U-Th dating of Lake Lisan (late Pleistocene dead sea) aragonite and implications for glacial east Mediterranean climate change
Introduction
The late Pleistocene and Holocene were characterized by rapid climate changes, reflected in the growth and retreat of large continental ice sheets and lakes, changes in sea level, and shifts in climatic zones. Many of these changes have been documented through analyses of deep-sea cores, coral reefs, and polar ice. Continental climate records are often complicated, discontinuous, and geographically scattered. However, they are more accessible than polar ice or the sea floor. They contain information on human living environments, and are key for understanding the interplay between marine and continental climate changes. Laminated lake sediments are important sources of climate information, often with a seasonal time resolution (cf. the early works of Broecker and Kaufman 1965, Kaufman 1971, Kaufman and Broecker 1965, Benson 1978).
Accurate ages are necessary for valid comparisons of continental records with marine sediments and ice cores. U-series dating by mass spectrometry is a primary means for obtaining precise ages of late Quaternary continental carbonates, as shown by studies on vein calcite Winograd et al 1988, Winograd et al 1992, and cave deposits (Kaufman et al., 1998). Obtaining accurate U-series ages on lake carbonates is problematic, because they typically contain significant amounts of detritus, whose Th and U must be considered in age determinations Ku and Liang 1984, Schwarcz and Latham 1989, Kaufman 1993, Bischoff and Fitzpatrick 1991, Luo and Ku 1991. Over the past two decades studies have discussed sample processing methods and corrections necessary to obtain accurate U-series ages of “dirty” carbonates. Recommendations have included leaching of the carbonate, whole rock dissolution, or mechanical separation of carbonate and detritus Ku and Liang 1984, Schwarcz and Latham 1989, Przybylowicz et al 1991, Kaufman 1993, Bischoff and Fitzpatrick 1991, Luo and Ku 1991, Henderson et al 2001. 238U-234U-230Th “isochron” techniques have been generally used to correct detrital contributions to the U and Th in samples. The independent presence of hydrogenous Th can cause additional age biases Luo and Ku 1991, Bischoff and Fitzpatrick 1991, Lin et al 1996.
To compare diverse climate records, it is also necessary to obtain ages at high resolution. This is possible using 14C from the Holocene to the last glacial maximum, but is problematic for ages over ∼25 ka, when the calendar calibration of 14C ages is not yet resolved well, and for ages greater than ∼35 ka when 14C age errors become significant. For dating of lake carbonates, generation of U-Th isochrons is labor intensive, requiring analyses of several samples for each age obtained to minimize error propagation due to small data populations (Ludwig, 2001).
This paper focuses on 238U-234U-230Th disequilibrium dating of aragonite from Lake Lisan, the late Pleistocene precursor of the Dead Sea (Fig. 1). The Lisan Formation consists mainly of laminated inorganic aragonite-detritus, plus gypsum, which precipitated from the lake water, and thicker layers of clastic material transported to the lake. The pioneering attempts to apply U-series dating to Lisan Formation sediments by Kaufman and colleagues Kaufman 1971, Kaufman 1993, Kaufman et al 1992 used α-counting techniques. They showed that reasonable ages can be obtained on Lisan Formation aragonite and that stratigraphic sections at different localities can be correlated. They found that Lake Lisan existed through most of the last glacial period from ∼63–18 ka.
We present a chronological record, determined by thermal ionization mass spectrometry (TIMS), of a measured and described stratigraphic section in the Perazim Valley (called PZ1), southwest of the Dead Sea (Fig. 1). In Lisan Formation aragonite samples the ages must take into account admixed detritus and initial hydrogenous Th. To reduce the number of analyses and still yield a high-resolution record, our strategy combined multiple analyses from a few stratigraphic heights, and fewer analyses from many heights in-between. The ages obtained are in stratigraphic order, and their accuracy is supported by concordance with the calendar age calibration of the 14C timescale by corals.
The Lisan Formation is a remarkable archive of the late Pleistocene history of the Earth. Recent studies address the region’s climatic Stein et al 1997, Machlus et al 2000, Abed and Yaghan 2000, Stein 2001, Bartov et al 2002, Bartov et al 2003, Landmann et al 2002, seismic (Marco et al., 1996), and paleomagnetic history (Marco et al., 1998). A precise chronology is the foundation of all of these studies, as well as the basis for comparison to other regional climate archives such as speleothems Bar-Matthews et al 1997, Bar-Matthews et al 1999, Bar-Matthews et al 2000, Kaufman et al 1998, Matthews et al 2000, Frumkin et al 1999, Frumkin et al 2000.
Section snippets
Lake Lisan: geological background
The ∼ 40-m-thick PZ1 section Fig. 1, Fig. 2 has been described at cm resolution Marco et al 1996, Marco et al 1998, Machlus et al 2000. It was selected for detailed geochemical, paleoseismic, and paleomagnetic studies Stein et al 1997, Machlus et al 1997, Marco et al 1996, Marco et al 1998 because of its apparently simple and conformable stratigraphy. Its top is 267 m below sea level (mbsl) and 150 m above the present Dead Sea (at 417 mbsl). Aragonite appears in thin (∼0.5–1 mm) laminae
Results
Sampling and analytical techniques are reported in Appendix A1. Locations are shown in Figure 1. U-series data are reported on PZ1 sample groups from 22 stratigraphic heights, averaging more than a sample group every 2 meters. Non-PZ1 samples include a ∼6-cm-thick aragonite crust on a piece of driftwood from the Dead Sea shore at Ein Gedi, and a Lisan Formation sample from Menahemya, south of the Sea of Galilee. Minor and trace element abundances are reported for some of the aragonite and
Discussion
The classic age equation for a carbonate with 234U/238U ratios out of secular equilibrium (Broecker, 1963, and Appendix A2, Eqn. A1) assumes no initial 230Th and closed system evolution. If a sample contains admixed detritus, then U-Th isochrons Ku and Liang 1984, Kaufman 1993, Bischoff and Fitzpatrick 1991, Luo and Ku 1991, designed to determine the detritus-free 234U/238U and 230Th/238U ratios of a set of coeval samples, can be used to determine ages. For a sample composed of detritus and
Conclusions
The primary findings of this study are summarized as follows.
- (1)
A high-resolution chronology is presented for the Lisan Formation, Jordan Valley, through U-230Th dating of inorganic aragonite by mass spectrometry, based on a strategy that combines multiple measurements from a few stratigraphic levels and fewer analyses from many levels in-between.
- (2)
The classical problem associated with U-series dating of carbonate sediments, the effects of admixed detritus, is addressed through analyses of soluble
Acknowledgements
This study has received support from the German-Israel Foundation for Scientific Research (GIF, Grant 332), the US-Israel Binational Science Foundation (BSF, Grant 96.227), and the Israel Scientific Foundation (ISF, Grant 694.95). The paper has benefited from reviews by Ken Ludwig and anonymous referees. Hagai Ron, Shmuel Marco, Malka Machlus, and Yuval Bartov joined the fieldwork at Perazim Valley. Discussions with Amitai Katz, Yehoshua Kolodny, and Abraham Starinsky were much appreciated.
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Present address: GEOMAR, Universität Kiel, Wischhofstrasse 1–3, 24148 Kiel, Germany
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Present address: Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel