Air-sea CO2 Flux Research Articles

Modelling terrigenous DOC across the north west European Shelf: Fate of riverine input and impact on air-sea CO2 fluxes

Terrigenous carbon in aquatic systems is increasingly recognised as an important part of the global carbon cycle. Despite this, the fate and distribution of terrigenous dissolved organic carbon (tDOC) in coastal and oceanic systems is poorly understood. We have implemented a theoretical framework for the degradation of tDOC across the land to ocean continuum in a 3D hydrodynamical-biogeochemical model on the North West European Shelf. A key feature of this model is that both photochemical and bacterial tDOC degradation rates are age dependant constituting an advance in our ability to describe carbon cycling in the marine environment. Over the time period 1986-2015, 182±17 Gmol yr−1 of riverine tDOC is input to the shelf. Results indicate that bacterial degradation is by far the most important process in removing tDOC on the shelf, contributing to 73±6 % (132±11 Gmol yr−1) of the total removal flux, while 21±3 % (39±6 Gmol yr−1) of riverine tDOC was advected away from the shelf and photochemical degradation removing 5±0.5 % of the riverine flux. Explicitly including tDOC in the model decreased the air-sea carbon dioxide (CO2) flux by 112±8 Gmol yr−1 (4±0.4 %), an amount approximately equivalent to the CO2 released by the UK chemical industry in 2020. The reduction is equivalent to 62 % of the riverine tDOC input to the shelf while approximately 17 % of riverine input is incorporated into the foodweb. This work can improve the assumptions of the fate of tDOC by Earth System Models and demonstrates that the inclusion of tDOC in models can impact ecosystem dynamics and change predicted global carbon budgets for the ocean.

Temporal and spatial variations of air-sea CO2 fluxes and their key influence factors in seagrass meadows of Hainan Island, South China Sea

Seagrass ecosystems have received a great deal of attention for contributing to uptake of atmospheric CO2, and thereby helping to mitigate global climate change (‘blue carbon’). Carbon budgets for seagrass ecosystems are developed by estimating air-sea CO2 fluxes. Data for air-sea CO2 flux for tropical seagrass ecosystems are lacking, which is problematic for constraining global seagrass carbon budgets. Here, we sought to address this important data gap for tropical seagrass ecosystems (dominated by Thalassia hemprichii and Enhalus acoroides) from the Hainan Island of South China Sea, while also testing what the main factors driving the variations of air-sea CO2 fluxes are. We found that air-sea CO2 fluxes exhibited a U-shape diurnal variability from 6 a.m. to 6 a.m. of the next day, with the highest and lowest air-sea CO2 fluxes values at early morning and afternoon, respectively. Biological processes were the driving force for mediating diurnal variations of seawater pCO2. The pCO2, sea in different seasons displayed a trend of increasing from spring, reaching maximum in summer and then a decreasing trend after summer, where water temperature, wind speed and seagrass growth mainly drove the variations. This resulted in net uptake of CO2 in all seasons except during summer in our study seagrass ecosystems, with greater negative values found in autumn (−3.63 ± 0.76 mmol m−2 d−1) than those in winter (−2.84 ± 0.60 mmol m−2 d−1). While the nutrient loading induced seagrass biomass changes (especially the seagrass T. hemprichii), which mediated the air-sea CO2 fluxes changes among different seagrass meadows. Net annual CO2 uptake potential under low nutrient loading (−0.77 ± 0.16 mol m−2 yr−1) was 23–54 % greater than high nutrient loading seagrass meadows, with the average annual air-sea CO2 flux of the three seagrass meadows as −0.64 ± 0.13 mol m−2 yr−1. These results suggest that tropical seagrass meadows of Hainan Island are a significant CO2 sink of atmospheric CO2, but this capacity can be diminished by nutrient loading. Scaling up, we estimate the annual atmospheric CO2 uptake by seagrass meadows of Hainan Island (total area 55.28 km2) was 1544 t of CO2 yr−1, equivalent to the annual emissions from the wholesale, retail, accommodation and catering industries of 164,000 tourists in Hainan Island. With carbon neutrality becoming an important part of global climate governance, this study provides timely information for capitalising on the ability of seagrasses to contribute to natural climate solutions.

