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deep dolomite formation mechanism
Marine Geology 275 (2010) 273–277
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Marine Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r g e o
Letter
Messinian salinity crisis: A novel unifying shallow gypsum/deep dolomite formation mechanism Gert J. de Lange a,⁎,1, Wout Krijgsman b,1 a b
Department of Earth Sciences — Geochemistry, Faculty of Geosciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands Paleomagnetic Laboratory ‘Fort Hoofddijk’, Faculty of Geosciences, Utrecht University, Budapestlaan 17, 3584 CD Utrecht, The Netherlands
a r t i c l e
i n f o
Article history: Received 6 February 2010 Received in revised form 22 May 2010 Accepted 27 May 2010 Available online 4 June 2010 Communicated by D.J.W. Piper Keywords: Mediterranean Messinian evaporite gypsum formation dolomite
a b s t r a c t The Messinian Salinity Crisis is a dramatic event that took place between 5.96 and 5.33 Ma, and resulted not only in the desiccation and reflooding of the Mediterranean but also in the deposition of 0.3–3 km thick evaporites at its seafloor. There has been considerable controversy on the modes of their formation. The first-stage gypsum deposits are considered to have only formed at silled marginal basins, whereas anoxic marls and dolostones are found at deeper settings. We agree with these observations but reject the coincidental presence of similar sills as being too fortuitous to explain gypsum formation in silled basins alone. Alternatively, we present a unifying mechanism in which gypsum precipitation takes place basin-wide at all shallow-water depths but its preservation is limited to silled marginal basins. Taking a realistic early Messinian scenario, including a moderate primary production rate, the sulphate-consumption rate in the deep water appears to exceed the sulphate supply rate as deduced from reported shallow-water gypsum deposition rates. The consequent lower dissolved sulphate content limits gypsum preservation but permits dolomite formation to take place in the deep water. The often observed concurring anoxic environment and absence of gypsum formation is thus related to diminished dissolved sulphate content via organic matter degradation and not to oxygen-free conditions as such. © 2010 Elsevier B.V. All rights reserved.
1. Introduction During the latest Messinian, the Mediterranean Sea transformed into a giant saline basin, one of the largest in the Earth's history and surely the youngest one (CIESM, 2008). This event, referred to as the Messinian Salinity Crisis (MSC), changed the chemistry of the ocean and had a permanent impact on both the terrestrial and marine ecosystems of the Mediterranean area (Cita et al., 1978; Hsü et al., 1973; Ryan, 2009). The onset of the MSC is marked by the deposition of gypsum-sapropel or dolomite-sapropel alternations of the “Lower Evaporites”, which were astronomically dated to have formed between 5.96 and ∼5.6 Ma (Krijgsman et al., 1999; Roveri et al., 2008). Earliest explanations were that evaporites were deposited in a deep and desiccated Mediterranean (Fig. 1a) (Hsü et al., 1973, 1978). Facies models for the gypsum deposits, however, point out impressive similarities in terms of thickness and overall trend, allowing bed-to-bed correlation across the western Mediterranean (Lugli et al., 2008). The “Lower Gypsum” units are now considered to be fully subaqueous with no evidence of a substantial sealevel fall and have an overall aggradational stacking pattern indicative of continuous creation and/or availability of accommodation space (CIESM, 2008; Roveri et al., 2008). Quantitative analyses on the Mediterranean–Atlantic water and salt fluxes indicate that continuing ⁎ Corresponding author. Tel.: +31 30 2535034. E-mail address: [email protected] (G.J. de Lange). 1 Both authors contributed equally. 0025-3227/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2010.05.003
oceanic inflow and restricted outflow must have persisted during its deposition (Krijgsman and Meijer, 2008). Increasing evidence shows that the Lower Gypsum deposits are only preserved at silled marginal basins (Fig. 1b) (Clauzon et al., 1996; Manzi et al., 2007; Roveri and Manzi, 2006), while dolostones or anoxic marls have been reported for deeper settings (Hilgen and Krijgsman, 1999; Roveri et al., 2008). If we reject the coincidental presence of sills for all marginal basins, then a unifying mechanism must exist that causes gypsum precipitation to occur only in shallow settings and dolostone mainly in deep settings (Fig. 1c). 2. Discussion In the following we will first introduce the general principles that govern sulphate and dolomite formation, then apply and interpret these for gypsum and dolomite in shallow and deep Messinian environmental settings. 2.1. Gypsum and dolomite formation and interactions Gypsum precipitation in evaporating seawater has been reported to take place at 3 to 7 times concentrated seawater (Fig. 2) (De Lange et al., 1990a; McCaffrey et al., 1987). Seawater is always largely oversaturated relative to dolomite but its formation has often been reported to be inhibited by the presence of dissolved sulphate (see Supplementary information; Baker and Kastner, 1981). This condition links it to gypsum
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G.J. de Lange, W. Krijgsman / Marine Geology 275 (2010) 273–277
Fig. 1. Schematic illustrations of MSC scenarios explaining the deposition of the Lower Gypsum (LG) units. a) Classic desiccation model (Hsü et al., 1973) that suggests evaporite formation in a deep-basin shallow-water configuration, possibly with diachronism during increasing evaporitic conditions in a system of interconnected silled subbasins of different depths (Rouchy and Caruso, 2006). b) Recently proposed scenario with preferential gypsum formation in silled marginal basins alone (CIESM, 2008; Roveri et al., 2008). The difference in sedimentary environment between marginal and deep settings implies similar physical properties for the sill configuration. c) New unifying scenario (this paper) with Mediterranean-wide gypsum precipitation within shallow-water levels, gypsum preservation in shallow but not in deep settings, and dolomite formation and resedimented gypsum in the deep basins.
