Global Biogeochemical Cycles最新文献

Fjord sediments are global sinks of organic carbon (OC), contributing to the long-term storage of atmospheric CO₂. Despite this recognition, the transfer and burial of OC in fjord sediments are still poorly quantified and suffer from a sampling bias toward distal environments where marine OC is dominant. Here we present organic geochemical data obtained on sediment samples (suspended river sediments, fjord sediment trap, surface fjord sediments, sediment core) collected in the Baker river-fjord system, with a particular focus on the Baker River submarine delta, which is fed by Chile's largest river. We measured total OC contents and OC stable isotope composition to quantify the amount and type of OC (marine or terrestrial) stored in the fjord submarine delta. We find that OC fluxes are twice higher in summer (106 ± 6 g OC/m²/yr) than in winter (53 ± 3 g OC/m²/yr) due to higher sediment discharge from meltwater. Sediment trap OC fluxes are on the same order of magnitude than those in the nearby sediment core (103 ± 15 g OC/m²/yr) during the last 35 years, suggesting rapid OC burial in sediments. Carbon isotopes suggest that the OC stored in the fjord submarine delta is predominantly of terrestrial origin. We calculate that the Baker submarine delta buries 3.8 ± 0.6 kt OC/yr, which corresponds to 26 ± 11% of the estimated OC annual flux delivered by the Baker River (14.4 ± 5.5 kt OC/yr). Fjord deltas should thus be considered in fjord OC budgets as they could significantly raise global estimates of terrestrial OC burial in marine sediments.

Silicon isotopes within silicic acid, δ³⁰Si, were measured on US GEOTRACES section GP15 from Alaska to Tahiti along 152°W. The distribution of silicic acid, Si(OH)₄, exhibited a double Si(OH)₄ maximum dominated by a midwater maximum that extended from 55°N latitude to the equator together with a bottom maximum north of 40°N latitude. The midwater maximum is dominated by North Pacific Deep Water and is consistent with the existence of a poorly ventilated North Pacific Shadow Zone between 1,500 and 3,000 m with an ideal mean age in excess of 1,300 years where regenerated Si(OH)₄ accumulates. A data-constrained model of the marine silicon cycle is used to assess the contribution of the Southern Ocean, North Pacific, and central latitudes to the preformed and regenerated forms of both Si(OH)₄ and δ³⁰Si along GP15. Regenerated Si(OH)₄ from the North Pacific and Southern Ocean are the dominant inputs to the midwater maximum. Silicic acid within the midwater maximum becomes lighter toward the north with a δ³⁰Si minimum at about 50°N. The model indicates that this feature is shaped by ready access to successive fractionations in the productive high-latitude oceans of both hemispheres. Silicon trapping within the North Pacific accounts for the upper part the δ³⁰Si minimum, while the Shadow Zone's interhemispheric connectivity with the Southern Ocean accounts for the deeper part. Isotope values in the bottom Si(OH)₄ maximum are slightly elevated relative to adjacent waters consistent with a sediment source, although this isotopic constraint lies within statistical uncertainty.

Thermal and biogeochemical states of the North Atlantic Ocean are affected on seasonal to decadal timescales by atmospheric forcing associated with the North Atlantic Oscillation (NAO). An NAO–based composite approach is applied to an Earth system model to reveal the fast and slow responses of the ocean to atmospheric impulse forcing. Over the seasonal boundary layer, the atmosphere induces a “fast”, seasonal ocean response driven by anomalies in the air–sea flux, vertical entrainment and Ekman transport. This fast response to an NAO$��+�$ anomaly results in negative temperature and positive carbon and nutrient anomalies over the subpolar gyre, and positive temperature and negative carbon and nutrient anomalies over the subtropical gyre. The “slow” response on inter-annual timescales involves changes in meridional overturning, tracer transport and entrainment. The slow response leads to a redistribution of anomalies between subpolar and subtropical regions, generating opposing–signed tracer anomalies in the subpolar and subtropical North Atlantic Oceans. These ocean responses also involve changes in the mixed-layer depth, as well as the associated changes in entrainment and turbulent mixing rates, which are particularly important for the carbon and nutrient responses given their large vertical gradients. Modifications in nutrient concentrations subsequently influence biological activity and biomass production. These thermal, carbon and nutrient responses to atmospheric events linked to the NAO can persist for up to a decade, often characterized by opposing–signed temperature and carbon anomalies, along with contrasting changes in the subtropical and subpolar gyres.

Large amounts of dissolved organic carbon (DOC) are transported laterally from uplands and wetlands to headwater streams; however, the global magnitude of this flux and its role in the carbon (C) budget remain unclear. By compiling 20,403 DOC concentration observations and applying machine learning models, we estimated the annual DOC flux from forested headwater streams north of 30°S to be 116.2 Tg C yr⁻¹ (90% CI: 86.5–145.2). Higher yields were observed in tropical regions (3.8 g C m⁻² yr⁻¹), where vegetation productivity is high, and in boreal regions (3.1 g C m⁻² yr⁻¹), where peatland cover and soil organic C stocks are substantial. When comparing our results with global terrestrial C flux estimates upscaled from eddy covariance data, we found that the fraction of net ecosystem production (NEP) lost annually as DOC ranged from negligible (<0.1%) to 20.7%, with discharge explaining 34% of the variation. Our study suggests that neglecting the lateral export of DOC could lead to an overestimation of NEP in forested headwater catchments, a bias that is further amplified by increased discharge.

