesis, which governs alkalinity, phosphate, and ammonia concentrations. In contrast to the complete utilization of sulfate in the upper part of the core, the reappearance of increasing sulfate levels in the middle of the section represents an approach to the original sulfate concentration during sediment deposition, possibly because of a lack of sufficient metabolizable organic matter and low sedimentation rates. Other significant chemical profiles are magnesium and calcium, from the pattern of which we deduce that lateral transport of magnesium-rich fluid occurred during the dissolution of carbonate. The general chemical zonations of interstitial waters at Site 1122 correspond to the lithostratigraphic units and paleontological age divisions. In particular, the sharp reduction of methane at 260 mbsf coincides with the base of the highly pyritized turbidites of the mud-wave sequence. The calcium carbonate concentration varies between 0 and 77%, indicating strong environmental changes during sediment deposition. Organic carbon content averages 0.24%, and shows also strong variations with the sedimentary facies. The organic carbonate concentration is low for marine environments and may reflect the dilution of the organic matter by either marine carbonate or terrigenous detritus. Alternatively, organic matter is likely to have decomposed to generate the methane.
Position: 41° 47.174'S, 171° 29.940'W
Start hole: 1848 hr, 13 September 1998
End hole: 2355 hr, 14 September 1998
Time on hole: 29.12 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 3301.40
Distance between rig floor and sea level (m): 11.30
Water depth (drill pipe measurement from sea level, m): 3290.10
Total depth (from rig floor, mbrf): 3459.50
Total penetration (mbsf): 158.10
Coring totals: type: APC; number: 17; cored: 158.10 m; recovered: 100.34%
Formation: lithostratigraphic Subunit IA (0-158.1 mbsf): greenish gray to white clayey nannofossil ooze
Position: 41° 47.1560'S, 171° 29.939'W
Start hole: 2355 hr, 14 September 1998
End hole: 0955 hr, 19 September 1998
Time on hole: 106.00 hr
Position: 41° 47.1598'S, 171° 29.9387'W
Seafloor (drill pipe measurement from rig floor, mbrf): 3301.10
Distance between rig floor and sea level (m): 11.30
Water depth (drill pipe measurement from sea level, m): 3289.80
Total depth (from rig floor, mbrf): 3790.10
Total penetration (mbsf): 489.00
Coring totals: type: APC; number: 17; cored: 155.40 m; recovered: 102.79%; type: XCB; number: 35; cored: 333.60 m; recovered: 87.43%
Formation: lithostratigraphic Subunit IA (0-181.9 mbsf): greenish gray to white clayey nannofossil ooze; lithostratigraphic Subunit IB (181.9-256.59 mbsf): white clayey nannofossil chalk interbedded with greenish gray clayey nannofossil chalk; lithostratigraphic Unit II (256.59-450.8 mbsf): light greenish gray clayey nannofossil chalk; lithostratigraphic Subunit IIIA (450.8-489.00 mbsf): light greenish gray and greenish gray clayey nannofossil chalk and nannofossil mudstone
Position: 41° 47.147'S, 171° 29.941'W
Start hole: 0955 hr, 19 September 1998
End hole: 1000 hr, 24 September 1998
Time on hole: 120.08 hr
Seafloor (drill pipe measurement from rig floor): 3301.50 mbrf
Distance between rig floor and sea level: 11.40 m
Water depth (drill pipe measurement from sea level): 3290.10 m
Total depth (from rig floor): 3934.30 mbrf
Total penetration: 632.80 mbsf
Coring totals: type: APC; number: 16; cored: 151.50 m; recovered: 101.45%; type: XCB; number: 17; cored: 158.40 m; recovered: 85.69%
Formation: lithostratigraphic Subunit IA (0-151.5 mbsf): greenish gray to white clayey nannofossil ooze; lithostratigraphic Subunit IIIA (484-542.9 mbsf): light greenish gray and greenish gray clayey nannofossil chalk and nannofossil mudstone; lithostratigraphic Subunit IIIB (542.9-550.5 mbsf): greenish gray plastically deformed clasts of clayey nannofossil chalk; lithostratigraphic Subunit IIIC (550.5-587.2 mbsf): light greenish gray and greenish gray nannofossil chalk and nannofossil mudstone; lithostratigraphic Unit IV (587.2-632.8 mbsf): white to light greenish gray micritic limestone
