Global Biogeochemical Cycles最新文献

Distributions of the natural radionuclide ²¹⁰Po and its grandparent ²¹⁰Pb along the GP15 Pacific Meridional Transect provide information on scavenging rates of reactive chemical species throughout the water column and fluxes of particulate organic carbon (POC) from the primary production zone (PPZ). ²¹⁰Pb is in excess of its grandparent ²²⁶Ra in the upper 400–700 m due to the atmospheric flux of ²¹⁰Pb. Mid-water ²¹⁰Pb/²²⁶Ra activity ratios are close to radioactive equilibrium (1.0) north of ∼20°N, indicating slow scavenging, but deficiencies at stations near and south of the equator suggest more rapid scavenging associated with a “particle veil” located at the equator and hydrothermal processes at the East Pacific Rise. Scavenging of ²¹⁰Pb and ²¹⁰Po is evident in the bottom 500–1,000 m at most stations due to enhanced removal in the nepheloid layer. Deficits in the PPZ of ²¹⁰Po (relative to ²¹⁰Pb) and ²¹⁰Pb (relative to ²²⁶Ra decay and the ²¹⁰Pb atmospheric flux), together with POC concentrations and particulate ²¹⁰Po and ²¹⁰Pb activities, are used to calculate export fluxes of POC from the PPZ. ²¹⁰Po-derived POC fluxes on large (>51 μm) particles range from 15.5 ± 1.3 mmol C/m²/d to 1.5 ± 0.2 mmol C/m²/d and are highest in the Subarctic North Pacific; ²¹⁰Pb-derived fluxes range from 6.7 ± 1.8 mmol C/m²/d to 0.2 ± 0.1 mmol C/m²/d. Both ²¹⁰Po- and ²¹⁰Pb-derived POC fluxes are greater than those calculated using the ²³⁴Th proxy, possibly due to different integration times of the radionuclides, considering their different radioactive mean-lives and scavenging mean residence times.

Soil organic carbon (SOC) stock exhibits substantial variability across spatial scales and depths. Drivers of such variability may be scale- and depth-dependent, but have been rarely systematically investigated. Assessing SOC measurements of 113,013 soil profiles worldwide, we show that climate, encompassing mean modern- and paleo-climate and climate extremes, is the predominant determinant across scales (from 50 km to the globe) and depths, explaining 34%–62% of the spatial variability of SOC stocks depending on the spatial scale and soil depth layer assessed. On finer scales (50–100 km), soil properties and mean modern- and paleo-climate are dominant in all soil depth layers. At broader scales (>100 km), the significance of climate extremes intensifies, alone explaining 27%–32% of the spatial variability of SOC stocks. Furthermore, we find nonlinear relationships of SOC stocks with most factors, while the relationship with the same factor is distinct across scales and depths. These results reinforce climate, particularly extremes, as the primary driving force of whole-soil carbon distribution across the globe, emphasizing the need to factor extremes into carbon management strategies.

Increasing anthropogenic CO₂ emissions to the atmosphere are partially sequestered into the global oceans through the air-sea exchange of CO₂ and its subsequent movement to depth, commonly referred to as the global ocean carbon sink. Quantifying this ocean carbon sink provides a key component for closing the global carbon budget, which is used to inform and guide policy decisions. These estimates are typically accompanied by an uncertainty budget built by selecting what are perceived as critical uncertainty components based on selective experimentation. However, there is a growing realization that these budgets are incomplete and may be underestimated, which limits their power as a constraint within global budgets. In this study, we present a methodology for quantifying spatially and temporally varying uncertainties in the air-sea CO₂ flux calculations for the fCO₂-product based assessments that allows an exhaustive assessment of all known sources of uncertainties, including decorrelation length scales between gridded measurements, and the approach follows standard uncertainty propagation methodologies. The resulting standard uncertainties are higher than previously suggested budgets, but the component contributions are largely consistent with previous work. The uncertainties presented in this study identify how the significance and importance of key components change in space and time. For an exemplar method (the UExP-FNN-U method), the work identifies that we can currently estimate the annual ocean carbon sink to a precision of ±0.70 Pg C yr⁻¹ (1σ uncertainty). Because this method has been built on established uncertainty propagation and approaches, it appears that applicable to all fCO₂-product assessments of the ocean carbon sink.

