Primary productivity and early diagenesis in the Toarcian Tethys on the example of the Mn-rich black shales of the Sachrang Formation, Northern Calcareous Alps

Organic, inorganic and isotope geochemical investigations have been performed on 25 samples — 20 laminated marl samples and 5 re-sediment samples — taken from a 27 m thick black shale section of early Toarcian age, cropping out in the Northern Calcareous Alps. On the basis of MnO, Fe, organic carbon (TOC) contents and Hydrogen Index (HI) values, the laminated marls of the section can be divided into two parts. TOC contents and HI values are below 2% and 500 mg HC/g TOC in the lower, Mn-rich (and Fe-rich) part (MnO between 2 and 16%, Fe between 3.9 and 6.4%), while they range between 5 and 9% and 600–700 mg HC/g TOC, respectively in the upper, Mn-poor (and Fe-poor) part (MnO <1%, Fe between 1.5 and 3.8%). Both original amount of organic carbon (TOCor), calculated from TOC, MnO and sulphur contents, and CaO concentration show a strong upward increase in the lower, Mn–Fe-rich part of the section. TOCor and CaO are believed to represent planktonic particles hence their simultaneous upward increases are interpreted as the result of an at least 2.5 fold increase of productivity during the first part of the “black shale event”. Following the same logic, in the second part of the “black shale event” productivity is believed to have slightly decreased from the previously reached high level. Stratigraphic variation of the sulphur isotopic ratio support this scenario. Comparison of organic geochemical and δ34S data of the re-sediments with those of the neighbouring marls suggests that increase and decrease (?) of productivity was paralleled by expansion and withdrawal of oxygen-depleted waters. During the “black shale event” the extent of the oxygen-depleted bottom water was governed by changes in intensity of productivity. Changes in rates of deposition of Mn and Fe were not related to those of the productivity but they deeply influenced nature and intensity of bacterial degradation of organic matter and especially the incorporation of sulphur into kerogen. The high contribution of Mn- and Fe-minerals in the case of the lower Mn-rich part prevented by both dilution and degradation of OM due to Mn-reduction an intense incorporation of sulphur into OM. In the upper, Mn- (and iron) poor part the high initial Corg/Fe ratio led to an important incorporation of sulphur into OM and an early termination of sulphate reduction.