Global Biogeochemical Cycles最新文献

Understanding the oceanic Copper (Cu) budget is essential for tracing nutrient pathways, interpreting ancient sediment records, and assessing global environmental changes. However, the global oceanic Cu cycle remains imbalanced, largely due to insufficient studies on the flux and isotopic composition of authigenic Cu in oxic pelagic sediments. Here, we present the Cu isotopic compositions of pelagic sediments collected from the western (non-hydrothermal area) and eastern South (hydrothermal area) Pacific Ocean. These results indicate that authigenic Cu in pelagic sediments is primarily hosted by iron-manganese (oxyhydr)oxides. The isotopic composition of authigenic Cu in pelagic sediments (0.01‰ ± 0.13‰, 2SD) is considerably lighter than the previously assumed value of ∼0.3‰, which was based on the Cu isotopic compositions of iron-manganese crusts and nodules. Using these new isotopic constraints, together with a newly calculated Cu flux to pelagic sediments of 15.2 × 10⁸ mol yr⁻¹, we propose a new, balanced oceanic budget for Cu isotopes. This study precisely defines the flux and isotopic composition of the largest oceanic Cu sink, placing new constraints on the marine Cu cycle.

Dissolved organic nitrogen (DON) is the dominant form of bioavailable nitrogen in the euphotic zone of subtropical gyres, where nitrate (NO₃⁻) concentrations are low. However, identifying regions where DON consumption may support surface ocean productivity remains challenging due to the relatively narrow range in euphotic zone DON concentrations. The stable isotopic composition (δ¹⁵N) of DON has recently emerged as a sensitive tool for identifying regions of DON production and consumption in the surface ocean. Here, we report DON concentration and δ¹⁵N measurements in the upper ∼300 m from a >10, 000 km zonal GO-SHIP transect along ∼30°S in the South Pacific (P06-2017 transect) from the Chilean to the Australian coasts. We observed higher upper 50 m DON concentrations in the east associated with DON production in regions with elevated primary productivity. Upper 50 m DON δ¹⁵N decreases from east to the west, coincident with a gradient in the δ¹⁵N of subsurface (i.e., 100–200 m) NO₃⁻, consistent with subsurface NO₃⁻ fueling DON production. Further, both the partial and complete assimilation of surface NO₃⁻ observed along the transect are imprinted on the δ¹⁵N of surface DON. An inverse relationship between the concentration and δ¹⁵N of surface DON was found in the western portion of the transect, consistent with DON consumption by phytoplankton. Combining concentrations and δ¹⁵N of DON, we identified regions of DON production and consumption across the largest subtropical ocean gyre.

The ocean plays a major role in controlling atmospheric carbon at decadal to millennial timescales, with benthic carbon representing the only geologic-scale storage of oceanic carbon. Despite its importance, detailed benthic ocean observations are limited and representation of the benthic carbon cycle in ocean and Earth system models (ESMs) is mostly empirical with little prognostic capacity, which hinders our ability to properly understand the long-term evolution of the carbon cycle and climate change-related feedbacks. The Benthic Ecosystem and Carbon Synthesis (BECS) working group, with the support of the US Ocean Carbon & Biogeochemistry Program (OCB), identified key challenges limiting our understanding of benthic systems, opportunities to act on these challenges, and pathways to increase the representation of these systems in global modeling and observational efforts. We propose a set of priorities to advance mechanistic understanding and better quantify the importance of the benthos: (a) implementing a model intercomparison exercise with existing benthic models to support future model development, (b) data synthesis to inform both model parameterizations and future observations, (c) increased deployment of platforms and technologies in support of in situ benthic monitoring (e.g., from benchtop to field mesocosm), and (d) global coordination of a benthic observing program (“GEOSed”) to fill large regional data gaps and evaluate the mechanistic understanding of benthic processes acquired throughout the previous steps. Addressing these priorities will help inform solutions to both global and regional resource management and climate adaptation strategies.

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.