Article
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

The importance of submarine groundwater discharge (SGD) is becoming increasingly recognized because of its potential significance as a source of dissolved species. To explore the probable coral geochemical signal of SGD and verify the validity of potential reliable proxies, multiple geochemical proxies over the last 137 years were identified from a Porites coral near a subterranean estuary in the northern South China Sea, where the SGD was 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.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... Previously, we hypothesized that submarine groundwater discharge (SGD) is an important, but poorly accounted for, source of rare earth elements (REE) and Nd isotopes to the ocean, and further that the SGD-flux of REEs may be sufficient to close the ocean REE budget and help resolve the Nd-paradox (Johannesson and Burdige, 2007). Several other studies have also investigated SGD fluxes of REEs to coastal waters (e.g., Duncan and Shaw, 2003;Prouty et al., 2009;Johannesson et al., 2011Johannesson et al., , 2017Kim and Kim, 2011, 2014Chevis et al., 2015a,b;Jiang et al., 2018;Paffrath et al., 2020). All these investigations have either inferred or demonstrated that SGD fluxes can be an important source of REEs to coastal waters. ...
... The REE concentration of groundwaters in subterranean estuaries, and hence SGD fluxes of REEs to the coastal ocean, are largely controlled by biogeochemical reactions that occur when fresh, and commonly anoxic, terrestrial groundwaters of meteoric origin mix with recirculating, saline and oxic waters of marine origin, and in the process of mixing, react with solid phases that make up the subterranean estuary (e.g., minerals, sedimentary organic matter; Duncan and Shaw, 2003;Johannesson et al., 2011Johannesson et al., , 2017Kim, 2011, 2014;Chevis et al., 2015a,b;Jiang et al., 2018;Paffrath et al., 2020). For example, REE concentrations and input-normalized fractionation patterns of groundwater discharging to the Pettaquamscutt River estuary in Rhode Island appear to reflect weathering of accessory minerals like apatite that are common in local bedrock and related glacial deposits, followed by precipitation of secondary phosphate minerals during mixing of groundwaters and local stream water with incoming marine waters from Rhode Island Sound (Chevis et al., 2015a;Adebayo et al., 2020). ...
... Similarly, dissolution of basaltic glass in fractured basalt flows, as well as release of REEs from suspended particles or colloids during mixing of terrestrial-source groundwater with marine waters in subterranean estuaries appears to be an important process that mobilizes REEs into groundwaters that subsequently discharge to the coastal ocean along the arid Kona Coast of Hawaii (Johannesson et al., 2017). Other studies have also demonstrated the importance of SGD from volcanic, oceanic islands as a major source of REEs to the coastal ocean (Kim and Kim, 2011;Jiang et al., 2018;Molina-Kescher et al., 2018). ...
Article
Full-text available
Rare earth elements (REE) and Nd isotope compositions of surface and groundwaters from the Indian River Lagoon in Florida were measured to investigate the influence of submarine groundwater discharge (SGD) on these parameters in coastal waters. The Nd flux of the terrestrial component of SGD is around 0.7±0.03 μmol Nd/day per m of shoreline across the nearshore seepage face of the subterranean estuary. This translates to a terrestrial SGD Nd flux of 4±0.2 mmol/day for the entire 5,880 m long shoreline of the studied portion of the lagoon. The Nd flux from bioirrigation across the nearshore seepage face is 1±0.05 μmol Nd/day per m of shoreline, or 6±0.3 mmol/day for the entire shoreline. The combination of these two SGD fluxes is the same as the local, effective river water flux of Nd to the lagoon of 12.7±5.3 mmol/day. Using a similar approach, the marine-sourced SGD flux of Nd is 31.4±1.6 μmol Nd/day per m of shoreline, or 184±9.2 mmol/day for the investigated portion of the lagoon, which is 45 times higher than the terrestrial SGD Nd flux. Terrestrial-sourced SGD has an εNd(0) value of −5±0.42, which is similar to carbonate rocks (i.e., Ocala Limestone) from the Upper Floridan Aquifer (−5.6), but more radiogenic than the recirculated marine SGD, for which εNd(0) is −7±0.24. Marine SGD has a Nd isotope composition that is identical to the εNd(0) of Fe(III) oxide/oxyhydroxide coated sands of the surficial aquifer (−7.15±0.24 and −6.98±0.36). These secondary Fe(III) oxides/oxyhydroxides formed during subaerial weathering when sea level was substantially lower during the last glacial maximum. Subsequent flooding of these surficial sands by rising sea level followed by reductive dissolution of the Fe(III) oxide/oxyhydroxide coatings can explain the Nd isotope composition of the marine SGD component. Surficial waters of the Indian River Lagoon have an εNd(0) of −6.47±0.32, and are a mixture of terrestrial and marine SGD components, as well as the local rivers (−8.63 and −8.14). Nonetheless, the chief Nd source is marine SGD that has reacted with Fe(III) oxide/oxyhydroxide coatings on the surficial aquifer sands of the subterranean estuary.
... As for coral Ba/Ca, the scenario is more complicated due to the complexity of biogeochemistry of Ba in coastal coral skeletons (Chen et al., 2011;Lewis et al., 2018;Sinclair, 2005;Tanzil et al., 2019). Monthly variability of coral Ba/Ca is not only always corresponded to river discharge peaks during wet season but would also show seasonal cycles or anomalously episodic spikes possibly linked to upwelling (Alibert & Kinsley, 2008;Lea et al., 1989), submarine groundwater (Alibert et al., 2003;Jiang et al., 2018), biological activity (Gillikin et al., 2006;Lewis et al., 2018;Sinclair, 2005;Wyndham et al., 2004), anomalously lower temperature (Chen et al., 2011), or sediment resuspension (Alibert et al., 2003;Esslemont et al., 2004). Similarly, coral δ 13 C also has limitations as an environmental proxy as drivers such as internal carbon source, coral-symbionts metabolism, kinetic effects, and other environmental variables may vary case by case (e.g., Allison & Finch, 2012;Deng et al., 2013;Grottoli, 2002;McConnaughey et al., 1997;McConnaughey, 2003;Swart, Healy, et al., 1996;Swart, Leder, et al., 1996). ...
... Coral Ba/Ca ratios exhibited regular seasonal peaks in austral autumn, fluctuating from~4.1 to~7.0 μmol/mol, with background values also exhibiting a marginal increase (p < 0.01) (Figure 3d). Such seasonal peaks and variation range are comparable to the previous reported Ba/Ca patterns in most corals across the Caribbean and Indo-Pacific regions, which are commonly attributed to the seasonal delivery of Ba by river runoff (Fallon et al., 1999;Jupiter et al., 2008;Lewis et al., 2018;McCulloch et al., 2003;Sinclair & McCulloch, 2004;Prouty et al., 2010;Saha, Rodriguez-Ramirez, et al., 2018), upwelling (Alibert & Kinsley, 2008;Lea et al., 1989;Reuer et al., 2003;Shen et al., 1992;Shen & Sanford, 1990), or groundwater (Alibert et al., 2003;Jiang et al., 2018). ...
Article
Full-text available
The geochemistry preserved in massive scleractinian corals has long been used as proxies for river runoff, but its reliability in naturally extreme environment (i.e., strong hydrodynamics and intensive thermal stress) has not been tested yet. Using Porites coral collected from the macrotidal nearshore Kimberley region of northwestern Australia, we assess the impacts of river runoff and associated changes in this extreme environment using elemental (Ba/Ca) and isotopic (δ¹⁸O and δ¹³C) compositions at both near monthly and annual resolutions. On the monthly timescales, significant terrestrial signatures were noted in skeletal Ba/Ca, ∆δ¹⁸O, and to a lesser extent in δ¹³C time series, highlighting their linkage to runoff input of Fitzroy River. However, all the geochemical time series as well as the observational coastal sea surface salinity exhibited a consistent ~1‐ to 2‐month lag with river discharge, possibly a manifestation of the sluggish water and sediments exchange in King Sound which are likely induced by strong tidal forcing. On the annual timescales, Ba/Ca follows the variation in river discharge, while freshwater supplied by both runoff and rainfall all contributed to δ¹⁸O variations. In contrast, annual δ¹³C is mainly dominated by the ¹³C Suess effect, showing a gradually downward trend. Importantly, we find that δ¹⁸O and Ba/Ca records exhibit consistent and significant long‐term trends, with δ¹⁸O being decreasing and Ba/Ca being increasing, coupled with the increased Australian monsoon, indicating that strengthened monsoon precipitation has likely brought more freshwater and sediment loads to the nearshore Kimberley region.
