A simple biomineralization model to explain Li, Mg, and Sr incorporation into aragonitic foraminifera and corals

Abstract

In contrast to Li/Ca and Mg/Ca, Li/Mg is strongly anticorrelated with growth temperature in aragonites precipitated by the benthic foraminifer Hoeglundina elegans and a wide range of scleractinian coral taxa. We propose a simple conceptual model of biomineralization that explains this pattern and that is consistent with available abiotic aragonite partition coefficients. Under this model the organism actively modifies seawater within its calcification pool by raising its [Ca 2+ ], using a pump that strongly discriminates against both Li + and Mg 2+. Rayleigh fractionation during calcification effectively reverses this process, removing Ca 2+ while leaving most Li + and Mg 2+ behind. The net effect of these two processes is that Li/Mg in the calcifying fluid remains very close to the seawater value, and temperature-dependent abiotic partition coefficients are expressed in the biogenic aragonite Li/Mg ratio. We further show that coral Sr/Ca is consistent with this model if the Ca 2+ pump barely discriminates against Sr 2+. In H. elegans the covariation of Sr/Ca and Mg/Ca requires either that the pump more strongly discriminates against Sr 2+ , or that cation incorporation is affected by aragonite precipitation rate via the mechanism of surface entrapment. In either case Li/Mg is minimally affected by such 'vital effects' which plague other elemental ratio paleotemperature proxies.

Long‐Term Impacts of the 1997–1998 Bleaching Event on the Growth and Resilience of Massive Porites Corals From the Central Red Sea

“…For instance, it has been suggested that Mg 2+ and Li + are both transported to the site of calcification via the direct paracellular transport of ambient seawater before being incorporated into the aragonite crystal by similar physicochemical mechanisms that still remain the subject of active debate (Case et al, 2010;Paolo Montagna et al, 2014;Rollion-Bard & Blamart, 2015). Regardless of the specific mechanisms, differences in biogenic crystal growth mechanics appear to have little direct effect on the Li/Mg ratio in the coral skeleton, thus making this a relatively robust proxy for ambient seawater temperature that is essentially independent of vital effects (Marchitto et al, 2018;Paolo Montagna et al, 2014;P. Regardless of the specific mechanisms, differences in biogenic crystal growth mechanics appear to have little direct effect on the Li/Mg ratio in the coral skeleton, thus making this a relatively robust proxy for ambient seawater temperature that is essentially independent of vital effects (Marchitto et al, 2018;Paolo Montagna et al, 2014;P.…”

“…While inorganic experiments suggest that Li + is incorporated into aragonite through heterovalent substitution for Ca 2+ (as opposed to homovalent substitution by Mg 2+ ; Marriott et al, 2004), other experimental and theoretical evidence suggests that both Li and Mg are incorporated through defects of the aragonite crystal (Paolo Montagna et al, 2014) and/or some other entrapment processes (Finch & Allison, 2008;Watson, 1996Watson, , 2004. Regardless of the specific mechanisms, differences in biogenic crystal growth mechanics appear to have little direct effect on the Li/Mg ratio in the coral skeleton, thus making this a relatively robust proxy for ambient seawater temperature that is essentially independent of vital effects (Marchitto et al, 2018;Paolo Montagna et al,…”

Calibration of Sr/Ca, Li/Mg and Sr‐U Paleothermometry in Branching and Foliose Corals

“…These issues have prompted the development of alternative paleothermometers, such as Li/Ca (Marriott et al, ) and Li/Mg (Case et al, ; Hathorne, Felis, et al, ; Lear et al, ; Montagna et al, ; see also Fowell et al, ; Marchitto et al, ). Since both Li + and Mg 2+ are not preferentially incorporated into the aragonite lattice ( K D << 1), the ratio of [Li + ]/[Mg 2+ ] remains relatively constant in the calcifying fluid as precipitation proceeds, even though [Ca 2+ ] would be expected to decline (Montagna et al, ).…”

“…Since both Li + and Mg 2+ are not preferentially incorporated into the aragonite lattice ( K D << 1), the ratio of [Li + ]/[Mg 2+ ] remains relatively constant in the calcifying fluid as precipitation proceeds, even though [Ca 2+ ] would be expected to decline (Montagna et al, ). Li/Mg is thought to be largely independent of Rayleigh fractionation and instead should reflect primarily the different temperature sensitivities of Li and Mg to incorporation (Hathorne, Felis, et al, ; Marchitto et al, ; Montagna et al, ). While a general exponential trend exists between Li/Mg and temperature (Marchitto et al, ; Montagna et al, ), some effects of species and location have been reported (Fowell et al, ).…”

“…Li/Mg is thought to be largely independent of Rayleigh fractionation and instead should reflect primarily the different temperature sensitivities of Li and Mg to incorporation (Hathorne, Felis, et al, ; Marchitto et al, ; Montagna et al, ). While a general exponential trend exists between Li/Mg and temperature (Marchitto et al, ; Montagna et al, ), some effects of species and location have been reported (Fowell et al, ). Accurate temperature constructions using Li/Mg may therefore require species and possibly site‐specific calibrations (Fowell et al, ).…”