Burial Fluxes Research Articles

Calcium isotopes reveal shelf acidification on southern Neotethyan margin during the Smithian-Spathian boundary cooling event

The Smithian-Spathian transition of the late Early Triassic was a critical period of environmental and biological upheavals, following the end-Permian mass extinction. Changes in carbonate deposition during this period have been attributed to intensified upwelling along shelf margins, but relevant studies are scarce. Here, we present calcium isotopes of bulk marine carbonate (δ44/40Cacarb) from a Smithian–Spathian boundary (SSB) succession (Guryul Ravine section, Kashmir) on the southern margin of the Neo-Tethys. Our smoothed δ44/40Cacarb curve reveals a ∼ 0.2‰ negative shift (from ∼ − 1.1‰ to ∼ − 1.3‰) across the SSB, concurrent with a ∼ +10‰ shift in δ13Ccarb. While increased Ca isotopic fractionation could play a role, we specifically examine potential impacts due to changes in marine Ca fluxes. Using a Ca-cycle mass balance model, we explore scenarios of decreased carbonate burial flux (Fcarb), decreased riverine flux (Friv), and a combination of these processes. The modeling suggest that a pulse decrease in Fcarb by 40% over ∼0.06 Myr match the negative shift in δ44/40Cacarb at Guryul Ravine. We infer that this decrease was likely related to intensified upwelling of acidic deep seawater due to invigorated global-oceanic circulation during the SSB cooling event. We suggest that the regionally diverse excursions in δ44/40Cacarb in the Tethyan region could be attributed to spatially varied upwellings in the shelf margin. The upwelling of acidic and anoxic deep seawater may have driven the second-order extinction of ammonoids and conodonts at the beginning of the SSB cooling event.

The Global Turbidity Current Pump and Its Implications for Organic Carbon Cycling.

Submarine turbidity currents form the largest sediment accumulations on Earth, raising the question of their role in global carbon cycles. It was previously inferred that terrestrial organic carbon was primarily incinerated on shelves and that most turbidity current systems are presently inactive. Turbidity currents were thus not considered in global carbon cycles, and the burial efficiency of global terrestrial organic carbon was considered low to moderate (∼10-44%). However, recent work has shown that burial of terrestrial organic carbon by turbidity currents is highly efficient (>60-100%) in a range of settings and that flows occur more frequently than once thought, although they were far more active at sea-level lowstands. This leads to revised global estimates for mass flux (∼62-90 Mt C/year) and burial efficiency (∼31-45%) of terrestrial organic carbon in marine sediments. Greatly increased burial fluxes during sea-level lowstands are also likely underestimated; thus, organic carbon cycling by turbidity currents could play a role in long-term changes in atmospheric CO2 and climate.

Transport and depositional processes of organic matter evaluated by biomarker profiles in Miocene turbiditic sequences from Hokkaido, Japan

Extrabasinal turbidity currents, also known as hyperpycnal flows, can deposit vast amounts of organic carbon to the marine realm. On the other hand, intrabasinal turbidity currents are commonly thought to resuspend and further oxidize organic matter (OM), creating deposits with poor preservation of terrestrial OM. However, recent studies have shown that intrabasinal turbidity currents can still preserve terrigenous OM after reworking previously deposited OM-rich sediments. As such, both types of turbidity currents may act as unconventional source rocks. Considering that there is a discrepancy between global carbon burial fluxes in the marine environment and the amount of organic carbon delivered by rivers, OM preservation within such sandy deposits should not be overlooked. We investigated both plant fragment-rich extrabasinal and intrabasinal turbidites of the Miocene Kawabata Formation (Hokkaido, Japan) by using biomarker distributions within different facies formed by changes in sediment concentration due to progressive deposition during waning flow. Clear distinctions in terms of sedimentological features and organic geochemical characteristics between different facies were found and are interpreted to be caused by different OM sources and sedimentological processes. The sandy extrabasinal turbidites have high amounts of organic carbon, high pristane/phytane ratios, and fewer amounts of marine algal biomarkers due to direct transport from the terrestrial environment without mixing with marine waters. Intrabasinal, Bouma-type turbidites generally have poor OM, and low pristane/phytane values but high concentrations of algal biomarkers since sediments are sourced from the marine environment. In both extrabasinal and intrabasinal turbidites of this study, the plant fragment-rich sandy layers deposited by flows with lower sediment concentrations preserve high amounts of terrigenous OM, possibly due to having densities allowing woody plant fragments to remain within the flow and corresponding deposit, while basal Ta sandstones and Te hemipelagic deposits contain more marine OM due to erosion plus incorporation of marine sediments and mixing with marine waters respectively. Mud clasts formed in basal flows such as surge-like intrabasinal turbidity currents may also enhance OM preservation in plant fragment-poor sandstones.