Rice cultivation produces methane (CH₄) due to anaerobic conditions induced by flood irrigation, significantly contributing to global warming. While most studies use national emission factors (EFs), our study synthesized 726 published measurements across India (the second largest methane emitter after China) to develop district-level water regime-specific EFs for estimating district-scale emissions and warming potential. CH₄ emissions from Indian rice fields increased from 3.7 (3.4–4.1) Tg to 4.8 (4.4–5.3) Tg during 1966–2017, driven by rice area and water-regime variations. Meanwhile, district-level emissions increased by ∼930%, influenced by management practices such as animal manure, fertilizer application, and water input, accurately reflecting regional variations compared to previous estimates. Employing a novel muti-output random forest mitigation model (R² ∼ 0.9), we found that a 25% warming reduction at the district-level requires curtailing animal manure, nitrogen fertilizer, and water input by 8.5%, 12.9%, and 10.9%, respectively. These curtailments nearly double for a 50% mitigation scenario. Comparing our emissions with previous bottom-up studies (used as inputs in global climate models) revealed discrepancies in prior national figures. With top-down estimates, our emissions correlated positively, suggesting higher reliability. Including our new regionally validated data in global climate models may provide more accurate climate projections at the Indian and global scales.

Large stocks of soil carbon (C) and nitrogen (N) in northern permafrost soils are vulnerable to remobilization under climate change. However, there are large uncertainties in present-day greenhouse gas (GHG) budgets. We compare bottom-up (data-driven upscaling and process-based models) and top-down (atmospheric inversion models) budgets of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) as well as lateral fluxes of C and N across the region over 2000–2020. Bottom-up approaches estimate higher land-to-atmosphere fluxes for all GHGs. Both bottom-up and top-down approaches show a sink of CO₂ in natural ecosystems (bottom-up: −29 (−709, 455), top-down: −587 (−862, −312) Tg CO₂-C yr⁻¹) and sources of CH₄ (bottom-up: 38 (22, 53), top-down: 15 (11, 18) Tg CH₄-C yr⁻¹) and N₂O (bottom-up: 0.7 (0.1, 1.3), top-down: 0.09 (−0.19, 0.37) Tg N₂O-N yr⁻¹). The combined global warming potential of all three gases (GWP-100) cannot be distinguished from neutral. Over shorter timescales (GWP-20), the region is a net GHG source because CH₄ dominates the total forcing. The net CO₂ sink in Boreal forests and wetlands is largely offset by fires and inland water CO₂ emissions as well as CH₄ emissions from wetlands and inland waters, with a smaller contribution from N₂O emissions. Priorities for future research include the representation of inland waters in process-based models and the compilation of process-model ensembles for CH₄ and N₂O. Discrepancies between bottom-up and top-down methods call for analyses of how prior flux ensembles impact inversion budgets, more and well-distributed in situ GHG measurements and improved resolution in upscaling techniques.

The abrupt warming events punctuating the Termination 1 (about 11.7–18 ka Before Present, BP) were marked by sharp rises in the concentration of atmospheric methane (CH₄). The role of permafrost organic carbon (OC) in these rises is still debated, with studies based on top-down measurements of radiocarbon (¹⁴C) content of CH₄ trapped in ice cores suggesting minimum contributions from old and strongly ¹⁴C-depleted permafrost OC. However, organic matter from permafrost can exhibit a continuum of ¹⁴C ages (contemporaneous to >50 ky). Here, we investigate the large-scale permafrost remobilization at the Younger Dryas-Preboreal transition (ca. 11.6 ka BP) using the sedimentary record deposited at the Lena River paleo-outlet (Arctic Ocean) to reflect permafrost destabilization in this vast drainage basin. Terrestrial OC was isolated from sediments and characterized geochemically measuring δ¹³C, Δ¹⁴C, and lignin phenol molecular fossils. Results indicate massive remobilization of relatively young (about 2,600 years) permafrost OC from inland Siberia after abrupt warming triggered severe active layer deepening. Methane emissions from this young fraction of permafrost OC contributed to the deglacial CH₄ rise. This study stresses that underestimating permafrost complexities may affect our comprehension of the deglacial permafrost OC-climate feedback and helps understand how modern permafrost systems may react to rapid warming events, including enhanced CH₄ emissions that would amplify anthropogenic climate change.