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Using portable OSL reader to obtain a time scale for soil accumulation and erosion in archaeological terraces, the Judean Highlands, Israel
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Research paper
Using portable OSL reader to obtain a time scale for soil accumulation and erosion in archaeological terraces, the Judean Highlands, Israel Naomi Porata,∗, Gloria I. Lópezb, Nadav Lenskya, Rotem Elinsonc, Yoav Avnia, Yelena Elgart-Sharonc, Galina Faershteina,d, Yuval Gadotc a
Geological Survey of Israel, 30 Malkhe Israel St., Jerusalem, 9550116, Israel Luminescence Dating Laboratory, CENIEH, Paseo Sierra de Atapuerca 3, 09002, Burgos, Spain c The Department of Archaeology and Ancient Near Eastern Cultures, Tel Aviv University, Tel Aviv, 6997801, Israel d Institute of Earth Sciences, The Hebrew University, Jerusalem, Israel b
A R T I C LE I N FO
A B S T R A C T
Keywords: Portable OSL reader OSL Soil erosion Bench terraces
Current conventions on the fate of agricultural bench terraces in the mountainous Mediterranean climatic zones, assume that after abandonment terraces undergo rapid degradation and soil loss. First walls crumble, followed by soil washed out of breaches in the walls by runoff, resulting in rapid erosion of soil from the slopes downhill into the streams. Additionally, soil erosion may explain why previous optically stimulated luminescence (OSL) dating of terrace soils in the Judean Highlands, Israel, found mostly young soils dating to the past 700 years, and only occasionally were older ages obtained for soils at the very base of these terraces. In contrast, observations made in the same region show that slopes with degraded terraces appear to still retain much soil even though only faint remains of the terraces exist. To test if terraces and soils indeed erode entirely and how long this might take, a relatively smooth hill slope showing remains of highly degraded sets of terraces was studied. Samples were collected from excavated pits for full OSL dating, in addition to samples densely collected for OSL measurements using a portable OSL reader (PR). Air-born photogrammetry was used to obtain high spatial resolution Digital Elevation Model. Results show that the main body of terraces was first built ∼800 years ago and maintained until 175-100 years age when they were abandoned and subsequently degraded. Since then 30–45% of soil volume was lost to erosion, however steady-state was reached at a relatively high slope of 65%, with stabilization by vegetation. The thick soil present on most of the slope suggests that after the first stage of rapid degradation the slopes reache equilibrium, most likely due to vegetation that reduces direct soil erosion, so most of the soil is retained on the slope. Finally, the PR allows for a much more nuanced understanding of terrace soil history.
1. Introduction
are a pre-requisite for sustainable living in mountainous areas (de Geus, 1975; Finkelstein, 1996; Gibson, 2001, but see already Hopkins, 1985: 180). There are several possible anthropogenic and/or natural factors that may have affected the OSL age range found in earlier studies. Previously we have dealt with the possibility that sampling was not comprehensive and elsewhere in the landscape older terraces do exist (Gadot et al., 2016a), and that continuous terrace maintenance exposes the sediment to sunlight and erases the OSL signal, thus obliterating older terraces ages (Porat et al., 2017). In this article we wish to address the possibility that earlier phases of terracing were entirely erased from the landscape due to intense soil erosion at times when terraces were not maintained. Soil movement that resulted from human exploitation is a world-
Man-made farming bench terraces cover large portions of the mountain area in Israel (Ron, 1966; Wikinson, 2003). Terraces increase the area of arable land, improve water infiltration and retention, and prevent soil erosion (Bevan et al., 2013; Arnaez et al., 2015). Extensive research on bench terraces in the Judean Highlands around Jerusalem (Fig. 1a and Fig. S1), which included mapping, trenching and optically stimulated luminescence (OSL) dating of soil infill, showed that most of the terraces were constructed in the last 700 years, and only occasionally are older soils found at the base of these terraces (Davidovich et al., 2012; Gadot et al., 2016a, 2016b; Porat et al., 2017). The relatively recent ages contrast with previous archaeological estimates for the age of the terraces that were based on the assumption that terraces
∗
Corresponding author. E-mail address: [email protected] (N. Porat).
https://doi.org/10.1016/j.quageo.2018.04.001 Received 30 November 2017; Received in revised form 9 April 2018; Accepted 11 April 2018 1871-1014/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Porat, N., Quaternary Geochronology (2018), https://doi.org/10.1016/j.quageo.2018.04.001
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Fig. 1. a. Location map, the study area shown with a star. b–d: Terraced slopes in the Judean Highlands. b. A fully terraced slope with a stepped profile (northern slopes of Nahal Halilim, terrace heights 0.5–1 m). c. A terrace undergoing erosion by wall collapse and soil wash (Nahal Rephaim, terrace height ∼ 0.7 m). d. Aerial photo of a slope with degraded terraces showing lines of white stones (marked by white arrows) tracing the remains of the collapsed terrace walls (Nahal Shmuel). Pits M1-M3 are visible as smooth, brown streaks. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
which induces the signal, is measured separately and the age is calculated from the ratio De/dose rate (Aitken, 1998). OSL dating is time- and labor-intensive, and at best several months pass between sample collection and obtaining an age. Over the years several approaches have been devised to shorten this time, either by reducing sample preparation time (e.g. Burbidge et al., 2006; Roberts et al., 2009; Durcan et al., 2010; Leighton and Bailey, 2015), or reducing measurement time by for example using a standardized growth curve (e.g. Roberts and Duller, 2004; Lai et al., 2007). One such means for obtaining rapid luminescence data is the SUERC PR (Sanderson and Murphy, 2010) which measures OSL and infrared stimulated luminescence (IRSL) signals directly on large, untreated samples. Signal intensity is a proxy of age, environmental dose rates and mineral sensitivity, and neither the De nor the dose rate is measured with the PR. Nonetheless intensities, depletion rates and the ratio between the OSL and IRSL signals can be used to estimate relative ages within a section (Stang et al., 2012), to identify sediments that contain a significant component of grains that were not bleached at the time of deposition, potential diagenesis and even mineralogical variations (Kinnaird et al., 2017; Muñoz-Salinas et al., 2011; Munyikwa et al., 2012; King et al., 2014; Bateman et al., 2015), or construct correlation charts to obtain numerical age estimates (Stone et al., 2015). Sample preparation is minimal (Burbidge et al., 2007) and measurement time short, and a preview of sediment luminescence properties can aid in more focused sampling for full OSL dating and in obtaining a rough depositional history of the sediments. When collected continuously, OSL signal intensities from bulk samples measured with a PR can be plotted as luminescence profiles (Sanderson et al., 2001, 2007; Stang et al., 2012), allowing for a nuanced visualization of changes along a stratigraphic profile. In this study we carried out full OSL dating on a selected set of samples in combination with a large number of
