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SCIENCE CHINA Earth Sciences Science China Press and Springer-Verlag Berlin Heidelberg 2015 *Corresponding author email zhangqiang210 RESEARCH PAPER May 2015 Vol.58 No.5 707–717 doi 10.1007/s11430-014-5049-3 Clay mineral assemblages at IODP Site U1340 in the Bering Sea and their paleoclimatic significance ZHANG Qiang1,2*, CHEN MuHong1, LIU JianGuo1, YU ZhaoJie3, ZHANG LanLan1 2 University of Chinese Academy of Sciences, Beijing 100049, China; 3 Marine Geochemistry and Sedimentology Laboratory of IDES, University of Paris XI, Orsay Cedex 91405, France Received April 10, 2014; accepted December 17, 2014; published online March 13, 2015 Clay mineral assemblages and crystallinities in sediments from IODP Site 1340 in the Bering Sea were analyzed in order to trace sediment sources and reconstruct the paleoclimatic history of the Bering Sea since Pliocene the last 4.3 Ma. The re- sults show that clay minerals at Site U1340 are dominated by illite, with a moderate amount of smectite and chlorite, and minor kaolinite. Sediment source studies suggest that the clay mineral assemblages and their sources in the studied core are controlled primarily by the climate conditions. During the warm periods, clay minerals originated mainly from the adjacent Aleutian Is- lands, and smectite/illitechlorite ratios increased. During the cold periods, clay minerals from the Alaskan region distinctly increased, and smectite/illitechlorite ratios declined. Based on smectite/illitechlorite ratios and clay mineral crystallinities, the evolutionary history of the paleoclimate was revealed in the Bering Sea. In general, the Bering Sea was characterized by warm and wet climate condition from 4.3 to 3.94 Ma, and then cold and dry condition associated with the enhanced volcanism from 3.94 to 3.6 Ma. Thereafter, the climate gradually became cold and wet, and then was dominated by a cold and dry condi- tion since 2.74 Ma, probably induced by the intensification of the Northern Hemisphere Glaciation. The interval from 1.95 to 1.07 Ma was a transitional period of the climate gradually becoming cold and wet. After the middle Pleistocene transition 1.07 to 0.8 Ma, the Bering Sea was governed mainly by cold and wet climate with several intervals of warm climate at 0.42 Ma MIS 11, 0.33 Ma MIS 9 and 0.12 Ma MIS 5, respectively. During the last 9.21 ka the Holocene, the Bering Sea was characterized primarily by relatively warm and wet climatic conditions. the Bering Sea, Site U1340, clay mineral, crystallinity, paleoclimate Citation Zhang Q, Chen M H, Liu J G, et al. 2015. Clay mineral assemblages at IODP Site U1340 in the Bering Sea and their paleoclimatic significance. Sci- ence China Earth Sciences, 58 707–717, doi 10.1007/s11430-014-5049-3 The Bering Sea, as a semi-enclosed marginal sea of the Pa- cific Rim with a surface area of 2.29106 km2, is the third largest marginal sea in the world, surpassed only by the Mediterranean and the South China Seas Hood, 1983. As a depositional basin with high primary production and an outlet to both the North Pacific and the Arctic Oceans, the Bering Sea plays a significant role in modern and past glob- al climatic systems, and well records the climatic and envi- ronmental changes of the subarctic regions in the sediments Takahashi, 1998, 1999. Given its unique location and piv- otal role, the Bering Sea is a critical place for the recon- struction of subarctic climate variability on various time scales Takahashi, 2005. Therefore, the comprehensive knowledge of the paleoclimatic and paleoceanographic fea- tures of the Bering Sea can improve our understanding of 708 Zhang Q, et al. Sci China Earth Sci May 2015 Vol.58 No.5 the evolutionary history of the paleoenvironments related to the climate changes in the subarctic Takahashi et al., 2011. By employing long-term sedimentary sequences from IODP Expedition 323, Pliocene-Pleistocene paleoceanog- raphy studies have been conducted preliminarily in the Bering Sea. Aiello et al. 2012 revealed the evolution of sedimentation on the basis of sedimentary constituents, par- ticle size and petrographic analyses. Mrz et al. 2013 re- ported a special period of high export productivity from 2.6 to 1.8 Ma on the basis of the biogenic silica record at Site U1341. Onodera et al. 2013 and Teraishi et al. 2013 re- spectively showed diatoms, silicoflagellates, and ebridians results at Site U1341 and Site U1343 that were used to de- cipher the general paleoceanographic conditions. Zhang et al. 2014a reconstructed the vertical water-mass conditions mainly on the basis of the radiolarian assemblages. Using the warm-water radiolarian species Spongodiscus bicon- cavus at Site U1340, Chen et al. 2014 revealed the wa- ter-mass exchanges between high and low latitudes and their relations to the global climate changes. However, these studied were focused