Development and application of new geochemical paleothermometers: How cold was the Little Ice Age in the North Atlantic?

Stan Hart, Geology & Geophysics
Anne Cohen, Geology and Geophysics
Simon Thorrold, Biology Department

OCCI Funded Project: 2003

Proposed Research

Accurate estimates of past sea surface temperatures (SST) are key to understanding the sensitivity of the earth's climate system to changing boundary conditions. For example, how did the North Atlantic surface ocean respond to decreased solar activity and increased volcanism that characterized the late stages of the Little Ice Age (~1790-1840 AD) ?

Paleoceanographers have developed a range of geochemical tools and analytical techniques to extract information from the preserved skeletons of marine organisms (forams, corals etc) that lived during these climatic events. However, different proxies often produce different paleotemperature estimates because physiological processes as well as environmental factors influence the geochemistry of the skeleton

In this project we will develop an alternative SST proxy based on magnesium and calcium isotopes in corals. Based on our understanding of how corals build their skeletons, we propose that coral physiological processes discriminate less against the isotopes of calcium and magnesium than they do against the elements themselves. Therefore, we expect the isotopic abundances of these elements in the coral skeleton to reflect SST variability rather than physiological or diagenetic processes.

We will use the microscale sampling capabilities of laser-ablation ICP MS to analyse the isotope composition of daily growth bands (inset in figure) in the skeleton of the massive brain coral Diploria labyrinthiformis collected on Bermuda (see figure). Two different paleothermometers previously applied to the 1790-1850 growth bands of this coral have yielded two different paleotemperature estimates for this time period. We will initially calibrate the temperature dependence of Mg and Ca isotopes using modern Diploria corals that have been stained in situ and have corresponding in situ instrumental temperature measurements. We will then reconstruct high-resolution SSTs from several decade long windows spanning important climate events over the past 320 years (1990-present, 1930-1950, 1890-1910, 1790-1850).

The ultimate goal of this work will be to apply these tracers to corals from crucial times in earth's climate history, including the Last Glacial Maximum (LGM) and the Little Ice Age (LIA).