18O increases that have been
attributed to a combination of ice growth and deep-water cooling (e.g., Shackleton and
Kennett, 1975). Multisensor track (MST) and color reflectance data (650-750 nm) collected from Holes 1092A-1092D were used to determine depth offsets in the composite section. Gamma-ray attenuation (GRA) bulk density and magnetic susceptibility data were collected at 2- to 4-cm intervals on cores recovered from Holes 1092A-1092D. Color reflectance data were collected at 4- to 6-cm intervals on cores from Holes 1092A-1092D (see "Physical Properties," "Lithostratigraphy,"; both for details about these MST and color reflectance data sets).
The composite data show that the cores from Site 1092 provide a continuous overlap to 188 mcd (base of Core 177-1092A-18H). The data used to construct the composite section and determine core overlaps are presented on a composite depth scale in Figures F6, F7, and F8. The depth offsets that comprise the composite section for Holes 1092A-1092D are given in Table T2 (also in ASCII format in the TABLES directory).
Stretching and compression of sedimentary features in aligned cores indicates distortion of the cored sequence. Because much of the distortion occurred within individual cores on depth scales of <9 m, it was not possible to align every feature in the MST and color reflectance records accurately by simply adding a constant to the mbsf core depth. Within-core scale changes will require postcruise processing to align smaller sedimentary features. Only after allowing variable adjustments of peaks within each core can an accurate estimate of core gaps be made.
Following construction of the composite depth section for Site 1092, a single spliced record was assembled for the aligned cores over the upper 188 mcd using cores from all four holes. The composite depths were aligned so that tie points between adjacent holes occurred at exactly the same depths in mcd. Intervals having significant disturbance or distortion were avoided if possible. The Site 1092 splice (Table T3, also in ASCII format in the TABLES directory) can be used as a sampling guide to recover a single sedimentary sequence between 0 and 188 mcd. Spliced records of magnetic susceptibility and GRA bulk density are shown in Figure F9.
Sediments recovered at Site 1092 provide a Pleistocene-early Miocene record. Pleistocene calcareous nannofossils are abundant to common, showing good to moderate preservation. The Pleistocene assemblages do not show a clear stratigraphic range of Emiliania huxleyi and Pseudoemiliania lacunosa in Hole 1092A, probably because of disturbance during recovery and/or reworking. However, the Pleistocene record of Hole 1092B provides good biostratigraphic resolution. Pliocene and Miocene intervals are characterized by the absence of biostratigraphic marker species, a fact which prevents an accurate biozonation. One or two samples per section were examined from Site 1092. Besides the standard zonations of Martini (1971) and Okada and Bukry (1980), we include Pleistocene events and age calibrations by Raffi et al. (1993) and Wei (1993). For the Miocene interval, the biochronology proposed by Berggren et al. (1995) is used. Tables T4 and T5 (both also in ASCII format in the TABLES directory), and Figure F10, summarize the main calcareous nannofossil biostratigraphic results.
The Pleistocene interval is represented from 0 to ~34 mcd at Site 1092 (Figs. F10, F11). The first occurrence (FO) of medium Gephyrocapsa (4-5.5 µm) approximates the Pliocene/Pleistocene boundary between 32.01 and 32.51 mcd (Fig. F10). The acme of E. huxleyi is recorded between 0.65 and 1.15 mcd (base of Subzone NN21b). The FO of E. huxleyi is present from 1.72 to 2.15 mcd (Subzone NN21a) in Hole 1092A, whereas it is recognized at the top of Hole 1092A (Table T4). The last occurrence (LO) of P. lacunosa is present between 2.75 and 3.31 mcd, defining the base of Zone NN20. The LO and FO of Reticulofenestra asanoi are recognized at Site 1092 from 7.55 to 8.69 and from 12.05 to 13.55 mcd, respectively. The reentrance of medium Gephyrocapsa (4-5.5 µm) is recognized from 8.69 to 10.90 mcd in Hole 1092B, although this event is not iden-tified in Hole 1092A because the species is almost absent above 14.0 mbsf (Table T4). The LO of Gephyrocapsa >5.5 µm is observed from 14.65 to 16.05 mcd, whereas the FO of this species is present from 24.54 to 25.54 mcd. The LO of Calcidiscus macintyrei is not identified at Site 1092 because the species is very rare and its stratigraphic range is discontinuous (Table T4; Fig. F10).
