Dissolved Strontium, Sr/Ca Ratios, and the Abundance of Acantharia in the Indian and Southern Oceans

We report measurements of strontium concentrations and Sr/Ca ratios in the Indian and Southern Oceans, which show that subtle geochemical variations along the main thermocline are the product of calcium carbonate (CaCO3) and celestite (SrSO4) precipitation and dissolution. Our calculations suggest that celestite skeletons precipitated by Acantharia contribute up to 10 mol % of the combined amount of carbonate and celestite shells precipitated in the Indian Ocean. The data suggest that the distribution of the concentration of strontium in the global ocean is governed by the different modes of deep-water formation in the Southern Ocean and North Atlantic. The formation of Antarctic bottom waters from strontium-rich, upwelled deep waters forms a southern end member contrasted with the strontium-depleted North Atlantic deep water. The difference in strontium concentrations and Sr/Ca ratios of the different water masses reported here is maintained by precipitation, export, and dissolution of CaCO3 and SrSO4. These preformed strontium concentrations correlate linearly with nitrate and phosphate concentrations in the Indian Ocean, but this correlation is weaker in low latitudes, where the mixotrophic lifestyle of Acantharia allows them to thrive in nutrient-depleted environments.