wide phenomenon (Certini and Scalenghe, 2011; Ackermann et al., 2015). In the Southern Levant soil erosion is known to have impacted the arid zones where the removal of the vegetation coverage by either intensive herding or the abandonment of agricultural plots, has exposed the soils to severe erosion (Avni, 2005). Although the phenomenon of terrace walls in the Mediterranean highlands is associated with the need to prevent soil erosion, not much is known about the effectiveness and rates of this erosion (Ackermann et al., 2015: 64–65). On the one hand the steep slopes and heavy rains foster the erosions of soil into the dry river beds below, but on the other hand the relatively fast growth of vegetation may be a significant stabilization factor that needs to be considered (Ackermann et al., 2004, 2013). In this limestone-dominated region, soil production is very low and the major source for soils is the slow accumulation of dust on the slopes (Ganor, 1975; Yaalon and Ganor, 1973). Hence, the consequences of possible rapid and thorough soil loss during periods with less intense habitation are a severe shortage of soil when following generations attempt to build new terraces, and the disappearance of soils that would have testified to early terrace building episodes. Here we describe the fate of soils on slopes after terrace abandonment using archaeological data, OSL dating and high-resolution air-born photogrammetry. For the conduct of this study we used the SUERC portable OSL reader (PR), a novel technology only recently applied in archaeological soil studies (Sanderson and Murphy, 2010; Kinnaird et al., 2017). OSL is used extensively for dating quartz grains that had been exposed to sunlight and then buried. The method uses a radiation-induced signal that grows with time and is rapidly reset by exposure to sunlight (Huntley et al., 1985; Wintle and Murray, 2006; Preusser et al., 2009). Conventionally, quartz is extracted and purified from the sediment in the laboratory, and the dose (or equivalent dose; De in Gy) absorbed by the grains is measured. The environmental dose rate (in Gy/ka), that 2
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(Sanderson and Murphy, 2010). The measurement was duplicated after turning the sample over into a second Petri dish, to measure fresh, unexposed grains. This preparation protocol resulted in highly reproducible measurements. To obtain more information on the composition of the terrace soil, organic and inorganic carbon contents were measured on the samples collected for full OSL dating (Table S1). The terraced slopes were scanned by means of air-born photogrammetry to obtain high spatial resolution (< 3 cm) Digital Elevation Model (DEM) and ortho-rectified air photos. Part of the scanned area is presented on Fig. S2. The area was scanned using Terrascan-B drone, a drone specifically built to conduct aerial survey and 3-D modeling in a single flight with over 220 images taken, using a 20.1 MP Exmor APS HD APS-C CMOS Sensor camera with mirrorless shutter. The camera has CMOS, 23.2 × 15.4 mm, with 5456 × 3632 pixels. For each image, camera location, angle and attitude was registered and later implemented in the 3D model reconstruction. The images were stitched together using Pix4D, Agisoft and Visual SFM software. The 3D mesh presented in Fig. S2a is a photorealistic model of the scanned terrain. It shows the DEM superposed on the rectified air photos to provide a 3-D image of the study area, along with a hillshade map of the study area with the location of the three pits.
measurements using the PR. The amalgamation of both data sets provides a detailed chronology of terrace construction, soil accumulation and subsequent terrace collapse, soil loss and abandonment histories. 1.1. Field and laboratory methods Existing and maintained terraced slopes in the Judean Highlands have a stepped topography, with nearly vertical stone walls that hold the soil in place, and horizontal treads used for cultivation (Fig. 1b). When abandoned, the stepped profile is degraded over time by failure of the walls (Fig. 1c) and runoff that washes the soil downslope; this results in a smoother slope profile. An east-facing slope in Nahal (stream) Shmuel, an ephemeral stream north-west of Jerusalem, was selected (Figs. S1 and S2a). The stream is characterized by steep and irregular slopes that are underlain by the marly to dolomitic Beit Meir Formation (Sneh and Avni, 2011). These usually create a smooth slope, in places covered with calcrete, and do not develop into a naturally stepped landscape. Currently the slope is steep (40–45%), has an undulating, relatively smooth profile and is not terraced, but several lines of evidence suggest that the slope was once fully terraced: On the slope there are contour-parallel lines of small stones (Fig. 1d), most likely the fallen remains of terrace walls; horizontal treads are still observable, bracketed by steeper, vegetated slope sections (Fig. S2a); and some of the treads continue north beyond a perpendicular wall into a plot with recently built and well maintained terraces (Fig. S2a). Aerial photography shows that this maintained plot was cultivated in the early 20th century if not before. Today the studied slope is stabilized by vegetation, particularly thorny burnet (Sarcopoterium spinosum), and no evidence for erosion by runoff such as gullies or rills were observed. In places free standing soil sections up to 2 m high with a slope of 65–70% are visible, with no wall holding it from collapse and washing into the stream. We assume that this soil used to be supported by terrace walls which had collapsed following their abandonment. The aim of this study was to evaluate until when were these terrace walls maintained and how much soil has eroded since their collapse. Three different treads were selected for further research (Fig. S2b) and in each a pit was excavated down to bedrock, found at a depth of 1.5–2 m (Fig. S3). Small colluvial aprons extending from the base of the terraces downwards onto the next terrace were also excavated. Samples for full, conventional OSL dating were collected at intervals of 0.5–0.7 m, a total of 13 samples (Table S1; Fig. S3). Additionally, two pits (M1 and M3) were sampled at 0.1 m intervals along their entire profile for PR measurements. For direct comparison, samples were collected also from the holes from which the conventional OSL samples were taken. Two surface (modern) samples were scraped (about 0.02 m thick) for PR measurements, one from the surface of a tread covered with lichen, and the second from a small colluvial apron depositing sediment from one of the treads into another. All samples were collected under a light-tight cover to prevent exposure to sunlight. For conventional OSL dating, very-find-sand quartz (75–125 μm) was extracted under amber lightings using routine laboratory procedures (Faershtein et al., 2016; see notes in Table S1). For each sample the De was measured using the SAR protocol (Murray and Wintle, 2000, 2003) on 19 2-mm aliquots. Dose rates were determined as in Gadot et al. (2016a) and the average De and errors were calculated using the central age model (CAM; Galbraith and Roberts, 2012). For PR measurements, 15–20 g of sample were collected from the main pit walls at 0.1 m intervals into black film canisters after brushing off the surficial, light-exposed material. Under amber lighting, the samples were oven-dried at 50 °C, gently crushed to disintegrate clumps and sieved to less than 0.5 mm. A 5 g sample was placed in a Petri dish 5 cm in diameter, compressed with a smaller dish to smooth the sediment surface and ensure constant distance to the PM tube, and measured using the CW Proxy protocol (Table S2), which included a double depletion of the IRSL signal followed by a 2-step OSL measurement