The Atlantic Meridional Overturning Circulation (AMOC) impacts temperatures, ecosystems, and the carbon cycle. However, AMOC effects on Earth's carbon cycle remains poorly understood, in part because contributions of different physical and biological mechanisms that impact carbon storage in the ocean are not typically diagnosed in climate models. Here, we explore modeled effects of AMOC shutdowns on ocean Dissolved Inorganic Carbon (DIC) by applying a new decomposition that explicitly calculates preformed and regenerated DIC components and separates physical and biological contributions. An extensive evaluation in transient simulations finds that the method is accurate, especially for basin-wide changes, whereas errors can be significant at global and local scales. In contrast, estimates of respired carbon based on Apparent Oxygen Utilization lead to large errors and are generally not reliable. In response to a shutdown of the AMOC under Last Glacial Maximum (LGM) background climate, ocean carbon increases and then decreases, leading to opposite changes in atmospheric carbon dioxide (CO₂). DIC changes are dominated by opposing changes in biological carbon storage. Whereas regenerated components increase in the Atlantic and dominate the initial increase in global ocean DIC until model year 1000, preformed components decrease in the other ocean basins and dominate the long-term DIC decrease until year 4000. Biological disequilibrium is an important contribution to preformed carbon changes. Biological saturation carbon decreases in the Pacific, Indian, and Southern Oceans due to a decrease in surface alkalinity. The spatial patterns of the DIC components and their changes in response to an AMOC collapse are presented.

The Arabian Sea experiences an intense perennial Oxygen Minimum Zone (OMZ) (150–1,200 m) in its northern and central regions. Earlier measurements during the 1990s (Joint Global Ocean Flux Study, India) revealed methane (CH₄) oversaturation in the upper 300–400 m in this region. The basin is reported to have experienced warming and OMZ intensification in recent years, while the particle flux is expected to remain moderately steady. In response to the above phenomena, we aimed to examine the change in CH₄ distribution in comparison to the 1990s by studying the depth profiles of CH₄ along 8–21°N over 68°E meridian from February 2017 to October 2024. The present study showed CH₄ oversaturation in the upper 135–350 m in the OMZ region and 100–150 m in the non-OMZ region. The upper OMZ CH₄ maxima exhibits moderate spatial variability with higher concentrations toward the north. However, there was no significant change in the CH₄ maxima in the OMZ in comparison to those during the 1990s. The waters in the OMZ core and below were undersaturated with CH₄ as observed in the 1990s. We hypothesize that the CH₄ build-up in the OMZ waters was possibly related to its in situ production from the anoxic micro-niches in sinking detrital particulate matter. The CH₄ undersaturation in the core of OMZ was possibly due to reduced particle fluxes, which led to low CH₄ production potential. However, the potential role of oxidants such as nitrite and oxygen on the CH₄ cycling in the Arabian Sea OMZ needs further research.

Plankton community structure influences biogeochemical and ecosystem processes, such as sequestration of atmospheric CO₂, carbon export to the ocean floor, and the productivity of higher trophic levels. One means of analyzing community structure is through the distribution of biovolume across size classes (the size spectrum), since size is a proxy for plankton functional traits. To understand how climate forcing affects plankton communities, we assessed the size spectra in the historical simulations of seven Earth System Models (ESMs) included in the 6th Coupled Model Intercomparison Project and analyzed projected changes under a high emissions scenario (SSP5-8.5). We compared historical estimates with the Pelagic Size Structure database (PSSdb), a novel size structure dataset from imaging systems. The median slope from models ranged from −1.66 to −1.07, with shallower slopes from this range approximating both the theoretical expectation and PSSdb observations (−1.05), with variations around the median representing differences in the total biovolume distribution across plankton functional groups. Consistent with the observations, most ESMs show steeper slopes and lower biovolume in oligotrophic subtropical gyres compared with productive ocean regions. Historical versus climate change simulations reveal increases in slope and biovolume at high latitudes, associated with greater biomass and productivity, and decreases at lower latitudes, consistent with nutrient limitation from stronger stratification. We emphasize the need for expanded observational data. Despite ESMs not being designed to simulate size, the plankton size spectra from models provide insights on large-scale ecological and biogeochemical processes, and how climate change could affect these dynamics in the future.

Early diagenetic forward and reverse weathering reactions play a significant role in controlling alkalinity fluxes and silica, alkali metal and alkaline earth metal cycling in coastal systems. In Kongsfjorden, Svalbard, the inputs of autochthonous biogenic debris (diatomaceous silica) and allochthonous lithogenic material of varying reactivity (dominated by clays, especially illite and chlorite, and primary aluminosilicates, mostly plagioclase) drive complex balances of diagenetic silicate reactions in sediments. The rapid dissolution of reactive silica results in the release of dissolved silica (Si_d) into pore-waters and sustains elevated benthic Si_d fluxes (−0.2 to −0.8 mmol m⁻² d⁻¹), which are on the upper end of values previously determined for Arctic environments. Increases with depth in pore-water lithium (Li⁺), potassium, magnesium, and barium concentrations within the top centimeters provided evidence for forward weathering of clays quickly upon burial. Due to the prevalent occurrence of forward weathering, the benthic net Li⁺ flux was associated with a light isotope signal. Decreases in pore-water rubidium concentrations with depth at the near-glacier station, elevated ratios of the authigenically altered silica to the biogenic silica pool at all sites, and small increases of pore-water δ⁷Li values with depth showed that reverse weathering also takes place. Anoxic incubation of diatom frustule probes provided further evidence for the neoformation of cation-rich clays. The superposition of reverse and forward weathering results in cryptic silica and cation cycling that muted net benthic fluxes. In deeper sediments, changes in pore-water solute patterns indicated an interconnected occurrence of reverse and forward weathering, potentially driven by reactive silica-limitation.