Next Article in Journal
Physics-Informed Dynamics Modeling: Accurate Long-Term Prediction of Underwater Vehicles with Hamiltonian Neural ODEs
Previous Article in Journal
An Affordable Wave Glider-Based Magnetometry System for Marine Magnetic Measurement
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
JMSE
Volume 13
Issue 11
10.3390/jmse13112090
jmse-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Diagenetic Barite Growths in the Mixing Zone of a Carbonate Coastal Aquifer

by
Fernando Sola
1,*,
Malva Mancuso
2 and
Ángela Vallejos
1
1
Water Resources and Environmental Geology, Department of Biology & Geology, University of Almería, 04120 Almería, Spain
2
Engineering and Environmental Technology Department, Universidade Federal de Santa Maria (UFSM), Santa Maria 97105-900, Brazil
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2025, 13(11), 2090; https://doi.org/10.3390/jmse13112090
Submission received: 8 October 2025 / Revised: 26 October 2025 / Accepted: 29 October 2025 / Published: 3 November 2025
(This article belongs to the Special Issue Marine Karst Systems: Hydrogeology and Marine Environmental Dynamics)
Browse Figures
Versions Notes

Abstract

Mixing zones in carbonate coastal aquifers are dynamic interfaces where freshwater and seawater converge, triggering complex biogeochemical processes. This study investigates diagenetic barite (BaSO4) precipitation within such a mixing zone in the dolomitic aquifer of the Sierra de Gádor (SE Spain). Three sectors were analyzed: two active mixing zones—one associated with submarine discharge and the other affected by marine intrusion—and an uplifted, fossilized Pleistocene mixing zone. Mineralogical, petrographic, and geochemical analyses reveal extensive dissolution of the dolomitic bedrock, forming polygonal voids and fracture-controlled porosity, frequently covered by Fe and Mn oxides. Barite crystals were identified exclusively in the Fe oxide precipitates at depths where 80% of seawater is reached. The saturation index for barite in groundwater suggests near-equilibrium conditions across the fresh–brackish–saline transition; however, barite precipitation is localized where Fe oxides act as a geochemical barrier, concentrating Ba and enabling nucleation. SEM imaging shows well-formed euhedral barite crystals up to 100 µm in size. This form of crystallization would be similar to the marine diagenetic barite formation models involving organic matter degradation and Ba remobilization, translated to a coastal aquifer setting in this study. Trace metal analyses show significant enrichment of Pb (up to 20 wt%) and other elements (Zn, Ni, and Co), suggesting potential for ore-forming processes if redox conditions shift. This work proposes a conceptual model for diagenetic barite formation in coastal aquifers, emphasizing the role of Fe and Mn oxides as reactive substrates in metal cycling at the land–sea interface.

1. Introduction

Mixing zones in coastal aquifers represent regions where a wide array of biogeochemical processes occur [1,2,3,4,5,6]. In these areas, fresh groundwater from the continent meets with saltwater that has infiltrated the aquifer from the marine environment. The differences between these two water masses extend beyond salinity, encompassing factors such as chemical composition, pH, Eh, dissolved oxygen, density, and other related parameters. The lower density of freshwater makes the water body overlaying the saline wedge enter from the sea. The contact between both water masses forms a mixing zone whose width will depend on the dispersion/diffusion processes occurring, which are primarily controlled by the permeability of the aquifer rock [7,8,9], along with the variations in the piezometric level that the aquifer undergoes due to phenomena such as recharge, wave action, tides, or atmospheric pressure [10,11,12].
Among others, geochemical processes that occur in the mixing zone of coastal aquifer include the following: (1) Mineral dissolution/precipitation. Soluble phases, such as gypsum or halite, can be dissolved if an increase in the piezometric level causes the saltwater wedge to retreat toward the sea, resulting in a volume of the coastal aquifer being invaded by less saturated groundwater. On the other hand, these minerals may precipitate, especially in the saltwater zone, when the solution reaches the saturation state in small pores of the aquifer. Of particular interest are the dissolution processes of phases such as calcite and dolomite, which constitute the matrix of coastal carbonate aquifers. Causes of these dissolution processes are related to nonlinear mixing effects. Thus, mixtures of calcite-saturated freshwater and calcite-supersaturated seawater can be undersaturated with respect to calcite [13,14]. This may result in the dissolution of calcite and development of porosity in coastal carbonate aquifers [15]. (2) Dolomitization. The dolomitization process of a pre-existing carbonatic rock in mixing zones was proposed in the 1970s by several authors [16,17,18,19]. However, this theory was later questioned due to the absence of dolomitized bands in current mixing zones [20]. However, researchers have observed a transformation from aragonite to Mg-calcite [21] or recrystallization processes of pre-existing dolomite crystals [22]. More recently, the original model has been revisited, incorporating other factors such as the alkalinity, redox potential (Eh), or terminal electron acceptors (TEAs) as precursors to dolomitization processes [5]. (3) Microbial metabolism. The dynamics of flow and nutrients in the mixing zone of a coastal aquifer can host various biogeochemical reactions mediated by microorganisms, including denitrification and sulfate reduction [23,24,25,26]. It is evident that microbial-mediated reactions play an important role in the genesis of multiple discrete hydrochemical zones, characterized by the presence of Fe2+, Mn2+, Sr2+, and SO4-rich groundwater. It has been hypothesized that microbial activity enhanced ferromanganese precipitation [27,28,29]. (4) Redox transformations of metals. Coastal aquifers exhibit strong redox gradients along the groundwater flowpath, transitioning from reducing conditions in the deeper inland zones to increasingly oxidizing environments near the discharge area. These gradients are primarily driven by microbial activity, organic matter degradation, and mixing with oxygenated seawater [1,30]. As a result, these systems function as reactive geochemical fronts where redox-sensitive elements undergo significant transformations. In the inland anoxic regions, metals such as iron (Fe) and manganese (Mn) are typically found in their reduced soluble forms (Fe2+ and Mn2+). However, as groundwater moves towards the discharge zone, increasing redox potential promotes the oxidation of these metals, leading to their precipitation as iron (hydr)oxides and manganese oxides near the coastline [31,32,33,34]. During this process, various metals and anions can be co-precipitated through adsorption and structural substitution mechanisms, which influence the distribution of trace elements in these systems [35]. Elements such as As, Pb, Zn, Cu, and Co can be adsorbed onto these incipient phases and become trapped in the mineral matrix as solid growth progresses [36,37].
Barium is another ion than can be subordinately present in Fe and Mn oxides [38]. Specifically, in submarine polymetallic manganese oxide nodules, barium may be present in the more hydrated phases [39]. This barium may originate from the incorporation of dissolved barium ions from seawater or the sedimentary material in which the nodules develop. Sediments rich in biogenic barite can lead to the formation of diagenetic barite layers [40,41,42,43]. This diagenetic barite is widely distributed in both current and ancient marine and oceanic environments [42,44]. It has also been described in continental settings, where it is believed to have formed through the recycling of pre-existing barite ores [45].
This study examines the diagenetic processes involved in the mixing zones, a transition environment between land and the ocean. For this purpose, a dolomitic coastal aquifer (SE Spain) with current and ancient diagenetic features, including barite growths, has been studied. Three sections of this aquifer have been analyzed. Two of these sections correspond to cores drilled near the coastline, showing the ongoing diagenetic processes: one in a discharge area with thick mixing zone and the other affected by marine intrusion with a thin transition zone. The third section reveals the record of an ancient mixing zone Pleistocene in age, uplifted by tectonic activity, and currently perched on the cliffs.

