[A-CG52] Science in the Arctic Region

[ACG52-05] Validation of alkalinity as a freshwater tracer in the Pacific Arctic Ocean

*Komura Tsukushi¹, Michiyo Yamamoto-Kawai¹, Kaede Goto¹, Mariko Hatta², Akihiko Murata² (1.Tokyo University of Marine Science and Technology, 2.Japan Agency for Marine-Earth Science and Technology)

Keywords:Arctic Ocean, Freshwater, Alkalinity

The Pacific side of the Arctic Ocean is known as the “freshwater reservoir” because it receives and stores large amounts of freshwater, including sea ice meltwater, river runoff and precipitation. Changes in the distribution of freshwater in this region impact water circulation, biogeochemical cycles, and sea ice formation in the Arctic Ocean, as well as global overturning circulation by altering NADW formation. In order to distinguish between sea ice meltwater and meteoric water (river + precipitation), chemical tracers such as oxygen isotope ratios (δ¹⁸O) and alkalinity have been used. Alkalinity is useful because it’s simple analytical method allows for the acquisition of large amounts of data compared to δ¹⁸O. However, validation of alkalinity as freshwater tracer is insufficient, as alkalinity of meteoric water can vary between ocean regions. Accordingly, the aim of this study is to validate alkalinity as a freshwater tracer in the Pacific Arctic Ocean, by comparing it with the well-established traditional method using δ¹⁸O.
We used δ¹⁸O and alkalinity data obtained during the cruises of CCGS Louis S. St-Laurent in 1997 and 2003-2022 in the basin area, and R/V Mirai in 2022 in coastal area.
First, we estimated the alkalinity of the meteoric water in each target area based on the combination of δ¹⁸O and alkalinity. Result showed that alkalinity of meteoric water end-member in the coastal area varied largely between regions and seasons, whereas in the basin area, it remained at a similar value. Therefore, occasional measurement of δ¹⁸O is necessary in coastal areas, while alkalinity alone can be used to estimate freshwater fractions in the basin area. If one assumes a constant alkalinity for meteoric water in coastal regions, it can result in a large error in the calculation results.
Based on these findings, we could estimate the fractions of sea ice melt water and meteoric water in the Pacific Arctic Ocea using alkalinity with error ranges similar to those of δ¹⁸O. Additionally, we demonstrated that high-frequency observations of alkalinity can identify freshwater sources with greater temoral and spatial resolution than δ¹⁸O.