Global Biogeochemical Cycles最新文献

The ocean's biological carbon pump (BCP) plays a key role in global carbon cycling by transporting biologically fixed carbon from the surface to the deep ocean. Prior analyses of the BCP in Earth System Model (ESM) simulations have typically evaluated particulate organic carbon (POC) flux at a fixed export depth horizon of 100 m. However, this overlooks spatial and temporal variations in the depth that sinking POC must penetrate to reach the mesopelagic or to sequester carbon from the atmosphere on climate-relevant timescales. We use depth-resolved POC flux output from eight Coupled Model Intercomparison Project Phase 6 (CMIP6) ESMs to compare global and regional changes in POC flux at five export depth horizons −100 m, the base of the euphotic zone (EZ depth), the particle compensation depth (PCD), the maximum annual mixed layer depth (MLD_max), and 1,000 m—under the high-emissions scenario SSP5-8.5. We also examine the relationship among net primary production, export efficiency from the surface ocean, and transfer efficiency to depth in key regions of the ocean, identifying model- and region-specific variations in the mechanistic drivers of POC flux changes in the deep ocean. Globally and spatially, trends in POC flux magnitude and decline are similar at the four surface export depth horizons, and multimodel variability in POC flux change by 2100 is greatest at the 1,000 m export depth horizon (+4% to −55%). This indicates the importance of improving model parameterizations of transfer efficiency and POC flux to the deep ocean.

Nitrous oxide (N₂O) is a potent greenhouse gas with its radiative forcing 265–298 times stronger than that of carbon dioxide (CO₂). Recent field studies show N₂O emissions from northern high latitude (north of 45°N) ecosystems have increased due to warming. However, spatiotemporal quantification of N₂O emissions remains inadequate in this region. Here we revise the Terrestrial Ecosystem Model to incorporate more detailed processes of soil nitrogen (N) biogeochemical cycling, permafrost thawing effects, and atmospheric N deposition. Terrestrial Ecosystem Model is then used to analyze N₂O emissions from natural terrestrial ecosystems in the region. Our study reveals that regional N₂O production and net emissions increased from 1969 to 2019. Production rose from 1.12 (0.82–1.46) to 1.18 (0.84–1.51) Tg N yr⁻¹, while net emissions increased from 0.98 (0.7–1.34) to 1.05 (0.72–1.39) Tg N yr⁻¹, considering permafrost thawing. Emissions from permafrost regions grew from 0.37 (0.2–0.57) to 0.41 (0.21–0.6) Tg N yr⁻¹. Soil N₂O uptake from the atmosphere remained relatively stable at 0.12 (0.1–0.15) Tg N yr ⁻¹. Atmospheric N deposition significantly increased N₂O emission by 37.2 ± 2.9%. Spatially, natural terrestrial ecosystems act as net sources or sinks of −12 to 900 mg N m⁻² yr⁻¹ depending on changing temperature, precipitation, soil characteristics, and vegetation types. Our findings underscore the critical need for more observational studies to reduce the uncertainty in N₂O budget.

As part of the REgional Carbon Cycle Assessment and Processes-2 (RECCAP-2) project of the Global Carbon Project, here we estimate the GHG budgets (anthropogenic and natural sources and sinks) for the South Asia (SA) region as a whole and each country (Afghanistan, Bangladesh, Bhutan, India, Nepal, Pakistan, and Sri Lanka) for the decade of 2010–2019 (2010s). Countries in the region are experiencing a rapid rise in fossil fuel consumption and demand for agricultural land, leading to increased deforestation and higher greenhouse gas emissions. This study synthesizes top-down (TD) and bottom-up (BU) dynamic global vegetation model results, BU GHG inventories, ground-based observation upscaling, and direct emissions for major GHGs. The fluxes for carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) analyzed include fossil fuel emissions, net biome productivity, land use change, inland waters, wetlands, and upland and submerged soils. Our analysis shows that the overall total GHG emissions contributed to a net increase of 34%–43% during the 2010s compared to the 2000s, primarily driven by industrial activities. However, terrestrial ecosystems acted as a notable exception by serving as a CO₂ sink in the 2010s, effectively sequestering atmospheric carbon. The sink was significantly smaller than overall carbon emissions. Overall, the 2010s GHG emissions based on BU and TD were 4,517 ± 639.8 and 4,532 ± 807.5 Tg CO₂ eq, with CO₂, CH₄, and N₂O emissions of 2165.2 ± 297.1, 1,404 ± 95.9, and 712 ± 466 Tg CO₂ eq based on BU models 2,125 ± 515.1, 1,531 ± 205.2, and 876 ± 446.0 Tg CO₂ eq based on TD models. Total emissions from SA in the 2010s accounted for approximately 8% of the global share. The terrestrial CO₂ sinks estimated by the BU and TD models were 462.9 ± 195.5 and 210.0 ± 630.4 Tg CO₂, respectively. Among the SA countries, India was the largest emitter contributing to 80% of the region's total GHG emissions, followed by Pakistan (10%) and Bangladesh (7%).