saturation, i.e. the conditions for formation of the one exclude those for the other (see Eqs. (1), (2)). 2þ
þ SO4
2þ
þ Mg
Ca Ca
2−
¼ N CaSO4
2þ
þ 2CO3
2−
ð1Þ
¼ N CaMgðCO3 Þ2
2−
ð2Þ −
þ
53SO4 þ C106 H263 O110 N16 P ¼ N39CO2 þ 67HCO3 þ 16NH4 2− − þ HPO4 þ 39H2 O þ 53HS
ð3Þ
(average marine OM) 2−
−
CO3 þ CO2 þ H2 O ¼ N2HCO3
ð4Þ
Another process that links the saturation states of gypsum and dolomite is that of organic matter (OM) decomposition by sulphate reduction. The common sequence of oxidants consumption for OM remineralization goes from oxygen, via Mn- and iron-oxides, to sulphate (e.g. Froelich et al., 1979; De Lange, 1986). The sulphate concentration in seawater being 100-times larger than oxygen, ultimately, i.e. if sufficient OM is available, sulphate will be the predominant oxidant for OM. The latter process will not only decrease the dissolved sulphate concentration but will also increase the dissolved carbonate content (see Eqs. (3), (4) and Supplementary information). Implicitly this means that lowsulphate conditions, i.e. unfavorable conditions for gypsum formation, always coincide with anoxic, i.e. oxygen-free conditions, although there is no direct relationship between the two. The processes described above give a novel, comprehensive explanation for the often observed coincidence of anoxia and the absence of gypsum formation (e.g. Nurmi and Friedman, 1977).
2.2. Gypsum formation and precipitation in shallow versus deep settings Early MSC gypsum deposits have been reported for silled marginal basins alone (e.g. Manzi et al., 2005; Roveri and Manzi, 2006), whereas anoxic marls and dolostones are found at deeper settings (e.g. Roveri et al., 2008; Hilgen and Krijgsman, 1999). We agree with these observations but do not attribute this to initial deposition but rather to subsequent preservation. In addition, the coincidental presence of similar sills seems rather fortuitous to explain gypsum formation in silled basins for different settings.
In general, it is hard to imagine that gypsum precipitation can only occur in shallow settings and not basin-wide: the shallow marginal basin waters would suffer more from evaporation than the open deep-basin waters, leading to a more dense fluid that would – without a sill – immediately flow towards the deeper parts of the basin (Fig. 3). This would not only result in the whole basin to be more saline but also to be stratified. The continued inflow of ocean surface water will remain on top of the deep high-density brine (similar to present-day deep-Ionian brine basins (De Lange et al., 1990b)), while outflowing water is necessary to counterbalance the salt influx. In any case, the Messinian Mediterranean would have been characterized by a reasonably well-mixed upper water mass (‘shallow waters’), and a strongly stratified lower ‘deep-water mass’. The MSC stratification with deep concentrated brines, is very stable, and can only be replaced by an even higher salinity water mass. Such severe stratification automatically results in deep water to become nutrient-enriched and oxygen-depleted due to ongoing decomposition of organic debris from surface water primary production. As a consequence of deep-water anoxia, most of the phosphate resulting from organic matter decomposition will be regenerated rather than buried (Slomp et al., 2004). The latter authors report that the assumed nearly stagnant deep-water conditions during sapropel formation and the consequent enhanced P regeneration have resulted in enhanced primary production thus leading to increased downward fluxes of organic debris (for a detailed discussion, see Slomp et al., 2004 and refs therein). Although stratific6ation may have been more severe during the MSC, given time, a similar scenario can easily be adopted for that time interval as well. Degradation of the settling organic debris via sulphate reduction leads to significantly decreased deep-water sulphate contents (Fig. 3). As gypsum precipitation is primarily controlled by its equilibrium with dissolved calcium and sulphate, sulphate reduction would lead to a decreased saturation or undersaturation with respect to gypsum for Mediterranean deep water, but not for the more regularly replenished shallow waters. The consequence of sulphate concentrations in the deep water to be lower than in the shallow water, is that only in the shallow-water gypsum formation may occur. These gypsum crystals (density of ∼2.3) will subsequently descend to the deep seafloor. In gypsum-saturated shallow settings, these will be preserved, whereas in the gypsumundersaturated deep settings these will be dissolved (Fig. 3). Potentially, this would raise the calcium and sulphate concentrations in the deep water. However, the dissolved deep-water sulphate concentration
G.J. de Lange, W. Krijgsman / Marine Geology 275 (2010) 273–277
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during primary production in the surface waters is thought to be remineralized in the deep water. In the case of high-productivity areas this number is higher (50%) (Sarmiento and Gruber, 2006), and due to decreased degradation rates of OM under anoxic conditions, an even higher fraction can make its way to the deep-water. A remineralization fraction of 25 to 75% for an anoxic brine water column seems therefore realistic for a low and high-productivity scenario respectively. Low to moderately high primary productivity ranges from 30 to 300 g C/m2yr (but can range to N500 g C/m2yr for high primary productivity). The range of deep-water OM remineralization is thus from 8 to 225 g C/ m2yr, corresponding to 0.7 to 19 M C/m2yr, which is equivalent to a sulphate reduction rate of 0.3 to 9.5 M SO4/m2yr. These calculations indicate that the potential sulphate flux into the deep water (2.6 M SO4/m2yr) falls within the range between highand low-estimates for sulphate reduction. We thus conclude that for an anoxic water column and moderate primary production, deepwater sulphate removal rates may exceed the sulphate supply rates, thus that deep water is likely to be undersaturated relative to gypsum. In contrast, for a sediment surface within a relatively shallow surface mixed (oxic) water column (taken to be ∼250 m) such deposit would remain stable as concentrations of dissolved Ca and SO4 remain on average near saturation values for gypsum. In other words, gypsum formation takes place in all shallow waters, whereas gypsum deposits in shallow settings are preserved but not in deep settings. 2.3. Deep-water dolomite formation
