Holocene climatic and hydrological changes in Big Soda Lake, Nevada

Abstract:

It is important to separate the role of hydrology from climate change when assessing the Holocene history of lake sediments. Big Soda Lake is a Holocene maar lake in the Great Basin of Nevada with a 9 m sediment record that covers the Holocene. The mm-scale laminations, diatoms, and δ¹⁸O and δ¹³C from bulk calcite in the core are consistent with a saline, closed-basin lake down to approximately 4.3 m below the sediment water interface. The top 7 cm of the core show the influence of human hydrologic modifications to the area. Below that, stable isotope data shows variable groundwater input that is mostly consistent with climate variation from other records in the western USA. Below 4.3 m depth, the laminations abruptly end and are replaced by unlaminated massive mud, sand, and gravel to the bottom. The isotopic composition of the calcite abruptly changes from covarying, to inversely varying compositions below this break. This break occurred at about 5,600 cal yr BP. In addition, the diatom assemblage below 4.3 m is similar to that found in modern Walker Lake; whereas the diatom assemblage above 4.3 m is similar to modern Mono Lake. The δ¹⁸O isotopic composition of the calcite is on average 6 ‰ more negative below 4.3 m than above the change, indicating that the lake contained fresher water before 5,600 cal yr BP ago. The cause for the abrupt change cannot be explained through climate shifts because climate in the Great Basin has been shown to be more arid between 8,000 and 5,000 cal yr BP. It has been hypothesized that the Walker River flowed to the Carson River Basin between about 14,000 and 5,000 cal yr BP, and this added water may have raised the groundwater table sufficiently in the Carson Basin to freshen Big Soda Lake. Once the Walker River diverted back to the Walker Basin, more saline conditions prevailed reflecting changes in climate and human influence at the top of the core.