Distribution of foraminifera and other components in ODP Leg 188 sites

During Leg 188 of the Ocean Drilling Program (ODP), employing JOIDES Resolution, we drilled holes at three sites in the southern Indian Ocean in and near Prydz Bay, East Antarctica, between 28 January and 29 February 2000. The objectives of the voyage were to: - Core through sediments deposited when Antarctica underwent the transition from "greenhouse" to the modern "icehouse" state late in the Eocene or early in the Oligocene, at sites obtaining their sediment from the currently subglacial Gamburtsev Mountains that probably were the site of nucleation of the ice sheet (principally Site 1166); - Obtain a sediment record from times at which major changes in the ice sheet volume and characteristics took place as judged from oxygen isotope records, especially at ~23.7 Ma (Oligocene/Miocene boundary), 12-16 Ma (middle Miocene), and 2.7 Ma (late Pliocene) (mainly Site 1165); and - Sample through the upper Pliocene and Quaternary in an attempt to document fluctuations in the extent of the ice sheet over the continental shelf during the Quaternary (especially Site 1167). Paleogene foraminifer-bearing marine sections were not intersected, and thus discussion of marine sections is restricted to the Neogene. Foraminifers are not major contributors to Leg 188 chronostratigraphy but contribute to paleoenvironmental interpretation, to issues such as carbonate compensation depth (CCD) effects and source and history of sediment, and provide a basis for Sr and d18O studies. Chronostratigraphy for the various sections was compiled from diatoms, radiolarians, and paleomagnetism (Shipboard Scientific Party, 2001, doi:10.2973/odp.proc.ir.188.101.2001). Foraminifers were sporadic rather than continuous except in short intervals; however, the Neogene foraminifers from the region are very poorly known and the new records proved to be of significant value in paleoenvironmental interpretation. Only at Site 1167 did drilling intersect a section that yielded foraminifers virtually throughout. Other than for the very young section at each site, there is virtually no continuity of assemblages between sites and thus each section is treated here as separate and unrelated.

Diatom datums and abundance in ODP Hole 188-1165B and 188-1166A sediments

The biostratigraphic distribution and qualitative relative abundance of Quaternary-Pliocene diatoms from Ocean Drilling Program Leg 188, Sites 1165 (64.380°S, 67.219°E) and 1166 (67.696°S, 74.787°E) offshore from East Antarctica, are documented in this report. The upper ~50 meters below seafloor (mbsf) of Hole 1165B consists of brown diatom-bearing silty clay spanning the upper Pleistocene to lower Pliocene. The diatom stratigraphy indicates a disconformity at ~17.1 mbsf of 0.5- to 0.6-m.y. duration. The integration of biostratigraphic and magnetostratigraphic data identified other disconformities at ~6.0, 14.4, 15.6, and 16.0 mbsf, but the duration of these hiatuses cannot be resolved through diatom biostratigraphy. In Hole 1166A, a narrow interval of diatomaceous Quaternary sediment is identified in the upper 2.92 mbsf and dated biostratigraphically at <0.38 Ma. The remaining Quaternary-Pliocene section is dominated by diamicton, except at ~114 mbsf, where two thin diatomaceous beds are present. The lower bed is ~65 cm thick, 2.5-2.7 to 2.7-3.2 Ma in age, and possibly disconformably overlain by the upper bed, which is ~15 cm thick and 1.8-2.0 to 2.1-2.5 Ma in age. The Pliocene assemblages in Hole 1166A contain components of both Southern Ocean and Antarctic continental shelf (Ross Sea) diatom floras.