2. Results Excavations exposed relatively homogenous soil sections. Visual inspection did not reveal any differences in the soils nature and we could not offer a stratigraphic sequence for the development of the soil coverage. The full OSL ages from the three pits range from 4500 to 220 years (before 2015) and are in stratigraphic order (Fig. S3). Over-dispersion values (OD; an indication of the scatter in De values remaining after all measurement uncertainties have been taken into account) vary from 3 to 29% (Table S1), however the higher OD resulted mostly from one or two outlying aliquots which were removed from the data and not used for the average De and age calculations (the source of these outliers is discussed by Porat et al., 2017). In all pits the soil profile can be divided into two or three phases, separated by unknown time intervals (Fig. S3). The sections exposed in the pits comprise soil rich in carbonate particles (72 ± 7% CaCO3), with a slight increase in CaCO3 with soil age. The dose rates (0.8–1.3 Gy/ka) are inversely correlated with the CaCO3 contents (Fig. 2c), suggesting that the carbonate particles act as a dilutant and could have been added deliberately as crushed bedrock to the soil. Organic contents vary, from 0.4 to 0.5% in the younger horizons to 0.1–0.4% in the older horizons (Table S1). Fig. 2a and b show the change in signal intensity (the post-IR OSL signal) measured by the PR as a function of depth (luminescence profiles) for pits M1 and M2, respectively. The OSL signal grows with depth, at first gradually, but from a depth of 1 m or 1.2 m downwards it increases rapidly, implying much older soils. This pattern follows the same trend observed in the full OSL ages (Fig. S3), but with greater detail. Most samples show excellent reproducibility of values, particularly in pit M3. The IRSL signal, measured in step 2 of Table S2, shows a very similar trend with depth, and the IRSL/OSL ratio is fairly constant, mostly in the range of 0.1–0.2. 3. Discussion To evaluate how well the PR signal can be used as a proxy for age, the full OSL ages from pits M1 and M3 were compared to the signal intensity measured for the small samples taken from the same spots (Fig. 2d). A very good correlation is observed between the full OSL ages and the signals measured with the PR (R2 = 0.993), most likely the result of the fine, homogeneous soil. The good correlation allows us to use the linear equation on Fig. 2d to calculate age estimates for all samples measured only by PR, and obtain a detailed history of soil accumulation in the pits. 3
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Fig. 2. a and b. Luminescence profiles for pits M1 and M3, respectively, constructed from blue stimulated luminescence (BSL) signal measured using the PR, plotted against depth in the pits. Two data points are plotted for each depth; note good replication for most samples. Blue diamonds - samples for PR measurements taken for direct comparison from the full OSL sampling holes. The full OSL ages are shown in their respective depth (ages are given in years before 2015; see also Fig. S3 and Table S1). c. Sample dose rates as a function of CaCO3 contents. d. Correlation between the BSL signals measured using PR and the full OSL ages, with the fitting equation. Brown squares - Pit M1; Blue diamonds - Pit M3 e. Age estimates calculated from the equation in panel d. for all PR samples, plotted against depth. Episodes of soil ages are boxed. Blue diamonds – pit M3; brown squares – pit M1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
here are the outskirts of this settlement. The older soils are overlain by a thin, 0.05–0.1 m interval with estimated ages of ∼1650–1900 yr, Roman to early Byzantine times (Fig. 2e). This event of soil manipulation and exposure to sunlight was found in the full OSL ages of samples mostly collected from colluvial aprons adjacent to the pits (Fig. S3). Notably, these old aprons have remained intact and were never entirely eroded down-slope. This age range is not exceptional as it is found at the base of other Ottoman terraces in the Jerusalem Highlands (Gadot et al., 2016a, 2016b). The top meter or so in the pits gradually becomes younger towards the surface, from 900-800 yr at the bottom to 100 yr at the top. In accordance with the addition of soil over time, terrace wall height must have also increased gradually, resulting in larger cultivation plots. The surface samples gave age estimates of 75 yr for the colluvial apron and 175 yr for the lichen-covered tread. Gadot et al. (2016a) found that cultivated and routinely plowed soil in a similar environment gave a
Fig. 2e shows the estimated ages with depth for all samples. The age estimates form a clear pattern, and overall the two pits have a very similar depositional history. At the bottom of the pits the age estimates range from 6200 to 3250 yr. The old ages and the relatively low TOC (Table S1) suggest that these might be the natural soils. Notably, early Holocene soils, such as those found in Har Eitan (Gadot et al., 2016a) or Ramat Rahel (Davidovich et al., 2012) were not found here, perhaps due to the smooth calcrete covering the bedrock that enhances erosion and the lack of crevices that preserved the early Holocene soils at those sites. Some stone blocks were found at the base of pit M3, underlying a soil with an age of 1880 ± 110 yr (bottom left on Fig. S3), so these older sediments might indicate the existence of a settlement site at this spot that was abandoned and later buried below the Roman age terraces. Archaeological excavations conducted some 300 m south of the study area revealed a village dating to the Middle Bronze age (∼3500 years ago; Weksler-Bdolah, 1997) and it could be that the soils exposed 4
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full OSL age of ∼40 yr, so an age of 75 yr for un-plowed soil could express the exposure of grains to sunlight under natural bioturbation. It appears that the establishment of lichen cover sealed the surface of the tread from mixing and thus from sunlight for the past175 years. The age estimate for the topmost sample from pit M3, taken from a depth of ∼0.1 m, is 100 yr, and the youngest full OSL age of 220 yr was obtained at a depth of 0.4 m, below the plowing depth of 0.15–0.2 m. Altogether this gives a range of 100–175 years for terrace abandonment. Note that in pit M1 the ages estimated for the top-most samples are not in stratigraphic order, giving ∼800 yr (Fig. 2e). This might indicate that at the latest stage in terrace maintenance, soil with a substantial residual OSL signal was added to the top of the tread. We used the high resolution topographic profiles of the terraces obtained from the photogrammetry survey together with field documentation, to estimate the volume of soil that was trapped behind terrace walls and the fraction that is now missing (Fig. S4; location of the profile is shown as a black line marked M3 on Fig. S2b). The position of the terrace walls was reconstructed from the lines of fallen stones located at the base of terrace risers (Fig. 1d). As the pits were excavated down to bedrock, the height of the terrace and the missing wall were estimated by the vertical distance from bedrock to tread remnants. After reconstructing the top of the wall (with an appropriate up-slope tilt), the original width of the cultivated tread was restored. Thus the total volume of the intact terrace infill was established. When imposing the total volume of the original terrace on the current topographic profile, the area of the missing, eroded soil can be calculated. Fig. S4 presents such an example of terrace soil content (for pit M3), including the reconstructed upper profile, the measured eroded profile, and observed bedrock profile (upper, middle and lower profiles, respectively). Calculations for the three excavated terraces on current steep slope provide soil loss values of ∼30% for terraces M1 and M3, and ∼45% for M2 (with estimated uncertainty of ∼10%). This is a maximum value as it does not take into account soil redeposited down-slope in the small aprons observed, which now cover the bottom of each terrace riser. The slope of the terrace risers, 65–70%, supports these relatively low soil losses: when terrace walls existed, the slope of the risers was ∼90%, as observed in standing terraces. The reduction in slope after the walls collapsed to 65–70% indicates a loss of 25–30%.
