Ocean pCO2 Research Articles

Impact of coastal upwelling dynamics on the pCO2 variability in the southeastern Arabian Sea

The southeastern Arabian Sea (SEAS) experiences moderate to weak upwelling along the southwest coast of India during the southwest monsoon (SM). The coastal upwelling initiates from mid-May along the southern-most tip of India, and with the advancement of SM, it propagates towards the northern latitude. This study examines the impact of coastal upwelling dynamics on the spatio-temporal variability of pCO2 in SEAS. It aims to identify the factors controlling the variability of pCO2 using high-resolution, regional ocean-ecosystem model-simulated outputs while available in situ and ship-based observations are used to establish the capability of the model. The cold deeper water that rises to the surface during upwelling decreases surface ocean pCO2 by 50 ± 2.4 µatm, whereas the presence of carbon-rich upwelling waters to a significantly shallower depth increases pCO2 by 52 ± 1.5 µatm of SEAS during SM. The salinity component increases surface ocean pCO2 by 2.0 ± 1.6 µatm. It has relatively less impact when compared with the individual effects of temperature and dissolved inorganic carbon components in controlling surface ocean pCO2 variability. The biological activities are profound only in the southern part of SEAS where biology-driven changes decrease surface ocean pCO2 by 4.0 ± 0.4 µatm during SM. The total biology-driven changes consist of both soft and hard tissues decrease the pCO2 level of SEAS by 2.0 ± 0.2 µatm during SM. Therefore, the upwelling-driven physical dynamics dominate the biological processes in controlling the spatio-temporal variability of surface ocean pCO2 in SEAS during SM, which contrasts the physical-biological dynamics of the sea east of Sri Lanka, where the biological processes dominate over physical dynamics in controlling surface ocean pCO2 variability.

Autonomous Wintertime Observations of Air‐Sea Exchange in the Gulf Stream Reveal a Perfect Storm for Ocean CO2 Uptake

AbstractA scarcity of wintertime observations of surface ocean carbon dioxide partial pressure (pCO2) in and near the Gulf Stream creates uncertainty in the magnitude of the regional carbon sink and its controlling mechanisms. Recent observations from an Uncrewed Surface Vehicle (USV), outfitted with a payload to measure surface ocean and lower atmosphere pCO2, revealed sharp gradients in ocean pCO2 across the Gulf Stream. Surface ocean pCO2 was lower by ∼50 μatm relative to the atmosphere in the subtropical mode water (STMW) formation region. This undersaturation combined with strong wintertime winds allowed for rapid ocean uptake of CO2, averaging −11.5 mmol m−2 day−1 during the February 2019 USV mission. The unique timing of this mission revealed active STMW formation. The USV proved to be a useful tool for CO2 flux quantification in the poorly observed, dynamic western boundary current environment.

Growth rate rather than temperature affects the B∕Ca ratio in the calcareous red alga <i>Lithothamnion corallioides</i>

Abstract. The B/Ca ratio in calcareous marine species is informative of past seawater CO32- concentrations, but scarce data exist on B/Ca in coralline algae. Recent studies suggest influences of temperature and growth rates on B/Ca, the effect of which could be critical for the reconstructions of surface ocean pH and atmospheric pCO2. In this paper, we present the first laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of Mg, Sr, Li, and B in the coralline alga Lithothamnion corallioides collected from different geographic settings and depths across the Mediterranean Sea and in the Atlantic Ocean. We produced the first data on putative temperature proxies (Mg/Ca, Li/Ca, Sr/Ca, Mg/Li) and B/Ca in a coralline algal species grown in different basins from across the photic zone (12, 40, 45, and 66 m depth). We tested the B/Ca correlation with temperature proxies and growth rates in order to evaluate their possible effect on B incorporation. Our results suggested a growth rate influence on B/Ca, which was evident in the sample with the lowest growth rate of 0.10 mm yr−1 (Pontian Isl., Italy; 66 m depth) and in Elba (Italy; 45 m depth), where the algal growth rate was the highest (0.14 mm yr−1). At these two sites, the measured B/Ca was the lowest at 462.8 ± 49.2 µmol mol−1 and the highest at 757.7 ± 75.5 µmol mol−1, respectively. A positive correlation between B/Ca and temperature proxies was found only in the shallowest sample from Morlaix (Atlantic coast of France; 12 m depth), where the amplitude of temperature variation (ΔT) was the highest (8.9 ∘C). Still, fluctuations in B/Ca did not mirror yearly seasonal temperature oscillations as for Mg/Ca, Li/Ca, and Sr/Ca. We concluded that growth rates, triggered by the different ΔT and light availability across depth, affect the B incorporation in L. corallioides.

Reconstruction of global surface ocean <i>p</i>CO<sub>2</sub> using region-specific predictors based on a stepwise FFNN regression algorithm

Abstract. Various machine learning methods were attempted in the global mapping of surface ocean partial pressure of CO2 (pCO2) to reduce the uncertainty of the global ocean CO2 sink estimate due to undersampling of pCO2. In previous research, the predictors of pCO2 were usually selected empirically based on theoretic drivers of surface ocean pCO2, and the same combination of predictors was applied in all areas except where there was a lack of coverage. However, the differences between the drivers of surface ocean pCO2 in different regions were not considered. In this work, we combined the stepwise regression algorithm and a feed-forward neural network (FFNN) to select predictors of pCO2 based on the mean absolute error in each of the 11 biogeochemical provinces defined by the self-organizing map (SOM) method. Based on the predictors selected, a monthly global 1∘ × 1∘ surface ocean pCO2 product from January 1992 to August 2019 was constructed. Validation of different combinations of predictors based on the Surface Ocean CO2 Atlas (SOCAT) dataset version 2020 and independent observations from time series stations was carried out. The prediction of pCO2 based on region-specific predictors selected by the stepwise FFNN algorithm was more precise than that based on predictors from previous research. Applying the FFNN size-improving algorithm in each province decreased the mean absolute error (MAE) of the global estimate to 11.32 µatm and the root mean square error (RMSE) to 17.99 µatm. The script file of the stepwise FFNN algorithm and pCO2 product are distributed through the Institute of Oceanology of the Chinese Academy of Sciences Marine Science Data Center (IOCAS, https://doi.org/10.12157/iocas.2021.0022, Zhong, 2021.

Improved Quantification of Ocean Carbon Uptake by Using Machine Learning to Merge Global Models and pCO2 Data

AbstractThe ocean plays a critical role in modulating climate change by sequestering CO2 from the atmosphere. Quantifying the CO2 flux across the air‐sea interface requires time‐dependent maps of surface ocean partial pressure of CO2 (pCO2), which can be estimated using global ocean biogeochemical models (GOBMs) and observational‐based data products. GOBMs are internally consistent, mechanistic representations of the ocean circulation and carbon cycle, and have long been the standard for making spatio‐temporally resolved estimates of air‐sea CO2 fluxes. However, there are concerns about the fidelity of GOBM flux estimates. Observation‐based products have the strength of being data‐based, but the underlying data are sparse and require significant extrapolation to create global full‐coverage flux estimates. The Lamont Doherty Earth Observatory‐Hybrid Physics Data (LDEO‐HPD) pCO2 product is a new approach to estimating the temporal evolution of surface ocean pCO2 and air‐sea CO2 exchange. LDEO‐HPD uses machine learning to merge high‐quality observations with state‐of‐the‐art GOBMs. We train an eXtreme Gradient Boosting (XGB) algorithm to learn a non‐linear relationship between model‐data mismatch and observed predictors. GOBM fields are then corrected with the predicted model‐data misfit to estimate real‐world pCO2 for 1982–2018. The resulting reconstruction by LDEO‐HPD is in better agreement with independent pCO2 observations than other currently available observation‐based products. Within uncertainties, LDEO‐HPD global ocean uptake of CO2 agrees with other products and the Global Carbon Budget 2020.