Transient Shoaling, Over‐Deepening and Settling of the Calcite Compensation Depth at the Eocene‐Oligocene Transition

AbstractThe major Cenozoic shift from a shallow (∼3–4 km) to deep (∼4.5 km) calcite compensation depth (CCD) occurred at the Eocene‐Oligocene Transition (∼34 Ma), suggesting a strong relationship between calcium carbonate (CaCO3) cycling and Antarctic glaciation. However, the linkages between these two events are debated. Here we present new records of bulk sediment stable isotope and carbonate composition from a depth transect of sites in the low‐latitude Pacific Ocean and one site from the South Atlantic Ocean, together with a new benthic foraminiferal stable isotope record (δ13Cb and δ18Ob) from the Pacific where the sedimentary sequence is most expanded. Our records reveal a short‐lived (∼300 Kyr) CCD shoaling event closely associated with a negative carbon isotope excursion in the latest Eocene. This event is immediately followed by CCD deepening which occurs in two rapid (∼40 Kyr‐long) steps. Our data show that the first of these deepening steps represents recovery from the latest Eocene shoaling event while the second was closely associated with a rapid increase in δ18Ob and shows a distinctive over‐deepening and settling pattern to >5 and 4.4 km, respectively. These results, together with good agreement between Pacific and South Atlantic records, strongly suggest that the carbon cycle was perturbed globally shortly before the inception of Antarctic glaciation. Once large‐scale Antarctic glaciation was initiated, rapid further change in global seawater chemistry triggered transitory deep ocean carbonate burial fluxes far exceeding their early Oligocene steady state values.

Different integrated mechanisms drove the two pulses of the Late Ordovician mass extinction

Across the Ordovician–Silurian transition, the first large-scale extinction event in geological history, the Late Ordovician mass extinction (LOME), occurred. The LOME was a double-pulse event, and the two extinction pulses occurred in the late Katian – early Hirnantian (LOME-1) and middle Hirnantian (LOME-2). At present, the comprehensive driving mechanism of the two extinction pulses is still controversial. In this study, two cores (JY87 and JY143) from the middle Yangtze shelf in South China were selected as the research objects. Combined with previous research results, four main stressors, namely, volcanism, ocean redox, climate and nutrient supply, were analyzed. On this basis, the two-pulse driving mechanism of the LOME was comprehensively studied. The comprehensive mechanisms driving the two extinction pulses were different. During the LOME-1, the intensity of volcanism was relatively weak. Volcanic silicate weathering, high nutrients and long-term accumulation of high organic carbon burial flux may have jointly caused cold conditions and led to the expansion of the anoxic sulfide zone and enhancement of sulfidation. The cold conditions had less effect on deep-water benthos, but enhanced euxinia led to their extinction. In contrast, shallow-water benthos were less affected by the enhanced euxinia, but the cold conditions led to their extinction. During the LOME-2, a sudden and brief increase in volcanism that was extremely intense occurred, bringing in abundant nutrients and SO2, resulting in more widespread and long-lasting euxinia. This occurrence led to the extinction of benthos that survived during the LOME-1.

