# Global Biogeochemical Cycles

Online ISSN: 1944-9224
Print ISSN: 0886-6236
Recent publications
We present long‐term changes in Norwegian lake water quality across regional gradients in atmospheric pollution, air temperature, hydrology, and vegetation using (a) a national representative lake survey carried out in 1995 and 2019 (ThousandLakes), and (b) an annual lake survey from acid‐sensitive catchments (78 lakes, TrendLakes) from 1990 to 2020. Our analysis encompasses all major chemical constituents, for example, anions and cations, dissolved organic matter (DOM), nutrients, iron (Fe), and silicate (SiO2). During these decades, environmental changes included declines in sulfur (S) and nitrogen (N) deposition, climate warming, and increase in forest biomass. Strong chemical recovery from acidification is found, attributed to large reductions in atmospheric deposition, moderated by catchment processing from land use and climate change. Browning counteracted chemical recovery in some regions, while Ca increased unexpectedly. We suggest that increased weathering, from enhanced terrestrial productivity, is an important driver of increased Ca—substantiated by widespread, substantial increases in SiO2. Light‐ and nutrient‐limitation has become more prevalent, indicated by higher DOM, lower nitrate (NO3), and lower NO3to total phosphorous ratios. Declines in lake NO3 occurred independently of N deposition, suggesting increased catchment N retention, possibly from increased terrestrial productivity. We conclude that decreased air pollution continues to be a dominant driver of long‐term trends in lake chemistry, but climate‐induced increase in terrestrial weathering processes, governed by increased biomass, is likely to have an increasing impact on future lake acidity, nutrient, and light status, that may cascade along the aquatic continuum from rivers to the coast.

Rising temperatures in the Arctic Ocean can cause considerable changes, such as decreased ice cover and increased water inflow from the Pacific/Atlantic sector, which may alter dissolved methane (CH4) cycles over the Arctic Ocean. However, the fate of dissolved CH4 in the Arctic remains uncertain. Here, we show that CH4 in the Chukchi Sea is enhanced in the shelf/slope areas, stored in the Upper Halocline (UHC), and transported to the central Arctic, contributing to the CH4 excess (△CH4) in the basins. The concentration of △CH4 in the UHC was increasing (0.1 nM per year) and the △CH4 has been distributed deeper and farther in the last decade than in the 1990s because of the intensification of Pacific water inflow due to oceanographic (currents) and atmospheric forcings (winds). We found heterogeneous CH4 (208.4%±131.7%) in the Polar Mixed Layer (PML) and CH4 supersaturation (1100.9%–1245.4%) in the below-ice seawater in the basins, which may indicate the effect of sea ice cycles with the support of sediment-origin CH4. We estimate the sea-to-air flux to be 1.1–2.4 μmol CH4 m-2 day-1 during the ice-free period in the Chukchi Sea, which suggests that the Chukchi Sea is currently a minor source (0.003 Tg in summer) of atmospheric CH4. Taken together, we propose a bottom-up mechanism for CH4 transport and emission and are concerned that the increases in the concentration of △CH4 and the transport distance/rate of △CH4 plume are occurring, with the potential to affect CH4 emissions in the Pacific sector of the Arctic Ocean.

Despite the Pacific being the location of the earliest seawater Cd studies, the processes which control Cd distributions in this region remain incompletely understood, largely due to the sparsity of data. Here, we present dissolved Cd and δ¹¹⁴Cd data from the US GEOTRACES GP15 meridional transect along 152°W from the Alaskan margin to the equatorial Pacific. Our examination of this region's surface ocean Cd isotope systematics is consistent with previous observations, showing a stark disparity between northern Cd‐rich high‐nutrient low‐chlorophyll waters and Cd‐depleted waters of the subtropical and equatorial Pacific. Away from the margin, an open system model ably describes data in Cd‐depleted surface waters, but atmospheric inputs of isotopically light Cd likely play an important role in setting surface Cd isotope ratios (δ¹¹⁴Cd) at the lowest Cd concentrations. Below the surface, Southern Ocean processes and water mass mixing are the dominant control on Pacific Cd and δ¹¹⁴Cd distributions. Cd‐depleted Antarctic Intermediate Water has a far‐reaching effect on North Pacific intermediate waters as far as 47°N, contrasting with northern‐sourced Cd signatures in North Pacific Intermediate Water. Finally, we show that the previously identified negative Cd* signal at depth in the North Pacific is associated with the PO4 maximum and is thus a consequence of an integrated regeneration signal of Cd and PO4 at a slightly lower Cd:P ratio than the deep ocean ratio (0.35 mmol mol⁻¹), rather than being related to in situ removal processes in low‐oxygen waters.

Increased greenhouse gas emissions are causing unprecedented climate change, which is, in turn, altering emissions and removals (referring to the oxidation of atmospheric CH4 by methanotrophs within the soil) of the atmospheric CH4 in terrestrial ecosystems. In the global CH4 budget, wetlands are the dominant natural source and upland soils are the primary biological sink. However, it is unclear whether and how the soil CH4 exchanges across terrestrial ecosystems and the atmosphere will be affected by warming and changes in precipitation patterns. Here, we synthesize 762 observations of in situ soil CH4 flux data based on the chamber method from the past three decades related to temperature and precipitation changes across major terrestrial ecosystems worldwide. Our meta‐analysis reveals that warming (average warming of +2°C) promotes upland soil CH4 uptake and wetland soil CH4 emission. Decreased precipitation (ranging from −100% to −7% of local mean annual precipitation) stimulates upland soil CH4 uptake. Increased precipitation (ranging from +4% to +94% of local mean annual precipitation) accelerates the upland soil CH4 emission. By 2100, under the shared socioeconomic pathway with a high radiative forcing level (SSP585), CH4 emissions from global terrestrial ecosystems will increase by 22.8 ± 3.6 Tg CH4 yr⁻¹ as an additional CH4 source, which may be mainly attributed to the increase in precipitation over 30°N latitudes. Our meta‐analysis strongly suggests that future climate change would weaken the natural buffering ability of terrestrial ecosystems on CH4 fluxes and thus contributes to a positive feedback spiral.

In arable systems, large amounts of nutrients, particularly of nitrogen (N) and phosphorus (P), are not efficiently converted into harvestable products and are lost from agricultural systems, with negative consequences for agricultural productivity and the environment. These nutrient losses are mediated by hydroclimatic processes causing nutrient leaching and volatilization. We quantify over the period 1987–2012 how water availability through the evaporative ratio (actual evapotranspiration divided by precipitation) and irrigation, agricultural practices, and edaphic conditions jointly affect nutrient use efficiencies in 110 agricultural catchments in the United States. We consider N and P use efficiencies (nitrogen use efficiency [NUE] and phosphorous use efficiency [PUE]) defined as ratios of catchment‐scale N and P in harvested products over their respective inputs, as well as the NUE/PUE ratio, as an indication of catchment‐scale N and P imbalance. Both efficiencies increase through time because of changes in climate and agronomic practices. Setting all else at the median value of the data set, NUE and PUE increased with evaporative ratio by 0.5% and 0.2% when increasing the evaporative ratio by 20% and by 4.9% and 18.8% in the presence of irrigation. NUE was also higher in catchments where maize and soybean were dominant (increasing by 2.3% for a 20% increase in maize and soybean fractional area). Soil properties, represented by mineral soil texture and organic matter content, had only small effects on the efficiencies. Our results show that both climatic conditions and crop choice are important drivers of nutrient use efficiencies in agricultural catchments.

