Coral Geochemical Record of Submarine Groundwater Discharge

Palaeogeography, Palaeoclimatology, Palaeoecology 507 (2018) 30–38

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Palaeogeography, Palaeoclimatology, Palaeoecology

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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 affected 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 Pacifi c 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 monsoon 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. All the samples were measured using an inductively the past century. coupled plasma mass spectrometer. The signals of the REE, Y isotopes (89Y, 139La, 140Ce, 141Pr, 146Nd, 149Sm, 153Eu, 159Tb, 160Gd, 161Dy, 165Ho, 167Er, 169Tm, 172Yb, 175Lu), other trace elements (40Ca, 55Mn, 2. Methods and materials 90Zr, and 137Ba), and the internal standard isotopes were simulta- neously monitored and measured in the same aliquots. The signals were 2.1. Sample collection and chronology further corrected for any oxide, hydroxide, and isobaric interferences and calibrated against the certified elemental abundances to calculate During May 2006, the core of a living and healthy colony of the the elemental concentrations in the coral samples. All samples were also hermatypic scleractinian coral Porites lutea labeled as XL2 was drilled measured four times, and the relative standard deviation of measure- using an underwater pneumatic drill at a water depth of 5 m from ments at each run was typically < 5%. Analyzed data were assessed for Xiaodonghai bay, Sanya, in southern Hainan Island within the northern accuracy and precision using a quality assurance and quality control SCS (Fig. 1). The sampling location is hundreds of meters away from the (QA/QC) program, which included reagent blanks, duplicate testing,

31 W. Jiang et al. Palaeogeography, Palaeoclimatology, Palaeoecology 507 (2018) 30–38

Fig. 2. Hydrogeological map of Hainan Island in the northern South China Sea (simpliﬁed from the “Hydrogeological atlas of the People's Republic of China” published by the Geological Publishing House).

and certified geochemical reference materials with deviations < 5%. that Pr/Pr* (=PrN/(0.5CeN + 0.5NdN)) must be greater than one if Ce The detailed process has been introduced by Nguyen et al. (2013) and is truly negative. All coral samples show true negative Ce anomalies and Song et al. (2014). positive La anomalies, as shown in Fig. S3, according to Bau et al. (1997) and Nothdurft et al. (2004). Their Ce/Ce* values range from 3. Results 0.30 to 0.87 (average 0.48, n = 137), which is compatible with modern oxygenated seawater in the SCS (Alibo and Nozaki (2000). However, it 3.1. REE geochemistry is noteworthy that some coral samples' Ce/Ce* values fall outside the range of modern seawater in the Pacific and Indian Oceans. 3) All coral Coral skeletons can record seawater REE compositions, even though samples show superchondritic Y/Ho molar ratios between 92 and 175 seawater REE abundances vary by an order of magnitude (Wyndham (average 127, n = 137). The variation trend of the REE/Ca ratios in the fl et al., 2004). The low concentrations of lithophile elements such as time series is similar to the uctuation of Ce/Ce* values and NdN/YbN fl Zirconium (Zr, ~12.5 ppb), which are generally insoluble in seawater, ratios, but opposite to the uctuation of Y/Ho ratios (Table 1). In fact, fi and the lack of a correlation between the REE/Ca and Zr/Ca ratio data there are signi cant correlations between REE/Ca ratios and the Ce/ in the coral XL1 (Fig. S1 in the Supporting Information) supports the Ce* values (r = 0.32, p < 0.01, n = 137), NdN/YbN ratios (r = 0.48, − assertion that the cleaned coral REE values reliably represent the sur- p < 0.01, n = 137), and Y/Ho ratios (r = 0.54, p < 0.01, n = 137). face seawater conditions. The REE levels in the XL1 coral core are in the − − range of ~56–334 ng·g 1 (mean 109 ng·g 1), which is within the range of those previously published from Porites corals collected from coastal areas in the SCS (Jiang et al., 2017). What is noteworthy is that the average REE level of the XL1 coral is higher than that from Longwan −1 Table 1 Bay (79 ng·g ) of Hainan Island, but lower than the estuarine zones, ffi −1 Pearson correlation coe cients on annual data between XL1 coral geochemical such as Hong Kong off the Pearl River (419 ng·g ) and Nha Trang Bay – fi ffi − proxies during 1870 2006 (the insigni cant coe cients were not presented). off the Cai River and Rac River (340 ng·g 1)(Liu et al., 2011; Nguyen et al., 2013). Mn/Ca REE/Ca Ba/Ca Ce/Ce* NdN/YbN Y/Ho

