Global Biogeochemical Cycles最新文献

Changes in precipitation and land use influence carbon (C), nitrogen (N), and phosphorus (P) exports from land to receiving waters. However, how these drivers differentially alter elemental inputs and impact subsequent ecosystem stoichiometry over time remains poorly understood. Here, we quantified long-term (1979–2020) trends in C, N, and P exports at three sites along the mainstem of a north temperate river in Québec, Canada, that successively drains forested, urban, and more agriculturally impacted land-use areas. Riverine N and to a lesser degree C exports tended to increase over time, with major inter-annual variation largely resolved by changes in precipitation. Historical increases in net anthropogenic N inputs on land (NANI) also explained increases in riverine N exports, with about 35% of NANI reaching the river annually. Despite higher Net anthropogenic P inputs, NAPI, over time, P exports tended to decrease at all riverine sites. This decrease in P at the forested site was more gradual, whereas a precipitous drop was observed at the downstream urban site following legislated P removal in municipal wastewater. Changes in historical ecosystem stoichiometry reflected the differential elemental exports due to natural and anthropogenic drivers and ranged from 174: 23: 1 to 547: 76: 1 over the years. Our work shows how C, N, and P have responded to different drivers in the same catchment over the last four decades, and how their differential riverine exports have influenced ecosystem stoichiometry.

Copper (Cu) is an important micronutrient for marine organisms, which can also be toxic at elevated concentrations. Here, we present a new model of global ocean Cu biogeochemical cycling, constrained by GEOTRACES observations, with key processes including sources from rivers, dust, and sediments, biological uptake and remineralization of Cu, reversible scavenging of Cu onto sinking particles, conversion of Cu between labile and inert species, and ocean circulation. In order for the model to match observations, in particular the relatively small increase in Cu concentrations along the global “conveyor belt,” we find it is necessary to include significant external sources of Cu with a magnitude of roughly 1.3 Gmol yr⁻¹, having a relatively stronger impact on the Atlantic Ocean, though the relative contributions of river, dust, and sediment sources are poorly constrained. The observed nearly linear increase in Cu concentrations with depth requires a strong benthic source of Cu, which includes the sedimentary release of Cu that was reversibly scavenged from the water column. The processes controlling Cu cycling in the Arctic Ocean appear to be unique, requiring both relatively high Cu concentrations in Arctic rivers and reduced scavenging in the Arctic. Observed partitioning of Cu between labile and inert phases is reproduced in the model by the slow conversion of labile Cu to inert in the whole water column with a half-life of ∼250 years, and the photodegradation of inert Cu to labile in the surface ocean with a minimum half-life of ∼2 years at the equator.

Coral reefs are subject to degradation by multiple environmental stressors which are predicted to intensify. Stress can alter ecosystem composition, with shifts from hard coral to macroalgae dominated reefs often accompanied by an increase in soft corals and sponges. Such changes may alter net ecosystem metabolism and biogeochemistry by shifting the balance between photosynthesis, respiration, calcification and dissolution. We deployed high temporal resolution pH and dissolved oxygen (DO) sensors at four Caribbean reef sites with varying covers of hard and soft corals, sponges and macroalgae. The resultant data indicated that the strength of the “metabolic pulse”, specifically the co-variation in daily pH and DO oscillations, was driven by the net balance of light -dependent and -independent metabolism. pH and DO were positively correlated over the diel cycle at coral dominated sites, suggesting that photosynthesis and respiration were the major controlling processes, and further indicated by agreement with a simple production:respiration model. Whereas, at a site with high macroalgal cover, pH and DO decoupling was observed during daylight hours. This indicates that an unidentified light-driven process altered the expected pH:DO relationship. We hypothesize that this could be mediated by the higher levels of macroalgae, which either stimulated bacterial-mediated carbonate dissolution via the production and release of allelopathic compounds or retained oxygen, evolved during photosynthesis, in the gaseous form in seawater (ebullition). Our work demonstrates that high resolution monitoring of pH and DO provides insight into coral reef biogeochemical functioning and can be key for understanding long-term changes in coral reef metabolism.

As ocean Carbon Dioxide Removal techniques are being considered, it is critical that they be evaluated against our scientific understanding of the global biological carbon pump. In a recent paper Nowicki et al. (2022, https://doi.org/10.1029/2021GB007083) provide an innovative and comprehensive breakdown of the different mechanistic pathways of carbon sequestration through the present-day biological pump but then speculate that “These results suggest that ocean carbon storage will weaken as the oceans stratify and the subtropical gyres expand due to anthropogenic climate change.” Essentially, the authors combine their steady state result that oligotrophic subtropical gyres have lower residence times than other areas with the climate change result of these areas increasing under climate warming and extrapolate—assuming “all else is equal”—that the overall ocean will suffer a reduction in carbon sequestration efficiency. Expressing global changes in carbon sequestered by the ocean's biological pump as the summation of local changes in the sequestered carbon, timescale of return to the surface, and biogeographical area, I discuss how all three terms are tightly coupled, and summarize decades of climate change modeling consistently indicating that the global scale physical sequestration response is an increase - in opposition of what one would infer from changes in subtropical area alone.

The gravitational sinking of organic debris from ocean ecosystems is a dominant mechanism of the biological carbon pump (BCP) that regulates the global climate. The fraction of primary production exported downward, the e-ratio, is an important but poorly constrained BCP metric. In mid- and high-latitude oceans, seasonal and local variations of sinking particle fluxes strongly modulate the e-ratio. These locally specific e-ratio variations and their ecological foundations are here encapsulated in the term “export systems” (ES). ES have been partly characterized for a few ocean locations but remain largely ignored over most of the ocean surface. Here, in a fully conceptual approach and with the primary aim to understand rather than to estimate ocean carbon export, we combine biogeochemical (BGC) modeling with satellite observations to map ES at fine spatio-temporal scales. We identify four plausible ES with distinct e-ratio seasonalities across mid- and high-latitude oceans. The ES map confirms the outlines of traditional BGC provinces and unveils new boundaries indicating where (and how) the annual relationship between carbon export and production changes markedly. At six sites where ES features can be partially inferred from in situ data, we test our approach and propose key ecological processes driving carbon export. In the light of our findings, a re-examination of 1,841 field-based e-ratios could challenge the conventional wisdom that e-ratios change strongly with latitude, suggesting a possible seasonal artifact caused by the timing of observations. By deciphering carbon export mechanistically, our conceptual ES map provides timely directions to emergent ocean robotic explorations of the BCP.

Iron (Fe) is a limiting micronutrient in many marine ecosystems. The lack of sufficient Fe can stunt marine productivity and limit carbon sequestration from the atmosphere to the ocean. Recent studies suggest that biomass burning represents an important Fe source to the marine environment because pyrogenic particles have enhanced solubility after atmospheric processing. We examined foliage representative of four distinct biomes subject to frequent burning events, including boreal/temporal forests, humid tropical, arid tropical, and grassland. We burned these samples in the absence of soil to isolate the Fe from the fine particle (PM_2.5) fraction that is derived directly from the burning foliage. We find that <1.5% of the Fe in plant matter is aerosolized throughout the burn in the fine fraction. We estimate that between 2% and 9% of the Fe released from biomass burning can be attributed to the fine fraction of the foliage itself, and <50% from the foliage overall. Most of the Fe aerosolized during biomass burning is accounted for by soil-suspended particles.

The Editors of the Global Biogeochemical Cycles express their appreciation to those who served as peer reviewers for the journal in 2022.

