Earthquake-induced barium anomalies in the Lisan Formation, Dead Sea Rift valley, Israel
Introduction
The present paper is a first report of prominent barium concentration anomalies in seismically perturbed, brecciated layers within the Lisan Formation in the Dead Sea basin (DSB). The purpose of our study is two-fold:
- 1.
Provision of geological and geochemical data pertinent to the occurrence and composition of the Ba-rich layers.
- 2.
Evaluation of a possible relationship between the anomalies and frequent earthquakes that occurred during their formation.
The Ba-enriched seismites are contained within the Pleistocene Lisan Formation exposed in the DSB (Fig. 1). The geology and hydrology of the basin, as well as the geochemistry of its waters and sediments have been thoroughly investigated during the last six decades (e.g. Bentor, 1961, Goldschmidt et al., 1967, Zak, 1967, Zak, 1997, Neev and Emery, 1967, Freund et al., 1970, Starinsky, 1974, Begin et al., 1974, Begin et al., 2004, Katz et al., 1977, Garfunkel, 1981, Stein et al., 1997, Stein et al., 2000, Stanislavsky and Gvirtzman, 1999, Moise et al., 2000, Krumgalz et al., 2000, Sagy et al., 2003, Klein-BenDavid et al., 2004, Klein-BenDavid et al., 2005, Kolodny et al., 2005, Bartov et al., 2006, Enzel et al., 2006, Torfstein, 2008, Katz and Starinsky, 2009, Torfstein et al., 2009).
Dead Sea basin sediments reflect two different environments of deposition that existed, sequentially, in the area since Neogene times. These are the Sedom marine evaporitic inland lagoon, and the lacustrine environment of the DSB Lake (Katz and Starinsky, 2009).
Recent studies shed light on the lacustrine sediment record deposited in the DSB Lake between ∼ 720 and 70 ka (Waldmann et al., 2007, Torfstein et al., 2009, Katz and Starinsky, 2009). The overlying Lisan Formation (∼ 70 ka to 14 ka, after Haase-Schramm et al., 2004) is up to ∼ 40 m thick and is exposed between the northern Arava Valley in the south and Lake Kinneret (Sea of Galilee) in the north. The principal facies consists of alternating aragonite and detritus laminae (“AAD”, after Marco, 1996), up to a few mm thick, where the former is perfectly preserved material which precipitated from the lake during summer, while the latter were washed into the lake during winters (Begin et al., 1974, Katz et al., 1977). The detritus consists of calcite, dolomite, quartz and clay. Laminated, massive and disseminated gypsum, and sandy beds make up the rest of the Lisan Fm. (Stein et al., 1997).
The Lisan Fm. is covered by the Zeelim Fm. deposited as of about 10.2 ka, containing similar detritus-evaporate laminated packages (Migowski et al., 2004, Migowski et al., 2006).
Evidence supporting deposition of the Lisan sediments from a saline to hypersaline water body includes their fine, unperturbed lamination, the high water-soluble (chloride) salt content and the ion ratios therein, the predominance of aragonite laminae and Sr/Ca and Sr isotope ratios therein (Begin et al., 1974, Begin et al., 2004, Katz et al., 1977, Katz and Kolodny, 1989, Stein et al., 1997, Katz and Starinsky, 2009).
While practically all of the H2O in DSB waters is of meteoric origin (Moise et al., 2000, Kolodny et al., 2005, Katz and Starinsky, 2009), the salts dissolved therein were inherited from ancient Mediterranean seawater and from upper Cretaceous carbonate rocks with which these waters interacted. The origin of the salts and later compositional modification are reflected by the Ca-chloridic nature (Ca > (SO4 +HCO3)) of the saline DSB waters. Starinsky (1974) proposed that the high salinity of the DSB brines was achieved by evaporation of seawater in the Sedom lagoon, from which they infiltrated into upper Cretaceous limestone aquifers. Most of the Mg in the brine was lost during dolomitization of the limestone, in exchange for Ca. The latter interacted with the brine's sulfate, precipitating gypsum and/or anhydrite. The low Na/Cl ratios in the brines resulted from halite crystallization in the original lagoon.
