Global Biogeochemical Cycles最新文献

Surface ocean marine dissolved organic matter (DOM) serves as an important reservoir of carbon (C), nitrogen (N), and phosphorus (P) in the global ocean, and is produced and consumed by both autotrophic and heterotrophic communities. While prior work has described distributions of dissolved organic carbon (DOC) and nitrogen (DON) concentrations, our understanding of DOC:DON:DOP stoichiometry in the global surface ocean has been limited by the availability of DOP concentration measurements. Here, we estimate mean surface ocean bulk and semi-labile DOC:DON:DOP stoichiometry in biogeochemically and geographically defined regions using newly available marine DOM concentration databases. Global mean surface ocean bulk (C:N:P = 387:26:1) and semi-labile (C:N:P = 179:20:1) DOM stoichiometries are higher than Redfield stoichiometry, with semi-labile DOM stoichiometry similar to that of global mean surface ocean particulate organic matter (C:N:P = 160:21:1) reported in a recent compilation. DOM stoichiometry varies across ocean basins, ranging from 251:17:1 to 638:43:1 for bulk and 83:15:1 to 414:49:1 for semi-labile DOM C:N:P, respectively. Surface ocean DOP concentration exhibits larger relative changes than DOC and DON, driving surface ocean gradients in DOC:DON:DOP stoichiometry. Inferred autotrophic consumption of DOP helps explain intra- and inter-basin patterns of marine DOM C:N:P stoichiometry, with regional patterns of water column denitrification and iron supply influencing the biogeochemical conditions favoring DOP use as an organic nutrient. Specifically, surface ocean marine DOM exhibits increasingly P-depleted stoichiometries from east to west in the Pacific and from south to north in the Atlantic, consistent with patterns of increasing P stress and alleviated iron stress.

We evaluate the impact of anthropogenic carbon (C_ant) accumulation on multiple ocean acidification (OA) metrics throughout the water column and across the major ocean basins using the GLODAPv2.2016b mapped product. OA is largely considered a surface-intensified process caused by the air-to-sea transfer of C_ant; however, we find that the partial pressure of carbon dioxide gas (pCO₂), Revelle sensitivity Factor (RF), and hydrogen ion concentration ([H⁺]) exhibit their largest responses to C_ant well below the surface (>100 m). This is because subsurface seawater is usually less well-buffered than surface seawater due to the accumulation of natural carbon from organic matter remineralization. pH and aragonite saturation state (Ω_Ar) do not exhibit spatially coherent amplified subsurface responses to C_ant accumulation in the GLODAPv2.2016b mapped product, though nonlinear characteristics of the carbonate system work to amplify subsurface changes in each OA metric evaluated except Ω_Ar. Regional variability in the vertical gradients of natural and anthropogenic carbon create regional hot spots of subsurface intensified OA metric changes, with implications for vertical shifts in biologically relevant chemical thresholds. C_ant accumulation has resulted in subsurface pCO₂, RF, and [H⁺] changes that significantly exceed their respective surface change magnitudes, sometimes by >100%, throughout large expanses of the ocean. Such interior ocean pCO₂ changes are outpacing the atmospheric pCO₂ change that drives OA itself. Re-emergence of these waters at the sea surface could lead to elevated CO₂ evasion rates and reduced ocean carbon storage efficiency in high-latitude regions where waters do not have time to fully equilibrate with the atmosphere before subduction.