Seasonal Variations and Drivers of Surface Ocean pCO2 in the Seasonal Ice Zone of the Eastern Indian Sector, Southern Ocean

AbstractTo quantitatively assess the inorganic carbon cycle in the eastern Indian sector of the Southern Ocean (80–150°E, south of 60°S), we measured ocean surface temperature, salinity, total alkalinity (TA), the partial pressure of carbon dioxide (pCO2), and concentrations of chlorophyll‐a (chl a), dissolved inorganic carbon (DIC), and nutrients during the KY18 survey (December 2018–January 2019). The sea–air CO2 flux in this region was −8.3 ± 12.7 mmol m−2 day−1 (−92.1 to +10.6 mmol m−2 day−1). The ocean was therefore a weak CO2 sink. Based on the DIC and TA in the temperature minimum layer, we estimated the change of pCO2 from winter to summer (δpCO2) due to changes in water temperature, salinity, and biological activity (photosynthesis). The spatial distribution of pCO2 in the western part (80–110°E) of the study area was mainly driven by biological activity, which decreased pCO2 from December to early January, and in the eastern part (110–150°E) by temperature, which increased pCO2 from January to February. We also examined the changes in the CO2 concentrations (xCO2) over time by comparing data from 1996 with our data (2018–2019). The oceanic and atmospheric xCO2 increased by 23 and 45 ppm in 23 years, respectively. These changes of ocean xCO2 were mainly driven by an increase in CO2 uptake from the atmosphere as a result of the rise in atmospheric xCO2 and increase in biological activity associated with the change in the water‐mass distribution.

Seasonal to decadal spatiotemporal variations of the global ocean carbon sink.

The global ocean has absorbed approximately 30% of anthropogenic CO2 since the beginning of the industrial revolution. However, the spatiotemporal evolution of this important global carbon sink varies substantially on all timescales and has not yet been well evaluated. Here, based on a reconstructed observation‐based product of surface ocean pCO2 and air–sea CO2 flux (the MPI‐SOMFFN method), we investigated seasonal to decadal spatiotemporal variations of the ocean CO2 sink during the past three decades using an adaptive data analysis method. Two predominant variations are modulated annual cycles and decadal fluctuations, which account for approximately 46% and 25% of all extracted components, respectively. Although the whole summer to non‐summer seasonal difference pattern is determined by the Southern Ocean, the non‐summer CO2 sink at mid‐latitudes in both hemispheres shows an increasing trend (a total increase of approximately 1.0 PgC during the period 1982–2019), while it is relatively stable in summer. On decadal timescales for the global ocean carbon sink, unlike the weakening decade (1990–1999) and the reinvigoration decade (2000–2009) in which the Southern Ocean plays the dominant role, the reinforcement decade (2010–2019) is mainly the result from the weakening source effect in the equatorial Pacific Ocean. Our results suggest that except for the Southern Ocean's role in the global ocean carbon sink, the strengthening non‐summer's sink at mid‐latitudes in both hemispheres and the decadal or longer timescales of equatorial Pacific Ocean dynamics should be fully considered in understanding the oceanic carbon cycle on a global scale.

A Surface pCO2 Increasing Hiatus in the Equatorial Pacific Ocean Since 2010

AbstractWe used multiple data sets to investigate the decadal and longer time scale variability of sea surface partial pressure of CO2 (pCO2) over the equatorial Pacific Ocean from 1990 to 2019. Unlike the increasing trend in the global oceanic pCO2, both the Niño 3.4 and warm pool regions featured distinct decadal variabilities. A pCO2 increasing stagnation, a hiatus stage, was identified in the Niño 3.4 region from 2010 to 2019. Further analysis demonstrated that low‐frequency pCO2 was negatively correlated with the Interdecadal Pacific Oscillation (IPO) index. Both the 2009/10 and 2015/16 El Niño events and an increasing phase of IPO contributed to this long pCO2 stagnation. Correspondingly, a weaker upwelling was induced in the central and eastern tropical Pacific as the trade winds weakened and thus stagnated the increase in pCO2 in the Niño 3.4 region. Our results imply a significant impact of climate variabilities on sea surface pCO2 in the equatorial Pacific.