The Post-Archean Australian Shale (PAAS)-normalized REE + Y Mn/Ca 1 distribution patterns (Taylor and McLennan, 1985) derived from the REE/Ca 1 XL1 coral samples are characterized by the following features (Fig. S2). Ba/Ca 0.566a 1 a 1) All coral samples are heavy-REE enriched, but have much lower and Ce/Ce* 0.319 1 Nd /Yb 0.475a 0.404b 1 Nd /Yb ratios (~0.35) than the coral from the estuarine zones (Liu N N N N Y/Ho −0.543a 1 et al., 2011; Nguyen et al., 2013). 2) Given that the anomalous abundance of La in seawater and marine precipitates can produce negative a Correlation is signiﬁcant at the 0.01 level (2-tailed). b Ce/Ce* values (=CeN/(0.5LaN + 0.5PrN)), Bau et al. (1997) argued Correlation is signiﬁcant at the 0.05 level (2-tailed).

32 W. Jiang et al. Palaeogeography, Palaeoclimatology, Palaeoecology 507 (2018) 30–38

Fig. 3. (A) Comparison between the 31-year running correlation between the XL1 coral REE/Ca ratios and Ba/Ca ratios and PDO index calculated by averaging the monthly January–February-March SST anomalies since 1900 in the North Pacific Ocean poleward of 20°N (data from the University of Washington Joint Institute for the Study of Atmosphere and Oceans, http://jisao.washington.edu/pdo/PDO.latest, Mantua et al. (1997)). Black and blue dots and solid lines indicate the correlation coefficients and PDO index, respectively. (B) Comparison between XL1 coral REE/Ca and Ba/Ca variability during 1870–2006 CE Black and wine dots and solid lines indicate the coral REE/Ca and Ba/Ca ratios, respectively. The gray zones represent the warm PDO periods, as well as the valleys of the correlation coefficients.

