Site Introduction 

San Francisco Bay is a shallow estuary on the west coast of California, which drains water from around 40 percent of the state. The iconic Golden Gate Bridge crosses the strait that connects the bay to the Pacific Ocean. It has long been an important area for humans: first to the Indigenous communities who lived in the area for many centuries, and then to the European colonizers who established it as a significant global port. Today, the bay bears the uneasy distinction of being one of the most reconfigured aquatic ecosystems on the planet, with more than 200 invasive species having taken up residence since the mid-nineteenth century. In some places, 99 percent of the bay’s biomass11The total mass of living organisms in an environment at a given time. is comprised of introduced, non-native species.

San Francisco Bay sits in a tectonic depression between two major faults—the San Andreas and the Hayward—and has fluctuated between a terrestrial and marine landscape over the last million years. Today’s brackish,22Brackish water is saltwater that is not as salty as seawater, often produced by the mixing of sea and freshwater in environments such as estuaries or by evaporation of freshwaters containing dissolved salts. muddy estuary is the latest manifestation of a landscape produced by a continual interplay of sea-level change, tectonic motions (as stresses and compression between multiple faults have uplifted and lowered a mosaic of crustal blocks33A block is a mass of rock or other matrix maintaining its original structure.), and the accumulation of river-borne sediments.

Indigenous populations (predominantly Ohlone) utilized the marine, estuarine, and freshwater habitats in the Bay Area from the late-Holocene (around 4,000 years ago).44The Holocene is the current geological epoch, beginning around 11,700 years ago and characterized by a relatively stable climate that allowed major human civilizations and the modern world to develop. The remains of shell mounds and village sites record prehistoric foraging habits and the extent of occupation in the region. Indigenous culture was decimated by the European colonizers who arrived in the mid-eighteenth century: the Indigenous population of California plummeted from around 310,000 in 1769 to 16,277 in 1880, as a result of disease, dislocation, starvation, and genocide at the hands of Europeans. The California Gold Rush (1848–55) sparked an immigration boom and turned San Francisco Bay into an important global seaport and continental rail terminus. A 1940s initiative to fill in parts of the bay for industrial use (the Reber Plan) was halted in the 1960s by the establishment of the nonprofit Save the Bay, which has since then worked to protect and restore the area.

Location of the Core 

The San Francisco Bay estuarine system has a rich sedimentary record thanks to a constant supply of clays, silts, and organic matter transported there by the surrounding rivers. The subtidal mudflat ecosystem is home to many species of bacteria, algae, various other microorganisms (including foraminifera55Foraminifera are a diverse group of single-celled marine organisms with a shell made of calcium carbonate. They live in a variety of habitats and have a diverse range of forms, and their fossils can be used to reconstruct past environments. and ostracods66Ostracods (also known as seed shrimps) are tiny marine and freshwater crustaceans with a shrimplike body enclosed in a bivalve shell.), mollusks, and crustaceans. As freshwater meets seawater, various conditions (including a greater concentration of dissolved ions) promote what is known as “flocculation,” where suspended particles stick together and sink, preserving organic and chemical matter in the sediment. While there are depositional gaps in the sedimentary record of San Pablo Bay (at the northern end of San Francisco Bay) possibly due to sediment mixing and resuspension, the South Bay (from where the GSSP-candidate77A GSSP is a Global Boundary Stratotype Section and Point, which defines the lower boundary of a chronostratigraphic unit on the geologic time scale. It is sometimes referred to as a “golden spike”, after the marker which is, in some cases, driven into the boundary. core was extracted) preserves a record of continuous deposition. To ensure a complete archive, the core extraction site was also selected to avoid the many areas of the bay disturbed by dredging and shipping lanes.

San Francisco Bay has become home to a large number of invasive species—some of which were deliberately introduced while others hitched a ride on ships from across the globe. Invasive species have been documented in the area since 1853, with a distinct acceleration in increase observable around 1950. While many of these neobiota88Organisms that are non-native to the environment in which they now reside. are soft-bodied and do not preserve well, some have hard shells that remain in the bay’s shallow subtidal sediments, waiting to become the fossils of the future. Decades of scientific study has established a chronology of species invasion in the area over the last two centuries, meaning that organism remains can be used to develop a biostratigraphic record that is temporally precise enough to define the Anthropocene.

The Core and Results

The 230-centimeter-long San Francisco Bay core holds about 70 years of sediments. Its dense, grayish mud is visibly flecked with coarse sand and shells, and contains geochemical and mineral traces of past environmental conditions. Remains of five hard-shelled invasive species were found, all first appearing in the bay within a narrow stratigraphic interval of twenty-one centimeters, above 128 centimeters depth. This represents a temporal range spanning the mid 1970s to late 1980s, marking a distinct signature of ecosystem change. First to appear—in the same one-centimeter interval around the mid 1970s, which suggests they may have arrived together—are two Japanese ostracods: Bicornucythere bisanensis and Spinileberis quadriaculeata.99Ostracods are tiny crustaceans that live inside a bivalved shell. Above the first occurrence of these ostracods, the Japanese foraminifera Trochammina hadai occurs at 119 centimeters depth. It is known to have arrived in San Francisco Bay in 1983. At a slightly higher level in the core, at 115 centimeters depth, is the first occurrence of the East Asian bivalve mollusc, Potamocorbula amurensis, which is colloquially known as the Amur River Clam. It invaded San Francisco Bay in 1986. Finally the ostracod Eusarsiella zostericola, an arrival from the US Atlantic Coast, appears a little way above the clam. This evidence of significant anthropogenic ecosystem reconfiguration corresponds to a larger global trend of species redistribution that will be legible in the geological record in the far future, just as ancient changes to the biosphere can be read in the rock record.

