|Ancient droplets. Images of a single water droplet (top) and multiple water droplets (far left), trapped in salt crystals, are illuminating how ocean chemistry changed over time.
Ancient seas are often preserved as limestone and, when they were drying up, as salt deposits. In the early 1980s, geologists noticed that the mineral content of these rocks varies according to their age. Some geologists theorized that this variation reflected shifting sea chemistry, disputing the assumption that seawater composition was static. Lawrence Hardie, a Johns Hopkins University geochemist, believed that ocean chemistry changed according to the movement of tectonic plates: The magma released when the plates move apart, he reasoned, absorbs magnesium, releases calcium, and thereby alters seawater. But although geologists thought that mid-oceanic ridge activity might have influenced seawater across the globe, they'd never managed to demonstrate that seawater had actually changed.
So Hardie teamed up with Tim Lowenstein and his colleagues at the State University of New York in Binghamton, who thought shifting ocean chemistry might reveal itself in ancient salt deposits. As they report in the 2 November issue of Science, the scientists collected salt crystals from around the world that contained densely packed droplets of water. As small as 30 micrometers, they were tiny to extract, so the fluid was frozen, sliced into sections, and examined with an electron microscope. They found that the magnesium-calcium ratio was the same around the world at any given time, but varied through history--an indication that seawater chemistry had changed.
Lowenstein wonders whether the oscillations may have affected the evolution of ocean organisms such as sponges and algae. Hardie points out that during the Cretaceous period calcium-absorbing planktonic algae thrived. When sea-flow spreading slowed, one such species went extinct.
The paper adds additional weight to the growing belief that seawater changed in composition, says Heinrich Holland, a Harvard University geochemist. But, he notes, "I think it's going to be a few more years before the whole question [of why ocean chemistry shifted] gets really sorted out."
Tim Lowenstein's home page
Lawrence Hardie's home page
The story of plate tectonics