Dynamic redox and nutrient cycling response to climate forcing in the Mesoproterozoic ocean

The Mesoproterozoic ocean (1.6–1.0 Ga) is commonly portrayed as stratified, with oxic surface waters overlying mid-depth euxinia and predominantly ferruginous deep waters. However, emerging records reveal substantial spatiotemporal variability in redox structure, including intervals of deeper oxyclines, dysoxia, or even oxic deeper waters, and transitions between ferruginous and euxinic states within single successions on multi-million-year timescales. Distinguishing temporal variability from spatial heterogeneity is challenging due to sparse, variably dated records, hindering understanding of nutrient-linked feedbacks, particularly phosphorus dynamics, and their implications for ocean oxygenation. This study targets two high-resolution sections from the ~1.4 Ga Xiamaling Formation (North China Craton) to resolve controls on redox heterogeneity and nutrient cycling, and to assess the role of climate forcing—especially orbital-scale variability and latitudinal shifts relative to the ITCZ—in shaping regional ocean biogeochemistry.

Prior work established widespread ferruginous conditions in the Proterozoic ocean with localized euxinia in productive regions, but also documented significant redox heterogeneity including pulsed oxygenation and deeper oxyclines. Geochemical paleoredox proxies (Fe speciation, trace metal enrichments such as U, Mo, Re, Mn) and sulfur isotopes have been widely applied to reconstruct depositional redox. Nutrient cycling studies emphasize phosphorus as a key control on productivity and oxygenation feedbacks, with redox-dependent P regeneration and sink-switching affecting P availability. Previous work on the Xiamaling Formation suggested orbital forcing of climate and variable redox states, but the mechanistic linkage between climate-driven weathering/aeolian inputs, OMZ dynamics, and P cycling remained unresolved. This study integrates high-resolution geochemistry with a biogeochemical box model to disentangle these influences.

Field sampling targeted two high-resolution sections within Units of the ~1.4 Ga Xiamaling Formation in the deepest part of the basin. Geochemical analyses included: (1) TOC by LECO analyzer after HCl removal of inorganic carbon (RSD 3–5%, accuracy >95%); (2) major/trace elements (Fe, P, Al, K, Ti by ICP-OES; U, Mo, Re, Mn by ICP-MS) following HNO3–HF–HClO4–H3BO3 digestion of ashed samples. Elemental enrichment factors (XEF) were calculated relative to average Upper Continental Crust as (X/Al)sample/(X/Al)UCC. (3) Iron speciation applied sequential extractions: sodium acetate (Fecarb), dithionite (Feox), ammonium oxalate (Femag), and chromous chloride for pyrite sulfur (Fepy). FeHR was computed as Fecarb+Feox+Femag+Fepy; FeHR/FeT thresholds of <0.22 indicate oxic and >0.38 indicate anoxic water-column conditions; Fepy/FeHR >0.6–0.8 indicates euxinia. Pyrite δ34S was measured on Ag2S precipitates using an Elemental PYRO cube with IsoPrime MS; standards ensured accuracy <0.33‰ and precision <0.3‰ (1σ). For Section B, no Ag2S was recovered. (4) Phosphorus phase partitioning employed a modified SEDEX scheme targeting PFe, Pauth (carbonate fluorapatite), Pdet (detrital apatite), and Porg, with spectrophotometric molybdate-blue detection (880 nm) or ICP-OES where reagents interfered. Replicate RSDs were <5% for most pools (PFe 18% due to low concentrations). Ratios used included P/Al, TOC/Porg, and TOC/Preac (sum of reactive P pools), compared to Redfield 106:1 and UCC benchmarks (P/Al=87 ppm/wt%, Ti/Al=0.05, K/Al≈0.3). (5) Weathering and aeolian input proxies: K/Al to infer chemical weathering intensity (higher K/Al implies lower weathering due to K retention); Ti/Al to infer aeolian contributions. (6) A multi-box biogeochemical model modified from a 4-box carbon–alkalinity framework added two surface shelf boxes with distinct weathering and upwelling fractions (Wfrac and Ufrac set to 0.99 for Shelf 1 and 0.01 for Shelf 2) to emulate ITCZ-influenced (runoff-dominated) versus arid, aeolian-influenced margins. Weathering fluxes were sinusoidally forced at ~15 kyr (Shelf 1) and ~60 kyr (Shelf 2) to emulate orbital-scale variability. Organic carbon burial scaled with shelf phosphate concentrations and assumed a 10% burial efficiency; remineralization partitioned a fixed fraction to Fe(III) reduction with the remainder via sulfate reduction. Model outputs tracked phosphate inventories, Corg delivery to sediments, and remineralization by Fe reduction and sulfate reduction. Interpretations integrated proxy data with model behavior to infer OMZ dynamics, euxinia intensity, and P recycling under varying climate/weathering regimes.

