Pleistocene dolomite from the Namibian shelf: High Sr-87/Sr-86 and delta O-18 values indicate an evaporative, mixed-water origin

Dolomites from Upper Pleistocene sandstone and diatomaceous mud recovered f rom a coast-parallel, 4-km-wide trough on the Namibian shelf between Hotten tot Bay and Spencer Bay have Sr-87/Sr-86 ratios between 0.70946 and 0.71309 . Marine mollusc shells associated with the dolomites have a modern seawate r Sir isotope ratio of 0.70915. The greater-than-seawater Sr ratios indicat e unequivocally that groundwaters were involved in dolomite formation. Grou ndwaters, containing radiogenic Sr derived from weathering of Precambrian b asement rocks, were focused into local troughs, mixed with seawater, and ev aporated in the and climate. The delta C-13(PDB) values of -4.0 to -2.4 par ts per thousand indicate uptake of bicarbonate derived in part from microbi al degradation of organic matter, and delta O-18(PDB) values of 1.4 to 6.4 parts per thousand, suggest that the dolomite cement precipitated from a va riably evaporated groundwater-seawater mixture. Dolomite cementation of the basal bioelastic 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 o nly in the deepest, central trough area and contain 26 to 45 wt % dissemina ted dolomite. The delta C-13(PDB), values of -0.3 to -0.5 parts per thousan d and delta O-18(PDB),, values of 5.7 to 6.4 parts per thousand suggest tha t the disseminated dolomite formed from an evaporated groundwater-seawater mixture largely by replacement of biogenic carbonate when the shelf was sub aerially exposed between 75 and 11 ka. Holocene transgressive Shelly gravel s and muddy sands contain only reworked Pleistocene dolomite, but Holocene dolomite may occur in coastal salt pans onshore.