Sink For Atmospheric CO2 Research Articles

Lithium isotope fractionation during the weathering of granite: Responses to pH

The weathering of silicate rock is a major sink of atmospheric CO2 in the global carbon cycle and thus controls global climate change. Lithium (Li) isotopes are considered to be a sensitive proxy to trace the continental weathering, but factors controlling the fractionation behaviors of Li isotopes during chemical weathering of granite have not been addressed explicitly. Here, we present new Li isotope data of bulk samples, primary minerals, and secondary minerals from a granite weathering profile in South China to constrain the mechanisms of Li isotope fractionation during chemical weathering. The bulk samples of weathering products have relatively negative τLiTiO2 values ranging from −6.0% to −73.8% and δ7Li values ranging from −11.4‰ to +2.4‰. From the bottom to the top of the weathering profile, the pH gradually decreases (from 9.54 to 4.88). With decreasing pH, the Li concentrations of the weathered saprolites gradually decrease, while the δ7Li values of the saprolites decrease first and then increase. Li concentrations and Li isotope compositions of the constituent minerals vary considerably. Interestingly, Li isotope ratios of both the bulk samples and minerals show two distinct fractionation behaviors during chemical weathering under different pH conditions. The heavy isotope (7Li) is preferentially released into the liquid phase under basic and weakly acidic pH conditions and the light isotope (6Li) is preferentially released into the liquid phase under acidic pH conditions. This finding helps to explain the inconsistent fractionation behaviors of Li isotopes between laboratory experiments and field studies. The influence of pH on Li isotope fractionation during silicate weathering implies that the evolution of oceanic pH over geological time scales may partly account for the variations in oceanic Li isotope compositions.

Remote sensing of dissolved CO2 concentrations in meso-eutrophic lakes using Sentinel-3 imagery

Satellite observation can significantly reduce the uncertainties in CO2 emission estimations compared to insufficient field data. However, big challenges remain in developing remote sensing-based models for mapping concentrations of dissolved carbon dioxide (cCO2) in lakes at regional or global scales. We developed a cCO2 estimation model using Sentinel-3-derived lake environmental variables and near-synchronous field cCO2 data from 16 lakes in the middle and lower reaches of the Yangtze and Huai River (ML_YHR) basins in Eastern China (N = 248). Stepwise quadratic polynomial regressions of several combinations of chlorophyll-a (Chl-a), water temperature (Tw), Secchi disk depth (ZSD), and photosynthetic active radiation (PAR)-related variables were tested and validated to select the best approach. The final model showed high performance in calibration and validation (R2 > 0.72, RMSE < 6.35 μmol L−1, MAPE < 30.31%). The model sensitivity analysis, based on Monte Carlo simulations, showed the model's estimated bias as <25% based on uncertainties of all input variables. Spatial and temporal dynamics of dissolved CO2 concentrations in 113 lakes (≥ 10 km2) in the ML_YHR basins were mapped from 2016 to 2021 using the Sentinel-3 data. The result showed that CO2 concentrations were low in the summer and autumn but high in the winter and spring with dramatic variations (e.g., mean coefficient of variation: 52.59%). The annual mean CO2 concentrations of lakes revealed that about 28% of the lakes acted as weak atmospheric CO2 sinks (14.96 ± 1.13 μmol L−1) while the rest were sources (19.22 ± 2.02 μmol L−1), compared with a mean concentration of CO2 atmospheric equilibrium (16.29 μmol L−1). CO2 concentrations decreased with increasing eutrophication and decreasing lake size (p < 0.05). This study advances current knowledge about CO2 emissions from emerging meso-eutrophic lakes and shows how satellite remote sensing can expand the spatiotemporal coverage of lake CO2.

Carbonate Mineral Dissolution and Its Carbon Sink Effect in Chinese Loess

The relationship between the source and sink of atmospheric CO2 has always been a widely discussed issue in global climate change research. Recent studies revealed that the chemical weathering of carbonate rocks contributed to 1/3 (~0.5 Pg C/yr) of the missing carbon sinks (MCS) globally, and there are still 2/3 of MCS (~0.5 Pg C/yr) that need to be explored. As one of the main overburdened parts of the earth, loess is one of the important driving factors for atmospheric CO2 consumption. Here, we elaborated on the dissolution process and the carbon sink effect from carbonate and silicate minerals in loess. The relationship between carbonate dissolution and carbon source/sink is elucidated, and the mechanism of carbon sink formation from secondary carbonates in loess is clarified. Additionally, the commonly used methods for the identification of primary and secondary carbonates are summarized, and the methods for the study of loess carbon sinks and the influencing factors of loess carbon sinks are also revealed. Based on the research results and progress interpretations, the prospects of loess carbon sinks are discussed to provide a scientific basis for further research on loess carbon sinks.

