Posts Tagged 'chemistry'

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Carbon-rich waters are becoming even more acidic as atmospheric CO2 levels rise

Published 20 November 2025 Press releases Closed
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The waters bordering North America could soon be inhospitable to critical marine creatures if the Northeastern Pacific Ocean continues to acidify at the current rate, a new study shows.

Earth’s oceans have become approximately 30% more acidic since the industrial revolution began more than 200 years ago. Acidification changes marine chemistry and depletes key minerals that calcifying organisms, such as corals and clams, need to build their skeletons and shells. The Northeastern Pacific is naturally more acidic than other oceans, fueling debate about how much its chemistry will change in the coming decades.

The study, published Nov. 13 in Nature Communications, shows that high baseline acidity makes the water more sensitive to additional carbon dioxide from human activities. Analyses of coral skeletons from the past century revealed that CO2 has been accumulating in North American waters faster than in the atmosphere, driving rapid acidification.

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“The findings implicate not only marine ecosystems, but all of the people who depend on them as well,” added lead author Mary Margaret Stoll, a UW doctoral student of oceanography.

The ocean becomes more acidified when carbon dioxide dissolves to form an acid that releases hydrogen and bicarbonate ions, lowering the water’s pH level. In North America, a powerful current system — the California Current — transports cool water south along the coast. The combination of current flow and wind creates optimal conditions for upwelling, a process that cycles deep water to the surface.

Organic matter — dead plants and animals — sinks to the bottom of the ocean, where it decomposes and releases carbon dioxide back into the water. Upwelling surfaces this CO2 rich water, increasing the acidity of subsurface and surface zones. These natural fluctuations complicate researchers’ efforts to predict how much acidification will occur from human activities.

This study helps resolve these questions with records kept by centuries old corals.

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Deep Pacific carbonate chemistry since the Last Glacial Maximum

Published 19 November 2025 Science Closed
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Quantitative constraints on deep ocean carbonate chemistry are critical for understanding the processes responsible for glacial-interglacial changes in atmospheric pCO2 and the ocean feedbacks that amplify carbon cycle change. Here, we present a new, high-resolution, B/Ca-based record of carbonate ion concentration (Δ[CO32−]) from central equatorial Pacific site ML1208-16BB spanning the last 35 kyr. This site, bathed by Pacific Deep Water, reveals a ∼24 ± 7 μmol/kg rise in deep ocean [CO32−] between ∼20 and 10 kyr, a larger change than previously reconstructed from sites in the western equatorial Pacific and those in the central equatorial Pacific bathed by Lower Circumpolar Deep Water. Our new reconstruction permits estimation of deep Pacific calcite saturation state (Ω), quantifying the degree of deep water undersaturation during the Last Glacial Maximum and implying a critical role for sedimentary porewater saturation state in resolving the Pacific carbonate preservation paradox. Finally, we pair our Δ[CO32−] reconstruction with previously-published benthic δ13C to present a process-oriented understanding of late glacial, deglacial, and Holocene deep Pacific carbonate chemistry changes. Our data suggest a larger role for glacial and deglacial alkalinity changes than previously suggested by records from the equatorial Pacific Ocean.

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A century of change in the California Current: upwelling system amplifies acidification

Published 19 November 2025 Science Closed
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Predicting the pace of acidification in the California Current System (CCS), a productive upwelling system that borders the west coast of North America, is complex because the anthropogenic contribution is intertwined with other natural sources. A central question is whether acidification in the CCS will follow the pace of increasing atmospheric CO2, or if climate effects and other biogeochemical processes will either amplify or attenuate acidification. Here, we apply the boron isotope pH proxy to cold-water orange cup corals to establish a historic level of acidification in the CCS and the Salish Sea, an associated marginal sea. Through a combination of complementary modeling and geochemical approaches, we show that the CCS and Salish Sea have experienced amplified acidification over the industrial era, driven by the interaction between anthropogenic CO2 and a thermodynamic buffering effect. From this foundation, we project future acidification in the CCS under elevated CO2 emissions. The projected change in pCO2 over the 21st century will continue to outpace atmospheric CO2, posing challenges to marine ecosystems of biological, cultural, and economic importance.

