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The Geochemistry and Bioturbation of Clay Sediments Associated with Amalgamated Crusts at the Gagua Ridge

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The Geochemistry and Bioturbation of Clay Sediments Associated with Amalgamated Crusts at the Gagua Ridge minerals Article The Geochemistry and Bioturbation of Clay Sediments Associated with Amalgamated Crusts at the Gagua Ridge Shuai Chen 1,2,3,*, Zhigang Zeng 1,2,3, Xiaoyuan Wang 1,2,3,*, Xuebo Yin 1,3, Bowen Zhu 1,4, Kun Guo 2,5 and Xin Huang 2,6 1 Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China; [email protected] (Z.Z.); [email protected] (X.Y.); [email protected] (B.Z.) 2 Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China; [email protected] (K.G.); [email protected] (X.H.) 3 Center for Ocean Mega-Science, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, China 4 University of Chinese Academy of Sciences, Beijing 100049, China 5 College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China 6 Guangdong Province Key Laboratory of Coastal Ocean Variation and Disaster Prediction, Guangdong Ocean University, Zhanjiang 524088, China * Correspondence: [email protected] (S.C.); [email protected] (X.W.); Tel.: +86-82898541 (S.C.) Received: 15 February 2019; Accepted: 8 March 2019; Published: 13 March 2019 Abstract: Based on the analysis of geochemical and mineralogical compositions, deep sea clay sediment characteristics and their material sources were examined in the eastern flank of the Gagua Ridge. The mineralogy mainly consists of detrital clay minerals, quartz, and authigenic phillipsite. There is scarce biogenic debris (siliceous or calcareous). The consolidated sediments are more enriched in Si, Al, K, Na, Li, Sc, Cr, Rb, and Cs than the associated crusts and nodules. The unmixed sediment samples were mainlycontributed by Asian eolian dust. The onset of the outer Fe-Mn crust growth nearly coincides with the Central Asia aridification event at ~3.5 Ma, which resulted in an abrupt increase in eolian flux of Asian dust. Intensified surface primary productivity is assumed to bring more metals to deep waters, and eventually facilitate the outer Fe-Mn crust formation. Authigenic phillipsite may come from the alteration of local basic volcanic glasses and cause excess Al, high Al/Ti, and low Si/Al ratios. However, phillipsites hardly affect the abundance of rare earth elements (REEs) and their patterns. In addition, the investigation of two kinds of burrows inside the consolidated sediments reveals that the inner nodules of the amalgamated crusts may remain on the oxic sediment surface, due to frequent benthic activities. Keywords: Gagua Ridge; amalgamated crusts; clay sediments; eolian dust; bioturbation 1. Introduction Fe-Mn nodules form by precipitation from both seawater and sediment pore waters on the sediment surface. Fe-Mn oxide crusts grow on rock surfaces throughout the global oceans, at water depths ranging from approximately 400–7000 m [1]. Crusts also grow on the hardground of consolidated or indurated sediments, such as the amalgamated crusts from the Shatsky Rise and Gagua Ridge [2,3], and the Fe-Mn crusts in the Central Indian Basin [4,5]. In addition, the amalgamated crusts also show a close proximity to the underlying nodules cemented by the consolidated sediments. Deep sea Fe-Mn crusts, nodules, and their associated sediments have been investigated by many Minerals 2019, 9, 177; doi:10.3390/min9030177 www.mdpi.com/journal/minerals Minerals 2019, 9, 177 2 of 16 researchers studying interelemental relationships and hoping to understand the possible contributions from different sources (i.e., [6–12]). In the Pacific Ocean, Elderfield et al. (1981) [6] observed that the highest nodule rare earth elements (REEs) are associated with the lowest REEs of the associated sediments, and the nodules with the largest positive Ce anomalies are found on sediments with the smallest negative Ce anomalies, indicating competitive scavenging of REEs between the nodules and sediments. However, such a relation is not found within Central Indian Ocean Basin nodule-sediment pairs [11] or in the southwest Pacific Ocean [7]. Pattan and Parthiban (2011) [11] concluded that it may not be appropriate to correlate elemental behavior between nodule and sediment pairs, because none of the major, trace, and rare earth elements exhibit any interelemental relationships between these pairs. Major and trace elements in nodules, micronodules, and abyssal clay on the central Clarion-Clipperton abyssal plain presented complex signatures, reflecting the influence of both the upper continental crust and MORB, as well as probable East Pacific Rise material transport via the Pacific North Equatorial Current [10]. Banakar et al. (2003) [13] also identified a Himalayan-derived silicate-detritus component in a Central Indian Ocean Fe-Mn crust that was located nearly 1000 km south of the Bengal Fan. Therefore, the Fe-Mn crusts are also expected