Fig. 2. Seawater evaporation path (De Lange et al., 1990a; McCaffrey et al., 1987). Gypsum formation starts at approx. 3× evaporated seawater, whereas halite formation starts at approx. 7× evaporated seawater (red line). From that point onward very little gypsum can precipitate due to the low Ca content.
ultimately depends on the relative magnitudes of the organic matter (OM) flux and the descending gypsum flux. Finding gypsum evaporites basin-wide in shallow marginal sites but not in the deep sites can therefore be easily explained by this unifying biogeochemical mechanism, provided that the shallow-waters gypsum formation rate, i.e. sulphate supply rate to the deep water, is counterbalanced by the sulphate-consumption rate from organic matter mineralization. The potential effects for different degrees of imbalance between deep-water sulphate-input and consumption, and the consequential concentrations developments have been sketched in Fig. 3 and are quantified in the next paragraph. If the deep water remains undersaturated relative to gypsum, then all descending gypsum will dissolve. As a result, the sulphate flux to the deep water can be taken to be similar to the gypsum formation rate in the shallow waters. The average deposition rate deduced from shallowwater Lower Gypsum deposits in Spain and Italy is ∼2.6 M SO4/m2yr (see Supplementary information). The OM flux settling into the deep water is related to its production in the surface water and the degree of remineralization prior to arriving to the deep water. For the present-day open ocean oligotrophic setting, some 10% of the initial OM formed
Decreasing sulphate and concomitantly increasing dissolved carbonate content, both resulting from OM mineralization (see above and Supplemental information), have a major impact on the potential for dolomite formation in the deep water. Sulphate inhibiting dolomite formation, or in other words low sulphate being a requisite for dolomite formation, has often been reported for a range of environments (Baker and Kastner, 1981; Middelburg et al., 1990). Accordingly, sulphatereduction and consequent low-sulphate content often coincide with inferred dolomite formation (Aloisi et al., 2002; Baker and Kastner, 1981; Burton, 1993; Middelburg et al., 1990). Recently, dolomite formation has been shown to occur in very shallow lagoonal, warm, sulphate-reducing environments and to be microbially mediated (Sánchez-Román et al., 2009). One of the rare examples where active dolomite formation could be demonstrated in a normal marine environment, also demonstrated the potential Ca limitation (Middelburg et al., 1990). The latter is unlikely in our scenario not only because the pool of Ca is much larger but also because continuing supply is taking place due to settling and dissolving gypsum. This is even permitting higher rates of dolomite formation, provided that the associated sulphate fluxes are counterbalanced by OM remineralization-related sulphate-consumption rates. In addition, OM remineralization not only consumes the sulphate but also produces ΣCO2 thus contributing two-fold to potential dolomite formation (Fig. 3). 2.4. Gypsum dynamics for marginal settings We have shown here that gypsum precipitation in shallow-water depths and dolomite formation in deep-water settings during the early phase of the Messinian Salinity Crisis in the Mediterranean can be explained by a unifying mechanism. Consequently, a thick rim of gypsum, acting as a cap on top of more liquid mud, would have formed around the Mediterranean margin. Any physical destabilization such as sea-level change or tectonic activity, may then easily have caused downslope transport of these marginal gypsum deposits. Such downslope transport has in fact been suggested and inferred (Martinez del Olmo, 1996; Roveri et al., 2001; Lofi et al., 2005; Ryan, 2008). The seismic-facies of the “Lower Evaporites” (the unit below the transparent halite) in the deep west-Mediterranean basin is interpreted to comprise large elements of clastic sediments and reworked evaporites (Lofi et al., 2005). This is in agreement with the common absence of gypsum and
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Fig. 3. Relative concentration developments for shallow and deep-basin waters along a hypothetical average salinity path (i.e. intermediate natural variability is ignored). SWS: SeaWater Saturation stage: I: representing normal seawater, III–IV: 3 to 4× evaporated seawater respectively. Sufficient re-supply of ions by continued inflow is assumed to maintain approximate surface waters levels, but the potential surface waters precipitation of carbonate at enhanced salinities has been ignored to better visualize the effects by target processes. To clearly illustrate potential processes, we have assumed salinity to increase up to 4SWS, then to remain constant. As gypsum formation takes place between SWS 3 and 7, we could have taken any value in this range. Importantly, the deep-water salinity remains higher than that of surface waters during sustained periods. Furthermore, we have indicated potential realistic ranges for deep-water SO4 content, and have taken into account reduced SO4-inhibition (dark blue shading) for potential dolomite formation (e.g. (Baker and Kastner, 1981)) but have also indicated effects (light blue shading) of potential microbial dolomite formation (Sánchez-Román et al., 2009). Note that illustrated deep-water concentrations are magnified to improve illegibility.
