Carbon Burial Research Articles

Warm plankton soup and red herrings: calcareous nannoplankton cellular communities and the Palaeocene-Eocene Thermal Maximum.

Past global warming events such as the Palaeocene–Eocene Thermal Maximum (PETM—56 Ma) are attributed to the release of vast amounts of carbon into the ocean, atmosphere and biosphere with recovery ascribed to a combination of silicate weathering and organic carbon burial. The phytoplanktonic nannoplankton are major contributors of organic and inorganic carbon but their role in this recovery process remains poorly understood and complicated by their contribution to marine calcification. Biocalcification is implicated not only in long-term carbon burial but also both short-term positive and negative climatic feedbacks associated with seawater buffering and responses to ocean acidification. Here, we use exceptional records of preserved fossil coccospheres to reconstruct cell size distribution, biomass production (particulate organic carbon, POC) and (particulate) inorganic carbon (PIC) yields of three contrasting nannoplankton communities (Bass River—outer shelf, Maud Rise—uppermost bathyal, Shatsky Rise—open ocean) through the PETM onset and recovery. Each of the sites shows contrasting community responses across the PETM as a function of their taxic composition and total community biomass. Our results indicate that nannoplankton PIC:POC had no role in short-term climate feedback and, as such, their importance as a source of CO2 to the environment is a red herring. It is nevertheless likely that shifts to greater numbers of smaller cells at the shelf site in particular led to greater carbon transfer efficiency, and that nannoplankton productivity and export across the shelves had a significant modulating effect on carbon sequestration during the PETM recovery.This article is part of a discussion meeting issue ‘Hyperthermals: rapid and extreme global warming in our geological past’.

Sediment microfabric records mass sedimentation of colonial cyanobacteria and extensive syndepositional metazoan reworking in Pliocene sapropels

AbstractThe sapropel record of the eastern Mediterranean provides unique insight into the primary climatic, oceanographic, and biological drivers of organic carbon enrichment in marine sediments. The dominant source of organic matter, timing of oxygen depletion at the sea floor, and extent of metazoan reworking of these deposits remain unclear. These questions are addressed by combining microbeam imaging with bulk and molecular geochemical characterization of several Pliocene sapropels, revealing four microfacies which record distinct palaeoceanographic conditions, phytoplankton assemblages, and degrees of postdepositional reworking. The most organic‐rich, carbonate‐lean sapropel intervals consist of alternating 10–60‐μm‐thick organic and detrital mineral laminae. Petrographical features consistent with a pelagic origin, δ15N <−1.8 and the presence of 2α‐methylhopanes indicate that organic laminae formed by mass sedimentation of N‐fixing colonial cyanobacteria (Trichodesmium). The association of framboidal pyrite with organic laminae suggests that mass sedimentation led to the abrupt development of sea floor anoxia. Unlike similar Quaternary sapropels, no petrographic or biomarker evidence is found for a mat‐forming diatom contribution to these organic‐rich, laminated intervals. However, episodic mass sedimentation was not found to be a universal ingredient of sapropel formation. Carbonate‐lean, weakly laminated intervals contain >50 μm organomineral aggregates, interpreted as marine snow, whereas carbonate microfossil‐rich intervals record periods of nitrogen fixation and moderately increased primary production by a diverse assemblage of calcareous, organic‐walled, and siliceous plankton. The results presented here further show that burrowing by microscopic meiofauna impacted most sapropels, extending into seemingly laminated intervals below obvious disruption from burrowing macrofauna, indicating that metazoans influence organic carbon burial in oxygen‐depleted settings even where physical displacement of sediment is not visible.

Ocean Circulation in the Toarcian (Early Jurassic): A Key Control on Deoxygenation and Carbon Burial on the European Shelf

