The Messinian evaporites in the Mediterranean: a box model study
R.P.M. Topper
1, P.Th. Meijer
1, R. Flecker
2and M.J.R. Wortel
11Department of Earth Sciences, Utrecht University, Utrecht, The Netherlands
2BRIDGE, School of Geographical Sciences, Bristol University, Bristol, UK e-mail: r.p.m.topper@uu.nl
1. Introduction
Many aspects of the Mediterranean massive evaporite layers, deposited during the Messini- an Salinity Crisis (MSC), remain controversial and unexplored.
Combining strontium isotope and salinity data with results from a box model, we investigate if and how the fresh water budget and the size of the Atlantic-Mediterranean gateway varied during the Late Miocene.
Dating of the onset of marginal gypsum forma- tion shows it is synchronous throughout the Mediterranean. The deep water halite depo- sits, on the other hand, are hard to date and are substantially thicker in the EMed than in the WMed. We examine, with another box mo- del, the spatial and temporal evolution of sa- linity in the Mediterranean with different fresh water budgets and gateway sizes in order to explain the observed synchroneity and diffe- rent halite thicknesses.
2. Models
Sr concentration, Sr-isotope ratio and a Late Miocene water budget are incorporated in a 2-box model.
Outflow (Q) from each basin is line- arly dependent on the salinity con- trast between the basins; Q=gXY (SX-
SY), where gXY is the linear exchange coefficient. gXY can be varied between 106 and 101, corresponding to an open and very restricted gateway, respectively.
This Sr box model allows for examination of the temporal evolution of salt and Sr-isotope ratio as a function of the individual hydrologic fluxes (Atlantic in and outflow, river input and evaporation).
After adding a parametrisation of watercolumn stratification and evaporite formation, the evaporite box model can be used
to examine timing and thickness of gypsum and halite.
Gypsum deposition commences in a basin once the salinity in one of the layers exceeds 145 g/l, halite depo- sition starts above 350 g/l.
3. Strontium isotopes
The Late Miocene Sr-curve
Pre-MSC 87Sr/86Sr values only deviate from coeval oceanic values (shaded area) in marginal basins and the Tyrrhenian Sea. During the MSC 87Sr/86Sr values drop significantly from ocean values to 0.7085 in the last stage of the MSC. After the Pliocene reflooding 87Sr/86Sr values shift back to oceanic values.
4. Primary Lower Gypsum
Taking into account the uncertainties associated with the dating, gypsum formation at the onset of the MSC should commence with a maximum difference of ~20 kyr between the basins to be considered synchronous.
The largest range of parameters with synchronous onset of gypsum formation is
found in settings with a large Strait of Sicily. This range increases by introducing stra- tification.
5. Halite
Because the Messinian halites are mainly found in the deep basins, their origin and timing of on- set are not well known. As for the PLG, for tho- se model results that show halite formation in both the WMed and EMed, we can determine whether the onset of halite formation is synchro- nous.
In the parameter range where halite is formed in both basins, halite formation is generally syn- chronous. Only when the Strait of Sicily is suf- ficiently restricted, is asynchronous formation possible.
While the observed thickness of gypsum in the WMed and EMed is similar, the halite thickness in the EMed (2 km) is more than twice that of the WMed (< 1 km). These differences are as yet unexplained. The duration of deep basinal halite formation is largely unconstrained, but extrapolating from the few marginal basin out- crops, it is placed in the 60 kyr interval between 5.61 and 5.55 Ma.
Halite thicknesses formed in the model in 60 kyr should approximate the observed thicknes- ses and have a ratio between EMed and WMed thickness of ~2. Both requirements are only met in a small parameter range (in the orange area within the black outline) in settings with stratifi- cation, a large Strait of Sicily and a highly res- tricted Atlantic-Mediterranean gateway.
The different thicknesses can be explained by a different ratio of E-P-R over basin volume, the main control on the net salt gain per unit volume for each basin.
References
R.P.M. Topper, R. Flecker, P.Th. Meijer and M.J.R. Wortel, 2011. A box model of the Late Miocene Mediterranean Sea: Implications from combined 87Sr/86Sr and salinity data. Paleoceanography 26, PA3223.
R.P.M. Topper and P.Th. Meijer, submitted. A modelling perspective on spatial and temporal variations in Messinian evaporite deposits.
6. MSC scenario
Before the MSC, oxygen and carbon isotopes, and the occurrence of sapropels indicate some restriction of the Atlantic-Mediterranean gateway and anoxia in the deep basins.
A restriction of the Atlantic-Mediterranean gateway causes the Mediterranean salinity to rise.
Neither the fresh water budget or the size of the Strait of Sicily changes. Gypsum starts to form in the marginal basins, while it is inhibited by anoxia in the deep basin.
A further restriction of the Atlantic-Mediterranean gateway forces the Mediterranean to halite saturation. Halite is precipitated in the deep basins from the denser deep water layer. Some-
time during halite deposition the Atlantic-Mediterranean gateway closes completely, leading to a large sea level drop and erosion along the margins. During the sea level fall, at most 33% of the observed halite deposits can be formed. A connection with the Atlantic is necessary during part of this phase to explain the whole halite volume.
Model results
Salinity and 87Sr/86Sr values are
known for three Late Miocene time intervals; pre-MSC,Primary Lower Gypsum (PLG) and halite (HL). Lo- cating the corresponding range in our model results we obtain an esti- mate of fresh water budget and ga- teway restriction in these intervals.
Pre-MSC: open marine salinities
and 87Sr/86Sr values correspond to a model with non-restricted gateway exchange with the Atlantic.
