Deep-ocean Oxygen Research Articles

Reconfiguring oxygenation at ∼1.4 Ga: New constraints as informed by the ancient oceanic sulfur cycle

The extent of atmospheric and oceanic oxygenation during the Mesoproterozoic remains an area of active debate. Here, we report major and trace elements, organic sulfur (OS) speciation, sulfur isotopes in multiple phases [pyrite (py), OS, and carbonate-associated sulfate (CAS)], and a numerical model of sulfate concentrations from one of the Mesoproterozoic's best-preserved geochemical archives, the Xiamaling Formation (ca. 1.4 Ga) in the Yanliao Basin. These data reveal a previously unrecognized ocean-atmosphere oxygenation process. Starting with Unit 3, intense sulfurization increased organic carbon burial and thus oxygen release, resulting in a deep-water oxidation event within the Yanliao Basin that could be regarded as a transient restricted oxygen oasis based on low pyrite content, elevated kerogen S:C ratios, higher δ34Spy than δ34SOS (Δδ34SOS-py < 0), and a low calculated sulfate concentration of ∼0.07–0.14 mM. During Unit 2, the basin was reconnected to the open ocean with the deposition of organic-rich shale across northern China and northern Australia, accompanied by a shift in Δδ34SOS-py to 5.0‰ and a calculated seawater sulfate concentration of ∼0.1–1.14 mM. Such large-scale burial of organic matter released oxygen into the atmosphere, accompanied by significant sulfate delivery to the basin and euxinic conditions characterized by Mo concentrations and 34S-depleted pyrite at the topmost of Unit 2. A subsequent globally synchronized atmospheric and deep ocean oxygenation event may have occurred during Unit 1. This event was recorded by a progressive increase in Δδ34SOS-py (up to +9.6‰), increased oxidized OS species, and positive Ce anomalies (attributed to the reductive dissolution of the Ce-enriched Mn oxides), all of which were caused by increased sulfate concentrations (calculated to be ∼1.15–2.31 mM) and a spatial descent of the redox interface. The combination of these lines of evidence illustrates the sensitivity of the oceanic sulfur cycle to the ocean-atmosphere system and provides novel constraints on oxygenation at ∼1.4 Ga.

Carbonate-hosted manganese deposits and ocean anoxia

Late Devonian (ca. 360 Ma), Early Carboniferous (ca. 330 Ma), and Early Triassic (ca. 250 Ma) manganese deposits in the South China Block support an emerging view that some Mn carbonates form through direct synsedimentary (authigenic) precipitation. These Mn carbonates accumulated on distal shelves and are interbedded with lime mudstone and heterozoan carbonates that accumulated in coastal upwelling environments. Lithofacies, Ce anomalies combined with vanadium, uranium, and molybdenum enrichments indicate that the Mn carbonates were primarily precipitated under anoxic conditions. We provide a robust dataset to clarify the precipitation pathway of these Mn carbonates and explore the implications of using them as proxies of regional shallow water and deep-ocean anoxia. In the studied deposits, Mn calcite encases dolomite rhombs (<10 μm) or aggregates, indicating heterogenous nucleation, which is supported by similar lattice parameters of both minerals with a small lattice mismatch (<2.1%). The presence of micro-sized euhedral rhombs and subhedral aggregates of rhombs, the non-stoichiometric compositions, and characteristic organic matter Raman spectroscopy peaks near the D and G bands in the dolomite nuclei suggest an organogenic origin. Authigenic organogenic dolomite precipitation is interpreted to have preconditioned the sediment with nucleation sites for the subsequent precipitation of Mn carbonates in the shallow seafloor. This is consistent with δ13C values (−2.61‰) of Mn carbonate bulk samples that are slightly lower than coeval seawater (0.31‰, from host rocks), suggesting incorporation of 12C during microbial respiration of sedimentary organic matter during the precipitation of organogenic dolomite and hydrothermal sourced inorganic carbon. The absence of Mn in shallow facies suggests terrestrial input of Mn was minimal. Geochemical and sedimentologic evidence suggests that coastal upwelling delivered manganous anoxic deep waters to the shelves. Eu/Eu⁎ ratios (<1.5) and decoupling of Fe and Mn imply long-range transport of hydrothermal Mn, during which hydrothermal fingerprints were muted through mixing with ambient seawater. The transport of hydrothermal Mn to shelves indicates that the regional deep oceans at the time must have been anoxic, as Mn2+ would be easily oxidized and become insoluble under oxygen-rich conditions, preventing its long-distance transport. This precipitation pathway and depositional model implies that Mn carbonates are a largely unrecognized proxy for tracking ancient oceanic redox fluctuations, particularly during times of well-oxygenated global oceans. Hence, these Mn deposits record at least transient regional deep-ocean oxygen depletion in the generally well-oxygenated Late Paleozoic and Early Triassic oceans. The recognition that this periodic anoxia was broadly coeval with the anoxia-related Hangenberg, Serpukhovian, and Permian-Triassic biotic crises suggests a potential relationship between Mn carbonate accumulation and extinction events.

