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

Little is known about the rate of increase of coastal seawater p CO 2 ( p CO 2 sea ), despite its necessity for assessing future oceanic CO 2 uptake capacity. We examined temporal changes in p CO 2 sea in central Sagami Bay during 2001–2009. Weekly p CO 2 sea was reconstructed using time series of particulate organic carbon isotope delta (POC-δ 13 C) of settling particles at 150 m from moored sediment-trap experiments. For p CO 2 sea estimation, an empirical relationship between suspended POC-δ 13 C and aqueous CO 2 concentration from repeat ship observations in 2007–2008 was applied to the trapped POC-δ 13 C. Air–sea CO 2 flux was calculated using the air–sea p CO 2 difference with gas transfer velocity. Estimated Bay p CO 2 sea varied by 190 μatm (mean 294 μatm) and was mostly below atmospheric p CO 2 ( p CO 2 air ). The mean oceanic CO 2 uptake was 82 mg m −2 d −1 , suggesting that Sagami Bay is an efficient sink for atmospheric CO 2 . Meanwhile, carbon sequestration to the mesopelagic layer by particulate carbon export accounted for 60–75% of the CO 2 uptake, with the rest likely removed horizontally via surface water exchange. The p CO 2 sea showed an increasing trend of + 3.9 µatm y −1 , approximately twice that of p CO 2 air , 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 p CO 2 sea increasing rate (+ 4.9 µatm y −1 ) was observed accompanied by POC concentration decreasing, which resulted in a decrease in CO 2 uptake over time. Long-term summer nutrient depletion and reduced primary production may increase p CO 2 sea in the Bay.