204 / 2025-04-14 21:59:27

High-Resolution Carbon Isotope and Redox fluctuations Provide Insights into Eukaryotic Evolution and Marine Oxygenation in the Early Mesoproterozoic Ocean

carbon isotope,redox condition,biogeochemistry,co-evolution of life and environment

Theme 3: Earth-Life Co-evolution Through Time > Session 3a: Precambrian Earth and Life Co-evolution

Decimeter-scale eukaryotic fossils and evidence of marine oxygenation preserved in the ~1.56 Ga Gaoyuzhuang Formation (North China Craton) offer critical insights into life–environment interactions during the early Mesoproterozoic. However, the precise nature of this relationship remains ambiguous due to limited basin-wide stratigraphic correlation and controversial redox conditions for the early eukaryotes. In this study, we integrate high-resolution carbon isotope analyses and multiple redox-sensitive geochemical proxies from two fossil-bearing sections (the Qianxi and Kuancheng sections), representing shallow- and deeper-water depositional settings, respectively, to develop robust δ¹³C_carb reference curves for the Gaoyuzhuang Formation and to refine our understanding of the spatial variability in oceanic redox states during this period.



  The δ¹³C_carb values obtained from members III and IV of the Gaoyuzhuang Formation are generally consistent with typical Mesoproterozoic records, varying within a narrow range of approximately –3‰ to +1‰. Our detailed stratigraphic analysis has identified four distinct δ¹³C_carb perturbations within these members, with similar patterns observed in both studied sections. Notably, the second perturbation (N2) may record a causal link between a transient marine oxygenation event occurring just below the fossil-preservation horizon. During the third perturbation (N3), a pronounced gradient in δ¹³C_carb is evident, with values of –1.6‰ in the shallow-water section contrasting sharply with –3.5‰ in the deeper-water section, resulting in a ~2‰ difference. In contrast, the corresponding δ¹³C_org values remain relatively stable throughout member III yet exhibit a significantly larger gradient (~7‰) between the shallow and deeper settings. This divergence implies decoupling behaviors of carbon cycling during this time period.



  We interpret these isotopic distinctions as reflecting different carbon reservoirs and biogeochemical processes operating in a redox-stratified ocean. Specifically, the δ¹³C_org variations are likely controlled by spatially distinct microbial communities under different redox conditions and influenced by a potential deep-water dissolved organic carbon (DOC) reservoir, whereas the δ¹³C_carb signatures were buffered by a large, relatively well-mixed dissolved inorganic carbon (DIC) pool. Our results further suggest that a short-lived oxygenation event in the deep-water setting may have driven enhanced remineralization of DOC, thereby triggering the observed negative δ¹³C_carb excursion and the ensuing gradient in the N3 interval.



Combining the chemostratigraphic correlation, our redox proxy analyses (including iron speciation and redox-sensitive elements) provide robust evidence for a redox-stratified structure in the early Mesoproterozoic ocean. The geochemical results point to ferruginous conditions dominating the deeper-water environment, while shallower settings experienced relatively higher levels of oxygenation. Interestingly, our data reveal an increasing deepening of the chemocline within the shallow-water domain of Gaoyuzhuang Member III (Qianxi section); initial conditions were characterized by ferruginous waters, accompanied with a pulsed oxygenation event, as documented by multiple independent geochemical research. Over time, this environment evolved into one characterized by highly dysoxic to weakly dysoxic conditions, indicating an overall increase in oxygen levels in the shallow waters. Despite the transient nature of the ‘pulsed’ oxygenation event, our integrated geochemical and chemostratigraphic records do not support a direct causal relationship between this oxygenation event and the evolution of macroeukaryotes. Rather, our observations emphasize that the prevailing ‘dysoxic’ conditions likely provided a critical redox framework that supported both the survival and the exceptional preservation of early macroeukaryotes.



  In the deeper-water settings, an interesting pattern emerges: with increasing oxygen levels in the overlying shallow waters, there is a corresponding intensification of euxinic conditions in deep waters or porewaters. This is supported by a coeval increase in the Fe_py/Fe_HR ratio in the Kuancheng carbonate but still within the ferruginous range. We propose that sulfate may have acted as a significant oxidant in the Mesoproterozoic ocean, thereby promoting spatial heterogeneity in redox conditions across the shelf regions and influencing the disparate responses to marine oxygen fluctuations.



  Overall, our high-resolution carbon isotope and redox geochemical analyses from the Gaoyuzhuang Formation provide new constraints on the carbon cycling and marine redox dynamics that prevailed during a critical interval of early eukaryotic evolution. By linking short-term carbon isotope perturbations and redox changes with the environmental requirements of early macroeukaryotes, our study reveals the dynamic interplay between biogeochemical processes and evolutionary innovation in the early Mesoproterozoic ocean and eukaryotes. These findings shed new lights on our understanding of the complex feedbacks between environmental change and biological evolution during the early Mesoproterozoic.