Carbonate veins reveal chemistry of ancient seawater

Feb 05, 2010
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. Credit: Christopher Smith-Duque (NOCS)

The chemical composition of our oceans is not constant but has varied significantly over geological time. In a study published this week in Science, researchers describe a novel method for reconstructing past ocean chemistry using calcium carbonate veins that precipitate from seawater-derived fluids in rocks beneath the seafloor. The research was led by scientists from the University of Southampton's School of Ocean and Earth Science (SOES) hosted at the National Oceanography Centre, Southampton (NOCS).

"Records of ancient seawater chemistry allow us to unravel past changes in climate, and evolution of life in the oceans. These processes affect and have shaped our planet over millions of years," said Dr Rosalind Coggon, formerly of NOCS now at Imperial College London.

"Reconstructing past ocean chemistry remains a major challenge for Earth scientists, but small calcium carbonate veins formed from warm seawater when it reacts with basalts from the provide a unique opportunity to develop such records," added co-author Professor Damon Teagle from SOES.

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 ocean crust.

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 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.

Explore further: Mexico's Volcano of Fire blows huge ash cloud

More information: Coggon, R. M., Teagle, D. A. H., Smith-Duque, C. E., Alt, J. C., & Cooper, M. J. Reconstructing past seawater Mg/Ca and Sr/Ca from mid-ocean ridge flank calcium carbonate veins. Science (published online 4 February 2010). Doi:10.1126/science.1182252

Provided by National Oceanography Centre, Southampton

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