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Palmer LTER: Paleohistory of the Palmer LTER region: Palmer Deep sedimentary record

Matthew LoPiccolo and Eugene Domack, Department of Geology, Hamilton College, Clinton, New York 13323

In 1992, the R/V Polar Duke cruise PD92-1 collected several piston cores from the Palmer Deep Basin, south of Anvers Island ( figure 1). Analysis of core PD92-30, retrieved at 64°51.720'S 64°12.506'W from Basin I, found that core 30 is an excellent indicator of paleoclimatic fluctuations as shown by magnetic susceptibility and total organic carbon (TOC) content (Leventer et al. 1996). Magnetic susceptibility and TOC both experience high-resolution cyclical fluctuations on a 200-300 year timescale, which correlates with studies by LoPiccolo (1996) and Mashiotta (1992) on core 22 collected from the central basin of Andvord Bay in 1988. The importance of PD92-30 as a climate indicator lies in the fact that it was collected along the polar/subpolar boundary. Domack and McClennen (1996) claim that this region experiences subtle changes in sea-ice extent that affect productivity, unlike areas to the north and south where sea ice is a less or more permanent feature.

Unfortunately, piston core operations often lose approximately the top meter of sediment. This missing top of the core is crucial for the complete determination of Holocene climate change. Thus, three short gravity cores were collected from Basin I during R/V Polar Duke cruise 1995-10 in an attempt to obtain the sediment water interface (LoPiccolo, unpublished report) and the youngest sediments in the basin. Correlation of magnetic susceptibility, TOC content, grain size data, and carbon-14 (14C) dates from PD92-PC30 and PD95-GC 1, 2, and 3 will determine how much sediment was lost from PD92-PC30, establish a detailed stratigraphic sequence of Basin I, and give more precise corrections for radiocarbon dates. This combined information will provide a more accurate picture of Holocene climate change in the Antarctic Peninsula region, and that information will help determine modern climate trends and possible anthropogenic influences on climate.

Three 8-centimeter (cm) diameter gravity cores of various lengths (188 cm, 261 cm, and 264 cm) were collected in Basin I of the Palmer Deep, Antarctica, during R/V Polar Duke cruise 95-10. The objective of this coring operation was to obtain a sediment core of the first meter of sediment in the Palmer Deep; at approximately 64°51.720'S 64°12.506'W, the site of PC 30, retrieved during PD92-2 (table 1). The cores were processed for magnetic susceptibility aboard the R/V Polar Duke using a Bartington magnetic susceptibility recorder, model MS-2C. Measurements were taken every 5 cm. Cores 1, 2, and 3 were split, photographed, described, and x-ray radiographed, at the Antarctic Marine Geology Research Facility at Florida State University where all three cores are curated. Cores 2 and 3 were subsampled every 5 cm for further analysis at Hamilton College, Clinton, New York. The x-ray radiographs were examined to determine lithologic structures and used to conduct a gravel concentration analysis.

Core 3 subsamples were analyzed for grain size, TOC, and radiocarbon dates. The grain size analysis was conducted using a Malvern Master Sizer E. Fifty-three samples were analyzed at the 0.1-80 micron range. The TOC content was determined by combustion in a LECO induction furnace. Samples were prepared by first soaking in a 2 normal hydrochloric acid solution followed by series of decants using distilled water. Several organic-rich samples required the use of a centrifuge to separate the water from the suspended sediment sample. Six sediment samples were collected from GC 3 for 14C analysis. Samples were taken at the sediment water interface, and at depths 25 cm, 50 cm, 115 cm, 200 cm, and 230 cm while on board the R/V Polar Duke and transferred chilled to Hamilton College where they were dried and acified in the 2 normal hydrochloric acid solution. 14C dates were determined at the University of Arizona by accelerator mass-spectrometry and are reported in table 2. Core 2 subsamples were weighed for the determination of sample water content.

Our preliminary stratigraphic correlations are illustrated in figure 2 where we compare the magnetic susceptibility and radiocarbon data from the four cores collected in Basin I of the Palmer Deep. In summary, we believe that stratigraphic correlation is good between the sites below the uppermost meter because both radiocarbon and magnetic susceptibility signatures indicate normal stratigraphic succession. Above this depth, however, the stratigraphy is less clear, and evidence indicates significant reworking of organic particulates because radiocarbon ages are inversed and are older than ages from lower strata. Although our sedimentologic study of these cores is far from complete, at this early stage, it is clear that sediment gravity flows are an important component of the most recent record in Basin I of the Palmer Deep. This presence contrasts dramatically with the last 4,000 years of deposition, which is marked by pelagic and hemipelagic sedimentation (Leventer et al. 1996). The cause for this change in sediment regime will be the focus of continued research in the Palmer Deep system.

This program was supported by a National Science Foundation Research Experience for Undergraduates grant to Hamilton College (OPP 94-18153, Earth Sciences). This work was carried out in cooperation with the Palmer LTER investigators and special word of thanks to Dave Karl and Ray Smith for arranging ship time on the Polar Duke. Appreciation to Antarctic Support Associates staff members Cole Mather and Greg Packard is also acknowledged.

References

Domack, E.W., and C.E. McClennen. 1996. Accumulation of glacial marine sediments in fjords of the Antarctic Peninsula and their use as paleoenvironmental indicators. In R. Ross, E. Hofmann, and L. Quetin (Eds.), Foundations for ecosystem research west of the Antarctic Peninsula (Antarctic Research Series, Vol. 70). Washington, D.C.: American Geophysical Union.

Leventer, A., E.W. Domack, S.E. Ishman, S. Brachefled, C.E. McClennen, and P. Manley. 1996. 200-300 year productivity cycles in the Antarctic Peninsula region: Understanding the linkages among the Sun, atmosphere, oceans, sea ice, and biota (Geological Society of America Bulletin, Vol. 108). Boulder: Geological Society of America.

LoPiccolo, M.H. 1996. Productivity and meltwater cycles in Andvord Bay, Antarctica: Evidence of high frequency paleoclimatic fluctuations. (B.A. thesis, Hamilton College, Clinton, New York.)

Mashiotta, T.A. 1992. Biogenic sedimentation in Andvord Bay, Antarctica: A 3,000-year record of paleoproductivity. (B.A. thesis, Hamilton College, Clinton, New York.)