Refractory dissolved organic carbon (RDOC) represents the second largest reservoir for ocean carbon storage, the bulk of which is held in the deep ocean, out of contact with the atmosphere on decadal to millennial timescales. Thus, understanding the mechanisms governing its production, delivery, and storage within the deep ocean is crucial for fully elucidating the oceanic carbon cycle and its impacts on global climate dynamics. Here we report observations of marine DOC across the Arctic, finding that the Eurasian Basin deep waters (>1,700 m) harbor the global maxima in deep water DOC concentrations. Given the basin's relatively long residence time (>150 years) and the absence of known RDOC delivery pathways into the ocean interior, we attempt to describe how the elevated Arctic Ocean deep water DOC is maintained. Using box model simulations, we find a significant role for brine rejection from continental shelf surface waters in delivering DOC to the abyss, which simultaneously ventilates Arctic Ocean deep waters. Comparison of kinetic loss rates for DOC consumption estimated as a function of subsurface temperatures demonstrates an elevated temperature sensitivity for Arctic RDOC relative to other ocean basins, possibly linked to its elevated terrigenous and/or “fresh” content, with the subzero temperatures of the Arctic currently suppressing DOC remineralization, helping to explain the deep water maxima. The Arctic Ocean currently stores ∼5.3 Pg C as DOC over the multi-centennial scale residence times of its deep waters, which may be reduced by ∼1%–4% over the next century of warming.

The differing contributions of phytoplankton groups to biological pump have been insufficiently explored. We evaluated the sinking of phytoplankton in the mesopelagic layer using 16S rRNA gene amplicon sequencing. Sinking particles were collected from June to August 2022 in the Sea of Japan using sediment traps moored at depths of 387 and 890 m. Morphologically categorized fecal pellets—ellipsoidal, cylindrical, spherical, and tabular types—were analyzed for their carbon content and phytoplankton assemblages as well as the bulk and non-fecal particles. Fecal pellets contributed ≤4.1% and ≤8.0% of the total particulate organic carbon (POC) flux at 387 and 890 m depths, respectively. Ellipsoidal pellets, likely of appendicularian origin, accounted for 59.3%–78.5% of the fecal pellets' carbon fluxes. Diatoms, particularly Chaetocerotales, were the dominant phytoplankton group across all sinking types and depths, as indicated by eukaryotic chloroplast and cyanobacteria gene proportions. Cyanobacteria Synechococcales were most prevalent in ellipsoidal and cylindrical fecal pellets at 890 m depth. Amplicon sequence variant richness positively correlated with fecal pellet's POC content, with Synechococcales and Chaetocerotales exhibiting the highest diversity in ellipsoidal fecal pellets at both depths. Non-Chaetocerotales diatoms showed comparable or lower diversity levels than the non-fecal particles. These findings suggest that Chaetocerotales and Synechococcales were the most effectively transported phytoplankton groups into the mesopelagic layer through zooplankton grazing and repackaging, particularly by appendicularians. In contrast, other phytoplankton groups, including non-Chaetocerotales diatoms, played a less significant role in this process.