Thus, after abandonment terrace walls do undergo rapid degradation and there is some soil loss, but overall most of the soil is retained on the slopes as a stable cover. In the case of the Judean Highlands terraces, it seems that the transient erosional phase lasted less than 100 years and from then onwards the slopes are stabile. Soil erosion in this area is not as severe as has previously been assumed, and the lack of older terrace ages cannot be explained by erosion. Degradation of terraces from stepped to smooth slopes is a diffusionlike process, similar to fault scarp degradation (Enzel et al., 1996). After a riser forms, erosion starts rapidly and material is transported from the top of an upper tread to the inner edge of a tread below it, smoothing the profile. As slope moderates, the rate of transport slows down until equilibrium is reached, with no erosion of the stable slope. Future work will use modelling to calculate diffusion rates from degraded terraces of known age and use these rates for estimating the time of terrace abandonment from topographic profile alone. Acknowledgements This research was supported by an Israel Science Foundation grant (Grant No. 1691/13) awarded to Y.G. and N.P. Field work was conducted under license G-80/2015 from The Israel Antiquities Authority. We wish to thank the following persons for their assistance in the field and lab: Nitsan Ben-Melech, Boaz Gross, Helena Roth, Maya Hadash, Roni Avidav, Nitsan Shalom, Shirad Galmor, Danilo Giordano, Yael Yakobi and Ayelet Feldman. We thank Erez Biton, director of Terrascanlabs (www.tmt.co.il) for providing the photogrammetry data, and an anonymous reviewer for constructive comments. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.quageo.2018.04.001. References Ackermann, O., Maeir, A.M., Bruins, H.J., 2004. Unique human-made catenary changes and their effect on soil and vegetation in the semi-arid Mediterranean zone: a case study on Sarcopoterium spinosum distribution near Tell es-Safi/Gath, Israel. Catena 57, 309–330. Ackermann, O., Zhevelev, H.M., Svoray, T., 2013. Sarcopoterium spinosum from mosaic structure to matrix structure: impact of calcrete (Nari) on vegetation in a Mediterranean semi-arid landscape. Catena 101, 79–91. Ackermann, O., Weiss, E., Zhevelev, H., Maeir, A., Frumin, S., Horwitz, K.L., 2015. Key points in the paleo-anthropocene period in Israel: past human activity as the designer of the present-day landscape. The geography network. Hebrew e Journal for Geographical Research 8, 61–74. Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, NewYork. Arnaez, J., Lana-Renault, N., Lasanta, T., Ruiz-Flano, P., Castroviejo, J., 2015. Effects of farming terraces on hydrological and geomorphological processes: a review. Catena 128, 122–134. Avni, Y., 2005. Gully incision as a key factor in desertification in an arid environment, the Negev highlands, Israel. Catena 63 (2), 185–220. Bateman, M.D., Stein, S., Ashurst, R.A., Selby, K., 2015. Instant luminescence Chronologies? High resolution luminescence profiles using a portable luminescence reader. Quat. Geochronol. 30, 141–146. Bevan, A., Conolly, J., Colledge, S., Frederick, C., Palmer, C., Siddall, R., Stellatou, A., 2013. The long-term ecology of Agricultural terraces and enclosed fields from Antikythera, Greece. Hum. Ecol. 41, 255–272. Burbidge, C.I., Duller, G.A.T., Roberts, H.M., 2006. De determination for young samples using the standardised OSL response of coarse-grain quartz. Radiat. Meas. 41, 278–288. Burbidge, C.I., Sanderson, D.C.W., Housley, R.A., Jones, P.A., 2007. Survey of Palaeolithic sites by luminescence profiling, a case study from Eastern Europe. Quat. Geochronol. 2, 296–302. Certini, G., Scalenghe, R., 2011. Anthropogenic soils are the golden spikes for the Anthropocene. Holocene 21, 1269–1274. Davidovich, U., Porat, N., Gadot, Y., Avni, Y., Lipschits, O., 2012. Archaeological investigations and OSL dating of terraces at Ramat Rahel, Israel. J. Field Archaeol. 37, 192–208. de Geus, C.H.J., 1975. The importance of archaeological research into the Palestinian agricultural terraces, with an excursus on the Hebrew word gbî. Palest. Explor. Q. 107, 65–74. Durcan, J.A., Roberts, H.M., Duller, G.A.T., Alizai, A.H., 2010. Testing the use of rangefinder OSL dating to inform field sampling and laboratory processing strategies. Quat.
