Large floods during late Oxygen Isotope Stage 3, southern Negev desert, Israel

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Quaternary Science Reviews 25 (2006) 704–719

Large floods during late Oxygen Isotope Stage 3, southern Negev desert, Israel Noam Greenbauma,b,, Naomi Poratc, Ed Rhodesd, Yehouda Enzele,f a

Department of Geography & Environmental Studies, University of Haifa, Mt. Carmel, Haifa 31905, Israel Department of Natural Resources & Environmental Management, University of Haifa, Mt. Carmel, Haifa 31905, Israel c Geological Survey of Israel, 30 Malkhe Israel St., Jerusalem 95501, Israel d Research School of Earth Sciences and Research School of Asian and Pacific Studies, The Australian National University, Canberra, ACT 0200, Australia e Institute of Earth Sciences, Hebrew University, Givat Ram, Jerusalem 91904, Israel f Department of Geography, Hebrew University, Mt. Scopus, Jerusalem 91905, Israel b

Received 20 December 2004; accepted 12 July 2005

Abstract Slackwater deposits were found in a cave in the Nahal Netafim catchment (35 km2), near the head of the Gulf of Aqaba in the southern Negev, Israel. The sedimentological record includes 27 large paleofloods, dated by infrared stimulated luminescence to 33,000–29,000 years ago. The scatter of the ages and their large uncertainties prevented an assessment of the exact duration of the record and the specific timing of each flood. Bayesian analysis was used to adjust the dense dating results so that the time interval between the first and last flood deposit preserved in the cave could be estimated. Minimum peak discharges were reconstructed based on the estimated elevation of the Late Pleistocene channel bed as indicated by fluvio-pedogenic layers found near the cave. The average frequency of these large floods (200–600 m3 s 1) for the period between 33,000 and 29,000 years ago is about 1 flood per 150 years, while for the mid–late Holocene it is only 1 large flood per 1000 years. Eight floods out of the 27 recorded deviate from the envelope curves of mid–late Holocene paleofloods and measured floods in the hyperarid Negev desert, indicating a different hydroloclimatological regime. The anomalous large floods are hypothesized to have resulted from an increase in regional rainfall intensity and/or duration, attributed to increased frequency of the Red Sea Trough low-pressure system that affects the region. Available records indicate that the northern Negev and areas farther north in Israel controlled by Mediterranean pressure systems were wetter 40–20 ka BP. At the same time, the southern Negev, probably in response to the Red Sea Trough system, also experienced short episodes of more and/or larger rainstorms. The timing of these episodes in both the northern and the southern Negev towards the Last Glacial Maximum points to a potential synchronous strengthening of both the Mediterranean and Red Sea systems, currently acting at different seasons. These episodes of increased storminess in the area were brief and were not able to alter the general hyperarid conditions in this area. r 2005 Elsevier Ltd. All rights reserved.

1. Introduction In a small basin at the head of the Gulf of Aqaba a series of frequent extremely large floods, much larger Corresponding author. Department of Geography & Environmental Studies, University of Haifa, Mt. Carmel, Haifa 31905, Israel. Tel.: +972 4 8240020; fax: +972 4 8249605. E-mail address: [email protected] (N. Greenbaum).

0277-3791/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2005.07.008

than expected from modern records, were identified. Here we document these floods, date them and show that they are consistent with other geomorphological evidence for episodes of large floods at the same period. Among the various aspects that comprise a paleoclimate, the most challenging to establish are the records of the extremes. One of the few ways available to generate such records is the determination of frequency and magnitudes of past floods with the assumption that

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they represent the extreme rainstorms in a region where snowmelt runoff is not a factor. Paleoflood hydrology can provide such long-term records. Reconstruction of past floods includes the determination of peak discharge and time of occurrence. Paleoflood studies, using slackwater deposits (SWD) and other paleostage indicators (PSI) were conducted in semi-arid and a few subhumid areas of the world including the United States (e.g., Patton et al., 1979; Kochel et al., 1982; Ely and Baker, 1985; Baker, 1987; Partridge and Baker, 1987; Webb et al., 1988; Enzel et al., 1994; O’Connor et al., 1994), Australia (e.g., Baker et al., 1983, 1987; Wohl, 1988), monsoonal peninsular India (e.g., Ely et al., 1996), Spain (Benito et al., 2003) and Southern France (Sheffer et al., 2003). Over 200 paleofloods from 19 sites in 14 drainage basins were reconstructed in the hyperarid (o100 mm yr 1) Negev Desert, Israel (Fig. 1), with drainage areas ranging between 6 and 2950 km2 (Greenbaum, 1996; Greenbaum et al., 2000, 2001). The best-preserved SWD records occur in the larger drainage basins, with areas 41000 km2. Such records were found in Nahal Zin, Nahal Paran and Nahal Neqarot catchments (Fig. 1). Smaller drainage basins yielded poorer preservation and smaller number of flood deposits (Greenbaum et al., 2001). Most of the SWD provided data on the largest floods that occurred in the specific basin during the length of record. The magnitudes of paleoflood peak discharges in the Negev Desert are usually between 2 and 4 times higher than the maximal measured peak discharge (Greenbaum et al., 2001). The length of the paleoflood records in the Negev is between few tens or a hundred years and up to 7600 years in Nahal Ashalim (Fig. 1). The latter is related to extraordinary preservation of the paleoflood deposits in the Ashalim cave (Greenbaum et al., 2001). The maximal peak discharges of the paleofloods in the wadis of the Negev Desert comprise a database for a regional paleoflood envelope curve (Fig. 2). This curve provides an estimation of the maximum flood magnitude to be expected in this hyperarid area at the late Holocene time scale (Greenbaum et al., 2001). Temporal patterns of paleoflood occurrences were used in determining flood frequency responses to the varying climate at 20th century to Holocene time scales (e.g., Knox, 1983, 1993, 2000; Enzel et al., 1989, 1992; Webb and Betancourt, 1992; Ely et al., 1993; Frumkin et al., 1998; Greenbaum et al., 2000; Redmond et al., 2002). Episodes of increased and decreased flooding in the northern Negev were suggested to be contemporaneous with rises and falls in the level of the Dead Sea, respectively (Greenbaum et al., 2000). During the early–mid Holocene and the late Holocene the levels of the Dead Sea, a terminal lake located at the northeastern Negev, were fluctuating in high and low

