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The Cryogenian Period (720–635 Million years ago, Ma) hosts sedimentary and geochronological evidence for two long-lived global-scale glaciations during the Sturtian (ca. 717–660 Ma) and Marinoan (ca. 650–635 Ma) cryochrons. Radiometric and chemostratigraphic data, in addition to climate modelling, support an approximately synchronous global deglaciation from the Sturtian cryochron, followed by a non-glacial interval (ca. 660–650 Ma) with abundant globally-distributed marine sedimentary successions. The palaeontological record of Cryogenian non-glacial successions is dominated by microfossils and problematic macrofossils, some of which have been interpreted as possible sponge-grade organisms. Biomarker analyses also hint at the rise to dominance of green algae and the possible first appearance of demospongiae during this interval. Oxygen and nutrient availability can fuel biotic complexity, however Cryogenian non-glacial palaeoredox and palaeonutrient (e.g. phosphorus, P) dynamics are poorly understood. Furthermore, while regional lithostratigraphic and chemostratigraphic correlations of carbonate-dominated Cryogenian non-glacial sedimentary successions are well documented, the temporal calibration of globally distributed carbonate and siliciclastic successions has not been attempted. Without a global chronostratigraphic age framework, the regional versus global nature of geochemical responses to Earth System perturbations and the sequence of biotic events throughout this interval remain obscured. Here we present new high resolution palaeoredox and P phase association data from five globally distributed Cryogenian non-glacial drill core successions. The combination of Fe speciation and trace element palaeoredox reconstructions with P speciation data clearly show dynamic changes to bioavailable P recycling in response to local and global scale nutrient-driven palaeomarine redox conditions. We also present a new global Cryogenian non-glacial chronostratigraphic framework for the calibration, in relative time, of geochemical and palaeontological data from carbonate and siliciclastic-dominated successions. This enables our new data to be interpreted in the context of the highly dynamic global C and S cycles and biotic record throughout this interval. This approach, in combination with new insights from climate models that constrain changes to atmospheric CO2 and temperature, sheds new light on the mechanisms for global changes to ocean redox and nutrients, and possible drivers for a possible increase in biotic diversity throughout this interval.

Fossilized lipids offer one of only few windows into ancient ecosystems. The utility of such biomarkers is determined by the phylogenetic distribution of lipid biosynthetic capabilities in extant organisms and extrapolation of this information into the past. 2-Methylhopanes in sedimentary rocks were once used to infer the importance of Cyanobacteria as primary producers in the geological past. However, the discovery of the hopanoid C-2 methyltransferase (HpnP) in Alphaproteobacteria led to the downfall of this molecular proxy. In the present study, we re-examined the distribution of HpnP in a new phylogenetic framework including novel candidate phyla, and re-interpreted a revised geological record of 2-methylhopanes based on contamination-free samples. We show that HpnP was likely present in the last common ancestor of Cyanobacteria, while the gene appeared in Alphaproteobacteria only around 750 million years ago. A subsequent rise of sedimentary 2-methylhopanes around 600 Ma likely reflects the expansion of Alphaproteobacteria that coincided with the rise of eukaryotic algae — possibly connected by algal dependency on microbially-produced vitamin B12. Our findings re-establish 2-methylhopanes as cyanobacterial biomarkers prior to 750 Ma and thus as a potential tool to measure the importance of oxygenic Cyanobacteria as primary producers on the early Earth. Our study illustrates how genetics can improve the diagnostic value of biomarkers and refine the reconstruction of early ecosystems.

The absence of unambiguous animal body fossils in rocks older than the late Ediacaran has rendered fossil lipids the most promising tracers of early organismic complexity. Yet much debate surrounds the various potential biological sources of putative metazoan steroids found in Precambrian rocks. Here we show that 26-methylated steranes—hydrocarbon structures currently attributed to the earliest animals—can form via geological alteration of common algal sterols, which carries important implications for palaeo-ecological interpretations and inhibits the use of such unconventional ‘sponge’ steranes for reconstructing early animal evolution. Via congruent observations in geological samples and pyrolysis experiments, the authors demonstrate that 26-alkylsteranes posited as sponge biomarkers can form during diagenesis of common algal sterols.

