Global Biogeochemical Cycles最新文献

Calcium carbonate (CaCO₃) dissolution is an integral part of the ocean's carbon cycle. However, laboratory measurements and ocean alkalinity budgets disagree on the rate and loci of dissolution. In situ dissolution studies can help to bridge this gap, but so far published studies have not been utilized as a whole because they have not previously been compiled into one data set and lack carbonate system data to compare between studies. Here, we compile all published measurements of CaCO₃ dissolution rates in the water column (11 studies, 752 data points). Combining World Ocean Atlas data (temperature, salinity) with the neural network CANYON-B (carbonate system variables), we estimate seawater saturation state (Ω) for each rate measurement. We find that dissolution rates at the same Ω vary by 2 orders of magnitude. Using a machine learning approach, we show that while Ω is the main driver of dissolution rate, most variability can be attributed to differences in experimental design, above all bias due to (diffusive) transport and the synthetic or biogenic nature of CaCO₃. The compiled data set supports previous findings of a change in the mechanism driving dissolution at Ω_crit = 0.8 that separates two distinct dissolution regimes: r_slow = 0.29 · (1 − Ω)^0.68(±0.16) mass% day⁻¹ and r_fast = 2.95 · (1 − Ω)^2.2(±0.2) mass% day⁻¹. Above the saturation horizon, one study shows significant dissolution that cannot solely be explained by established theories such as zooplankton grazing and organic matter degradation. This suggests that other, non-biological factors may play a role in shallow dissolution.

In this work, we investigated the seasonal cycle of atmospheric potential oxygen (APO), a unique tracer of air-sea gas exchanges of molecular oxygen (O₂) and carbon dioxide (CO₂), expressed as APO = O₂ + 1.1 × CO₂. APO data were obtained from flask air samples collected since the late 1990s at three Japanese ground stations and on commercial cargo ships sailing between Japan and Australia/New Zealand, North America, and Southeast Asia. We also analyzed the APO spatial distribution and seasonal cycles with simulations from an atmospheric transport model using climatological oceanic O₂ fluxes from an empirical product that relate O₂ flux to ocean heat as input. Model simulations reproduced the observed APO seasonal cycles generally well, but with larger amplitudes and earlier occurrence of seasonal minima and maxima than in the observations. Moreover, the observed seasonal cycles exhibited larger APO enhancements than the simulations in autumn and early winter, especially in the North Pacific at 20°N–60°N. These enhancements remained when refining the comparison by adjusting the simulated APO peak-to-peak amplitudes and seasonal phases to the observations. This suggests additional O₂ emissions in the North Pacific, not well expressed in the air-sea O₂ fluxes used as input for our model simulations. The average autumn enhancement at 40°N–60°N was approximately twice that measured at 20°N–40°N. Confirming previous studies, our results indicate two distinct mechanisms possibly contributing to the additional oceanic O₂ emissions: outgassing from a subsurface shallow oxygen maximum at 20°N–40°N and autumn phytoplankton bloom at 40°N–60°N.

Measurements of the surface ocean fugacity of carbon dioxide (fCO₂) provide an important constraint on the global ocean carbon sink, yet the gap-filling products developed so far to cope with the sparse observations are relatively coarse (1° × 1° by 1 month). Here, we overcome this limitation by using a novel combination of machine learning-based methods and target transformations to estimate surface ocean fCO₂ and the associated sea-air CO₂ fluxes (FCO₂) globally at a resolution of 8-day by 0.25° × 0.25° (8D) over the period 1982 through 2022. Globally, the method reconstructs fCO₂ with accuracy similar to that of low-resolution methods (∼19 μatm), but improves it in the coastal ocean. Although global ocean CO₂ uptake differs little, the 8D product captures 15% more variance in FCO₂. Most of this increase comes from the better-represented subseasonal scale variability, which is largely driven by the better-resolved variability of the winds, but also contributed to by the better-resolved fCO₂. The high-resolution fCO₂ is also capable of capturing the signal of short-lived regional events such as hurricanes. For example, the 8D product reveals that fCO₂ was at least 25 μatm lower in the wake of Hurricane Maria (2017), the result of a complex interplay between the decrease in temperature, the entrainment of carbon-rich waters, and an increase in primary production. By providing new insights into the role of higher frequency variations of the ocean carbon sink and the underlying processes, the 8D product fills an important gap.