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amplitudes, respectively (Frumkin et al., 1991; Enzel et al., 2003). The levels of the Dead Sea are excellent indicators of the climatic conditions from northern Israel to the northern Negev (Klein, 1981; Klein and Flohn, 1987; Greenbaum, 1996; Enzel et al., 2003). Increased frequency of Mediterranean lows that penetrate the northern part of Israel and cause wetter average conditions in the northern drainage basin of the Dead Sea, and larger floods in the northern Negev can explain this association (Greenbaum, 1996; Kahana et al., 2002; Redmond et al., 2002). It was also proposed that the levels of the earlier late Pleistocene lake Lisan that occupied this basin are associated mainly with rainfall in northern and central Israel generated from low pressure systems approaching the Middle East from the eastern Mediterranean (e.g., Bartov et al., 2002, 2003). Dead Sea levels serve here as a paleoclimatic indicator for climate conditions in the northern subhumid part of Israel. The specific flood and climate records for the southern Negev are not well established. Therefore, associating an increased frequency of large floods with an average wetter climate in central and northern Israel for any period is difficult, especially for Late Pleistocene episodes. In this study we report evidence of increased flood magnitude and frequency in the southern Negev during oxygen isotope stage (MIS) 3 transition into MIS 2. We suggest that this increase is contemporaneous with the largest documented level rise of Lake Lisan between 30 and 27 ka, the Late Pleistocene predecessor of the Dead Sea, towards its maxima during 27–25 ka, although the causal hydroclimatic systems are different—Mediterranean system versus Red Sea system.

2. The study area Nahal Netafim is a small (36 km2) catchment, which flows into Nahal Roded that drains the western rim of the Arava valley to the Gulf of Aqaba-Elat (Figs. 1 and 3). The upper reaches of Nahal Netafim are entrenched in carbonate rocks and form a large canyon. Downstream, near the upper study site (NET-2, Fig. 3) Nahal Netafim narrows and cuts a gorge with vertical walls and bedrock channel—5–10 m wide within igneous and metamorphic rocks. Downstream of the Netafim cave (Fig. 3) the canyon widens again to 30–50 m and to 4100 m near the confluence with Nahal Roded. At its lower reach Nahal Netafim cuts into igneous and metamorphic rocks and into Early Pleistocene, wellcemented, coarse-grained conglomerate. Nahal Yael, a tributary of 0.5 km2, which has operated as field laboratory for the last 35 years (Schick, 2000), joins Nahal Roded 1 km downstream of the confluence with Nahal Netafim (Fig. 3).

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Fig. 1. Location map of the Negev and Sinai deserts, mean annual rainfall, paleoflood sites and caves in the Negev desert, Israel.

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Fig. 2. Magnitude of the largest late Pleistocene floods in Nahal Netafim (500–600 m3 s 1) in relation to the mid–late Holocene (Greenbaum et al., 2001) (A) and the measured (B) envelope curves (Meirovich et al., 1998).

Fig. 3. Nahal Netafim catchment and study sites.

Most of the Negev is hyperarid (o100 mm yr 1; Fig. 1). Mean annual rainfall decreases from 250 mm yr 1 in the northern Negev to 20–30 mm yr 1 in the Southern Negev, near the study area. The majority of the rainstorms in the southern and central Negev originate from localized convective storms in association with the Active Red Sea Trough (ARST) system (Kahana et al., 2002). The system is characterized by surface lowpressure trough and an upper-level cold trough aloft. The difference between the extensive heating on the surface and the upper level colder air causes thermal instability. The source of moisture for these storms is distant (41000 km) tropical air transported at high altitudes by south winds, but the moisture amounts are relatively limited (Dayan et al., 2001). This synoptic feature typically occurs during autumn (October– November), but also during spring (March–May).

