Messinian Seawater Research Articles

Multi-proxy investigation of the post-evaporitic succession of the Piedmont Basin (Pollenzo section, NW Italy): A new piece in the Stage 3 puzzle of the Messinian Salinity Crisis

Paleoenvironmental reconstruction of the Mediterranean Basin at the end of the Messinian Salinity Crisis is contentious. One section that records this final phase (Stage 3) is the Pollenzo Section in the Piedmont Basin (NW Italy). Here, we present new stratigraphic, sedimentological, petrographic, micropaleontological (ostracods, calcareous nannofossils, foraminifera, dinoflagellates) and geochemical ( 87 Sr/ 86 Sr ratios) data from the Cassano Spinola Conglomerates (CSC) and interpret the paleoenvironment of this northernmost tip of the Mediterranean Basin. The CSC comprise three depositional units: members A and C, which were deposited subaqueously, and the intervening member B, which is continental. The CSC is topped by a ~ 50 cm-thick black layer, which is directly overlain by the open marine Argille Azzurre Formation of early Zanclean age. Our investigation reveals that member A is largely barren of autochtonous microfossils, except for an almost monospecific ostracod assemblage of Cyprideis torosa at the top, which indicates shallow-water (<30 m) conditions. Paratethyan ostracods and, possibly, taxa of calcareous nannofossils adapted to low-salinity water first occur in member C. 87 Sr/ 86 Sr ratios measured on ostracod valves from the member A/B transition (0.708871–0.708870) and member C (0.708834–0.708746) are lower than the coeval Messinian seawater values (~0.709024) and the 87 Sr/ 86 Sr ratios of a hypothetical lake filling Piedmont (>0.7090) estimated by means of the present-day 87 Sr/ 86 Sr signature of the Po river, the main drainage system of Northern Italy that receives the weathering products (including ions) of the Alps and Apennines. These values are likely to reflect the mixing of local high 87 Sr/ 86 Sr river water with low 87 Sr/ 86 Sr from the Mediterranean, which at the time was dominated by inputs from Eastern Paratethys, circum-Mediterranean rivers and Atlantic Ocean. Our results suggest that, at times during the final stage of the Messinian Salinity Crisis, the Piedmont Basin was hydrologically connected with the main Mediterranean Basin. At regional scale, this implies that the water level in the Mediterranean Basin was relatively high. • Piedmont Basin alternated subaqueous and continental conditions during MSC Stage 3. • 87 Sr/ 86 Sr values are inconsistent with subaqueous sedimentation in a local river-fed endorheic lake • Piedmont Basin-Mediterranean connection during times of Paratethyan ostracods spreading. • The Mediterranean water-level was occasionally relatively high during Stage 3.

The ore genesis of the Jebel Mecella and Sidi Taya F[sbnd]Ba (Zn[sbnd]Pb) Mississippi Valley-type deposits, Fluorite Zaghouan Province, NE Tunisia, in relation to Alpine orogeny: Constraints from geological, sulfur, and lead isotope studies

Jebel Mecella and Sidi Taya F–(BaPbZn) deposits are located within the Fluorite Zaghouan Province (NE Tunisia). The mineralization occurs along the unconformity surface between the Jurassic limestones and Upper Cretaceous rocks. The mineralization consists mainly of fluorite, barite, sphalerite, and galena. The δ34S values of barite at Jebel Mecella (14.8–15.4‰) and at Sidi Taya (21.6–22.2‰) closely match those of the Triassic evaporites and Messinian seawater, respectively. The range of δ34S values of galena and sphalerite in both deposits (−6.9 to +2.4‰) suggests the involvement of thermochemical sulfate reduction and possibly organically-bound sulfur in the generation of sulfur. Lead isotope data with 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.893–18.903, 15.684–15.699, and 38.850–38.880, respectively suggests a single homogeneous source reservoir of Paleozoic age and/or the homogenization of the Paleozoic–Cretaceous multireservoir-derived fluids along their long migration paths to the loci of deposition during the Alpine orogeny.

The Messinian marine to nonmarine gypsums of Jumilla (Northern Betic Cordillera, SE Spain): Isotopic and Sr concentration constraints on the origin of parent brines

The origin of the Messinian Hoya de la Sima (HS) gypsum (Betic foreland) is constrained using 87Sr/86Sr, δ34S, Sr concentration, and petrographic data. The Lower and Middle HS units consist of subaqueous vertically-aligned and stromatolitic selenites, the latter containing unusual microbial depositional textures. The Upper Unit consists of very-shallow-water bioturbated lenticular gypsum with Paracamelus ichnites. 87Sr/86Sr and δ34S indicate precipitation from predominantly marine waters, with upward increasing continental influence. Mixing models between Messinian seawater and continental water that dissolved Triassic evaporites show that the percentages of seawater required to explain the measured 87Sr/86Sr are analogous to the percentages obtained using δ34S, supporting precipitation from such mixtures. 87Sr/86Sr and δ34S of Lower HS selenites resemble those of the Primary Lower Gypsum (PLG) of the Messinian Salinity Crisis (MSC), in both cases indicating precipitation from seawater–continental water mixtures in which most Sr and SO4 were supplied by Messinian seawater. In the Lower HS selenites, Sr concentrations indicate contributing continental waters with Sr/Ca ratios similar to seawater. However, Sr concentrations of PLG selenites from other Betic basins (Bajo Segura, Sorbas indicate parent waters with Sr/Ca ratios lower than seawater. If the Sr contents of the betic PLG selenites are representative, it is unlikely that the Lower HS selenites represent the PLG. However, we cannot completely discard that option since different LPG subbasins could have had variable Sr/Ca. The HS gypsums formed coevally to diapirism of Triassic evaporites, in a restricted lagoonal basin developed during or slightly after a phase of strike-slip faulting in the Betic Cordillera. More general implications of this work are that Sr concentrations, combined with 87Sr/86Sr and δ34S data, provide key constraints on the origin of parent brines, and using Sr concentrations as paleosalinity proxy in gypsum precipitated from seawater–continental water mixtures is inconclusive because they largely depend on the initial Sr/Ca ratio of the brine. This study also illustrates that the electron microprobe is ideal to obtain reliable, spatially-resolved, Sr concentration data in selenitic gypsum. These data, in combination with fluorescence petrography, provide valuable criteria to discriminate primary depositional from diagenetic gypsum, a critical step in the interpretation of geochemical data.

The Touissit-Bou Beker Mississippi Valley-Type District of Northeastern Morocco: Relationships to the Messinian Salinity Crisis, Late Neogene-Quaternary Alkaline Magmatism, and Buoyancy-Driven Fluid Convection

