HIGH-RESOLUTION FAUNAL AND ISOTOPIC RECORDS FROM HOLOCENE ESTUARINE DEPOSITS OF SAN FRANCISCO BAY, CALIFORNIA

A 3.52 m core (DJ6-93SF-6) recovered from the western portion of south San Francisco Bay, CA (37º37.83’ N, 122º21.99’ W) near the San Francisco International Airport has yielded a high resolution, Holocene climatic history of the region. The faunal and isotopic evidence provided by estuarine benthic foraminifers suggests that three distinct climatic periods (two cool, one warm) have taken place in the bay during the last 3800 years.

A modern, shallow water depth (<10 m), foraminiferal assemblage consisting of abundant, subtidal estuarine taxa (e.g., Elphidium excavatum, Ammonia beccarii, Elphidium gunteri, and Elphidiella hannai) characterizes the core sediments. Within the faunal assemblage, two distinct associations affected by climate change were observed. In the first association, Elphidium excavatum is the dominant taxa, often comprising 70-90% of the fauna. Today, this species typically resides in cold, estuarine waters. Its dominance in the core between 352-150 cm (~1870 B.C. to ~A.D 680; dated by AMS¹⁴C), and 88-18 cm (~A.D. 1270 to ~A.D. 1990), suggests that during these two periods, this area of the bay has remained relatively cold and shallow (i.e., for ~3300 years out of the last four millennia). An Ammonia beccarii – Elphidium gunteri association occurs between 150-88 cm (~A.D. 680 to ~A.D. 1270), and its presence is interpreted as representing a period of warmer climatic conditions. In contrast, in the upper 18 cm of the core (~A.D. 1990 to present), the fauna changes dramatically due not to climate, but to the appearance of an invasive foraminifer from Japan.

Oxygen isotope values were measured in specimens of Elphidium excavatum at ~10 cm intervals downcore. From 150-88 cm (~A.D. 680 to ~A.D. 1270), d¹⁸O values (mean=-3.81 mil) average 0.3 and 0.2 mil lighter than below and above this interval, respectively, corresponding to an increase in water temperature of ~1°C in the bay at a time that correlates well with records of the Medieval Warm Period. At 69 cm, following the culmination of this interval of increased water temperature, the d¹⁸O value is the heaviest recorded in the core (-2.31 mil). These data, associated with a 2.5 times increase in sedimentation rate from 88 to 58 cm, suggest that the drought had ended and increased river runoff contributed to a heightened sedimentation rate at the core site.