Burial Rate Of Organic Matter Research Articles

Processes and forcing mechanisms of the carbon cycle perturbation during Cretaceous Oceanic Anoxic Event 2

<p indent="0mm">Oceanic Anoxic Event 2 (OAE2, ~94 Ma) was one of two global oceanic anoxic events in the Cretaceous and has been one of the most studied paleoceanographic events of the Mesozoic. Owing to the hyperthermal climate state during OAE2, it serves as a key event in the study of Earth system evolution during extreme greenhouse warmth. Recently, high temporal and spatial resolution studies and the application of globally representative paleoenvironmental proxies have permitted major advances in our understanding of the triggering mechanisms and feedbacks of the Earth system during OAE2. By reviewing these findings, this paper analyzes the detailed processes involved in the OAE2 carbon cycle perturbation and associated environmental changes to elucidate the mechanisms that forced the evolution of carbon isotopes (<italic>δ</italic>¹³C) in seawater through the event. The most remarkable feature of OAE2 is a sharp positive <italic>δ</italic>¹³C excursion recorded globally, indicating a major perturbation of carbon cycle during the event. Outgassing from massive magmatism associated with large igneous province (LIP) emplacement was the major trigger of this carbon cycle perturbation. Increased primary productivity and extensive anoxia were responsible for the long-term positive <italic>δ</italic>¹³C excursion during OAE2. Interactions between paleoenvironmental parameters, such as weathering intensity, paleotemperature, CO₂ concentration, primary productivity and seawater redox condition controlled the short-term <italic>δ</italic>¹³C changes, which are superimposed on the long-term positive excursion of the entire OAE2 event. In this paper, we present a comprehensive model for the interactions between paleoenvironment changes and <italic>δ</italic>¹³C fluctuations through OAE2. The <italic>δ</italic>¹³C curve across the OAE2 has been previously divided into six segments by Li et al. (2017), labeled as C1 to C6, among which the major carbon cycle perturbation interval spans segments C2 to C5. During segment C2, a minor negative excursion in <italic>δ</italic>¹³C with a magnitude of ~1‰ is observed in open marine settings and relates to outgassing from LIP magmatic activity. However, enhanced productivity and deoxygenation of seawater during C2, especially in restricted marine settings, promoted the preservation of organic matter and this in turn led to an increase of <italic>δ</italic>¹³C. Based on simple mass balance calculation, we suggest that a 25% increase in the burial rate of organic carbon could diminish the effects of LIP outgassing on <italic>δ</italic>¹³C values in seawater and lead to relatively stable <italic>δ</italic>¹³C values during C2 in restricted seas. Segment C3 is characterized by a rapid increase in <italic>δ</italic>¹³C values that lasted for <sc>~280 ka.</sc> Significantly enhanced biological fixation of carbon is suggested to be responsible for the positive excursion in <italic>δ</italic>¹³C. A globally recorded negative excursion in <italic>δ</italic>¹³C has been found in the middle of this segment, which is consistent with an interval of maximum weathering intensity and global cooling. We suggest that reoxygenation of bottom water owing to cooling and consequent respiration of organic carbon from enhanced continental input and organic rich sediments could be the major cause of this decrease in <italic>δ</italic>¹³C in the middle of segment C3. Within the lower part of segment C4, LIP magmatism remained active and weathering intensity decreased, which led to a return to anoxic conditions and a highly fertilized state within the oceans. Consequent high burial rate of organic matter resulted in the positive trend in <italic>δ</italic>¹³C. Meanwhile, the sediment retention potential for nutrients (e.g., P) decreased under anoxic conditions, which led to sustained high primary productivity and high <italic>δ</italic>¹³C (<italic>δ</italic>¹³C plateau) that lasted for <sc>~370 ka.</sc> After this peak in <italic>δ</italic>¹³C, nutrient supply from weathering and LIP magmatism reached a minimum threshold to maintain a productivity-anoxia feedback, which in turn led to the inevitable recovery of <italic>δ</italic>¹³C (segment C5) to pre-OAE values and the termination of OAE2.

Carbon cycle evolution before and after the Great Oxidation of the atmosphere

The rock record of organic carbon abundance and its isotopic composition is consistent with the evolution of life more than 3800 million years ago (Ma). Despite this, there are very few insights as to the ecology of this ancient biosphere or to its level of activity. One possible insight, however, comes from the isotopic composition of inorganic and organic carbon in ancient rocks. This isotope record can be used, in principle, to determine the proportion of inorganic carbon entering the oceans that was buried in sediments as organic matter, and thus it helps constrain the activity level of the ancient biosphere. A quantitative analysis of this isotope record, however, requires that we understand how the Earth-surface carbon reservoir has evolved over time, as burial rates of organic matter in marine sediments depend on the input rates of carbon to the oceans. We must also know how organic matter is weathered as a function of atmospheric oxygen concentrations, thus indicating how much of the organic matter in sediments is newly formed or recycled. To explore these issues, a carbon cycle model is developed here that includes an evolving Earth-surface carbon reservoir as well as the oxygen dependency of the organic matter weathering in rocks. The model also allows for the release of CO₂ from organic matter during metamorphism and it contains a rock cycle with young and old reservoirs with appropriate transfer fluxes between them. The model shows that before the Great Oxidation Event (GOE) about 2400 Ma, only about 5 percent to 10 percent as much organic matter was buried into marine sediments as compared with today. Such low rates of organic matter burial would be consistent with a subdued marine biosphere. Such a subdued biosphere could possibly be consistent with primary production driven by anoxygenic photosynthesis coupled to an iron cycle. In association with, and in the aftermath of, the GOE, the biosphere likely increased its activity level by an order of magnitude. This large increase would have completely transformed the biology of the Earth and could have resulted from either the evolution and/or expansion of oxygen-producing cyanobacteria or a dramatic increase in the availability of nutrients to fuel oxygenic phototrophs.

