Compound extremes of temperature and acidity that extend over substantial fractions of the water column can be particularly damaging to marine organisms, as they experience not only additional stress by the potentially synergistic effects of these two stressors, but also a reduction in habitable vertical space. Here, we detect and analyse such column-compound extremes (CCX) in the Southern Ocean between 1980 and 2019, and characterise their duration, intensity, and spatial extent. To this end, we use daily output from a hindcast simulation of the Regional Ocean Modeling System (ROMS), coupled with the Biological Elemental Cycling (BEC) model. We first detect extremes in temperature and acidity ([H+]) within the top 300m using a relative threshold of 95% and then identify CCX where conditions are extreme for both stressors for at least 50m of the water column. When analysed on a fixed baseline, positive trends in ocean warming and acidification caused CCX to last longer, intensify, and expand throughout the Southern Ocean. In the Antarctic zone, CCX expanded between 1980 and 2020 by more than 10 times in volume, lasted up to 120 days longer, and doubled in anomaly. Some of the largest and longest events occurred in Antarctic Marine Protected Areas (MPAs), covering more than 200 000 km2 and persisting for over 500 days. CCX in the Subantarctic and Northern zones quadrupled in volume and increased by more than 30% in anomaly. Across the Southern Ocean, the increasing occurrence of CCX exacerbates the risks to marine ecosystems from warming and acidification.
Continue reading ‘Compound marine heatwaves and acidity extremes in the Southern Ocean’Posts Tagged 'regionalmodeling'
Compound marine heatwaves and acidity extremes in the Southern Ocean
Published 5 May 2025 Science ClosedTags: Antarctic, chemistry, modeling, regionalmodeling
Disentangling the effects of global and regional drivers on diverse long‐term pH trends in coastal waters
Published 29 April 2025 Science ClosedTags: chemistry, modeling, North Pacific, regionalmodeling
Abstract
Unlike declines of pH in the open ocean on the total scale (pHT), coastal systems have shown complex long‐term trends in pHT due to a multitude of global and regional drivers. These drivers include changes in nutrient loading, human‐accelerated chemical weathering of watersheds, acid‐rain and land‐use changes, and ocean acidification due to atmospheric CO2 increase. We lack understanding of how these co‐occurring processes have influenced long‐term pHT changes in coastal waters. To address this knowledge gap, a coupled hydrodynamic‐biogeochemical‐carbonate chemistry model was used to conduct a hindcast simulation and scenario analyses of carbonate chemistry in the Chesapeake Bay between 1951 and 2010. Trend analysis reveals increasing pHT in the upper Bay due to river alkalinization but decreasing pHT in the bottom waters of the mid‐and lower Bay due to ocean acidification. No trend is detected in the surface waters of the mid‐ and lower Bay due to competition between the two drivers. The effect of river alkalinization on the acidic volume in the estuary is twice that of ocean acidification. Our findings show that river alkalinization provides an important buffer against acidification while eutrophication plays a secondary role. Our results also suggest ocean alkalinity enhancement could be effective in mitigating acidification in coastal waters.
Plain Language Summary
Ocean acidification due to uptake of atmospheric CO2 is a concern in the open ocean. In contrast, pH in coastal systems has shown both decreasing and increasing long‐term trends. A number of global and regional processes drive these diverse trends, including changes in nutrient loading, human‐accelerated chemical weathering in watersheds, changes in acid‐rain and land‐use, as well as atmospheric CO2 increases. Using the Chesapeake Bay as a model coastal system, we conducted a 60‐year model simulation and scenario analysis of carbonate chemistry between 1951 and 2010. We found pH increased in the upper Bay, decreased in the bottom waters of the mid‐and lower Bay, and displayed no trend in the surface waters of the mid‐ and lower Bay. River alkalinization drove the pH increase in the upper Bay, whereas ocean acidification drove the pH decline in the lower Bay. The two drivers canceled each other in the surface waters of the mid‐ and lower Bay and resulted in no long‐term trends in pH there. As ocean alkalinity enhancement (OAE) is being considered as an approach for marine carbon dioxide removal, results of this analysis strongly suggest that OAE could be an effective way to mitigate ocean acidification in coastal waters.
Continue reading ‘Disentangling the effects of global and regional drivers on diverse long‐term pH trends in coastal waters’Modeling terrestrial dissolved organic carbon and its effect on the carbonate system in the Sunda Shelf seas, Southeast Asia
Published 17 April 2025 Science ClosedTags: biogeochemistry, chemistry, modeling, North Pacific, regionalmodeling, South Pacific
Abstract
The flux of dissolved organic carbon (DOC) from land to sea is an important transfer within the global carbon cycle. The biogeochemical fate of this terrestrial DOC (tDOC) remains poorly understood and is usually neglected in ocean models. Southeast Asia accounts for around 10% of global tDOC flux, mostly from tropical peatland-draining rivers discharging onto the Sunda Shelf. We developed a new light-driven parameterization of tDOC remineralization that accounts for photochemical, microbial, and interactive photochemical–microbial degradation, and simulated the transport and remineralization of tDOC through the Sunda Shelf seas using the regional 3D hydrodynamical HAMSOM and biogeochemical ECOHAM models (only for the carbonate system). Our realistic hindcast simulations for 1958–2022 show that about 50% of riverine tDOC is remineralized before leaving the shelf. This lowers seawater pH across the entire inner Sunda Shelf by an average of 0.005 (by up to 0.05 in the Malacca Strait). Correspondingly, seawater pCO2 is raised, increasing yearly CO2 outgassing from the shelf by 19% (3.1 Tg C yr−1, 0.14 mol m−2 yr−1) during 2013–2022. Even regional ocean acidification trends increase, because river discharge and tDOC flux increase. Our model reveals large spatial variability with greatest inputs and remineralization of tDOC close to major peatlands, especially off Sumatra and Borneo. The interannual variability in tDOC input and the monsoonal current reversal lead to strong temporal variability in carbonate system parameters in these areas. Our results highlight the importance of representing tDOC in ocean models, and reveal the fate of tropical peatland tDOC.
