Organic Matter In Marine Sediments Research Articles

New Insight into Factors Controlling Organic Matter Distribution in Lower Cambrian Source Rocks: A Study from the Qiongzhusi Formation in South China

Sedimentary organic matter (OM) is a major reservoir of organic carbon in the global carbon cycle. Despite many studies, there still exist many debates on the mechanism of OM accumulation and preservation in marine sediments. We present a new field study of a Lower Cambrian shallow marine shelf sequence in the northern edge of the Yangtze Plate, China. Our results show that palynological OM and biogenic silica (Bio-Si) could be used alongside more conventional redox and paleo-productivity proxies to study the distribution of OM in marine sediments. The qualitative and quantitative study of palynological OM provides more detailed information on the nature of sedimentary organic carbon, which can be helpful in the assessment of primary productivity and OM preservation. In addition, the presence of Bio-Si stimulates the physical preservation of OM. Further analysis indicates that an increase in Bio-Si can promote OM preservation. This case-study provides insight into the intertwined factors controlling OM accumulation in the Early Cambrian.

Bound to rock

Geochemistry Organic matter binds to phyllosilicates, a process which affects both its transport and chemical stability. How does that affect the fate of terrestrial organic carbon that enters the ocean? Blattmann et al. show that organic carbon derived from soils is stripped from mineral surfaces upon discharge and dispersal into the ocean, whereas organic matter derived from ancient rocks is preserved there. Their results show that preservation of continentally derived organic matter in marine sediments is controlled largely by phyllosilicate mineralogy. Science , this issue p. [742][1] [1]: /lookup/doi/10.1126/science.aax5345

Mineralogical control on the fate of continentally derived organic matter in the ocean

First-order relationships between organic matter content and mineral surface area have been widely reported and are implicated in stabilization and long-term preservation of organic matter. However, the nature and stability of organomineral interactions and their connection with mineralogical composition have remained uncertain. In this study, we find that continentally derived organic matter of pedogenic origin is stripped from smectite mineral surfaces upon discharge, dispersal, and sedimentation in distal ocean settings. In contrast, organic matter sourced from ancient rocks that is tightly associated with mica and chlorite endures in the marine realm. These results imply that the persistence of continentally derived organic matter in ocean sediments is controlled to a first order by phyllosilicate mineralogy.

Isotopic and Geochemical Study of Organic Matter in Marine Sediments from the Indigirka Delta to the Ice Shelf Border of the East-Siberian Sea

Various hydrocarbon gases (СН4, С2Н4, С2Н6, С3Н6, С3Н8, С4Н8, n-С4Н10) and CO2 have been extracted in low concentrations from different layers of marine sediments up to 2 m thick on the continental shelf of the East Siberian Sea along the profile from the mouth of the Indigirka River to the ice shelf border. A freshwater complex supplied by the Indigirka and other rivers has the dominant influence on the composition of organic matter (OM) in the almost entire studied area of the East Siberian Sea. Concentrations of most common gases CH4 and CO2 increase with increasing sampling depth in sediment core collected at station 5602-2, while the carbon isotopic composition of CO2 decreases, reaching a value of –24‰. A sulfate reduction area was found at sediment depth of 48–70 cm. It was shown for the first time that the diagenesis of OM results in a systematic change in isotope fractional characteristics (IFC). IFC was obtained for the low-maturation OM, for OM from the sulfate reduction area, and for the high-maturation OM. OM in the low horizons of the sediment (below 77 cm from the surface) is much more evolved compared with the immature sediments of the upper horizons.

Uncultured Gammaproteobacteria and Desulfobacteraceae Account for Major Acetate Assimilation in a Coastal Marine Sediment.