Mechanistic strategies of microbial communities regulating lignocellulose deconstruction in a UK salt marsh

BackgroundSalt marshes are major natural repositories of sequestered organic carbon with high burial rates of organic matter, produced by highly productive native flora. Accumulated carbon predominantly exists as lignocellulose which is metabolised by communities of functionally diverse microbes. However, the organisms that orchestrate this process and the enzymatic mechanisms employed that regulate the accumulation, composition and permanence of this carbon stock are not yet known. We applied meta-exo-proteome proteomics and 16S rRNA gene profiling to study lignocellulose decomposition in situ within the surface level sediments of a natural established UK salt marsh.ResultsOur studies revealed a community dominated by Gammaproteobacteria, Bacteroidetes and Deltaproteobacteria that drive lignocellulose degradation in the salt marsh. We identify 42 families of lignocellulolytic bacteria of which the most active secretors of carbohydrate-active enzymes were observed to be Prolixibacteracea, Flavobacteriaceae, Cellvibrionaceae, Saccharospirillaceae, Alteromonadaceae, Vibrionaceae and Cytophagaceae. These families secreted lignocellulose-active glycoside hydrolase (GH) family enzymes GH3, GH5, GH6, GH9, GH10, GH11, GH13 and GH43 that were associated with degrading Spartina biomass. While fungi were present, we did not detect a lignocellulolytic contribution from fungi which are major contributors to terrestrial lignocellulose deconstruction. Oxidative enzymes such as laccases, peroxidases and lytic polysaccharide monooxygenases that are important for lignocellulose degradation in the terrestrial environment were present but not abundant, while a notable abundance of putative esterases (such as carbohydrate esterase family 1) associated with decoupling lignin from polysaccharides in lignocellulose was observed.ConclusionsHere, we identify a diverse cohort of previously undefined bacteria that drive lignocellulose degradation in the surface sediments of the salt marsh environment and describe the enzymatic mechanisms they employ to facilitate this process. Our results increase the understanding of the microbial and molecular mechanisms that underpin carbon sequestration from lignocellulose within salt marsh surface sediments in situ and provide insights into the potential enzymatic mechanisms regulating the enrichment of polyphenolics in salt marsh sediments.2Bi9cdUnFhfykPyYYU2FLjVideo

Multiple S-isotopic constraints on paleo-redox and sulfate concentrations across the Ediacaran-Cambrian transition in South China

Changes in oceanic chemical conditions are frequently invoked as a primary driver of biodiversification and mass extinction events in Earth history. During the Ediacaran–Cambrian transition, two major biological events, including the extinction of the Ediacaran biota and the Cambrian explosion, occurred approximately coincident with perturbations of the carbon and sulfur cycle. These dramatic biological changes and coeval geochemical events are commonly thought to be linked, but their causal relationships remain controversial. Here we present multiple S-isotopic compositions of pyrite (δ34Spy and Δ33Spy) from the Xiaotan section spanning the Ediacaran-Cambrian transition interval in South China. Our isotopic results show that negative Δ33Spy values, together with positive δ34Spy values, occurred across the Ediacaran-Cambrian boundary. The negative Δ33Spy data suggest mixing of sulfide from deep waters during the upwelling of sulfidic waters into oxygenated shallow waters. The coincidence between the negative Δ33Spy and the approximate timing of the extinction of the Ediacaran biota implies that paleo-redox perturbations may have contributed to the disappearance of the Ediacaran biota. Negative Δ33Spy values coupled with positive δ34Spy values were also observed in the Cambrian Zhongyicun Member when the phosphorites were deposited, suggesting a strong linkage between the upwelling of nutrient-rich sulfidic deep waters and the deposition of the phosphorites in oxygenated shelf areas. In addition, using box models and stoichiometry calculations, we quantitatively estimated elevated burial rates of organic matter and pyrite based on the parallel positive δ13Ccarb, δ34Spy and Δ33Spy excursions observed at Xiaotan within Cambrian Stage 2. Our mass balance modelling results provide an independent constraint on sulfate concentrations which were 4 mM or lower in the early Cambrian oceans. The probable increase in oxygen levels resulting from the enhanced burial of pyrite and organic matter could have provided a more habitable environment for early animals and thus paved the way for the radiation of early Cambrian animals.

Sources and burial of organic carbon in the middle Okinawa Trough during late Quaternary paleoenvironmental change

The sediments from a piston core ECS12A recovered from the middle Okinawa Trough in the East China Sea were measured for total organic carbon (TOC), total nitrogen (TN), and other biogenic elements to provide constraints on the sources and burial rates of depositional organic matter (OM) and on the changes in primary productivity since 19 ka. The last glacial sediments (ca. 17–19ka) are characterized by low contents of biogenic elements and well-developed turbidite layers, suggesting low primary productivity but a high component of terrigenous sediment. With rising sea level and enhanced monsoons during the deglacial period, the proportion of marine OM gradually increased. The least negative δ13Corg values and the smallest grain size of sediments deposited ca. 10–14.5ka indicate high primary productivity and a sedimentary environment dominated by the marine component. The source and burial rates of OM in the Holocene sediments (ca. 5.4–10ka) were largely controlled by the intensification of the Kuroshio Current, which caused a slight decrease in primary productivity, but strengthened the oceanic circulation in the East China Sea. Overall, the source-to-sink process of OM in the Okinawa Trough is governed by complex interactions between sea level, climate and ocean circulation.

