11:00 AM - 1:00 PM
[AOS18-P03] Distribution of dissolved organic radiocarbon and dynamics of dissolved organic matter in the northwestern North Pacific
Keywords:Dissolved organic matter, Radiocarbon, North Pacific, Bering Sea, Okhotsk Sea, Carbon cycle
Dissolved organic carbon (DOC) forms the largest reservoir of organic carbon in the ocean. In the deep layer (>1,500 m) of the North Pacific, it is known that DOC shows more than 6,000 years of 14C age and remain in the ocean for several times longer than the global seawater circulation. Since the 14C age of the ocean DOC provides information on the timescale of its sources and geochemical cycles, it is required to understand the distribution at the global scale. In this study, we firstly report the distribution of the 14C isotope ratio of DOC (DOC-Δ14C) in seawater of the northwestern marginal regions of the North Pacific and give an overview the dynamics of DOC in this area.
Observations were carried out from 2018 to 2019 at four stations: Sta K2 (47.00°N, 160.00°E) inside the North Pacific Subarctic Gyre, Sta B03 (54.03°N, 163.00°E) on the northwestern edge, Sta E03 (55.73°N, 164.77°E) on the northwestern edge of the Bering Sea, and Sta BF (46.44°N, 151.12°E) in the Bussol’ Strait in the southern Okhotsk Sea (Fig. 1) with the cooperation of R/V Multanovskiy (FERHRI, Russia) Mu18 and R/V Mirai (JAMSTEC) MR19-2 cruises. We also used DOC-Δ14C data at Sta K2 in 2006 (Tanaka et al., 2010) for comparison. Seawater samples were collected with Niskin-X samplers, and salinity and temperature were measured by CTD at the time of water sampling. For the analysis of DOC-Δ14C in seawater, DOC was oxidized to carbon dioxide by an improved UV irradiation method (Otosaka et al., 2021), and the generated carbon dioxide was recovered in a vacuum glass line, and graphitized. The 14C isotope ratio was measured with a single-stage accelerator mass spectrometer at the Atmosphere and Ocean Research Institute, the University of Tokyo.
The DOC-Δ14C value in seawater obtained in this study ranged between -499 and -236‰, which was generally high in the surface layer and decreased with depth (Fig. 2). The DOC-Δ14C value at the deep layer (>1500 m) of Sta K2 converges to -480±8‰, which was not significantly different from the value at the same depth in 2006 (-488±8 ‰). The DOC concentration in the deep layer of Sta K2 was 37±1 µmol/kg, and the relationship between the DOC concentration and Δ14C was consistent with the that was reported from deep water in other open oceans (Druffel et al., 2016). This suggests that the characteristics of DOC in deep water of Sta K2 represent the result of DOC alteration associated with deep circulation on a global scale.
At all stations, DOC-Δ14C was explained by a two-component mixing model of “modern” DOC produced in the surface layers and “old” DOC that circulates in the deep layers. Interestingly, in Sta K2, the estimated concentration of “old” DOC showed a maximum in the intermediate layers from 200m to 800m. The concentration of “old” DOCs in the upper layers was particularly high in the northern stations of this study (Stas E03 and B03), suggesting that DOC from this area has been transported to the open ocean through the intermediate layers. In addition, the contribution of “old” DOCs in the intermediate layers at Sta K2 was more remarkable in 2006, suggesting that the horizontal DOC flux through the intermediate layer may fluctuate over time.
References
Druffel ERM, Griffin S, Coppola AI, Walker BD (2016): Geophys. Res. Lett., 43, 5279-5286.
Otosaka S, Jeon H, Hou Y et al (2021): The 15th International Conference on Accelerator Mass Spectrometry (AMS-15), T5-13.
Tanaka T, Otosaka S, Wakita M, Amano H, Togawa O (2010): Nucl. Inst. and Methods in Physics. Res. B, 268, 1219-1221.
Acknowledgements
This work was supported by the Japan Society for the Promotion of Science grants KAKENHI Nos JP16K00527, JP19H04260, JP15H05817, 15H05818, and JP15H05820. This work was also supported by the Arctic Challenge for Sustainability (ArCS) project, and the Grant for Joint Research Program of the Institute of Low Temperature Science, Hokkaido University.
Observations were carried out from 2018 to 2019 at four stations: Sta K2 (47.00°N, 160.00°E) inside the North Pacific Subarctic Gyre, Sta B03 (54.03°N, 163.00°E) on the northwestern edge, Sta E03 (55.73°N, 164.77°E) on the northwestern edge of the Bering Sea, and Sta BF (46.44°N, 151.12°E) in the Bussol’ Strait in the southern Okhotsk Sea (Fig. 1) with the cooperation of R/V Multanovskiy (FERHRI, Russia) Mu18 and R/V Mirai (JAMSTEC) MR19-2 cruises. We also used DOC-Δ14C data at Sta K2 in 2006 (Tanaka et al., 2010) for comparison. Seawater samples were collected with Niskin-X samplers, and salinity and temperature were measured by CTD at the time of water sampling. For the analysis of DOC-Δ14C in seawater, DOC was oxidized to carbon dioxide by an improved UV irradiation method (Otosaka et al., 2021), and the generated carbon dioxide was recovered in a vacuum glass line, and graphitized. The 14C isotope ratio was measured with a single-stage accelerator mass spectrometer at the Atmosphere and Ocean Research Institute, the University of Tokyo.
The DOC-Δ14C value in seawater obtained in this study ranged between -499 and -236‰, which was generally high in the surface layer and decreased with depth (Fig. 2). The DOC-Δ14C value at the deep layer (>1500 m) of Sta K2 converges to -480±8‰, which was not significantly different from the value at the same depth in 2006 (-488±8 ‰). The DOC concentration in the deep layer of Sta K2 was 37±1 µmol/kg, and the relationship between the DOC concentration and Δ14C was consistent with the that was reported from deep water in other open oceans (Druffel et al., 2016). This suggests that the characteristics of DOC in deep water of Sta K2 represent the result of DOC alteration associated with deep circulation on a global scale.
At all stations, DOC-Δ14C was explained by a two-component mixing model of “modern” DOC produced in the surface layers and “old” DOC that circulates in the deep layers. Interestingly, in Sta K2, the estimated concentration of “old” DOC showed a maximum in the intermediate layers from 200m to 800m. The concentration of “old” DOCs in the upper layers was particularly high in the northern stations of this study (Stas E03 and B03), suggesting that DOC from this area has been transported to the open ocean through the intermediate layers. In addition, the contribution of “old” DOCs in the intermediate layers at Sta K2 was more remarkable in 2006, suggesting that the horizontal DOC flux through the intermediate layer may fluctuate over time.
References
Druffel ERM, Griffin S, Coppola AI, Walker BD (2016): Geophys. Res. Lett., 43, 5279-5286.
Otosaka S, Jeon H, Hou Y et al (2021): The 15th International Conference on Accelerator Mass Spectrometry (AMS-15), T5-13.
Tanaka T, Otosaka S, Wakita M, Amano H, Togawa O (2010): Nucl. Inst. and Methods in Physics. Res. B, 268, 1219-1221.
Acknowledgements
This work was supported by the Japan Society for the Promotion of Science grants KAKENHI Nos JP16K00527, JP19H04260, JP15H05817, 15H05818, and JP15H05820. This work was also supported by the Arctic Challenge for Sustainability (ArCS) project, and the Grant for Joint Research Program of the Institute of Low Temperature Science, Hokkaido University.