Site 1123



Hole 1123A
Position: 41°47.1743´S, 171°29.9401´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 (0–158.62 mbsf): greenish gray to white clayey nannofossil ooze

Hole 1123B
Position: 41°47.1598´S, 171°29.9387´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 (450.8–488.66 mbsf): light greenish gray and greenish gray clayey nannofossil chalk and nannofossil mudstone

Hole 1123C
Position: 41°47.1466´S, 171°29.9405´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 (0–151.79 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): greenish gray plastically deformed clasts of clayey nannofossil chalk
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. The site was drilled in a water depth of 3290 m. The holes penetrated a major sediment drift occurring 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 one (707 ms) was interpreted to be of middle Oligocene age. The upper sediments at this site comprise a 0.2 s thick sequence of (hemi)pelagic drape. On 3.5-kHz records, at the surface the drape formed a series of irregular, vertically-climbing mud waves. Parallel reflectors within this drape probably represent muddy calcareous pelagites and purer calcareous pelagites. The anticipated presence of a substantial carbonate record back to the middle Oligocene made Site 1123 a prime site at which to evaluate the evolution of the DWBC system, including information on the NADW component of flow. It was expected that the upper part of the sequence would contain a record of volcanic ashes derived from North Island, New Zealand. The objectives of Site 1123 were thus to (1) test the coherence of the paleoclimatic record with Milankovitch cyclicity; (2) 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) evaluate grain-size signals (flow speed) to determine water-mass movement to estimate the velocity behavior of the DWBC; and (4) determine paleoproductivity and location of the Sub-Tropical Convergence and paleohydrography of Circumpolar Deep Water (including the NADW component).

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 5). Seventeen cores were taken at Hole 1122A 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 containing the major volcanic ashes from New Zealand of the last 4.2 m.y. (Fig. 11). Splicing was based on color reflectance and magnetic susceptibility. The ashes are surprisingly variable in thickness over the 20-m distance between holes, a possible coring artifact.
The stratigraphic sequence is remarkably uniform over the upper 450 mbsf. Subdivision is made only on the basis of more frequent ashes in the upper section and a change from ooze to chalk lower down. The well-known seismic reflector 'Y' is here evidenced by only a small step in velocity and density at around 145 mbsf depth caused by compaction. The dominant lithology 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 cyclicty detected visually and by spectrophotometer can be closely matched to isotopic stratigraphy and dated thereby. This agrees with magnetostratigraphy in placing the Matuyama/Brunhes boundary at 32 mbsf (Fig. 11) with implied sedimentation rate of 4.1 cm/k.y. The entire sequence shows cyclic properties, magnetic susceptibility, and/or reflectance, and/or GRAPE, sometimes all three. It is also apparent in the lower resolution gamma radiation record. The lower part of the drift sequence has higher magnetic susceptibility, is darker green from chlorite input, but retains the clear cyclicity seen in the Pleistocene.

The sequence is extraordinarily well dated by 113 microfossil datum points and every single magnetic polarity shift (plus two new ones) back to Chron C6n or 20 Ma on the scale of Berggren et al. (1995). We have ~100% 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. This is the Marshall Paraconformity, regional marker of the middle Oligocene environmental and sea level change. Beneath that is early Oligocene to Eocene micritic limestone, deposited at an average sedimentation rate of 2 cm/k.y., about half the Neogene rate. Only the upper Pleistocene foraminiferal assemblages show little influence of dissolution. Miocene and Pliocene samples are somewhat impoverished in planktonic and also in benthic taxa, with many test fragments at some levels suggesting the area was swept by corrosive bottom waters. This was not the case in the Eocene and early Oligocene where planktonic-dominated assemblages are found. The Neogene planktonic foraminifers are clearly from the warmer waters north of the Subtropical Convergence. The light/dark sedimentary cyclicity is reflected as warm/cold cycles in planktonic foraminifers and possible productivity changes in diatom abundances. Upper 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 DWBC starting after 30 Ma.

The physical properties records are excellent, showing cyclicity at several long and short wavelengths in magnetic susceptibility, density, gamma radiation, and color reflectance. Overall properties are uniform or gently increasing down to 450 mbsf, then increase sharply in a unit overlying the unconformity at 587 mbsf. This uniformity and small scale variability are also seen in the downhole logs, from which good information was obtained 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. 11). Organic carbon contents are twice the average of 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 sufficient metabolizable organic matter is virtually absent, indicated by 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. Sulfate reduction occurs moderately in the upper part of the hole, probably related to the relatively high organic carbon content (~0.5%) compared to normal deep-sea carbonate sediments. Sulfate 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 possibly imply changes of paleoproductivity.
Downhole logging measurements were taken in Hole 1123B, using the triple combination, the FMS-sonic, and the GHMT. Borehole conditions were good and the data quality is excellent. 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, in order 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 this site is set to become the Neogene standard for the southwest Pacific and to define the properties of the water flowing into this ocean for the last 20 m.y.



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