CYCLIC VARIATIONS IN PHYSICAL PROPERTIES WITHIN THE ANDVORD AND SCHOLLAERT DRIFTS: ANTARCTIC PENINSULA

Patricia L. Manley (patmanley@middlebury.edu) and Peter C. Ryan (pryan@middlebury.edu), Geology Department, Middlebury College, Middlebury, VT 05753; Stefanie A. Brachfeld (brachfeld.2@osu.edu), Byrd Polar Research Center, Ohio State University, 108 Scott Hall, 1090 Carmack Road, Columbus, OH 43210; Amy Leventer (aleventer@mail.colgate.edu), Geology Department, Colgate University, 13 Oak Drive, Hamilton, NY 13346; Eugene Domack (edomack@hamilton.edu), Geology Department, Hamilton College, Clinton, NY, 13323

High-resolution seismic profiling has identified large sediment deposits within the Gerlache Strait (Schollaert Drift, Canals et al., 1998) and the mouth of Andvord Bay (Andvord Drift, Harris et al., 1999). Jumbo piston cores taken at each of these drifts (JPC28 and JPC18 respectively) have been analyzed for physical properties at 5-cm intervals; e.g. water content, bulk density, porosity, voids ratio, compressional-wave velocities (Vp), electrical resistivity and magnetic susceptibility. Quantitative diatom studies, percent biogenic silica and quantitative XRD analyses of mineral content were done for JPC28.

Synthetic seismograms generated (using Vp and bulk density) show that reflectors are associated with small-scale variations in bulk density and water content of the hemipelagic/pelagic sediments. In turn the bulk density and water content values are dependent on the percentage of biogenic silica present in the sediment. Electrical resistivity measurements vary inversely with water content-which in turn is associated with the amount of biogenic silica present.

Quantitative XRD analyses indicate that the main minerals are albitic plagioclase (most abundant), chlorite, quartz, iron-rich mica and hornblende. Carbonate and iron oxides are present in minor amounts (<2%). Hematite and magnetite both appear to be present but are either poorly crystalline and/or present only in trace amounts, and are thus difficult to quantify. Rock-magnetic analyses confirm the presence of magnetite throughout core JPC28, even in low-susceptibility interval below 1100 cm. There appear to be no products of chemical weathering and all minerals present are primary igneous or metamorphic minerals. The sum of biogenic silica and XRD-determined mineral % in most cases is 90-100%, indicating that (1) the biogenic silica is either amorphous or poorly crystalline, and (2) detailed QXRD can be used to estimate the percent biogenic silica. The XRD results accurately reproduced the percent biogenic silica obtained through a quantitative selective dissolution analysis.

Quantitative diatom studies on JPC28 show that within the upper 13.6 meters of the core (~last 3300 years) high magnetic susceptibility regions (high bulk density and low electric resistivity) contain a low percent of diatom abundance. This contrasts with the low magnetic susceptibility sections (low bulk density and high electric resistivity) which contain an order of magnitude higher percentage of diatoms. The biogenic component of the sediment is the dominant control over the physical properties of the marine sediment within these drifts. Below 13.6 m and between 13.6 m and 18.4 m (~3300 - 3600 years) rhythmic laminations appear related to primary productivity events having an annual cycle.

AMS dates allow for spectral analyses on the various physical properties of these cores for comparison. Initial results show comparable decadal to millennial-scale cycles are present in biogenic silica, electric resistivity, density and mineral content and demonstrate that geophysical properties can be used to investigate paleoclimate change.