Thick sulfate evaporite accumulations marking a mid-Neoproterozoic oxygenation event (Ten Stone Formation, Northwest Territories, Canada)

Thick sulfate evaporite accumulations are absent from Proterozoic strata between ca. 2000 and ca. 1000 Ma, and detailed sedimentologic, stratigraphic, and geochemical data for the oldest Neoproterozoic thick marine sulfate evaporite successions are largely lacking. The middle Neoproterozoic Ten Stone Formation (Little Dal Group, Northwest Territories, Canada) consists of ~500 m of pelagic lagoonal gypsite and anhydritite (rocks consisting of the minerals gypsum and anhydrite) deposited shortly before the ca. 811 Ma Bitter Springs carbon isotope anomaly in an intracratonic basin that developed prior to breakup of Rodinia. The thickness of regional stratigraphic subdivisions of this formation, defined by subtle silt- and carbonate-bearing intervals, indicates a minor terrigenous source in the southeast and a silled connection to the open ocean in the northwest. Deposition of the Ten Stone Formation began with abrupt, tectonically triggered subsidence and restriction, and ended equally abruptly, as shown by stratigraphic contacts across which lithofacies corresponding to strikingly different paleoenvironments change sharply, with no evidence for hiatus or erosion. Stratigraphic cyclicity in the evaporite succession is minimal owing to isolation of bottom-hugging, dense lagoonal brine from overlying waters. Deposition of the Ten Stone Formation in a basin that experienced intermittent, basin-scale tectonic adjustments, as recorded by details of its stratigraphy, supports the interpretation that the Mackenzie Mountains Supergroup accumulated in an extensional, tectonically active intracratonic basin whose structure resembled a lower-plate extensional system. The absence of halite from the Ten Stone Formation contrasts with its abundance in the stratigraphically lower, gypsum-free Dodo Creek Formation, suggesting that deposition of the lower to middle Little Dal Group spanned a major oxygenation event, during which the sequence of evaporite mineral precipitation from seawater changed from halite-first to sulfate-first in response to rapid accumulation of atmospheric oxygen and concomitant increase in the global marine sulfate reservoir. The limited range of sulfur isotope values in a new data set spanning hundreds of meters of gypsite indicates a strongly and persistently oxidizing mid-Neoproterozoic atmosphere, an abundance of sulfate in seawater, and marine oxygenation extending below storm wave base. The mineralogy, sedimentology, stratigraphy, and geochemistry of the Ten Stone Formation are virtually indistinguishable from those of thick, Phanerozoic “deep-water” (below wave-base) evaporite successions, and indicate that the tectonic, climatic, and geochemical conditions required for deposition of thick successions of marine sulfate evaporites were well established prior to ca. 811 Ma. Thick sulfate evaporite successions in equivalent stratigraphic positions just below the Bitter Springs carbon isotope excursion elsewhere in Laurentia, as well as on the Congo craton, and in South Australia attest to the global impact of the rapidly increased seawater sulfate reservoir prior to Rodinia’s breakup. High relative burial rates of organic matter prevailed before the breakup of Rodinia and led to oxygenation of the atmosphere-ocean system, growth of the seawater sulfate reservoir, and, in association with a warm and arid climate, deposition for the first time in Earth’s history of thick sulfate evaporites in the middle Neoproterozoic, ~100 m.y. before the first Cryogenian glacial episode. The Neoproterozoic Oxygenation Event may have taken place in several steps, the first of which preceded the Bitter Springs anomaly.

Beyond the Fe-P-redox connection: preferential regeneration of phosphorus from organic matter as a key control on Baltic Sea nutrient cycles

Abstract. Patterns of regeneration and burial of phosphorus (P) in the Baltic Sea are strongly dependent on redox conditions. Redox varies spatially along water depth gradients and temporally in response to the seasonal cycle and multidecadal hydrographic variability. Alongside the well-documented link between iron oxyhydroxide dissolution and release of P from Baltic Sea sediments, we show that preferential remineralization of P with respect to carbon (C) and nitrogen (N) during degradation of organic matter plays a key role in determining the surplus of bioavailable P in the water column. Preferential remineralization of P takes place both in the water column and upper sediments and its rate is shown to be redox-dependent, increasing as reducing conditions become more severe at greater water-depth in the deep basins. Existing Redfield-based biogeochemical models of the Baltic may therefore underestimate the imbalance between N and P availability for primary production, and hence the vulnerability of the Baltic to sustained eutrophication via the fixation of atmospheric N. However, burial of organic P is also shown to increase during multidecadal intervals of expanded hypoxia, due to higher net burial rates of organic matter around the margins of the deep basins. Such intervals may be characterized by basin-scale acceleration of all fluxes within the P cycle, including productivity, regeneration and burial, sustained by the relative accessibility of the water column P pool beneath a shallow halocline.

Formation mechanism of high quality marine source rocks—Coupled control mechanism of geological environment and organism evolution

High quality marine source rock (HQMSR) is the key prerequisite for medium to large hydrocarbon accumulations. However, the forming mechanism remains unclear. On the basis of the investigation for the geodynamic setting of the Middle-Upper Yangtze during the Early Cambrian in different spatial scales and the analysis of trace elements, the main controlling factors of the development of high quality marine source rock are discussed, with specific consideration of the burial rate of the organic matter. The formation of high quality marine source rocks is suggested to be the result of a coordinated development and the interaction between geological environments and organism evolution during the major geological transition periods. We perceived that the burial rate of organic matter was influenced by the primary productivity and its burial conditions. The abundance of autogenetic molybdenum gained directly by the chemical speciation analysis of rocks could be used as a proxy for the burial organic matter. The burial rate of autogenetic molybdenum and the sedimentary organics in modern marine environments could be used to estimate the sedimentary organics in ancient environments effectively.