2. Geological Setting

The Sierra de Gádor is a mountain massif composed of detrital and carbonatic materials from the Middle Triassic, belonging to the Alpujárride Complex of the Betic Cordillera (SE Spain). These materials have undergone regional metamorphism during the Alpine orogeny. The metasedimentary sequence that constitutes this mountain range, from base to top, consists of at least 600 m of phyllites, quartzites, and schists of Anisian age, and a thick sequence of up to 1500 m of limestones, dolomites, and calcareous schists ranging from Anisian to Ladinian age [46,47,48]. Overlying these materials, heterozoan carbonates from the Upper Tortonian were deposited in a marine platform environment in the higher parts of the mountain range [49,50] or as resedimented deposits on its margins [51,52].
South of the Sierra de Gádor, an extensive coastal plain extends, shaped by coastal marine sedimentation, which led to the formation of a Plio-Quaternary marine terrace now uplifted by tectonics [53]. This surface was subsequently partially buried by Pleistocene and Holocene alluvial deposits sourced from the erosional dismantling of the mountain range. The southeasternmost end of the Sierra directly connects to the Mediterranean Sea, forming a carbonate coastal aquifer that develops along a cliff-lined coastline. This aquifer extends between the cities of Aguadulce and Almería (Figure 1). The name Aguadulce (Spanish for ‘freshwater’) is derived from the historic groundwater discharges from the Sierra de Gádor along the coastline, which have now disappeared due to intensive exploitation [54]. Directly above this former discharge zone, a series of dissolution conduits of the Triassic dolomitic rock are observed in the cliff. This cliffed area and the diagenetic processes currently acting in its mixing zone, as well as those that have acted in the past, are the subject of this study.

3. Methods

A total of five cores were drilled across the study area. The purpose of these drillings are twofold: first, to determine whether the dissolution features continue at depth, and second, to intercept the current groundwater discharge zone. Core recovery was carried out in two of them. The recovery rate was very high, owing to the coherence of the dolomitic rock, but the recovery significantly decreased in the final core, where the dolomitic rock was intensely dissolved. The degree of rock dissolution was calculated based on a count of a digital photograph on a regular grid with 300 points. The seawater was not reached in one of them because the degree of dissolution and fragmentation of the rock did not allow progress in drilling due to problems of wall collapse. The cores were photographed, described, and archived. Rock samples were taken from the mineralized zones in both cores as well as from the cliff wall. These samples were analyzed by X-ray diffraction using a BRUKER D8 ADVANCE diffractometer by Bruker AXS GmbH (Karlsruhe, Germany) operating in Da Vinci geometry and equipped with an X-ray tube (Cu-Kα1 radiation: λ = 1.54 Å) and Thermo X Series II ICP-MS by Thermo Fisher Scientific (Bremen, Germany). A Nikon polarizing LV 100ND microscope with a DS-Fi2-U3 Nikon digital camera by Nikon Corporation (Nanjing, China) and Zeiss Sigma 300-VP SEM by Carl Zeiss AG (Oberkochen, Germany) were used to observe and analyze samples and for photomicroscopy. All of these techniques were performed at the Scientific Instrumentation Centre of the University of Almeria.
For this study, two samples in the form of crushed fragments were used for U/Th geochronology at SGS Isobar (Miami, FL, USA). High-precision uranium and thorium isotope analysis was performed on a Thermo Fischer Neptune PlusTM MC-ICP-MS by Thermo Fisher Scientific (Bremen, Germany). Homogeneous samples free from clay particles and other detritus were selected because they would be rich in calcite. These samples were ground in an agate mortar until they were reduced to a fine, homogeneous powder. Samples were digested and then screened for uranium and thorium.
The multi-level sampling technique was used to collect groundwater samples from the top to the bottom of the boreholes using a discrete interval sampler (Solinst Mod. 425 by Solinst Canada Ltd., Georgetown, ON, Canada). Temperature, electrical conductivity (EC), concentration of dissolved oxygen, pH, and Eh were determined in situ. Alkalinity (as HCO3) was determined by titration at the time of sampling. Samples were taken in duplicate, filtered using a 0.45 μm Millipore filter, and stored in polyethylene bottles at 4 °C. For cation analysis, one bottle of each sample was acidified to pH < 2 with environmental-grade (ultra-pure) nitric acid to avoid problems of absorption or precipitation. Sample composition of major anions and cations was determined using METROHM 930 Compact IC Flex by Metrohm AG (Herisau, Switzerland), and minor and trace elements were determined using Thermo Scientific iCAP TQ ICP-MS by Thermo Fisher Scientific (Bremen, Germany) (Scientific Instrumentation Centre, University of Almeria).
Assuming chloride is a conservative ion (Cl), the percentage of seawater in the samples was calculated using the mass balance, assuming conservative mixing of seawater (SW) and freshwater (FW) [1], described by the following formula:
Seawater (%) = [Clsample − ClFW]/[ClSW − ClFW] × 100
The saturation indices of minerals in groundwater were calculated using the USGS geochemical/thermodynamic modeling code PHREEQC version 3 [55].

4. Results

In order to determine the biogeochemical and diagenetic processes occurring in the carbonate coastal aquifer, two sectors were selected along the cliffed coastline of Sierra de Gádor: the Aguadulce site, known for being a discharge zone of groundwater from the Sierra de Gádor massif, and the Palmer site, a ravine that cuts through the cliffs, thus facilitating access for study (Figure 1). At the Aguadulce site, there are two observation points. The first one corresponds to a current thick mixing zone, while the second one is an ancient mixing zone on the cliff uplifted by tectonic activity.

4.1. Aguadulce Site

4.1.1. Sedimentary Record

The Aguadulce discharge zone was located, prior to being dewatered due to overexploitation, at the base of the cliff, discharging directly onto the shoreline and into the sea [54]. In the same area, just above this discharge zone, and attached to the middle of the dolomitic cliff (Figure 2a), there exists a level of heterometric, well-rounded, coarse-grained conglomerates with frequent Lithophaga borings (Figure 2b). This conglomerate, with a thickness of 4 m, is situated at 18 m above sea level (a.s.l.). The clasts, up to 60 cm in diameter, consist of the same dolomites that form the substrate of the cliff. Between the clasts, there is a little matrix, which consists of medium-coarse siliciclastic sand. Towards the west, this conglomerate transitions laterally into a thin layer, approximately 80 cm thick, predominantly composed of fossilized remains of oysters (Ostreidae) and a smaller amount of red algae (Figure 2c). Above the conglomerate and the oyster bed are matrix-supported breccia outcrops containing dolomitic clasts embedded in a red clay matrix (Figure 2d). The clast sizes are heterometric, reaching up to 1.5 m in diameter. Locally, fossilized remains of the pulmonate gastropod Iberus gualtieranus, an endemic species from the Sierra de Gádor, can be found within this matrix (Figure 2d).

4.1.2. Diagenetic Features

Beneath the conglomerate layer, the dolomitic substrate exhibits notable dissolution, which is distributed in a diffuse banding of meter-thick layers (Figure 3a). Within these bands, there are areas where the degree of dissolution is so high that it results in voids and conduits, some of which may exceed one meter in diameter (Figure 3b). The dissolution of the dolomitic rock generates, in many of these bands, polygonal voids, millimetric in height and centimeter-scale in length (Figure 3c). The degree of dissolution in the carbonate rock generally exceeds 60%. Covering the geometric surfaces of the dissolution cavities, millimetric manganese oxides have precipitated, giving the entire assemblage a blackish hue (Figure 3d). White calcite precipitates are observed covering these oxides, ranging in thickness from millimetric to centimeter-scale. Two samples of these carbonate precipitates were dated by U/Th geochronology in order to determine the age of their formation. The ages obtained were >500,000 y BP, the maximum limit that can be measured using this technique.
A borehole (AG) with continuous core recovery was drilled at the base where the dissolution features of the previously described dolomitic rock are no longer observed at the surface. The recovered core, measuring 47 m in length, does not exhibit significant dissolution features in the first 37 m (Figure 4a). However, beyond this depth, the dolomitic rock shows increased dissolution, with both dissolution surfaces and pre-existing fractures coated mainly by iron (hydr)oxides (Figure 4b).

