DYNAMICS OF BIOTIC AND ABIOTIC MARKERS OF THE ANTHROPOCENE AT JASPER RIDGE BIOLOGICAL PRESERVE, CALIFORNIA, USA

Humans have altered our planet throughout the Holocene, but the scale of impacts increased dramatically in the mid-20^th century, representing the start of the proposed Anthropocene Epoch. These pervasive anthropogenic impacts are comparable in magnitude, uniqueness, and geologic perseverance to the global changes that mark previous major geologic time intervals. To identify the geologic signals that characterize the Anthropocene, we studied two sediment cores from Searsville Reservoir, located at Jasper Ridge Biological Preserve in the eastern foothills of the San Francisco Peninsula, California. These cores have distinct seasonal layers and exceptionally high sedimentation rates—they are 944.5 and 852.5 cm long, and span from 1900 to 2018 CE—which has allowed detailed investigation of the Holocene-Anthropocene transition. One of these cores is being considered as a Global Boundary Stratotype Section and Point for the Anthropocene.

We analyzed concentrations of mercury, lead, and other heavy metals in the cores and found a temporal sequence of increases and declines which closely match measured atmospheric concentrations and historic records of global mining intensity and peak commercial uses. Stable isotope analyses show depletion of ¹³C caused by burning of fossil fuels, and depletion in ¹⁵N caused by global increases in reactive nitrogen—signals which track known atmospheric patterns over the last century. We also analyzed the radionuclides Plutonium-239,240 and Cesium-137, generated by nuclear bomb testing which began in 1945, peaked in 1962, then declined sharply beginning in 1963. In Searsville, the first appearance of ^239,240Pu was in 1947, and both ^239,240Pu and ¹³⁷Cs peaked in 1963, consistent with a lag of 1-2 yrs between ejection into the atmosphere and deposition. Local indications of human impacts are evident in the pollen and plankton microfossil assemblages and are linked to well-documented historic events in the watershed. Our analyses document the expression of local and global human impacts at Searsville and are providing context and critical data for a proposed watershed restoration project. Searsville is particularly emblematic as a geological record of the Anthropocene because the section is itself a direct consequence of human activity—the emplacement of a dam.