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The associated rainstorms are characterized by shortduration (several minutes up to a few tens of minutes), high-intensity rainfall that can reach up to 150 mm h 1 for few minutes (Katsnelson, 1979; Sharon, 1972, 1979; Sharon and Kutiel, 1986). The area covered by such rainstorms is limited to a few tens up to a few hundreds square kilometers with remarkable spatial variability during the storm (Sharon, 1972). The Red Sea Trough system dominates the southern Negev floods (Schick, 1988; Greenbaum, 1996; Kahana et al., 2002). The ARST generally produces floods in every large drainage basin in the southern and central Negev and its impact decreases northward (Kahana et al., 2002). The largest floods in the Negev generated by ARST are relatively well documented and analyzed (Greenbaum et al., 1998; Dayan et al., 2001). The majority of the rainstorms in the northern Negev are generated by regional depressions from the Eastern Mediterranean. The effect of the Mediterranean weather systems is greater in the northern Negev and decreases farther south in the central and southern Negev. These storms are characterized by long duration and relatively constant and low intensities of about 10 mm h 1 in the southern Negev. They are continuous, uniform and cover wide areas and typically occur in the winter months (December–February; Sharon, 1972; Katsnelson, 1979). The typical soils in this region are ‘‘Reg soils’’. These gravelly soils develop in stable, coarse-texture (coarse gravel and sand) alluvial surfaces in the hyperarid environment of the Negev and Sinai (Gerson et al., 1985; Amit and Gerson, 1986; Gerson and Amit, 1987; Amit et al., 1993). Reg soils accrete airborne dust and salts with time and are characterized by stony desert pavement overlying a thin (1–2 cm), silty, vesicular A horizon. The underlying gravel-free B and top of C horizons have silty-loamy texture (50–60% silt) and are dominated by 10–30% gypsum. The C horizon occupies most of the soil profile and includes 3–10% salt. The majority of the gravel (60–80%) in the C horizon is shattered by salt weathering. Holocene Reg soils exceed a epth of 70 cm whereas Pleistocene Reg soils exceed a depth of 80–160 cm (Gerson and Amit, 1987). Two sites in Nahal Netafim were selected for detailed studies, one representing subrecent to recent accumulation of SWD and the second with much older deposits. 2.1. The Netafim NET-2 site The upper study site—NET-2 (Fig. 3) is located within the igneous bedrock reach of Nahal Netafim where the canyon is 35-–45 m wide and the average channel slope is 0.025. The drainage area at the site is 26 km2. SWD are deposited as a sequence over a small relict of an alluvial terrace on the right bank. The stratigraphic evidence at the site includes five flood deposits (units 1–5, Table 1) ranging in texture from

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granules to silt. Unit 3 of this sequence contains 4100% percent modern Carbon (PMC) and therefore, postdates 1956 AD (Baker et al., 1985). Two floods, the 1994 and the 1997 floods (units 6 and 7, respectively, Table 1) have both stratigraphic evidence in the form of driftwood lines and observational evidence (Greenbaum et al., 2001). This is supported by the freshness of the organic debris of the driftwood line and the similarity between the reconstructed peak discharges of 70 and 30 m3 s 1, respectively, and the estimations of the peak discharge of the flood downstream. The computed discharge for the 1997 flood was used to estimate the magnitudes of peak discharges for the SWD in site NET-2, where all peak discharges were found to range between 70 and 150 m3 s 1 (Table 1). The entire sequence is short and records only the largest floods during the last decades or the last 100 years at the most (Greenbaum et al., 2001).

2.2. The Netafim cave The lower site is located 2.5 km downstream of site NET-2 (Fig. 3) within a cave at the channel’s left bank, which is constructed of well-cemented conglomerate (Fig. 4). The right bank is composed of granite rocks. The alluvial channel at this site is 20–30 m wide and the average slope is 0.02. The drainage area at this site is 31 km2. Remnants of a low, discontinuous alluvial terrace—0.3 to 0.4 m above channel bed occur along the study reach. The opening of the cave is 1.7 m above current channel bed (Fig. 4) and its size is approximately 3 by 4 m. The maximum height of the cave opening is 3.6 m and it is filled almost to its top with 3.3 m of flood SWD (Figs. 5 and 6). These flood deposits are 1.7–5 m above the present-day channel bed (Fig. 6). The

Table 1 Paleofloods in Nahal Netafim site NET-2 Flood no.

Discharge (m3 s 1)

1 2 3 4 5 6 7

150 70 80 90 120 70 30 a

Age (years, AD)

1956a

1994 (?) 1997

Radiocarbon sample: Beta–099941–100.570.9% PMC.

Fig. 5. Fine grained flood SWD within the Netafim cave.

Fig. 4. The lower section of the study reach near the Netafim cave formed within well-cemented conglomerate which filled the bedrock canyon probably during Early Pleistocene. The elevation of the cave opening (1.7 m) above the channel bed serves as a threshold for paleoflood deposition within the cave.

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Fig. 6. Netafim Cave—cross section geometry, peak discharges and Fluvio-Pedogenic Layer (FPL). Source of FPL ages after Lekach et al. (1999) and Schick et al. (2001).

sediments are stratified and the texture of each sedimentary unit is fining upward with well-sorted, coarser particles, from granules up to fine gravel (1–2 cm in size) at the base of the unit (Fig. 7a). The deposits in the cave were distinguished and separated into 27 floods (Fig. 7a and Table 2).

3. Methods 3.1. Paleohydrology Fine-grained SWD and PSI such as driftwood lines represent minimal to exact high stages of former large floods and provide excellent natural records of their magnitudes. These fine-grained sandy and silty sediments are deposited rapidly from suspension at sites where flow velocities are significantly reduced. Caves and alcoves in canyon walls are among the best SWD sites as they accumulate flood-related sedimentation far away from the main flow (Baker, 1987). Ideal paleoflood sites are located in bedrock canyons and preserve

multiple flood stratigraphic records which can be separated into individual flow events, using several well established sedimentological criteria (Baker, 1987). The flood stratigraphy at the Netafim cave was determined from sedimentological evidence that signified breaks in depositional sequences, which became the basis for separating individual flood deposits (Patton et al., 1979; Kochel and Baker, 1982; Ely and Baker, 1985; Baker, 1987; Partridge and Baker, 1987). Numerical dating was carried out to establish the chronology of the paleofloods. The section of the flood deposits in the cave was separated into 27 sedimentary layers, ranging in texture from granules to clay. Only small remnants, stuck to the lower parts of the top of the cave, were preserved from the uppermost deposits (units 25–27, Fig. 7a). 3.2. Hydraulic methods Paleodischarge estimates were obtained by the HECRAS hydraulic computer model (Hydrologic Engineering Center, 1997), which generates water surface