4.1.3. Microscopy and Analysis of Precipitated Minerals

The observation of samples, both under optical and scanning electron microscopy (SEM), of the Mn oxides collected from the cliff and the Fe oxides and hydroxides present in the core, reveals that they consist of microcrystalline botryoidal growths. Due to their small crystallographic size, these phases cannot be identified by X-ray diffraction. On the Fe oxide precipitates recovered from the core, euhedral barite (BaSO4) crystals have grown. These crystals are with their 001 crystallographic axis randomly oriented with respect to their dissolution surface and reach sizes of up to 100 µm (Figure 4c–e). Locally, Fe oxide precipitates are observed on the crystal faces of the barite (Figure 4e,f).
The chemical analysis of the mineral precipitates of Fe and Mn oxides shows that, in addition to these metals, a significant number of other elements have been fixed in relatively abundant concentrations (Table 1). Particularly notable are the Pb concentrations recorded in the Mn oxide samples taken from the cliff. Pb values can reach up to 20% by weight in these samples. In contrast, the Pb content in the Fe oxide sample from the core is significantly lower, with values around 1.5% by weight. For Ba concentrations in all samples collected from the Aguadulce site, the obtained values are high, around 5%. Other metals, such as Zn, Cu, Ni, and Co, are also present in smaller amounts.

4.1.4. Hydrochemical Analysis

Groundwater samples were collected along the borehole drilled at the Aguadulce site. The depth of the piezometric head is located 1 m above sea level, with the base of the borehole at an elevation of −20 m a.s.l. The salinity record, expressed as the chloride ion concentration (Figure 5a), shows that up to −7 m, the water is fresh, with seawater percentages not exceeding 13%. From this point onwards, there is a rapid increase in groundwater salinity, reaching 63% seawater at an elevation of −11 m. Salinity continues to rise more gradually with depth until the base of the borehole, where seawater constitutes 80% of the sample.
The content of metal ions, in µg/L, from these groundwater samples is presented in Figure 5b. The distribution of these ions with depth does not follow the same trend as the Cl ion (Figure 5a). The concentration of Pb is fairly uniform, approximately 2 µg/L, as is the concentration of Ba, around 30 µg/L. In the case of Fe, Mn, and Zn ions, an anomaly is observed at −7 m a.s.l., resulting from an increase in the concentration of these ions at that depth.

4.2. Palmer Site

At the Palmer site, unlike the Aguadulce site, there is no preserved sedimentary record in the cliff. All evidence related to potential mixing processes has been studied using boreholes that intersect the current freshwater–saltwater interface. A total of four boreholes were drilled, aligned perpendicularly to the coastline along the Palmer ravine (Figure 1). Core recovery was achieved only in the borehole closest to the sea (PAL-1).

4.2.1. Diagenetic Features

The core recovered from the piezometer closest to the shoreline in this area has a total length of 34 m (Figure 6a). Manganese oxides have precipitated on the dissolution and fracture surfaces within this section (Figure 6b).

4.2.2. Microscopy and Analysis of Precipitated Minerals

As observed at the Aguadulce site, manganese oxide precipitates recovered in core PAL-1 exhibit botryoidal microcrystalline morphologies (Figure 6c,d). Chemical analysis of these Mn oxides indicates a substantial iron content, with weight percentages exceeding 15% (Table 1). Other elements identified in the precipitates, in decreasing order of abundance, include Zn, Ba, Ni, Ti, and Pb.

4.2.3. Hydrochemical Analysis

Hydrogeochemical characterization of the groundwater at the El Palmer site was conducted through sampling at various depths across four boreholes located in the area. From these data, vertical profiles of selected physicochemical parameters were constructed along a sea-to-inland transect (Figure 7). Salinity, represented by the chloride ion concentration, indicates that the freshwater lens thickens inland from the coast. The interface is a relatively thin subhorizontal zone, and the saline water is below a depth of approximately −10 m.
The redox potential (Eh) profile shows higher values in the shallow freshwater strip of the aquifer, while negative Eh values were recorded at the inland interface zone. Iron and manganese ion concentrations are lower in the freshwater domain and peak in the saline zone, especially in the inner sectors of the aquifer. These concentrations significantly exceed those measured in seawater samples (Figure 7).
Sulfate ion distribution follows a trend similar to that of chloride, distinguishing a freshwater zone with low sulfate levels, an intermediate interface, and a saline zone with concentrations comparable to seawater. Conversely, barium exhibits an inverse pattern, with maximum concentrations in the freshwater and interface zones, and minimum values in the saline water. Lead concentrations peak near the coastline, with values similar to seawater, and gradually decrease inland. Zinc shows the highest concentrations in the saline zone and lowest in the freshwater; in the deeper, inland parts of the aquifer, Zn concentrations can exceed 25 ppb, compared to less than 5 ppb in seawater.

5. Discussion

5.1. Interpretation of the Sedimentary Record and Diagenetic Features

The heterometric, rounded conglomerate layer with Lithophaga borings identified at the Aguadulce site was likely deposited at the base of a paleo-cliff, representing the approximate sea level at the time of deposition. Regional neotectonic activity has since uplifted this unit to its current position at approximately 40 m above sea level. Laterally, this conglomeratic layer transitions into a lag deposit composed of oyster shells and red algae, interpreted as a shoreface facies developed distally from the cliff base. Overlying these units are heterometric breccias with a red clay matrix, indicative of alluvial fan deposits (Figure 2). Conglomeratic deposits interpreted as marine terraces extend across the coastal plain west of Aguadulce [56,57]. These terraces reach elevations of up to 100 m a.s.l. as a result of Quaternary tectonic uplift [58,59].
Beneath this sedimentary succession, the dolomitic rock forming the paleo-cliff substrate shows significant dissolution. Most dissolution cavities, typically polygonal in shape, are arranged in meter-thick bands. Similar dissolution features observed in ancient marine carbonate platforms, formerly acting as paleo-cliffs, have been interpreted as resulting from freshwater–seawater mixing processes [13,60,61,62,63]. In contrast to the rounded dissolution morphologies typically seen in limestones, the cavities in the Aguadulce paleo-cliff exhibit polyhedral geometries, attributed to the dissolution behavior of dolomitic rocks. In both lithologies, Fe and Mn oxide precipitates are commonly found coating the dissolution surfaces [13,61].
The precipitation of such oxides has also been documented in modern coastal aquifer settings [6,64,65]. Iron oxide formation is often attributed to purely chemical processes driven by the mixing of oxygen-rich seawater with groundwater [66,67,68,69]. Iron oxidation mechanisms in groundwater can be complex and are often dynamic, particularly in freshwater environments [70]. Nonetheless, microbial activity can play a significant role in promoting oxide precipitation under certain conditions [24,28].
In the vertical profile from borehole AG (Figure 5b), Fe concentrations in groundwater decline progressively from the water table down to −5 m, followed by a local increase down to −7 m, and a further marked increase below −14 m, reaching peak concentrations at the piezometer base. Fe oxide precipitates observed in the core from this borehole are located between −18 and −21 m, a depth where groundwater salinity reaches approximately 75–80% of seawater (Figure 5a). In contrast, Mn oxides in the PAL-1 core were found at depths between −5 and −9 m, where salinity corresponds to 99% seawater. At this depth, Mn concentrations in groundwater are about 50 ppb, similar to seawater, while values of up to 250 ppb are recorded in the more inland, reducing zones of the aquifer (Figure 7). This indicates that Mn comes from the dissolution of the Sierra de Gador carbonate massif, as does Ba. This massif contains numerous Pb and Zn (F, Ba) mineralizations that have been mined until recently [71].
Chemical analyses of the Fe and Mn oxides reveal that, in addition to these metals, other trace elements such as Zn, Ba, Ni, Ti, and Pb are co-precipitated. Ref. [29] shows that marine Fe and Mn oxide precipitates act as traps for the fixation of other metals. In the Aguadulce and Palmer deposits, the concentrations of some of these metals are significant, with values reaching up to 20 wt% Pb and 6 wt% Ba in some samples (Table 1). Coastal aquifers are recognized as a major source of trace elements and nutrients to the marine environment, surpassing riverine inputs in some cases [72,73,74,75]. However, the precipitation of certain oxides within coastal aquifers could act as a geochemical barrier, effectively retaining and accumulating dissolved species before they reach the ocean [24,73].

5.2. Diagenetic Barite Formation Model

Barite (BaSO4) in marine environments forms in mixing zones between two fluids—one enriched in barium and the other in sulfate—resulting in supersaturation with respect to BaSO4. It is one of the few minerals that can form authigenically within the water column, in sediments, and at hydrothermal vents [42]. This mineral has also been shown to precipitate within aggregates of organic matter and extracellular polymeric substances, which is why it is commonly used as a proxy for paleoproductivity [76]. Once deposited on the seafloor, the degradation of organic matter creates microenvironments enriched in Ba [77,78,79,80,81]. The dissolution of biogenic barite in these settings leads to barium enrichment in pore waters. This barium diffuses through the sediment and, upon interacting with sulfate-rich fluids, precipitates again as diagenetic barite [82].
River and estuarine waters are typically more enriched in Ba than seawater which, in contrast, has high sulfate concentrations. Coastal aquifers serve as transition zones between continental and marine waters, where Ba-rich groundwater meets sulfate-rich seawater. Saturation index analyses for barite along boreholes AG and PAL-1 indicate that barite is near saturation throughout the freshwater, mixing zone, and saline intervals (Table 2). However, barite was not detected in any section of the PAL-1 core and was only observed between −18 and −21 m a.s.l. in AG core. This depth corresponds to the interval where iron precipitates were also found, with barite crystals nucleating on them. The local saturation of BaSO4 at this depth appears to be linked to the high Ba concentrations associated with iron oxides, a process analogous to that observed in deep-sea sediments enriched in biogenic barite. On the ocean floor, partial dissolution of biogenic barite followed by reprecipitation can lead to the neoformation of barite crystals. In marine sediments, such recrystallization from aggregates of sub-micrometer grains of biogenic barite may result in well-formed euhedral crystals up to 5 mm in size. The crystals analyzed in this study also exhibit euhedral morphologies, reaching up to 100 µm in diameter.
Therefore, the proposed model for diagenetic barite formation in coastal carbonate aquifers involves an initial phase of Fe and/or Mn (oxyhydr)oxide precipitation in the more oxygenated zone of the mixing interface. These precipitates act as geochemical barriers that, besides concentrating Fe and Mn, also accumulate other ions and metals, most notably Ba. High localized Ba concentrations within oxides lining fractures and dissolution cavities in the dolomitic rock interact with sulfate-rich seawater to facilitate the crystallization of barite (Figure 8).
A similar model, albeit without the prerequisite of Fe/Mn oxide precipitation, has been proposed to explain barite mineralization in carbonate formations of northeastern British Columbia [83]. In that scenario, Ba-rich continental groundwater interacting with SO42−-rich marine water infiltrating the coastal aquifer results in barite precipitation at the mixing zone.
The Fe and Mn oxide precipitates identified in the studied coastal aquifer not only exhibit elevated Ba contents but also anomalously high concentrations of Pb, Zn, Ni, and other metals. For instance, Pb concentrations reach up to 20 wt% in these precipitates (Table 1).

6. Conclusions

The study of modern and Pleistocene mixing zones between freshwater and marine groundwater in a coastal dolomitic aquifer in southeastern Spain reveals common features across all settings: intense dissolution of dolomitic rock, accompanied by the precipitation of iron and/or manganese (oxyhydr)oxides on dissolution surfaces. Manganese oxides are primarily found within the saline water zone, just below the freshwater–seawater interface. In contrast, iron oxides form within the mixing zone, where salinity reaches approximately 75–80% of typical seawater salinity.
Hydrochemical profiles measured along a cross-section perpendicular to the coastline indicate that Fe and Mn are present in soluble form towards the most reductive area inland of the aquifer. These ions subsequently precipitate, likely mediated by microbial activity, as oxides in the more oxidizing conditions closer to the coastline. These metal oxides also co-precipitate with significant concentrations of other trace metals such as Ba, Pb, and Zn, with concentrations in some cases reaching up to 20 wt% in the oxide phases. The interaction of this barium with marine sulfate has led to the formation of abundant, well-developed euhedral barite crystals up to 100 µm in diameter.
This newly proposed model of diagenetic barite formation may represent an early stage in the development of economic barite mineral deposits. Moreover, under favorable conditions involving the circulation of reducing fluids, Pb and Zn co-precipitated within the iron/manganese oxides could potentially be remobilized to form sulfide mineralizations such as galena and/or sphalerite within the dolomitic matrix. This process may ultimately lead to the formation of Mississippi Valley-type ore deposits.

Author Contributions

Conceptualization, F.S.; Methodology, Á.V.; Software, M.M.; Validation, M.M.; Investigation, F.S., M.M. and Á.V.; Writing—original draft, Á.V.; Writing—review & editing, F.S. and Á.V. All authors have read and agreed to the published version of the manuscript.