Sulphate and d34S in pore waters and d13C, d18O and mineral composition of carbonate nodules from ODP Leg 188 sites

Carbon and oxygen isotopic compositions of authigenic carbonate nodules or layers reflect the diagenetic conditions at the time of nodule growth. The shallowest samples of carbonate nodules and dissolved inorganic carbon of pore water samples beneath the sulfate reduction zone (0-160 meters below seafloor [mbsf]) at Site 1165 have extremely negative d13C values (-50 per mil and -62 per mil, respectively). These negative d13C values indicate nodule formation in association with anaerobic methane oxidation coupled with sulfate reduction. The 34S of residual sulfate at Site 1165 shows only minor 34S enrichment (+6 per mil), even with complete sulfate reduction. This small degree of apparent 34S enrichment is due to extreme "open-system" sulfate reduction, with sulfate abundantly resupplied by diffusion from overlying seawater. Ten calcite nodules from Site 1165 contain minor quartz and feldspar and have d13C values ranging from -49.7 per mil to -8.2 per mil. The nodules with the most negative d13C values currently are at depths of 273 to 350 mbsf and must have precipitated from carbonate largely derived from subsurface anaerobic methane oxidation. The processes of sulfate reduction coupled with methane oxidation in sediments of Hole 1165B are indicated by characteristic concentration and isotopic (d34S and d13C) profiles of dissolved sulfate and bicarbonate. Three siderite nodules from Site 1166 contain feldspar and mica and one has significant carbonate-apatite. The siderite has d13C values ranging from -15.3 per mil to -7.6 per mil. These siderite nodules probably represent early diagenetic carbonate precipitation during microbial methanogenesis.

Ice-rafted debris in ODP Holes 177-1090D and 188-1165B

Constraining the nature of Antarctic Ice Sheet (AIS) response to major past climate changes may provide a window onto future ice response and rates of sea level rise. One approach to tracking AIS dynamics, and differentiating whole system versus potentially heterogeneous ice sheet sector changes, is to integrate multiple climate proxies for a specific time slice across widely distributed locations. This study presents new iceberg-rafted debris (IRD) data across the interval that includes Marine Isotope Stage 31 (MIS 31: 1.081-1.062 Ma, a span of ~19 kyr; Lisiecki and Raymo, 2005), which lies on the cusp of the mid-Brunhes climate transition (as glacial cycles shifted from ~41,000 yr to ~100,000 yr duration). Two sites are studied - distal Ocean Drilling Program (ODP) Leg 177 Site 1090 (Site 1090) in the eastern subantarctic sector of the South Atlantic Ocean, and proximal ODP Leg 188 Site 1165 (Site 1165), near Prydz Bay, in the Indian Ocean sector of the Antarctic margin. At each of these sites, MIS 31 is marked by the presence of the Jaramillo Subchron (0.988-1.072 Ma; Lourens et al., 2004) which provides a time-marker to correlate these two sites with relative precision. At both sites, records of multiple climate proxies are available to aid in interpretation. The presence of IRD in sediments from our study areas, which include garnets indicating a likely East Antarctic Ice Sheet (EAIS) origin, supports the conclusion that although the EAIS apparently withdrew significantly over MIS 31 in the Prydz Bay region and other sectors, some sectors of the EAIS must still have maintained marine margins capable of launching icebergs even through the warmest intervals. Thus, the EAIS did not respond in complete synchrony even to major climate changes such as MIS 31. Further, the record at Site 1090 (supported by records from other subantarctic locations) indicates that the glacial MIS 32 should be reduced to no more than a stadial, and the warm interval of Antarctic ice retreat that includes MIS 31 should be expanded to MIS 33-31. This revised warm interval lasted about 52 kyr, in line with several other interglacials in the benthic d18O records stack of Lisiecki and Raymo (2005), including the super-interglacials MIS 11 (duration of 50 kyr) and MIS 5 (duration of 59 kyr). The record from Antarctica-proximal Site 1165, when interpreted in accord with the record from ANDRILL-1B, indicates that in these southern high latitude sectors, ice sheet retreat and the effects of warming lasted longer than at Site 1090, perhaps until MIS 27. In the current interpretations of the age models of the proximal sites, ice sheet retreat began relatively slowly, and was not really evident until the start of MIS 31. In another somewhat more speculative interpretation, ice sheet retreat began noticeably with MIS 33, and accelerated during MIS 31. Ice sheet inertia (the lag-times in the large-scale responses of major ice sheets to a forcing) likely plays an important part in the timing and scale of these events in vulnerable sectors of the AIS.