Biomineralization and Tentative Links to Plate Tectonics
Tectonic processes appear to be a principal forcing function controlling some aspects of the sedimentary cycling of materials during the geologic past. Chemical trends observed in the sedimentary rock record for the past 80 Ma substantiate the hypothesis that plate tectonics play a major role in influencing, on a long-time scale, several variables of ocean-atmosphere-sediment chemistry. The apparent long-term decrease in sea level since the Late Cretaceous (80 Ma) was probably caused by a global decrease in sea-floor spreading rate and ridge volume. This waning of tectonic activity may have resulted in reduced volcanic and diagenetic-metamorphic CO2 release to the earth’s surface environment, giving rise to lower atmospheric CO2 level and temperatures. The carbonate saturation state and the Mg/Ca, Sr/Ca, Na/Ca, 87Sr/86Sr, and 143Nd/144Nd ratios of sea water may have progressively increased during the sea level fall. It is postulated that the global environmental response to increased tectonic activity probably is opposite to that proposed for the past 80 Ma.
Physical and chemical changes in earth’s surface environment induced by plate tectonic processes may have been the cause of some observed changes in the composition of skeletal and non-skeletal carbonates during the Phanerozoic. The inferred mineralogy of carbonate ooids and cements appears to vary in an oscillatory manner between aragonite and calcite, perhaps reflecting cyclic variations in sea water carbonate saturation state and Mg/Ca ratio. Inferred variations in skeletal carbonate mineralogy suggest a progressive increase with decreasing geologic age in the deposition of skeletal high-magnesian calcite and aragonite, marked by the appearance of two principal groups of modern reef-building organisms, hermatypic corals (aragonite) and coralline algae (high-magnesian calcite), in the mid to late Mesozoic (200 Ma). This evolutionary mineralogical change may reflect a long-term trend of decreasing atmospheric CO2 and increasing sea water carbonate saturation state and Mg/Ca ratio.
Experimental evidence indicates that the solubility of both skeletal and non-skeletal Mg-calcites is a function of the Mg content of the calcite. The Mg content of the coralline alga, Porolithon gardineri, grown in controlled microcosm environments, increased with increasing temperature and calcite saturation state of sea water. Experimental data confirm the hypothesis that skeletal and non-skeletal carbonate compositions can be a function of sea water carbonate saturation state, Mg/Ca ratio, and temperature.
It appears that the fossil record of biomineralization may be linked to internal earth processes through the influence of plate tectonic events on the physico-chemistry of the surface environment. Gaian and extraterrestrial processes as forcing functions for biological change need to be considered in light of earth’s plate tectonic history.
KeywordsPlate Tectonic Benthic Foraminifera Coralline Alga Carbonate Compensation Depth Carbonate Saturation State
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