High-resolution carbon isotopic records from the Ordovician of South China: Links to climatic cooling and the Great Ordovician Biodiversification Event (GOBE)

The Great Ordovician Biodiversification Event (GOBE) represented the largest increase in diversity in the marine biosphere during the Phanerozoic Eon, yet its causes and consequences remain poorly understood. Patterns of isotopic variation in high-resolution δ 13C carb and δ 13C org records from a well-exposed section at Honghuayuan in South China may provide important insights regarding the GOBE. The Honghuayuan isotopic profiles, which can be correlated with C-isotopic records from contemporaneous sections globally, reveal large perturbations to the global carbon cycle during the Ordovician. A + 8‰ increase in δ 13C org values in the Floian implies a large, albeit transient increase in the burial rate of organic matter during the mid-Early Ordovician that may have contributed to climatic cooling and played an important role in triggering the GOBE. A + 4‰ increase in δ 13C carb and high-frequency variation in δ 13C org in the Darriwilian to Sandbian suggest a second episode of elevated organic carbon burial rates accompanied by substantial instability in the global carbon cycle during the late Middle and early Late Ordovician. This pattern may mark the onset of climate changes culminating in the end-Ordovician Hirnantian glaciation and mass extinction event that terminated the GOBE.

Sedimentary manganese metallogenesis in response to the evolution of the Earth system

The concentration of manganese in solution and its precipitation in inorganic systems are primarily redox-controlled, guided by several Earth processes most of which were tectonically induced. The Early Archean atmosphere–hydrosphere system was extremely O 2-deficient. Thus, the very high mantle heat flux producing superplumes, severe outgassing and high-temperature hydrothermal activity introduced substantial Mn 2+ in anoxic oceans but prevented its precipitation. During the Late Archean, centered at ca. 2.75 Ga, the introduction of Photosystem II and decrease of the oxygen sinks led to a limited buildup of surface O 2-content locally, initiating modest deposition of manganese in shallow basin-margin oxygenated niches (e.g., deposits in India and Brazil). Rapid burial of organic matter, decline of reduced gases from a progressively oxygenated mantle and a net increase in photosynthetic oxygen marked the Archean–Proterozoic transition. Concurrently, a massive drawdown of atmospheric CO 2 owing to increased weathering rates on the tectonically expanded freeboard of the assembled supercontinents caused Paleoproterozoic glaciations (2.45–2.22 Ga). The spectacular sedimentary manganese deposits (at ca. 2.4 Ga) of Transvaal Supergroup, South Africa, were formed by oxidation of hydrothermally derived Mn 2+ transferred from a stratified ocean to the continental shelf by transgression. Episodes of increased burial rate of organic matter during ca. 2.4 and 2.06 Ga are correlatable to ocean stratification and further rise of oxygen in the atmosphere. Black shale-hosted Mn carbonate deposits in the Birimian sequence (ca. 2.3–2.0 Ga), West Africa, its equivalents in South America and those in the Francevillian sequence (ca. 2.2–2.1 Ga), Gabon are correlatable to this period. Tectonically forced doming-up, attenuation and substantial increase in freeboard areas prompted increased silicate weathering and atmospheric CO 2 drawdown causing glaciation on the Neoproterozoic Rodinia supercontinent. Tectonic rifting and mantle outgassing led to deglaciation. Dissolved Mn 2+ and Fe 2+ concentrated earlier in highly saline stagnant seawater below the ice cover were exported to shallow shelves by transgression during deglaciation. During the Sturtian glacial-interglacial event (ca. 750–700 Ma), interstratified Mn oxide and BIF deposits of Damara sequence, Namibia, was formed. The Varangian (≡ Marinoan; ca. 600 Ma) cryogenic event produced Mn oxide and BIF deposits at Urucum, Jacadigo Group, Brazil. The Datangpo interglacial sequence, South China (Liantuo–Nantuo ≡ Varangian event) contains black shale-hosted Mn carbonate deposits. The Early Paleozoic witnessed several glacioeustatic sea level changes producing small Mn carbonate deposits of Tiantaishan (Early Cambrian) and Taojiang (Mid-Ordovician) in black shale sequences, China, and the major Mn oxide–carbonate deposits of Karadzhal-type, Central Kazakhstan (Late Devonian). The Mesozoic period of intense plate movements and volcanism produced greenhouse climate and stratified oceans. During the Early Jurassic OAE, organic–rich sediments host many Mn carbonate deposits in Europe (e.g., Úrkút, Hungary) in black shale sequences. The Late Jurassic giant Mn Carbonate deposit at Molango, Mexico, was also genetically related to sea level change. Mn carbonates were always derived from Mn oxyhydroxides during early diagenesis. Large Mn oxide deposits of Cretaceous age at Groote Eylandt, Australia and Imini-Tasdremt, Morocco, were also formed during transgression–regression in greenhouse climate. The Early Oligocene giant Mn oxide–carbonate deposit of Chiatura (Georgia) and Nikopol (Ukraine) were developed in a similar situation. Thereafter, manganese sedimentation was entirely shifted to the deep seafloor and since ca. 15 Ma B.P. was climatically controlled (glaciation–deglaciation) assisted by oxygenated polar bottom currents (AABW, NADW). The changes in climate and the sea level were mainly tectonically forced.