AbstractThe Toarcian Oceanic Anoxic Event (T‐OAE, ∼183 Myr) was a long‐lasting episode of ocean deoxygenation during the Early Jurassic. The event is related to a period of global warming and characterized by major perturbations to the hydrological and carbon cycles with high rates of organic matter burial in shelf seas. Ocean circulation during the Toarcian and its influence on marine biogeochemical cycles are still not fully understood. Here we assess the spatial extent of anoxia in the NW Tethys Ocean during the T‐OAE, the relationship with ocean circulation and the impact on organic carbon burial, using new and existing sedimentary records from the European Epicontinental Shelf in combination with general circulation model results. We demonstrate that bottom waters on the southwestern part of the shelf were mainly oxic during the T‐OAE, while those in the northeastern basins were mostly anoxic or even sulfidic. Results for two ocean‐atmosphere models (Fast Ocean‐Atmosphere Model and Massachusetts Institute of Technology general circulation model) suggest the presence of a strong clockwise gyre over the European Epicontinental Shelf, which brought oxygenated equatorial waters from the Tethys Ocean to the southern shelf. The northward limb of the gyre was significantly weakened due to the rough bathymetry of the northern shelf, making this relative small region highly sensitive to local ocean stratification. These sluggish ocean dynamics promoted bottom water anoxia and enhanced burial of organic carbon in the northeastern basins, which accounted for 3–5% of the total carbon extracted from the ocean‐atmosphere system as recorded by the positive carbon isotope shift.

Burial of Organic Carbon in the Taiwan Strait

AbstractOrganic carbon (OC) burial in nondeltaic continental shelves is an important part of the global carbon cycle. Of these, the Taiwan Strait (TWS) is a typical, nondeltaic shelf region that is influenced by a variety of factors including strong ocean currents, coastal upwelling, and small river inputs. To better understand how these factors influence OC burial in the TWS, we measured total organic carbon and total nitrogen contents as well as the stable carbon isotopes of sedimentary organic matter (δ13Corg) from the TWS. We also measured glycerol dialkyl glycerol tetraethers in surface sediments collected from the TWS. The concentrations of sedimentary total organic carbon and total nitrogen along the coast were high but decreased in areas more central to the TWS. This gradient was controlled by both hydrological conditions and coastal upwelling. The calculated accumulation rate of organic carbon ranged from 1.9 to 47.2 g C/m2/year and was comparable to other areas of the Chinese marginal seas. The source contribution calculated from the three‐end‐member model revealed that mostly marine OC was buried in the upwelling regions. In comparison, OC derived from terrestrial plant and soil was buried in the estuaries. Collectively, these results showed that the TWS may serve as a CO2 sink in the global carbon cycle. This is due to the preservation of the labile portions of the OC derived from plant and marine source, which formed the main component of the buried OC in the TWS.

Paired δ13Ccarb and δ13Corg records of the Ordovician on the Yangtze platform, South China

During the Ordovician, huge biological revolutions and environmental changes happened in Earth’s history, including the Great Ordovician Biodiversification Event, global cooling and so on, but the cause of these events remains controversial. Herein, we conducted a paired carbon isotopic analysis of carbonate (δ13Ccarb) and organic matter (δ13Corg) through the Ordovician in the Qiliao section on the Yangtze platform of South China. Our results showed that the δ13Ccarb trend of the Qiliao section can be correlated with local and global curves. The δ13Corg trend seems is less clear than the δ13Ccarb trend for stratigraphic correlations, but some δ13Corg positive excursions in the Middle and Upper Ordovician may be used for correlation studies. These carbon isotopic records may have global significance rather than local significance, revealing several fluctuations to the global carbon cycle during the Ordovician. Several known δ13Ccarb and δ13Corg negative and positive excursions have been recognised in this study, including the early Floian Negative Inorganic Carbon (δ13Ccarb) Excursion (EFNICE), as well as the early Floian Positive Organic Carbon (δ13Ccarb) Excursion, the mid-Darriwilian Inorganic Carbon (δ13Ccarb and δ13Corg) Excursion (MDICE), and the early Katian Guttenberg Inorganic Carbon (δ13Ccarb and δ13Corg) Excursion (GICE). These positive excursions and a smooth decline trend of δ13Corg values during the early to mid-Floian may imply multiple episodes of enhanced organic carbon burial that began at the early Floian stage, probably resulting in further decline in atmospheric pCO2 and then global cooling.

Warming enhances sedimentation and decomposition of organic carbon in shallow macrophyte-dominated systems with zero net effect on carbon burial.