The core also records various traces of human industry. Between 190 centimeters and 130 centimeters, there is a significant rise in mercury levels—perhaps evidence of mining. The elevated concentrations may represent the increased demand for mercury in the 1960s and 1970s, as well as mercury emitted into the atmosphere by fossil fuel combustion. Spheroidal carbonaceous particles (SCPs)1010A spheroid-shaped form of black carbon particle produced during high temperature oil and coal combustion. SCPs are a distinct marker of human influence on the planet, and can now be found in environmental archives on every continent, including very remote areas such as the high Arctic and Antarctica. occur at relatively low concentrations throughout the core, likely first appearing in the 1960s.

Collection and Analysis 

A 230-centimeter-long sediment core (SFB-20A) was collected from the south of San Francisco Bay in April 2019, using a vibracorer. This is a system comprising a thin metal cylinder that is lowered to the seafloor (in this case from a boat) and then vibrated into the loosely compacted sediment. The sealed tube is then taken back to the laboratory and cut in half lengthwise to reveal the intact sediment.

The core was subjected to the following analyses: microfossils (foraminifera, ostracods, and mollusks), SCPs, mercury, radionuclides, carbon isotopes, nitrogen isotopes, pollen, microplastics, and XRF scanning.

The Research Team 

The research team is comprised of Stephen Himson, Mark Williams, Jan Zalasiewicz, Colin Waters, Mary McGann, Arnoud Boom, Sue Sampson, Cerin Pye, Juan Carlos Berrio, Genevieve Tyrrell, Christopher Stocker, Ian Wilkinson, Neil Rose, Andy Cundy, Irka Hajdas, and Juliana Ivar do Sol. 

The research is led by Stephen Himson and Mark Williams. Himson is a PhD student whose work focuses on understanding biological change in San Francisco Bay and its geological signature. He is supervised by Mark Williams, Jan Zalasiewicz, and Colin Waters, who have extensive research experience studying paleontological and sedimentological records both in deep time and in recent sedimentary successions. The research has been supported by Mary McGann, who was instrumental in organizing the logistics of the coring expedition. McGann has conducted groundbreaking work examining the biostratigraphic and ecological implications of the introduction of Trochammina hadai to San Francisco Bay.

The analytical team includes researchers with expertise in radiogenic, stable isotope, and elemental analyses in addition to anthropogenic marker and microfossil processing and analysis. Arnoud Boom and Genevieve Tyrrell undertook stable carbon and nitrogen analysis, Sue Sampson undertook the mercury analysis, Juan Carlos Berrio and Cerin Pye undertook the pollen analysis, Ian Wilkinson aided in the identification and interpretation of the ostracod fauna Neil Rose supported the analysis of SCPs, Andy Cundy undertook the analysis of plutonium isotopes, Irka Hajdas undertook the analysis of radiogenic carbon, Christopher Stocker undertook the sampling and preparation of microplastic samples, and Juliana Ivar do Sol undertook the analysis of microplastics.

Principal investigators (listed alphabetically):
Mark Williams, University of Leicester
Stephen Himson, University of Leicester

Contributing Scientists/Researchers (listed alphabetically):
Arnoud Boom, University of Leicester, Carbon and Nitrogen isotopes
Juan Carlos Berrio, University of Leicester, Pollen analysis
Peter Dal Ferro, United States Geological Survey, Core collection
Theresa Fregoso, United States Geological Survey, Core collection/surveying
Irka Hajdas, ETH Zurich, Radiocarbon analysis
Rachael Holmes, University of Leicester, Pollen analysis
Juliana Ivar do Sul, IOW Leibniz, Microplastic analysis
Bruce Jaffe, United States Geological Survey, Core collection/surveying
Mary McGann, United States Geological Survey, Coring and micropalaeontology
Jennifer McKee, United States Geological Survey, Core collection
Daniel Powers, United States Geological Survey, Core collection
Cerin Pye, University of Leicester, Pollen analysis
Neil Rose, University College London, SCP analysis (supervision)
Sue Sampson, University of Leicester, Mercury analysis
Colin Waters, University of Leicester, PhD advisor to Stephen Himson
Ian Wilkinson, British Geological Survey, Ostracod analysis
Handong Yang, University College London, 210Pb dating (UCL)
Jan Zalasiewicz, University of Leicester, Stratigraphical analysis