Characteristic late Pliocene assemblages are not observed at Site 1092. As a result, the standard biozones (NN16-18) are not identified because marker species such as Discoaster pentaradiatus, Discoaster brouweri, and Discoaster tamalis are absent. The LO of Reticulofenestra pseudoumbilicus (base of Zone NN16) is present from 61.39 to 62.39 mcd, and the FO of P. lacunosa (within Zones NN14-15; Rio et al., 1990) is recorded between 64.19 and 65.19 mcd (Fig. F10). The acme of small Gephyrocapsa is recognized between 62.39 and 64.19 mcd, close to the LO of R. pseudoumbilicus (Table T4). According to Marino (1994), the acme of small Gephyrocapsa approximates the early/late Pliocene boundary.
A transitional interval between the Pliocene and Miocene is defined between 70 and 75 mcd at Site 1092 (Figs. F10, F11). No calcareous nannofossil markers are identified that constrain the Miocene/Pliocene boundary. The closest middle/late Miocene boundary nannofossil event is the LO of Coccolithus miopelagicus (Zone NN8) that is present between 170.22 and 174.15 mcd, below the base of Chron 5n (156.04-157.04 mcd) (Table T4; Fig. F11). The presence of the last common occurrence (LCO) of Cyclicargolithus floridanus, the LO of Coronocyclus nitescens, and the LO of Calcidiscus premacintyrei in a short interval (between 176.73 and 179.85 mcd) suggests a hiatus of ~3 m.y. spanning the interval between Zones NN7 and NN5 (Fig. F10). The early/middle Miocene boundary is approximated by the FO of C. premacintyrei (Zone NN4) between 194.84 and 196.34 mcd. A lower Miocene assemblage is observed from below 196.34 mcd to the bottom of Hole 1092A. Reticulofenestra bisecta, a species whose disappearance was recognized at Site 1090 (23.9 Ma, base of Zone NN1), is absent in the deepest samples studied at Site 1092 (210.69 mcd). This indicates that the interval recovered is younger than 23.9 Ma (Zones NN1-2) (Table T4; Fig. F10).
The planktic foraminifer content is high in all studied core-catcher (CC) samples and the planktic foraminifers are, with only a few exceptions, well preserved. The planktic foraminifer fauna at Site 1092 can be divided into two major assemblages: a Pliocene-Pleistocene assemblage and a Miocene assemblage. The Pliocene-Pleistocene assemblage is dominated by Neogloboquadrina pachyderma (sinistral) with major contributions from Globigerina bulloides, Globigerina quinqueloba, Globorotalia puncticulata, and Globorotalia puncticuloides (Table T6, also in ASCII format in the TABLES directory). The Miocene assemblage is dominated by G. bulloides, Globorotalia scitula, and Neogloboquadrina continousa. The taxonomic concept of N. continuosa applied to the Miocene of Site 1092 includes four- and five-chambered specimens. Future shore-based work will show if this group may be subdivided into N. continuosa and Neogloboquadrina mayeri. Zonal subdivision is difficult because of the absence of several marker species; it is clear, however, that Sample 177-1092A-20H-CC, 13-18 cm (188.35 mbsf), is of early Miocene age (>17.3 Ma; Berggren et al., 1995) because of the presence of Catapsydrax dissimilis.
Benthic foraminifers at Site 1092 are generally abundant and vary from poor to good in their state of preservation. Well preserved Bolboforma specimens are found intermittently downhole from Sample 177-1092A-14H-CC, 10-15 cm (131.11 mbsf, 147.23 mcd), notably within the middle Miocene interval.
Benthic foraminifers typically constitute less than 5% of the total foraminifer fauna from the >63-µm fraction studied. Benthic foraminifer abundances are variable, reaching a maximum of 1265 specimens/cm3 in Sample 177-1092B-1H-CC, 7-12 cm (10.55 mcd) (Table T7, also in ASCII format in the TABLES directory). Higher abundances below ~170 mcd coincide with a general decrease in sediment accumulation rates throughout the middle Miocene (Fig. F11). No barren intervals occur, suggesting that a continuous benthic foraminifer isotopic record should be relatively easy to obtain from this site.
