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.