Quaternary paleo-oceanography, pteropod preservation and stable-isotope record of the Red Sea

An abundance of pteropod species, their mode of preservation, and stable isotope ratios of carbon and oxygen in benthic and planktic foraminifera have been determined in core MD76-140 from the northern Red Sea. Isotopic data and benthic foraminiferal species indicate changes in O₂ concentration in the bottom sediments. The glacial stages are characterized by an almost anoxic bottom, while the interglacial bottom was generally well ventilated. In general very well preserved pteropod shells occur in glacial oxygen-depleted intervals, and also on recent well ventilated bottoms. In the interglacial stages the preservation deteriorates, and in the most extreme cases there is a complete dissolution of the aragonitic shells. In these cases most of the assemblage is lost and the only traces of the original shells are their internal molds. These are cemented with high magnesium calcite (HMC) which is a more stable carbonate phase in bottom sediments of the interglacial. Dissolution of aragonite discriminates between large and small shells, the latter tend to be removed completely, while the larger ones may be preserved as internal HMC molds. Limacina inflata is more readily molded than L. trochiformis, and thus its relative abundance increases in poorly preserved assemblages. The state of aragonitic shell preservation depends on the degree of organic matter combustion in the sediments. Well preserved shells occur in sediments where the supply of organic matter is limited and O₂ levels are high. Preservation deteriorates with increasing supply of organic carbon, O₂ consumption and increasing production of CO₂. In this case the dissolved carbonate is reprecipitated as HMC cement. With further increase in organic matter supply and reduced ventilation, nearly all oxygen is consumed and NO₃^- and subsequently SO₄^2- become the oxidizing agents. This is followed by an increasing carbonate alkalinity and excellent aragonitic shell preservation.