Date of Award

May 2018

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor(s)

Christopher Scholz

Keywords

lacustrine studies, Nevada, paleolimnology, radiocarbon, seismic stratigraphy, Walker Lake

Subject Categories

Physical Sciences and Mathematics

Abstract

The western USA has experienced heightened sensitivity to drought in recent years, and these conditions are compounded by a rapidly expanding population. Lake deposits are sensitive archives useful for reconstructing past hydroclimate, and such paleorecords can provide constraints for future planning. Walker Lake, Nevada is a terminal basin in the westernmost Basin and Range province. It contains a long sediment record extending at least to the late Pleistocene. Ten sediment cores, and ~300 km of CHIRP seismic reflection data were acquired from the lake in 2013 to develop a detailed environmental history from the lake’s stratigraphic record. Seven sequences were identified in the high-resolution seismic reflection data, with the deepest sequence observed 30 m below lake floor. Sequences 1-5 exhibit an external wedge shaped geometry, whereas sequences 6 and 7 drape the lower sequences and are sub-parallel to the water-bottom. Four pairs of samples of bulk organic carbon and organic macrofossils, such as seeds and wood, from the same stratigraphic depths were analyzed to assess influences of older carbon on radiocarbon ages. These pair samples yielded age disparities between ~400 years to ~1900 years indicating that bulk material may be unreliable. Instead I used fossil pollen rather than bulk or discrete macrofossils to date the sediment. Unlike lacustrine carbonates and mixed bulk organic samples, pollen is terrestrial and is not directly influenced by lake water dissolved DIC. Consequently, the pollen ages are used to develop the core chronologies. The oldest age from the analysis of fossil pollen is ~7431 cal. years BP, which allows for reassessment of the timing and magnitude of hydroclimate events in the region. Our integrated seismic stratigraphic and sediment core analysis reveals a basin-wide angular unconformity at 3540 cal. years B.P., indicating a sustained period of low lake level (or desiccation) with localized erosion. The geochemical, XRF, and physical property data from the sediment cores show limited variability during this lowstand, suggesting protracted stability of the Walker Lake water column and depositional system; only minor changes in values are observed at the depth of the angular unconformity. Using these integrated seismic stratigraphic and sediment core analyses, Walker Lake underwent three periods of lake-level drop during the Holocene: ~8600 cal. year B.P., 7630 cal. year B.P., and 3540 cal. year B.P.

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Open Access

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