Seasonal cycle of surface ocean pCO2 and pH in the northern Indian Ocean and their controlling factors

This paper examines the seasonal variability of surface ocean pCO2and pH in the northern Indian Ocean. It aims to identify their controlling factors using a high-resolution, regional ocean-ecosystem model simulation and available surface ocean carbon observations. The seasonal variability of pCO2 and pH of the northern Indian Ocean is attributed to the changes in surface temperature (T), dissolved inorganic carbon (DIC), total alkalinity (ALK), and salinity (S). The western Arabian Sea has an enormous seasonal variance of pCO2 due to coastal upwelling dynamics. In contrast, the seasonal variance of pCO2 in the Bay of Bengal is governed by the upper ocean mixed layer dynamics, albeit with smaller amplitudes. The contribution of T (DIC) in the seasonal variance of pCO2 and pH at the western Arabian Sea is ±90 (∓100) μatm and ±0.18 (∓0.20) pH units, respectively. The contribution of ALK and S is complementary to each other in inducing seasonal variances of pCO2 and pH in the southeastern Arabian Sea with a magnitude of ±5∼10 μatm and ±0.02 pH units, respectively. In the northern Bay of Bengal, salinity plays a significant role in controlling seasonal variability of pCO2 and pH with amplitudes of roughly ±20 µatm and ±0.18 pH units, respectively, along the pathways of freshwater spreading. The maxima of seasonal amplitudes of pCO2 and pH occur during April-May and August-September for both basins. This study highlights the hotspots of pCO2 and pH in the northern Indian Ocean, where observational efforts to monitor pCO2 and pHbecome an essential requirement.

Observation system simulation experiments in the Atlantic Ocean for enhanced surface ocean <i>p</i>CO<sub>2</sub> reconstructions

Abstract. To derive an optimal observation system for surface ocean pCO2 in the Atlantic Ocean and the Atlantic sector of the Southern Ocean, 11 observation system simulation experiments (OSSEs) were completed. Each OSSE is a feedforward neural network (FFNN) that is based on a different data distribution and provides ocean surface pCO2 for the period 2008–2010 with a 5 d time interval. Based on the geographical and time positions from three observational platforms, volunteering observing ships, Argo floats and OceanSITES moorings, pseudo-observations were constructed using the outputs from an online-coupled physical–biogeochemical global ocean model with 0.25∘ nominal resolution. The aim of this work was to find an optimal spatial distribution of observations to supplement the widely used Surface Ocean CO2 Atlas (SOCAT) and to improve the accuracy of ocean surface pCO2 reconstructions. OSSEs showed that the additional data from mooring stations and an improved coverage of the Southern Hemisphere with biogeochemical ARGO floats corresponding to least 25 % of the density of active floats (2008–2010) (OSSE 10) would significantly improve the pCO2 reconstruction and reduce the bias of derived estimates of sea–air CO2 fluxes by 74 % compared to ocean model outputs.

Analysis of Environmental Controls on the Quasi-Ocean and Ocean CO2 Concentration by Two Intelligent Algorithms

Previous studies have demonstrated CO2 absorption by soils in arid regions, where the absorbed CO2 is conjectured to be finally sequestrated in the “subterranean ocean”—groundwater. This study compares environmental controls of ocean CO2 concentration (surface ocean pCO2) and quasi-ocean CO2 concentration (deep-soil pCO2). We aim to explore the latent relationship, both linear and nonlinear between the environmental variables, and CO2 concentration, utilizing two intelligent algorithms—the partial least linear regression (PLSR) algorithm and the artificial neural network (ANN) algorithm. For quasi-ocean CO2 concentration, RPD <1.4 and R2 <40%. While for ocean CO2 concentration, RPD >1.4 and R2 is 99.7%. Linear relationships between the considered environmental controls and ocean CO2 concentration are proved; however, there is no evident relationship between most of the considered environmental controls and quasi-ocean CO2 concentration. Groundwater level is proved to be a relatively important environmental control on the quasi-ocean CO2 concentration, suggesting groundwater discharge/recharge as a significant modulator of soil CO2 absorption in arid regions.

