Paleoceanographic processes associated with the deposition of organic-rich sapropels and black shales.

摘要

A multi-proxy geochemical study was undertaken on organic-rich Mediterranean sapropels and Cretaceous black shales in order to explore the relative importance of enhanced primary productivity versus widespread anoxia and, more broadly, which paleoceanographic processes are involved in the formation of these deposits.; Sedimentary sequences containing two sapropel layers were compared along a longitudinal transect in the Mediterranean Sea using nitrogen and organic carbon stable isotopes and trace metal contents as paleoceanographic proxies. The investigation of the isotopic composition of sapropels reveals the occurrence of higher nutrient availability and primary productivity rates coupled with high rates of nitrogen fixation during times of sapropel formation. The enhanced carbon export promoted the establishment of dysoxic conditions in the water column and sediments that appear to reach levels of severe anoxia at least sporadically in the eastern basin. A dramatic difference in sapropel expression exists between the eastern and western Mediterranean, suggesting the role of the eastern basin as a focal point for the initiation of sapropel formation.; A sedimentary sequence containing two sapropel layers in the expanded Vrica Section was investigated for indications of variations in the structure and stability of the water column during sapropel deposition using high-resolution oxygen and carbon isotopic measurements of two planktonic foraminifera species. Sapropels coincide with a freshening of the upper water column due to enhanced precipitation and continental runoff, which causes greater surface stratification.; The origin of the organic matter contained within several black shale intervals that were identified as possibly related to Oceanic Anoxic Events was investigated in a Cretaceous sequence recovered from the western North Atlantic using a suite of organic-geochemical, trace-elemental, and isotopic proxies. These layers are associated with increased carbon burial, preferential recycling of nitrogen, high nitrogen fixation rates, and anoxic/suboxic conditions that likely expanded into the water column. Although sapropels and black shales have important differences, sapropels are possible analogs to black shale intervals and can be helpful in understanding the role of nitrogen fixation, denitrification, and water column anoxia in Cretaceous black shale deposition.