The Southern Ocean is the largest region in which iron limits the growth of phytoplankton. However, a phytoplankton bloom thousands of square kilometres in area forms each spring-summer in the Indian sector of the Southern Ocean, both above and to the east of the Kerguelen Plateau. The central region of the Kerguelen Plateau hosts the volcanically active islands, Heard and McDonald (HIMI), the former of which is largely covered by glaciers. The sources and processes governing supply of iron from HIMI to the region are relatively unknown. In the austral summer of 2016, the first voyage to focus on biogeochemical cycling in the HIMI region was undertaken (GEOTRACES process study GIpr05). Using iron redox measurements, we show here that each of the adjacent islands are strong sources of dissolved iron(II) (DFe(II)), though controlled by different supply mechanisms. At Heard Island, the greatest DFe(II) concentrations (max 0.57 nmol L) were detected north of the island. An inverse correlation of DFe(II) concentrations with salinity suggests the origin is from a sea-terminating glacier on the island. At McDonald Islands, the greatest DFe(II) concentrations (max 1.01 nmol L) were detected east of the islands which, based on DFe(II) profiles from five targeted stations, appears likely to originate from shallow diffuse hydrothermalism. Elevated DFe(II) around HIMI may increase Fe availability for biota and indicate slower oxidation kinetics in the region, which has implications for transport of Fe away from the islands to the broader northern Kerguelen Plateau where the annual plankton bloom is strongest.
Dissolved iron (dFe) plays an important role in regulating marine biological productivity. In high nutrient, low chlorophyll (HNLC) regions (> 33% of the global ocean) iron is the primary growth limiting nutrient, and elsewhere can regulate nitrogen fixation and growth by diazotrophs. Overall, dFe supply potentially impacts half of global ocean productivity. The link between iron availability and carbon export is strongly dependent on the phytoplankton iron quotas, or cellular Fe:C ratios. This ratio can vary by more than an order of magnitude in the open ocean and is positively correlated with ambient dFe concentration in sparse field observations. The Community Earth System Model (CESM) ocean component has been modified to simulate dynamic, group-specific, phytoplankton iron quotas (Fe:C) that vary as a function of ambient iron concentration. The simulated Fe:C ratios match the spatial trends in the observations and improve the correlation with global-scale, observed nutrient distributions. Acclimation of phytoplankton Fe:C ratios dampens the biogeochemical response to varying atmospheric deposition fluxes of soluble iron, compared to a model with fixed Fe:C. However, varying atmospheric soluble iron supply still has first order impacts on global carbon and nitrogen fluxes, and on the spatial patterns of nutrient limitation; both of which are strongly sensitive to changes in pyrogenic sources of iron. Accounting for dynamic, phytoplankton iron quotas is critical for capturing the ocean biogeochemical responses to varying atmospheric soluble iron inputs, including expected changes in both the mineral dust and pyrogenic sources with climate warming and anthropogenic activity.
In stratified oligotrophic waters, phytoplankton communities forming the deep chlorophyll maximum (DCM) are isolated from atmospheric iron sources above and remineralized iron below. Reduced supply leads to a minimum in dissolved iron (dFe) near 100 m, but it is unclear if iron limits growth at the DCM. Here, we propose that natural iron addition events occur regularly with the passage of mesoscale eddies, which alter the supply of dFe and other nutrients relative to the supply of light, and can be used to test for iron limitation at the DCM. This framework is applied to two eddies sampled in the North Pacific Subtropical Gyre. Observations in an anticyclonic eddy center indicated downwelling of iron-rich surface waters, leading to increased dFe at the DCM but no increase in productivity. In contrast, uplift of isopycnals within a cyclonic eddy center increased supply of both nitrate and dFe to the DCM, and led to dominance of picoeukaryotic phytoplankton. Iron addition experiments did not increase productivity in either eddy, but did enhance leucine incorporation at ambient light in the cyclonic eddy, a potential indicator of iron stress among Prochlorococcus. Rapid cycling of siderophores and low dFe:nitrate uptake ratios also indicate that a portion of the microbial community was stressed by low iron. However, near-complete nitrate drawdown in this eddy, which represents an extreme case in nutrient supply compared to nearby Hawaii Ocean Time-series observations, suggests that recycling of dFe in oligotrophic ecosystems is sufficient to avoid iron limitation in the DCM under typical conditions.