Ocean iron fertilization (OIF) aims to remove carbon dioxide (CO₂) from the atmosphere by stimulating phytoplankton carbon-fixation and subsequent deep ocean carbon sequestration in iron-limited oceanic regions. Transdisciplinary assessments of OIF have revealed overwhelming challenges around the detection and verification of carbon sequestration and wide-ranging environmental side-effects, thereby dampening enthusiasm for OIF. Here, we utilize five requirements that strongly influence whether OIF can lead to atmospheric CO₂ removal (CDR): The requirement (a) to use preformed nutrients from the lower overturning circulation cell; (b) for prevailing iron-limitation; (c) for sufficient underwater light for photosynthesis; (d) for efficient carbon sequestration; (e) for sufficient air-sea CO₂ transfer. We systematically evaluate these requirements using observational, experimental, and numerical data in an “informed back-of-the-envelope approach” to generate circumpolar maps of OIF (cost-)efficiency south of 60°S. Results suggest that (cost-)efficient CDR is restricted to locations on the Antarctic Shelf. Here, CDR costs can be <100 US$/tonne CO₂ while they are mainly >>1,000 US$/tonne CO₂ in offshore regions of the Southern Ocean, where mesoscale OIF experiments have previously been conducted. However, sensitivity analyses underscore that (cost-)efficiency is in all cases associated with large variability and are thus difficult to predict, which reflects our insufficient understanding of the relevant biogeochemical and physical processes. While OIF implementation on Antarctic shelves appears most (cost-)efficient, it raises legal questions because regions close to Antarctica fall under three overlapping layers of international law. Furthermore, the constraints set by (cost-)efficiency reduce the area suitable for OIF, thereby likely reducing its maximum CDR potential.

Geochemical analyses of trace elements in the ocean water column have suggested that pelagic clay-rich sediments are a major source of various elements to bottom-waters. However, corresponding high-quality measurements of trace element concentrations in porewaters of pelagic clay-rich sediments are scarce, making it difficult to evaluate the contributions from benthic processes to global oceanic cycles of trace elements. To bridge this gap, we analyzed porewater and bulk sediment concentrations of vanadium, chromium, cobalt, nickel, copper, arsenic, molybdenum, barium and uranium, as well as concentrations of the major oxidants nitrate, manganese, iron, and sulfate in the top 30 cm of cores collected along a transect from Hawaii to Alaska. The data show large increases in porewater concentrations of vanadium, manganese, cobalt, nickel, copper, and arsenic within the top cm of the sediment, consistent with the release of these elements from remineralized organic matter. The sediments are a sink for sulfate, uranium, and molybdenum, even though conditions within the sampled top 30 cm remain aerobic. Porewater chromium concentrations generally increase with depth due to release from sediment particles. Extrapolated to the global aerial extent of pelagic clay sediment, the benthic fluxes in mol yr⁻¹ are Ba 3.9 ± 3.6 × 10⁹, Mn 3.4 ± 3.5 × 10⁸, Co 2.6 ± 1.3 × 10⁷, Ni 9.6 ± 8.6 × 10⁸, Cu 4.6 ± 2.4 × 10⁹, Cr 1.7 ± 1.1 × 10⁸, As 6.1 ± 7.0 × 10⁸, V 6.0 ± 2.5 × 10⁹. With the exception of vanadium, calculated fluxes across the sediment–water interface are consistent with the variability in bottom-water concentrations and ocean residence time of the studied elements.

Climate warming increases carbon assimilation by plant growth and also accelerates permafrost CO₂ emissions; however, the overall ecosystem CO₂ balance in permafrost regions and its economic impacts remain largely unknown. Here we synthesize in situ measurements of net ecosystem CO₂ exchange to assess current and future carbon budgets across the northern permafrost regions using the random forest model and calculate their economic implications under the Shared Socio-economic Pathways (SSPs) based on the PAGE-ICE model. We estimate a contemporary CO₂ emission of 1,539 Tg C during the nongrowing season and CO₂ uptake of 2,330 Tg C during the growing season, respectively. Air temperature and precipitation exert the most control over the net ecosystem exchange in the nongrowing season, while leaf area index plays a more important role in the growing season. This region will probably shift to a carbon source after 2,057 under SSP5-8.5, with a net emission of 17 Pg C during 2057–2100. The net economic benefits of CO₂ budget will be $4.5, $5.0, and $2.9 trillion under SSP1-2.6, SSP2-4.5, and SSP5-8.5, respectively. Our results imply that a high-emission pathway will greatly reduce the economic benefit of carbon assimilation in northern permafrost regions.

