Until recently, most earth scientists held that the seas have remained chemically constant over hundreds of millions of years. But a new analysis of ancient seawater has confirmed what some scientists began to suspect several years ago: During the last half-billion years or so, the oceans have fluctuated between varying chemistries.
The result could explain why some ancient organisms flourished in the oceans at certain times and not others, comments Steven M. Stanley of Johns Hopkins University in Baltimore.
In the Nov. 2 Science, Tim K. Lowenstein of the State University of New York in Binghamton and his colleagues report that the shifts in ocean chemistry correlate with rates of movements in Earth’s massive tectonic plates. During periods of rapid versus slow seafloor spreading, different dissolved ions become prevalent in the waters, says Lowenstein.
The fluctuations also correlate with periods when certain salts precipitated out of the oceans and when mineral-making animals, such as sponges and corals, switched between the production of two crystal forms of calcium carbonate. One, called aragonite, develops more readily when abundant magnesium is available. The other, calcite, forms in the presence of abundant calcium.
Stanley and Lawrence A. Hardie, an earth scientist at Johns Hopkins and a coauthor of the report, had conjectured that fluctuating ocean chemistries are responsible for oscillating calcite and aragonite reef production by simple marine organisms.
To identify the ions in ancient oceans, Lowenstein’s team obtained salt crystals from drilled cores and salt mines around the world and examined seawater inclusions trapped within them. After freezing the samples, the researchers could identify ions in cubic inclusions as small as 30 micrometers on a side using a technique called environmental scanning electron microscopy.
Scientists have more confidence in their knowledge of how and when inclusions form if they are small rather than large. However, researchers who had previously studied ancient seawater inclusions had required samples large enough to extract with a pipette. The results of chemical analyses of large inclusions may not accurately reflect the sea chemistry at any one time, notes Robert H. Goldstein of the University of Kansas in Lawrence.
By using the electron microscopy technique to analyze small inclusions, Lowenstein and his colleagues found that seawater from different time periods had “extraordinary” differences, Lowenstein says.
Water from the Late Precambrian, Permian, and Tertiary periods, for example, had a chemistry similar to today’s oceans, the researchers report.
Seas at these times had high concentrations of sodium ions and far more magnesium ions than calcium ions. That chemistry is consistent with the production of aragonite, rather than calcite, during those geologic periods.
During other periods, including the Cambrian, Silurian, and Cretaceous, ocean water was lower in sodium than it is today and the ratio of magnesium to calcium was also lower, the team found. During these times, simple organisms generally made calcite.
The new results provide the first confirmation of the theory that sea chemistry can explain oscillating calcite and aragonite production, Stanley says.