In the framework of the RECCAP2 initiative, we present the greenhouse gas (GHG) and carbon (C) budget of Europe. For the decade of the 2010s, we present a bottom-up (BU) estimate of GHG net-emissions of 3.9 Pg CO₂-eq. yr⁻¹ (using a global warming potential on a 100 years horizon), which are largely dominated by fossil fuel emissions. In this decade, terrestrial ecosystems acted as a net GHG sink of 0.9 Pg CO₂-eq. yr⁻¹, dominated by a CO₂ sink that was partially counterbalanced by net emissions of CH₄ and N₂O. For CH₄ and N₂O, we find good agreement between BU and top-down (TD) estimates from atmospheric inversions. However, our BU land CO₂ sink is significantly higher than the TD estimates. We further show that decadal averages of GHG net-emissions have declined by 1.2 Pg CO₂-eq. yr⁻¹ since the 1990s, mainly due to a reduction in fossil fuel emissions. In addition, based on both data driven BU and TD estimates, we also find that the land CO₂ sink has weakened over the past two decades. A large part of the European CO₂ and C sinks is located in Northern Europe. At the same time, we find a decreasing trend in sink strength in Scandinavia, which can be attributed to an increase in forest management intensity. These are partly offset by increasing CO₂ sinks in parts of Eastern Europe and Northern Spain, attributed in part to land use change. Extensive regions of high CH₄ and N₂O emissions are mainly attributed to agricultural activities and are found in Belgium, the Netherlands and the southern UK. We further analyzed interannual variability in the GHG budgets. The drought year of 2003 shows the highest net-emissions of CO₂ and of all GHGs combined.

Trichodesmium plays a key role in the biogeochemical cycling of carbon, nitrogen and phosphorus in the Tropical Atlantic Ocean. A complex interplay of physicochemical factors control the growth of Trichodesmium. However, owing to the large spatial and temporal variability, the relative influence of these factors in controlling Trichodesmium distribution and abundance remains unclear. In this study, we examined the basin-scale distribution pattern of Trichodesmium in the upper 200 m water column of the Atlantic Ocean (25°N–30°S and 70°W–20°E) using a large data set (n = 33,235) and tried to constrain the distribution based on various physicochemical parameters. We suggest that the combined effect of warm temperatures and phosphate (PO₄³⁻) availability determines the zonal spatial extent and the abundance of Trichodesmium in the Tropical North Atlantic Ocean. However, the availability of dissolved iron, along with high sea surface temperatures and meteorological parameters such as the wind direction and precipitation, likely govern the meridional distribution of Trichodesmium across the Atlantic Ocean. Excess PO₄³⁻ at the surface rules out the possibility of PO₄³⁻ limitation in regulating the meridional distribution of the Trichodesmium. Depth-integrated nitrogen fixation rates, based on a multiple linear regression, vary from 0.07 to 306 μmol N m⁻² d⁻¹. The presence of Trichodesmium colonies down to a depth of 200 m and the depth-integrated nitrogen fixation rates reflect the pivotal role of Trichodesmium in the nitrogen budget of this region.

Anthropogenic inputs of nitrogen (N) and phosphorus (P) to terrestrial ecosystems alter soil nutrient cycling. However, the global-scale responses of soil P fractions to N and P inputs and their underlying mechanisms remain elusive. We conducted a global meta-analysis based on 818 observations of soil P fractions from 99 field N and P addition experiments in forest, grassland, and cropland ecosystems ranging from temperate to tropical zones. Our global meta-analysis revealed distinct responses of soil P fractions to N and P enrichment. For studies using the Chang and Jackson inorganic (Pi) method, we found that high N addition promoted the transformation of immobile Pi fractions into Ferrum/Aluminum-bound Pi and available Pi in surface soils through soil acidification. However, this acid-induced transformation of Pi fractions by N addition was observed only in Calcium-rich soils, while in acidic soils, further acidification led to increase P binding. In contrast, additions of P alone or combined with N significantly increased all soil Pi fractions. Regarding the Hedley P fractions, N addition generally decreased labile organic P by enhancing soil acid phosphatase activity. The responses of other P fractions were influenced by soil pH, fertilization rates, ecosystem type, and other factors. P addition increased most soil P fractions. Overall, both P fractionation methods consistently demonstrate that N inputs deplete soil P and accelerate P cycling, while P inputs increase most soil P fractions, alleviating P limitation. These findings are crucial for predicting the effects of future atmospheric N and P deposition on P cycling processes.

Vertical migrants are a diverse group of organisms, which includes crustaceans, cephalopods and mesopelagic fishes. They play an active role in the biogeochemical cycles but are in general not included in numerical models. In this study we introduce a fully coupled Earth system model that represents vertical migration and with this resolves the key components of the mesopelagic ecosystem, namely migrating zooplankton and mesopelagic fish, including their feedbacks on biogeochemical cycles. The redistribution of nutrients in the water column by vertical migration results in a reduction of the net primary production of 14%–21%, as well as in an asymmetric response in the low oxygenated waters in the tropical Pacific (an increase in the northern and a decrease in the southern oxygen minimum zone). On a global scale, we find the active transport of carbon out of the surface layer to be equivalent to ∼25% of the total export (∼30% relative to passive sinking). In the low latitudes, migration results regionally in a reduction of the shallow export by 2%–10% and an increase of the deep carbon export by 6%–15%. In our simulations, mesopelagic fish, with a biomass of 3–3.4 Gt wet weight, have a slightly larger impact on active carbon flux than migrating zooplankton.