As a contribution to the Regional Carbon Cycle Assessment and Processes phase 2 (RECCAP2) project, we present synthesized estimates of Arctic Ocean sea-air CO₂ fluxes and their uncertainties from surface ocean pCO₂-observation products, ocean biogeochemical hindcast and data assimilation models, and atmospheric inversions. For the period of 1985–2018, the Arctic Ocean was a net sink of CO₂ of 116 ± 4 TgC yr⁻¹ in the pCO₂ products, 92 ± 30 TgC yr⁻¹ in the models, and 91 ± 21 TgC yr⁻¹ in the atmospheric inversions. The CO₂ uptake peaks in late summer and early autumn, and is low in winter when sea ice inhibits sea-air fluxes. The long-term mean CO₂ uptake in the Arctic Ocean is primarily caused by steady-state fluxes of natural carbon (70% ± 15%), and enhanced by the atmospheric CO₂ increase (19% ± 5%) and climate change (11% ± 18%). The annual mean CO₂ uptake increased from 1985 to 2018 at a rate of 31 ± 13 TgC yr⁻¹ dec⁻¹ in the pCO₂ products, 10 ± 4 TgC yr⁻¹ dec⁻¹ in the models, and 32 ± 16 TgC yr⁻¹ dec⁻¹ in the atmospheric inversions. Moreover, 77% ± 38% of the trend in the net CO₂ uptake over time is caused by climate change, primarily due to rapid sea ice loss in recent years. Furthermore, true uncertainties may be larger than the given ensemble standard deviations due to common structural biases across all individual estimates.

We assess the Southern Ocean CO₂ uptake (1985–2018) using data sets gathered in the REgional Carbon Cycle Assessment and Processes Project Phase 2. The Southern Ocean acted as a sink for CO₂ with close agreement between simulation results from global ocean biogeochemistry models (GOBMs, 0.75 ± 0.28 PgC yr⁻¹) and pCO₂-observation-based products (0.73 ± 0.07 PgC yr⁻¹). This sink is only half that reported by RECCAP1 for the same region and timeframe. The present-day net uptake is to first order a response to rising atmospheric CO₂, driving large amounts of anthropogenic CO₂ (C_ant) into the ocean, thereby overcompensating the loss of natural CO₂ to the atmosphere. An apparent knowledge gap is the increase of the sink since 2000, with pCO₂-products suggesting a growth that is more than twice as strong and uncertain as that of GOBMs (0.26 ± 0.06 and 0.11 ± 0.03 Pg C yr⁻¹ decade⁻¹, respectively). This is despite nearly identical pCO₂ trends in GOBMs and pCO₂-products when both products are compared only at the locations where pCO₂ was measured. Seasonal analyses revealed agreement in driving processes in winter with uncertainty in the magnitude of outgassing, whereas discrepancies are more fundamental in summer, when GOBMs exhibit difficulties in simulating the effects of the non-thermal processes of biology and mixing/circulation. Ocean interior accumulation of C_ant points to an underestimate of C_ant uptake and storage in GOBMs. Future work needs to link surface fluxes and interior ocean transport, build long overdue systematic observation networks and push toward better process understanding of drivers of the carbon cycle.

The Great Calcite Belt (GCB) is a band of high concentrations of suspended particulate inorganic carbon (PIC) spanning the subantarctic Southern Ocean and plays an important role in the global carbon cycle. The key limiting factors controlling coccolithophore growth supporting this high PIC have not yet been well-characterized in the remote Pacific sector, the lowest PIC but largest area of the GCB. Here, we present in situ physical and biogeochemical measurements along 150°W from January to February 2021, where a coccolithophore bloom occurred. In both months, PIC was elevated in the Subantarctic Zone (SAZ), where nitrate was >1 μM and temperatures were ∼13°C in January and ∼14°C in February, consistent with conditions previously associated with optimal coccolithophore growth potential. The highest PIC was associated with a relatively narrow temperature range that increased about 1°C between occupations. A fresher water mass had been transported to the 150°W meridian between occupations, and altimetry-informed Lagrangian backtracking estimates show that most of this water was likely transported from the southeast within the SAZ. Applying the observations in a coccolithophore growth model for both January and February, we show that the ∼1.7°C increase in temperature can explain the rise in PIC between occupations.

