Global Biogeochemical Cycles最新文献

This paper is the first comprehensive synthesis of what is currently known about the different natural and anthropogenic fluxes of rhenium (Re) on Earth's surface. We highlight the significant role of anthropogenic mobilization of Re, which is an important consideration in utilizing Re in the context of a biogeochemical tracer or proxy. The largest natural flux of Re derives from chemical weathering and riverine transport to the ocean (dissolved = 62 × 10⁶ g yr⁻¹ and particulate = 5 × 10⁶ g yr⁻¹). This review reports a new global average [Re] of 16 ± 2 pmol L⁻¹, or 10 ± 1 pmol L⁻¹ for the inferred pre-anthropogenic concentration without human impact, for rivers draining to the ocean. Human activity via mining (including secondary mobilization), coal combustion, and petroleum combustion mobilize approximately 560 × 10⁶ g yr⁻¹ Re, which is more than any natural flux of Re. There are several poorly constrained fluxes of Re that merit further research, including: submarine groundwater discharge, precipitation (terrestrial and oceanic), magma degassing, and hydrothermal activity. The mechanisms and the main host phases responsible for releasing (sources) or sequestrating (sinks) these fluxes remain poorly understood. This study also highlights the use of dissolved [Re] concentrations as a tracer of oxidation of petrogenic organic carbon, and stable Re isotopes as proxies for changes in global redox conditions.

Afforestation is widely believed to sequester carbon (C) in soil. However, the effect of afforestation on soil organic C (SOC) accumulation is still debated due to the contrasting features of particulate and mineral-associated organic C (POC and MAOC). We conducted a field investigation of 144 paired sampling sites by comparing afforested and non-afforested lands to investigate the POC and MAOC dynamics after afforestation across the Danjiangkou basin in subtropical China, where forests are dominated by Platycladus orientalis, Quercus variabilis and Pinus massoniana. The average contents of SOC, POC, and MAOC were significantly increased by afforestation; however, POC and MAOC responded differently to afforestation type. All afforestation types promoted the POC content, and MAOC also showed positive responses to afforestation except that afforestation with P. massoniana from shrubland significantly reduced the MAOC content. With increasing SOC content, the POC grew at a faster rate than MAOC at high SOC levels. Afforestation hindered the growth rate of POC, while it promoted the growth rate of MAOC as SOC accrued, which potentially obscured the distinct patterns of C accumulation triggered by afforestation. The variation partitioning suggests that, under afforestation, microbial traits had a higher contribution to both POC and MAOM variations compared with non-afforested land. These results suggest that the robust buildup of microbial biomass due to increased plant C input following afforestation could contribute to soil C accumulation by promoting microbial necromass.

The gravitational settling of organic particles from the surface to the deep ocean is an important export pathway and one of the largest components of the ocean carbon pump. The strength and efficiency of the gravitational pump are often measured using metrics reliant on reference depths and empirical formulations that parameterize the relationship between depth and the flux or concentration of particulate organic carbon (POC). Here, BGC-Argo profiles were used to identify the isolume where POC concentration, [POC], starts to decline, revealing attenuation trends below this isolume that are remarkably consistent across the global ocean. We developed a simple empirical approach that uses observations from the first optical depth to predict [POC] from the surface ocean to the base of the mesopelagic (1,000 m), allowing assessments of spatial and temporal variability in gravitational pump efficiencies. We find that rates of [POC] attenuation are high in areas of high biomass and low in areas of low biomass, supporting the view that bloom events sometimes result in a relatively weak deep biological pump that is characterized by low transfer efficiency to the base of the mesopelagic. Our isolume-based attenuation model was applied to satellite data to yield the first remote sensing-based estimate of integrated global POC stock of 3.02 Pg C over the top 1,000 m, with an uncertainty of 0.69 Pg C. Of this total stock, approximately 1.02 Pg was located above the reference isolume where [POC] begins to attenuate.

Runoff from rapidly melting mountain glaciers is a dominant source of riverine organic carbon in many high-latitude and high-elevation regions. Glacier dissolved organic carbon is highly bioavailable, and its composition likely reflects internal (e.g., autotrophic production) and external (i.e., atmospheric deposition) sources. However, the balance of these sources across Earth's glaciers is poorly understood, despite implications for the mineralization and assimilation of glacier organic carbon within recipient ecosystems. We assessed the molecular-level composition of dissolved organic matter from 136 mountain glacier outflows from 11 regions covering six continents using ultrahigh resolution 21 T mass spectrometry. We found substantial diversity in organic matter composition with coherent and predictable (80% accuracy) regional patterns. Employing stable and radiocarbon isotopic analyses, we demonstrate that these patterns are inherently linked to atmospheric deposition and in situ production. In remote regions like Greenland and New Zealand, the glacier organic matter pool appears to be dominated by in situ production. However, downwind of industrial centers (e.g., Alaska and Nepal), fossil fuel combustion byproducts likely underpin organic matter composition, resulting in older and more aromatic material being exported downstream. These findings highlight that the glacier carbon cycle is spatially distinct, with ramifications for predicting the dynamics and fate of glacier organic carbon concurrent with continued retreat and anthropogenic perturbation.