primarily on the paleoceanophic changes and their responses to global climate change events. Systematic studies on regional climatic characteris- tics and their evolutionary history on long timescales in the Bering Sea and adjacent continents remained lacking until today. Clay minerals, which are provided by physical or chemi- cal weathering processes from various lithological rocks on land, are one of the most important components in sediment of the world’s oceans. Because the clay mineral types and the proportions of the individual clay minerals are closely related to paleoclimatic conditions of continental source area and the nature of the source rocks e.g., Singer, 1984; Chamley, 1989; Liu Z F et al., 2005, 2007, 2012; Liu J G et al., 2013, clay mineral assemblages have been widely ap- plied to studying sediment source and paleoclimate evolu- tion in various regions e.g., Chamley and Diester-Haass, 1979; Ehrmann, 1998; Franke and Ehrmann, 2010. This study is aimed to present a detailed record of clay mineral assemblages at IODP Site U1340, and then discuss the source areas for clay minerals and the paleoclinatic condi- tions in the Bering Sea and adjacent continents. 1 Materials and s The samples used in the present study were obtained from the sedimentary sequence of Site U1340, IODP Expedition 323, which is located on the Bowers Ridge at 5324.0008N and 17931.2973W at a water depth of 1295 m Figure 1. Figure 1 The location of Site U1340, and surface water circulation in the Bering Sea Stabeno et al., 1994; Coachman et al., 1999; Takahashi, 2005. The thick green dashed line denotes the potential position of the coastline during the glacial periods according to the global sea-level change in the LGM Hop- kins, 1973; Bintanja et al., 2005. KC, Kamchatka Current; BSC, Bering Slope Current; ANSC, Aleutian North Slope Current. Zhang Q, et al. Sci China Earth Sci May 2015 Vol.58 No.5 709 The studied core with a maximum composite depth of 604 m was obtained by establishing the stratigraphic correla- tions among the three holes cored at Site U1340. The sedi- ments in the studied core consisted mostly of dark green- gray diatom ooze or diatom silt, except for a few volcanic ash layers and siliciclastic sediments Takahashi et al., 2011. A total of 120 samples were taken with a sampling interval of 1.5 m from the top 15m and 6 m for the rest of the core. Clay minerals 2 μm were separated based on the Stoke’s settling velocity principle after removing carbonate and organic matter with acetic acid 15 and hydrogen peroxide 10, respectively Wan et al., 2006, 2007, 2008. Clay mineralogy determinations were pered by standard X-ray diffraction XRD on an X’Pert Pro MPD diffractometer with CuKα radiation 40 kV, 40 mA in Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences. Three XRD runs were conducted, following drying at room temperature, ethylene-glycol solvation for 12 h, and heating at 490C for 2 h. Scans were pered from 3to 30 2θ with a speed of 0.008 2θ per second. Identification and interpretation of clay minerals were made mainly using the position of the 001 series of basal reflections on the three XRD diagrams of air drying, ethylene glycol solvation, and heating conditions Moore and Reynolds, 1997. Relative percentages of the four main clay mineral groups were estimated by weighting integrated peak areas of characteristic basal reflections smectite –17 , illite –10 , and kaolinite/chlorite –7 in the glycolat- ed state using the Topas 2P software with the empirical factors of Biscaye 1965. Relative proportions of kaolinite and chlorite were determined from the ratio of 3.57/3.54 peak areas. The crystallinity is an effective measure of the lattice or- dering and crystallite size for clay minerals Biscaye, 1965; Ehrmann, 1998. The illite and smectite crystallinities are generally expressed as the integral breadth 2θ of the illite 10 and smectite 17 peaks Petschikc et al., 1996; Ehr- mann, 1998. The integral breadth is the width of the rec- tangle, which has the same height and the same area as the measured peak. The illite chemistry index is a parameter to measure the weathering degree of illite. It is estimated using the value of the 5 and 10 illite peak areas ratio of eth- ylene-glycolated samples. Values below 0.4 indicate Fe- Mg-rich illites biotites and micas that are characteristic of physically weathering rocks. Conversely, values above 0.4 represent Al-rich illites, which are indicative of strong hy- drolysis Ehrmann, 1998; Liu et al., 2005; Wan et al., 2006. Therefore, the illite chemistry index is generally used to trace the natures of the weathering processes and climate conditions of the source areas Ehrmann, 1998; Liu et al., 2005; Wan et al., 2006. 