Experimental determination of factors controlling U/Ca of aragonite precipitated from seawater: Implications for interpreting coral skeleton

Abstract

The U/Ca ratio of aragonite coral skeleton exhibits coherent patterns of seasonal and interannual variability. In field-sampled corals and those grown in controlled culture experiments, strong correlations have been found between coral skeleton U/Ca and water temperature, pH, carbonate ion concentration, and salinity. However, the mechanism(s) underlying these different correlations remain unclear. We performed abiogenic precipitation experiments designed to evaluate the sensitivity of U partitioning between aragonite and seawater to temperature, pH, and the concentration of carbonate ion in seawater. Aragonite was precipitated from seawater by addition of carbonate alkalinity at rates set to maintain stable carbonate chemistry during precipitation. Experiments were conducted at 20-40°C, with pH 7.8-9.0 and carbonate ion concentrations of 600-2600 lmol kg À1 . U/Ca ratios of the bulk precipitate and fluid were determined by inductively coupled plasma mass spectrometry. Our results show that the U/Ca ratio of aragonite precipitated from seawater decreases with increasing carbonate ion concentration, and is independent of pH and temperature. We use these results as a framework to interpret the skeletal composition of coral aragonite precipitated from a calcifying fluid that is semi-isolated from the external seawater environment. Accordingly, coral U/Ca ratios are consistent with calcifying fluid carbonate ion concentrations that are several times greater than those of ambient seawater. Correlations between coral U/Ca ratios and seawater temperature, carbonate chemistry, and other environmental variables arise indirectly, via the impacts of these variables on the carbonate ion concentration of the coral calcifying fluid.

The Origin and Role of Organic Matrix in Coral Calcification: Insights From Comparing Coral Skeleton and Abiogenic Aragonite

“…Raman spectroscopy was used to analyze abiogenic aragonites precipitated from 0.2-µm filtered Vineyard Sound (Massachusetts, USA) seawater (described in DeCarlo et al, 2015;Holcomb et al, 2016) and the JCp-1 Porites coral skeleton (Okai et al, 2002), both before and after thermal annealing. Briefly, the abiogenic aragonites were precipitated by addition of NaHCO 3 and Na 2 CO 3 solutions to seawater, combined with air/CO 2 bubbling, which together increases Ar and induces crystal nucleation.…”

“…Following precipitation, the aragonites were rinsed with ethanol and then 18 M ohm deionized water, stored in glass vials that were washed with deionized water, and always handled with latex gloves. Subsamples of the same abiogenic samples have previously been analyzed with Raman spectroscopy using an infrared (785 nm) laser (DeCarlo et al, 2015, which reduces or eliminates fluorescence from organic molecules.…”

“…These analyses require >20 mg of aragonite powder, which exceeds the amount of material remaining for most of the abiogenic samples after previous measurements (DeCarlo et al, 2015;Holcomb et al, 2016). Therefore, we chose to mix three abiogenic samples (f02, f03, and f06) that had enough material remaining and that precipitated across the full range of Ar .…”

Coral calcifying fluid aragonite saturation states derived from Raman spectroscopy

“…Raman spectroscopy was used to analyse the abiogenic aragonite precipitates described in DeCarlo et al (2015) and Holcomb et al (2016) (Supplement Table S1). Briefly, aragonite was precipitated by addition of Na 2 CO 3 and NaHCO 3 solutions to seawater.…”

“…Observing this fluid has proved difficult though, due to its small size and isolation beneath the living polyp (Clode and Marshall, 2002). Estimates of fluid carbonate chemistry have so far been derived from micro-electrodes, pH-sensitive dyes, boron isotopes, B/Ca, U/Ca, and bulk calcification rates (AlHorani et al, 2003;Trotter et al, 2011;Venn et al, 2011;DeCarlo et al, 2015;Cai et al, 2016;Holcomb et al, 2016;Raybaud et al, 2017). However, these approaches do not always agree (Ries, 2011;Holcomb et al, 2014), and they have so far focused on calcifying fluid carbonate chemistry without considering the effect of [Ca 2+ ] on Ar .…”