Recent reduced abyssal overturning and ventilation in the Australian Antarctic Basin

Dense water formed near Antarctica, known as Antarctic bottom water (AABW), drives deep ocean circulation and supplies oxygen to the abyssal ocean. Observations show that AABW has freshened and contracted since the 1960s, yet the drivers of these changes and their impact remain uncertain. Here, using observations from the Australian Antarctic Basin, we show that AABW transport reduced by 4.0 Sv between 1994 and 2009, during a period of strong freshening on the continental shelf. An increase in shelf water salinity between 2009 and 2018, previously linked to transient climate variability, drove a partial recovery (2.2 Sv) of AABW transport. Over the full period (1994 to 2017), the net slowdown of −0.8 ± 0.5 Sv decade−1 thinned well-oxygenated layers, driving deoxygenation of −3 ± 2 μmol kg−1 decade−1. These findings demonstrate that freshening of Antarctic shelf waters weakens the lower limb of the abyssal overturning circulation and reduces deep ocean oxygen content.

Enhanced ocean oxygenation during Cenozoic warm periods

Dissolved oxygen (O2) is essential for most ocean ecosystems, fuelling organisms’ respiration and facilitating the cycling of carbon and nutrients. Oxygen measurements have been interpreted to indicate that the ocean’s oxygen-deficient zones (ODZs) are expanding under global warming1,2. However, models provide an unclear picture of future ODZ change in both the near term and the long term3–6. The paleoclimate record can help explore the possible range of ODZ changes in warmer-than-modern periods. Here we use foraminifera-bound nitrogen (N) isotopes to show that water-column denitrification in the eastern tropical North Pacific was greatly reduced during the Middle Miocene Climatic Optimum (MMCO) and the Early Eocene Climatic Optimum (EECO). Because denitrification is restricted to oxygen-poor waters, our results indicate that, in these two Cenozoic periods of sustained warmth, ODZs were contracted, not expanded. ODZ contraction may have arisen from a decrease in upwelling-fuelled biological productivity in the tropical Pacific, which would have reduced oxygen demand in the subsurface. Alternatively, invigoration of deep-water ventilation by the Southern Ocean may have weakened the ocean’s ‘biological carbon pump’, which would have increased deep-ocean oxygen. The mechanism at play would have determined whether the ODZ contractions occurred in step with the warming or took centuries or millennia to develop. Thus, although our results from the Cenozoic do not necessarily apply to the near-term future, they might imply that global warming may eventually cause ODZ contraction.

REE Anomalies Changes in Bottom Sediments Applied in the Western Equatorial Atlantic Since the Last Interglacial

We reconstruct paleoredox conditions in the Western Equatorial Atlantic (WEA) over the glacial-interglacial cycle (~130 ka) by using new high-resolution REEs data and their anomalies from a marine sediment core (GL-1248) collected from the equatorial margin off the continental shelf of NE Brazil. This approach aims to improve the understanding of the dynamics of paleoclimatic and sedimentary inputs on the coast of northeastern Brazil. Marine sediments were analyzed via Mass Spectrometry (ICP-MS) after total digestion with HF/HNO3. REEs proxies are a useful tool in understanding the transport and origin of sediments due to their physicochemical properties. Our data showed the Parnaíba River was the main source of REEs content in the western South Atlantic. Fe minerals (Fe-oxyhydroxides) produced via weathering of continental and tropical soils were the principal REE-carrier phase during transportation and ultimate deposition at core site GL-1248. Several regional climatic factors mainly rainfall changes contributed significantly to continental-REEs erosion of sedimentary layers of the Parnaíba Basin, and transport and deposition of the mobilized REEs from the continent to the study site. Furthermore, changes in the negative Ce-anomaly showed low variation along the core indicating a reduction in deep ocean oxygenation during the interglacial relative to the last glacial period. That variation, probably, was associated with glacial-interglacial variations in sea level with the exposure of the continental shelf. The origin of positive Eu anomalies in siliciclastic sediment, also observed in the core, was explained by preferential retention by feldspars such as plagioclases and potassium feldspars mostly from the assimilation of felspar during fractionation crystallization of felsic magma in the Parnaíba basin since the Last Interglacial.