The Touissit-Bou Beker Mississippi Valley-type (MVT) district of northeastern Morocco includes five major strata-bound Pb-Zn-Ag deposits that produced, over a century of mining, more than 70 Mt of ore at an average grade of 4.0 wt % Pb, 3.5 wt % Zn, ~1 wt % Cu, and 120 g/t Ag. Economic orebodies are hosted by a 25-m-thick sequence of unmetamorphosed, flat-lying, diagenetically and hydrothermally dolomitized carbonate platform rocks of Aalenian-Bajocian (174–168 Ma) age. The sulfide mineralization consists principally of galena and sphalerite, and occurs as open-space fillings and partial to massive replacements of medium- to coarse-grained dolostones. Throughout the district and surrounding prospects, sulfur isotope compositions of sulfide minerals range from −8.6 to 12.9‰ CDT ( n = 194) and display spatial and temporal variations characterized by an overall progressive decrease of δ 34S values: (1) with advancing paragenetic sequence, and upward stratigraphic position; (2) toward distal prospects; and (3) within single grains from core to rim. Similarly, the distribution of 3He/4He ratios shows an overall lowering from 0.5 RA to <0.01 RA with advancing paragenetic stage. Conversely, lead isotope compositions of galena display fairly uniform 206Pb/204Pb (18.31–18.37), 207Pb/204Pb (15.61–15.66), and 208Pb/204Pb (38.45–38.62) ratios, which contrast significantly with the highly heterogeneous isotopic compositions (206Pb/204Pb = 17.76–18.49, 207Pb/204Pb = 15.61–15.70, 208Pb/204Pb = 37.76–39.55) shown by potential metal source rocks. The large variations in δ 34S values are explained in terms of inorganic and/or bacteriogenic reduction, at different rates, of seawater sulfate in seawater or pore fluids, in reservoirs both open and closed with respect to sulfate. The highly variable 3He/4He values reflect different degrees of mixing between mantle and crustal He components along the fluid flow path. Estimated contributions of the mantle-derived (4He) component involved in the mineralizing fluids of the paragenetically early Zn-rich ore-stage I, and in the later Pb-rich main ore-stage II, are in the range of ~3 to 8%. With time, the mantle-derived input to the mineralizing fluids decreased, and contemporaneously the involvement of surface waters increased, consistent with the lowering of δ 34S with advancing paragenetic stage. This geochemical evolution toward lower isotopic values indicates that the Touissit-Bou Beker hydrothermal system became exclusively dominated by crustally derived fluids, and probably incorporated at shallower levels the influx of progressively larger volumes of cooler, dilute, and oxidized meteoric waters. Whole-rock and galena lead isotope compositions are consistent with a model in which metals were extracted from the local country rocks, particularly the basement Visean rhyodacites and associated siliciclastic rocks, and from the overlying Triassic to Upper Bajocian sediments. The extraction of lead likely occurred during the late Neogene, coincident with the Messinian salinity crisis and anorogenic alkaline magmatism. Conventional MVT genetic models cannot satisfactorily account for Pb-Zn-Ag ore deposition in the Touissit-Bou Beker district. Accordingly, we propose an alternative model involving a genetic link among extensional tectonics, Neogene-Quaternary mafic magmatism, the Messinian salinity crisis, and formation of the Touissit-Bou Beker deposits. We suggest that this mineralization resulted from the regional circulation of warm saline brines, sourced within the Rif corridor, during Messinian-Tortonian time (11.6–5.3 Ma). Messinian paleogeography would have favored the evaporation of seawater and development of a reflux-circulation system, synchronous with the emplacement of Neogene-Quaternary alkaline magmas. The associated elevated heat flow and subsequent increased geothermal gradient initiated buoyancy-driven fluid convection of downward-flowing Messinian seawater, which ultimately promoted the mobilization of older, high-temperature, rock-buffered basement dense brines stored within the Paleozoic basement, and the formation of base-metal–bearing chloride complexes. Mixing of Messinian seawater and basement-derived hydrothermal brines triggered deposition of Pb- and Zn-rich stage I and II mineralization. The resulting mixed brines were mainly centered on the Touissit Shelf and its flanks, and then flowed laterally away from the basement high, giving rise to the lower grade mineralization of the distal prospects. The predominance of a high heat flow regime centered on a positive paleogeographic structure (i.e., basement high) and more importantly the availability of appropriate sulfur nutrients (i.e., “fertile” vs. biogenic sulfur), may have constituted the critical ore controls that governed the size of the ore deposits. Finally, during late Pb-rich cuboctahedral stage III mineralization, interpreted to have been emplaced during a magmatic interlude contemporaneous with the closure of the Rif corridor and its final exhumation, the hydrothermal system became open to the incursion of meteoric waters; subsequent mixing with deep-seated, reduced, H2S-rich, saline waters led to precipitation of the sulfides of stage III mineralization.

