Magazine article Oceanus

Ground-Truthing the Paleoclimate Record: Sediment Trap Observations Aid Paleooceanographers

Magazine article Oceanus

Ground-Truthing the Paleoclimate Record: Sediment Trap Observations Aid Paleooceanographers

Article excerpt

The geological record contains a wealth of information about Earth's past environmental conditions. During its long geological history the planet has experienced changes in climate that are much larger than those recorded during human history; these environmental conditions range from periods when large ice sheets covered much of the northern hemisphere, as recently as 20,000 years ago, to past atmospheric concentrations of greenhouse gases that warmed Earth's polar regions enough to melt all of the ice caps 50 million years ago. Since human civilization has developed during a fairly short period of unusually mild and stable climate, humans have yet to experience the full range of variability that the planer's natural systems impose. Thus, the geological record has become an extremely important archive for understanding the range of natural variability in climate, the processes that cause climate change on decadal and longer time scales, and the background variability from which greenhouse warming must be detected.

One of the principal archives of climate information is the fossil record in deep sea sediments. For many years it has been known that seawater temperature plays an important role in the biogeography of many organisms living in the oceans: Tropical and polar regions foster different species. Using the fossilized remains of these organisms in deep sea sediment cores, marine geologists can reconstruct past changes in sea surface temperature and produce records that document past variations in sea surface conditions caused by changes in the earth-sun orbital geometry and by abrupt changes in ocean circulation. The major fossil-producing organisms for this type of research have been the foraminifera, single-celled animals with planktonic (floating) and benthonic (bottom-dwelling) varieties. These organisms produce an easily identified sand-grain-sized structure made of calcium carbonate (called a test) that is unique for each species and is well preserved in sediments found in nearly all of the major ocean basins.

Early 1980s studies of the geographic occurrence of the various species of foraminifera suggested that temperature was a primary control on their distribution. The observations were based on plankton tows of the upper water column and the distribution of fossilized remains in surface sediments of the seafloor below. The species could easily be grouped into tropical, subtropical, and polar assemblages ([ILLUSTRATION OMITTED] below), and the parallels with the sea surface temperature distribution were obvious. By mapping the distributions of these species, estimates of past sea surface temperatures were made and paleoceanography was born.

The accuracy of these reconstructions, however, is difficult to assess because biological systems are affected by many more environmental variables than temperature alone and because many of the environmental factors co-vary. For instance, salinity and temperature are highly correlated in the oceans because warmer waters are more likely to have experienced the evaporation that causes greater salinity. Without culturing these organisms in the laboratory (a difficult procedure that has been successful for only a few species), it is very hard to differentiate the effects of salinity changes from temperature changes. We are left with finding alternative ways to document their usefulness as temperature indicators. Using modern observational systems, including sediment traps and satellites, we are evaluating the reliability of foraminifera for temperature estimates under realistic conditions in today's oceans.

Sediment traps are suspended on moorings to sample debris settling through the water column, debris that includes the fossil remains of planktonic foraminifera. Since sediment traps can collect time series samples with preprogrammed temporal resolutions of several weeks to several months, foraminifers, with several-week life spans, are ideally suited for collection using these moored instruments. …

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