... In the tropical and subtropical regions around the Pacific Ocean, calcareous sand is distributed in shallow water and near the coast, such as Guam, Hawaii, Haiti, and some other places. In these areas, without proper foundation treatment, calcareous sand was observed to be liquefied and lost stability by seismic hazards, posing a threat to the offshore structures, wharves, and ports [7]. Besides, many engineering accidents of marine construction have been caused by the special mechanical properties of calcareous sand [8]. ...
Article
Full-text available
Calcareous sand is widespread around Nansha Islands, South China Sea. In oceanic and coastal engineering, calcareous sand is usually used as a building foundation and backfill material for airport runway embankments. The engineering characteristics of calcareous sand is different from terrigenous sand because of its irregular grain shape, lower particle strength, and internal voids, which have caused many engineering problems in the last decades. Cement-stabilized soil, as a common foundation reinforcement method, can solve these engineering problems and improve the foundation strength effectively. Therefore, it is very important to estimate the engineering characteristics of cement-stabilized calcareous sand foundations. In this paper, the basic engineering characteristics, bearing capacity, and deformational behavior of calcareous sand were studied by carrying out a series of tests on cement-stabilized calcareous sand. It is found that: (1) the uniaxial compression strength of calcareous sand is higher than that of Guangzhou soft soil but lower than that of filter medium quartz sand; (2) the deformation of the calcareous sand under compression is mainly plastic, and the elastic deformation gradually increases with increasing cement content; (3) the apparent cohesion of calcareous sand increases, while internal friction angle decreases with increasing cement content; (4) cement-stabilized method can significantly improve the bearing capacity of calcareous sand foundation, especially for the saturated state. A cement content equal to or more than 15% and a thickness of 1/8 of the foundation can effectively improve the bearing capacity of the foundation; and (5) the ultimate bearing capacity of the foundation by numerical calculation is higher than that by experiments, while the settlement by calculating is lower.
... They found strong correlations between annual precipitation and the Ba/Ca coral time-series and concluded that Ba/Ca ratios can be used as a proxy of precipitation, which in the Yucatan is a direct indicator of SGD as there is minimal surface runoff (Young et al., 2008). Jiang et al. (2018) measured variations of REE/Ba and REE/Ca ratios in coral samples from Hainan Island, China, and used these as a long-term (>100 year) proxy for SGD in the study area. They observed an increase in the reconstructed SGD corresponding to the escalating trend of local rainfall and suggested that "coral paleohydrological records can contribute to model-data comparisons in SGD models under different pumping scenarios." ...
Article
Full-text available
Intrusion of saltwater into freshwater coastal aquifers poisons an essential resource. Such intrusions are occurring along coastlines worldwide due largely to the over-pumping of freshwater and sea level rise. Saltwater intrusion impacts drinking water, agriculture and industry, and causes profound changes in the biogeochemistry of the affected aquifers, the dynamic systems called subterranean estuaries. Subterranean estuaries receive freshwater from land and saltwater from the ocean and expose this fluid mixture to intense biogeochemical dynamics as it interacts with the aquifer and aquiclude solids. Increased saltwater intrusion alters the ionic strength and oxidative capacity of these systems, resulting in elevated concentrations of certain chemical species in the groundwater, which flows from subterranean estuaries into the ocean as submarine groundwater discharge (SGD). These highly altered fluids are enriched in nutrients, carbon, trace gases, sulfide, metals, and radionuclides. Seawater intrusion expands the subterranean estuary. Climate change amplifies sea level variations on short and seasonal time scales. These changes may result in higher SGD fluxes, further accelerating release of nutrients and thus promoting biological productivity in nutrient-depleted waters. But this process may also adversely affect the environment and alter the local ecology. Research on saltwater intrusion and SGD has largely been undertaken by different groups. We demonstrate that these two processes are linked in ways that neither group has articulated effectively to date.
... The usability of the groundwater is immensely significant in regions that are geographically isolated systems like coral islands. Hence the submarine groundwater discharge has received rapid attention in recent decades, especially the coral reef regions of tropical countries (Jiang et al., 2018). ...
... One of the most commonly studied invertebrates in this respect are corals. Sometimes the impact of SGD on corals causes no apparent physical stress, but it can affect the isotopic signature, such as by altering the δ 18 O and δ 15 N signatures or rare earth element ratios of corals on century to Holocene scales [101,108,110]. More often, the presence of SGD in a coral ecosystem results in a reduction of coral cover and coral species diversity [26,74,94]. ...
Article
Full-text available
Submarine groundwater discharge (SGD) is a global and well-studied geological process by which groundwater of varying salinities enters coastal waters. SGD is known to transport bioactive solutes, including but not limited to nutrients (nitrogen, phosphorous, silica), gases (methane, carbon dioxide), and trace metals (iron, nickel, zinc). In addition, physical changes to the water column, such as changes in temperature and mixing can be caused by SGD. Therefore SGD influences both autotrophic and heterotrophic marine biota across all kingdoms of life. This paper synthesizes the current literature in which the impacts of SGD on marine biota were measured and observed by field, modeling, or laboratory studies. The review is grouped by organismal complexity: bacteria and phytoplankton, macrophytes (macroalgae and marine plants), animals, and ecosystem studies. Directions for future research about the impacts of SGD on marine life, including increasing the number of ecosystem assessment studies and including biological parameters in SGD flux studies, are also discussed.
Article
Rare earth element and yttrium (REY) concentrations of carbonates can be used as potential proxies to trace the geochemistry of precipitating waters and physico-chemical environments. However, it is difficult to retrieve pristine REY signatures of carbonates because they can be contaminated by non-carbonate phases and modified by post-depositional processes. In this paper, we review the controlling factors of the REY signatures, such as the total REE concentrations, REY patterns, as well as elemental anomalies (mainly La, Ce, Eu and Gd), and Y/Ho ratios from published information. The non-carbonate phases, including clastic detritus, Fe-Mn(oxyhydr)oxides and phosphates, and post-depositional modification are the important factors that contaminate pristine REY signatures. The useful proxies and relationships among element concentrations, anomalies, and ratios can be used to evaluate the non-carbonate contamination or post-depositional modification. The selection of suitable normalization standards is also critical in certain cases. We evaluate the appropriate analytical methods, dissolution processes and pre-leaches to minimize the influence of non-carbonate contamination and post-depositional modification. We conclude that partial dissolution with acetic acid may be a robust approach to retrieve pristine REY signatures from the bulk carbonates, whereas LA-ICP-MS technique is appropriate for in-situ analyses of pure carbonate microfacies.
Article
Full-text available
Microplastic pollution in marine environ- ments has increased rapidly during the last decades. These microplastic particles are transported into the coral reef environment from terrestrial origin. Recent investigations suggest that microplastic ingestion can adversely affect marine invertebrates, especially in the coral reef ecosystem. The present attempt has been carried out to identify the coral growth band impreg- nated microplastics from polished coral slabs and acid leached insoluble residues through microscopic exam- ination. The impregnated microplastics have been present in the following descending order: Nylon [Polyester [Polypropylene [Polyethylene. The irregular-shaped black and red microplastic particles have been dominantly observed. The annual growth band impregnated microplastics are most probably derived from tourist activities and chiefly controlled by oceanic currents and sediment re- suspension. The maximum distribution of microplas- tics has been observed in the coral annual growth bands of 1964 and 2005.
Article
Full-text available
Study region: The study region encompasses the nearshore, coastal waters off west Maui, Hawaii. Here abundant groundwater—that carries with it a strong land-based fingerprint—discharges into the coastal waters and over a coral reef. Study focus: Coastal groundwater discharge is a ubiquitous hydrologic feature that has been shown to impact nearshore ecosystems and material budgets. A unique combined geochemical tracer and oceanographic time-series study addressed rates and oceanic forcings of submarine groundwater discharge at a submarine spring site off west Maui, Hawaii. New hydrological insights for the region: Estimates of submarine groundwater discharge were derived for a primary vent site and surrounding coastal waters off west Maui, Hawaii using an excess 222Rn (t1/2 = 3.8 d) mass balance model. Such estimates were complemented with a novel thoron (220Rn, t1/2 = 56 s) groundwater discharge tracer application, as well as oceanographic time series and thermal infrared imagery analyses. In combination, this suite of techniques provides new insight into the connectivity of the coastal aquifer with the near-shore ocean and examines the physical drivers of submarine groundwater discharge. Lastly, submarine groundwater discharge derived constituent concentrations were tabulated and compared to surrounding seawater concentrations. Such work has implications for the management of coastal aquifers and downstream nearshore ecosystems that respond to sustained constituent loadings via this submarine route.
