Rapid increase of surface water pCO2 revealed by settling particulate organic matter carbon isotope time series during 2001–2009 in Sagami Bay, Japan

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Little is known about the rate of increase of coastal seawater pCO2 (pCO2sea), despite its necessity for assessing future oceanic CO2 uptake capacity. We examined temporal changes in pCO2sea in central Sagami Bay during 2001–2009. Weekly pCO2sea was reconstructed using time series of particulate organic carbon isotope delta (POC-δ13C) of settling particles at 150 m from moored sediment-trap experiments. For pCO2sea estimation, an empirical relationship between suspended POC-δ13C and aqueous CO2 concentration from repeat ship observations in 2007–2008 was applied to the trapped POC-δ13C. Air–sea CO2 flux was calculated using the air–sea pCO2 difference with gas transfer velocity. Estimated Bay pCO2sea varied by 190 μatm (mean 294 μatm) and was mostly below atmospheric pCO2 (pCO2air). The mean oceanic CO2 uptake was 82 mg m−2 d−1, suggesting that Sagami Bay is an efficient sink for atmospheric CO2. Meanwhile, carbon sequestration to the mesopelagic layer by particulate carbon export accounted for 60–75% of the CO2 uptake, with the rest likely removed horizontally via surface water exchange. The pCO2sea showed an increasing trend of + 3.9 µatm y−1, approximately twice that of pCO2air, and the two converged. Concurrently, a decreasing trend in POC export flux and an increasing trend in nitrogen isotope delta of trapped particles were found. Particularly, a large summer pCO2sea increasing rate (+ 4.9 µatm y^−1) was observed accompanied by POC concentration decreasing, which resulted in a decrease in CO2 uptake over time. Long-term summer nutrient depletion and reduced primary production may increase pCO2sea in the Bay.

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