Robert B. Dunbar, Stanford University
Department of Geological and Environmental Sciences
Stanford CA 94305-2115

The Antarctic Peninsula is highly sensitive to climate change and is currently experiencing rapid and unusual warming. In 1998, the Ocean Drilling Program triple-cored the Palmer Deep (site 1098: 64°51'S, 64°12'W), a large depression off the west coast of the Antarctic Peninsula, to examine Holocene oceanographic variability. More than 45 m of diatomaceous muds and oozes and muddy diamictons were recovered, comprising the first high resolution, continuous, Late Pleistocene through Holocene sediment record from the Antarctic continental margin. As part of a multi-proxy analysis of the site 1098 cores, we analyzed biogenic opal, organic C and N concentrations, and ¹³C/¹²C and ¹⁵N/¹⁴N isotopic ratios of sedimentary organic matter every 2.5 cm downcore (1600 samples, sample interval of about 8 years). Our chronology is based on 36 AMS radiocarbon dates. The upper 42 meters of core spans approximately the past 12,500 years with sedimentation rates ranging from 0.3 to 1 cm/year. Biogenic opal contents range from 2 to 10% during the deglacial section of the core (>11,500 yr BP) and vary from 15 to 40% in the section younger than about 8500 yr BP. Opal content varies significantly at several timescales, including 70, 140, and 220 years. Large-scale rhythmicity with a period of about 1600-2000 years is also observed throughout the Holocene. Sedimentary organic δ¹³C ranges from -26 to -21 ppmil, consistent with diatom-derived organic matter from Antarctic coastal seas. Enriched ¹³C values generally, but not always, correspond with high opal contents, suggesting the occurrence of several different types of open water/sea ice algal bloom systems. Sedimentary δ¹⁵N increases downcore (from about 3 to about 5.5 ppmil), consistent with diagenetic loss of ¹⁴N. Rhythmic inversions suggest an environmental signal as well, in this case relative nutrient utilization in surface waters. δ¹³C and δ¹⁵N are highly coherent at periods of about 60 and 140 years. Most proxies are coherent with opal content in the ~200-year period band. We interpret multidecadal through millennial variability in these 5 biological tracers as resulting from significant changes in marine productivity, probably modulated by a combination of changing sea ice regime (extent and seasonality) and upper water column properties (e.g. mixed layer depth and circulation). A mid-Holocene interval of significantly enhanced productivity occurred between about 3500 and 7500 calibrated ¹⁴C yr B.P. with a maximum at 6000 to 7000 yr B.P. This event in the Antarctic Peninsula coincides precisely with the largest moisture balance anomaly of the Holocene on the Andean Altiplano, as inferred from the sediments of Lake Titicaca. A likely mechanism that explains variability at both sites is insolation seasonality. We also relate our Holocene Antarctic record with other similar resolution climate records from the Antarctic Ice Sheet and the western Pacific.