Wetlands are the largest natural source of global atmospheric methane (CH₄). Despite advances to our understanding of changes in temperature and precipitation extremes, their impacts on carbon-rich ecosystems such as wetlands, remain significantly understudied. Here, we quantify the impacts of extreme temperature, precipitation, and dry events on wetland CH₄ dynamics by investigating the effects of both compound and discrete extreme-events. We use long-term climate data to identify extreme-events and 45 eddy covariance sites data sets sourced from the FLUXNET-CH₄ database and Ameriflux project to assess impacts on wetland CH₄ emissions. These findings reveal that compound hot + dry extreme-events lead to large increases in daily CH₄ emissions. However, per event, discrete dry-only extreme-events cause the largest total decrease in CH₄ emissions, due to their long duration. Despite dry-only extreme-events leading to an overall reduction in CH₄ emissions, enhanced fluxes are often observed for the first days of dry-only extreme-events. These effects differ depending on wetland type, where marsh sites tend to be sensitive to most types of extreme-events. Lagged impacts are significant for at least the 12 months following several types of extreme-events. These findings have implications for understanding how extreme-event impacts may evolve in the context of climate change, where changes in the frequency and intensity of temperature and precipitation extreme-events are already observed. With increasing occurrences of enhanced CH₄ fluxes in response to hot-only extreme-events and hot + wet extreme-events and fewer occurrences of reduced CH₄ fluxes during cold-only extreme-events, the impact of wetland CH₄ emissions on climate warming may be increasing.

Global marine anthropogenic CO₂ inventories have traditionally emphasized the North Atlantic's role in the carbon cycle, while Southern hemisphere processes are less understood. The South Subtropical Convergence (SSTC) in the South Atlantic, a juncture of distinct nutrient-rich waters, offers a valuable study area for discerning the potential impacts of climate change on the ocean's biological carbon pump (C_soft). Using discrete observations from GLODAPv2.2022 and BGC-Argo at 40°S in the Atlantic Ocean from 1972 to 2023, an increase in dissolved inorganic carbon (DIC) of +1.44 ± 0.11 μmol kg⁻¹ yr⁻¹ in surface waters was observed. While anthropogenic CO₂ played a role, variations in the contribution of C_soft were observed. Discrepancies emerged in assessing C_soft based on the tracers employed: when using AOU, C_soft(AOU) recorded an increase of +0.20 ± 0.03 μmol kg⁻¹ yr⁻¹, while using nitrate as the reference, C_soft(NO3) displayed an increase of +0.85 ± 0.07 μmol kg⁻¹ yr⁻¹. Key processes such as water mass composition shifts, changes in oxygenation, remineralization in the Southern Ocean, and the challenges they pose in accurately representing the evolving C_soft are discussed. These findings highlight that while global studies primarily attribute DIC increase to anthropogenic CO₂, observations at 40°S reveal an intensified biological carbon pump, showing that regional DIC changes are more complex than previously thought and emphasizing the need for better parameterizations to compute the BCP in the marine carbon budget.

Marine mesozooplankton play an important role for marine ecosystem functioning and global biogeochemical cycles. Their size structure, varying spatially and temporally, heavily impacts biogeochemical processes and ecosystem services. Mesozooplankton exhibit size changes throughout their life cycle, affecting metabolic rates and functional traits. Despite this variability, many models oversimplify mesozooplankton as a single, unchanging size class, potentially biasing carbon flux estimates. Here, we include mesozooplankton ontogenetic growth and reproduction into a 3-dimensional global ocean biogeochemical model, PISCES-MOG, and investigate the subsequent effects on simulated mesozooplankton phenology, plankton distribution, and organic carbon export. Utilizing an ensemble of statistical predictive models calibrated with a global set of observations, we generated monthly climatologies of mesozooplankton biomass to evaluate the simulations of PISCES-MOG. Our analyses reveal that the model and observation-based biomass distributions are consistent ($��r_{\mathrm{pearson}}�$ = 0.40, total epipelagic biomass: 137 TgC from observations vs. 232 TgC in the model), with similar seasonality (later bloom as latitude increases poleward). Including ontogenetic growth in the model induced cohort dynamics and variable seasonal dynamics across mesozooplankton size classes and altered the relative contribution of carbon cycling pathways. Younger and smaller mesozooplankton transitioned to microzooplankton in PISCES-MOG, resulting in a change in particle size distribution, characterized by a decrease in large particulate organic carbon (POC) and an increase in small POC generation. Consequently, carbon export from the surface was reduced by 10%. This study underscores the importance of accounting for ontogenetic growth and reproduction in models, highlighting the interconnectedness between mesozooplankton size, phenology, and their effects on marine carbon cycling.