Acetate is a key intermediate in anaerobic mineralization of organic matter in marine sediments. Its turnover is central to carbon cycling, however, the relative contribution of different microbial populations to acetate assimilation in marine sediments is unknown. To quantify acetate assimilation by in situ abundant bacterial populations, we incubated coastal marine sediments with 14C-labeled acetate and flow-sorted cells that had been labeled and identified by fluorescence in situ hybridization. Subsequently, scintillography determined the amount of 14C-acetate assimilated by distinct populations. This approach fostered a high-throughput quantification of acetate assimilation by phylogenetically identified populations. Acetate uptake was highest in the oxic-suboxic surface layer for all sorted bacterial populations, including deltaproteobacterial sulfate-reducing bacteria (SRB), which accounted for up to 32% of total bacterial acetate assimilation. We show that the family Desulfobulbaceae also assimilates acetate in marine sediments, while the more abundant Desulfobacteraceae dominated acetate assimilation despite lower uptake rates. Unexpectedly, members of Gammaproteobacteria accounted for the highest relative acetate assimilation in all sediment layers with up to 31–62% of total bacterial acetate uptake. We also show that acetate is used to build up storage compounds such as polyalkanoates. Together, our findings demonstrate that not only the usual suspects SRB but a diverse bacterial community may substantially contribute to acetate assimilation in marine sediments. This study highlights the importance of quantitative approaches to reveal the roles of distinct microbial populations in acetate turnover.

Anaerobic Ammonium Oxidation Linked to Microbial Reduction of Natural Organic Matter in Marine Sediments

Identification of microbial processes driving the loss of nitrogen from the oceans is of paramount relevance as these processes affect primary productivity in these ecosystems, which ultimately affects global biogeochemical cycles. Denitrification and anammox (anaerobic ammonium oxidation coupled to nitrite reduction) are the only identified processes so far that lead to nitrogen loss in marine environments. Here we provide stoichiometric and spectroscopic evidence, as well as tracer analysis with [15N]ammonium, revealing that anaerobic ammonium oxidation linked to the microbial reduction of natural organic matter (NOM) fuels nitrogen loss in marine sediments from the eastern tropical North Pacific coast. Tracer analysis revealed that the NOM-dependent anammox process was responsible for producing ∼1.5 μg of 15N2 (g of sediment)−1 day–1 after incubation for 27 days in sediment incubations amended with Pahokee peat, while intrinsic NOM present in the sediment promoted the production of ∼0.4 μg of 15N2 (g o...

Transient exposure to oxygen or nitrate reveals ecophysiology of fermentative and sulfate-reducing benthic microbial populations.

SummaryFor the anaerobic remineralization of organic matter in marine sediments, sulfate reduction coupled to fermentation plays a key role. Here, we enriched sulfate‐reducing/fermentative communities from intertidal sediments under defined conditions in continuous culture. We transiently exposed the cultures to oxygen or nitrate twice daily and investigated the community response. Chemical measurements, provisional genomes and transcriptomic profiles revealed trophic networks of microbial populations. Sulfate reducers coexisted with facultative nitrate reducers or aerobes enabling the community to adjust to nitrate or oxygen pulses. Exposure to oxygen and nitrate impacted the community structure, but did not suppress fermentation or sulfate reduction as community functions, highlighting their stability under dynamic conditions. The most abundant sulfate reducer in all cultures, related to Desulfotignum balticum, appeared to have coupled both acetate‐ and hydrogen oxidation to sulfate reduction. We describe a novel representative of the widespread uncultured candidate phylum Fermentibacteria (formerly candidate division Hyd24‐12). For this strictly anaerobic, obligate fermentative bacterium, we propose the name ‘USabulitectum silens’ and identify it as a partner of sulfate reducers in marine sediments. Overall, we provide insights into the function of fermentative, as well as sulfate‐reducing microbial communities and their adaptation to a dynamic environment.

CO2-dependent carbon isotope fractionation in the dinoflagellate Alexandrium tamarense