4. Conclusions This study shows that correlating a limited number of conventional full OSL dating with a large number of rapid PR measurements on small bulk samples is a powerful tool. Here it was used to construct a detailed chronology of agricultural terrace building and the history of soil accumulation, use and abandonment of terraces on the western slope of Nahal Shmuel in the Judean Highlands. In mid-Holocene the slopes were covered with natural soils. Cultivation and soil manipulation by man started in the study area during Roman times and the first phase lasted perhaps up to the early Byzantine period. That phase is represented by a thin layer in the pits and also in the colluvial aprons which could be the remains of eroded Roman-Byzantine terraces. The major episode of terrace construction began ∼800 years ago, in agreement with finds elsewhere in the Judean Highlands (Gadot et al., 2016a, 2016b), and continued uninterrupted until 175-100 years ago. It seems that terraces were maintained throughout that time as the estimated ages cover the entire period and decrease very gradually upsection. 175-100 years ago the terraces were abandoned and no longer maintained. Terrace walls crumbled, followed by soil erosion, where up to 45% of the soil originally behind the stone walls was lost downhill (but is perhaps still partially stored on the slope). However, the slope stabilized rapidly by lichen and vegetation, and is currently not undergoing much if any erosion. Possibly, the fenced plot, still operational today, was built to the north as an alternative. 5
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Earth Surf. Process. Landforms 36, 651–660. Munyikwa, K., Brown, S., Kitabwala, Z., 2012. Delineating stratigraphic breaks at the bases of postglacial dunes in central Alberta, Canada. Earth Surf. Process. Landforms 27, 1603–1614. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiat. Meas. 32, 57–73. Murray, A.S., Wintle, A.G., 2003. The single aliquot regenerative dose protocol: potential for improvements in reliability. Radiat. Meas. 37, 377–381. Porat, N., Davidovich, U., Avni, Y., Avni, G., Gadot, Y., 2017. Using OSL measurements to decipher soil history in archaeological terraces, Judean Highlands, Israel. Land Degrad. Dev. http://dx.doi.org/10.1002/ldr.2729. Preusser, F., Chithambo, M.L., Götte, T., Martini, M., Ramseyer, K., Sendezera, E.J., Susino, G.J., Wintle, A.G., 2009. Quartz as a natural luminescence dosimeter. Earth Sci. Rev. 97, 184–214. Roberts, H., Durcan, J.A., Duller, G.A.T., 2009. Exploring procedures for the rapid assessment of optically stimulated luminescence range-finder ages. Radiat. Meas. 44, 582–587. Roberts, H.M., Duller, G.A.T., 2004. Standardised growth curves for optical dating of sediment using multiple-grain aliquots. Radiat. Meas. 38, 241–252. Ron, Z., 1966. Agricultural terraces in the judean mountains. Isr. Explor. J. 16 (33–49), 111–122. Sanderson, D.C.W., Bishop, P., Houston, I., Boonsener, M., 2001. Luminescence characterisation of quartz-rich cover sands from NE Thailand. Quat. Sci. Rev. 20, 893–900. Sanderson, D.C.W., Bishop, P., Stark, M., Alexander, S., Penny, D., 2007. Luminescence dating of canal sediments from angkor borei, mekong delta, southern Cambodia. Quat. Geochronol. 2, 322–329. Sanderson, D.C.W., Murphy, S., 2010. Using simple portable OSL measurements and laboratory characterisation to help understand complex and heterogeneous sediment sequences for luminescence dating. Quat. Geochronol. 5, 299–305. Sneh, A., Avni, Y., 2011. Geological Map of Israel 1:50,000, Jerusalem Sheet. Geological Survey of Israel. Stang, D.M., Rhodes, E.J., Heimsath, A.M., 2012. Assessing soil mixing processes and rates using a portable OSL-IRSL reader: preliminary determinations. Quaternary Gechronology 10, 314–319. Stone, A.E.C., Bateman, M.D., Thomas, D.S.G., 2015. Rapid age assessment in the Namib Sand Sea using portable luminescence reader. Quat. Geochronol. 30, 134–140. Weksler-Bdolah, S., 1997. Alona. Excavations and Surveys in Israel 19 68*–70*. Wikinson, T.K., 2003. Archaeological Landscapes of the Near East. (Tucson). Wintle, A.G., Murray, A.S., 2006. A Review of Quartz Optically Stimulated Luminescence Characteristics and Their Relevance in Single-aliquot Regeneration Dating Protocols. Yaalon, D.H., Ganor, E., 1973. The influence of dust on soils during the Quaternary. Soil Sci. 116, 146–155.
Geochronol. 5, 86–90. Enzel, Y., Amit, R., Porat, N., Zilberman, E., Harrison, J.B., 1996. Estimating the ages of fault scarps in the Arava, Israel. Tectonophysics 253, 305–317. Faershtein, G., Porat, N., Avni, Y., Matmon, A., 2016. Aggradation-incision transition in arid environments at the end of the Pleistocene: an example from the Negev Highlands, southern Israel. Geomorphology 253, 289–304. Finkelstein, I., 1996. Ethnicity and origins of iron I settlers in the highlands of canaan, can the real Israel stand up? Biblic. Archaeol. 59, 198–212. Gadot, Y., Davidovich, U., Avni, G., Avni, Y., Piasetzky, M., Faerstein, G., Golan, D., Porat, N., 2016a. The formation of a mediterranean terraced landscape: mount eitan, judean highlands, Israel. Journal of Archaeological Science Reports 6, 397–417. Gadot, Y., Davidovich, U., Avni, Y., Avni, G., Porat, N., 2016b. The formation of terraced landscapes in the Judean Highlands in Israel, and its implications for Biblical agricultural history. Hebrew Bible and Ancient Israel. http://dx.doi.org/10.1628/ 219222717X14991542936068. Galbraith, R.F., Roberts, R.G., 2012. Statistical aspects of equivalent dose and error calculation and display in OSL dating: an overview and some recommendations. Quat. Geochronol. 11, 1–27. Ganor, E., 1975. Atmospheric Dust in Israel: Sedimentological and Meteorological Analysis of Dust Deposition. Ph.D. thesis. Hebrew Univ. of Jerusalem, Jerusalem, pp. 224. Gibson, S., 2001. Agricultural terraces and settlement expansion in the highlands of early Iron Age Palestine : is there any correlation between the two? In: Mazar, A. (Ed.), Studies in the Archaeology of the Iron Age in Israel and Jordan. Journal for the Study of the Old Testament Supplementary Series 331. Sheffield University Press, Sheffield, pp. 113–146. Hopkins, D., 1985. “The Highlands of Canaan.” Agricultural Life in the Early Iron Age. vol. 3 Burns & Oates. Huntley, D.J., Godfrey-Smith, D.I., Thewalt, M.L.W., 1985. Optical dating of sediments. Nature 313, 105–107. King, G.E., Sanderson, D.C.W., Robinson, R.A.J., Finch, A.A., 2014. Understanding processes of sediment bleaching in glacial settings using a portable OSL reader. Boreas 43, 955–972. Kinnaird, T., Bolòs, J., Turner, A., Turner, S., 2017. Optically-stimulated luminescence profiling and dating of historic agricultural terraces in Catalonia (Spain). J. Archaeol. Sci. 78, 66–77. Lai, Z.-P., Brückner, H., Zöller, L., Fülling, A., 2007. Existence of a common growth curve for silt-sized quartz OSL of loess from different continents. Radiat. Meas. 42, 1432–1440. Leighton, C.L., Bailey, R.M., 2015. Investigating the potential of HCl-only treated samples using range-finder OSL dating. Quat. Geochronol. 25, 1–9. Muñoz-Salinas, E., Bishop, P., Sanderson, D.C.W., Zamorano, J.J., 2011. Interpreting luminescence data from a portable OSL reader: three case studies in fluvial settings.