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Anthropogenic forcing and upwelling accelerate aragonite undersaturation in the Prydz Bay, East Antarctica

Published 18 November 2025 Science Closed
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Abstract

The Southern Ocean is one of the rapidly acidifying regions globally, yet direct observational constraints on its carbonate chemistry remain scarce. Here, we combine shipboard measurements of pH and aragonite saturation state (Ωarag) from the summer 2015 Chinese National Antarctic Research Expedition with reconstructed wintertime conditions to characterize acidification in Prydz Bay. We report the first observation-based occurrence of surface aragonite undersaturation (Ωarag < 1.0) in the northern basin—emerging nearly two decades earlier than model projections. Surface Ωarag undersaturation is primarily driven by accelerated uptake of anthropogenic CO2 and shoaling of the aragonite saturation horizon, fueled by persistent upwelling of CO2-rich Circumpolar Deep Water. In the ocean interior, organic matter remineralization, CaCO3 dissolution, and continued anthropogenic CO2 intrusion further lower pH and Ωarag. Our results demonstrate that Prydz Bay is at the forefront of Southern Ocean acidification, highlighting the urgent need to incorporate both anthropogenic and natural biogeochemical feedbacks into high-resolution models to better predict future ecological impacts.

Plain Language Summary

The Southern Ocean is acidifying faster than most of the oceans on Earth, but most of what we know comes from computer models rather than direct observations. In Prydz Bay, East Antarctica, we combine shipboard data collected in summer 2015 with reconstructed winter conditions to track changes in seawater acidity (pH) and aragonite saturation (Ωarag), a key factor for shell-building marine life. We discover that seasonal expansion of low pH and nearly undersaturated aragonite led to the unforeseen early emergence of surface aragonite undersaturation in the northern basin. Strikingly, conditions that models predicted would not occur until around year 2038 have appeared nearly 20 years earlier. This rapid shift is driven by increased uptake of atmospheric CO2 and the rising influence of deep CO2-rich waters. In deeper layers, ongoing CO2 intrusion and the breakdown of organic material make the water even more acidic. These findings reveal that Prydz Bay is on the front lines of ocean acidification, underscoring the urgent need to reduce CO2 emissions and to better understand natural feedbacks in order to protect Antarctic marine ecosystems.

Key Points

  • Surface near-undersaturated Ωarag emerged in Prydz Bay during summer, nearly two decades ahead of model predictions
  • Enhanced atmospheric CO2 uptake and rapid shoaling of the aragonite saturation horizon dominate surface Ωarag undersaturation
  • Persistent upwelling of CO2-rich deep water combined with anthropogenic CO2 penetration shoal the Ωarag saturation horizon
Continue reading ‘Anthropogenic forcing and upwelling accelerate aragonite undersaturation in the Prydz Bay, East Antarctica’

Monsoon-driven biogeochemical shifts and acidification risk in tropical estuarine ecosystems: a case study from the Indian coast

Published 17 November 2025 Science Closed
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Tropical estuaries serve as biogeochemical hotspots where the interactions between monsoon hydrology and human activities significantly impact ecosystem health. However, limited information exists on their carbonate chemistry, which is crucial for assessing climate vulnerability. This study provides the first seasonal assessment of hydrography, nutrients, and carbonate system dynamics in the Haripur estuary, Bay of Bengal. Seasonal evaluation revealed significant variations in pH, carbonate system indicators, and nutrients (p < 0.001). During the monsoon, pH declined to 7.12 ± 0.17, dissolved oxygen dropped to near-hypoxic levels (2.95 ± 0.35 mg L−1), and nutrient enrichment was observed with elevated dissolved inorganic nitrogen (6.07 ± 0.74 μM) and phosphate (1.61 ± 0.39 μM). Carbonate saturation states remained persistently corrosive, reaching minima of ΩAr (0.03 ± 0.01) and ΩCa = 0.04 ± 0.01) among the lowest reported for Indian estuaries. Multivariate analysis identified nutrient enrichment and carbonate imbalance as the dominant stressors, explaining 32.4 % of the total variance. These findings clearly indicate that the Haripur estuary functions as a regional hotspot of monsoon-driven acidification and a global outlier exhibiting year-round carbonate undersaturation. Urgent management interventions are recommended to mitigate hypoxia and acidification risks in this vulnerable tropical estuary through nutrient load reduction, enhanced tidal flushing, and ecosystem-based adaptation. The results further provide a valuable basis for developing best management practices in the context of regional and global climate change, thereby supporting the objectives of Sustainable Development Goal 14 (Life Below Water).

