Sedimentology and Environmental Conditions of Sapropels

Abstract

Color (lightness), organic carbon content, total nitrogen content, carbonate content, and bulk and clay mineralogy were determined for most of the sapropelic sediments recovered by Leg 42A and for some of their adjacent normal sediments.Maximum values of organic carbon and nitrogen reach 16.7% and 1%, respectively. Carbon/nitrogen ratios generally increase with greater organic carbon content and with increasing age. Very high ratios of some sapropel material suggest a considerable supply of terrigenous organic material.The color of the sediment becomes darker with increasing organic carbon content. This is interpreted as the result of monosulfide formation which is controlled by the original content of finely dispersed organic matter. In contrast, the formation of pyrite in sapropels seems to depend upon the presence or absence of lumps of organic matter.Total carbonate content ranges between almost zero and 78% in the sapropels. In comparison to normal sediments, no major carbonate dissolution could be observed. Calcite is the dominant carbonate mineral, while the content of detrital dolomite reaches a maximum of 16%. Occasionally, appearances of calcite and aragonite are attributed to turbiditic supply of shallow water carbonates. In the Messinian evaporitic sequence of Site 374, primary or early diagenetic dolomite is the only carbonate mineral.Calcite, mainly derived from the tests of foraminifers and coccolithophorids; has no relation to the content of organic carbon. This, together with the abundance of plant debris, suggests that the main part of the organic material in sapropels does not originate from plankton.Gypsum is a common component of sapropels, while anhydrite is restricted to the evaporitic sediments of Messinian age. For these strata, contemporaneous formation of sulfides (pyrite) and sulfates (gypsum, anhydrite) is assumed.The clay mineralogy of the sapropel shows considerable differences from that of the normal host sediments: greater contents of organic matter result in increased mineral alteration. The alteration leads to a degradation of the minerals towards mixed-layer types or chlorite, or to their complete destruction. The process increases in the series: kaolinite, illite, chlorite, smectite, attapulgite. The degree of alteration is greatest at the bases of sapropels. It appears to occur when a rich supply of detrital fragile materials has been available to sites in the center of the sapropelic area in the deeper parts of the basin. It is clear that the mechanisms of sapropel formation operate at the sediment/seawater interface and do not depend on depth of burial.

Nitrogen isotopes in chlorophyll and the origin of Eastern Mediterranean sapropels

“…Hole 969C The fraction of terrestrial organic matter in sapropels has been widely debated, with some studies concluding it is significant (Calvert, 1983;Deroo, et al, 1978;Shaw and Evans, 1984;Sigl, et al, 1978) and others that it is minor Poutanen and Morris, 1985;Smith, et al, 1986;ten Haven, et al, 1987). Evidence for the existence of a significant terrestrial component to sapropel organic matter carne from elemental ratios (i.e., high C/N; Calvert, 1983;Sigl, et al, 1978), terrestrial biomarkers (i.e., high n-alkane carbon numbers and a high odd/ even n-alkane predominance; Deroo, et al, 1978), and bulk characterisitics of the organic matter (i.e., high concentrations of humic material (Deroo, et al, 1978) and high abundances of pollen and higher plant debris (Shaw and Evans, 1984;Sigl, et al, 1978)).…”

“…Evidence for the existence of a significant terrestrial component to sapropel organic matter carne from elemental ratios (i.e., high C/N; Calvert, 1983;Sigl, et al, 1978), terrestrial biomarkers (i.e., high n-alkane carbon numbers and a high odd/ even n-alkane predominance; Deroo, et al, 1978), and bulk characterisitics of the organic matter (i.e., high concentrations of humic material (Deroo, et al, 1978) and high abundances of pollen and higher plant debris (Shaw and Evans, 1984;Sigl, et al, 1978)). Evidence against a significant terrestrial component to sapropel organic matter carne from elemental ratios (i.e., low C/N; Sutherland, et al, 1984), carbon isotopes (i.e., high 813c…”

Clay Mineralogy of Site 397, South of Canary Islands (DSDP Leg 47A)

“…This was revealed by the relatively light color of decarbonized clay suspensions. No systematic relationship exists between the color of the sediment and the amount or crystallinity of smectite, in contrast to Mediterranean sapropelic sediments (Sigl et al, 1978). All these observations point to a weak or nonexistent clay diagenesis being influenced by the organic matter.…”

“…Another explanation of the Cretaceous assemblages might be based on the organic matter content of the sediments (Unit 5 contains organic material, primarily plant remains). The organic matter sometimes is opposed to the conservation of clay particles such as smectite, which are small and characterized by high-exchange properties (Chamley, 1971;Sigl et al, 1978). One could envisage a partial destruction of smectite by organic acids in this special environment.…”

Clay mineral assemblages of Mesozoic pelagic and flysch sediments of the Lombardian Basin (Southern Alps): implications for palaeotectonics, palaeoclimate and diagenesis

“…HERBERT & FISCHER, 1986). Alternatively, the smectites could have been destroyed by organic acids as observed in Neogene sapropels of the Eastern Mediterranean Sea (SIGL et al, 1978). However, further research is needed to explain the illite-rich layers.…”