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The ocean has absorbed anthropogenic carbon dioxide (C_anthro) from the atmosphere and played an important role in mitigating global warming. However, how much C_anthrois accumulated in coastal oceans and where it comes from have rarely been addressed with observational data. Here, we use a high-quality carbonate dataset (1996–2018) in the U.S. East Coast to address these questions. Our work shows that the offshore slope waters have the highest C_anthroaccumulation changes (ΔC_anthro) consistent with water mass age and properties. From offshore to nearshore, ΔC_anthrodecreases with salinity to near zero in the subsurface, indicating no net increase in the export of C_anthrofrom estuaries and wetlands. Excesses over the conservative mixing baseline also reveal an uptake of C_anthrofrom the atmosphere within the shelf. Our analysis suggests that the continental shelf exports most of its absorbed C_anthrofrom the atmosphere to the open ocean and acts as an essential pathway for global ocean C_anthrostorage and acidification. 

Abstract Dissolved inorganic carbon (DIC) and its stable carbon isotope (δ¹³C‐DIC) are valuable parameters for studying the aquatic carbon cycle and quantifying ocean anthropogenic carbon accumulation rates. However, the potential of this coupled pair is underexploited as only 15% or less of cruise samples have been analyzed forδ¹³C‐DIC because the traditional isotope analysis is labor‐intensive and restricted to onshore laboratories. Here, we improved the analytical precision and reported the protocol of an automated, efficient, and high‐precision method for ship‐based DIC andδ¹³C‐DIC analysis based on cavity ring‐down spectroscopy (CRDS). We also introduced a set of stable in‐house standards to ensure accurate and consistent DIC andδ¹³C‐DIC measurements, especially on prolonged cruises. With this method, we analyzed over 1600 discrete seawater samples over a 40‐d cruise along the North American eastern ocean margin in summer 2022, representing the first effort to collect a large dataset ofδ¹³C‐DIC onboard of any oceanographic expedition. We evaluated the method's uncertainty, which was 1.2 μmol kg⁻¹for the DIC concentration and 0.03‰ for theδ¹³C‐DIC value (1σ). An interlaboratory comparison of onboard DIC concentration analysis revealed an average offset of 2.0 ± 3.8 μmol kg⁻¹between CRDS and the coulometry‐based results. The cross‐validation ofδ¹³C‐DIC in the deep‐ocean data exhibited a mean difference of only −0.03‰ ± 0.07‰, emphasizing the consistency with historical data. Potential applications in aquatic biogeochemistry are discussed. 

The western Arctic Ocean is rapidly acidifying due to sea ice loss. 

Rapid climate warming and sea-ice loss have induced major changes in the sea surface partial pressure of CO2 ( pCO2I). However, the long-term trends in the western Arctic Ocean are unknown. Here we show that in 1994–2017, summer pCO2I in the Canada Basin increased at twice the rate of atmospheric increase. Warming and ice loss in the basin have strengthened the pCO2I seasonal amplitude, resulting in the rapid decadal increase. Consequently, the summer air–sea CO2 gradient has reduced rapidly, and may become near zero within two decades. In contrast, there was no significant pCO2I increase on the Chukchi Shelf, where strong and increasing biological uptake has held pCO2I low, and thus the CO2 sink has increased and may increase further due to the atmospheric CO2 increase. Our findings elucidate the contrasting physical and biological drivers controlling sea surface pCO2I variations and trends in response to climate change in the Arctic Ocean. 

Abstract The acidification of coastal waters is distinguished from the open ocean because of much stronger synergistic effects between anthropogenic forcing and local biogeochemical processes. However, ocean acidification research is still rather limited in polar coastal oceans. Here, we present a 17‐year (2002–2019) observational data set in the Chukchi Sea to determine the long‐term changes in pH and aragonite saturation state (Ω_arag). We found that pH and Ω_aragdeclined in different water masses with average rates of −0.0047 ± 0.0026 years⁻¹and −0.017 ± 0.009 years⁻¹, respectively, and are ∼2–3 times faster than those solely due to increasing atmospheric CO₂. We attributed the rapid acidification to the increased dissolved inorganic carbon owing to a combination of ice melt‐induced increased atmospheric CO₂invasion and subsurface remineralization induced by a stronger surface biological production as a result of the increased inflow of the nutrient‐rich Pacific water. 