Article
Full-text available
Variations in discharge and turbulent kinetic energy (TKE) were studied at a point-source submarine groundwater discharge (SGD), within a fringing reef lagoon, from quadrature (neap) to syzygy (spring) tides. The principal factors affecting discharge and TKE variations were tides and waves. Field data indicated discharge and TKE varied with high and low tides, and with quadrature and syzygy. Maximum discharge and TKE values were observed during low tides when the hydrostatic pressure over the jet was minimal, while the lowest discharge and TKE values were found at high tides. Syzygy tides produced consistent saltwater intrusion during high tides, while quadrature tides produced the greatest TKE values. In general as the discharge intensified during low tides, jet temperatures decreased suggesting that waters originated further within the aquifer. At the same time jet salinities increased, suggesting a mixing of aquifer and seawater. To reconcile these two seemingly opposing views, it is proposed that the jet conduit is connected to a stratified chamber with seawater below brackish water. The greatest subtidal discharge occurred during quadrature tides. Syzygy produced low subtidal discharge driven by flow reversals (flow into the aquifer) observed throughout syzygy high tides in conjunction with the peak wave setup (>5cm) observed during a storm. While tides were the primary driving force of the discharge, waves played a non-negligible role. Wave effects on the discharge were most evident during syzygy high tides combined with a storm, when the subtidal spring discharge was weakest and salt intrusion developed. This article is protected by copyright. All rights reserved.
Article
Full-text available
Small mountainous rivers (SMRs), despite their disproportionate size relative to larger rivers, may contribute as much as one third of the total terrigenous material exported to the coastal ocean. Corals growing near the mouths of tropical SMRs have the potential to provide records of terrigenous material fluxes because they are long lived and elemental and isotope signatures within their skeletons are useful proxies of palaeoceanographic variability. Here, a 56 year record of coral skeletal Ba/Ca, Mn/Ca, Y/Ca, δ13C, and Δ14C measurements is presented from a Montastraea faveolata colony growing ˜1 km from the mouth of an SMR in eastern Puerto Rico. Coral Ba/Ca was coherent with river discharge and coral skeletal δ13C at annual periodicity corresponding to synchronous depletions of both δ13C and Δ14C in the coral skeleton, and Mn/Ca and Y/Ca were not coherent with river discharge at annual periodicity. Coherence between river discharge and Ba/Ca and the concurrent timing of increases in Ba/Ca with decreases in δ13C and Δ14C indicate that river discharge is simultaneously recorded by all three geochemical tracers. A reconstruction of Rio Fajardo discharge is presented for a period in which no instrumental discharge records exists shows good fidelity with the timing of increased discharge during the wet season. Thus, coral-based multiple-proxy records can be valuable tools for reconstructing the timing and variability of SMR discharge to the coastal ocean. Such records are critical to understanding changes in material fluxes and biogeochemical cycles in coastal environments due to land use and climate change.
Article
Full-text available
Submarine groundwater discharge (SGD) is neither well understood nor commonly investigated in Hawai'i, but it is recognized as a potential pollution source to coastal environments. Between 1998 and 2000, this study located and quantified both total SGD and the terrestrial SGD fraction (ftgw) in Kahana Bay, O'ahu. CTD casts were used to profile the water structure and identify potential areas of SGD impact in the bay. Lee-type seepage meters were used to measure SGD rates and collect samples of SGD directly. Radon-222, Si, Cl-, and total alkalinity (Ta) were used as natural tracers to measure the terrestrial groundwater fraction within SGD. Nutrient concentrations were also measured to calculate total nutrient fluxes into the bay via SGD. Ninety percent of the SGD in Kahana Bay occurs in the inner bay within 1 km of the shoreline. The average total SGD flux measured was 90 × 106 L d-1, 16% of which was terrestrial groundwater. By comparison, the average annual surface runoff from Kahana River is 90.7 × 106 L d-1. Estimated fluxes of total dissolved phosphorus and nitrogen by SGD to the bay were 500 and 200% greater than fluxes via surface runoff, respectively. Thus, SGD in Kahana Bay has proved to be a significant source of both freshwater and total nutrient input comparable to that from the surface runoff of Kahana River.
Article
Full-text available
Nha Trang Bay (NTB) is located on the Central Vietnam coast, western South China Sea. Recent coastal development of Nha Trang City has raised public concern over an increasing level of pollution within the bay and degradation of nearby coral reefs. In this study, multiple proxies (e.g., trace metals, rare earth elements (REEs), and Y/Ho) recorded in a massive Porites lutea coral colony were used to reconstruct changes in seawater conditions in the NTB from 1995 to 2009. A 14-year record of REEs and other trace metals revealed that the concentrations of terrestrial trace metals have increased dramatically in response to an increase in coastal development projects such as road, port, and resort constructions, port and river dredging, and dumping activities since 2000. The effects of such developmental processes are also evident in changes in REE patterns and Y/Ho ratios through time, suggesting that both parameters are critical proxies for marine pollution.
Article
Full-text available
To evaluate the role of submarine groundwater discharge (SGD) as a source of rare earth elements (REEs) in the coastal ocean, we estimated the SGD associated discharge of REEs into two semi-enclosed coastal bays off a volcanic island, Jeju, Korea. The coastal brackish groundwater showed pronounced enrichments of middle REEs (MREE) relative to light REEs (LREE) and heavy REEs (HREE) when normalized against the upper continental crust (UCC), whereas seawater samples outside the bays showed a HREE enrichment pattern. The enrichment of both MREE and HREE was clearly identified in bay waters, resulting from mixing between groundwater and offshore seawater. The mass balances of REEs demonstrated that the REE fluxes through SGD were two to three orders of magnitude higher than those that occurred through the other sources, such as diffusion from bottom sediments and atmospheric dust fallout. The SGD-driven Nd flux from the entire Jeju Island during this summer was approximately 120±60mold−1, which is comparable to the Nd fluxes from major rivers (i.e., Mississippi River). Our results imply that highly permeable oceanic islands are particularly important for REE fluxes to the ocean.
Article
Full-text available
Evidence gleaned from the instrumental record of climate data identifies a robust, recurring pattern of ocean–atmosphere climate variability centered over the midlatitude North Pacific basin. Over the past century, the amplitude of this climate pattern has varied irregularly at interannual-to-interdecadal timescales. There is evidence of reversals in the prevailing polarity of the oscillation occurring around 1925, 1947, and 1977; the last two reversals correspond to dramatic shifts in salmon production regimes in the North Pacific Ocean. This climate pattern also affects coastal sea and continental surface air temperatures, as well as streamflow in major west coast river systems, from Alaska to California.
Article
Full-text available
Based on groundwater geochemistry, stratigraphy, and surficial and tectonic characteristics, the northern Yucatan Peninsula, Mexico, a possible analog for ancient carbonate platforms, is divided into six hydrogeochemical/physiographic regions: (1) Chicxulub Sedimentary Basin, a Tertiary basin within the Chicxulub impact crater; (2) Cenote Ring, a semicircular region of sinkholes; (3) Pockmarked Terrain, a region of mature karst; (4) Ticul fault zone; (5) Holbox Fracture Zone-Xel-Ha Zone; and (6) Evaporite Region. Regional characteristics result from tectonics, rock type, and patterns of sedimentation, erosion, and rainfall. The Cenote Ring, characterized by high groundwater flow, outlines the Chicxulub Basin. Most groundwater approaches saturation in calcite and dolomite but is undersaturated in gypsum. Important groundwater parameters are: SO4/Cl ratios related to seawater mixing and sulfate dissolution; Sr correlation with SO4 and saturation of Lake Chichancanab water with celestite, indicating celestite as a major source of Sr; high Sr in deep water of cenotes, indicating deep circulation and contact of groundwater with evaporite; and correlation of Ca, Mg, and SO4, probably related to gypsum dissolution and dedolomitization. Based on geochemistry we propose: (1) a fault between Lake Chichancanab and Cenote Azul; (2) deep seaward movement of groundwater near Cenote Azul; and (3) contribution of evaporite dissolution to karst development in the Pockmarked Terrain. Chemical erosion by mixing-zone dissolution is important in formation of Estuario Celestun and other estuaries, but is perhaps inhibited at Lake Bacalar where groundwater dissolves gypsum, is high in Ca, low in CO3, and does not become undersaturated in calcite when mixed with seawater.