The major-ion composition of Cenozoic seawater: The past 36 million years from fluid inclusions in marine halite

Fluid inclusions from ten Cenozoic (Eocene-Miocene) marine halites are used to quantify the major-ion composition (Mg^2+^, Ca^2+^, K^+^, Na^+^, SO~4~^2−^, and Cl^−^) of seawater over the past 36 My. Criteria used to determine a seawater origin of the halites include: (1) stratigraphic, sedimentologic, and paleontologic observations; (2) Br^−^ in halite; (3) δ^34^S of sulfate minerals; (4) ^87^Sr/^86^Sr of carbonates and sulfates; and (5) fluid inclusion brine compositions and evaporation paths, which must overlap from geographically separated basins of the same age to confirm a "global" seawater chemical signal. Changes in the major-ion chemistry of Cenozoic seawater record the end of a systematic, long term (\>150 My) shift from the Ca^2+^-rich, Mg^2+^- and SO~4~^2−^-poor seawater of the Mesozoic ("CaCl~2~ seas") to the "MgSO~4~ seas" (with higher Mg^2+^ and SO~4~^2−^\>Ca^2+^) of the Cenozoic. The major ion composition of Cenozoic seawater is calculated for the Eocene-Oligocene (36-34 Ma), Serravallian-Tortonian (13.5-11.8 Ma) and the Messinian (6-5 Ma), assuming chlorinity (565 mmolal), salinity, and the K^+^ concentration (11 mmolal) are constant and the same as in modern seawater. Fluid inclusions from Cenozoic marine halites show that the concentrations of Mg^2+^and SO~4~^2−^ have increased in seawater over the past 36 My and the concentration of Ca^2+^ has decreased. Mg^2+^ concentrations increased from 36 mmolal in Eocene-Oligocene seawater (36-34 Ma) to 55 mmolal in modern seawater. The Mg^2+^/Ca^2+^ ratio of seawater has risen from ∼2.3 at the end of the Eocene, to 3.4 and 4.0, respectively, at 13.5 to 11.8 Ma and 6 to 5 Ma, and to 5 in modern seawater. Eocene-Oligocene seawater (36-34 Ma) has estimated ranges of SO~4~^2−^ = 14--23 mmolal and Ca^2+^ = 11--20 mmolal. If the (Ca^2+^)(SO~4~^2−^) product is assumed to be the same as in modern seawater (∼300 mmolal^2^), Eocene-Oligocene seawater had Ca^2+^ ∼16 mmolal and SO~4~^2−^ ∼19 mmolal. The same estimates of Ca^2+^ and SO~4~^2−^ for Serravallian-Tortonian seawater (13.5-11.8 Ma) are SO~4~^2−^ = 19--27 mmolal and Ca^2+^ = 8--16 mmolal and SO~4~^2−^ ∼24 mmolal and Ca^2+^ ∼ 13 mmolal if the (Ca^2+^)(SO~4~^2−^) product is equal to that in modern seawater. Messinian seawater has an estimated range of SO~4~^2−^ ∼21--29 mmolal and Ca^2+^ ∼7--15 mmolal with SO~4~^2−^ ∼26 mmolal and Ca^2+^ ∼12 mmolal assuming the (Ca^2+^)(SO~4~^2−^) product is equal to that in modern seawater. Regardless of the estimation procedure, SO~4~^2−^ shows progressively increasing concentrations from 36 Ma to the present values, which are the highest of the Cenozoic.

Messinian Ca–Cl Brines from Mediterranean Basins: Tracing Diagenetic Effects by Ca/Mg Versus Ca/Sr Diagram