Carbon and oxygen isotopic composition of Lower to Middle Cambrian sediments at Taijiang, Guizhou Province, China

Secular variations in the carbon isotopic composition of organic and carbonate carbon characterize the Lower to Middle Cambrian transition that is exposed on the Yangtze Platform at Taijiang, Guizhou Province, southern China. δ13C values for organic matter range between −33.4 and −26.5‰. The carbon isotopic composition for carbonate carbon fluctuates between −2.7 and +3.1‰. A progressive decrease in the isotopic difference (Δδ) between these two isotope records reflects a decrease in the proportional contribution of bacterial biomass to the total sedimentary organic matter. In general, the observed changes are interpreted to reflect primary depositional values, notably variations in the burial rates of organic matter. These, in turn, are linked to biological changes across the Lower to Middle Cambrian transition. No distinct shift in the carbon isotopic composition marks the proposed Lower–Middle Cambrian boundary.

Chemostratigraphy and correlation of the late Pennsylvanian Madera Formation, Cañon de San Diego, New Mexico, U.S.A.

Evaluation of unaltered brachiopod geochemistry from the Virgilian Madera Formation of northern New Mexico suggests that strontium isotopes are the proxy of choice for correlating sections. Based on isotopes, a diachronous relationship describes the sequences of the upper Madera Formation exposed in Canyon de San Diego. After consideration of this fact, carbon and oxygen isotopes of unaltered Madera brachiopods correlate well between the two sections in Canyon de San Diego. Late Pennsylvanian unaltered brachiopods from the North American Midcontinent and the European-Russian Platform define a relatively constant trend for87Sr/86Sr during the Missourian (0.708260) with minor oscillations marking the Virgilian (0.708267), possibly heralding the onset of early Permian glaciation. Carbon isotope values support a greater burial rate of organic matter for the Late Pennsylvanian (+3.21‰, PDB) relative to modern times (+1.6‰, PDB) with some regional differences in oceanographic conditions between the North American Midcontinent and the European-Russian Platform. Although regional oceanographic differences characterize the epeiric seas of the North American Midcontinent and the European-Russian Platform they are within natural variations for geographically separate provinces and modern counterparts. Oxygen isotopes of unaltered brachiopods with a global average value of −3.32‰ for the Late Pennsylvanian support the concept of an ice-free Earth, generally warm epeiric sea temperatures (about 17–24°C) with a possible secular shift in seawater-18O composition of about 0.7‰ for the Missourian-Virgilian (Stephanian).

Burial rates of organic matter along the eastern Canadian margin and stable isotope constraints on its origin and diagenetic evolution

Organic carbon (OC) and nitrogen (N) contents and δ 13 C and δ 15 N values in total organic matter (OM) were measured in sub-surface sediments (0–30 cm sub-bottom) from 21 cores raised from the Laurentian Channel of the Gulf of St. Lawrence and the Labrador Sea, to document OM fluxes and storage along the eastern Canadian margin. Storage rates as high as ∼2.5 g m −2 yr −1 for OC and ∼0.2 g m −2 yr −1 for N are observed in the Laurentian Channel, suggesting that the shelf plays a significant role in terms of OM storage (from 1 to 2% of the primary production). Based on the isotopic composition of the essentially marine OM of the Labrador Sea ( δ 13 C/V-PDB =−21.9±0.4‰; δ 15 N/AIR =7.6±0.6‰; n=12), there is no isotopic evidence for a significant relative input of terrestrial OM along the Laurentian Channel ( δ 13 C/V-PDB =−21.9±0.4‰; δ 15 N/AIR =8.0±0.9‰; n=10), either due to high relative fluxes of marine OM and/or to the trapping of continental OM in the estuary and upstream. High storage rates of OM are also observed on the continental rise of the Labrador Sea (as high as 1.1 g C m −2 yr −1 and ∼0.09 g N m −2 yr −1). They contrast with one order of magnitude lower rates on the slope, due to low sedimentation rates (SR) and sediment winnowing by the Western Boundary Undercurrent (WBUC). Reduced early diagenetic alteration of OM is observed, particularly in the Laurentian Channel. It results in discrete (i) losses of OC and N, (ii) shifts in C/N ratios, suggesting preferential removal of N-bearing OM also highlighted by losses in total hydrolysable amino acids (HAA). In the Labrador Sea slope records, due to low SR, OM concentration changes linked to long term temporal variations may superimpose on these diagenetic trends, and some influence of the WBUC is noticeable.

Enhanced preservation of organic matter in sediments deposited within the oxygen minimum zone in the northeastern Arabian Sea

The presence of a strongly developed oxygen minimum zone (OMZ; [O 2]<2 μM) in the northeastern Arabian Sea affords the opportunity to investigate whether oxygen deficiency in bottom waters enhances the preservation of organic matter in the underlying sediments. We explored if the observed patterns of organic matter accumulation could be explained by differences in productivity, sedimentation rate, water depth, and mineral texture. The differences in the burial rates of organic matter in sediments deposited within or below the OMZ could not be explained on the basis of these factors. All collected evidence points to a coupling of low oxygen concentrations and enhanced organic matter preservation. Under more oxygenated conditions bioturbation as well as the presence of labile manganese and iron oxides are probably important factors for a more efficient microbially mediated degradation of organic matter. Pore water profiles of dissolved Mn 2+ and Fe 2+ show that reduction of manganese and iron oxides plays a minor role in sediments lying within the OMZ and a larger role in sediments lying below the OMZ.