Temperatures have been rising throughout recent decades and are predicted to rise further in the coming century. Global warming affects carbon cycling in freshwater ecosystems, which both emit and bury substantial amounts of carbon on a global scale. Currently, most studies focus on the effect of warming on overall carbon emissions from freshwater ecosystems, while net effects on carbon budgets may strongly depend on burial in sediments. Here, we tested whether year-round warming increases the production, sedimentation, or decomposition of particulate organic carbon and eventually alters the carbon burial in a typical shallow freshwater system. We performed an indoor experiment in eight mesocosms dominated by the common submerged aquatic plant Myriophyllum spicatum testing two temperature treatments: a temperate seasonal temperature control and a warmed (+4°C) treatment (n=4). During a full experimental year, the carbon stock in plant biomass, dissolved organic carbon in the water column, sedimented organic matter, and decomposition of plant detritus were measured. Our results showed that year-round warming nearly doubled the final carbon stock in plant biomass from 6.9±1.1g C in the control treatment to 12.8±0.6g C (mean±SE), mainly due to a prolonged growing season in autumn. DOC concentrations did not differ between the treatments, but organic carbon sedimentation increased by 60% from 96±9.6 to 152±16gCm-2 yaer-1 (mean±SE) from control to warm treatments. Enhanced decomposition of plant detritus in the warm treatment, however, compensated for the increased sedimentation. As a result, net carbon burial was 40±5.7gCm-2 year-1 in both temperature treatments when fluxes were combined into a carbon budget model. These results indicate that warming can increase the turnover of organic carbon in shallow macrophyte-dominated systems, while not necessarily affecting net carbon burial on a system scale.

Methane fluxes in marine sediments quantified through core analyses and seismo-acoustic mapping (Bornholm Basin, Baltic Sea)

Methane is a terminal product of anaerobic degradation of organic matter in subsurface marine sediments depleted of reactive oxidants. The depth and age of the sediment where sulfate is depleted determines the extent of methane production relative to the burial of organic carbon. We aimed to understand how this methane production is controlled and distributed in an apparently uniform sediment basin. We combined seismo-acoustic surveys and geochemical analyses of sediment cores to explore the distribution of methane fluxes in brackish-marine mud deposits of the Bornholm Basin, southern Baltic Sea. Geophysical mapping revealed the depth distribution of (a) the thickness of the Holocene organic-rich mud layer overlaying organic-poor Postglacial clay, and (b) the upper boundary of methane gas bubbles trapped in the Holocene sediment. By correlating these two parameters with the methane distributions in sediment cores from 44 stations we developed algorithms to estimate and map, at high spatial resolution, the diffusive methane fluxes in the 75–95-m deep and >4000 km2 large Bornholm Basin. The two approaches, termed the FGD (Free Gas Depth) model and the HML (Holocene Mud Layer) model, yielded similar budgets for the total upwards methane flux through the sediment column in the Bornholm Basin, about 17 ton methane C day−1. Complete sulfate depletion at depth, and thus onset of methane production, required a minimum threshold thickness of the Holocene mud layer of 4 m. Although the Bornholm Basin has an even depth contour and uniform surface sediments, methane production was strongly focused in hotspots where the HML has greatest thickness. This heterogeneity could not be predicted from bathymetry or from properties of the surface sediment but was related to the topography of the glacial landscape buried underneath the Holocene mud blanket. This demonstrates the importance of including seismo-acoustic mapping of subsurface stratigraphy for the interpretation and geographic extrapolation of sediment core data for biogeochemical processes such as methane production and methane flux. The two approaches, which were here combined for the first time, may thereby be applied to map methane production also in other coastal and shelf sediments with shallow gas and distinct Holocene deposits.

Lac des Bois, a locality in the northern Western Interior Seaway (Canada) with Tethyan faunal connections during the Cenomanian/Turonian Thermal Maximum

Early Turonian fishes are known from only a few localities in the Northern Hemisphere, including three in the present-day European/Mediterranean region and three in northern Canada. One of the latter localities, Lac des Bois (66°52.087ʹ N), preserves a diverse marine fauna composed of trace fossils, foraminifera, molluscs and actinopterygian fishes including early acanthomorphs. The molluscan fauna, as well as a radiometric date from bentonites recovered from the site, support an age of latest Cenomanian/early Turonian for the locality. This interval spans the global Oceanic Anoxic Event 2, a time characterized globally by high temperatures and high carbon burial. Although the Cenomanian/Turonian boundary interval has been documented in North America in the Arctic Sverdrup Basin and the southern Interior Seaway, the present study provides the first account of a faunal assemblage that inhabited the northern Western Interior Seaway during this interval. Organic-rich shales and sandstones indicate limited organic decomposition and low oxygen conditions, although the presence of trace fossils (Skolithos, Planolites, Taenidium and Alcyonidopsis, as well as fugichnia) indicates that the sea floor was not completely anoxic. The presence of coiled planktic foraminifera indicates warmer water and extends the known migration limit of these Tethyan-derived forms some 6° further north. Surprisingly, the fish fauna of localities in the Bohemian Cretaceous Basin (Czech Republic) bears a close resemblance to that of Lac des Bois, although acanthomorph fish have not been recorded from the former. This suggests faunal connections between the two regions, either through the northern Polar Sea, or through the postulated Hudson Strait.

