Cambrian sea water preserved as inclusions in marine low-magnesium calcite cement

THE existence of temporal changes in the chemical composition of the oceans, which could provide constraints on the potential variability of the ocean-atmosphere; system, remains an open question. Assessments of the chemistry of ancient oceans have relied largely on analysis of marine precipitates, generally carbonate and evaporite minerals¹. These studies suggest that, whereas marine salinity has remained relatively stable over Phanerozoic time¹, magnesium, calcium and sulphate concentrations of ancient oceans and CO₂ partial pressure of ancient atmospheres may have changed^2-4. The ratios of isotopes of carbon, oxygen, sulphur and strontium also appear to have varied^3,5,6. Here we present analyses of primary, one-phase fluid inclusions in Cambrian and Ordovician marine cements, which appear to represent aliquots of early Palaeozoic oceans. The cements have trace element, stable isotope and strontium isotope contents that are consistent with their having been precipitated in a Cambrian-Ordovician marine environment, and the fluids have marine salinities. As these (apparently primary) cements are low-magnesium calcite, unlike the predominantly high-magnesium calcite and aragonite of today's carbonate precipitates, the chemistry of the Cambrian ocean-atmosphere system seems to have been different from that of today.