presence of erosional unconformities at the sill-less Mediterranean passive margins (Lofi et al., 2005; Rouchy and Caruso, 2006). Evidence for large-scale resedimentation of gypsum has also been found for deep Messinian settings in the Northern Apennines and Sicily, whereas Lower Gypsum preservation took place only at those marginal settings that were silled (Manzi et al., 2005; Roveri et al., 2008). Stratigraphic correlations combined with magnetostratigraphic dating suggest that in these regions the main phase of gypsum re-sedimentation post-dates the Lower Gypsum units (Manzi et al., 2007; Roveri et al., 2008). Tectonic activity, possibly related to isostatic response to massive MSC halite deposition and/or sea level lowering (Govers et al., 2009), may have intensified this erosional phase by gravitational instability. The only places where the Lower Gypsum units were not removed by erosion are the settings that were protected by tectonic structures such as sills (Krijgsman and Meijer, 2008; Roveri et al., 2008). It is thus ‘preservation’ in the widest sense that has resulted in gypsum deposits rather than a differentiation related to the environment of formation.
3. Conclusions During the Messinian Salinity Crisis thick evaporite units have been deposited at the Mediterranean seafloor. Considerable controversy existed and a range of different mechanisms had been suggested on the mode of their formation, in particular for the observed shallow gypsum versus deepwater dolomite deposits. All observations can be explained by a single unifying mechanism in which gypsum saturation and thus formation takes place at all shallow-water depths and consequently is preserved at a shallow seafloor only. Organic matter remineralization combined with stagnant deep-water conditions results in dissolved sulphate depletion and thus prevents gypsum to be deposited. In other words, the coincidence of anoxic conditions and observed nondeposition of gypsum is not fortuitous but is a logical consequence of common biogeochemical processes. This remineralization not only reduces the dissolved sulphate content but also enhances dissolved carbonate content thus promoting dolomite formation. Gypsum initially
G.J. de Lange, W. Krijgsman / Marine Geology 275 (2010) 273–277
deposited at all shallow settings, may be redeposited into deeper settings due to gravitational instability unless protected by a sill. Acknowledgements CIESM is thanked for the MSC Workshop organization, and all participants to the Almeria CIESM-Workshop are acknowledged for constructive discussions and exchange of views. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.margeo.2010.05.003. References Aloisi, G., Bouloubassi, I., Heijs, S.K., Pancost, R.D., Pierre, C., Sinninghe Damsté, J.S., Gottschal, J.C., Forney, L.J., Rouchy, J.-M., 2002. CH4-consuming microorganisms and the formation of carbonate crusts at cold seeps. Earth and Planetary Science Letters 203, 195–203. Baker, P.A., Kastner, M., 1981. Constraints on the formation of sedimentary dolomite. Science 213 (4504), 214–216. Burton, E.A., 1993. Controls on marine carbonate cement mineralogy: review and reassessment. Chemical Geology 105, 163–179. CIESM, 2008. The Messinian Salinity Crisis from mega-deposits to microbiology — a consensus report. CIESM Workshop Monographs, 33. Monaco. 