Continue reading ‘Monsoon-driven biogeochemical shifts and acidification risk in tropical estuarine ecosystems: a case study from the Indian coast’

Newly discovered CO2 (carbon dioxide) vent cave drives r-strategy shift in a Mediterranean aphotoendosymbiotic coral

Published 14 November 2025 Science Closed
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Highlights

  • Characterization of an unexplored CO2 vent cave
  • CO2 vents chemical-physical parameters affect ecological traits of calcifiers
  • Aphotoendosymbiotic solitary coral naturally inhabiting a CO2-rich gas environment.
  • Prolonged acidified conditions did not affect C. inornata growth rate
  • Shift towards an r-demographic strategy in response to acidified conditions

Abstract

Submarine CO2 volcanic vents represent peculiar environments with varying seawater chemical-physical parameters that may affect the ecological traits of calcifying organisms, such as growth and demographic characteristics. The present study focused on exploring the growth and population dynamics of a temperate, solitary and aphotoendosymbiotic coral Caryophyllia inornata (Duncan, 1878) living in a CO2 vent cave at 14 m depth. The volcanic emissions in and around the cave led high levels of pCO2, resulting in lower calcium carbonate saturation state (Ωa: 2.1–2.2) values compared to those observed in the ambient seawater of the Mediterranean Sea, not affected by venting activity. Prolonged acidified conditions (pHT: 7.5) did not affect C. inornata growth rate but resulted in a population with higher percentage of juvenile individuals, lower average ages and a lower age at maximum biomass percentage, thus suggesting a transition in its population dynamics towards an r-demographic strategy. This study provides a detailed characterization of a previously unexplored CO2 vent cave, highlighting the importance of these sites as natural laboratories to offer valuable insights into understanding the full ecological impact of aphotoendosymbiotic corals under ocean acidification.

Continue reading ‘Newly discovered CO2 (carbon dioxide) vent cave drives r-strategy shift in a Mediterranean aphotoendosymbiotic coral’

Spaces of anthropogenic CO2 emissions compatible with climate boundaries

Published 14 November 2025 Science Closed
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Climate boundaries are planetary boundaries for the climate system: limits within which humanity can sustainably prosper. Here we introduce a modelling framework to analyse global warming, ocean acidification, sea-level rise and Arctic sea-ice melt. Using a reduced-form model, we map out anthropogenic CO2 emissions, carbon dioxide removal and solar radiation management pathways compatible with these boundaries. We define safety levels as the probability to stay within one or several boundaries considering physical uncertainty. If CO2 emissions peak in 2030, net-zero CO2 is reached in 2050, and carbon dioxide removal capacity is 10 PgC yr−1, without solar radiation management, remaining within the global warming boundary of 2 °C exhibits a safety level of 80%. When all four boundaries are considered together, the safety level drops to 35%. Our results highlight key trade-offs in mitigation options and suggest a need to assess climate boundaries holistically to develop sustainable future strategies.

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Sediment topography enhances the response of coral reef carbonate sediment dissolution to ocean acidification

Published 13 November 2025 Science Closed
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The interaction between water flow and sediment topography (e.g., surface ripples) in shallow, permeable coral reef carbonate sediments establishes pressure gradients that increase the rate of sediment–water solute exchange relative to water flow along a flat bottom. It is unknown how this effect from surface ripples may modify the rate at which the sediment porewater is exposed to future chemical changes in the overlying water column, such as elevated pCO2 that is causing ocean acidification (OA). To address this question, this study used a series of 22-h incubations in flume aquaria with live permeable calcium carbonate sediment communities and examined the interactive effect of pCO2 (400 and 1000 µatm) and surface topography (flat and rippled sediments) on invertebrate infaunal activity, carbonate sediment microbial metabolism, and inorganic carbonate dissolution. Results show that the introduction of oxygen into flat sediments was largely driven by infaunal activity, whereas introduction of oxygen into rippled sediments was largely driven by physical flow processes. Rippled sediments exhibited rates of respiration and gross primary production that were ~ 45% and ~ 50% higher, respectively, than flat sediments. An increase in pCO2 shifted the sediments in the flat flumes from net calcifying to net dissolving, an effect that was amplified an additional ~ 60% in rippled sediments. These results suggest that current estimates of coral reef carbonate sediment calcification may be underestimating the dissolution response to OA where the carbonate sediment environment exhibits ripples in the topography.

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Extinguishing the saturation horizon

Published 13 November 2025 Science Closed
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The buffering capacity of the world’s oceans has helped offset the effects of CO2 emissions on climate; however, it is not without cost. Bruce Gibb discusses the causes, the chemistry and the consequences of ocean acidification.

Regular assessments of anthropogenic CO2 emissions and their (re)distribution within the atmosphere, hydrosphere, biosphere and lithosphere are critical to a better understanding of the global carbon cycle and climate change predictions1. Given that CO2 levels have increased from a pre-industrial level of 280 ppm to a current level of 447 ppm, it is crucial to track where this all goes.