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Fig. 7. (Continued)

Fig. 7. (a) Stratigraphic section of the SWDs, associated peak discharges and IRSL ages at the Netafim Cave. (b) The Bayesian correction of the IRSL ages.

profiles, using step-backwater calculations for various discharge values (O’Connor and Webb, 1988; Webb and Jarrett, 2002; Fig. 8). Comparing the elevations of the SWDs and PSIs to the computed elevations provides a minimal estimation of the peak discharge. Flume experiments suggest that the elevation of the deposits is close to the peak stage of the flood (Kochel and Ritter, 1987; Baker and Kochel, 1988). However, a comparison of SWD elevations with other field observations such as the elevation of driftwood lines of the same floods,

historical water levels, and gauged discharges, indicates that heights of the flood deposits commonly are lower than the actual peak stage (Kochel, 1980; Ely and Baker, 1985; O’Connor et al., 1986; Baker, 1987; Partridge and Baker, 1987; Greenbaum, 1996). For example, in Nahal Zin, a large ephemeral stream channel in the Central Negev, the observed differences between flood deposits and peak stage for large floods are 50–70 cm (Greenbaum, 1996). This would lead to an underestimation of the peak discharge (Kochel et al., 1982; Baker, 1987) and they should, therefore be treated as a minimum value. The water surface profiles for the study reaches, calculated by the HEC-RAS, indicate subcritical flow regime (Fig. 9), typical of large floods in bedrock-controlled streams (e.g., Ely and Baker, 1985; O’Connor et al., 1986; Baker, 1987; Partridge and Baker, 1987). Supercritical conditions are reached only rarely and at very few locations (Greenbaum et al., 2000, 2001) where channel gradient increases abruptly. Inaccuracies in the discharge estimations may also result from the selected hydraulic parameters. Manning’s n is considered to be of low sensitivity when calculating flow discharges of large floods (O’Connor and Webb, 1988). Enzel et al. (1994) show that a change of 720% in n values produces a change ofo75% in the corresponding discharge. 3.3. Luminescence dating No organic material was found within the Netafim Cave deposits. However the fine, sandy sediment of the

ARTICLE IN PRESS N. Greenbaum et al. / Quaternary Science Reviews 25 (2006) 704–719 Table 2 Paleofloods, peak discharges, IRSL ages and corrected ages using Bayesian analysis at the Netafim cave Flood no.

Peak discharge (m3 s 1)

IRSL age years (ka)

Bayesian age years (ka)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

200–300 200–300 200–300 200–300 300–400 300–400 300–400 300–400 300–400 300–400 300–400 300–400 400–500 400–500 400–500 400–500

32.171.3 34.371.2 29.871.2 31.971.3

33.072.1 32.571.6 32.071.6 31.571.6

34.371.4

30.571.6

28.671.2

30.571.6

32.171.0 27.171.0

29.571.6

17 18 19 20 21 22 23 24 25 26 27

400–500 400–500 400–500 500–600 500–600 500–600 500–600 500–600 500–600 500–600 500–600

30.071.1

29.572.1

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SWD is suitable for luminescence dating (Aitken, 1998). Nine samples were collected from the section at intervals, starting about 0.3 m from the base of the section in event 1 (sample NTF-1, Fig. 7a and Table 3) and ending about 0.6 m from the roof of the cave in event #22 (sample NTF-8; Fig. 7a). Event 16 was sampled twice, about 1 m apart (NTF-7 and NTF-7a; Fig. 7a and Table 3) to test for reproducibility. Fine sand-size alkali feldspars were extracted using routine laboratory procedures (Porat et al., 1999). Equivalent doses (De) were determined using the infrared stimulated luminescence (IRSL) and the single aliquot added dose protocol (Duller, 1994) and 6–8 aliquots were measured for each sample. Dose rates were evaluated both in the field and in the laboratory. Gamma and cosmic dose rates were measured in the field using a Rotem P-11 gamma scintillator, calibrated to include cosmic rays. A representative sample of the sediments was analyzed for the radioelements U, Th and K, measured by ICP. Table 3 lists the field and laboratory data. Quartz was also extracted for two samples and measured using the optically stimulated luminescence (OSL) and the single aliquot regenerative (SAR) dose protocol (Murray and Wintle, 2000). However there was a large inter-aliquot scatter and the ages were in an obvious reversed stratigraphic order (Table 3). This mineral was therefore not used any further. Fading tests were carried out on a feldspar sample from the nearby Nahal Roded catchment, using the

Fig. 8. Water surface profiles for various peak discharges along the present and late Pleistocene channel of Nahal Netafim at the Netafim Cave site. Note the elevation and associated peak discharges of the SWD stratigraphic section.

