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Please check this box if you have no corrections to make to the PDF file Thank you for your assistance. GCA 8649 No. of Pages 1, Model 5+ 12 February 2014 Highlights • Chemical cycling of Ba, Sr, Ca, U, Mn and Ra within a karst subterranean estuary is evaluated. • Terrestrial groundwater reflects aquifer rock trace element content for Sr, Ca, Ba, Mn and Ra. • Ra and Ba chemical cycling is decoupled; both demonstrate low salinity desorption. • Spring mixing zone Ba, Sr, Ca and Mn cycling is largely conservative between terrestrial groundwater and the ocean. • Karst subterranean estuaries are a significant source of Ra, Sr and U to the ocean. 1 GCA 8649 No. of Pages 20 12 February 2014 Available online at www.sciencedirect.com 1 ScienceDirect Geochimica et Cosmochimica Acta xxx (2014) xxx–xxx www.elsevier.com/locate/gca 2 Trace element geochemistry of groundwater in a 3 karst subterranean estuary (Yucatan Peninsula, Mexico) a,⇑ a b 4 Q1 Meagan Eagle Gonneea , Matthew A. Charette , Qian Liu , c c 5 Jorge A. Herrera-Silveira , Sara M. Morales-Ojeda 6 a Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, USA 7 b State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China 8 c CINVESTAV-IPN, Unidad Me´rida, A.P. 73 CORDEMEX, Me´rida, Yucata´n, Mexico 109 Received 7 August 2013; accepted in revised form 28 January 2014; available online xxxx 11 12 Abstract 13 Trace element cycling within subterranean estuaries frequently alters the chemical signature of groundwater and may ulti- 14 mately control the total chemical load to the coastal ocean associated with submarine groundwater discharge. Globally, karst 15 landscapes occur over 12% of all coastlines. Subterranean estuaries in these regions are highly permeable, resulting in rapid 16 infiltration of precipitation and transport of groundwater to the coast, and the predominant carbonate minerals are readily 17 soluble. We studied the chemical cycling of barium (Ba), strontium (Sr), manganese (Mn), uranium (U), calcium (Ca) and 18 radium (Ra) within the carbonate karst subterranean estuary of the Yucatan Peninsula, which is characterized by a terrestrial 19 groundwater lens overlying marine groundwater intrusion with active submarine discharge through coastal springs. Terres- 20 trial groundwater calcium (1–5 mmol kgÀ1) and alkalinity (3–8 mmol kgÀ1) are enriched over that predicted by equilibrium 21 between recharging precipitation and calcite, which can be accounted for by groundwater organic matter respiration and 22 subsequent dissolution of calcite, dolomite and gypsum. There is a close agreement between the observed terrestrial ground- 23 water Sr/Ca, Mn/Ca, Ba/Ca and Ra/Ca and that predicted by equilibrium dissolution of calcite, thus the trace element 24 content of terrestrial groundwater is largely determined by mineral dissolution. Subsequent mixing between terrestrial ground- 25 water and the ocean within the actively discharging springs is characterized by conservative mixing of Sr, Mn, Ba and Ca, 26 while U is variable and Ra displays a large enrichment (salinity: 1.9–34.9, Ba: 60–300 nmol kgÀ1, Sr: 15–110 lmol kgÀ1,U: 27 0.3–35 nmol kgÀ1, Mn: 0.3–200 nmol kgÀ1, Ca: 4.3–12.9 mmol kgÀ1, 226Ra: 18–2140 dpm 100 LÀ1). The deep groundwater 28 sampled through cenotes, local dissolution features, is typified by elevated Ba, Sr, Ca, Mn and Ra and the absence of U within 29 marine groundwater, due to enhanced dissolution of the aquifer matrix following organic matter degradation and redox pro- 30 cesses including sulfate reduction (salinity: 0.2–36.6, Ba: 7–1630 nmol kgÀ1, Sr: 1.3–210 lmol kgÀ1,U:0.3–18 nmol kgÀ1, Mn: 31 0.6–2600 nmol kgÀ1, Ca: 2.1–15.2 mmol kgÀ1, 226Ra 20–5120 dpm 100 LÀ1). However, there is no evidence in the spring geo- 32 chemistry that deep marine groundwater within this reaction zone exchanges with the coastal ocean via spring discharge. 