Site 1123 is located 410 km northeast of the Chatham Islands, on the deep northeastern slopes of Chatham Rise at a water depth of 3290 m (Fig. F1 in the "Site 1123" chapter). The holes penetrated the North Chatham sediment drift, which is located between 169° W and 175° W at depths of 2200-4500 m. The drift is thicker than 0.6 s above 3500-m water depth and has been deposited where the DWBC decelerates after passing through Valerie Passage. The drift is well defined by three seismic reflectors. Before drilling, the basal reflector (707 ms two-way traveltime [TWT]) was interpreted to be of middle Oligocene age. The upper sediments at this site comprise a 0.2-s-thick sequence of sediments, which on 3.5-kHz records comprise a series of irregular, vertically climbing mud waves. Parallel reflectors within this drape probably represent hemipelagites and calcareous pelagites. The anticipated presence of a substantial carbonate record back to the middle Oligocene made Site 1123 a prime location at which to evaluate the evolution of the DWBC system, including information on the NADW component of flow. It was also expected that the upper part of the sequence would contain a record of volcanic tephras derived from North Island, New Zealand. The objectives of Site 1123 were thus (1) to test the coherence of the paleoclimatic record with Milankovitch cyclicity; (2) to determine the evolution of circum-Antarctic ocean circulation, with particular reference to periods of tectonic opening of critical seaways and climatic events (e.g., growth of Antarctic ice at 15-14 Ma); (3) to evaluate isotopic and grain-size signals to determine the paleohydrography of CDW and to estimate the volume transport of the DWBC; and (4) to determine the paleoproductivity associated with the nearby STC.
Three holes that recovered a sedimentary section spanning the last 20 m.y. were cored with the APC/XCB at Site 1123 to a maximum depth of 632.8 mbsf (Table T5, also in ASCII format. Seventeen cores were taken at Hole 1123A with the APC to 158.1 mbsf. Hole 1123B was cored with the APC to 155.4 mbsf and deepened with the XCB to 489.0 mbsf. Logging was conducted from the bottom of the hole at 489 mbsf to the bit at 84 mbsf. Three standard tool-string configurations were run: the triple combination, the FMS-sonic (two passes), and the GHMT. The condition of the borehole was good and the quality of the data is excellent. Hole 1123C was cored with the APC to 151.5 mbsf and deepened by drilling ahead to 484.0 mbsf. One XCB core was obtained from 230.0 to 239.6 mbsf to provide overlap with an interval of poor recovery in Hole 1123B. XCB coring resumed and advanced from 484.0 mbsf to the modified depth objective of 632.8 mbsf with excellent recovery.
Triple APC coring resulted in a complete composite section for the upper 150 mbsf, which contained the major volcanic tephras erupted from New Zealand over the last 4.2 m.y. (Fig. F14). Splicing between cores was based on color reflectance and magnetic susceptibility data. The tephras are surprisingly variable in thickness over the 20-m distance between holes, which is probably a result of coring disturbance.
The stratigraphic sequence at Site 1123 is remarkably uniform over the upper 450 mbsf. Subdivision into Units I and II was made using the presence of tephras in the upper section (above 182 m). Diffuse reflector R1, which before drilling was correlated with regional reflector Y, is in the drill core evidenced by only a small step in velocity and density at around 145 mbsf depth (i.e., corresponds to a lithification front). The dominant lithology of Unit I is a pale greenish gray ooze/chalk with a carbonate content oscillating around 65% (±15%). In the upper 40 mbsf of the section, the color cyclicity (both visual and spectrophotometrically measured) is likely to be closely matched to isotopic stratigraphy and thereby dated. This cyclostratigraphic dating is consistent with the magnetostratigraphy, which recognizes the Matuyama/Brunhes boundary at 32 mbsf (Fig. F14). Units I and II, which have an average sedimentation rate of 4.1 cm/k.y., both show cyclicity in lithology, magnetic susceptibility, and/or reflectance, and/or GRAPE, and/or natural gamma-ray intensity. The lower part of the drift sequence, Unit III, has higher magnetic susceptibility and is darker green because of the presence of chlorite, but it retains the clear cyclicity seen in the Pleistocene. Unit III is punctuated by a 7.6-m-thick, plastically deformed mass flow subunit between 542.9 and 550.5 mbsf.
The Unit I-III sequence is extraordinarily well dated by 113 microfossil datum points and every magnetic polarity shift (plus two new ones) back to Chron C6n or 20 Ma on the scale of Berggren et al. (1995). There is ~100% core recovery over the 0-11 and 16-20 Ma intervals. Recovery of this sequence ended with a substantial hiatus from Chrons C6n to C12r, at ~32 Ma, with 12.5 m.y. missing.
Unit III rests with profound paraconformity on early Oligocene to Eocene micritic limestone, which was deposited at an average sedimentation rate of 2 cm/k.y. The contact between Units III and IV comprises an abrupt, Chondrites-burrowed surface, and represents the Marshall Paraconformity, a regional marker of middle Oligocene environmental and stratigraphic change (Carter and Landis, 1972; Carter, 1985; Fulthorpe et al., 1996). Late Pleistocene foraminiferal assemblages show little influence of dissolution. Miocene and Pliocene samples are somewhat impoverished in planktonic and benthic taxa, and at some levels abundant test fragments occur, suggesting that the area was swept by corrosive cold bottom waters. In contrast, the Eocene and early Oligocene microfaunas from Unit IV comprise well-preserved planktonic-dominated assemblages. The Neogene planktonic foraminifers are clearly derived from the warmer waters north of the STC (Globigerinoides conglobatus, G. ruber). The light/dark sedimentary cyclicity reflects warm/cold cycles in planktonic foraminifers and possible productivity changes are mirrored in diatom abundances. Late Neogene diatoms show a mixture of local (warm water) forms and subantarctic forms from Bounty Trough or further south. This aspect to the flora is lacking in the beds below the Marshall Paraconformity. The evidence all points to the occurrence of major environmental change in the middle Oligocene, with the DWBC starting after ~30 Ma.
The physical properties records from Site 1123 are excellent and show cyclicity at several long and short wavelengths in magnetic susceptibility, density, gamma radiation, and color reflectance. Overall, most properties are uniform or gently increase down to 450 mbsf, and then increase again sharply in a unit overlying the unconformity at 587 mbsf. These characteristics occur in the downhole logs, which contain very good data on acoustic velocities, magnetic susceptibility, resistivity, gamma radiation, and density down to 486 mbsf.
Most of the recovered sediments are carbonate dominated, with carbonate concentrations between 10.3% and 84.3% and an average of 57.5% (Fig. F14). Organic carbon contents are twice the average for deep-sea sediments. The organic material seems to be heavily oxidized, probably by microbial reworking during sedimentation or early diagenesis. As a result of these processes, metabolizable organic matter is virtually absent, which results in low sedimentary methane concentrations and moderate sulfate concentrations in the pore water.
Interstitial water compositions are dominantly controlled by the high carbonate content of the sediments. Moderate reduction occurs in the upper part of the hole, probably because of a relatively high organic carbon content (~0.5%) there compared to normal deep-sea carbonate sediments. Sulfate content decreases gradually with depth to 13 mM at ~200 mbsf, below which it remains almost constant. Alkalinity shows a small maximum value of 8.5 mM at 107 mbsf. Carbonate diagenetic reactions are inferred from the profiles of dissolved calcium, magnesium, and strontium. The variation of dissolved silica in the lower part of the hole may imply changes of paleoproductivity.
Downhole logging measurements were taken in Hole 1123B, using the triple combination, the FMS-sonic, and the GHMT tools. Borehole conditions were good and the data were of high quality. A successful correlation was made between core and log-based magnetic susceptibility. The results were used to position missing sections of core. The sonic velocity data were used to construct a set of integrated travel times, to calculate the depth to major seismic reflectors. Distinct logging units were recognizable within the downhole measurements, reflecting fluctuating sedimentary conditions through time.