to have recorded the past changes in sediment supply from the Ganges and Brahmaputra River systems to the Bengal Fan. The north-south oriented Gagua Ridge is a continuous bathymetric high ridge that extends over 300 km from the Island of Luzon in the Philippines to the north and intersects the Ryukyu subduction zone east of Taiwan [14,15]. The Gagua Ridge separates the Western Philippine Basin (WPB) to the east from the Huatung Basin (HB) to the west. Amalgamated Fe-Mn crusts were first recovered in 2016 at the eastern flank of the Gagua Ridge, and these crusts have been analyzed for their geochemical and mineralogical compositions in order to understand the formation process [3]. The amalgamated crust was composed of three layers: an outer crust, a middle crust, and inner nodules. All samples were mainly composed of Fe-rich vernadite, amorphous FeOOH and associated quartz, plagioclase, pyroxene and Mg-titanomagnetite. Consolidated sediments filled in the space between the outer crust and inner nodules, and the middle crust grew into the sediments [3]. There are also many burrows in the sediment samples with different sizes and fillings. Previous studies have reported the occurrence of ubiquitous benthic faunal activities on crust and nodule-covered deep sea sediments, such as tracks, trails, and burrowing structures [4,5,16–21]. The intense and prolonged bioturbation activity of the benthic fauna may have facilitated the occurrence of Fe-Mn nodules and micronodules [5,20]. In the present study, major, trace and, rare earth elemental analyses and mineralogical examinations were conducted for clay sediments covered by an outer Fe-Mn crust. The objectives of this study were to understand the possible sediment provenance, their significance to elemental behavior in sediments and associated crusts and nodules, and to explore the role of bioturbation in the formation of the amalgamated crusts in the Gagua Ridge. 2. Background The study area is located on the eastern flank of the Gagua Ridge, which lies on the westernmost edge of the WPB (Figure1). The Gagua Ridge is affected by two water masses off the coast of southeastern Taiwan (Figure1). One water mass is the northward-flowing Kuroshio Current (KC), and the other water mass is the southward-flowing Luzon Undercurrent (LUC). At 22◦ N–25◦ N, the KC is approximately 300 m deep and 170 km wide, with a maximum velocity of 1 m/s and a volume transport between 15 and 25 Sv [22]. Therefore, the transport of sediments delivered by rivers from mainland China and the Taiwan mountain belt would not easily reach the Gagua Ridge. Although thick sedimentary units (~4 km) exist in the HB [23], these units have become ponded within the basin, due to the presence of the Gagua Ridge. The VM33-95 core collected in the HB was estimated to have a sedimentation rate of 6 cm/kyr [24]. The sedimentation rate to the east of the Gagua Ridge in the WPB is low (approximately 5.3–5.4 mm/kyr) according to the nearest stations, PH-6 and St 6, from Huh (1992) [25]. Asian continental dust may contribute approximately 10–50% of the detrital fraction of Minerals 2019, 9, 177 3 of 16 Minerals 2019, 9, x FOR PEER REVIEW 3 of 16 sediments on the Benham Rise, which lies ~300 km south of the Gagua Ridge in the WPB [26]. Thus, the main source of material in the Gagua Ridge would be a combination of eolian dust and volcanic [26]. Thus, the main source of material in the Gagua Ridge would be a combination of eolian dust debris [3]. and volcanic debris [3]. FigureFigure 1.1. LocationLocation map map of ofthe the sampling sampling site site(TVG12-4, (TVG12-4, indicated indicated by red by star) red modified star) modified from Chen from et Chenal. (2018) et al. [3]. (2018) The yellow [3]. The circle yellow depicts circle the depicts sediment the core sediment location core of stations location PH-6 of stations and St PH-66 (depth and > St5500 6 (depth m; [25]). > 5500 The m flow;[25 ]).paths The of flow the pathsmain currents of the main are shown currents by are the shown gray and by thedashed gray black and dashedarrows. blackNEC-North arrows. Equatorial NEC-North Current, Equatorial KC-Kuroshio Current, Current, KC-Kuroshio LUC-Luzon Current, Undercurrent, LUC-Luzon SCS-South Undercurrent, China SCS-SouthSea, BR-Benham China Sea, Rise, BR-Benham WPB-West Rise, Philippine WPB-West Ba Philippinesin, GR-Gagua Basin, Ridge, GR-Gagua HB-Huatung Ridge, HB-Huatung Basin, OT- Basin,Okinawa OT-Okinawa Trough, Trough,and EAWM-East and EAWM-East Asian winter Asian wintermonsoon. monsoon. The bathymetric The bathymetric map is map sourced is sourced from fromGeoMapApp GeoMapApp (http://www.geomapapp.org). (http://www.geomapapp.org ). 3.3. Materials Materials and and Methods Methods TheThe amalgamated amalgamated Fe-Mn Fe-Mn crusts crusts were were collected collected at aat water a water depth depth of 4071of 4071 m by m TV-grabby TV-grab during during the HOBAB4the HOBAB4 cruise cruise in 2016.
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