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.

No abstract

The clay mineralogy of Messinian sediments from several sections of Caltanissetta and Ciminna basins (Sicily) show the large extension of smectite facies in comparison with the under-and over-lying series (Tortonian, Pliocene). Messinian smectites are not related to the lithology (major and minor sequences, evaporites, tripoli, volcanic ash) and seem to be chiefly inherited from Messinian soils. Such an origin suggests a temperate-warm climate with contrasting wet and dry seasons. The rythmical character of the climate can contribute for explain the variable periodicity of Messinian deposits during the time.

Pétrographie des évaporites messiniennes de la Méditerranée. Comparaison des forages JOIDES-DSDP et des depots du bassin de Sicile.

Die Tonfraktion eiszeitlicher Bildungen am Ende des Präkambriums präsentiert eine grosse mineralogische Vielfalt : Illit, Chlorit, Vermiculit, Smektit, Kaolinit, unregelmässige Wechsellagerungen und manchmal regelmässige. Der Einfluss der Diagenese kann im allgemeinen ausser Acht gelassen werden mit Ausnahme der porenreichen Sande, in denen sich sekundärer Kaolinit bilden konnte. Die Tonminerale, deren Vergesellschaftungen sich im Laufe der Zeit ändern, tragen dazu beijede sedimentäre Formation zu charakterisieren und helfen, die mit den Ablagerungen zeitgleichen paläogeographischen Phänomene zu rekonstruieren : die meteorische Verwitterung ist im Substratum schwächer als im Gletscher, in dessen Inneren sich interglaziale Niveaus durch eine stärkere Hydrolyse der Tonminerale auszeichnen ; die zeitgleiche Entwicklung kationenreicher Milieus begünstigt das Kristallwachstum (Chlorit) ;die nord-oder nordost-wärts Verlagerung der Minerale der Glazialbildungen in denen eine an die Dynamik der Gletscher gebundene Komgrössensortierung der Tonminerale zu beobachten ist.