Coral Geochemical Record of Submarine Groundwater Discharge
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Palaeogeography, Palaeoclimatology, Palaeoecology 507 (2018) 30–38 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo Coral geochemical record of submarine groundwater discharge back to 1870 in the northern South China Sea T ⁎ Wei Jianga, Kefu Yua, , Yinxian Songb, Jian-xin Zhaoc, Yue-xing Fengc, Yinghui Wanga, Shendong Xua a Guangxi Key Laboratory on the Study of Coral Reefs in the South China Sea, Coral Reef Research Center of China, School of Marine Sciences, Guangxi University, Nanning 530004, PR China b College of Environment and Engineering, Hohai University, Nanjing 210098, PR China c Radiogenic Isotope Facility, School of Earth and Environmental Sciences, The University of Queensland, Brisbane, QLD 4072, Australia ARTICLE INFO ABSTRACT Keywords: The importance of submarine groundwater discharge (SGD) is becoming increasingly recognized because of its Rare earth element potential significance as a source of dissolved species. To explore the probable coral geochemical signal of SGD Barium and verify the validity of potential reliable proxies, multiple geochemical proxies over the last 137 years were Submarine groundwater discharge identified from a Porites coral near a subterranean estuary in the northern South China Sea, where the SGD was Karst reported to be the predominant flux of terrestrial waters to the coastal ocean. Results indicated that the SGD in the coastal zone was the dominant source of trace elements, especially REE and Ba, due to the various dissolution reactions occurring during groundwater flow in the karst terrain. The time- and frequency-domain comparison between the coral geochemical proxy and the local/regional precipitation indicated that coral REE/Ca ratios are predominantly impacted by the SGD associated with local precipitation, while coral Ba/Ca ratios are also af- fected by the primary productivity and allochthonous seawater Ba from surrounding areas. The REE signal from coral allows us to reconstruct the coastal surface seawater REE concentrations and the SGD rates on the coast of Sanya during 1870–2006. In a novel approach to developing a proxy for historic SGD to coastal waters, this study provides evidence that the coral REE/Ca record from the karst coast with large SGD has potentials to be a promising paleohydrological indicator. 1. Introduction and radon isotopes) that would help determine annual SGD fluxes (Kwon et al., 2014; Street et al., 2008; Wu et al., 2013), a lack of direct Defined as any fluid flow of water from the seabed to the overlying measurements has made it difficult to estimate the SGD over timespans marine water column (Burnett et al., 2003), submarine groundwater of decades to centuries, which hampers our understanding of the his- discharge (SGD) has received increasing attention in recent decades in torical evolution of SGD. coastal zones, especially in coral reef regions (Cyronak et al., 2014; Among numerous tracers of SGD, barium (Ba) (Gonneea et al., 2014; Garrison et al., 2003; Nelson et al., 2015; Prouty et al., 2017; Street Gonneea et al., 2013; Horta-Puga and Carriquiry, 2012; Santos et al., et al., 2008; Wang et al., 2014; Wang et al., 2017). SGD has been 2011; Shaw et al., 1998) and rare earth element (REE) (Chevis et al., considered a significant and continuous source of nutrients and metals 2015a; Chevis et al., 2015b; Johannesson and Burdige, 2007; to the coastal ocean (Moore, 2010), with potential negative impacts to Johannesson et al., 2017; Kim and Kim, 2011, 2014; Prouty et al., coral abundance and diversity (Fabricius, 2005; Prouty et al., 2017) due 2009) have been well investigated. Fortunately, the temporal variations to its higher levels of terrestrial compounds than those of river waters of surface seawater in coral reef regions can be inferred from corals; (Boehm et al., 2006; Santos et al., 2008; Slomp and Cappellen, 2004) these variations have been demonstrated in various ocean basins in and comparable fluxes to the river discharge in many coastal areas (e.g., response to Ba or REE input, including in the North Paci fic Ocean Burnett et al., 2003; Moore, 1996; Moore et al., 2008). Despite its po- (Prouty et al., 2010; Prouty et al., 2009), the East Atlantic Ocean tential importance in understanding coastal biogeochemical cycles and (Carriquiry and Horta-Puga, 2010; Horta-Puga and Carriquiry, 2012), the availability of multiple tracers of groundwater input (e.g., radium the South China Sea (SCS) (Jiang et al., 2017; Liu et al., 2011; Nguyen ⁎ Corresponding author at: School of Marine Sciences, Guangxi University, No.100, Daxuelu Road, Nanning 530004, PR China. E-mail address: [email protected] (K. Yu). https://doi.org/10.1016/j.palaeo.2018.05.045 Received 29 January 2018; Received in revised form 19 May 2018; Accepted 30 May 2018 Available online 21 June 