Methane-derived carbonate cementation of marine sediments from the Kattegat, Denmark: Geochemical and geological evidence

Abstract

Methane-derived carbonate cementation of Holocene marine sediment occurs at several locations in the Kattegat, Denmark. Generally, the carbonate-cemented sandstones occur at the seafloor as individual slabs or widely distributed, thinly lithified pavements. In some areas, the lithified sandstones take the form of large and spectacular, vertical pillar and mushroom-like bodies standing up from the seafloor. The carbonate cement consists of aragonite, high-Mg calcite, and dolomite. The high-Mg calcite is the most widespread carbonate solid phase found. Aragonite is primarily associated with shell accumulations in the sediments. So far, dolomite has only been recorded as cement in the large, vertical lithified sandstone bodies. The carbonates are remarkably depleted in 13C, i.e. generally in the range of −55% to −45% PDB and are therefore interpreted as being the result of methane oxidation by sulphate-reducing bacteria in the anaerobic diagenetic zone of the sediments. However, the oxygen isotopic composition and the concentrations of minor elements indicate that carbonate precipitation took place under relatively constant temperature and salinity conditions in normal marine pore fluids. The habit and mineralogy of the carbonate-cemented sandstone observed most likely are controlled by the effect of bacterial activity and upward gas migration on the interstitial geochemical microenvironment. The lithified sediments were subsequently exposed as spectacular sandstone formations on the seafloor by submarine erosion. Data from onshore exploration wells reveal that methane is encountered in Eemian-Early Weichselian (Late Quaternary) marine deposits. The chemical composition and the isotopic signatures of the methane suggest a gas of microbial origin, and it is thought that the gas is generated within the host marine Quaternary sediment. The locations of submarine gas seeps, along with those of the carbonate-cemented sandstone formations and the onshore gas seeps, show a linear distribution which largely coincides with the extension of the Fennoscandian border zone. Thus the seeps may be controlled by the structural framework in the northeastern part of the Danish Subbasin.