Effects of Water Masses and Circulation on the Surface Water Partial Pressure of Carbon Dioxide in Summer in Eastern Beibu Gulf, China

A gulf is a typical ecological zone where carbon cycle is jointly affected by complex environmental factors and strong human activities, and the Beibu Gulf has complex water masses and circulation structures. In this study, we used underway, continuous observational data of the surface water partial pressure of carbon dioxide (pCO2), temperature (SST), salinity (SSS), dissolved oxygen (DO), and chlorophyll α along with vertical profile observations of temperature, salinity, carbonate system parameters and nutrients to determine the spatiotemporal variations and research effects of water masses and circulation on summer pCO2 in the eastern part of Beibu Gulf. In the summers of 2011 and 2014, the mean pCO2 in the eastern part of Beibu Gulf was 417 μatm and 405 μatm, respectively, and the mean sea–air CO2 flux was 3.3 mmol m−2 d−1 and 1.6 mmol m−2 d−1, respectively. In the summer of 2011, the northern part of the Beibu Gulf was controlled by a cyclonic circulation, and pCO2 at the center of the cyclonic circulation increased by more than 15 μatm to a mean value of more than 10 μatm above that of the surrounding waters. The southern part of the Beibu Gulf was affected by an anticyclonic circulation and western coastal water masses, with a high temperature, low salinity, low pCO2, and downwelling surface waters. In the summer of 2011, the mean pCO2 was approximately 17 μatm lower than that in the surrounding waters, and no clear downwelling was observed in summer 2014. The eastern part of Beibu Gulf was a source of atmospheric CO2 in the summer, only the region affected by the northern coastal water in the eastern part of Beibu Gulf was a sink of atmospheric CO2, and pCO2 had distinctly different spatiotemporal distributions under the influence of complex water masses and circulation structures.

Late Cretaceous evolution of chemical weathering at the northeastern South American margin inferred from mineralogy and Hf-Nd isotopes

The origin of the global temperature decrease that initiated the last greenhouse-icehouse transition about 90 million years ago still remains debated. Among the potential processes driving long-term climate evolution over million-year timescales, this study investigates the possible impact of the uplift-weathering connection on the late Cretaceous atmospheric CO2 drawdown and associated cooling.We analysed a marine sediment record from the Demerara margin, using Nd and Hf isotopes in clay-size detrital fractions (ΔɛHf(t)clay) together with bulk and clay mineralogy and major element abundances to reconstruct the evolution of tectonic uplift and chemical weathering intensity in northeastern South America during the late Cretaceous. Our data indicate that silicate weathering intensified on the northeastern South American margin during the middle Campanian and the Maastrichtian, concomitant with an uplift phase of the Guiana craton. We propose that the tectonic pulse highlighted by apatite fission track data, argon/argon dating and primary silicate mineral evolution, was accompanied by an accelerated chemical weathering, which presumably acted as a sink for atmospheric CO2. By contrast, during the earlier late Turonian to early Campanian, period of relative tectonic quiescence, climate most likely acted as the main driver controlling the evolution of chemical weathering intensity. The main phase of enhanced weathering induced by the uplift of the northeastern South American margin occurred during the Campanian, thereby postdating the onset of global seawater temperature decrease (late Turonian). By inference, this regional event is unlikely to have triggered late Cretaceous cooling but it may have acted as an additional factor stabilizing colder climate conditions at that time.

