SEASONAL RESOLUTION SEA SURFACE TEMPERATURES FROM LOTTIA GIGANTEA SHELL GEOCHEMISTRY

Producing seasonal-resolution records of past sea surface temperature is important if we are to better understand the role of seasonality and interannual climate modes, such as the El Niño Southern Oscillation (ENSO), in controlling mean climate on diverse timescales. Such records would be of particular use along the west coast of North America where seasonal upwelling and ENSO events have significant effects on the regional climate and ecology. However, finding suitable seasonal-resolution archives of climate information has proved difficult outside the range of tropical surface corals. At mid-to-high latitudes, marine mollusk shells provide an archive with significant potential. Using a New Wave Micromill, successive samples were drilled from the cross sections of modern Lottia gigantea (Owl limpet) shells from the California coastline and analysed to produce stable isotope (δ¹⁸O and δ¹³C) and trace element (Mg/Ca, Sr/Ca) profiles. These profiles are then compared with nearby instrumental records of seawater temperature and salinity. The oxygen isotope composition of the shells accurately record absolute sea surface temperatures and capture more than 80% of the full seasonal range. Trace element ratios in Lottia gigantea shells are more complex but are also investigated in an attempt to identify useful trace element proxies of environmental variables. Finding and dating sequences of marine mollusk shells spanning long periods of time is difficult due to sea-level change and the destructional nature of most coastal environments. It is possible to avoid these problems by using the shells of marine mollusks collected for food by humans in North America during the Holocene and left behind in middens. Fossil Lottia gigantea shells dating from 10,000 years B.P. to recent, obtained from archaeological sites on San Miguel and Santa Rosa Islands off California, were analysed to produce snapshots of past sea surface temperature seasonality.