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Quaternary Geochronology journal homepage: www.elsevier.com/locate/quageo
Research paper
Using portable OSL reader to obtain a time scale for soil accumulation and erosion in archaeological terraces, the Judean Highlands, Israel Naomi Porata,∗, Gloria I. Lópezb, Nadav Lenskya, Rotem Elinsonc, Yoav Avnia, Yelena Elgart-Sharonc, Galina Faershteina,d, Yuval Gadotc a
Geological Survey of Israel, 30 Malkhe Israel St., Jerusalem, 9550116, Israel Luminescence Dating Laboratory, CENIEH, Paseo Sierra de Atapuerca 3, 09002, Burgos, Spain c The Department of Archaeology and Ancient Near Eastern Cultures, Tel Aviv University, Tel Aviv, 6997801, Israel d Institute of Earth Sciences, The Hebrew University, Jerusalem, Israel b
A R T I C LE I N FO
A B S T R A C T
Keywords: Portable OSL reader OSL Soil erosion Bench terraces
Current conventions on the fate of agricultural bench terraces in the mountainous Mediterranean climatic zones, assume that after abandonment terraces undergo rapid degradation and soil loss. First walls crumble, followed by soil washed out of breaches in the walls by runoff, resulting in rapid erosion of soil from the slopes downhill into the streams. Additionally, soil erosion may explain why previous optically stimulated luminescence (OSL) dating of terrace soils in the Judean Highlands, Israel, found mostly young soils dating to the past 700 years, and only occasionally were older ages obtained for soils at the very base of these terraces. In contrast, observations made in the same region show that slopes with degraded terraces appear to still retain much soil even though only faint remains of the terraces exist. To test if terraces and soils indeed erode entirely and how long this might take, a relatively smooth hill slope showing remains of highly degraded sets of terraces was studied. Samples were collected from excavated pits for full OSL dating, in addition to samples densely collected for OSL measurements using a portable OSL reader (PR). Air-born photogrammetry was used to obtain high spatial resolution Digital Elevation Model. Results show that the main body of terraces was first built ∼800 years ago and maintained until 175-100 years age when they were abandoned and subsequently degraded. Since then 30–45% of soil volume was lost to erosion, however steady-state was reached at a relatively high slope of 65%, with stabilization by vegetation. The thick soil present on most of the slope suggests that after the first stage of rapid degradation the slopes reache equilibrium, most likely due to vegetation that reduces direct soil erosion, so most of the soil is retained on the slope. Finally, the PR allows for a much more nuanced understanding of terrace soil history.
1. Introduction
are a pre-requisite for sustainable living in mountainous areas (de Geus, 1975; Finkelstein, 1996; Gibson, 2001, but see already Hopkins, 1985: 180). There are several possible anthropogenic and/or natural factors that may have affected the OSL age range found in earlier studies. Previously we have dealt with the possibility that sampling was not comprehensive and elsewhere in the landscape older terraces do exist (Gadot et al., 2016a), and that continuous terrace maintenance exposes the sediment to sunlight and erases the OSL signal, thus obliterating older terraces ages (Porat et al., 2017). In this article we wish to address the possibility that earlier phases of terracing were entirely erased from the landscape due to intense soil erosion at times when terraces were not maintained. Soil movement that resulted from human exploitation is a world-
Man-made farming bench terraces cover large portions of the mountain area in Israel (Ron, 1966; Wikinson, 2003). Terraces increase the area of arable land, improve water infiltration and retention, and prevent soil erosion (Bevan et al., 2013; Arnaez et al., 2015). Extensive research on bench terraces in the Judean Highlands around Jerusalem (Fig. 1a and Fig. S1), which included mapping, trenching and optically stimulated luminescence (OSL) dating of soil infill, showed that most of the terraces were constructed in the last 700 years, and only occasionally are older soils found at the base of these terraces (Davidovich et al., 2012; Gadot et al., 2016a, 2016b; Porat et al., 2017). The relatively recent ages contrast with previous archaeological estimates for the age of the terraces that were based on the assumption that terraces
∗
Corresponding author. E-mail address: [email protected] (N. Porat).
https://doi.org/10.1016/j.quageo.2018.04.001 Received 30 November 2017; Received in revised form 9 April 2018; Accepted 11 April 2018 1871-1014/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Porat, N., Quaternary Geochronology (2018), https://doi.org/10.1016/j.quageo.2018.04.001
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Fig. 1. a. Location map, the study area shown with a star. b–d: Terraced slopes in the Judean Highlands. b. A fully terraced slope with a stepped profile (northern slopes of Nahal Halilim, terrace heights 0.5–1 m). c. A terrace undergoing erosion by wall collapse and soil wash (Nahal Rephaim, terrace height ∼ 0.7 m). d. Aerial photo of a slope with degraded terraces showing lines of white stones (marked by white arrows) tracing the remains of the collapsed terrace walls (Nahal Shmuel). Pits M1-M3 are visible as smooth, brown streaks. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
which induces the signal, is measured separately and the age is calculated from the ratio De/dose rate (Aitken, 1998). OSL dating is time- and labor-intensive, and at best several months pass between sample collection and obtaining an age. Over the years several approaches have been devised to shorten this time, either by reducing sample preparation time (e.g. Burbidge et al., 2006; Roberts et al., 2009; Durcan et al., 2010; Leighton and Bailey, 2015), or reducing measurement time by for example using a standardized growth curve (e.g. Roberts and Duller, 2004; Lai et al., 2007). One such means for obtaining rapid luminescence data is the SUERC PR (Sanderson and Murphy, 2010) which measures OSL and infrared stimulated luminescence (IRSL) signals directly on large, untreated samples. Signal intensity is a proxy of age, environmental dose rates and mineral sensitivity, and neither the De nor the dose rate is measured with the PR. Nonetheless intensities, depletion rates and the ratio between the OSL and IRSL signals can be used to estimate relative ages within a section (Stang et al., 2012), to identify sediments that contain a significant component of grains that were not bleached at the time of deposition, potential diagenesis and even mineralogical variations (Kinnaird et al., 2017; Muñoz-Salinas et al., 2011; Munyikwa et al., 2012; King et al., 2014; Bateman et al., 2015), or construct correlation charts to obtain numerical age estimates (Stone et al., 2015). Sample preparation is minimal (Burbidge et al., 2007) and measurement time short, and a preview of sediment luminescence properties can aid in more focused sampling for full OSL dating and in obtaining a rough depositional history of the sediments. When collected continuously, OSL signal intensities from bulk samples measured with a PR can be plotted as luminescence profiles (Sanderson et al., 2001, 2007; Stang et al., 2012), allowing for a nuanced visualization of changes along a stratigraphic profile. In this study we carried out full OSL dating on a selected set of samples in combination with a large number of
wide phenomenon (Certini and Scalenghe, 2011; Ackermann et al., 2015). In the Southern Levant soil erosion is known to have impacted the arid zones where the removal of the vegetation coverage by either intensive herding or the abandonment of agricultural plots, has exposed the soils to severe erosion (Avni, 2005). Although the phenomenon of terrace walls in the Mediterranean highlands is associated with the need to prevent soil erosion, not much is known about the effectiveness and rates of this erosion (Ackermann et al., 2015: 64–65). On the one hand the steep slopes and heavy rains foster the erosions of soil into the dry river beds below, but on the other hand the relatively fast growth of vegetation may be a significant stabilization factor that needs to be considered (Ackermann et al., 2004, 2013). In this limestone-dominated region, soil production is very low and the major source for soils is the slow accumulation of dust on the slopes (Ganor, 1975; Yaalon and Ganor, 1973). Hence, the consequences of possible rapid and thorough soil loss during periods with less intense habitation are a severe shortage of soil when following generations attempt to build new terraces, and the disappearance of soils that would have testified to early terrace building episodes. Here we describe the fate of soils on slopes after terrace abandonment using archaeological data, OSL dating and high-resolution air-born photogrammetry. For the conduct of this study we used the SUERC portable OSL reader (PR), a novel technology only recently applied in archaeological soil studies (Sanderson and Murphy, 2010; Kinnaird et al., 2017). OSL is used extensively for dating quartz grains that had been exposed to sunlight and then buried. The method uses a radiation-induced signal that grows with time and is rapidly reset by exposure to sunlight (Huntley et al., 1985; Wintle and Murray, 2006; Preusser et al., 2009). Conventionally, quartz is extracted and purified from the sediment in the laboratory, and the dose (or equivalent dose; De in Gy) absorbed by the grains is measured. The environmental dose rate (in Gy/ka), that 2
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(Sanderson and Murphy, 2010). The measurement was duplicated after turning the sample over into a second Petri dish, to measure fresh, unexposed grains. This preparation protocol resulted in highly reproducible measurements. To obtain more information on the composition of the terrace soil, organic and inorganic carbon contents were measured on the samples collected for full OSL dating (Table S1). The terraced slopes were scanned by means of air-born photogrammetry to obtain high spatial resolution (< 3 cm) Digital Elevation Model (DEM) and ortho-rectified air photos. Part of the scanned area is presented on Fig. S2. The area was scanned using Terrascan-B drone, a drone specifically built to conduct aerial survey and 3-D modeling in a single flight with over 220 images taken, using a 20.1 MP Exmor APS HD APS-C CMOS Sensor camera with mirrorless shutter. The camera has CMOS, 23.2 × 15.4 mm, with 5456 × 3632 pixels. For each image, camera location, angle and attitude was registered and later implemented in the 3D model reconstruction. The images were stitched together using Pix4D, Agisoft and Visual SFM software. The 3D mesh presented in Fig. S2a is a photorealistic model of the scanned terrain. It shows the DEM superposed on the rectified air photos to provide a 3-D image of the study area, along with a hillshade map of the study area with the location of the three pits.
measurements using the PR. The amalgamation of both data sets provides a detailed chronology of terrace construction, soil accumulation and subsequent terrace collapse, soil loss and abandonment histories. 1.1. Field and laboratory methods Existing and maintained terraced slopes in the Judean Highlands have a stepped topography, with nearly vertical stone walls that hold the soil in place, and horizontal treads used for cultivation (Fig. 1b). When abandoned, the stepped profile is degraded over time by failure of the walls (Fig. 1c) and runoff that washes the soil downslope; this results in a smoother slope profile. An east-facing slope in Nahal (stream) Shmuel, an ephemeral stream north-west of Jerusalem, was selected (Figs. S1 and S2a). The stream is characterized by steep and irregular slopes that are underlain by the marly to dolomitic Beit Meir Formation (Sneh and Avni, 2011). These usually create a smooth slope, in places covered with calcrete, and do not develop into a naturally stepped landscape. Currently the slope is steep (40–45%), has an undulating, relatively smooth profile and is not terraced, but several lines of evidence suggest that the slope was once fully terraced: On the slope there are contour-parallel lines of small stones (Fig. 1d), most likely the fallen remains of terrace walls; horizontal treads are still observable, bracketed by steeper, vegetated slope sections (Fig. S2a); and some of the treads continue north beyond a perpendicular wall into a plot with recently built and well maintained terraces (Fig. S2a). Aerial photography shows that this maintained plot was cultivated in the early 20th century if not before. Today the studied slope is stabilized by vegetation, particularly thorny burnet (Sarcopoterium spinosum), and no evidence for erosion by runoff such as gullies or rills were observed. In places free standing soil sections up to 2 m high with a slope of 65–70% are visible, with no wall holding it from collapse and washing into the