Abstract
Abstract. Synchronized transitions in the polymorph mineralogy of the major reef-building and sediment-producing calcareous marine organisms and abiotic CaCO3 precipitates (ooids, marine cements) throughout Phanerozoic time are believed to have been caused by tectonically induced variations in the Mg/Ca ratio of seawater (molar Mg/Ca>2="aragonite seas", <2="calcite seas"). Here, I assess the geological evidence in support of secular variation in seawater Mg/Ca and its effects on marine calcifiers, and review a series of recent experiments that investigate the effects of seawater Mg/Ca (1.0–5.2) on extant representatives of calcifying taxa that have experienced variations in this ionic ratio of seawater throughout the geologic past. Secular variation in seawater Mg/Ca is supported by synchronized secular variations in (1) the ionic composition of fluid inclusions in primary marine halite, (2) the mineralogies of late stage marine evaporites, abiogenic carbonates, and reef- and sediment-forming marine calcifiers, (3) the Mg/Ca ratios of fossil echinoderms, molluscs, rugose corals, and abiogenic carbonates, (4) global rates of tectonism that drive the exchange of Mg2+ and Ca2+ along zones of ocean crust production, and (5) additional proxies of seawater Mg/Ca including Sr/Mg ratios of abiogenic carbonates, Sr/Ca ratios of biogenic carbonates, and Br concentrations in marine halite. Laboratory experiments have revealed that aragonite-secreting bryopsidalean algae and scleractinian corals and calcite-secreting coccolithophores exhibit higher rates of calcification and growth in experimental seawaters formulated with seawater Mg/Ca ratios that favor their skeletal mineral. These results support the assertion that seawater Mg/Ca played an important role in determining which hypercalcifying marine organisms were the major reef-builders and sediment-producers throughout Earth history. The observation that primary production increased along with calcification within the bryopsidalean and coccolithophorid algae in mineralogically favorable seawater is consistent with the hypothesis that calcification promotes photosynthesis within some species of these algae through the liberation of CO2. The experiments also revealed that aragonite-secreting bryopsidalean algae and scleractinian corals, and bacterial biofilms that secrete a mixture of aragonite and high Mg calcite, began secreting an increased proportion of their calcium carbonate as the calcite polymorph in the lower-Mg/Ca experimental seawaters. Furthermore, the Mg/Ca ratio of calcite secreted by the coccolithophores, coralline red algae, reef-dwelling animals (crustacea, urchins, calcareous tube worms), bacterial biofilms, scleractinian corals, and bryopsidalean algae declined with reductions in seawater Mg/Ca. Notably, Mg fractionation in autotrophic organisms was more strongly influenced by changes in seawater Mg/Ca than in heterotrophic organisms, a probable consequence of autotrophic organisms inducing a less controlled mode of calcification simply through the removal of CO2 via photosynthesis. These results indicate that biomineralogical control can be partially overridden by ambient seawater Mg/Ca and suggest that modern aragonite-secreting organisms may have secreted a mixture of aragonite and low Mg calcite, and that modern high Mg calcite-secreting organisms probably secreted low Mg calcite, in calcite seas of the past. These effects of seawater Mg/Ca on the polymorph mineralogy and calcite Mg/Ca ratio of calcareous skeletons should be accounted for in thermal-chemical reconstructions of seawater that are based upon skeletal Mg/Ca. Lastly, by identifying how marine calcifiers respond to changes in seawater Mg/Ca and absolute Ca2+ concentration, this work should enhance our interpretation of parallel studies investigating the effects of anthropogenic CO2-induced ocean acidification on marine calcification.
Highlights
Chemical analyses of fluid inclusions derived from ancient primary marine halite indicate that the Mg/Ca ratio1 of seawater has varied between 1.0 and 5.2 throughout the Phanerozoic Eon (Lowenstein et al, 2001, 2003, 2005; Horita et al, 2002; Brennan et al, 2004; Timofeeff et al, 2006)
I evaluate the geological evidence in support of secular variation in seawater Mg/Ca and its effects on marine calcification and review experiments that explore the effects of seawater Mg/Ca on the biomineralization of extant representatives of the calcifying taxa that were subjected to the alternating calcite and aragonite seas of Phanerozoic time
The discovery that coral skeleton polymorph mineralogy varies with the Mg/Ca of seawater suggests that scleractinian corals may have secreted at least part of their skeleton as low Mg calcite in the calcite seas of Late Cretaceous and early Cenozoic time, when they existed as isolated colonies and did not build the massive, widespread reefs that they do today
Summary
Chemical analyses of fluid inclusions derived from ancient primary marine halite indicate that the Mg/Ca ratio of seawater has varied between 1.0 and 5.2 throughout the Phanerozoic Eon (Lowenstein et al, 2001, 2003, 2005; Horita et al, 2002; Brennan et al, 2004; Timofeeff et al, 2006) These secular variations in seawater Mg/Ca are thought to be responsible for systematic changes in the polymorph mineralogy of ooids and marine cements throughout Phanerozoic time (Sandberg, 1983). Stanley and Hardie (1998, 1999) observed that the carbonate mineralogy of simple, hypercalcifying organisms has varied in concert with Sandberg’s (1983) aragonite and calcite seas throughout Phanerozoic time (Fig. 1) They conclude that certain hypercalcifying organisms were only able to function as major reef builders (corals, sponges and algae) and carbonate sediment producers (algae) when their CaCO3 polymorph mineralogy was favored by the Mg/Ca ratio of seawater.
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