Global Biogeochemical Cycles最新文献

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.

We analyzed the in situ diel variability in dissolved O₂ from a compilation of quality-controlled measurements collected with underwater gliders in the North Pacific Subtropical Gyre over a period spanning 14 years (2008−2021) and totaling >10,500 vertical profiles. We derived metabolic rates at different depths (0−110 m) from the diel oscillations in O₂ anomaly (ΔO₂), calculated here as the difference between instantaneous and 2-day mean O₂ concentrations, using all assembled measurements. Depth-integrated annual gross oxygen production and community respiration (CR) derived from this study (± standard deviation) were 87 ± 3 mmol O₂ m⁻² d⁻¹ and 89 ± 4 mmol O₂ m⁻² d⁻¹, respectively. Gross oxygen production and CR decreased with depth and were lower in winter and spring than in summer and fall. Hourly rates of O₂ change indicated relatively constant nighttime respiration, but enhanced net O₂ production in the morning compared with the afternoon. We found that when ΔO₂ was calculated by subtracting the O₂ concentration at equilibrium (ΔO₂-eq), as proposed in other studies, metabolic rates were overestimated by approximately 31%–36% in this oligotrophic oceanic region, due to diel variations in O₂ solubility driven by temperature oscillations. By demonstrating the reliability of depth-resolved characterization of ocean metabolism, we hope to pave the way for the use of autonomous underwater observations to characterize primary productivity and respiration in understudied ocean regions, improving the spatiotemporal resolution provided by conventional methods.

The ocean's biological pump, a critical component of the Earth's carbon cycle, transports organic matter from the surface ocean to depth and is dominated by sinking particles, often in the form of marine snow-sized (diameter ≥0.5 mm) aggregates. Controls of sinking particle carbon export are thought to be driven largely using ecological processes that create and transform sinking particles. We diagnose the importance of both biotic and abiotic processes in the dynamics of marine snow and other suspended particles using image-based determination of their size distribution. These observations were made during the demise of the North Atlantic spring bloom in May 2021 as part of the Export Processes in the Ocean from RemoTe Sensing-North Atlantic (EXPORTS-NA) field campaign. We show that intense storm events generated high turbulent mixing rates in the upper ocean that impacted the abundance, size distribution, porosity and sinking of marine snow. Mixed-layer turbulence levels both created and destroyed marine snow and the sequence of entrainment and detrainment of the mixed layer induced by repeated storm forcings enhanced the vertical transport of aggregates to depth. Evidence of biological transformations was also observed at mesopelagic depths, both for the consumption of particulate matter and in the creation of smaller particles from larger ones, likely due to interactions with zooplankton. Collectively, these results illustrate the complex interplay of physical and biological processes regulating the dynamics of marine snow and suggest their inclusion in predictive models of the ocean's biological pump.