Microbial mat development and dolomite formation under pre-evaporitic conditions during the Atlantic in a temperate area: the Sarliève Lake (French Massif Central)

The Sarliève marsh in the Limagne plain, in the heart of the French Massif Central, functioned as an endorheic lake during the Late Glacial and the Early and Middle Holocene. During the Late Boreal and the Atlantic it experienced drastic lowering of the water level as a result of dry and warm climatic episodes. Then, pre-evaporitic conditions triggered the deposition of sediment rich in organic matter (OM) and in carbonates including dolomite. Fifty one samples from a ca. 1.8 m sediment core section covering the period were analysed using Rock-Eval pyrolysis and gas chromatography-mass spectrometry (GC-MS). Throughout the interval, the organic matter content remained notable to high [up to 13.35% total organic carbon (TOC)] and of good quality as indicated by low Oxygen Index (OI) values (<200mg. CO2 g -1 TOC) and high Hydrogen Index (HI) values (160-660 mg HC g -1 ) which, as a rule, increased with increasing TOC content. In contrast to the acid fractions, which sometimes contained notable proportions of n-C16 or n-C18 fatty acids (FAs; analysed as the methyl esters, FAMEs), the neutral fractions were almost devoid of low molecular weight compounds. The latter were probably biodegraded during early diagenesis. Conversely, high molecular weight compounds were abundant in both fractions and were dominated by n-alkanols, n-alkanes, steroids and hopanoids in the neutrals and evennumbered FAs and hopanoids in the acid fractions. The hopanoids were dominated by regular in su -0 06 16 30 1, v er si on 1 31 A ug 2 01 1 Author manuscript, published in "Organic Geochemistry 42, 9 (2011) 1089-1098" DOI : 10.1016/j.orggeochem.2011.06.015