- Section B (lower, oligotrophic): Geochemical signatures (enriched Re with low U; very low MnEF; low TOC ≈ 0.070 ± 0.006 wt%) indicate dysoxic bottom waters and mobilization of Mn, consistent with a ferruginous OMZ. Low TOC/Porg and TOC/Preac ratios (well below Redfield) reflect oxidation of limited organic matter, enhanced bacterial P storage, and Fe-associated P drawdown/sink-switching to authigenic carbonate fluorapatite, promoting sustained oligotrophy. High K/Al and elevated, cyclic Ti/Al imply low, stable chemical weathering with significant, orbitally modulated aeolian inputs and likely variable upwelling under arid conditions driving OMZ expansion/contraction. - Section A (upper, eutrophic): Elevated FeHR/FeT and UEF indicate persistent anoxic bottom waters; Fepy/FeHR >0.6 and elevated MoEF denote euxinia, though at relatively low Mo concentrations compared with modern euxinic basins, consistent with low Mesoproterozoic sulfate. MoEF and Fepy/FeHR cyclicity, coupled with pyrite δ34S variations (higher δ34S during weakly euxinic intervals), indicate oscillations between weakly and strongly sulfidic states tied to variable sulfate availability. TOC/Porg ratios exceed Redfield and ~300:1, evidencing preferential P release during anaerobic remineralization, especially under stronger sulfate reduction. TOC/Preac > Redfield across Section A shows net P recycling to the water column; despite similar recycled P fluxes under weakly and strongly euxinic intervals, higher TOC and Preac during highly euxinic phases imply enhanced surface P bioavailability and productivity. - Weathering and input controls: In Section A, K/Al peaks during weakly euxinic intervals indicate lower chemical weathering; transitions to lower K/Al correspond to more intense weathering, increasing riverine phosphate and sulfate delivery, thereby boosting productivity and sulfide production (more highly euxinic conditions). Low, stable Ti/Al indicates limited aeolian input and dominance of runoff. In Section B, high Ti/Al cyclic peaks indicate enhanced aeolian fluxes; high K/Al supports low weathering intensity and aridity. - Modeling results: A two-shelf model with contrasting weathering/upwelling inputs reproduces coexisting, asynchronous biogeochemical states: Shelf 1 (high P input) shows about tenfold higher phosphate concentrations, high and variable organic carbon burial, and high, variable sulfate reduction rates consistent with oscillatory euxinia; Shelf 2 (low P input) maintains low, relatively stable productivity, strong Fe(III) reduction relative to sulfate reduction, and limited sulfide production, consistent with a ferruginous OMZ whose extent fluctuates. The model demonstrates that regional differences in weathering/upwelling, plausibly driven by ITCZ position and orbital forcing, can generate the observed redox and nutrient cycling heterogeneity. - Paleogeographic context: Reconstructions place the site near ~15°N at ~1.4 Ga near the variable northern limit of the ITCZ. Section B records arid, aeolian-dominated conditions outside ITCZ influence (Hadley cell limb), whereas Section A records runoff-dominated conditions under direct ITCZ influence. Distinct redox-nutrient regimes likely reflect shifts in ITCZ position and/or NCC latitudinal migration (ca. 1.44–1.35 Ga).

The study addresses the origin of Mesoproterozoic redox heterogeneity by demonstrating that local to regional climate forcing, rather than global stepwise redox transitions, can produce pronounced temporal fluctuations in redox state and nutrient cycling. In the Xiamaling basin, arid, wind-driven conditions with enhanced aeolian fluxes and variable upwelling expanded a ferruginous OMZ and promoted phosphorus drawdown, maintaining oligotrophy (Section B). Subsequent intervals influenced by stronger continental runoff and variable chemical weathering supplied more sulfate and phosphate, enhancing productivity, phosphorus recycling, and cyclic transitions between weakly and strongly euxinic waters (Section A). Proxy cyclicity (MoEF, Fepy/FeHR, δ34S, K/Al, Ti/Al, TOC/P ratios) coupled with model outputs shows that shifts in ITCZ position and orbitally forced weathering inputs can synchronize biogeochemical feedbacks, modulating P regeneration, organic carbon burial, and the balance between Fe reduction and sulfate reduction. These findings emphasize that spatially heterogeneous, climate-controlled boundary conditions can underpin the Mesoproterozoic redox mosaic, complicating extrapolation from single sections to the global ocean and informing interpretations of oxygenation feedbacks linked to phosphorus availability and primary productivity.

High-resolution geochemical data and biogeochemical modeling from the ~1.4 Ga Xiamaling Formation reveal that regional climate forcing—via orbitally paced aridity/wind strength and shifts in ITCZ position—drove pronounced, cyclic changes in ocean redox and nutrient cycling. An arid, aeolian-influenced regime produced a ferruginous OMZ with strong P drawdown and persistent oligotrophy, whereas a runoff-dominated regime with variable chemical weathering produced oscillations between weakly and strongly euxinic states, enhanced P recycling, and persistent eutrophy. The multi-box model corroborates that contrasting shelf settings can sustain markedly different phosphate inventories, organic carbon burial rates, and remineralization pathways despite ocean connectivity. Consequently, much of the Mesoproterozoic redox heterogeneity likely reflects climate-driven regional variability rather than global step-changes. Future work should integrate paleogeographic context, climate dynamics, and improved chronological control to distinguish spatial from temporal signals and to refine nutrient–oxygen feedbacks across Precambrian oceans.

- Chronological uncertainties: Orbital periods for observed cycles are difficult to constrain; constant sedimentation rates were assumed in some interpretations and in model forcing for demonstration but are unlikely in reality. - Proxy constraints: No pyrite (Ag2S) was recovered for δ34S in Section B, limiting sulfur cycle constraints there; some proxies (e.g., MoEF) may be influenced by basin-scale metal inventories and global anoxia. - Model simplifications: The two-shelf model uses highly idealized circulation (strong shelf–deep coupling), fixed burial efficiency (10%), and fixed partitioning between Fe(III) and sulfate reduction; upwelling is routed through shelves without explicit open-ocean upwelling, potentially damping the effects of weathering variability and synchronizing responses. - Spatial representativeness: Results derive from a single basin setting; while the approach suggests general mechanisms, extrapolation to global conditions is limited and requires broader spatial coverage. - Palaeogeographic uncertainties: Reconstructions of NCC latitude and ITCZ position at 1.4 Ga carry uncertainties that affect inferred climatic regimes.