Resilience of benthic ecosystem C-cycling to future changes in dissolved oxygen availability

In marine sediments, the availability, cycling and burial of organic carbon (OC), the size and composition of the faunal community, and the availability of dissolved oxygen (DO) are closely coupled. In light of expected expansions in marine hypoxia and of oxygen minimum zones (OMZs) in particular, it is now necessary to de-convolve DO from the frequently co-varying factors OC concentration and faunal biomass, in order to understand the effect of changing dissolved oxygen (DO) concentrations on the magnitude and pattern of biological processing of organic carbon (OC). This is especially important on the continental slope, a significant location for C cycling and burial.In this study, stable isotope tracer experiments were conducted at three sites with contrasting ambient DO concentrations of 0.5, 2.8 and 21.2 μM (at depths of 530m, 812m and 1140m respectively) on the Indian continental margin. Experiments were conducted both at ambient DO concentrations, and also, for the first time, under manipulated DO concentrations both 5% above and below ambient. The 13C label was added as algal detritus, and traced through the processes of respiration, and uptake into bacterial biomass, and into metazoans and foraminifera.Total C biological processing under ambient DO conditions was similar across all three sites, suggesting that benthic communities are well adapted to local conditions, such that OC processing is optimised even at severely hypoxic sites.DO manipulation produced changes in the pattern of OC processing by the benthic community. Oxygen manipulations in both directions resulted in decreases in total community respiration, except at the most hypoxic site. Bacterial uptake, in contrast, increased in response to all DO manipulations. Faunal 13C uptake tended to increase with increased DO. At the most hypoxic site (0.5 μM) this was attributable to increased foraminiferal activity, whereas at the most oxygenated site (21.2 μM) it was the metazoans that showed increased biomass-specific 13C uptake. Similarly, decreases in DO tended to reduce faunal 13C uptake, with metazoans disproportionately affected where they were already living at the lower end of their DO tolerance (i.e. 2.8 μM). Thus, the taxa most affected by DO manipulation depended on antecedent DO conditions. The total capacity of the benthic community to process freshly deposited OC (i.e. respiration plus uptake by bacterial and different fauna) increased following upwards manipulation of DO at the 0.5 μM site, but was not adversely affected by downwards manipulation of DO. Thus, results suggest that benthic communities possess some functional resilience, and that future expansion of marine hypoxia, while impacting benthic ecosystem structure, may not have as marked an effect on biological C processing.

Dynamics of sediment flux to a bathyal continental margin section through the Paleocene–Eocene Thermal Maximum

Abstract. The response of the Earth system to greenhouse-gas-driven warming is of critical importance for the future trajectory of our planetary environment. Hyperthermal events – past climate transients with global-scale warming significantly above background climate variability – can provide insights into the nature and magnitude of these responses. The largest hyperthermal of the Cenozoic was the Paleocene–Eocene Thermal Maximum (PETM ∼ 56 Ma). Here we present new high-resolution bulk sediment stable isotope and major element data for the classic PETM section at Zumaia, Spain. With these data we provide a new detailed stratigraphic correlation to other key deep-ocean and terrestrial PETM reference sections. With this new correlation and age model we are able to demonstrate that detrital sediment accumulation rates within the Zumaia continental margin section increased more than 4-fold during the PETM, representing a radical change in regional hydrology that drove dramatic increases in terrestrial-to-marine sediment flux. Most remarkable is that detrital accumulation rates remain high throughout the body of the PETM, and even reach peak values during the recovery phase of the characteristic PETM carbon isotope excursion (CIE). Using a series of Earth system model inversions, driven by the new Zumaia carbon isotope record, we demonstrate that the silicate weathering feedback alone is insufficient to recover the PETM CIE, and that active organic carbon burial is required to match the observed dynamics of the CIE. Further, we demonstrate that the period of maximum organic carbon sequestration coincides with the peak in detrital accumulation rates observed at Zumaia. Based on these results, we hypothesise that orbital-scale variations in subtropical hydro-climates, and their subsequent impact on sediment dynamics, may contribute to the rapid climate and CIE recovery from peak-PETM conditions.