2018 0031-0182/ © 2018 Elsevier B.V. All rights reserved. W. Jiang et al. Palaeogeography, Palaeoclimatology, Palaeoecology 507 (2018) 30–38 coast with no rivers emptying into the sea, except for a very small river, Sanya River, which is separated from the sampling location by the geographical barrier of Luhuitou Peninsula. The SCS is the biggest marginal sea in China, covering the tropics and subtropics, in which most coral species are distributed. Sanya has a tropical oceanic mon- soon climate with an average of 1347.5 mm annual precipitation. In- terannual and interdecadal rainfall variation in the study area is mainly influenced by well-known ocean-atmosphere systems, such as the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) (Deng et al., 2013; Yan et al., 2011). On Hainan Island, karst landforms are mainly distributed in the west and south, including Sanya, where karst water and pore phreatic water dominate the groundwater types (Zhou, 2005). The hydrogeologic conditions of the Hainan Island, including Sanya, can be observed in Fig. 2. The study area is characterized by carbonate rocks with karst fracture-aquifer. Large amounts of SGD can be observed around the Sanya coast, and no record of volcanic eruption has been reported in recent centuries. The coral sample was washed with freshwater and cut into an 8-mm thick slab along the major growth axis by a water-lubricated, diamond- bit masonry saw. X-ray radiography was used to identify the annual growth bands, which indicated the growth procedure of coral aragonite (see Song et al., 2014). Each high- and low-density band constitutes an annual couplet, generally representing one year of growth (Knutson et al., 1972). To remove surface contaminants and organic matters, the coral slabs were soaked and sterilized with 10% H2O2 for 48 h, cleaned three times in an ultrasonic bath using Milli-Q water to remove any detrital materials, and then air-dried in the oven at 60 °C for 48 h. Sample XL1 contained growth bands growing between 1870 and 2006 CE, covering 137 years of the record. Before collecting the coral samples, preliminary milling along the designated sample track was undertaken to remove the upper ~1 mm of the surface. Fine powder was milled from ~2 mm wide sampling grooves continuously along the Fig. 1. Map of Hainan Island in the northern South China Sea indicating the maximum growth axis, homogenized to avoid any seasonal signatures sampling location of Porites coral XL1 from Xiaodonghai Bay. and obtain annual subsamples for elemental analysis. The method of chronology used in this study was introduced in detail by Song et al. et al., 2013; Song et al., 2014), the South Pacific Ocean (Fallon et al., (2014). 2002), the Great Barrier Reef (McCulloch et al., 2003), and the Western Indian Ocean (Fleitmann et al., 2007). Specifically, the Ba and REE 2.2. Geochemical analysis inputs recorded by corals in SGD areas were closely associated with local hydrological parameters, indicating that these tracers had the All the laboratory analyses were completed at the Radiogenic fl strongest relationships to the SGD (Horta-Puga and Carriquiry, 2012; Isotope Facility at the University of Queensland. The Te on and plastic Prouty et al., 2009). containers used during the experimental processes were pre-cleaned Hainan Island is located in the northern SCS with a coastline length using strict acid-cleaning procedures. All the samples and carbonate of 1550 km. The coast of Hainan Island, including the coastal sea area reference material (Jcp-1) were digested in 2% HNO3, which was pre- of Sanya (Wang et al., 2014; Wang et al., 2017), the Laoye Lagoon (Ji pared by diluting double-quartz-distilled 70% HNO3 with Milli-Q water. et al., 2013), and the eastern coastal areas (Su et al., 2011a; Su et al., The 2% HNO3 stock solution of 60 ppb was prepared with internal 6 61 103 115 187 2011b), has been proven to contain abundant SGD. Here, we selected standard isotopes Li, Ni, Rh, In, and Re to correct for the ff fi the Xiaodonghai Bay, near Sanya in southern Hainan Island, as our matrix e ects of calcium (Ca) and instrumental drift. Certi ed geo- study area to attempt to quantify its historical SGD fluxes. To explore chemical reference materials W-2, Jcp-1, and BIR-1 were prepared by the probable coral geochemical signal of SGD and verify their validity adding 60 ppb solution and diluting it to 6 ppb using 2% HNO3. – as reliable proxies, Ba and REE, as well as other elements, were ana- Approximately 2.5 3.0 mg coral samples randomly chosen from the lyzed from a long-lived, near-shore Porites coral core collected in the completely grounded and mixed samples (~50 mg total) were weighed study area (Fig. 1). The coral record allowed us to investigate the re- into low-density polyethylene tubes and dissolved using 10 mL of 6 ppb sponse of surface seawater to the SGD and address its significance over 2% HNO3 solution.