U-series and oxygen isotope chronology of the mid-Pleistocene Lake Amora (Dead Sea basin)

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Abstract

This study establishes for the first time the chronology and limnological history of Lake Amora (Dead Sea basin, Israel), whose deposits (the Amora Formation) comprise one of the longest exposed lacustrine records of the Pleistocene time. The Amora Formation consists of sequences of laminated primary aragonite and silty-detritus, Ca-sulfate minerals, halite and clastic units. This sedimentary sequence was uplifted and tilted by the rising Sedom salt diapir, exposing ∼320 m of sediments on the eastern flanks of Mt. Sedom (the Arubotaim Cave (AC) section).

The chronology of the AC section is based on U-disequilibrium dating (230Th–234U and 234U–238U ages) combined with floating δ18O stratigraphy and paleomagnetic constraints. The determination of the 230Th–234U ages required significant corrections to account for detrital Th and U. These corrections were performed on individual samples and on suites of samples from several stratigraphic horizons. The most reliable corrected ages were used to construct an age-elevation model that was further tuned to the oxygen isotope record of east Mediterranean foraminifers (based on the long-term similarity between the sea and lake oxygen isotope archives).

The combined U-series-δ18O age-elevation model indicates that the (exposed) Amora sequence was deposited between ∼740 and 70 ka, covering seven glacial–interglacial cycles (Marine Isotope Stages (MIS) 18 to 5).

Taking the last glacial Lake Lisan and the Holocene Dead Sea lacustrine systems as analogs of the depositional–limnological environment of Lake Amora, the latter oscillated between wet (glacial) and more arid (interglacial) conditions, represented by sequences of primary evaporites (aragonite and gypsum that require enhanced supply of freshwater to the lakes) and clastic sediments, respectively. The lake evolved from a stage of rapid shifts between high and low-stand conditions during ∼740 to 550 ka to a sabkha-like environment that existed (at the AC site) between 550 and 420 ka. This stage was terminated by a dry spell represented by massive halite deposition at 420 ka (MIS12-11). During MIS10-6 the lake fluctuated between lower and higher stands reaching its highest stand conditions at the late glacial MIS6, after which a significant lake level decline corresponds to the transition to the last interglacial (MIS5) low-stand lake, represented by the uppermost part of the Formation.

δ18O values in the primary aragonite range between 6.0 and −1.3, shifting cyclically between glacial and interglacial intervals. The lowest δ18O values are observed during interglacial stages and may reflect short and intense humid episodes that intermittently interrupted the overall arid conditions. These humid episodes, expressed also by enhanced deposition of travertines and speleothems, seem to characterize the Negev Desert, and in contrast to the overall dominance of the Atlantic–Mediterranean system of rain patterns in the Dead Sea basin, some humid episodes during interglacials may be traced to southern sources.

Introduction

The Quaternary climate is characterized by cyclic shifts between different combinations of cold, warm, humid and dry conditions at various sites and time scales. These changes are recorded in many geological archives such as ice cores, deep-sea sediments, coral reefs, speleothems and lacustrine sedimentary sequences.

The paleo-climatic exploration of these archives, requires the establishment of reliable and precise chronologies, which beyond the radiocarbon time scale (∼50 ka) are usually obtained by U-series methods (e.g., Broecker, 1963, Edwards et al., 1987, Ludwig et al., 1992, Winograd et al., 1992, Stein et al., 1993, Kaufman et al., 1998) or luminescence-based techniques (e.g., Stokes, 1999, Lian and Roberts, 2006). The best targets for U-series dating are probably corals (Edwards et al., 2003) and speleothems (e.g., Wang et al., 2001, Bar-Matthews et al., 2003, Richards and Dorale, 2003) although even there, contamination by detrital material and diagentic effects should be considered (Thompson et al., 2003, Villemant and Feuillet, 2003, Lazar et al., 2004). In the case of lacustrine carbonates, U–Th dating is hampered by the need to correct for both detrital additions of U and Th and for the presence of initial (hydrogenous) Th (Kaufman, 1971, Kaufman, 1993, Lin et al., 1996, Israelson et al., 1997, Kaufman et al., 1992, Haase-Schramm et al., 2004, Stein and Goldstein, 2006). Contrary to the 230Th–234U and 231Pa–235U methods, which are only applicable for sediments younger than ∼500 and 200 ka, respectively, the 234U–238U disequilibrium method can be used to extend the datable range up to 1 Ma (Cherdyntsev, 1971, Ludwig et al., 1992, Richards and Dorale, 2003), although it depends on a good knowledge of the initial 234U/238U activity ratio. Additional chronological constraints can be obtained from the reconstruction of changes in the stable isotopes, chemical ratios or paleontological marine records, and correlating them to the well-known changes in Earth’s orbital parameters. In the case of continental records (e.g., Winograd et al., 1992, Tzedakis et al., 2003) such a correlation is not as straightforward because the sea–land climate connection is not always understood or is not strong. Studying the nature of this connection is a pre-requisite for the correlation of continental and marine records (e.g., Kolodny et al., 2005).