The chemical composition of subducting sediment and its consequences for the crust and mantle

Subducted sediments play an important role in arc magmatism and crust–mantle recycling. Models of continental growth, continental composition, convergent margin magmatism and mantle heterogeneity all require a better understanding of the mass and chemical fluxes associated with subducting sediments. We have evaluated subducting sediments on a global basis in order to better define their chemical systematics and to determine both regional and global average compositions. We then use these compositions to assess the importance of sediments to arc volcanism and crust–mantle recycling, and to re-evaluate the chemical composition of the continental crust. The large variations in the chemical composition of marine sediments are for the most part linked to the main lithological constituents. The alkali elements (K, Rb and Cs) and high field strength elements (Ti, Nb, Hf, Zr) are closely linked to the detrital phase in marine sediments; Th is largely detrital but may be enriched in the hydrogenous Fe–Mn component of sediments; REE patterns are largely continental, but abundances are closely linked to fish debris phosphate; U is mostly detrital, but also dependent on the supply and burial rate of organic matter; Ba is linked to both biogenic barite and hydrothermal components; Sr is linked to carbonate phases. Thus, the important geochemical tracers follow the lithology of the sediments. Sediment lithologies are controlled in turn by a small number of factors: proximity of detrital sources (volcanic and continental); biological productivity and preservation of carbonate and opal; and sedimentation rate. Because of the link with lithology and the wealth of lithological data routinely collected for ODP and DSDP drill cores, bulk geochemical averages can be calculated to better than 30% for most elements from fewer than ten chemical analyses for a typical drill core (100–1000 m). Combining the geochemical systematics with convergence rate and other parameters permits calculation of regional compositional fluxes for subducting sediment. These regional fluxes can be compared to the compositions of arc volcanics to asses the importance of sediment subduction to arc volcanism. For the 70% of the trenches worldwide where estimates can be made, the regional fluxes also provide the basis for a global subducting sediment (GLOSS) composition and flux. GLOSS is dominated by terrigenous material (76 wt% terrigenous, 7 wt% calcium carbonate, 10 wt% opal, 7 wt% mineral-bound H 2O +), and therefore similar to upper continental crust (UCC) in composition. Exceptions include enrichment in Ba, Mn and the middle and heavy REE, and depletions in detrital elements diluted by biogenic material (alkalis, Th, Zr, Hf). Sr and Pb are identical in GLOSS and UCC as a result of a balance between dilution and enrichment by marine phases. GLOSS and the systematics of marine sediments provide an independent approach to the composition of the upper continental crust for detrital elements. Significant discrepancies of up to a factor of two exist between the marine sediment data and current upper crustal estimates for Cs, Nb, Ta and Ti. Suggested revisions to UCC include Cs (7.3 ppm), Nb (13.7 ppm), Ta (0.96 ppm) and TiO 2 (0.76 wt%). These revisions affect recent bulk continental crust estimates for La/Nb and U/Nb, and lead to an even greater contrast between the continents and mantle for these important trace element ratios. GLOSS and the regional sediment data also provide new insights into the mantle sources of oceanic basalts. The classical geochemical distinction between `pelagic' and `terrigenous' sediment sources is not valid and needs to be replaced by a more comprehensive understanding of the compositional variations in complete sedimentary columns. In addition, isotopic arguments based on surface sediments alone can lead to erroneous conclusions. Specifically, the Nd/Hf ratio of GLOSS relaxes considerably the severe constraints on the amount of sediment recycling into the mantle based on earlier estimates from surface sediment compositions.

Carbon, nitrogen, and sulfur geochemistry of Archean and Proterozoic shales from the Kaapvaal Craton, South Africa

