Uptake Of Atmospheric CO2 Research Articles

Trace and rare earth element distribution in hyperalkaline serpentinite-hosted spring waters and associated authigenic carbonates from the Ronda peridotite

The Ronda peridotite in southern Spain is subject to low-temperature serpentinization by circulating groundwaters that produces hyperalkaline fluids of high pH and low dissolved inorganic carbon content (DIC). When these waters resurface as hyperalkaline springs they take up atmospheric carbon dioxide (CO2), which triggers the precipitation of travertine carbonates. Spring waters at Baños del Duque, Fuente Romana, and Fuente Amargosa show typical chemical features of hyperalkaline springs such as high pH and low DIC, yet exhibit an unusual distribution of rare earth elements (REEs) enriched in lanthanum (La) and cerium (Ce). In order to constrain changes in spring water chemistry and carbonate precipitation during atmospheric CO2 uptake, a reaction path model was set up to illustrate that the distribution of REE species in the high pH and low DIC water is strongly dependent on REE complexation by hydroxide and carbonate species. Reaction path modeling results show that during spring fluid emission and CO2 uptake, REE speciation in the fluids shifts from a dominance of hydroxide to carbonate complexes. The complexation of rare earth elements in the high pH, low DIC fluids strongly depends on the DIC content of the respective fluid. The REE distribution of travertine deposits associated with the hyperalkaline springs deviates from that in the fluids, suggesting that travertines precipitated from fluids different to those currently emitted from the springs.

Decadal trends in Ocean Acidification from the Ocean Weather Station M in the Norwegian Sea

The Ocean Weather Station M (OWSM) is situated at a fixed position in the Norwegian Sea, one of the major basins of the Nordic Seas, which represents an important area for uptake of atmospheric CO 2 as well as deep water formation. At OWSM, the inorganic carbon cycle has been regularly monitored since 2001, and significant interannual changes of the carbonate system have been determined. Data collected at this site since the 1990s have been included, and over the 28 last years the surface fugacity of CO 2 ( f CO 2 ) has increased by 2.92 ± 0.37 μatm yr −1 , while surface pH and aragonite saturation (Ω Ar ) have decreased by −0.0033 ± 0.0005 yr −1 and −0.018 ± 0.003 yr −1 , respectively. This corresponds to a surface pH change of −0.092 over 28 years, which is comparable to the global mean pH decrease of −0.1 since the onset of the industrial revolution. Our estimates suggest that 80% of the surface pH trend at OWSM is driven by uptake of CO 2 from the atmosphere. In the deepest layer, Ω Ar has decreased significantly (−0.006 ± 0.001 yr −1 ) over the last 28 years, now occasionally reaching undersaturated values (Ω Ar < 1). As a rough estimate, the saturation horizon has shoaled by 7 m yr −1 between 1994 and 2021. The increase in surface f CO 2 is confirmed by semi-continuous measurements of CO 2 from the site (2.69 ± 0.14 μatm yr −1 ), and thus, the area has become less of a net sink for atmospheric CO 2 , taking into consideration an atmospheric CO 2 increase at OWSM of 2.27 ± 0.08 μatm yr −1 . • pH of the Norwegian Sea surface has decreased significantly over the last two decades. • ocean acidification is affecting the full water column (0–2100 m). • the acidification is primarily due to uptake of CO 2 from the atmosphere. • the area is a decreasing sink for atmospheric CO 2. • the saturation horizon, which is currently around 2000 m depth, is shoaling.

Air-water CO2 and water-sediment O2 exchanges over a tidal flat in Tokyo Bay

Despite the potential for carbon storage in tidal flats, little is known about the details of relevant processes because of the complexity of intertidal physical and chemical environments and the uniqueness of the biota. We measured air-water carbon dioxide (CO2) fluxes and water-sediment oxygen (O2) fluxes over a tidal flat in Tokyo Bay by the eddy covariance method, which has the potential to facilitate long-term, broad-scale, continuous monitoring of carbon flows in tidal flats. The results indicated that throughout the tidal flat in Tokyo Bay, CO2 was taken up from the atmosphere at a rate of 6.05 ± 7.14 (mean ± SD) mmol m−2 hour−1, and O2 was taken up from the water into the sediment at a rate of 0.62 ± 1.14 (mean ± SD) mmol m−2 hour−1. The fact that the CO2 uptake rate was about 18 times faster than the previously reported average uptake rate in the whole area of Tokyo Bay was attributable to physical turbulence in the water column caused by bottom friction. Statistical analysis suggested that light intensity and water temperature were the major factors responsible for variations of CO2 and O2 exchange, respectively. Other factors such as freshwater inputs, atmospheric stability, and wind speed also affected CO2 and O2 exchange. High rates of O2 uptake from the water into the sediment surface and high rates of atmospheric CO2 uptake into the water column occurred simultaneously (R2 = 0.44 and 0.47 during day and night, respectively). The explanation could be that photosynthetic consumption of CO2 and production of O2 in the water column increased the downward CO2 (air to water) and O2 (water to sediment) fluxes by increasing the concentration gradients of those gases. Resuspension of sediment in the low-O2 layer by physical disturbance would also increase the O2 concentration gradient and the O2 flux in the water.

Stable carbon isotopes of dissolved inorganic carbon in the Western North Pacific Ocean: Proxy for water mixing and dynamics

The uptake of atmospheric CO2 and the cycle of dissolved inorganic carbon (DIC) in the ocean are the major mechanisms and pathways controlling global climate change and carbon cycling. The stable carbon isotope (δ13C) of DIC, therefore, provides an important tracer for processes such as air-sea exchange, photosynthesis, and water dynamics in the ocean. Here, we present new δ13C-DIC data on water samples collected from a north-south transect (13°N–40°N, 150°E) in the western North Pacific (NP) Ocean in November 2019 and compare the results with those previously reported for similar transects (149.3°E) during WOCE and CLIVAR projects over the past three decades. The values of δ13C-DIC, ranging from -0.83‰ to 0.86‰, were higher in the surface waters and decreased with depth. The high δ13C-DIC values in the surface waters were influenced primarily by isotopic fractionation during air-sea exchange and photosynthesis. With depth, the movement of different water masses and mixing, as well as bathypelagic respiration in the dark water of the ocean, all play important roles in influencing the distribution and isotopic signatures of δ13C-DIC in the western NP Ocean. The δ13C-DIC values of the 0–200 m water layer varied from -0.17‰ to 0.86‰, with lower values at high latitudes, affected by the low δ13C-DIC values carried by the Oyashio Current to the Kuroshio Extension (KE) region. A downward trend was present in the δ13C-DIC signature from north to south in the North Pacific Intermediate Water (NPIW) and Pacific Deep Water (PDW) in the western NP, which reflected the remineralization of organic matter with a horizontal transport of NPIW and PDW. We found a strong 13C Suess Effect in the upper 2,000 m in the western NP Ocean, and δ13C-DIC at the surface (&amp;lt;50 m) has decreased by 0.60‰-0.85‰ since 1993. The mean δ13C-DIC change in the surface ocean was estimated at 0.28‰ per decade between 1993 and 2019. The air-sea exchange and water mixing in the study area may have accelerated the absorption of anthropogenic CO2 in recent years, which likely caused a slightly faster rate of decrease in the δ13C-DIC from 2005–2019 than that observed from 1993–2005.

Erosion of carbonate-bearing sedimentary rocks may close the alkalinity budget of the Baltic Sea and support atmospheric CO2 uptake in coastal seas

High alkalinity values observed in coastal seas promote the uptake of CO2 from the atmosphere. However, the alkalinity budget of coastal areas and marginal seas is poorly understood, even though some of the recently observed alkalinity enhancement can be ascribed to riverine fluxes and anaerobic processes in shelf sediments. Here, we investigate the alkalinity budget of the Baltic Sea to identify previously unrecognized alkalinity sources. We quantify the generation of alkalinity and dissolved calcium (Ca) in this marginal sea applying simple mass balance calculations. Using this approach, we identify alkalinity and Ca sources of approximately 324 Gmol yr-1 and 122 Gmol yr-1, respectively, that cannot be ascribed to the riverine input. The magnitude of the Ca source suggests that a major fraction of the excess alkalinity (244 Gmol yr-1) is induced by the dissolution of calcium carbonate (CaCO3). A review of available field data shows that carbonate-bearing rocks at the coast and the seabed of the Baltic Sea are rapidly eroded and may provide sufficient CaCO3 to close the Ca budget. Hence, dissolution of eroded CaCO3 is the most likely source for the Ca enrichment observed in Baltic Sea water. This hypothesis is supported by mass accumulation rates of sediments derived from radioisotope data that are evaluated to derive a basin-wide rate of mud to muddy sand accumulation at the bottom of the Baltic Sea. The resulting value (139 Tg yr-1) exceeds current estimates of riverine particle fluxes into the Baltic Sea by more than one order of magnitude and confirms that rates of till erosion are sufficiently high to account for the Ca and most of the alkalinity excess in Baltic Sea water. Finally, we show that deliberate addition of CaCO3 to sediments deposited in the Baltic Sea could neutralize significant amounts of CO2 and help to achieve net-zero greenhouse gas emissions in the Baltic region.

Seasonal Water Mass Evolution and Non-Redfield Dynamics Enhance CO2 Uptake in the Chukchi Sea.

The Chukchi Sea is an increasing CO2 sink driven by rapid climate changes. Understanding the seasonal variation of air-sea CO2 exchange and the underlying mechanisms of biogeochemical dynamics is important for predicting impacts of climate change on and feedbacks by the ocean. Here, we present a unique data set of underway sea surface partial pressure of CO2 (pCO2) and discrete samples of biogeochemical properties collected in five consecutive cruises in 2014 and examine the seasonal variations in air-sea CO2 flux and net community production (NCP). We found that thermal and non-thermal effects have different impacts on sea surface pCO2 and thus the air-sea CO2 flux in different water masses. The Bering summer water combined with meltwater has a significantly greater atmospheric CO2 uptake potential than that of the Alaskan Coastal Water in the southern Chukchi Sea in summer, due to stronger biological CO2 removal and a weaker thermal effect. By analyzing the seasonal drawdown of dissolved inorganic carbon (DIC) and nutrients, we found that DIC-based NCP was higher than nitrate-based NCP by 66%-84% and attributable to partially decoupled C and N uptake because of a variable phytoplankton stoichiometry. A box model with a non-Redfield C:N uptake ratio can adequately reproduce observed pCO2 and DIC, which reveals that, during the intensive growing season (late spring to early summer), 30%-46% CO2 uptake in the Chukchi Sea was supported by a flexible stoichiometry of phytoplankton. These findings have important ramification for forecasting the responses of CO2 uptake of the Chukchi ecosystem to climate change.

Ocean alkalinity enhancement – avoiding runaway CaCO3precipitation during quick and hydrated lime dissolution

Abstract. Ocean alkalinity enhancement (OAE) is a method that can remove carbon dioxide (CO2) from the atmosphere and counteract ocean acidification through the dissolution of alkaline minerals. Currently, critical knowledge gaps exist regarding the dissolution of different minerals suitable for OAE in natural seawater. Of particular importance is to understand how much alkaline mineral can be dissolved before secondary precipitation of calcium carbonate (CaCO3) occurs, since secondary CaCO3 precipitation reduces the atmospheric CO2 uptake potential of OAE. Using two types of mineral proposed for OAE, quick lime (CaO) and hydrated lime (Ca(OH)2), we show that both (&lt;63 µm of diameter) dissolved in seawater within a few hours. No CaCO3 precipitation occurred at a saturation state (ΩA) of ∼5, but CaCO3 precipitation in the form of aragonite occurred above an ΩA value of 7. This limit is lower than expected for typical pseudo-homogeneous precipitation, i.e. in the presence of colloids and organic matter. Secondary precipitation at low ΩA (∼ 7) was the result of heterogeneous precipitation onto mineral surfaces, most likely onto the added CaO and Ca(OH)2 particles. Most importantly, runaway CaCO3 precipitation was observed, a condition where significantly more total alkalinity (TA) was removed than initially added. Such runaway precipitation could reduce the OAE CO2 uptake efficiency from ∼ 0.8 mol of CO2 per mole of added TA down to 0.1 mol of CO2 per mole of TA. Runaway precipitation appears to be avoidable by dilution below the critical ΩA threshold of 5, ideally within hours of the mineral additions to minimise initial CaCO3 precipitation. Finally, OAE simulations suggest that for the same ΩA threshold, the amount of TA that can be added to seawater would be more than 3 times higher at 5 ∘C than at 30 ∘C. The maximum TA addition could also be increased by equilibrating the seawater to atmospheric CO2 levels (i.e. to a pCO2 of ∼ 416 µatm) during addition. This would allow for more TA to be added in seawater without inducing CaCO3 precipitation, using OAE at its CO2 removal potential.