168 pp. Cita, M.B., Wright, R.C., Ryan, W.B.F., Longinelli, A., 1978. Messinian paleoenvironments. In: Hsü, K.J. (Ed.), Initial Reports of the Deep Sea Drilling Project, pp. 1003–1035. Clauzon, G., Suc, J.P., Gautier, F., Berger, A., Loutre, M.F., 1996. Alternate interpretation of the Messinian Salinity Crisis: controversy resolved? Geology 24 (4), 363–366. De Lange, G.J., 1986. Early diagenetic reactions in interbedded pelagic and turbiditic sediments in the Nares Abyssal Plain (western North Atlantic): consequences for the composition of sediment and interstitial water. Geochimica et Cosmochimica Acta 50, 2543–2561. De Lange, G.J., Boelrijk, N.A.I.M., Catalano, G., Corselli, C., Klinkhammer, G.P., Middelburg, J.J., Müller, D.W., Ullman, W.J., Van Gaans, P., Woittiez, J.R.W., 1990a. Sulphate-related equilibria in the hypersaline brines of the Tyro and Bannock Basins, eastern Mediterranean. Marine Chemistry 31, 89–112. De Lange, G.J., Middelburg, J.J., Van der Weijden, C.H., Catalano, G., Luther III, G.W., Hydes, D.J., Woittiez, J.R.W., Klinkhammer, G.P., 1990b. Composition of anoxic hypersaline brines in the Tyro and Bannock Basins, eastern Mediterranean. Marine Chemistry 31, 63–88. Froelich, p.N., Klinkhammer, G.P., Bender, M.L., Luedtke, N.A., Heath, G.R., Cullen, D., Dauphin, P., Hammond, D., Hartman, B., Maynard, V., 1979. Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis. Geochimica et Cosmochimica Acta 43, 1075–1090. Govers, R., Meijer, P., Krijgsman, W., 2009. Regional isostatic response to Messinian Salinity Crisis events. Tectonophysics 463 (1–4), 109–129. Hilgen, F.J., Krijgsman, W., 1999. Cyclostratigraphy and astrochronology of the Tripoli diatomite Formation (pre-evaporite Messinian, Sicily, Italy). Terra Nova 11, 16–22. Hsü, K.J., Ryan, W.B.F., Cita, M.B., 1973. Late Miocene desiccation of the Mediterranean. Nature 242, 240–244.
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Marine Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r g e o
Letter
Messinian salinity crisis: A novel unifying shallow gypsum/deep dolomite formation mechanism Gert J. de Lange a,⁎,1, Wout Krijgsman b,1 a b
Department of Earth Sciences — Geochemistry, Faculty of Geosciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands Paleomagnetic Laboratory ‘Fort Hoofddijk’, Faculty of Geosciences, Utrecht University, Budapestlaan 17, 3584 CD Utrecht, The Netherlands
a r t i c l e
i n f o
Article history: Received 6 February 2010 Received in revised form 22 May 2010 Accepted 27 May 2010 Available online 4 June 2010 Communicated by D.J.W. Piper Keywords: Mediterranean Messinian evaporite gypsum formation dolomite
a b s t r a c t The Messinian Salinity Crisis is a dramatic event that took place between 5.96 and 5.33 Ma, and resulted not only in the desiccation and reflooding of the Mediterranean but also in the deposition of 0.3–3 km thick evaporites at its seafloor. There has been considerable controversy on the modes of their formation. The first-stage gypsum deposits are considered to have only formed at silled marginal basins, whereas anoxic marls and dolostones are found at deeper settings. We agree with these observations but reject the coincidental presence of similar sills as being too fortuitous to explain gypsum formation in silled basins alone. Alternatively, we present a unifying mechanism in which gypsum precipitation takes place basin-wide at all shallow-water depths but its preservation is limited to silled marginal basins. Taking a realistic early Messinian scenario, including a moderate primary production rate, the sulphate-consumption rate in the deep water appears to exceed the sulphate supply rate as deduced from reported shallow-water gypsum deposition rates. The consequent lower dissolved sulphate content limits gypsum preservation but permits dolomite formation to take place in the deep water. The often observed concurring anoxic environment and absence of gypsum formation is thus related to diminished dissolved sulphate content via organic matter degradation and not to oxygen-free conditions as such. © 2010 Elsevier B.V. All rights reserved.