Global oceanic anoxia linked with the Capitanian (Middle Permian) marine mass extinction

The timing and causation of the Capitanian (late Middle Permian) biocrisis remain controversial. Here, a detailed uranium-isotopic (δ238U) profile was generated for the mid-Capitanian to lower Wuchiapingian of the Penglaitan section (the Guadalupian/Lopingian Permian global stratotype) in South China for the purpose of investigating relationships between the biocrisis and coeval oceanic anoxic events (OAEs). Negative δ238U excursions indicate two distinct OAEs, a mid-Capitanian (OAE-C1) and an end-Capitanian (OAE-C2) event. Mass balance modeling shows that the anoxic sink of uranium (Fanox; i.e., the fraction of the total U burial flux) and anoxic seafloor area (Farea; i.e., the fraction of total seafloor area) increased during each OAE. A dynamic mass balance model yields increases of Fanox from <30% to >60% and Farea from ∼1% to ∼4-7% during each OAE. These two OAEs coincided with two extinction episodes during the Capitanian biocrisis, supporting a causal relationship between oceanic anoxia and mass extinction during the Middle Permian. The most likely driver of middle to late Capitanian global warming and oceanic anoxia was episodic magmatism of the Emeishan Large Igneous Province.

Biogenic silica cycling in the Skagerrak

Dissolved silicate (H4SiO4) is essential for the formation of the opaline skeletal structures of diatoms and other siliceous plankton. A fraction of particulate biogenic silica (bSi) formed in surface waters sinks to the seabed, where it either dissolves and returns to the water column or is permanently buried. Global silica budgets are still poorly constrained since data on benthic bSi cycling are lacking, especially on continental margins. This study describes benthic bSi cycling in the Skagerrak, a sedimentary depocenter for particles from the North Sea. Biogenic silica burial fluxes, benthic H4SiO4 fluxes to the water column and bSi burial efficiencies are reported for nine stations by evaluating data from in-situ benthic landers and sediment cores with a diagenetic reaction-transport model. The model simulates bSi contents and H4SiO4 concentrations at all sites using a novel power law to describe bSi dissolution kinetics with a small number of adjustable parameters. Our results show that, on average, 1100 mmol m-2 yr-1 of bSi rains down to the Skagerrak basin seafloor, of which 50% is released back to overlying waters, with the remainder being buried. Biogenic silica cycling in the Skagerrak is generally consistent with previously reported global trends, showing higher Si fluxes and burial efficiencies than deep-sea sites and similar values compared to other continental margins. A significant finding of this work is a molar bSi-to-organic carbon burial ratio of 0.22 in Skagerrak sediments, which is distinctively lower compared to other continental margins. We suggest that the continuous dissolution of bSi in suspended sediments transported over long distances from the North Sea leads to the apparent decoupling between bSi and organic carbon in Skagerrak sediments.

Carbon stocks and fluxes in Asia-Pacific mangroves: current knowledge and gaps

Mangrove forest plays a key role in regulating climate change, earth carbon cycling and other biogeochemical processes within blue carbon ecosystems. Therefore, mangrove forests should be incorporated into Earth system climate models with the aim of understanding future climate change. Despite multiple carbon stock and flux assessments taking place over the past couple of decades, concrete knowledge of carbon source/sink patterns is largely lacking, particularly in the biodiversity-rich Asia-Pacific (AP) region with its 68 493 km2 of mangrove area. Thus, to understand the gaps in mangrove blue carbon research in the AP region, we summarize a recent decade-long inventory of carbon stock pools (aboveground, belowground and soil) and biogeochemical flux components (burial, export/import, soil-air and water-air CO2 flux) across 25 AP countries to understand the current knowledge and gaps. While carbon stock assessments of individual components are available for all 25 countries, whole ecosystem carbon stocks—including live and standing dead aboveground and belowground, downed woody debris and soil carbon stocks—are often lacking, even in highly researched countries like Indonesia. There is restricted knowledge around biogeochemical carbon fluxes in 55% of the countries, suggesting poor carbon flux research across the region. Focusing on flux components, reports on sediment-to-sea carbon exports are extremely limited (coming from just nine countries in the AP region). There is notable scarcity of data on carbon export fluxes in Indonesian mangroves. Given the key role AP mangroves play in climate change mitigation worldwide, more detailed and methodologically comparable investigation of biogeochemical source/sink processes is required to better understand the role of this large carbon source in global carbon stocks and fluxes, and hence, global climate.