Remote, harsh conditions of the Southern Ocean challenge our ability to observe the region’s influence on the climate system. Southern Ocean air-sea CO2 flux estimates have significant uncertainty due to the reliance on limited ship-dependent observations in combination with satellite-based and interpolated data products. We utilize a new approach, making direct measurements of air-sea CO2, wind speed, and surface ocean properties on an Uncrewed Surface Vehicle (USV). In 2019 the USV completed the first autonomous circumnavigation of Antarctica providing hourly CO2 flux estimates. Using this unique data set to constrain potential error in different measurements and propagate those through the CO2 flux calculation, we find that different wind speed products and sampling frequencies have the largest impact on CO2 flux estimates with biases that range from -4% to +20%. These biases and poorly-constrained interannual variability could account for discrepancies between different approaches to estimating Southern Ocean CO2 uptake.
Water scarcity becomes more severe and there will need to be a concerted effort to ensure that water supply remains secure. To mitigate the danger of water scarcity using an energy efficient method for water desalination is crucial. This review objectively compares the most practical thermal and reverse osmosis desalination technologies and the latest energy recovery opportunities for each process. This analysis reveals that thermal and reverse osmosis desalination should not be considered as a competitor since (mostly) the feed water quality and the final permeate water standards are the main parameters that impose choosing either of the most appropriate desalination method or hybridization of both approaches. Desalination of seawater using improved thermal desalination in which pre-heated helium gas instead of air is used to increase evaporation efficiency up to 3 times, is a promising technique that could make this sub-boiling thermal desalination approach the future of thermal desalination. The effect of Renewable energy resources, BCE, multiple stage units and optimizing liquid height in the dehumidifier were particularly evaluated in the HDH process which could perfectly handle the sub-boiling thermal desalination approach. Results of this comprehensive review can aid decision-makers by manifesting the main developments in desalination techniques to a reasonable degree of accuracy.
Stable isotope ratios of carbon and nitrogen (δ13C and δ15N) in the particulate organic matter (POM) of the euphotic layer were seasonally investigated in the vicinity of the Kuroshio from 2008 to 2019 (n = 490). Generalized linear models (GLMs) showed significant seasonality of δ13C and δ15N of POM in the coastal (between Japan Main Island and the northern edge of the Kuroshio) and offshore (Kuroshio and more southern parts) areas. Seasonal climatological δ13C estimated based on the GLMs and seasonal median values of the environmental parameters was the highest in summer (-22.4 ± 0.2‰ and -22.9 ± 0.2‰ in the coastal and offshore areas, respectively) and lowest in winter in both areas (-23.9 ± 0.2‰ and -24.3 ± 0.2‰ in the coastal and offshore areas, respectively). Seasonal climatological δ15N showed different spatial variations from spring to summer. The δ15N value was the lowest during winter (0.8 ± 0.4‰), and increased to a similar level during the other three seasons in the coastal area (~3‰), suggesting that nitrate originating in the deep-sea water was the main source of new production from spring to autumn. In contrast, δ15N in the offshore areas decreased from spring (2.6 ± 0.4‰) to summer (0.9 ± 0.4‰), suggesting that the significant contributions of atmospheric deposition and nitrogen fixation in the summer were similar to those around Hawaii. Therefore, the nitrogen sources for biological production were different between the areas and seasons in the vicinity of the Kuroshio.