Biospheric particulate organic carbon (POC_bio) burial and rock petrogenic particulate organic carbon (POC_petro) oxidation are opposing long-term controls on the global carbon cycle, sequestering and releasing carbon, respectively. Here, we examine how watershed glacierization impacts the POC source by assessing the concentration and isotopic composition (δ¹³C and Δ¹⁴C) of POC exported from four watersheds with 0%–49% glacier coverage across a melt season in Southeast Alaska. We used two mixing models (age-weight percent and dual carbon isotope) to calculate concentrations of POC_bio and POC_petro within the bulk POC pool. The fraction POC_petro contribution was highest in the heavily glacierized watershed (age-weight percent: 0.39 ± 0.05; dual isotope: 0.42 (0.37–0.47)), demonstrating a glacial source of POC_petro to fjords. POC_petro was mobilized via glacier melt and subglacial flow, while POC_bio was largely flushed from the non-glacierized landscape by rain. Flow normalized POC_bio concentrations exceeded POC_petro concentrations for all streams, but surprisingly were highest in the heavily glacierized watershed (mean: 0.70 mgL⁻¹; range 0.16–1.41 mgL⁻¹), suggesting that glacier rivers can contribute substantial POC_bio to coastal waters. Further, the most heavily glacierized watershed had the highest sediment concentration (207 mgL⁻¹; 7–708 mgL⁻¹), and thus may facilitate long-term POC_bio protection via sediment burial in glacier-dominated fjords. Our results suggest that continuing glacial retreat will decrease POC concentrations and increase POC_bio:POC_petro exported from currently glacierized watersheds. Glacier retreat may thus decrease carbon storage in marine sediments and provide a positive feedback mechanism to climate change that is sensitive to future changes in POC_petro oxidation.

The increasing atmospheric nitrous oxide (N₂O) concentration stems from the development of agriculture. However, N₂O emissions from global rice-based ecosystems have not been explicitly and systematically quantified. Therefore, this study aims to estimate the spatiotemporal magnitudes of the N₂O emissions from global rice-based ecosystems and determine different contribution factors by improving a process-based biogeochemical model, TRIPLEX-GHG v2.0. Model validation suggested that the modeled N₂O agreed well with field observations under varying management practices at daily, seasonal, and annual steps. Simulated N₂O emissions from global rice-based ecosystems exhibited significant increasing trends from 0.026 ± 0.0013 to 0.18 ± 0.003 TgN yr⁻¹ from 1910 to 2020, with ∼69.5% emissions attributed to the rice-growing seasons. Irrigated rice ecosystems accounted for a majority of global rice N₂O emissions (∼76.9%) because of their higher N₂O emission rates than rainfed systems. Regarding spatial analysis, Southern China, Northeast India, and Southeast Asia are hotspots for rice-based N₂O emissions. Experimental scenarios revealed that N fertilizer is the largest global rice-N₂O source, especially since the 1960s (0.047 ± 0.010 TgN yr⁻¹, 35.24%), while the impact of expanded irrigation plays a minor role. Overall, this study provides a better understanding of the rice-based ecosystem in the global agricultural N₂O budget; further, it quantitively demonstrated the central role of N fertilizer in rice-based N₂O emissions by including rice crop calendars, covering non-rice growing seasons, and differentiating the effects of various water regimes and input N forms. Our findings emphasize the significance of co-management of N fertilizer and water regimes in reducing the net climate impact of global rice cultivation.