Global Biogeochemical Cycles最新文献

The Southern Ocean accounts for ∼30% of the ocean's CO₂ sink, partly due to its biological pump that transfers surface-produced organic carbon to deeper waters. To estimate large-scale Southern Ocean carbon export potential and characterize its drivers, we measured the carbon and nitrogen isotope ratios of surface suspended particulate matter (δ¹³C_SPM, δ¹⁵N_SPM) for samples collected in summer 2016/2017 during the Antarctic Circumnavigation Expedition (364 stations). Concurrent measurements of phytoplankton community composition revealed the dominance of large diatoms in the Antarctic and nano-phytoplankton (mainly haptophytes) in open Subantarctic waters. As expected, δ¹³C_SPM was strongly dependent on pCO₂, with local deviations in this relationship explained by phytoplankton community dynamics. δ¹⁵N_SPM reflected the nitrogen sources consumed by phytoplankton, with higher inferred nitrate (versus recycled ammonium) dependence generally coinciding with higher micro-phytoplankton abundances. Using δ¹⁵N_SPM and a two-endmember isotope mixing model, we quantified the extent of nitrate- versus ammonium-supported growth, which yields a measure of carbon export potential. We estimate that across the Southern Ocean, 41 ± 29% of the surface-produced organic carbon was potentially exported below the seasonal mixed layer during the growth season, with maximum export potential (50%–99%) occurring in the Antarctic Circumpolar Current's southern Boundary Zone and near the (Sub)Antarctic islands, reaching a minimum in the Subtropical Zone (<33%). Alongside iron, phytoplankton community composition emerged as an important driver of the Southern Ocean's biological pump, with large diatoms dominating regions characterized by high nitrate dependence and elevated carbon export potential and smaller, mainly non-diatom taxa proliferating in waters where recycled ammonium supported most productivity.

The subantarctic zone is an important region in the Southern Ocean with respect to its influence on air-sea CO₂ exchange and the global ocean carbon cycle. However, understanding of the magnitude and drivers of the flux are still being refined. Using observations from the Southern Ocean Time Series (SOTS) station (∼47°S, 142°E) and auxiliary data, we developed a multiple linear regression model to compute the sea surface partial pressure of CO₂ (pCO₂) over the past two decades. The mean amplitude of the pCO₂ seasonal cycle between 2004 and 2021 was 44 μatm (range 30–54 μatm). Summer minima ranged from 310 to 370 μatm and winter maxima were near equilibrium with the atmosphere. The non-thermal (i.e., biological processes and mixing) contribution to the seasonal variability in pCO₂ was several times larger than the thermal contribution. The SOTS region acted as a net carbon sink at annual time scales, with mean magnitude of 6.0 mmol m⁻² d⁻¹. The positive phase of the Southern Annular Mode (SAM) increased ocean carbon uptake primarily through an increase in wind speed at zero time lag. Increased surface pCO₂ was correlated with a positive SAM with a lag of 4 months, mainly due to reduced biological uptake and increased mixing. During the autotrophic season, pCO₂ was predominantly impacted by primary productivity, whereas water mass movement, inferred by temperature and salinity anomalies, had a larger impact on the heterotrophic season. In general, mesoscale processes such as eddies and frontal movement impact the local biogeochemical features more than the SAM.

The availability of silicon (Si) in the ocean plays an important role in regulating biogeochemical and ecological processes. The Si budget of the Arctic Ocean appears balanced, with inputs equivalent to outputs, though it is unclear how a changing climate might aggravate this balance. In this study, we focus on Si cycling in Arctic coastal areas and continental shelf sediments to better constrain the Arctic Ocean Si budget. We provide the first estimate of amorphous Si (ASi) loading from erosion of coastal Yedoma deposits (30–90 Gmol yr⁻¹), demonstrating comparable rates to particulate Si loading from rivers (10–90 Gmol yr⁻¹). We found a positive relationship between surface sediment ASi and organic matter content on continental shelves. Combining these values with published Arctic shelf sediment properties and burial rates we estimate 70 Gmol Si yr⁻¹ is buried on Arctic continental shelves, equivalent to 4.5% of all Si inputs to the Arctic Ocean. Sediment dissolved Si fluxes increased with distance from river mouths along cruise transects of shelf regions influenced by major rivers in the Laptev and East Siberian seas. On an annual basis, we estimate that Arctic shelf sediments recycle approximately up to twice as much DSi (680 Gmol Si) as is loaded from rivers (340–500 Gmol Si).

