9 - Environments

Summary

A major achievement of low-temperature geochemistry is its ability to provide estimates of variables such as ocean temperature, atmospheric composition and pressure, erosion intensity, and biological productivity. These estimates come through geochemical observables, known as proxies, which can be related with some confidence to a variety of parameters of our environment. The understanding of ancient climates, oceans, atmospheres, and biological activity would be very poor in the absence of these proxies and would remain qualitative and highly speculative. The derived environmental information, however uncertain it may be, can always be tested against predictions and with the help of improved observations can be continuously improved.

Let us first briefly review some of the most important environmental proxies for modern environments (<65 Ma).

1. As shown by Dansgaard in 1964, mean annual air temperature can be determined (or estimated) from the mean δD or δ¹⁸O value of the local precipitation (rain or snow).

2. The amount of ice locked up in polar regions is derived from the average δ¹⁸O value of seawater.

3. The temperature of deep oceanic water can be obtained from the δ¹⁸O values of benthic foraminifera.

4. The surface ocean δ¹⁸O is perturbed by evaporation, precipitation, and continental run-off. The sea-surface temperature (SST) can instead be obtained from the Mg/Ca and Sr/Ca ratios in the carbonates produced by organisms living in the photic zone, typically corals, pelagic foraminifera, and coccolithophores. The δ¹⁸O values of fish tooth enamel (phosphate), which is more resistant to diagenetic modification than carbonates, are a useful temperature proxy. Sea-surface temperature is also obtained from the relative abundances of alkenones extracted from sediments.

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