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Long term variability of temperature and pH in the Bay of Bengal: an investigation on acoustic perspective

Published 12 November 2025 Science Closed
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This study comprehensively assesses the long-term variability of temperature, ocean acidity changes, and their implications on sound absorption and acoustic propagation in the Bay of Bengal. The analysis reveals a persistent warming trend in the Indian Ocean over the past 50 years, with a significant increase in temperature observed during the Sagar Maitri cruise in 2019. Thermal structure analysis using HadleySST EN4 data indicates warming in the upper 50m but a cooling trend in the 100-200m depth range. Oceanic Heat Content analysis highlights an increasing tendency of heat storage in the upper 50m, indicative of global warming.

In the context of surface ducted propagation, Sonic Layer Depth (SLD) and gradients in the Sound Speed Profile (SSP) were crucial factors influencing acoustic energy behavior. The study revealed a decreasing trend in in-layer gradient (Gr_SL) since 1990, intensifying after that period. The below-layer gradient (Gr_BL) also exhibited a decreasing trend, implying complex dynamics in the sonic layer with potential implications for sound propagation in the surface duct.

The investigation into pH changes spanning 65 years demonstrates a declining trend, particularly since the 1990s, attributed to increased atmospheric CO2 dissolution. The study linked this decrease to anthropogenic activities, aligning with global trends. The analysis of sound absorption illustrated a nonlinear relationship between absorption, frequency, and pH, emphasizing a significant impact of ocean acidification on sound absorption in the Bay of Bengal. The acoustic propagation modeling further highlighted a decrease in transmission loss with reducing pH, leading to increased sound travel and potentially noisier oceans. Salinity variations play a more significant role than temperature in influencing sound absorption.

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Biogeochemical controls on the co-occurrence of mid-depth pH and DO minima in the inner shelf of the East China Sea

Published 12 November 2025 Science Closed
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Highlights

  • Mid-depth pH minima co-occur with DO minima and nitrate maxima near the thermocline.
  • Mid-depth pH minima are driven by both organic matter respiration and upwelling.
  • Low carbonate buffering capacity amplifies mid-depth pH and pCO2 signals.

Abstract

While ocean acidification in coastal oceans is well documented, mid-depth pH dynamics remains largely understudied. In August 2017, we conducted high-resolution vertical profiling of temperature, salinity, pH, dissolved oxygen (DO), and nitrate using in situ biogeochemical sensors in the inner East China Sea shelf. Additionally, vertical distributions of dissolved inorganic carbon (DIC), total alkalinity (TA), partial pressure of CO2 (pCO2), and aragonite saturation state (Ωa) were also calculated. Our observations revealed that mid-depth pH minima (<7.85) co-occurred with DO minima (<60 μmol L−1) and nitrate maxima within or just below the seasonal thermocline. The DO–pH relationships at these stations followed Redfield stoichiometry, indicating organic matter respiration as a primary driver of mid-depth pH minima. High chlorophyll a concentrations (>5.0 μg L−1) at these sites suggested recent phytoplankton blooms fueling the mid-depth oxygen and pH decrease. Although temperature-salinity relationships indicated upwelled water masses contribute to mid-depth pH minima at some stations, their low-pH signature is fundamentally caused by aerobic respiration. A synthesis of five years of cruise data showed that mid-depth pH and DO minima, as well as nitrate maxima, were consistently located along the margins of upwelling zones or salinity fronts—regions of high biological productivity. These patterns underscore the coupled effects of physical transport and biogeochemical processes on mid-depth pH dynamics. Additionally, waters at mid-depth exhibited the lowest carbonate buffer capacity and highest DIC/TA ratios in vertical profiles, amplifying pH declines and pCO2 elevations. Such mid-depth pH minima may negatively affect upper-layer coastal ecosystems, including shellfish aquaculture.

Continue reading ‘Biogeochemical controls on the co-occurrence of mid-depth pH and DO minima in the inner shelf of the East China Sea’

Biogeochemical properties of shallow-water CO2 seeps on Himeshima Island and Showa Iwojima Island, Japan

Published 7 November 2025 Science Closed
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Volcanic gases erupt from the seafloor in several regions around Japan. Volcanological and geochemical gas seep studies have mainly focused on coastal shallow-water areas that are relatively accessible and important to human society. Shallow-water CO2 seeps are thought to foreshadow future marine environments that may develop if CO2 emissions are not drastically reduced. Thus, CO2 seeps provide important insights for assessing and projecting the impacts of ocean acidification on marine ecosystems. This study is the first to investigate two shallow-water CO2 seeps near Japan from the perspective of ocean acidification. We observed biotic transitions and reduced biodiversity around these CO2 seeps, as well as high CO2 concentrations, low pH, and low calcium carbonate saturation—conditions expected to occur by the end of this century unless anthropogenic CO2 emissions are significantly reduced. These results suggest that, from a marine life conservation perspective, it is essential to mitigate ocean acidification through substantial reductions in anthropogenic CO2. Shallow-water CO2 seeps serve as natural experimental sites that illustrate ocean acidification and its effects on marine ecosystems. Given that the shallow-water CO2 seeps examined in this study are both located in geoparks, study tours and ecotourism field trips should utilize these sites to enhance awareness of the consequences of ocean acidification and climate change.

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Climate refugia could disappear from Australia’s marine protected areas by 2040

Published 3 November 2025 Science Closed
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Abstract

Climate change manifests in the ocean as chronic stressors, including warming, acidification and deoxygenation, and as acute stressors such as marine heatwaves. While marine protected areas (MPAs) are often designed to mitigate local stressors such as fishing and mining, their design seldom considers climate change. Using the Australian marine estate as a case study, we use projections from 11 CMIP6 Earth System Models to assess the climate exposure of Australian waters, and implications for the MPA network. We find that, under scenarios that exceed 1.8°C of global surface warming this century, ocean climate is projected to surpass recent variability (1995–2014) from mid-century. This results in the disappearance of climate analogs—where future ocean conditions remain within recent variability—and of climate refugia—regions with slowest rates of environmental change, most likely to retain biodiversity—by 2040. Australian MPAs and unprotected areas exhibit similar patterns of exposure to warming, acidification, deoxygenation, and marine heatwaves, suggesting that MPA placement with respect to future climate is no better than random. Despite potential re-emergence of climate refugia after 2060 under lower-emissions scenarios, continued emissions under current Nationally Determined Contributions (SSP2–4.5) risk ecosystem collapse from chronic and acute thermal stress across protected and unprotected waters. While cutting emissions can partially cap or delay climate impacts, even under lower-emissions scenarios, effective conservation requires adaptive strategies that protect biodiversity in place and on the move.

Plain Language Summary

Marine protected areas (MPAs) are designed to safeguard ocean biodiversity from threats like fishing, but their design rarely considers climate change impacts. We assessed the future exposure of Australia’s MPAs to climate change using projections of ocean climate. Our findings reveal that if global surface warming exceeds 1.8°C this century, Australian marine ecosystems will face entirely novel ocean conditions beyond recent historical variability (1995–2014) by mid-century. This results in the Australia-wide disappearance of regions with slowest rates of climate change—climate refugia—representing a substantial threat to marine biodiversity. Our results suggest that MPAs are no better off than unprotected areas, facing the same risks from warming, acidification, deoxygenation, and marine heatwaves as unprotected waters. We found that reducing emissions could facilitate the reappearance of some climate refugia after 2060, but continuing along current emissions trends risks ecosystem collapse from warming throughout Australia’s protected and unprotected waters. Effective marine conservation requires both emissions reductions and adaptive strategies to protect biodiversity as species respond to a changing ocean climate.

Key Points

  • Ocean climate in Australia will reach a climate horizon by mid-century, representing novel conditions beyond recent variability (1995–2014)
  • Under global warming scenarios exceeding 1.8°C this century, climate refugia are projected to disappear from Australian waters by 2040
  • Existing MPAs and unprotected areas exhibit equivalent patterns of exposure to multiple ocean climate metrics, suggesting a lack of climate-smart design
Continue reading ‘Climate refugia could disappear from Australia’s marine protected areas by 2040′

Rapid ocean acidification and coral calcification response in the northern South China Sea: insights from δ11B and B/Ca records in Porites coral

Published 30 October 2025 Science Closed
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Abstract

Ocean acidification (OA) threatens coral calcification by reducing the carbonate ion concentration that corals need to build their skeletons. However, assessments of the impacts of long-term OA are scarce, limiting our understanding of the response and acclimatization of corals to high pCO2 levels. Here we present a 42-year (1968–2010) seasonal δ11B and B/Ca records from Porites corals at Dongsha Atoll, located in the northern South China Sea. Our results reveal a rapid decline in seawater pH over this period, at a rate of −0.0021 ± 0.0008 pH units per year. Of special interest is that the interannual variability in seawater pH appears to be primarily co-regulated by hydrological changes in the Pearl River and fluctuations in the strength of Kuroshio intrusion. These factors are linked to large-scale climate systems and interannual-to-decadal variability, including the Pacific Decadal Oscillation, El Nino-Southern Oscillation, and East Asian Winter Monsoon. Meanwhile, reconstructed carbonate chemistry from the coral calcifying fluid suggests that Porites corals at Dongsha Atoll are able to physiologically modulate their internal pH. This up-regulation of internal pH not only buffers seasonal fluctuations in the aragonite saturation state and sustains stable calcification rates year-round, but also aids in long-term resistance to the detrimental effects of OA.

