Hypoxia in the Eemian: mollusc faunas and sediment mineralogy from Cyprina Clay in the southern Baltic region

The Cyprina Clay at Mommark, southern Denmark, contains a unique record of a shallow threshold‐controlled fjord environment through an entire interglacial. Today, the area suffers from periodic severe hypoxia, and the Eemian mollusc faunas (dominated by the hypoxia‐tolerant Corbula gibba and Arctica islandica ), the sediments' varying content of pyrite, calcite and gypsum, as well as evidence of shell‐dissolution all show that the area also suffered from oxygen stress during the Eemian. A comparison with previous faunal studies on Cyprina Clay indicates that the entire southwestern Baltic region suffered from oxygen stress of varying degree throughout the Eemian. At Mommark, the severity of hypoxia varies inversely with water depth, indicating control by sea level and water depth over the basin threshold. After an early phase of eustatic sea level rise came a prolonged period of isostatic emergence and regression. Although this isostasy pattern is different from the Holocene, the present isostatic uplift of the Baltic entranceways may have the same effect and add to the anthropogene hypoxia threat in the Baltic region.