Abstract The Chukchi Sea is an increasing CO₂sink driven by rapid climate changes. Understanding the seasonal variation of air‐sea CO₂exchange and the underlying mechanisms of biogeochemical dynamics is important for predicting impacts of climate change on and feedbacks by the ocean. Here, we present a unique data set of underway sea surface partial pressure of CO₂(pCO₂) and discrete samples of biogeochemical properties collected in five consecutive cruises in 2014 and examine the seasonal variations in air‐sea CO₂flux and net community production (NCP). We found that thermal and non‐thermal effects have different impacts on sea surfacepCO₂and thus the air‐sea CO₂flux in different water masses. The Bering summer water combined with meltwater has a significantly greater atmospheric CO₂uptake potential than that of the Alaskan Coastal Water in the southern Chukchi Sea in summer, due to stronger biological CO₂removal and a weaker thermal effect. By analyzing the seasonal drawdown of dissolved inorganic carbon (DIC) and nutrients, we found that DIC‐based NCP was higher than nitrate‐based NCP by 66%–84% and attributable to partially decoupled C and N uptake because of a variable phytoplankton stoichiometry. A box model with a non‐Redfield C:N uptake ratio can adequately reproduce observedpCO₂and DIC, which reveals that, during the intensive growing season (late spring to early summer), 30%–46% CO₂uptake in the Chukchi Sea was supported by a flexible stoichiometry of phytoplankton. These findings have important ramification for forecasting the responses of CO₂uptake of the Chukchi ecosystem to climate change. 

Abstract To examine seasonal and regional variabilities in metabolic status and the coupling of net community production (NCP) and air‐sea CO₂fluxes in the western Arctic Ocean, we collected underway measurements of surface O₂/Ar and partial pressure of CO₂(pCO₂) in the summers of 2016 and 2018. With a box‐model, we demonstrate that accounting for local sea ice history (in addition to wind history) is important in estimating NCP from biological oxygen saturation (Δ(O₂/Ar)) in polar regions. Incorporating this sea ice history correction, we found that most of the western Arctic exhibited positive Δ(O₂/Ar) and negativepCO₂saturation, Δ(pCO₂), indicative of net autotrophy but with the relationship between the two parameters varying regionally. In the heavy ice‐covered areas, where air‐sea gas exchange was suppressed, even minor NCP resulted in relatively high Δ(O₂/Ar) and lowpCO₂in water due to limited gas exchange. Within the marginal ice zone, NCP and CO₂flux magnitudes were strongly inversely correlated, suggesting an air to sea CO₂flux induced primarily by biological CO₂removal from surface waters. Within ice‐free waters, the coupling of NCP and CO₂flux varied according to nutrient supply. In the oligotrophic Canada Basin, NCP and CO₂flux were both small, controlled mainly by air‐sea gas exchange. On the nutrient‐rich Chukchi Shelf, NCP was strong, resulting in great O₂release and CO₂uptake. This regional overview of NCP and CO₂flux in the western Arctic Ocean, in its various stages of ice‐melt and nutrient status, provides useful insight into the possible biogeochemical evolution of rapidly changing polar oceans. 

Abstract The Arctic Ocean has turned from a perennial ice‐covered ocean into a seasonally ice‐free ocean in recent decades. Such a shift in the air‐ice‐sea interface has resulted in substantial changes in the Arctic carbon cycle and related biogeochemical processes. To quantitatively evaluate how the oceanic CO₂sink responds to rapid sea ice loss and to provide a mechanistic explanation, here we examined the air‐sea CO₂flux and the regional CO₂sink in the western Arctic Ocean from 1994 to 2019 by two complementary approaches: observation‐based estimation and a data‐driven box model evaluation. ThepCO₂observations and model results showed that summer CO₂uptake significantly increased by about 1.4 ± 0.6 Tg C decade⁻¹in the Chukchi Sea, primarily due to a longer ice‐free period, a larger open area, and an increased primary production. However, no statistically significant increase in CO₂sink was found in the Canada Basin and the Beaufort Sea based on both observations and modeled results. The reduced sea ice coverage in summer in the Canada Basin and the enhanced wind speed in the Beaufort Sea potentially promoted CO₂uptake, which was, however, counteracted by a rapidly decreased air‐seapCO₂gradient therein. Therefore, the current and future Arctic Ocean CO₂uptake trends cannot be sufficiently reflected by the air‐seapCO₂gradient alone because of the sea ice variations and other environmental factors.