Funding

This research is part of the general research lines promoted by the CEIMAR Campus of International Excellence and was supported by grant PID2023-148816OB-I00 from Ministry of Science, Innovation and Universities (MICIU).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Appelo, C.A.J.; Postma, D. Geochemistry, Groundwater and Pollution; CRC Press: Boca Raton, FL, USA, 2005. [Google Scholar]
  2. McAllister, S.M.; Barnett, J.M.; Heiss, J.W.; Findlay, A.J.; MacDonald, D.J.; Dow, C.L.; Luther, G.W.; Michael, H.A.; Chan, C.S. Dynamic hydrologic and biogeochemical processes drive microbially enhanced iron and sulfur cycling within the intertidal mixing zone of a beach aquifer. Limnol. Oceanogr. 2015, 60, 329–345. [Google Scholar] [CrossRef]
  3. Heiss, J.W.; Post, V.E.; Laattoe, T.; Russoniello, C.J.; Michael, H.A. Physical controls on biogeochemical processes in intertidal zones of beach aquifers. Water Resour. Res. 2017, 53, 9225–9244. [Google Scholar] [CrossRef]
  4. Moore, W.S.; Joye, S.B. Saltwater intrusion and submarine groundwater discharge: Acceleration of biogeochemical reactions in changing coastal aquifers. Front. Earth Sci. 2021, 9, 600710. [Google Scholar] [CrossRef]
  5. Petrash, D.A.; Bialik, O.M.; Staudigel, P.T.; Konhauser, K.O.; Budd, D.A. Biogeochemical reappraisal of the freshwater–seawater mixing-zone diagenetic model. Sedimentology 2021, 68, 1797–1830. [Google Scholar] [CrossRef]
  6. Goyetche, T.; Luquot, L.; Carrera, J.; Martínez-Pérez, L.; Folch, A. Identification and quantification of chemical reactions in a coastal aquifer to assess submarine groundwater discharge composition. Sci. Total. Environ. 2022, 838, 155978. [Google Scholar] [CrossRef]
  7. Abarca, E.; Clement, T.P. A novel approach for characterizing the mixing zone of a saltwater wedge. Geophys. Res. Lett. 2009, 36, L06402. [Google Scholar] [CrossRef]
  8. Chang, S.W.; Clement, T.P. Experimental and numerical investigation of saltwater intrusion dynamics in flux-controlled groundwater systems. Water Resour. Res. 2012, 48, W09527. [Google Scholar] [CrossRef]
  9. Lu, C.; Chen, Y.; Zhang, C.; Luo, J. Steady-state freshwater–seawater mixing zone in stratified coastal aquifers. J. Hydrol. 2013, 505, 24–34. [Google Scholar] [CrossRef]
  10. Ataie-Ashtiani, B.; Volker, R.E.; Lockington, D.A. Tidal effects on sea water intrusion in unconfined aquifers. J. Hydrol. 1999, 216, 17–31. [Google Scholar] [CrossRef]
  11. Cartwright, N.; Li, L.; Nielsen, P. Response of the salt–freshwater interface in a coastal aquifer to a wave-induced groundwater pulse: Field observations and modelling. Adv. Water Resour. 2004, 27, 297–303. [Google Scholar] [CrossRef]
  12. Vallejos, A.; Sola, F.; Pulido-Bosch, A. Processes Influencing groundwater level and the freshwater-saltwater Interface in a coastal aquifer. Water Resour. Manag. 2015, 29, 679–697. [Google Scholar] [CrossRef]
  13. Smart, P.L.; Dawans, J.M.; Whitaker, F.F. Carbonate dissolution in a modern mixing zone, South Andros, Bahamas. Nature 1988, 335, 811–813. [Google Scholar] [CrossRef]
  14. Rezaei, M.; Sanz, E.; Raeisi, E.; Ayora, C.; Vázquez-Suñé, E.; Carrera, J. Reactive transport modeling of calcite dissolution in the fresh-salt water mixing zone. J. Hydrol. 2005, 311, 282–298. [Google Scholar] [CrossRef]
  15. Sanford, W.E.; Konikow, L.F. Simulation of Calcite Dissolution and Porosity Changes in Saltwater Mixing Zones in Coastal Aquifers. Water Resour. Res. 1989, 25, 655–667. [Google Scholar] [CrossRef]
  16. Hanshaw, B.B.; Back, W.; Deike, R.G. A geochemical hypothesis for dolomitization by ground water. Econ. Geol. 1971, 66, 710–724. [Google Scholar] [CrossRef]
  17. Badiozamani, K. The Dorag Dolomitization Model, Application to the Middle Ordovician of Wisconsin. J. Sediment. Res. 1973, 43, 965–984. [Google Scholar] [CrossRef]
  18. Land, L.S.; Salem, M.R.I.; Morrow, D.W. Paleo-hydrology of ancient dolomites: Geochemical evidence. AAPG Bull. 1975, 59, 1602–1625. [Google Scholar]
  19. Magaritz, M.; Goldenberg, L.; Kafri, U.; Arad, A. Dolomite formation in the seawater–freshwater interface. Nature 1980, 287, 622–624. [Google Scholar] [CrossRef]
  20. Machel, H.-G.; Mountjoy, E.W. Chemistry andenvironments of dolomitization—A reappraisal. Earth-Sci. Rev. 1986, 23, 175–222. [Google Scholar] [CrossRef]
  21. Melim, L.; Swart, P.K.; Eberli, G.P. Mixing-Zone Diagenesis in the Subsurface of Florida and the Bahamas. J. Sediment. Res. 2004, 74, 904–913. [Google Scholar] [CrossRef]
  22. Gaswirth, S.B.; Budd, D.A.; Farmer, G.L. The role and impact of freshwater–seawater mixing zones in the maturation of regional dolomite bodies within the proto Floridan Aquifer, USA. Sedimentology 2007, 54, 1065–1092. [Google Scholar] [CrossRef]
  23. Spiteri, C.; Slomp, C.P.; Charette, M.A.; Tuncay, K.; Meile, C. Flow and nutrient dynamics in a subterranean estuary (Waquoit Bay, MA, USA): Field data and reactive transport modeling. Geochim. Cosmochim. Acta 2008, 72, 3398–3412. [Google Scholar] [CrossRef]
  24. Ruiz-González, C.; Rodellas, V.; Garcia-Orellana, J. The microbial dimension of submarine groundwater discharge: Current challenges and future directions. FEMS Microbiol. Rev. 2021, 45, fuab010. [Google Scholar] [CrossRef]
  25. Seibert, S.L.; Massmann, G.; Meyer, R.; Post, V.E.A.; Greskowiak, J. Impact of mineral reactions and surface complexation on the transport of dissolved species in a subterranean estuary: Application of a comprehensive reactive transport modeling approach. Adv. Water Resour. 2024, 191, 104763. [Google Scholar] [CrossRef]
  26. Seibert, S.L.; Massmann, G.; Meyer, R.; Post, V.E.A.; Greskowiak, J. Reactive transport modeling to reveal the impacts of beach morphodynamics, storm floods and seasonal groundwater recharge on the biogeochemistry of sandy subterranean estuaries. Adv. Water Resour. 2025, 196, 104884. [Google Scholar] [CrossRef]
  27. Penny, E.; Lee, M.-K.; Morton, C. Groundwater and microbial processes of Alabama coastal plain aquifers. Water Resour. Res. 2003, 39, 1320. [Google Scholar] [CrossRef]
  28. Vallejos, A.; Sola, F.; Vargas-García, M.C.; Mancuso, M. Microbial-induced MnO2 precipitation in a carbonate coastal aquifer. Sci. Total. Environ. 2024, 915, 169968. [Google Scholar] [CrossRef]
  29. Majamäki, R.; Wasiljeff, J.; Purkamo, L.; Hultman, J.; Asmala, E.; Yli-Hemminki, P.; Jørgensen, K.S.; Koho, K.; Kuva, J.; Virtasalo, J.J. Microbially Enhanced Growth and Metal Capture by Ferromanganese Concretions in a Laboratory Experiment. Geobiology 2025, 23, e70010. [Google Scholar] [CrossRef]
  30. Chapelle, F. Geochemistry of Groundwater. In Treatise on Geochemistry; Elsevier: Amsterdam, The Netherlands, 2003; Volume 5, pp. 425–449. [Google Scholar]
  31. Stumm, W.; Morgan, J.J. Aquatic Chemistry, Chemical Equilibria and Rates in Natural Waters, 3rd ed.; John Wiley & Sons, Inc.: New York, NY, USA, 1996. [Google Scholar]