Quantitative estimates of relative
species abundance were made from Hole 1092A, with counts of up to 231 specimens/sample.
Species richness is variable, with a maximum of 37 taxa recorded in Sample
177-1092A-20H-CC, 13-18 cm, and a minimum of 13 taxa in Sample 177-1092A-3H-CC, 9-14 cm
(Table T7). Not all of this variability can be
accounted for by sample size (see "Biostratigraphy" in the "Explanatory Notes"
chapter), and a trend of increasing species richness is evident downhole (Table T7), notably in the early to middle Miocene. Such
trends in species richness have been noted previously (e.g., at ODP Sites 747, 748, and
751 by Mackensen, 1992) and coincide with deep-water benthic foraminifer 18O increases that have been
attributed to a combination of ice growth and deep-water cooling (e.g., Shackleton and
Kennett, 1975).
Three main benthic foraminifer assemblages are recognized at Site 1092, although there is clearly potential to further subdivide the assemblages after more detailed sampling.
Assemblage 1 is present from the mudline down to Sample 177-1092A-7H-CC, 15-20 cm (73.51 mcd), and is dominated by Globocassidulina subglobosa, Melonis barleeanum, Melonis pompiliodes, and Pullenia bulloides. The large, well-sorted foraminifer tests and high abundances throughout the upper 30 mcd at Site 1092 suggest that the Pleistocene sediments have undergone some winnowing.
Assemblage 2 is present between Samples 177-1092A-8H-CC, 11-16 cm (83.17 mcd), and 18H-CC, 0-7 cm (188.35 mcd), and is dominated by Cibicidoides mundulus, Epistominella exigua, and Gyroidinoides soldanii. Important additional taxa include Laticarinina pauperata, Pullenia subcarinata, Stilostomella lepidula, and Uvigerina hispidocostata. The LO of a single specimen of Rectuvigerina senni in Sample 177-1092A-13H-CC, 10-15 cm (135.77 mcd), indicates a middle Miocene age and does not fit well with the other stratigraphic evidence, which indicates a late Miocene age. Closer sampling in this interval will determine whether or not this age assignment for R. senni should be revised.
Abundant, well-preserved Bolboforma specimens are recorded in Samples 177-1092A-14H-CC, 10-15 cm (147.23 mcd), and 16H-CC, 14-19 cm (169.09 mcd), and are tentatively assigned to the B. compressispinosa Zone of Qvale and Spiegler (1989). The LCO of B. compressispinosa at Site 747 corresponds to an age of ~11.5 Ma (Mackensen and Spiegler, 1992), again suggesting that this interval at Site 1092 requires further investigation, because the other stratigraphic evidence clearly indicates younger ages.
Assemblage 3 is present in Samples 177-1092A-19H-CC and 20H-CC, and is dominated by S. lepidula, Martinotiella sp., G. subglobosa, G. soldanii, and C. mundulus. Additional taxa which are not observed uphole include Rectuvigerina multicosta, Bulimina cf. simplex, and an unidentified species of Melonis that is relatively abundant. The significant changes in the benthic foraminifer assemblages below ~190 mcd support the interpreted hiatus at this depth. An early Miocene age, however, is indicated by the co-occurrence of U. hispidocostata and Nuttallides umbonifera, similar to the middle early Miocene assemblages described by Mackensen (1992) from ODP Leg 120.
For biostratigraphic age assignments, we used the zonations proposed by Gersonde and Bárcena (1998) and Gersonde et al. (1998) for the last 2 m.y. and the Neogene, respectively. All diatom stratigraphic information from the four holes was combined and converted to the mcd scale (Tables T5, T8, T9; all also in ASCII format in the TABLES directory).
Diatoms are generally common to abundant and presentation is moderate to good in the top ~45 mcd, an interval that is Pleistocene in age. Below this level we observe strongly variable abundance and preservation ranging from rare to abundant and poor to good, respectively, until ~65 mcd. The following ~10-m-thick interval, representing the Pliocene/Miocene transition, contains generally well-preserved diatom assemblages with common to abundant diatoms. The calcareous Miocene sequences below ~75 contain only trace to rare amounts of diatoms. However, acid-cleaned samples from this interval contain moderate to well-preserved assemblages that are useful for biostratigraphic age assignment and paleo-environmental interpretation (Table T8). During examination of the diatom assemblages, we also encountered silicoflagellates in rare to trace numbers, as well as sporadic occurrences of Actiniscus species (Table T8).