The carbon isotopic composition of marine sedimentary organic matter is used to resolve long-term histories of pCO2 based on studies indicating a CO2-dependence of photosynthetic carbon isotope fractionation (εP). It recently was proposed that the δ13C values of dinoflagellates, as recorded in fossil dinocysts, might be used as a proxy for pCO2. However, significant questions remain regarding carbon isotope fractionation in dinoflagellates and how such fractionation may impact sedimentary records throughout the Phanerozoic. Here we investigate εP as a function of CO2 concentration and growth rate in the dinoflagellate Alexandrium tamarense. Experiments were conducted in nitrate-limited chemostat cultures. Values of εP were measured on cells having growth rates (μ) of 0.14–0.35d−1 and aqueous carbon dioxide concentrations of 10.2–63μmolkg−1 and were found to correlate linearly with μ/[CO2(aq)] (r2=0.94) in accord with prior, analogous chemostat investigations with eukaryotic phytoplankton. A maximum fractionation (εf) value of 27‰ was characterized from the intercept of the experiments, representing the first value of εf determined for an algal species employing Form II RubisCO—a structurally and catalytically distinct form of the carbon-fixing enzyme. This value is larger than theoretical predictions for Form II RubisCO and not significantly different from the ∼25‰ εf values observed for taxa employing Form ID RubisCO. We also measured the carbon isotope contents of dinosterol, hexadecanoic acid, and phytol from each experiment, finding that each class of biomarker exhibits different isotopic behavior. The apparent CO2-dependence of εP values in our experiments strengthens the proposal to use dinocyst δ13C values as a pCO2 proxy. Moreover, the similarity between the εf value for A. tamarense and the consensus value of ∼25‰ indicates that the CO2-sensitivity of carbon isotope fractionation saturates at similar CO2 levels across all three ecologically prominent clades of eukaryotic phytoplankton. This continuity of εf across taxa may help to explain why there is no coherent signature of phytoplankton evolutionary succession in Phanerozoic carbon isotope records.

Preservation of organic matter in marine sediments by inner-sphere interactions with reactive iron

Interactions between organic matter and mineral matrices are critical to the preservation of soil and sediment organic matter. In addition to clay minerals, Fe(III) oxides particles have recently been shown to be responsible for the protection and burial of a large fraction of sedimentary organic carbon (OC). Through a combination of synchrotron X-ray techniques and high-resolution images of intact sediment particles, we assessed the mechanism of interaction between OC and iron, as well as the composition of organic matter co-localized with ferric iron. We present scanning transmission x-ray microscopy images at the Fe L3 and C K1 edges showing that the organic matter co-localized with Fe(III) consists primarily of C=C, C=O and C-OH functional groups. Coupling the co-localization results to iron K-edge X-ray absorption spectroscopy fitting results allowed to quantify the relative contribution of OC-complexed Fe to the total sediment iron and reactive iron pools, showing that 25–62% of total reactive iron is directly associated to OC through inner-sphere complexation in coastal sediments, as much as four times more than in low OC deep sea sediments. Direct inner-sphere complexation between OC and iron oxides (Fe-O-C) is responsible for transferring a large quantity of reduced OC to the sedimentary sink, which could otherwise be oxidized back to CO2.

Unraveling signatures of biogeochemical processes and the depositional setting in the molecular composition of pore water DOM across different marine environments

Dissolved organic matter (DOM) in marine sediment pore waters derives largely from decomposition of particulate organic matter and its composition is influenced by various biogeochemical and oceanographic processes in yet undetermined ways. Here, we determine the molecular inventory of pore water DOM in marine sediments of contrasting depositional regimes with ultrahigh-resolution mass spectrometry and complementary bulk chemical analyses in order to elucidate the factors that shape DOM composition. Our sample sets from the Mediterranean, Marmara and Black Seas covered different sediment depths, ages and a range of marine environments with different (i) organic matter sources, (ii) balances of organic matter production and preservation, and (iii) geochemical conditions in sediment and water column including anoxic, sulfidic and hypersaline conditions. Pore water DOM had a higher molecular formula richness than overlying water with up to 11,295 vs. 2114 different molecular formulas in the mass range of 299–600Da and covered a broader range of element ratios (H/C=0.35–2.19, O/C=0.03–1.19 vs. H/C=0.56–2.13, O/C=0.15–1.14). Formula richness was independent of concentrations of DOC and TOC. Near-surface pore water DOM was more similar to water column DOM than to deep pore water DOM from the same core with respect to formula richness and the molecular composition, suggesting exchange at the sediment–water interface. The DOM composition in the deeper sediments was controlled by organic matter source, selective decomposition of specific DOM fractions and early diagenetic molecule transformations. Compounds in pelagic sediment pore waters were predominantly highly unsaturated and N-bearing formulas, whereas oxygen-rich CHO-formulas and aromatic compounds were more abundant in pore water DOM from terrigenous sediments. The increase of S-bearing molecular formulas in the water column and pore waters of the Black Sea and the Mediterranean Discovery Basin was consistent with elevated HS- concentrations reflecting the incorporation of sulfur into biomolecules during early diagenesis. Sulfurization resulted in an increased average molecular mass of DOM and higher formula richness (up to 5899 formulas per sample). In sediments from the methanogenic zone in the Black Sea, the DOM pool was distinctly more reduced than overlying sediments from the sulfate-reducing zone. Bottom and pore water DOM from the Discovery Basin contained the highest abundances of aliphatic compounds in the entire dataset; a large fraction of abundant N-bearing formulas possibly represented peptide and nucleotide formulas suggesting preservation of these molecules in the life inhibiting environment of the Discovery Basin. Our unique data set provides the basis for a comprehensive understanding of the molecular signatures in pore water DOM and the turnover of sedimentary organic matter in marine sediments.