The C, N, and S contents and VC and δ 13Cδ 34S values were analyzed for 100 shale samples from ten formations, 3.0 to 2.1 Ga in age, in the central and eastern regions of the Kaapvaal Craton, South Africa. The Kaapvaal shales are characterized by generally low contents of organic C (range 0.06–2.79 wt%, average 0.47 wt%), N (range <0.01–0.09 wt%, average 0.1 wt%), and S (range <0.01–1.63 wt%, average 0.1 wt%). The low N/C (<0.005) and H/C (mostly ∼0.2) atomic ratios in kerogens from the shales indicated that the Kaapvaal shales lost considerable amounts of N, C, S, and H during diagenesis and regional metamorphism (up to the greenschist facies). From the theoretical relationships between the H/C ratios of kerogen and organic C contents of shales, the original C contents of the Archean and Proterozoic shales from the Kaapvaal Craton are estimated to be on average ∼2 wt%. These values are similar to the average organic C content of modern marine sediments. This suggests that the primary organic productivity and the preservation of organic matter in the ocean during the period of 3.0 to 2.1 Ga were similar to those in the Phanerozoic era, provided the flux of clastic sediments to the ocean was similar. This would also imply that the rate of O 2 accumulation in the atmosphere-ocean system, which has equaled the burial rate of organic matter in sediments, has been the same since ∼3.0 Ga. The δ 34S values of bulk-rock sulfides (mostly pyrite) range from +2.7 to +7.4%‰ for seven sulfide-rich samples of ∼2.9 Ga to ∼2.6 Ga. These values are consistent with a suggestion by Ohmoto (1992) and Ohmoto et al. (1993) that most pyrite crystals in Archean shales were formed by bacterial reduction of seawater sulfate with δ 34S values between +2 and +10‰, and that the Archean seawater was sulfate rich. Changes in the δ 13C org values during maturation of kerogen were evaluated with theoretical calculations from the experimental data of Peters et al. (1981) and Lewan (1983), and from the observations by Simoneit et al. (1981) on natural samples. These evaluations suggest that the magnitudes of δ 13C org increase are much less than those estimated by Hayes et al. (1983) and Des Marais et al. (1992), and only about 2 to 3%‰ for the kerogens that decreased their H/C ratios from 1.5 to less than 0.3. Based on the relationships among sulfide-S contents, organic-C contents, and δ 13C org values, four different types of depositional environments are identified for the Archean and early Proterozoic shales in the Kaapvaal Craton: (I) euxinic marine basins, characterized by normal marine organisms with δ 13C org= −33 ± 3%‰ (II) near-shore, oxic marine environment, characterized by normal marine organisms with δ 13C org = −31 ± 3%‰; (III) hypersaline, low-sulfate lakes, characterized by organisms with δ 13C org= −2 ± 3%‰; and (IV) euxinic, marine basins which supported the activity of methanogenic and methanotrophic bacteria and accumulated organic matter with δ 13C org= −43 ± 3%‰. In contrast to the currently popular model positing a global anoxic ocean prior to ∼2.2 Ga (e.g., Des Marais et al, 1992; Hayes, 1994; Logan et al., 1995), this study suggests that the development of anoxic basins, which accumulated Group II and IV sediments, occurred only regionally and episodically during the period between 3.0 Ga and 2.1 Ga. This further suggests that the normal ocean has been oxic since at least ∼3.0 Ga. Diversifications of environments, as well as of biological species, had already occurred ∼3.0 Ga. The carbon isotope mass balance calculation suggests that the removal rates of organic C and carbonate C from the ocean and the weathering rates of organic C and carbonate C on the continents during the 3.0–2.1 Ga period were basically the same as those in the Phanerozoic era. This would have been possible only if the atmospheric P O 2 level had been basically constant since at least 3.0 Ga. The results of this study, therefore, add to a growing list of evidence that the atmosphere has been oxic (i.e., P O 2 > 1%PAL) since at least 3.0 Ga. The list of evidence includes the sulfur isotope data on Archean sedimentary rocks ( Ohmoto and Felder, 1987; Ohmoto et al., 1993), the Fe 3+Ti ratios of paleosols ( Ohmoto, 1996), and the paragenesis of minerals in the “detrital” gold-uranium ores in pre-2.0 Ga quartz pebble beds that suggests nondetrital origins for uraninite and pyrite in these deposits ( Barnicoat et al., 1997).

Early diagenetic processes in Labrador Sea sediments: uranium-isotope geochemistry

The concentration and isotopic composition of U dissolved in pore waters from hemipelagic sediments of the Labrador Sea were determined by thermal ionization mass spectrometry in two 30 cm long box cores. The present fluxes of seawater U that diffuses across the sediment–seawater interface are on the order of 2–4 μg/(cm2∙ka). This diffusion imposes decreasing gradients of dissolved U downwards, but the U concentration in pore waters immediately below the surface is much lower than that of open-ocean seawater. This is a primary feature that cannot be explained by carbonate precipitation due to decompression during core retrieval. More likely, it reflects the presence of a stagnant benthic boundary layer above the sediment–water interface, in which molecular diffusion of U is slower than in the overlying turbulently mixed seawater, and (or) of microzones near the interface where U is bioaccumulated. Uranium is adsorbed at depths onto the solid phase in response to changes in the redox conditions within the sediments. In the Labrador Sea, this occurs at the onset of iron reduction and corresponds to a colour transition from brown to grey. Adsorption of U is sufficiently large to alter the initial content and the isotopic composition of U in the detrital component. Accumulation of authigenic U on the solid phase does not proceed at a steady state. This is due to the uneven burial rates of organic matter, which is essential to the establishment of redox conditions appropriate for U reduction, and concomitant stepwise displacement of the redox fronts. This indicates that discrete periods of enhanced primary productivity recurred over the last millenium in the Labrador Sea, inducing U fluxes to the sediments greater than they are now. Measured pore-water U concentrations are greater than the overlying seawater at depth in the cores, despite the fact that none of the conditions necessary to release U under reducing conditions are present in the sediments. More likely, U-bearing particles &lt; 0.45 μm were transferred with the solution phase through the filtering device, artificially increasing the pore-water U content.

Biogeochemical cycles of carbon and sulfur and their effect on atmospheric oxygen over phanerozoic time