Low sedimentation rates in the upper and middle Pleistocene intervals (~10 m/m.y.) and broad sample spacing did not allow us to subdivide the Thalassiosira lentiginosa Zone into its three subzones. The base of the T. lentiginosa Zone was identified at 4.81 mcd. Because Thalassiosira elliptipora, the marker taxon for the Actinocyclus ingens Subzone c, was observed only intermittently, we combined Subzones b and c of the A. ingens zone. The FO of Fragilariopsis barronii, which defines the boundary between Subzones a and b of the A. ingens Zone, is noted at 21.74 mcd. Assemblages assigned to the upper Pliocene Proboscia barboi Zone, which corresponds with the Olduvai Subchron, were found between 35.8 and 38.3 mcd. Below this interval, the Thalassiosira kolbei-Fragilariopsis matuyamae Zone, which ranges from 2 to 2.5 Ma, is recognized to 49.1 mcd. The sediments representing the lower Pleistocene A. ingens Zone and the upper Pliocene P. barboi and T. kolbei-F. matuyamae Zones were deposited at a significantly higher sedimentation rate (~29 m/m.y.) than the mid- and upper Pleistocene sequences (Fig. F11). The intervals 49.1-50.1 and 50.1-54.2 mcd are marked by lower sedimentation rates, corresponding with the Thalassiosira vulnifica and the Thalassiosira insigna Zones, respectively. However, the presence of one or more hiatuses cannot be ruled out. Hiatuses punctuating this time interval have also been observed at Site 1091 (see "Chronostratigraphy" in the "Site 1091" chapter). The FO of Fragilariopsis interfrigidaria, which defines the base of the F. interfrigidaria Zone, was identified at 64.3 mcd. This zone straddles the early/late Pliocene boundary and has its base at 3.8 Ma. The diatom assemblages found below the F. interfrigidaria Zone to ~71 mcd do not allow a clear biostratigraphic age assignment. The nominate taxon of the underlying Fragilariopsis barronii Zone, which has its FO at 4.4 Ma, was not encountered in the samples studied. However, Thalassiosira inura, which has its FO at ~4.8 Ma, was noted to a depth of 67.2 mcd. This could be interpreted as indicating the base of the T. inura Zone at that depth. Alternatively, it might indicate the presence of a hiatus between the sediments assigned to the F. interfrigidaria and the T. inura Zones. We could not identify the underlying Thalassiosira oestrupii Zone, which straddles the Pliocene/Miocene boundary, because the nominate taxon whose LO defines the base of this zone was not present. However, the assemblages between 67.2 and 75.5 mcd contain taxa such as Fragilariopsis praeinterfrigidaria, F. reinholdii, F. aurica, and F. clementia, all known to occur in the earliest Pliocene and the latest Miocene. The uppermost portion of the F. reinholdii Zone, which is below the T. oestrupii Zone, is marked by the LO of Hemidiscus triangularus, which was placed by Harwood and Maruyama (1992) below the Miocene/Pliocene boundary at ~5.9 Ma. This taxon, which has a distinctive morphology, was found in Samples 177-1092C-8H-CC, 13-20 cm (75.45 mcd), and 177-1092A-8H-1, 120-122 cm (75.65 mcd). These samples are also characterized by common occurrences of fragments of the large diatom Neobrunia mirabilis. A similar assemblage was reported from ODP Site 701 (Shipboard Scientific Party, 1988), from a ~30-m-thick sedimentary interval tentatively correlated with the middle portion of late Miocene Chron C3A (Clement and Hailwood, 1991). This indicates that the Miocene/Pliocene transition falls in the interval between 67.2 and 75.6 mcd and is marked by one or more hiatuses, which is also indicated by magnetostratigraphy (see "Paleomagnetism").