In this study we set out to establish a chronological framework for the mid to late Pleistocene Amora Formation (Fm), deposited in the lacustrine environment of the Dead Sea basin (DSB) (Fig. 1, Fig. 2). Together with its successors (Lake Lisan and the Dead Sea), this sequence of sediments comprises one of the longest records of its kind.

Most investigations of lacustrine records have focused on late Pleistocene and Holocene time scales (i.e., <50 ka). There are only a few examples of lacustrine sequences extending several hundreds of thousands of years back in time such as the Burmester core in the Bonneville basin, Utah (Eardley et al., 1973, Oviatt et al., 1999) and Lake Baikal (Prokopenko et al., 2006). These records, however, were obtained from drill cores that provide limited information on field relations and spatial variability. In the case of Lake Amora, a unique tectonic setting (see below) resulted in the uplift and surface exposure of the sedimentary record over a wide area, enabling a better impression of the sequence, high accessibility and relatively easy sampling.

The study of the limnological history of the DSB lakes is important because they are “amplifier lakes”, i.e., their water level responds sharply to climatic changes (Street-Perrott and Harrison, 1985), therefore reflecting changes in regional climatic–hydrologic conditions that can be correlated to global changes (Bartov et al., 2003, Haase-Schramm et al., 2004). Lake level fluctuations are identified through changes in the sedimentary and chemical composition of the sediments (Neev and Emery, 1967, Begin et al., 1974, Katz et al., 1977, Stein et al., 1997) and their mode of sedimentation (Machlus et al., 2000, Bartov et al., 2002, Bartov et al., 2003). The Dead Sea Group (Sedom, Amora, Lisan and Ze’elim Formations; Fig. 1) was so far dated by radiocarbon (Schramm et al., 2000, Ken-Tor et al., 2001, Migowski et al., 2004, Migowski et al., 2006) and the 230Th–234U method (Kaufman, 1971, Kaufman et al., 1992, Schramm et al., 2000, Haase-Schramm et al., 2004, Waldmann et al., 2007). The lake’s chronology is presently available for the past ∼130 ka, with a few ages reaching ∼300 ka. Beyond that time, the DSB lacustrine bodies have not been directly dated. Based on stratigraphic considerations, Zak (1967) suggested that the Sedom lagoon existed between the late Pliocene and early Pleistocene, and that the age of the Amora Fm. is between ∼1 Ma and ∼100 ka.

Section snippets

The Pleistocene Lake Amora

The history of water bodies in the DSB began with the ingression of the Mediterranean Sea during Pliocene times (the Sedom Lagoon; Zak, 1967). The interaction of the evaporated seawater filling the Sedom lagoon with the carbonate wall-rocks of the Dead Sea rift valley together with sulfate reduction led to the production of a Ca-chloride brine, which played a major role in the subsequent evolution of the lacustrine bodies (Starinsky, 1974). After the disconnection of the Sedom Lagoon from the

Field work and sample preparation

Field samples approximately 20 × 20 × 20 cm3 in size were collected from the Amora Fm. exposures at AC and PZ2 sites, wrapped in plastic coating to avoid disintegration and transported to the laboratory.

Approximately 5 g of selected laminae (aragonite, detritus, gypsum and anhydrite) were scraped separately and collected. Following grinding and homogenization, the mineralogical composition of selected samples (Table 1) was identified by XRD.

Chemistry

Most samples were processed at the Geological Survey of

230Th–234U ages

The isotopic composition of U and Th was analyzed in 91 samples. In eight more samples only U isotopic compositions were measured (Table 2). Only samples that met the following arbitrarily-determined requirements were considered reliable for single sample age calculation: (1) (230Th/232Th) > 10, (2) (238U/232Th) >10, (3) [Th] < 0.5 ppm (Fig. 6). Samples that did not pass this test were considered as containing a high-detrital component and therefore not sufficiently clean. 232Th-rich samples display

Conclusions

  • 1.

    The chronology of the Amora Fm. was established by combining U-series, δ18O wiggle matching, stratigraphic correlations to dated sequences, extrapolated sedimentation rates and complementary paleomagnetic data. This effort extends the chronology of the Dead Sea basin fill, thus establishing one of the longest records of its kind. Each of the chronological tools discussed in this study could not independently suffice for the determination of the chronology of the Amora Fm., and it is only their

Acknowledgments

We thank Y. Enzel (HUJI) and A. Ayalon (GSI) for their critical reading of an earlier version of this work, as well as A. Starinsky (HUJI), I. Gavrieli and M. Bar-Matthews (GSI), and S. Goldstein (LDEO) for fruitful discussions. A. Hofmann (MPI, Mainz) was one of the initiators of this project and provided assistance along the way. E. Shelef, B. Tatarsky, N. Jesselson, U. Shaanan, U. Kodington and S. Reuveni helped in field and lab work. We also thank E. Barkan for performing the oxygen isotope

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    Present address: Lamont-Doherty Earth Observatory, Columbia University, USA.

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