Carbon sinks/sources in the Yellow and East China Seas—Air-sea interface exchange, dissolution in seawater, and burial in sediments

The sinks/sources of carbon in the Yellow Sea (YS) and East China Sea (ECS), which are important continental shelf seas in China, could exert a great influence on coastal ecosystem dynamics and the regional climate change process. The CO2 exchange process across the seawater-air interface, dissolved and particulate carbon in seawater, and carbon burial in sediments were studied to understand the sinks/sources of carbon in the continental shelf seas of China. The YS and the ECS generally have different patterns of seasonal air-sea CO2 exchange. In the YS, regions west of 124°E can absorb CO2 from the atmosphere during spring and winter, and release CO2 to the atmosphere during summer and autumn. The entire YS is considered as a CO2 source throughout the year with respect to the atmosphere, but there are still uncertainties regarding the exact air-sea CO2 exchange flux. Surface temperature and phytoplankton production were the key controlling factors of the air-sea CO2 exchange flux in the offshore region and nearshore region of the YS, respectively. The ECS can absorb CO2 during spring, summer, and winter and release CO2 to the atmosphere during autumn. The annual average exchange rate in the ECS was −4.2±3.2 mmol m−2 d−1 and it served as an obvious sink for atmospheric CO2 with an air-sea exchange flux of 13.7×106 t. The controlling factors of the air-sea CO2 exchange in the ECS varied significantly seasonally. Storage of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in the YS and the ECS were 425×106 t and 1364×106 t, and 28.2×106 t and 54.1×106 t, respectively. Long-term observation showed that the DOC content in the YS had a decreasing trend, indicating that the “practical carbon sink” in the YS was decreasing. The total amount of particulate organic carbon (POC) stored in the YS and ECS was 10.6×106 t, which was comparable to the air-sea CO2 flux in these two continental shelf seas. The amounts of carbon sequestered by phytoplankton in the YS and the ECS were 60.42×106 t and 153.41×106 t, respectively. Artificial breeding of macroalgae could effectively enhance blue carbon sequestration, which could fix 0.36×106–0.45×106 t of carbon annually. Organic carbon (OC) buried in the sediments of the YS was estimated to be 4.75×106 t, and OC of marine origin was 3.03×106 t, accounting for 5.0% of the TOC fixed by phytoplankton primary production. In the ECS, the corresponding depositional flux of OC in the sediment was estimated to be 7.4×106 t yr−1 , and the marine-origin OC was 5.5×106 t, accounting for 5.4% of the phytoplankton primary production. Due to the relatively high average depositional flux of OC in the sediment, the YS and ECS have considerable potential to store a vast amount of “blue carbon.”

Seagrass organic matter transfer in Posidonia oceanica macrophytodetritus accumulations

Seagrass ecosystems are net autotrophic systems which contribute to organic carbon burial in marine sediment. Dead seagrass leaves are often exported outside the seagrass beds and may form accumulations (exported macrophytodetritus accumulations, hereafter EMAs) from littoral zones to deepest canyons. Understanding how seagrass organic matter is channeled in its associated trophic web is necessary to assess the role of the seagrass ecosystem as blue carbon service providers. We used gut content and stable isotope analyses to delineate the Posidonia oceanica EMA food web structure and to determine the importance of detrital material in the diets of macrofauna. Evidence from gut contents and stable isotopes showed that this food web is fuelled mainly by two food sources found in the detritus accumulations: 1) P. oceanica detritus itself and 2) epiphytes and drift macroalgae. Dead leaves of P. oceanica enter the diet of dominant species, representing more than 60% of animal abundance. The food web is structured in five trophic levels with a numerical dominance of detritivore/herbivore species at the first consumer level. Animals act as a vector for seagrass organic matter transfer to upper trophic levels and this “dead seagrass signal” is followed through the entire food web. Seagrass primary production and seagrass organic matter processing by animals are spatially decoupled and this should be taken into account in assessments of seagrass ecosystems as key actors in C cycles in coastal areas.