33 Total submarine groundwater discharge rates calculated from radium tracers are 40–95 m3 mÀ1 dÀ1, with terrestrial discharge 34 contributing 75 ± 25% of the total. Global estimates of chemical loading from karst subterranean estuaries suggest Sr and U 35 fluxes are potentially 15–28% and 7–33% of total ocean inputs (8.2–15.3 mol yÀ1 and 4.0–7.7 mol yÀ1), respectively. Radium- 36 226 inputs from karst subterranean estuaries are 34–50 times river inputs (6.7–9.9 Â 1016 dpm yÀ1). 37 Ó 2014 Published by Elsevier Ltd. 38 39 Abbreviations: SGD, submarine groundwater discharge; STE, subterranean estuary; SI, saturation index. ⇑ Corresponding author. Tel.: +1 508 289 3236. E-mail addresses: [email protected] (M.E. Gonneea), [email protected] (M.A. Charette), [email protected] (Q. Liu), [email protected] (J.A. Herrera-Silveira), [email protected] (S.M. Morales-Ojeda). 0016-7037/$ - see front matter Ó 2014 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.gca.2014.01.037 Please cite this article in press as: Gonneea M. E., et al. Trace element geochemistry of groundwater in a karst subterranean estuary (Yucatan Peninsula, Mexico). Geochim. Cosmochim. Acta (2014), http://dx.doi.org/10.1016/j.gca.2014.01.037 GCA 8649 No. of Pages 20 12 February 2014 2 M.E. Gonneea et al. / Geochimica et Cosmochimica Acta xxx (2014) xxx–xxx 40 1. INTRODUCTION a proxy of ocean redox state (Tribovillard et al., 2006). 99 More recently, studies have shown SGD associated fluxes 100 41 Submarine groundwater discharge (SGD) is the trans- and chemical transformations within the subterranean estu- 101 42 port of terrestrially sourced groundwater and groundwater ary should be considered in oceanic element budgets (Dunk 102 43 of marine origin across the ocean-aquifer interface (Burnett et al., 2002; Santos et al., 2011; Holmden et al., 2012; Beck 103 44 et al., 2006). Many studies have shown that SGD is an et al., 2013). To this end, SGD-derived chemical fluxes are 104 45 important source of trace elements to the coastal ocean determined for the Yucatan Peninsula. We then extrapolate 105 46 (Shaw et al., 1998; Basu et al., 2001; Moore et al., 2006; Beck the karst end members reported here to calculate potential 106 47 et al., 2007; Bone et al., 2007; Moore, 2010; Santos et al., global SGD derived chemical fluxes through karst subterra- 107 48 2011; Beck et al., 2013; Gonneea et al., 2013). However, nean estuaries and compare them to ocean element budgets. 108 49 uncertainty remains concerning the total chemical flux via 50 SGD, due in part to groundwater transit through the subter- 2. METHODS 109 51 ranean estuary (STE), the region of the coastal aquifer where 52 sharp gradients in salinity, oxygen and redox state facilitate 2.1. Field site 110 53 the biogeochemical cycling of trace elements (Moore, 1999; 54 Charette and Sholkovitz, 2006; Kaleris, 2006; Beck et al., The Yucatan Peninsula is a large karst platform (165,000 111 55 2007; Gonneea et al., 2008; Santos et al., 2011). To fully km2) that is bordered by the Gulf of Mexico along the west 112 56 understand the impact SGD has on the coastal ocean and and north coasts and by the Caribbean Sea on the east coast 113 57 the role SGD plays in global ocean element budgets, we need (Fig. 1). The peninsula is characterized by a humid tropical 114 58 to quantify how chemical cycling within the subterranean climate, with maximum daily temperatures ranging from 25 115 59 estuary impacts transport of dissolved species to the ocean. to 35 °C and average rainfall that varies spatially across the 116 60 Geochemical transformations within the salinity-mixing Peninsula from 555 to –1500 mm yÀ1.