Air-sea CO2 fluxes and cross-shelf exchange of inorganic carbon in the East China Sea from a coupled physical-biogeochemical model

The East China Sea (ECS) has been reported to be a significant sink of atmospheric CO2, but less is known about horizontal transport of dissolved inorganic carbon (DIC) across the shelf. A coupled physical-biogeochemical model has been implemented for the ECS to simulate the inorganic carbon system and estimate CO2 fluxes and cross-shelf DIC transport in the ECS. A 6-year model hindcast (2013–2018) was performed and assessed. Multiple existing datasets from in-situ observations are used to constrain and validate the model. The model reproduces the spatial and temporal patterns of nitrogen, chlorophyll and CO2 parameters in general agreement with observations. Modeling estimation reveals that the ECS takes up CO2 at an annual mean rate of about 8.20 ± 3.13 mmol m−2 d−1, and experiences substantial seasonal variability. The total annual CO2 uptake in the ECS is about 21.55 Tg C yr−1. Modeling estimation suggests that the biological processes contribute to about 15 % of the shelf CO2 uptake in the ECS, leaving ~80 % of the shelf uptake contributed by other physical-chemical processes, e.g., physical pump and/or solubility pump. The horizontal fluxes of DIC between the ECS and the adjacent ocean are more than two orders of magnitude larger than the air-sea CO2 flux on the ECS and result in a net DIC export of about ~33.8 ± 14.87 Tg C yr−1 from the shelf area. Modeling results suggest that this conveyance of DIC to the open ocean is equivalent to about 70 % of the inorganic carbon inflow from riverine and atmospheric pathways in the annual scale.

Mechanisms and drivers controlling spatio-temporal evolution of pCO2 and air-sea CO2 fluxes in the southern Java coastal upwelling system

Upwelling systems are known for their complex dynamic processes spanning a wide variety of spatio-temporal scales, due to strong coupling between the ocean and atmosphere. One such upwelling system is found at the eastern boundary of the Indian Ocean along the southern coast of Java during the southeast monsoon (SM). This study examines the mechanisms and drivers involved in the spatial and temporal evolutions of surface pCO2 in this upwelling system over seasonal to inter-annual timescales using a coupled, high-resolution regional model. A decomposition analysis was used to quantify the changes in pCO2 in response to changes in surface-temperature (T), surface-salinity (S), dissolved inorganic carbon (DIC), and total alkalinity (ALK). The upwelling of deeper, cooler waters decreases the surface pCO2 by 50 ± 1.48 μatm from June to September. However, the presence of DIC-rich waters at significantly shallower depths increases the surface pCO2 by 27 ± 0.9 μatm. The surface pCO2 increases by 5 ± 0.50 μatm due to changes in salinity, which exerts less control than T-driven changes. Biological changes increase the surface pCO2 by 17.54 ± 2.74 μatm. The maximum seasonal amplitudes of the variations in pCO2 caused by seawater solubility and biology are in counterbalance during the SM. Therefore, the upwelling-driven combined effect of physical mechanisms dominates the effect of biological mechanisms in reducing surface pCO2 under these conditions. During the SM, the southern coast of Java acts as a seasonal CO2 sink (−1.36 ± 6.48 g C m−2 year−1), although consideration of the mean flux over 2006–2017 shows that the region acts as a source of CO2 (1.31 ± 5.03 g C m−2 year−1). Furthermore, following a negative Indian Ocean Dipole (IOD) year, a strong sink of atmospheric CO2 is observed during the SM.

Estimating marine carbon uptake in the northeast Pacific using a neural network approach