In natural resource exploration, Ca–Cl basinal brines are important for understanding the origin and spatial and temporal distribution of hydrocarbons and sedimentary ore deposits. Little attention has been paid to the possible connection between fossil basinal brines and paleo-seawaters and to the implications for reconstructing paleo-seawater compositions. Secular variations of Ca/Mg and Ca/Sr ratios in seawater have been documented mainly using fluid inclusions in halite, calcareous fossils and mineral analyses. However, brines and other sedimentary records connected to paleo-seawater or its evaporated residues may be chemically affected by burial diagenesis or the effects of continental waters of meteoric origin, thus complicating interpretations of the analytical results. To investigate these effects on fluids and minerals related to the Messinian salinity crisis of the Mediterranean basin, we re-evaluate published data from: (1) brackish-to-brine waters from onshore (Northern Apennine foredeep; Levantine basin) and offshore (porewaters from the Deep Sea Drilling Project); (2) Messinian parental seawater deduced from calcareous fossils, fluid inclusions and sulfate minerals; (3) meteoric waters dissolving evaporites. The compositional trends related to seawater evaporation, diagenesis and mixing that affect the Ca/Mg and Ca/Sr molar ratios of the basinal brines are effectively discriminated on a binary plot depicting the proper fields for seawater and meteoric-derived fluids. Brines showing stronger dolomitization start from Ca/Mg and Ca/Sr molar ratios of Messinian seawater deduced from the published analysis of fluid inclusions and open ocean fossils, that are therefore here validated ex post.

The origin of sulfate mineralization and the nature of the BaSO4–SrSO4 solid-solution series in the Ain Allega and El Aguiba ore deposits, Northern Tunisia

Ain Allega and El Aguiba are among the largest barite–celestite deposits attributed to the Triassic period in Tunisia. They are located in the flysch zone on the eastern edge of the Triassic diapir of Jebel Hamra. The ore body consists of dolomite intensely brecciated and surrounded by marls, clay, gypsum and dolomite which forms the hanging wall of the deposit, and rimmed by Paleocene marls. The ore minerals show a cap-rock type mineralization with various forms and types, in particular impregnation in dolomite, breccia cement, replacement of carbonate cap-rock and open space-filling in the dissolution cavities and fractures. Ore minerals include sphalerite, galena, marcasite and pyrite. Principal gangue minerals are barite, celestite, calcite, dolomite and quartz. The ore minerals are hosted by the Triassic carbonate rocks, which show hydrothermal alteration, dissolution and brecciation.The study of barite–celestite solid-solution shows a bimodal distribution in which the composition is not continuous. Some intermediate compositions are missing; 20–80% SrSO4 for the El Aguiba ore deposit and 50–70% SrSO4 for the Ain Allega ore deposit.X-ray diffraction peaks in the BaSO4–SrSO4 series display considerable broadening at intermediate compositions. The unit cell volume varies from 346.97Å3 in barite (100% BaSO4) to 308.29Å3 in celestite (100% SrSO4). This variation in cell parameters is attributed to the degree of substitution of barium by strontium since there is less variation between ionic radii of these elements. Consequently, these substitutions caused variation in morphologies, position and d-spacing of peaks (200, 011, 113, 312,122, 112, and 111). Some barite crystals have small-scale, strontium compositional banding which suggests that the crystals responded to regular fluctuations in physiochemical conditions during deposition. The degree of supersaturation in Ba2+ and Sr2+ controls the growth mechanism and consequently the surface structure, which in turn controls the mechanism of adsorption and incorporation of minor and trace elements into the growing crystal. For a solid solution, the transitional supersaturation for different growth mechanisms can be significantly different for the two end members. One of the most interesting and intriguing phenomena observed in natural crystals of the (Ba,Sr)SO4 solid solution from this study area is the development of compositional oscillatory zoning, consisting of alternating Ba-rich and Sr-rich layers.The sulfur isotope analyses show significant variability for barite and celestite (from 16.2 to 23‰). These values are interpreted as the result of the mixing of two sulfur end-members in the mineralizing fluids, corresponding to thermochemical sulfate reduction of Messinian seawater, together with Triassic sulfate, as sulfur sources.Fluid inclusion studies of celestite show that the BaSO4–SrSO4 solid-solution in both deposits was precipitated from hot saline solution (Th=190±20°C; 16.37wt.% NaCl equivalent in Ain Allega and 8.2wt.% NaCl equivalent in El Aguiba). Consequently, the precipitation could be from mixing of basinal brines with magmatic–meteoric fluid.