Manganese (Mn) is an essential plant micronutrient that plays a critical role in the litter decomposition by oxidizing and degrading complex organic molecules. Previous studies report a negative correlation between Mn concentrations and carbon (C) storage in organic horizons and suggest that high Mn concentrations in leaf litter reduce soil C storage in forest ecosystems, presumably by stimulating the oxidation of lignin by fungal enzymes. Yet, the relationship between Mn and C in the litter layer and organic soil remains poorly understood and restricted to a few biomes, hampering our ability to improve mechanistic understanding of soil C accumulation. To examine plant‐soil interactions that underlie observed relationships between Mn and C across a wide range of biomes, we extracted biogeochemical data reported for plants and soils from the National Ecological Observatory Network (NEON) database. We found that increased C and nitrogen (N) storage in organic horizons were associated with declines in Mn concentrations across diverse ecosystems at the continental scale, and this relationship was associated with the degree of organic matter decomposition (i.e., Oi, Oe, and Oa). Carbon and N stocks were more strongly correlated with Mn than with climatic variables (i.e., temperature and precipitation). Foliar Mn was strongly correlated with foliar lignin, and both these parameters increased with a decrease in soil pH, indicating links between soil pH, foliar chemistry, and litter decomposability. Our observations suggest that increased Mn bioavailability and accumulation in foliage under moderately acidic soil conditions support fungal decomposition of lignin‐rich litter and contributes to lower soil C stocks.

Long‐term carbon (C) sequestration in terrestrial vegetation and soil is mediated by soil nitrogen (N) supply. Afforestation is regarded as a global‐scale solution to climate change; thus, resolving the role of N in either facilitating or reducing the long‐term C benefits of this practice has essential implications to maximize its C sink potential. The impacts of afforestation on soil C, N, and their stoichiometric ratio have been widely explored but what regulates these impacts remains unclear at regional and global scales. In this study, we conducted an intensive field sampling investigation including 610 pairs of afforested and control plots in northern China and extensively compiled a global data set containing 211 afforested‐control pairs worldwide to evaluate responses of soil N concentrations and C:N ratios to afforestation and further explored their major regulator. We identified a soil N threshold, the inflection point where afforestation changes from increasing to decreasing soil C and N, which was 0.86 (95% CI: 0.81–0.91) kg N m⁻² in 0–1 m depth. Changes in soil C:N ratios with afforestation were mediated by initial relative abundance of soil C and N and types of mycorrhiza associated with planted trees. Increases in soil C:N were mostly driven by trees with ectomycorrhizal associations but did not change for those associated with arbuscular mycorrhizal fungi. These results provide a data‐based understanding on soil C and N dynamics following afforestation and its underlying mechanisms and further highlight the importance of site selection based on initial soil properties in future afforestation.

Northern high‐latitude lakes are hotspots for cycling dissolved organic carbon (DOC) inputs from allochthonous sources to the atmosphere. However, the spatial distribution of lake dissolved organic matter (DOM) is largely unknown across Arctic‐boreal regions with respect to the surrounding landscape. We expand on regional studies of northern high‐latitude DOM composition by integrating DOC concentrations, optical properties, and molecular‐level characterization from lakes spanning the Canadian Taiga to the Alaskan Tundra. Lakes were sampled during the summer from July to early September to capture the growing season. DOM became more optically processed and molecular‐level aromaticity increased northward across the Canadian Shield to the southern Arctic and from interior Alaska to the Tundra, suggesting relatively greater DOM incorporation from allochthonous sources. Using water isotopes (δ¹⁸O‐H2O), we report a weak overall trend of increasing DOC and decreasing aromaticity in lakes that were hydrologically isolated from the landscape and enriched in δ¹⁸O‐H2O, while within‐region trends were stronger and varied depending on the landscape. Finally, DOC correlated weakly with chromophoric dissolved organic matter (CDOM) across the study sites, suggesting that autochthonous and photobleached DOM were a major component of the DOC in these regions; however, some of the northernmost and wetland‐dominated lakes followed pan‐Arctic riverine DOC‐CDOM relationships, indicating strong contributions from allochthonous inputs. As many lakes across the North American Arctic are experiencing changes in temperature and precipitation, we expect the proportions of allochthonous and autochthonous DOM to respond with aquatic optical browning with greater landscape connectivity and more internally produced DOM in hydrologically isolated lakes.

In organic soils, the availability of terminal electron acceptors (TEAs) determines the ratio of CO2 to CH4 formation under anoxic conditions. While the importance of electron accepting capacities (EACs) of organic matter is increasingly acknowledged, redox properties of organic matter have only been investigated in a limited set of peat and reference materials. Therefore, we incubated 60 peat samples from 15 sites covering a variety of both bog and poor to intermediate fen samples and characterized their capacities to serve as TEA for anaerobic CO2 formation. We quantified CO2 and CH4 formation and changes in available EAC in anoxic incubations of 56 days. In our experiment, on average 36.5% of CO2 could be attributed to CH4 formation, assuming an CO2:CH4 ratio for methanogenesis of 1:1. Regarding the remaining CO2 formed, for which a corresponding TEA would be needed, we could on average explain 70.8% by corresponding consumption of EAC from both organic and inorganic TEAs, the latter contributing less than 0.1%. When the initial EAC was high, CO2 formation from the apparent consumption of EAC was high and outweighed CO2 formation from methanogenesis. Rapid depletion of available EAC, especially in reactive peat, resulted in a higher share of CO2 from CH4 formation. Our study demonstrates that EAC provides the most important redox buffer for competitive suppression of methanogenesis in peat soils, particularly under fluctuating water table levels, when EAC is repeatedly regenerated. Moreover, electron budgets including EAC of organic matter could largely explain anaerobic CO2 production.

Periodic blooms of salps (pelagic tunicates) can result in high export of organic matter, leading to an “outsized” role in the ocean's biological carbon pump (BCP). However, due to their episodic and patchy nature, salp blooms often go undetected and are rarely included in measurements or models of the BCP. We quantified salp‐mediated export processes in the northeast subarctic Pacific Ocean in summer of 2018 during a bloom of Salpa aspera. Salps migrated from 300 to 750 m during the day into the upper 100 m at night. Salp fecal pellet production comprised up to 82% of the particulate organic carbon (POC) produced as fecal pellets by the entire epipelagic zooplankton community. Rapid sinking velocities of salp pellets (400–1,200 m d⁻¹) and low microbial respiration rates on pellets (<1% of pellet C respired day⁻¹) led to high salp pellet POC export from the euphotic zone‐up to 48% of total sinking POC across the 100 m depth horizon. Salp active transport of carbon by diel vertical migration and carbon export from sinking salp carcasses was usually <10% of the total sinking POC flux. Salp‐mediated export markedly increased BCP efficiency, increasing by 1.5‐fold the proportion of net primary production exported as POC across the base of the euphotic zone and by 2.6‐fold the proportion of this POC flux persisting 100 m below the euphotic zone. Salps have unique and important effects on ocean biogeochemistry and, especially in low flux settings, can dramatically increase BCP efficiency and thus carbon sequestration.

Continental margins play an essential role in global ocean biogeochemistry and the carbon cycle; however, global assessments of this role remain highly uncertain. This uncertainty arises from the large variability over a broad range of temporal and spatial scales of the processes that characterize these environments. High‐resolution simulations with ocean biogeochemical models have emerged as essential tools to advance biogeochemical assessments at regional scales. Here, we examine the processes and balances for carbon, oxygen, and nitrogen cycles along the U.S. West Coast in an 11‐year hindcast simulation with a submesoscale‐permitting oceanic circulation coupled to a biogeochemical model. We describe and quantify the biogeochemical cycles on the continental shelf, and their connection to the broader regional context encompassing the California Current System. On the shelf, coastal and wind stress curl upwelling drive a vigorous overturning circulation that supports biogeochemical rates and fluxes that are approximately twice as large as offshore. Exchanges with the proximate sediments, submesoscale shelf currents, bottom boundary layer transport, and intensified cross‐shelf export of shelf‐produced materials further impact coastal and open‐ocean balances. While regional variability prevents extrapolation of our results to global margins, our approach provides a powerful tool to identify the dominant dynamics in different shelf setting and quantify their large‐scale consequences.

Plant invasion and aquaculture activities have drastically modified the landscape of coastal wetlands in many countries, but their impacts on soil organic carbon (SOC) mineralization and greenhouse gas production remain poorly understood. We measured SOC mineralization rate and soil CO2 and CH4 production rates in three habitat types from 21 coastal sites across the tropical and subtropical zones in China: native mudflats (MFs), Spartina alterniflora marshes (SAs) and aquaculture ponds (APs). Landscape change from MFs to SAs or APs increased total and labile fraction of SOC, as well as carbon mineralization rate and greenhouse gas production, but there were no discernible differences in SOC source-sink dynamics between SAs and APs. SOC mineralization rate was highest in SAs (20.4 μg g-1 d-1), followed by APs (16.9 μg g-1 d-1) and MFs (11.9 μg g-1 d-1), with CO2 as the dominant by-product. Bioavailable SOC was less than 2% and was turned over within 60 days in all three habitat types. Proliferation of S. alterniflora marshes and expansion of aquaculture pond construction had resulted in a net increase in soil CO2-eq production of 0.4–4.3 Tg yr-1 in the last three decades. Future studies will benefit from better census and monitoring of coastal habitats in China, complementary in situ measurements of greenhouse gas emissions, and more sampling in the southern provinces to improve spatial resolution.

The Eastern Tropical North Pacific (ETNP), like the other marine oxygen deficient zones (ODZs), is characterized by an anoxic water column, nitrite accumulation at the anoxic core, and fixed nitrogen loss via nitrite reduction to N2O and N2 gases. Here, we constrain the relative contribution of biogeochemical processes to observable features such as the secondary nitrite maximum (SNM) and local pH maximum by simultaneous measurement of inorganic nitrogen and carbon species. High‐resolution sampling within the top 1 km of the water column reveals consistent chemical features previously unobserved in the region, including a tertiary nitrite maximum. Dissolved inorganic carbon measurements show that pH increases with depth at the top of the ODZ, peaking at the potential density of the SNM at σθ = 26.15 ± 0.06 (1 s.d.). We developed a novel method to determine the relative contributions of anaerobic ammonium oxidation (anammox), denitrification, nitrite oxidation, dissimilatory nitrate reduction to nitrite, and calcium carbonate dissolution to the nitrite cycling in the anoxic ODZ core. The calculated relative contributions of each reaction are slightly sensitive to the assumed C:N:P ratio and the carbon oxidation state of the organic matter sinking through the ODZ. Furthermore, we identify the source of the pH increase at the top of ODZ as the net consumption of protons via nitrite reduction to N2 by the denitrification process. The increase in pH due to denitrification impacts the buffering effect of calcite and aragonite dissolving in the ETNP.

Marine dissolved organic phosphorus (DOP) serves as an organic nutrient to marine autotrophs, sustaining a portion of annual net community production (ANCP). Numerical models of ocean circulation and biogeochemistry have diagnosed the magnitude of this process at regional to global scales but have thus far been validated against DOP observations concentrated within the Atlantic basin. Here we assimilate a new marine DOP data set with global coverage to optimize an inverse model of the ocean phosphorus cycle to investigate the regionally variable role of marine DOP utilization by autotrophs contributing to ANCP. We find ∼25% of ANCP accumulates as DOP with a regionally variable pattern ranging from 8% to 50% across nine biomes investigated. Estimated mean surface ocean DOP lifetimes of ∼0.5–2 years allow for transport of DOP from regions of net production to net consumption in subtropical gyres. Globally, DOP utilization by autotrophs sustains ∼14% (0.9 Pg C yr⁻¹) of ANCP with regional contributions as large as ∼75% within the oligotrophic North Atlantic and North Pacific. Shallow export and remineralization of DOP within the ocean subtropics contributes ∼30%–80% of phosphate regeneration within the upper thermocline (<300 m). These shallow isopycnals beneath the subtropical gyres harboring the preponderance of remineralized DOP outcrop near the poleward edge of each gyre, which when combined with subsequent lateral transport equatorward by Ekman convergence, provide a shallow overturning loop retaining phosphorus within the subtropical biome, likely helping to sustain gyre ANCP over multiannual to decadal timescales.

Earth System Models project a decline of dissolved oxygen in the oceans due to climate warming. Observational studies suggest that the ratio of O2 inventory to ocean heat content is several fold larger than what can be explained by solubility alone, but the ratio remains poorly understood. In this work, models of different complexity are used to understand the factors controlling the air‐sea O2 flux to heat flux ratio (O2/heat flux ratio) during deep convection. Our theoretical analysis based on a one‐dimensional convective adjustment model indicates that the vertical stratification and distribution of oxygen before the convective mixing determines the upper bound for the O2/heat flux ratio. Two competing rates, the mean entrainment rate of deeper waters into the mixed layer and the rate of air‐sea gas exchange, determine how much the actual ratio departs from the upper bound. The theoretical predictions are tested against the outputs of a regional ocean model. The model sensitivity experiments broadly agree with the theoretical predictions. Our results suggest that the relative vertical gradients of temperature and oxygen at sites of deep water formation are an important local metric to quantify the marginal changes between years with high and lower heat loss.

We present high-resolution profiles of dissolved, labile and total particulate trace metals (TMs) on the Northeast Greenland shelf from GEOTRACES cruise GN05 in August 2016. Combined with radium isotopes, stable oxygen isotopes, and noble gas measurements, elemental distributions suggest that TM dynamics were mainly regulated by the mixing between North Atlantic-derived Intermediate Water, enriched in labile particulate TMs (LpTMs), and Arctic surface waters, enriched in Siberian shelf-derived dissolved TMs (dTMs; Co, Cu, Fe, Mn, and Ni) carried by the Transpolar Drift. These two distinct sources were delineated by salinity-dependent variations of dTM and LpTM concentrations and the proportion of dTMs relative to the total dissolved and labile particulate ratios. Locally produced meltwater from the Nioghalvfjerdsbrae (79NG) glacier cavity, distinguished from other freshwater sources using helium excess, contributed a large pool of dTMs to the shelf inventory. Localized peaks in labile and total particulate Cd, Co, Fe, Mn, Ni, Cu, Al, V, and Ti in the cavity outflow, however, were not directly contributed by submarine melting. Instead, these particulate TMs were mainly supplied by the re-suspension of cavity sediment particles. Currently, Arctic Ocean outflows are the most important source of dFe, dCu and dNi on the shelf, while LpTMs and up to 60% of dMn and dCo are mainly supplied by subglacial discharge from the 79NG cavity. Therefore, changes in the cavity-overturning dynamics of 79NG induced by glacial retreat, and alterations in the transport of Siberian shelf-derived materials with the Transport Drift may shift the shelf dTM-LpTM stoichiometry in the future.

