VARIATIONS IN ABUNDANCES AND DISTRIBUTIONS OF ISOPRENOID CHROMANS ANDLONG-CHAIN ALKYLBENZENES IN SEDIMENTS OF THE MULHOUSE BASIN - A MOLECULAR SEDIMENTARY RECORD OF PALEOSALINITY

The abundances and distribution patterns of mono-, di- and trimethylat ed 2-methyl-2-(4,8,12-trimethyltridecyl) chromans (MTTCs) and long-cha in alkylbenzenes in extracts of marl (66 samples), anhydrite (15) and halite (1) strata of the Salt IV Formation of the Oligocene Mulhouse B asin are reported. The distributions of the methylated MTTCs indicate salinity changes of the upper part of a density stratified water colum n of the basin. These variations are explained by a tectonically or cl imatically induced change in the supply of water of relative lower sal inity to form a layer overlying the deeper water brine. Hence, it is s uggested that mesohaline (3.5-15%) conditions in the surface waters we re established as a result of periodic incursions of marine water and subsequent evaporation. Conversely, during periods when the surface wa ter was derived mainly from fresh water from the hinterland, lower ave rage salinity in the surface layer resulted. The distributions of long -chain alkylbenzenes also appear to record these changes.