Decoupled oxygenation of the Ediacaran ocean and atmosphere during the rise of early animals

The Ediacaran Period (∼635 to 541 Ma) witnessed the early diversification and radiation of metazoans, in the form of the Ediacaran Biota. This biological revolution, beginning at ∼575 Ma, has been widely attributed to a temporally restricted episode of deeper ocean oxygenation, potentially caused by a contemporaneous rise in atmospheric oxygen levels. However, quantitative geochemical-record-driven estimates of Ediacaran atmospheric and oceanic redox evolution are lacking, and hence possible links between oceanic and atmospheric oxygenation remain speculative. Here, after screening for possible post-depositional alteration, we utilize paleogeographically-diverse carbon and sulfur isotope records from South China, Oman and USA-Mexico, to develop a biogeochemical isotope mass balance model to quantify Ediacaran atmospheric oxygen and oceanic sulfate evolution. Model results from all three continents indicate that Ediacaran atmospheric oxygen levels rose monotonically between ∼630 Ma and ∼590 Ma, and subsequently remained relatively stable at around 0.6 present atmospheric level for the remainder of the Ediacaran. By contrast, the marine sulfate reservoir appears to have remained relatively stable before ∼575 Ma, with a subsequent large pulse where sulfate concentrations rose to ∼8 mM. These quantitative results indicate that Ediacaran oceanic and atmospheric oxygenation were decoupled, which is consistent with published geochemical records. We propose that the early Ediacaran rise of atmospheric oxygen levels, driven by increased net burial of organic carbon and pyrite, may not have established widespread deep-ocean oxygenation. Instead, later pulsed input of oxidizing power (mainly sulfate) from the continents drove transient episodes of seafloor oxygenation that accompanied radiations of the Ediacaran Biota.

The role of the solid earth in regulating atmospheric O2 levels

Solid-earth processes act as both sources and sinks for atmospheric O₂. They act as sinks by introducing reduced minerals and gases to the earth9s surface that can remove O₂ from the atmosphere and ocean. They act as sources by exporting organic carbon and sedimentary pyrite to the mantle via subduction. Here we examine the relative sizes of igneous source and sinks of O₂ for the modern earth to determine their magnitudes and if they are in balance today. We find that igneous sinks for O₂ remove 1.83×10¹² mol O₂/yr (±0.43, 1σ) while subduction indirectly releases 1.56×10¹² mol/O₂ yr (±0.33, 1σ). This indicates that today igneous O₂ sinks are balanced by solid earth sources. We propose this balance is achieved by negative feedbacks associated with either low-temperature hydrothermal sinks for O₂, which are sensitive to deep-ocean O₂ concentrations, or the amount of organic carbon and pyrite buried in sediments and subducted, which are sensitive to dissolved O₂ concentrations. We also explore how igneous sinks for O₂ may have varied in the Neoproterozoic when atmospheric O₂ concentrations are thought to have been lower and the deep ocean anoxic. We find that despite these changes, the igneous O₂ sink was essentially the same as today: 1.78×10¹² (±0.43, 1σ) mol O₂/yr. We explore how this sink would change as the deep ocean accumulated sulfate, became oxygenated, and began oxidizing oceanic crust such that there was an increase in the subduction flux of oxidants to the subarc mantle. We propose that significant changes to the O₂ cycle, both in terms of positive and negative feedbacks could occur during these transitions. For example, accumulation of sulfate in the deep ocean would increase the oxidation state of high-temperature hydrothermal fluids, decreasing the size of this O₂ sink and thus promoting an increase in atmospheric O₂. In contrast, the oxygenation of the deep ocean would have allowed hydrothermally derived H₂S to react with and consume O₂ instead of being titrated out via reactions with dissolved Fe²⁺. Additionally, deep-ocean oxygenation would have initiated the oxidation of oceanic crust at low temperatures, creating new sinks for O₂. Finally, the oxidation of the subarc mantle via subduction of newly oxidized sediments and altered oceanic crust would have increased the oxygen fugacity of arc volcanic gases, decreasing their overall demand for O₂, allowing yet more O₂ to accumulate in the atmosphere. We place these changes into a conceptual framework and discuss their potential impact on the history of atmospheric and marine O₂ concentrations from the Neoproterozoic to late Paleozoic.

