Review: geological and experimental evidence for secular variation in seawater Mg/Ca (calcite-aragonite seas) and its effects on marine biological calcification

Abstract

Abstract. Synchronized transitions in the polymorph mineralogy of the major reef-building and sediment-producing calcareous marine organisms and abiotic CaCO 3 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 Mg 2+ and Ca 2+ 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 aragonitesecreting 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/CaCorrespondence to: J. B. Ries (jries@unc.edu) 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 CO 2 .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 ...

Skeletal mineralogy of geniculate corallines: providing context for climate change and ocean acidification research

“…greater than 8−12% MgCO 3 ) is more soluble than aragonite in seawater . Species depositing this polymorph are therefore likely to be more susceptible to the initial effects of OA (Gao et al 1993, Morse et al 2007, Kuffner et al 2008, Ries et al 2009, Ries 2010, Lombardi et al 2011.…”

“…crustose coralline algae, or free-living nodules, known as rhodoliths or maerl (Irvine & Chamberlain 1994, Nelson 2009). Coralline algae have limited control over their calcification processes in that they are able to specify deposition of the calcite polymorph, as opposed to aragonite, but are unable to actively control the degree of Mg incorporation into their calcite skeletons (Ries 2010). Variation in Mg content is controlled by mechanisms including the Mg/Ca ratio of seawater, which is only applicable over geological timescales (Ries 2006(Ries , 2010, and factors that influence growth rate, e.g.…”

“…Within the marine environment, different biogenic polymorphs of CaCO 3 are deposited, each with different solubility in seawater (Ries 2011). Aragonite, the polymorph deposited by, for example, scleractinian coral species, is more soluble than pure calcite; however, the solubility of calcite increases with increasing magnesium ion (Mg 2+ ) content substituting for calcium (Ca 2+ ) ions , Ries 2010, 2011. High-Mg biogenic calcite (i.e.…”

Long-term exposure to elevated pCO2 more than warming modifies early-life shell growth in a temperate gastropod

“…The magnesium: calcium (Mg 2þ :Ca 2þ ) ratio in seawater influences organic calcification processes on a microscopic level, so acidification can tip calcification towards the deposition of specific forms (Ries, 2010). Low levels of Mg 2þ favour the formation of calcite, and high levels favour the deposition of aragonite (Ries, 2010). Juvenile molluscs preferentially deposit aragonite, possibly due to weaker controls over the early biomineralisation processes (Weiss et al, 2002), and on approaching maturity, calcite deposition increases.…”

“…In addition, seawater magnesium carbonate (MgCO 3 ) may also become under-saturated because of carbonate chemistry changes in seawater. The magnesium: calcium (Mg 2þ :Ca 2þ ) ratio in seawater influences organic calcification processes on a microscopic level, so acidification can tip calcification towards the deposition of specific forms (Ries, 2010). Low levels of Mg 2þ favour the formation of calcite, and high levels favour the deposition of aragonite (Ries, 2010).…”