Key Points
- We modeled terrestrial dissolved organic carbon (tDOC) using a new scheme for photo-, bio-, and interactive photo-bio-degradation
- TDOC input to the Sunda Shelf in 2013–2022 is 15.9 Tg C yr−1. 50% is remineralized on the shelf, 28% directly exported to Indian Ocean
- This drives shelf-wide outgassing of 3.1 Tg C yr−1 in 2013–2022, lowers pH and aragonite saturation, increases ocean acidification trends
Plain Language Summary
The transport of terrestrial dissolved organic carbon (tDOC) from land to sea via rivers is an important part within the global carbon cycle. The majority of this tDOC is remineralized by sunlight and marine bacteria, which produces CO2 leading to ocean acidification and CO2 outgassing into the atmosphere. The degradation process is poorly understood, usually neglected in ocean models. Southeast Asia accounts for around 10% of global tDOC flux, mostly from rivers with peatland areas in their catchment. We developed an equation of tDOC remineralization that depends on sunlight and accounts for photochemical, microbial, and interactive photochemical–microbial degradation. With this, we simulated the fate of tDOC using a 3D computer model system for the Southeast Asian region. Our realistic results for 1958–2022 show: 50% of the tDOC is remineralized before leaving the Sunda Shelf. As a result, the shelf water acidifies and emits more CO2 to the atmosphere. Because of increasing river freshwater runoff (from climate change), more tDOC is transported in time into the sea and more is remineralized, increasing the ocean acidification. This happens mostly in coastal seas close to rivers with much peatland in their catchment. This is harmful for calcifying marine organisms like corals.
Continue reading ‘Modeling terrestrial dissolved organic carbon and its effect on the carbonate system in the Sunda Shelf seas, Southeast Asia’An improved long-term high-resolution surface pCO2 data product for the Indian Ocean using machine learning
Published 16 April 2025 Science ClosedTags: chemistry, Indian, modeling, regionalmodeling
Accurate estimation of surface ocean pCO2 is crucial for understanding the ocean’s role in the global carbon cycle and its response to climate change. In this study, we employ a machine learning algorithm to correct the deviations in high-resolution (1/12°) model simulations of surface pCO2 from the INCOIS-BIO-ROMS model (pCO2model) for the period 1980–2019, using available observations (pCO2obs). We train the XGBoost model to generate spatio-temporal deviations (pCO2obs − pCO2model) of pCO2model. The interannually and climatologically varying deviations are then added back to the original model separately, which results in an improved surface pCO2 data product. A comparison of our surface pCO2 data product with moored observations, gridded SOCAT, CMEMS-LSCE-FFNN, and OceanSODA demonstrates an improvement by approximately 40% ± 3.31% in RMSE. Further analysis reveals that adding climatological deviations to pCO2model results in greater improvements than adding interannual deviations. This analysis underscores the ability of machine learning algorithms to enhance the accuracy of model-simulated surface pCO2 outputs.
Continue reading ‘An improved long-term high-resolution surface pCO2 data product for the Indian Ocean using machine learning’Physical and biogeochemical controls of sea surface pCO2 along the North American Atlantic coastal ocean margins on different time scales
Published 24 March 2025 Science ClosedTags: biogeochemistry, chemistry, modeling, North Atlantic, regionalmodeling
Coastal ocean margins play a crucial role in the global carbon cycle, acting as significant carbon sinks and highly productive ecosystems while being particularly vulnerable to climate change and ocean acidification. Despite their importance, there remains a critical knowledge gap in understanding the complex carbon dynamics and acidification processes in these regions, especially in the North American Atlantic Coastal Ocean Margin (NAACOM), which encompasses diverse and economically vital coastal ecosystems. This dissertation aims to investigate the complex physical and biogeochemical processes controlling inorganic carbon chemistry and ocean acidification processes along the NAACOM on different spatial and temporal scales. Through a series of studies, I address critical gaps in understanding the spatial and temporal variability of surface ocean partial pressure of CO2 (pCO2) and air-sea CO2 fluxes in these regions.
I first evaluate the reliability of multiple pCO2 products in the Gulf of Mexico (GoMx). Our analysis reveals significant spatial and seasonal variations in pCO2 due to complex local non-thermal dynamics, particularly in the Louisiana Shelf and Western Florida Shelf. I find that the GoMx is CO2-neutral overall, but with notable spatial differences in pCO2 trends. However, despite the fact that all the reconstructed data products are capable of providing a reasonable estimated annual mean pCO2 climatology for the GoMx, they are unable to simulate the long-term trend of pCO2 within the GoMx.
To address the limitations in existing pCO2 products, I develop a new regional reconstructed pCO2 product (ReCAD-NAACOM-pCO2) for the entire NAACOM. This product uses a novel two-step approach combining random forest and linear regression. Trained on Surface Ocean CO2 Atlas (SOCAT) observations and various environmental variables, ReCAD-NAACOM-pCO2 demonstrates high accuracy and robustness in capturing regional-scale variations, seasonal cycles, and decadal trends of pCO2 from 1993 to 2021.