Plain Language Summary

Ocean acidification poses a major threat to coral reefs by reducing the concentration of carbonate ions essential for coral skeleton formation. However, long-term assessments of ocean acidification impacts on coral calcification are scarce, limiting our understanding of coral resilience and their potential for acclimation to ocean acidification. Using a 42-year Porites coral δ11B and B/Ca records, we investigated long-term variability in pHsw and the carbonate chemistry of the coral calcifying fluid (e.g., pHcf and Ωcf) in the northern South China Sea. Our results reveal a significant decline in pHsw over the past four decades. The interannual variability in pHsw is primarily co-modulated by hydrological changes in the Pearl River system and variations in the strength of Kuroshio intrusion. Physiological modulation of pHcf up-regulation in Dongsha corals plays a key role in minimizing seasonal fluctuations in Ωcf, maintaining stable year-round calcification rates, and contributing to their long-term resistance to the adverse effects of ocean acidification over the past 42 years.

Key Points

  • A 42-year δ11B and B/Ca records from Dongsha corals reveals rapid OA and physiological adjustment of calcifying fluid carbonate chemistry
  • Interannual pHsw variability is primarily co-modulated by hydrological changes in the Pearl River and variations in the Kuroshio intrusion
  • Under a rapid OA rate (−0.0021 ± 0.0008 pHsw yr−1), calcifying fluid Ωcf remains constant, suggesting that DSA corals are resistant to OA
Continue reading ‘Rapid ocean acidification and coral calcification response in the northern South China Sea: insights from δ11B and B/Ca records in Porites coral’

Unprecedented carbon accumulation in the Indian Ocean during 2016–2017

Published 29 October 2025 Science Closed
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Abstract

During 2016–2017, the Indian Ocean experienced a pronounced increase in dissolved inorganic carbon (∼0.39 PgC/yr), approximately four times greater than the annual mean air–sea CO2 flux. Using a reconstructed data product and a state-of-the-art ocean biogeochemical model, we attribute this anomaly to an enhanced Southern Ocean inflow and a weakened Indonesian Throughflow associated with an El Niño event accompanied by a positive Indian Ocean Dipole (IOD), and followed by a negative IOD during the El Niño-to-La Niña transition. The resulting carbon accumulation leads to a decline in aragonite saturation and a shoaling of the aragonite saturation horizon in the southeastern Indian Ocean. This subsurface acidification may pose risks to deep-water calcifying organisms. Our findings demonstrate that ocean carbon storage and acidification are strongly modulated by circulation-driven transport processes, highlighting the need for improved subsurface observations and model capabilities to better capture the interior carbon response to climate variability.

Plain Language Summary

Between 2016 and 2017, the Indian Ocean stored a much larger amount of carbon than usual—about four times more than the typical annual exchange of carbon between the ocean and atmosphere. Using reconstructed observations and an advanced ocean model, we show that this unusual carbon buildup was caused by stronger inflow from the Southern Ocean and a weaker Indonesian Throughflow, driven by El Niño and negative Indian Ocean Dipole events. This extra carbon made the water more acidic and caused the depth at which aragonite (a mineral important for shell-building organisms) remains stable to rise by nearly 20 m in the southeastern Indian Ocean. These chemical changes could threaten deep-water organisms that rely on stable chemical conditions. Our results highlight how ocean currents can strongly affect carbon storage and acidification, and point to the need for better subsurface measurements and models to understand how climate variability impacts the ocean interior.

Key Points

  • Indian Ocean carbon storage varied unprecedentedly in 2016–2017, driven by circulation anomalies linked to climate variability
  • Anomalous dissolved inorganic carbon inventory was mainly due to increased Southern Ocean inflow and weakened Indonesian Throughflow
  • Anomalous carbon redistribution caused subsurface acidification, shoaling aragonite saturation depth by ∼20 m in the southeast Indian Ocean
Continue reading ‘Unprecedented carbon accumulation in the Indian Ocean during 2016–2017’

Regulation of air-sea CO2 flux and aragonite saturation state in coral reef ecosystems along the eastern coast of Hainan Island, China

Published 28 October 2025 Science Closed
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Highlights

  • Sea surface pCO2 and Ωarag in coral reefs exhibit significant regional variability.
  • The influences of different processes on their dynamics have been clarified.
  • DO, SRP, salinity, and turbidity serve as indicators of biological carbon metabolism.
  • Coral reefs are at considerable risk of acidification by the end of this century.

Abstract

Coral reefs are highly sensitive to environmental stressors and human disturbance, leading to significant variability in seawater carbonate parameters, including partial pressure of CO2 (pCO2) and aragonite saturation state (Ωarag). Although coral reefs extensively occupy the eastern coast of Hainan Island, existing research is insufficient for a comprehensive understanding of their carbonate dynamics and acidification status. In this study, we characterize carbonate chemistry across six coral reef ecosystems along the east Hainan coast. The observed ranges for sea surface pCO2 (342–584 μatm), air-sea CO2 flux (−0.65–2.28 mmol C m−2 d−1), and Ωarag (2.64–3.77) exhibit substantial regional variability, which correlates with wide ranges of dissolved inorganic carbon (DIC, 1848–1968 μmol kg−1) and total alkalinity (TA, 2170–2285 μmol kg−1). A quantitative analysis indicates that the distributions of these carbonate parameters are primarily influenced by the upwelling of CO2-enriched subsurface waters. Additional factors such as coastal riverine plumes, sea surface CO2 outgassing, and biological carbon metabolism significantly shape the overall carbonate chemistry. Correlation analyses suggest that dissolved oxygen (DO) and soluble reactive phosphorus (SRP) serve as indicators of biological organic metabolism, while salinity and turbidity reflect inorganic carbon metabolism. Currently, there appears to be no immediate risk from acidification affecting calcified organisms within these environments, however, they may face substantial threats from seawater acidification by the end of this century due to global warming effects, alongside elevated atmospheric CO2 concentrations as well as regional disturbances arising from both biological organic and inorganic carbon metabolism.

Continue reading ‘Regulation of air-sea CO2 flux and aragonite saturation state in coral reef ecosystems along the eastern coast of Hainan Island, China’

Two decades of pHT measurements along the GO-SHIP A25 section

Published 27 October 2025 Science Closed
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The North Atlantic (NA) GO-SHIP A25 OVIDE-BOCATS section is a long-term repeat hydrographic transect extending from Portugal to Greenland. Since 2002, physical and biogeochemical measurements have been carried out biennially along the OVIDE-BOCATS section, contributing to a better understanding of water mass properties, mixing, circulation, carbon storage, and climate change impacts such as ocean acidification (OA) in the NA. In particular, the high-precision pH measurements on the total hydrogen ion scale (pHT) from the OVIDE-BOCATS program represent a key milestone in monitoring OA in this particularly climate sensitive region. The method used for pHT determination relies on adding meta-cresol purple (mCP) dye to the seawater sample and spectrophotometrically measuring its absorbances at specific wavelengths. The OVIDE-BOCATS program has used unpurified mCP dye, which impurities have been proven to bias pHT values. Here we quantified the bias induced by these impurities in pHT measurements. We found that measurements carried out using the unpurified mCP dye tend to be, on average, 0.011 ± 0.002 pHT units higher than those obtained using the purified mCP dye, with this difference slightly decreasing at higher pHT values. Moreover, we tested independent methods to correct the effect of impurities in both the historical and recent OVIDE-BOCATS pHT data, demonstrating that the correction is consistent across methods. The long-term pHT dataset has been updated to include newly acquired data and absorbance measurements, and to standardize corrections for mCP dye impurities. This effort results in a twenty-year dataset of pHT corrected for mCP dye impurities, that demonstrates the possibility of a global effort to improve the reliability and coherency of spectrophotometric pHT measurements made with unpurified mCP dye. The corrections applied to our pHT dataset have negligible implications for the OA rates previously reported, but they do affect the depth of the aragonite saturation horizon, implying a shoaling of approximately 150 m.

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Seasonal and interannual variability of Atlantidae heteropods along the west coast of Baja California, Mexico

Published 23 October 2025 Science Closed
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Highlights

  • Atlantid species richness was higher in winter than in spring.
  • Maximum species diversity was associated with the 2013–2016 anomalous warm period.
  • Distribution associated with seawater masses, hypoxia, and aragonite saturation.
  • Atlantid species are potential biological indicators of environmental changes.

Abstract

The Atlantidae are holoplanktonic gastropods with aragonitic shells that inhabit the epipelagic habitat primarily in tropical and subtropical oceans, as well as in certain transitional and temperate regions, such as the California Current System. However, there is limited knowledge about how their diversity, distribution, and abundance respond to environmental changes over different time scales. The strongest seasonal changes of zooplankton species composition and environmental conditions in the southern California Current System occur between winter and spring. El Niño Southern Oscillation and marine heat waves are two additional environmental change drivers of interannual scale. Our aim was to infer the effect of the seasonal (winter-spring) and interannual (2012–2016) environmental variability on the diversity, distribution, and abundance of the Atlantidae species assemblage along the Pacific coast off the Baja California peninsula, Mexico. Atlantidae diversity was higher during winters than during springs. Their horizontal distribution recorded during winter was statistically correlated with temperature, salinity, and the seawater masses distribution, and during spring was correlated with the depth of hypoxic conditions (<60 μmol O2/kg oxyline) and the depth of Ω aragonite saturation horizon. Atlanta californiensis was the most abundant species, mainly during spring and its relative abundance decreased during anomalously warm periods, while tropical/subtropical species showed an opposite abundance pattern. The maximum species richness was associated with the 2013–2015 marine heat wave and El Niño 2015–2016 events, when tropical species were observed in the study area. Differences in the species community structure, their response to Ω aragonite undersaturated waters and hypoxia, and their seawater mass affinity showed that atlantids are useful biological indicators of environmental changes, ocean acidification, and deoxygenation conditions.