MSC (PLG): gypsum saturation and
87Sr/86Sr values below the oceanic range correspond to a model with restricted gateways and an average river input and evaporation
MSC (HL): halite saturation and low 87Sr/86Sr values correspond to a model with almost closed gateways and a slightly wetter climate.
The pathway between these stages delineates simple gateway restric- tion with a slightly wetter climate during the HL stage.
Sr box model
Evaporite box model
Salinity and 87Sr/86Sr results with inter- pretation from a model with a late Mio- cene geometry and water budget as a function of gateway size (gWA) and
river input (x R)
Relative timing of the onset of gypsum deposition in the
western and eastern Mediterranean as a function of gateway efficiencies (gWA and gEW) and river inputs
(RWest and REast). Similar to the figure on the left. Relative timing of the onset of halite deposition in the western and eastern Mediterranean as a function of ga- teway efficiencies and river inputs.
Colours indicate whether the ratio between
EMed and WMed halite thicknesses is correct.
The outlined areas indicate where modelled halite thicknesses fit with observations in both basins.
PP31A-2011
Rhone
Nile (0.706)
PLG
Halite
0.7085 0.7086 0.7087 0.7088 0.7089 0.7090 0.7091 0.7092
87 Sr/86 Sr
4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0
Age [Ma]
Ocean curve
Sardella/M.dei Corvi, Italy DSDP, Balearic
Pissouri/Cyprus Gavdos, Greece DSDP, Ionian DSDP, Levantine
Northern Apennines, Italy
M. del Casino, Italy San Severino, Italy Sicily
Sorbas Basin, Spain Southern Turkey Tyrrhenian Sea
Vena del Gesso, Italy Vera Basin, Spain
MSC
Pre−MSC
PLG HL
1
100 101 102 103 104 105 106
g WA [m3 /s per g/l]
0.5 1.0 1.5 2.0
fR
50 100 150 200 250 300
Salinity [g/l]
3
fR
Increasing river input in Med Increasing size of connection between Atlantic and Med
Pre−MSC
PLG HL
1
100 101 102 103 104 105 106
g WA [m3 /s per g/l]
0.5 1.0 1.5 2.0
fR
0.7084 0.7086 0.7088 0.7090 87Sr/86Sr
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.00.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.00.5 1.0 1.5 2.0
Weak stratificationStrong stratificationNo stratification x R Eastx R East
x RWest x RWest West x RWestWest
x R Eastx R East
x RWest x RWest x RWest x RWestWest
x R Eastx R East
x RWest x RWest x RWest x RWestWest Correct thickness ratio Wrong thickness ratio Correct halite thickness in both basins
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
x R East x R East
x R East
x RWest x R West x R West
x R East
x R WestWest
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0
Weak stratificationStrong stratificationNo stratification x R East x R East
x R East
x R West x RWest x R West
x R East
x R WestWest
x R East x R East
x R East
x R West x RWest x R West
x R East
x R WestWest
0.5 1.0 1.5 2.0
x R East
0.5 1.0 1.5 2.0
x R East
0.5 1.0 1.5 2.0
x RWest 0.5 1.0 1.5 2.0
x RWest 0.5 1.0 1.5 2.0
x R West 0.5 1.0 1.5 2.0
x RWest
0.5 1.0 1.5 2.0
West
0.5 1.0 1.5 2.0
x R East
0.5 1.0 1.5 2.0
x R East
0.5 1.0 1.5 2.0
x RWest 0.5 1.0 1.5 2.0
x RWest 0.5 1.0 1.5 2.0
x R West 0.5 1.0 1.5 2.0
x RWest
0.5 1.0 1.5 2.0
West
0.5 1.0 1.5 2.0
x R East
0.5 1.0 1.5 2.0
x R East
0.5 1.0 1.5 2.0
x RWest 0.5 1.0 1.5 2.0
x RWest 0.5 1.0 1.5 2.0
x R West 0.5 1.0 1.5 2.0
x RWest
0.5 1.0 1.5 2.0
West
Increasing size of connection between WMed and EMed (Sicily Strait) Increasing size of connection between Atlantic and WMed
Increasing river input in EMed
Increasing river input in WMed
Weak stratificationStrong stratificationNo stratification
100 102 104 106 m3/s (g/L)-1
gEW
100
101 gWA
Synchronous Not synchronous Deposition in one /none of the basins
gWA gEW
E-P RWest E-P REast
Atl WMed EMed
QAW
QWA QWE
QEW S/G/HW SURF
S/G/HW DEEP
S/G/HE SURF S/G/HE DEEP S/G/HA
gWA
E-P RWest E-P REast
Atl WMed EMed
QAW
QWA QWE
QEW SE [Sr]E
87Sr/86SrE SA
[Sr]A
87Sr/86SrA
Sw [Sr]W
87Sr/86SrW
Salinity
~35
Pre-MSC (pre 5.96 Ma)
Anoxia
WMed EMed
marginal basin
silled marginal
basin
Atl
Salinity
~145
PLG (5.96 - 5.61 Ma)
Salinity
>350
Salinity
>350
1 - 1.5 km Halite (5.61 - ... Ma)
Halite (... - 5.55 Ma)
gWA gEW
E-P RWest E-P REast
Atl WMed EMed
QAW
QWA QWE
QEW S/G/HW SURF
S/G/HW DEEP
S/G/HE SURF S/G/HE DEEP S/G/HA
gWA
E-P RWest E-P REast
Atl WMed EMed
QAW
QWA QWE
QEW SE [Sr]E
87Sr/86SrE SA
[Sr]A
87Sr/86SrA
Sw [Sr]W
87Sr/86SrW