Climatic and geomorphic drivers of plant organic matter transport in the Arun River, E Nepal

Fixation of atmospheric CO2 in terrestrial vegetation, and subsequent export and deposition of terrestrial plant organic matter in marine sediments is an important component of the global carbon cycle, yet it is difficult to quantify. This is partly due to the lack of understanding of relevant processes and mechanisms responsible for organic-matter transport throughout a landscape. Here we present a new approach to identify terrestrial plant organic matter source areas, quantify contributions and ascertain the role of ecologic, climatic, and geomorphic controls on plant wax export in the Arun River catchment spanning the world's largest elevation gradient from 205 to 8848 m asl, in eastern Nepal. Our approach takes advantage of the distinct stable hydrogen isotopic composition (expressed as δD values) of plant wax n-alkanes produced along this gradient, transported in river waters and deposited in flood deposits alongside the Arun River and its tributaries. In mainstem-flood deposits, we found that plant wax n-alkanes were mostly derived from the lower elevations constituting only a small fraction (15%) of the catchment. Informed by remote sensing data, we tested four differently weighted isotopic mixing models that quantify sourcing of tributary plant-derived organic matter along the Arun and compare it to our field observations. The weighting parameters included catchment area, net primary productivity (NPP) and annual rainfall amount as well as catchment relief as erosion proxy. When weighted by catchment area the isotopic mixing model could not explain field observations on plant wax δD values along the Arun, which is not surprising because the large arid Tibetan Plateau is not expected to be a major source. Weighting areal contributions by annual rainfall and NPP captured field observations within model prediction errors suggesting that plant productivity may influence source strength. However weighting by a combination of rainfall and catchment relief also captured the observed δD value pattern suggesting dominantly erosive control. We conclude that tributaries at the southern Himalayan front with high rainfall, high productivity, high relief and high erosion rates dominate plant wax exports from the catchment.

New drilling of the early Aptian OAE1a: the Cau core (Prebetic Zone, south-eastern Spain)

Abstract. The Cretaceous was punctuated by several episodes of accelerated global change, defined as Oceanic Anoxic Events (OAEs), that reflect abrupt changes in global carbon cycling. The Aptian Oceanic Anoxic Event (OAE1a; 120 Ma) represents an excellent example, recorded in all major ocean basins, and associated with massive burial of organic matter in marine sediments. The OAE1a is concomitant with the "nannoconid crisis", which is characterized by a major biotic turnover, and a widespread demise of carbonate platforms. Many studies have been published over the last decades on OAE1a's from different sections in the world, and provide a detailed C-isotope stratigraphy for the event. Nevertheless, new high-resolution studies across the event are essential to shed light on the precise timing and rates of the multiple environmental and biotic changes that occurred during this critical period of Earth history. Here we present a new drill core recovering an Aptian section spanning the OAE1a in southern Spain. The so-called Cau section was drilled in the last quarter of 2015. The Cau section is located in the easternmost part of the Prebetic Zone (Betic Cordillera), which represents platform deposits of the southern Iberian palaeomargin. The lower Aptian deposits of the Cau section belong to a hemipelagic unit (Almadich Formation), deposited in a highly subsident sector of the distal parts of the Prebetic Platform. Previous work on the early Aptian of the Cau succession has focused on stratigraphy, bioevents, C-isotope stratigraphy, and organic and elemental geochemistry. A more recent study based on biomarkers has presented a detailed record of the pCO2 evolution across the OAE1a (Naafs et al., 2016). All these studies reveal that the Cau section represents an excellent site to further investigate the OAE1a, based on its unusually high sedimentation rate and stratigraphic continuity, the quality and preservation of fossils, and the well-expressed geochemical signatures.

