Our understanding of the Precambrian ocean's chemical evolution has significantly advanced. For the Mesoproterozoic (1.6–1.0 billion years ago; Ga), a stratified ocean redox model has emerged, featuring oxic surface waters overlying euxinic (sulfidic) mid-depth waters in productive areas, with deeper waters primarily ferruginous (anoxic and iron-bearing). However, recent geochemical evidence reveals greater variability in Mesoproterozoic ocean redox chemistry than this generalized model suggests. Some areas show predominantly ferruginous conditions with only very shallow oxygenation, while others indicate oxycline deepening or the development of dysoxic or oxic deeper waters. Transitions between ferruginous and euxinic conditions have also been observed within the same stratigraphic succession over multi-million-year timescales. The factors driving this variability remain largely unknown, partly due to difficulties distinguishing temporal and spatial variability in limited and often poorly-dated Mesoproterozoic records. This also hinders understanding of potential oxygenation feedbacks linked to nutrient availability, particularly phosphorus, which was significantly impacted by the Mesoproterozoic water column's redox state. This study aims to investigate the dynamic interplay between ocean redox conditions, nutrient cycling (specifically phosphorus), and climate forcing during the Mesoproterozoic, utilizing high-resolution geochemical data and biogeochemical modeling to address the significant gaps in our understanding of this critical period in Earth's history.

Previous research on Mesoproterozoic ocean redox conditions has presented a somewhat simplified picture. Studies have highlighted the prevalence of ferruginous conditions in deeper waters and oxic conditions near the surface, sometimes with a layer of euxinic waters in between. However, this model has been challenged by findings showing greater variability in redox conditions, both spatially and temporally. Some studies point to the presence of oxic deeper waters, while others emphasize transitions between ferruginous and euxinic conditions within single stratigraphic sequences. The role of nutrients, particularly phosphorus, in influencing these redox changes has also been a topic of investigation, with evidence suggesting strong links between phosphorus availability and primary productivity, which in turn could have impacted the oxygenation state of the ocean. However, a comprehensive understanding of the factors that controlled this variability, including the influence of climate, remains elusive. This study builds upon this existing research by providing high-resolution geochemical data from a specific location, coupled with biogeochemical modeling, to offer new insights into the complex interplay between redox conditions, nutrient cycling, and climate during the Mesoproterozoic.

This study analyzed two high-resolution sections from the ~1.4 Ga Xiamaling Formation in the North China Craton. Total organic carbon (TOC) concentrations were determined using a LECO carbon analyzer after inorganic carbon removal. Major and trace element concentrations (Fe, P, Al, K, Ti, U, Mo, Re, Mn) were measured using inductively coupled plasma-optical emission spectrometry (ICP-OES) and inductively coupled plasma-mass spectrometry (ICP-MS). Enrichment factors (EFs) were calculated relative to average Upper Continental Crust (UCC). Sequential Fe extractions were performed to determine carbonate-associated Fe (Fe_carb), Fe (oxyhydr)oxide minerals (Fe_ox), magnetite (Fe_mag), and pyrite (Fe_py). Highly reactive Fe (Fe_HR) was calculated as the sum of these phases. Pyrite δ³⁴S isotope analyses were conducted using an Elemental PYRO cube coupled to an IsoPrime continuous flow mass spectrometer. Phosphorus phase partitioning was carried out using a sequential extraction scheme targeting iron-bound P (P_Fe), authigenic carbonate fluorapatite-associated P (P_auth), detrital apatite (P_det), and organic-bound P (P_org). A multi-box biogeochemical model, modified from a 4-box atmosphere-ocean carbon-alkalinity system, was used to simulate the interaction between weathering inputs, nutrient cycling, and ocean redox conditions. The model included two surface ocean boxes overlying different continental shelves with unique weathering inputs to explore the possibility of different carbon cycle dynamics based on ITCZ influence. Weathering fluxes were allowed to oscillate at periods of 15 kyr and 60 kyr, reflecting orbital forcing. Remineralization was simplified, assuming a constant fraction of available carbon processed through Fe(III) and SO₄ reduction. This integrated approach, combining high-resolution geochemical data and biogeochemical modeling, enabled a detailed investigation into the dynamic interplay between redox conditions, nutrient cycling, and climate forcing in the Mesoproterozoic.