Abstract. The global ocean takes up nearly a quarter of anthropogenic CO2 emissions annually, but the variability in this uptake at regional scales remains poorly understood. Here we use a neural network approach to interpolate sparse observations, creating a monthly gridded seawater partial pressure of CO2 (pCO2) data product from January 1998 to December 2019, at 1/12∘ × 1/12∘ spatial resolution, in the northeast Pacific open ocean, a net sink region. The data product (ANN-NEP; NCEI Accession 0277836) was created from pCO2 observations within the 2021 version of the Surface Ocean CO2 Atlas (SOCAT) and a range of predictor variables acting as proxies for processes affecting pCO2 to create nonlinear relationships to interpolate observations at a spatial resolution 4 times greater than leading global products and with better overall performance. In moving to a higher resolution, we show that the internal division of training data is the most important parameter for reducing overfitting. Using our pCO2 product, wind speed, and atmospheric CO2, we evaluate air–sea CO2 flux variability. On sub-decadal to decadal timescales, we find that the upwelling strength of the subpolar Alaskan Gyre, driven by large-scale atmospheric forcing, acts as the primary control on air–sea CO2 flux variability (r2=0.93, p<0.01). In the northern part of our study region, divergence from atmospheric CO2 is enhanced by increased local wind stress curl, enhancing upwelling and entrainment of naturally CO2-rich subsurface waters, leading to decade-long intervals of strong winter outgassing. During recent Pacific marine heat waves from 2013 on, we find enhanced atmospheric CO2 uptake (by as much as 45 %) due to limited wintertime entrainment. Our product estimates long-term surface ocean pCO2 increase at a rate below the atmospheric trend (1.4 ± 0.1 µatm yr−1) with the slowest increase in the center of the subpolar gyre where there is strong interaction with subsurface waters. This mismatch suggests the northeast Pacific Ocean sink for atmospheric CO2 may be increasing.

High-resolution study of the air-sea CO2 flux and net community oxygen production in the Ligurian Sea by a fleet of gliders

Intense glider monitoring was conducted in the Ligurian Sea for five months to capture the Net Community Production (NCP) variability in one of the most dynamic and productive regions of the Mediterranean Sea. Using the SeaExplorer glider technology, we were able to observe continuously from January to the end of May 2018 the physical and biogeochemical variables during the last period of intense convection observed in this region. High-frequency measurements from these gliders provided valuable information for determining dissolved O2 (DO) concentrations between coastal and open sea waters. Our DO balance approach provided an estimate of NCP fluxes complemented by the prediction of air-sea CO2 fluxes based on a neural network adapted to the Mediterranean Sea (CANYON-MED). Based on our NCP calculation method, our results show that the air-sea O2 flux and DO inventory have contributed largely to the NCP variability. The NCP values also suggest that heterotrophic conditions were predominant in winter and became autotrophic in spring, with strong variability in coastal waters due to the occurrence of sub-mesoscale structures. Finally, CO2 fluxes at the air-sea interface reveal that during the convection period, the central zone of the Ligurian Sea acted as a CO2 sink from January to March with little impact on NCP fluxes counterbalanced by a thermal effect of seawater.

Satellite-estimated air-sea CO2 fluxes in the Bohai Sea, Yellow Sea, and East China Sea: Patterns and variations during 2003–2019

The Bohai Sea (BS), Yellow Sea (YS), and East China Sea (ECS) together form one of the largest marginal sea systems in the world, including enclosed and semi-enclosed ocean margins and a wide continental shelf influenced by the Changjiang River and the strong western boundary current (Kuroshio). Based on in situ seawater pCO2 data collected on 51 cruises/legs over the past two decades, a satellite retrieval algorithm for seawater pCO2 was developed by combining the semi-mechanistic algorithm and machine learning method (MeSAA-ML-ECS). MeSAA-ML-ECS introduced semi-analytical parameters, including the temperature-dependent seawater pCO2 (pCO2,therm) and upwelling index (UISST), to characterise the combined effect of atmospheric CO2 forcing, thermodynamic effects, and multiple mixing processes on seawater pCO2. The best-selected machine learning algorithm is XGBoost. The satellite-derived pCO2 achieved excellent performance in this complicated marginal sea, with low root mean square error (RMSE = 20 μatm) and mean absolute percentage deviation (APD = 4.12 %) for independent in situ validation dataset. During 2003–2019, the annual average CO2 sinks in the BS, YS, ECS, and entire study area were 0.16 ± 0.26, 3.85 ± 0.68, 14.80 ± 3.09, and 18.81 ± 3.81 Tg C/yr, respectively. Under continuously increasing atmospheric CO2 concentration, the BS changed from a weak source to a weak sink, the YS experienced interannual fluctuations but did not show significant trend, while the ECS acted as a strong sink with CO2 absorption increased from ∼10 Tg C in 2003 to ∼19 Tg C in 2019. In total, CO2 uptake in the entire study area increased by 85 % in 17 years. For the first time, we present the most refined variation in the satellite-derived pCO2 and air-sea CO2 flux dataset. These complete ocean carbon sink statistics and new insights will benefit further research on carbon fixation and its potential capacity.

The controversial Southern Ocean air-sea CO2 flux in the era of autonomous ocean observations

The controversial Southern Ocean air-sea CO2 flux in the era of autonomous ocean observations

Seagrass Blue Carbon Stock and Air–Sea CO2 Fluxes in the Karimunjawa Islands, Indonesia during Southeast Monsoon Season

Research focusing on seagrass ecosystems as carbon storage has been conducted in various Indonesian waters. However, an essential aspect that remains unexplored is the simultaneous analysis of blue carbon storage in seagrass alongside carbon dioxide (CO2) flux values, particularly within Karimunjawa waters. This study aims to assess the organic carbon stock and sea–air CO2 flux in seagrass ecosystems in Karimunjawa. Our hypothesis posits that although seagrass ecosystems release CO2 into the water, their role as blue carbon ecosystems enables them to absorb and accumulate organic carbon within seagrass biomass and sediments. This investigation took place in Karimunjawa waters, encompassing both vegetated (seagrass meadows) and unvegetated (non-seagrass meadows) areas during August 2019, 2020, and 2022. Over this period, the organic carbon stock in seagrass and sediment displayed an increase, rising from 28.90 to 35.70 gCorg m−2 in 2019 and from 37.80 to 45.25 gCorg m−2 in 2022. Notably, the expanse of seagrass meadows in Karimunjawa dwindled by 328.33 ha from 2019 to 2022, resulting in a total carbon stock reduction of the seagrass meadows of 452.39 tC to 218.78 tC. Sediment emerges as a pivotal element in the storage of blue carbon in seagrass, with sedimentary organic carbon outweighing seagrass biomass in storage capacity. The conditions in Karimunjawa, including a high A:B ratio, low dry bulk density, and elevated water content, foster a favorable environment for sediment carbon absorption and storage, facilitated by the waters’ CO2 emission during the southeast monsoon season. Notably, our findings reveal that CO2 release within vegetated areas is lower compared to unvegetated areas. This outcome underscores how seagrass ecosystems can mitigate CO2 release through their adeptness at storing organic carbon within biomass and sediment. However, the presence of inorganic carbon in the form of calcium carbonate introduces a nuanced dynamic. This external source, stemming from allochthonous origins like mangroves, brown algae like Padina pavonica, and calcareous epiphytes, leads to an increase in sedimentary organic carbon stock of 53.2 ± 6.82 gCorg m−2. Moreover, it triggers the release of CO2 into the atmosphere, quantified at 83.4 ± 18.26 mmol CO2 m−2 d−1.

An assessment of air-sea CO2 flux parameterizations during tropical cyclones in the Bay of Bengal

The exchange of air-sea CO2 plays a significant role in regulating the Earth’s climate. The errors associated with the estimations of air-sea CO2 fluxes during extreme transient events like tropical cyclones (TCs) are important for climate research. In this study, we assess the estimates of CO2 gas transfer velocity and the corresponding air-sea flux derived by employing five wind-dependent and two wave-dependent parameterizations for eight TCs in the Bay of Bengal using mooring observations and reanalysis datasets. To start with, we analyze drag coefficient (CD) and associated frictional velocity (u*) derived from two globally very commonly used bulk flux algorithms, COARE 3.0 and the updated version COARE 3.6, with the estimates from wind-wave tank experiments for moderate and high wind speeds for all TCs. The analysis indicates that COARE 3.6 provides the best estimate of drag coefficient. Further, we find that the wave-dependent parameterization by Woolf (2005) provides the best estimates of CO2 gas transfer velocity compared to existing estimates of laboratory-based wind-wave tank experiments for high winds. Among all wind-only parameterizations, the hybrid parameterization proposed by Nightingale et al. (2000) performs best for high winds. We find that for winds < 20 m/s, the resultant fluxes of CO2 estimated using these seven parameterizations vary within 5 mmol CO2 m-2 d-1. However, for winds > 20 m/s, the difference between wind- and wave-parameterized fluxes are significant (∼50 mmol CO2 m-2 d-1). The percentage of variation in CO2 flux explained by transfer velocity (difference in sea and air pCO2) during TC conditions is nearly 78 (15)%.