Article
Full-text available
Strong correlations between Corg and Ba fluxes indicate a link between upper ocean biological processes and barium flux to the seafloor. The ratio of organic carbon to barium decreases systematically with water depth. Data from 10 sites indicate that organic debris settling from the 200-m depth has a Corg/Ba ratio of approximately 200. The systematic decrease in this ratio with increasing water depth results from the simultaneous decay of organic matter and uptake of Ba in settling particles. This behavior provides additional evidence that the formation of barite in oceanic particles is a consequence of decomposition/uptake in microenvironments rather than the secretion of barite by specific organisms. -Authors
Article
The composition and concentrations of trace metals in coastal seawater have changed in parallel with variations in geochemical processes, climate and anthropogenic activities. To evaluate the response of trace metals in coastal seawater to climatic changes and human disturbances, we report annual-resolution trace element data for a Porites coral core covering ~100years of continuous growth from a fringing reef in Xiaodonghai Bay in the northern South China Sea. The results suggested that the trace metal contents in the coral skeleton demonstrated decadal to interdecadal fluctuations with several large or small peaks in certain years with remarkable environmental significances. All of the trace metals in coastal surface seawater, especially Cr and Pb (related to industrial or traffic emissions), were impacted by terrestrial inputs, except for Sr and U, which were impacted by the surface seawater temperature (SST). Moreover, Mn, Ni, Fe and Co were also contributed by weapons and military supplies during wars, and Cu, Cd and Zn were further impacted by upwelling associated with their biogeochemical cycles. Ba and rare earth element (REE) in coastal surface seawater were dominated by runoff and groundwater discharge associated with precipitation. This study provided the potential for some trace metals (e.g., REE, Ba, Cu, Cd, and Zn) in coral skeletons to be used as proxies of natural (e.g., upwelling and precipitation) and anthropogenic (e.g., war and coastal construction) variability of seawater chemistry to enable the reconstruction of environmental and climatic changes through time.
Article
To investigate variation in nitrite, nitrate, phosphate, and silicate in a spring–neap tide in a coral reef system influenced by groundwater discharge, we carried out a time-series observation of these nutrients and 228Ra, a tracer of groundwater discharge, in the Luhuitou fringing reef at Sanya Bay in the South China Sea. The maximum 228Ra, 45.3 dpm 100 L−1, appeared at low tide and the minimum, 14.0 dpm 100 L−1, appeared during a flood tide in the spring tide. The activity of 228Ra was significantly correlated with water depth and salinity in the spring–neap tide, reflecting the tidal-pumping feature of groundwater discharge. Concentrations of all nutrients exhibited strong diurnal variation, with a maximum in the amplitude of the diel change for nitrite, nitrate, phosphate, and silicate in the spring tide of 0.46, 1.54, 0.12, and 2.68 µM, respectively. Nitrate and phosphate were negatively correlated with water depth during the spring tide but showed no correlation during the neap tide. Nitrite was positively correlated with water depth in the spring and neap tide due to mixing of nitrite-depleted groundwater and nitrite-rich offshore seawater. They were also significantly correlated with salinity (R2 ≥ 0.9 and P
Article
Panamá's extreme hydroclimate seasonality is driven by Intertropical Convergence Zone rainfall and resulting runoff. River discharge (Q) carries terrestrially-derived barium to coastal waters that can be recorded in coral. We present a Ba/Ca record (1996–1917) generated from a Porites coral colony in the Gulf of Chiriquí near Coiba Island (Panamá) to understand regional hydroclimate. Here coral Ba/Ca is correlated to instrumental Q (R = 0.67, p < 0.001), producing a seasonally-resolved Reduced Major Axis regression of Ba/Ca (μmol/mol) = Q (m³/s) × 0.006 ± 0.001 (μmol/mol)(m³/s)− 1 + 4.579 ± 0.151. Our results support work in the neighboring Gulf of Panamá that determined seawater Ba/Ca, controlled by Q, is correlated to coral Ba/Ca (LaVigne et al., 2016). Additionally, the Coiba coral Ba/Ca records at least 5 El Niño events and identified 22 of the 37 wet seasons with below average precipitation. These data corroborate the Q proxy and provide insight into the use of coral Ba/Ca as an El Niño and drought indicator.
Article
Groundwater and seawater samples were collected from nearshore wells and offshore along the Kona Coast of the Big Island of Hawaii to investigate rare earth element (REE) behavior in local subterranean estuaries. Previous investigations showed that submarine groundwater discharge (SGD) is the predominant flux of terrestrial waters to the coastal ocean along the arid Kona Coast of Hawaii. Groundwater and seawater samples were filtered through 0.45 μm and 0.02 μm pore-size filters to evaluate the importance of colloidal and soluble (i.e., truly dissolved ionic species and/or low molecular weight [LMW] colloids) fractions of the REEs in the local subterranean estuaries. Mixing experiments using groundwater collected immediately down gradient from a wastewater treatment facility (WWTF) proximal to the Kaloko-Hanokohau National Historic Park, and more “pristine” groundwater from a well constructed in a lava tube at Kiholo Bay, were mixed with local seawater to study the effect of solution composition (i.e., pH, salinity) on the concentrations and fractionation behavior of the REEs as groundwater mixes with seawater in Kona Coast subterranean estuaries. The mixed waters were also filtered through 0.45 or 0.02 μm filters to ascertain the behavior of colloidal and soluble fractions of the REEs across the salinity gradient in each mixing experiment. Concentrations of the REEs were statistically identical (two-tailed Student t-test, 95% confidence) between the sequentially filtered sample aliquots, indicating that the REEs occur as dissolved ionic species and/or LMW colloids in Kona Coast groundwaters. The mixing experiments revealed that the REEs are released to solution from suspended particles or colloids when Kona Coast groundwater waters mix with local seawater. The order of release that accompanies increasing pH and salinity follows light REE (LREE) > middle REE (MREE) > heavy REE (HREE). Release of REEs in the mixing experiments is driven by decreases in the free metal ion activity in solution and the concomitant increase in the amount of each REE that occurs in solution as dicarbonato complexes [i.e., Ln(CO3)2⁻] as pH increases across the salinity gradient. Input-normalized REE patterns of Kona Coast groundwater and coastal seawater are nearly identical and relatively flat compared to North Pacific seawater, indicating that SGD is the chief source of these trace elements to the ocean along the Kona Coast. Additionally, REE concentrations of the coastal seawater are between 10 and 50 times higher than previously reported open-ocean seawater values from the North Pacific, further demonstrating the importance of SGD fluxes of REEs to these coastal waters. Taken together, these observations indicate that large-scale removal of REEs, which characterizes the behavior of REEs in the low salinity reaches of many surface estuaries, is not a feature of the subterranean estuary along the Kona Coast. A large positive gadolinium (Gd) anomaly characterizes groundwater from the vicinity of the WWTF. The positive Gd anomaly can be traced to the coastal ocean, providing further evidence of the impact of SGD on the coastal waters. Estimates of the SGD fluxes of the REEs to the coastal ocean along the Kona Coast (i.e., 1.3 – 2.6 mmol Nd day⁻¹) are similar to recent estimates of SGD fluxes of REEs along Florida’s east coast and to Rhode Island Sound, all of which points to the importance of SGD as significant flux of REEs to the coastal ocean.
