Global Biogeochemical Cycles最新文献

Dissolved organic carbon (DOC) constitutes the largest pool of reduced carbon in the global ocean, with important contributions from both recently formed and aged, biologically refractory DOC (RDOC). The mechanisms regulating RDOC transformation and removal remain uncertain though hydrothermal vents have been identified as sources and sinks. This study examines RDOC sinks in the deep Pacific Ocean, highlighting the role of submarine hydrothermal systems. Geochemical survey data from GO-SHIP and GEOTRACES projects, alongside specific investigations of Pacific hydrothermal systems, suggest that particulate iron introduced by hydrothermal systems plays a key role in scavenging DOC and delivering it to the seafloor, leaving a deficit in the RDOC of the deep ocean. Dilution of the oceanic water column by hydrothermal fluids exhibiting low DOC concentrations likely plays a secondary role.

The transport of trace metals (TMs) in and out of the upper ocean is largely controlled by particulate phases, but the seasonal variability and residence time of these particles are not well known. Over three years and 21 cruises, we measured upper ocean particulate trace metal concentrations and export fluxes at 150 m at Station ALOHA in the North Pacific Subtropical Gyre. Vertical profiles for most metals were highest near the surface, but Al, Fe, Ni, and Cu also showed evidence of scavenging below the mixed layer. In contrast, labile particulate Mn and Cd had unique subsurface maxima driven by the photoinhibition of Mn-oxides in the euphotic zone and uptake of Cd near the base of the euphotic zone. Our sampling period captured pulses of lithogenic dust input, evident in high export fluxes of particulate Al, Ti, and Fe, which were exported on a timescale of ∼10 days. The mean export-based residence time for labile particulate Fe was 36 days, three times longer than that for recalcitrant Fe, indicating that labile particulate Fe is recycled several times before export. Particulate Cu and Co had mean residence times of 6 months, similar to particulate C, N, and P, suggesting that their export is controlled by the export of biomass. Labile particulate Mn, Ni, and Cd appear to be exported more efficiently with mean residence times of 1.8–3 months. The range of TM residence times underlines the differences in the recycling, biotic utilization, and scavenging of these metals in the upper ocean.

Dissolved organic carbon (DOC) is a dynamic component of riverine carbon pools that plays a vital role in determining regional carbon balance. However, owing to limitations in observational data and methodologies, the spatiotemporal dynamics of riverine DOC at a regional scale and their underlying driving factors remain poorly understood. In this study, we compiled riverine DOC concentration measurements for China, using which we analyzed the spatial and temporal patterns of DOC concentrations from 1982 to 2020, and examined the potential driving factors, including climate, vegetation, soil, and hydrology. The results revealed that the average annual DOC concentration in Chinese rivers for the assessed period was 4.06 mg L⁻¹, with the highest concentrations found in Northeast China (i.e., the Songliao River). We also found that there had been a significant reduction in annual DOC concentrations in Chinese rivers from 1982 to 2020, associated with significant declines in DOC in spring and summer. Further analyses revealed that these reductions in DOC concentrations could mainly be attributed to the synergistic effect of climate warming and soil drying. In addition, the total flux of DOC from major rivers in China and the average DOC yield were estimated at 8.15 Tg yr⁻¹ and 1.16 g m² yr⁻¹, respectively. Our findings in this study provide foundational data support for the accurate assessment of regional carbon budgets and offer theoretical insights for developing a regional land-ocean-aquatic continuum (LOAC) carbon cycling model for China.

Dissolved organic carbon (DOC) is a ubiquitous component of freshwater ecosystems that is sensitive to global change. In turn, DOC controls fundamental biogeochemical processes and functions. These controls depend on both the amount and composition of organic molecules comprising the dissolved organic matter (DOM) pool, which reflects the relative contributions of catchment-derived terrestrial inputs and in situ production. Stream DOM fluctuates with land use, soil mobility, and hydrology; however, few studies have monitored long-term changes in DOM composition to investigate links with climate. Here, we characterized 17-year trends in DOC and DOM in 48 streams across a land use gradient and modeled patterns therein with climatic and hydrological conditions. Across streams, Mann-Kendall trend analyses showed that DOC decreased through time, while DOM became fresher, more aromatic, and contained an increased proportion of urban-derived DOM from the terrestrial catchment. Using generalized additive models, we observed significant linear, unimodal, and multimodal patterns in DOM composition with precipitation and soil temperature. Generally, precipitation increased terrestrial DOM, whereas soil temperature increased urban-derived DOM, particularly in catchments characterized by increasing levels of urbanization. Our study highlights the importance of long-term monitoring in understanding dynamic interactions between terrestrial—fluvial carbon transfer and biogeochemical effects of global climate change and urbanization. Altogether, our results show that interactions between climate change and urbanization will shape future DOM dynamics in streams.

