REEFAL MICROBIAL CARBONATE ABUNDANCE: COMPARISONS WITH PHANEROZOIC METAZOAN DIVERSITY, MASS EXTINCTION EVENTS, AND SEAWATER SATURATION STATE

Reefal microbial carbonate (RMC) abundance (Kiessling 2002, SEPM Spec. Publ. 72: 625-690) shows an oscillating and declining Phanerozoic trend; it varies inversely with metazoan diversity (Sepkoski 1997, J. Paleo. 71: 533-539) and directly with calculated seawater saturation state (Riding & Liang 2004, Goldschmidt Conference, Copenhagen, Geochim. Cosmochim. Acta Suppl.: A354). RMCs responded most to metazoan diversity and saturation state when these influences reinforced one another. Thus, when metazoan diversity was low and saturation state high, RMCs increased; e.g., during the Cambrian-Early Ordovician and Late Silurian, and following end-Frasnian, end-Permian, and end-Jurassic extinctions. Conversely, when metazoan diversity was high and saturation state low, RMCs declined, e.g., during the Early Silurian, Late Carboniferous, Mid-Jurassic, and Cenozoic. When metazoan diversity and saturation state trends did not correspond, their effects were less marked, and this was often the case following Mass Extinction (ME) events. Changes in RMC abundance immediately following the five major MEs were: end-Ordovician, 51% decrease; end-Frasnian, 370% increase; end-Permian, 17% increase; end-Triassic, 17% decrease; end-Cretaceous, no change. Very substantial increase in RMC abundance only occurred in the aftermath of the end-Frasnian event, when saturation state was high, whereas at other MEs RMCs either decreased or did not alter very significantly. Thus, (i) end-Ordovician extinction affected metazoans and RMCs alike, and RMC decrease may reflect low temperatures that reduced saturation state; (ii) prior to the end-Permian ME RMCs were already abundant, possibly due to elevated saturation state, and increase in the aftermath was relatively slight; (iii) just prior to the end-Cretaceous ME, RMCs were at a minimal level possibly due to low saturation state, and were not noticeably affected by metazoan extinctions. Present-day coincidence of high metazoan diversity and low saturation state results in RMCs being scarcer than at almost any other time in the Phanerozoic. Although RMC presence was noticeable in all major ME aftermaths apart from the end-Cretaceous, these data indicate that their abundance did not increase very significantly except in the end-Frasnian aftermath.