Growth performance of the sea cucumber Holothuria scabra and the seaweed Eucheuma denticulatum: integrated mariculture and effects on sediment organic characteristics

Deposit-feeding sea cucumbers play a key role in marine ecosystems through biotur- bation, burrowing and feeding on organic matter in marine sediments. Many deposit-feeding holothurians have therefore been recommended for integrated multitrophic aquaculture systems (IMTA). We set up an integrated mariculture system of sea cucumber Holothuria scabra and sea- weed Eucheuma denticulatum in Bweleo, Unguja Island of Zanzibar, Tanzania, to investigate the effect of stocking density on the growth and survival of culture species, total organic matter (TOM) and total organic carbon (TOC) content in the sediment. Treatments that included a fixed stocking density (500 g, ca. 200 g m �2 ) of E. denticulatum and 4 sea cucumber stocking densities (monoculture, low, medium and high density; 0, 150 ± 5, 236 ± 24, 345 ± 48 g m �2 , mean ± SD) of medium-sized H. scabra (114 ± 37 g) were established. Stocking density of H. scabra did not influ- ence survival of either species. Seaweed cultured under high stocking density of H. scabra had a higher specific growth rate of 2.33% d �1 than that cultured at the medium or low densities or with- out sea cucumbers. Sea cucumbers cultured at low stocking density had a higher mean growth rate of 0.80 g d �1 compared to those cultured at medium or high densities. TOM and TOC in sed- iments decreased over the experimental period at medium sea cucumber stocking density, while at low and high stocking densities, organic matter accumulated. The study demonstrates that the integration of E. denticulatum and H. scabra at 200 g m �2 enhances seaweed growth and can reduce organic matter content in the sediments.

Geochemical constraints on the distribution and rates of debromination in the deep subseafloor biosphere

Organic matter in marine sediments is degraded through a range of diverse metabolic pathways which are dependent on substrate availability, environmental conditions, and microbial ecology. The rates and systematics of these metabolic pathways affect long-term global geochemical cycles and the degradation of organic matter in the subsurface marine environment. Organohalide respiration is one of these pathways that has been hypothesized to be widely active in the deep biosphere, with the carbon–halogen bonds being broken through microbially-mediated redox reactions. Besides directly providing energy to microbes in marine sediments and allowing bromine to cycle back into the overlying ocean, organobromine respiration may also be closely linked to nitrogen and carbon cycling in anoxic marine sediments. Here we investigate the distribution and rates of debromination by tracking the production of dissolved bromide (Br−) with depth in pore water sampled at several continental margins. Pore water profiles of Br− and ammonium (NH4+) concentrations from the Krishna–Godavari (K-G) basin on the southeastern margin of India indicate a common distribution of rates of debromination and NH4+ production in continental margin sediments, and suggest that the pools of bioavailable nitrogen and organobromine compounds are likely geochemically associated at these sites. Dissolved Br− and total solid-phase bromine concentration profiles from the K-G basin and Costa Rica margin indicate the most rapid debromination occurs in the upper 10–20m of the sediment column. The rates of debromination in the sediment column from the Costa Rica, Cascadia, and Nankai convergent margins are estimated using numerical reaction-transport modeling of pore water Br− concentration profiles to constrain the maximum amount of metabolic energy that could be provided to the microbial communities through organobromine respiration. The modeled rates of debromination provide an upper limit to organobromine respiration activity because other debromination processes may also be responsible for an unknown fraction of these geochemically-derived rates. Modeled rates of debromination on the order of 101–103μmolm−2y−1 indicate that the maximum amount of energy that is potentially provided through organobromine respiration is low relative to other metabolic pathways such as sulfate reduction and methanogenesis, however organobromine respiration may still serve an important niche in the microbial community and debromination is an important part of the oceanic bromine cycle.

