Rare earth element behavior during groundwater–seawater mixing along the Kona Coast of Hawaii
Introduction
Submarine groundwater discharge (SGD) is increasingly recognized as an important flux of water, nutrients, and metals to the coastal ocean (Moore, 2010). Although the volume of fresh SGD to the ocean is commonly small compared to river discharge, geochemical reactions in subterranean estuaries (e.g., Moore, 1999) can generate substantial fluxes of nutrients, carbon, and metals to the coastal ocean (Moore, 1997, Cai et al., 2003, Charette and Buesseler, 2004, Slomp and Van Cappellen, 2004, Charette et al., 2005, Kim et al., 2005, Windom et al., 2006, Roy et al., 2010, Roy et al., 2011, Johannesson et al., 2011, Chevis et al., 2015a, Chevis et al., 2015b). Submarine groundwater discharge includes all flow of water from the seafloor into the overlying marine water column within continental margin regions, regardless of chemical composition and physical driving force (Church, 1996, Burnett et al., 2003, Moore, 2010). Thus, SGD includes “fresh submarine groundwater discharge” that originates from meteoric recharge of terrestrial aquifers, as well as “recirculated saline submarine groundwater discharge”, which includes water that cycles through coastal aquifers caused by wave set-up, tidal pumping, geothermal or density gradients, and/or bioirrigation (Taniguchi et al., 2002). These components of SGD are commonly referred to as “terrestrial SGD” and “marine SGD”, respectively, and their sum represents the total SGD (Martin et al., 2007, Roy et al., 2010).
The rare earth elements (REEs, also known as lanthanides) have been utilized to study the petrogenesis of magmatic rocks and for tracing the circulation and mixing of oceanic water masses (Jakeš and Gill, 1970, Hanson, 1980, Piepgras and Wasserburg, 1987, McKenzie and O’Nions, 1991). The effectiveness of the REEs as tracers of geochemical reactions and processes chiefly reflects their uniform trivalent charge (Ce4+ and Eu2+ can also occur), and the gradual decrease in their ionic radii with increasing atomic number (i.e., the lanthanide contraction) that accompanies the progressive filling of the 4f-electron shell across the REE series. Consequently, the REEs exhibit strong fractionation as a group due to size and charge, as well as important “within-group” fractionation that result from the lanthanide contraction (see Johannesson et al., 2005, Johannesson et al., 2014; and references therein). These distinctive features of the REEs can accordingly provide insight into complex and subtle geochemical processes that other, single element tracers cannot readily discriminate (Quinn et al., 2004, Johannesson et al., 2005, Johannesson et al., 2014).
Investigations of the REEs in surface water estuaries reveal substantial removal of dissolved REEs (i.e., filtrates that passed through 0.45 μm or 0.22 μm pore size filters; Sholkovitz, 1992) where river waters first mix with seawater (Martin et al., 1976, Goldstein and Jacobsen, 1988a, Elderfield et al., 1990, Sholkovitz, 1993, Sholkovitz, 1995, Sholkovitz and Szymczak, 2000). Removal of REEs in the low salinity reaches of estuaries reflects salt-induced coagulation and flocculation of Fe-rich, organic colloids, which scavenge and fractionate the REEs such that the order of removal is light REE (LREE) > middle REE (MREE) > heavy REEs (HREE; Sholkovitz, 1995). At higher salinities the REEs are released back to solution owing to diagenetic reactions within estuarine sediments and/or displacement from surface sites on suspended particles by more abundant competing cations (Sholkovitz, 1995, Sholkovitz and Szymczak, 2000). Here, the order of release also follows LREE > MREE > HREE. Consequently, the HREE enriched, shale-normalized REE fractionation pattern that characterizes seawater (which originates in part during chemical weathering of continental rocks) is further modified by the competitive effects of aqueous and surface complexation reactions acting on REEs during their transport to the ocean in rivers and estuaries (i.e., Nesbitt, 1979, Hoyle et al., 1984, Goldstein and Jacobsen, 1988b, Elderfield et al., 1990, Sholkovitz, 1995, Byrne and Liu, 1998).
