To reconstruct the salinity, the WHOI team analyzed sediments containing highly resistant organic compounds called alkenones, which are uniquely produced by Emiliania huxleyi the same photosynthetic organism oceanographers study to determine past sea surface temperatures. By examining the ratio of two hydrogen isotopes in the alkenones, they were able to map the salinity trend in the Black Sea over the last 6,500 years.
"One of the isotopes, deuterium, is not very common in nature," explains WHOI marine paleoecologist Marco Coolen, "and it doesn't evaporate as easily as other isotopes. Higher ratios of deuterium are indicative of higher salinity."
Salinity began to rise in the Black Sea about 9000 years ago, when the ocean invaded the previously freshwater lake through the Straits of Bosporus, and continued to increase until approximately 3000 years ago, when the levels approached normal ocean values.
"But the trend since then is counter-intuitive," says Giosan. "The entire basin freshened, especially in the last 1,500 years." The likely explanation points to a increase in the river input combined with a reduced evaporation as the climate cooled at the time, he adds.
Such an influx of nutrient-rich river water would affect the composition of the ecosystem, and would be reflected by the ecological history of major phytoplankton groups in the Black Sea, such as diatoms and dinoflagellates.
Researchers traditionally conduct paleo studies of phytoplankton by using a microscope to count the fossil skeletons found in sediment cores. But, this method is limited because some phytoplankton leave no fossils, so, instead, Coolen looked for sedimentary genetic remains of the past inhabitants of the Black Sea water column.
"DNA offers the best opportunity to learn the past ecology of the Black Sea," says Coolen. "Calcareous and organic-walled dinocysts are frequently used to reconstruct past environmental condi
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Woods Hole Oceanographic Institution