The FO of F. reinholdii, the nominate taxon of the latest Miocene diatom zone, was found in a sample at 86.04 mcd. This interval was tentatively placed in a normal polarity interval of Chron C4, probably C4n.2n, which ranges from 7.65 to 8.07 Ma (see "Paleomagnetism"). This suggests that the FO of F. reinholdii at Site 1092 is not at ~6.4 Ma as proposed by Harwood and Maruyrama (1992) for southern high-latitude areas, but it is close to its FO in the equatorial Pacific at 8.2 Ma, a datum reported by Barron (1992). This is supported by the proximity of the FO of Actinocyclus ingens var. ovalis and FO of F. reinholdii at ~86.8 mcd. The datum of the FO of A. ingens var. ovalis has been placed in the reversed polarity interval of Chron C4 (Gersonde and Burckle, 1990). However, because of the broad spacing of acid-cleaned samples available for shipboard investigations, we have not been able to accurately identify the range of the FO of A. ingens var. ovalis at Site 1092. The same is true for the diatom age assignments for the Miocene interval below 90 mcd. Despite the broad sample spacing, it was possible to identify diatom zones in relation to the magnetostratigraphy. The base of the early late Miocene Asteromphalus kennettii and Denticulopsis hustedtii Zones was placed at ~137 and 151 mcd on the basis of the FO of A. kennettii and the LO of Denticulopsis dimorpha, respectively. The underlying D. dimorpha Zone was identified based on the presence of D. meridionalis and the abundant to dominant occurrence of the nominate species between ~150 and ~172 mcd. According to Harwood and Maruyama (1992), D. meridionalis is restricted to the upper portion of the D. dimorpha Zone. The age assignment on the basis of the three late Miocene diatom zones agrees well with the paleomagnetic correlation of these zones with Chrons C4A and C5 see "Biostratigraphy" in the "Explanatory Notes" chapter).
Below the interval assigned to the D. dimorpha Zone, we could not find markers of the late middle Miocene Denticulopsis praedimorpha-Nitzschia denticuloides and D. praedimorpha Zones. This indicates the presence of a hiatus that spans a time interval from the lower D. dimorpha Zone, close to the late/middle Miocene boundary, to the middle Miocene time period represented by the N. denticuloides and the Denticulopsis hustedtii-Nitzschia grossepunctata Zones. This interpretation agrees with the paleomagnetic stratigraphy as well as the radiolarian and calcareous nannofossil biostratigraphic age assignments. Physical properties measurements indicate distinct changes in resistivity and blue reflectance (450-550 nm) at 178-179 mcd, which may indicate a hiatus in the record of Site 1092 (Fig. F16).
Below the hiatus in the upper portion of the middle Miocene, diatoms indicate ages in the N. denticuloides, D. hustedtii-N. grossepunctata, and Actinocyclus ingens var. nodus Zones to a depth of 187 mcd. These zones represent the middle and lower portion of the middle Miocene between ~12.8 and ~14.4 Ma. They are underlain by sediments assigned to the lowermost middle Miocene A. ingens-Denticulopsis maccollumii and D. maccollumii Zones, which straddle the middle/early Miocene boundary between ~192.5 and ~194 mcd. This assignment is based on the presence of the nominate taxa. The co-occurrence of Thalassiosira fraga, Nitzschia maleinterpretaria, and Azpeitia tabularis between 202.5 and 209 mcd, places this interval in the middle T. fraga Zone, from ~18.3 to 19 Ma. The broad spacing of acid-cleaned samples makes it difficult to determine if the lower Miocene record is punctuated by hiatuses, or if this section was deposited at low sedimentation rates.
Radiolarian biostratigraphy at Site 1092 is based on the examination of 34 CC samples (Table T10, also in ASCII format in the TABLES directory). Radiolarians at Site 1092 are highly variable both in abundance and preservation. Samples from above ~80 mcd yield well-preserved and abundant radiolarian assemblages, whereas those from the lower portion generally contain poorly to moderately preserved radiolarian assemblages of rare to common abundance. Radiolarian assemblages indicate that the recovered sequence is Pleistocene to early Miocene in age and that hiatuses occur at ~70 and ~180 mcd.