  32. Hansel, C.; Benner, S.; Neiss, J.; Dohnalkova, A.; Kukkadapu, R.; Fendorf, S. Secondary Mineralization Pathways Induced by Dissimilatory Iron Reduction of Ferrihydrite under Advective Flow. Geochim. Cosmochim. Acta 2003, 67, 2977–2992. [Google Scholar] [CrossRef]
  33. Wehrmann, L.M.; Riedinger, N.; Brunner, B.; Kamyshny, A., Jr.; Herbert, L.C.; Brüchert, V.; Jørgensen, B.B.; Ferdelman, T.G.; Formolo, M.J. Iron-controlled oxidative sulfur cycling recorded in the distribution and isotopic composition of sulfur species in glacially influenced fjord sediments of west Svalbard. Chem. Geol. 2017, 466, 678–695. [Google Scholar] [CrossRef]
  34. Zhao, Z.; Zhang, C.; Cao, W.; Hofmann, H.; Wang, T.; Li, L. Oxidative precipitation of Fe (II) in porous media: Laboratory experiment and numerical simulation. ACS ES&T Water 2023, 3, 963–973. [Google Scholar] [CrossRef]
  35. Senn, A.-C.; Hug, S.J.; Kaegi, R.; Hering, J.G.; Voegelin, A. Arsenate co-precipitation with Fe(II) oxidation products and retention or release during precipitate aging. Water Res. 2018, 131, 334–345. [Google Scholar] [CrossRef]
  36. Fernex, F.; Février, G.; Bénaïm, J.; Arnoux, A. Copper, lead and zinc trapping in Mediterranean deep-sea sediments: Probable coprecipitation with Mn and Fe. Chem. Geol. 1992, 98, 293–306. [Google Scholar] [CrossRef]
  37. Tebo, B.M.; Bargar, J.R.; Clement, B.G.; Dick, G.J.; Murray, K.J.; Parker, D.; Verity, R.; Webb, S.M. Biogenic manganese oxides: Properties and mechanisms of formation. Annu. Rev. Earth Planet. Sci. 2004, 32, 287–328. [Google Scholar] [CrossRef]
  38. Sugiyama, M.; Hori, T. Geochemical behavior of barium in the vicinity of a MnO2/Mn2+ redox front formed in a eutrophic lake. Jpn. J. Limnol. 1994, 55, 27–37. [Google Scholar] [CrossRef]
  39. Ren, Y.; Guan, Y.; Sun, X.; Xu, L.; Xiao, Z.; Deng, Y.; He, W. Nano-mineralogy and growth environment of Fe-Mn polymetallic crusts and nodules from the South China Sea. Front. Mar. Sci. 2023, 10, 1141926. [Google Scholar] [CrossRef]
  40. Torres, M.E.; Brumsack, H.J.; Bohrmann, G.; Emeis, K.C. Barite fronts in continental margin sediments: A new look at barium remobilization in the zone of sulfate reduction and formation of heavy barites in diagenetic fronts. Chem. Geol. 1996, 127, 125–139. [Google Scholar] [CrossRef]
  41. Riedinger, N.; Kasten, S.; Gröger, J.; Franke, C.; Pfeifer, K. Active and buried authigenic barite fronts in sediments from the Eastern Cape Basin. Earth Planet. Sci. Lett. 2006, 24, 876–887. [Google Scholar] [CrossRef]
  42. Griffith, E.M.; Paytan, A. Barite in the ocean–occurrence, geochemistry and palaeoceanographic applications. Sedimentology 2012, 59, 1817–1835. [Google Scholar] [CrossRef]
  43. Zhou, X.; Chen, D.; Dong, S.; Zhang, Y.; Guo, Z.; Wei, H.; Yu, H. Diagenetic barite deposits in the Yurtus Formation in Tarim Basin, NW China: Implications for barium and sulfur cycling in the earliest Cambrian. Precambrian Res. 2015, 263, 79–87. [Google Scholar] [CrossRef]
  44. Jewell, P. Bedded barite in the geologic record. SEPM Spec. Public 2000, 66, 147–161. [Google Scholar]
  45. Laurent, D.; Lopez, M.; Combes, P.J.; Guerrot, C.; Spangenberg, J.E.; Gaucher, E.C. Synsedimentary to early diage-netic rejuvenation of barite-sulfides ore deposits: Example of the Triassic intrakarstic mineralization in the Lodève basin (France). Mar. Pet. Geol. 2020, 119, 104464. [Google Scholar] [CrossRef]
  46. Baena, J.; Voermans, F. Mapa Geológico de España, 1:50.000. Plan MAGNA. Hoja 1044 (Alhama de Almería); IGME: Madrid, Spain, 1983. [Google Scholar]
  47. Martin-Rojas, I.; Somma, R.; Delgado, F.; Estévez, A.; Iannace, A.; Zamparelli, V. The Triassic platform of the Gador-Turon unit (Alpujarride complex, Betic Cordillera, southeast Spain): Climate versus tectonic factors controlling platform architecture. Facies 2012, 58, 297–323. [Google Scholar] [CrossRef]
  48. Ros-Franch, S.; Sola, F.; Braga, J.C.; Márquez-Aliaga, A. Bivalve shell beds in the Triassic of the Sierra De Gádor (Al-pujárride complex, Betic Cordillera, SE Spain). J. Iber. Geol. 2025, 1–21. [Google Scholar] [CrossRef]
  49. Sælen, G.; Braga, J.C.; Sola, F. Oyster shells, bulk carbonate sediment, and meteoric calcite cement as recorders of oceanic and radiogenic 87Sr/86Sr in mixed heterozoan carbonates and terrigenous sediment. J. Sediment. Res. 2022, 92, 50–65. [Google Scholar] [CrossRef]
  50. Sola, F.; Braga, J.C.; Sælen, G. Contradictory coeval vertical facies changes in upper Miocene heterozoan carbonate–terrigenous deposits (Sierra de Gádor, Almería, SE Spain). J. Sediment. Res. 2022, 92, 257–274. [Google Scholar] [CrossRef]
  51. Sola, F.; Puga-Bernabéu, Á.; Aguirre, J.; Braga, J.C. Heterozoan carbonate deposition on a steep basement escarpment (Late Miocene, Almería, south-east Spain). Sedimentology 2017, 64, 1107–1131. [Google Scholar] [CrossRef]
  52. Sola, F.; Puga-Bernabéu, Á.; Aguirre, J.; Braga, J.C. Origin, evolution and sedimentary processes associated with a late Miocene submarine landslide, southeast Spain. Sediment. Geol. 2018, 364, 351–366. [Google Scholar] [CrossRef]
  53. Zazo, C.; Goy, J.L.; Dabrio, C.J.; Lario, J.; González-Delgado, J.A.; Bardají, T.; Hillaire-Marcel, C.; Cabero, A.; Ghaleb, B.; Borja, F.; et al. Retracing the Quaternary history of sea-level changes in the Spanish Mediterranean–Atlantic coasts: Geomorphological and sedimentological approach. Geomorphology 2013, 196, 36–49. [Google Scholar] [CrossRef]
  54. Díaz-Puga, M.A.; Vallejos, A.; Daniele, L.; Sola, F.; Rodríguez-Delgado, D.; Molina, L.; Pulido-Bosch, A. An oceanographic survey for the detection of a possible submarine groundwater discharge in the coastal zone of Campo de Dalias, SE Spain. In Advances in the Research of Aquatic Environment; Springer: Berlin/Heidelberg, Germany, 2011; Volume 1, pp. 417–424. [Google Scholar]
  55. Parkhurst, D.L.; Appelo, C.A.J. Description of input and examples for PHREEQC version 3—A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. In Modeling Techniques; U.S. Geological Survey Techniques and Methods: Denver, CO, USA, 2013; Chapter A43; 497p. [Google Scholar]
  56. Goy, J.L.; Zazo, C. Synthesis of the quaternary in the Almeria littoral neotectonic activity and its morphologic features, Western Betics, Spain. Tectonophysics 1986, 130, 259–270. [Google Scholar] [CrossRef]
  57. Zazo, C.; Goy, J.L.; Dabrio, C.J.; Bardají, T.; Hillaire-Marcel, C.; Ghaleb, B.; González-Delgado, J.A.; Soler, V. Pleistocene raised marine terraces of the Spanish Mediterranean and Atlantic coasts: Records of coastal uplift, sea-level highstands and climate changes. Mar. Geol. 2003, 194, 103–133. [Google Scholar] [CrossRef]
  58. Marín-Lechado, C.; Galindo-Zaldívar, J.; Rodríguez-Fernández, L.R.; Serrano, I.; Pedrera, A. Active faults, seismicity and stresses in an internal boundary of a tectonic arc (Campo de Dalías and Níjar, southeastern Betic Cordilleras, Spain). Tectonophysics 2005, 396, 81–96. [Google Scholar] [CrossRef]