1. Introduction During the latest Messinian, the Mediterranean Sea transformed into a giant saline basin, one of the largest in the Earth's history and surely the youngest one (CIESM, 2008). This event, referred to as the Messinian Salinity Crisis (MSC), changed the chemistry of the ocean and had a permanent impact on both the terrestrial and marine ecosystems of the Mediterranean area (Cita et al., 1978; Hsü et al., 1973; Ryan, 2009). The onset of the MSC is marked by the deposition of gypsum-sapropel or dolomite-sapropel alternations of the “Lower Evaporites”, which were astronomically dated to have formed between 5.96 and ∼5.6 Ma (Krijgsman et al., 1999; Roveri et al., 2008). Earliest explanations were that evaporites were deposited in a deep and desiccated Mediterranean (Fig. 1a) (Hsü et al., 1973, 1978). Facies models for the gypsum deposits, however, point out impressive similarities in terms of thickness and overall trend, allowing bed-to-bed correlation across the western Mediterranean (Lugli et al., 2008). The “Lower Gypsum” units are now considered to be fully subaqueous with no evidence of a substantial sealevel fall and have an overall aggradational stacking pattern indicative of continuous creation and/or availability of accommodation space (CIESM, 2008; Roveri et al., 2008). Quantitative analyses on the Mediterranean–Atlantic water and salt fluxes indicate that continuing ⁎ Corresponding author. Tel.: +31 30 2535034. E-mail address: [email protected] (G.J. de Lange). 1 Both authors contributed equally. 0025-3227/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2010.05.003
oceanic inflow and restricted outflow must have persisted during its deposition (Krijgsman and Meijer, 2008). Increasing evidence shows that the Lower Gypsum deposits are only preserved at silled marginal basins (Fig. 1b) (Clauzon et al., 1996; Manzi et al., 2007; Roveri and Manzi, 2006), while dolostones or anoxic marls have been reported for deeper settings (Hilgen and Krijgsman, 1999; Roveri et al., 2008). If we reject the coincidental presence of sills for all marginal basins, then a unifying mechanism must exist that causes gypsum precipitation to occur only in shallow settings and dolostone mainly in deep settings (Fig. 1c). 2. Discussion In the following we will first introduce the general principles that govern sulphate and dolomite formation, then apply and interpret these for gypsum and dolomite in shallow and deep Messinian environmental settings. 2.1. Gypsum and dolomite formation and interactions Gypsum precipitation in evaporating seawater has been reported to take place at 3 to 7 times concentrated seawater (Fig. 2) (De Lange et al., 1990a; McCaffrey et al., 1987). Seawater is always largely oversaturated relative to dolomite but its formation has often been reported to be inhibited by the presence of dissolved sulphate (see Supplementary information; Baker and Kastner, 1981). This condition links it to gypsum
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Fig. 1. Schematic illustrations of MSC scenarios explaining the deposition of the Lower Gypsum (LG) units. a) Classic desiccation model (Hsü et al., 1973) that suggests evaporite formation in a deep-basin shallow-water configuration, possibly with diachronism during increasing evaporitic conditions in a system of interconnected silled subbasins of different depths (Rouchy and Caruso, 2006). b) Recently proposed scenario with preferential gypsum formation in silled marginal basins alone (CIESM, 2008; Roveri et al., 2008). The difference in sedimentary environment between marginal and deep settings implies similar physical properties for the sill configuration. c) New unifying scenario (this paper) with Mediterranean-wide gypsum precipitation within shallow-water levels, gypsum preservation in shallow but not in deep settings, and dolomite formation and resedimented gypsum in the deep basins.
saturation, i.e. the conditions for formation of the one exclude those for the other (see Eqs. (1), (2)). 2þ
þ SO4
2þ
þ Mg
Ca Ca
2−
¼ N CaSO4
2þ
þ 2CO3
2−
ð1Þ
¼ N CaMgðCO3 Þ2
2−
ð2Þ −
þ
53SO4 þ C106 H263 O110 N16 P ¼ N39CO2 þ 67HCO3 þ 16NH4 2− − þ HPO4 þ 39H2 O þ 53HS
ð3Þ
(average marine OM) 2−
−
CO3 þ CO2 þ H2 O ¼ N2HCO3
ð4Þ
Another process that links the saturation states of gypsum and dolomite is that of organic matter (OM) decomposition by sulphate reduction. The common sequence of oxidants consumption for OM remineralization goes from oxygen, via Mn- and iron-oxides, to sulphate (e.g. Froelich et al., 1979; De Lange, 1986). The sulphate concentration in seawater being 100-times larger than oxygen, ultimately, i.e. if sufficient OM is available, sulphate will be the predominant oxidant for OM. The latter process will not only decrease the dissolved sulphate concentration but will also increase the dissolved carbonate content (see Eqs. (3), (4) and Supplementary information). Implicitly this means that lowsulphate conditions, i.e. unfavorable conditions for gypsum formation, always coincide with anoxic, i.e. oxygen-free conditions, although there is no direct relationship between the two. The processes described above give a novel, comprehensive explanation for the often observed coincidence of anoxia and the absence of gypsum formation (e.g. Nurmi and Friedman, 1977).
2.2. Gypsum formation and precipitation in shallow versus deep settings Early MSC gypsum deposits have been reported for silled marginal basins alone (e.g. Manzi et al., 2005; Roveri and Manzi, 2006), whereas anoxic marls and dolostones are found at deeper settings (e.g. Roveri et al., 2008; Hilgen and Krijgsman, 1999). We agree with these observations but do not attribute this to initial deposition but rather to subsequent preservation. In addition, the coincidental presence of similar sills seems rather fortuitous to explain gypsum formation in silled basins for different settings.
