Paper No. 8
Presentation Time: 9:00 AM-6:30 PM
PAST AND PRESENT WEATHERING ENVIRONMENTS IN THE DRY VALLEYS, ANTARCTICA BASED ON SEDIMENT CHEMISTRY
STEFFEN, Ashley L., Dept. of Geosciences, North Dakota State University, PO Box 6050, Fargo, ND 58108, SAINI-EIDUKAT, Bernhardt, Department of Geosciences, North Dakota State Univ, NDSU Dept 2745, P.O Box 6050, Fargo, ND 58108-6050, LEWIS, Adam R., Department of Geosciences, North Dakota State University, P.O. Box 6050 Dept. 2745, Fargo, ND 58108-6050, ZAMORA, Felix J., Geosciences, North Dakota State University, NDSU Dept 2745, PO Box 6050, Fargo, ND 58108-6050 and LEPPER, Kenneth, Department of Geosciences, North Dakota State University, P.O. Box 6050, Dept. 2745, Fargo, ND 58108-6050, alsteffen1@gmail.com
We are analyzing glacial and stream sediments from the Dry Valleys of East Antarctica as part of an ongoing project to better understand past weathering environments and sediment provenance. A major climate cooling at 14 Ma is known from both terrestrial and offshore records. The weathering environment is hypothesized to have changed from one in which glacier melting and surface water was common to cold and hyper-arid. Sample sets include modern streams, Holocene-aged alluvial fans, Miocene-aged post-climate transition sublimation tills, and Miocene pre-transition lacustrine sediments and tills, the oldest of which are 19.76 Ma. Sediments were dry sieved in the field to remove >16 mm clasts, sieved in the laboratory to < 2 mm, pulverized to 250 micrometers, fused into glass beads and analyzed by XRF for major and trace elements. Sample splits were analyzed for inorganic and organic carbon. We discriminated samples based on the chemical index of alteration (CIA) and other geochemical characteristics.
CIA values for the tills, regardless of age, are similar. However, fluvial samples show an age-related difference. Modern stream sediments had an average CIA of 42, ranging from 40 to 44. Holocene alluvial fans, dated using OSL, range from CIA of 36 - 50 (avg = 41). These values suggest a relatively weak weathering environment. Fluvial sediments dated using volcanic ash to before the climate transition have CIA values ranging from 42 to 77, with an average of 69. This suggests a more intense weathering regime at that time. On a CIA plot, samples from the Friis Hills show slight variation from the expected trend toward K.
The very low CIA values observed in select samples may be due to the contribution of mafic volcanic dikes and sills to the sediment matrix (e.g., Ferrar dolerite CIA = 43). Aerial deposit of salts from the nearby ocean may also contribute to low CIA values. The combination of various bedrock contributions and weathering regimes results in distinct geochemical groupings. We are evaluating whether chemical fingerprinting of each sediment type, related to weathering environment, age, and provenance, is possible.