The lungs of the Earth: Review of the carbon cycle and mass extinction of species

The ability of carbon to combine with oxygen or/and hydrogen, leading to the formation of complex molecules such as amino acids, carbohydrates, lipids, proteins and nucleic acids, in the presence of water, forms the basis of the chemistry of advanced life. The carbon, oxygen, nitrogen and sulphur cycles, mediated by the atmosphere-ocean-land system, constitute the “lungs of the biosphere”, allowing the exchange of essential components of biological molecules. The capture of atmospheric carbon dioxide through photosynthesis, release of oxygen, respiration and burial of carbon produce the balance on which the biosphere depends. The atmospheric concentration of carbon-dominated greenhouse gases plays a key role regulating terrestrial temperatures. The mean global temperature of ~14.9 °C allows the existence on the Earth surface of aqueous media where metabolic microbiological processes are performed, among other by chemo-bacteria, microbes and algae. The geological record displays a close correspondence between paleo-CO2 levels and paleo-temperature trends, allowing the identification of environmental factors that underlie the evolution and extinction of species. Unoxidizing atmospheric and low-pH hydrosphere conditions on the early Earth, dominated by methane, CO2 and CO, constrained the appearance of oxygenating organisms, with the exception of minor oxygen release by stromatolites. An increase in photosynthetic oxygen about 2.45 Ga was associated with proliferation of phytoplankton. Glaciation followed by the “Cambrian Explosion” of life at 543 Ma is considered responsible for development of complex proteins and abundant marine life. The anthropogenic extraction and transfer from the Earth’s crust to the atmosphere of carbon, including coal, oil, tar sand, shale oil, methane gas, coal seam gas and other forms of hydrocarbon, constitutes the most significant shift in composition of the atmosphere since the PETM hyperthermal event (~56 Ma) and the K-T boundary extinction (~66 Ma), with worrying consequences for the planetary habitat.

Review on ocean acidification and carbon cycling in the Arctic Ocean

The uptake of anthropogenic CO2 by the ocean decreases seawater pH and carbonate mineral aragonite saturation state ( Ω arag), a process known as Ocean Acidification (OA). This could be detrimental to marine organisms and ecosystems. It is well known that the Arctic Ocean is a Bellwether for prospective changes in the global ocean acidification. Over the past two decades, global warming and climate change have caused rapid changes in the Arctic, especially in the western Arctic Ocean. These changes include rapid sea ice retreat and increases in sea surface temperatures, Pacific water inflow, freshwater storage, primary production, and surface CO2 concentrations. Projected climate change processes are thought to amplify OA in the Arctic Ocean, and absorption of CO2 and carbon burial in the arctic ocean makes it more rapid and significant in response to climate change than other basins. Based on the advantages of high-level China-US cooperation over the past decade, thorough and complete work of data collection, sorting and analysis for the 7 Chinese Arctic Research Expedition (CHINARE-Arc) cruises from 1999 to 2016 with the icebreaker R/V Xuelong, and a supplement of other US database collected during 1994−2011 which have provided opportunities for Chinese and U.S. scientists to investigate the status and changes of the Arctic Ocean. In this review, we first summarize the climate and environmental change in the arctic over the past 20 years. These changes have important implications for the carbon cycle and ocean acidification. With respect to carbon cycle, we summarize influences of rapidly changing sea-ice and growing Pacific inflow on the absorption of atmospheric CO2 in the Chukchi Sea, the response of deep-ocean carbon sequestration (continental shelf pump) to the changing environment, and to summarize a hypothesis of low-low-high variation trend in sea surface water p CO2 along a decreasing ice-cover gradient across three different zones in the western Arctic Ocean under a rapid sea ice melting scenario in the summer. In particular, high resolution survey of sea surface CO2 level (high p CO2) were reported across the Canada Basin, showing a great increase over earlier observations. Contrary to the current prediction, the Arctic Ocean basin would not become a large atmospheric CO2 sink under ice-free conditions. In addition, we review the rate and scale of ocean acidification in the western Arctic Ocean and evaluate links to environmental and climate change over the past two decades. Our new research indicates that, between the 1990s and 2010, acidified waters has increased in the upper 250 m from 5% to 31% of the total area north of 70°N. The Arctic Ocean is the first open ocean where we see such a large-scale increase in acidification, more than two times faster than that observed in the Pacific or Atlantic oceans. Tracer data and model simulations suggest that increased Pacific Winter Water (PWW), driven by retreating sea ice and anomalous circulation patterns in the summer season, is primarily responsible for this OA expansion. Finally, the future changes of carbon cycling and ocean acidification in the context of climate change are outlined.