Despite numerous investigations of surface water estuaries, much less is known about the processes that control REE concentrations and fractionation in subterranean estuaries, as well as the importance of SGD fluxes of REEs to the ocean on a global basis (Duncan and Shaw, 2003, Johannesson and Burdige, 2007, Johannesson et al., 2011, Kim and Kim, 2011, Chevis et al., 2015a, Chevis et al., 2015b). Interest in SGD fluxes of REEs to the ocean also stems in part from the possibility that SGD could be an important component of the “missing Nd flux” that would balance the global Nd budget and address the “Nd paradox” (Goldstein and Hemming, 2003, Johannesson and Burdige, 2007). The “Nd paradox” refers to the apparent decoupling of dissolved Nd concentrations, whereby Nd concentrations exhibit nutrient-like profiles in the ocean (indicating vertical cycling and, hence, long residence times, ∼104 years), and Nd isotope ratios that exhibit inter- and intra-oceanic variations that preclude vertical cycling and support Nd residence times that are similar to or less than the oceanic mixing time (∼500–1500 years; Bertram and Elderfield, 1993, Jeandel et al., 1995, Jeandel et al., 1998, Tachikawa et al., 1997, Tachikawa et al., 1999a, Tachikawa et al., 1999b, Tachikawa et al., 2003, Lacan and Jeandel, 2001, Goldstein and Hemming, 2003, Siddall et al., 2008, Arsouze et al., 2009). Current estimates of riverine and atmospheric fluxes appear to be roughly an order of magnitude too low to balance the global ocean Nd budget and preserve the inter- and intra-oceanic Nd isotope ratios. The source of the missing Nd flux is thought to reflect “boundary exchange”, which is a broadly defined process that involves exchange of Nd between the continental shelf and the ocean (Lacan and Jeandel, 2005, Jeandel et al., 2013), and likely includes SGD (Johannesson and Burdige, 2007, Chevis et al., 2015a, Chevis et al., 2015b). Indeed, recent studies demonstrate that SGD associated with small volcanic islands, including the Hawaiian Islands, have a measureable impact on the REE concentrations and Nd isotope signatures of coastal waters as well as surface waters in the open ocean (Kim and Kim, 2011, Kim and Kim, 2014, Fröllje et al., 2016).
In this contribution we expand on our investigations of REE cycling in subterranean aquifers by presenting REE concentration data for groundwaters and seawater from the arid, Kona Coast of Hawaii where previous investigations have demonstrated the existence of large, nutrient-rich plumes of groundwater, and extensive diffuse seeps of SGD to the coastal ocean (e.g., Johnson et al., 2008, Street et al., 2008, Knee et al., 2010). We employ a modification of the product approach for mixing experiments pioneered by Sholkovitz (1976) to study processes that control REE concentrations and fractionation across the salinity gradient of Kona Coast subterranean estuaries.
Section snippets
Setting
The study site is located on the arid western coast of the island of Hawaii (Fig. 1), where the overwhelming majority of meteoric water that falls in the region is discharged to the adjacent coastal ocean via SGD (Oki et al., 1999, Duarte et al., 2006, Johnson et al., 2008, Tillman et al., 2014a, Prouty et al., 2016). Precipitation within the immediate study region (e.g., Kaloko – Honokohau National Historic Park) ranges from 500 to 760 mm a−1, whereas annual pan evaporation exceeds 1700 mm (Ekern
Sample collection
All sampling and laboratory plasticware (HDPE, Teflon®), including filters, were cleaned prior to use by following standard trace element cleaning procedures (e.g., Johannesson et al., 2004, Fitzsimmons and Boyle, 2012, Fitzsimmons and Boyle, 2014). Groundwater samples were collected from wells in the vicinity of Honokohau Harbor, the Kaloko-Honokohau National Historic Park (i.e., KAHO wells), and from the Hind “well” located near Kiholo Bay (Fig. 1; Table 1). The Hind “well” is essentially a
Geochemistry of Kona groundwaters
The major solute compositions of the Kona groundwaters are presented in Table 2 along with nutrient concentrations and other ancillary geochemical parameters. The groundwater are chiefly Na–HCO3 type waters, although groundwater from the KAHO 2 well is a Na–Cl water with major ion ratios similar to coastal seawater (Fig. 2). The Kona groundwaters are brackish as demonstrated by their measured salinities and electrical conductivities, as well as their computed ionic strengths (Table 2).
REEs in Kona Coast groundwaters
The nearly identical shale-normalized REE fractionation patterns of Kona Coast groundwaters and the local seawater provides strong evidence that SGD is the chief source of REEs to the coastal ocean (Fig. 4). Furthermore, the positive Eu anomalies that are apparent in REE patterns when Kona Coast groundwaters are normalized to the coastal seawater values (Fig. 12) likely reflect a weathering signature of the local basalts (e.g., Tanaka et al., 2008, Ren et al., 2009, Fröllje et al., 2016). The
Conclusions
The similarity in the REE fractionation patterns between the Kona Coast groundwater and coastal seawater, the 10- to 50-fold enrichment in REEs concentrations of Kona Coast seawater over open-ocean, North Pacific seawater, and the lack of surface streams along the Kona Coast, all point to SGD being the chief source of REEs to the coastal ocean along the Kona Coast of the Big Island of Hawaii. This notion is further supported by the nearly identical shale-normalized REE pattern of local
Acknowledgements
This project was supported by NSF grants OCE-0825920 to Johannesson and OCE-0825895 to Burdige, as well as USGS Natural Resources Preservation Program (NRPP) and Park Oriented Biological Support (POBS) Award to Prouty. The authors wish to thank C. H. Conaway at the U.S. Geological Survey in Menlo Park, California, for the ion chromatography analyses of the anion concentrations, and D. Gross and S. Beavers (NPS) for logistical support. The lead author is indebted to Michael and Mathilda Cochran
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