Organic matter deposition and preservation in ancient deep‐sea levée sediments: Implications for Neoproterozoic trends in carbon burial

AbstractDeep‐marine levées are areally extensive features that experience high rates of sedimentation, making them ideal sites for significant carbon burial and preservation. Although modern deep‐marine levées have been shown to sequester a large proportion of the world's total buried organic carbon, few studies have attempted to assess carbon deposition and preservation in ancient deep‐marine levée deposits. Observations of well‐exposed levée deposits of the Neoproterozoic Windermere Supergroup in British Columbia, Canada, have shown that depositional processes in levées can result in the concentration and enrichment of sedimentary marine organic matter. In contrast to many previous studies where organic‐rich strata occur as black shales formed in anoxic conditions, organic matter in this study occurs mostly in banded, mud‐rich sandstones deposited under oxic conditions. Scanning electron microscopy shows that this organic carbon occurs primarily as nano‐scale coatings on clay particles, but also as uncommon sand‐sized organomineralic aggregates and discrete sand‐sized amorphous grains. As flows overspilled the channel margins, the rapid collapse of the turbulent suspension resulted in elevated rates of sediment fallout that promoted aggregation of organic matter and clay particles, increasing cohesive forces in the flow, and ultimately en masse deposition. Rapid burial plus association with clay mineral surfaces prevented organic matter degradation, thereby effectively sequestering significant amounts of carbon in the sediment and therefore a net sink for atmospheric CO2. However, the mechanisms and efficiency by which organic matter is buried and preserved on geological timescales is influenced by many factors, including glacial, sea‐level and tectonic cycles. This work elucidates the fundamental physical and chemical processes that control organic matter accumulation and preservation in deep‐marine levées, and how these processes have evolved throughout geological time.

Emerging trends in the pretreatment of microalgal biomass and recovery of value-added products: A review

Microalgae are a promising source of raw material (i.e., proteins, carbohydrates, lipids, pigments, and micronutrients) for various value-added products and act as a carbon sink for atmospheric CO2. The rigidity of the microalgal cell wall makes it difficult to extract different cellular components for its applications, including biofuel production, food and feed supplements, and pharmaceuticals. To improve the recovery of products from microalgae, pretreatment strategies such as biological, physical, chemical, and combined methods have been explored to improve whole-cell disruption and product recovery efficiency. However, the diversity and uniqueness of the microalgal cell wall make the pretreatment process more species-specific and limit its large-scale application. Therefore, advancing the currently available technologies is required from an economic, technological, and environmental perspective. Thus, this paper provides a state-of-art review of the current trends, challenges, and prospects of sustainable microalgal pretreatment technologies from a microalgae-based biorefinery concept.

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.

The buffering of a riverine carbonate system under the input of acid mine drainage: Example from a small karst watershed, southwest China

In a karstic area affected by acid mine drainage (AMD), hydrochemical conditions, such as temperature, salinity, alkalinity, DIC, dissolved oxygen, and nutrients, may affect the buffering capacity of carbonate systems in freshwater systems. The resulting pH fluctuation is larger than that of a marine system. Therefore, this study focuses on the buffering of a riverine carbonate system under the input of AMD and discusses the variations in a series of buffering factors, including the Revelle factor, γDIC, γAlk βDIC, βAlk, ωDIC, and ωAlk. The results revealed that the Revelle factor could reflect the buffering process effectively; in addition, the maximum value of the Revelle factor appeared at pH = 8.5. The data points for pH greater than this value indicated that the Huatan River had the ability to absorb atmospheric CO2 in spring. Conversely, the data for pH less than this value reflected the buffering of H+ during CO2 degassing in summer and autumn. In winter, the data were around the maximum value, indicating the weakest buffering capacity. As a result, the dynamics of the carbonate system caused the most sensitive response to pH. In addition, the maximum Revelle factor value did not always indicate the carbonate system had reached equilibrium; the presence of strong CO2 degassing was still a possibility. Under acidic conditions, as CO2(aq) increased, the absolute values of γDIC, βDIC, ωDIC, and γAlk increased correspondingly, indicating the enhanced buffering capacity of H+ during CO2 degassing. Under the four Representative Concentration Pathways scenarios (RCPs) included in the IPCC’s fifth assessment report, the degassing rate of the Huatan River would decrease by 5%, 15%, 26%, or 48%, depending on the scenario. Even though the Huatan River revealed CO2 degassing characteristics in winter and spring under current conditions, it will eventually become a sink for atmospheric CO2 as atmospheric CO2 concentration increases. In this light, the carbon sink effect in karst areas will become increasingly important.