Dynamics of the Seawater Carbonate System in the East Siberian Sea: The Diversity of Driving Forces

The East Siberian Sea (ESS) is a large and the shallowest part of the Arctic Ocean. It is characterized by high biogeochemical activity, but the seawater carbonate system remains understudied, especially during the late autumn season. Data from the research vessel (RV) “Professor Multanovsky” cruise were used to assess the dynamics of the seawater carbonate system, air–sea CO2 fluxes, and the calcium carbonate corrosive waters in the two biogeochemical provinces of the ESS shortly before freeze-up. The ESS waters were mainly a sink for atmospheric CO2 due to the limited dispersion of river waters, autumn water cooling, and phytoplankton blooms in its eastern autotrophic province. The mean value of the CO2 air–sea flux was 11.2 mmol m−2 day−1. The rate of CO2 uptake in the eastern ESS was an order of magnitude larger than that in the western ESS. The specific waters and ice cover dynamics determined intensive photosynthesis processes identified on the eastern shelf and in the northern deep oligotrophic waters. A part of the surface and most of the bottom ESS waters were corrosive with respect to calcium carbonate, with the lowest saturation state of aragonite (0.22) in the bottom layer of the eastern ESS. The eastern ESS was the main source of these waters into the deep basin. The observed export of corrosive shelf waters to the deep sea can have a potential impact on the ocean water ecosystem in the case of mixing with layers inhabited by calcifying organisms.

Wastewater inputs reduce the CO2 uptake by coastal oceans

Every year a large quantity of wastewater is generated worldwide, but its influence on the carbon dioxide (CO2) uptake by coastal oceans is not well understood. Here, sea surface CO2 partial pressure (pCO2) and air-sea CO2 flux were examined in the Jiaozhou Bay (JZB), a temperate coastal bay strongly disturbed by wastewater inputs. Monthly surveys from April 2014 through March 2015 showed that surface pCO2 in the JZB substantially varied both temporally and spatially between 163 μatm and 1222 μatm, with an annual average of 573 μatm. During April–December, surface pCO2 was oversaturated with respect to the atmosphere, with high values exceeding 1000 μatm in the northeastern part of the bay, where seawater salinity was low mainly due to the inputs of wastewater with salinity close to zero. During January–March, surface pCO2 was undersaturated, with the lowest value of <200 μatm also mainly in the northeastern part because of low water temperature and strong biological production. Over an annual cycle, apparently sea surface temperature dominated the monthly variation of surface pCO2 in this shallow bay, while wastewater inputs and related biological production/respiration dominated its spatial variability. Overall, the JZB was a net CO2 source to the atmosphere, emitting 9.6 ± 10.8 mmol C m−2 d−1, unlike its adjacent western part of the Yellow Sea and most of the temperate coastal oceans which are a net CO2 sink. This was possibly associated with wastewater inputs that cause high sea surface pCO2 via direct inputs of CO2 and degradation of organic matter. Thus, from this viewpoint reducing wastewater discharge or lowering CO2 levels in discharged wastewater may be important paths to enhancing the CO2 uptake by coastal oceans in the future.

Underestimation of oceanic carbon uptake in the Arctic Ocean: ice melt as predictor of the sea ice carbon pump

Abstract. The Arctic Ocean is generally undersaturated in CO2 and acts as a net sink of atmospheric CO2. This oceanic uptake is strongly modulated by sea ice, which can prevent air–sea gas exchange and has major impacts on stratification and primary production. Moreover, carbon is stored in sea ice with a ratio of alkalinity to dissolved inorganic carbon that is larger than in seawater. It has been suggested that this storage amplifies the seasonal cycle of seawater pCO2 and leads to an increase in oceanic carbon uptake in seasonally ice-covered regions compared to those that are ice-free. Given the rapidly changing ice scape in the Arctic Ocean, a better understanding of the link between the seasonal cycle of sea ice and oceanic uptake of CO2 is needed. Here, we investigate how the storage of carbon in sea ice affects the air–sea CO2 flux and quantify its dependence on the ratio of alkalinity to inorganic carbon in ice. To this end, we present two independent approaches: a theoretical framework that provides an analytical expression of the amplification of carbon uptake in seasonally ice-covered oceans and a simple parameterization of carbon storage in sea ice implemented in a 1D physical–biogeochemical ocean model. Sensitivity simulations show a linear relation between ice melt and the amplification of seasonal carbon uptake. A 30 % increase in carbon uptake in the Arctic Ocean is estimated compared to ice melt without amplification. Applying this relationship to different future scenarios from an earth system model that does not account for the effect of carbon storage in sea ice suggests that Arctic Ocean carbon uptake is underestimated by 5 % to 15 % in these simulations.

A CO2 and H2O Gas Analyzer with Reduced Error due to Platform Motion

Abstract One long-standing technical problem affecting the accuracy of eddy correlation air–sea CO2 flux estimates has been motion contamination of the CO2 mixing-ratio measurement. This sensor-related problem is well known but its source remains unresolved. This report details an attempt to identify and reduce motion-induced error and to improve the infrared gas analyzer (IRGA) design. The key finding is that a large fraction of the motion sensitivity is associated with the detection approach common to most closed- and open-path IRGA employed today for CO2 and H2O measurements. A new prototype sensor was developed to both investigate and remedy the issue. Results in laboratory and deep-water tank tests show marked improvement. The prototype shows a factor of 4–10 reduction in CO2 error under typical at-sea buoy pitch and roll tilts in comparison with an off-the-shelf IRGA system. A similar noise reduction factor of 2–8 is observed in water vapor measurements. The range of platform tilt motion testing also helps to document motion-induced error characteristics of standard analyzers. Study implications are discussed including findings relevant to past field measurements and the promise for improved future flux measurements using similarly modified IRGA on moving ocean observing and aircraft platforms.

Seasonal variation of inorganic carbon parameters and air–sea exchange of CO2 in Marian Cove, King George Island, Western Antarctic Peninsula

Inorganic carbon parameters were observed in Marian Cove, King George Island, Western Antarctic Peninsula, to assess the impact of the Antarctic coastal regions on air–sea CO2 exchange. The variations in total alkalinity (TA) and dissolved inorganic carbon (DIC) were caused by ice melting, formation, and biological activities. The net annual air–sea CO2 flux (5.6 ± 11.8 mmol m−2 d−1) indicated that Marian Cove was a CO2 source in the atmosphere, suggesting the opposite role of the Antarctic coastal regions to the Southern Ocean in CO2 flux estimates. Finally, this study identified the controlling factors of the annual variation of TA and DIC for the first time through direct field observations in King George Island. This study indicated that Antarctic coastal regions can act as a CO2 source to the atmosphere. Thus, further investigations and continuous monitoring are required in the coastal areas to improve our understanding of global carbon cycles.

Rapid fCO2 rise in the northern Barents Sea and Nansen Basin

Maps of surface water fugacity of CO2 (fCO2) over eastern Fram Strait, south-western Nansen Basin, and the north-western Barents Sea (73–84°N, 5–46°E) from September 1997 to December 2020 were made and used to investigate seasonal and temporal trends. The mapping utilized a neural network technique, the self-organizing map (SOM), that was trained with different combinations of satellite/observational/model data of sea surface temperature (SST), sea surface salinity (SSS), mixed layer depth (MLD), chlorophyll a (Chl a), sea ice concentration, and atmospheric mole fraction of CO2 (xCO2). The trained SOM was labelled with available surface ocean fCO2 data, and the labelled SOM was subsequently used to map the fCO2. The produced maps reveal that fCO2 in northern Barents Sea, at the border of the Nansen Basin, has increased significantly over the last decades by between 4.2 and 5.5 ± 0.6–1.1 µatm yr−1 over the winter to summer seasons. These rates are twice the rate of atmospheric CO2 increase, which was about 2 µatm yr−1. The spatial pattern coincides with the strongest decreases in sea ice concentration as well as with a salinification of the surface water. The former allows for a prolongation of the air-sea CO2 flux with resultant oceanic CO2 uptake in previously ice-covered waters, and the latter is caused by a shift from Arctic Water dominance to more saline waters containing more dissolved inorganic carbon, most likely of Atlantic Water origin although brine-release influenced deep water may also contribute.