Methane (CH₄) emissions from tropical wetlands represent half of the global wetland emissions, but uncertainties remain concerning the extent of tropical methane sources. One limitation is to conceive tropical wetlands as a single ecosystem, especially in global land surface models. We estimate CH₄ emissions and assess their environmental and anthropogenic drivers. We created a data set with 101 studies involving 328-point measurements, classified the sites into four wetland types, and included relevant biological and environmental information. We estimate the global CH₄ emission rate from tropical wetlands as 35 (5–160) mg CH₄ m⁻² d⁻¹ (median, first and third quartile) and an annual global rate of 94 (56, 158) Tg y⁻¹. Fluxes differed among the wetland types, but except for anthropogenic factors, significant environmental drivers at the global scale could not be quantitatively identified because of high flux variability, even within wetland types. Coastal wetlands generate median emissions of 12 (5–23) Tg y⁻¹. Inland deep-water wetlands emit 53 (32–114) Tg y⁻¹, with highly variable areal extent. Emissions from inland shallow-water wetlands are 52 (33–78) Tg y⁻¹ with variation due to seasonal changes in water table level. Human-made wetlands emit 17 (10⁻⁴) Tg y⁻¹. Pollution and N inputs from agriculture are significant anthropogenic drivers of emissions from tropical wetlands. Specific drivers of change need to be considered according to wetland type when estimating global emissions as well as their specific vulnerability to global change. Additionally, these differences should be considered when implementing wetland management practices aimed at decreasing methane emissions.

Coastal oceans, the transition zones between terrestrial and oceanic systems, are susceptible to anthropogenic mercury (Hg) inputs and are regarded as critical dynamic interfaces of the global Hg cycle. However, the extent to which coastal oceans are accountable for sequestering Hg remains largely unknown owing to the lack of data on high-resolution Hg accumulation in marine sediments. Synthesizing the results of this study (eight cores and 212 surface sediments) and the literature (three cores and 149 surface sediments), we provide a quantitative evaluation of the biogeochemical cycle of sedimentary Hg in the East China Marginal Seas (ECMS), including the response of the coastal marine sediments to anthropogenic disturbance as well as both human-derived and natural Hg burial fluxes. We find a linear increase in Hg accumulation since the 1950s (2.0 ± 2.5% yr⁻¹) and a decline in Hg accumulation between 2010 and 2016. Modern burial fluxes of total and anthropogenic Hg in the ECMS (covering ∼4.8 × 10⁵ km² of sea surface) were estimated to be 89.1 ± 48.3 and 35.9 ± 33.1 Mg yr⁻¹, respectively. Using a compilation of 688 surface sediments and 131 sediment cores (819 samples in total) distributed globally in coastal oceans, we estimate that approximately 1,590 (range: 1,190–2,760) Mg yr⁻¹ (Method 1) and 540 (range: 310–960) Mg yr⁻¹ (Method 2) Hg are accumulated in coastal ocean regions. Our findings suggest that coastal oceans are likely the largest global marine sinks for Hg and play a dominant role in regulating the oceanic Hg cycle and budgets.

The production by microorganisms of nitrous oxide (N₂O), a trace gas contributing to global warming and stratospheric ozone depletion, is enhanced around the oceanic oxygen minimum zones (OMZs). The production constitutes an important source of atmospheric N₂O. Although an OMZ is found in the northern part of the eastern Indian Ocean, the Bay of Bengal (BoB), two earlier studies conducted during the later phase of winter monsoon (February) and spring intermonsoon (March–April) found quite different magnitudes of N₂O accumulation. This study found two- to ten-fold greater accumulation of N₂O during the autumn intermonsoon (November) than for other seasons described in earlier reports. The maximum N₂O concentration (136 nmol kg⁻¹ at 16°N, 88°E) is comparable to those observed around the OMZ in the Arabian Sea or eastern tropical Pacific. Isotopic signatures suggest that bacterial denitrification and archeal nitrification play important roles in N₂O production, but earlier studies using nitrate or nitrite analysis did not confirm denitrification in the BoB. Large seasonal variation of N₂O implicates the BoB as an important N₂O source, similar to the Arabian Sea and eastern tropical Pacific, if the accumulated N₂O is emitted to the atmosphere during the subsequent monsoon season.