Pb–Zn mineralization in a Miocene regional extensional context: The case of the Sidi Driss and the Douahria ore deposits (Nefza mining district, northern Tunisia)

The Sidi Driss and Douahria sulfide ore deposits and showings are located in the Nefza mining district, northern Tunisia. The ores are hosted within Upper Miocene (Messinian) basins, within carbonate lenses composed of Fe–Mn-enriched dedolomite partially or totally replaced by early barite and celestite. The ore mainly consists of galena and spherulitic/colloform sphalerite (partially replaced by later Fe-sulfides). It is present as disseminated or banded ores and has been deposited within the host-rock through dissolution/replacement process or through void infilling. The remaining voids are filled by late calcite/barite locally associated with a late oxidation event. In situ ion microprobe sulfur isotope analyses show significant variability for sphalerite (from − 43.9 to 1.2‰), galena (from − 30.3 to − 2‰) and marcasite δ 34S (from − 35.9 to 25.8‰). These values are interpreted as the result of (i) the mixing of two sulfur end-members in the mineralizing fluids, corresponding to different reduction processes (bacterially-mediated sulfate reduction and thermochemical sulfate reduction, with early celestite and barite and/or contemporaneous Messinian seawater, together with Triassic sulfate, as sulfur sources, and (ii) Rayleigh fractionation process in a closed system. Several arguments (Fe–Mn enrichment in early diagenetic carbonates, involvement of Messinian seawater as a sulfur source, low temperature of deposition and soft-style sedimentary figures within sulfides) suggest that the Sidi Driss and Douahria Pb–Zn ore deposits are syn-diagenetic. Moreover, the properties of the ore deposits, with Fe–Mn carbonates having a wider distribution than that of the base metal sulfides, and the characteristics of the mineralization, in which diagenetic sulfates are a sulfur source and Cd enrichment in sphalerite, support the classification of the Sidi Driss and Douahria Pb–Zn deposits as SHMS-Sedex type. Late Miocene extension, comprising rifting dynamics and basalt emplacement, favored a thermally-driven fluid circulation origin for the Sidi Driss and Douahria deposits, enhancing inception of small-scale shallow convection cells. Such sulfide emplacement is significantly different from that of southernmost MVT ore deposits of Tunisia, which are associated with diapirs and are interpreted as the result of a Serravalian–Tortonian gravity-driven fluid circulation event related to late Alpine convergence.

Sr, S, and OSO4 Isotopes and the Depositional Environments of the Upper Miocene Evaporites, Spain

ABSTRACT Upper Miocene evaporites in the Nijar Basin, southeastern Spain, contain thick, massive and coarse-crystalline selenites of the middle Messinian Yesares Formation. On the basis of their sedimentary fabrics and structures, such as euhedral terminations of crystals, well preserved algal filament inclusions, and standing-growth orientations, these selenites are interpreted to be primary, bottom-growth deposits. Most of these selenites have Sr concentrations of about 1000 ppm (range from 700 to 1800 ppm). 87Sr/86Sr values of Yesares selenites vary between 0.70890 and 0.70896 (n = 18), which are all within the known range of Messinian seawater, suggesting a marine origin. However, quantitative modeling of 87Sr/86Sr vs. Sr/Ca of fluids demonstrates that a minor amount of admixed nonmarine fluid (e.g., 20%) might have contributed to the Yesares marine brines. 34S and 18OSO4 values of Yesares selenites range from 21 to 24‰ CDT and from 10 to 16‰ SMOW, respectively. These ranges are more variable and most values are heavier than those expected for normal Miocene marine gypsum. On the basis of quantitative modeling, these heavy 34S and 18OSO4 values have most likely resulted from reduction and oxidation reactions of sulfur species that occurred in brines rather than from contributions of nonmarine water or from reservoir (crystallization) effects. The use of Sr, S, and O isotopes provides a possible means of distinguishing whether an ancient evaporite was deposited by marine, nonmarine, or hybrid brines. Sr isotopes, however, are insensitive to minor nonmarine contributions on the basis of quantitative modeling, and consequently marine Sr isotopes for evaporites (e.g., gypsum) do not necessarily indicate pure marine depositional environments. Compared to Sr isotopes, 34S and 18OSO4 of marine gypsum are even more insensitive to minor nonmarine contributions because of much higher SO4 concentrations in seawater and seawater-evaporated brines than in most freshwaters. Therefore, Sr, O, and S isotopes should be used with caution to interpret depositional environments and to determine parent fluids of ancient evaporites.