No seawater incursions postdating the Sedom lagoon are known, leaving its chloride and accompanying marine solutes as the dominant source of salts in the DSB waters. Hydrological changes, Dead Sea transform tectonics and climatic fluctuations left their impact on the brines in the subsurface, as well as on the saline DSB Lake. A major feature of the DSB Lake was the intermittent stratification of its water column (Katz et al., 1977, Stein et al., 1997) brought about by the density difference between the feeding Ca-chloride brines (ρ ≤ 1.3 g cm− 3) and freshwater runoff (ρ ≈ 1 g cm− 3). Stratification of the lake must have occurred during lake level rises responding to “wetter” periods, when the freshwater input exceeded the evaporative loss of water. Katz and Starinsky (2009) showed that the characteristic AAD facies of the Lisan Fm. evolved during stratified periods of the lake. Independent observations demonstrating anoxic conditions that existed in the lake's deeper waters (Gavrieli et al., 2001, Torfstein et al., 2005, Torfstein et al., 2008) and the stratification of the modern Dead Sea that lasted well into the 20th century (Neev and Emery, 1967) support the same conclusion.
The Lisan Formation, extending along a 220 km active plate boundary, provides an outstanding sediment sequence for paleoseismic studies (Seilacher, 1984, Marco and Agnon, 1995, Marco et al., 1996, Marco, 1996, Begin et al., 2005, Heifetz et al., 2005). It displays many well-preserved exposures and sensitive stratigraphic markers, as well as a continuous, densely-dated stratigraphic record (Schramm, 1997, Schramm et al., 2000, Haase-Schramm et al., 2004).
The Ba-anomalies occur within well-defined breccia layers, or seismites, contained within orderly-laminated sediment layers of the Lisan Fm. (Fig. 2). Field relations show that each seismite was formed by earthquake shaking (Marco and Agnon, 1995, Marco and Agnon, 2005, Agnon et al., 2006). Such deformation structures were found also in younger (up to ≈ 10 ka) sediments around the Dead Sea (Enzel et al., 2000, Ken-Tor et al., 2001, Migowski et al., 2004). While all of the Ba anomalies found are confined to the Lisan seismites, similar seismites without Ba anomalies also appear in the studied area. An explanation of the first observation must be consistent also with the second.
Here we argue that the Ba anomalies were formed by stripping of Ba2+ ions from the DSB Lake water column, into Ba(Sr)SO4 crystallites. The latter nucleated on the surfaces of grains suspended from the bottom into the lake by earthquake shaking, and during their settling and accumulation as seismites on the lake's bottom.
Section snippets
Field work
Sampling of the seismic breccias was carried out in the Lisan Fm. outcrops east of Masada (Fig. 1). Fresh 0.5–1.5 g samples were collected from vertical cliffs by manually pushing a 5 mm ID cylindrical steel corer into the soft, horizontal sediment layers. The vertical sampling profile included also a few centimeters of the undisturbed laminae beneath and above the seismite. Two larger samples (≈ 30 × 20 × 20 cm) from the Lisan Masada outcrop were carved from the outcrop to obtain better resolution
Results and discussion
Chemical analyses of 90 samples collected from a vertical profile through seismite “B” (Fig. 2), are presented in Tables S1 (water extracts) and S2 (acid extracts of the water extracted samples). More aragonite than detritus laminae are listed in the tables because the latter are frequently too thin for separation. Characteristic Ba concentration and Ba/Ca ratio profiles across the seismite are displayed in Fig. 3, Fig. 4. The slight difference in the seismite thickness between the exposure
Summary and conclusions
Seismites, in the form of breccia layers in the Lisan Fm. along the Dead Sea western shore display conspicuous Ba concentration anomalies, well above background values measured in underlying and overlying sediments. Ba concentrations in the seismites gradually increase upwards, attaining maxima close to, or at the upper boundary of the breccias. HR-SEM and EMP analyses show that the excess Ba resides within micron-scale, Ba(Sr)SO4 crystallites anchored to the matrix grains.
The following
Acknowledgements
We are grateful to Dr. Elad Israeli, Mrs. Tamar Shalev, and Dr. Inna Popov from the Hebrew University for their professional assistance in SEM and EMP analysis. Mrs. Ahuva Agranat and Enat Kasher carried out the elaborate water and acid extractions and analytical preparations in the Hebrew University geochemical laboratory. Ofra Klein-BenDavid was very helpful in the fieldwork and in the meticulous separation of the Lisan aragonite and detritus laminae from the bulk samples collected in the
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