Salinization alters the elemental balance of wetlands and induces variations in plant survival strategies. Sulfur (S) plays vital roles in serving regulatory and catalytic functions in stress resistance of plants. Yet, how plant S and its relationships with nitrogen (N) vary across natural environmental gradients are not well documented. We collected 1,366 plant samples and 230 water and sediment samples from 230 wetlands in Tibetan Plateau and adjacent arid regions of western China, to analyze the effects of environmental variables on plant S accumulation and N‐S correlations. We found that plant S correlated with N in unimodal patterns. Salinity, rather than temperature or nutrient supply, promoted disproportionate accumulation of S but limited N uptake, inducing decoupling of N‐S correlation in plants. Toward high salinity, the faster increasing rates of total S than that of glutathione, the most abundant organic‐S compound in plant resistance, provided potential evidences explaining the decoupled plant N‐S correlation. A salinity of 3.9‰ was calculated to be a threshold at which substantial changes in plant N‐S correlation occurred. We designed a conceptual model to illustrate the mechanisms driving variations of N‐S correlation in plants and environments along salinity gradient. Furthermore, high salinity filtered out the salt‐sensitive species and reassembled the communities. In conclusion, increased salinity affected wetland plants by inducing S accumulation in plants and selecting salt‐tolerant species with high S concentrations at community level, providing evidences for plant adaptive mechanisms to salinity in arid regions.

Global warming triggers permafrost thaw, which increases the release of terrigenous organic matter (terr‐OM) to the Arctic Ocean by coastal erosion and rivers. Terrigenous OM degradation in the Arctic Ocean contributes to greenhouse gas emissions and severe ocean acidification, yet the vulnerability of different terr‐OM components is poorly resolved. Here, terr‐OM degradation dynamics are studied with unprecedented spatial coverage over the World's largest shelf sea system—the East Siberian Arctic Shelf (ESAS), using a multi‐proxy molecular biomarker approach. Mineral‐surface‐area‐normalized concentrations of terr‐OM compounds in surface sediments decreases offshore. Differences between terr‐OM compound classes (lignin phenols, high‐molecular weight [HMW] n‐alkanes, n‐alkanoic acids and n‐alkanols, sterols, 3,5‐dihydroxybenzoic acids, cutin acids) reflect contrasting influence of sources, propensity to microbial degradation and association with sedimenting particles, with lignin phenols disappearing 3‐times faster than total terr‐OM, and twice faster than other biomarkers. Molecular degradation proxies support substantial terr‐OM degradation across the ESAS, with clearest trends shown by: 3,5‐dihydroxybenzoic acid/vanillyl phenol ratios, acid‐to‐aldehyde ratios of syringyl and vanillyl phenols, Carbon Preference Indices of HMW n‐alkyl compounds and sitostanol/β‐sitosterol. The combination of terr‐OM biomarker data with δ¹³C/Δ¹⁴C‐based source apportionment indicates that the more degraded state of lignin is influenced by the relative contribution of river‐transported terr‐OM from surface soils, while HMW n‐alkanoic acids and stigmasterol are influenced by erosion‐derived terr‐OM from Ice Complex deposits. Our findings demonstrate differences in vulnerability to degradation between contrasting terr‐OM pools, and underscore the need to consider molecular properties for understanding and modeling of large‐scale biogeochemical processes of the permafrost carbon‐climate feedback.

Glacier meltwater supplies silicon (Si) and iron (Fe) sourced from weathered bedrock to downstream ecosystems. However, the extent to which these nutrients reach the ocean is regulated by the nature of the benthic cycling of dissolved Si and Fe within fjord systems, given the rapid deposition of reactive particulate fractions at fjord heads. Here, we examine the benthic cycling of the two nutrients at four Patagonian fjord heads through geochemical analyses of sediment pore waters, including Si and Fe isotopes (δ³⁰Si and δ⁵⁶Fe), and reaction‐transport modeling for Si. A high diffusive flux of dissolved Fe from the fjord sediments (up to 0.02 mmol m⁻² day⁻¹) compared to open ocean sediments (typically <0.001 mmol m⁻² day⁻¹) is supported by both reductive and non‐reductive dissolution of glacially‐sourced reactive Fe phases, as reflected by the range of pore water δ⁵⁶Fe (−2.7 to +0.8‰). In contrast, the diffusive flux of dissolved Si from the fjord sediments (0.02–0.05 mmol m⁻² day⁻¹) is relatively low (typical ocean values are >0.1 mmol m⁻² day⁻¹). High pore water δ³⁰Si (up to +3.3‰) observed near the Fe(II)‐Fe(III) redox boundary is likely associated with the removal of dissolved Si by Fe(III) mineral phases, which, together with high sedimentation rates, contribute to the low diffusive flux of Si at the sampled sites. Our results suggest that early diagenesis promotes the release of dissolved Fe, yet suppresses the release of dissolved Si at glaciated fjord heads, which has significant implications for understanding the downstream transport of these nutrients along fjord systems.

Preservation of organic carbon (OC) in marine and terrestrial deposits is enhanced by bonding with reactive iron (FeR). Association of OC with FeR (OC‐FeR) provides physical protection and hinders microbiological degradation. Roughly 20% of all OC stored in unconsolidated marine sediments and 40% of all OC present in Quaternary terrestrial deposits is preserved as OC‐FeR, but this value varies from 10% to 80% across global depositional environments. Here, we provide a new assessment of global OC‐FeR burial rates in both marine and terrestrial environments, using published estimates of OC associated with FeR, carbon burial, and probabilistic modeling. We estimate the marine OC‐FeR sink between 31 and 70 Mt C yr⁻¹ (average 52 Mt C yr⁻¹), and the terrestrial OC‐FeR sink at between 146 and 917 Mt C yr⁻¹ (average 446 Mt C yr⁻¹). In marine environments, continental shelves (average 17 Mt C yr⁻¹) and deltaic/estuarine environments (average 11 Mg C yr⁻¹) are the primary settings of OC‐FeR burial. On land, croplands (279 Mt C yr⁻¹) and grasslands (121 Mt C yr⁻¹) dominate the OC‐FeR burial budget. Changes in the Earth system through geological time impact the OC‐FeR pools, particularly in marine settings. For example, periods of intense explosive volcanism may lead to increased net OC‐FeR burial in marine sediments. Our work highlights the importance of OC‐FeR in marine carbon burial and demonstrates how OC‐FeR burial rates may be an order of magnitude greater in terrestrial environments, but here OC‐FeR stocks are most sensitive to the anthropogenic impacts of climatic change.

Direct exploitation and use of arsenic resources has diminished in recent years, but inadvertent mobilizations of As from mineral extractions (metal ores, coal, and phosphate rock) are now as much as ten‐fold greater (1,500–5,600 × 10⁹ g/yr) than the As released by the natural rate of rock weathering at the Earth's surface (60–544 × 10⁹ g/yr). Although some As from mining activities enters global cycling through leaching and spills, the amount of dissolved As in rivers (23 × 10⁹ g/yr) is similar to the theoretical mobilization of As from chemical weathering. Anthropogenic emissions to the atmosphere (17–38 × 10⁹ g As/yr) are double the natural background sources (10–25 × 10⁹ g As/yr), largely as a result of the smelting of Cu and other non‐ferrous ores. This results in increased atmospheric deposition near regions with high mining and industrial activities, with potential consequences to human health, natural ecosystems and agriculture. Using median values for As, the ratio of anthropogenic to natural emissions to the atmosphere (1.57) suggests a human impact on the global As cycle that rivals those for V, Hg and Pb.

The change in anthropogenic CO2 (Canth) in the Atlantic Ocean is linked to the Atlantic Meridional Overturning Circulation (AMOC), that redistributes Canth meridionally and in depth. We have employed direct biogeochemical measurements and hydrographic data from the last 30 years, adjusted using inverse models for each decade with both physical and biogeochemical constraints. We then have computed the meridional transports and the vertical transports between two sections at the interphases by advection and diffusion. We have focused on the repeated sections at three latitudes—30°S, 24, and 55°N, dividing the Atlantic into two boxes. We have divided the net transport into upper, deep and abyssal layers, with an upper and abyssal northward transport of Canth and a southward component in deep layers. The change in time in the net transports of Canth appears to be mainly due to modifications in the transport of upper layers. The lower layer of the AMOC, a combination of deep and abyssal waters, maintain more consistent transports in time. Vertical advection plays an important role in the North Atlantic, exporting Canth from upper to deep layers. In the South Atlantic, the newly formed Antarctic Bottom Water exports Canth from abyssal to deep layers. The strong gradient in Canth concentration at the interphase of upper and deep layers results in a strong vertical diffusion.