Peak discharge computations for the Late Pleistocene paleofloods at the Netafim cave are complex. Estimating the elevation of channel bed during the Late Pleistocene is crucial for determining the paleoflood stages and peak discharges. The assumption of no significant change in channel geometry through time may be valid for the resistant bedrock walls but not for the alluvial channel bed, particularly over long periods. Late Pleistocene channel bed elevation was reconstructed using the Fluvio-Pedogenic layers (FPL) found and studied in alluvial sections at the nearby channel of Nahal Yael (Lekach et al., 1998, 1999; Fig. 3) and at the

4 6

11 16 16 22

2.70 3.15

3.60 3.95 3.95 4.50

1122 1122 1067 1036 1036 1038 1568 997

10173.2 11273.1 9673.3 116716 9873.1 9473.1 120730 9473.0 8971.8 8672.1 9772.9

11.6 11.4 11.2 — 11.9 11.4 — 11.7 11.3 11.6 11.9

K KF (%) 1.5 1.8 1.8 1.8 1.7 1.4 1.4 1.6 1.4 1.5 1.7

K (%) 1.91 1.41 1.37 1.37 1.37 1.29 1.29 1.51 1.35 1.91 1.47

U (ppm) 4.82 4.81 4.51 4.51 3.96 3.59 3.59 6.24 3.81 4.99 4.60

Th (ppm)

532 522 513— 545 522 — 536 518 532 545

Int. b (mGy/a)

252 215 205 5 192 178 4 255 187 256 214

Ext. a (mGy/a)

1485 1657 1645 1486 1526 1310 1186 1544 1322 1497 1576

Ext. b (mGy/a)

891(85)

865(45) 882(45)

Calc. g (mGy/a)

3234 3276 3230 2613 3079 2735 2226 3271 2789 3190 3227

32.171.3 34.071.2 29.871.2 44.276.3 31.971.3 34.371.4 54.1713.7 28.671.2 32.171.0 27.171.0 30.071.1

Total dose Age (ka) (mGy/a)

33.072.1 32.571.6 32.071.6 — 31.571.6 30.572.1 — 30.571.6 29.572.1 29.571.6 29.572.1

Bayesian age (ka)

Infrared stimulated luminescence ages for alkali feldspar and optically stimulated luminescence for quartz (Qz) extracted from the slackwater deposits. Height was measured from the base of stream channel. Grain size for all samples: 149–177 mm. Estimated water contents 171%. External (Ext.) g dose rates were measured in the field. Calculated (calc.) g is from radioisotope contents, in parenthesis the estimated cosmic contribution included in the calculated g (with an overburden of 4 m granite).

1 2 2

1.75 2.00 2.30

NTF-1 NTF-2 NTF-3 NTF-3 Qz NTF-4 NTF-5 NTF-5 Qz NTF-6 NTF-7 NTF-7a NTF-8

De (Gy)

4.1. Paleoflood magnitude reconstructed at the Netafim cave

Ext. g (mGy/a)

4. Results

Flood#

approach presented by Huntley and Lamothe (2001). Within errors no fading was detected over a period of 5 months.

Height (m)

Fig. 9. Paleofloods in Nahal Netafim and other paleoclimatic records: Dead Sea levels (Bartov et al., 2002) northern Israel and Negev; Lacustrine deposition in Wadi Firan (Rogner and Smykatz-Kloss, 1991) in Sinai.

Sample

712

Table 3 Results of luminescence dating in Netafim cave

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Netafim cave site (Schick et al., 2001; Fig. 6). These red, continuous and compacted layers, which occur at an average depth of 40–50 cm, form a relatively stable, nonactive surface and underlie the gray, non-cohesive, active alluvium. The depth of these layers is related to cumulative water availability before and during floods. The depth of the FPLs form a lower limit for scour and fill processes, and therefore reflects the local hydroclimatically controlled flood regime (Lekach et al., 1998). Similar FPLs were found in the channels of many other wadis in the hyperarid region of the southern Arava (Schick et al., 2001). Alluvial terraces and fans along Nahal Yael expose a series of buried FPLs (Lekach et al., 1999; Schick et al., 2001). Two FPLs found near the Netafim cave (Lekach et al., 1999; Amit et al., in press) were used in this study to represent the elevation of the channel bed: (a) in the present channel at a depth of about 1 m. This layer was dated by luminescence to about 9 ka (Fig. 6); and (b) within an exposed low alluvial terrace located 0.3 m above present channel bed, which is stratigraphically older and topographically higher than the current channel bed FPL. This unit was not dated because of its poor preservation, but it is similar in the degree of development, character and elevation to the FPLs in the nearby Nahal Yael—another tributary of the Nahal Roded drainage basin (Fig. 3). The FPLs there were dated by luminescence to about 35–20 ka (Lekach et al., 1999; Schick et al., 2001). Based on the above we assume that the 0.3 m high terrace with the FPL represents the Late Pleistocene channel bed of Nahal Netafim, and its level was used for the paleo-discharge calculations. HEC-RAS water surface profiles were run through the study reach for peak discharge determinations of the Netafim cave SWD (Fig. 8). Discharges similar to the modern maximum (150 m3 s 1) could not have formed the highest SWD in the cave even if the channel bed was at the level of the opening of the cave, currently located 1.7 m above the channel. Also, in such a case coarse bed load gravel (larger than pebbles) would have entered the cave, whereas no such gravel was found. The only gravel found within the cave are fine pebbles or finer at the base of the deposits, which are a part of the slackwater sedimentary sequence. Such fine gravels were previously reported in suspension during large floods in the Negev. These gravels are completely different from the typical bedload, which characterizes active channels in this environment. Therefore we can safely assume that channel bed level during the deposition of the SWD was not much higher than the modern level. The entire record contains 27 paleofloods. If our assumption that the elevation of the late Pleistocene channel is valid, then the minimum calculated peak