High interannual surface pCO2 variability in the southern Canadian Arctic Archipelago's Kitikmeot Sea

Abstract. Warming of the Arctic due to climate change means the Arctic Ocean is now free from ice for longer, as sea ice melts earlier and refreezes later. Yet, it remains unclear how this extended ice-free period will impact carbon dioxide (CO2) fluxes due to scarcity of surface ocean CO2 measurements. Baseline measurements are urgently needed to understand spatial and temporal air–sea CO2 flux variability in the changing Arctic Ocean. There is also uncertainty as to whether the previous basin-wide surveys are representative of the many smaller bays and inlets that make up the Canadian Arctic Archipelago (CAA). By using a research vessel that is based in the remote Inuit community of Ikaluqtuutiak (Cambridge Bay, Nunavut), we have been able to reliably survey pCO2 shortly after ice melt and access previously unsampled bays and inlets in the nearby region. Here we present 4 years of consecutive summertime pCO2 measurements collected in the Kitikmeot Sea in the southern CAA. Overall, we found that this region is a sink for atmospheric CO2 in August (average of all calculated fluxes over the four cruises was −4.64 mmol m−2 d−1), but the magnitude of this sink varies substantially between years and locations (average calculated fluxes of +3.58, −2.96, −16.79 and −0.57 mmol m−2 d−1 during the 2016, 2017, 2018 and 2019 cruises, respectively). Surface ocean pCO2 varied by up to 156 µatm between years, highlighting the importance of repeat observations in this region, as this high interannual variability would not have been captured by sparse and infrequent measurements. We find that the surface ocean pCO2 value at the time of ice melt is extremely important in constraining the magnitude of the air–sea CO2 flux throughout the ice-free season. However, further constraining the air–sea CO2 flux in the Kitikmeot Sea will require a better understanding of how pCO2 changes outside of the summer season. Surface ocean pCO2 measurements made in small bays and inlets of the Kitikmeot Sea were ∼ 20–40 µatm lower than in the main channels. Surface ocean pCO2 measurements made close in time to ice breakup (i.e. within 2 weeks) were ∼ 50 µatm lower than measurements made &gt; 4 weeks after breakup. As previous basin-wide surveys of the CAA have focused on the deep shipping channels and rarely measure close to the ice breakup date, we hypothesize that there may be an observational bias in previous studies, leading to an underestimate of the CO2 sink in the CAA. These high-resolution measurements constitute an important new baseline for gaining a better understanding of the role this region plays in the uptake of atmospheric CO2.

Spatial and temporal variations in sea surface pCO2 and air-sea flux of CO2 in the Bering Sea revealed by satellite-based data during 2003–2019

The understanding of long-time-series variations in air-sea CO2 flux in the Bering Sea is critical, as it is the passage area from the North Pacific Ocean water to the Arctic. Here, a data-driven remote sensing retrieval method is constructed based on a large amount of underway partial pressure of CO2 (pCO2) data in the Bering Sea. After several experiments, a Gaussian process regression model with input parameters of sea surface temperature, sea surface height, mixed-layer depth, chlorophyll a concentration, dry air mole fractions of CO2, and bathymetry was selected. After validation with independent data, the root mean square error of pCO2 was&amp;lt; 24 μatm (R2 = 0.94) with satisfactory performance. Then, we reconstructed the sea surface pCO2 in the Bering Sea from 2003 to 2019 and estimated the corresponding air-sea CO2 fluxes. Significant seasonal variations were identified, with higher sea surface pCO2 in winter/spring than in summer/autumn in both the basin and shelf area. Semiquantitative analysis reveals that the Bering Sea is a non-temperature-dominated area with a mean temperature effect on pCO2 of 12.7 μatm and a mean non-temperature effect of −51.8 μatm. From 2003 to 2019, atmospheric pCO2 increased at a rate of 2.1 μatm yr−1, while sea surface pCO2 in the basin increased rapidly (2.8 μatm yr−1); thus, the CO2 emissions from the basin increased. However, the carbon sink in the continental shelf still continuously increased. The whole Bering Sea exhibited an increasing carbon sink with the area integral of air-sea CO2 fluxes increasing from 6 to 19 TgC over 17 years. Meanwhile, the seasonal amplitudes in pCO2 in the shelf area also increased, approaching 14 μatm per decade. The reaction of the continuously added CO2 in continental seawater reduced the ocean CO2 system capacity. This is the first study to present long-time-series satellite data with high resolution in the Bering Sea, which is beneficial for studying the changes in ocean ecosystems and carbon sink capacity.

The distribution and emission of CO2, CH4 and light hydrocarbons in an anticyclonic eddy of the Kuroshio extension

Mesoscale eddies are energetic and swirling circulations that frequently occur in the open ocean. The effects of mesoscale eddies on the biogeochemical cycling of climate-relevant gases remain poorly constrained. We investigated the distribution and air-sea fluxes of CO2, methane, and five non–methane hydrocarbons (NMHCs) in an anticyclone eddy of Kuroshio Extension during September 2019. Within eddy core, intense stratification hindered the replenishment of nutrients and favored the growth of small-size phytoplankton, such as Prochlorococcus. Seawater pCO2 decreased from 406.1 μatm at the eddy outside to 377.5 μatm at the eddy core, accompanied by a decrease in surface seawater temperature from 26.7 °C to 25.2 °C. The vertical distribution of methane (0.3-9.9 nmol L-1) was influenced by the eddy process, with a maximum at 80 m in the eddy core, which might be attributed to the degradation of phosphonates sustained by Prochlorococcus. The concentrations of five NMHCs (ethane, ethylene, propane, propylene, and isoprene) ranged from 17.2-126.2, 36.7-168.1, 7.5-29.2, 22.6-64.1, 54.5-172.1, 3.5-27.9 pmol L-1, respectively. Isoprene correlated well with Chl-a concentrations at the eddy core, while no significant correlation was observed at the eddy outside. Air-sea fluxes of CO2 and isoprene associated with the eddy core were higher than those of the eddy outside, while the maximum ventilation of methane and other NMHCs (ethane, ethylene, propane, and propylene) was found at the eddy outside. Collectively, physical processes such as eddies impact the production and distribution of light hydrocarbons in seawater and further influence their regional emissions to the atmosphere.

Assessment of austral autumn air–sea CO2 exchange in the Pacific sector of the Southern Ocean and dominant controlling factors

The factors that control the partial pressure of carbon dioxide (pCO2) in the Pacific sector of the Southern Ocean were investigated in April 2018, onboard the icebreaker, ARAON. The mean (± 1σ) of the sea surface pCO2 was estimated to be 431 ± 6 μatm in the north of the Ross Sea (NRS), 403 ± 18 μatm in the Amundsen–Bellingshausen Sea (ABS), and 426 ± 16 μatm in the western Antarctic Peninsula and Weddell Sea (WAP/WS). The controlling factors for pCO2 in the NRS appeared to be meridionally different based on the southern boundary of the Antarctic Circumpolar Current (SB; ~62.5°S in the Ross Sea). The sea surface pCO2 exhibited a strong correlation with salinity and the difference between the O2/Ar (ΔO2/Ar) values of the sample and air-saturated water in the north and south of the SB, respectively. The pCO2 in the ABS and western WAP/WS displayed a strong correlation with salinity. Furthermore, ΔO2/Ar and sea ice formation appear to be the dominant factors that control pCO2 in the Confluence Zone (CZ) and northern parts of WAP/WS. The estimated air–sea CO2 fluxes (positive and negative values indicate the source and sink for atmospheric CO2, respectively) range from 3.1 to 18.8 mmol m−2 d−1 in the NRS, −12.7 to 17.3 mmol m−2 d−1 in the ABS, and −59.4 to 140.8 mmol m−2 d−1 in the WAP/WS. In addition, biology-driven large variations in the air–sea CO2 flux were observed in the CZ. Our results are the most recent observation data acquired in austral autumn in the Southern Ocean.