Characteristics of soil CO2 emission and ecosystem carbon balance in wheat-maize rotation field with 4-year consecutive application of two lignite-derived humic acids

Humic acid originating from lignite is a popular resource of organic fertilizer. The effects of humic acid application on crop biomass and soil CO2 emission charged the regional agro-ecosystem carbon balance. Two kinds of humic acid, obtained from lignite via H2O2-oxidation (OHA) and KOH-activation (AHA), were applied in a wheat-maize rotation located field at three levels of 500 (OHA1; AHA1), 1000 (OHA2; AHA2), and 1500 kg hm−2 (OHA3; AHA3), only chemical fertilizer treatment (CF) as control to investigate the change of soil CO2 emission, crop yield and ecosystem carbon balance in 2016–2019. During the four experimental years, the trend of cumulative efflux of soil CO2 was increasing in medium and high dosage humic acid treatments. The grain yield of wheat and maize had the same trend as the cumulative efflux of soil CO2 due to the increase of soil NO3−-N and soil available P directly affected by humic acid application. The main factor of cumulative soil CO2 efflux improvement was soil NO3−-N and soil available P in 2016, while soil available potassium became key factor in 2019 with the step regression. Net ecosystem productivity (NEP) was used to assess ecosystem carbon balance, which was positive values showed atmospheric CO2 sink under all the fertilization treatments and increased with the increase of humic acid use level. AHA2 and AHA3 treatments charged the higher NEP in 2019 than 2016. Meanwhile, AHA treatment presented a higher NEP average than OHA treatment with the same applied level. Crop yield and soil available P was the directly positive factor to NEP over four years under the fertilization by SEM analysis. It is recommended that AHA be applied at 1000 kg hm−2 together with chemical fertilizers to achieve the higher crop yield and a sink of the atmospheric CO2 in agricultural fields in North China.

CO2 dynamics in a small and old subtropical reservoir in East Asia: Environmental controls driving seasonal and spatial variability

Inland waters have been increasingly viewed as hotspots for greenhouse gas (GHG) emissions owing to their strong capability to intercept and mineralize carbon from the terrestrial environment. Although small waterbodies in humid subtropical climates have the potential to emit considerable amounts of GHG, their emission patterns have remained understudied. This study involved intensive measurements of carbon dioxide (CO2) emissions from a small reservoir and its upstream and downstream reaches located in subtropical Hong Kong. Our results revealed that a variety of metabolic, hydrological, and hydrochemical processes play a critical role in regulating its CO2 dynamics. The reservoir was an overall source of CO2 to the atmosphere with an average areal flux of 24.6 mmol m−2 d−1, and it occasionally functioned as a sink for atmospheric CO2 under intense solar radiation when primary productivity was high. This flux is on the low side relative to that of global (sub)tropical reservoirs, which was likely attributable to the prolonged history of the reservoir (>150 years) and the occasional undersaturation of CO2 in the water column. We also noticed pronounced differences in the underlying controls of CO2 dynamics between the reservoir and its upstream and downstream reaches, emphasizing the importance of taking into account the distinct characteristics of both lentic and lotic waters when evaluating catchment-scale CO2 fluxes.

Identifying the biological control of the annual and multi-year variations in South Atlantic air–sea CO2 flux

Abstract. The accumulation of anthropogenic CO2 emissions in the atmosphere has been buffered by the absorption of CO2 by the global ocean, which acts as a net CO2 sink. The CO2 flux between the atmosphere and the ocean, which collectively results in the oceanic carbon sink, is spatially and temporally variable, and fully understanding the driving mechanisms behind this flux is key to assessing how the sink may change in the future. In this study a time series decomposition analysis was applied to satellite observations to determine the drivers that control the sea–air difference of CO2 partial pressure (ΔpCO2) and the CO2 flux on seasonal and inter-annual timescales in the South Atlantic Ocean. Linear trends in ΔpCO2 and the CO2 flux were calculated to identify key areas of change. Seasonally, changes in both the ΔpCO2 and CO2 flux were dominated by sea surface temperature (SST) in the subtropics (north of 40∘ S) and were correlated with biological processes in the subpolar regions (south of 40∘ S). In the equatorial Atlantic, analysis of the data indicated that biological processes are likely a key driver as a response to upwelling and riverine inputs. These results highlighted that seasonally ΔpCO2 can act as an indicator to identify drivers of the CO2 flux. Inter-annually, the SST and biological contributions to the CO2 flux in the subtropics were correlated with the multivariate El Niño–Southern Oscillation (ENSO) index (MEI), which leads to a weaker (stronger) CO2 sink in El Niño (La Niña) years. The 16-year time series identified significant trends in ΔpCO2 and CO2 flux; however, these trends were not always consistent in spatial extent. Therefore, predicting the oceanic response to climate change requires the examination of CO2 flux rather than ΔpCO2. Positive CO2 flux trends (weakening sink for atmospheric CO2) were identified within the Benguela upwelling system, consistent with increased upwelling and wind speeds. Negative trends in the CO2 flux (intensifying sink for atmospheric CO2) offshore into the South Atlantic gyre were consistent with an increase in the export of nutrients from mesoscale features, which drives the biological drawdown of CO2. These multi-year trends in the CO2 flux indicate that the biological contribution to changes in the air–sea CO2 flux cannot be overlooked when scaling up to estimates of the global ocean carbon sink.

