Diatom isotope data from ODP Hole 178-1098A for the last deglaciation (12.2-12.8 ka BP)

Understanding the response of the Antarctic ice sheets during the rapid climatic change that accompanied the last deglaciation has implications for establishing the susceptibility of these regions to future 21st Century warming. A unique diatom d18O record derived from a high-resolution deglacial seasonally laminated core section off the west Antarctic Peninsula (WAP) is presented here. By extracting and analysing single species samples from individual laminae, season-specific isotope records were separately generated to show changes in glacial discharge to the coastal margin during spring and summer months. As well as documenting significant intra-annual seasonal variability during the deglaciation, with increased discharge occurring in summer relative to spring, further intra-seasonal variations are apparent between individual taxa linked to the environment that individual diatom species live in. Whilst deglacial d18O are typically lower than those for the Holocene, indicating glacial discharge to the core site peaked at this time, inter-annual and inter-seasonal alternations in excess of 3 per mil suggest significant variability in the magnitude of these inputs. These deglacial variations in glacial discharge are considerably greater than those seen in the modern day water column and would have altered both the supply of oceanic warmth to the WAP as well as regional marine/atmospheric interactions. In constraining changes in glacial discharge over the last deglaciation, the records provide a future framework for investigating links between annually resolved records of glacial dynamics and ocean/climate variability along the WAP.

(Table T1) Abundance of radiolarian skeletons in scrape samples of ODP Hole 178-1098B

Palmer Deep is a series of three glacially overdeepened basins on the Antarctic Peninsula shelf, ~20 km southwest of Anvers Island. Site 1098 (64°51.72'S, 64°12.48'W) was drilled in the shallowest basin, Basin I, at 1012 m water depth. The sediment recovered was primarily laminated, siliceous, biogenic, pelagic muds alternating with siliciclastic hemipelagic sediments (Barker, Camerlenghi, Acton, et al., 1999). Sedimentation rates of 0.1725 cm/yr in the upper 25 m and 0.7-0.80 cm/yr in the lower 25 m of the core have been estimated from 14C (Domack et al., 2001). The oldest datable sediments have an age of ~13 ka and were underlain by diamicton sediments of the last glacial maximum (Domack et al., 2001). The large-scale water-mass distribution and circulation in the vicinity of Palmer Deep is dominated by Circumpolar Deep Water (CDW) below 200 m (Hofmann et al., 1996). Palmer Deep is too far from the coast to be influenced by glacial meltwater and cold-tongue generation associated with it (Domack and Williams, 1990; Dixon and Domack, 1991). Circulation patterns in the Palmer Deep area are not well understood, but evidence suggests southward flow across Palmer Deep from Anvers Island to Renaud Island (Kock and Stein, 1978). The water south of Anvers Island is nearly open with loose pack ice from February through May. The area is covered with sea ice beginning in June (Gloersen et al., 1992; Leventer et al., 1996). Micropaleontologic data from the work of Leventer et al. (1996) on a 9-m piston core has revealed circulation and climate patterns for the past 3700 yr in the Palmer Deep. The benthic foraminifer assemblage is dominated by two taxa, Bulimina aculeata and Bolivina pseudopunctata, which are inversely related. High relative abundances of B. aculeata occur cyclically over a period of ~230 yr. The assemblage associated with high abundance of B. aculeata in Palmer Deep resembles that from the Bellingshausen shelf, which is associated with CDW. In addition to the faunal evidence, hydrographic data indicate incursions of CDW into Palmer Deep (Leventer et al., 1996). A distinctive diatom assemblage dominated by a single genus was associated with peaks in B. aculeata, whereas a few different assemblages were associated with lows in B. aculeata. Leventer et al. (1996) interpreted the variability in diatom assemblages as an indication of changes in productivity associated with changes in water column stability. Abelmann and Gowing (1997) studied the horizontal and vertical distributions of radiolarians in the Atlantic sector of the Southern Ocean. They show that the spatial distribution of radiolarian assemblages reflects hydrographic boundaries. In a transect from the subtropical Atlantic to polar Antarctic zones, radiolarians in the upper 1000 m of the water column occurred in distinct surface and deep-living assemblages related to water depth, temperature, salinity, and nutrient content. Living assemblages resembled those preserved in underlying surface sediments (Abelmann and Gowing, 1997). Circumantarctic coastal sediments from neritic environments contained a distinctive assemblage dominated by the Phormacantha hystrix/Plectacantha oikiskos group and Rhizoplegma boreale (Nishimura et al., 1997). Low diversity and species compositions distinguished the coastal sediments from the typical pelagic Antarctic assemblages. Factors that controlled the assemblages were water depth, proximity to the coast, occurrence of sea ice, and steepness of topography, rather than temperature and salinity. Nishimura et al. (1997) found a gradient of sorts from deep-water sites containing diverse assemblages typical of pelagic environments to coastal sites with low diversity assemblages dominated by P. hystrix/P. oikiskos group and R. boreale. In general, sites between these two extremes had increased proportions of the coastal assemblage with decreasing water depth (Nishimura et al., 1997). At a site near Hole 1098 (GC905), they showed that the relative abundance of the coastal assemblage increased downcore (Nishimura et al., 1997). The purpose of the research presented here was to make a cursory investigation into the radiolarian assemblages as possible paleoenvironmental indicators.