Article
Coral reefs maintain extraordinary biodiversity and provide protection from tsunamis and storm surge, but inshore coral reef health is degrading in many regions due to deteriorating water quality. Deconvolving natural and anthropogenic changes to water quality is hampered by the lack of long term, dated water quality data but such records are required for forward modelling of reef health to aid their management. Reef corals provide an excellent archive of high resolution geochemical (trace element) proxies that can span hundreds of years and potentially provide records used through the Holocene. Hence, geochemical proxies in corals hold great promise for understanding changes in ancient water quality that can inform broader oceanographic and climatic changes in a given region. This article reviews and highlights the use of coral-based trace metal archives, including metal transported from rivers to the ocean, incorporation of trace metals into coral skeletons and the current ‘state of the art’ in utilizing coral trace metal proxies as tools for monitoring various types of local and regional source-specific pollution (river discharge, land use changes, dredging and dumping, mining, oil spills, antifouling paints, atmospheric sources, sewage). The three most commonly used coral trace element proxies (i.e., Ba/Ca, Mn/Ca, and Y/Ca) are closely associated with river runoff in the Great Barrier Reef, but considerable uncertainty remains regarding their complex biogeochemical cycling and controlling mechanisms. However, coral-based water quality reconstructions have suffered from a lack of understanding of so-called vital effects and early marine diagenesis. The main challenge is to identify and eliminate the influence of extraneous local factors in order to allow accurate water quality reconstructions and to develop alternate proxies to monitor water pollution. Rare earth elements have great potential as they are self-referencing and reflect basic terrestrial input.
Article
Material fluxes (e.g., nutrients) from coastal waters to offshore areas play an important role in controlling the water quality of the adjacent sea areas not only by increasing nutrient concentration but also by changing nutrient structures. In this study, naturally occurring isotopes, 226Ra and 228Ra, were measured with the alpha spectrometry in the Wenjiao-Wenchang and Wanquan estuaries and adjacent sea areas along the east coast of Hainan Island. The excess 226Ra and 228Ra activities were observed by comparison with the values derived from the conservative mixing of freshwater and seawater end-members in both estuaries. Using a one-dimensional diffusion model, the horizontal eddy diffusion coefficient of 3.16 × 10 5 cm 2/s, for nutrients diffusing from their sources, was derived from 228Ra activities. Consequently, the corresponding nutrient fluxes flowing into the coastal waters were assessed. The results can provide useful information for the study of the mixing and exchange processes of coastal waters as well as dissoluble pollutant transport in this sea area.
Article
Due to limited in situ data and diagnostic numerical models, the summer circulation structure and formation mechanism in the Beibu Gulf have always been in controversy in the past 50 years. Therefore, a new three-dimensional hindcast model was built within the northwestern South China Sea (SCS), forced with the daily averaged wind, heat flux, lateral flux, as well as tidal harmonic and eight major rivers discharges. And the east boundary was set up far away off the Qiongzhou Strait (QS). Lastly, the model results were consistent with not only the synchronous observation data from the project 908 but also the historical observed data. As a result, the summer circulation structure was revealed that the southern Gulf was occupied by an anticyclonic eddy whereas the northern Gulf was dominated by a cyclonic gyre. Although the circulation major structure was stable, its area and strength had yearly and monthly oscillation. The other three sensitive experiments indicated that the circulations in the southern and northern Gulf were driven by the SCS circulation and monsoon wind, respectively. After the theoretical analysis of the potential vorticity budget, it was further revealed the circulation in the northern Gulf was driven by the positive wind stress curl in summer. Besides, the river discharge was also significant as the vertical circulation had two layer structures outside the mouth of the Red River. Generally, this work calls for the further research on other subjects, such as ocean biogeochemical or marine fisheries.
Article
Geochemical cycles of trace metals are important influences on the composition and function of the marine ecosystem. Although spatial distributions of most trace metals have now been determined in at least some parts of the oceans, temporal variations have barely been studied on account of data limitations. In this paper, we report on a 159-year record of trace metal concentrations from a Porites coral from the northern South China Sea (SCS), and discuss how oceanic and climatic processes control variations in Mn, Cu, and V concentrations in this region. Our results show that trace metal concentrations in the coral skeleton demonstrate decadal to interdecadal fluctuations, and that their variations are controlled by different mechanisms. The input of Mn to reef water is partly controlled by the Pacific Decadal Oscillation (PDO), which controls precipitation and river runoff. Surface-water concentrations of the nutrient-like element Cu are controlled by summer upwelling to the east of Hainan Island. The concentrations of V show complex inter-relationships, and are linked to riverine input prior to the 1990 and to upwelling after the 1990. Our results imply that in the northern SCS, ocean-atmosphere climate fluctuations, such as the PDO and the East Asian Summer Monsoon (EASM), are important factors that influence long-term variability of Mn, Cu, and V concentrations in seawater, by controlling precipitation-related river runoff and the strength of upwelling systems. This article is protected by copyright. All rights reserved.
Article
We identified a barely noticed contributor, submarine groundwater discharge (SGD), to acidification of a coastal fringing reef system in Sanya Bay in the South China Sea based on time-series observations of Ra isotopes and carbonate system parameters. This coastal system was characterized by strong diel changes throughout the spring to neap tidal cycle of dissolved inorganic carbon (DIC), total alkalinity, partial pressure of CO 2 (pCO 2) and pH, in the ranges of 1851−2131 μmol kg −1 , 2182−2271 μmol kg −1 , 290−888 μatm and 7.72−8.15, respectively. Interestingly, the diurnal amplitudes of these parameters decreased from spring to neap tides, governed by both tidal pumping and biological activities. In ebb stages during the spring tide, we observed the lowest salinities along with the highest DIC, pCO 2 and Ra isotopes, and the lowest pH and aragonite saturation state. These observations were consistent with a concurrent SGD rate up to 25 and 44 cm d −1 , quantified using Darcy's law and 226 Ra, during the spring tide ebb, but negligible at flood tides. Such tidal-driven SGD of low pH waters is another significant contributor to coastal acidification, posing additional stress on coastal coral systems, which would be even more susceptible in future scenarios under higher atmospheric CO 2 .
Article
Along the continental margins, rivers and submarine groundwater supply nutrients, trace elements, and radionuclides to the coastal ocean, supporting coastal ecosystems and, increasingly, causing harmful algal blooms and eutrophication. While the global magnitude of gauged riverine water discharge is well known, the magnitude of submarine groundwater discharge (SGD) is poorly constrained. Using an inverse model combined with a global compilation of 228Ra observations, we show that the SGD integrated over the Atlantic and Indo-Pacific Oceans between 60°S and 70°N is (12 ± 3) x 1013 m3 yr-1, which is 3 to 4 times greater than the freshwater fluxes into the oceans by rivers. Unlike the rivers, where more than half of the total flux is discharged into the Atlantic, about 70% of SGD flows into the Indo-Pacific Oceans. We suggest that SGD is the dominant pathway for dissolved terrestrial materials to the global ocean, and this necessitates revisions for the budgets of chemical elements including carbon.
Article
There is increasing evidence that submarine groundwater discharge (SGD) is an important source of water and dissolved materials to the ocean. One of the primary tracers of this process is the quartet of radium isotopes (223Ra, 224Ra, 226Ra and 228Ra), whereby excess activities in surface waters can often be attributed to an input supplied via SGD. This approach requires the radium end member activity to be well constrained, however, natural variability in groundwater radium may span several orders of magnitude. Therefore, this variability is usually the main driver of uncertainties in volumetric SGD estimates. To investigate the physical and biogeochemical controls on groundwater radium activities, we conducted a three-year time series of radium and barium, a chemical analogue for radium, within the subterranean estuary of a coastal aquifer (Waquoit Bay, MA, USA). Gonneea et al. (2013) demonstrated that movement of the salinity interface within the subterranean estuary is driven by changes in the hydraulic gradient between groundwater level and sea level height. For Waquoit Bay, seasonal scale sea level change, not groundwater level, was the main driver in hydraulic gradient fluctuations. Seasonal changes in groundwater chemistry can be attributed to the resulting movement of the salinity transition zone between terrestrial and marine groundwater. Landward movement of the interface results in a large release of radium isotopes (226Ra = 1400 dpm 100 L-1) and barium (3000 nmol kg-1) associated with an increase in groundwater salinity. The magnitude of these releases cannot be explained by in situ production or weathering alone, but is likely due to salinity driven desorption from surface-bound sediment inventory. The timing of these peak concentrations is not always in phase with model-derived estimates of SGD; as a result, the groundwater concentration rather than the water flux is the main driver of Ra and Ba inputs to Waquoit Bay surface waters. The behavior of the subterranean estuary as an ion exchange reservoir has important implications for the timing and flux of various nutrients and pollutants that transit this region prior to discharge. In addition to modulating chemical fluxes via submarine groundwater discharge on seasonal time scales, transgression of the subterranean estuary may alter the input of chemicals to the ocean on decadal and longer time scales. During this study, the observed excess flux of 226Ra and Ba from the subterranean estuary can be accounted for with sorbed sediment pools and accelerating rates of sea level rise in this region.
