Contenuto scaricabileScarica il pdf
Holocene-aged lake sediments from the southeastern margin of Tulare Lake, CA
Holocene-aged, high resolution lacustrine stratigraphy from the Tulare Lake basin in the San Joaquin Valley paired with oceanic surface temperature data form the components necessary to inform models that forecast regional precipitation in this important agricultural center. Over the past 15,000 years, lake-level changes based on previous work from both trench exposures and core from the northwestern margin of Tulare Lake correlate well with ocean temperatures over this time period suggesting the sea-surface temperature of the adjacent Pacific Ocean drive precipitation in the Sierra Nevada. These interesting results are tested here using exposures from the SE margin of the lake. Quaternary lacustrine sediments from three new trench localities within the Tulare Lake Basin at relatively high elevations were described and sampled for quantitative geochemical and geophysical analyses. They were dated using a combination of 14C methods on anodonta, gastropods, bulk organic carbon, and charcoal found within the layers and paleomagnetic secular variation. The highest elevation locality exposed on the sides of a borrow pit near the Pixley National Wildlife Refuge at the easternmost extent of the lake has a more or less uniform grain-size distribution and fossil hash assemblage suggestive of a prolonged shoreline facies. Grain-size analysis yields a typically bimodal distribution for the individual samples and shows five coarse grained depositional events in both mean (460μm) and coarse fraction (500μm) grain size when all the sample sizes are plotted versus depth. Total organic carbon varies from 0-1.2% by weight and total inorganic carbon varies from 0-1.7%. Radiocarbon dates from these sediments range in age from 1cal ka B.P. to 401± 28 cal yr B.P. suggesting that the lake shoreline reached a level of at least 64 m several times during the late Holocene based on depositional events. Poso Canal, a second locality 4.5 km SW of Alpaugh, CA contains distinct stratigraphic units varying from deepwater clay deposits to shallow water near shore sands similar in age to those found in the trench exposures in the previous work near the NW shore of the lake basin, thus supporting these initial results. Seven anodonta, two gastropods, seven charcoal, and seven bulk sediment samples from the second trench locality were dated from 7.2 cal ka B.P. to 225 cal yr B.P. There is an unconformity here from 6.2 to 2.4 cal ka B.P. representing a shallow lake level during this 3.8 ka hiatus. A third locality on the Atwell Island Restoration Project land operated by the Bureau of Land Management, exposes stratigraphy related to a large sand spit that was deposited by longshore drift of Kern River sediments at times of relatively high lake level. This exposure contains layers of well sorted fine sands intercalated with fine-grain lake muds that likely reflect changing discharge of the Kern River through time. The lithology consist of well-defined layering characterized by well sorted, clay rich lake silt, very fine grain sand, and fine to medium grain oxidized sand. Radiocarbon dates were obtained from two bulk sediment samples from this trench locality, 14.5 cal ka B.P. and 15.1 cal ka B.P. Overall, results agree with those of previous detailed works. For example the lake does not appear to have risen to high elevation in the middle Holocene (Davis, 1999; Negrini et al., 2006; Kirby et al., 2012; Blunt, 2013). One exception is that the Atwell Island exposure suggests that latest Pleistocene lake level was higher than that reported by Negrini et al., 2006 and more in line with later results by Blunt and Negrini (in review).
- In Collection: