Sedimentary molybdenum and uranium : Improving proxies for deoxygenation in coastal depositional environments

Sedimentary molybdenum (Mo) and uranium (U) enrichments are widely used to reconstruct changes in bottom water oxygen conditions in aquatic environments. Until now, most studies using Mo and U have focused on restricted suboxic-euxinic basins and continental margin oxygen minimum zones (OMZs), leaving mildly reducing and oxic (but eutrophic) coastal depositional environments vastly understudied. Currently, it is un-known: (1) to what extent Mo and U enrichment factors (Mo-and U-EFs) can accurately reconstruct oxygen conditions in coastal sites experiencing mild deoxygenation, and (2) to what degree secondary (depositional environmental) factors impact Mo-and U-EFs. Here we investigate 18 coastal sites with varying bottom water redox conditions, which we define by means of five "redox bins", ranging from persistently oxic to persistently euxinic, from a variety of depositional environments. Our results demonstrate that Mo-and U-EF-based redox proxies and sedimentary Mo and U contents can be used to differentiate bottom water oxygen concentration among a range of modern coastal depositional environments. This is underpinned by the contrasting EFs of Mo and U along the redox gradient, which shows a substantial difference of Mo-EFs between redox bins 3-5 (ir/ regularly suboxic - ir/regularly dysoxic - persistently oxic) and of U-EFs between redox bins 1-2 (persistently euxinic - ir/regularly euxinic). Surprisingly, we observe comparatively low redox proxy potential for U in en-vironments of mild deoxygenation (redox bins 3-5). Further, we found that secondary factors can bias Mo-and U-EFs to such an extent that EFs do not reliably reflect bottom water redox conditions. We investigate the impact of limited Mo sedimentary sequestration in sulfidic depositional environments (i.e., the "basin reservoir effect", equilibrium with FeMoS4), Fe/Mn-(oxy)(hydr)oxide "shuttling", oxidative dissolution, the sulfate methane transition zone in the sediment, sedimentation rate, and the local Al background on Mo-and U-EFs.Peer reviewe

Gene-Based Modeling of Methane Oxidation in Coastal Sediments: Constraints on the Efficiency of the Microbial Methane Filter

Methane is a powerful greenhouse gas that is produced in large quantities in marine sediments. Microbially mediated oxidation of methane in sediments, when in balance with methane production, prevents the release of methane to the overlying water. Here, we present a gene-based reactive transport model that includes both microbial and geochemical dynamics and use it to investigate whether the rate of growth of methane oxidizers in sediments impacts the efficiency of the microbial methane filter. We focus on iron- and methane-rich coastal sediments and, with the model, show that at our site, up to 10% of all methane removed is oxidized by iron and manganese oxides, with the remainder accounted for by oxygen and sulfate. We demonstrate that the slow growth rate of anaerobic methane-oxidizing microbes limits their ability to respond to transient perturbations, resulting in periodic benthic release of methane. Eutrophication and deoxygenation decrease the efficiency of the microbial methane filter further, thereby enhancing the role of coastal environments as a source of methane to the atmosphere

Sedimentary molybdenum and uranium: Improving proxies for deoxygenation in coastal depositional environments

Sedimentary molybdenum (Mo) and uranium (U) enrichments are widely used to reconstruct changes in bottom water oxygen conditions in aquatic environments. Until now, most studies using Mo and U have focused on restricted suboxic-euxinic basins and continental margin oxygen minimum zones (OMZs), leaving mildly reducing and oxic (but eutrophic) coastal depositional environments vastly understudied. Currently, it is unknown: (1) to what extent Mo and U enrichment factors (Mo- and U-EFs) can accurately reconstruct oxygen conditions in coastal sites experiencing mild deoxygenation, and (2) to what degree secondary (depositional environmental) factors impact Mo- and U-EFs. Here we investigate 18 coastal sites with varying bottom water redox conditions, which we define by means of five “redox bins”, ranging from persistently oxic to persistently euxinic, from a variety of depositional environments. Our results demonstrate that Mo- and U-EF-based redox proxies and sedimentary Mo and U contents can be used to differentiate bottom water oxygen concentration among a range of modern coastal depositional environments. This is underpinned by the contrasting EFs of Mo and U along the redox gradient, which shows a substantial difference of Mo-EFs between redox bins 3–5 (ir/regularly suboxic – ir/regularly dysoxic – persistently oxic) and of U-EFs between redox bins 1–2 (persistently euxinic – ir/regularly euxinic). Surprisingly, we observe comparatively low redox proxy potential for U in environments of mild deoxygenation (redox bins 3–5). Further, we found that secondary factors can bias Mo-and U-EFs to such an extent that EFs do not reliably reflect bottom water redox conditions. We investigate the impact of limited Mo sedimentary sequestration in sulfidic depositional environments (i.e., the “basin reservoir effect”, equilibrium with FeMoS4), Fe/Mn-(oxy)(hydr)oxide “shuttling”, oxidative dissolution, the sulfate methane transition zone in the sediment, sedimentation rate, and the local Al background on Mo- and U-EFs

Deoxygenation and organic carbon sequestration in the Tethyan realm associated with the middle Eocene climatic optimum

The middle Eocene climatic optimum (ca. 40 Ma) stands out as a transient global warming phase of ~400 k.y. duration that interrupted long-term Eocene cooling; it has been associated with a rise in atmospheric CO2 concentrations that has been linked to a flare-up in Arabia-Eurasia continental arc volcanism. Increased organic carbon burial in the Tethys Ocean has been proposed as a carbon sequestration mechanism to bring the middle Eocene climatic optimum to an end. To further test these hypotheses, we assessed the sedimentary and geochemical expression of the middle Eocene climatic optimum in the northern Peri-Tethys, specifically, the organic-rich Kuma Formation of the Belaya River section, located on the edge of the Scythian Platform in the North Caucasus, Russia. We constructed an age-depth model using nannofossil chronobiostratigraphy. Throughout the studied middle Eocene interval (41.2–39.9 Ma), we documented sea-surface temperatures of 32–36 °C based on the tetraether index of tetraethers consisting of 86 carbons (TEX86), depending on proxy calibration, and during the early middle Eocene climatic optimum, we observed sea-surface warming of 2–3 °C. Despite the proximity of the section to the Arabia-Eurasia volcanic arc, the hypothesized source of volcanic CO2, we found no evidence for enhanced regional volcanism in sedimentary mercury concentrations. Sedimentary trace-element concentrations and iron speciation indicate reducing bottom waters throughout the middle Eocene, but the most reducing, even euxinic, conditions were reached during late middle Eocene climatic optimum cooling. This apparent regional decoupling between ocean warming and deoxygenation hints at a role for regional tectonics in causing basin restriction and anoxia. Associated excess organic carbon burial, extrapolated to the entire regional Kuma Formation, may have been ~8.1 Tg C yr–1, comprising ~450 Pg C over this ~55 k.y. interval. Combined with evidence for enhanced organic carbon drawdown in the western Peri-Tethys, this supports a quantitatively significant role for the basin in the termination of the middle Eocene climatic optimum by acting as a large organic carbon sink, and these results collectively illustrate that the closing Tethys Ocean might have affected global Paleogene climate

Facies Distribution, Sequence Stratigraphy, Chemostratigraphy, and Diagenesis of the Middle-Late Triassic Al Aziziyah Formation, Jifarah Basin, NW Libya

This study presents the depositional facies, sequence stratigraphy, chemostratigraphy and diagenetic evolution of the Middle-Late Triassic Al Aziziyah Formation, Jifarah Basin northwest Libya. Eight measured sections were sampled and analyzed. High-resolution stable carbon isotope data were integrated with an outcrop-based sequence stratigraphic framework, to build the stratigraphic correlation, and to provide better age control of the Al Aziziyah Formation using thin section petrography, cathodoluminescence (CL) microscopy, stable isotope, and trace element analyses. The Al Aziziyah Formation was deposited on a gently sloping carbonate ramp and consists of gray limestone, dolomite, and dolomitic limestone interbedded with rare shale. The Al Aziziyah Formation is predominantly a 2nd-order sequence (5-20 m.y. duration), with shallow marine sandstone and peritidal carbonate facies restricted to southernmost sections. Seven 3rd-order sequences were identified (S1-S7) within the type section. North of the Ghryan Dome section are three mainly subtidal sequences (S8-S10) that do not correlate to the south. Shallowing upward trends define 4th-5th order parasequences, but correlating these parasequences between sections is difficult due to unconformities. The carbon isotope correlation between the Ghryan Dome and Kaf Bates sections indicates five units of δ13C depletion and enrichment (sequences 3-7). The enrichment of δ13C values in certain intervals most likely reflects local withdrawal of 12C from the ocean due to increased productivity, as indicated by the deposition of organic-rich sediment, and/or whole rock sediment composed of calcite admixed with aragonite. The depletion of δ13C is clearly associated with exposure surfaces and with shallow carbonate facies. Heavier δ18O values are related to evaporetic enrichment of 18O, whereas depletion of δ18O is related to diagenesis due to freshwater input. Al Aziziyah Formation diagenetic events indicate: 1) initial meteoric and shallow burial; 2) three types of dolomite D1, D2 and D3 were most likely formed by microbial, seepage reflux and burial processes, respectively; and 3) diagenetic cements cannot be related to the arid, mega-monsoonal climate of the Triassic and most likely formed subsequently in a humid, meteoric setting

Hypoxia in the Holocene Baltic Sea : Comparing modern versus past intervals using sedimentary trace metals

