OCEANIC-ATMOSPHERIC CHANGE AND THE EVOLUTION OF THE PRECAMBRIAN CARBONATE SYSTEM

Carbonate minerals are intimately linked to the evolution of the Earth’s biosphere. Clearly, the enzymatic production of carbonate biominerals stands as one of the most remarkable events witnessed by the Earth’s carbonate system. This clear association between biology and carbonate mineral formation has, however, also resulted in a view of mineral evolution in which biological evolution, biological mediation, and biological control of mineralization play primary roles in carbonate fabric development through space and time. In contrast to this bio-centric viewpoint, there is substantial evidence from the Earth’s Precambrian record that biotic innovation likely played a far less important role in the determination of carbonate minerals and carbonate fabrics than did fundamental changes in the abiotic parameters that affect mineral precipitation.

Many fundamental changes in mineral evolution have resulted from long-term changes in the elemental make-up of Earth systems associated with the progressive oxygenation of the Earth’s ocean-atmosphere system. These trends are apparent, as well, in examination of the carbonate system. Independent evidence derived from the non-steady state behavior of carbon and sulfur isotope systems demands that the Precambrian ocean-atmosphere system experienced a progressive evolution from CO2-rich and O2 [SO4]-poor, to CO2-poor and O2 [SO4]-rich. Whereas CO2-availability fundamentally affects carbonate saturation state, redox evolution appears to have played a fundamental role in regulating both carbonate mineralogy and the fabric-controlling elements of nucleation and growth. As a result, it is within the protracted “boring billion” (~1.0 Ga to 2.0 Ga; Hazen & Ferry 2010) that the greatest complexity of fabric within the carbonate system is preserved. During this interval, endmember carbonate fabrics (e.g., herringbone carbonate, cyanobacterial calcification), as well as novel, non-actualistic fabrics (e.g., molar-tooth carbonate) are distributed spatially within sedimentary environments. Deciphering relationships among these environments permits attribution of carbonate fabrics to specific geochemical conditions within the water column and provides critical feedback for understanding the temporal evolution of the ocean-atmosphere system.