Paleoceanography

An Uncompromised Carbonate Record

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Science  26 Oct 2001:
Vol. 294, Issue 5543, pp. 747
DOI: 10.1126/science.294.5543.747d

The strong dependence of atmospheric partial pressure of CO2 (pCO2) on the carbonate ion content of the deep ocean has made reconstruction of the temporal evolution of deep-water carbonate chemistry over glacial cycles an important goal for paleoceanographers. Past variations of deep-ocean CO32− concentrations are recorded in many marine sediments, but most records are compromised by problems such as low sedimentation rates, chemical erosion, and bioturbation.

Hodell et al. demonstrate that the carbonate record from a single site in the deep South Atlantic (Ocean Drilling Program Site 1089) represents a qualitative, high-resolution record of the temporal evolution of the carbonate saturation state of the deep sea. From the phase relations between the site 1089 carbonate signal and benthic δ18O, seawater Sr/Ca, and pCO2, they conclude that sea level helps control carbonate compensation (the accumulation or dissolution of carbonate sediments in response to changes in the distribution of alkalinity and dissolved inorganic carbon) in the deep sea, and that carbonate compensation plays an important role in amplifying pCO2 variations caused by other mechanisms. Additional measurements of dissolution indices as a function of depth may make it possible to determine quantitatively deep-sea carbonate ion changes and to evaluate their impact on pCO2 variability over glacial cycles.—HJS

Earth Planet. Sci. Lett.192, 109 (2001).

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