Elemental and isotopic compositions of trench-slope black shales, Bohemian Massif, with implications for oceanic and atmospheric oxygenation in early Cambrian

This study examines a lower Cambrian pyrite-bearing black shale–graywacke succession in the Czech Republic interpreted as infill of a deep-marine, extensional trench-slope basin on top of an accretionary wedge that developed during subduction of an oceanic plate beneath the northern margin of Gondwana. The new U–Pb detrital zircon geochronology of the graywacke constrains the maximum age of deposition to 533 +5/−6 Ma, whereas the Re–Os dating of pyrite yielded a younger age of 507 ± 16 Ma. Distribution of major and trace elements and calculated enrichment factors (EF) indicate that this succession was deposited under changing redox conditions over a short time span and that the basin was presumably controlled by tectonic subsidence and varying supply of terrigenous arc-derived material. This depositional setting is reflected by largely variable EFs and δ98Mo and δ53Cr values. Euxinic conditions detected at the base of the sampled black shale interval are documented by the highest values of EFs of redox-sensitive metals (e.g., Mo, U, V, Ni, Co, As) and also the Corg/P (>1000) and DOPT (>0.7) values. Black shales show lower δ56Fe values due to the excess of authigenic pyrite-Fe with a mean δ56Fe value of −0.02‰ over detrital Fe in graywacke with a mean δ56Fe value of +0.25‰. However, the Fe isotopic signatures of the black shales are not consistent with iron shuttling, mixing of authigenic and detrital sources, or hydrothermal metal enrichment. Instead, they most likely resulted from partial oxidation of pyrite through the syndepositional oxidizing hydrothermal fluids (Si–Ba enrichment), which resulted in precipitation of isotopically heavy Fe-oxyhydroxides. We propose that our maximal recorded δ98Mo value (+ 0.98‰) might represent the best estimate for the ancient local seawater Mo composition at around 533 Ma and argues against deep-ocean oxygenation in the early Cambrian. On the other hand, the Se/Co ratios of synsedimentary pyrite indicate a mean atmosphere O2 value of ~27%.

Glacial heterogeneity in Southern Ocean carbon storage abated by fast South Indian deglacial carbon release

Past changes in ocean 14C disequilibria have been suggested to reflect the Southern Ocean control on global exogenic carbon cycling. Yet, the volumetric extent of the glacial carbon pool and the deglacial mechanisms contributing to release remineralized carbon, particularly from regions with enhanced mixing today, remain insufficiently constrained. Here, we reconstruct the deglacial ventilation history of the South Indian upwelling hotspot near Kerguelen Island, using high-resolution 14C-dating of smaller-than-conventional foraminiferal samples and multi-proxy deep-ocean oxygen estimates. We find marked regional differences in Southern Ocean overturning with distinct South Indian fingerprints on (early de-)glacial atmospheric CO2 change. The dissipation of this heterogeneity commenced 14.6 kyr ago, signaling the onset of modern-like, strong South Indian Ocean upwelling, likely promoted by rejuvenated Atlantic overturning. Our findings highlight the South Indian Ocean’s capacity to influence atmospheric CO2 levels and amplify the impacts of inter-hemispheric climate variability on global carbon cycling within centuries and millennia.

Deep-Ocean Oxygen May Increase with Climate Change

A millennial-scale ocean simulation indicates that oxygen gains in the deep ocean may offset oxygen losses in the upper water layer under a protracted climate change scenario.

