The aerobic diagenesis of Mesoproterozoic organic matter

Abstract

The Xiamaling Formation in the North China Block contains a well-preserved 1400 Ma sedimentary sequence with a low degree of thermal maturity. Previous studies have confirmed the dynamic and complex nature of this evolving marine setting, including the existence of an oxygen-minimum zone, using multi-proxy approaches, including iron speciation, trace metal dynamics, and organic geochemistry. Here, we investigate the prevailing redox conditions during diagenesis via the biomarkers of rearranged hopanes from the finely laminated sediments of the organic-rich black shales in Units 2 and 3 of the Xiamaling Formation. We find that rearranged hopanes are prominent in the biomarker composition of the oxygen-minimum zone sediment, which is completely different from that of the sediment in the overlying anoxic strata. Since the transition process from hopanes to rearranged hopanes requires oxygen via oxidation at the C-l6 alkyl position of 17α(H)-hopanes, we infer that dissolved oxygen led to the transformation of hopane precursors into rearranged hopanes during the early stages of diagenesis. The use of hopanoid hydrocarbons as biomarkers of marine redox conditions has rarely been previously reported, and the hydrocarbon signatures point towards oxic bottom waters during the deposition of Unit 3 of the Xiamaling Formation, which is consistent with the earlier oxygen-minimum zone environmental interpretation of this Unit.

Microbial assemblage and palaeoenvironmental reconstruction of the 1.38 Ga Velkerri Formation, McArthur Basin, northern Australia

“…The ca . 1.44 Ga Hongshuizhang Formation and the 1.38 Ga Xiamaling Formation, both in northern China, are inferred to have been deposited under anoxic, but not sulphidic conditions (Luo, George, Xu, & Zhong, ; Luo, Hallmann, Xie, Ruan, & Summons, ), with the possibility of oxygenation for example in unit three of the Xiamaling Formation (Wang et al., ; Zhang et al., ). In those studies, water column chemistry was reconstructed using redox‐sensitive trace metals and biomarkers, including the absence of aryl isoprenoids, reducing the probability of photic zone euxinia (Luo et al., , ).…”

“…Pr and Ph are in relatively higher concentration in the Hongshuizhang, Nonesuch and Xiamaling formations compared with the Velkerri Formation. Additionally, the Xiamaling contains trace concentrations of diahopanes and increased concentration of dibenzofuran interpreted to be biomarkers for aerobic diagenesis (Wang et al., ). Luo et al.…”

On the co‐evolution of surface oxygen levels and animals

“…This effort builds from the principle that the surface oceans are largely in equilibrium with the atmosphere (despite some disequilibrium from oxygen production). Reports of mid‐Proterozoic basin scale ocean oxygenation exist (Planavsky, Slack, et al, ; Sperling et al, ; Wang et al, ) which, barring very low levels of primary productivity, are most easily interpreted as intervals with oxygen above 1% PAL. On the other end of traditional estimates, there is general agreement that marine anoxia was more widespread than in the modern and the oceans were likely not fully ventilated until the Phanerozoic (or at least the Neoproterozoic) (Ozaki & Tajika, ; reviewed by Lyons, Reinhard, & Planavsky, ), suggesting lower than modern atmospheric oxygen levels.…”

The Biomarkers in the Mesoproterozoic Organic‐rich Rocks of North China Craton: Implication for the Precursor and Preservation of Organism in the Prokaryotic Realm

“…Diagenesis under oxidising condition plays an important role in the Ts/Tm ratio, which is often low in sub-oxic environments (Moldowan et al, 1986;Peters et al, 2005), and the formation of C 30 DiaH is considered to be related to the acid-catalysis of clay minerals during the early diagenetic process (Peters et al, 2005;Baydjanova and George, 2019). The oxic environment is also considered to be favourable for the formation of C 30 DiaH (Asif et al, 2010;Jiang et al, 2018;Wang et al, 2018), and a positive correlation is often observed between the C 29 Ts and C 30 DiaH contents (Jiang et al, 2018). Additionally, thermal maturity has also been found to affect the ratios of the parameters, whereby the ratios of Ts/Tm (Chandra et al, 1994), C 29 Ts/ C 30 H, and C 30 DiaH/C 30 H (Moldowan et al, 1991;Jiang and Zhang, 2015;Fang et al, 2019) are considered to be positively correlated with thermal maturity.…”