Fluorine incorporation into calcite, aragonite and vaterite CaCO3: Computational chemistry insights and geochemistry implications

The abundant occurrence of calcium carbonate minerals in marine sediments and their high fluorine content suggests that fluorine is a good candidate for reconstructing paleoceanographic parameters. However, the potential of fluorine as a paleoproxy had hardly been explored, and fundamental insights into the behaviour of fluorine in biogenic carbonates and marine sediments is required. A first-principles modelling approach is used here to analyse the incorporation mechanisms of fluorine into crystalline calcium carbonates. We compute F incorporation into the CaCO3 lattice via a number of mechanisms, but concentrate on comparison of the energetics of the two easiest substitution mechanisms: replacing one oxygen atom within the carbonate group to form a (CO2F)− group as against a substitution involving replacement of the CO3 group by two fluorine ions to form a CaF2 defect. These incorporation mechanisms are fundamentally different from that of iodine into calcium carbonates, where a carbon atom is replaced. Our simulations suggest that the substitution of CO32- by F22- is the most favoured and that fluorine is preferentially incorporated into the three naturally-occurring polymorphs of calcium carbonate in the order vaterite ⪆ aragonite ≫ calcite. These results explain the previously-reported preponderance of fluorine in aragonite corals, and lend support to the use of F/Ca as a proxy for ocean pCO2.

Impacts of Multiple Stressors on a Benthic Foraminiferal Community: A Long-Term Experiment Assessing Response to Ocean Acidification, Hypoxia and Warming

Ocean chemistry is changing as a result of human activities. Atmospheric carbon dioxide (CO2) concentrations are increasing, causing an increase in oceanic pCO2 that drives a decrease in oceanic pH, a process called ocean acidification (OA). Higher CO2 concentrations are also linked to rising global temperatures that can result in more stratified surface waters, reducing the exchange between surface and deep waters; this stronger stratification, along with nutrient pollution, contributes to an expansion of oxygen-depleted zones (so called hypoxia or deoxygenation). Determining the response of marine organisms to environmental changes is important for assessments of future ecosystem functioning. While many studies have assessed the impact of individual or paired stressors, fewer studies have assessed the combined impact of pCO2, O2, and temperature. A long-term experiment (∼10 months) with different treatments of these three stressors was conducted to determine their sole or combined impact on the abundance and survival of a benthic foraminiferal community collected from a continental-shelf site. Foraminifera are well suited to such study because of their small size, relatively rapid growth, varied mineralogies and physiologies. Inoculation materials were collected from a ∼77-m deep site south of Woods Hole, MA. Very fine sediments (<53 μm) were used as inoculum, to allow the entire community to respond. Thirty-eight morphologically identified taxa grew during the experiment. Multivariate statistical analysis indicates that hypoxia was the major driving factor distinguishing the yields, while warming was secondary. Species responses were not consistent, with different species being most abundant in different treatments. Some taxa grew in all of the triple-stressor samples. Results from the experiment suggest that foraminiferal species’ responses will vary considerably, with some being negatively impacted by predicted environmental changes, while other taxa will tolerate, and perhaps even benefit, from deoxygenation, warming and OA.

An observing system simulation experiment for Indian Ocean surface pCO2 measurements

An observing system simulation experiment (OSSE) is conducted to identify potential locations for making surface ocean pCO2 measurements in the Indian Ocean using the Bayesian Inversion method. As of the SOCATv3 release, the pCO2 data is limited in the Indian Ocean. To improve our modeling of this region, we need to identify where and what observation systems would produce the most good or benefit for their cost. The potential benefits of installing pCO2 sensors in the existing RAMA and OMNI moorings of the Indian Ocean, the potential of Bio-Argo floats (with pH measurements), and the implementation of the ship of opportunity program (SOOP) for underway sampling of pCO2 are evaluated. A cost function of dissolved inorganic carbon as a model state vector and CO2 flux mismatch as the source of error is minimized, and the basin-wide CO2 flux uncertainty reduction is estimated for different seasons. The maximum flux uncertainty reduction achievable by installing pCO2 sensors in the existing RAMA and OMNI moorings is limited to 30% during different seasons. One may consider that around 20 Bio-Argos are still the right choice over installing mooring based pCO2 sensors and achieve uncertainty reduction up to 50% with additional benefit of profiling the sub-surface upto 1000–2000 m. However, a single track SOOP has the potential to reduce the uncertainty by approximately 62%. This study identifies vital RAMA and OMNI moorings and SOOP tracks for observing Indian Ocean pCO2.Plain Language Summary.Surface ocean partial pressure of CO2 (pCO2) information is vital for estimating sea-to-air CO2 exchanges. This parameter is least available from the Indian Ocean as compared to other global tropical and southern oceans. There has been no effort made so far to measure surface ocean pCO2 in the Indian Ocean with routine monitoring such as by mounting instruments to moorings or by underway sampling via any ship of opportunity program. Therefore there is a considerable demand to start pCO2 observations in the Indian Ocean. However, one key question that emerges is where to deploy pCO2 instruments in the Indian Ocean to learn the most with limited resources. This study addresses this question with inverse modeling techniques. The study finds that the existing moorings of the Indian Ocean are capable of hosting pCO2 sensors, and data from those are useful to reduce the uncertainty in the surface sea-to-air CO2 flux estimation by a quarter magnitude. In contrast, the Bio-Argo floats with pH sensors, and the ship of opportunity underway sampling of pCO2 may benefit from reducing the same up to 50% and 62%, respectively.

Quantifying the Atmospheric CO2 Forcing Effect on Surface Ocean pCO2 in the North Pacific Subtropical Gyre in the Past Two Decades

Despite the well-recognized importance in understanding the long term impact of anthropogenic release of atmospheric CO2 (its partial pressure named as pCO2air) on surface seawater pCO2 (pCO2sw), it has been difficult to quantify the trends or changing rates of pCO2sw driven by increasing atmospheric CO2 forcing (pCO2swatm_forced) due to its combination with the natural variability of pCO2sw (pCO2swnat_forced) and the requirement of long time series data records. Here, using a novel satellite-based pCO2sw model with inputs of ocean color and other ancillary data between 2002 and 2019, we address this challenge for a mooring station at the Hawaii Ocean Time-series Station in the North Pacific subtropical gyre. Specifically, using the developed pCO2sw model, we differentiated and separately quantified the interannual-decadal trends of pCO2swnat_forced and pCO2swatm_forced. Between 2002 and 2019, both pCO2sw and pCO2air show significant increases at rates of 1.7 ± 0.1 μatm yr–1 and 2.2 ± 0.1 μatm yr–1, respectively. Correspondingly, the changing rate in pCO2swnat_forced is mainly driven by large scale forcing such as Pacific Decadal Oscillation, with a negative rate (-0.5 ± 0.2 μatm yr–1) and a positive rate (0.6 ± 0.3 μatm yr–1) before and after 2013. The pCO2swatm_forced shows a smaller increasing rate of 1.4 ± 0.1 μatm yr–1 than that of the modeled pCO2sw, varying in different time intervals in response to the variations in atmospheric pCO2. The findings of decoupled trends in pCO2swatm_forced and pCO2swnat_forced highlight the necessity to differentiate the two toward a better understanding of the long term oceanic absorption of anthropogenic CO2 and the anthropogenic impact on the changing surface ocean carbonic chemistry.

Role of river discharge and warming on ocean acidification and pCO2 levels in the Bay of Bengal

Shifts in surface ocean pCO2 and pH are important controls governing global climate. Based on the linear relationship of observed surface pH and pCO2 with sea surface temperature (SST), sea surface salinity (SSS) and Chlorophyll-a (Chl-a) multiple linear regression equations were developed. Based on remote sensing SST, Chl-a and model-derived SSS, pH and pCO2 data were derived from 1998 to 2015. Overall warming of BoB is noticed at the rate of 0.004° to 0.03 °C/y whereas cooling is found in the northwestern BoB during winter and spring seasons associated with an increase in atmospheric dust. Decrease in SSS is noticed during all seasons due to melting of Himalayan ice cover associated with increase in fresh water flux due to increase in atmospheric temperature. Increase in pH is observed in the eastern and southern Bay during all seasons associating with warming and decrease in salinity. In contrast, decrease in pH (−0.001 y−1) and pCO2 increase (+0.1 to +0.7 µatm y−1) is noticed in the western and head Bay during winter and spring seasons due to deposition of atmospheric pollutants. This study suggests that increase in freshwater input due to melting of Himalayan ice cover and deposition of atmospheric pollutants are dominant controlling factors on surface ocean pH and pCO2 in the BoB between 1998 and 2015 and this region is acting as a stronger sink for the atmospheric CO2 in the present than that in the past two decades. The global coastal regions are significantly influenced by river discharge and atmospheric deposition of pollutants and they are not part of the global models leading to ill-reproduction of seasonal variability in pH and pCO2. Inclusion of these processes may improve prediction of pH and pCO2 in the regions heavily influenced by discharge/deposition from land and atmosphere.

Irradiance, photosynthesis and elevated pCO2 effects on net calcification in tropical reef macroalgae

Calcifying tropical macroalgae produce sediment, build three-dimensional habitats, and provide substrate for invertebrate larvae on reefs. Thus, lower calcification rates under declining pH and increasing ocean pCO2, or ocean acidification, is a concern. In the present study, calcification rates were examined experimentally under predicted end-of-the-century seawater pCO2 (1116 μatm) and pH (7.67) compared to ambient controls (pCO2 409 μatm; pH 8.04). Nine reef macroalgae with diverse calcification locations, calcium carbonate structure, photophysiology, and site-specific irradiance were examined under light and dark conditions. Species included five from a high light patch reef on the Florida Keys Reef Tract (FKRT) and four species from low light reef walls on Little Cayman Island (LCI). Experiments on FKRT and LCI species were conducted at 500 and 50 μmol photons m−2 s−1 in situ irradiance, respectively. Calcification rates independent of photosystem-II (PSII) were also investigated for FKRT species. The most consistent negative effect of elevated pCO2 on calcification rates in the tropical macroalgae examined occurred in the dark. Most species (89%) had net calcification rates of zero or net dissolution in the dark at low pH. Species from the FKRT that sustained positive net calcification rates in the light at low pH also maintained ~30% of their net calcification rates without PSII at ambient pH. However, calcification rates in the light independent of PSII were not sustained at low pH. Regardless of these low pH effects, most FKRT species daily net calcification rates, integrating light/dark rates over a 24h period, were not significantly different between low and ambient pH. This was due to a 10-fold lower dark, compared to light, calcification rate, and a strong correspondence between calcification and photosynthetic rates. Interestingly, low-light species sustained calcification rates on par with high-light species without high rates of photosynthesis. Low-light species' morphology and physiology that promote high calcification rates at ambient pH, may increase their vulnerability to low pH. Our data indicate that the negative effect of elevated pCO2 and low pH on tropical macroalgae at the organismal level is their impact on dark net calcification, probably enhanced dissolution. However, elevated pCO2 and low pH effects on macroalgae daily calcification rates are greatest in species with lower net calcification rates in the light. Thus, macroalgae able to maintain high calcification rates in the light (high and low irradiance) at low pH, and/or sustain strong biotic control with high [H+] in the bulk seawater, are expected to dominate under global change.

Consistency and Challenges in the Ocean Carbon Sink Estimate for the Global Carbon Budget

Based on the 2019 assessment of the Global Carbon Project, the ocean took up on average, 2.5+/-0.6PgCyr-1 or 23+/-5% of the total anthropogenic CO2 emissions over the decade 2009-2018. This sink estimate is based on global ocean biogeochemical models (GOBMs) and is compared to data-products based on surface ocean pCO2 (partial pressure of CO2) observations accounting for the outgassing of river-derived CO2. Here we evaluate the GOBM simulations by comparing the simulated pCO2 to observations. The simulations are well suited for quantifying the global ocean carbon sink on the time-scale of the annual mean and its multi-decadal trend (RMSE <20 μatm), as well as on the time-scale of multi-year variability (RMSE <10 μatm), despite the large model-data mismatch on the seasonal time-scale (RMSE of 20-80 μatm). Biases in GOBMs have a small effect on the global mean ocean sink (0.05 PgC yr−1), but need to be addressed to improve the regional budgets and model-data comparison. Accounting for non-mapped areas in the data-products reduces their spread as measured by the standard deviation by a third. There is growing evidence and consistency among methods with regard to the patterns of the multi-year variability of the ocean carbon sink, with a global stagnation in the 1990s and an extra-tropical strengthening in the 2000s. GOBMs and data-products point consistently to a shift from a tropical CO2 source to a CO2 sink in recent years. On average, the GOBMs reveal less variations in the sink than the data-based products. Despite the reasonable simulation of surface ocean pCO2 by the GOBMs, there are discrepancies between the resulting sink estimate from GOBMs and data-products. These discrepancies are within the uncertainty of the river flux adjustment, increase over time, and largely stem from the Southern Ocean. Progress in our understanding of the global ocean carbon sink necessitates significant advancement in modelling and observing the Southern Ocean including (i) a game-changing increase in high-quality pCO2 observations, and (ii) a critical re-evaluation of the regional river flux adjustment.

