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

Abstract

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. CO 2 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-C 16 or n-C 18 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 even- suggest that all the hopanoids derive from the same microbial precursors, slight shifts in the acid/alcohol ratio being governed by limited changes in redox conditions during early diagenesis.

6700 yr sedimentary record of climatic and anthropogenic signals in Lake Aydat (French Massif Central)

“…Along the whole core, as is generally observed (e.g. Disnar et al, 2011), TOC and HI remain correlated, whereas HI and OI are anticorrelated. Thus, the simultaneous variation in these different parameters delineates different sedimentation phases.…”

Refining the Sarliève Paleolake (France) Neolithic Chronology by Combining Several Radiocarbon Approaches

“…The sediments deposited during this time are composed of fine-grained silty marls, greenish gray to brown, mainly calcitic, that display numerous laminated layers rich in pristine Ca-dolomite, with small amounts of aragonite, which are clearly autochthonous (Bréhéret et al 2003). These layers correspond to the fossilization of microbial mats developed at the bottom of a saline pan environment (Bréhéret et al 2008;Disnar et al 2011) as a consequence of excess evaporation from the restricted system. The homogeneous, physically mixed, and bioturbated intervals that separate these laminated layers result from more humid episodes that could have induced the opening of the system.…”

“…Lipids extracted from the Sarliève lake sediments, in the SARL 17b core, are mainly higher plants in origin, indicated by the predominance of long-chain n-alkyl lipids (n-alkanes, n-alkanols, and fatty acids), with minor macrophyte and cyanobacterial contributions (Disnar et al 2011). Additionally, the predominance of n-alkanols over n-alkanes was interpreted as excellent preservation of higher plant remains that resulted from their short residence time in soils, accompanied by rapid transport to the lake via runoff or aerial transport and limited postdepositional decomposition.…”