[H-TT16] Development and application of environmental traceability methods

[HTT16-P05] Evaluation of specificity of atmospheric and oceanic circulation associated with sea ice lost in the Western Arctic Ocean

*Takahiro Miyazawa¹, Keiji Horikawa¹, Ki-Cheol Shin², Masanobu Yamamoto³, Kenta Suzuki⁴ (1.University of Toyama, 2.Research Institute for Humanity and Nature, 3.Faculty of Environmental Earth Science, Hokkaido University, 4.Chiba Institute of Technology)

Keywords:Primary roduction, Pacific seawater, Neodimium Isotope ratio, Lead Isotope ratio

In recent years, the effects of global warming have been significantly evident in the Arctic Ocean, such as the change of sea ice from perennial ice to seasonal sea ice and the halving of sea ice extent in summer (compared to the 1970s–80s) (Kashiwase et al., 2017). In particular, in the Western Arctic Ocean (WAO), the inflow of warm, low-salinity Pacific seawater into the WAO has delayed sea ice formation and depletion (Weingartner et al., 1999; Woodgate et al., 2005). Pacific seawater flows into the WAO from the Bering Strait and then diverges into the Chukchi Slope Current (CSC) and the Beaufort Shelf Breakjet (BSJ) at Barrow Canyon. CSC has an average annual westward flow and mainly transports Bering seawater (Corlett and Pickart, 2017). On the other hand, BSJ has an eastward flow on average per year, and mainly transports Alaskan coastal waters. However, according to 10-year observations from 2002~2014, the intensification of easterly winds over Beaufort Sea has reduced the transport volume of BSJ by 77% (Brugler et al., 2014), and the circulation of the atmosphere and ocean has changed. Long-term environmental data are essential for assessing whether recent changes in Pacific seawater inflows and atmospheric-oceanic conditions are unprecedented in the past several hundred to thousand years. In this study, we reconstruct the inflow variability of Pacific seawater over the past 1,300 years and evaluate the specificity of atmospheric and oceanic circulation in the WAO. The samples used were marine sediment (MR22-06C BC2-GC08, 71.9102 °N, 154.1604 °W, depth: 223 m and MR22-06C BC2-Box1, 71.9099 °N, 154.1577 °W, depth: 224 m) collected at the Barrow Canyon BC2 site. BC2-Box1 has been found to retain continuous deposits for the past 40 years, and BC2-GC08 has been found to hold continuous deposits for the past 1300 years due to the 14C age of the interposed shellfish (Yamamoto et al. (submitted)). For these samples, the clastic material and Fe-Mn hydroxide fractions were obtained by sequential dissolution method. We used a solution prepared by adding Sodium hydroxide to a solution of 0.016 M Hydroxylamine Hydrochloride + 0.8 M Acetic Acid), referring to Du et al. (2016). The dissolution time was 5 min, and we treated as a Fe-Mn hydroxide fraction. In order to reconstruct the inflow of Pacific seawater, we analyzed the concentrations of 34 elements such as Ca, REEs, and Mn, as well as the isotope ratios of Nd and Pb. The Ca concentration of clastic materials in the BC2-GC08 sample doubled after 1950 compared to the previous period and showed a remarkable increase compared to the medieval warm period (MWP). This is thought to reflect an increase in primary production due to recent changes in the WAO. The Nd isotope ratio of the clastic material fluctuated from –7 to –9.5 _εNd over the past 1300 years, and the Fe-Mn hydroxide fluctuated from –4 ~ –6.5 _εNd. In the MWP (900 to 1300), the εNd value was low, suggesting that the contribution of the Alaskan coastal water originating from the Yukon River. On the other hand, the Little Ice Age (1350 to 1900) showed high εNd values, suggesting that Bering seawater contributed a large amount. The Pb isotope ratio of the clastic material (²⁰⁶Pb/²⁰⁴Pb) varied from 19.03 to 19.15 overall. From the past to the present, the Pb isotope ratio has shown an increasing trend, and is characterized by a shift to a high value of ²⁰⁶Pb/²⁰⁴Pb since 1900. Therefore, these results suggest that the contribution of Bering seawater decreased after 1900, and that the contribution of Alaskan coastal waters increased relatively. This result supports the recent strengthening of Beaufort Gyre (BG), anticyclonic wind circulation in the WAO, and the reduction in eastward transport of BSJ in Brugler et al., (2014). It indicates that the increase in primary production is unique in the WAO during the past 1300 years, although the inflow of Pacific seawater cannot be decided to be unique.