Geoscience Reference
In-Depth Information
9
Environments
A major achievement of low-temperature geochemistry is its ability to provide estimates of
variables such as ocean temperature, atmospheric composition and pressure, erosion inten-
sity, 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 biologi-
cal 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).
estimated) from the mean
18
O value of the local precipitation (rain or snow).
2. The amount of ice locked up in polar regions is derived from the average
δ
δ
Dor
18
O value
δ
of seawater.
3. The temperature of deep oceanic water can be obtained from the
18
O values of benthic
δ
foraminifera.
4. The surface ocean
18
O 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
δ
18
O values of fish
δ