Utilizing this new product, I investigate the spatial and seasonal variability of pCO2 and air-sea fluxes across the NAACOM. I reveal a south-to-north decreasing gradient in pCO2, primarily driven by the meridional temperature gradient and modulated by regional processes. In southern areas, seasonal thermal cycles dominate pCO2 seasonality, while in northern areas, biological processes play a more significant role. Our analysis identifies the NAACOM as a CO2 sink, with a mean flux of -0.63 ± 0.19 mol C m-2 yr-1 (-10.14 ± 3.00 Tg C yr-1) in the narrow margin (depth < 200m) and -0.60 ± 0.21 mol C m-2 yr-1 (-24.24 ± 8.31 Tg C yr-1) in the wide margin (distance from coast < 400km). This wide-margin CO2 uptake is around 60% lower than previously reported values, emphasizing the importance of accurate regional assessments.
Finally, I explore the impact of Gulf Stream variability on coastal acidification along the U.S. East Coast. Our findings show unexpectedly rapid acidification, with pCO2 increasing faster than atmospheric CO2 in both the Mid and South Atlantic Bights (MAB and SAB) during 2002-2021, accompanied by a rapid decline in the dissolution saturation of pH and calcium carbonate saturation state (Ω). Specifically, during this period, the sea surface pCO2 of the SAB shelf grew 60% faster than the atmosphere pCO2 and 28% faster than the MAB. I attribute this acceleration to an overall extraordinary regional warming background, and additionally to reduced biological carbon removal and decreased buffer capacity on the SAB shelf — all linked to Gulf Stream dynamics. By incorporating Gulf Stream dynamic parameters into a multiple linear regression model and Earth System Models, I provide revised estimates for past and future pCO2 levels in these regions. I show that the rapid pCO2 contrast between SAB and MAB can be traced back to the 1990s and will continue to be larger in the future due to a weakening Gulf Stream.
This dissertation significantly advances our understanding of carbon dynamics in the NAACOM, providing crucial insights for improved coastal carbon cycle modeling. Our findings highlight the complex interplay of physical, biological, and chemical processes in shaping coastal carbon dynamics and acidification patterns. The development of the ReCAD-NAACOM-pCO2 product offers a valuable tool for future research in this area.
Furthermore, our research underscores the urgent need for enhanced ocean acidification monitoring in Gulf Stream-influenced areas and other Western Boundary Current regions. The rapid coastal acidification observed along the U.S. East Coast has important implications for marine ecosystem management and policy-making. As coastal regions face increasing pressures from climate change, our work provides critical information to support informed decision-making and adaptation strategies.
Continue reading ‘Physical and biogeochemical controls of sea surface pCO2 along the North American Atlantic coastal ocean margins on different time scales’Spatiotemporal dynamics of marine heatwaves and ocean acidification affecting coral environments in the Philippines
Published 21 March 2025 Science ClosedTags: biological response, BRcommunity, chemistry, corals, modeling, North Pacific, regionalmodeling
The coral reefs in the Philippines are facing an unprecedented crisis. This study, based on a comprehensive analysis of marine heatwaves (MHWs), degree heating weeks (DHWs), and ocean acidification (OA) indices derived from satellite observations and reanalysis data, reveals how thermal stress and OA have progressively eroded coral ecosystems from 1985 to 2022. This study analyzed 12 critical coral habitats adjacent to the Philippines. The monthly average sea surface temperature (SST) in the study area ranged from 26.6 °C to 29.3 °C. The coast of Lingayen Gulf was identified as the most vulnerable coral reef site in the Philippines, followed by Davao Oriental and Polillo Island. The coast of Lingayen Gulf recorded the highest total MHW days in 2022, amounting to 293 days. The coast of Lingayen Gulf also reached the highest DHW values in July and August 2022, with 8.94 °C weeks, while Davao Oriental experienced the most extended average duration of MHWs in 2020, lasting 90.5 days per event. Large-scale climate features such as the El Niño–Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) significantly influenced the study area’s SST anomalies and MHW events. High-risk coral bleaching periods, such as 1988–1989, 1998–1999, 2007–2008, and 2009–2010, were characterized by transitions from El Niño and positive PDO phases, to La Niña and negative PDO phases. However, since 2015, global warming has led to high cumulative heat stress without specific climate background patterns. We propose a Coral Marine Environmental Vulnerability Index (CoralVI) to integrate the spatiotemporal dynamics of warming and acidification and their impacts on coral habitats. The data show a rapid increase in the marine environmental vulnerability of coral habitats in the Philippines in recent years, extending to almost the entire coastline, posing significant threats to coral survival.
Continue reading ‘Spatiotemporal dynamics of marine heatwaves and ocean acidification affecting coral environments in the Philippines’Regression-based characterization of the marine carbonate system across shelf and nearshore waters of Queen Charlotte Sound
Published 21 March 2025 Science ClosedTags: chemistry, modeling, North Pacific, regionalmodeling
Highlights
- Carbonate system parameters were predicted by linear regressions in coastal waters.
- Oxygen concentrations were key predictive variables in most linear regressions.
- Regression uncertainty was always greater in coastal waters than in offshore waters.
- The aragonite saturation horizon initially shoaled at the onset of winter winds.
- Persistent downwelling favourable winds subsequently deepened the saturation horizon.
Abstract
Marine carbonate system measurements are essential for understanding ocean acidification and CaCO3 saturation states, and their response to oceanographic and anthropogenic processes. Acquiring such measurements in remote coastal areas is limited by challenges in the development and deployment of autonomous sensors for these parameters, and by the complexity and costs of directly measuring them. We address this challenge by extending an established method of estimating carbonate system parameters through proxy variables to the remote waters of Queen Charlotte Sound, British Columbia. Paired regressions are developed from bottle samples and common hydrographic measurements for both offshore and coastal waters that intersect in this region but retain distinct relationships between total alkalinity and salinity. Application of these regression models to regional autonomous glider data demonstrates that established features of this shelf system can be characterized from a carbonate system perspective along a contiguous transect from land to the shelf break, despite greater regression uncertainties in coastal water. These results also provide new insight into the dynamics of aragonite saturation on the shelf in relation to regional winds. This approach extends a tool to gain knowledge of the marine carbonate system to a coastal region of the northeast Pacific Ocean where hydrographic data used as proxies are more commonly available.
Continue reading ‘Regression-based characterization of the marine carbonate system across shelf and nearshore waters of Queen Charlotte Sound’Influences of global warming and upwelling on the acidification in the Beaufort Sea
Published 7 March 2025 Science ClosedTags: Arctic, chemistry, modeling, regionalmodeling
Over the past three decades, increasing atmospheric CO2 (AtmCO2) has led to climate warming, sea ice reduction and ocean acidification in the Beaufort Sea (BS). Additionally, the effects of upwelling on the carbon cycle and acidification in the BS are still unknown. The Regional Arctic System Model (RASM) adequately reflects the observed long-term trends and interannual variations in summer sea ice concentration (SIC), temperature, partial pressure of CO2 (pCO2) and pH from 1990 to 2020. Multiple linear regression results from a control case show that surface (0–20 m) pH decline is significantly driven by AtmCO2 and SIC, while AtmCO2 dominates in subsurface (20–50 m) and deep layers (50–120 m). Regression results from a sensitivity case show that even if the AtmCO2 concentration remained at 1990 levels, the pH would still exhibit a long-term decline trend, being significantly driven by SIC only in the surface layers and by SIC and net primary production (NPP) in the subsurface layers. In contrast to the nearly linearly increasing AtmCO2 over the last three decades, the ocean pH shows more interannual variations that are significantly affected by SIC and mixed layer depth (MLD) in the surface, NPP and Ekman pumping velocity (EPV) in the subsurface and EPV only in the deep layer. The comparison of results from high and low SIC years reveals that areas with notable pH differences are overlapping regions with the largest differences in both SIC and MLD, and both cause a statistically significant increase in pCO2 and decrease in pH. Comparison of results from high and low EPV years reveals that although stronger upwelling can lift up more nutrient-rich seawater in the subsurface and deep layers and lead to higher NPP and pH, this effect is more than offset by the higher DIC lifted up from deep water, leading to generally lower pH in most regions.
Continue reading ‘Influences of global warming and upwelling on the acidification in the Beaufort Sea’Ocean acidification around the UK and Ireland
Published 5 March 2025 Newsletters and reports ClosedTags: biological response, chemistry, field, modeling, North Atlantic, regionalmodeling, review
KEY FACTS
What is already happening
- Atmospheric CO2 exceeded 420 ppm in 2024 and has continued to increase by approximately 2.5 ppm per year over the last decade. The global ocean absorbs approximately a quarter of anthropogenic carbon dioxide (CO2) emissions annually.
- The North Atlantic Ocean contains more anthropogenic CO2 than any other ocean basin, and surface waters are experiencing an ongoing decline in pH (increasing acidity). Rates of acidification in bottom waters are occurring faster at some locations than in surface waters.
- Some species are already showing effects from ocean acidification when exposed to short-term fluctuations and could be used as indicator species for long-term impacts on marine ecosystems.
What could happen in the future
- Models project that the average continental shelf seawater pH will continue to decline to year 2050 at similar rates to the present day, with rates then increasing in the second half of the century, depending on the emissions scenario.
- The rate of pH decline in coastal areas is projected to be faster in some areas (e.g. Bristol Channel) than others, such as the Celtic Sea.
- Under high-emission scenarios, it is projected that bottom waters on the North-West European Shelf seas will become corrosive to moresoluble forms of calcium carbonate (aragonite). Episodic undersaturation events are projected to begin by 2030.
- By 2100, up to 90% of the north-west European shelf seas may experience undersaturation for at least one month of each year.
- High levels of nearshore variability in carbonate chemistry may mean that some coastal species have a higher adaptative capacity than others. However, all species are at increased risk from extreme exposure episodes.
Spatiotemporal variability of seawater carbonate chemistry in diverse coral reef environments in South East Asia and Australia
Published 26 February 2025 Science ClosedTags: biogeochemistry, chemistry, field, modeling, regionalmodeling, South Pacific
Coral reefs are under threat from global environmental perturbations including ocean warming, acidification, and deoxygenation. The degree to which these perturbations will affect coral reefs is dependent on a range of factors including the local hydrodynamics and biogeochemical processes, both which vary widely across space and time. Consequently, not all coral reefs will be affected equally due to differences in local properties and processes. To develop predictive capacity for how coral reefs will be affected by ocean acidification (OA) it is necessary to first understand the current seawater CO2 chemistry variability and drivers. In this dissertation, autonomous sensors and discrete seawater samples were used to characterize the natural spatial and temporal variability of seawater CO2 chemistry across different reef scales and habitats in Australia and South East Asia (i.e., Heron Island, Great Barrier Reef; Dongsha Atoll and Taiping Island, South China Sea; and Onna-son Reef, Okinawa). In Heron Island, the largest spatial and temporal variability in seawater chemistry was associated with the most shallow, western region of the platform that also had the longest residence time. Interactions between reef geomorphology and timing of the tidal cycle greatly influenced chemical gradients and variability leading to some unexpected trends and patterns. On Dongsha, elevated pH and aragonite saturation state (ΩAr) were observed inside a semi-enclosed lagoon during both day and night. Future projections showed that this environment will not cross, detrimental aragonite thresholds (e.g., ΩAr < 2.92) as frequently as patch reefs and the large-scale lagoon of Dongsha. However, concurrent high water temperature and hypoxia indicated that this environment will not offer respite to taxa sensitive to OA. In Onna-son, the influence of seaweed cultivation on seawater chemistry during spring was compared to times of no cultivation during fall and winter. pH elevation was observed during both spring (+0.13 units) and fall (+0.10 units), but it was not possible to separate the role of cultivated seaweed from natural taxa. This dissertation demonstrates current coral reef seawater CO2 chemistry conditions and biochemical function, information that will be critical for making rigorous projections for the future.
Continue reading ‘Spatiotemporal variability of seawater carbonate chemistry in diverse coral reef environments in South East Asia and Australia’Climate-driven shifts in Southern Ocean primary producers and biogeochemistry in CMIP6 models
Published 26 February 2025 Science ClosedTags: Antarctic, biogeochemistry, biological response, BRcommunity, chemistry, community composition, modeling, otherprocess, phytoplankton, regionalmodeling
As a net source of nutrients fuelling global primary production, changes in Southern Ocean productivity are expected to influence biological carbon storage across the global ocean. Following a high-emission, low-mitigation pathway (SSP5-8.5), we show that primary productivity in the Antarctic zone of the Southern Ocean is predicted to increase by up to 30 % over the 21st century. The ecophysiological response of marine phytoplankton experiencing climate change will be a key determinant in understanding the impact of Southern Ocean productivity shifts on the carbon cycle. Yet, phytoplankton ecophysiology is poorly represented in Coupled Model Intercomparison Project phase 6 (CMIP6) climate models, leading to substantial uncertainty in the representation of its role in carbon sequestration. Here we synthesise the existing spatial and temporal projections of Southern Ocean productivity from CMIP6 models, separated by phytoplankton functional type, and identify key processes where greater observational data coverage can help to improve future model performance. We find substantial variability between models in projections of light concentration (>15 000 (µE m−2 s−1)2) across much of the iron- and light-limited Antarctic zone. Projections of iron and light limitation of phytoplankton vary by up to 10 % across latitudinal zones, while the greatest increases in productivity occurs close to the coast. Temperature, pH and nutrients are less spatially variable – projections for 2090–2100 under SSP5-8.5 show zonally averaged changes of +1.6 °C and −0.45 pH units and Si* ([Si(OH)4]–[NO3–]) decreases by 8.5 µmol L−1. Diatoms and picophytoplankton and/or miscellaneous phytoplankton are equally responsible for driving productivity increases across the subantarctic and transitional zones, but picophytoplankton and miscellaneous phytoplankton increase at a greater rate than diatoms in the Antarctic zone. Despite the variability in productivity with different phytoplankton types, we show that the most complex models disagree on the ecological mechanisms behind these productivity changes. We propose that a sampling approach targeting the regions with the greatest rates of climate-driven change in ocean biogeochemistry and community assemblages would help to resolve the empirical principles underlying the phytoplankton community structure in the Southern Ocean.
Continue reading ‘Climate-driven shifts in Southern Ocean primary producers and biogeochemistry in CMIP6 models’Effect of thermal and non-thermal processes on the variability of ocean surface pCO2 and buffering capacity in the North Indian Ocean
Published 24 February 2025 Science ClosedTags: biogeochemistry, chemistry, Indian, modeling, regionalmodeling
Highlights
- A coupled atmosphere–ocean-biogeochemistry model is customised for NIO.
- High surface pCO2 in upwelling regions of NIO are controlled by non-thermal processes.
- Surface pCO2 changes in upwelling regions of NIO are more sensitive to changes in DIC.
- Diffusion, CO2 flux and Phytoplankton uptake primarily control DIC variability in NIO.
Abstract
The oceans have absorbed nearly 30% of the anthropogenic CO2 that alters the ocean carbon chemistry. The oceanic processes are highly complex, which mandate approaches that couple its physical, chemical and biological states. Here, we use a coupled atmosphere–ocean-biogeochemistry model, incorporating spatially and temporally varying atmospheric CO2 to simulate the north Indian Ocean (NIO) carbon dynamics for the period 2013–2020. We assess the seasonal variability of Dissolved Inorganic Carbon (DIC), total Alkalinity (ALK), ocean surface pCO2 and buffering capacity. To assess the mechanisms that control carbon dynamics in the region, we segregate the ocean surface pCO2 into temperature-driven (thermal) and bio-physical processes induced (non-thermal) pCO2. We find that the thermally driven pCO2 is dominant in summer (June, July, August and September; JJAS), but the non-thermal component in winter (December, January and February; DJF) in the northern Arabian Sea (AS). The northern AS is characterised by a deep mixed layer and convection-induced vertical mixing during winter. DIC from the subsurface layer is uplifted to the surface, which results in high ocean surface pCO2 in winter. Off the Oman coast, the non-thermal processes control the surface pCO2 in summer. In the northern bay, the thermal component of pCO2 is dominant in summer and non-thermal component is prominent in winter as in northern AS, but their magnitudes are lower due to large riverine flux. The budget analysis reveals strong influence of diffusion, CO2 flux and biological processes in controlling DIC variability in NIO. Low buffering capacity in upwelling regions indicates that pCO2 changes are more sensitive to changes in DIC, primarily due to the upwelled DIC-rich surface waters. Therefore, it results in a reduced ability to absorb CO2. This warrants the need to address recent changes in carbon dynamics in response to the increased levels of atmospheric CO2.
Continue reading ‘Effect of thermal and non-thermal processes on the variability of ocean surface pCO2 and buffering capacity in the North Indian Ocean’Long-term biogeochemical variations in the southern South China Sea and adjacent seas: A model data analysis
Published 17 February 2025 Science ClosedTags: biogeochemistry, chemistry, modeling, North Atlantic, regionalmodeling
Highlights
- Sea surface temperature in the southern South China Sea is rising by 0.2°C per decade, highlighting climate change’s impact on the region.
- Significant declines in Dissolved Oxygen and pH levels highlight broader impacts on ocean health in the southern SCS.
- SST anomalies strongly influence biogeochemical changes, highlighting biogeochemistry’s sensitivity to climate variability.
Abstract
The southern portion of the South China Sea (SCS), which constitutes the largest semi-enclosed marginal sea in Southeast Asia, is currently undergoing unusual surface warming, which may significantly impact its marine ecosystems. This study examines the long-term variations of biogeochemistry in the southern SCS and its adjacent seas from 1993 to 2020, using data from the Copernicus Marine Environment Monitoring Service (CMEMS) model to better understand the implications of these warming trends on nutrient dynamics and overall marine health. Analysis of interannual variations highlights a strong relationship between sea surface temperature (SST) anomalies and the occurrence of El Niño-Southern Oscillation and Indian Ocean Dipole events, leading to significant alterations in biogeochemistry in the southern SCS. These findings emphasize the sensitivity of biogeochemistry to climatic variability. Over the study period, a region-wide increasing SST trend (0.2 °C per decade) indicates the influence of climate change, while surface Chlorophyll concentration and Phytoplankton biomass show decreasing trends, suggesting potential shifts in primary productivity. Additionally, surface Dissolved Oxygen and pH levels have significantly declined, pointing to broader impacts on ocean health. These observed declining trends in biogeochemical variables are particularly pronounced in the southeastern Vietnam, East Coast of Peninsular Malaysia, and Java Sea regions, which are primarily driven by significant changes in SST and related wind patterns. The weakening of summer southwesterly winds and winter northeasterly winds, evidenced by linear trends in wind stress, has reduced nutrient supply through upwelling and decreased ocean heat loss, respectively. This disruption exacerbates surface ocean warming and limits the replenishment of essential nutrients, leading to diminished primary productivity and altering marine ecosystem dynamics. This study provides valuable insights into present biogeochemical trends under climate change, forming a crucial basis for future projection studies in this region.
Continue reading ‘Long-term biogeochemical variations in the southern South China Sea and adjacent seas: A model data analysis’The effect of carbonate mineral additions on biogeochemical conditions in surface sediments and benthic–pelagic exchange fluxes (update)
Published 11 February 2025 Science ClosedTags: biogeochemistry, chemistry, dissolution, mitigation, modeling, regionalmodeling, sediment
Coastal sediments are hotspots of biogeochemical processes that are impacting subsurface and overlying water conditions. Fluid composition in sediments is altered through the mineralization of organic matter which, under oxic conditions, further lowers both pH and the carbonate saturation state. As a potential mitigation strategy for this sediment acidification, we explored the effects of mineral additions to coastal sediments. We experimentally quantified carbonate mineral dissolution kinetics of carbonate shells suitable for field application and then integrated these data into a reactive transport model that represents early diagenetic cycling of C, O, N, S, and Fe and traces total alkalinity, pH, and saturation state of CaCO3. Model simulations were carried out to delineate the impact of mineral type and amount added, porewater mixing, and organic matter mineralization rates on sediment alkalinity and its flux to the overlying water. Model results showed that the added minerals undergo initial rapid dissolution and generate saturated conditions demonstrating the potential of alkalinity enhancement in mitigating surface sediment acidification. Aragonite dissolution led to higher total alkalinity concentrations than calcite. Simulations of carbonate mineral additions to sediment environments with low rates of organic matter mineralization exhibited a substantial increase in mineral saturation state compared to sediments with high CO2 production rates, highlighting the environment-specific extent of the effect of mineral addition. Our work indicates that carbonate additions have the potential to effectively buffer surficial sediments over multiple years, yielding biogeochemical conditions that counteract the detrimental effect of low-pH sediment conditions on larval recruitment and potentially increase benthic alkalinity fluxes to support marine carbon dioxide removal (mCDR) in the overlying water.
Continue reading ‘The effect of carbonate mineral additions on biogeochemical conditions in surface sediments and benthic–pelagic exchange fluxes (update)’Understanding and quantifying carbon export to coastal oceans through deltaic systems
Published 7 February 2025 Science ClosedTags: chemistry, modeling, North Atlantic, regionalmodeling
The ocean is the second largest carbon reservoir on Earth and a significant carbon sink to mitigate the adverse effects of climate change. However, ocean carbon sink is expected to decrease in the future with continuing seawater acidification, more stable ocean stratification, and intensified extreme weather events. Estuaries, the connection between land and the coast, undergo complex carbon reforming and pose the greatest uncertainty in global carbon budgets. Coupled physical-biogeochemical models over the Gulf of Mexico (GoM) were configured using biogeochemical boundary conditions from a global climate model (GCM), preserving the long-term climate variability and the conservation of carbon pools in the regional model (RM). Subsequently, a nested domain over the Barataria Basin was built to quantify the carbon export in the eroding estuary. Intensive model-data comparison confirmed that the RM could reliably simulate the ocean carbon system in the GoM, and added values (AVs) were achieved both at the GCM scale and the RM scale. Model results reveal that the GoM water has been experiencing a ~ 0.0016 yr-1 decrease in surface pH over the past two decades, accompanied by a ~ 1.66 µatm yr-1 increase in sea surface pCO2. The river-dominated northern GoM (NGoM) is a substantial carbon sink, and the open GoM is a carbon source during summer and a carbon sink for the rest of the year. Based on the sensitivity test for higher river discharge scenarios, the NGoM was predicted to become a stronger ocean CO2 sink in the future. Stronger fluvial influence will increase the ecosystem resilience after hurricanes’ impact but reduce its resistance at the same time. The model estimated that Barataria Basin exports approximately 52.5 ×106 kg yr-1 dissolved inorganic carbon (DIC) and 9.68 ×106 kg yr-1 total organic carbon (TOC) to the coastal water. The biggest carbon loss term of the Barataria Basin was in the form of CO2 outgassing in quantity of 98.8×106 kg yr-1.
