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Journal of Sedimentary Research

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Pleistocene Dolomite from the Namibian Shelf: High ₈₇Sr/₈₆Sr and ₁₈O Values Indicate an Evaporative, Mixed-Water Origin

¹ Department of Geological Sciences, University of Cape Town, Rondebosch 7701 South Africa; compton{at}geology.uct.ac.za
² Department of Geological Sciences, University of Cape Town, Rondebosch 7701 South Africa
³ Department of Geological Sciences, University of Cape Town, Rondebosch 7701 South Africa

Dolomites from Upper Pleistocene sandstone and diatomaceous mud recovered from a coast-parallel, 4-km-wide trough on the Namibian shelf between Hottentot Bay and Spencer Bay have ₈₇Sr/₈₆Sr ratios between 0.70946 and 0.71309. Marine mollusc shells associated with the dolomites have a modern seawater Sr isotope ratio of 0.70915. The greater-than-seawater Sr ratios indicate unequivocally that groundwaters were involved in dolomite formation. Groundwaters, containing radiogenic Sr derived from weathering of Precambrian basement rocks, were focused into local troughs, mixed with seawater, and evaporated in the arid climate. The ₁₃C_PDB values of -4.0 to -2.4 indicate uptake of bicarbonate derived in part from microbial degradation of organic matter, and ₁₈O_PDB values of 1.4 to 6.4 suggest that the dolomite cement precipitated from a variably evaporated groundwater-seawater mixture. Dolomite cementation of the basal bioclastic sand and gravel occurred during emergence when sea level was below the trough sill depth of 65 m, most likely between 500 and 130 ka. Overlying calcareous and diatomaceous Eemian mud and sand are dolomitic only in the deepest, central trough area and contain 26 to 45 wt % disseminated dolomite. The ₁₃C_PDB values of -0.3 to -0.5 and ₁₈O_PDB values of 5.7 to 6.4 suggest that the disseminated dolomite formed from an evaporated groundwater-seawater mixture largely by replacement of biogenic carbonate when the shelf was subaerially exposed between 75 and 11 ka. Holocene transgressive shelly gravels and muddy sands contain only reworked Pleistocene dolomite, but Holocene dolomite may occur in coastal salt pans onshore.