The study revealed distinct contrasts in ocean biogeochemistry between two sections of the Xiamaling Formation. Section B, characterized by consistently high K/Al ratios and high Ti/Al ratios, suggests low and stable chemical weathering rates and a significant aeolian contribution. Low TOC/Porg and TOC/Preac ratios indicate extensive oxidation of a limited organic matter supply and P drawdown associated with Fe minerals, promoting persistent oligotrophic conditions. Section A, in contrast, shows fluctuating K/Al ratios indicating variable chemical weathering rates, along with elevated TOC/Porg ratios reflecting preferential P release during anaerobic organic matter remineralization, consistent with cyclicity in sulfide availability (indicated by MoEF values and pyrite δ³⁴S data). Elevated TOC and Preac concentrations in Section A suggest a productive setting with enhanced P recycling. The cyclicity in redox and nutrient dynamics in both sections is consistent with orbital climate forcing. In Section B, peaks in Ti/Al ratios around the midpoint of green mudstone intervals likely reflect maxima in aeolian source flux, suggesting a wind-driven orbital control in an arid environment. In Section A, peaks in K/Al ratios during weakly euxinic intervals suggest less intense chemical weathering, while higher K/Al ratios during highly euxinic intervals reflect more intense weathering, leading to increased phosphate and sulfate delivery and consequently, enhanced productivity and more intense euxinia. A biogeochemical model confirmed that varying weathering inputs can drive substantially different carbon cycle and redox signatures on different parts of the continental shelf. The model shows that changes in phosphate input result in substantially different phosphorus concentrations and cyclic variations in organic carbon reaching the sediment. In the oligotrophic setting, organic carbon is depleted via Fe reduction, and little sulfide is produced, while in the eutrophic setting, sulfate reduction rates are high and variable. The paleogeographic reconstruction suggests that the study site was located at about 15°N at 1.4 Ga, near the variable northern limit of the ITCZ. Section B likely represents deposition outside the ITCZ, while Section A reflects a low-latitude tropical setting under the ITCZ's direct influence. The findings suggest that the observed redox heterogeneity in the Mesoproterozoic was strongly influenced by regional climate dynamics, driven by variations in ITCZ position and atmospheric circulation cells.

The results of this study significantly advance our understanding of Mesoproterozoic ocean redox heterogeneity. The findings demonstrate that regional climate forcing, specifically variations in the position of the ITCZ and associated atmospheric circulation cells, played a crucial role in shaping the observed dynamic interplay between redox conditions, nutrient cycling, and primary productivity. The contrasting biogeochemical signatures in the two sections of the Xiamaling Formation highlight the importance of considering spatial variations in interpreting the Mesoproterozoic ocean's redox evolution. The study emphasizes that simply extrapolating observations from a single location to the global scale is inadequate, as regional climate dynamics imposed significant controls on local biogeochemistry. The integrated approach of high-resolution geochemical analysis and biogeochemical modeling provided a robust framework for interpreting the complex interplay of factors governing the Mesoproterozoic ocean's chemical evolution.

This study provides compelling evidence for the significant influence of climate forcing on Mesoproterozoic ocean redox and nutrient cycling. The contrasting biogeochemical signatures between the two sections of the Xiamaling Formation highlight the importance of considering regional climate dynamics when interpreting the Mesoproterozoic ocean's redox evolution. Future studies should incorporate paleogeographic location and climate forcing into their analyses to improve our understanding of the global-scale processes influencing ocean oxygenation and its impact on biological evolution. The high-resolution geochemical data and biogeochemical modeling presented here provide a valuable framework for future research aimed at refining our understanding of this critical period in Earth's history.

The study's interpretation relies on the assumption of relatively constant sedimentation rates, which may not have been entirely accurate. The biogeochemical model, while helpful, simplifies several complex processes, such as remineralization pathways and upwelling dynamics. The model's simplified representation of the ocean circulation might not fully capture the nuances of the actual conditions. While the study provides strong evidence for climate forcing, precisely quantifying the contributions of various orbital timescales to the observed cyclicity remains challenging.