Carbon cycling in the East Sea (Japan Sea): A review

The East Sea (also known as the Japan Sea; hereafter, EJS) is a semi-enclosed marginal sea surrounded by the Korean Peninsula, Russia, and the Japanese Islands. The EJS is connected to the Pacific through shallow straits. Thus, the EJS has its own thermohaline circulation and the characteristic biogeochemistry. The deep overturning circulation plays a critical role in carbon cycling including absorption of atmospheric CO2 and its sequestration into the interior of the sea. The turnover time of the deep EJS (&amp;gt;1000 m) is ~ hundred years and probably varies depending on physical climate forcing. Thus, the effect of climate change on oceanic processes may be more easily detected in the EJS. In this paper, we summarize the current understanding of carbon cycling in the EJS. We focus especially on the Ulleung Basin in the southwestern EJS, from which more extensive data are available. Notable features of carbon cycling in the EJS include the following: primary productivity and the export/production ratio are higher than in the adjacent Pacific; the EJS is a net sink of atmospheric CO2 and anthropogenic CO2 content is ~1% of the dissolved inorganic carbon inventory; dissolved inorganic carbon in the sea interior is mostly supplied by organic matter decomposition rather than CaCO3 dissolution and thus, the deep waters are vulnerable to acidification; N:P molar ratio of the deep waters is ~13, lower than the Redfield ratio; concentration of dissolved organic carbon is significantly higher than in the oceans; and sediment resuspension and lateral transport is an important component of sinking particulate organic carbon (POC) flux. Another important feature is the temporal trends observed for the last few decades. For example, pH, calcium carbonate saturation status, and dissolved oxygen concentration in the sea interior have decreased, whereas dissolved inorganic carbon and likely, the inventory of anthropogenic CO2 have increased. These temporal trends have an implication on better understanding of the processes occurring more slowly in the oceans. Brief suggestions for future research that will improve our understanding of carbon cycling and its variability are provided at the end of the paper.

Nitrogen-based symbioses, phosphorus availability, and accounting for a modern world more productive than the Paleozoic.

Evolution of high-productivity angiosperms has been regarded as a driver of Mesozoic ecosystem restructuring. However, terrestrial productivity is limited by availability of rock-derived nutrients such as phosphorus for which permanent increases in weathering would violate mass balance requirements of the long-term carbon cycle. The potential reality of productivity increases sustained since the Mesozoic is supported here with documentation of a dramatic increase in the evolution of nitrogen-fixing or nitrogen-scavenging symbioses, including more than 100 lineages of ectomycorrhizal and lichen-forming fungi and plants with specialized microbial associations. Given this evidence of broadly increased nitrogen availability, we explore via carbon cycle modeling how enhanced phosphorus availability might be sustained without violating mass balance requirements. Volcanism is the dominant carbon input, dictating peaks in weathering outputs up to twice modern values. However, times of weathering rate suppression may be more important for setting system behavior, and the late Paleozoic was the only extended period over which rates are expected to have remained lower than modern. Modeling results are consistent with terrestrial organic matter deposition that accompanied Paleozoic vascular plant evolution having suppressed weathering fluxes by providing an alternative sink of atmospheric CO2 . Suppression would have then been progressively lifted as the crustal reservoir's holding capacity for terrestrial organic matter saturated back toward steady state with deposition of new organic matter balanced by erosion of older organic deposits. Although not an absolute increase, weathering fluxes returning to early Paleozoic conditions would represent a novel regime for the complex land biota that evolved in the interim. Volcanism-based peaks in Mesozoic weathering far surpass the modern rates that sustain a complex diversity of nitrogen-based symbioses; only in the late Paleozoic might these ecologies have been suppressed by significantly lower rates. Thus, angiosperms are posited to be another effect rather than proximal cause of Mesozoic upheaval.