Widespread sediment resuspension and transport processes on continental margins can modify deposits and influence the preservation of particulate organic carbon (POC) in marine sediments. However, it remains unclear how post-depositional processes interact with physical mineral protection to affect the transport and fate of terrestrial POC along the river-estuary-shelf paths. Here, we synthesized literature data and newly obtained results from multiple analyses of sedimentology, mineralogy, and inorganic and organic geochemical tracers. Our goal was to quantitatively evaluate the impact of sediment reworking on the redistribution and further transformations of terrestrial POC at the Yangtze River-ocean interface. Our results reveal that sediment resuspension resulting from physical forces along with mineral protection of phyllosilicates plays a crucial role in regulating the recycling and fate of terrestrial POC during its transport across the coastal ocean continuum. Physical processes lead to the resuspension of sequestered POC from suboxic/anoxic muddy sediments into the overlying water column. Concurrently, the interplay of energetic forcing and elevated oxygen levels has the potential to disrupt the organo-mineral associations. The decrease in mineral-carbon stabilization increases the likelihood that reactive POC inclusion/aggregation with minerals becomes accessible to surrounding microorganisms, making it susceptible to microbial/oxidative degradation. Consequently, mostly phyllosilicate-protected ¹⁴C-depleted POC (primarily soil-derived) in <63 μm suspended sediment (>90% of the total mass) remains available for export and reburial in continental shelf sediments. The lateral transport of resuspended sediments from estuaries, previously underestimated, represents a potential contributor to the remobilized millennial-aged POC components involved in active biogeochemical cycling on continental margins.

The biological gravitational pump (BGP) and particle injection pumps (PIPs) are significant export pathways for particulate organic carbon from the surface ocean to the interior. Part of this exported carbon fuels remineralization in the mesopelagic ocean and part is sequestered in the deep ocean. Using observations from Biogeochemical-Argo, we characterized the seasonality and magnitude of the BGP and two PIPs: the mixed layer pump (MLP) and eddy subduction pump (ESP), in the Australian sector of the Subantarctic Zone (SAZ sector). For the first time, float-based estimates were rigorously combined with sediment trap flux (F₁₀₀₀) observations from the Southern Ocean Time Series (SOTS), to investigate these pumps' relative and cumulative contributions to carbon export. The BGP exports about 28.6 g C m⁻² year⁻¹, mostly during the productive season and dominates the F₁₀₀₀ seasonality. The MLP exports about 7.6 g C m⁻² year⁻¹, mostly while the mixing layer seasonally shoals; the ESP sporadically exports up to 100 mg C m⁻² day⁻¹, such that these two PIPs have a short but intense impact on the F₁₀₀₀. The carbon transfer efficiency is 3.6% in the SOTS region. An oxygen-based annual net community production estimate (∼50 g C m⁻² year⁻¹) further strengthens this study, and suggests the BGP and MLP make the dominant contribution to the mesopelagic carbon budget. This is representative of the broader SAZ sector in terms of the magnitude and seasonality of carbon export, the consumption of organic material in the mesopelagic, and the organic carbon sequestration in the deep sea.

The terrestrial biosphere plays a major role in the global carbon cycle, and there is a recognized need for regularly updated estimates of land-atmosphere exchange at regional and global scales. An international ensemble of Dynamic Global Vegetation Models (DGVMs), known as the “Trends and drivers of the regional scale terrestrial sources and sinks of carbon dioxide” (TRENDY) project, quantifies land biophysical exchange processes and biogeochemistry cycles in support of the annual Global Carbon Budget assessments and the REgional Carbon Cycle Assessment and Processes, phase 2 project. DGVMs use a common protocol and set of driving data sets. A set of factorial simulations allows attribution of spatio-temporal changes in land surface processes to three primary global change drivers: changes in atmospheric CO₂, climate change and variability, and Land Use and Land Cover Changes (LULCC). Here, we describe the TRENDY project, benchmark DGVM performance using remote-sensing and other observational data, and present results for the contemporary period. Simulation results show a large global carbon sink in natural vegetation over 2012–2021, attributed to the CO₂ fertilization effect (3.8 ± 0.8 PgC/yr) and climate (−0.58 ± 0.54 PgC/yr). Forests and semi-arid ecosystems contribute approximately equally to the mean and trend in the natural land sink, and semi-arid ecosystems continue to dominate interannual variability. The natural sink is offset by net emissions from LULCC (−1.6 ± 0.5 PgC/yr), with a net land sink of 1.7 ± 0.6 PgC/yr. Despite the largest gross fluxes being in the tropics, the largest net land-atmosphere exchange is simulated in the extratropical regions.