  59. Pedrera, A.; Marín-Lechado, C.; Stich, D.; Ruiz-Constán, A.; Galindo-Zaldívar, J.; Rey-Moral, C.; de Lis Mancilla, F. Nucleation, linkage and active propagation of a segmented Quaternary normal-dextral fault: The Loma del Viento fault (Campo de Dalías, Eastern Betic Cordillera, SE Spain). Tectonophysics 2012, 522, 208–217. [Google Scholar] [CrossRef]
  60. Back, W.; Hanshaw, B.B.; Herman, J.S.; Van Driel, J.N. Differential dissolution of a Pleistocene reef in the ground-water mixing zone of coastal Yucatan, Mexico. Geology 1986, 14, 137–140. [Google Scholar] [CrossRef]
  61. Baceta, J.; Wright, V.; Pujalte, V. Palaeo-mixing zone karst features from Palaeocene carbonates of north Spain: Criteria for recognizing a potentially widespread but rarely documented diagenetic system. Sediment. Geol. 2001, 139, 205–216. [Google Scholar] [CrossRef]
  62. Calner, M.; Lehnert, O.; Nõlvak, J. Palaeokarst evidence for widespread regression and subaerial exposure in the middle Katian (Upper Ordovician) of Baltoscandia: Significance for global climate. Palaeogeogr. Palaeoclim. Palaeoecol. 2010, 296, 235–247. [Google Scholar] [CrossRef]
  63. Li, Z.; Goldstein, R.H.; Franseen, E.K. Meteoric calcite cementation: Diagenetic response to relative fall in sea-level and effect on porosity and permeability, Las Negras area, southeastern Spain. Sediment. Geol. 2017, 348, 1–18. [Google Scholar] [CrossRef]
  64. Charette, M.A.; Sholkovitz, E.R. Oxidative precipitation of groundwater-derived ferrous iron in the subterranean estuary of a coastal bay. Geophys. Res. Lett. 2002, 29, 85-1–85-4. [Google Scholar] [CrossRef]
  65. Cao, W.; Hofmann, H.; Yan, G.; Scheuermann, A. Porewater exchange and iron transformation in a coastal groundwater system: A field investigation, driving mechanisms analysis, and conceptual model. Front. Mar. Sci. 2024, 11, 1385517. [Google Scholar] [CrossRef]
  66. Charette, M.A.; Sholkovitz, E.R.; Hansel, C.M. Trace element cycling in a subterranean estuary: Part 1. Geochemistry of the permeable sediments. Geochim. Cosmochim. Acta 2005, 69, 2095–2109. [Google Scholar] [CrossRef]
  67. Charette, M.A.; Sholkovitz, E.R. Trace element cycling in a subterranean estuary: Part 2. Geochemistry of the pore water. Geochim. Cosmochim. Acta 2006, 70, 811–826. [Google Scholar] [CrossRef]
  68. Cao, W.; Yan, G.; Hofmann, H.; Scheuermann, A. State of the Art on Fe Precipitation in Porous Media: Hydrogeochemical Processes and Evolving Parameters. J. Mar. Sci. Eng. 2024, 12, 690. [Google Scholar] [CrossRef]
  69. Cao, W.; Hofmann, H.; Scheuermann, A. Iron Curtain Formation in Coastal Aquifers: Insights from Darcy-Scale Experiments and Reactive Transport Modelling. J. Mar. Sci. Eng. 2025, 13, 1909. [Google Scholar] [CrossRef]
  70. Testa, J.M.; Charette, M.A.; Sholkovitz, E.R.; Allen, M.C.; Rago, A.; Herbold, C.W. Dissolved iron cycling in the subterranean estuary of a coastal bay: Waquoit Bay, Massachusetts. Biol. Bull. 2002, 203, 255–256. [Google Scholar] [CrossRef]
  71. Díaz-Puga, M.A.; Pulido-Bosch, A.; Vallejos, A.; Sola, F.; Daniele, L.; Simón, M.; García, I. Impact of mine leachates on a carbonate aquifer (SE Spain). Mine Water Environ. 2021, 40, 225–234. [Google Scholar] [CrossRef]
  72. Burnett, W.C.; Bokuniewicz, H.; Huettel, M.; Moore, W.S.; Taniguchi, M. Groundwater and pore water inputs to the coastal zone. Biogeochemistry 2003, 66, 3–33. [Google Scholar] [CrossRef]
  73. Moore, W.S. The effect of submarine groundwater discharge on the ocean. Annu. Rev. Mar. Sci. 2010, 2, 59–88. [Google Scholar] [CrossRef]
  74. Sawyer, A.H.; Michael, H.A.; Schroth, A.W. From soil to sea: The role of groundwater in coastal critical zone processes. WIREs Water 2016, 3, 706–726. [Google Scholar] [CrossRef]
  75. Santos, I.R.; Chen, X.; Lecher, A.L.; Sawyer, A.H.; Moosdorf, N.; Rodellas, V.; Tamborski, J.; Cho, H.-M.; Dimova, N.; Sugimoto, R.; et al. Submarine groundwater discharge impacts on coastal nutrient biogeochemistry. Nat. Rev. Earth Environ. 2021, 2, 307–323. [Google Scholar] [CrossRef]
  76. Martinez-Ruiz, F.; Paytan, A.; Gonzalez-Muñoz, M.T.; Jroundi, F.; Abad, M.D.M.; Lam, P.J.; Bishop, J.K.B.; Horner, T.J.; Morton, P.L.; Kastner, M. Barite formation in the ocean: Origin of amorphous and crystalline precipitates. Chem. Geol. 2019, 511, 441–451. [Google Scholar] [CrossRef]
  77. Goldberg, E.D.; Arrhenius, G. Chemistry of pelagic sediments. Geochim. Cosmochim. Acta 1958, 13, 153–212. [Google Scholar] [CrossRef]
  78. Bishop, J.K.B. The barite-opal-organic carbon association in oceanic particulate matter. Nature 1988, 332, 341–343. [Google Scholar] [CrossRef]
  79. Bernstein, R.E.; Byrne, R.H.; Betzer, P.R.; Greco, A.M. Morphologies and transformations of celestite in seawater: The role of acantharians in the strontium and barium geochemistry. Geochim. Cosmochim. Acta 1992, 56, 3273–3279. [Google Scholar] [CrossRef]
  80. Bernstein, R.E.; Byrne, R.H.; Schijf, J. Acantharians: A missing link in the oceanic biogeochemistry of barium. Deep-Sea Res. I 1998, 45, 491–505. [Google Scholar] [CrossRef]
  81. Ganeshram, R.S.; François, R.; Commeau, J.; Brown-Leger, S.L. An experimental investigation of barite formation in seawater. Geochim. Cosmochim. Acta 2003, 67, 2599–2605. [Google Scholar] [CrossRef]
  82. Breheret, J.-G.; Brumsack, H.-J. Barite concretions as evidence of pauses in sedimentation in the Marnes Bleues Formation of Vocontian Basin (SE France). Sediment. Geol. 2000, 130, 205–228. [Google Scholar] [CrossRef]
  83. Morrow, D.W.; Krouse, H.R.; Ghent, E.D.; Taylor, G.C.; Dawson, K.R. A hypothesis concerning the origin of barite in Devonian carbonate rocks of northeastern British Columbia. Can. J. Earth Sci. 1978, 15, 1391–1406. [Google Scholar] [CrossRef]
Jmse 13 02090 g001
Figure 1. Location of the study area on a simplified geological map (1: Triassic dolostone, 2: Neogene deposits, 3: Quaternary alluvial). The two monitoring sites, Aguadulce and Palmer, are indicated. The locations of the drilled boreholes (AG, PAL-1, PAL-2, PAL-3, and PAL-4) are shown in the photographs below.
Figure 1. Location of the study area on a simplified geological map (1: Triassic dolostone, 2: Neogene deposits, 3: Quaternary alluvial). The two monitoring sites, Aguadulce and Palmer, are indicated. The locations of the drilled boreholes (AG, PAL-1, PAL-2, PAL-3, and PAL-4) are shown in the photographs below.
Jmse 13 02090 g001
Jmse 13 02090 g002
Figure 2. Aguadulce site. (a) Panoramic view of the cliff excavated in Triassic dolostone bedrock. (b) Close-up of the conglomerate perched on the cliff where Lithophaga borings are visible. (c) Accumulation of fossil remains, mainly composed of bivalves and red algae. (d) Dolomitic breccia supported by a reddish clay matrix. Gastropod fragments are locally present within the matrix.