Ocean acidification (OA) threatens coral reef persistence by decreasing calcification and accelerating the dissolution of reef frameworks. The carbonate chemistry of coastal areas where many reefs exist is strongly influenced by the metabolic activity of the underlying benthic community, contributing to high spatiotemporal variability. While characterizing this variability is difficult, it has important implications for the progression of OA and the persistence of the ecosystems. Here, we characterized the carbonate chemistry at 38 permanent stations located along 10 inshore-offshore transects spanning 250 km of the Florida Coral Reef (FCR), which encompass four major biogeographic regions (Biscayne Bay, Upper Keys, Middle Keys, and Lower Keys) and four shelf zones (inshore, mid-channel, offshore, and oceanic). Data have been collected since 2010, with approximately bi-monthly periodicity starting in 2015. Increasing OA, driven by increasing DIC, was detected in the mid-channel, offshore, and oceanic zones in every biogeographic region. In the inshore zone, however, increasing TA counteracted any measurable OA trend. Strong seasonal variability occurred at inshore sites and included periods of both exacerbated and mitigated OA. Seasonality was region-dependent, with greater variability in the Lower and Middle Keys. Elevated pH and aragonite saturation states (Ω_Ar) were observed in the Upper and Middle Keys, which could favor reef habitat persistence in these regions. Offshore reefs in the FCR could be more susceptible to global OA by experiencing open-ocean-like water chemistry conditions. By contrast, higher seasonal variability at inshore reefs could offer a temporary OA refuge during periods of enhanced primary production.

Ocean warming is associated with a decline in the global oxygen (O₂) inventory, but the ratio of O₂ loss to heat gain is poorly understood. We analyzed historical variability in temperature (T), O₂, and nitrate $��\left(�N�O_3^{-}�\right)�$ in hydrographic observations and model simulations of the North Atlantic, a relatively well-sampled region that is important for deep ocean ventilation. Multidecadal fluctuations of O₂ concentrations in subpolar thermocline waters (100–700 m) are correlated with changes in their heat content, with a slope 35% steeper than that expected from thermal solubility. Variations of O₂ in excess of the solubility effect are correlated with observed decadal changes in $��N�O_3^{-}�$ in the surface layer (0–50 m), which declines by ∼1 mmol N m⁻³ per degree of temperature anomaly. Enhanced biologically mediated drawdown of nutrients from the photic zone and associated respiration in deeper water account for the additional depletion of thermocline O₂ during warm years. In model simulations, increased nutrient consumption in warm periods is driven by an early start of the phytoplankton growing season and faster phytoplankton growth rates at higher temperatures. Our results highlight a role for phytoplankton T-dependent growth rates in amplifying ocean O₂ loss.

Tropical lands play an important role in the global carbon cycle yet their contribution remains uncertain owing to sparse observations. Satellite observations of atmospheric carbon dioxide (CO₂) have greatly increased spatial coverage over tropical regions, providing the potential for improved estimates of terrestrial fluxes. Despite this advancement, the spread among satellite-based and in-situ atmospheric CO₂ flux inversions over northern tropical Africa (NTA), spanning 0–24°N, remains large. Satellite-based estimates of an annual source of 0.8–1.45 PgC yr⁻¹ challenge our understanding of tropical and global carbon cycling. Here, we compare posterior mole fractions from the suite of inversions participating in the Orbiting Carbon Observatory 2 (OCO-2) Version 10 Model Intercomparison Project (v10 MIP) with independent in-situ airborne observations made over the tropical Atlantic Ocean by the National Aeronautics and Space Administration (NASA) Atmospheric Tomography (ATom) mission during four seasons. We develop emergent constraints on tropical African CO₂ fluxes using flux-concentration relationships defined by the model suite. We find an annual flux of 0.14 ± 0.39 PgC yr⁻¹ (mean and standard deviation) for NTA, 2016–2018. The satellite-based flux bias suggests a potential positive concentration bias in OCO-2 B10 and earlier version retrievals over land in NTA during the dry season. Nevertheless, the OCO-2 observations provide improved flux estimates relative to the in situ observing network at other times of year, indicating stronger uptake in NTA during the wet season than the in-situ inversion estimates.