Continue reading ‘Seasonal and interannual variability of Atlantidae heteropods along the west coast of Baja California, Mexico’

On the measurement of ocean acidity with ambient sound

Published 22 October 2025 Science Closed
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Abstract

The volume-integrated pH of seawater can be determined from the frequency and depth dependence of wind-generated ambient noise in the ocean. Over the 1–10 kHz frequency band, three main processes contribute to the acoustic attenuation in seawater: the chemical relaxation of boric acid and magnesium carbonate (<3 kHz, related to pH) and of magnesium sulfate (>3 kHz, unrelated to pH). When local winds are strong (>10 m/s), the ambient noise is dominated by locally generated surface noise, which exhibits a depth-independent directionality and weak frequency and depth-dependent intensity. By measuring the depth dependence of the spectral slope, the pH may be estimated from a comparison of the experimental data with an analytical model of ambient noise. Wideband (5 Hz–30 kHz) vertical ambient sound profiles were recorded using two- and four-channel free-falling acoustic profilers at depths ranging from 500 m to 10 km during nine deployments in the Philippine Sea, Mariana Trench, and Tonga Trench from 2009 to 2021. Two analytical models of the depth dependence of ambient noise were developed: a simplified linear model valid at depths <1,500 m and a full nonlinear model valid for the deep ocean. Estimates of pH were found by minimizing the mean absolute percent error between the measurements and the models. This method of passive acoustic absorption spectroscopy demonstrates the potential and sources of uncertainty in determining the depth-averaged value of pH. The method could be suitable for the long-term passive acoustic monitoring of ocean acidity.

Plain Language Summary

In this work, we demonstrate that ambient sound in the ocean can be used to measure local, depth-averaged ocean pH. This is possible because the absorption of sound in seawater depends on chemical processes, including the relaxation of boric acid and magnesium carbonate, and has a frequency-dependent sensitivity to the pH. By analyzing the depth dependence of ambient sound over the wind-driven noise-dominated band (1–10 kHz), we can estimate pH through a comparison of measured power spectral slopes with an analytical model. Using measurements from the Philippine Sea, Mariana Trench, and Tonga Trench, carried out from 2009 to 2021 with a free-falling autonomous instrument platform, Deep Sound, we estimated the depth-averaged pH in each location. This technique can be used for long-term passive acoustic monitoring of ocean acidity.

Key Points

  • The depth dependence of the spectral slope of wind-generated noise provides a measurement of the differential acoustic attenuation between 1 and 10 kHz
  • The differential attenuation is used to estimate the depth-integrated pH
  • The proposed method enables long-term volumetric (order of km3) monitoring of ocean acidity
Continue reading ‘On the measurement of ocean acidity with ambient sound’

From global emissions to local impacts: spatially explicit modeling of ocean acidification in life cycle assessment

Published 22 October 2025 Science Closed
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Ocean acidification poses a critical threat to marine ecosystems. While life cycle assessment frameworks provide a method for assessing and combatting many anthropogenic impacts, marine impact models remain underdeveloped compared to their terrestrial counterparts. This study presents the first spatially explicit characterization model for quantifying the impacts of ocean acidification that includes both midpoint and endpoint characterization factors (CFs). Midpoint CFs were spatially delineated by using marine ecoregions and Food and Agriculture Organization fishing areas, leveraging spatially explicit fate and fate sensitivity factors. Endpoint CFs were calculated using species sensitivity distributions that include species across a range of calcification levels, climate zones, and trophic levels. Results demonstrate significant geographic variability in ocean acidification impacts, with polar regions showing heightened vulnerability. Our findings emphasize the need for spatially explicit modeling to account for the diverse biogeochemical and ecological responses to ocean acidification. This work advances marine impact assessment by integrating spatial and biological complexity, providing critical tools for quantifying ocean acidification’s global ecological and economic consequences.

Continue reading ‘From global emissions to local impacts: spatially explicit modeling of ocean acidification in life cycle assessment’

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