Improved efficiency of the biological pump as a trigger for the Late Ordovician glaciation

The first of the ‘Big Five’ Phanerozoic mass extinctions occurred in tandem with an episode of glaciation during the Hirnantian Age of the Late Ordovician. The mechanism or change in the carbon cycle that promoted this glaciation, thereby resulting in the extinction, is still debated. Here we report new, coupled nitrogen isotope analyses of bulk sediments and chlorophyll degradation products (porphyrins) from the Vinini Creek section (Vinini Formation, Nevada, USA) to show that eukaryotes increasingly dominated marine export production in the lead-up to the Hirnantian extinction. We then use these findings to evaluate changes in the carbon cycle by incorporating them into a biogeochemical model in which production is increased in response to an elevated phosphorus inventory, potentially caused by enhanced continental weathering in response to the activity of land plants and/or an episode of volcanism. The results suggest that expanded eukaryotic algal production may have increased the community average cell size, leading to higher export efficiency during the Late Katian. The coincidence of this community shift with a large-scale marine transgression increased organic carbon burial, drawing down CO2 and triggering the Hirnantian glaciation. This episode may mark an early Palaeozoic strengthening of the biological pump, which, for a short while, may have made eukaryotic algae indirect killers. Enhanced algal productivity during the Late Ordovician may have led to carbon drawdown and the inception of the Hirnantian glaciation, according to sediment geochemistry and carbon cycle modelling.

Past Carbonate Preservation Events in the Deep Southeast Atlantic Ocean (Cape Basin) and Their Implications for Atlantic Overturning Dynamics and Marine Carbon Cycling

AbstractMicropaleontological and geochemical analyses reveal distinct millennial‐scale increases in carbonate preservation in the deep Southeast Atlantic (Cape Basin) during strong and prolonged Greenland interstadials that are superimposed on long‐term (orbital‐scale) changes in carbonate burial. These data suggest carbonate oversaturation of the deep Atlantic and a strengthened Atlantic Meridional Overturning Circulation (AMOC) during the most intense Greenland interstadials. However, proxy evidence from outside the Cape Basin indicates that AMOC changes also occurred during weaker and shorter Greenland interstadials. Here we revisit the link between AMOC dynamics and carbonate saturation in the deep Cape Basin over the last 400 kyr (sediment cores TN057‐21, TN057‐10, and Ocean Drilling Program Site 1089) by reconstructing centennial changes in carbonate preservation using millimeter‐scale X‐ray fluorescence (XRF) scanning data. We observe close agreement between variations in XRF Ca/Ti, sedimentary carbonate content, and foraminiferal shell fragmentation, reflecting a common control primarily through changing deep water carbonate saturation. We suggest that the high‐frequency (suborbital) component of the XRF Ca/Ti records indicates the fast and recurrent redistribution of carbonate ions in the Atlantic basin via the AMOC during both long/strong and short/weak North Atlantic climate anomalies. In contrast, the low‐frequency (orbital) XRF Ca/Ti component is interpreted to reflect slow adjustments through carbonate compensation and/or changes in the deep ocean respired carbon content. Our findings emphasize the recurrent influence of rapid AMOC variations on the marine carbonate system during past glacial periods, providing a mechanism for transferring the impacts of North Atlantic climate anomalies to the global carbon cycle via the Southern Ocean.

Sequestration of macroalgal carbon: the elephant in the Blue Carbon room.

Macroalgae form the most extensive and productive benthic marine vegetated habitats globally but their inclusion in Blue Carbon (BC) strategies remains controversial. We review the arguments offered to reject or include macroalgae in the BC framework, and identify the challenges that have precluded macroalgae from being incorporated so far. Evidence that macroalgae support significant carbon burial is compelling. The carbon they supply to sediment stocks in angiosperm BC habitats is already included in current assessments, so that macroalgae are de facto recognized as important donors of BC. The key challenges are (i) documenting macroalgal carbon sequestered beyond BC habitat, (ii) tracing it back to source habitats, and (iii) showing that management actions at the habitat lead to increased sequestration at the sink site. These challenges apply equally to carbon exported from BC coastal habitats. Because of the large carbon sink they support, incorporation of macroalgae into BC accounting and actions is an imperative. This requires a paradigm shift in accounting procedures as well as developing methods to enable the capacity to trace carbon from donor to sink habitats in the ocean.

