NEW RELATIVE HOLOCENE SEA-LEVEL CURVE FOR SAN FRANCISCO BAY, CALIFORNIA, USA

The first well-dated sea-level curve proposed for the San Francisco Bay Area was developed more than 37 years ago by Atwater et al. (1977), based mainly on the elevation and age of 13 uncalibrated radiocarbon dates. For this study, 333 radiocarbon dates from the San Francisco Bay and Delta estuary system were compiled and analyzed and 250 were used to estimate the rate of Holocene sea-level rise. Two different model were developed as a result. One is a local polynomial regression called the “Punctuated Model” because the fitted data do not form a continuous curve, and the second model is a 3rd-degree polynomial regression known as the “Curve Model” because the fitted data form a relatively continuous curve over the entire sequence.

When the two models are plotted against one another, the results are very similar, with any differences of one being within 2-sigma error range of the other. Here again, the only notable difference between these models occurs in the Middle Holocene, where some flattening of the curve is indicated by the Punctuated model. This model identifies three “flat” periods, the first extending roughly from 7400 to 6600 cal BP, the second from about 6000 to 5600 cal BP, and the third between 5400 and 4600 cal BP, which may represent significant decreases in the overall rate of sea-level rise, or possibly, “still stands” or “near still stands” of the sea during the Middle Holocene.

Based on the new sea level curve, a large part of the landscape once available for prehistoric human populations is now buried or submerged due to Holocene sea-level rise in the San Francisco Bay and estuary system. The discovery in early 2014 of an intact human skeleton at an elevation of 12.8 meters (42 feet) below sea level in downtown San Francisco is the fourth such example found below sea level in the San Francisco Bay over the past 45 years. These discoveries underscore the need for accurate Holocene sea-level reconstructions to predict where buried or submerged prehistoric archaeological resources are likely located, and to evaluate long-term relationships between land, sea, and human occupation in the San Francisco Bay Area.