Figure 2. Aguadulce site. (a) Panoramic view of the cliff excavated in Triassic dolostone bedrock. (b) Close-up of the conglomerate perched on the cliff where Lithophaga borings are visible. (c) Accumulation of fossil remains, mainly composed of bivalves and red algae. (d) Dolomitic breccia supported by a reddish clay matrix. Gastropod fragments are locally present within the matrix.
Jmse 13 02090 g002
Jmse 13 02090 g003
Figure 3. Aguadulce site. (a) Panoramic view of the cliff excavated in Triassic dolostone bedrock. (b) Meter-scale dissolution cavities in the dolostone. (c) Centimeter-scale dissolution features with polygonal morphologies. (d) Mn oxides with botryoidal growth forms precipitated on the dissolution surface of the dolostone.
Figure 3. Aguadulce site. (a) Panoramic view of the cliff excavated in Triassic dolostone bedrock. (b) Meter-scale dissolution cavities in the dolostone. (c) Centimeter-scale dissolution features with polygonal morphologies. (d) Mn oxides with botryoidal growth forms precipitated on the dissolution surface of the dolostone.
Jmse 13 02090 g003
Jmse 13 02090 g004
Figure 4. Aguadulce site. (a) Synthetic lithostratigraphic column showing the position of Fe oxide precipitates. Adjacent to the column, the water table and bodies of fresh groundwater (FW) and mixed water (Mx) are indicated. (b) Details of core recovered from the zone with Fe oxide mineralization on the dolostone dissolution surface. (c,d) SEM images of barite crystals growing on Fe oxides in the mineralized zone with compositional analyses. (e,f) SEM image of a euhedral barite crystal and its compositional spectrum.
Figure 4. Aguadulce site. (a) Synthetic lithostratigraphic column showing the position of Fe oxide precipitates. Adjacent to the column, the water table and bodies of fresh groundwater (FW) and mixed water (Mx) are indicated. (b) Details of core recovered from the zone with Fe oxide mineralization on the dolostone dissolution surface. (c,d) SEM images of barite crystals growing on Fe oxides in the mineralized zone with compositional analyses. (e,f) SEM image of a euhedral barite crystal and its compositional spectrum.
Jmse 13 02090 g004
Jmse 13 02090 g005
Figure 5. Aguadulce site. (a) Profile of Cl− ion concentration (mg/L) in groundwater along the AG borehole, correlated with the percentage of seawater. Freshwater (FW) and mixing zone are identified. (b) Profile of trace element concentrations (µg/L) in groundwater.
Figure 5. Aguadulce site. (a) Profile of Cl− ion concentration (mg/L) in groundwater along the AG borehole, correlated with the percentage of seawater. Freshwater (FW) and mixing zone are identified. (b) Profile of trace element concentrations (µg/L) in groundwater.
Jmse 13 02090 g005
Jmse 13 02090 g006
Figure 6. Palmer site. (a) Synthetic lithostratigraphic column showing the position of Mn oxide precipitates. Adjacent to the column, the water table and bodies of fresh groundwater (FW), mixed water (Mx), and seawater (SW) are indicated. (b) Details of core recovered from the zone with Mn oxide mineralization on the dolostone dissolution surface. (c,d) SEM images of botryoidal Mn oxide precipitates and its compositional spectrum.
Figure 6. Palmer site. (a) Synthetic lithostratigraphic column showing the position of Mn oxide precipitates. Adjacent to the column, the water table and bodies of fresh groundwater (FW), mixed water (Mx), and seawater (SW) are indicated. (b) Details of core recovered from the zone with Mn oxide mineralization on the dolostone dissolution surface. (c,d) SEM images of botryoidal Mn oxide precipitates and its compositional spectrum.
Jmse 13 02090 g006
Jmse 13 02090 g007
Figure 7. Palmer site. Profiles of physicochemical parameters measured in the El Palmer aquifer (SW: seawater values). Borehole positions (PAL-1, PAL-2, PAL-3, and PAL-4) are shown in Figure 1.
Figure 7. Palmer site. Profiles of physicochemical parameters measured in the El Palmer aquifer (SW: seawater values). Borehole positions (PAL-1, PAL-2, PAL-3, and PAL-4) are shown in Figure 1.
Jmse 13 02090 g007
Jmse 13 02090 g008
Figure 8. Diagenetic model of barite (BaSO4) crystal formation in a carbonate coastal aquifer from former precipitates of iron and/or manganese oxy-hydroxides.
Figure 8. Diagenetic model of barite (BaSO4) crystal formation in a carbonate coastal aquifer from former precipitates of iron and/or manganese oxy-hydroxides.
Jmse 13 02090 g008
Table 1. Concentrations (g/Kg) and weight percent (wt%) of metallic elements determined by ICP-MS in samples collected from the cliff and cores at Aguadulce and Palmer sites.
Table 1. Concentrations (g/Kg) and weight percent (wt%) of metallic elements determined by ICP-MS in samples collected from the cliff and cores at Aguadulce and Palmer sites.
Aguadulce Site Palmer Site
CliffAG (Core)PAL-1 (Core)
g/Kgwt%g/Kgwt%g/Kgwt%
P0.410.205.461.240.330.25
Ti0.110.050.320.070.830.64
Mn140.4067.930.430.10102.5879.43
Fe1.740.84390.9988.6018.6214.42
Co3.971.920.290.070.550.43
Ni0.450.220.870.201.010.78
Cu2.381.150.980.220.170.13
Zn0.910.446.391.451.871.45
As0.590.296.581.490.650.50
Sr0.660.320.250.060.310.24
Ba12.235.9221.894.961.451.12
Pb42.8420.736.861.550.790.61
Table 2. Saturation indices of mineral phases in groundwater samples from boreholes AG and PAL-1. The percentage of seawater (SW) in the samples is included.
Table 2. Saturation indices of mineral phases in groundwater samples from boreholes AG and PAL-1. The percentage of seawater (SW) in the samples is included.
DepthSaturation IndexSW
(m a.s.l.)CalciteDolomiteBariteHematitePyrolusite%
AG1−0.43−0.59−0.0413.87−14.060
−1−0.130.02−0.1415.07−12.770
−3−0.16−0.04−0.1214.50−13.040
−5−0.100.18−0.1214.24−12.803
−7−0.030.46−0.0715.45−11.8514
−9−0.160.40−0.0714.04−13.5651
−11−0.170.42−0.0714.12−13.4663
−13−0.160.47−0.1014.36−13.2969
−15−0.140.50−0.0614.84−13.1274
−17−0.100.59−0.0515.02−13.1875
−19−0.160.49−0.0715.27−13.2180
PAL-1−1−0.85−1.160.1716.89−12.0012
−20.371.25−0.3615.31−10.8016
−3−0.320.030.0414.27−11.6439
−40.090.94−0.0616.32−11.6274
−70.251.33−0.1816.50−10.6499
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Sola, F.; Mancuso, M.; Vallejos, Á. Diagenetic Barite Growths in the Mixing Zone of a Carbonate Coastal Aquifer. J. Mar. Sci. Eng. 2025, 13, 2090. https://doi.org/10.3390/jmse13112090

AMA Style

Sola F, Mancuso M, Vallejos Á. Diagenetic Barite Growths in the Mixing Zone of a Carbonate Coastal Aquifer. Journal of Marine Science and Engineering. 2025; 13(11):2090. https://doi.org/10.3390/jmse13112090

Chicago/Turabian Style

Sola, Fernando, Malva Mancuso, and Ángela Vallejos. 2025. "Diagenetic Barite Growths in the Mixing Zone of a Carbonate Coastal Aquifer" Journal of Marine Science and Engineering 13, no. 11: 2090. https://doi.org/10.3390/jmse13112090

APA Style

Sola, F., Mancuso, M., & Vallejos, Á. (2025). Diagenetic Barite Growths in the Mixing Zone of a Carbonate Coastal Aquifer. Journal of Marine Science and Engineering, 13(11), 2090. https://doi.org/10.3390/jmse13112090

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop