"These processes affect ocean chemistry and have shaped our planet over millions of years," said Dr Rosalind Coggon, a professor Imperial College London.
Calcium carbonate veins record the chemical evolution of seawater as it flows through the ocean crust and reacts with the rock. The composition of past seawater can therefore be determined from suites of calcium carbonate veins that precipitated millions of years ago in ancient
Calcium carbonate veins are common in upper ocean crust, where they precipitate from low temperature (<100 C) seawater-derived hydrothermal fluids that have reacted with the basaltic lavas that form the ocean floor.
These fluids are modified by chemical exchange with the ocean crust as they heat up. The veins precipitate from the basement fluid at compositions that have evolved away from that of the initial seawater, but from analyses of suites of these veins the chemistry of seawater at that time can be estimated.
(Photo Credit: Christopher Smith-Duque (NOCS))
The researchers reconstructed records of the ratios of strontium to calcium (Sr/Ca) and magnesium to calcium (Mg/Ca) over the last 170 million years. To do this, they analysed calcium carbonate veins from basaltic rocks recovered by several decades of scientific deep-ocean drilling by the Integrated Ocean Drilling Program (IODP) and its predecessors.
"The carbonate veins indicate that both the Sr/Ca and Mg/Ca ratios of seawater were significantly lower than at present prior to about 25 million years ago. We attribute the increases in seawater Sr/Ca and Mg/Ca since then to the long-term effects of decreased seafloor volcanism and the consequent reduction in chemical exchange between seawater and the ocean crust," said Professor Teagle.
Citation: Coggon et al., 'Reconstructing Past Seawater Mg/Ca and Sr/Ca from Mid-Ocean Ridge Flank Calcium Carbonate Veins', Science, February 2010; doi:10.1126/science.1182252