In general, it is hard to imagine that gypsum precipitation can only occur in shallow settings and not basin-wide: the shallow marginal basin waters would suffer more from evaporation than the open deep-basin waters, leading to a more dense fluid that would – without a sill – immediately flow towards the deeper parts of the basin (Fig. 3). This would not only result in the whole basin to be more saline but also to be stratified. The continued inflow of ocean surface water will remain on top of the deep high-density brine (similar to present-day deep-Ionian brine basins (De Lange et al., 1990b)), while outflowing water is necessary to counterbalance the salt influx. In any case, the Messinian Mediterranean would have been characterized by a reasonably well-mixed upper water mass (‘shallow waters’), and a strongly stratified lower ‘deep-water mass’. The MSC stratification with deep concentrated brines, is very stable, and can only be replaced by an even higher salinity water mass. Such severe stratification automatically results in deep water to become nutrient-enriched and oxygen-depleted due to ongoing decomposition of organic debris from surface water primary production. As a consequence of deep-water anoxia, most of the phosphate resulting from organic matter decomposition will be regenerated rather than buried (Slomp et al., 2004). The latter authors report that the assumed nearly stagnant deep-water conditions during sapropel formation and the consequent enhanced P regeneration have resulted in enhanced primary production thus leading to increased downward fluxes of organic debris (for a detailed discussion, see Slomp et al., 2004 and refs therein). Although stratific6ation may have been more severe during the MSC, given time, a similar scenario can easily be adopted for that time interval as well. Degradation of the settling organic debris via sulphate reduction leads to significantly decreased deep-water sulphate contents (Fig. 3). As gypsum precipitation is primarily controlled by its equilibrium with dissolved calcium and sulphate, sulphate reduction would lead to a decreased saturation or undersaturation with respect to gypsum for Mediterranean deep water, but not for the more regularly replenished shallow waters. The consequence of sulphate concentrations in the deep water to be lower than in the shallow water, is that only in the shallow-water gypsum formation may occur. These gypsum crystals (density of ∼2.3) will subsequently descend to the deep seafloor. In gypsum-saturated shallow settings, these will be preserved, whereas in the gypsumundersaturated deep settings these will be dissolved (Fig. 3). Potentially, this would raise the calcium and sulphate concentrations in the deep water. However, the dissolved deep-water sulphate concentration
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during primary production in the surface waters is thought to be remineralized in the deep water. In the case of high-productivity areas this number is higher (50%) (Sarmiento and Gruber, 2006), and due to decreased degradation rates of OM under anoxic conditions, an even higher fraction can make its way to the deep-water. A remineralization fraction of 25 to 75% for an anoxic brine water column seems therefore realistic for a low and high-productivity scenario respectively. Low to moderately high primary productivity ranges from 30 to 300 g C/m2yr (but can range to N500 g C/m2yr for high primary productivity). The range of deep-water OM remineralization is thus from 8 to 225 g C/ m2yr, corresponding to 0.7 to 19 M C/m2yr, which is equivalent to a sulphate reduction rate of 0.3 to 9.5 M SO4/m2yr. These calculations indicate that the potential sulphate flux into the deep water (2.6 M SO4/m2yr) falls within the range between highand low-estimates for sulphate reduction. We thus conclude that for an anoxic water column and moderate primary production, deepwater sulphate removal rates may exceed the sulphate supply rates, thus that deep water is likely to be undersaturated relative to gypsum. In contrast, for a sediment surface within a relatively shallow surface mixed (oxic) water column (taken to be ∼250 m) such deposit would remain stable as concentrations of dissolved Ca and SO4 remain on average near saturation values for gypsum. In other words, gypsum formation takes place in all shallow waters, whereas gypsum deposits in shallow settings are preserved but not in deep settings. 2.3. Deep-water dolomite formation
Fig. 2. Seawater evaporation path (De Lange et al., 1990a; McCaffrey et al., 1987). Gypsum formation starts at approx. 3× evaporated seawater, whereas halite formation starts at approx. 7× evaporated seawater (red line). From that point onward very little gypsum can precipitate due to the low Ca content.
