Late Quaternary sea-level fluctuation and the development of coastal estuaries such as San Francisco Bay Jonathan Najarro, Hide Takahashi, & Marisela Mendoza GEOL 120 (Sources: USGS)

Outline Define key terms Timeframe and setting Processes contributing to estuary formation Proxy methods used to identify past conditions Explanation of cross sections in SF Bay

Key Terms What is an estuary? What is a proxy? An estuary is a place where fresh and salt water mix, such as a bay, salt marsh, or where a river enters an ocean. Many species such as various types of birds rely on the estuary ecosystem. What is a proxy? In paleoclimatology, proxies are preserved physical features that stand in for direct measurements to enable scientists to reconstruct past climactic conditions

Timeframe and Setting Focus: From the late Pleistocene to the present (Around 125 ka to current process) Study Area: San Francisco Bay estuaries **Relevant study site near to Point Reyes Peninsula**

Processes Occurring Estuaries: Formed through repeated oscillation glacial and interglacial periods, eroding a valley and forming a connection between a body of freshwater and a body of seawater. Interglacial period: Sea level is higher, suspended sediment settles to the bottom and forms a layer of mud Glacial period: The absence of marine fossils in a sediment layer (Examples: Foraminifera, Shells) Post-Wisconsinan Glaciation: sea level rose 2 cm/yr, and estuary expansion was 30 m/yr until 8,000 years ago

Proxy Methods Core sampling – using a hollow steel tube called a core drill to extract stratigraphic layers Use of stable carbon isotopes to determine paleosalinity levels Palynology: The study of fossil pollen

Ice Core Drill Diagram

Explanation of Layers in SF Bay Cross Sections Qhe: Modern SF Bay (interglacial period) Qphw & Qpha: Non-marine sediments deposited during the last glacial period (~20ka) Qpe: Paleo-SF Bay (Last interglacial, ~125ka) Qpa: Glacial period prior to 125 ka.

Late Pleistocene According to Atwater B.F. (1977): Sediment samples collected in South San Francisco Bay Methods: Cross-section analysis and Geologic Map

Late Pleistocene Findings from Atwater et al (1977): Sangamon: Clay & Silty clay, fluvial deposits At least 20-40 m depths of water Holocene: Diatoms, Pelecypods, Foraminiferas

Holocene According to Malamud-Roam and Lynn Ingram (2004): Late Holocene δ13 C and Pollen Records *Relative proportion of C3 vs C4 plants *Proxy of vegetation change Based on four tidal marshes in the S.F. Bay Estuary *Inference of overall estuary paleosalinity Higher than mean salinity Lower than mean salinity 1600 – 1300 cal yr B.P. Before 2000 cal yr B.P. 1000 – 800 cal yr B.P. 1300 – 1200 cal yr B.P. 300 – 200 cal yr B.P. 150 cal yr B.P. – A.D. 1950 A.D. 1950 - Present Low = More Fresh Water Inflow

Malamud & Ingram (2004)

Summary Estuaries are a body of water formed by salt and fresh water mixing They are formed by sedimentary deposits during oscillating glacial and interglacial periods Proxy methods such as core sampling and usage of stable carbon isotopes help determine past conditions

Literature References * Atwater, B.F., Hedel, C.W., and Helley, E.J., 1977, Late Quaternary depositional history, Holocene sea-level changes, and vertical crustal movement, southern San Francisco Bay, California, U.S. Geological Survey Professional Paper 1014, p. 1-15. * Malamud-Roam, F., and Lynn Ingram, B., 2004, Holocene δ13C and pollen records of paleosalinity from tidal marshes in the San Francisco Bay estuary, California, Quaternary Research, 62, p. 134-145. * Malamud-Roam, F.P., Lynn Ingram, B., Hughes, M., and Florsheim, J.L., 2006, Holocene paleoclimate records from a large California estuarine system and its watershed region: linking watershed climate and bay conditions, Quaternary Science Review, 25, p. 1570-1598. * Wicander, R., Historical Geology, 7th Edition, 2012, Brooks Cole