Carbon (δ13C) and hydrogen (δ2H) compound-specific isotope analyses on sedimentary hydrocarbons are widely used for ecological reconstructions and oil-source rock or oil-oil correlations. However, the effects of thermal alteration on isotopic composition are not fully understood, potentially imparting a bias on interpretation of older and more mature sedimentary sequences. We measured δ13C and δ2H of n-alkanes in 23 extracted bitumens from the 1.64 Ga Paleoproterozoic Barney Creek Formation in the southern McArthur Basin, Australia. The samples cover a wide range of thermal maturities with calculated vitrinite reflectance (Rc) values from 0.4% to 1.3%. Our results illustrate that while δ13C of kerogen remains relatively constant, the δ13C and δ2H of n-alkanes have a strong positive correlation with thermal maturity. Average δ13Calk increase by 6.8‰ and δ2Halk by 69‰ among the samples in the analysed maturity range. At the same time, the carbon isotopic offset between n-alkanes and kerogen (Δδ13Calk–ker) climbs from 1.3‰ to 8.5‰ with increasing maturity. Therefore, the substantial maturity influence on stable carbon and hydrogen isotopes of n-alkanes must be considered in palaeoecological and petroleum correlation studies. In the initial stages of maturation, n-alkanes from the Barney Creek Formation display increasingly positive “isotope slopes” in plots of δ13Calk against carbon number. However, with further maturation, the slopes became increasingly negative. The isotope slope inversion indicates that the dominant mechanisms for n-alkane generation and degradation changed during the progression from early diagenesis to metagenesis. Numerical models suggest that the formation of positive and negative isotope slopes may be driven by the balance of the formation of n-alkanes from kerogen and their subsequent degradation, and by dependence of the degradation rate constant k on n-alkane chain length.

Eukaryotic algae rose to ecological relevance after the Neoproterozoic Snowball Earth glaciations, but the causes for this consequential evolutionary transition remain enigmatic. Cap carbonates were globally deposited directly after these glaciations, but they are usually organic barren or thermally overprinted. Here we show that uniquely-preserved cap dolostones of the Araras Group contain exceptional abundances of a newly identified biomarker: 25,28-bisnorgammacerane. Its secular occurrence, carbon isotope systematics and co-occurrence with other demethylated terpenoids suggest a mechanistic connection to extensive microbial degradation of ciliate-derived biomass in bacterially dominated ecosystems. Declining 25,28-bisnorgammacerane concentrations, and a parallel rise of steranes over hopanes, indicate the transition from a bacterial to eukaryotic dominated ecosystem after the Marinoan deglaciation. Nutrient levels already increased during the Cryogenian and were a prerequisite, but not the ultimate driver for the algal rise. Intense predatory pressure by bacterivorous protists may have irrevocably cleared self-sustaining cyanobacterial ecosystems, thereby creating the ecological opportunity that allowed for the persistent rise of eukaryotic algae to global importance., It remains unclear when and why the world’s oceans, once largely occupied by bacteria, became dominated by photosynthetic algae. Here, using fossil lipids in million year old rocks, the authors show that predation after the Snowball Earth glaciations created the opportunity for a global shift to algal ecosystems.

Hoshino, Yosuke et al., Sedimentary hydrocarbon remnants of eukaryotic C26–C30 sterols can be used to reconstruct early algal evolution. Enhanced C29 sterol abundances provide algal cell membranes a density advantage in large temperature fluctuations. Here, we combined a literature review with new analyses to generate a comprehensive inventory of unambiguously syngenetic steranes in Neoproterozoic rocks. Our results show that the capacity for C29 24-ethyl-sterol biosynthesis emerged in the Cryogenian, that is, between 720 and 635 million years ago during the Neoproterozoic Snowball Earth glaciations, which were an evolutionary stimulant, not a bottleneck. This biochemical innovation heralded the rise of green algae to global dominance of marine ecosystems and highlights the environmental drivers for the evolution of sterol biosynthesis. The Cryogenian emergence of C29 sterol biosynthesis places a benchmark for verifying older sterane signatures and sets a new framework for our understanding of early algal evolution., This study was principally funded by the Max Planck Society (Max Planck Research Group grant to C.H.); S.N. was supported by the Deutsche Forschungsgemeinschaft (Na1172/2-1). D.M. was supported by the Petroleum Branch of the Department of Primary Industries and Resources, South Australia., We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).