Although zooplankton play a substantial role in the biological carbon pump and serve as a crucial link between primary producers and higher trophic level consumers, the skillful representation of zooplankton is not often a focus of ocean biogeochemical models. Systematic evaluations of zooplankton in models could improve their representation, but so far, ocean biogeochemical skill assessment of Earth system model (ESM) ensembles have not included zooplankton. Here we use a recently developed global, observationally based map of mesozooplankton biomass to assess the skill of mesozooplankton in six CMIP6 ESMs. We also employ a biome‐based assessment of the ability of these models to reproduce the observed relationship between mesozooplankton biomass and surface chlorophyll. The combined analysis found that most models were able to reasonably simulate the large regional variations in mesozooplankton biomass at the global scale. Additionally, three of the ESMs simulated a mesozooplankton‐chlorophyll relationship within the observational bounds, which we used as an emergent constraint on future mesozooplankton projections. We highlight where differences in model structure and parameters may give rise to varied mesozooplankton distributions under historic and future conditions, and the resultant wide ensemble spread in projected changes in mesozooplankton biomass. Despite differences, the strength of the mesozooplankton‐chlorophyll relationships across all models was related to the projected changes in mesozooplankton biomass globally and in regional biomes. These results suggest that improved observations of mesozooplankton and their relationship to chlorophyll will better constrain projections of climate change impacts on these important animals.

Understanding the response of terrestrial ecosystem water use efficiency (WUE) to climate change is critical to accurately represent the carbon‐water cycle processes. However, how WUE dynamics are evolving under seasonal climate variations and biome‐specific characteristics remains still unclear. In this study, we integrated two state‐of‐the‐art retrieval algorithms to estimate gross primary productivity (GPP) and evapotranspiration (ET) from satellite MODIS records. Such metrics served as input to quantify ecosystem WUE, expressed as the ratio of GPP to ET, and explore its dynamics during the dry and wet season from 2001 to 2018 in China's key tropical to subtropical transitional zones, that is, Yunnan Province. Results show large spatial and seasonal variability in WUE over the observational period. During the dry season, the increasing trends in GPP and ET have led to contrasting WUE patterns in forest and non‐forest biomes, leading to positive and negative WUE trends, respectively. During the wet season, the declining trends in GPP occurring in combination with opposite trends in ET, have caused decreasing WUE consistently across all biomes except croplands, likely further modulated by human factors. The observed changes in WUE appear primarily driven by variations in air temperature (Ta) and vapor pressure deficit (VPD) during both dry and wet seasons. Overall, these results contribute to a better understanding of the carbon‐water interplay in tropical‐subtropical transitional zones and provide new insights to improve our capacity to predict the terrestrial ecosystem's response to climate change.

Approximately 40% of earth's carbon (C) stored in land vegetation and soil is within the boreal region. This large C pool is subjected to substantial removals and transformations during periodic wildfire. Fire‐altered C, commonly known as pyrogenic carbon (PyC), plays a significant role in forest ecosystem functioning and composes a considerable fraction of C transport to limnic and oceanic sediments. While PyC stores are beginning to be quantified globally, knowledge is lacking regarding the drivers of their production and transport across ecosystems. This study used the chemo‐thermal oxidation at 375°C (CTO‐375) method to isolate a particularly refractory subset of PyC compounds, here called black carbon (BC), finding an average increase of 11.6 g BC m⁻² at 1 year postfire in 50 separate wildfires occurring in Sweden during 2018. These increases could not be linked to proposed drivers, however BC storage in 50 additional nearby unburnt soils related strongly to soil mass while its proportion of the larger C pool related negatively to soil C:N. Fire approximately doubled BC stocks in the mineral layer but had no significant effect on BC in the organic layer where it was likely produced. Suppressed decomposition rates and low heating during fire in mineral subsoil relative to upper layers suggests potential removals of the doubled mineral layer BC are more likely transported out of the soil system than degraded in situ. Therefore, mineral soils are suggested to be an important storage pool for BC that can buffer short‐term (production in fire) and long‐term (cross‐ecosystem transport) BC cycling.

We present labile (L‐pTM) and refractory (R‐pTM) particulate trace metal distributions of Fe, Mn, Al, Ti, Co, Zn, Cd, Ni, Pb, Cu, and P for a transect along the southwest African shelf and an off‐shore section at 3°S of the GEOTRACES GA08 section cruise. Particle sources and biogeochemical cycling processes are inferred using particle‐type proxies and elemental ratios. Enhanced concentrations of bio‐essential L‐pTMs (Zn, Cu, Ni, Cd, Co, and P) were observed in the Benguela upwelling region, attributed to enhanced primary production. Bio‐essential pTM stoichiometric ratios (normalized to pP) were consistent with phytoplankton biomass across the transect, except for Fe and Mn, which included adsorbed and labile oxide phases. Low pP lability (∼41%) suggests a potential refractory biogenic source on the Benguela shelf. Variable labilities observed between stations along the transect indicated potentially different biogenic pP labilities among different plankton groups. Benthic resuspension was prevalent in (near‐)bottom waters along the transect and formed an important source of Fe and Mn oxides. Lithogenic particles along the entire shelf were Mn deficient and particles on the Benguela shelf were enriched in Fe, consistent with regional sediment compositions. Enhanced available‐Fe (dissolved + labile particulate Fe) concentrations (up to 39.6 nM) were observed in oxygen‐deficient (near‐)bottom waters of the Benguela shelf coinciding with low L‐pMn. This was attributed to the faster oxidation kinetics of Fe, allowing Fe‐oxide precipitation and retention on the shelf, while Mn oxidation was slower. Enhanced L‐pFe in the Congo River plume, which comprised as much as 93% of the available‐Fe pool, was attributed to increased scavenging and formation of Fe oxides. Increased scavenging of other particle‐reactive trace metals (TMs) (Mn, Al, and Pb) was also apparent in Congo‐influenced waters. However, particles did not play a significant role in transporting TMs off‐shelf within Congo plume waters.

Understanding potential response of forest carbon (C) and nutrient storage to warming is important for climate mitigation policies. Unfortunately, those responses are difficult to predict in seasonally dry forests, in part, because ecosystem processes are highly sensitive to both changes in temperature and precipitation. We investigated how warming might alter stocks of C, nitrogen (N), and phosphorus (P) in vegetation and the entire regolith (soil + weathered bedrock or “saprock”) using a space‐for‐time substitution along a bioclimatic gradient in the Sierra Nevada, California. The pine‐oak and mixed‐conifer forests between 1,160–2,015 m elevation have more optimal climates (not too dry or hot) for ecosystem productivity, soil weathering, and cycling of essential elements than the oak savannah (405 m) and subalpine forest (2,700 m). We found decreases in overstory vegetation nutrient stocks with decreasing elevation because of enhanced water limitation and greater occurrence of disturbances. Stocks of C, N, and P in the entire regolith peaked at the pine‐oak and mixed‐conifer forests across the bioclimatic gradient, driven by thicker regolith profiles and greater nutrient input rates. These observations suggest long‐term warming will decrease ecosystem nutrient storage at the warmer, transitional pine‐oak zone, but will increase nutrient storage at the colder, subalpine zone. Assuming steady‐state conditions, we found the mean residence time of ecosystem C decreased with projected rising air temperatures and increased following a major drought event across the bioclimatic gradient. Our study emphasizes potentially elevation‐dependent changes in nutrient storage and C persistence with warming in seasonally dry forests.

