Developing Ultra High-Resolution Foraminiferal Biofacies and Stable Isotope (δ13C and δ18O) Records from Planktonic and Benthic Foraminifers from IODP Expedition 318: Wilkes Land, Hole U1357A

Ultra-high resolution (~6 yr) foraminiferal assemblage and δ¹³C and δ¹⁸O records from planktonic and benthic foraminifers are being developed for the upper 90 meters of IODP Expedition 318 Hole U1357A to investigate paleoceanographic changes that occurred during the last 6,000 years off of Antarctica. Site U1357 is located ~50 km off the Adélie coastline of East Antarctica (66°24.8‧S, 140°25.7‧E) in the Adélie Trough, which is a glacially scoured valley on the continental shelf. An exceptional sedimentary archive of Holocene changes was triple cored during the expedition penetrating nearly 190 mbsf. Carbon-14 age dates from U1357A provide a firm chronology, suggesting an average sedimentation rate of nearly 2-cm/ yr, with the base of the core dated as approximately 11,000 years old. Samples at 10 cm spacing were collected during the expedition for isotopic and foraminiferal studies. During the Late Holocene (0-3.0 kya), foraminiferal assemblages are generally characterized by low diversity, low abundances and are dominated by agglutinated forms. This is interpreted to be the result of lower oxygen and more corrosive bottom waters, suggesting weaker bottom water circulation in the Adélie Trough. These intervals with high abundances of agglutinated forms correlate well with increased terrigenous input throughout both Hypsithermal and Neoglacial periods. During the Hypsithermal (3.5-5.5 kya), foraminiferal assemblages are more diverse, have higher abundances, and are dominated by calcareous forms such as Epistominella spp. and Quinqueloculina spp. Planktonic foraminiferal abundances are also highest during this time interval. Taken together these results indicate higher oxygen levels and less corrosive bottom waters occurred during this time, suggesting stronger bottom water circulations bathed the trough. Results thus far indicate that planktonic foraminifers occur in nearly 70% of the samples below 12 mbsf, suggesting bottom water conditions were generally conducive for the preservation of the calcareous tests. Nearly all planktonic foraminifers were dominated (as expected) by N. pachyderma (sinstral). The size distribution of N. pachyderma specimens were trimodal, with a majority being relatively large (> 250 μm), with two groups consisting of small (63-125 μm in diameter) specimens and a mid size ranged (150 and < 250 μm). In this study, only the large subset of N. pachyderma were used for the stable isotopic records. Preliminary isotopic results indicate generally heavy δ¹⁸O values for the Mid-Holocene Climatic Optimum (5-6 kya) and lighter for the Neoglacial (2 kya). We interpret this as a possible decrease in iceberg melt waters during the Hypsithermal. Lighter δ¹³C values occurred during the Hypsithermal than in the Neoglacial, which could be due to either lower productivity or increased surface to deep water mixing during the Hypsithermal.