Continue reading ‘Understanding and quantifying carbon export to coastal oceans through deltaic systems’An observation-based method to estimate carbonate system variations in the Labrador Sea
Published 6 February 2025 Science ClosedTags: chemistry, field, modeling, North Atlantic, regionalmodeling
The ocean carbonate system consists of pH, alkalinity, inorganic carbon and the partial pressure of carbon dioxide, and during the current era of anthropogenic change, its dynamics are key for understanding changes in the ocean and its ecosystem over time. The focus of this study is to estimate the carbonate system in the Labrador Sea with time series methods, using direct observations from the ocean surface and interior, and chemical relationships between variables. Interior ocean observations are minimal for some of these variables, however, connections between the variables rooted in chemistry were used to create pseudo-observations using CO2SYS, increasing the information available. A state space model was designed that combined GLODAP and SOCAT observations along with pseudo-observations in a time series estimate of the carbonate system. The Labrador Sea between 1993 and 2016 shows increasing rates for DIC (0.57-1.16 µmol kg−1 year−1) and fCO2 (0.70-2.45 µatm year−1), as well as acidification via pH trends (0.0007-0.0018 year−1). These ranges describe the scale of rates that are occurring at various depths through the water column, though they do not change linearly with depth. Largest rates are found at the surface for DIC, 500-1500 m for fCO2, and 500-1500 m for pH. Total alkalinity also decreased and is correlated with the freshening of salinity. With the core carbonate variables estimated, other aspects of the carbonate system are calculated using CO2SYS, such as the aragonite and calcite saturation states, the Revelle factor, and the carbonate species. Our method also calculates uncertainties that vary over time and depth based on the availability of observations and their variance, which has lowered the uncertainty for pH by 71% and for fCO2 by 64% compared to time-independent methods.
Continue reading ‘An observation-based method to estimate carbonate system variations in the Labrador Sea’Satellite-derived ocean color data for monitoring pCO2 dynamics in the North Indian Ocean
Published 27 January 2025 Science ClosedTags: chemistry, Indian, modeling, regionalmodeling
Highlights
- A Multiparametric Linear Regression (MLR) model was developed using in-situ and satellite observations to accurately estimate pCO2 in the NIO region.
- Validation of the MLR model showed significant low errors (MRE = 0.08, MNB = 0.013, RMSE = 7.26 μatm) and a high correlation coefficient (R2 = 0.96), demonstrating superior performance.
- The interannual (2012-2022) variability of pCO2 in the NIO region shows an increasing trend.
- The study reveals seasonal variability in pCO2 in the NIO, peaking pre and post-monsoon, influencing marine ecosystems.
Abstract
The partial pressure of carbon dioxide (pCO2) in the North Indian Ocean (NIO) undergoes significant variations due to factors such as biological activity, ocean circulation patterns, and atmospheric influences. Understanding these variations is crucial for assessing the ocean role in the global carbon cycle and their impact on climate change. Estimating pCO2 through in-situ platforms is challenging due to the time-consuming, expensive, and complex nature of water sample collection, particularly under rough oceanic conditions. Conversely, remote sensing technology offers high spatiotemporal resolution data over extensive synoptic scales, making it a valuable tool for pCO2 estimation. Current models for estimating pCO2 in the NIO region are limited due to the improper selection of model parameters and the scarcity of in-situ measurements, highlighting the need for a more accurate approach. This study develops a Multiparametric Linear Regression (MLR) method, integrating satellite and in-situ observations of sea surface temperature (SST), sea surface salinity (SSS), and chlorophyll-a (Chla) concentration. To develop and validate this model, in-situ data were sourced from the Global Ocean Data Analysis Project (GLODAP). Validation results showed that the proposed MLR approach outperformed existing global models, achieving low mean relative error (MRE = 0.08), mean normalized bias (MNB = 0.013), and root mean square error (RMSE = 7.26 μatm), with a high correlation coefficient (R2 = 0.96). This study has the potential to improve understanding of carbon dynamics in the NIO region and its contribution to the global carbon cycle. The pCO2 maps generated in this study improve climate modeling and monitoring, supporting predictions and mitigation efforts. This accurate model also aids policy-making, environmental management, and ecological assessments.
