Wetlands within the Dunes: Paleoecological Reconstructions of Prehistoric San Francisco

Authors: Michelle Goman*, Sonoma State University
Topics: Paleoenvironmental Change, Biogeography, Environment
Keywords: Paleoecology, San Francisco, Wetlands
Session Type: Paper
Day: 4/9/2020
Start / End Time: 11:10 AM / 12:25 PM
Room: Grand Ballroom 1, Sheraton, IM Pei Tower, Second Floor Level
Presentation File: No File Uploaded

Expansive dune fields once covered the region we now call San Francisco. Recent excavation for building foundations at 200 Van Ness Avenue exposed a dune sequence with three interbedded organic rich horizons. Organic rich material was processed and examined for pollen and macro-botanical analysis and radiocarbon ages determined. Paleoecological analysis indicates diverse wetland habitats existed within the dune fields over the past ~2000 years.

The oldest sample lies 12 m below modern surface (bms) and dates to 2060 cal B.P., at this time a moist seep prevailed with Morella spp. and an understory of ferns grew. The site was buried by dunal sand until 1930 cal yr B.P. (10 m bms) when a well-developed freshwater wetland persisted. The vegetation at this time was dominated by species in the Cyperaceae family (S. cf. americanus, S. cf. acutus, Cyperus spp. and Carex spp.) other wetland taxa (Ranunculus and Typha) were also present. Seeds of Potamogeton were identified indicating the presence of standing water. The youngest sample dates to 1220 cal yr B.P. (6.4 m bms), the pollen record indicates that Salix and Morella were present in significant percentages. Nutlets of Morella were also identified indicating a local presence of the shrub; no seeds from the Cyperaceae family were present.

The variation in wetland paleoecology will be assessed for potential insights in reconstructing water table fluctuations, likely associated with climate variability and/or local geomorphic processes.

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