Some of the best fossils to use as proxy data are forams and diatoms. They have been used worldwide to piece together the picture of past climate. Forams and diatoms are shelled microorganisms found in aquatic and marine environments. One of the factors that makes them so versatile is that there are benthic types (those that are bottom dwellers) and planktonic types (those that float in the water), enabling them to represent a large range of the ocean environment.
The shells of forams are composed of calcium carbonate (CaCO3). Diatom shells are composed of silicon dioxide (SiO2). The shells of these microorganisms are important for recording past climates. Paleoclima-tologists study the remains of foram and diatom shells deposited in sediment cores removed from lakes and oceans. (When the microorganisms die, their shells fall to the bottom sediment and are buried.)
Through analysis of the shells, the chemistry of the water can be derived. The oxygen isotope ratios (18O-16O) in their shells can be analyzed to determine what the water temperature was. As discussed earlier, warm water has less 16O, which means that shells that lived in warm water are higher in 18O. Through the collection of thousands of samples of forams and diatoms worldwide, scientists have been able to reconstruct past surface and bottom water temperatures.
In addition to water temperature, it is also possible to make inferences about the environment based on the foram and diatom record. For instance, the species of foram or diatom as well as their abundance
tells its own story. Scientists know that in warm climates the microorganisms multiply. Even more helpful, each species exists in its own set of desirable conditions, so when that species appears in the fossil record, it can be inferred that that particular condition existed at the time. Because tropical, subtropical, and polar species differ, their fossilized remains in deep-sea sediments allow marine geologists to map and date their relative distributions, thereby inferring variations in sea surface conditions caused by climate change and global warming.
Deposits of forams have also enabled paleoclimatologists to make assumptions about ocean circulation and major ocean currents. One specific foram, Neogloboquadrina pachyderma, is found only in polar regions. One form of the species is generally abundant in the coldest waters of the North Atlantic and lives only where the water is colder than 46°F (8°C). The other form exists only where the water is warmer than 46°F (8°C). This distinction enables scientists to track the 46°F (8°C) isotherm (the boundary marking the dividing line between where it is warmer or colder). This is important because it is directly related to the location where deepwater convection occurs in the North Atlantic's heat-carrying currents. Through mapping the foram fossil record, scientists have concluded that ocean circulation is indeed closely linked to rapid changes in climate.
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