The Arctic Ocean covers 3 % of the Earth's surface but is estimated to contribute 5–14 % to the global ocean carbon sink. Sparse and unevenly distributed observations complicate our understanding of the size and the controlling mechanisms of this carbon sink. We adopt and advance the two-step neural network approach of Landschützer et al. (2016, https://doi.org/10.1002/2015gb005359; Self Organizing Map—Feed Forward Network) to improve region-specific reconstructions of the surface ocean partial pressure of carbon dioxide ($��p�$($��{\text{CO}}_2�$)) in the Arctic Ocean and subsequently estimate the air-sea $��{\text{CO}}_2�$ flux. Our study shows that biogeochemical properties previously selected as predictor variables for $��p�$($��{\text{CO}}_2�$) at the global scale are not well suited to the Arctic Ocean and a sensitivity study reveals large differences in the size of the Arctic Ocean carbon sink depending on the choice of air-sea $��{\text{CO}}_2�$ flux parameterization. This is most acute for those relating to sea ice cover, leading to a difference of up to 25 % (9.2–13.3 Pg C) in the total size of the Arctic Ocean carbon sink over the 32-year duration of the study.

Tropical forests play a critical role in global climate regulation by taking up and storing significant amounts of carbon. Soil nitrogen (N) dynamics is the key to understanding forest productivity. However, we have only a very rudimentary understanding of in situ soil N transformations in the Amazon Basin, the largest intact tropical forest. This study investigated gross soil N dynamics across 12 locations in the Brazilian Amazon Basin using ¹⁵N labeling to quantify gross N transformations in situ. Our results revealed significant variation in both gross N mineralization and nitrification at the local and basin-scales. Unexpectedly, no spatial patterns were observed across locations, nor were gross N rates correlated with any measured soil properties. Our results suggest a stochastic and unpredictable nature of gross N transformation rates in the rainforests of the Amazon basin. These findings highlight the need for a more nuanced understanding of N cycling in tropical forests, which could improve ecological models and inform strategies for managing these ecosystems in the face of climate change and deforestation.

Submarine groundwater discharge (SGD) serves as a crucial pathway for terrestrial carbon transport to the ocean. However, our understanding of SGD's contribution to carbon dynamics and biogeochemical processes remains limited. Here, we used the radium quartet to estimate SGD in Daya Bay (China) across seasons and then applied dissolved carbon budget models for dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) to assess carbon sources and sinks and quantify SGD-derived carbon fluxes. The buffering capacity against ocean acidification and associated biogeochemical processes within the carbonate system was analyzed. SGD-derived DIC flux was 19–39 times that of local riverine input in autumn, and 27–66 times that of local riverine input in spring. SGD-derived DOC flux ranged from 2 to 6 times that of local riverine input in autumn and from 2 to 8 times in spring. Further, the biogeochemical processes regulating carbon components in seawater exhibited significant seasonal characteristics. Primary production and CO₂ outgassing were predominant in spring, associated with higher biological activity and calmer wind conditions. With lower primary productivity and enhanced remineralization in autumn, 37.5% of seawater samples might have undergone organic matter degradation and carbonate dissolution. Moreover, groundwater exhibited a buffering capacity across different seasons, with higher values observed in nearshore seawater during autumn and offshore seawater during spring. The buffering capacity of nearshore seawater was affected by coastal groundwater, exhibiting significant deviations relative to offshore seawater. This study emphasizes the essential role of SGD in coastal carbonate systems and reveals the seasonal characteristics in biogeochemical processes, buffering capacity, and environmental implications.