A deep-learning estimate of the decadal trends in the Southern Ocean carbon storage

Uptake of atmospheric carbon by the ocean, especially at high latitudes, plays an important role in offsetting anthropogenic emissions. At the surface of the Southern Ocean south of 30∘S, the ocean carbon uptake, which had been weakening in 1990s, strengthened in the 2000s. However, sparseness of in-situ measurements in the ocean interior make it difficult to compute changes in carbon storage below the surface. Here we develop a machine-learning model, which can estimate concentrations of dissolved inorganic carbon (DIC) in the Southern Ocean up to 4 km depth only using data available at the ocean surface. Our model is fast and computationally inexpensive. We apply it to calculate trends in DIC concentrations over the past three decades and find that DIC decreased in the 1990s and 2000s, but has increased, in particular in the upper ocean since the 2010s. However, the particular circulation dynamics that drove these changes may have differed across zonal sectors of the Southern Ocean. While the near-surface decrease in DIC concentrations would enhance atmospheric CO2 uptake continuing the previously-found trends, weakened connectivity between surface and deep layers and build-up of DIC in deep waters could reduce the ocean’s carbon storage potential.

The effect of ocean acidification on the escape behaviour of the sea star Parvulastra exigua to its sea star predator Meridiastra calcar

Ocean acidification (OA) driven by sea water uptake of anthropogenic atmospheric CO2 has broad deleterious effects on marine species including modified behavioural interactions such as between predators and prey. Predatory sea stars are key species in many marine ecosystems and often elicit defensive responses in their prey. This study investigated the effect of elevated CO2 on the escape response of the sea star Parvulastra exigua to its sea star predator Meridiastra calcar. In response to touch by M. calcar, P. exigua exhibits a distinct fleeing response. The escape response of P. exigua with respect to velocity and escape trajectory was investigated after both species were acclimated in OA conditions. At pHT 7.6 and 7.8 velocity and escape trajectory of the fleeing response of P. exigua did not differ from that seen in the ambient treatment. However, there was a delay in the time that P. exigua started to flee with the initiation time being 2.8 times slower (10 vs 28 s) at pHT 7.6. This delay may increase the vulnerability of P. exigua to predation by M. calcar and have ecological effects with respect to the role of this species as an algal grazer on rocky shores of southeast Australia where these sea star species co-occur.

Phycosphere pH of unicellular nano- and micro- phytoplankton cells and consequences for iron speciation

Surface ocean pH is declining due to anthropogenic atmospheric CO2 uptake with a global decline of ~0.3 possible by 2100. Extracellular pH influences a range of biological processes, including nutrient uptake, calcification and silicification. However, there are poor constraints on how pH levels in the extracellular microenvironment surrounding phytoplankton cells (the phycosphere) differ from bulk seawater. This adds uncertainty to biological impacts of environmental change. Furthermore, previous modelling work suggests that phycosphere pH of small cells is close to bulk seawater, and this has not been experimentally verified. Here we observe under 140 μmol photons·m−2·s−1 the phycosphere pH of Chlamydomonas concordia (5 µm diameter), Emiliania huxleyi (5 µm), Coscinodiscus radiatus (50 µm) and C. wailesii (100 µm) are 0.11 ± 0.07, 0.20 ± 0.09, 0.41 ± 0.04 and 0.15 ± 0.20 (mean ± SD) higher than bulk seawater (pH 8.00), respectively. Thickness of the pH boundary layer of C. wailesii increases from 18 ± 4 to 122 ± 17 µm when bulk seawater pH decreases from 8.00 to 7.78. Phycosphere pH is regulated by photosynthesis and extracellular enzymatic transformation of bicarbonate, as well as being influenced by light intensity and seawater pH and buffering capacity. The pH change alters Fe speciation in the phycosphere, and hence Fe availability to phytoplankton is likely better predicted by the phycosphere, rather than bulk seawater. Overall, the precise quantification of chemical conditions in the phycosphere is crucial for assessing the sensitivity of marine phytoplankton to ongoing ocean acidification and Fe limitation in surface oceans.

Kinetics of Olivine Weathering in Seawater: An Experimental Study

Enhanced weathering of mafic and ultra-mafic minerals has been suggested as a strategy for carbon dioxide removal (CDR) and a contribution to achieve a balance between global CO2 sources and sinks (net zero emission). This study was designed to assess CDR by dissolution of ultramafic sand (UMS) in artificial seawater (ASW). Fine grained UMS with an olivine content of ~75% was reacted in ASW for up to 134 days at 1 bar and 21.5–23.9°C. A decline in total alkalinity (TA) was observed over the course of the experiments. This unexpected result indicates that TA removal via precipitation of cation-rich authigenic phases exceeded the production of TA induced by olivine dissolution. The TA decline was accompanied by a decrease in dissolved inorganic carbon and Ca concentrations presumably induced by CaCO3 precipitation. Temporal changes in dissolved Si, Ca, Mg, and TA concentrations observed during the experiments were evaluated by a numerical model to identify secondary mineral phases and quantify rates of authigenic phase formation. The modeling indicates that CaCO3, FeOOH and a range of Mg-Si-phases were precipitated during the experiments. Chemical analysis of precipitates and reacted UMS surfaces confirmed that these authigenic phases accumulated in the batch reactors. Nickel released during olivine dissolution, a potential toxic element for certain organisms, was incorporated in the secondary phases and is thus not a suitable proxy for dissolution rates as proposed by earlier studies. The overall reaction stoichiometry derived from lab experiments was applied in a box model simulating atmospheric CO2 uptake in a continental shelf setting induced by olivine addition. The model results indicate that CO2 uptake is reduced by a factor of 5 due to secondary mineral formation and the buffering capacity of seawater. In comparable natural settings, olivine addition may thus be a less efficient CDR method than previously believed.

COASTAL OCEAN ACIDIFICATION IN BRAZIL: A BRIEF OVERVIEW AND PERSPECTIVES

This perspective paper aims at presenting the current knowledge on the processes of ocean acidification (OA) and coastal acidification (CA) in the Brazilian coastal ocean. We define and differentiate the processes of OA and CA: the first driven by the actual global increase of atmospheric carbon dioxide (CO2); the second driven by a combination of ocean uptake of atmospheric CO2 and other local/regional chemical additions or subtractions in aquatic ecosystems at the land-ocean interface. Regarding OA, we have centered our analysis on the data available for the main water masses along the Brazilian coast: South Atlantic Central Water (SACW), Tropical Water (TW), and Coastal Water (CW). The few data available for the shallow coastal waters (&lt; 200 m depth) of the continental shelf reveal an increase in the anthropogenic component of the total dissolved inorganic carbon (DIC) pool in the SACW, with a decline in the ocean pH (over two decades; 1993-2013), and in the saturation state of calcium carbonate (CaCO3) minerals. We could not find OA trends for TW and CW because no data was available. Overall, the colder water masses (SACW, Plata Plume) have lower buffering capacity and simulations show that will potentially experience earlier negative OA impacts than the warmer waters masses (TW, Amazon Plume). Regarding CA, we have identified some local/regional studies investigating the carbonate chemistry in nearshore/estuarine ecosystems, particularly on the quantification of sources and sinks of CO2 , and determining short-term variabilities. Apparently, spreading coastal eutrophication in Brazil can enhances or reduces the process of OA, depending on the net ecosystem metabolism in combination with other chemical alterations. However, we could not find medium-long term acidification trends due to the limited data. There is a limited capacity to produce long time-series of carbonate chemistry parameters in key ecosystems and regions along the Brazilian coast. This lack of past information hinders and impairs the scientific community for identifying potential patterns of acidification along the Brazilian coast. We call for an urgent action in Brazil, with emphasis on the establishment of moored buoys/stations and/or scientific programs in the long term with continuous, real-time measurements of the main carbonate chemistry parameters.&#x0D; Keywords: ocean acidification, coastal acidification, coastal eutrophication, continental shelf, Southwestern Atlantic Ocean

Temporal dynamics of surface ocean carbonate chemistry in response to natural and simulated upwelling events during the 2017 coastal El Niño near Callao, Peru

Abstract. Oxygen minimum zones (OMZs) are characterized by enhanced carbon dioxide (CO2) levels and low pH and are being further acidified by uptake of anthropogenic atmospheric CO2. With ongoing intensification and expansion of OMZs due to global warming, carbonate chemistry conditions may become more variable and extreme, particularly in the eastern boundary upwelling systems. In austral summer (February–April) 2017, a large-scale mesocosm experiment was conducted in the coastal upwelling area off Callao (Peru) to investigate the impacts of ongoing ocean deoxygenation on biogeochemical processes, coinciding with a rare coastal El Niño event. Here we report on the temporal dynamics of carbonate chemistry in the mesocosms and surrounding Pacific waters over a continuous period of 50 d with high-temporal-resolution observations (every second day). The mesocosm experiment simulated an upwelling event in the mesocosms by addition of nitrogen (N)-deficient and CO2-enriched OMZ water. Surface water in the mesocosms was acidified by the OMZ water addition, with pHT lowered by 0.1–0.2 and pCO2 elevated to above 900 µatm. Thereafter, surface pCO2 quickly dropped to near or below the atmospheric level (405.22 µatm in 2017; Dlugokencky and Tans, 2021; NOAA/Global Monitoring Laboratory (GML)) mainly due to enhanced phytoplankton production with rapid CO2 consumption. Further observations revealed that the dominance of the dinoflagellate Akashiwo sanguinea and contamination of bird excrements played important roles in the dynamics of carbonate chemistry in the mesocosms. Compared to the simulated upwelling, natural upwelling events in the surrounding Pacific waters occurred more frequently with sea-to-air CO2 fluxes of 4.2–14.0 mmol C m−2 d−1. The positive CO2 fluxes indicated our site was a local CO2 source during our study, which may have been impacted by the coastal El Niño. However, our observations of dissolved inorganic carbon (DIC) drawdown in the mesocosms suggest that CO2 fluxes to the atmosphere can be largely dampened by biological processes. Overall, our study characterized carbonate chemistry in nearshore Pacific waters that are rarely sampled in such a temporal resolution and hence provided unique insights into the CO2 dynamics during a rare coastal El Niño event.

Mitigating the ecological footprint of alkali-activated calcined clays by waste marble addition

Low-CO2, Portland-free cement binders can be produced by reaction of calcined clays in alkaline solutions, although the environmental footprint associated with alkaline activators may represent a point of concern. The amount of alkaline activator, and its embodied CO2, can be reduced by partial replacement of calcined clay with waste marble. Fresh and hardened state properties, as well as compositional and microstructural evolution of alkali-activated blends of calcined clay and waste marble, were investigated by combining experimental methods with thermodynamic modelling. It was shown that substitution of calcined clay with 30% waste marble was beneficial to the workability and mechanical properties of alkali-activated binders and can mitigate drying shrinkage. The physical and chemical role of calcium carbonate and the composition of the reaction products were discussed, along with the role of carbonation reactions occurring by uptake of atmospheric CO2. This approach can contribute to both reducing the CO2 footprint of cement and upcycling potentially hazardous waste from the dimension stone industry.

A CO2 sink in a tropical coastal lagoon impacted by cultural eutrophication and upwelling

Tropical coastal lagoons are recognized as very productive ecosystems. However, very little is known on the direction and magnitude of their CO2 exchange with the atmosphere. We report on air-water CO2 fluxes and carbon concentrations in a tropical shallow lagoon impacted by cultural eutrophication. During two contrasting field surveys, the first in Nov. 2019 during dry conditions and the second in Feb. 2020 during wet summer condition and intrusion of coastal upwelling waters inside the lagoon. At both seasons, spatial in situ monitoring of temperature, salinity and partial pressure of CO2 (pCO2) reveal the presence of undersaturated pCO2 values (between 60 and 300 ppmv), uptake of atmospheric CO2 (between 0.9 and 9.2 mmol m−2 h−1) inside the lagoon, at salinities between 10 and 32. Temporal monitoring data at the mouth of the lagoon throughout tidal cycles revealed the entrance of cold South Atlantic Central Waters (SACW) from the Cabo Frio's upwelling during flood tides in Feb. 2020 but not in Nov. 2019. The cold SACW with temperatures down to 16 °C were supersaturated in CO2 (pCO2 about 520 ppmv) and a CO2 source to the atmosphere (between 0.2 and 1.4 mmol m−2 h−1). During mixing of this SACW in the most marine compartment of the lagoon, the biological CO2 uptake and enhanced autotrophy overcame the effect of degassing SACW waters by heating. For both cruises, a consistent landward trend consisted in a decrease in pCO2 and dissolved inorganic carbon (DIC) mirrored by an increase of dissolved organic carbon (DOC) (between 4.6 and 19.8 mg L−1), particulate organic carbon (POC) (between 3.3 and 12.3 mg L−1), and chlorophyll a (Chl a) (between 7.6 and 217 μg L−1). POC, DOC and Chl a and the POC:Chl a ratio suggest that, organic carbon in the lagoon is a mixture of phytoplankton biomass produced in the lagoon, domestic wastewater loaded from multiple point sources and pulsated inputs from small rivers. Our study in the Saquarema lagoon confirms recent works in other densely populated tropical coastal ecosystems, revealing that eutrophication enhances the autotrophy of shallow tropical lagoons, making them act as efficient factories for atmospheric CO2 absorption and organic carbon storage in sediments and export to the ocean.

Geochemical and C‐O Isotopic Study of Ophiolite‐Derived Carbonates of the Barzaman Formation, Oman: Evidence of Natural CO2 Sequestration Via Carbonation of Ultramafic Clasts

AbstractCarbonate precipitation through atmospheric CO2 uptake by alkaline‐hyperalkaline waters offers a potential approach to mitigating anthropogenic CO2 emissions. The Oman Ophiolite produces high‐pH water characterized by continuous sequestration of CO2 at the air‐water interface. The geochemical and isotopic data of carbonates from the Barzaman Formation is used to assess the amount of atmospheric CO2 stored in the dolomite‐calcite assemblage. Post Archean Australian Shale ‐normalized rare earth elements patterns, with the exception of La and Ce anomalies, are similar to those of the bulk oceanic and lower crusts, with increasing LREE and flat HREE trends, and a positive Eu anomaly. The δ13CVPDB and δ18OVSMOW isotope values of the analyzed samples show two distinct end‐members, in which dolomite (−7.77‰ and +27.3‰) is isotopically heavier than calcite (−9.93‰ and +21.5‰). The estimated carbonate growth temperatures (18°C–56°C) are indistinguishable from the previously reported range (18°C–66°C). The C‐O isotope model for calcite, groundwater, and atmospheric CO2 shows that an ophiolite‐derived calcite sample absorbed an unequivocal amount of atmospheric CO2 (78% ± 11%) during precipitation. At the same time, dissolved inorganic carbon (DIC) in water accounts for the remaining carbon contribution (22% ± 9%). DIC is closely associated with different carbonate lithofacies and ophiolite‐derived soil, exhibiting large variations in C‐O isotopic compositions caused by isotopic disequilibrium. Taken together, geochemical and isotopic properties confirm that the carbonates were formed under oxic conditions triggered by the water‐rock interaction. For a reliable estimate of CO2 sequestered by carbonates of the Barzaman Formation, a systematic groundwater analysis is recommended to determine the contribution of CO2 in DIC.

Diagnosing CO2 emission-induced feedbacks between the Southern Ocean carbon cycle and the climate system: A multiple Earth System Model analysis using a water mass tracking approach

AbstractAnthropogenic CO2emission-induced feedbacks between the carbon cycle and the climate system perturb the efficiency of atmospheric CO2uptake by land and ocean carbon reservoirs. The Southern Ocean is a region where these feedbacks can be largest and differ most among Earth System Model projections of 21stcentury climate change. To improve our mechanistic understanding of these feedbacks, we develop an automated procedure that tracks changes in the positions of Southern Ocean water masses and their carbon uptake. In an idealised ensemble of climate change projections, we diagnose two carbon–concentration feedbacks driven by atmospheric CO2(due to increasing air-sea CO2partial pressure difference,dpCO2, and reducing carbonate buffering capacity) and two carbon–climate feedbacks driven by climate change (due to changes in the water mass surface outcrop areas and local climate impacts). Collectively these feedbacks increase the CO2uptake by the Southern Ocean and account for one-fifth of the global uptake of CO2emissions. The increase in CO2uptake is primarily dpCO2-driven, with Antarctic intermediate waters making the largest contribution; the remaining three feedbacks partially offset this increase (by ~25%), with maximum reductions in Subantarctic mode waters. The process dominating the decrease in CO2uptake is water mass-dependent: reduction in carbonate buffering capacity in Subtropical and Subantarctic mode waters, local climate impacts in Antarctic intermediate waters, and reduction in outcrop areas in circumpolar deep waters and Antarctic bottom waters. Intermodel variability in the feedbacks is predominatelydpCO2–driven and should be a focus of efforts to constrain projection uncertainty.

Effect of Cations on the Oxidation and Atmospheric Corrosion of Iron Interfaces to Minerals.

Surface corrosion involves a series of redox reactions that are catalyzed by the presence of ions. On infrastructure surfaces and in complex and natural environments, iron surfaces readily undergo redox reactions, impacting chemical processes. In this study, the effect of how cations influence the formation of the mineral scale on iron surfaces and its connection to surface corrosion was investigated in CaCl2(aq) and NaCl(aq) electrolytes. Polarized modulated-infrared reflection absorption spectroscopy (PM-IRRAS) measurements were used to measure the oxidation and formation of carbonates at the air/electrolyte/iron interface, which confirmed that the iron surface oxidized faster in CaCl2(aq) than in NaCl(aq). PM-IRRAS, attenuated total reflectance-Fourier transformed infrared spectroscopy, and X-ray photoelectron spectroscopy show that after the adsorption of atmospheric O2 and CO2, calcium carbonate (CaCO3) in the form of calcite and aragonite was produced on iron in the presence of CaCl2(aq), whereas siderite (FeCO3) was produced on the surface of iron in the presence of NaCl(aq). However, in either solution without gradual O2 and CO2 exposure, a heterogeneous mixture of lepidocrocite (γ-FeOOH) and an iron hydroxy carbonate (Fex(OH)yCO3) was grown on the iron surface. In situ liquid AFM was used to measure the surface roughness in CaCl2(aq) and NaCl(aq), as an estimation of the corrosion rate. In CaCl2(aq), Fe was found to corrode faster than Fe in NaCl(aq) due to more ions at equimolar concentrations. Surface physical changes, as measured by ex situ AFM, confirmed the presence of a heterogeneous mixture of γ-FeOOH and an Fex(OH)yCO3 in the submerged region. This indicates that the cation does not affect the type of mineral grown on the Fe surface in the region completely submerged in the electrolyte. These results suggest that the cations play a unique role in the initial stages of corrosion at the interface region, influencing the uptake of atmospheric CO2 and mineral nucleation. The knowledge gained from these interfacial reactions are important for understanding the connection between surface corrosion, mineral grown, and CO2 capture for sequestration.

Importance of resilient pastures for New Zealand’s agricultural soil carbon stocks

New Zealand’s agricultural pastures contain significant soil carbon (C) stocks that are susceptible to change when impacted by management and natural processes (e.g., climate). Inputs of C to these pastoral soils is through photosynthetic uptake of atmospheric CO2 either on-site or elsewhere. Changes in soil C stocks are in response to the management of the system that alters the input-output balance. Increasing the resilience of pastures to climatic events such as hot and dry summers or cool and wet winters can increase inputs of C to the soil while sustaining above-ground production and so provide an opportunity for C sequestration. Furthermore, increased pasture for grazing can reduce the need for management practices identified as detrimental for soil C stocks such as irrigation or the production of cropped supplemental feed. A reduction in the need for renewal and its associated soil C losses, and the establishment of a more diverse sward, especially if deeper-rooting species are included, has the potential for increasing soil C stocks provided the diversity can be maintained. From a soil C perspective, a resilient pasture maximises CO2 uptake to ensure adequate above- and below-ground inputs to maintain or increase soil C stocks and minimise the need for management activities detrimental to soil C.

The Impact of the Amazon on the Biological Pump and the Air‐Sea CO2 Balance of the Western Tropical Atlantic

AbstractThe Amazon River strongly modifies the biogeochemistry of the Western Tropical Atlantic (WTA). To disentangle the different mechanisms driving these modifications, we conduct a series of modeling experiments with a high‐resolution regional ocean model (ROMS) coupled to a biogeochemical/ecological model (BEC) that we augmented to include Diatom‐Diazotroph‐Assemblages (DDAs). In our model, the Amazon River increases net primary production (NPP) in the WTA by almost 10%, exceeding the stimulation expected from the supplied inorganic nitrogen and phosphorus by a factor of two. This amplification is fueled by new nitrogen stemming from DDA‐driven N2 fixation in the plume region, supported, in part, by the consumption of riverine dissolved organic phosphorus. The vertical export of organic carbon is enhanced by a shift of the phytoplankton community toward diatoms induced by the large amount of Si(OH)4 delivered by the Amazon. These changes in NPP and export production induce a strong uptake of atmospheric CO2. In contrast, the remineralization of the river‐delivered terrestrial organic matter leads to a release of CO2 over the WTA, which is partially offset by a net uptake induced by the riverine dissolved inorganic carbon and alkalinity. Overall, the Amazon reduces the strong outgassing of the WTA in our simulations by more than 50%. Our study demonstrates how rivers modify the marine biological pump and the air‐sea CO2 fluxes in the downstream ocean through a myriad of cascading effects, highlighting the need to fully consider the land‐ocean aquatic continuum in the modeling of the Earth System.