Early Wintertime CO2 Uptake in the Western Arctic Ocean

AbstractTo investigate CO2 uptake in the western Arctic Ocean (north of 65°N), we conducted underway, ship‐based observations of the partial pressures of CO2 (pCO2) and total dissolved inorganic carbon (TCO2) in surface seawater in early winter (November 2018). From these two properties of the seawater inorganic carbon system, we calculated total alkalinity (TA). In the early winter, surface seawater pCO2 in most places was lower than atmospheric pCO2. The weighted mean of the air‐sea influxes of CO2 were calculated to be 7.5 ± 1.6 mmol m−2 d−1. The calculated influxes imply that the area acted as a moderate sink for atmospheric CO2 in early winter, and its rate of CO2 uptake was comparable to that (8.0 ± 1.7 mmol m−2 d−1) in summer (late August−September 2017). Spatial variations of surface seawater pCO2 in the early winter could be attributed mostly to conservative mixing changes of TCO2 and TA, which together accounted for more than 70% of the pCO2 variations. In the marginal ice zone, however, there was a decrease of surface seawater pCO2 by 70–90 μatm because of horizontal advection of water with an anomalously high temperature from the Pacific Ocean and its subsequent cooling. We concluded that mixing of water masses was as important as biological processes in causing spatial variations of pCO2 and CO2 uptake in the western Arctic Ocean, especially during seasons and in areas associated with little biological activity.

Daily dynamics of contrasting spring algal blooms in Santa Monica Bay (central Southern California Bight).

Protistan algae (phytoplankton) dominate coastal upwelling ecosystems where they form massive blooms that support the world's most important fisheries and constitute an important sink for atmospheric CO2 . Bloom initiation is well understood, but the biotic and abiotic forces that shape short-term dynamics in community composition are still poorly characterized. Here, high-frequency (daily) changes in relative abundance dynamics of the metabolically active protistan community were followed via expressed 18S V4 rRNA genes (RNA) throughout two algal blooms during the spring of 2018 and 2019 in Santa Monica Bay (central Southern California Bight). A diatom bloom formed after wind-driven, nutrient upwelling events in both years, but different taxa dominated each year. Whereas diatoms bloomed following elevated nutrients and declined after depletion each year, a massive dinoflagellate bloom manifested under relatively low inorganic nitrogen conditions following diatom bloom senescence in 2019 but not 2018. Network analysis revealed associations between diatoms and cercozoan putative parasitic taxa and syndinean parasites during 2019 that may have influenced the demise of the diatoms, and the transition to a dinoflagellate-dominated bloom.

Persistent Continental Shelf Carbon Sink at the Ieodo Ocean Research Station in the Northern East China Sea

Hourly (2017–2021) to seasonal (2015–2021) inorganic C data were collected at the Ieodo Ocean Research Station (32.07°N and 125.10°E) in the northern East China Sea (ECS), located under the influence of the nutrient-rich Changjiang Diluted Water (CDW). An increase in phytoplankton biomass from April to mid-August (the warming period) equalized much of the temperature-driven increase in the surface pCO2 and thus, made the northern ECS a moderate sink of atmospheric CO2. From November to March (the cooling period), a large pCO2 reduction, driven by a temperature reduction, and a high air–sea CO2 exchange rate, because of high windspeeds, transformed the basin into a substantial CO2 sink, yielding an annual net C uptake of 61.7 g C m–2 yr–1. The effects of biological production and temperature change on seawater pCO2 (and thus, the net air–sea CO2 flux) were decoupled each season and acted in concert to increase the net annual CO2 sink by the region. The present study provided the observational and mechanistic lines of evidence for confirming “continental shelf C pump”—a mechanism in the shallow waters of the continental shelves that accumulate a significant amount of C (via reinforced cooling and promoted biological C uptake) that is transported from the basin surface waters to the interior of the adjacent deep ocean. In the future, an increasing input of anthropogenic nutrients into the northern ECS is likely to make the region a stronger CO2 sink.

