Understanding the timing and trajectory of atmospheric oxygenation remains fundamental to deciphering its causes and consequences. Given its origin in oxygen-free photochemistry, mass-independent sulfur isotope fractionation (S-MIF) is widely accepted as a geochemical fingerprint of an anoxic atmosphere. Nevertheless, S-MIF recycling through oxidative sulfide weathering—commonly termed the crustal memory effect (CME)—potentially decouples the multiple sulfur isotope (MSI) record from coeval atmospheric chemistry. Herein, however, after accounting for unrecognised temporal and spatial biases within the Archaean–early-Palaeoproterozoic MSI record, we demonstrate that the global expression of the CME is barely resolvable; thereby validating S-MIF as a tracer of contemporaneous atmospheric chemistry during Earth’s incipient oxygenation. Next, utilising statistical approaches, supported by new MSI data, we show that the reconciliation of adjacent, yet seemingly discrepant, South African MSI records requires that the rare instances of post-2.3-billion-year-old S-MIF are stratigraphically restricted. Accepting others’ primary photochemical interpretation, our approach demands that these implied atmospheric dynamics were ephemeral, operating on sub-hundred-thousand-year timescales. Importantly, these apparent atmospheric relapses were fundamentally different from older putative oxygenation episodes, implicating an intermediate, and potentially uniquely feedback-sensitive, Earth system state in the wake of the Great Oxidation Event.

The degree to which ocean deoxygenation will alter the function of marine communities remains unclear but may be best constrained by detailed study of intervals of rapid warming in the geologic past. The Paleocene–Eocene Thermal Maximum (PETM) was an interval of rapid warming that was the result of increasing contents of greenhouse gases in the atmosphere that had wide ranging effects on ecosystems globally. Here, we present stable nitrogen isotope data from the Eastern Peri-Tethys Ocean that record a significant transition in the nitrogen cycle. At the initiation of the PETM, the nitrogen isotopic composition of sediments decreased by ~6‰ to as low as −3.4‰, signaling reorganization of the marine nitrogen cycle. Warming, changes in ocean circulation, and deoxygenation caused a transition to nitrogen cycle to conditions that were most similar to those experienced during Oceanic Anoxic Events of the Mesozoic.

Significance: The permanent disappearance of mass-independent sulfur isotope fractionation (S-MIF) from the sedimentary record has become a widely accepted proxy for atmospheric oxygenation. This framework, however, neglects inheritance from oxidative weathering of pre-existing S-MIF–bearing sedimentary sulfide minerals (i.e., crustal memory), which has recently been invoked to explain apparent discrepancies within the sulfur isotope record. Herein, we demonstrate that such a crustal memory effect does not confound the Carletonville S-isotope record; rather, the pronounced Δ ³³ S values identified within the Rooihoogte Formation represent the youngest known unequivocal oxygen-free photochemical products. Previously observed ³³ S-enrichments within the succeeding Timeball Hill Formation, however, contrasts with our record, revealing kilometer-scale heterogeneities that highlight significant uncertainties in our understanding of the dynamics of Earth’s oxygenation.

Putative metazoan body fossils from the Precambrian are curiously lacking morphological characteristics that link them unambiguously to extant animal phyla, including sponges. Chemical fossils such as the rare C ₃₀ hydrocarbons 24- iso -propylcholestane (24- i pc) and 26-methylstigmastane (26-mes), however, have been proposed as evidence for the Neoproterozoic emergence of the Demospongiae (Porifera) due to their prevalence in rocks of this age and the occurrence of their sterol precursors in contemporary demosponges. However, there are alternative hypotheses which posit that diagenetic alteration products of algal sterols, or those from Rhizaria or other protists, account for the enigmatic steroid distributions observed in these ancient sedimentary rocks. Here, we report additional support for the Neoproterozoic rise of demosponges through the chemical characterization of two previously unrecognized C ₃₁ hydrocarbons—24- n -butylcholestane (24- n bc) and 24- sec -butylcholestane (24- sec bc). Precursor C ₃₁ sterols from contemporary demosponges, as well as a suite of synthesized C ₃₁ sterol standards were reduced to their sterane counterparts. Gas chromatography-tandem mass spectrometry analysis and collisionally activated dissociation mass spectra confirmed the presence of 24- n bc and 24- sec bc in well-preserved early Ediacaran rocks, and the coelution of these compounds with synthetic standards enhances the robustness of these findings. Co-occurrence of abundant 24- i pc and 24- sec bc was found for numerous Neoproterozoic-Cambrian rock/oil samples, closely mimicking the abundance patterns and high structural selectivity of major C ₃₀ and C ₃₁ sterols detected in numerous species of modern demosponges. These findings support the hypothesized first emergence of sponges during the Neoproterozoic Era.