Article
The impact of groundwater on pCO2 variability was assessed in two coral reef lagoons with distinct drivers of submarine groundwater discharge (SGD). Diel variability of pCO2 in the two ecosystems was explained by a combination of biological drivers and SGD inputs. In Rarotonga, a South Pacific volcanic island, SGD was driven primarily by a steep terrestrial hydraulic gradient, and the water column was influenced by the high pCO2 (5,501 μatm) of the fresh groundwater. In Heron Island, a Great Barrier Reef coral cay, SGD was dominated by seawater recirculation in permeable sediments (i.e. tidal pumping) and pCO2 was mainly impacted through the stimulation of biological processes. The Rarotonga water column had a relatively higher average pCO2 (549 μatm) than Heron Island (471 μatm), however, pCO2 exhibited a greater diel range in Heron Island (778 μatm) than in Rarotonga (507 μatm). SGD flux rates were quantified using a radon (222Rn) mass balance. The Rarotonga water column received 29.0 ± 8.2 mmol free-CO2 m-2 d-1 from SGD, while the Heron Island water column received 12.1 ± 4.2 mmol free-CO2 m-2 d-1. Both systems were sources of carbon dioxide to the atmosphere (averaging 8.8 ± 3.4 and 2.5 ± 2.1 mmol CO2 m- 2 d-1 in Rarotonga and Heron Island, respectively), with SGD-derived free-CO2 most likely contributing to the outgassing of CO2. Studies measuring the metabolism of coral reefs via changes in carbonate chemistry (e.g. photosynthesis, respiration, calcification, and calcium carbonate (CaCO3) dissolution rates) may need to consider the effects of groundwater seepage on water column carbonate chemistry and greenhouse gas evasion. Local drivers of coral reef carbonate chemistry such as SGD may offer more approachable management solutions to mitigating the effects of ocean acidification (OA) on coral reefs.
Article
Trace element cycling within subterranean estuaries frequently alters the chemical signature of groundwater and may ultimately control the total chemical load to the coastal ocean associated with submarine groundwater discharge. Globally, karst landscapes occur over 12% of all coastlines. Subterranean estuaries in these regions are highly permeable, resulting in rapid infiltration of precipitation and transport of groundwater to the coast, and the predominant carbonate minerals are readily soluble. We studied the chemical cycling of barium (Ba), strontium (Sr), manganese (Mn), uranium (U), calcium (Ca) and radium (Ra) within the carbonate karst subterranean estuary of the Yucatan Peninsula, which is characterized by a terrestrial groundwater lens overlying marine groundwater intrusion with active submarine discharge through coastal springs. Terrestrial groundwater calcium (1–5 mmol kg−1) and alkalinity (3–8 mmol kg−1) are enriched over that predicted by equilibrium between recharging precipitation and calcite, which can be accounted for by groundwater organic matter respiration and subsequent dissolution of calcite, dolomite and gypsum. There is a close agreement between the observed terrestrial groundwater Sr/Ca, Mn/Ca, Ba/Ca and Ra/Ca and that predicted by equilibrium dissolution of calcite, thus the trace element content of terrestrial groundwater is largely determined by mineral dissolution. Subsequent mixing between terrestrial groundwater and the ocean within the actively discharging springs is characterized by conservative mixing of Sr, Mn, Ba and Ca, while U is variable and Ra displays a large enrichment (salinity: 1.9–34.9, Ba: 60–300 nmol kg−1, Sr: 15–110 μmol kg−1, U: 0.3–35 nmol kg−1, Mn: 0.3–200 nmol kg−1, Ca: 4.3–12.9 mmol kg−1, 226Ra: 18–2140 dpm 100 L−1). The deep groundwater sampled through cenotes, local dissolution features, is typified by elevated Ba, Sr, Ca, Mn and Ra and the absence of U within marine groundwater, due to enhanced dissolution of the aquifer matrix following organic matter degradation and redox processes including sulfate reduction (salinity: 0.2–36.6, Ba: 7–1630 nmol kg−1, Sr: 1.3–210 μmol kg−1, U: 0.3–18 nmol kg−1, Mn: 0.6–2600 nmol kg−1, Ca: 2.1–15.2 mmol kg−1, 226Ra 20–5120 dpm 100 L−1). However, there is no evidence in the spring geochemistry that deep marine groundwater within this reaction zone exchanges with the coastal ocean via spring discharge. Total submarine groundwater discharge rates calculated from radium tracers are 40–95 m3 m−1 d−1, with terrestrial discharge contributing 75 ± 25% of the total. Global estimates of chemical loading from karst subterranean estuaries suggest Sr and U fluxes are potentially 15–28% and 7–33% of total ocean inputs (8.2–15.3 mol y−1 and 4.0–7.7 mol y−1), respectively. Radium-226 inputs from karst subterranean estuaries are 34–50 times river inputs (6.7–9.9 × 1016 dpm y−1).
Article
Submarine groundwater discharge (SGD) with inputs of nutrients in certain regions may play a significant role in controlling water quality in the coastal regions. In this paper, we have determined four naturally occurring radium isotope (223Ra, 224Ra, 226Ra and 228Ra) activities and nutrient concentrations in surface water, coastal groundwater and river water in the mixing zone of Laoye Lagoon to estimate the fluxes of SGD by several models. The activities of the four radium isotopes of ground water were considerably greater than those in surface water samples. Using a 224Ra/228Ra activity ratio (AR) model, we estimated the average lagoon water age to be 3.2 days, which was comparable with the flushing time of 4.0 days. Based on the excess radium isotopes and the water age of the lagoon, the estimated fluxes of SGD (in 106 m3/d) ranged from 2.64 to 5.32 with an average of 4.11. Moreover, we used Si balance to evaluate the flux of SGD (4.8 × 106 m3/d) which was close to the result calculated by radium. The SGD-derived nutrient fluxes (in mol/d) were DIN = 1.7 × 105, PO43 − = 5.2 × 102, and SiO3 = 5.3 × 104. Furthermore, we applied the biogeochemical budget approach using SiO3 as a tracer to evaluate the impact of SGD. The differences between the results estimated by radium and SiO3 may indicate different pathways for the input of nutrients.
Article
The Yucatan Peninsula consists of a karstic terrain that allows the aquifer to directly recharge from rainfall. Due to the various dissolution/precipitation reactions occurring during groundwater flow, the groundwater discharge in the coastal zone becomes a source of trace elements including Ba. The aim of this study was to use the coralline Ba/Ca record as a proxy of precipitation under the consideration that rainfall rates vary at inter-annual time scales. Annual Ba/Ca ratios, both the total content (Ba/CaTC) and the Ca-substitutive fraction (Ba/CaCaF), were quantified in a 52-a old coral colony of Montastraea annularis from the Punta Nizuc Reef, Mexican Caribbean. Average Ba/CaTC (5.90 ± 0.56 μmol/mol) was ∼20% higher than Ba/CaCaF (4.85 ± 0.33 μmol/mol) indicating that Ba is also incorporated in other fractions. Correlation between annual precipitation and Ba/CaTC time-series is significant (r = 0.77, p < 0.05), allowing the use of the Ba/CaTC ratio as a proxy of precipitation, and hence, enabling the reconstruction of precipitation patterns through time. Likewise, the Ba/CaCaF ratio can be used for the reconstruction of dissolved Ba in coastal seawater.
Article
Interdecadal variations in the early (May-June) summer monsoon rainfall over South China (SCMR) are found to be related to the ENSO (El Niño/Southern Oscillation) and the PDO (Pacific Decadal Oscillation). An interdecadal variation in SCMR can be identified, with more dry (wet) monsoon years during the periods of high (low) PDO index. Such variations are also related to ENSO in association with PDO. When ENSO and PDO are in phase, i.e. high PDO phase/El Niño events, or low PDO phase/La Niña events, the SCMR tends to be below or above normal respectively more often. But when the ENSO and PDO are out-of-phase, the SCMR has no wet or dry preference. Such relationships appear to be related to the intensity of the subtropical high determined by the superposition of the effects of ENSO and PDO.
