- Email: [email protected]
Residual ages of modern sediments in an hyperarid region, Israel
Quaternary Science Reviews 20 (2001) 795}798
Residual ages of modern sediments in an hyperarid region, Israel夽 Naomi Porat *, Ezra Zilberman , Rivka Amit , Yehouda Enzel Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel Institute of Earth Sciences, The Hebrew University, Jerusalem 91904, Israel
Abstract Modern sediments from a variety of depositional environments in the hyperarid southern Arava region, Israel, were analyzed for their residual equivalent dose (D ). Colluvial and #uvial sediments from drainage systems of di!erent sizes and parent materials were collected and alkali feldspars were measured using the single-aliquot additive-dose protocol. All samples showed a large scatter in the D s. For most types of the sediments, the average D is between 2 and 11 Gy, equivalent to residual ages of 1}5 ka. The lowest D is usually in the range of 0.1}4 Gy, con"rming that the single-aliquot protocol can identify the best bleached grains. Newly weathered material from active channels gave the largest D 's of up to 30 Gy. Reworked #uvial sediments, derived from terraces and fans, gave progressively smaller D 's with increasing level of recycling. The degree of bleaching of colluvial sediments strongly depends on the face of the slope. The high D 's and the large scatter result from the mode of sediment transport. Brief rain episodes result in #ash #oods, where large volumes of sediment are transported over short distances and only a small part of the sediment is exposed at any time to the sun. Only repeated cycles of transportation can homogenize the sediment and fully expose all its particles to the sun. Single-grain or small-aliquot protocols can overcome the high and variable residual D values and enable better dating of sediments in hyperarid environments. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The southern Arava valley, Israel (Fig. 1) is located in an hyperarid region, having about 350 days of sunshine in a year. The average annual rainfall is &30 mm, with large inter-annual variability, and the ephemeral streams (or wadis) experience an average of less than one #ash #ood event per year. The extensive exposure to sunlight should reset the luminescence signal of sediments being transported in this region. In recent years, a large number of sediments from wadi terraces in the region were dated using IRSL on sand-size alkali feldspar grains (Porat et al., 1996, 1997, Amit et al., 1999). In many cases a large scatter in the equivalent dose (D ) was found, regardless of the measurement protocol used (single- or multiple-aliquot additive dose). One possible source for this scatter is heterogeneous bleaching of the sediment particles at deposition (Clarke, 1996). This will produce residual D values of di!ering magnitudes, which will augment the more or less uniform D accrued after deposition. As a result, there will be large di!erences in D 's between grains and aliquots. 夽
Paper published in December 2000. * Corresponding author. E-mail address: [email protected] (N. Porat).
To test this hypothesis, we sampled modern sediments from di!erent depositional settings in the region, such as colluvial mantles and active channels. Incision and terrace formation in the southern Arava valley has been active since the Pliocene (Garfunkel, 1970; Gerson et al., 1993), and terrace sequences are composed of alluvial material derived from the late Precambrian basement rocks exposed nearby as well as from the Mesozoic platform carbonate rocks. The source of the feldspars has been uniform since the Pliocene; they were weathered from the igneous basement. This rather uniform source of sediments over a long time enables comparison of di!erent variables such as the depositional environment and the distance of transport.
2. Field and laboratory work Modern sediments were collected from colluvial and #uvial depositional environments (Table 1). We also sampled slackwater deposits and sediments along di!erent types of streams reworking primary slope material or older terraces. The in#uence of transport distance was tested along a transect.
0277-3791/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 2 1 - 4
796
N. Porat et al. / Quaternary Science Reviews 20 (2001) 795}798
head of the tributary towards well-sorted sand at the con#uence with the main active channel and in the active alluvial fan. E Well-sorted slackwater deposits, with post-bomb C ages, were sampled from N. Neta"m. E A well-sorted sandy sediment was collected from the stream cutting across the Avrona playa, which forms the base level for many streams. The samples were scraped from the tope 1}2 cm of the sediment and placed immediately in black bags. Fine sand size alkali feldspar grains (150}177 or 178}210 lm; the same size that was previously used for dating the terraces) were separated using routine lab procedures (Porat et al., 1999). The D was measured using a Ris+ automated reader equipped with infrared diodes (880*80 nm). Detection was through a Schott BG-39 and a Corning 7}59 "lter pack with maximum transmission at 380 nm. The single-aliquot additive-dose protocol with preheat corrections was used (Duller, 1994); preheat was at 2203C for 10 min and 6}12 aliquots were measured for each sample.
3. Results
Fig. 1. Location map.
2.1. Colluvial samples All colluvial samples are formed on old alluvial fans and terraces. E A mantle covering a 25 year old road cut in a &60,000 year old terrace, facing southeast. E Mantles covering south- and north-facing cuts in a quarry, cutting into the same terrace. These cuts are recent, about 2}3 years old. E A mantle covering a north-facing meander cut in N. Avrona, weathering a late Pleistocene terrace. 2.2. Alluvial samples of diwerent sources E The channels of two small streams, transporting coarse sediments from recently weathered colluvium on the igneous bedrock. E A small tributary of N.(Nahal) Shehoret, incising into a &60,000 year old terrace and reworking its sediment, was chosen for a transect to test the a!ect of the distance traveled by the grains. The sorting of the sediment changes from poorly sorted gravel at the
Fourteen samples were analyzed in this study and the results are given in Table 1 and in Fig. 2. Most samples, regardless of depositional environment, show a large scatter in the D 's. There are large di!erences between the colluvial samples, depending on the slope, faces direction and perhaps also on the duration of colluviation. Comparison between north- and south-facing scarps formed at the same time on the same parent material (on two sides of a quarry;Cs 21 & 22) shows that after 2}3 years of colluviation the D 's of colluvium formed on the south facing scarp is 6}11 Gy, whereas that from the northfacing scarp is higher, 10}15 Gy. Only colluvium on older scarps (&25 year old road cut;C7) which faces southeast is well bleached, with D 's of 0.4}3 Gy. The transect along a stream incising into a &60 ka terrace (Fig. 2, center) shows that the D 's do not decrease in a consistent manner downstream, over a distance of about 800 m. Sand extracted from a gravely sample, from the head of the stream (C11), gave D 's in the range of 0.5}6 Gy, whereas the sandy slackwater deposit from nearby (C12) gave D 's of 1}10 Gy. Downstream, gravel to cobble bar sediments (C13) gave D 's of 8}19 Gy. Further down, where the stream joins other tributaries (C14), the D 's were down to 0.1}4 Gy, but sandy sedi ments from the active fan of N. Shehoret (C8), deposited during the most recent #ood, gave values of 6}12 Gy. Only at N. Avrona in the Avrona playa (C15), which is the local base level for the regional drainage, the sediments were well bleached, with D 's of 0.2}0.4 Gy.
N. Porat et al. / Quaternary Science Reviews 20 (2001) 795}798
797
Table 1 A list of samples analyzed in this study. Fine-sand alkali feldspars were separated from the samples for IRSL measurements. Sample
Location
Depositional environment
1 5
N. Mekorot N. Yael
Active channel Active channel
7
N. Shehoret
8
N. Shehoret
Weathered granite Weathered gneiss and late Pleistocene terrace Colluvium on a 25-year &60 ka terrace old road cut Active fan Various terraces
9
N. Neta"m
Slackwater deposit
10
N. Neta"m
Slackwater deposit
11 12 13 14 15
N. N. N. N. N.
Channel Slackwater deposit Bar in active channel Active channel Active channel
16
Avrona playa
21
N. Shehoret
22
N. Shehoret
Shehoret Shehoret Shehoret Shehoret Avrona
Parent material
Sediment type
Pleistocene terraces and weathered granite Pleistocene terraces and weathered granite &60 ka terrace &60 ka terrace &60 ka terrace &60 ka terrace Various terraces
Colluvium on a meander Late Pleistocene terrace cut Colluvium on a 2-year &60 ka terrace old quarry cut Colluvium on a 2-year &60 ka terrace old quarry cut
Comments
Gravel Gravel Coarse sand
D (Gy) 9.5}30.2 7.3}17.3
Facing south}east
0.4}2.8
Well-sorted medium Deposited during most sand recent #ood Well-sorted "ne sand Post-bomb C age
5.9}12.3
Well-sorted "ne sand Post-bomb C age
3.8}12.1
Coarse gravel Sand Cobbles and gravel Gravel Well-sorted "ne sand Deposited during most recent #ood Sand North facing
0.5}6.2 0.9}10.0 7.9}19.1 0.1}3.9 0.2}0.4
Coarse sand
South facing
5.6}10.8
Coarse sand
North facing
9.6}15.1
2.1}9.1
4.8}15.3
4. Discussion and conclusions
Fig. 2. D values for all samples. The transect along N. Shehoret is presented in the center.
Slackwater deposits from N. Neta"m (Cs 9 & 10) gave highly scattered D 's from 2 to 12 Gy, similar to the slackwater deposit from N. Shehoret. The highest values were obtained from sediments directly weathering igneous basement rocks to the active channel. Two such small streams draining igneous slopes (Cs 1 & 5) gave D 's from 7 to 30 Gy.
The highest D values with the largest scatter were measured for grains from streams transporting colluvium on igneous and metamorphic rocks. These grains are weathered for the "rst time and their luminescence properties such as the susceptibility to bleaching and luminescence sensitivity to radiation di!er from that of a material that has gone through several weathering bleaching and deposition cycles (Richardson, 1994). All analyzed slackwater deposits have high and scattered D 's. These are suspended sediments and when transported are expected to travel the longest distance and get the best exposure to sunlight. Currently, we have no explanation for these unexpected results. The transect along a stream in N. Shehoret did not show any systematic decrease in the equivalent dose along the entire length of the stream nor do the residual values depend on the speci"c depositional environment (Fig. 2). High values may be expected in bars, where gravel and pebbles shield the sand grains, but the wellsorted sand from the active fan and slackwater deposits are also not well bleached. The distance of transport along this transect, about 800 m, is apparently not su$cient to increase the level of bleaching. This most likely results from the mode of sediment transport. Since the beginning of the Holocene rainfall occurs in intensive storms resulting in #ash
798
N. Porat et al. / Quaternary Science Reviews 20 (2001) 795}798
#oods. These #oods carry very high coarse sediment loads over short distances, and the sand grains are rapidly buried. In that mode not all grains are exposed to sunlight and this causes the large and scattered residual doses observed in the di!erent depositional environments. The only well-bleached sample is from the Avrona playa, and its residual D 's are equivalent to 80}200 years, most of which can be assigned to instrumental noise. These sediments were e$ciently bleached because N. Avrona is the local base level; the sediments are sandy and well sorted, they have traveled the longest distance and were recycled several times. Currently, most of the sediments are locally derived from the uplifted Holocene playa sediments (Amit et al., 1999). There are no bars to trap and cover the sediment. This modern analogue can be used to assess the youngest samples datable in the region. Although within the transect there is no systematic decrease in D 's, there is a clear di!erence between "rst cycle erosion, the streams draining the terraces (N. Shehoret) and the "nal base level (N. Avrona). Only repeated exposures to sunlight over long transport distances allow for good resetting of the luminescence signal. Moreover, it appears that overall, repeated transport improves the bleachability of the samples and their luminescence sensitivity. As the variability in D within a single stream is large, there is no `averagea residual dose. It is not possible to subtract some &average' modern analogue to correct for the residual signal at the time of deposition. Moreover, the poor resetting results from the current `#ash-#ooda regime, therefore it cannot be used for Pleistocene sediments, for which there is evidence that the rainfall pattern may have been di!erent. As for colluvial sediments, the extent and intensity of exposure to sunlight, and hence the degree of solar resetting of the luminescence signal, is slope-face dependent. At the latitude of the study area, 30 N, north-facing slopes receive only a limited exposure to direct sunlight, and that only in the summer mornings. All other faces receive long hours of exposure, facilitating a more thorough bleaching of the grains. Therefore, colluvium on north-facing scarps is not well bleached, regardless of the length of exposure. Annual dose rates in the Holocene and Pleistocene fans and terraces in the region range from 2 to 3 Gy/ka. Therefore, the maximum residual ages of the modern
sediments sampled for this study may be as high as 15 ka, but for most samples it is between 1 and 6 ka. Nonetheless, the lowest D for each modern analogue is the closest to being fully bleached, accounting for 1}2 ka. It is thus very important to identify the lowest D for a geo logical sample by using a single-aliquot or single-grain protocols for a large number of aliquots or grains. Only the lowest D values may give the best age estimate for that deposit.
Acknowledgements This research was supported by the Israel Science Foundation funded by the Israel Academy of Science and Humanities. We would like to thank R. Madmon for "eld assistance and S. Ze!ran for sample preparation. An earlier version of this manuscript bene"ted from comments by an anonymous reviewer.
References Amit, R., Zilberman, E., Porat, N., Enzel, Y., 1999. Relief inversion in the Avrona playa as evidence of large-magnitude historical earthquakes, southern Arava Valley, Dead Sea Rift. Quaternary Research 52, 76}91. Clarke, M.L., 1996. IRSL dating of sands: bleaching characteristics at deposition inferred from the use of single aliquots. Radiation Measurements 26, 611}620. Duller, G.A.T., 1994. Luminescence dating of sediments using single aliquots: new procedures. Quaternary Science Reviews (Quaternary Geochronology) 13, 149}156. Garfunkel, Z., 1970. The tectonics of the western margins of the Southern Arava. Ph.D Dissertation, Hebrew University, Jerusalem. Gerson, R., Grossman, S., Amit, R., Greenbaum, N., 1993. Indicators of faulting events and periods of quiescence in desert alluvial fans. Earth Surface Processes and Landforms 18, 181}202. Porat, N., Wintle, A.G., Amit, R., Enzel, Y., 1996. Late Quaternary earthquake chronology from luminescence dating of alluvial deposits of the Arava Valley, Israel. Quaternary Research 46, 107}117. Porat, N., Amit, R., Zilberman, E., Enzel, Y., 1997. Luminescence dating of fault-related alluvial fan sediments in the southern Arava Valley, Israel. Quaternary Science Reviews (Quaternary Geochronology) 16, 397}402. Porat, N., Zhou, L-P., Chazan, M., Noy, T., Horwitz, L.K., 1999. Dating the Lower Paleolithic open-air site of Holon, Israel by luminescence and ESR techniques. Quaternary Research 51, 328}341. Richardson, C.A., 1994. E!ects of bleaching on the sensitivity to dose of the infrared-stimulated luminescence of potassium-rich feldspars from Ynyslas, Wales. Radiation Measurements 23, 587}592.
Residual ages of modern sediments in an hyperarid region, Israel夽 Naomi Porat *, Ezra Zilberman , Rivka Amit , Yehouda Enzel Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel Institute of Earth Sciences, The Hebrew University, Jerusalem 91904, Israel
Abstract Modern sediments from a variety of depositional environments in the hyperarid southern Arava region, Israel, were analyzed for their residual equivalent dose (D ). Colluvial and #uvial sediments from drainage systems of di!erent sizes and parent materials were collected and alkali feldspars were measured using the single-aliquot additive-dose protocol. All samples showed a large scatter in the D s. For most types of the sediments, the average D is between 2 and 11 Gy, equivalent to residual ages of 1}5 ka. The lowest D is usually in the range of 0.1}4 Gy, con"rming that the single-aliquot protocol can identify the best bleached grains. Newly weathered material from active channels gave the largest D 's of up to 30 Gy. Reworked #uvial sediments, derived from terraces and fans, gave progressively smaller D 's with increasing level of recycling. The degree of bleaching of colluvial sediments strongly depends on the face of the slope. The high D 's and the large scatter result from the mode of sediment transport. Brief rain episodes result in #ash #oods, where large volumes of sediment are transported over short distances and only a small part of the sediment is exposed at any time to the sun. Only repeated cycles of transportation can homogenize the sediment and fully expose all its particles to the sun. Single-grain or small-aliquot protocols can overcome the high and variable residual D values and enable better dating of sediments in hyperarid environments. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The southern Arava valley, Israel (Fig. 1) is located in an hyperarid region, having about 350 days of sunshine in a year. The average annual rainfall is &30 mm, with large inter-annual variability, and the ephemeral streams (or wadis) experience an average of less than one #ash #ood event per year. The extensive exposure to sunlight should reset the luminescence signal of sediments being transported in this region. In recent years, a large number of sediments from wadi terraces in the region were dated using IRSL on sand-size alkali feldspar grains (Porat et al., 1996, 1997, Amit et al., 1999). In many cases a large scatter in the equivalent dose (D ) was found, regardless of the measurement protocol used (single- or multiple-aliquot additive dose). One possible source for this scatter is heterogeneous bleaching of the sediment particles at deposition (Clarke, 1996). This will produce residual D values of di!ering magnitudes, which will augment the more or less uniform D accrued after deposition. As a result, there will be large di!erences in D 's between grains and aliquots. 夽
Paper published in December 2000. * Corresponding author. E-mail address: [email protected] (N. Porat).
To test this hypothesis, we sampled modern sediments from di!erent depositional settings in the region, such as colluvial mantles and active channels. Incision and terrace formation in the southern Arava valley has been active since the Pliocene (Garfunkel, 1970; Gerson et al., 1993), and terrace sequences are composed of alluvial material derived from the late Precambrian basement rocks exposed nearby as well as from the Mesozoic platform carbonate rocks. The source of the feldspars has been uniform since the Pliocene; they were weathered from the igneous basement. This rather uniform source of sediments over a long time enables comparison of di!erent variables such as the depositional environment and the distance of transport.
2. Field and laboratory work Modern sediments were collected from colluvial and #uvial depositional environments (Table 1). We also sampled slackwater deposits and sediments along di!erent types of streams reworking primary slope material or older terraces. The in#uence of transport distance was tested along a transect.
0277-3791/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 2 1 - 4
796
N. Porat et al. / Quaternary Science Reviews 20 (2001) 795}798
head of the tributary towards well-sorted sand at the con#uence with the main active channel and in the active alluvial fan. E Well-sorted slackwater deposits, with post-bomb C ages, were sampled from N. Neta"m. E A well-sorted sandy sediment was collected from the stream cutting across the Avrona playa, which forms the base level for many streams. The samples were scraped from the tope 1}2 cm of the sediment and placed immediately in black bags. Fine sand size alkali feldspar grains (150}177 or 178}210 lm; the same size that was previously used for dating the terraces) were separated using routine lab procedures (Porat et al., 1999). The D was measured using a Ris+ automated reader equipped with infrared diodes (880*80 nm). Detection was through a Schott BG-39 and a Corning 7}59 "lter pack with maximum transmission at 380 nm. The single-aliquot additive-dose protocol with preheat corrections was used (Duller, 1994); preheat was at 2203C for 10 min and 6}12 aliquots were measured for each sample.
3. Results
Fig. 1. Location map.
2.1. Colluvial samples All colluvial samples are formed on old alluvial fans and terraces. E A mantle covering a 25 year old road cut in a &60,000 year old terrace, facing southeast. E Mantles covering south- and north-facing cuts in a quarry, cutting into the same terrace. These cuts are recent, about 2}3 years old. E A mantle covering a north-facing meander cut in N. Avrona, weathering a late Pleistocene terrace. 2.2. Alluvial samples of diwerent sources E The channels of two small streams, transporting coarse sediments from recently weathered colluvium on the igneous bedrock. E A small tributary of N.(Nahal) Shehoret, incising into a &60,000 year old terrace and reworking its sediment, was chosen for a transect to test the a!ect of the distance traveled by the grains. The sorting of the sediment changes from poorly sorted gravel at the
Fourteen samples were analyzed in this study and the results are given in Table 1 and in Fig. 2. Most samples, regardless of depositional environment, show a large scatter in the D 's. There are large di!erences between the colluvial samples, depending on the slope, faces direction and perhaps also on the duration of colluviation. Comparison between north- and south-facing scarps formed at the same time on the same parent material (on two sides of a quarry;Cs 21 & 22) shows that after 2}3 years of colluviation the D 's of colluvium formed on the south facing scarp is 6}11 Gy, whereas that from the northfacing scarp is higher, 10}15 Gy. Only colluvium on older scarps (&25 year old road cut;C7) which faces southeast is well bleached, with D 's of 0.4}3 Gy. The transect along a stream incising into a &60 ka terrace (Fig. 2, center) shows that the D 's do not decrease in a consistent manner downstream, over a distance of about 800 m. Sand extracted from a gravely sample, from the head of the stream (C11), gave D 's in the range of 0.5}6 Gy, whereas the sandy slackwater deposit from nearby (C12) gave D 's of 1}10 Gy. Downstream, gravel to cobble bar sediments (C13) gave D 's of 8}19 Gy. Further down, where the stream joins other tributaries (C14), the D 's were down to 0.1}4 Gy, but sandy sedi ments from the active fan of N. Shehoret (C8), deposited during the most recent #ood, gave values of 6}12 Gy. Only at N. Avrona in the Avrona playa (C15), which is the local base level for the regional drainage, the sediments were well bleached, with D 's of 0.2}0.4 Gy.
N. Porat et al. / Quaternary Science Reviews 20 (2001) 795}798
797
Table 1 A list of samples analyzed in this study. Fine-sand alkali feldspars were separated from the samples for IRSL measurements. Sample
Location
Depositional environment
1 5
N. Mekorot N. Yael
Active channel Active channel
7
N. Shehoret
8
N. Shehoret
Weathered granite Weathered gneiss and late Pleistocene terrace Colluvium on a 25-year &60 ka terrace old road cut Active fan Various terraces
9
N. Neta"m
Slackwater deposit
10
N. Neta"m
Slackwater deposit
11 12 13 14 15
N. N. N. N. N.
Channel Slackwater deposit Bar in active channel Active channel Active channel
16
Avrona playa
21
N. Shehoret
22
N. Shehoret
Shehoret Shehoret Shehoret Shehoret Avrona
Parent material
Sediment type
Pleistocene terraces and weathered granite Pleistocene terraces and weathered granite &60 ka terrace &60 ka terrace &60 ka terrace &60 ka terrace Various terraces
Colluvium on a meander Late Pleistocene terrace cut Colluvium on a 2-year &60 ka terrace old quarry cut Colluvium on a 2-year &60 ka terrace old quarry cut
Comments
Gravel Gravel Coarse sand
D (Gy) 9.5}30.2 7.3}17.3
Facing south}east
0.4}2.8
Well-sorted medium Deposited during most sand recent #ood Well-sorted "ne sand Post-bomb C age
5.9}12.3
Well-sorted "ne sand Post-bomb C age
3.8}12.1
Coarse gravel Sand Cobbles and gravel Gravel Well-sorted "ne sand Deposited during most recent #ood Sand North facing
0.5}6.2 0.9}10.0 7.9}19.1 0.1}3.9 0.2}0.4
Coarse sand
South facing
5.6}10.8
Coarse sand
North facing
9.6}15.1
2.1}9.1
4.8}15.3
4. Discussion and conclusions
Fig. 2. D values for all samples. The transect along N. Shehoret is presented in the center.
Slackwater deposits from N. Neta"m (Cs 9 & 10) gave highly scattered D 's from 2 to 12 Gy, similar to the slackwater deposit from N. Shehoret. The highest values were obtained from sediments directly weathering igneous basement rocks to the active channel. Two such small streams draining igneous slopes (Cs 1 & 5) gave D 's from 7 to 30 Gy.
The highest D values with the largest scatter were measured for grains from streams transporting colluvium on igneous and metamorphic rocks. These grains are weathered for the "rst time and their luminescence properties such as the susceptibility to bleaching and luminescence sensitivity to radiation di!er from that of a material that has gone through several weathering bleaching and deposition cycles (Richardson, 1994). All analyzed slackwater deposits have high and scattered D 's. These are suspended sediments and when transported are expected to travel the longest distance and get the best exposure to sunlight. Currently, we have no explanation for these unexpected results. The transect along a stream in N. Shehoret did not show any systematic decrease in the equivalent dose along the entire length of the stream nor do the residual values depend on the speci"c depositional environment (Fig. 2). High values may be expected in bars, where gravel and pebbles shield the sand grains, but the wellsorted sand from the active fan and slackwater deposits are also not well bleached. The distance of transport along this transect, about 800 m, is apparently not su$cient to increase the level of bleaching. This most likely results from the mode of sediment transport. Since the beginning of the Holocene rainfall occurs in intensive storms resulting in #ash
798
N. Porat et al. / Quaternary Science Reviews 20 (2001) 795}798
#oods. These #oods carry very high coarse sediment loads over short distances, and the sand grains are rapidly buried. In that mode not all grains are exposed to sunlight and this causes the large and scattered residual doses observed in the di!erent depositional environments. The only well-bleached sample is from the Avrona playa, and its residual D 's are equivalent to 80}200 years, most of which can be assigned to instrumental noise. These sediments were e$ciently bleached because N. Avrona is the local base level; the sediments are sandy and well sorted, they have traveled the longest distance and were recycled several times. Currently, most of the sediments are locally derived from the uplifted Holocene playa sediments (Amit et al., 1999). There are no bars to trap and cover the sediment. This modern analogue can be used to assess the youngest samples datable in the region. Although within the transect there is no systematic decrease in D 's, there is a clear di!erence between "rst cycle erosion, the streams draining the terraces (N. Shehoret) and the "nal base level (N. Avrona). Only repeated exposures to sunlight over long transport distances allow for good resetting of the luminescence signal. Moreover, it appears that overall, repeated transport improves the bleachability of the samples and their luminescence sensitivity. As the variability in D within a single stream is large, there is no `averagea residual dose. It is not possible to subtract some &average' modern analogue to correct for the residual signal at the time of deposition. Moreover, the poor resetting results from the current `#ash-#ooda regime, therefore it cannot be used for Pleistocene sediments, for which there is evidence that the rainfall pattern may have been di!erent. As for colluvial sediments, the extent and intensity of exposure to sunlight, and hence the degree of solar resetting of the luminescence signal, is slope-face dependent. At the latitude of the study area, 30 N, north-facing slopes receive only a limited exposure to direct sunlight, and that only in the summer mornings. All other faces receive long hours of exposure, facilitating a more thorough bleaching of the grains. Therefore, colluvium on north-facing scarps is not well bleached, regardless of the length of exposure. Annual dose rates in the Holocene and Pleistocene fans and terraces in the region range from 2 to 3 Gy/ka. Therefore, the maximum residual ages of the modern
sediments sampled for this study may be as high as 15 ka, but for most samples it is between 1 and 6 ka. Nonetheless, the lowest D for each modern analogue is the closest to being fully bleached, accounting for 1}2 ka. It is thus very important to identify the lowest D for a geo logical sample by using a single-aliquot or single-grain protocols for a large number of aliquots or grains. Only the lowest D values may give the best age estimate for that deposit.
Acknowledgements This research was supported by the Israel Science Foundation funded by the Israel Academy of Science and Humanities. We would like to thank R. Madmon for "eld assistance and S. Ze!ran for sample preparation. An earlier version of this manuscript bene"ted from comments by an anonymous reviewer.
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