The Greenland Ice Sheet (GrIS) is experiencing significant mass loss and freshwater discharge at glacier fronts. The freshwater input from Greenland will impact the physical properties of adjacent coastal seas, including important regions of deep water formation and contribute to global sea level rise. However, the biogeochemical impact of increasing freshwater discharge from the GrIS is less well constrained. Here, we demonstrate the use of bio-optical sensors on ocean gliders to track biogeochemical properties of meltwaters off Southwest Greenland. Our results reveal that fresh, coastal waters, with an oxygen isotopic composition characteristic of glacial meltwater, are distinguished by a high optical backscatter and high levels of fluorescing dissolved organic matter (FDOM), representative of the overall coloured dissolved organic matter pool. Reconstructions of geostrophic velocities are used to show that these particle and FDOM-enriched coastal waters cross the strong boundary currents into the Labrador Sea. Meltwater input into the Labrador Sea is likely driven by mesoscale processes, such as eddy formation and local bathymetric steering, in addition to wind-driven Ekman transport. Ocean gliders housing bio-optical sensors can provide the high-resolution observations of both dissolved and particulate glacially-derived material that are needed to understand meltwater dispersal mechanisms and their sensitivity to future climatic change.
The Eocene–Oligocene transition (~34 Ma), is marked by the rapid development of a semi-permanent Antarctic ice-sheet, as indicated by ice-rafted debris. Proxy reconstructions indicate a drop in atmospheric CO₂ and global cooling. How these changes affected sea surface temperatures in the North Atlantic and ocean water stratification remains poorly constrained. In this study, we apply clumped-isotope thermometry to well-preserved planktic foraminifera, that are associated with mixed-layer and thermocline dwelling depths from the drift sediments at IODP Site 1411, Newfoundland, across four intervals bracketing the EOT. The mixed-layer dwelling foraminifera record a cooling of 2.2 ± 2.4 °C (mean ± 95% CI) across the EOT. While the cooling amplitude is similar to previous SST reconstructions, absolute temperatures (Eocene 20.0 ± 2.7 °C, Oligocene 18.0 ± 2.1 °C) appear colder than what is expected for this location based on previously reconstructed SSTs for the northernmost Atlantic. We discuss seasonal bias, recording depth, and appropriate consideration of paleolatitudes, all of which complicate the comparison between SST reconstructions and model output. Thermocline dwelling foraminifera record a larger cooling across the EOT (Eocene 19.0 ± 3.4 °C, Oligocene 14.0 ± 3.1 °C, cooling of 5.2 ± 3.2 °C), than foraminifera from the mixed layer, consistent with an increase in ocean stratification which may be related to the onset or intensification of the Atlantic meridional overturning circulation.
The elemental stoichiometry of particulate organic carbon (C), nitrogen (N), and phosphorus (P) connects the C fluxes of biological production to the availability of the limiting nutrients in the ocean. It also influences the marine food-web by modulating the feeding behavior of zooplankton and the decomposition of organic matter by bacteria and viruses. Despite its importance, there is a general paucity of information on how the global C:N:P ratio evolves seasonally and interannually, and large parts of the global ocean remain devoid of observational data. Here, we developed a new method that combines satellite ocean-color data with a cellular trait-based model to characterize the spatio-temporal variability of the phytoplankton stoichiometry in the surface mixed layer of the ocean. Here, we demonstrated this method specifically for the C:P ratio. The approach was applied to phytoplankton growth rates and chlorophyll-to-carbon ratios derived from MODIS-Aqua and to maps of temperature-dependent nutrient limitation in order to generate global and seasonal maps of upper-ocean phytoplankton C:P. Taking it a step further, we determined the C:P of the bulk particulate organic matter, using MODIS-Aqua estimates of particulate organic carbon and phytoplankton biomass. A reasonably good comparison of our results with available data, both horizontal distributions and time series, indicates the viability of our new method in accurately quantifying seasonally resolved global ocean bulk C:P. We anticipate that the new hyperspectral capabilities of the NASA’s PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) mission will facilitate the determination of phytoplankton stoichiometry for different size classes and can further enhance the predictability of marine ecosystem stoichiometry from space.