Article
Continuous radon ((222)Rn) monitoring was conducted at two stations (site A and site B) with different perpendicular distance from the shoreline in Xiangshan Bay, East China Sea. Based on a (222)Rn balance model (various sources and sinks of (222)Rn in coastal water), the average rate of SGD was estimated to be 0.69cm/day and 0.23cm/day for site A and site B, respectively. The results from a nutrient analysis of the groundwater indicate that the associated nutrients fluxes loading through the SGD pathway were 4.27×10(6)mol/day for DIN, 2.24×10(4)mol/day for DIP and 1.82×10(6)mol/day for DSi, respectively, which were comparable to or even higher than the levels observed in the local streams. Therefore, adequate attention should be paid to the importance of SGD as one source of nutrients during the eutrophication control process in this area.
Article
In this study we have used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), to produce a high resolution coral record of rare earth elements (REE), Mn and Ba from coastal Porites corals from the Great Barrier Reef of Australia. Validation of the LA-ICP-MS technique indicated that the method provides accurate and reproducible (RSD = 13–18%) analysis of low concentration REE in corals (∼1 to 100 ppb). The REE composition in coral samples was found to closely reflect that of the surrounding seawater and distribution coefficients of ∼1–2 indicated minimal fractionation of the series during incorporation into coral carbonate. To explore the idea that coral records of REE can be used to investigate dissolved seawater composition, we analyzed two coastal corals representing a total of ∼30 yr of growth, including a 10-yr overlapping period. Comparable results were obtained from the two samples, particularly in terms of elemental ratios (Nd/Yb) and the Ce anomaly. Based on this evidence and results from the determination of distribution coefficients, we suggest that useful records of seawater REE composition can be obtained from coral carbonates. When compared to the REE composition of a mid shelf coral, coastal corals showed a significant terrestrial influence, characterized by higher REE concentrations (greater than 10 times) and light REE enrichment. The REE composition of coastal seawater inferred from the coral record was dependent on seasonal factors and the influence of flood waters. REE fractionation displayed a strong seasonal cycle that correlated closely with Mn concentration. We suggest that higher Nd/Yb ratios and higher Mn concentrations in summer result from scavenging of heavy REE by particulate organic ligands and Mn reductive dissolution respectively, both processes displaying higher rates during periods of high primary productivity. The Ce anomaly also displayed a strong seasonal cycle showing an enhanced anomaly during summer and during flood events. This is consistent with the Ce anomaly being primarily controlled by the abundance of Ce oxidizing bacteria. Based on these arguments, we suggest that the coral record of dissolved REE and Mn may be regarded as a useful proxy for biological activity in coastal seawater.
Article
The geochemistry and isotopic composition of a karstic coastal aquifer in western Ireland has shed light on the effect of sea-water/groundwater interactions on the water quality of Ireland’s Atlantic coastal zone. The use of stable isotope data from the IAEA precipitation station in Valentia, located in SW Ireland has facilitated the characterization of groundwater recharge conditions in the western part of Ireland and suggests that groundwater is mostly replenished by the isotopically light winter precipitation. The dissolved SO42- in the karstic groundwater that was collected during baseflow conditions with δ34S values between 4.6‰ and 18‰ may be composed of S stemming from three principal sources: SO42- derived from precipitation which is composed of both sea-spray S (δ34S: 20‰) and an isotopically light anthropogenic source (δ34S: 1–5‰), SO42-stemming from animal slurries (δ34S: ∼5‰), and intruding sea-water SO42- (δ34S: 20.2‰). The isotopic composition of δ18O in dissolved groundwater SO42- collected during baseflow conditions is interpreted as reflecting sea-water intrusion to the karstic coastal groundwater system. The highest δ18O values in dissolved groundwater SO42- were in samples collected near the coast (4.8±0.4‰) and the lowest (2±0.5‰) were collected further inland. The δ15N and δ18O values of groundwater NO3- were between 3.4‰ and 11.4‰ and approximately 7.7‰, respectively, and reflect geochemical conditions in the aquifer that do not promote attenuation of NO3- through denitrification. As a result N loading to Kinvara Bay that is controlled by submarine groundwater discharge (SGD) was calculated as 5tons/day on average compared to an estimated N-input that derives from precipitation of approximately 2.5tons/a. SGD into the bay may result in near coastal sea-water quality changes. These results represent one of the first studies addressing the effect of groundwater quality in Ireland on the European Atlantic coastal zone.
Article
Concentrations of rare earth elements (REE) and Nd isotopic ratios were analyzed for seawater, filtered suspension and sediment trap samples collected in the tropical Atlantic Ocean (EUMELI program, EUtrophic, MEsotrophic and oLIgotrophic sites, 20°N, 18°–21°W). This is the first REE/Nd dataset on solution and different-sized particles collected at the same site. We present direct evidence of the Nd isotopic exchange between particulate lithogenic fraction and seawater without significant mass transfer. This exchange is probably one of the main factors that simultaneously constrains the Nd concentration and isotopic ratio budget. We propose a new approach to estimate the residence time of Nd in the ocean (τNd) based on isotopic exchange: 200 yr < τNd< 1000 yr. The exchange requires a partial dissolution of lithogenic Nd. We estimate that the fraction of soluble Nd proportion in atmospheric dust is of the order of 20% based on the isotopic ratios. We suggest that the partial dissolution of atmospheric fallout is probably one of the main REE sources of the ocean.
Article
Groundwater is a major resource in Hawaii and is the principal source of water for municipal, agricultural, and industrial use. With a growing population, a long-term downward trend in rainfall, and the need for proper groundwater management, a better understanding of the hydroclimatological system is essential. Proxy records from corals can supplement long-term observational networks, offering an accessible source of hydrologic and climate information. To develop a qualitative proxy for historic groundwater discharge to coastal waters, a suite of rare earth elements and yttrium (REYs) were analyzed from coral cores collected along the south shore of Moloka'i, Hawaii. The coral REY to calcium (Ca) ratios were evaluated against hydrological parameters, yielding the strongest relationship to base flow. Dissolution of REYs from labradorite and olivine in the basaltic rock aquifers is likely the primary source of coastal ocean REYs. There was a statistically significant downward trend (-40%) in subannually resolved REY/Ca ratios over the last century. This is consistent with long-term records of stream discharge from Moloka'i, which imply a downward trend in base flow since 1913. A decrease in base flow is observed statewide, consistent with the long-term downward trend in annual rainfall over much of the state. With greater demands on freshwater resources, it is appropriate for withdrawal scenarios to consider long-term trends and short-term climate variability. It is possible that coral paleohydrological records can be used to conduct model-data comparisons in groundwater flow models used to simulate changes in groundwater level and coastal discharge.
Article
The present day distributions of Ba in the water columns at three anoxic marine sites, namely the Cariaco Trench, Framvaren Fjord, and Black Sea, are presented. Dissolved Ba levels generally increase with depth, ranging from 45–85, 64–280, and 180–460 nM in surface and bottom waters for the three basins, respectively. Small maxima are observed in the vicinity of the in redox interface in both the Framvaren Fjord and Black Sea. Comparison of the dissolved and particulate Ba, Fe, and Mn distributions show that the maxima do not result from adsorption onto freshly precipitated Fe and/or Mn oxyhydroxides. As for the open ocean, Ba cycling in all three basins is dominated by its uptake, primarily in the form of barite, into particulate matter associated with productivity in surface waters, followed by its regeneration at depth or in the sediments. Microbiological activity near the redox interface promotes the breakdown of settling particulate matter and the release of barite just above the interface in the Black Sea, and most likely in the Framvaren Fjord, thus providing in part for the observed maxima. Dissolution of such barite in the marginal sediments of these basins probably also contributes to the maxima. Thermodynamic calculations show deep Black Sea Ba concentrations exceed saturation with respect to pure barite by at least a factor of 2. However, the uniformity of the deep water concentrations suggests thermodynamic control by some phase; it is likely that impurities, incorporated into barite during its rapid formation near the surface in microenvironments provided by decaying organisms, are responsible for the levels observed. Additional factors controlling the distributions of Ba in each basin are also discussed.
Article
Rare earth element (REE) concentrations were measured in 5 well water samples and 3 springs located along a groundwater flow path in a shallow, tuffaceous alluvial aquifer from southern Nevada, USA. The REE concentrations in these groundwaters decrease in the direction of groundwater flow. A previous investigation demonstrated that REE solid-liquid phase partitioning coefficients (i.e., Kd’s) for groundwaters from tuffaceous alluvial aquifers in southern Nevada are relatively high (mean Kd = 102.6). Our groundwater REE data, in conjunction with these Kd’s, support strong sorption of aqueous REEs to aquifer surface sites as the primary removal mechanism of REEs from these groundwaters. In addition, relatively high aqueous REE concentrations occur at distinct locations along the groundwater flow path. The elevated REE concentrations are explained by addition of deeper groundwaters, influx of geothermal waters from a hot spring system, differences in solution complexation, and/or mixtures of regional and local recharge sources. Solution complexation modelling of REEs in the groundwaters indicate that carbonate complexes account for more than 99% of each REEs in solution. Moreover, groundwater Yb/Nd ratios (a measure of REE fractionation) are associated with alkalinity (HCO3− + CO32−; r = 0.71). The data and speciation model results indicate that REE fractionation (i.e., the observed heavy REE, HREE, enrichments compared to rock-sources) is controlled by formation of progressively stronger carbonate complexes in solution with increasing atomic number, which inhibits HREE sorption compared to light REEs (LREE); and a greater affinity for the LREEs to sorb to surface sites in the local tuffaceous alluvial aquifers compared to the HREEs.
Article
The rare earth elements (REE) were analyzed in a groundwater system from south-central Nevada (i.e., Ash Meadows National Wildlife Refuge, the Spring Mountains, Pahranagat National Wildlife Refuge, and the Nevada Test Site) in order to investigate their potential use as tracers of regional groundwater flow. Previous investigations using conservative tracers (e.g., deuterium and uranium isotopes) identified recharge in local mountains as the primary source (60–70% of total discharge) for the springs in the regional discharge zone (i.e., Ash Meadows) with the remaining contribution being interbasin flow from the northeast. Initial mixing calculations for these groundwaters using shale-normalized REE patterns agreed well with the previous studies; however, because the REEs are not expected to behave conservatively in natural waters, the effect of both solution complexation, which acts to enhance the stability of the REES in solution, as well as surface complexation, responsible for the particle reactive behavior of the REES, were examined in subsequent mixing calculations. In order to assess the roles of solution and surface complexation, relative partitioning coefficients were estimated for each REE in each groundwater by evaluating the ratio of the ionic strength corrected co3β1REE, co3β2REE, and [CO32−]F to the first hydrolysis binding constants for the REES. The relative partitioning coefficients were then used to calculate REE patterns expected to develop and persist in solution as a consequence of solution and surface complexation. The calculated REE values closely resembled the actual measured REE concentrations, suggesting that the REEs are, in fact, controlled by solution and surface complexation in these groundwaters. The calculated REE concentrations were subsequently used to determine mixing ratios, the results of which coincided with the initial calculations as well as the previous studies. The results of this study suggest that solution complexation of the REEs is sufficient to overcome, to a certain degree, the affinity of the REEs to be adsorbed onto surface sites in the aquifers such that distinctive REE signatures develop and persist in solution in groundwaters from different aquifers. The ability of solution complexation to overcome surface complexation is likely related to the formation of the negatively charged dicarbonato complex [i.e., Ln(CO3)2−, where Ln is any REE], which accounts for significant fractions of each REE in these groundwaters.
Article
The Pacific Decadal Oscillation (PDO) has been shown to have significant climatic and environmental impacts across the Pan-Pacific Basin; however, there are no records of PDO activity from the South China Sea (SCS), the largest marginal sea in the northwest Pacific Ocean. This study suggests that a series of geochemical profiles obtained from a modern coral in the northern SCS records annual PDO activity dating back to 1853. These geochemical data are significantly correlated with the PDO index, and their patterns of variation closely match those of the PDO index over the last century. The relationship between the PDO and coral geochemistry may be related to the influence of the PDO on rainfall on Hainan Island. Rainfall patterns influence the volume of terrestrial runoff, which, in turn, is a primary determinant of δ18O and Δδ18O values in coral; however, coral δ13C values are also influenced by the 13C Suess effect. The results indicate that Sr/Ca ratios in coral are affected by a combination of sea surface temperature and terrestrial runoff.
Book
This book describes the composition of the present upper crust, and deals with possible compositions for the total crust and the inferred composition of the lower crust. The question of the uniformity of crustal composition throughout geological time is discussed. It describes the Archean crust and models for crustal evolution in Archean and Post-Archean time. The rate of growth of the crust through time is assessed, and the effects of the extraction of the crust on mantle compositions. The question of early pre-geological crusts on the Earth is discussed and comparisons are given with crusts on the Moon, Mercury, Mars, Venus and the Galilean Satellites.
Article
Submarine groundwater discharge is defined as any flow of water at continental margins from the seabed to the coastal ocean, regardless of fluid composition or driving force1. The flux of submarine groundwater discharge has been hypothesized to be a pathway for enriching coastal waters in nutrients, carbon and metals2. Here, we estimate the submarine groundwater flux from the inventory of 228Ra in the upper Atlantic Ocean, obtained by interpolating measurements at over 150 stations. Only 46% of the loss in 228Ra from radioactive decay is replenished by input from dust, rivers and coastal sediments. We infer that the remainder must come from submarine groundwater discharge. Using estimates of 228Ra concentrations in submarine groundwater discharge, we arrive at a total flux from submarine groundwater discharge of 2–41013 m3 yr- 1, between 80 and 160% of the amount of freshwater entering the Atlantic Ocean from rivers. Submarine groundwater discharge is not a freshwater flux, but a flux of terrestrial and sea water that has penetrated permeable coastal sediments. Our assessment of the volume of submarine groundwater discharge confirms that this flux represents an important vehicle for the delivery of nutrients, carbon and metal to the ocean.
Article
The contribution of submarine groundwater discharge (SGD) to oceanic metal budgets is only beginning to be explored. Here, we demonstrate that biogeochemical processes in a northern Florida subterranean estuary (STE) significantly alter U and Ba concentrations entering the coastal ocean via SGD. Tidal pumping controlled the distribution of dissolved metals in shallow beach groundwater. Hourly observations of intertidal groundwaters revealed high U and low Ba concentrations at high tide as a result of seawater infiltration into the coastal aquifer. During ebb tide. U decreased and Ba increased due to freshwater dilution and, more importantly, biogeochemical reactions that removed U and added Ba to solution. U removal was apparently a result of precipitation following the reduction of U(VI) to U(IV). A significant correlation between Ba and dissolved organic carbon (DOC) in shallow beach groundwaters implied a common source, likely the mineralization of marine particulate organic matter driven into the beach face by tidal pumping. In deeper groundwaters, where the labile organic matter had been depleted, Ba correlated with Mn. We estimate that net SGD fluxes were -163 and + 1660 mu mol m(-1) d(-1) for U and Ba, respectively (or -1 and +8 mu mol m(-2) d(-1) if a 200-m wide seepage area is considered). Our results support the emerging concept that subterranean estuaries are natural biogeochemical reactors where metal concentrations are altered relative to conservative mixing between terrestrial and marine endmembers. These deviations from conservative mixing significantly influence SGD-derived trace metal fluxes.
Article
LATTICE-BOUND cadmium in scleractinian corals has been shown to be a sensitive tracer of historical changes in the nutrient content of surface waters1,2. Barium also substitutes into the lattice of aragonite reef-building corals because there is solid solution between orthorhombic BaCO3 (witherite) and CaCO3(aragonite)3. It is expected that the substitution should be proportional to the Ba content of sea water, which increases from low values in warm surface waters to higher values in cold deep waters. Here we present a high-resolution coralline Ba record from the Galapagos Islands spanning the period 1965-1978. Coralline Ba/Ca tracks historical sea surface temperatures, reflecting the vertical displacement of warm nutrient-poor surface waters by cold, nutrient-rich source waters. Differences between coralline Ba and Cd records may be due to preferential uptake of Cd by phytoplankton during times of lower surface nutrients.