2:15 PM - 2:30 PM
[MIS18-02] Changes in oceanic uptake of anthropogenic CO2 revealed by carbon isotopic and morphological analysis in North Pacific coral skeletons
Keywords:Suess effect, anthropogenic CO2, carbon isotope ratio, global warming, ocean acidification
Since the industrial revolution the increase of atmospheric carbon dioxide (CO2) due to the human activities such as the use of fossil fuels and deforestation has contributed to accelerating global warming. Previous studies have pointed out that the increase in atmospheric CO2concentration accelerates the uptake of CO2 into the ocean, resulting in decrease in carbon isotope ratio of dissolved inorganic carbon (δ13CDIC) in sea water (Oceanic 13C Suess effect (Gruber et al, 1999)). Nevertheless, the observation records CO2 concentration are still limited at Mauna Loa Observatory in Hawaii since 1958 for atmospheric CO2, and oceanic CO2 (e.g. pCO2, DIC concentration, pH) has been observed at Station ALOHA since 1989.
Carbon isotope ratio of coral skeletons (δ13Cc) has been used to evaluate the oceanic uptake of anthropogenic CO2 and Oceanic 13C Suess effect. However, the factors that cause the variation of δ13Cc are not only environmental factors (e.g. solar radiation, DIC) but also physiological factors (e.g. photosynthesis, respiration, spawning. In addition, previous studies have often used coral cores collected from sea where CO2 environmental data is scarce, so the impact of anthropogenic CO2 on marine ecosystems and its future predictions are under discussion. In this study, we analyzed δ13Cc over the past 100 years collected in Hawaii where pCO2 and CO2 concentrations have been observed, and compared it with the observed records. In addition, isotope analysis and skeletal morphological analysis inhabiting tropical and temperate areas along the coast of Japan were conducted to clarify the factors that cause changes in δ13Cc.
We collected Porites evermanni ( 1.5m length) core from Makai Pier on the east coast of Oahu, Hawaii (water depth 1~2m) and analyzed δ13Cc using the isotope ratio mass spectrometry coupled with the carbonate device (Kiel Device IV and Thermo Scientific 253 Plus). As environmental data for comparison with the results, we mainly used the observation records of Station ALOHA. Skeletal morphology of Plesiastrea versipora collected from 7 sites in Kuroshio current was quantitatively measured using a digital microscope (KEYENCE VHX-2000) and discussed together with the δ13Cc results of the same samples (Koyama, 2020 MS).
The δ13Cc in coral cores changed with seasonal variability (2.09‰~-3.99‰), decreasing at a rate -0.022‰/year since the early 1950s, and the decadal variability has been converging since the 1990s. From this study and previous studies, δ13Cc average tended to decrease toward higher latitudes, especially in the Northwest Pacific where the value is ~-6.0‰. In addition, skeletal morphological analysis showed that high-latitude corals in the northwestern Pacific Ocean have longer septa and larger corallite sizes.
A positive correlation found betweenδ13Cc monthly average and DIC concentration in Hawaii coral (high in summer, low in winter). The δ13Cc decrease rate after 1989 (-0.021‰/year) is similar to the δ13CDIC decrease rate (-0.026‰/year) observed at Station ALOHA, suggesting that δ13Cc decrease rate records an Oceanic 13C Suess effect. However, skeletal morphological analysis suggested that high-latitude corals may changes their morphology to maintain photosynthesis by developing tissues with high photosynthetic activity, and δ13Cc of high-latitude corals strongly reflects the influence of the Oceanic 13C Suess effect. No significant correlation was found between δ13Cc variability in Hawaii over the past 100 years and PDO, NPGO, ENSO, which drive climate change on decadal scale. A comparison of δ13Cc variability between Hawaii and high latitudes off the coast of Japan showed different patterns. We infer that the 100-year scale of δ13Cc change records the variability of oceanic CO2 uptake unique to each region.
Carbon isotope ratio of coral skeletons (δ13Cc) has been used to evaluate the oceanic uptake of anthropogenic CO2 and Oceanic 13C Suess effect. However, the factors that cause the variation of δ13Cc are not only environmental factors (e.g. solar radiation, DIC) but also physiological factors (e.g. photosynthesis, respiration, spawning. In addition, previous studies have often used coral cores collected from sea where CO2 environmental data is scarce, so the impact of anthropogenic CO2 on marine ecosystems and its future predictions are under discussion. In this study, we analyzed δ13Cc over the past 100 years collected in Hawaii where pCO2 and CO2 concentrations have been observed, and compared it with the observed records. In addition, isotope analysis and skeletal morphological analysis inhabiting tropical and temperate areas along the coast of Japan were conducted to clarify the factors that cause changes in δ13Cc.
We collected Porites evermanni ( 1.5m length) core from Makai Pier on the east coast of Oahu, Hawaii (water depth 1~2m) and analyzed δ13Cc using the isotope ratio mass spectrometry coupled with the carbonate device (Kiel Device IV and Thermo Scientific 253 Plus). As environmental data for comparison with the results, we mainly used the observation records of Station ALOHA. Skeletal morphology of Plesiastrea versipora collected from 7 sites in Kuroshio current was quantitatively measured using a digital microscope (KEYENCE VHX-2000) and discussed together with the δ13Cc results of the same samples (Koyama, 2020 MS).
The δ13Cc in coral cores changed with seasonal variability (2.09‰~-3.99‰), decreasing at a rate -0.022‰/year since the early 1950s, and the decadal variability has been converging since the 1990s. From this study and previous studies, δ13Cc average tended to decrease toward higher latitudes, especially in the Northwest Pacific where the value is ~-6.0‰. In addition, skeletal morphological analysis showed that high-latitude corals in the northwestern Pacific Ocean have longer septa and larger corallite sizes.
A positive correlation found betweenδ13Cc monthly average and DIC concentration in Hawaii coral (high in summer, low in winter). The δ13Cc decrease rate after 1989 (-0.021‰/year) is similar to the δ13CDIC decrease rate (-0.026‰/year) observed at Station ALOHA, suggesting that δ13Cc decrease rate records an Oceanic 13C Suess effect. However, skeletal morphological analysis suggested that high-latitude corals may changes their morphology to maintain photosynthesis by developing tissues with high photosynthetic activity, and δ13Cc of high-latitude corals strongly reflects the influence of the Oceanic 13C Suess effect. No significant correlation was found between δ13Cc variability in Hawaii over the past 100 years and PDO, NPGO, ENSO, which drive climate change on decadal scale. A comparison of δ13Cc variability between Hawaii and high latitudes off the coast of Japan showed different patterns. We infer that the 100-year scale of δ13Cc change records the variability of oceanic CO2 uptake unique to each region.