Understanding changes in the ocean carbonate system is central to understanding ocean and coastal acidification and the effects these phenomena will have in the future. To create a more complete overview of the recent history of the carbonate system in the nearshore Northeastern United States, several recently published or in-development statistical models have used simple ocean chemistry parameters of salinity, temperature dissolved oxygen, and nitrate, or these variables plus the addition of other input parameters: sea surface temperature, chlorophyll a, sea surface height, bathymetry, and atmospheric pCO2 to generate estimates of dissolved inorganic carbon (DIC) and total alkalinity (TA). Both a Random Forest Regression model and a multiple linear regression model predicting carbonate chemistry parameters was tested for accuracy in predicting fugacity of CO2 (fCO2) by comparing them with the publicly available fCO2 data from the Surface Ocean CO2 Atlas (SOCAT) database. Comparisons revealed a bias by the models to overestimate fCO2, which was also observed when comparing the SOCAT dataset to collocated discrete observations. To resolve these biases in fCO2, a correction was fitted to the modeled datasets. This investigation suggests that models that accurately predict carbonate parameters of DIC and TA, may be limited in their ability to reproduce fCO2 conditions in coastal areas without correction. This study suggests that extrapolating ocean carbonate system models based on parameters outside their intended uses should be considered for their potential limitations.
Large positive anomalies in lower troposphere methane (CH4) in early fall of nearly every year (2003 to 2015) led to an average atmospheric CH4 growth of 3.06 to 3.49 ppb yr-1 for the Barents and Kara Seas (BKS). At the same time, sea surface temperature (SST) increased from 0.0018 to 0.15 °C yr-1 while sea ice coverage decreased. Large positive CH4 anomalies were discovered around Franz Josef Land (FJL) and offshore west Novaya Zemlya with smaller CH4 enhancement and growth near Svalbard, downstream and north of known seabed CH4 seepage. The strongest SST increase each year was in the southeast Barents Sea in June due to strengthening of the warm Murman Current (MC) and in the south Kara Sea in September. We propose that atmospheric CH4 increase is occurring due to seepage from the petroleum reservoirs underlying the BKS and thawing of subsea permafrost and hydrates which then ventilates to the atmosphere from seasonal deepening of the surface ocean mixed layer and also from “methane shoaling” where currents transport deep water CH4 into shallower waters. Continued strengthening heat transfer by the MC to the BKS will contribute to further warming (with the Barents Sea projected ice-free around 2030) and marine CH4 emissions to the atmosphere.
Marine dissolved organic phosphorus (DOP) can serve as an organic nutrient to marine autotrophs, helping to sustain a portion of annual net community production (ANCP). Numerical models of ocean circulation and biogeochemistry have diagnosed the magnitude of this process at regional to global scales but have thus far been validated against DOP observations concentrated within the Atlantic basin. Here we assimilate a new marine DOP dataset with global coverage to optimize an inverse model of the ocean phosphorus cycle to investigate the regionally variable role of marine DOP utilization by autotrophs contributing to ANCP. We find ~25% of ANCP accumulates as DOP with a regionally variable pattern ranging from 8 – 50% across nine biomes investigated. Estimated mean surface ocean DOP lifetimes of ~0.5 – 2 years allow for transport of DOP from regions of net production to net consumption in subtropical gyres. Globally, DOP utilization by autotrophs sustains ~14% (0.9 Pg C yr-1) of ANCP with regional contributions as large as ~75% within the oligotrophic North Atlantic and North Pacific. Shallow export and remineralization of DOP within the ocean subtropics contributes ~30 – 80% of phosphate regeneration within the upper thermocline (< 300 m). These shallow isopycnals beneath the subtropical gyres harboring the preponderance of remineralized DOP outcrop near the poleward edge of each gyre, which when combined with subsequent lateral transport equatorward by Ekman convergence, provide a shallow overturning loop retaining phosphorus within the subtropical biome, likely helping to sustain gyre ANCP over multi-annual to decadal timescales.