Benthic-pelagic coupling and isotopic fractionation of barium in Kiel Bight, SW Baltic Sea

Barium (Ba) isotopes are a promising new tracer for riverine freshwater input to the ocean and marine biogeochemical cycling. However, many processes that affect Ba cycling at continental margins have not yet been investigated with respect to Ba isotope fractionation. Here, we present a comprehensive data set of Ba concentration and isotope data for water column, pore water and sediment samples from Kiel Bight, a seasonally stratified and hypoxic fjord in the southwestern Baltic Sea. The surface water Ba concentration and Ba isotope inventory of the water column can generally be explained by mixing of riverine freshwater and Atlantic seawater. However, the deep-water below the seasonal pycnocline (10 - 15 m water depth) is characterized by a pronounced positive Ba concentration anomaly (up to 915 nM) that is accompanied by a δ138Ba of ~+0.25 ‰, which is lighter than expected from the seawater-freshwater mixing line (Ba: 77 nM, δ138Ba: +0.32 ‰ at a salinity of 18). Pore water profiles indicate a Ba flux across the sediment-water interface, which contributes to the enrichment in isotopically light Ba in the deep-water. Pore waters of surface sediments and deep-waters are oversaturated with respect to barite. Therefore, barite dissolution is unlikely to account for the benthic Ba flux. Water column Ba concentrations closely correlate with those of the nutrients phosphate and silica, which are removed from surface waters by biological processes and recycled from the sediment by diffusion across the sediment-water interface. As nutrient-to-Ba ratios differ among sites and from those observed in open-marine systems, we propose that Ba is removed from surface waters by adsorption onto biogenic particles (rather than assimilation) and regenerated within surface sediments upon organic matter degradation. Pore water data for subsurface sediments in Kiel Bight indicate preferential transfer of isotopically heavy Ba into an authigenic phase during early diagenesis. Quantifying the burial flux associated with this authigenic Ba phase along continental margins could potentially help to settle the isotopic imbalance between known Ba source and sink fluxes in the ocean.

Carbon Cycle Responses to Changes in Weathering and the Long‐Term Fate of Stable Carbon Isotopes

AbstractThe causes of CO2 variations over the past million years remain poorly understood. Imbalances between the input of elements from rock weathering and their removal from the atmosphere‐ocean‐biosphere system to the lithosphere likely contributed to reconstructed changes. We employ the Bern3D model to investigate carbon‐climate responses to step‐changes in the weathering input of phosphorus, alkalinity, carbon, and carbon isotope ratio (δ13C) in simulations extending up to 600,000 years. CO2 and climate approach a new equilibrium within a few ten thousand years, whereas equilibrium is established after several hundred thousand years for δ13C. These timescales represent a challenge for the initialization of sediment‐enabled models and unintended drifts may be larger than forced signals in simulations of the last glacial–interglacial cycle. Changes in dissolved CO2 change isotopic fractionation during marine photosynthesis. This causes distinct spatio‐temporal perturbations in δ13C and affects the burial flux of 13C. We force a cost‐efficient emulator, based on the Bern3D results, with contrasting literature‐based weathering histories over the last 800 thousand years. Glacial–interglacial amplitudes of up to 30 ppm in CO2, 0.05‰ in δ13C, and ∼15 mmol m−3 in deep ocean are emulated for changes in carbonate rock weathering. Plausible input from the decomposition of organic matter on shelves causes variations of up to 10 ppm in CO2, 0.09‰ in δ13C, and 5 mmol m−3 in , highlighting the non‐negligible effect of weathering‐burial imbalances.

Climate fluctuations during the Ordovician-Silurian transition period in South China: Implications for paleoenvironmental evolution and organic matter enrichment