3.2. Temporal variability of REE/Ca and Ba/Ca ratios in coral 4. Discussion

Throughout the coral record, the coral REE/Ca and Ba/Ca ratios are 4.1. Control of REE and Ba in coastal seawaters relatively stable, except for several peaks, and they both show similar varying tendencies with significant positive correlations (r = −0.57, The REE record in coral skeletons has great potential to provide a p < 0.01) as shown in Fig. 3. From 1870 to 1907, the coral REE/Ca promising tool for monitoring the temporal and spatial gradients of (Ba/Ca) ratio was relatively low and stable, at 0.22 ± 0.04 terrigenous effects to coastal seawater (Saha et al., 2016). Some studies − (14.27 ± 2.39) ng·g 1. The REE/Ca (Ba/Ca) ratio increased and then have used variations in coral skeletal REE concentrations to understand − plateaued at 0.27 ± 0.03 (19.17 ± 4.96) ng·g 1 during 1908–1937. magnitude and timing of various factors affecting the reef environment, After 1938, the REE/Ca ratio decreased sharply, then increased steadily including natural and human activities (Liu et al., 2011; Fallon et al., − at 0.29 ± 0.06 ng·g 1 during 1938–1976, and then plateaued at 2002; Nguyen et al., 2013). Traditionally, REE in marine environments − 0.35 ± 0.05 ng·g 1 during 1976–2006. In a slightly different manner, are primarily derived from hydrothermal fluids and rivers (Boynton and the coral Ba/Ca ratio decreased sharply then increased steadily during Henderson, 1984; Henderson, 2013); however, in the global ocean, 1938–2006. However, the coral Ba/Ca ratios during 1938–1976 over 90% of the REE sources are still unidentified based on the Nd − (15.09 ± 3.59 ng·g 1) were lower than they were isotope mass balance (Lacan and Jeandel, 2005; Tachikawa et al., − (27.73 ± 6.03 ng·g 1) during 1977–2006. The correlations among 1999). Johannesson and Burdige (2007) suggested that REE fluxes coral geochemical indicators, local precipitation, and the ENSO index through SGD may be the missing source of REE in ocean water, which published by NOAA (https://www.esrl.noaa.gov/psd/enso/mei/) could be an order of magnitude higher than river-driven fluxes. Duncan during 1961–2006 are exhibited in Table 2. The highly significant and Shaw (2003) inferred the importance of REEs in SGD from the REE correlations between them indicate that the REE parameters should be concentration gradients in groundwater within the surficial aquifer to associated with the local rainfall. Indeed, the spectral power analysis conclude that groundwater is a major source to local coastal waters. also indicates that our coral REE/Ca and Ba/Ca ratio time series aligns Johannesson et al. (2011) further advanced this notion by actually with the periodicity of the ENSO and PDO (Fig. S4), which contain computing the REE SGD fluxes to the overlying waters of the Indian significant periodicities of 2–7 and 20–30 years. River Lagoon, Florida. Besides, Kim and Kim (2011, 2014) showed that SGD was a major source of REE in local coastal waters. Given the lack of covariance between the REE/Ca and Mn/Ca ratios (Chen et al., 2015; Fallon et al., 2002; Lewis et al., 2007)(Table 1), as well as the absence of large surface runoff from rivers and volcanic eruptions near the study area, SGD may be the primary controlling factor for REE variability in

Table 2 Pearson correlation coefficients on annual data between XL1 coral geochemical proxies and the local/regional rainfall indicators based on a 3 year running averages during 1961–2004 (the insignificant coefficients were not presented).

Ba/Ca Ce/Ce* REE/Ca NdN/YbN Y/Ho Precipitation ENSO index PDO index

Ba/Ca 1 Ce/Ce* −0.539a 1 REE/Ca 1 a a NdN/YbN −0.510 0.688 1 Y/Ho −0.682a 1 Precipitation 0.410a −0.367b 1 ENSO index 0.299b 0.328b −0.355b −0.297b 1 PDO index −0.302b 0.629a 1

a Correlation is signiﬁcant at the 0.01 level (2-tailed). b Correlation is signiﬁcant at the 0.05 level (2-tailed).