From all points of view, Site 1123 has the potential to become the Neogene standard section, and it will certainly provide an important reference site for the integration of Southwest Pacific microfaunas into tropical zonation schemes. Results from the site will also help define the properties of the water flowing into the Pacific Ocean for the last 20 m.y.
Position: 39° 29.901'S, 176° 31.894'W
Start hole: 1748 hr, 24 September 1998
End hole: 0305 hr, 25 September 1998
Time on hole: 9.28 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 3979.00
Distance between rig floor and sea level (m): 11.50
Water depth (drill pipe measurement from sea level, m): 3967.50
Total depth (from rig floor, mbrf): 3988.50
Total penetration (mbsf): 9.50
Coring totals: type: APC; number: 1; cored: 9.50 m; recovered: 100.11%
Formation: lithostratigraphic Subunit IA (0-9.5 mbsf): nannofossil silty clay
Position: 39° 29.901'S, 176° 31.894'W
Start hole: 0305 hr, 25 September 1998
End hole: 1745 hr, 26 September 1998
Time on hole: 38.67 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 3978.10
Distance between rig floor and sea level (m): 11.50
Water depth (drill pipe measurement from sea level, m): 3966.60
Total depth (from rig floor, mbrf): 3988.00
Total penetration (mbsf): 9.90
Coring totals: type: APC; number: 2; cored: 9.90 m; recovered: 99.90%
Formation: lithostratigraphic Subunit IA (0-9.88 mbsf): nannofossil silty clay
Position: 39° 29.901'S, 176° 31.894'W
Start hole: 1745 hr, 26 September 1998
End hole: 1630 hr, 30 September 1998
Time on hole: 94.75 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 3978.00
Distance between rig floor and sea level (m): 11.50
Water depth (drill pipe measurement from sea level, m): 3966.50
Total depth (from rig floor, mbrf): 4451.10
Total penetration (mbsf): 473.10
Coring totals: type: APC; number: 14; cored: 132.00 m; recovered: 102.12%; type: XCB; number: 35; cored: 333.10 m; recovered: 89.76%
Formation: lithostratigraphic Subunit IA (8-60.6 mbsf): nannofossil silty clay; lithostratigraphic Subunit IB (60.6-178.4 mbsf): silty clay grading into nannofossil silty clay intercalated with clayey nannofossil ooze grading into nannofossil ooze; lithostratigraphic Subunit IC (178.4-302.5 mbsf): clay-bearing nannofossil chalk intercalated with clayey nannofossil chalk and nannofossil mudstone; lithostratigraphic Unit II (302.5-411.5 mbsf): nannofossil chalk with interbeds and laminae of clay-bearing nannofossil chalk that grades downcore through a nannofossil-bearing mudstone to a plain mudstone; lithostratigraphic Unit III (411.5-419.3 mbsf): clayey nannofossil chalk; lithostratigraphic Unit IV (419.3-429.09 mbsf): mudstone; lithostratigraphic Unit V (429.09-467.4 mbsf): nannofossil bearing mudstone; lithostratigraphic Unit VI (467.4-473.1 mbsf): nannofossil-bearing mudstone
Position: 39° 29.884'S, 176° 31.892'W
Start hole: 1630 hr, 30 September 1998
End hole: 2330 hr, 1 October 1998
Time on hole: 31.00 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 3978.00
Distance between rig floor and sea level (m): 11.50
Water depth (drill pipe measurement from sea level, m): 3966.50
Total depth (from rig floor, mbrf): 4133.60
Total penetration (mbsf): 155.60
Coring totals: type: APC; number: 14; cored: 133.00 m; recovered: 99.13%
Formation: lithostratigraphic Subunit IA (22.5-60.6 mbsf): nannofossil silty clay; lithostratigraphic Subunit IB (60.6-155.6 mbsf): silty clay grading into nannofossil silty clay intercalated with clayey nannofossil ooze grading into nannofossil ooze
Site 1124 is located ~600 km due east of New Zealand's North Island, on the 250-km-long north-south-trending ridge of Rekohu Drift (Fig. F1 from the "Site 1124" chapter). The main Rekohu sequence, consisting principally of inferred Miocene drift sediments, overlies older sediments beneath seismic reflector R4 (= regional reflector X), and is onlapped from the west by overbank turbidites from the Hikurangi Channel. Rekohu Drift has clearly acted as an effective barrier to eastward dispersal of terrigenous sediment from Hikurangi Channel, which turns abruptly to the left (north) against the drift. The channel is thought to have originally flowed to the north along the New Zealand margin toward Kermadec Trench and then to have been diverted eastward by a major slide off Hawke Bay in the early to mid-Pleistocene (Lewis et al., 1998). The drift sediments at this site were believed to be calcareous pelagites before drilling. It was hoped that Site 1124 would yield a mainly carbonate record of the Miocene paleohydrography of the DWBC, and, if it penetrated reflector R4, important information on the middle Cenozoic initiation deep-water circulation. The objectives of Site 1124 were thus to determine: (1) the Miocene evolution of the DWBC and associated water masses, (2) the provenance of sediment in the DWBC system, and (3) the Neogene volcanic history of the North Island.
