Sedimentary records of late Pleistocene-Holocene paleoenvironments from Convict Lake (California, USA)

Abstract

We document the deglacial and postglacial history of Convict Creek Canyon using sedimentary and palynological datasets from Convict Lake, a hydrologically open terminal moraine lake in the Sherwin Range (Mono County, California). High-resolution seismic reflection data were used to clarify the three-unit (I-III, oldest to youngest) depositional architecture of Convict Lake and guide sediment core acquisition within conformable strata of the basin axis. Fourteen radiocarbon (¹⁴C) dates established the depositional timeline of the upper ∼615 cm of the core (∼9.3 cal ka to present), while the lower ∼336 cm is mostly undated due to a dearth of organic materials. Seismic unit I consists of coarse and poorly sorted gray-green detrital sediments interpreted as glacial till and outwash from the deglaciation of the Convict Creek Canyon. Lake development occurred in seismic unit II, marked by the deposition of black muds. Both seismic units II and III are interpreted as lacustrine paleoenvironments affected by slope margin instability that resulted in gravity flows. Lake level change drove the transition from unit II to III at ∼4.6–3.9 cal ka (mean = ∼4.3 cal ka); while basin center strata are conformable, unconformities at the margins suggest partial subaerial exposure. Peaks in grass and herb pollen indicate relatively dry conditions in the watershed during much of seismic unit II. Seismic unit III (∼4.3 cal ka-present) consists of relatively fine-grained black muds punctuated by numerous tephras. Organic and inorganic carbon concentrations become highly variable after ∼2.2 cal ka, marking the onset of climatic and tectonic instability that is likewise captured by montane forest, woodland tree, and herb pollen. The effects of climate changes in the last millennium (e.g., Medieval Climate Anomaly, Little Ice Age) appear to have had minimal effect on Convict Lake, perhaps owing to watershed hydrogeomorphology. This study reveals the timing of postglacial lake formation in Convict Creek Canyon and illustrates how terrestrial and aquatic proxies can be used to clarify Quaternary landscape-lakescape evolution in the eastern Sierra Nevada, a region threatened by climate change and natural hazards.