1 HW – Physical Climatology Implication of biogeochemical change driven by global warming in the East Sea IL NAM KIM Department of Marine Science, University of Texas, Austin, TX 78705, USA Abstract: The East Sea is undergoing physical changes caused by global warming: deepening oxygen minimum layer and increasing temperature. However, any correspondence to biogeochemical change was not noticed yet. Here, we present indirect evidences of denitrification in response to a global warming and estimate the amount of denitrification by the linear inverse model. N/P ratio of the East Sea below 300m is estimated as 12.4, quite lower than the Redfield ratio of 16. Intense N/P ratio and oxygen minimum zones are founded between 900m and 2200m in the Ulleung Basin. Two stations are strongly expected to occur denitrification, where is located within the Ulleung Basin, show a series of phenomena at specific depths that nitrate profile is reversed, N/P ratio is less than 12.4, and nitrite shows a peak. Evidently, these results indicate that denitrification is occurring in the East Sea, in particular, in the Ulleung Basin. Through linear inverse modeling, the mean flux of denitrification in the UB is estimated as 9.0 ~ 16.0 daymmol ⋅⋅−2µ, and the mean rate is ranged from 1441035.11~1038.6−⋅×× dmol . However, note that the East Sea is still high concentration in oxygen compared with the condition denitrified. Nevertheless, considered it in relation to the recently radical changes of the East Sea, the results might be involved in biogeochemical feedback loop returning as positive effect on the climate change. Therefore, we need to note the future change of the East Sea. Keywords: the East Sea, linear inverse model, N/P ratio and oxygen minimum, denitrification, biogeochemical feedback loop, the climate change.2 1. Introduction The East Sea is a semi-enclosed marginal sea in the North Western Pacific surrounded by Korea, Russia, and Japan. It has four straits (Korea, Tsugaru, Soya, and Tatar), three major basins (Ulleung, Yamato, and Japan), and one rise (Yamato). In the upper part of the East Sea, four straits play a role to exchange seawaters between the East Sea and the neighboring oceans, which are Okhotsk, Pacific Ocean, and the South Sea. [Fig. 1]. The East Sea is frequently cited as “Miniature Ocean”, because it has a lot of oceanic characteristics- for instances, water formation, front, eddy, upwelling, and etc. In view of the time that the anthropogenic CO2 has been emitted in the atmosphere since pre-industry, the East Sea plays an optimal place as a natural model to study modern climate change [Kim et al., 2001], because the residence time of the East Sea is about 100 years [Kim and Kim, 1996; Seung and Kim, 1997; Yanagi, 2002]. Recently, many studies supported the fact that physical changes deduced from global warming are occurring in the East Sea: for examples, increases of air and sea temperature [Gamo, 1999; Min and Kim, 2006], deepening of the oxygen minimum layer [Kim and Kim, 1996; Chen et al., 1999, Kim et al., 2001; Kang et al., 2004, Kim et al., 2004], and the change of deep water formation from bottom to intermediate mode [Gamo, 1999; Gamo et al., 2001; Kang et al., 2003b, Kim et al., 2003; Chae et al., 2005]. Also, some scenarios on the possibility that3 the East Sea will become an anoxic sea were represented through modeling [Chen et al., 1999; Kang et al., 2004]. It is necessary that biogeochemical change must be accompanied instantaneously with physical changes; however, any biogeochemical change in the East Sea is not noticed yet. Just, the previous literatures have been reporting that the East Sea shows quite low N/P ratio, which is ranged from 11.30 to 14.70 [Table 1]. The reason is now unclear, but recently a few scientists carefully assume the reason that denitrification is occurring in the East Sea – for examples, Yanagi [2002] suggested the possibility through nitrogen budget in terms of mass balance, and Tischenko et al. [2006] pointed out nitrite maximum, which is an indirect evidence of denitrification, near the bottom. In general, denitrification is occurred in the restricted condition that oxygen is absent or very low concentrations, and reduces N/P ratio because nitrate is used as oxidant instead of O2 [Anderson and Sarmiento, 1994]. Ironically, the East Sea is well known as high oxygen condition in the northern Pacific Ocean, because it has own water formation system [Kim et al., 1996; Talley et al., 2006]. Considered it in relation to the recent changes of the East Sea, however, some phenomena seem to be related to biogeochemical change. Here, we will therefore deal with some symptoms that are suspected as a result of denitrification, despite high oxygen condition, estimate the amount of denitrification by linear4 mixing inverse model including biogeochemical changes, and construct a feedback loop based on the previous literatures that reported warming evidences in the East Sea. 2. Material and Method 2.1 Data In 1999, two cruises were carried out enough to cover the whole East Sea for summer, except for the territorial waters of North Korea [Fig. 1]. The project, which is shortly named as CREAMS (Circulation Research of East Asian Marginal Seas)Ⅱ, was firstly collaborated by three countries: United States, Russia, and South Korea. More specific cruise information is available in Talley et al. [2004], and the source of data used is from http://sam.ucsd.edu/onr_data/hydrography.html. The East Sea has three basins: Japan Basin (JB), Ulleung Basin (UB), and Yamato Basin (YB). As shown in Fig. 1, to facilitate the presentation of our analysis we have further divided the Japan Basin at 135E into the Western Japan Basin (WJB) and the Eastern Japan Basin (EJB). 2.2 Method Originally, Tomczak and Large [1988] developed the OMP analysis, not considering biogeochemical changes. Then, Karstensen and Tomczak [1998] introduced extended OMP analysis including the redfield ratio to involve biogeochemical changes. Again, To assess5
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