Water stress regulates land-atmosphere carbon dioxide (CO₂) exchanges in the tropics; however, its role remains poorly characterized due to the confounding roles of radiation, temperature and canopy dynamics. In particular, uncertainty stems from the relative roles of plant-available water (supply) and atmospheric water vapor deficit (demand) as mechanistic drivers of photosynthetic carbon (C) uptake variability. Using satellite measurements of gravity, CO₂ and fluorescence to constrain a mechanistic carbon-water cycle model from 2001 to 2018, we found that the interannual variability (IAV) of water stress on photosynthetic C uptake was 52% greater than the combined effects of other factors. Surprisingly, the dominance of water stress on C uptake IAV was greater in the wet tropics (94%) than in the dry tropics (26%). Plant-available water supply and atmospheric demand both contributed to the IAV of water stress on photosynthetic C uptake across the tropics, but the IAV of demand effects was 21% greater than the IAV of supply effects (33% greater in the wet tropics and 6% greater in the dry tropics). We found that the IAV of water stress on C uptake was 24% greater than the IAV of the combination of other factors in the net land-atmosphere C sink in the whole tropics, 26% greater in the wet tropics, and 7% greater in the dry tropics. Given the recent trends in tropical precipitation and atmospheric humidity, our findings indicate that water stress——from both supply and demand——will likely dominate the climate response of land C sink across tropical ecosystems in the coming decades.

Atmospheric nitrogen (N) deposition is a key process influencing plant-soil N processes and associated functions of forest ecosystems. However, the N deposition into soils based on open-field precipitation observations remains inaccurate due to the unconstrained precipitation-canopy N exchanges, which prevents a better evaluation of N deposition effects on forest N cycles and functions. Nitrate ($��{{\text{NO}}_3}^{-}�$) is a major form of reactive N. Based on a data synthesis of fluxes and isotopes (¹⁵N, ¹⁷O, ¹⁸O) of atmospheric $��{{\text{NO}}_3}^{-}�$ inputs in forests, here we constructed a new method to quantify fractions and fluxes of throughfall $��{{\text{NO}}_3}^{-}�$ ($���t�-��{{\text{NO}}_3}^{-}�$) contributors (nitrification ($���n�-��{{\text{NO}}_3}^{-}�$) and particulates ($���$

Particulate phases transport trace metals (TM) and thereby exert a major control on TM distribution in the ocean. Particulate TMs can be classified by their origin as lithogenic (crustal material), biogenic (cellular), or authigenic (formed in situ), but distinguishing these fractions analytically in field samples is a challenge often addressed using operational definitions and assumptions. These different phases require accurate characterization because they have distinct roles in the biogeochemical iron cycle. Particles collected from the upper 2,000 m of the northwest subtropical Atlantic Ocean over four seasonal cruises throughout 2019 were digested with a chemical leach to operationally distinguish labile particulate material from refractory lithogenics. Direct measurements of cellular iron (Fe) were used to calculate the biogenic contribution to the labile Fe fraction, and any remaining labile material was defined as authigenic. Total particulate Fe (PFe) inventories varied <15% between seasons despite strong seasonality in dust inputs. Across seasons, the total PFe inventory (±1SD) was composed of 73 ± 13% lithogenic, 18 ± 7% authigenic, and 10 ± 8% biogenic Fe above the deep chlorophyll maximum (DCM), and 69 ± 8% lithogenic, 30 ± 8% authigenic, and 1.1 ± 0.5% biogenic Fe below the DCM. Data from three other ocean regions further reveal the importance of the authigenic fraction across broad productivity and Fe gradients, comprising ca. 20%–27% of total PFe.

Regional carbon budget assessments attribute and track changes in carbon sources and sinks and support the development and monitoring the efficacy of climate policies. We present a comprehensive assessment of the natural and anthropogenic carbon (C-CO₂) fluxes for Australasia as a whole, as well as for Australia and New Zealand individually, for the period from 2010 to 2019, using two approaches: bottom-up methods that integrate flux estimates from land-surface models, data-driven models, and inventory estimates; and top-down atmospheric inversions based on satellite and in situ measurements. Our bottom-up decadal assessment suggests that Australasia's net carbon balance was close to carbon neutral (−0.4 ± 77.0 TgC yr⁻¹). However, substantial uncertainties remain in this estimate, primarily driven by the large spread between our regional terrestrial biosphere simulations and predictions from global ecosystem models. Within Australasia, Australia was a net source of 38.2 ± 75.8 TgC yr⁻¹, and New Zealand was a net CO₂ sink of −38.6 ± 13.4 TgC yr⁻¹. The top-down approach using atmospheric CO₂ inversions indicates that fluxes derived from the latest satellite retrievals are consistent within the range of uncertainties with Australia's bottom-up budget. For New Zealand, the best agreement was found with a national scale flux inversion estimate based on in situ measurements, which provide better constrained of fluxes than satellite flux inversions. This study marks an important step toward a more comprehensive understanding of the net CO₂ balance in both countries, facilitating the improvement of carbon accounting approaches and strategies to reduce emissions.