An increasing body of work has shown the potential impacts of subglacial discharge from marine‐terminating glaciers on the marine environment around Greenland. Upwelling of nutrients associated with rising buoyant plumes near the front of marine‐terminating glaciers plays a key role in maintaining the high productivity of connected fjords. The response of protist communities to subglacial discharges into fjords nevertheless remains poorly understood. Here we show data of water properties, nutrients, and protist communities during two summers in 2018 and 2019 in a Greenlandic fjord system fed by marine‐terminating glaciers. This study included the period of intense summer melting of the Greenland Ice Sheet in 2019. The data revealed high nutrient concentrations in 2019 that were attributed to intensified upwelling of nutrients and dissolved iron into the subsurface layer. The source of the iron and the nutrients was subglacial discharge and deep fjord water, respectively. Intense glacial discharges in 2019 mitigated nitrate and phosphate limitations of phytoplankton in the fjord and resulted in an increase of chlorophyll a in the subsurface layer and growth of large diatoms. Heterotrophic protists such as dinoflagellates, tintinnids, and nanoflagellates were more abundant in the summer of 2019. We concluded that nutrient upwelling by subglacial discharges was the major driver of the structure of lower trophic levels in fjord ecosystems. We hypothesize that the more intense melting of glaciers and related increase in subglacial discharge will enhance nutrient upwelling, and increased summer productivity in fjords until the glaciers retreat and terminate on land.

The sediments within fjords are critical components of the mid- to high-latitude coastal carbon (C) cycle, trapping and storing more organic carbon (OC) per unit area than other marine sedimentary environments. Located at the land-ocean transition, fjord sediments receive OC from both marine and terrestrial environments; globally, it has been estimated that 55% to 62% of the OC held within modern fjord sediments originates from terrestrial environments. However, the mid-latitude fjords of the Northern Hemisphere have largely been omitted from these global compilations. Here we investigate the mechanism driving the distribution of OC originating from different sources within the sediments of 38 Scottish fjords. From an array of fjord characteristics, the tidal range and outer sill depth were identified as the main drivers governing the proportions of marine and terrestrial OC in the sediments. Utilizing this relationship, we estimate that on average 52 ± 10% of the OC held within the sediments of all Scotland’s fjords is terrestrial in origin. These findings show that the Scottish fjords hold equivalent quantities of terrestrial OC as other global fjord systems. However, the analysis also highlights that the sediments within 29 % of Scottish fjords are dominated by marine derived OC, which is driven by local fjord geomorphology and oceanography.

Present estimates of the biogeochemical cycles of calcium, strontium, and potassium in the ocean reveal large imbalances between known input and output fluxes. Using pore fluid, incubation, and solid sediment data from North Pacific multi‐corer cores we show that, contrary to the common paradigm, the top centimeters of abyssal sediments can be an active site of authigenic precipitation of clay minerals. In this region, clay authigenesis is the dominant sink for potassium and strontium and consumes nearly all calcium released from benthic dissolution of calcium carbonates. These observations support the idea that clay authigenesis occurring over broad regions of the world ocean may be a major buffer for ocean chemistry on the time scale of the ocean overturning circulation, and key to the long‐term stability of Earth's climate.

Although iron and light are understood to regulate the Southern Ocean biological carbon pump, observations have also indicated a possible role for manganese. Low concentrations in Southern Ocean surface waters suggest manganese limitation is possible, but its spatial extent remains poorly constrained and direct manganese limitation of the marine carbon cycle has been neglected by ocean models. Here, using available observations, we develop a new global biogeochemical model and find that phytoplankton in over half of the Southern Ocean cannot attain maximal growth rates because of manganese deficiency. Manganese limitation is most extensive in austral spring and depends on phytoplankton traits related to the size of photosynthetic antennae and the inhibition of manganese uptake by high zinc concentrations in Antarctic waters. Importantly, manganese limitation expands under the increased iron supply of past glacial periods, reducing the response of the biological carbon pump. Overall, these model experiments describe a mosaic of controls on Southern Ocean productivity that emerge from the interplay of light, iron, manganese and zinc, shaping the evolution of Antarctic phytoplankton since the opening of the Drake Passage.

Accurate estimation of regional and global patterns of ecosystem respiration (ER) is crucial to improve the understanding of terrestrial carbon cycles and the predictive ability of the global carbon budget. However, large uncertainties still exist in regional and global ER estimation due to the drawbacks of modeling methods. Based on eddy covariance ER data from 132 sites in China from 2002 to 2020, we established Intelligent Random Forest (IRF) models that integrated ecological understanding with machine learning techniques to estimate ER. The results showed that the IRF models performed better than semiempirical models and machine learning algorithms. The observed data revealed that gross primary productivity (GPP), living plant biomass, and soil organic carbon (SOC) were of great importance in controlling the spatiotemporal variability of ER across China. An optimal model governed by annual GPP, living plant biomass, SOC, and air temperature (IRF‐04 model) matched 93% of the spatiotemporal variation in site‐level ER, and was adopted to evaluate the spatiotemporal pattern of ER in China. Using the optimal model, we obtained that the annual value of ER in China ranged from 5.05 to 5.84 Pg C yr⁻¹ between 2000 and 2020, with an average value of 5.53 ± 0.22 Pg C yr⁻¹. In this study, we suggest that future models should integrate process‐based and data‐driven approaches for understanding and evaluating regional and global carbon budgets.

Polar regions are undergoing dramatic, rapid, and possibly irreversible changes. Substantial shifts in patterns of sea ice extent and thickness have cascading effects on polar ecosystems (including phytoplankton), with implications for carbon cycling and global climate. Phytoplankton growth is closely tied to environmental variables such as light and nutrient availability, which are sensitive to climate‐induced changes in upper ocean circulation, stratification, and sea ice cover. Recently, Prend et al. (2022, https://doi.org/10.1029/2022GB007329) investigated temporal and spatial scales of chlorophyll (a proxy for phytoplankton biomass) variability in the Southern Ocean. They demonstrated that the dominant temporal scale of variability is sub‐seasonal (∼0.5–3 months). The implications of this are two‐fold: first, climate oscillations (such as the Southern Annular Mode) are not major drivers of year‐to‐year variation in chlorophyll; second, intermittent bursts of chlorophyll, generated by small‐scale processes such as storms and eddies, dictate the annual mean chlorophyll concentration. Additionally, spatial autocorrelation for chlorophyll concentration varied by time scale: seasonal chlorophyll variability was correlated over much larger areas than were variations in year‐to‐year chlorophyll concentration. Based on Prend et al. (2022, https://doi.org/10.1029/2022GB007329), future work should be cognizant of (a) the spatio‐temporal scales over which chlorophyll is averaged and (b) the need to focus on small‐scale, sub‐seasonal events (rather than large‐scale climate oscillations) to mechanistically explain chlorophyll variability.

The deuterium excess (d-excess) of precipitation varies seasonally at sites across the globe, an observation that has often been linked to seasonal changes in oceanic evaporation conditions, continental moisture recycling, and sub-cloud raindrop re-evaporation. However, there have been very few studies to quantify and evaluate the relative importance of these processes. Here, we revisit the mechanisms of precipitation d-excess seasonality in low- and mid-latitudes through a new analysis of precipitation isotope databases along with climate reanalysis products and moisture tracking models. In low-latitudes, the raindrop re-evaporation effect, indicated by local relative humidity, exerts a strong and prevalent control on observed d-excess seasonality and overprints the effect of oceanic evaporation conditions. In mid-latitudes, the effect of oceanic evaporation conditions becomes stronger and seems dominant in the observed d-excess seasonality. However, the ultimate d-excess signals are produced after complex modulations by several reinforcing or competing processes, including prior distillations, moisture recycling, supersaturation in snow formation, and raindrop re-evaporation. Among these processes, substantial increases in the proportion of recycled moisture during the warm and dry season do not produce higher precipitation d-excess in mid-latitude continental interiors. We develop a simple seasonal water storage model to show that contributions of previously-evaporated residual water storage and higher transpiration fractions may lead to relatively low d-excess in evapotranspiration fluxes during periods of enhanced continental moisture recycling. This study underscores the ubiquitous non-conservative behavior in d-excess throughout the water cycle, as opposed to using d-excess as a simple tracer for remote conditions at oceanic moisture sources.

The coastal ocean and marginal sea play a disproportionally important role in the release of nitrous oxide (N2O) into the atmosphere. The spatial and temporal distribution of N2O in these important source regions remains highly uncertain due to the scarcity of N2O measurements. Here we present a large data set of N2O concentrations and fluxes obtained from 10 cruises covering four seasons in the Northern South China Sea (NSCS). The study area is overall a net source of atmospheric N2O with an annual flux of 1.9 ± 1.2 × 10⁸, 0.8 ± 0.5 × 10⁸ and 1.2 ± 0.7 × 10⁸ mol N2O yr⁻¹ in the shelf, slope and basin regions, respectively. In terms of global warming potentials, the N2O emissions offset 27.8% of the CO2 sink on the shelf, and are equivalent to 3.5 and 0.2‐fold of the CO2 emission in the slope and basin of the NSCS. On the seasonal time scale, N2O flux was significantly higher in autumn and winter than in the warm seasons. The spatial variability was contrastingly less pronounced. The seasonality of N2O distribution in the shelf region was modulated by the riverine discharge, while intrusion of the Kuroshio Current exerted profound control on N2O distribution in the open waters of the NSCS. The variable relationships between N2O excess, apparent oxygen utilization, and nitrate in the shelf, NSCS basin and the Luzon Strait indicated a regional difference in N2O cycling pathways along with the impact of water mass mixing. Our study establishes a robust baseline to understand N2O distribution and flux in the NSCS.

Atmospheric deposition of dissolved organic carbon (DOC) to terrestrial ecosystems is a small, but rarely studied component of the global carbon (C) cycle. Emissions of volatile organic compounds (VOC) and organic particulates are the sources of atmospheric C and deposition represents a major pathway for the removal of organic C from the atmosphere. Here, we evaluate the spatial and temporal patterns of DOC deposition using 70 data sets at least one year in length ranging from 40° south to 66° north latitude. Globally, the median DOC concentration in bulk deposition was 1.7 mg L⁻¹. The DOC concentrations were significantly higher in tropical (<25°) latitudes compared to temperate (>25°) latitudes. DOC deposition was significantly higher in the tropics because of both higher DOC concentrations and precipitation. Using the global median or latitudinal specific DOC concentrations leads to a calculated global deposition of 202 or 295 Tg C yr⁻¹ respectively. Many sites exhibited seasonal variability in DOC concentration. At temperate sites, DOC concentrations were higher during the growing season; at tropical sites, DOC concentrations were higher during the dry season. Thirteen of the thirty‐four long‐term (>10 years) data sets showed significant declines in DOC concentration over time with the others showing no significant change. Based on the magnitude and timing of the various sources of organic C to the atmosphere, biogenic VOCs likely explain the latitudinal pattern and the seasonal pattern at temperate latitudes while decreases in anthropogenic emissions are the most likely explanation for the declines in DOC concentration.

Our knowledge of the factors that can influence the stock of organic carbon (OC) that is stored in the soil of seagrass meadows is evolving, and several causal effects have been used to explain the variation of stocks observed at local to national scales. To gain a global‐scale appreciation of the drivers that cause variation in soil OC stocks, we compiled data on published species‐specific traits and OC stocks from monospecific and mixed meadows at multiple geomorphological settings. Species identity was recognized as an influential driver of soil OC stocks, despite their large intraspecific variation. The most important seagrass species traits associated with OC stocks were the number of leaves per seagrass shoot, belowground biomass, leaf lifespan, aboveground biomass, leaf lignin, leaf breaking force and leaf OC plus the coastal geomorphology of the area, particularly for lagoon environments. A revised estimate of the global average soil OC stock to 20 cm depth of 15.4 Mg C ha⁻¹ is lower than previously reported. The largest stocks were still recorded in Mediterranean seagrass meadows. Our results specifically identify Posidonia oceanica from the Mediterranean and, more generally, large and persistent species as key in providing climate regulation services, and as priority species for conservation for this specific ecosystem service.

The oceanic biological carbon pump modulates atmospheric CO2 concentrations by transporting carbon from the sunlit surface to greater depths. The efficiency of the biological pump and its response to warming temperatures are of great importance to future projections of global change. Here, we investigate a time series of organic carbon fluxes from a monthly resolved sediment trap mooring in the Gulf of Aqaba (GOA), northern Red Sea, between 2014 and 2016. We evaluate the attenuation of sinking organic carbon in the context of the seasonally changing euphotic zone and provide the first estimates of biological pump efficiency in this region. The base of the euphotic zone changed seasonally as the system transitioned from oligotrophic and stratified conditions in summer to mesotrophic conditions during the winter mixing period. Carbon attenuation assessed using a power law fit yields an average coefficient of b = 0.80 ± 0.37, lower than expected based on the warm temperatures in the GOA. Estimates of export efficiency decreased from 40% in summer to 20% in winter, and show the opposite seasonal pattern as transfer efficiencies, which increased from 50% in summer to ∼95% in winter. Overall, the efficiency of the carbon pump was close to ∼20% in both seasons. These observations challenge the notion of a globally uniform positive correlation between increasing temperature and increasing carbon attenuation in the ocean and imply that warm subtropical ecosystems can support moderately enhanced carbon pump efficiencies, possibly also related to increased, dust‐driven, mineral ballasting in low latitude regions such as the GOA.

Two decades of research has shown that the global river network emits significant amounts of greenhouse gas. Despite much progress, there is still large uncertainty in the temporal dynamics of gas exchange and thus carbon emissions to the atmosphere. Much of this uncertainty stems from a lack of existing tools for studying the spatiotemporal dynamics of gas exchange velocity k600 ${k}_{600}$ (the rate of this diffusive transport). We propose that the NASA/CNES/UKSA/CSA Surface Water and Ocean Topography (SWOT) satellite can provide new insights to fluvial gas exchange modeling upon launch and subsequent data collection in 2022. Here, we exploit the distinct geomorphology of SWOT‐observable rivers (>50 m wide) to develop a physical model of gas exchange that is remotely sensible and explains 50% of log‐transformed variation across 166 field measurements of k600 ${k}_{600}$. We then couple this model with established inversion techniques to develop BIKER, the “Bayesian Inference of the k600 ${k}_{600}$ Exchange Rate” algorithm. We validate BIKER on 47 SWOT‐simulated rivers without an in‐situ calibration, yielding an algorithm that predicts the k600 ${k}_{600}$ timeseries solely from SWOT observations with a by‐river median Kling‐Gupta Efficiency of 0.21. BIKER is better at inferring the temporal variation of gas exchange (median correlation coefficient of 0.91), than reproducing the absolute rates of exchange (median normalized RMSE of 51%). Finally, BIKER is robust to measurement errors implicit in the SWOT data. We suggest that BIKER will be useful in mapping global‐scale fluvial gas exchange and improving CO2 $\left[\mathrm{C}{\mathrm{O}}_{2}\right]$ emissions estimates when coupled with river CO2 $\left[\mathrm{C}{\mathrm{O}}_{2}\right]$ models.