2 Amended age model Takahashi et al. 2011 showed several biostratigraphic and magnetostratigraphic events at Site U1340 in the IODP Ex- pedition 323 Preliminary Report and Proceedings volumes. However, there were some certain errors for the core depths and ages of these events because of the relatively low reso- lution for shipboard investigations. Zhang et al. 2014b studied radiolarian biostratigraphy at Site U1340, and con- structed the age-depth plot on the basis of the synthetical datum of the effective radiolarian, diatom, silicoflagellata and magnetostratigraphic events. In the age-depth plot es- tablished by Zhang et al. 2014b, the average radiolarian date 2.9 Ma/312.05 m of the last occurrence of Phor- mostichoartus fistula 2.8–4.4 Ma/314.4 m and the first occurrence of Phormostichoartus pitomorphus 2.15–3.63 Ma/309.7 m was adopted as one of the age-control points. However, the ages of these two radiolarian events varied greatly and showed wide ranges in the subarctic, which was probably resulted from the regional differences, and thus were not the ideal biostratigraphic data for the age-control point. Therefore, this age point 2.9 Ma/312.05 m was re- moved, and because the last occurrence of silicoflagellate Ebriopsis antiqua antiqua was approximately consistent 2.47–2.48 Ma in the subarctic Pacific Ling, 1992, the average date 2.475 Ma/305.5 m of this silicoflagellate event at Site U1340 was taken as a new age point in the amended age model Figure 2. Age estimation of various depths was pered by a linear interpolation between two control points. The base age of the studied core was calculated at approximately 4.3 Ma i.e., the late Early Pliocene Figure 2. Figure 2 The age model of Site U1340. 710 Zhang Q, et al. Sci China Earth Sci May 2015 Vol.58 No.5 3 The results The clay mineral assemblages at Site U1340 consist mainly of illite, smectite, and chlorite Figure 3. Illite is the domi- nant clay mineral with an average of 70, followed by smectite and chlorite with average contents of 17 and 10, respectively. Kaolinite is a minor component with average content of 3 Figure 3. Among the content varia- tions of the four clay minerals, illite and chlorite have a similar variation pattern, whereas smectite content shows an opposite pattern with those of illite and chlorite. Based on the variations in smectite and illite contents, which syn- chronously showed three remarkable changes at 3.6, 2.74, and 1.07 Ma respectively, the evolution of the clay mineral assemblages can be subdivided into four intervals from Stages I to IV, with Stage I, Stage III divided into Substages Ia and Ib, and Substages IIIa and IIIb, and Stgae IV divided into Substages IVa, IVb and IVc, respectively Figure 3. Smectite exhibited relatively high content average of 22 during stage Ia 4.3 to 3.94 Ma, and became more abundant average of 36 during stage Ib 3.94 to 3.6 Ma. Then, the smectite content average of 9 showed an overall decreasing trend with relatively large fluctuations and low peak values at 3.14, 2.99 and 2.77 Ma during stage II 3.6 to 2.74 Ma. Synchronously, the illite content displayed an increasing trend and presented relatively high values. The clay mineral assemblages during stage III 2.74 to 1.07 Ma were characterized by stably low smectite con- tent average of 4 and high illite content average of 80, with a slight increase in the smectite content average of 8 during stage IIIb 1.95 to 1.07 Ma. The smectite content distinctly increased with the decrease in illite values during stage IV after 1.07 Ma. Stage IVa 1.07 to 0.8 Ma was characterized by large fluctuations of the smectite and illite contents. Stage IVb was an interval of distinct changes in the smectite content with peaks of 49 at 0.42 Ma, 41 at 0.33 Ma, and 40 at 0.12 Ma, respectively. Stage IVc was a period of overall high smectite contents average of 48 and relatively low illite values average of 36, and the clay mineral assemblages during this stage were ap- proximately consistent with those of the surface sediments smectite 43 and illite 30 near the Aleutian Island Naidu et al., 1995. The integral breadth is an inverse measure of the clay mineral crystallinity. High values of the integral breadth indicate poor crystallinities; low values indicate good crys- tallinities. According to the crystallinity categories for smectite and illite proposed by Ehrmann 1998, smectite Figure 3 Fluctuation patterns of clay mineral assemblages at Site U1340. Zhang Q, et al. Sci China Earth Sci May 2015 Vol.58 No.5 711 are Well crystalline 2.0. illite are Very well crystal- line 0.8. The values of the smecite crystallinity integral breadth, 2θ vary between 0.14 and 3.782θ with an average of 1.152θ. These val- ues indicated that smecite was characterized mainly by moderate to poor crystallinity before 1.07 Ma, and then good to moderate crystallinity after 1.07 Ma Figure 4. The values of illite crystallinity at Site U1340 range from 0.23 to 0.792θ with an average of 0.522θ, and indicate that il
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