Global Biogeochemical Cycles最新文献

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.

The strength of the Biological Carbon Pump (BCP) to sequester atmospheric CO₂ in the East Indian Ocean is unclear due to lack of studies. Here, we estimated Particulate Organic Carbon (POC) export flux by using ²³⁴Th as a flux proxy in the upper Indian Ocean (0–300 m depth), including the East Indian Ocean. In seawater, the soluble parent radionuclide, ²³⁸U (t_1/2 = 4.47 × 10⁹ yr) decays to produce a particle-reactive daughter, ²³⁴Th (t_1/2 = 24.1 d), which surface adsorbs onto particles, and sinks from the euphotic zone to the sea bottom. Disequilibrium between ²³⁸U and ²³⁴Th in seawater and POC/²³⁴Th ratio in sinking particles is used to estimate POC export flux. In this study, euphotic depth integrated ²³⁴Th deficit fluxes and the estimated POC export flux varied from negligible to 2,025 ± 87 dpm m⁻² d⁻¹ and negligible to 6.6 ± 0.6 mmol C m⁻² d⁻¹, respectively. The BCP efficiency varied from negligible (in coastal Arabian Sea) to 14% (near equator), except for the Andaman Sea (0%–80%). Temporal decoupling of primary productivity and POC export flux in the Andaman Sea resulted in a high export ratio. Compilation of spring intermonsoon ²³⁴Th based POC export flux and export efficiency from JGOFS and GEOTRACES showed high export flux and efficiency in the open Arabian Sea and in the Equatorial Indian Ocean but low POC export flux and efficiency in the Bay of Bengal, Andaman Sea, East Indian Ocean, and South Indian Ocean. Although low in magnitude, the Equatorial Indian Ocean sequesters atmospheric CO₂ like the equatorial- Atlantic and Pacific Oceans.

Artificial reservoirs significantly alter the natural transport of suspended particulate matter (SPM) from rivers to oceans, thereby reshaping the global carbon cycle through changes in particulate organic carbon (POC) dynamics over decadal to millennial timescales. Here, we investigate dam-induced perturbation of POC composition, transport, and fate within the Changjiang (CJ) River basin in response to the operation of cascade mega-reservoirs (CMRs) along the Jinshajiang (JSJ) in the upper CJ. The CMRs have introduced new perturbations to SPM and POC delivery, compounding the effects of the Three Gorges Dam (TGD). We analyzed elemental, stable, and radiogenic isotopic compositions of POC, as well as the inorganic chemistry of SPM collected from both the upper and lower CJ. Since the construction of CMRs, POC sequestration in artificial reservoirs reaches approximately 6.6 megatons carbon per year (MtC yr⁻¹), 3.8 MtC yr⁻¹ of which being POC of biospheric origin (POC_bio). Notably, the flux of POC trapped in the TGD declined from 1.6 to 0.4 MtC yr⁻¹, while CMRs sequestered 0.7 MtC yr⁻¹. This shift highlights the relocation of POC burial sites from the TGD and estuary to upstream reservoirs. The rapid burial of terrestrial POC in large mountainous river reservoirs is expected to enhance POC preservation by minimizing mineralization caused by prolonged transport to estuaries. The significant reduction in sediment load and the increased proportion of POC_bio due to reservoir retention have substantially altered the composition and flux of exported POC, impacting downstream and estuarine carbon cycles.

Nitrate is an essential nutrient for phytoplankton growth and is a primary component of ocean carbon cycling. In this study, we developed a neural network constrained by the high spatial and temporal coverage of BGC-Argo floats to predict nitrate in a consistent way throughout space and time in the Southern Ocean, a key area for ocean carbon uptake and controlling global ocean nutrient distributions. After correcting for physical and sampling biases using the Biogeochemical Southern Ocean State Estimate model, we show that annual net community production (ANCP), originally calculated from seasonal nitrate drawdown, reveals the greatest production around the 45–55°S meridional band, and an average basin-wide ANCP of 3.91 ± 0.13 PgC y⁻¹ with a significant increase of 0.67% y⁻¹ from 2004 to 2022. We also highlight that using the common nitrate seasonal drawdown method to derive ANCP might underestimate the true carbon export at depth by about one third. Our findings align with previous studies, which indicate an increase in surface satellite chlorophyll-a and model export fluxes. Our results demonstrate the potential of leveraging machine learning constrained by BGC-Argo observations to study long-term changes of biogeochemical processes in the ocean.

The ocean accounts for ∼20%–30% of global nitrous oxide (N₂O) emissions, with coastal upwelling systems estimated to contribute disproportionately to the sea-air flux of this potent greenhouse gas. To investigate the mechanisms of and controls on N₂O production in coastal upwelling systems, we measured the concentration and nitrogen and oxygen isotopic composition of N₂O (δ¹⁵N-N₂O and δ¹⁸O-N₂O) along a cross-shelf transect in the Southern Benguela Upwelling System (SBUS). At the shelf bottom, N₂O concentrations increased from the outer shelf toward the shore (11–32 nM) inversely to dissolved oxygen (182 ± 17 to <1 μM) and in concert with the remineralization tracers, apparent oxygen utilization (108 ± 21 to 221 ± 33 μM) and nitrogen (N)-deficit (up to 20.4 μM). These observations suggest that both nitrification and denitrification may be involved in N₂O production on the SBUS shelf. The δ¹⁵N-N₂O implicates both processes as potential N₂O sources on the shelf, with high δ¹⁸O-N₂O values (≤57.2‰) specifically incriminating sediments as the primary N₂O source to the water column. Isotopic changes across the shelf delineate three discrete domains with distinct N₂O sources. Sedimentary nitrification and/or denitrification dominate N₂O production on the midshelf, while coupled nitrification-denitrification explains N₂O production on the inner-shelf. At the shallow inner-shelf where oxygen is depleted, both water column and sedimentary denitrification account for the production and partial consumption of N₂O. This study illuminates the disproportionate contribution of sedimentary N cycling to N₂O production on the SBUS shelf.