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JOURNAL OF QUATERNARY SCIENCE (2012) 27(9) 891–900
ISSN 0267-8179. DOI: 10.1002/jqs.2578
Reconstruction of environmental changes using a multi-proxy approach in the Ulleung Basin (Sea of Japan) over the last 48 ka JIANJUN ZOU,1 XUEFA SHI,1* YANGUANG LIU,1 JIHUA LIU,1 KANDASAMY SELVARAJ2,3 and SHUH-JI KAO2,3 1 Key Laboratory of State Oceanic Administration for Marine Sedimentology and Environmental Geology, First Institute of Oceanography, Qingdao 266061, China 2 State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China 3 Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan Received 30 September 2011; Revised 6 May 2012; Accepted 26 July 2012
ABSTRACT: Based on elemental geochemical data, we reconstructed the sediment provenance, surface productivity and bottom water redox conditions for the last 48 ka in the Ulleung Basin (Sea of Japan) and inferred the factors controlling them. Al2O3/TiO2 ratio and chemical index of alteration (CIA) suggest that sediment provenance changed during the glacial period (48–18 ka) compared to the deglacial (ca. 18–11 ka) and Holocene. Mass accumulation rates of total organic carbon (TOC), CaCO3, phosphorus, cadmium and excess barium reveal low paleoproductivity during low sea stand. During 18–11 ka, productivity increased due to increasing inflow of nutrient-rich water masses – the Oyashio and the East China Sea coastal water – in tandem with the rising sea level. Maximum productivity occurred during Younger Dryas and Pre-boreal periods when sea level was at 60 m and then gradually decreased as the Tsushima Warm Current inflow kicked off at ca. 9.3 ka, consistent with other paleoredox proxies, which reveal the presence of anoxic bottom water during ca. 12–9 ka. With the changes in paleoredox proxies and their ratios (TOC, Mo, U, Mn, C/S ratio and Uauthigenic and Mo contents), we hypothesized that the redox changes were mainly ventilation driven and were superimposed on the influence of circulation-induced productivity changes. The global climate and sea-level changes on a millennial timescale play a major role in enhancing paleoproductivity and restrict bottom water advection, subsequently driving the oxygenation of bottom water in the Ulleung Basin. Copyright # 2012 John Wiley & Sons, Ltd. KEYWORDS: sediment geochemistry; provenance; paleoproductivity; paleoredox; Ulleung Basin.
Introduction Marginal seas are recognized as important contributors and regulators of the global carbon cycle (Jahnke, 2010; Seitzinger et al., 2005; Walsh et al., 1981) and are thus intimately linked to climate change on a glacial–interglacial timescale. The Sea of Japan, a typical semi-enclosed marginal sea in the northwestern Pacific Ocean, is surrounded by the Eurasian continent and North Pacific Ocean. Previous reports showed that the laminated layers with alternate dark organic-rich and light organic-poor sediment deposition were ubiquitous in the Sea of Japan (Itaki et al., 2007; Oba et al., 1991; Tada, 1994). This phenomenon was deemed to be correlated with the changes in surface productivity and oxygen content in the bottom water. Geochemical studies on sediment cores from the Ocean Drilling Program further indicated that millennial-scale cyclic dark–light layering was the consequence of fluctuating physicochemical conditions of seawater that was associated with Dansgaard–Oeschger (D-O) cycles (Tada and Irino, 1999). The sedimentary record of various redox-sensitive elements (e.g. U, Mn, Mo) may reflect the redox conditions prevailing in the water column at the time of sediment deposition (Elbaz-Poulichet et al., 2005) and therefore can be used to discover paleoredox in the marine environment (Algeo and Lyons, 2006; Nameroff et al., 2002; Russell and Morford, 2001; Tribovillard et al., 2006). This is mainly because these trace elements tend to have significant solubility reduction under reducing conditions, resulting in authigenic enrichment in oxygen-depleted sedimentary facies (Tribovillard et al., 2006). Furthermore, biogenic elements such as cadmium, *Correspondence: Xuefa Shi, as above. E-mail: xfshi@fio.org.cn
Copyright ß 2012 John Wiley & Sons, Ltd.
phosphorus and barium show significant correlation with productivity changes, and have therefore been used to evaluate past productivity changes in different ocean basins (Dymond et al., 1992; Schenau et al., 2005; Tribovillard et al., 2006). Based on the development of laminated layers in core KCES-1 retrieved from the Ulleung Basin, Liu et al. (2010) inferred that basin-wide changes in surface productivity and oxygen content in the bottom water occurred in the past. In this paper, using high-resolution geochemical analysis with the well-defined age model of core KCES-1, we reconstruct the paleoproductivity and paleoredox history of the Ulleung Basin for the last 48 000 calendar years before the present (48 ka). Furthermore, we use additional proxies such as chemical index of alteration and degree of pyritization in core KCES-1 for the first time to substantiate our inferences. The Sea of Japan is composed of three deep basins – Ulleung, Yamato and Japan – and exchanges water with other adjoining seas and oceans through only four narrow and shallow passages, namely Tartar (15 m), Soya (55 m), Tsugaru (130 m) and Tsushima (140 m), from north to south (Piper and Isaacs, 1996) (Fig. 1a). The maximum sill depth for the four straits is 140 m in Tsushima Strait and the minimum depth is just 15 m in Tartar Strait (Piper and Isaacs, 1996). Therefore, on a millennial timescale, the past eustatic sea-level changes exert a strong control on the oceanographic regime and environmental history of the Sea of Japan (Kim et al., 2001; Oba et al., 1991; Takei et al., 2002). Among the four straits, Tsushima plays the most important role since the Tsushima Warm Current (TWC), a branch of the Kuroshio Current (KC) flowing northeastward from the East China Sea, is responsible for major heat and water inflow to the Sea of Japan (Fig. 1b). The total volume transport of the modern-day TWC through the strait is 2.64 Sv (Sv ¼ 106 m3 s1) (Takikawa et al.,
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Figure 1. (a) Physiography of the Sea of Japan and adjacent seas with four straits. The dashed curve stands for the 120 m isobaths. (b) Location of sediment core KCES-1 (*), site 16 (&) and 97PC-19 (*) with dominant currents illustrated. Arrows denote simplified surface currents in the study region.
