Global Biogeochemical Cycles最新文献

Observations of dissolved cadmium (dCd) and phosphate (PO₄) suggest an unexplained loss of dCd to the particulate phase in tropical oxyclines. Here, we compile existing observations of particulate Cd and phosphorus (P), and present new data from the US GEOTRACES GP15 Pacific Meridional Transect to examine this phenomenon from a particulate Cd perspective. We use a simple algorithm to reproduce station depth profiles of particulate Cd and P via regeneration and possible subsurface accumulation. Our examination of regeneration reveals decoupling of particulate Cd and P driven by variable partitioning between two particulate pools with differing labilities. Further, we identify evidence for subsurface particulate Cd accumulation at 31 stations. Subsurface particulate Cd accumulation occurs most consistently in the mesopelagic tropical Pacific but can be found in all examined ocean basins. This accumulation is not well-correlated with dissolved oxygen or particulate sulfide concentration. Instead, we observe that particulate Cd accumulation occurs in regions where the concentration of dCd is relatively high compared to dissolved zinc (dZn) and speculate that it is the result of enhanced dCd biological uptake in response to the subsurface micronutrient balance.

Discharge of calved ice, runoff and mixing driven by subglacial discharge plumes likely have consequences for marine biogeochemistry in Disko Bay, which hosts the largest glacier in the northern hemisphere, Sermeq Kujalleq. Glacier retreat and increasing runoff may impact the marine silica cycle because glaciers deliver elevated concentrations of dissolved silica (dSi) compared to other macronutrients. However, the annual flux of dSi delivered to the ocean from the Greenland Ice Sheet is poorly constrained because of difficulties distinguishing the overlapping influence of different dSi sources. Here we constrain silica dynamics around Disko Bay, including the Ilulissat Icefjord and four other regions receiving glacier runoff with contrasting levels of productivity and turbidity. Both dissolved silica and Si* ([dSi]-[NO_x ^-]) concentrations indicated conservative dynamics in two fjords with runoff from land-terminating glaciers, consistent with the results of mixing experiments. In three fjords with marine-terminating glaciers, macronutrient-salinity distributions were strongly affected by entrainment of nutrients in subglacial discharge plumes. Entrainment of dSi from saline waters explained 93 ± 51% of the dSi enrichment in the outflowing plume from Ilulissat Icefjord, whereas the direct contribution of freshwater to dSi in the plume was likely 0%-3%. Whilst not distinguished herein, other minor regional dSi sources include icebergs and dissolution of amorphous silica (aSi) in either pelagic or benthic environments. Our results suggest that runoff around Greenland is supplemented as a dSi source by minor fluxes of 0.25 ± 0.67 Gmol yr^-1 dSi from icebergs and ∼1.9 Gmol year^-1 from pelagic aSi dissolution.

This case study of Kongsfjorden, western coastal Svalbard, provides insights on how freshwater runoff from marine- and land-terminating glaciers influences the biogeochemical cycles and distribution patterns of carbon, nutrients, and trace elements in an Arctic fjord system. We collected samples from the water column at stations along the fjord axis and proglacial river catchments, and analyzed concentrations of dissolved trace elements, together with dissolved nutrients, as well as alkalinity and dissolved inorganic carbon. Statistical tools were applied to identify and quantify biogeochemical processes within the fjord that govern the constituent distributions. Our results suggest that the glacier type affects nutrient availability and, therefore, primary production. Glacial discharge from both marine-terminating glaciers and riverine discharge from land-terminating glaciers are important sources of dissolved trace elements (dAl, dMn, dCo, dNi, dCu, and dPb) that are involved in biological and scavenging processes within marine systems. We identified benthic fluxes across the sediment-water interface to supply fjord waters with silicate, dFe, dCu, and dZn. Our data show that intensive carbonate weathering in proglacial catchments supplies fjord waters with additional dissolved carbonates and, therefore, attenuates reduced buffering capacities caused by glacial runoff. Our study provides valuable insight into biogeochemical processes and carbon cycling within a climate-sensitive, high-latitude fjord region, which may help predict Arctic ecosystem changes in the future.

Heat and drought events are increasing in frequency and intensity, posing significant risks to natural and agricultural ecosystems with uncertain effects on the net ecosystem CO₂ exchange (NEE). The current Vegetation Photosynthesis and Respiration Model (VPRM) was adjusted to include soil moisture impacts on the gross ecosystem exchange (GEE) and respiration (R _ECO) fluxes to assess the temporal variability of NEE over south-western Europe for 2001-2022. Warming temperatures lengthen growing seasons, causing an increase in GEE, which is mostly compensated by a similar increment in R _ECO. As a result, there is a modest increase in the net carbon sink of 0.69 gC m^-2 yr^-1 but with high spatial and annual variability. The heatwave of 2022 reduced net carbon uptake by 91.7 TgC, a 26.4% decrease from the mean. The interannual variability of NEE is more influenced by drought in temperate humid regions than in Mediterranean semi-arid regions. These results emphasize the vulnerability of the net carbon sink as drying trends could revert the NEE trends, as it is happening for croplands in the French Central Massif.