Accurate assessments of soil organic carbon (SOC) storage beneath impervious surface areas (ISAs, SOCISA) are key for understanding the urbanization impact on carbon pools, but previous studies either ignored the SOCISA pool or overlooked the urbanization impacts on the SOC pool. Based on observations from 152 sampling sites in 43 cities, we show that mainland China has a SOCISA stock of 1,016 Tg to 100‐cm depth, with a mean density of 6.21 ± 3.90 kg m⁻². Comparison between SOCISA and the SOC of background soil (SOCbackground) indicates ∼19% SOC loss due to ISA conversion, similar to the effect of cropland conversion. Unlike the vertical pattern of SOCbackground, which declined faster in the upper soil layers, SOCISA decreased linearly with depth. Moreover, the SOCISA is uncorrelated with SOCbackground and decreases with precipitation. These unique patterns indicate the SOC loss mainly caused by topsoil removal during land conversion. The fact that the SOCISA of older ISA was not lower than that of newly converted ISA further confirmed that soil sealing preserved SOC. Finally, both the high correlation between SOCISA and SOCPSA (SOC of the adjacent pervious surfaces) and the converging SOC densities in urban soils showed strong influences from urban greenspace on SOCISA. Based on the findings, we recommend improving greenspace management to increase the presealing SOC stock, preventing ISA construction in wet/warm seasons to mitigate SOC loss, and developing technology to seal carbon stock under ISA. We conclude that SOCISA is a major component of the urban carbon pool. It has a unique spatial pattern and cannot be estimated using surrogate data such as SOCbackground or SOCPSA.

A key challenge for current‐generation Earth system models (ESMs) is the simulation of the penetration of sinking particulate organic carbon (POC) into the ocean interior, which has implications for projections of future oceanic carbon sequestration in a warming climate. This paper presents a new, cost‐efficient, mechanistic 1D model that prognostically calculates POC fluxes by carrying four component particles in two different size classes. Gravitational settling and removal/transformation processes are represented explicitly through parameterizations that incorporate the effects of particle size and density, dissolved oxygen, calcite and aragonite saturation states, and seawater temperature, density, and viscosity. The model reproduces the observed POC flux attenuation at 22 locations in the North Atlantic and North Pacific. The model is applied over a global ocean domain with seawater properties prescribed from observation‐based climatologies in order to address an important scientific question: What controls the spatial pattern of mesopelagic POC transfer efficiency? The simulated vertical POC transfer is more efficient at high latitudes than at low latitudes with the exception of oxygen minimum zones, which is consistent with recent inverse modeling and neutrally buoyant sediment trap studies. Here, model experiments show that the relative abundance of large‐sized, rapidly sinking particles and the slower rate of remineralization at high latitudes compensate for the region's lack of calcium carbonate ballast and the cold‐water viscous resistance, leading to higher transfer efficiencies compared to low‐latitude regions. The model could be deployed in ESMs in order to diagnose the impacts of climate change on oceanic carbon sequestration and vice versa.

Two centuries of anthropogenic CO2 emissions have increased the CO2 concentration of the atmosphere and the dissolved inorganic carbon (DIC) concentration of the ocean compared to preindustrial times. These anthropogenic carbon perturbations are often equated to the amount of anthropogenically emitted carbon in the atmosphere or ocean, which ignores the possibility of a shift of natural carbon between the oceanic and atmospheric carbon reservoirs. Here we use a data‐assimilated ocean circulation model and numerical tracers akin to ideal isotopes to label carbon when it is emitted by anthropogenic sources. We find that emitted carbon accounts for only about 45% of the atmospheric CO2 increase since preindustrial times, the remaining 55% being natural CO2 that outgassed from the ocean in response to anthropogenically emitted carbon invading the ocean. This outgassing is driven by the order‐10 seawater carbonate buffer factor which causes increased leakage of natural CO2 as DIC concentrations increase. By 2020, the ocean had outgassed ∼159 Pg of natural carbon, which is counteracted by the ocean absorbing ∼347 Pg of emitted carbon, about 1.8 times more than the net increase in oceanic carbon storage of ∼188 PgC. These results do not challenge existing estimates of anthropogenically driven changes in atmospheric or oceanic carbon inventories, but they shed new light on the composition of these changes and the fate of anthropogenically emitted carbon in the Earth system.

Upwelling of subsurface waters injects macronutrients (fixed N, P, and Si) and micronutrient trace metals (TMs) into surface waters supporting elevated primary production in Eastern Boundary Upwelling Regions. The eastern South Atlantic features a highly productive shelf sea transitioning to a low productivity N‐Fe (co)limited open ocean. Whilst a gradient in most TM concentrations is expected in any off‐shelf transect, the factors controlling the magnitude of cross‐shelf TM fluxes are poorly constrained. Here, we present dissolved TM concentrations of Fe, Co, Mn, Cd, Ni, and Cu within the Benguela Upwelling System from the coastal section of the GEOTRACES GA08 cruise. Elevated dissolved Fe, Co, Mn, Cd, Ni, Cu and macronutrient concentrations were observed near shelf sediments. Benthic sources supplied 2.22 ± 0.99 μmol Fe m⁻² day⁻¹, 0.05 ± 0.03 μmol Co m⁻² day⁻¹, 0.28 ± 0.11 μmol Mn m⁻² day⁻¹ and were found to be the dominant source to shallow shelf waters compared to atmospheric depositions. Similarly, off‐shelf transfer was a more important source of TMs to the eastern South Atlantic Ocean compared to atmospheric deposition. Assessment of surface (shelf, upper 200 m) and subsurface (shelf edge, 200–500 m) fluxes of Fe and Co indicated TM fluxes from subsurface were 2–5 times larger than those from surface into the eastern South Atlantic Ocean. Under future conditions of increasing ocean deoxygenation, these fluxes may increase further, potentially contributing to a shift toward more extensive regional limitation of primary production by fixed N availability.

Realistic prediction of the near‐future response of Arctic Ocean primary productivity to ongoing warming and sea ice loss requires a mechanistic understanding of the processes controlling nutrient bioavailability. To evaluate continental nutrient inputs, biological utilization, and the influence of mixing and winter processes in the Laptev Sea, the major source region of the Transpolar Drift (TPD), we compare observed with preformed concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP), silicic acid (DSi), and silicon isotope compositions of DSi (δ³⁰SiDSi) obtained for two summers (2013 and 2014) and one winter (2012). In summer, preformed nutrient concentrations persisted in the surface layer of the southeastern Laptev Sea, while diatom‐dominated utilization caused intense northward drawdown and a pronounced shift in δ³⁰SiDSi from +0.91 to +3.82‰. The modeled Si isotope fractionation suggests that DSi in the northern Laptev Sea originated from the Lena River and was supplied during the spring freshet, while riverine DSi in the southeastern Laptev Sea was continuously supplied during the summer. Primary productivity fueled by river‐borne nutrients was enhanced by admixture of DIN‐ and DIP‐rich Atlantic‐sourced waters to the surface, either by convective mixing during the previous winter or by occasional storm‐induced stratification breakdowns in late summer. Substantial enrichments of DSi (+240%) and DIP (+90%) beneath the Lena River plume were caused by sea ice‐driven redistribution and remineralization. Predicted weaker stratification on the outer Laptev Shelf will enhance DSi utilization and removal through greater vertical DIN supply, which will limit DSi export and reduce diatom‐dominated primary productivity in the TPD.

Top-cited authors
• University of British Columbia - Vancouver
• Max Planck Institute for Biogeochemistry Jena
• University of Virginia
• ETH Zurich
• Marine Biological Laboratory