The biogechemical cycles of carbon and sulfur contain within them the principles processes that control the level of atmospheric oxygen over Phanerozoic time. These are the production, deposition, burial and eventual oxidative weathering of organic matter and the diagenetic formation, burial and eventual oxidative weathering of sedimentary pyrite. Observations of modern sedimentary environments indicate that: (1) most organic matter, on a worldwide basis, is deposited in marine shelf-deltaic sediments, rather than in high-productivity upwelling regions or anoxic basins; (2) burial rate and the degree of preservation of organic matter are positively correlated with total sedimentation rate; (3) the rate of decomposition of organic matter in natural waters and sediments is highly dependent on the source and degree of aging of the organic matter itself, and is largely unaffected by the concentration of the principal oxidants, O 2 and SO 4 2-; (4) pyrite sulfur concentration in normal marine environments correlates well with organic carbon concentration due to the limitation of bacterial sulfate reduction by reactive organic matter that escapes aerobic destruction during deposition and bioturbation; (5) depth-integrated sulfate reduction in normal marine sediments correlates positively with total sedimentation rate; (6) in euxinic sediments excess pyrite is formed from sulfidic bottomwaters leading to a lack of correlation between organic carbon and pyrite sulfur, a good correlation between pyrite sulfur and total iron, and high degrees of pyritization of iron; and (7) very low concentrations of pyrite are found in continental fresh water sediments, regardless of organic matter or iron content, due to limitation of pyrite formation by the low concentration of dissolved sulfate in fresh water. Through the use of carbon and sulfur isotope mass balance models, it is possible to calculate original rates of burial of organic carbon (C) and pyrite sulfur (S) over Phanerozoic time. Unusually high calculated rates of organic carbon burial during Permo-Carboniferous time are attributed to: (1) the rise of vascular plants on the continents; (2) the availability of vast flooded lowlands (swamps) on the emergent continent of Pangea where deposited organic matter could be preserved; and (3) the high C/P ratio of vascular plant material which enabled greater organic carbon burial, per unit of phosphorus burial, than was the case previously for marine organic matter. High C/S ratios for Permo-Carboniferous time are attributed to a large proportion of total worldwide organic matter burial occurring in fresh water swamps and lakes. Low C/S ratios for the early Paleozoic are attributed to a much greater abundance of euxinic basins than at present and to the lack of deposition of vascular plant-derived organic matter at that time. There is little doubt that atmospheric O 2 has varied over Phanerozoic time. However, straightforward calculation of atmospheric O 2 levels, from rates of organic matter burial etc. derived from the isotope mass balance models, leads to geologically unreasonable fluctuations, even when linear O 2 negative feedback is introduced to the modelling. There is a need for additional quantifiable and geologically reasonable feedback mechanisms that constrain the modeling of atmospheric O 2 variation. Three possible candidates discussed here are: (1) splitting of the reservoirs of materials undergoing weathering into old and young sub-reservoirs, which enables rapid isotope recycling via more rapid weathering of younger rocks; (2) changing rates of worldwide erosion and total sediments burial which simultaneously enables variations in O 2 production, due to changes in burial rates of organic matter and pyrite, and in O 2 consumption, due to changes in rates of exposure of organic matter and pyrite to oxidative weathering; and (3) changing the stoichiometry of O 2 uptake during weathering by making it dependent on atmospheric O 2 level.

Biogeochemical cycles of carbon and sulfur and their effect on atmospheric oxygen over phanerozoic time

The biogechemical cycles of carbon and sulfur contain within them the principles processes that control the level of atmospheric oxygen over Phanerozoic time. These are the production, deposition, burial and eventual oxidative weathering of organic matter and the diagenetic formation, burial and eventual oxidative weathering of sedimentary pyrite. Observations of modern sedimentary environments indicate that: (1) most organic matter, on a worldwide basis, is deposited in marine shelf-deltaic sediments, rather than in high-productivity upwelling regions or anoxic basins; (2) burial rate and the degree of preservation of organic matter are positively correlated with total sedimentation rate; (3) the rate of decomposition of organic matter in natural waters and sediments is highly dependent on the source and degree of aging of the organic matter itself, and is largely unaffected by the concentration of the principal oxidants, O 2 and SO 4 2-; (4) pyrite sulfur concentration in normal marine environments correlates well with organic carbon concentration due to the limitation of bacterial sulfate reduction by reactive organic matter that escapes aerobic destruction during deposition and bioturbation; (5) depth-integrated sulfate reduction in normal marine sediments correlates positively with total sedimentation rate; (6) in euxinic sediments excess pyrite is formed from sulfidic bottomwaters leading to a lack of correlation between organic carbon and pyrite sulfur, a good correlation between pyrite sulfur and total iron, and high degrees of pyritization of iron; and (7) very low concentrations of pyrite are found in continental fresh water sediments, regardless of organic matter or iron content, due to limitation of pyrite formation by the low concentration of dissolved sulfate in fresh water. Through the use of carbon and sulfur isotope mass balance models, it is possible to calculate original rates of burial of organic carbon (C) and pyrite sulfur (S) over Phanerozoic time. Unusually high calculated rates of organic carbon burial during Permo-Carboniferous time are attributed to: (1) the rise of vascular plants on the continents; (2) the availability of vast flooded lowlands (swamps) on the emergent continent of Pangea where deposited organic matter could be preserved; and (3) the high C/P ratio of vascular plant material which enabled greater organic carbon burial, per unit of phosphorus burial, than was the case previously for marine organic matter. High C/S ratios for Permo-Carboniferous time are attributed to a large proportion of total worldwide organic matter burial occurring in fresh water swamps and lakes. Low C/S ratios for the early Paleozoic are attributed to a much greater abundance of euxinic basins than at present and to the lack of deposition of vascular plant-derived organic matter at that time. There is little doubt that atmospheric O 2 has varied over Phanerozoic time. However, straightforward calculation of atmospheric O 2 levels, from rates of organic matter burial etc. derived from the isotope mass balance models, leads to geologically unreasonable fluctuations, even when linear O 2 negative feedback is introduced to the modelling. There is a need for additional quantifiable and geologically reasonable feedback mechanisms that constrain the modeling of atmospheric O 2 variation. Three possible candidates discussed here are: (1) splitting of the reservoirs of materials undergoing weathering into old and young sub-reservoirs, which enables rapid isotope recycling via more rapid weathering of younger rocks; (2) changing rates of worldwide erosion and total sediments burial which simultaneously enables variations in O 2 production, due to changes in burial rates of organic matter and pyrite, and in O 2 consumption, due to changes in rates of exposure of organic matter and pyrite to oxidative weathering; and (3) changing the stoichiometry of O 2 uptake during weathering by making it dependent on atmospheric O 2 level.