Coral skeletal growth is sensitive to environmental change and may be adversely impacted by an acidifying ocean. However, physiological processes can also buffer biomineralization from external conditions, providing apparent resilience to acidification in some species. These same physiological processes affect skeletal composition and can impact paleoenvironmental proxies. Understanding the mechanisms of coral calcification is thus crucial for predicting the vulnerability of different corals to ocean acidification and for accurately interpreting coral-based climate records. Here, using boron isotope (δ11B) measurements on cultured cold-water corals, we explain fundamental features of coral calcification and its sensitivity to environmental change. Boron isotopes are one of the most widely used proxies for past seawater pH, and we observe the expected sensitivity between δ11B and pH. Surprisingly, we also discover that coral δ11B is independently sensitive to seawater dissolved inorganic carbon (DIC). We can explain this new DIC effect if we introduce boric acid diffusion across cell membranes as a new flux within a geochemical model of biomineralization. This model independently predicts the sensitivity of the δ11B-pH proxy, without being trained to these data, even though calcifying fluid pH (pHCF) is constant. Boric acid diffusion resolves why δ11B is a useful proxy across a range of calcifiers, including foraminifera, even when calcifying fluid pH differs from seawater. Our modeling shows that δ11B cannot be interpreted unequivocally as a direct tracer of pHCF. Constant pHCF implies similar calcification rates as seawater pH decreases, which can explain the resilience of some corals to ocean acidification. However, we show that this resilience has a hidden energetic cost such that calcification becomes less efficient in an acidifying ocean
The Blob was a marine heat wave in the Northeast Pacific from 2013 to 2016. While the upper ocean temperature in the Blob has been well described, the impacts on marine biogeochemistry have not been fully studied. Here, we characterize and develop understanding of Eastern North Pacific upper ocean biogeochemical properties during the Winter of 2013-14 using in situ observations, an observation-based product, and reconstructions from a collection of ocean models. We find that the Blob is associated with significant upper ocean biogeochemical anomalies: a 5% increase in aragonite saturation state (temporary reprieve of ocean acidification) and a 3% decrease in oxygen concentration (enhanced deoxygenation). Anomalous advection and mixing drives the aragonite saturation anomaly, while anomalous heating and air-sea gas exchange drive the oxygen anomaly. Marine heatwaves do not necessarily serve as an analogue for future change as they may enhance or mitigate long-term trends.
Coastal vegetated habitats like seagrass meadows can mitigate anthropogenic carbon emissions by sequestering CO2 as “blue carbon” (BC). Already, some coastal ecosystems are actively managed to enhance BC storage, with associated BC stocks included in national greenhouse gas inventories or traded on international markets. However, the extent to which BC burial fluxes are enhanced or counteracted by other carbon fluxes, especially air-water CO2 flux (FCO2) remains poorly understood. To this end, we synthesized all available direct FCO2 measurements over seagrass meadows made using a common method (atmospheric Eddy Covariance), across a globally-representative range of ecotypes. Of the four sites with seasonal data coverage, two were net CO2 sources, with average FCO2 equivalent to 44 - 115% of the global average BC burial rate. At the remaining sites, net CO2 uptake was 101 - 888% of average BC burial. A wavelet coherence analysis demonstrates that FCO2 was most strongly related to physical factors like temperature, wind, and tides. In particular, tidal forcing appears to shape global-scale patterns in FCO2, likely due to a complex suite of drivers including: lateral carbon exchange, bottom-driven turbulence, and pore-water pumping. Lastly, sea-surface drag coefficients were always greater than prediction for the open ocean, supporting a universal enhancement of gas-transfer in shallow coastal waters. Our study points to the need for a more comprehensive approach to BC assessments, considering not only organic carbon storage, but also air-water CO2 exchange, and its complex biogeochemical and physical drivers.