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discharges are 200–600 m3 s 1 (Fig. 8a and Table 2). Four floods (units 1–4) have peak discharges between 200 and 300 m3 s 1, 8 floods (units 5–12) have peak discharges of 300–400 m3 s 1, 7 floods (units 13–19) have peak discharges of 400–500 m3 s 1, and 8 floods (units 20–27) have peak discharges of 500–600 m3 s 1. These calculated magnitudes are the minimum values for these paleofloods and they rise with threshold increase. The calculated minimum specific peak discharges for these floods range between 6.5 and 19.4 m3 s 1 km 2, which are relatively large. The apparent increase in peak discharge over time in the Netafim cave sediments results from the cave filling up with the SWD. Most of the deposits are only a few tens of centimeters thick whereas the related floods were large. The lowest deposits with the smallest reconstructed peak discharges (200 m3 s 1) are only minimum values and the actual magnitudes could have been larger than later floods. Only larger and larger floods exceeded the elevation of the top surface of SWDs deposition in the cave with time, but this does not mean that the floods in general became larger towards the end of the period. However, the observation that the upper deposits (units 16–19) contain fine gravel at their base may indicate larger discharges. After the cave filled, no matter how high a flood was, its SWD were not preserved inside the cave. Inset SWD relicts, which may record smaller floods (but still4200 m3 s 1) during this period, were not found. Thus we assume that our record is only partial and includes the beginning of this flood episode but not its end (although it may well be the entire period). Floods with peak dischargeso200 m3 s 1 are not represented in this record at all because they did not exceed the threshold of the cave entrance, even when it was still unfilled. 4.2. Luminescence dating The ages of the SWD section in the cave fall within a narrow range, from 34.371.4 to 27.171.0 ka (Fig. 7a and Tables 2 and 3). The ages are not in strict stratigraphic order, and taking into account the errors on the ages, it is not possible to determine the time of deposition for individual SWD units. It is also not possible to determine the time span during which the stack of sediments accumulated (Fig. 7a), only that it was within several thousands of years. To better estimate the time that had elapsed between each flood event and the time span in which the entire section was deposited, statistical analysis using Bayesian methods was employed (Rhodes et al., 2003). The samples from the SWD have very low scatter on the De values, in the range of 2–3 Gy or 3%. According to criteria developed by Clarke (1996) this indicates that the samples are homogenous and were

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well bleached at the time of deposition. Two samples were collected from the same level (NTF-7 and NTF-7a; Table 3) within a horizontal distance of 2 m. Although their De values are similar (89 and 86 Gy, respectively) the dose rates for the two samples differ, mainly because of a higher g dose rate measured for sample NTF-7a. The hole from which this sample was collected was further extended to accommodate the g counter, resulting in the detector being very close to the granite wall on the side of the cave and a high-count rate. As a result, the ages for NTF-7 and NTF-7a are 32.171.0 and 27.171.0 ka, respectively. However if the age for NTF-7a is calculated with the g dose rates of NTF-7, the age is 30.871.2 ka, in a satisfactory agreement with 32.171.0 ka. 4.3. Bayesian analysis A Bayesian age model was used to estimate the age of the youngest and oldest samples, assuming that the true depositional ages lie in stratigraphic order. The Bayesian analysis was performed using OxCal (Bronk Ramsey 1995) using the methods described in Rhodes et al. (2003). The estimated systematic error component of 2% was first stripped out from the overall uncertainty associated with each age estimate. The age estimates were then analyzed in their correct stratigraphic order within a sequence using the OxCal software. An additional fractional error was added in quadrature to every age estimate, referred to by Rhodes et al. (2003) as the unshared systematic error (USS), so that the variation between the individual age estimates was consistent with the widened errors and the stratigraphic constraint. For these data, a USS value of 6% was found satisfactory, although somewhat on the high side. The age model output was used to calculate the midpoint. As the errors are approximately symmetrically distributed, this represents a good estimate of the most likely age for each sample. The systematic error of 2% was then recombined to provide overall age uncertainties. Although these overall age uncertainties are relatively high, they represent a better age estimate for each dated SWD horizon and for the base and top of the section (Fig. 7b). The ages thus calculated are given in Tables 2 and 3. The age of the oldest and youngest samples are 33.072.1 and 29.572.1 ka, respectively. If we extrapolate these ages to the base and top of the section, it spans from 33.0 to 28.8 ka, or roughly 4200 years. When taking into account the 1s errors, the time span could be from several hundred years and up to 8400 years. However, it is unlikely that an age difference of 8400 years between the bottom and top of the section would not be detected for samples that are 30,000 years old and have very small intra-sample scatter.