33 W. Jiang et al. Palaeogeography, Palaeoclimatology, Palaeoecology 507 (2018) 30–38 the coral records. 2010b; Perry et al., 2002). Considering the distribution of karst coast in Interestingly, SGD in coastal zones is also the main source of Ba in the study area, the covarying behavior (Table 1) between the ground- coastal seawater (Horta-Puga and Carriquiry, 2012; Moore, 1997; Shaw water trace metals, particularly REE (Johannesson and Burdige, 2007; et al., 1998), especially in karst terrain. In the absence of surface runoff Kim and Kim, 2011; Kim and Kim, 2014; Prouty et al., 2009) and Ba from streams, Wei et al. (2000) determined that the precipitation over (Gonneea et al., 2014; Gonneea et al., 2013; Horta-Puga and Carriquiry, southern Hainan Island and SGD should be the main sources of Ba near 2012; Santos et al., 2011; Shaw et al., 1998), indicated that the mineral the Sanya coast, and the surrounding areas (e.g., the central Vietnam dissolution may be a main source-not the continental weathering. Ob- coast) may also contribute to the Ba flux through the surface current, viously, this may also be the key reason that REE and Ba were able to be according to the seasonal periodical variations of coralline Ba/Ca ratios. distinguished from the other metals (e.g., Pb, Cr, Co, Fe, Ni, and Mn) in Recently, our research based on the annual time series of multiple the XL2 coral from our previous research in this region (Jiang et al., metals from a Porites coral hundreds of meters away from the study area 2017). revealed that the REE and Ba shared similar sources distinguished from other metals (e.g., Cu, Cd, Zn, Mn, Ni, Fe, and Co), and they were 4.2. Reliability of coral REE/Ca (Ba/Ca) ratios as SGD proxies dominated by fluvial input or SGD associated with precipitation (Jiang et al., 2017). Additionally, the Ba supply to the river is typically limited In karst coasts, rainwater can easily penetrate the aquifer through in the tropics (Edmond et al., 1978), and this limited Ba supply has also permeable carbonate outcrops and fractures. Since groundwater inside been demonstrated by the low effective river end-member Ba con- the aquifer dissolves the limestone and creates preferential conduits, centration for floods and negative trend with river discharge recorded groundwater may flow more rapidly than in porous or detritic homo- by calibrated coral records (Sinclair and McCulloch, 2004). On Hainan geneous aquifers (Cable et al., 2002; Garcia-Solsona et al., 2010a). SGD Island, several interacting environmental factors have changed over the is widespread and occurs where the aquifer is connected hydraulically past few decades, including urbanization, land cover, and infrastructure with the sea through permeable sediments or rocks. Though the coral construction, which, through surface runoff (Grove et al., 2010; Jupiter reef ecosystem is always relatively far away from the SGD's directly et al., 2008; Lewis et al., 2012; McCulloch et al., 2003; Moyer et al., influential areas, the groundwater residence times and biogeochemical 2012; Prouty et al., 2010), has the potential to impact the Ba and REE nature of the coastal aquifer can vastly transform terrestrial con- loads to the coastal waters. However, there was no coherence among stituents en route to the sea (Swarzenski et al., 2017). Varying rainfall Ba/Ca (REE/Ca) ratios and such factors (Fig. 4), indicating the insig- rates at annual to decadal timescales on the coastal karstic terrain was nificant contribution of surface runoff to the Ba and REE loads, which is always directly related to the groundwater recharge variability of the in agreement with the results determined by Wei et al. (2000). What is aquifer. Concomitantly, the changes occurring during groundwater re- noteworthy is that the coral Ba/Ca and REE/Ca records diverge in the charge promote fluid-rock interactions, such as carbonate dissolution, several years, e.g., 1969, 1974, 1985 and 2003–2006, the cause may be whose products, such as trace metals, are delivered into reef waters via the strengthening of surface current (Gao et al., 2015) or significant SGD to ultimately become incorporated in the coral skeletal records difference between local and the whole northern SCS region. Further- (Horta-Puga and Carriquiry, 2012). One of the most important simila- more, the synchronous dramatic increase of urban population re- rities of REE and Ba is that they can be quickly incorporated into the presenting urbanization and coral Ba/Ca ratios since 2003 may also submarine groundwater solution and then transported to the coastal imply the possible relationship between them. In an isolated case, it is waters right away. In fractured rock aquifers, REE concentrations have noteworthy that the peaks of coral geochemical proxies may correspond been shown to be relatively constant along groundwater paths, and are to the railway construction in Sanya during 1955–1957. believed to be in steady state with host rocks for the quickly achieving Besides affecting rainfall quantity, trace element geochemical cy- of REE during infiltration of slightly acidic rainfall (Prouty et al., 2009; cling can also alter the chemical signatures of groundwater and may Tweed et al., 2006). Similar to REE, coral Ba/Ca have also been con- ultimately control the total chemical load to the coastal ocean asso- firmed as reliable proxy of SGD on the coastal karstic terrain (Horta- ciated with SGD (Gonneea et al., 2014). Thus, the trace element content Puga and Carriquiry, 2012) for the high permeability permiting of SGD is largely determined by mineral dissolution (e.g., the equili- groundwater to outflow to the coastal zone immediately after pre- brium dissolution of calcite) (Gonneea et al., 2014). Tovar-Sanchez cipitation due to the high hydraulic pressures (Santos et al., 2012). et al. (2014) considered whether SGD-derived metals were conditioned Coral REE/Ca ratios have been linked to changes in ocean chemistry, by the hydrogeological formations of the aquifer and discharge type. with their variability implicated as a climate proxy of river discharge The karst coast with high permeability always yields abundant SGD (Fallon et al., 2002; Lewis et al., 2012; Lewis et al., 2007; Wyndham through single- or multiple-vent features, due to the rapid infiltration of et al., 2004), sea level variation in estuaries (Liu et al., 2011), and SGD precipitation (Bakalowicz, 2015; Gonneea et al., 2014; Kim et al., 2003; in a volcanic island (Prouty et al., 2009), but the ratios cannot respond Parra et al., 2015; Swarzenski et al., 2017; Swarzenski et al., 2001), and to upwelling due to their tiny and insensitive vertical gradients the predominant carbonate minerals are readily soluble (Beddows (Sholkovitz and Shen, 1995). Likewise, coral Ba/Ca ratios have also et al., 2007; Einsiedl, 2012; Fleury et al., 2007; Garcia-Solsona et al., been applied as proxies of river discharge (Brenner et al., 2017; Grove