Atmospheric deposition delivers carbon to glacier surfaces, including from fossil fuel and biomass combustion. Nonetheless, spatial variation in the sources of organic and black carbon deposited on glaciers is poorly understood, along with their role in driving glacier outflow dissolved organic matter (DOM) composition and fate. Here, we used bulk and compound-specific carbon isotopic analyses to constrain the sources of dissolved organic carbon (DOC) and dissolved black carbon (DBC) in 10 glacier outflows across four regions. To understand the relationships between glacier DOM composition and sources of DOC and DBC, isotopic data were used in conjunction with ultrahigh resolution molecular-level analyses. Globally, a substantial yet variable component of DOC was sourced from anthropogenic aerosols (12%–91%; median 50%), influencing regional DOM composition (aliphatics 26.9%–58.4% relative abundance; RA). Relatively older radiocarbon ages (i.e., larger fossil-derived component) of glacier DOC were correlated with more ¹³C depleted DOC and DBC signatures, where DOM had higher aromaticity, elevated RA of condensed aromatics, and a lower RA of aliphatic compounds. This study highlights that anthropogenic deposition is pervasive, but its extent varies spatially with ramifications for DOM composition, and thus reactivity and fate.

The physical and biogeochemical properties of the western Arctic Ocean are rapidly changing, resulting in cascading shifts to the local ecosystems. The nutrient-rich Pacific water inflow to the Arctic through the Bering Strait is modified on the Chukchi and East Siberian shelves by brine rejection during sea ice formation, resulting in a strong halocline (called the Upper Halocline Layer (UHL)) that separates the cold and relatively fresh surface layer from the warmer and more saline (and nutrient-poor) Atlantic-derived water below. Biogeochemical signals entrained into the UHL result from Pacific Waters modified by sediment and river influence on the shelf. In this synthesis, we bring together data from the 2015 Arctic U.S. GEOTRACES program to implement a multi-tracer (dissolved and particulate trace elements, radioactive and stable isotopes, macronutrients, and dissolved gas/atmospheric tracers) approach to assess the relative influence of shelf sediments, rivers, and Pacific seawater contribution to the Amerasian Arctic halocline. For each element, we characterized their behavior as mixing dominated (e.g., dCu, dGa), shelf-influenced (e.g., dFe, dZn), or a combination of both (e.g., dBa, dNi). Leveraging this framework, we assessed sources and sinks contributing to elemental distributions: shelf sediments (e.g., dFe, dZn, dCd, dHg), riverine sources, (e.g., dCu, dBa, dissolved organic carbon), and scavenging by particles originating on the shelf (e.g., dFe, dMn, dV, etc.). Additionally, synthesized results from isotopic and atmospheric tracers yielded tracer age estimates for the Upper Halocline ranging between 1 and 2 decades on a spatial gradient consistent with cyclonic circulation.

Accurate accounting of greenhouse-gas (GHG) emissions and removals is central to tracking progress toward climate mitigation and for monitoring potential climate-change feedbacks. GHG budgeting and reporting can follow either the Intergovernmental Panel on Climate Change methodologies for National Greenhouse Gas Inventory (NGHGI) reporting or use atmospheric-based “top-down” (TD) inversions or process-based “bottom-up” (BU) approaches. To help understand and reconcile these approaches, the Second REgional Carbon Cycle Assessment and Processes study (RECCAP2) was established to quantify GHG emissions and removals for carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), for ten-land and five-ocean regions for 2010–2019. Here, we present the results for the North American land region (Canada, the United States, Mexico, Central America and the Caribbean). For 2010–2019, the NGHGI reported total net-GHG emissions of 7,270 TgCO₂-eq yr⁻¹ compared to TD estimates of 6,132 ± 1,846 TgCO₂-eq yr⁻¹ and BU estimates of 9,060 ± 898 TgCO₂-eq yr⁻¹. Reconciling differences between the NGHGI, TD and BU approaches depended on (a) accounting for lateral fluxes of CO₂ along the land-ocean-aquatic continuum (LOAC) and trade, (b) correcting land-use CO₂ emissions for the loss-of-additional-sink capacity (LASC), (c) avoiding double counting of inland water CH₄ emissions, and (d) adjusting area estimates to match the NGHGI definition of the managed-land proxy. Uncertainties remain from inland-water CO₂ evasion, the conversion of nitrogen fertilizers to N₂O, and from less-frequent NGHGI reporting from non-Annex-1 countries. The RECCAP2 framework plays a key role in reconciling independent GHG-reporting methodologies to support policy commitments while providing insights into biogeochemical processes and responses to climate change.