Continue reading ‘Satellite-derived ocean color data for monitoring pCO2 dynamics in the North Indian Ocean’Carbon cycling and ocean acidification studies in Baffin Bay and Nares Strait
Published 20 January 2025 Science ClosedTags: Arctic, biogeochemistry, chemistry, field, modeling, regionalmodeling
The marine carbon cycle in Canadian Arctic waters, particularly in Nares Strait and Baffin Bay, is undergoing rapid change due to a shifting climate. Despite this, there’s been a paucity of research into the carbonate chemistry and biogeochemical processes in these regions. This thesis addresses this gap by investigating the complex carbon dynamics in these waters, critical for understanding their role in the global carbon cycle. The first research chapter evaluates the strength of the biological carbon pump during the spring ice-edge bloom in Baffin Bay. We found stark differences in springtime net community production (NCP) between Arctic and Atlantic water domains, in western and eastern Baffin Bay, respectively. Arctic outflow waters exhibited low spring NCP (< 1 mol C m-2) due to persistent sea-ice cover and strong stratification of the upper water column, whereas the Atlantic water domain displayed high NCP rates (up to 5.7 mol C m-2). The first comprehensive examination of the marine carbon dynamics in Nares Strait is also presented. Using a multi-tracer linear mixing model, we distinguished the role of physical and biological processes on the distribution of dissolved inorganic carbon in Nares Strait. We identified water mass mixing as the dominant control on marine carbon dynamics, with primary production also playing an important role in decreasing surface pCO2. Importantly, this investigation also provided the first documented evidence of Siberian river waters arriving in Nares Strait. The final research chapter of this thesis investigates the biogeochemical processes affecting aragonite saturation states (ΩAr), and the state of ocean acidification in Baffin Bay, with a focus on the west Greenland continental shelf region, which has remained under-studied in terms of its marine biogeochemistry. We identify two main depth-dependent processes shaping the ΩAr distribution throughout Baffin Bay; within the upper 200 metres, lower ΩAr coincides with increasing fractions of Arctic-outflow waters, while below 200 metres organic matter respiration is responsible for decreasing ΩAr. Surprisingly, substantial Arctic-outflow waters were identified on the west Greenland shelf, challenging what is currently known of circulation patterns in the bay, and underscoring the need for further research.
Continue reading ‘Carbon cycling and ocean acidification studies in Baffin Bay and Nares Strait’A machine-learning reconstruction of sea surface pCO2 in the North American Atlantic Coastal Ocean Margin from 1993 to 2021
Published 15 January 2025 Science ClosedTags: chemistry, modeling, North Atlantic, regionalmodeling
Insufficient spatiotemporal coverage of observations of the surface partial pressure of CO2 (pCO2) has hindered precise carbon cycle studies in coastal oceans and justifies the development of spatially and temporally continuous pCO2 data products. Earlier pCO2 products have difficulties in capturing the heterogeneity of regional variations and decadal trends of pCO2 in the North American Atlantic Coastal Ocean Margin (NAACOM). This study developed a regional reconstructed pCO2 product for the NAACOM (Reconstructed Coastal Acidification Database-pCO2, or ReCAD-NAACOM-pCO2) using a two-step approach combining random forest regression and linear regression. The product provides monthly pCO2 data at 0.25 degrees spatial resolution from 1993 to 2021, enabling investigation of regional spatial differences, seasonal cycles, and decadal changes in pCO2. The observation-based reconstruction was trained using Surface Ocean CO2 Atlas (SOCAT) observations as observational values, with various satellite-derived and reanalysis environmental variables known to control sea surface pCO2 as model inputs. The product shows high accuracy during the model training, validation, and independent test phases, demonstrating robustness and a capability to accurately reconstruct pCO2 in regions or periods lacking direct observational data. Compared with all the observation samples from SOCAT, the pCO2 product yields a determination coefficient of 0.92, a root-mean-square error of 12.70 mu atm, and an accumulative uncertainty of 23.25 mu atm. The ReCAD-NAACOM-pCO2 product demonstrates its capability to resolve seasonal cycles, regional-scale variations, and decadal trends of pCO2 along the NAACOM. This new product provides reliable pCO2 data for more precise studies of coastal carbon dynamics in the NAACOM region. The dataset is publicly accessible at https://doi.org/10.5281/zenodo.14038561 (Wu et al., 2024a) and will be updated regularly.
Continue reading ‘A machine-learning reconstruction of sea surface pCO2 in the North American Atlantic Coastal Ocean Margin from 1993 to 2021’Ocean acidification and its regulating factors in the East China Sea off the Yangtze River estuary
Published 14 January 2025 Science ClosedTags: chemistry, field, modeling, North Pacific, regionalmodeling
Highlights
- Summer surface photosynthesis counteracts acidification from anthropogenic CO₂.
- Summer bottom respiration contributes more to acidification than anthropogenic CO₂.
- Winter acidification mainly driven by rising atmospheric CO₂; minimal biological impact.
- Buffering capacity and DIC:TA ratio crucially impact future pH and Ωar decline rates.
Abstract
This study examines the seasonal variations in carbonate system parameters in the East China Sea (ECS) off the Yangtze River estuary (YRE) and analyzes the contributions of anthropogenic CO₂ and eutrophication to acidification. Carbonate parameters data were collected during summer 2019 and combined winter 2011. During winter, acidification is primarily driven by rising atmospheric CO₂, with minimal impact from biological processes. In contrast, summer presents a different pattern: enhanced photosynthesis due to eutrophication in surface waters helps mitigate the acidification effects of atmospheric CO₂ increases, while in bottom waters, the combined pressures of atmospheric CO₂ and intensified aerobic respiration leads to more severe acidification. Notably, biological processes now contribute more to acidification than increasing atmospheric CO₂ in the bottom waters. Our projections indicate that the summer bottom waters will experience the most pronounced acidification, with average pH levels expected to decline from 8.04 to 7.82 and aragonite saturation state (Ωar) values decreasing from 2.24 to 1.38 between 2000 and 2100. Additionally, our study indicates that winter acidification trends are also concerning, with pH only slightly higher than in summer bottom waters. The buffering capacity and the DIC:TA ratio play significant roles in determining the rate of future pH and Ωar declines. The strong buffering capacity in summer surface waters mitigates the pH decline, while the low DIC:TA ratio results in a rapid drop in Ωar.
Continue reading ‘Ocean acidification and its regulating factors in the East China Sea off the Yangtze River estuary’