Hole 1124A consists of a full core that failed to recover the mudline. The bit was raised by 5 m and Hole 1124B was spudded. The core barrel was retrieved with a shattered liner and no core. The second APC core achieved an incomplete stroke and required retrieval of the bottom-hole assembly with the stuck core barrel to the surface. Hole 1124C was washed to 8.0 mbsf where XCB coring was initiated. The hole was cored with the XCB from 8.0 to 27.2 mbsf, was deepened with the APC to 159.2 mbsf, and with the XCB to 473.1 mbsf (Table T6, also in ASCII format). The hole was logged from total depth to 78 mbsf with the triple combination, the FMS-sonic, and the GHMT. Hole 1124D was drilled ahead to 22.6 mbsf with the XCB and deepened with the APC to refusal at 155.6 mbsf.
A high-quality and complete spliced record was obtained for 17.7 to 174 mcd from Holes 1124C and 1124D. The uppermost 11.7 mcd is also complete in Cores 181-1124A-1H and 181-1124B-1H and 2H, but coring difficulties caused by tephras resulted in the loss of a small section between 11.7 and 17.7 mcd. The Brunhes/Matuyama boundary is located at 33 mcd (Fig. F15), however, and it should therefore be straightforward to estimate the resulting time gap in the middle Pleistocene, which may be only ~100 k.y. long. The coring problems suggest that considerable lateral variability occurs in tephra thickness and cementation over the short distances between holes (~20 m).
The sequence at Site 1124 has been divided into six lithostratigraphic units. Units I and II, which compose the drift sequence, occupy the top 412 mbsf. These units are pale greenish gray ooze and chalk showing cyclicity in color, GRAPE, and magnetic susceptibility. Tephra beds are increasingly common from the upper Miocene upward, over the top 200 mbsf of the hole. The drift sequence is broken by a hiatus of ~4 m.y. within the lower Miocene (23-19 Ma). Beneath this, a thick (110 m) upper Oligocene section (Unit II) overlies the Marshall Paraconformity, which occurs at 412 mbsf and has similar characteristics to its occurrence at Site 1123 (see "Lithostratigraphy" in the "Site 1123" chapter) Microfaunas indicate that there is a 5-m.y. hiatus (32-27 Ma) across the Marshall Paraconformity at Site 1124. Beneath the paraconformity occur four thin units of contrasting character: lower Oligocene nannofossil chalk (Unit III), middle Eocene brown to dark brown mudstone (Unit IV), Paleocene nannofossil chalk with zeolitic interbeds (Unit V), and Upper Cretaceous cherty zeolitic mudstone with nannofossils (Unit VI), the first three being separated by two further profound paraconformities and the last two by the Cretaceous/Tertiary (K/T) boundary. Unfortunately, the boundary section itself is missing between cores, though it shows clearly on the FMS and other downhole logs as a resistivity high and magnetic susceptibility low. Carbonate contents are variable at Site 1124, averaging 36% but ranging from 0% to 88%. The lowest values are found in the middle and upper Miocene, between 100 and 300 mbsf. The organic carbon content is normal for deep-sea sediment, averaging 0.31%. The Eocene brown mudstone (Unit IV) is similar to the Waipawa Black Shale facies known from eastern North Island (e.g., Moore, 1988), but has a low organic carbon content of 0.26% to 0.44%.
