A re-evaluation of aragonite versus calcite seas

Abstract

Some workers have argued that the mineralogy of ancient carbonates may have been different from that of modern sediments, with calcite being considered the dominant mineral during the Ordovician, Devonian-mid Carboniferous, and Jurassic-Cretaceous to Early/Middle Cenozoic (e.g. Sandberg 1983; Wilkinson and Algeo 1989). Variation in carbonate mineralogy has been related to the position of global sea level (emergent or submergent modes, Wilkinson et al. 1985), change in rates of seafloor spreading (e.g. Mackenzie and Pigott 1981; Hardie 1996), PCO2 level (e.g. Sandberg 1985; Mackenzie and Morse 1992; Hallock 1997) and Mg/Ca ratios related to spreading rate (e.g. Stanley and Hardie 1998). However, other researchers suggest that the assumption of change of original carbonate mineralogy through time needs to be re-evaluated in the light of mineralogical change that is related to water temperature or latitudes (e.g. Nelson 1988). Evaluation of Ordovician (Arenig to Ashgill) Gordon Group carbonates of Tasmania (Australia), based on petrographic (e.g. abundant Chlorozoan biota, and oomold texture) and geochemical criteria (such as high Sr/Na ratios) indicated that aragonite (not calcite) was the predominant mineral in these warm water, subtropical carbonates (Rao 1990). Petrographic (e.g. presence or absence of aragonite relicts, abundant acicular to fibrous isopachous marine cement, presence of diffuse laminae and a number of spalled ooids) and geochemical evidences (such as elevated Sr) in the Upper Jurassic Mozduran limestone, in Kopet-Dagh Basin in northeast Iran, showed variation in carbonate mineralogy, in spite of similar atmospheric PCO2 level, global sea-level and tectonic setting. This is evidenced by aragonite occurring in the shallowest part of the basin (Adabi and Rao 1991) and mainly calcite with some aragonite forming in the relatively deeper water (below wave base) (Adabi 1997). Carbonate mineralogy in Recent shallow marine carbonates, and in experimental studies, varies with seawater temperature. In the Recent, aragonite is the predominant mineral in warm, shallow marine carbonates and calcite the dominant mineral in marine cool water carbonates (James and Clarke 1997). Therefore, variations in carbonate mineralogy in the Mozduran limestone are attributed to seawater temperature assuming invariant seawater chemistry prevailed in the Upper Jurassic. Several Jurassic examples show variations in ooid carbonate mineralogy, such as the Upper Jurassic Smackover oolite of the Gulf Coast region (southern Arkansas and northern Louisiana) and Upper Jurassic ooids in the Purbeck limestones of Swiss and French Jura. These results are not in agreement with the concept of a "calcite sea" during the Ordovician and the Upper Jurassic periods. Very recently, Westphal and Munnecke (2003) showed that in spite of the tendency of abiotic precipitates (Sandberg 1983) and skeletal mineralogy (Stanley and Hardie 1999) to follow the general trend of calcite seas and aragonite seas, organisms with calcite and aragonite mineralogy coexisted throughout the Phanerozoic. They have examined the temporal and spatial distribution of limestone-marl alternations in Ordovician, Jurassic and Cretaceous (times of calcite seas). Limestone-marl alternations are most abundant in settings that favored aragonite production and accumulation analogous to the modern environment (Westphal and Munnecke 2003). If the above observations confirmed, the proposed secular variation in Phanerozoic carbonate mineralogy needs to be re-evaluated.