Sr/Ca, Mg/Ca ratios and Sr and stable isotopes of biogenic carbonates from the Late Miocene Velona Basin (central Apennines, Italy) provide evidence of unusual non-marine Messinian conditions

The Sr/Ca and Mg/Ca ratios of fossil ostracod and mollusc shells from a Late Miocene (Early Messinian) lacustrine sequence in the Velona Basin (central Apennines, Italy) were used to calculate the Sr/Ca and Mg/Ca of waters in which these taxa lived. Sr/Ca of these waters were always higher than the Sr/Ca calculated for the Messinian seawater, despite the broad salinity ranges (fresh water-oligo/mesohaline) inferred from the paleontological analyses. The estimated Mg/Ca water from the Velona samples are below the Mg/Ca value for coeval seawater even though they represent relatively high values when compared with many non-marine waters. The salinity variations of the Velona lacustrine system, recorded by changes in faunal assemblages, are linked to variable inputs of Na–Cl from deep groundwaters and are not reflected in significant changes in Sr/Ca and Mg/Ca of the waters calculated from shell chemistry. The combination of δ 18O signatures and Sr isotopic ratios from molluscs from the Velona Basin suggests a non-marine environment. During some episodes of the lake history, given the saline inputs, the waters in the lake would attain a large solute concentration (oligo to mesohaline), although they retained the meteoric δ 18O signature. We compare the paleohydrochemical inferences from the studied sequence with other Messinian records, and we show the importance of combining paleoecological and geochemical proxies to obtain consistent paleoenvironmental reconstructions.

Dolomitization by Mixed Evaporative Brines and Freshwater, Upper Miocene Carbonates, Nijar, Spain

