Abstract
Abstract. The isotopic and elemental systematics of boron in aragonitic coral skeletons have recently been developed as a proxy for the carbonate chemistry of the coral extracellular calcifying fluid. With knowledge of the boron isotopic fractionation in seawater and the B∕Ca partition coefficient (KD) between aragonite and seawater, measurements of coral skeleton δ11B and B∕Ca can potentially constrain the full carbonate system. Two sets of abiogenic aragonite precipitation experiments designed to quantify KD have recently made possible the application of this proxy system. However, while different KD formulations have been proposed, there has not yet been a comprehensive analysis that considers both experimental datasets and explores the implications for interpreting coral skeletons. Here, we evaluate four potential KD formulations: three previously presented in the literature and one newly developed. We assess how well each formulation reconstructs the known fluid carbonate chemistry from the abiogenic experiments, and we evaluate the implications for deriving the carbonate chemistry of coral calcifying fluid. Three of the KD formulations performed similarly when applied to abiogenic aragonites precipitated from seawater and to coral skeletons. Critically, we find that some uncertainty remains in understanding the mechanism of boron elemental partitioning between aragonite and seawater, and addressing this question should be a target of additional abiogenic precipitation experiments. Despite this, boron systematics can already be applied to quantify the coral calcifying fluid carbonate system, although uncertainties associated with the proxy system should be carefully considered for each application. Finally, we present a user-friendly computer code that calculates coral calcifying fluid carbonate chemistry, including propagation of uncertainties, given inputs of boron systematics measured in coral skeleton.
Highlights
Quantifying the carbonate chemistry of the fluid from which corals accrete their skeletons is essential for understanding the mechanisms of skeletal growth and the sensitivity of skeletal composition to environmental variability
Evidence from skeletal geochemistry and fluorescent dye experiments suggests that while seawater is the initial source of the calcifying fluid (McConnaughey, 1989; Adkins et al, 2003; Cohen and McConnaughey, 2003; Gagnon et al, 2012; Tambutté et al, 2012), the carbonate chemistry of the calcifying fluid is subject to substantial modifications that enhance the rapid nucleation and growth of aragonite crystals (Al-Horani et al, 2003; Venn et al, 2011)
Our analysis suggests that this KD formulation is poorly suited for accurately reconstructing fluid [CO23−] from boron systematics (Figs. 3d, 4)
Summary
Quantifying the carbonate chemistry of the fluid from which corals accrete their skeletons is essential for understanding the mechanisms of skeletal growth and the sensitivity of skeletal composition to environmental variability. Allison et al (2014) and Allison (2017) considered exchange reactions in which borate substitutes for bicarbonate (HCO−3 ), with the partition coefficient [B/Ca]aragonite This approach resolves the issue of charge balance and would account for CO23− reacting with H+, removing the pH dependence expected from Eq (8). The sensitivity of KD to [CO23−] or Ar is consistent with a surface kinetic model (DePaolo, 2011), in which trace element partitioning depends on the net rate of precipitation relative to dissolution Both the surface entrapment and kinetic models offer potential explanations as to why the low- Ar experiments of Mavromatis et al (2015) produced lower KD than the higher- Ar experiments of Holcomb et al (2016)
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