Fig. 4. Comparison of XL1 coral REE/Ca (black dotted line) and Ba/Ca (blue dotted line) ratios with contemporary local factors (data from Hainan Statistical Yearbooks published by Hainan Provincial Bureau of Statistics). The red solid line represents the trend of urban population in Hainan Island, and the wine solid line represents the trend of agricultural acreage of Hainan Island. The gray zones represent the period of mass construction near Sanya.

34 W. Jiang et al. Palaeogeography, Palaeoclimatology, Palaeoecology 507 (2018) 30–38 et al., 2010; Jupiter et al., 2008; Lewis et al., 2012; McCulloch et al., REE/Ca and Ba/Ca ratios during the period of instrumental observa- 2003), upwelling (Lea et al., 1989; Montaggioni et al., 2006), and SGD tion, a correlation analysis following a 3-year running average was (Horta-Puga and Carriquiry, 2012). According to Falkner et al. (1993), applied. It is worth noting that the coral Ba/Ca ratios were more sen- − the dissolved Ba content is ~34 nmol·kg 1 at a water depth of 30 m and sitive to the regional precipitation controller (e.g., PDO) than the coral − ~50 nmol·kg 1 at a water depth of 500 m in the Western Pacific. Given REE/Ca ratios. However, the Ba/Ca ratios were not in a manner similar the maximum water depth of 200 m in our study area (Wei et al., 2000), to the REE/Ca ratios, and they exhibited no highly significant correla- anomalous Ba spikes are too large to be caused by the Ba-rich upwel- tions with the local hydrological parameters (Table 2). Meanwhile, the ling. Thus, because the study region belongs to a non-estuarine area, the coral REE/Ca ratios were significantly correlated with the instrumental coral REE/Ca ratios and Ba/Ca ratios may be reliable proxies of local precipitation data from the Sanya meteorological station, and both the SGD. coral REE/Ca ratios and Ba/Ca ratios had relatively weak correlations The utility of the coral Ba/Ca and REE/Ca ratio proxy stems from with the ENSO/PDO index as proxies of regional precipitation. How- the fact that the SGD has elevated Ba and REE concentrations compared ever, despite their close relationship with the coral REE/Ca ratios, the to seawater. The coastal surface seawaters are two- to five-fold enriched coral Ba/Ca ratios showed no significant correlations with local pre- in Ba when compared to open sea surface waters (Lea and Boyle, 1991; cipitation. Strangely enough, Ba always exhibited more sensitivity to Moyer et al., 2012; Shaw et al., 1998). In fact, the estimated average Ba terrestrial input than did REE (Lewis et al., 2007; Prouty et al., 2009). concentration in our study was approximately twice as high as that To explore the relationships between the coral REE/Ca and Ba/Ca ratios measured in the open sea, as reported by Song et al. (2014) through on annual and decadal time scales, a 31-year (~30 year is the major relatively rough conversions from the coral Ba contents. This disparity cycle of the PDO according to Deng et al. (2013)) running correlation between coastal and open ocean Ba encouraged us to attempt to use the between the ratios was calculated (Fig. 3). Interestingly, a visual re- coral Ba/Ca ratio as a proxy of terrigenous input. Compared to the Ba verse trend can be observed based on the negative relationships concentration in the river endmember, the groundwater was found to (r = −0.340, p < 0.01) between the correlation coefficients and the be enriched in Ba at an order of magnitude higher (Shaw et al., 1998). PDO index introduced by Mantua et al. (1997) in Fig. 3. Because the Considering the lack of rivers in sampling location and abundant Karst interdecadal variations of precipitation in South China, including the ground water, in that sense, coral Ba may be a reliable proxy of SGD Beibu Gulf, are controlled by the PDO (Chan and Zhou, 2005; Mao rather than river discharge in study area. The REE concentrations in et al., 2011), the PDO index should represent regional rainfall rather most of the groundwater samples are substantially higher than their than local rainfall in Sanya. During warm PDO periods, heavy pre- concentrations in the surface waters, which is, on average, 100-fold cipitation should appear in the entire Beibu Gulf region. Thus, the lower than REE in the groundwater (Chevis et al., 2015b). The mass