The Ordovician-Silurian (OS) transition witnessed dramatic tectonic, climatic, marine, and biological coevolution, during which organic-rich black shale was widely deposited and is considered an important source rock. However, the potential links between the differential enrichment pattern of organic matter and climate fluctuations and paleoenvironmental evolution during this period are still unclear. In this paper, the Late Ordovician-early Silurian black shale in a JieLong (JL) section in South China was identified by graptolitic fossils, total organic carbon (TOC) content, X-ray diffraction (XRD), scanning electron microscopy (SEM) and geochemical analysis. The results show that the provenances of sedimentary rocks are all granodiorite-like. According to the variations in the TOC content and geochemical indicators, four layers are delineated. Under warm and humid climatic conditions, sea level rise and unstable seasonal upwelling led to Unit 1 organic matter enrichment controlled by high primary productivity, anoxic conditions and effective smectite production. Unit 2 organic matter burial was controlled by cold, arid and oxic environmental conditions, but primary productivity was still relatively high. After the end of the glacial period, warm and humid climatic conditions and rapid transgression were the main factors affecting the burial of organic matter in Unit 3. Under hot and humid climate conditions, higher terrigenous detrital inputs and unfavourable preservation environments, the dilution of organic matter in Unit 4 was accelerated. The corrected chemical index of alteration (CIAcorr) values show that the study area experienced a climate change of alternating cold and warm conditions in response to the climate fluctuation during the glacial-interglacial period. The CIAcorr value is negatively correlated with the TOC content, while the generation of available clay minerals (smectite) in warm and humid climates is usually consistent with the enhanced burial flux of organic matter, supporting the important role of climate fluctuations in the accumulation of organic matter. Finally, a comparison of the data from the present study with geochemical data from different profiles in South China reveals the role of climatic fluctuations during the OS transition in controlling paleoenvironmental evolution and organic matter enrichment, while such a heterogeneous factor is usually associated with global or regional geological events. Our study highlights this differential enrichment mechanism as a combined response of multiple factors and provides important implications for glacial-interglacial organic matter enrichment mechanisms.

Revisiting the biogenic silica burial flux determinations: A case study for the East China seas

The Coastal and Continental Margin Zones (CCMZs) contribute to 40% of the total burial flux of biogenic silica (bSi) of the world ocean. However, the accurate determination of the bSi content (bSiO2%) in marine sediments remains a challenge. The alkaline methods commonly used to quantitatively determine bSiO2% can completely digest the amorphous silica of diatoms but are less effective at digesting radiolarians and sponge spicules. In addition, the lithogenic silica (lSi) found in sediments is partly digested during these alkaline extractions, which can bias the accuracy of the determined bSiO2%. This is of importance in CCMZs where the lSi:bSi ratio is high. In this study, we examined sediments collected in the CCMZs of East China seas, an environment of peculiar interest given the large amount of lSi deposited by the Yellow River and the Yangtze River. The results show that alkaline digestions using stronger solutions and pretreatment steps resulted in an overestimate of the bSiO2% due to increased leaching of silica mainly from authigenic silicates and clays, whereas weak digestions underestimated the bSiO2% owing to incomplete digestion of sponge spicules. We found that the use of the Si/Al method accurately corrects for the lSi fraction in marine sediments, and thereby improves the determinations of bSiO2% in the sediments of East China seas CCMZs. Ensuring full digestion of all bSi remains challenging, in particular for sponge spicules, motivating both verifications via microscopy and longer extraction times. To emphasize the influence of these methodological differences, we revised the bSi burial flux in the East China seas and provide a new estimate of 253 (± 286) Gmol-SiO2 yr-1, which is one third of the previous estimates. We discuss the potential contribution from radiolarian and sponges and we propose a new general protocol for the determination of bSi in sediments that minimizes the methodological bias in bSi determination.

Variations in the concentration, source and flux of polycyclic aromatic hydrocarbons in sediments of the Pearl River Estuary: Implications for anthropogenic impacts

Variations in the distribution, source composition, mass inventory and burial flux of polycyclic aromatic hydrocarbons (PAHs) in surface sediments from the Pearl River Estuary (PRE) collected in 2011 and 2019 were analyzed to investigate the influence of the anthropogenic activities. Total concentrations of 16 priority PAHs in 2019 (200.40 ± 188.86 ng g−1 dry weight on average) were at the medium level among global bays/estuaries/coastal areas. In 2019, PAH concentrations have decreased by about 50% compared to 2011 and the dominant composition has changed from low- to high-molecular-weight PAHs. The qualitative and quantitative source apportionment analysis indicates that the dominant source of PAHs has shifted from petroleum (40.33%) in 2011 to traffic emission (44.17%) in 2019. The source variation in the PRE can be attributed to the transformation of the energy source structure from petrogenic to pyrogenic in the Pearl River Delta. The estimated PAH mass inventory of the top 5-cm sediment was 38.70 metric tons in 2019, which was about 41 metric tons lower than that in 2011. The average deposition fluxes have dropped from 418.91 ± 261.02 ng cm−2 yr−1 in 2011 to 215.52 ± 246.63 ng cm−2 yr−1 in 2019. The decreasing PAH concentration is related to the sediment coarsening and decline of total organic carbon. These findings in the PRE can be applied to other estuarine environments influenced by anthropogenic activities.

