Distribution of biologic, anthropogenic, and volcanic constituents as a proxy for sediment transport in the San Francisco Bay coastal system, California

The biological, anthropogenic, and volcanic constituents of more than 300 samples collected from 1995 to 2010 in San Francisco Bay and the nearby coastal area were analyzed to discern patterns of sediment transport and deposition. The biological constituents investigated include microfauna (benthic and planktic foraminifers, ostracods, radiolarians, diatoms, and thecamoebians), macrofauna (bivalve mollusks, gastropods, barnacles, bryozoa, worm tubes, echinoids, crabs, and fish fragments), and flora (woody stems, roots, spores, and seeds). The distributional pattern of the benthic foraminifers was further refined by Q-mode cluster analysis into four assemblages that reflect where the taxa reside: the Brackish Shallow Subtidal Assemblage, the Estuarine Shallow Subtidal Assemblage, the Estuarine Intermediate/Deep Subtidal Assemblage, and the Nearshore Marine Assemblage. The anthropogenic objects recovered in this study include welding slag and glass microspheres most likely used to increase roadway line reflectivity. The volcanic constituents are glass shards of the Pliocene (3.27 Ma) Nomlaki Tuff, and Miocene tephra mostly derived from the Great Central Valley (including from the 23-19 Ma Valley Springs Formation, among others) and from the Sonoma Volcanic Field of California. The presence of allochthonous sedimentological constituents in this study was used to identify pathways of sediment transport and deposition within the San Francisco Bay coastal system. Several patterns have been identified: 1) volcanic glass shards are transported from the Great Central Valley through the delta to all regions of the bay, including the extreme end of south bay, as well as along the coast outside the bay and southward to Pedro Point; 2) microorganisms (benthic and planktic foraminifers, ostracods, diatoms, and radiolaria) from the marine realm outside San Francisco Bay are found in the estuary at the southern end of south bay, commonly in the middle of San Pablo Bay, and occasionally as far east as Honker and Grizzly bays; 3) estuarine ostracods and benthic foraminifers that lived within the bay are present outside the bay on the San Francisco bar and along the coast; 4) freshwater gastropods, ostracods, and thecamoebians from rivers feeding the bay are present in the middle of central bay and other subembayments of San Francisco Bay; 5) marsh benthic foraminifers that lived along the margins of the bay are also present in the middle of all of the subembayments; and 6) welding slag and glass microspheres are found far from their presumed origin of docks or roads. The distribution of constituents found in this study is consistent with the interpretation that sediment is transported from the delta to all regions of the bay and through the Golden Gate into the offshore. The reverse is also true, that sediment is carried from the marine realm into San Francisco Bay and reaches as far as the extreme end of south bay and nearly to the delta in north bay. Channels in north, central, and south bays, as well as the Golden Gate, are conduits for sediment movement and typically are sites where scouring occurs. The primary sites of deposition in the San Francisco Bay coastal system are in Honker, Grizzly, and Richardson bays, along the margins of Suisun, San Pablo and south bays, and in eastern central bay.