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doi:10.2204/iodp.sp.346.2013

Land-ocean linkage through monsoon variability in the East Asian continental margin

With increasing awareness of the human impact on our environment, the public is paying more attention to the impact of discharge from large rivers on the environment of continental margins and marginal seas. Society is also becoming increasingly concerned about the impact of the consequent oceanographic changes on terrestrial climate. For example, the effect of the construction of the Three Gorges Dam, and the consequent decrease in Yangtze River discharge, on the oceanographic conditions in the East China Sea and the Japan Sea and regional climate in their surrounding area has been noted (e.g., Chen, 2002; Yang et al., 2011). As described above, EASM intensity varied significantly on orbital and millennial scales (Tada et al., 1999; Wang et al., 2001, 2008; Tada, 2004; Cheng et al., 2009), which should have caused significant variation in the discharges from the Yangtze and Yellow Rivers. Tada et al. (1999) speculated that such variation in Yangtze River discharge could have been responsible for the dramatic changes in paleoceanography of the Japan Sea, leading to deposition of distinct alternations of the organic carbon–rich dark laminated layers and organic carbon–poor light bioturbated layers. However, this hypothesis has never been tested rigorously. Thus, the process(es) and mechanism(s) linking the EASM, Yangtze River discharge, and Japan Sea paleoceanography are still not fully understood in spite of their importance to evaluate the ongoing and near-future changes in the oceanography of the Japan and East China Seas as well as to climate in the surrounding land area.

The oceanography of the Japan Sea is very sensitive to the nature and amount of the influx of seawater through the Tsushima Strait. This is important because in the present East China Sea, dissolved phosphorous is mainly supplied through upwelling of the subsurface Kuroshio water to the edge of the East China Sea shelf. The upwelling is basically induced by outflow of the low-salinity water from the shelf area (Chen et al., 1999) (Fig. F5). Consequently, the increase in freshwater discharge from the Yangtze River enhances nutrient supply to the East China Sea through enhancement of upwelling of the nutrient-rich subsurface Kuroshio water. Because nutrient supply to the Japan Sea is dominantly carried by the TWC, it is reasonable to consider that surface productivity in the Japan Sea is controlled by nutrient flux through the Tsushima Strait, especially in timescales greater than 100 y (Fig. F6). In this way, variations in the summer monsoon intensity may be recorded as variations in organic phosphorous and carbon burial rate in the Japan Sea, and documenting variability in this system may provide an important additional constraint on behavior of the EASM.

Sedimentary deposits in the Japan Sea indicate that such changes in nutrient cycling have also happened in the past and indeed may have changed in concert with abrupt climate change. For example, Quaternary hemipelagic sediments of the Japan Sea are characterized by centimeter- to decimeter-scale alternations of organic carbon–rich dark and organic carbon–poor light layers (e.g., Tada et al., 1992) that are associated with DOCs (Tada et al., 1995, 1999) (Fig. F7). Based on the increase in the relative abundance of Paralia sulcata, a sublittoral diatom species characteristic of ECSCW, in these dark layers, Tada et al. (1999) argued that deposition of these dark layers resulted from the increasing contribution of low-salinity and nutrient-rich ECSCW to the TWC. This would in turn have caused an increase in surface biological productivity and a reduction in vertical mixing. Tada et al. (1999) further argued that the increase in relative contribution of the ECSCW to the TWC could have been caused by increased discharge from the Yangtze and Yellow Rivers in response to intensified summer monsoon. This proposed mechanism is consistent with high-resolution records of δ¹⁸O in the Hulu Cave stalagmite, which suggests summer monsoon intensification during the DOC (Wang et al., 2001; Ijiri et al., 2005). These collected observations suggest an additional link between monsoonal dynamics (rainfall into the Yangtze/Yellow watersheds), ocean currents (ECSCW to the TWC), paleoproductivity (nutrients and upwelling), and sediment deposition (organic cycles).

Glacio-eustatic sea level changes may also have a profound influence on the paleoceanographic condition of the Japan Sea from orbital to millennial timescales. The sill depth of the sea became shallower than 20 m and the surface water salinity decreased drastically as a result of increasing contribution from precipitation and, thus, of the freshwater input from the surrounding rivers relative to the seawater influx through the Tsushima Strait during the Last Glacial Maximum (e.g., Oba et al., 1991, 1995; Matsui et al., 1998) (Fig. F8). The low-salinity surface water strengthened density stratification, causing euxinic deepwater conditions similar to the present Black Sea and resulted in deposition of a finely laminated thick, dark layer (Oba et al., 1991; Tada et al., 1999). According to a salt- and water-budget calculation using a simple box model, freshening of the surface water salinity of the Japan Sea becomes evident when the sill depth of the Tsushima Strait decreased below 30 m (Matsui et al., 1998). These studies further confirm the linkage among glacio-eustatic sea level decreases, the decrease in the surface-water salinity, development of euxinic deep water, and deposition of finely laminated dark layers.

On the other hand, periods other than glacial maxima are characterized by deposition of centimeter- to decimeter-scale alternations of dark laminated layers and light bioturbated layers, with the dark layers corresponding to the DOC interstadials (Tada et al., 1999), suggesting a link between the EASM, Yangtze River discharge, and paleoceanographic conditions of the Japan Sea. Thus, deconvolving sea level influences, driven by global climate variability (that is, ice volume), from those variables driven by EASM (such as local nutrient supply) will be critical. This requires an increased understanding of the physical dynamics of the oceanic circulation and the relationship to biogeochemical responses.

Potential contributions from Expedition 346

We aim to examine the paleoceanographic responses of the Japan Sea to variations in glacio-eustatic sea level changes and Asian monsoon dynamics through a multiproxy approach that will assess changes in surface and deepwater parameters. Site locations have been selected to record water masses in the eastern and western sides of the Japan Sea and through a near-complete depth gradient. Such reconstructions can be based on Mg/Ca and δ¹⁸O (and other isotopic systems) of benthic and planktonic foraminifers, organic biomarkers (e.g., alkenones, TEX86, etc.), and other proxies, including CaCO₃ concentration and preservation. Deepwater oxygenation levels can be assessed by documenting the degree of lamina preservation, C/S ratio, concentrations and accumulations of redox-sensitive elements (e.g., Mo), S, U isotopes, and δ¹³C (planktonic–benthic) (e.g., Tada et al., 1999; Crusius et al., 1999). We plan to reconstruct temporal variation in the carbonate compensation depth based on the comparison of burial flux of biogenic carbonate between the shallower and deeper sites. We also plan to reconstruct burial fluxes of biogenic silica, biogenic carbonate, and organic carbon to examine temporal changes in the nature and intensity of surface production. Geochemical proxies of export production (e.g., Ba) and terrigenous provenance are also likely to prove beneficial.

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