ABSTRACT Upper Miocene carbonates, Nijar, Spain have been studied in order to test a model for massive dolomitization of western Mediterranean reefal carbonates invoking evaporite brines formed during the Messinian salinity crisis. Tortonian-Messinian reef-bearing carbonates at Nijar comprise a kilometers-wide shelf fringing the Sierra Alhamilla highland, and dip southward into the evaporite-bearing Nijar Basin. In the Nijar platform, pervasive dolomites are present in greater than 75% of Tortonian-Messinian carbonates. Dolomites transect intra-Miocene sequence boundaries and occur in the youngest Messinian unit at Nijar, the oolitic Terminal Carbonate Complex (TCC). Dolomites do not transect the Miocene-Pliocene unconformity, but do occur as clasts in basal Pliocene lime grainstones. This constrains the dolomitization as syn- or post-TCC and pre-Pliocene in age, and suggests that dolomitization occurred within 600,000 years and possibly in less than 200,000 years. Carbon and oxygen isotopes of Nijar dolomites show marked positive covariation, with most values ranging from +5.4 to -1.2% d18O PDB and +2.5 to -4.3% d13C PDB. The highest d18O values are heavier than reasonable estimates of Messinian normal-salinity seawater dolomites, and therefore imply involvement of evaporative brines. Calculations of covariations of O and C isotopes during water-rock interaction and fluid-fluid mixing argue against recrystallization of Nijar dolomites, an interpretation supported by their Ca richness (49-57 mol %). These mixing calculations show that about 80% of Nijar ata are consistent with dolomitization in mixtures of freshwater and a few tens of percent of seawater-derived evaporative brine, and about 20% are consistent with mixtures of freshwater and normal-salinity seawater. 87Sr/86Sr of pure Nijar dolomites range from Messinian seawater values (0.7089) to high values (0.70928), and show distinct negative covariations with d18O, d13C, and Sr. Calculations of covariations of 87Sr/86Sr with d18O and d13C during water-rock interaction and fluid-fluid mixing argue strongly against recrystallization, and argue for their formation by mixtures of freshwater with a few tens of percent of evaporative brines, with seawater, and with seawater-brine mixtures. Modeling ovariations of 87Sr/86Sr and Sr/Ca ratios during mixing further supports involvement of evaporative brine, and suggests that the freshwaters had low Sr/Ca ratios and relatively high Sr and Ca concentrations. Na, Cl, and SO4 concentrations in Nijar dolomites (200-1700 ppm, 300-600 ppm, and 600-6500 ppm, respectively) argue for involvement of evaporative brines, and of mixtures of freshwater and seawater, when compared with other well-characterized dolomites. The proposed model is that most dolomitization occurred after reef formation, and during and possibly after TCC deposition, during multiple relative sealevel changes. The model invokes TCC deposition on the Nijar shelf during high sealevels, and evaporite deposition in Nijar basin during low sealevels, during which brines in the 5x- to 6x-seawater range were formed. During sealevel rises these brines mixed with seawater or remained as dense basin bottom water. In either case, the brines mixed with freshwater mainly in a groundwater setting, the freshwaters having radiogenic Sr and depleted d18O and d13C derived from a weathered mantle on the Sierra Alhamill . The brines and sea water-brine mixtures were the sources of Mg for the dolomites, and their circulation through platform rocks was driven primarily by buoyant circulation of the mixing zone beneath freshwater lenses. The Nijar model may be applicable to dolomitization in other partially land-locked basins having adjoining highlands (e.g., Sorbas Basin) that acted as freshwater recharge areas. The Mallorcan model, which invokes brines and seawater with no freshwater, may be appropriate for settings open to the deep Mediterranean and distant from highlands (e.g., Santa Pola, Spain; Mellilla, Morocco).

Origin and age of the Mediterranean Messinian evaporites: implications from Sr isotopes

Strontium isotope ratios from the Messinian (late Miocene; 6.4 to 4.85 Ma) evaporites of the Mediterranean basin [1,2] support a subdivision into stratigraphically lower, predominantly marine deposits (lower evaporites = LEV: lower gypsum and main salt) characterized by Sr isotopic ratios generally equivalent to those of Messinian seawater, and stratigraphically higher deposits (upper evaporites = UEV), with 87Sr/ 86Sr ratios lower than Messinian seawater. The range of 87Sr/ 86Sr ratios of the LEV unit from the eastern Mediterranean overlap with values obtained from essentially coeval Red Sea evaporites [3]. Strontium isotopic ratios measured from the upper evaporites, however, are lower, and a high proportion of this Sr was probably derived from low 87Sr/ 86Sr continental runoff that characterizes much of this basin [4]. These results, therefore, strongly support depositional models utilizing a deep desiccated basin [1] with a constant marine inflow during the early stages, including salt formation, followed by a series of long-lasting and sometimes isolated desiccation events that produced a strong, but variable, continental influence on the Sr isotopic compositions of the UEV. Strontium isotopic measurements can also be used to asses the actual timing of the deposition of the LEV. Ratios obtained from oyster fragments and foraminifers from pre-evaporitic units at ODP Site 654 (Tyrrhenian Sea) suggest that the major rifting phase in this region occurred between 8 and 6.5 Ma. Ratios obtained from the early LEV from the Mediterranean Sea suggest that initiation of evaporite deposition began about 5.65 Ma ago. Results from units immediately overlying the UEV in Sicily suggest termination of evaporite deposition about 4.9 Ma ago. Errors associated with these age estimates are about 0.5 Ma (2 σ).