Zinc isotopic evidence for enhanced continental weathering and organic carbon burial during the late Cambrian SPICE event

The globally recognized late Cambrian Steptoean Positive Carbon Isotope Excursion (SPICE; ca. 497–494 Ma) is one of the most prominent carbon isotope excursions during the Phanerozoic eon, yet its primary cause remains debatable. To provide new insights into this issue, here we investigate zinc stable isotopes (expressed as δ66Zn) on two coeval carbonate successions spanning the Miaolingian-Furongian transition in North China. Similar stratigraphic δ66Zn trends are observed in the two sections, indicating a widespread perturbation to the marine Zn cycling. An abrupt decline of δ66Zn values by up to 0.4‰ and a concomitant increase of Zn/Ca ratios are observed at the pre-SPICE to the early SPICE interval, which can be best explained by the enhanced influx of isotopically light Zn from continental weathering. Enhanced continental weathering increased the inputs of terrestrial-sourced nutrients to the oceans, promoting the primary productivity and burial fluxes of organically bound Zn. At the middle stage of the SPICE, the coupled positive shifts of δ66Zn and δ13Ccarb indicate a phase of marine Zn cycling fluctuations driven by massive organic matter burial. Enhanced sequestration of organic carbon caused a rise in atmospheric oxygen levels, favoring the remobilization of previously buried Zn at shallower marine settings. With the development of muted continental weathering during the falling limb of the SPICE, the return of δ66Zn to initial background values marks the reestablishment of the stable seawater chemistry condition. Our results constrain the important roles of enhanced continental weathering and organic matter burial in micronutrient fluctuations during the SPICE, which were most likely the primary trigger for the SPICE.

Anthropogenic-driven chronological increase of sediment organic carbon burial in a river-lake system

Total organic carbon (TOC) in lake sediments from upstream catchments is deposited and buried in substrate, recording historical environmental changes. However, the linkage among natural variability, anthropogenic activity, and TOC burial for has not yet been clarified. This study examined the lake sediments of five 200-cm-deep dated depositional cores in west Dongting lake, China to quantify the magnitude, allocation, and amplitude of TOC burial. 44.47–59.36% of TOC burial flux was buried at 100–200 cm, suggesting lake sediments at deep layers stored considerable carbon. TOC burial rate (BRTOC) decreased along the lake entrance to its body, which was explained by the geochemical differences. Since 1900, BRTOC presented an increasing with a 4–7 times uptrend, showing three sedimentary stages with the increased human disturbance, such as deforestation, hydroelectric facilities. Moreover, the coefficient of variation of BRTOC in the third stage was lower than that in the second stage for the implementation of watershed reforestation and reservoir construction. Our findings stressed that natural variations of lake sedimentation background induced the change of TOC burial among the depositional sites, and enhanced that anthropogenic perturbation drove its chronological increases. This research unveiled the linkage between TOC burial, natural variability, and human disturbance from the perspective of burial evolutions in a lacustrine sedimentary environment.

Spatiotemporal quantification of organic matter accumulation in the Eocene Green River Formation, Bridger Basin, Wyoming