Iron phosphate in the Ediacaran Doushantuo Formation of South China: A previously undocumented marine phosphate sink

Ediacaran phosphorites capture the dynamics of the ultimate biolimiting nutrient, phosphorus, during perhaps the most critical transition of Earth's climatic and ecological history. Concomitant with the Neoproterozoic Oxygenation Event was the deposition of the first extensive phosphorites across marine shelves, typically interpreted as a basinward shift in the locus of phosphogenesis facilitated by deep-ocean oxygenation. Petrographic and spectroscopic analyses of proximal phosphorites from the Ediacaran Doushantuo Formation near the Yangtze Gorges area of South China reveal the presence of interlaminated pristine, muddy, and granular phosphorites that were cemented by early diagenetic iron phosphate, an additional, previously undocumented seafloor P sink. Transmitted- and reflected-light microscopy, cathodoluminescence microscopy, and back-scattered electron scanning electron microscopy were used to reconstruct the paragenesis of phosphorites at the Wanjiagou section near Zhangcunping (Hubei Province) and demonstrate that pristine phosphorite precipitated authigenically as francolite via microbially-mediated phosphogenesis as evidenced by preserved microbial laminations along with filamentous and coccoidal microfossils. Subsequent tidal reworking of pristine hardgrounds and phosphatic mudstones produced minimally-coated intraclasts and peloidal phosphatic grains that were cemented by iron phosphate soon after deposition but before compaction. Raman spectroscopy, bulk-sample and micro-X-ray diffractometry indicate the iron phosphate cement is composed of phosphosiderite/strengite (Fe+3PO4•2H2O). The presence of phosphosiderite can be attributed to either syndepositional oxidation of phosphorites or thermal stabilization of vivianite (Fe2+3(PO4)2•8H2O). In both cases, the occurrence of phosphosiderite represents the first direct evidence for iron phosphate minerals as a P sink during the Ediacaran and suggests phosphogenesis may have partially proceeded in ferruginous porewaters in contrast to the redox-independent process of francolite precipitation. Integration of petrographic and geochemical techniques, such as those employed in this study, offers the ability to corroborate P speciation analyses and provides an independent determination of P phase partitioning. Ultimately, this approach is paramount in testing hypotheses that suggest ferrous iron phosphate minerals played a regulatory role in global Proterozoic oxygenation.

Contrasting Upper and Deep Ocean Oxygen Response to Protracted Global Warming

AbstractIt is well established that the ocean is currently losing dissolved oxygen (O2) in response to ocean warming, but the long‐term, equilibrium response of O2 to a warmer climate is neither well quantified nor understood. Here we use idealized multimillennial global warming simulations with a comprehensive Earth system model to show that the equilibrium response in ocean O2 differs fundamentally from the ongoing transient response. After physical equilibration of the model (&gt;4,000 years) under a two times preindustrial CO2 scenario, the deep ocean is better ventilated and oxygenated compared to preindustrial conditions, even though the deep ocean is substantially warmer. The recovery and overshoot of deep convection in the Weddell Sea and especially the Ross Sea after ~720 years causes a strong increase in deep ocean O2 that overcompensates the solubility‐driven decrease in O2. In contrast, O2 in most of the upper tropical ocean is substantially depleted owing to the warming‐induced O2 decrease dominating over changes in ventilation and biology. Our results emphasize the millennial‐scale impact of global warming on marine life, with some impacts emerging many centuries or even millennia after atmospheric CO2 has stabilized.

Surface ocean nitrate-limitation in the aftermath of Marinoan snowball Earth: Evidence from the Ediacaran Doushantuo Formation in the western margin of the Yangtze Block, South China

Geochemical and paleontological studies suggest that a rapid rise in atmospheric O2 level in the termination of the Marinoan global glaciation (635 Ma) might have triggered the diversification of eukaryotes and caused the deep ocean oxygenation. It is proposed that the Earth’s surface redox condition was directly linked to the marine primary productivity, which represented the only major O2 source before the evolution of land plants. The marine primary productivity was mainly controlled by the bioavailability of macronutrients, phosphorus (P) and nitrogen (N). Thus, reconstruction of ancient P and N cycles can provide direct constraints on marine organic matter production and the redox landscape of the ocean. Previous studies of the Ediacaran Doushantuo Formation in the Yangtze Block, South China, suggested that the Ediacaran marine N cycle was highly dynamic and was characterized by dramatic fluctuations in the intensity of denitrification. However, these studies were mainly focused on sections deposited in the offshore marine environment, and little is known about the N cycle in the nearshore regions. In this study, we report high-resolution organic carbon (δ13Corg) and nitrogen (δ15NTN) isotopes of the Doushantuo Formation at the E-Shan section in the western margin of the Yangtze Block. The Doushantuo Formation at the E-Shan section is composed of alternating deposition of thin bedded sandstone and mudstone, and was deposited in a nearshore delta environment. The δ13Corg profile displays two prominent negative excursions, which are coincident with two positive excursions in δ15NTN. The positive excursion in δ15NTN implies the enhanced denitrification, resulting in the loss of N (nitrate and ammonium). In addition, there are negative correlations between δ15NTN and TOC content and between δ15NTN and δ13Corg. Such relationships suggest that the reduction of N supply would lower primary productivity and accordingly decrease the carbon isotope of dissolved inorganic carbon (δ13CDIC) in the surface ocean. In addition, this interpretation also implies that N-fixation was not active in the nearshore region. We speculate that the muted N-fixation in the nearshore regions could be attributed to the Fe limitation in oxic shallow seawater and general Mo deficiency of the Ediacaran ocean. Thus, our study indicates that the surface ocean productivity and marine redox landscape were coupled in the Ediacaran ocean.