Formate, acetate, and propionate as substrates for sulfate reduction in sub-arctic sediments of Southwest Greenland.

Volatile fatty acids (VFAs) are key intermediates in the anaerobic mineralization of organic matter in marine sediments. We studied the role of VFAs in the carbon and energy turnover in the sulfate reduction zone of sediments from the sub-arctic Godthåbsfjord (SW Greenland) and the adjacent continental shelf in the NE Labrador Sea. VFA porewater concentrations were measured by a new two-dimensional ion chromatography-mass spectrometry method that enabled the direct analysis of VFAs without sample pretreatment. VFA concentrations were low and surprisingly constant (4–6 μmol L−1 for formate and acetate, and 0.5 μmol L−1 for propionate) throughout the sulfate reduction zone. Hence, VFAs are turned over while maintaining a stable concentration that is suggested to be under a strong microbial control. Estimated mean diffusion times of acetate between neighboring cells were <1 s, whereas VFA turnover times increased from several hours at the sediment surface to several years at the bottom of the sulfate reduction zone. Thus, diffusion was not limiting the VFA turnover. Despite constant VFA concentrations, the Gibbs energies (ΔGr) of VFA-dependent sulfate reduction decreased downcore, from −28 to −16 kJ (mol formate)−1, −68 to −31 kJ (mol acetate)−1, and −124 to −65 kJ (mol propionate)−1. Thus, ΔGr is apparently not determining the in-situ VFA concentrations directly. However, at the bottom of the sulfate zone of the shelf station, acetoclastic sulfate reduction might operate at its energetic limit at ~ −30 kJ (mol acetate)−1. It is not clear what controls VFA concentrations in the porewater but cell physiological constraints such as energetic costs of VFA activation or uptake could be important. We suggest that such constraints control the substrate turnover and result in a minimum ΔGr that depends on cell physiology and is different for individual substrates.

Terrestrial acidification during the end-Permian biosphere crisis?

Excessive acid rainfall associated with emplacement of the Siberian Traps magmatic province is increasingly accepted as a major contributing factor to the end-Permian biosphere crisis. However, direct proxy evidence of terrestrial acidification is so far not available. In this paper, we seek to determine the probability that relative proportions of extractable monophenolic components from soil-derived organic matter in marine sediments provide a molecular proxy for estimating soil acidity. Intermittently low and high ratios of vanillic acid to vanillin detected in latest Permian and earliest Triassic deposits of the southern Alps, Italy, support concepts of pulses of severe acidification (pH <4) during the main phase of the biosphere crisis.

Predictive isotope model connects microbes in culture and nature.

In PNAS, Wing and Halevy (1) present a new model that quantitatively describes the magnitude of sulfur isotope fractionation produced by dissimilatory microbial sulfate reduction (MSR). MSR is a major player in the global biogeochemical cycles and is responsible for the respiration of up to 30% of organic matter in marine sediments (2). This metabolism produces large isotope effects, in which the product, sulfide, is depleted in the heavy isotopes (33S, 34S, and 36S) relative to the most abundant isotope 32S (3), enriching modern seawater sulfate in 34S by about 21‰ (parts per thousand) compared with mantle sulfur. Sedimentary sulfur minerals preserve a record of this effect and are used to track changes in the sulfur isotope composition of seawater and the biogeochemical sulfur, carbon, and oxygen cycles through geologic time (4). Such reconstructions require an understanding of factors that control the magnitude of sulfur isotope effects and dictate the fractionation of sulfur isotopes by sulfate reducers under a range of growth conditions.

