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This detailed geologic record is one of twelve sites in consideration to be ratified as a "golden spike” marking the transition into the Anthropocene, the geological era in which human activity has become the dominant influence on the planet. 

Researchers from Northern Arizona University, Stanford University, and USGS collected sediment cores from two lakes in Jasper Ridge Biological Preserve near Stanford, CA. Searsville Lake is a reservoir created in 1892 by the construction of Searsville Dam on San Francisquito Creek. Over the last 125 years, more than 35 feet of sediments have accumulated behind the dam in a seasonally predictable pattern, creating layers known as varves. These varves provide a year-by-year, season-by-season look at changes in vegetation, land use, and climate. 

“Searsville Lake is the only ‘golden spike’ site being considered right now that exists solely because of human activity,” said Allison Stegner, a research scientist at Stanford University who led the Searsville Lake research and is a co-author of the study. “By impounding water with Searsville Dam, we created a geological archive.” 

Cores were collected from Searsville Lake and nearby Upper Lake Marsh—which is a natural pond hydrologically connected to Searsville Lake—using the USGS shallow-water coring platform Hadai, which was designed and constructed by Dan Powers and Pete Dal Ferro from the USGS Pacific Coastal and Marine Science Center Marine Facility. 

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Using pollen as their proxy data, the researchers compared sediment records at Searsville Lake to those from Upper Lake to track changes in vegetation going back 1,500 years. While the sediment records differed in distinct ways—the heavily impacted Searsville Dam site had higher sedimentation rates and a more finely resolved sediment record compared to Upper Lake Marsh, which showed relatively few human impacts and lower sedimentation rates—the records are complementary and together provide a detailed look at how pollen type and abundance on the Peninsula shifted through time. 

These changes in the pollen record occurred alongside human activities such as logging, agricultural land use change, shifts in wildfire regimes, and the introduction and spread of non-native plant species. Their research is part of a larger project that aims to define geologic records marking the Anthropocene.

“The Searsville cores are exceptional because they record the last 125 years of human impacts to this watershed in over 30 feet of sediment,” said SeanPaul LaSelle, USGS Geologist and co-author of the study. “That means each year, on average, is represented by almost three inches of sediment. This allowed us to pinpoint the geologic Anthropocene marker to late fall/early winter in 1948, based on the first detection of Plutonium isotopes from fallout following nuclear testing.” 

By comparing the geological, paleoecological, and historical records of both lakes, the scientists were able to reconstruct the previously undocumented impacts of socio-ecological influences on the area. These human-caused changes were found to overlap known climate change effects, highlighting the significant impact humans have had on the environment. The findings provide valuable insights into long-term changes on the San Francisco Peninsula, which could inform future resource management efforts.

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