Geobiology of bituminous carbonates from the Ediacaran Shibantan Member (Dengying Formation, South China)

Abstract

The Shibantan Member is one of only two known carbonate settings characterised by a 'typical' Ediacaran fossil association including Ediacara-type fossils. Aim of this project was a comprehensive geobiological characterisation of the Shibantan Member, addressing both sedimentological and biogeochemical issues in order to gain a better understanding of these palaeoecosystems in which early metazoans thrived. Since organic biomarker analyses are problematical if applied on thermally mature outcrop samples, contamination paths were critically assessed and the potential of the direct use of extraction residues for catalytic hydropyrolysis (HyPy) of the non-extractable portion of organic matter (kerogen) was evaluated. Sedimentological analysis revealed that the depositional system of the Shibantan Member can be best explained with a carbonate ramp model linked to an intra-platform basin situated on the Dengying carbonate platform. The Dengying carbonate platform was generally characterised by evaporitic dolomites deposited in sub- to supratidal inner-ramp environments as represented by the Hamajing Member. The black bituminous and laminated limestones of the lower Shibantan Member, which overlay the Hamajing Member with a sharp contact, were in contrast deposited in a subtidal lower- to middle-ramp environment. This strong facies contrast point to a rapid deepening event that was most likely due to a tectonically-induced increase in local subsidence. Sedimentation was probably condensed during lower Shibantan times as evidenced by common occurrences of early diagenetic chert bands and -concretions. However, the basin became then subsequently filled as indicated by a gradual transition to the dark wavy dolomites of the upper Shibantan Member, which was deposited in a subtidal middle-ramp environment and, ultimately, the Baimatuo Member, which essentially represents the same facies as the Hamajing Member (i.e. sub- to supratidal inner-ramp environments). A distinct slumping horizon as well as mass-flow deposits evidence that the sedimentation in the Shibantan basin was highly dynamic, partly due to wave affection and, most likely, tectonic processes. The microbial mat associated biota (i.e. Ediacara-type fossils, horizontal traces, and Vendotaenia-like fossils) flourished in the environments represented by the lower Shibantan Member. Given the close spatial relationship between the autochthonous fossil associations and allochthonous event deposits in this setting, the event deposits were probably not only important for the preservation of fossils but also for the supply of nutrients. This close linkage between abiotic and biotic processes was further constrained by the comprehensive biogeochemical analysis of a sample from the lower Shibantan Member. Trace elements demonstrate that the water body was temporarily stratified so that free molecular O2 was restricted to the upper water column during fair weather conditions. However, frequent mixing during storm events, as revealed by sedimentological analysis, probably led to the recurrent oxidation of the entire water column. Consequently, biogeochemical conditions were probably not stable during lower Shibantan times. Organic biomarker analyses were conducted to gain further insights into the biosphere of the Shibantan Member but are problematical since the organic matter is highly mature (Tmax ≥ 500°C) and bitumens in outcrop samples of the Shibantan Member are affected by external, most likely anthropogenic contamination. However, abundant sulphurised compounds in the bitumen and HyPy-treated extraction residue of the Shibantan carbonates hint at a high activity of sulphate reducing bacteria. Considering the evidences for mixing of the water column by storm events, sulphide oxidising bacteria were possibly temporarily favoured whenever oxygen was available at the sediment-water interface. Assuming that not all Vendotaenia-like fossils in the Shibantan Member represent shreds of microbial mats disrupted by storms and/or event deposition, an interpretation as sulphide oxidising bacteria appears reasonable. Stable carbon isotopes of carbonate phases (+3.3 to +4.0‰, VPDB) and syngenetic n-alkanes in the extraction residue after applying HyPy (-31.7 to -36.3‰, VPDB) point to the presence of phototrophic primary producers (cyanobacteria and/or algae) within the mat facies of the Shibantan Member and would also be in line with a changeable setting. Repeated cycles of mat-growth, reworking and re-deposition led to an effective burial of primary produced organic matter, and such withdrawal of 12C enriched organic matter caused an enrichment of newly precipitated carbonate phases in residual 13C. Thus, palaeoenvironmental conditions dynamically changed through a complex interplay of biogenic and abiogenic processes. Dependent on the changeable biogeochemical conditions, particularly the availability of oxygen, the microbial mat associated biota including Ediacara-type organisms temporarily thrived in this setting. This implies that stable environmental conditions (with respect to oxygen) were not necessarily a crucial prerequisite for the early evolution and diversification of early metazoan as commonly assumed. In case of the Shibantan Member one could even speculate that environmental instability has provoked innovative lifestyles and -strategies, allowing organisms to cope with the changeable conditions. However, this hypothesis has to be tested in future studies on other settings with Ediacara-type fossils (e.g. the Khatyspyt Formation in Siberia) and first calcifying metazoans.