Early Cambrian ocean mixing recorded by phosphorite successions in the Nanhua Basin, South China

Phosphorite successions in the nearshore Meishucun section and the offshore Gezhongwu section of the Nanhua Basin, South China, were investigated to decipher the circulation pattern of the early Cambrian ocean. The 539–535 Ma phosphorites from the Meishucun section contain authigenic phosphorus-rich mineral aggregates (collophanite grains) with much lower bio-limiting elements, Zn, Cd and Ni, than those from the Gezhongwu section. In contrast, collophanite grains in the 535–526 Ma phosphorites from both sections have similar concentrations of these elements. This change indicates that the ocean likely transformed from a chemically stratified state to a homogeneous state at ~535 Ma. Phosphorites from both sections have constant 87Sr/86Sr ratios and εNd(t) values, supporting the interpretation of a relatively constant terrestrial input into the Nanhua Basin between 539 Ma and 526 Ma. Instead, negative δ13C excursions and similar 208Pb/204Pb ratios of the 535–526 Ma phosphorites are indicative of an extensive upwelling of the deep oceanic water. In combination with previous studies of deep ocean oxygenation, our work provides evidence for the vertical mixing of the ocean between 535 Ma and 526 Ma, which introduced O2-rich water at the ocean bottom to promote early Cambrian biodiversity.

Neoproterozoic to early Phanerozoic rise in island arc redox state due to deep ocean oxygenation and increased marine sulfate levels

A rise in atmospheric O2 levels between 800 and 400 Ma is thought to have oxygenated the deep oceans, ushered in modern biogeochemical cycles, and led to the diversification of animals. Over the same time interval, marine sulfate concentrations are also thought to have increased to near-modern levels. We present compiled data that indicate Phanerozoic island arc igneous rocks are more oxidized (Fe3+/ΣFe ratios are elevated by 0.12) vs. Precambrian equivalents. We propose this elevation is due to increases in deep-ocean O2 and marine sulfate concentrations between 800 and 400 Ma, which oxidized oceanic crust on the seafloor. Once subducted, this material oxidized the subarc mantle, increasing the redox state of island arc parental melts, and thus igneous island arc rocks. We test this using independently compiled V/Sc ratios, which are also an igneous oxybarometer. Average V/Sc ratios of Phanerozoic island arc rocks are elevated (by +1.1) compared with Precambrian equivalents, consistent with our proposal for an increase in the redox state of the subarc mantle between 800 and 400 Ma based on Fe3+/ΣFe ratios. This work provides evidence that the more oxidized nature of island arc vs. midocean-ridge basalts is related to the subduction of material oxidized at the Earth's surface to the subarc mantle. It also indicates that the rise of atmospheric O2 and marine sulfate to near-modern levels by the late Paleozoic influenced not only surface biogeochemical cycles and animal diversification but also influenced the redox state of island arc rocks, which are building blocks of continental crust.

Long-term integrated biogeochemical budget driven by circulation in the eastern subpolar North Atlantic

The eastern subpolar North Atlantic (eSPNA) is a key region in the Atlantic Meridional Overturning Circulation (AMOC), playing an important role in biogeochemical cycles and climate regulation. Quantitative basin-scale biogeochemical budgets are still scarce despite the current need of establishing baselines of knowledge in a changing ocean. The physico-chemical data from the eight repetitions of the OVIDE section (2002–2016) are an unique opportunity to develop a novel evaluation of biogeochemical budgets in the eSPNA by combining robust and well established decadal mean mass transports with carbon variables, oxygen and inorganic nutrients in a full-depth inverse box model. The net balance between the carbon fixation and the respiration throughout the whole water column shows that the eSPNA is an important dissolved inorganic carbon (DIC) sink area where 119 ± 43 kmol·s−1 and 49 ± 31 kmol·s−1 of organic and inorganic carbon, respectively, are currently exported. The uptake due to mixed layer depth oxygenation of 807 ± 114 kmol·s−1 of oxygen from the atmosphere and its subsequent southward export are responsible for deep Atlantic Ocean oxygenation. Deep water formation processes connect the northward upper limb with the southward lower limb of the AMOC leading to tracer export to the deep ocean. With regard to the net macronutrient budgets, all element consumptions are balanced within uncertainties. The results presented here for carbon export and oxygen uptake are in agreement with the upper range of previous observations based on different methods. The findings of this integrated budget driven by circulation in the highly dynamic region of the eSPNA can be taken as a reference in future biogeochemical evaluations of the North Atlantic.

A New biological proxy for deep-sea paleo-oxygen: Pores of epifaunal benthic foraminifera

The negative consequences of fossil fuel burning for the oceans will likely include warming, acidification and deoxygenation, yet predicting future deoxygenation is difficult. Sensitive proxies for oxygen concentrations in ancient deep-ocean bottom-waters are needed to learn from patterns of marine deoxygenation during global warming conditions in the geological past. Understanding of past oxygenation effects related to climate change will better inform us about future patterns of deoxygenation. Here we describe a new, quantitative biological proxy for determining ocean paleo-oxygen concentrations: the surface area of pores (used for gas exchange) in the tests of deep-sea benthic foraminifera collected alive from 22 locations (water depths: 400 to 4100 m) at oxygen levels ranging from ~ 2 to ~ 277 μmol/l. This new proxy is based on species that are widely distributed geographically, bathymetrically and chronologically, and therefore should have broad applications. Our calibration demonstrates a strong, negative logarithmic correlation between bottom-water oxygen concentrations and pore surface area, indicating that pore surface area of fossil epifaunal benthic foraminifera can be used to reconstruct past changes in deep ocean oxygen and redox levels.

Response of a comprehensive climate model to a broad range of external forcings: relevance for deep ocean ventilation and the development of late Cenozoic ice ages

Over the past few million years, the Earth descended from the relatively warm and stable climate of the Pliocene into the increasingly dramatic ice age cycles of the Pleistocene. The influences of orbital forcing and atmospheric CO2 on land-based ice sheets have long been considered as the key drivers of the ice ages, but less attention has been paid to their direct influences on the circulation of the deep ocean. Here we provide a broad view on the influences of CO2, orbital forcing and ice sheet size according to a comprehensive Earth system model, by integrating the model to equilibrium under 40 different combinations of the three external forcings. We find that the volume contribution of Antarctic (AABW) vs. North Atlantic (NADW) waters to the deep ocean varies widely among the simulations, and can be predicted from the difference between the surface densities at AABW and NADW deep water formation sites. Minima of both the AABW-NADW density difference and the AABW volume occur near interglacial CO2 (270–400 ppm). At low CO2, abundant formation and northward export of sea ice in the Southern Ocean contributes to very salty and dense Antarctic waters that dominate the global deep ocean. Furthermore, when the Earth is cold, low obliquity (i.e. a reduced tilt of Earth’s rotational axis) enhances the Antarctic water volume by expanding sea ice further. At high CO2, AABW dominance is favoured due to relatively warm subpolar North Atlantic waters, with more dependence on precession. Meanwhile, a large Laurentide ice sheet steers atmospheric circulation as to strengthen the Atlantic Meridional Overturning Circulation, but cools the Southern Ocean remotely, enhancing Antarctic sea ice export and leading to very salty and expanded AABW. Together, these results suggest that a ‘sweet spot’ of low CO2, low obliquity and relatively small ice sheets would have poised the AMOC for interruption, promoting Dansgaard–Oeschger-type abrupt change. The deep ocean temperature and salinity simulated under the most representative ‘glacial’ state agree very well with reconstructions from the Last Glacial Maximum (LGM), which lends confidence in the ability of the model to estimate large-scale changes in water-mass geometry. The model also simulates a circulation-driven increase of preformed radiocarbon reservoir age, which could explain most of the reconstructed LGM-preindustrial ocean radiocarbon change. However, the radiocarbon content of the simulated glacial ocean is still higher than reconstructed for the LGM, and the model does not reproduce reconstructed LGM deep ocean oxygen depletions. These ventilation-related disagreements probably reflect unresolved physical aspects of ventilation and ecosystem processes, but also raise the possibility that the LGM ocean circulation was not in equilibrium. Finally, the simulations display an increased sensitivity of both surface air temperature and AABW volume to orbital forcing under low CO2. We suggest that this enhanced orbital sensitivity contributed to the development of the ice age cycles by amplifying the responses of climate and the carbon cycle to orbital forcing, following a gradual downward trend of CO2.

Molybdenum record from black shales indicates oscillating atmospheric oxygen levels in the early Paleoproterozoic

The early Paleoproterozoic witnessed Earth9s first major oxygenation, referred to as the Great Oxidation Event or GOE. The GOE began around 2.45 billion years ago (Ga) and progressed over hundreds of millions of years, as evidenced by multiple redox indicators, before coming to an abrupt end by ca. 2.06 Ga. The details of the GOE and the extent of oxygenation are still not resolved, however, and it is not clear how redox conditions across the GOE compare with those during the middle Proterozoic. In order to investigate the evolution of deep-ocean redox conditions during the GOE, we present Mo concentration and isotope data together with Fe speciation values for three key organic matter-rich shale units of the early Paleoproterozoic age (2.32–2.06 Ga). In addition, we present a new graphical representation of modeling suggesting that the oceanic Mo isotope system is highly sensitive to the balance between anoxic/suboxic and euxinic conditions until deep-ocean oxygenation, similar in scale to modern ocean oxygenation, is reached. Our approach indicates rising, yet oscillating atmospheric oxygen at 2.32 Ga, leading to an abrupt increase in Mo supply to the oceans and large Mo isotope variations under non-steady state conditions. The low seawater δ98Mo value based on the ca. 2.32 Ga black shales (0.32 ± 0.58‰) suggests that the oceans were still largely anoxic with locally developed euxinic conditions. Between 2.2 and 2.1 Ga, during the peak of the Lomagundi carbon isotope excursion, we observe higher δ98MoSW values (1.23 ± 0.36‰) together with lower Mo concentrations in euxinic shales ([Mo] = 6.3 ± 9.0 ppm). We suggest that a decrease in the continental Mo input flux in the later part of the GOE was the main cause of this trend. Lower sulfide availability on the continents after protracted sulfide weathering associated with the early stages of the GOE, and efficient Mo removal in poorly oxygenated oceans under weakly euxinic conditions would both have contributed to the contraction of the Mo oceanic reservoir. By ca. 2.06 Ga, the Mo isotope composition of seawater, as inferred from euxinic black shale intervals, became significantly lighter (0.70 ± 0.21‰), reflecting an increased rate of quantitative Mo removal due to the more widespread development of strongly euxinic conditions. Counterintuitively, seawater Mo concentrations recovered, likely due to an increase in the Mo input, which in turn might reflect enhanced weathering of organic carbon-rich shales deposited during the Lomagundi Event.

A record of deep-ocean dissolved O2 from the oxidation state of iron in submarine basalts.

The oxygenation of the deep ocean in the geological past has been associated with a rise in the partial pressure of atmospheric molecular oxygen (O2) to near-present levels and the emergence of modern marine biogeochemical cycles. It has also been linked to the origination and diversification of early animals. It is generally thought that the deep ocean was largely anoxic from about 2,500 to 800 million years ago, with estimates of the occurrence of deep-ocean oxygenation and the linked increase in the partial pressure of atmospheric oxygen to levels sufficient for this oxygenation ranging from about 800 to 400 million years ago. Deep-ocean dissolved oxygen concentrations over this interval are typically estimated using geochemical signatures preserved in ancient continental shelf or slope sediments, which only indirectly reflect the geochemical state of the deep ocean. Here we present a record that more directly reflects deep-ocean oxygen concentrations, based on the ratio of Fe3+ to total Fe in hydrothermally altered basalts formed in ocean basins. Our data allow for quantitative estimates of deep-ocean dissolved oxygen concentrations from 3.5 billion years ago to 14 million years ago and suggest that deep-ocean oxygenation occurred in the Phanerozoic (541 million years ago to the present) and potentially not until the late Palaeozoic (less than 420 million years ago).