| Tomales Bay (~50 km north of San Francisco, CA) is a long (20.4 km), narrow (0.7 to 1.7 km) and shallow (2.0 to 6.0 m) estuary located along the Northern California coast. It exhibits seasonal flow patterns and long residence times. Here we investigate foraminiferal assemblages from recent (Rose Bengal stained) and geologic records, geochemistry (delta18O, delta 13C), and total organic matter, to reconstruct geochemical conditions in Tomales Bay for the last ~550 years. Sediment accumulation rates based upon radiocarbon dating of benthic foraminifera (7 samples) range from 4.6 mm/yr to 17.1 mm/yr. The benthic foraminiferal assemblages are dominated by Elphidium hannai, Elphidium excavatum, Nonionella basispinatus, Ammonia tepida, and Buccella frigida. A recent (past ~110 years) increase in agglutinated foraminifera and decrease in calcareous species may be indicative of increased freshwater inputs into the Bay. Investigations of Rose Bengal stained surface foraminifera reveal that recently living specimens are present during spring to fall, and not during winter. An established delta18O calcite - temperature relationship is used to understand the deviation of modern (living) specimens from existing empirical relationships. At a minimum, the change in water temperature from spring to summer estimated for the past several hundred years is 2.4°C while the change in water salinity is 3.3 psu. A series of three events (fresh or warm) are identified in the cores from evidence in the foraminifera assemblages and isotopic analyses (delta18O max. change: -0.51 /, and delta13C max. change: -1.52 /), which likely reflect both anthropogenic and natural changes in the hydrology of Tomales Bay. Agricultural activities (e.g. logging, farming, changes in land-use) as well as infrastructural modifications to regulate water discharge, are the likely anthropogenic activities that influenced foraminiferal populations and geochemistry in the Bay. |