5. Discussion The Late Pleistocene record at the Netafim cave includes 27 large paleofloods. Due to present limitations of the luminescence dating methods, the ages of each flood as well as the time gap between two consecutive floods cannot be well constrained. Nevertheless, by using Bayesian analysis of the ages it is apparent that the time span of this flooding episode is restricted to a relatively short period. The results indicate that the extrapolated time span of the entire section can be very brief—several hundreds years and up to a maximum of 8400 years with an average duration of 4200 years. The resulting average flood frequency ranges between 427 and 3.2 large floods (4200 m3 s 1) per 1000 years—i.e. an estimated recurrence interval of between 450 and 300 years, respectively. The average value –6.4 large floods per 1000 years provides an estimated recurrence interval of about 150 years. The present flood frequency indicates only two 100–150 m3 s 1 floods during the last hundred years. Comparing the Late Pleistocene paleofloods in Nahal Netafim catchment at the Netafim Cave to the present floods represented at site NET-2 points to a large difference. All sub-recent and present floods areo200 m3 s 1 and therefore do not reach the cave opening threshold (Fig. 6). Similarly, low magnitude floods (o 200 m3 s 1) that occurred during the Late Pleistocene are not recorded at the Netafim cave. The flood record at the Netafim cave represents the extreme floods, and as the cave filled up it is probable that only part of the record was preserved. This implies that the average frequency is a minimum value and the floods4200 m3 s 1 could have been more frequent. Under present day hydrological conditions such flood magnitudes occur only in drainage basins that are at least two times larger than Nahal Netafim catchment (Fig. 2). In the hyperarid region at present, maximal peak discharges for drainage basins at the size of Nahal Netafim (35 km2) do not exceed 400 m3 s 1, which is still only about 2/3 of the maximum Late Pleistocene peak discharges (Fig. 2). The only other paleoflood record in the entire Negev desert, which is close in its temporal range and the number of floods to Nahal Netafim, is the Holocene record of Nahal Ashalim catchment (Figs. 1 and 2; Greenbaum et al., 2001). It has similar drainage area and mean annual precipitation, but different channel and basin morphometry and possibly also different type of storms. Comparing the Late Pleistocene record at Nahal Netafim to that of Nahal Ashalim, shows striking differences in the magnitude and estimated frequency of large floods. Nahal Ashalim has a record of 8 large floods (210–350 m3 s 1) during the last 7600 years and additional 12 smaller

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floods. The specific peak discharges range between 7.3 and 10.6 m3 s 1 km 2. The peak discharges of the largest paleofloods in Nahal Netafim are much higher than those of Nahal Ashalim (600 versus 350 m3 s 1, respectively). In Nahal Netafim 75% of the paleofloods have magnitudes 4350 m3 s 1 and about 30% of the paleofloods have magnitudes 4500 m3 s 1. The most striking observation is that the Nahal Netafim floods have magnitudes that significantly deviate from the paleoflood envelope curve of the Negev (Greenbaum et al., 2001; Fig. 2). The average frequency of the large paleofloods (4200 m3 s 1) in Nahal Netafim is 6–7 times higher than in Nahal Ashalim (1 flood in 1000 years versus 1 flood in 150 years, respectively). These differences in magnitude and frequency of large floods probably reflect differences in the hydrological regime between episodes in the Late Pleistocene and the Holocene. The extreme large floods in Nahal Netafim can be explained only by higher frequency of rainstorms characterized by higher rainfall intensities and/or durations and may also reflect a slightly higher annual rainfall. 5.1. Paleohydrological and paleoclimatological records The Late Pleistocene and Holocene paleohydrology and paleoclimatology of the northern Israel is relatively well documented (e.g. Bar-Mattews et al., 1997; BarMatthews et al., 1999; Frumkin et al., 1999, 2000; Enzel et al., 2003; Vaks et al., 2003) when compared to southern Israel. Most studies indicate that the area from northern Israel southward to the northern Negev experienced relatively wetter climatic conditions between 40 and 20 ka BP. At the same time the southern Negev as a part of the northern fringe of the ArabiaSahara desert, was probably hyperarid (e.g. Bull and Schick, 1979; Gerson, 1982a, b; Gerson et al., 1985; Gerson and Amit, 1987; Grossman and Gerson, 1987; Schick et al., 2001; Amit et al., 2004, in press; Vaks et al., 2004). 5.1.1. The southern Negev During the Late Pleistocene the southern Negev was probably hyperarid and dominated by the Red Sea Trough system. In the Nahal Yael watershed near Nahal Netafim (Fig. 3), minor average channel aggradation rates of a few centimeters per 1000 years were calculated for the Late Pleistocene, whereas the period 35–20 ka is characterized by the lowest rates of channel aggradation (Lekach et al., 1998, 1999; Schick et al., 2001). This indicates relative channel stability during this period. Changes between periods of aggradation and sediment evacuation were proposed to represent climate changes in this region (Bull and Schick, 1979; Bull, 1991; Clapp et al., 2000). Bull and Schick (1979) related the colluviation that produced the material for the

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aggradation of the channel to wetter climate conditions during Late Pleistocene while the subsequent erosion of debris from the slopes and channel alluviation were related to drier and stormier climate. Using cosmogenic nuclides Clapp et al. (2000) indicated that stripping of the slopes and sediment evacuation occurred later in the Late Pleistocene. Farther south of Nahal Netafim in the Sinai Peninsula (Fig. 1) Issar and Eckstein (1969) and Rogner and Smykatz-Kloss (1991) documented evidence for wetter conditions in the wide valley of Wadi Firan based on a thick sequence of well-bedded, marly deposits interpreted as lacustrine sediments and dated between 27.074.0 and 11.370.9 ka (Rogner and Smykatz-Kloss, 1991). The deposition of these sediments partially overlaps the timing of high magnitude floods in Nahal Netafim (Fig. 9). The formation mechanism of these deposits is not clear but it may be related to blockage of the wide channel and different hydrological regime with more water in the drainage basin, possibly due to increased recharge to the very shallow alluvial aquifer and/or to increased flood frequency and magnitudes. This may be related to increased storminess with higher rainfall intensities and/ or duration. Much farther to the south microfaunal analyses in a core from the central Red Sea (Fig. 1) Hemleben et al. (1996) and Almogi-Labin et al. (1998) documented enhanced monsoon activity and more humid climatic conditions during late MIS 3, which could have lead to increased activity of the Red Sea Trough. The episode of increased flood magnitudes must have been brief, as it did not alter the gypsic and salic properties of the Reg soils in the area. Pedological evidence from these soils in the southern Negev, dated by luminescence up to 100 ka (Porat et al., 1996, 1997; Amit et al., 2004) indicates that hyperarid climate (o 100 mm yr 1) prevailed in this region since Late Pleistocene (Gerson et al., 1985; Amit and Gerson, 1986; Gerson and Amit, 1987; Amit et al., 1993, 2004, in press). The relatively short heavy storms and resulting large floods were not recorded in the long-term formation of the soils, which are the product of the averaged effect of climate on soil hydrology regime over longer periods. Also, because infiltration during high intensity rainfall decreases significant larger portions of the rainwater are transformed into runoff and do not affect the soil pedogenic processes. Also, the low dissection of these alluvial surfaces indicates minor erosion. This is probably attributed to the stony, dense desert pavement, which covers the surface and provides excellent protection to the underlying soil. The heavy rainstorms associated with the floods recorded at the Netafim cave were probably rare and their contribution to the average conditions recorded by the soils was minor.

