A detrital signal of glaciation in the Sierra Nevada, California, U.S.A.

ABSTRACT Alpine glaciers are sensitive indicators of regional environmental conditions over a variety of temporal scales, from interdecadal to glacial–interglacial timescales. However, cycles of major alpine glaciation events are often difficult to define through time due to the inherent tendency of later glacial advances to erase the records of previous cycles. More distal sedimentary records of detritus shed from glaciated ranges offer integrated archives of glacial cycles through time if a region produces a distinct detrital signal of glaciation. The Sierra Nevada range in California, where regional latitudinal climate shifts closely align with latitudinal trends in bedrock geology, offers an excellent example of such a distinct glaciation signal in marine sediment provenance signatures. In this study, we examine changes in sand provenance signatures across samples from the deepwater continental slope (Last Glacial Maximum sediment), outer continental shelf (late Pleistocene–early Holocene sediment), and the central (mixed late Pleistocene to Holocene sediment) and bayhead (mid.–late Holocene sediment) regions of San Francisco Bay. Sediment from slope and shelf samples shows a strong central-southern Sierra Nevada Batholith affinity suggesting derivation from the Sierra Nevada when the range was glaciated during the latest Pleistocene. In contrast, mid- to late-Holocene bayhead samples show a strong northern Sierran affinity indicating that they were originally eroded from the Sierras after deglaciation. These results are consistent with the locus of erosion in the Sierras shifting from higher-elevation glaciated areas in the central and southern range when a rangewide ice cap was present through the latest Pleistocene toward the lower-elevation northern Sierras after rangewide deglaciation around 13 ka as precipitation patterns shifted north through the Holocene. This detrital signal of Sierra Nevada glacial cycles could prove useful in future efforts to better define the currently incomplete history of Quaternary glaciations in the region and serve as an analog to other regions globally.