(Table T1) Iridium concentrations in selected sediment samples of ODP Site 178-1096

Concentrations of Ir have been measured in 87 sediment samples from Ocean Drilling Program Site 1096 in search of evidence of fallout from the impact of the Eltanin asteroid, which occurred at 2.15 Ma, ~1300 km northwest of the site. An additional six samples were measured from a unique sand layer and adjacent sediments that are dated at ~1.6 Ma. These 93 sediment samples are all silts and muds that were deposited on a continental rise drift of the Antarctic Peninsula. No evidence of the Eltanin impact deposit was found in this study.

Occurrence of microtextures recorded from quartz sand grains of ODP Hole 178-1101A (Table 1)

Sediment drifts on the continental rise are located proximal to the western side of the Antarctic Peninsula and recorded changes in glacial volume and thermal regime over the last ca. 15 m.y. At Ocean Drilling Program (ODP) Site 1101 (Leg 178), which recovered sediments back to 3.1 Ma, glacial-interglacial cyclicity was identified based on the biogenic component and sedimentary structures observed in X-radiographs, magnetic susceptibility and lithofacies descriptions. Glacial intervals are dominated by fine-grained laminated mud and interglacial units consist of bioturbated muds enriched in biogenic components. From 2.2 to 0.76 Ma, planktonic foraminifera and calcareous nannofossils dominate in the interglacials suggesting a shift of the Antarctic Polar Front (APF) to the south near the drifts. Prior to 2.2 Ma, cyclicity cannot be identified and diatoms dominate the biogenic component and high percent opal suggests warmer conditions south of the APF and reduced sea ice over the drifts. Analyses of the coarse-grained terrigenous fraction (pebbles and coarse sand) from Sites 1096 and 1101 record glaciers at sea-level releasing iceberg-rafted debris (IRD) throughout the last 3.1 m.y. Analyses of quartz sand grains in IRD with the scanning electron microscope (SEM) show an abrupt change in the frequency of occurrence of microtextures at ~1.35 Ma. During the Late Pliocene to Early Pleistocene, the population of quartz grains included completely weathered grains and a low frequency of crushing and abrasion, suggesting that glaciers were small and did not inundate the topography. Debris shed from mountain peaks was transported supraglacially or englacially allowing weathered grains to pass through the glacier unmodified. During glacial periods from 1.35-0.76 Ma, glaciers expanded in size. The IRD flux was very high and dropstones have diverse lithologies. Conditions resembling those at the Last Glacial Maximum (LGM) have been episodically present on the Antarctic Peninsula since ~0.76 Ma. Quartz sand grains show high relief, fracture and abrasion common under thick ice and the IRD flux is low with a more restricted range of dropstone lithologies.

(Table 2) U-channel paleomagnetic record of ODP Site 178-1098

Thick Holocene sedimentary sections (>45 m) cored in the Palmer Deep by the United States Antarctic Program (USAP) and during Ocean Drilling Program (ODP) Leg 178 provide the first opportunity to examine past geomagnetic field behavior at high southern latitudes. After removal of a low-coercivity drilling overprint the sediments display a stable, single-component remanent magnetization. Two short cores that recovered the uppermost 2.6 m of sediment have inclinations that fluctuate about the present day inclination (-57°) measured at Faraday Station, and several features with wavelengths of 10 to 20 cm appear to be correlative. However, shipboard measurements of inclination fluctuations on split-core samples from three holes drilled at ODP Site 1098 do not correlate well with each other, even though the intensity and susceptibility data correlate very well and the overall mean inclination for cores from each hole is consistent with the expected geocentric axial dipole (GAD) inclination. The correlation is improved dramatically by using inclinations measured on u-channels taken from the pristine center of a split core. Consequently, the anomalous directions and the resulting poor between-hole correlation of inclinations obtained from shipboard data can be attributed to coring-induced deformation, which is common on the outer edge of ODP piston cores, and/or measurement artifacts in the split-core data. Our preferred inclination record is thus derived from u-channel results. The upper ~25 m represents continuous sedimentation over the past 9000 yr, with an average sedimentation rate exceeding 250 cm/kyr (0.25 cm/yr). Given that remanence measurements on u-channels average over an interval <7 cm long, we obtained independent measurements of the paleo-geomagnetic field that average over only ~30 yr. This high-resolution record is characterized by an inclination that fluctuates within +/-15° of the current GAD inclination.