Acantharians are planktonic protists (abundant in today oceans), sister-group to polycystine Radiolarians, that have the particularity of biomineralizing complex skeletons in strontium sulfate SrSO4 (i. e. celestine). Those protists have no (as far as I know) fossil record for the good reason that celestine is soluble in seawater once the organism decayed. However it has been speculated (e. g. Bernstein et al. 1992) that acantharian-derived celestine could be at the origin of the formation of Sr-rich barite.

What is the status of this hypothesis? If this is still considered true, is there a way to differentiate unambiguously acantharian-derived barite from other kind of barite?

This would of course have interesting paleontological implications.

Bernstein, R. E., Byrne, R. H., Betzer, P. R., Greco, A. M., 1992. Morphologies and transformations of celestite in seawater: The role of acantharians in strontium and barium geochemistry. Geochimica et Cosmochimica Acta, 56: 3273-3279.

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    $\begingroup$ The correct mineral name would be celestine, not celestite. It is the official name recognised by the IMA. $\endgroup$ – Gimelist Oct 11 '14 at 9:20

There is still some uncertainty, but some recent research is detailed below:

In research reported in the article Barite in the ocean – occurrence, geochemistry and palaeoceanographic applications (Griffith and Paytan, 2012), recent observations and research found that

Using the non-carbonate Sr content in suspended particles to track the presence of acantharia, van Beek et al. (2007) suggest that acantharian dissolution does contribute signifi- cantly to barite formation in the upper 500 m of the water column

However, a lack of a definitive correlation between the barite concentrations and acantharian distributions suggest that

these organisms are not required for the formation of barite in the water column (Bertram & Cowen, 1997)

and that further observations

provided direct evidence to support the water column micro-environment mechanism of marine barite formation and to verify that barite can form in the absence of acantharia.

The concentration of strontium in barite is a record of the Sr isotopic composition of the formation fluids, which according to the article has its source from sea water, sediments (both marine and terrigenous) and hydrothermal fluids.

It is this last point, particularly about the Sr concentrations in sea-water, which according to the article Sedimentation of acantharian cysts in the Iceland Basin: Strontium as a ballast for deep ocean particle flux, and implications for acantharian reproductive strategies (Martin et al. 2010) state can fluctuate by several percent

By precipitating celestite, Acantharia can reduce surface seawater strontium (Sr) concentrations by up to 5%

Experimentally the authors found that

Sr fluxes we report are too high to be attributed to nonacantharian sources.

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    $\begingroup$ Thanks for the articles! Van Beek et al. 2007 in particular is quite interesting: Ra/Ba ratio seems a good lead on determining acantharian-derived barite from other barite. $\endgroup$ – plannapus Oct 14 '14 at 6:42

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