Despite poor preservation of all groups in the Oligocene and Miocene, 66 microfossil datum points (49 above the middle Oligocene) have been recognized at Site 1124, giving a reasonably well-constrained age-depth curve. Planktonic foraminifers are too poorly preserved--only thick-shelled species have survived dissolution--for an assessment to be made of warm/cold assemblages. However, the diatoms represent a warm subtropical flora throughout. The upper Neogene, but not the upper Oligocene/lower Miocene, contains reworked Eocene taxa. As other indicators suggest the DWBC was flowing vigorously at this time, this may point to opening of a source to the south supplying Bounty Trough or scouring of Chatham Rise. Corrosion of foraminifers clearly intensifies after the formation of the Marshall Paraconformity compared with specimens in underlying strata, which is strong evidence for a new cold bottom-water source.
The magnetostratigraphy at Site 1124 was particularly good for the spliced interval of Holes 1124C and 1124D (30-170 mcd). All subchrons of the Matuyama, Gauss, Gilbert, and C3r-C4r inclusive were recognized in the upper part of the record. However, a strong magnetic overprint was encountered between 180 and 280 mbsf, which prevented unambiguous polarity determination over that interval. Magnetic intensities were very weak in the Oligocene, but increased in the Paleocene and around the Cretaceous/Tertiary boundary, where C29r was identified spanning the K/T boundary interval. Shore-based research should allow polarity and environmental magnetism to be determined for the whole record at Site 1124.
The Upper Cretaceous-Paleocene siliceous zeolitic mudstones were deposited at an average rate of 5 m/m.y. However, the rates for the succeeding two unconformity-bounded Eocene and lower Oligocene sections are indeterminate. Above the Marshall Paraconformity, the thick upper Oligocene-lower Miocene (~27-23 Ma) section accumulated at ~27 m/m.y. The middle and upper Miocene sections accumulated steadily at 10 m/m.y., slowing down (or possibly a brief hiatus) around 9.5 to 8.5 Ma. A sharp increase in sedimentation at 1.2 Ma to ~38 m/m.y. possibly records the Hikurangi Channel switching toward the drift and contributing fine tails of turbidity currents to it. This provides a possible age for the emplacement of the very large Ruatoria slide off eastern New Zealand.
Physical properties at Site 1124 are very uniform in two intervals, 20-178 and 178-280 mbsf, corresponding to lithostratigraphic Subunits IB and IC, respectively. The early drift and subdrift sequence shows downhole increases in density and compaction. The brown mudstone unit, however, is of lower density than the chalk above and below. Temperature measurements yield a gradient of 51.9° C/km and an estimated heat flow of 0.049 W/m2.
Good logging results were obtained with all tools except the NMRS magnetic intensity instrument. All show signals of useful dynamic range, except for magnetic susceptibility between 318 and 419 mbsf, which is of very low amplitude, paralleling the core values. These values correlate very well between core and log over the whole hole. The brown mudstone unit stands out sharply in gamma, porosity, seismic velocity, photoelectric effect, and magnetic susceptibility. A 30-cm-thick layer at the correct position for the K/T boundary is evident in resistivity, magnetic susceptibility, and on the Formation MicroScanner display. The logs are vital for filling in the major features of a 17-m unrecovered interval just above 300 mbsf. The integrated travel times based on the sonic log suggest that reflector R4 (= regional reflector X) probably corresponds to the top of the brown mudstone.
The organic carbon concentrations average 0.3% and are in the normal range for deep-sea sediments. Carbonate contents show a high variability with values between 0.1% and 88.3%, and thus reflect a varying combination of fluctuating biological productivity, dilution by noncarbonate hemipelagic sedimentary components, and postdepositional carbonate dissolution.
The dominant chemical reactions that control the interstitial water element concentrations include organic matter degradation, carbonate dissolution/precipitation, silica dissolution, chert formation, and reactions with clay minerals. The element profiles of alkalinity, phosphate, and ammonia are typical of a situation without active sulfate reduction, and reflect organic matter oxidation and carbonate precipitation. The behavior of Ca, Mg, and Sr in the bottom of the section reflects a chemical reaction other than carbonate diagenesis. The decrease of Sr is similar to the pattern of Li, which is related to Si utilization to form the chert in the lowermost part of the core. The low Si concentration in the middle of the section is attributed to poor preservation of biogenic siliceous sediments, probably caused by low paleoproductivity. The general chemical zonation of interstitial waters at Site 1124 can be related to the lithostratigraphic units, paleontological age divisions, and hiatuses.
Site 1124 will provide well dated and characterized material for paleoceanographic studies of the deeper levels of flow entering the Southwest Pacific Ocean, though bulk carbonate isotopic measurements may be necessary for some parts of the core. The stratigraphy of Site 1124 will be of particular importance for integrating the results from Leg 181 with the known onland geology of eastern North Island, New Zealand.
Position: 42° 32.996'S, 178° 9.989'W
Start hole: 2248 hr, 2 October 1998
End hole: 2000 hr, 3 October 1998
Time on hole: 21.20 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 1376.20
Distance between rig floor and sea level (m): 11.60
Water depth (drill pipe measurement from sea level, m): 1364.60
Total depth (from rig floor, mbrf): 1579.70
Total penetration (mbsf): 203.50
Coring totals: type: APC; number: 22; cored: 203.50 m; recovered: 102.74%
Formation: lithostratigraphic Subunit IA (0-70.8 mbsf): clayey nannofossil ooze interbedded with nannofossil-bearing silty clay; lithostratigraphic Subunit IB (70.8-203.52 mbsf): nannofossil-bearing silty clay intercalated clay-bearing nannofossil ooze
Site 1125 lies 610 km east of South Island, at 1359 m depth on the north slope of Chatham Rise (Fig. F1 from the "Site 1125" chapter). The site lies within the STC, a zone of high productivity, and is swept by the ECC, which runs east along the northern flank of Chatham Rise. The ECC is also supplied with water (and sediment) by a branch of the Southland Current, which flows up the eastern South Island and through Mernoo Saddle to intercept the ECC (Fig. F2). The Site 1125 area is thus richly supplied with both biopelagic and terrigenous material, as well as with tephra from the central North Island volcanoes.
The major targets of Site 1125 were to retrieve an unaltered sequence of lower Neogene and perhaps Paleogene sediments from which to deduce the history of AAIW activity on the margin (i.e., penetrate back to the Oligocene). Based on the occurrence of an apparently thin upper Neogene section in seismic profiles, it appeared likely that a high-quality oxygen isotopic record could be established, which would span the period of inception of Antarctic glaciation and the consequent delivery of cold water into the circulation system. In the event, a thick upper Neogene sequence was drilled, which will provide a record of AAIW paleohydrography, and of the changing paleoproductivity and position of the Subtropical Convergence. Site 1125 is a counterpart to DSDP Site 594, which lies on the south side of Chatham Rise and at a similar depth (1204 m). Site 1125 presently lies at the base of AAIW. Material from Site 1125 will be used for
13C and trace-element analysis (e.g., Cd/Ca in calcite) to allow ocean paleochemistry to be used to determine whether during glaciations the site lay under severely nutrient-depleted AAIW or enriched CDW.
Site 1125 yielded the thickest double-piston-cored section (Table T7, also in ASCII format) of the leg, with a complete composite section for 238 mcd, which extends back almost to the Miocene/Pliocene boundary (Fig. F16). Correlation between cores was, however, difficult, being based on very low amplitude signals in reflectance and magnetic susceptibility. The correlations are therefore not as firmly based as at other sites, and the record may prove difficult to tune.
The recovered sequence is divided into two units, which are distinguished primarily on inferred carbonate content and tephra. The upper Unit I is of Pliocene to Pleistocene age, shows cyclic alternations of more and less calcareous beds, and has increasing numbers of tephra. An upper subunit (Subunit IA, 0-74.8 m) contained occasional thin sandy glauconitic layers. Below 245 m, Unit II has higher carbonate content and is more uniform in appearance. Subunit IIB, from 333 mbsf to the bottom of the hole at 552 mbsf, comprises clayey nannofossil chalk with numerous tephra layers. Organic carbon values are yet to be determined, but, judging from the sulfate and methane curves, they are probably relatively high. The lower part of the section represents rapid deposition of dominantly biogenic material under oxygenated conditions. Biogenic sediment declined until rapid deposition resumed at 6 Ma. Halfway through the subsequent period of rapid deposition, at ~5.4 Ma, terrigenous sediment increased and biogenic material declined. This may reflect a change in the currents moving sediment that had been abundantly available since the 6 Ma change of plate motion and uplift of the Southern Alps, from the South Island coast.
Well-preserved and rich calcareous nannofossil, foraminiferal, and radiolarian floras and faunas are present throughout Site 1125, and rich diatom floras are present also below 160 mbsf. These microfossils have yielded 47 age datums between the early Miocene and late Pleistocene, thus providing a robust age-depth curve. The floras indicate subtropical high-productivity conditions, whereas the faunas show some mixing of warm and cool subtropical species. Some cyclicity in warm/cold faunas in light/dark layers is also apparent, but no subantarctic species have been found.
Magnetic intensities are weak in the upper 200 mbsf, and deeper in the core they decline to the noise level of the machine. A reasonable magnetic stratigraphy can be detected in the upper 200 mbsf, with most reversals down to the base of C3n.4n at 5.23 Ma being present. After shore-based work, the paleomagnetic data will significantly aid the development of a tuned age model for the Pliocene-Pleistocene sediments at this site.
Sedimentation rates show two periods of very rapid deposition, the early late Miocene (10.1-10.6 Ma; early Tongaporutuan) at 190 m/m.y., and the late late Miocene (6-5 Ma; Kapitean) at 150 m/m.y., separated by intervals of lower accumulation rates. The Pliocene to Holocene rate declined from the Kapitean high to a Quaternary average of 20 m/m.y., though these rates do not take into account a possible a 0.5-m.y. hiatus.
Physical properties at Site 1125 are very uniform, with slow and slight increases in density with depth. The magnetic susceptibility record is relatively featureless. The thermal gradient is 64.9° C/km, and the calculated heat flow is 0.071 W/m2.
Because of severe time constraints, just one logging tool was run at Site 1125: the triple combination, comprising gamma radiation, resistivity, and porosity/density sensors. Resistivity and density show little change over the top 500 m of the hole, but the latter jumps from 1950 to 2200 kg/m3 from around 510 mbsf to the bottom of the hole, accounting for the very slow drilling encountered there. The gamma record exhibits more character, which will be examined by spectral methods.
The upper part of the hole is marked by sulfate reduction, with a decline of sulfate to zero at 200 mbsf and an increase of methane to 500 mbsf in the zone of methanogenesis below. Most other geochemical properties are related to these processes: alkalinity and ammonia increase in the upper 200 m, and calcium decreases as a result of carbonate precipitation, followed by calcium increase and carbonate dissolution in the zone of methanogenesis. Silica concentrations increase steadily downhole, but sharply decrease in the bottom 40 m where particularly hard mudstone was encountered, probably caused by incipient silica cementation.
Site 1125 proved the equal of the other sites on Leg 181 in its capacity to surprise the shipboard scientific party. Two intervals with an astonishing sedimentation rate of over 150 m/m.y. ensured that middle Miocene sediment had not been reached even after 550 m of core penetration. The prospect for high-resolution study of productivity and intermediate water masses in the Southwest Pacific for the last 11 m.y. using Site 1125 materials is excellent. Comparisons between this site, located under subtropical water, and Site 594, just south of Chatham Rise and under subantarctic water, promise to yield a scientific cornucopia.