2005). This transport has a strong seasonal variation as the inflow of low-salinity nutrient-replete water mass driven by freshwater discharge from the rivers around the Yellow Sea and East China Sea dominates from August to October every year. The deep water in the basin, the Japan Sea Proper Water, is formed in the northern part of the sea in winter due to surface cooling and/or evaporation/freezing and then sinks toward the bottom (Sudo, 1986). The formation of this deep water may control the oxygen supply to deep and bottom water, which in turn is responsible for basin-wide ventilation. Variable primary production-induced oxygen consumption in the deep-water column may thus regulate organic matter burial, sedimentary and water column redox states. In addition, the environmental condition in the Sea of Japan was influenced by river discharge, the strength of Kuroshio and Oyashio currents, intensity of the East Asian monsoon and global climate changes on a glacial– interglacial timescale (Kido et al., 2007; Yokoyama et al., 2007). The studied site, Ulleung Basin, is situated beneath the entrance of the TWC and is located at the southernmost margin of the deep water. The sedimentary geochemistry retrieved from this basin would reveal insightful information regarding sediment provenance, productivity and redox history of the Sea of Japan. Previous studies have suggested that the paleoredox of bottom water in the Ulleung Basin has taken place since the last deglacial, based on the record of sedimentology and sedimentary geochemistry (Lim et al., 2011; Liu et al., 2010). In this study, we illustrate more comprehensive paleoenvironmental evolutional information since the last glacial using multiple proxies.
Material and methods Core KCES-1 (358 56.1500 N, 1308 41.9150 E; Fig. 1) recovered at a water depth of 1464 m from the southeastern Ulleung Basin in the Sea of Japan was provided by the Korean Ocean Research and Development Institute (KORDI). The region is influenced largely by the TWC in the present interglacial (Fig. 1). The length of core KCES-1 is 10.2 m and mainly consists of clayey silt and silt with occasional occurrence of plant debris at some depth intervals. Liu et al. (2010) also observed four layers of volcanic ash with different thicknesses. In the upper section of core (0 400 cm), sediment is dominated by clayey silt and silt, while alternate light and dark layers are seen in the middle section (400 730 cm). Below 730 cm, the sediment texture is foul-up and this is likely caused either by turbidity currents or shelf collapse due to slumping. In the present study, we therefore analyze the subsamples from the core top through 730 cm of core KCES-1, with the sampling interval ranging from Copyright ß 2012 John Wiley & Sons, Ltd.
6 to 20 cm based on the sediment texture. In total, 71 subsamples were analyzed for total contents of organic carbon (TOC), nitrogen (TN) and sulfur (TS), as well as selected major and trace elements.
TOC, TN, TS and CaCO3 Each sediment subsample was oven dried at 608C for 2 h and homogenized by grinding in an agate mortar. Total content of carbon (TC), TN and TS were determined with an elemental analyzer (EA; Vario EL III) at the First Institute of Oceanography, State Oceanic Administration, China. In addition, 2 g of each powdered sample was treated with 12 mL of 1 M HCl for 24 h at 608C to remove carbonate, and the residue was centrifuged and oven dried. TOC in these decarbonated samples was determined by using the same instrument. The content of calcium carbonate (CaCO3) was calculated using the equation CaCO3 ¼ ðTC TOCÞ 8:33
(1)
Quality assurance and control for the analytical process were evaluated with the help of reference material (GSD-9) and blank correction. The relative standard deviation of the GSD-9 for TC, TN, TOC and TS was 1.2%, 1.8%, 2.6% and 12%, respectively.
Major and minor elements For major and trace elemental analyses, each sediment subsample was oven dried at 1058C for 3 h. 50 mg of each aliquot transferred to Teflon beakers was digested with ultrapure HF and HNO3. Selected major (Al, Ca, Fe, Mg, Mn, Na, K, P and Ti) and minor (Ba, Cu, Ni, Sr, Th, V and Zn) elements were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES; Thermo Scientific iCAP 6000). In addition, specific redox-sensitive trace elements including uranium (U), molybdenum (Mo) and cadmium (Cd) were analyzed with inductively coupled plasma mass spectrometry (ICP-MS; Thermo Scientific XSERIES 2, located at the Key Laboratory of State Oceanic Administration for Marine Sedimentology and Environmental Geology, First Institute of Oceanography, China). Precision for most elements at the concentrations present in the reference material GSD-9 was 30%) at ca. 21, 17 and 11.5 ka, consistent with similar high DOP values found in core MD012404 from the Okinawa Trough (Kao et al., 2006). A distinct decrease of DOPT from ca. 11.5 ka to 8.6 ka and a continuous decrease since 6.8 ka to the present indicate a better ventilation of deep waters in the coring site, mainly resulting from the inflow of the warm Tsushima Current along with the East China Sea Coastal Current during these intervals. Consistently, previous studies have indicated that the Sea of Japan was dominated by cold surface water and anoxic bottom conditions throughout the last glacial period and by warm surface waters coupled with oxidizing bottom conditions during the Holocene (Oba et al., 1991). Algeo and Lyons (2006) reported that Mo/TOC ratios are useful for distinguishing degrees of restriction of the subchemoclinal water mass in anoxic marine environments, with ratios of >35, 15–35 and
