Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 179 (2016) 203–216 www.elsevier.com/locate/gca Multi-colony calibrations of coral Ba/Ca with a contemporaneous in situ seawater barium record Miche`le LaVigne a,⇑, Andre´a G. Grottoli b, James E. Palardy c, Robert M. Sherrell d,e a Department of Earth and Oceanographic Science, Bowdoin College, Brunswick, ME, USA b School of Earth Sciences, Ohio State University, Columbus, OH, USA c Environment and Natural Resources Division, Abt Associates Inc., Bethesda, MD, USA d Department of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ, USA e Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ, USA Received 23 April 2015; accepted in revised form 19 December 2015; available online 3 February 2016 Abstract The coral skeleton barium to calcium ratio (Ba/Cacoral), a proxy for seawater barium concentrations (BaSW), has been interpreted as a tracer of upwelling based on the characteristic ‘‘nutrient like” depth profile of BaSW. However, in some trop- ical regions, such as the Gulf of Panama´, substantial influence of terrestrial runoff inputs and differences between the vertical distribution of BaSW and that of the major nutrients (nitrate and phosphate) in the upper water column can complicate the interpretation of Ba/Cacoral as an upwelled nutrient proxy. In the Gulf of Panama´, contemporaneous Ba/Cacoral records from multiple colonies of Porites lobata, Pavona gigantea, and Pavona clavus corals record a nearly twofold change in surface water BaSW as a 20–70% increase in skeletal Ba/Ca with excellent correlation among Ba/Ca records from co-located colonies (r = 0.86–0.99). These results provide, for the first time, an absolute calibration of the coral Ba proxy with a contemporaneous BaSW record. Compiling the Ba/Cacoral records from three co-located colonies of each species into taxon-specific composite regressions reveals strong statistically significant correlations with the BaSW time-series record (p < 0.001). Differences among taxa in regression slope, y-intercept, and average distribution coefficient, as well as a demonstration of the application of the P. clavus calibration to a previously published Ba/Cacoral record, emphasize the necessity of using taxon-specific calibrations to reconstruct changes in BaSW with accuracy. These results support the application of Ba/Cacoral to reconstruct past changes in absolute BaSW concentrations, adding an important tool to the collection of geochemical proxies for reconstructing surface ocean biogeochemical processes in the past. Ó 2016 Elsevier Ltd. All rights reserved. 1. INTRODUCTION (sub-seasonal) resolution. Isotopic and trace element ratios preserved in the banded aragonitic skeleton of surface Surface-dwelling reef building corals provide the dwelling coral colonies have been used as proxies for opportunity to reconstruct continuous centennial scale tropical sea surface temperature and salinity (e.g. Beck proxy records of environmental conditions at high et al., 1992; Gagan et al., 1994; Nurhati et al., 2011; Carilli et al., 2014). Variations in seawater composition dri- ven by upwelling, coastal runoff, and oceanographic circu- ⇑ Corresponding author at: Department of Earth and Oceano- lation, have also been reconstructed using radiocarbon and 14 graphic Science, Bowdoin College, 6800 College Station, Bruns- trace elemental ratios in tropical coral skeleton (D C, wick, ME 04011, USA. Tel.: +1 (207) 798 4283. Cd/Ca, P/Ca, and Ba/Ca: e.g. Shen et al., 1987; E-mail address: [email protected] (M. LaVigne). http://dx.doi.org/10.1016/j.gca.2015.12.038 0016-7037/Ó 2016 Elsevier Ltd. All rights reserved. 204 M. LaVigne et al. / Geochimica et Cosmochimica Acta 179 (2016) 203–216 = Lea et al., 1989; Guilderson and Schrag, 1998; Montaggioni Ba CaCaCO3 DBa ¼ ð1Þ et al., 2006; Fleitmann et al., 2007; Matthews et al., 2008; Ba=CaSW LaVigne et al., 2010; Moyer et al., 2012; Grottoli et al., 2013). Values for D have been found to be roughly similar for The ratio of barium to calcium in tropical surface coral Ba various species of aragonitic surface-dwelling and deep sea skeleton (Ba/Ca ) has been shown to track upwelling coral corals as well as inorganic aragonite (1.2–1.4), suggesting and/or coastal runoff, depending upon the local sources that barium incorporation occurs via relatively simple ionic of barium to surface waters (e.g. Lea et al., 1989; Sinclair substitution and is not affected by the presence of sym- and McCulloch, 2004b; Sinclair, 2005; Montaggioni et al., bionts or other vital effects (Lea and Boyle, 1989; Lea 2006; Fleitmann et al., 2007; Prouty et al., 2010; Maina et al., 1989; Lea and Spero, 1992; Alibert et al., 2003; et al., 2012; Moyer et al., 2012). Barium is removed from Dietzel et al., 2004; Anagnostou et al., 2011; Ho¨nisch the dissolved phase in the upper water column by the pre- et al., 2011; LaVigne et al., 2011). Values of D that are cipitation of micron-scale barite crystals, formed in Ba estimated from coral analysis coupled to regional estimates microenvironments of decaying organic matter and proba- of seawater barium concentrations, disconnected in time or bly by other poorly described mechanisms as well (Dehairs space from the coral growth location, may lead to inaccura- et al., 1980; Bishop, 1988; Ganeshram et al., 2003). Barium cies in the reconstruction of absolute Ba concentration in is then returned to the dissolved phase in the thermocline SW the past. For example, acknowledging the lack of a direct and below, such that the resultant vertical concentration calibration, Lea et al. (1989) used D = 1.3, estimated profile resembles that of silicate, which is remineralized dee- Ba from mean Ba/Ca measured in Bermuda Montastrea annu- per than nitrate and phosphate. Because of this ‘‘nutrient- laris and Diploria labyrinthiformis corals and Ba from a like” Ba water column profile, Ba/Ca in surface corals SW SW single GEOSECS station located 1700 km northeast of has been used to reconstruct changes in the upwelling of Bermuda to reconstruct Ba variations in Gala´pagos nutrient-rich thermocline waters (Lea et al., 1989; Fallon SW from a Pavona clavus Ba/Ca record. Little progress in et al., 1999; Montaggioni et al., 2006; Allison and Finch, coral refining measurements of D or exploring taxonomic or 2007). However, the increase of Ba with depth is rela- Ba SW other dependencies of the Ba/Ca proxy has been made in tively subtle compared to the major nutrients nitrate and the ensuing 25 years. phosphate. Therefore, terrestrial runoff inputs of Ba to Recent work on previously established hermatypic coral coastal upwelling sites can compromise the interpretation proxies (d18O, Sr/Ca, and Cd/Ca) has shown that averaged of Ba/Ca as an upwelling proxy in some locations. In multi-colony records can yield more accurate seawater coastal regions affected by riverine input, skeletal Ba/Ca reconstructions (DeLong et al., 2007, 2011, 2013; has been shown to track changes in riverine sediment dis- Goodkin et al., 2007; Linsley et al., 2008; Matthews et al., charge, as barium desorbs from suspended sediment in 2008; Grottoli et al., 2013). Therefore, the natural variabil- low salinity estuarine waters, increasing Ba and thus, SW ity in Ba/Ca records among co-located colonies and skeletal Ba/Ca (Li and Chan, 1979; Alibert et al., coral coral taxa must be assessed to evaluate the reproducibility and 2003; McCulloch et al., 2003; Sinclair and McCulloch, accuracy of downcore Ba reconstructions. 2004a; Fleitmann et al., 2007; Prouty et al., 2008, 2010; SW A field-based study carried out in the Gulf of Panama´ in Carriquiry and Horta-Puga, 2010; Horta-Puga and 2003 led to the first direct calibration of two trace elemental Carriquiry, 2012; Maina et al., 2012; Moyer et al., 2012; upwelling tracers in surface-dwelling corals: Cd/Ca and P/ Mallela et al., 2013). Ca (Matthews et al., 2008; LaVigne et al., 2010; Grottoli While the previous work discussed above has correlated et al., 2013). This work revealed good intercolony repro- Ba/Ca variations to the occurrence of upwelling and coral ducibility for P/Ca in Pavona gigantea and Porites lobata terrestrial runoff events, the lack of a direct calibration of corals, however, Cd variability among co-located colonies Ba/Ca against Ba has limited interpretations to rela- coral SW suggests that Cd records derived from a single coral may tive changes in Ba/Ca rather than quantitative recon- coral not yield reliable upwelling reconstructions (Matthews structions of absolute Ba . Quantitative estimates of SW et al., 2008; Grottoli et al., 2013). In this study, we take Ba concentration from modern high-resolution coral SW advantage of this unique set of seawater and coral samples records would allow for modeling seasonal-centennial vari- to develop the first multi-colony calibrations for Ba/Ca ations in upwelling depth, riverine flux, or removal via pri- coral using a contemporaneous Ba/Ca record. mary production and barite formation (Lea et al., 1989). SW Previous studies of foraminifera, deep sea corals, and 2. METHODS inorganic precipitation of calcium carbonate have demon- strated that barium incorporates into calcite and aragonite 2.1. Samples in proportion to the Ba/Ca ratio of seawater (Kitano et al., 1971; Lea and Boyle, 1989; Lea et al., 1989; Lea and Spero, An archived set of replicate coral colonies and a time 1992; Dietzel et al., 2004; Anagnostou et al., 2011; Ho¨nisch series of in situ seawater samples from a previous study in et al., 2011; LaVigne et al., 2011). Based on the assumption the Gulf of Panama´ were analyzed to assess inter-colony that cations in general incorporate into carbonates via inor- Ba/Ca reproducibility (Matthews et al., 2006, 2008; ganic substitution for calcium, distribution coefficients (D) coral LaVigne et al., 2010; Grottoli et al., 2013).