Comparison of Geochemical, Grain-Size, and Magnetic Proxies for Rock Flour and Ice- Rafted Debris in the Late Pleistocene Mono Basin, CA
Abstract
Advance and retreat of mountain glaciers are important indicators of climate variability, but the most direct proxy record, mapping and dating of moraines, is by nature discontinous. The Sierra Nevada form the western boundary of the Mono Lake basin, and the proximity of the large Pleistocene lake to the glacial canyons of the Sierra presents a rare opportunity to examine glacial variability in a continuous, well-dated lacustrine sequence. We have applied a geochemical proxy for rock flour to the glacial silts of the late Pleistocene Wilson Creek Formation, but because it is time- and sample-intensive, another method is required for a high-resolution record. Previous microscopic examination, thermomagnetic measurements, XRD analysis, and new isothermal remnant magnetization (IRM) acquisition curves show that the magnetic mineralogy is dominated by fine-grained, unaltered magnetite. Bulk measurements show strong susceptibility (mean ~ 16 x 10- 6 m3/kg) and remanent magnetization (mean IRM ~ 10-2 Am2/kg) compared to diluting components (carbonate, smectite, rhyolitic ash). The Wilson Creek type section sediments also contain a coarse lithic fraction, quantified by counting the >2cm clasts in outcrop and the >425 μm fraction in the bulk sediment. Susceptibility, IRM, and ARM (anhysteretic remnant magnetization) are quite similar throughout the type section, with the abundance of coarse lithic fraction correlative to the ratio k/IRM. Because the magnetic fraction of the rock flour is fine-grained magnetite, IRM should capture the changes in concentration of flour through time, and the major features of the (low-resolution) geochemical flour proxy record are identifiable in the IRM record. Flux-correction of the IRM results in a rock flour proxy record with major peaks between 36 and 48 ka, similar to a rock flour record from neighboring Owens Lake. This regional glacial signal contrasts with peaks in coarse lithics between 58 and 68 ka in the Wilson Creek record; coupled with coeval high lake levels and a lack of geomorphic evidence of glacier-lake interaction, this is taken to indicate that the rafting was due to shore ice, rather than glacial icebergs.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMGP23A..03Z
- Keywords:
-
- 1051 Sedimentary geochemistry;
- 1512 Environmental magnetism;
- 1637 Regional climate change;
- 3344 Paleoclimatology (0473;
- 4900);
- 9345 Large bodies of water (e.g.;
- lakes and inland seas) (0746)