valleys of correlation coefficients corresponding to the warm PDO may balances of REE demonstrated that the REE fluxes through SGD were imply the interfusion of the allochthonous seawater Ba or REE from the two to three orders of magnitude higher than those through the other surrounding areas (e.g., the central Vietnam coast) through the surface sources in the coastal bays of an island in the Western Pacific(Kim and current. The summer circulation structure of Beibu Gulf, revealed by Kim, 2011, 2014). Also, Johannesson et al. (2011) reported that the net Gao et al. (2015), suggests that the southern gulf was occupied by an flux of Nd through SGD to the Indian River Lagoon was approximately anticyclonic eddy while the northern gulf was dominated by a cyclonic seven times that from the local surface runoff to the coastal waters gyre, offering a reasonable explanation for the allochthonous seawater based on a few REE measurements and a simple three-box model. Ba and REE. Based on the seasonal periodical variations of coralline Ba/ Chevis et al. (2015a) has a much larger data set as well as performed a Ca ratios near the study area, Wei et al. (2000) have proposed that the more rigorous flux calculation using a modified form of the diagenetic SGD and the surrounding areas (e.g. the central Vietnam coast) both equation that accounted for bioirrigation, demonstrating that the SGD contribute to the Ba flux. Meanwhile, the low coral Ba/Ca ratio flux of REEs is equal to that of the stream fluxes. Anyhow, the SGD has anomalies during summer months with high surface seawater tem- been shown to be an important source of REE to coastal waters (Duncan peratures also indicate that the seawater Ba/Ca ratios were significantly and Shaw, 2003). Johannesson and Burdige (2007) and Kim and Kim reduced though bounding to biogenic carbonates or depositing as barite (2014) even suggested that the SGD-driven REE fluxes may contribute due to the enhanced biomass during the high primary productivity considerably to the global budget of REE in the ocean, and dissolved period (Dymond et al., 1992). Thus, the coral Ba/Ca ratios are the result concentrations of REE have been used as tracers of groundwater flow of various complicated processes. paths in the hydrogeological environment (Banner et al., 1989; In contrast, the REE diluted by seawater are thought to follow a Johannesson et al., 1999; Johannesson et al., 1997). The coral REE conservative mixing pattern and are generally recognized to be useful contents in our study were much higher than in the Porites coral col- for tracing oceanic water masses, because the relative abundance of lected from Longwan Bay on the eastern coast of Hainan Island (Liu REE in the oceans is controlled by regional sources and biogeochemical et al., 2011). According to the hydrogeological conditions as shown in removal rates (Elderfield, 1988). The great value of REE as environ- Fig. 2, the study area is characterized by carbonate rocks with karst mental proxies is that they are self-normalising and apart fromusing fracture-aquifer, while the Longwan Bay is characterized by loose rocks their concentrations and ratios, anomalies and systematic changes in with pore phreatic water. As Kim and Kim (2011) noted, the karst coast shale-normalized patterns can be utilized to trace specific events. with high permeability should be responsible for the diversity of coral Though Wyndham et al. (2004) pointed out that the NdN/YbN ratios REE and the REE flux to the ocean. It is noteworthy that Longwan Bay is and Mn concentrations were closely connected to the scavenging of close to the Wanquan River, one of the largest rivers on Hainan Island, heavy REE by particulate organic ligands and Mn-reductive dissolution, but it belongs to the non-karst coast (Zhou, 2005). Obviously, our re- respectively, with both processes related to the primary productivity, sults support the conclusion that SGD, not surface runoff, may be the the lack of statistical relationship between the NdN/YbN ratios and Mn/ dominant source of REE on the coast of Hainan Island. Thus, the REE/ Ca ratios (Table 1) still indicates that the effects of the primary pro- coral ratios should be good proxies of SGD in the study area. ductivity should not be the main factors controlling the surface sea-

