An extraordinary record of Anthropocene stratigraphy from the San Francisco Peninsula, California, USA

Anthropogenic activities changed our planet over the course of the Holocene, but the scale of human impacts increased dramatically around the mid-20^th century, representing the start of the Anthropocene. Global signals of these impacts and the resulting biotic and abiotic changes are captured in the geologic record. These pervasive anthropogenic impacts, including pollution, road-building, the rise of plastics, etc., are comparable in magnitude, uniqueness, and geologic preservational longevity to global changes that mark previous major geologic time intervals. To identify global and local geological signals that can be used to characterize the Anthropocene, we examine sediment cores from Searsville Reservoir, a 126-year-old reservoir located in the eastern foothills of the San Francisco Peninsula. We collected four sediment cores ranging from 7.4 to 8.5 meters in length, indicating average sedimentation rates of 6 to 7 cm per year. This exceptional resolution allows us to explore the Anthropocene geologic record on a sub-annual temporal scale.

Computed tomography scans of the cores revealed >300 distinct layers ranging in thickness from <1mm to ~30mm. Many of the thicker laminae show upward fining, which we interpret as indicative of individual storm events. Sediment density generally decreases from the bottom to the top of the cores, consistent with both increasing organic inputs as the reservoir filled with sediment and began eutrophication, and with sediment compaction. X-ray fluorescence scans show overall declines in titanium (Ti) and iron (Fe), both major components of the detrital inputs, which is inferred to represent reduced catchment runoff, signaling reservoir infilling, reduced precipitation over the life of the reservoir, and/or increases in organic versus detrital inputs. Changes in source sediment, reflective of sedimentary inputs from different drainages within the watershed, are evidenced by consistent trade-offs between suites of elements. Searsville is located on the San Andreas Fault, and so we will track earthquake events in the stratigraphy. Global markers of the Anthropocene that will be analyzed include carbon (C) and nitrogen (N) isotopes, atmospheric pollutants like lead (Pb), components of black carbon, radionuclides from nuclear testing, and biologic indicators.