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5.1.2. Central Israel to the northern Negev At the same time, in the Mediterranean climate of central Israel, continuous 18O and 13C isotope analyses of speleothems from the Judea Mountains caves (BarMattews et al., 1997, 1999; Frumkin et al., 1999, 2000; Fig. 1) indicate that before the Last Glacial Maximum (LGM) (36–26 ka) the area was probably colder and wetter. Vaks et al. (2003) documented episodes of speleothem growth in a cave (Fig. 1) at the desert fringe where the mean annual rainfall is currently 250–300 mm. Relative high growth rates and low isotopic values characterize the period 38–28 ka indicating wetter conditions. Vaks et al. (2004) also reported no speleothem growth in ten caves in the central and northern Negev during the Last Glacial because climate conditions were too dry. Between 36 and 29 ka the Lake Lisan level rose moderately. The steep rise in Lake Lisan levels occurred at 29–27 ka, when it reached its highest level at 165 m below msl (Bartov et al., 2002; Fig. 9). The paleofloods in Nahal Netafim between 33 and 29 ka (Fig. 9) parallel this rise in Lake Lisan level, and may indicate that wetter conditions in the northern Israel paralleled more frequent large floods in the southern Negev. This is in spite of the fact that the Mediterranean depressions that are responsible for Lake Lisan level hardly bring flood-producing rains to the southern Negev, whereas the largest floods are formed by Active Red Sea Troughs (Kahana et al., 2002). Periods of rising Dead Sea levels in the Holocene are related to wetter climate in the northern subhumid headwater region of the Dead Sea basin (Klein 1981; Enzel et al., 2003). Such periods were characterized by high frequency of large floods in both Nahal Zin in the northeastern Negev (Fig. 1; Greenbaum et al., 2000) and Mt. Sedom at the southern margin of the Dead Sea (Frumkin et al., 1998). This higher flood frequency was attributed to higher frequency of large rainstorms with higher rainfall intensities and/or durations. The combination of higher magnitude and frequency of rainstorms in Nahal Netafim, in the hyperarid southern Negev, represents stormier climatic conditions during MIS 3. Each of these storms was probably characterized by higher rainfall intensities and/or longer durations as experienced and expected from the modern data. Higher frequency and magnitude of such storms may reflect short-term changes in climate, which cause large changes in flood magnitudes (Knox, 1993). In Nahal Netafim, even if annual rainfall amounts were three times higher than present values for a brief period, climate would still be hyperarid. Such climate would have produced larger and more frequent floods but would not affect the long-term average hydroclimatic conditions manifested in soil forming processes.

6. Conclusions Sedimentological evidence for 27 very large paleofloods during the period 33–29 ka were identified in a cave in Nahal Netafim catchment near the Gulf of Aqaba in the southern Negev Desert, Israel. Peak discharges were reconstructed using the elevation of channel bed during the Late Pleistocene as indicated by FPL found near the cave. The average recurrence interval of these large floods (200–600 m3 s 1) for this period is about 150 years. The average recurrence interval of such floods (200–350 m3 s 1) in drainage basins of similar size in the Negev, during climate conditions existing since the mid–late Holocene is only about 1000 years. The magnitude of the mid–late Holocene floods in the Negev is 50% larger than present floods peaks. At least eight out of the 27 paleofloods were larger than the envelope curve for flood magnitude established for the hyperarid Negev desert. This indicates a very different hydroclimatic regime compared with the mid-Holocene to present day regime. This proposed regime may be related to an increase in rainfall intensities and/or duration attributed to increased frequency of an intensified Red Sea Trough responsible for the majority of present-day large floods. Most paleoclimate records show that climate in the northern part of Israel and the northwestern Negev was relatively wetter 40–20 ka in response to Mediterranean frontal climate systems. At the same time, episodes of stormier conditions associated with higher intensities and/or durations of storm-rainfall prevailed in the southern Negev related to Red Sea Trough systems. The synchronity of these climatic conditions in both the northern and the southern Negev indicates a parallel strengthening of the two phenomena during the Last Glacial period. The episodes of storms punctuated the generally hyperarid climate conditions.

Acknowledgments NG and YE were funded by the Israel Water Authority. Michal Kidron from the cartography laboratory of the Hebrew University and Noga Yoselevich from the Department of Geography University of Haifa drew the figures. Sara Zeffren assisted in fieldwork and sample preparation. Jim Knox and Gerardo Benito provided constructive comments in their reviews.

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