Anthropogenic nutrient input has caused a rapid expansion of bottom water hypoxia in the Baltic Sea over the past century. Two earlier intervals of widespread hypoxia, coinciding with the Holocene Thermal Maximum (HTMHI; 8–4 ka before present; BP) and the Medieval Climate Anomaly (MCAHI; ~1200–750 years BP), have been identified from Baltic Sea sediments. Here we present sediment records from two sites in the Baltic Sea, and compare the trace metal (As, Ba, Cd, Cu, Mo, Ni, Pb, Re, Sb, Tl, U, V, Zn) enrichments during all three hypoxic intervals. Distinct differences are observed between the intervals and the various elements, highlighting the much stronger perturbation of trace metal cycles during the modern hypoxic interval. Both Mo and U show a strong correlation with Corg and very high absolute concentrations, indicative of frequently euxinic bottom waters during hypoxic intervals. During the modern hypoxic interval (ModernHI) comparatively less Mo is sequestered relative to Corg than in earlier intervals. This suggests partial drawdown of the water column Mo inventory in the modern water column due to persistent euxinia and only partial replenishment of Mo through North Sea inflows. Molybdenum contents in modern sediments are likely also affected by the recent slowdown in input of Mo in association with deposition of Fe and Mn oxides. Strong enrichments of U in recent sediments confirm that the ModernHI is more intense than past intervals. These results suggest that U is a more reliable indicator for the intensity of bottom water deoxygenation in the Baltic Sea than Mo. Sedimentary Re enrichment commences under mildly reducing conditions, but this element is not further enriched under more reducing conditions. Enrichments of V are relatively minor for the MCAHI and ModernHI, possibly due to strong reservoir effects on V in the water column, indicating that V is unreliable as an indicator for the intensity of bottom water hypoxia in this setting. Furthermore, Ba profiles are strongly influenced by post-depositional remobilization throughout the Holocene. The strong relationship between Corg and Ni, Tl and particularly Cu suggests that these trace metals can be used to reconstruct the Corg flux into the sediments. Profiles of As, Sb and Cd and especially Pb and Zn are strongly influenced by anthropogenic pollution

Quantifying volcanism and organic carbon burial across Oceanic Anoxic Event 2

Oceanic Anoxic Event 2 (ca. 94 Ma; OAE2) was one of the largest Mesozoic carbon cycle perturbations, but associated carbon emissions, primarily from the Caribbean large igneous province (LIP) and marine burial fluxes, are poorly constrained. Here, we use the carbon cycle box model LOSCAR-P to quantify the role of LIP volcanism and enhanced marine organic carbon (Corg) burial as constrained by the magnitude and shape of the positive stable carbon isotope (δ13C) excursion (CIE) in the exogenic carbon pool and atmospheric pCO2 reconstructions. In our best fit scenario, two pulses of volcanic carbon input—0.065 Pg C yr–1 over 170 k.y. and 0.075 Pg C yr–1 over 40 k.y., separated by an 80 k.y. interval with an input of 0.02 Pg C yr–1—are required to simulate observed changes in δ13C and pCO2. Reduced LIP activity and Corg burial lead to pronounced pCO2 reductions at the termination of both volcanic pulses, consistent with widespread evidence for cooling and a temporal negative trend in the global exogenic δ13C record. Finally, we show that observed leads and lags between such features in the records and simulations are explained by differences in the response time of components of the carbon cycle to volcanic forcin

Quantifying volcanism and organic carbon burial across Oceanic Anoxic Event 2

Oceanic Anoxic Event 2 (ca. 94 Ma; OAE2) was one of the largest Mesozoic carbon cycle perturbations, but associated carbon emissions, primarily from the Caribbean large igneous province (LIP) and marine burial fluxes, are poorly constrained. Here, we use the carbon cycle box model LOSCAR-P to quantify the role of LIP volcanism and enhanced marine organic carbon (Corg) burial as constrained by the magnitude and shape of the positive stable carbon isotope (δ13C) excursion (CIE) in the exogenic carbon pool and atmospheric pCO2 reconstructions. In our best fit scenario, two pulses of volcanic carbon input—0.065 Pg C yr–1 over 170 k.y. and 0.075 Pg C yr–1 over 40 k.y., separated by an 80 k.y. interval with an input of 0.02 Pg C yr–1—are required to simulate observed changes in δ13C and pCO2. Reduced LIP activity and Corg burial lead to pronounced pCO2 reductions at the termination of both volcanic pulses, consistent with widespread evidence for cooling and a temporal negative trend in the global exogenic δ13C record. Finally, we show that observed leads and lags between such features in the records and simulations are explained by differences in the response time of components of the carbon cycle to volcanic forcin

Corrigendum: Sediments as a Source of Iron, Manganese, Cobalt and Nickel to Continental Shelf Waters (Louisiana, Gulf of Mexico)(Front. Mar. Sci., (2022), 9, (811953), 10.3389/fmars.2022.811953)

In the original article, the support by the Netherlands Initiative Changing Oceans Program(NICO) was not mentioned in the acknowledgments. The corrected Acknowledgments are shown below: “We thank the captain, crew, technicians, (co-)chief scientists D. Rush and Z. Erdem and all other scientists aboard R/V Pelagia during cruise 64PE434 of the Netherlands Initiative Changing Oceans Program (NICO) in March 2018, for their assistance. We also thank H. de Waard, C. Mulder, M. Séguret, and A. van Dijk for analytical assistance in Utrecht.” The authors apologize for this error and state that this does not change the scientific conclusions of the article in any way. The original article has been update