The ECCO‐Darwin Data‐Assimilative Global Ocean Biogeochemistry Model: Estimates of Seasonal to Multidecadal Surface Ocean pCO2 and Air‐Sea CO2 Flux

AbstractQuantifying variability in the ocean carbon sink remains problematic due to sparse observations and spatiotemporal variability in surface ocean pCO2. To address this challenge, we have updated and improved ECCO‐Darwin, a global ocean biogeochemistry model that assimilates both physical and biogeochemical observations. The model consists of an adjoint‐based ocean circulation estimate from the Estimating the Circulation and Climate of the Ocean (ECCO) consortium and an ecosystem model developed by the Massachusetts Institute of Technology Darwin Project. In addition to the data‐constrained ECCO physics, a Green's function approach is used to optimize the biogeochemistry by adjusting initial conditions and six biogeochemical parameters. Over seasonal to multidecadal timescales (1995–2017), ECCO‐Darwin exhibits broad‐scale consistency with observed surface ocean pCO2 and air‐sea CO2 flux reconstructions in most biomes, particularly in the subtropical and equatorial regions. The largest differences between CO2 uptake occur in subpolar seasonally stratified biomes, where ECCO‐Darwin results in stronger winter uptake. Compared to the Global Carbon Project OBMs, ECCO‐Darwin has a time‐mean global ocean CO2 sink (2.47 ± 0.50 Pg C year−1) and interannual variability that are more consistent with interpolation‐based products. Compared to interpolation‐based methods, ECCO‐Darwin is less sensitive to sparse and irregularly sampled observations. Thus, ECCO‐Darwin provides a basis for identifying and predicting the consequences of natural and anthropogenic perturbations to the ocean carbon cycle, as well as the climate‐related sensitivity of marine ecosystems. Our study further highlights the importance of physically consistent, property‐conserving reconstructions, as are provided by ECCO, for ocean biogeochemistry studies.

Amplification of the Ocean Carbon Sink During El Niños: Role of Poleward Ekman Transport and Influence on Atmospheric CO2

AbstractAmplification of the ocean carbon sink during El Niño events partially offset terrestrial biosphere carbon losses to the atmosphere, but uncertainties in the magnitude, timing, and spatial extent of the ocean response confound our understanding of the global carbon budget and its sensitivity to climate. Here, we examine the mechanisms controlling the anomalous tropical Pacific Ocean CO2 drawdown during El Niño events harnessing multidecadal ocean pCO2 observations in conjunction with a state‐of‐the‐art ocean biogeochemical model. We show that poleward Ekman transport dramatically amplifies the near‐equatorial pCO2 anomaly identified in prior work and that this amplification varies considerably between El Niño events. During central Pacific events, the CO2 flux anomaly can vary up to fourfold between events (2002/2003 vs. 2015/2016), but it always recedes during the boreal winter to spring transition and the poleward transport of the anomaly mostly extends into the Northern Hemisphere. During eastern Pacific events characterized by an equatorially centered intertropical convergence zone (e.g., 1997/1998), the intense CO2 flux anomaly persists into boreal summer and can extend further into the Southern Hemisphere. The hemispheric asymmetry (northward vs. southward extension) and the termination (boreal winter/spring for central Pacific events vs. boreal summer for eastern Pacific events) of the ocean CO2 response are tied to the El Niño dynamics in the near‐equatorial region and also how it is amplified by poleward Ekman transport. Finally, we evaluate how differences in the ocean response influence atmospheric CO2 and discuss the potential of atmospheric CO2 satellite data to provide observational constraints.