Meteorological studies have suggested that interdecadal variations water REE contents. It's worth noting that the NdN/YbN ratios and Ce/ in rainfall over South China are associated with the PDO and ENSO Ce* values during the period of instrumental observation after a 3-year (Chan and Zhou, 2005; Mao et al., 2011). Though the local instrumental running average both showed significant correlations with the Ba/Ca precipitation and the ENSO index share similar trends (Fig. S5), they ratios, perhaps suggesting non-significant impacts to certain REE showed no highly significant correlations, indicating a regional dis- parameters (e.g., NdN/YbN ratios and Ce/Ce* values) from the primary parity between Sanya and the entirety of South China. To explore the productivity. The significant correlation between coral REE/Ca ratios relationship between the local/regional precipitation and the coral and LREE/HREE ratios (r = 0.522, p < 0.01) in time series (Fig. S6)

35 W. Jiang et al. Palaeogeography, Palaeoclimatology, Palaeoecology 507 (2018) 30–38 indicated an additional simple source for this binary mixing in the estuarine kasrst regions where large SGD can be observed. Given un- subterranean estuarine region. Considering the strong relationship be- certainties associated with groundwater ﬂow paths and residence times tween coral REE/Ca ratios and the local instrumental precipitation, as in study area, this approach for constructing SGD, however, is not well as the ﬁndings mentioned above, coral REE/Ca ratios should be without limitations, and further research is still required to understand more suitable proxies than coral Ba/Ca ratios or other REE parameters the quantitative relationships among precipitation, SGD, and coral REE for SGD in the study area. geochemistry.

4.3. Reconstructed seawater REE concentrations and SGD variations 5. Conclusions

Since the REE are incorporated into coral's aragonite lattice in The temporal variabilities of multiple geochemical proxies over the proportion to their concentrations in ambient seawater, their levels last 137 years were inferred from a Porites coral in a subterranean es- along the annual growth bands in corals may be good representatives of tuary near a karst coast in the northern SCS. The changes occurring their historical levels in ambient seawater (Sholkovitz and Shen, 1995; during groundwater recharge promoted fluid-rock interactions, such as Wyndham et al., 2004). Considering that the average empirical dis- carbonate dissolution, and produced abundant REE and Ba, which are tribution coefficient reported for REE/Ca molar ratios in Porites coral is delivered into the reef waters via SGD to ultimately become incorporated into the coral skeletal records. Coral REE/Ca ratios were (REE/Cacoral)/(REE/Caseawater) = ~1.98 (Sholkovitz and Shen, 1995), and that the concentration of Ca in seawater is conservative highly correlated with both local precipitation and regional rainfall −1 proxies (e.g., the ENSO index) at annual to decadal timescales, but coral (Caseawater = 0.412 g·kg )(Bruland, 1983), the significance of the REE/Ca ratios in the annual bands of coral as proxies for the dissolved Ba/Ca ratios were only poorly correlated with regional rainfall proxies. concentration of REE in seawater was investigated. Assuming that Meanwhile, the decrease in correlation coefficients between the coral salinity is constant, the concentration of dissolved REE in seawater REE/Ca and Ba/Ca ratios were synchronous with the timing of a warm − − varied from 34.3 pmol·kg 1 to 110.1 pmol·kg 1, with an average con- PDO with heavy rainfall. Time- and frequency-domain comparisons − centration of 61.1 ± 13.9 pmol·kg 1 during 1870–2006. These values between the coral geochemical proxy and the local/regional pre- are higher than the average concentration of REE in the oceanic surface cipitation indicated that coral REE/Ca ratios are predominantly im- waters in the Western Pacific(Alibo and Nozaki, 1999; Alibo and pacted by the SGD associated with local precipitation, while coral Ba/ Nozaki, 2000) but agree well with those expected for coastal surface Ca ratios were also affected by the primary productivity and al- seawater (Akagi et al., 2004). lochthonous seawater Ba from surrounding areas. The REE signal from Coral REE/Ca records can be applied to conduct model-data com- coral allows us to reconstruct the coastal surface seawater REE con- parisons in groundwater flow models for simulating changes in centrations and the SGD rates on the coast of Sanya from 1870 to 2006. groundwater discharge (Prouty et al., 2009). Since coral REE/Ca ratios The reconstructed REE concentrations in surface seawater in the study were considered suitable proxies of SGD fluxes in study area, this re- area were higher than the average concentrations of REE in the oceanic lationship was used for reconstructing the local historical annual SGD surface waters in the Western Pacific, but they agreed well with those fluxes. Due to the absence of SGD data during 1870–2006, the esti- expected for coastal surface seawater. The reconstructed SGD rate rose − − mated average SGD rate (~15 cm·d 1 or ~54.75 m·a 1) using 226Ra as a whole, corresponding to the escalating trend of local rainfall on near the Sanya coast by Wang et al. (2014) in 2012 was utilized. Due to Hainan Island. This study provides evidence that the coralline REE/Ca the similar average annual rainfall (~1832 mm) of 1963 and 2012 record of the coral can be reliably used as a long-term proxy for SGD in based on the Sanya meteorological station, the SGD rates were con- the study area. Therefore, it is possible that coral paleohydrological sidered to be identical. Based on the linear regression relationship records can contribute to model-data comparisons in SGD models under converted by rainfall in 2012 in study area between the REE/Ca ratios different pumping scenarios. and SGD rates, the SGD rates were reconstructed (Fig. 5). According to the reconstructed data in the time series, the SGD rate in the study area Acknowledgments − − − − varied from 6.13 cm·d 1 (22.36 m·a 1) to 19.67 cm·d 1 (71.79 m·a 1), − with an average concentration of 11 ± 2.49 cm·d 1 Thanks are due to the responsible editor and two anonymous re- − (39.82 ± 9.08 m·a 1) during 1970–2006. As shown in Fig. 5, the SGD viewers for their critical reviews and constructive comments, which rate rose as a whole during 1870–2006, corresponding to the escalating helped us improve significantly our paper. This work was financially trend of local rainfall on Hainan Island reconstructed from the Porites supported by the National Natural Science Foundation of China coral Δδ18ObyDeng et al. (2013). Thus, the local rainfall should be (91428203, 41603091, and 41663001) and the National Basic Research responsible for the variations of the SGD rate on the Sanya coast. Program of China (2013CB956102). Jiang thanks the support of the Considering the strong storage capacity of the karst network, the coral Guangxi Natural Science Foundation (2016GXNSFBA380113) and the REE/Ca ratios should be ideal proxies of SGD rate or flux in non- China Postdoctoral Science Foundation funded project

Fig. 5. Comparison of reconstructed SGD rate with contemporary precipitation proxy (the coral Δδ18O from Deng et al., (2013), the high/low Δδ18O values represent light/heavy rainfall intensity). The black dots and solid line represent the reconstructed SGD rate in the study area, and the blue dots and solid line represent the Δδ18O values from Porites coral in the eastern coast of Hainan Island, which represent the regional rainfall intensity of Hainan Island. Both the arrows represent the trend.

36 W. Jiang et al. Palaeogeography, Palaeoclimatology, Palaeoecology 507 (2018) 30–38

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