Recent high magnesian calcite/aragonite cementation of beach and submarine sediments from Denmark

Carbonate cementation of recent submarine and littoral sediments is recorded from several locations in the Kattegat Sea and the adjoining coasts. Two different types of cement related in mode of formation are described from the localities: 1) Aragonite cement restricted to sediment bodies of accumulated shell material mixed with quartz sand and 2) cement of magnesian calcite associated with sediment bodies of almost pure quartz sand. Both types of cement are present at all localities studied. The magnesian calcite shows a variable magnesium content in the range of 10-25 wt% MgCO (sub 3) . Several larger concretions of almost pure gypsum were found at a single locality, but minor amounts of gypsum associated with the cement of magnesian calcite have been detected by X-rays in several samples analysed. The carbon which forms part of both the aragonite and the magnesian calcite displays extreme negative delta C (super 13) values in the range of -25 to -55 per thousand PDB. The values are of the same order of magnitude as those typical for the carbon of methane, which frequently is recorded in the interval of -40 to -80 per thousand PDB. Furthermore, the carbon of the cement is radiocarbon-dated to be approximately 18,000 years old, whereas the cemented skeletal carbonate is recent to about 4,000 years old. Tires it is suggested that the carbon of the cement originates from oxidized methane from Quaternary deposits. The precipitation of calcium carbonate is thought to take place by mixing outflow of bicarbonate-enriched meteoric water with seawater. Magnesian calcite is believed to be the primary phase formed by the precipitating solution and the contemporary formation of aragonite most likely is caused by the higher nucleation rate of that polymorph when proper nuclei are present ( i.e ., mollusc shells).