The uppermost sample (177-1092C-1H-CC, 7-12 cm [3.99 mcd]), is correlative to the Psi Zone based on the presence of Stylatractus universus. The boundary between the Psi and underlying Chi Zone can be placed above Sample 177-1092B-1H-CC, 0-7 cm (7.90 mcd). The base of the Chi Zone is placed at ~39 mcd (Table T9). Below the base of the Chi Zone, Eucyrtidium calvertense, diagnostic of the Phi Zone, and Helotholus vema, defining the Upsilon Zone, co-occur to a depth of ~63 mcd. This suggests reworking of H. vema into the Phi Zone, so that the distinction of the Phi and Upsilon Zones is not possible at Site 1092. The latest Miocene to earliest Pliocene Tau Zone, defined by the absence of H. vema and Amphymenium challengerae, is not recognized. This agrees with the diatom evidence that indicates a hiatus in this interval. Sample 177-1092A-7H-CC, 15-20 cm (73.51 mcd), contains few specimens of A. challengerae, which has a short range from 6.10 to 6.58 Ma. This sample is, therefore, correlative to the late Miocene Amphymenium challengerae Zone, and the base of the zone at 6.58 Ma can be placed at 78.34 mcd (Table T9). Zonal assignment of Samples 177-1092A-8H-CC through 177-1092C-10H-CC (80.11-97.42 mcd), characterized by the co-occurrence of Lamprocyclas aegles and Stichocorys peregrina, remains uncertain because of the lack of index species. Samples 177-1092A-10H-CC, 15-20 cm (104.17 mcd), and 177-1092B-13H-CC, 10-15 cm (126.97 mcd), yield Cycladophora spongo-thorax, and Sample 177-1092C-14H-CC, 15-20 cm (142.74 mcd), contains Acrosphaera australis. Thus, the interval from 104.17 to 142.74 mcd can be assigned to the late Miocene Acrosphaera australis Zone. The underlying interval from Sample 177-1092A-14H-CC, 10-15 cm (147.23 mcd), to Sample 177-1092B-17H-CC, 23-28 cm (174.82 mcd), is characterized by continuous occurrences of Cyrtocapsella japonica, which is common in middle Miocene strata of the northwestern Pacific and was also found at Site 1088. Two samples in this interval, 177-1092A-16H-CC, 14-19 cm (169.09 mcd), and 177-1092B-17H-CC, 23-28 cm (174.82 mcd), contain Actinomma golownini, which ranges in age from 13.61 to 10.77 Ma and suggests that this interval is correlative to the middle Miocene Actinomma golownini Zone or middle-late Miocene Cycladophora spongothorax Zone. The interval from 184.56 to 210.69 mcd, below Sample 177-1092B-18H-CC, 25-30 cm, is characterized by the presence of Cycladophora golli regipileus, Cyrtocapsella longithorax, and C. tetrapera. These species indicate that the interval is of early Miocene age, younger than 19.11 Ma, and possibly correlative to the Cycldophora golli regipileus Zone. As a result, it is possible to infer the presence of a hiatus from the early middle to late early Miocene, in agreement with other biostratigraphic data (Fig. F10).
The radiolarian assemblages from the Pleistocene Psi Zone to the late Miocene Amphymenium challegerae Zone are composed of characteristic species of the Antarctic region. However, those from the middle to upper Miocene interval below the Amphymenium challegerae Zone are different from hitherto known Antarctic faunas. These assemblages are composed of the genus Didymocyrtis and Cyrtocapsella japonica, indicating warmer conditions than are typical for the Antarctic region.
Archive halves of APC cores recovered at Site 1092 were measured using the shipboard pass-through magnetometer. Measurements were made at 5-cm intervals. Sections obviously affected by drilling disturbance were not measured. Hole 1092A was measured after alternating-field demagnetization at peak fields of 0 (natural remanent magnetization [NRM]), 5, 10, 15, 20, and 25 mT. Holes 1092B and 1092D were measured after peak fields of 0, 10, 20, and 25 mT. Hole 1092C was measured after peak fields of 0, 10, and 20 mT.
At Site 1092, NRM intensities vary around 5 × 10-4 A/m for most of the cored intervals. Two intervals of higher intensities were found between 42 and 53 mbsf (~3 × 10-3 A/m) and below 165 mbsf in Hole 1092A (~2 × 10-3 A/m).
The inclination records are highly discontinuous in the upper 60 mbsf at all holes due to drilling-induced disturbance in the poorly consolidated nannofossil oozes. Below 70 mbsf in Holes 1092A-1092C, the inclination records indicate relatively well-defined polarity zones to a depth of 150 mbsf (Fig. F12; Table T5). The tentative polarity interpretation given in Figure F12 is based on the records from Holes 1092A and 1092D. However, unambiguous interpretations of the exact depth of polarity transitions cannot be made from shipboard data. Therefore, correlation of polarity zones to the geomagnetic polarity time scale must await detailed shore-based magnetic and biostratigraphic studies to confirm the exact position and identity of the polarity transitions.
A 210.74-mcd thick Pleistocene to early Miocene record was recovered at Site 1092. Holes 1092A-1092D were cored with the APC to 188.5, 168.9, 165.5, and 64.9 mbsf, respectively. Hole 1092D is a spot core drilled to ensure a continuous sedimentary section in the upper part of the cored interval. The combined MST and color reflectance data provide a nearly continuous section to 188 mcd (base of Core 177-1092A-18H) (Figs. F6, F7, F8).
All biostratigraphic datums, including calcareous nannofossil, diatom, and radiolarian events, and available magnetostratigraphic interpretations yield consistent age assignments throughout the record at Site 1092. The sedimentation rates in the carbonate-dominated sequences range between ~10 and ~29 m/m.y. for the Pliocene-Pleistocene, and between ~4 and ~38 m/m.y. for the Miocene (Fig. F11; Table T9). The upper and mid-Pleistocene sediments are restricted to the upper 5 mcd and were deposited at 10 m/m.y. This low sedimentation rate might be related to the occurrence of short hiatuses and/or sediment winnowing, as indicated by the presence of well-sorted foraminifer assemblages. A distinct increase in sedimentation rates is observed to a depth of ~45 mcd, and can be related to sediments of early Pleistocene and latest Pliocene age. In the late Pliocene (below 45 mcd), estimated sedimentation rates drop to ~12 m/m.y (Fig. F11B). A similar Pleistocene-upper Pliocene sedimentation pattern was also observed at Sites 1090 and 1091. The early Pliocene is disturbed by a hiatus at ~65 mcd, which spans approximately from 3.8 to 4.6 Ma (Figs. F10, F11). Disturbance caused by one or more hiatuses is also observed in the sediments spanning the Pliocene/Miocene boundary. For this reason, a preliminary Miocene/Pliocene boundary has been placed in a transition zone between 70 and 75 mcd. Below ~80 mcd, the magnetostratigraphic data show distinct variations in magnetic inclination that have been interpreted using the biostratigraphic record (Fig. F10). Resulting sedimentation rates are ~15 m/m.y. between ~7 and 10 Ma, and increase to ~38 m/m.y. in the early late Miocene (Fig. F11). A hiatus at ~178 mcd, which spans the earliest late Miocene to middle Miocene time interval, lasted from ~11 to 13 Ma. The middle and lower Miocene sediments below this hiatus were deposited at an average rate of 4 m/m.y. However, it is possible that one or more hiatuses punctuate the early middle and early Miocene record at Site 1092. The base of Site 1092 is in the lower Miocene, representing an age of ~19 Ma.
The sedimentation pattern at Site 1092 closely resembles that observed at Site 704, located only 61 km to the southeast of Site 1092. However, the sedimentation rates at Site 1092 are considerably lower throughout the entire section. Late and mid-Pleistocene sedimentation rates at Site 704 are ~45 m/m.y. and increase to as high as ~100 m/m.y. in the early Pleistocene and latest Pliocene (Hailwood and Clement, 1991). At Site 1092, we observe an increase from ~10 to ~29 m/m.y. during this time period. Between the middle late Pliocene and the base of the early Pliocene, sedimentation rates are ~20 m/m.y. at Site 704, whereas the incomplete record at Site 1092 precludes the estimation of sedimentation rates at present. At both Sites 704 and 1092, the Pliocene/Miocene transition is disturbed by one or more hiatuses. Sedimentation rates at Site 704 average 27 m/m.y. for the late and middle late Miocene and increase to ~48 m/m.y. in the early late Miocene. A similar two-fold increase of sedimentation rate was observed at Site 1092, where late-middle late Miocene and early late Miocene rates are ~15 and ~38 m/m.y., respectively. At both sites, a prominent hiatus is found in the middle Miocene. However, sedimentation rates in the early middle and early Miocene at Site 704 (~30 m/m.y.) are distinctly higher than those calculated for Site 1092 (~4 m/m.y.). This might be further indication that the oldest record at Site 1092 is also disturbed by one or more hiatuses.