Methane-rich cold seeps are oases of life in the deep sea, where microbial chemosynthesis of organic matter sustains thriving ecosystems independent of sunlight-derived energy. Here, we reveal a previously overlooked role of chemoautotrophy at seeps as powerful recyclers of scarce nutrients iron (Fe) and phosphorus (P). Investigations of sediments at Haima cold seeps (1,300–1,500 m deep) across varying methane seepage intensities showed that seep sediments released orders of magnitude more dissolved Fe and phosphate than background sediments, despite comparable organic matter remineralization rates. At Haima seeps with high methane, sediment phosphate effluxes reached 2.00–15.8 µmol m⁻² d⁻¹and dissolved Fe effluxes reached 2.24–47.4 µmol m⁻² d⁻¹, compared to background phosphate efflux of 1.21 µmol m⁻² d⁻¹ and dissolved Fe efflux of 0.412 µmol m⁻² d⁻¹. This enhancement in nutrient recycling stems from a cascade of coupled biogeochemical processes driven by the anaerobic oxidation of methane (AOM). Methane oxidation reduces Fe oxides, releasing both dissolved Fe and Fe-bound P. AOM also reduces sulfate to sulfide, precipitates dissolved Fe and suppresses the regeneration of P-binding Fe oxides, further promoting P release. These mechanisms maintained the disproportionately high benthic Fe and P recycling at seeps, which may significantly impact regional and global nutrient budgets, given the thousands of documented seeps and potentially orders of magnitude more undiscovered in the global ocean.

We report stable silicon isotope ratio (δ³⁰Si) of over 80 groundwater samples collected along the Indian coast, spanning a wide range of aquifer lithologies (alluvial, basalt, metamorphic, laterite and limestone), climate (semi-arid to tropical wet) and land use settings. Indian coastal groundwater exhibits large spatial variability in dissolved silicon (DSi) (80–1350 μM) and δ³⁰Si values (−1.1‰ to 4.5‰). On average, the δ³⁰Si value of the Indian coastal groundwater (0.8 ± 1.1‰, 1SD, n = 85) is comparable to published groundwater globally (0.8 ± 0.8‰, n = 117), and significantly lower than Indian riverine δ³⁰Si composition. The coastal groundwater δ³⁰Si values do not show any dependence on regional aquifer lithology. However, the permeable coastal alluvial groundwaters exhibit the highest variability in DSi and δ³⁰Si, likely acquiring signatures of shallow surface/subsurface processes through mixing. A broad negative correlation between δ³⁰Si values and the Ge/Si ratio is best explained by the partitioning of Si into secondary minerals phases within the weathering zone. The majority of coastal groundwater follows a steady-state model evolution, indicating a dynamic equilibrium between Si supply and the formation of secondary phases. In regions of low annual rainfall, groundwater irrigation can lead to infiltration of return flow water to aquifer systems, leading to their heavy δ³⁰Si values. The fresh submarine groundwater discharge along the Indian coast is estimated to be 2.1 GmolSi yr⁻¹, which is less than 1% of the riverine Si flux to the North Indian Ocean and 0.3% of the global fresh groundwater Si flux.

Deposition of volcanic ash is thought to impact marine biogeochemical cycling by adding soluble iron (Fe) to the surface ocean. The magnitude of this input is a function of the amount of ash deposited, the total Fe content in the ash, and ash-derived Fe's fractional solubility. However, the relative importance of chemical composition, acidic processing by the volcanic plume, and ash particle size in determining solubility is unclear. We paired an aerosol leach meant to provide an upper limit for fractional Fe solubility with chemical analyses of ash from the Cumbre Vieja (CV) and La Soufrière eruptions, which both impacted the North Atlantic in 2021. Fe in the ash samples is <6% soluble, but Fe fractional solubility in CV ash is approximately triple that of La Soufrière ash. Compared to La Soufrière, a larger proportion of the Fe in CV ash is in silicate rather than oxide minerals, which release more soluble Fe. Elevated levels of surficial fluorine (F) also suggest that CV ash was subjected to a more fluorine-rich eruption plume and underwent more acidic processing. Particle size does not appear to be a primary control on Fe release. We estimate that the CV eruption had a much larger impact on dissolved Fe (DFe) concentration in the surface ocean than the La Soufrière eruption because of differences in soluble Fe content and particle deposition velocity. These differences may help explain why some eruptions elicit a biological response in the ocean while others do not.