It has long been recognized that lakes can bury large amounts of organic carbon (CORG) in their sediment, with important consequences for conventional and unconventional petroleum resources and potentially for the global carbon cycle. The detailed distribution of lacustrine organic carbon through space and time is important to understanding its commercial and climatic implications, but has seldom been documented in detail. The Green River Formation offers a unique opportunity to improve this understanding, due to extensive Fischer assay analyses of its oil generative potential and to recently published radioisotopic age analyses of intercalat ed volcanic tuffs. Fischer assay analyses reveal distinctly different patterns of organic matter enrichment that correlate with different lacustrine facies associations. Histograms of oil generative potential for evaporative facies of the Wilkins Peak Member exhibit an approximately exponential distribution. This pattern is interpret ed to result from episodic expansion and contraction of Eocene Lake Gosiute across a low-gradient basin floor that experienced frequent desiccation. In contrast, histograms for fluctuating profundal facies of the upper Rife Bed of the Tipton Member and the lower LaClede Bed of the Laney Member exhibit an approximately normal or log normal distribution, with modes as high as 16–18 gallons per ton. This pattern is interpreted to reflect generally deeper conditions when the lake often intersected basin-bounding uplifts. Within the Bridger basin, burial of CORG was greatest in the south during initial Wilkins Peak Member deposition, reflecting greater rates of accommodation near the Uinta uplift. The locus of CORG burial shifted north during upper Wilkins Peak Member deposition, coincident with a decrease in differential accommodation. CORG burial during deposition of the upper Rife and lower LaClede Beds was greatest in the southeast, due either to greater accommodation or localized influx of river-borne nutrients. Average CORG burial fluxes are consistently ~4-5 g/m2 yr for each interval, which is an order of magnitude less than fluxes reported for small Holocene lakes in the northern hemisphere. Maximum rates of CORG burial during deposition of organic-rich mudstone beds (oil shale) were likely similar to Holocene lakes however. Deposition of carbonate minerals in the Bridger basin resulted in additional, inorganic carbon burial. Overall it appears that carbon burial by Eocene lakes could have influenced the global carbon cycle, but only if synchronized across multiple lake systems.

Differential mobilization and sequestration of sedimentary black carbon in the East China Sea

Black carbon (BC) derived from incomplete combustion of biomass and fossil fuels on land can be mobilized and transported to the ocean. Burial of BC in the ocean sequesters atmospheric CO2 into a long-term carbon sink, likely exerting a positive influence on mitigating global warming. However, the abundances, sources, and burial of sedimentary BC in marine sediments remain poorly constrained, hindering us from accurately understanding the mobilization and sequestration of BC and its roles in the ocean carbon cycle. Here, we investigate concentrations and isotopes (13C and 14C) of BC among grain size-fractionated surface sediments along a across-shelf transect from the Yangtze River prodelta to the Okinawa Trough to decipher the fate of BC in the East China Sea (ECS). Our results show that the bulk BC concentrations decrease firstly from the Yangtze River prodelta to the outer shelf and then increase to the Okinawa Trough. Grain size-fractionated BC concentrations vary along the transect, which we mainly attribute to the differential mobilization of BC driven by hydrodynamic processes. We argue that BC is aged during the mobilization, which results in an older 14C ages of BC found seaward. After considering biomass- and fossil-derived BC apportionments based on 14C balance calculation, we think that BC aging may be verified by more fossil-derived BC burial in the Okinawa Trough. We estimate that BC may account for ∼15% of sedimentary organic carbon (SOC), and up to ∼30% of terrestrial SOC buried in the ECS. BC burial fluxes decrease along the transect, and are heterogeneous in different size fractions, indicating differential sequestration of BC in the shelf and trough. We further estimate that 685 Gg/yr of BC is sequestered in the ECS, and 491 Gg/yr in the prodelta area, with ∼30% being continental biomass-derived BC. We suggest that increasing biomass-derived BC production on land and burying it in the ocean may serve as a powerful means for sequestrating atmospheric CO2, potentially contributing to carbon neutrality.

Beyond anoxia: Exploring sedimentary thallium isotopes in paleo-redox reconstructions from a new core top collection

