Significance of enigmatic blue ice moraine gypsum, Sør Rodane, East Antarctica

Joachim.Jacobs [ at ]


During the international GEA-II expedition to Sør Rondane (East Antarctica), large aggregates of crystalline gypsum were found within blue-ice moraines on the south side of the Sør Rondane Mountains at an elevation of ca. 2000 m and ca. 250 km inland of the present coastline. Gypsum aggregates reach one meter in diameter with individual crystal length up to 20 cm. Apparently, the gypsum grows in-situ and as a result of sublimation of the blue ice on the gravel of the moraine. Individual cobbles and sand are incorporated into the gypsum aggregates. The gypsum does not show signs of transport. In thin section, the gypsum shows an irregular growth zoning and in part bent cleavage planes. Abundant primary fluid inclusions are present parallel to the growth zoning as well as parallel to cleavage plains. This type of gypsum formation is enigmatic and has never been described before. It might indicate an important and thus far unknown process, characterising a special way of interaction between the lithosphere and the cryosphere. Preliminary Sr-isotope data indicate relatively high values and suggest crustal affinities. A preliminary set of sulfate sulfur (δ34SSO4) and oxygen (δ18OSO4) isotope data reveals values ranging from +7.0 and +29.6 ‰ (δ34SSO4) and between -16.7 and +0.3 ‰ (δ18OSO4). Samples display a positive correlation between δ34S and δ18O. Thus, Sulfur isotope values are highly variable and offer different conclusions with respect to potential sulfate sources. None of the gypsum samples displays a sulfur isotopic composition reflecting a pure seawater sulfate (i.e. sea spray) origin. Two samples yielded δ34SSO4 values of +7.0 and +9.0 ‰, most others are distinctly more positive than modern seawater sulfate (i.e., above +21.0 ‰), with a maximum value of +29.6 ‰. In general, the sulfur isotope values more positive than modern seawater sulfate suggest bacterial sulfate reduction. The low δ34SSO4 values are comparable to values measured previously for continental sulfate and are similar to a single pyrite sample from a black schist sample, ca. 150 km to the W, Steingarden area (this study). The oxygen isotopic composition for the blue ice gypsum samples is also characterized by rather variable including strongly negative δ18OSO4 values. Negative sulfate oxygen isotope values possibly reflect an 18O depleted continental source such as Antarctic glacial waters that could have fostered the oxidative weathering of sulfide sulfur. The observed variability could reflect different stages of bacterial sulfate reduction.

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2014 SCAR OPEN SCIENCE CONFERENCE, 22 Aug 2014 - 28 Aug 2014, Auckland, New Zealand.
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Jacobs, J. , Strauss, H. , Kipfstuhl, S. , Weikusat, C. , Elburg, M. and Läufer, A. (2014): Significance of enigmatic blue ice moraine gypsum, Sør Rodane, East Antarctica , 2014 SCAR OPEN SCIENCE CONFERENCE, Auckland, New Zealand, 22 August 2014 - 28 August 2014 .

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