This paper aims to study the changes in the Indian Ocean seawater pH in response to the changes in sea-surface temperature, sea-surface salinity, dissolved inorganic carbon (DIC), and total alkalinity (ALK) over the period 1980–2019 and its driving mechanisms using a high-resolution regional model outputs. The analysis indicates that the rate of change of declining pH in the Arabian Sea (AS), the Bay of Bengal (BoB), and the Equatorial Indian Ocean (EIO) is −0.014 $��\pm �$ 0.002, −0.014 $��\pm �$ 0.001, and −0.015 $��\pm �$ 0.001 unit dec⁻¹, respectively. Both in AS and BoB (EIO), the highest (lowest) decadal DIC trend is found during 2000–2009. The surface acidification rate has accelerated throughout the IO region during 2010–2019 compared to the previous decades. Further, our analysis indicates that El Ninõ and positive Indian Ocean Dipole events lead to an enhancement of the Indian Ocean acidification. The increasing anthropogenic CO₂ uptake by the ocean dominantly controls 80% (94.5% and 85.7%) of the net pH trend (1980–2019) in AS (BoB and EIO), whereas ocean warming controls 14.4% (13.4% and 7.0%) of pH trends in AS (BoB and EIO). The changes in ALK contribute to enhancing the pH trend of AS by 5.0%. ALK dominates after DIC in the EIO and, similar to the AS, contributes to increasing the negative pH trend by 10.7%. In contrast, it has a buffering effect in the BoB, suppressing the pH trend by −5.4%.

Continental margins receive, process and sequester most of the terrestrial organic carbon (terrOC) released into the ocean. In the Arctic, increasing fluvial discharge and collapsing permafrost are expected to enhance terrOC release and degradation, leading to ocean acidification and translocated CO₂ release to the atmosphere. However, the processes controlling terrOC transport beyond the continental shelf, and the amount of terrOC that reaches the slope and the rise are poorly described. Here we study terrOC transport to the Laptev Sea continental slope and rise by probing surface sediments with dual-isotope (δ¹³C/Δ¹⁴C) source apportionment, degradation-diagnostic terrestrial biomarkers (n-alkanes, n-alkanoic acids, lignin phenols) and ²¹⁰Pb_xs-based mass accumulation rates (MAR). The MAR-terrOC (g m⁻² yr⁻¹) decrease from 14.7 ± 12.2 on the shelf, to 7.0 ± 5.8 over the slope, to 2.3 ± 0.3 for the rise. Scaling this to the respective regimes yields that 80% of the terrOC accumulates on the shelf, while 11% and 9% of the accumulation occurs in slope and rise sediments, respectively. TerrOC remineralization is evidenced by biomarker degradation proxies (CPI of n-alkanes and 3,5Bd/V) indicating 40% and 60% more terrOC degradation from slope to rise, consistent with a decline in terrOC concentrations by 57%. TerrOC degradation only partially explains this decline. An updated Laptev Sea terrOC budget suggests that sediment transport dynamics such as turbidity currents may drive terrOC shelf-basin export, contributing to the observed accumulation pattern. This study quantitatively demonstrates that Arctic shelf seas are key receptor systems for remobilized terrOC, emphasizing their importance in the carbon cycle of the rapidly changing Arctic.

Aerosols continuously transport trace elements (TEs) across long distances to the ocean, fueling marine primary production and affecting global carbon cycles. Given the multiple sources and complex transport mechanisms, field investigations of aerosol TEs on a global scale are significant for understanding their role in marine biogeochemical cycles. Here, aerosol samples were collected along a 50,000-km route covering subtropical Northwest Pacific (NWP) marginal seas, Indian Ocean, Southern Ocean, Drake Passage, and South Pacific. Samples were analyzed for the concentrations of Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Cd, Tl, and Pb. Aerosol TEs were distributed heterogeneously, with significantly lower concentrations over remote oceans compared to coastal seas. Meanwhile, TE concentrations were generally high in the Indian Ocean, moderate in the Southern Ocean, and low in the South Pacific. Cr, Ni, Cu, Zn, Cd, As, and Pb were widely enriched, primarily originating from anthropogenic sources, while Al, Ti, V, Mn, Fe, and Co were mainly from crustal sources in remote oceans. Moreover, specific sources of TEs were clarified, for example, Cr and Ni were mainly from vehicle emissions. The estimated bulk TE deposition fluxes also varied spatially. For instance, the greatest deposition of Fe occurs in the NWP marginal sea, followed by the Drake Passage, Indian Ocean Sector of Southern Ocean, Pacific Sector of Southern Ocean, and South Pacific. This study contributes to a deeper understanding of the complex dynamics of aerosol TEs in the global ocean, providing valuable information for future studies and policy making regarding climate change.