The thallium isotopic composition (ε205Tl) of seawater has been identified as a promising proxy for global oceanic redox conditions due to its close association with Mn oxide burial. Currently the preferred archives for past seawater Tl isotope reconstructions are from anoxic (no oxygen detected) and/or sulfidic (no oxygen detected and free sulfide present) environments that may suffer from basin restriction. Here we conduct a comprehensive modern calibration of authigenic sedimentary ε205Tl with core tops covering a wide range of bottom water oxygenation and sediment compositions. We show that quantitative Tl removal in the reducing porewaters (e.g., Mn reduction) at/near the sediment–water interface could allow the sediments to track the water column ε205Tl. Even if the bottom water is not anoxic, a shallow oxygen penetration depth under high export productivity and/or high sedimentation rates may still allow rapid oxygen consumption and buildup of reducing porewaters conducive for complete Tl removal. Thus, applications of the Tl isotope redox proxy can be greatly expanded beyond the anoxic conditions into places with open-ocean connections and high mass accumulation rates for high-resolution temporal reconstructions. We present a decision tree using the sediment enrichment factors of Mn, Ba, and U to determine whether sediment archives are likely to record the seawater ε205Tl. We additionally provide an estimate of authigenic Tl burial fluxes in environments with quantitative Tl removal, which is roughly double the size of the ‘euxinic/anoxic’ sink in previously published global marine Tl mass balance estimates.

Sulfur isotope fractionation derived from reaction-transport modelling in the Eastern Equatorial Pacific

Microbial sulfate reduction in subseafloor sediments regulates a significant portion of the marine organic matter burial flux. Over secular timescales, sulfate reduction is the fundamental process connecting the biogeochemical cycles of sulfur, carbon, oxygen and phosphorus. Similar to carbon reduction, sulfate reduction is associated with a strong isotope fractionation process that allows us to track this process through time. It depends on a variety of factors and it has been argued previously that systematic differences between shallow and deep-sea environments might explain secular changes in the marine S-isotope ratio. However, observational data of in situ fractionation from deep-sea areas are scarce. Here we use a reaction-transport model to analyse the S-isotope fractionation during microbial sulfate reduction in the interstitial water of Ocean Drilling Program (ODP) Site 1226 (Leg 201). We find that the upper 100 m below seafloor are best explained with fractionation around −76‰, whereas below this depth the degree of fractionation drops to around −42‰. We propose that this shift is caused by changes in the ratio of the rate of microbial sulfate reduction relative to the rate of abiotic sulfide oxidation. Because large parts of deep oceans are characterized by exceedingly low sulfate reduction rates, this process may be widespread and possibly explain why pyrite S-isotope data suggest that the average S-isotope fractionation is around −50‰, rather than the theoretically predicted value below −70‰. Supplementary material: Supplementary figures and model results are available at https://doi.org/10.6084/m9.figshare.c.5976772 Thematic collection: This article is part of the Sulfur in the Earth system collection available at: https://www.lyellcollection.org/cc/sulfur-in-the-earth-system

The zinc isotope composition of late Holocene open-ocean marine sediments

The controls on Zn burial fluxes into marine sediments are not well constrained by existing datasets. To address this problem, new Zn-isotope data have been generated from a globally distributed array of late-Holocene age sediments that accumulated in open ocean settings underneath a diverse range of depositional conditions (Namibian margin, West African margin, Arabian margin, Californian margin, Peruvian margin, Southern Ocean, Indian Ocean, North and South Atlantic Oceans, South China Sea). The mean non-detrital isotopic composition of Zn in the whole dataset (δ66/64Zn = 0.37 ± 0.25 ‰, 2. S.D., n = 33) is only slightly lower than the modern deep-ocean composition (0.45 ± 0.14 ‰). Locations with total organic carbon >1 wt% have greater non-detrital enrichments of Zn and higher Zn isotope compositions than organic-poor sites, although the uncertainties overlap. Zinc associated with organic matter burial (using Zn/C ratios) and/or oxyhydroxide burial (using Zn/Fe and Zn/Mn ratios) account for a combined total of ~16–73% of the total non-detrital Zn content of the investigated sediments, therefore requiring an additional burial flux of Zn to balance the sedimentary budget. This flux is probably related to ZnS formation although Zn incorporation into authigenic clays is also possible. The new data indicate that open-ocean marine sediments track the deep-ocean Zn isotope composition to within 0.1–0.2 ‰, except in areas where isotopically distinct Zn fractions dominate the non-detrital Zn budget. The fraction of Zn removed into different sedimentary components is used to calculate Zn residence times ranging from ~2.7 kyr to 13.5 kyr that are within the range of previous estimates based on input fluxes to the ocean.