ultimately depends on the relative magnitudes of the organic matter (OM) flux and the descending gypsum flux. Finding gypsum evaporites basin-wide in shallow marginal sites but not in the deep sites can therefore be easily explained by this unifying biogeochemical mechanism, provided that the shallow-waters gypsum formation rate, i.e. sulphate supply rate to the deep water, is counterbalanced by the sulphate-consumption rate from organic matter mineralization. The potential effects for different degrees of imbalance between deep-water sulphate-input and consumption, and the consequential concentrations developments have been sketched in Fig. 3 and are quantified in the next paragraph. If the deep water remains undersaturated relative to gypsum, then all descending gypsum will dissolve. As a result, the sulphate flux to the deep water can be taken to be similar to the gypsum formation rate in the shallow waters. The average deposition rate deduced from shallowwater Lower Gypsum deposits in Spain and Italy is ∼2.6 M SO4/m2yr (see Supplementary information). The OM flux settling into the deep water is related to its production in the surface water and the degree of remineralization prior to arriving to the deep water. For the present-day open ocean oligotrophic setting, some 10% of the initial OM formed
Decreasing sulphate and concomitantly increasing dissolved carbonate content, both resulting from OM mineralization (see above and Supplemental information), have a major impact on the potential for dolomite formation in the deep water. Sulphate inhibiting dolomite formation, or in other words low sulphate being a requisite for dolomite formation, has often been reported for a range of environments (Baker and Kastner, 1981; Middelburg et al., 1990). Accordingly, sulphatereduction and consequent low-sulphate content often coincide with inferred dolomite formation (Aloisi et al., 2002; Baker and Kastner, 1981; Burton, 1993; Middelburg et al., 1990). Recently, dolomite formation has been shown to occur in very shallow lagoonal, warm, sulphate-reducing environments and to be microbially mediated (Sánchez-Román et al., 2009). One of the rare examples where active dolomite formation could be demonstrated in a normal marine environment, also demonstrated the potential Ca limitation (Middelburg et al., 1990). The latter is unlikely in our scenario not only because the pool of Ca is much larger but also because continuing supply is taking place due to settling and dissolving gypsum. This is even permitting higher rates of dolomite formation, provided that the associated sulphate fluxes are counterbalanced by OM remineralization-related sulphate-consumption rates. In addition, OM remineralization not only consumes the sulphate but also produces ΣCO2 thus contributing two-fold to potential dolomite formation (Fig. 3). 2.4. Gypsum dynamics for marginal settings We have shown here that gypsum precipitation in shallow-water depths and dolomite formation in deep-water settings during the early phase of the Messinian Salinity Crisis in the Mediterranean can be explained by a unifying mechanism. Consequently, a thick rim of gypsum, acting as a cap on top of more liquid mud, would have formed around the Mediterranean margin. Any physical destabilization such as sea-level change or tectonic activity, may then easily have caused downslope transport of these marginal gypsum deposits. Such downslope transport has in fact been suggested and inferred (Martinez del Olmo, 1996; Roveri et al., 2001; Lofi et al., 2005; Ryan, 2008). The seismic-facies of the “Lower Evaporites” (the unit below the transparent halite) in the deep west-Mediterranean basin is interpreted to comprise large elements of clastic sediments and reworked evaporites (Lofi et al., 2005). This is in agreement with the common absence of gypsum and
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Fig. 3. Relative concentration developments for shallow and deep-basin waters along a hypothetical average salinity path (i.e. intermediate natural variability is ignored). SWS: SeaWater Saturation stage: I: representing normal seawater, III–IV: 3 to 4× evaporated seawater respectively. Sufficient re-supply of ions by continued inflow is assumed to maintain approximate surface waters levels, but the potential surface waters precipitation of carbonate at enhanced salinities has been ignored to better visualize the effects by target processes. To clearly illustrate potential processes, we have assumed salinity to increase up to 4SWS, then to remain constant. As gypsum formation takes place between SWS 3 and 7, we could have taken any value in this range. Importantly, the deep-water salinity remains higher than that of surface waters during sustained periods. Furthermore, we have indicated potential realistic ranges for deep-water SO4 content, and have taken into account reduced SO4-inhibition (dark blue shading) for potential dolomite formation (e.g. (Baker and Kastner, 1981)) but have also indicated effects (light blue shading) of potential microbial dolomite formation (Sánchez-Román et al., 2009). Note that illustrated deep-water concentrations are magnified to improve illegibility.
presence of erosional unconformities at the sill-less Mediterranean passive margins (Lofi et al., 2005; Rouchy and Caruso, 2006). Evidence for large-scale resedimentation of gypsum has also been found for deep Messinian settings in the Northern Apennines and Sicily, whereas Lower Gypsum preservation took place only at those marginal settings that were silled (Manzi et al., 2005; Roveri et al., 2008). Stratigraphic correlations combined with magnetostratigraphic dating suggest that in these regions the main phase of gypsum re-sedimentation post-dates the Lower Gypsum units (Manzi et al., 2007; Roveri et al., 2008). Tectonic activity, possibly related to isostatic response to massive MSC halite deposition and/or sea level lowering (Govers et al., 2009), may have intensified this erosional phase by gravitational instability. The only places where the Lower Gypsum units were not removed by erosion are the settings that were protected by tectonic structures such as sills (Krijgsman and Meijer, 2008; Roveri et al., 2008). It is thus ‘preservation’ in the widest sense that has resulted in gypsum deposits rather than a differentiation related to the environment of formation.
3. Conclusions During the Messinian Salinity Crisis thick evaporite units have been deposited at the Mediterranean seafloor. Considerable controversy existed and a range of different mechanisms had been suggested on the mode of their formation, in particular for the observed shallow gypsum versus deepwater dolomite deposits. All observations can be explained by a single unifying mechanism in which gypsum saturation and thus formation takes place at all shallow-water depths and consequently is preserved at a shallow seafloor only. Organic matter remineralization combined with stagnant deep-water conditions results in dissolved sulphate depletion and thus prevents gypsum to be deposited. In other words, the coincidence of anoxic conditions and observed nondeposition of gypsum is not fortuitous but is a logical consequence of common biogeochemical processes. This remineralization not only reduces the dissolved sulphate content but also enhances dissolved carbonate content thus promoting dolomite formation. Gypsum initially
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