Role of seafloor production versus continental basalt weathering in Middle to Late Ordovician seawater 87Sr/86Sr and climate

The global climate of the Ordovician Period (486.9 to 443.1 Ma) is characterized by cooling that culminated in the Hirnantian glaciation. Chemical weathering of Ca- and Mg-bearing silicate minerals and the subsequent trapping of carbon in marine carbonates act as a sink for atmospheric CO2 on multi-million-year time scales, with basaltic rocks consuming CO2 at a greater rate than rocks of granitic composition. The oceanic Sr isotope ratio (87Sr/86Sr) can act as a geochemical proxy for the relative proportion of basaltic versus granitic weathering. Oxygen isotopes (δ18O) act as a proxy for paleotemperature and ice volume, providing a useful complement to 87Sr/86Sr in studies of ancient climate. Previous studies have reported stepwise cooling (increasing δ18O) during the Middle to Late Ordovician. Combined with Sr and C cycle models, this has led to the hypothesis that continental silicate weathering of mafic material drove Ordovician cooling (e.g., the Taconic Orogeny). However, Sr and C cycle models have not accounted for an apparent rise in sea level and seafloor production in the Middle Ordovician (Darriwilian), which would increase the hydrothermal Sr flux as well as degassing along continental volcanic arcs. Furthermore, some Ordovician studies contain temporal uncertainty between 87Sr/86Sr and δ18O curves if they are not based on paired analyses, which can obscure the relationship between silicate weathering and cooling. Here, we present new paired 87Sr/86Sr and δ18O data from conodont apatite and integrate this with both a deterministic (forward) and stochastic (reverse) modeling approach to argue that increased hydrothermal weathering played a role in driving marine 87Sr/86Sr, specifically an inflection occurring in the Pygoda serra conodont zone of the mid-Darriwilian Stage (∼ 460.9 Ma ± 1 My). This 87Sr/86Sr inflection is accompanied by an increase in δ18O, consistent with climate cooling. Clarifying the role of seafloor production for marine 87Sr/86Sr and the implications for Ordovician cooling allows for a more nuanced understanding of the factors that drive multi-million-year shifts in climate.

Monthly and seasonal variations in the surface carbonate system and air–sea CO2 flux of the Yellow Sea

Surface carbonate chemistry in the Yellow Sea was investigated based on discrete seawater samples collected from 2017 to 2020 at the Socheongcho Ocean Research Station (S-ORS; 37.423°N, 124.738°E). Records of carbon parameters, including seawater CO2 partial pressure (pCO2), revealed considerable seasonal variations, with amplitudes comparable to those observed across the western part of the Yellow Sea. The study site acted as a modest sink (−0.13 mol C m−2 yr−1) for atmospheric CO2. Biological processes (primary production and respiration) and physical conditions (temperature and degree of stratification) determined seawater pCO2, which fluctuated on an intraseasonal timescale between oversaturated and undersaturated with respect to atmospheric pCO2. Variation in pCO2 was significant in summer, depending on the biological carbon drawdown and tidal mixing-induced upwelling (increased pCO2 up to ~1000 μatm). The intraseasonal variability in seawater pCO2 may bias estimated air–sea CO2 fluxes, if measurements with a coarser (seasonal) time resolution are used.

Rates of atmospheric CO2 capture using magnesium oxide powder

Magnesium oxide (MgO) looping is a proposed technology for gigatonne-scale carbon dioxide (CO2) removal from the atmosphere. Here, we determined CO2 removal rates by measuring CO2 fluxes and total inorganic carbon (TIC) using a deposit of MgO powder (10 cm thickness; 76 kg MgO/m2). The majority of the MgO powder (∼87 wt%) was transformed to brucite [Mg(OH)2] after the first two wetting and drying cycles. CO2 fluxes varied depending on water content and porosity: −3.9 (saturated/wet), −12.5 (optimal), and −2.2 (dry) kg CO2/m2/yr. At these rates, only 3–18% of the MgO would react in 1 yr and require areas of 80,000–455,000 km2 to sequester 1 Gt CO2/yr. Gas-filled porosity of ∼39–58% and water contents of 7–17 wt% were optimal for CO2 ingress and carbonation. Dypingite [Mg5(CO3)4(OH)2·∼5H2O] was the sink of atmospheric CO2 and TIC increased from 0.2% to 7.5% CO2. The δ13C values of the solids (avg. −14.9‰; VPDB) were well below those expected for dypingite precipitated in equilibrium with laboratory CO2, demonstrating that CO2 supply was rate-limiting, despite daily tilling of the deposit. A linear extrapolation of the CO2 fluxes shows that 90% carbonation of MgO will require 5–27 yr. CO2 removal rates slowed down with time over 1 yr, in agreement with the expectation that the progressive depletion of reactants (MgO/brucite) is likely to substantially slow CO2 removal rates and greatly extend the time needed to achieve complete carbonation.