Environmental and biological controls on Mg and Li in deep-sea scleractinian corals

Abstract

Deep-sea scleractinian corals precipitate aragonite skeletons that provide valuable archives of past ocean conditions. During calcification biological mediation causes variability in trace metal incorporation and isotopic ratios of the aragonite such that signals caused by environmental controls can be overwhelmed. This complicates the interpretation of geochemical proxies used for paleo-reconstructions. In this study we examine the environmental controls on the Mg/Li ratio of 34 individuals from seven genera of deep-sea scleractinian corals: Desmophyllum, Balanophyllia, Caryophyllia, Enallopsammia, Flabellum, Trochocyanthus, and Lophelia. In addition we examine the distributions of Mg and Li in Desmophyllum and Balanophyllia using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Both Mg/Ca and Li/Ca ratios increased by more than a factor of 2 in the center of calcification regions compared to the outer, fibrous regions of the coral skeleton. As a result, replicate ~10 mg subsamples of coral show less variability in the Mg/Li ratio than Mg/Ca. Microscale Mg and Li results are consistent with Rayleigh-type incorporation of trace metals with additional processes dominating composition within centers of calcification. Comparison of Mg/Li to seawater properties near the site of collection shows that the ratio is not controlled by either carbonate ion or salinity. It appears that temperature is the major control on the Mg/Li ratio. For all 34 samples the temperature correlation (R2=0.62) is significantly better than for Mg/Ca (R2=0.06). For corals of the family Caryophyllidae the R2 value increases to 0.82 with the exclusion of one sample that was observed to have an altered, chalky texture. Despite this excellent correlation the scatter in the data suggests that the Mg/Li ratio of deep-sea corals cannot be used to reconstruct temperature to better than approximately ±1.6°C without better temperature control and additional calibration points on modern coral samples.