Uptake of Barium from Vermiculite by the Bacterium Pseudomonas fluorescens - New Indications for Barium as a Palaeoproxy

The apparent association of biogenic barite (BaSO 4 ) and organic matter in marine sediments has given support to the idea of using Ba as a proxy for palaeoproductivity. Despite this general acceptance only a few unicellular organisms (acantharian, diatoms) are known to incorporate barium. In this work, we show that the bacterium Pseudomonas fluorescens is able to take up barium. Because natural seawater harbors numerous bacteria many of which may take up Ba, this uptake of Ba explains why it may be considered a palaeoproxy. The introduction of this article reviews briefly what is known to date of Ba as a palaeoproxy, and what is known about bacterial interaction with Ba. Like other trace elements such as Fe, Mn and Sr, Ba can be extracted from vermiculite by Pseudomonas fluorescens and some mutants derived from it. Barium, therefore, seems to be as important a factor in bacterial metabolism as iron or other trace elements. The incorporation of Ba by living bacterial cells in the marine environment and its release from these cells upon their death and subsequent decay can contribute to Ba accumulation in marine sediments.

Stabilization of the coupled oxygen and phosphorus cycles by the evolution of bioturbation

Animal burrowing and sediment-mixing (bioturbation) began during the run up to the Ediacaran/Cambrian boundary1, 2, 3, initiating a transition4, 5 between the stratified Precambrian6 and more well-mixed Phanerozoic7 sedimentary records, against the backdrop of a variable8, 9 global oxygen reservoir probably smaller in size than present10, 11. Phosphorus is the long-term12 limiting nutrient for oxygen production via burial of organic carbon13, and its retention (relative to carbon) within organic matter in marine sediments is enhanced by bioturbation14, 15, 16, 17, 18. Here we explore the biogeochemical implications of a bioturbation-induced organic phosphorus sink in a simple model. We show that increased bioturbation robustly triggers a net decrease in the size of the global oxygen reservoir—the magnitude of which is contingent upon the prescribed difference in carbon to phosphorus ratios between bioturbated and laminated sediments. Bioturbation also reduces steady-state marine phosphate levels, but this effect is offset by the decline in iron-adsorbed phosphate burial that results from a decrease in oxygen concentrations. The introduction of oxygen-sensitive bioturbation to dynamical model runs is sufficient to trigger a negative feedback loop: the intensity of bioturbation is limited by the oxygen decrease it initially causes. The onset of this feedback is consistent with redox variations observed during the early Cambrian rise of bioturbation, leading us to suggest that bioturbation helped to regulate early oxygen and phosphorus cycles.

Decoupling of the carbon cycle during Ocean Anoxic Event 2

The Cenomanian to Turonian boundary transition (ca. 95–93 Ma) represents one of the most profound global perturbations in the carbon cycle of the past 140 m.y. This interval is characterized by widespread deposition of organic-rich fine-grained sediment marked by a globally recognized positive carbon isotope excursion (CIE) reflecting the widespread removal of 12 C-enriched organic matter in marine sediments under global anoxic greenhouse conditions. However, the exact timing and trigger of this inferred global phenomenon, termed Oceanic Anoxic Event 2 (OAE-2), is still debated, with recent studies showing diachroneity between the deposition of the organic-rich sediment and the CIE, and conflicting interpretations on detailed redox analyses in several of these inferred anoxic settings. Here we present the first evidence for widespread and persistent oxygenation during OAE-2 based primarily on the distribution of redox-sensitive trace metals and biota preserved in sedimentary rocks from the Western Interior Seaway of North America. Our data indicate anoxic-euxinic conditions in the mid- to late Cenomanian, but improved bottom-water oxygenation prior to and during the CIE. Trace metal enrichments support large volumes of mafic volcanism possibly from the High Arctic large igneous province (HALIP), which occur within the middle of the CIE indicating that the emplacement of LIPs was not the primary trigger of the Cenomanian-Turonian CIE. The apparent paradox of an oxygenated phase within OAE-2 suggests a much more complex carbon cycle during these global perturbations than previously thought. These findings have important implications for greenhouse carbon cycle changes over time scales of 0.1–10 m.y.