Climate change is expected to alter the input of nitrogen (N) sources in the Eastern Canadian Arctic Archipelago and Baffin Bay due to increased discharge from glacial meltwater and permafrost thaw. Since dissolved inorganic N is generally depleted in surface waters, dissolved organic N (DON) could represent a significant N source fueling phytoplankton activity in Arctic ecosystems. Yet, few DON data for this region exist. We measured concentrations and stable isotope ratios of DON (δ¹⁵N) and nitrate (NO₃⁻; δ¹⁵N and δ¹⁸O) to investigate the sources and cycling of dissolved nitrogen in regional rivers and marine samples collected in the Eastern Canadian Arctic Archipelago and Baffin Bay during the summer of 2019. The isotopic signatures of NO₃⁻ in rivers could be reproduced in a steady state isotopic model by invoking mixing between atmospheric NO₃⁻ and nitrified ammonium as well as NO₃⁻ assimilation by phytoplankton. DON concentrations were low in most rivers (≤4.9 μmol N L⁻¹), whereas the concentrations (0.54–12 μmol N L⁻¹) and δ¹⁵N of DON (−0.71–9.6‰) at the sea surface were variable among stations, suggesting dynamic cycling and/or distinctive sources. In two regions with high chlorophyll-a, DON concentrations were inversely correlated with chlorophyll-a and the δ¹⁵N of DON, suggesting net DON consumption in localized phytoplankton blooms. We derived an isotope effect of 6.9‰ for DON consumption. Our data helps establish a baseline to assess future changes in the nutrient regime for this climate-sensitive region.

Reservoirs influence the global climate by exchanging greenhouse gases (GHGs) of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) with the atmosphere. Few studies, however, quantify emissions of all three GHGs from reservoirs, particularly in permafrost-affected mountain regions where ecosystems are highly vulnerable to climate change. This study presents three-year direct measurements of CO₂, CH₄, and N₂O concentrations and fluxes upstream, within, and downstream from two reservoirs draining permafrost catchments on the Qinghai-Tibet Plateau, including periods of reservoir drawdown. Comparing GHG fluxes across space and time exhibits a general pattern of lower fluxes at the two reservoirs relative to up- and downstream channels. Ebullitive fluxes contributed to 36.7% and 9.4% of total CH₄ and N₂O fluxes, respectively. CO₂ has no response to drawdown, but CH₄ and N₂O display synchronous drawdown-associated increase within the reservoir, constituting 57.5% and 32.8% of the annual reservoir emissions in just 2 months, respectively. Riverine emissions from up- and downstream channels accounted for an outsized fraction (55.5% for CH₄, 17.3% for CO₂ and 16.5% for N₂O) of the system-wide GHG budget. Compared with global reservoirs, the two reservoirs have high CO₂ and N₂O but low CH₄ fluxes in CO₂ equivalents. Upscaling shows that the two reservoirs emit the same magnitude of carbon as thermokarst lakes, and four times higher N₂O than Finnish lakes on an areal basis. This article shows that alpine reservoirs draining permafrost catchments are unrecognized atmospheric sources in current reservoir GHG inventories, but also emphasizes the importance of system-wide emissions when assessing total GHG evasion from reservoir systems.

The present study explored the dynamics of total dissolved Cobalt (dCo) in the Indian Ocean, revealing different distribution patterns in the different sub-basins, nutrient-type in the southern sector, hybrid-type in the Arabian Sea to scavenged-type in the Bay of Bengal (BoB). The dCo in the coastal water of the Arabian Sea displays elevated (0.12–0.13 nmol L⁻¹) abundance and diminishes gradually toward the central Arabian Sea. Similarly, in the BoB, dCo concentrations are notably higher in the northern region (0.11 nmol L⁻¹) and gradually decrease toward the south (0.03 nmol L⁻¹ at 5°N). The Arabian Sea with higher biological uptake and remineralization in the oxycline supports a higher abundance of dCo in the intermediate oxygen minimum zone (OMZ), much a like the OMZs of the Atlantic and the Pacific Oceans. The influence of the phytoplankton community shift and uptake on the dCo distribution in the Indian Ocean could be inferred from the association between Co and phosphate in the photic waters. Our observation demonstrates a scavenging type dCo profile in the BoB due to its higher riverine as well as dust inputs in addition to its supply from continental shelf sediments. Such a higher concentration of dCo in the surface waters of the northern BoB masks the dCo signal associated with nitrite maxima. dCo gets removed by its scavenging with Mn oxides at deeper depths, as reflected by higher particulate Co in the BoB. Subduction fluids contribute significantly to the dCo inventory of the deep water in the Indian Ocean near the Java-Sumatra subduction zone.

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.