Carbonate Veins Reveal Chemistry of Ancient Seawater

Subject:	Carbonate Veins Reveal Chemistry of Ancient Seawater
Date:	Tue, 9 Feb 2010 03:45:56 -0800 (PST)

Photo of calcium carbonate veins at citation

Carbonate Veins Reveal Chemistry of Ancient Seawater

Calcium carbonate veins are common in upper ocean crust, where they
precipitate from low temperature ( (Credit: Christopher Smith-Duque
(NOCS))

ScienceDaily (Feb. 8, 2010) =97 The chemical composition of our oceans
is not constant but has varied significantly over geological time. In
a study published 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, plate tectonics and evolution of life in the
oceans. These processes affect ocean chemistry 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 oceanic crust 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 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.

The research was supported by the United Kingdom's Natural Environment
Research Council and used samples provided by the Ocean Drilling
Program (ODP) and Integrated Ocean Drilling Program (IODP). ODP was
sponsored by the US National Science Foundation (NSF) and
participating countries under management of Joint Oceanographic
Institutions Inc. IODP is supported by NSF; Japan's MEXT; ECORD; and
the People's Republic of China, Ministry of Science and Technology.

http://www.sciencedaily.com/releases/2010/02/100205091823.htm

Published Online February 4, 2010
Science DOI: 10.1126/science.1182252

Science Express Index
Reports
Reconstructing Past Seawater Mg/Ca and Sr/Ca from Mid-Ocean Ridge
Flank Calcium Carbonate Veins
Rosalind M. Coggon,1 Damon A. H. Teagle,2,* Christopher E. Smith-Duque,
2 Jeffrey C. Alt,3 Matthew J. Cooper2

Proxies for past seawater chemistry such as Mg/Ca and Sr/Ca ratios
provide a record of the dynamic exchanges of elements between the
solid Earth, atmosphere and hydrosphere, and the evolving influence of
life. Here, we estimate past oceanic Mg/Ca and Sr/Ca ratios from
suites of 1.6 to 170-million-year-old calcium carbonate veins
precipitated from seawater-derived fluids in ocean ridge flank
basalts. Our data indicate that prior to the Neogene, oceanic Mg/Ca
and Sr/Ca were lower than in the modern ocean. Decreased ocean
spreading since the Cretaceous and the resulting slow reduction in
ocean crustal hydrothermal exchange throughout the early Tertiary may
explain the recent rise in these ratios.

1 Department of Earth Science and Engineering, Imperial College
London, South Kensington Campus, Exhibition Road, London, SW7 2AZ, UK.
2 School of Ocean and Earth Science, National Oceanography Centre,
University of Southampton, Southampton, SO14 3ZH, UK.
3 Department of Geological Sciences, University of Michigan, Ann
Arbor, MI 48109=961005, USA.

http://www.sciencemag.org/cgi/content/abstract/science.1182252v1