Scleractinian Fossil Corals as Archives of Seawater δ26Mg

Abstract:

The recovery of environmental signatures from coral skeletons is often made difficult by ‘vital effects’, which cause skeletal chemistry to deviate from the expected composition of aragonite in equilibrium with seawater. Recent studies show that Mg isotopes in scleractinian corals are subject to vital effects, which appear as a departure of the δ²⁶Mg coral temperature dependence from that of inorganic aragonite [1]. However, different from the case for Mg/Ca or δ⁴⁴Ca in coral, the magnitude of the observed Mg-isotope vital effect is small (on the order of 0.1 ‰ or less). In addition, measurements of different species of modern coral show similar fractionations, suggesting that coral δ²⁶Mg is not species dependent [2]. Together, these observations indicate that corals should faithfully record the seawater Mg-isotope composition, and that vital effects will not bias reconstructions.

We measured Mg isotopes in a set of extremely well-preserved fossil scleractinian corals, ranging in age from Jurassic through Recent, to reconstruct past seawater δ²⁶Mg. Well-preserved fossil corals of similar age show a range in δ²⁶Mg of ~0.2 ‰, pointing to the presence of vital effects. However, our results show little variability in the δ²⁶Mg of fossil corals across different geologic ages, suggesting that seawater δ²⁶Mg has remained relatively constant throughout the Cenozoic and Mesozoic. This pattern has implications for our understanding of the mechanisms driving secular variations in seawater Mg/Ca. In particular, our data imply that dolomitization rates have not changed enough during the Mesozoic and Cenozoic to account for secular variations in seawater Mg/Ca.

Our coral δ²⁶Mg record agrees with a Cenozoic record from bulk foraminifera, further supporting the faithfulness of the coral archive. However, both of these records disagree with a third Cenozoic Mg-isotope record, derived from species-specific planktic forams [3].

[1] Saenger, C. et al. (2014) Chem. Geol. 360-361, 105-117.

[2] Wombacher, F. et al. (2011) Geochim. Cosmochim. Acta 75, 5797-5818.

[3] Pogge von Strandmann, P.A.E. et al. (2014) Biogeosciences Discuss. 11, 7451-7484.