Pore water freshening (i.e., decreases in dissolved Cl) has been documented in marine sediments along most active margins, with the migration of deep fluids or methane hydrate dissociation often invoked as sources of freshening in the sediment column. During D/V JOIDES Resolution Expedition 379T in 2019, two new sites (J1005 and J1006) were cored near ODP Site 1233 (41°S), adjacent to a seafloor mound venting structure. The three sites are less than 10 km apart but show marked differences in pore water chemistry and methane hydrate occurrence. The extent of Cl decrease is a function of distance from the mound, with the strongest freshening occurring at the closest site (J1006), which is the only site where methane hydrate was observed. Methane fluxes follow the same pattern, suggesting a common control. Increasing oxygen and decreasing hydrogen isotopes point to deep mineral bound water as the primary source of freshening near the mound, with fluids originating ~2.5 km below seafloor near the décollement. Secondary influences from methane hydrate dissociation and ash diagenesis also appear to influence regional pore water chemistry. The variability in pore water freshening suggests that fluid migration and eventual expulsion at the venting structure follows narrow pathways, likely along faults within the forearc complex. The migration of deep, gas-charged fluids may also support methane hydrate saturations greater than in situ organic carbon diagenesis would allow, but nonetheless consistent with geophysical estimates. Together, the data highlight an important link between fluid migration and methane hydrate formation on the Chilean Margin.
Particle sinking velocity is an important determinant of carbon transport and sequestration to the deep-sea. It is however technically challenging to measure in situ particle sinking velocities. Recently, methods based on the radioactive pairs, (234Th-238U and 210Po-210Pb) were developed to estimate average sinking velocity (ASV), along the classical carbon export flux estimates. The influence of ASVs on key metrics of the biological carbon pump such as (i) the particle export efficiency (defined as the proportion of PP being exported below the surface ocean) (ii) carbon export fluxes and attenuation, still remain uncertain and need to be further evaluated. ASVs are calculated in five biogeochemically contrasting sites: high latitude (Irminger Basin; Scotia Sea), temperate (PAP site) and oligotrophic (BATS, Equator) North Atlantic. ASVs are also calculated for different bloom stages (bloom - post bloom) in the North Atlantic and at the start of the bloom in two contrasting sites in the Southern Ocean (Scotia Sea). A systematic increase of ASVs with depth, inversely correlated to carbon flux attenuation, is detected. We assess whether the increase in ASV with depth is correlated with either temperature or community structure (phytoplankton and/or zooplankton). Evidences of ASV correlation with carbon export efficiency are detected, but they vary strongly with season and location, e.g. very distinct relationships are found for the results from Scotia Sea, likely driven by zooplankton abundance.
Coastal tropical waters are experiencing rapid increases in anthropogenic pressures, yet coastal biogeochemical dynamics in the tropics are poorly studied. We present a multi-year biogeochemical time series from the Singapore Strait in Southeast Asia’s Sunda Shelf Sea. Despite being highly urbanised and a major shipping port, the strait harbours numerous biologically diverse habitats, and is a valuable system for understanding how tropical marine ecosystems respond to anthropogenic pressures. Our results show strong seasonality driven by the semi-annual reversal of ocean currents: dissolved inorganic nitrogen (DIN) and phosphorus varied from ≤0.05 µmol l-1 during the intermonsoons to ≥4 µmol l-1 and ≥0.25 µmol l-1, respectively, during the southwest monsoon. Si(OH)4 exceeded DIN year-round. Based on nutrient concentrations, their relationships to salinity and coloured dissolved organic matter, and the isotopic composition of NOx-, we infer that terrestrial input from peatlands is the main nutrient source. This input delivered dissolved organic carbon (DOC) and nitrogen, but was notably depleted in dissolved organic phosphorus. In contrast, particulate organic matter showed little seasonality, and the δ13C of particulate organic carbon (-21.0 ± 1.5‰) is consistent with a primarily autochthonous origin. Diel changes in dissolved O2 varied seasonally with a pattern that suggests that light availability controls primary productivity more than nutrient concentrations. However, diel changes in pH were greater during the southwest monsoon, when remineralisation of terrestrial DOC lowers the seawater buffer capacity. We conclude that terrestrial input results in mesotrophic conditions, and that the strait might be vulnerable to further eutrophication if nutrient inputs increase during seasons when light availability is high. Moreover, the seasonality of diel pH variation suggests that coral reefs exposed to terrestrial organic matter in the Sunda Shelf may be at significant risk from future ocean acidification.