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doi:10.2204/iodp.proc.346.204.2018

Introduction

To better understand how climatic and oceanic changes in the Japan Sea/East Sea and East China Sea affected primary production in surface water, a high-resolution record of organic matter content is needed. Calcium carbonate (%CaCO₃), total organic carbon (%TOC), total nitrogen (%N), carbon stable isotope (δ¹³C), and nitrogen stable isotope (δ¹⁵N) contents of sediment samples are all widely used proxies for primary productivity in surface water as a result of paleoceanographic changes. These geochemical proxies are functions of nutrient availability (Mahaffey et al., 2003), primary productivity rates (Fry and Sherr, 1984), and differing organic matter source areas (O’Leary, 1981; Peterson and Howarth, 1987).

However, degradation of organic matter in both the water column and sediment surface can cause substantial alteration of organic matter from its original composition (Canuel and Martens, 1996; Lehmann et al., 2002), especially in the case of denitrification in anoxic water columns (Cline and Kaplan, 1975; Mariotti et al., 1981; Montoya et al., 1991; Altabet et al., 1999). Although the total amount of organic matter deposited in the sedimentary record is only a small portion of the original organic matter produced, it typically still contains accurate geochemical records (Meyers and Eadie, 1993) and therefore can be used as a proxy to determine past paleoceanographic and paleoenvironmental conditions.

Paleoproductivity proxies such as %CaCO₃, %TOC, %N, δ¹³C, and δ¹⁵N are best used in conjunction with other proxies such as benthic foraminifer assemblages, biogenic silica concentrations, and trace element geochemistry. Therefore, these reported data should be used as a complementary guide to paleoproductivity levels using additional productivity proxies.

Here, we evaluated the CaCO₃, TOC, N, δ¹³C, and δ¹⁵N concentrations in sediment cores retrieved from Integrated Ocean Drilling Program (IODP) Expedition 346 in the Japan Sea/East Sea and East China Sea. Expedition 346 drilling objectives were successfully completed with the retrieval of continuous sedimentary sequences at seven sampling sites in the Japan Sea/East Sea and two sampling sites in the East China Sea during the summer of 2013 (Fig. F1; see the “Expedition 346 summary ” chapter [Tada et al., 2015a]).

IODP Site U1426 is in the Yamato Basin near the top of the Oki Ridge. It is located in the same location as Ocean Drilling Program Site 798 (37°2.00′N, 134°48.00′E) at 903 meters below sea level (mbsl). Four holes were drilled at this location. Hole U1426A had the best recovery of 396.7 m, extending from the Pliocene to the Holocene. IODP Site U1427 is also in the Yamato Basin near the outer margin of the continental shelf near the coast of Honshu Island. It is located at 35°57.92′N, 134°26.06′E and is the shallowest site at 330.3 mbsl. Three holes were cored at Site U1427 with Hole U1427A recovering 548.6 m of sediment, extending from the lower Pleistocene to the Holocene. IODP Site U1429 is located in the northernmost part of the East China Sea in the Okinawa Trough at 31°37.04′N, 128°59.85′E and 732 mbsl. Three holes were cored at Site U1429, with Hole U1429A recovering the greatest amount (190.3 m) of sediment, extending from the middle Pleistocene to the Holocene.

Sediments recovered during drilling were predominantly fine-grained biogenic siliceous and carbonate oozes with distinct light–dark sedimentary cycles. The dark layers were mostly laminated, whereas the light layers were homogeneous to bioturbated. Meter-scale alternations in the light and dark sedimentary units record orbital-scale variations in surface and deep-water circulations, whereas centimeter-scale alternations in the light and dark sequences record millennial-scale climatic oscillations (see the “Expedition 346 summary” chapter [Tada et al., 2015a]). Previous studies have demonstrated that the light–dark sedimentary cycles were synchronous basin wide and that they can be correlated between sites in the study area (Tada et al., 1992; Watanabe et al., 2007).

The lowering of glacioeustatic sea level during glacial periods limited influx of the Tsushima Warm Current (TWC) into the Japan Sea/East Sea through the Tsushima Strait. With less input from the TWC and increased contribution from precipitation, the sea became nearly isolated and a low-salinity surface layer developed (Oba et al., 1991; Tada et al., 1999; Kitamura et al., 2001; Khim et al., 2008). This low-salinity layer limited deep-water ventilation resulting from increased water column stratification (Oba et al., 1991) and caused suboxic to anoxic bottom water conditions, which are associated with organic-rich, dark sedimentary layers (Watanabe et al., 2007; Khim et al., 2008). Light layers are associated with oxic conditions during interglacial and interstadial periods when there was a significant influx of the TWC through the Tsushima Strait (Tada et al., 1992, 1999).

The purpose of this research was to determine how glacioeustatic sea level changes and the resulting variation in the influx of water though the Tsushima Strait affected primary productivity rates and organic matter sources in the Japan Sea/East Sea on an orbital timescale. By comparing the Japan Sea/East Sea sites to the East China Sea sites, a better understanding of the timing and degree of isolation of the Japan Sea/East Sea during glacial lowstands can be developed.

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