High pCO2-induced exopolysaccharide-rich ballasted aggregates of planktonic cyanobacteria could explain Paleoproterozoic carbon burial

The contribution of planktonic cyanobacteria to burial of organic carbon in deep-sea sediments before the emergence of eukaryotic predators ~1.5 Ga has been considered negligible owing to the slow sinking speed of their small cells. However, global, highly positive excursion in carbon isotope values of inorganic carbonates ~2.22–2.06 Ga implies massive organic matter burial that had to be linked to oceanic cyanobacteria. Here to elucidate that link, we experiment with unicellular planktonic cyanobacteria acclimated to high partial CO2 pressure (pCO2) representative of the early Paleoproterozoic. We find that high pCO2 boosts generation of acidic extracellular polysaccharides (EPS) that adsorb Ca and Mg cations, support mineralization, and aggregate cells to form ballasted particles. The down flux of such self-assembled cyanobacterial aggregates would decouple the oxygenic photosynthesis from oxidative respiration at the ocean scale, drive export of organic matter from surface to deep ocean and sustain oxygenation of the planetary surface.

Did climate changes trigger the Late Devonian Kellwasser Crisis? Evidence from a high-resolution conodont [formula omitted] record from South China

The impact of climate change on biodiversity has been the focus of studies on the Phanerozoic biotic crises. However, it is still controversial whether climate change has caused the Frasnian–Famennian (F–F) biotic crisis since there is no unequivocal view on the global climate change during this critical period. In order to reconstruct palaeotemperatures during the F–F transition in South China (eastern Palaeotethys), a high-resolution oxygen isotope (δ18OPO4) record was obtained based on 104 measurements of conodont apatite from the Yangdi section. The oxygen isotope record based on mono-generic samples reveals an increase in δ18OPO4 by 0.7‰ in the Lower Kellwasser horizon (LKH) and 1.4‰ in the Upper Kellwasser horizon (UKH), translating into low-latitude surface water cooling of ∼3 °C and ∼6 °C, respectively. These two δ18OPO4 shifts agree well with the time-equivalent δ18OPO4 records from the western Palaeotethys, suggesting that two climate cooling pulses in conjunction with the deposition of the Kellwasser horizons are a global signal during the F–F transition. The positive shift of δ18OPO4 coincides with positive excursion in carbon isotope of carbonates (δ13Ccarb) in the UKH, indicating that enhanced burial of organic carbon resulted in a drop in atmospheric pCO2 and thus global climate cooling. Cooling started immediately before the Upper Kellwasser biotic crisis, with the lowest temperature documented at the top of the UKH. Climate cooling during the deposition of the LKH is observed in conjunction with the sharp decline in metazoan reefs. The coincidence of cooling and the Kellwasser biotic crisis suggests that global cooling played a major role in the collapse of ecosystems.

Sulfur isotope change across the Early Mississippian K–O (Kinderhookian–Osagean) δ13C excursion

Paired carbonate associate sulfate (CAS) sulfur isotopes (δ34SCAS), pyrite sulfur isotopes (δ34SPY) and CAS oxygen isotopes (δ18OCAS) across the Early Mississippian K–O δ13C excursion are documented from two sections of a west-dipping carbonate ramp in the southern Great Basin, western U.S.A. A 4–6‰ positive δ34SCAS anomaly, accompanied by negative shifts in δ34SPY and δ18OCAS, is found within the K–O δ13C excursion. In the section with a broader δ13C excursion, Δ34S (Δ34S=δ34SCAS–δ34SPY) increases from 15‰ to 45‰ and δ13Ccarb drops from 7‰ to 4‰ at the same stratigraphic interval. If this δ34SCAS anomaly represents a global phenomenon, the large magnitude (4–6‰) and short duration (shorter than that of δ13C) suggest an unusual pyrite burial event that expanded from sediments to the ocean water column. In this scenario, the areal and volumetric expansion of sulfate reduction and pyrite burial was likely triggered by abundantly available organic matter near the peak of the K–O δ13C excursion, during which organic carbon production and burial may have reached a maximum, thus substantially expanding the oxygen minimum zone (OMZ). Numerical simulations suggest that pyrite burial rates 2.5–5 times higher than that of the modern ocean followed by sulfide oxidation are required to produce the observed δ34SCAS anomaly in a sulfate-rich ([SO4] ≥28 mM) Early Mississippian ocean. Alternatively, the sulfur and CAS oxygen isotope anomalies may record local sulfur cycling in a foreland basin where changes in weathering input and bottom-water redox conditions in response to sea-level fall and cooling resulted in isotope changes. In both scenarios (either local or global), the integrated carbon, sulfur, and CAS-oxygen isotope data suggest a much more dynamic sulfur cycle across the K–O δ13C excursion than has been previously suggested.