stream. We assume that this soil used to be supported by terrace walls which had collapsed following their abandonment. The aim of this study was to evaluate until when were these terrace walls maintained and how much soil has eroded since their collapse. Three different treads were selected for further research (Fig. S2b) and in each a pit was excavated down to bedrock, found at a depth of 1.5–2 m (Fig. S3). Small colluvial aprons extending from the base of the terraces downwards onto the next terrace were also excavated. Samples for full, conventional OSL dating were collected at intervals of 0.5–0.7 m, a total of 13 samples (Table S1; Fig. S3). Additionally, two pits (M1 and M3) were sampled at 0.1 m intervals along their entire profile for PR measurements. For direct comparison, samples were collected also from the holes from which the conventional OSL samples were taken. Two surface (modern) samples were scraped (about 0.02 m thick) for PR measurements, one from the surface of a tread covered with lichen, and the second from a small colluvial apron depositing sediment from one of the treads into another. All samples were collected under a light-tight cover to prevent exposure to sunlight. For conventional OSL dating, very-find-sand quartz (75–125 μm) was extracted under amber lightings using routine laboratory procedures (Faershtein et al., 2016; see notes in Table S1). For each sample the De was measured using the SAR protocol (Murray and Wintle, 2000, 2003) on 19 2-mm aliquots. Dose rates were determined as in Gadot et al. (2016a) and the average De and errors were calculated using the central age model (CAM; Galbraith and Roberts, 2012). For PR measurements, 15–20 g of sample were collected from the main pit walls at 0.1 m intervals into black film canisters after brushing off the surficial, light-exposed material. Under amber lighting, the samples were oven-dried at 50 °C, gently crushed to disintegrate clumps and sieved to less than 0.5 mm. A 5 g sample was placed in a Petri dish 5 cm in diameter, compressed with a smaller dish to smooth the sediment surface and ensure constant distance to the PM tube, and measured using the CW Proxy protocol (Table S2), which included a double depletion of the IRSL signal followed by a 2-step OSL measurement
2. Results Excavations exposed relatively homogenous soil sections. Visual inspection did not reveal any differences in the soils nature and we could not offer a stratigraphic sequence for the development of the soil coverage. The full OSL ages from the three pits range from 4500 to 220 years (before 2015) and are in stratigraphic order (Fig. S3). Over-dispersion values (OD; an indication of the scatter in De values remaining after all measurement uncertainties have been taken into account) vary from 3 to 29% (Table S1), however the higher OD resulted mostly from one or two outlying aliquots which were removed from the data and not used for the average De and age calculations (the source of these outliers is discussed by Porat et al., 2017). In all pits the soil profile can be divided into two or three phases, separated by unknown time intervals (Fig. S3). The sections exposed in the pits comprise soil rich in carbonate particles (72 ± 7% CaCO3), with a slight increase in CaCO3 with soil age. The dose rates (0.8–1.3 Gy/ka) are inversely correlated with the CaCO3 contents (Fig. 2c), suggesting that the carbonate particles act as a dilutant and could have been added deliberately as crushed bedrock to the soil. Organic contents vary, from 0.4 to 0.5% in the younger horizons to 0.1–0.4% in the older horizons (Table S1). Fig. 2a and b show the change in signal intensity (the post-IR OSL signal) measured by the PR as a function of depth (luminescence profiles) for pits M1 and M2, respectively. The OSL signal grows with depth, at first gradually, but from a depth of 1 m or 1.2 m downwards it increases rapidly, implying much older soils. This pattern follows the same trend observed in the full OSL ages (Fig. S3), but with greater detail. Most samples show excellent reproducibility of values, particularly in pit M3. The IRSL signal, measured in step 2 of Table S2, shows a very similar trend with depth, and the IRSL/OSL ratio is fairly constant, mostly in the range of 0.1–0.2. 3. Discussion To evaluate how well the PR signal can be used as a proxy for age, the full OSL ages from pits M1 and M3 were compared to the signal intensity measured for the small samples taken from the same spots (Fig. 2d). A very good correlation is observed between the full OSL ages and the signals measured with the PR (R2 = 0.993), most likely the result of the fine, homogeneous soil. The good correlation allows us to use the linear equation on Fig. 2d to calculate age estimates for all samples measured only by PR, and obtain a detailed history of soil accumulation in the pits. 3
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Fig. 2. a and b. Luminescence profiles for pits M1 and M3, respectively, constructed from blue stimulated luminescence (BSL) signal measured using the PR, plotted against depth in the pits. Two data points are plotted for each depth; note good replication for most samples. Blue diamonds - samples for PR measurements taken for direct comparison from the full OSL sampling holes. The full OSL ages are shown in their respective depth (ages are given in years before 2015; see also Fig. S3 and Table S1). c. Sample dose rates as a function of CaCO3 contents. d. Correlation between the BSL signals measured using PR and the full OSL ages, with the fitting equation. Brown squares - Pit M1; Blue diamonds - Pit M3 e. Age estimates calculated from the equation in panel d. for all PR samples, plotted against depth. Episodes of soil ages are boxed. Blue diamonds – pit M3; brown squares – pit M1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
here are the outskirts of this settlement. The older soils are overlain by a thin, 0.05–0.1 m interval with estimated ages of ∼1650–1900 yr, Roman to early Byzantine times (Fig. 2e). This event of soil manipulation and exposure to sunlight was found in the full OSL ages of samples mostly collected from colluvial aprons adjacent to the pits (Fig. S3). Notably, these old aprons have remained intact and were never entirely eroded down-slope. This age range is not exceptional as it is found at the base of other Ottoman terraces in the Jerusalem Highlands (Gadot et al., 2016a, 2016b). The top meter or so in the pits gradually becomes younger towards the surface, from 900-800 yr at the bottom to 100 yr at the top. In accordance with the addition of soil over time, terrace wall height must have also increased gradually, resulting in larger cultivation plots. The surface samples gave age estimates of 75 yr for the colluvial apron and 175 yr for the lichen-covered tread. Gadot et al. (2016a) found that cultivated and routinely plowed soil in a similar environment gave a
Fig. 2e shows the estimated ages with depth for all samples. The age estimates form a clear pattern, and overall the two pits have a very similar depositional history. At the bottom of the pits the age estimates range from 6200 to 3250 yr. The old ages and the relatively low TOC (Table S1) suggest that these might be the natural soils. Notably, early Holocene soils, such as those found in Har Eitan (Gadot et al., 2016a) or Ramat Rahel (Davidovich et al., 2012) were not found here, perhaps due to the smooth calcrete covering the bedrock that enhances erosion and the lack of crevices that preserved the early Holocene soils at those sites. Some stone blocks were found at the base of pit M3, underlying a soil with an age of 1880 ± 110 yr (bottom left on Fig. S3), so these older sediments might indicate the existence of a settlement site at this spot that was abandoned and later buried below the Roman age terraces. Archaeological excavations conducted some 300 m south of the study area revealed a village dating to the Middle Bronze age (∼3500 years ago; Weksler-Bdolah, 1997) and it could be that the soils exposed 4
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full OSL age of ∼40 yr, so an age of 75 yr for un-plowed soil could express the exposure of grains to sunlight under natural bioturbation. It appears that the establishment of lichen cover sealed the surface of the tread from mixing and thus from sunlight for the past175 years. The age estimate for the topmost sample from pit M3, taken from a depth of ∼0.1 m, is 100 yr, and the youngest full OSL age of 220 yr was obtained at a depth of 0.4 m, below the plowing depth of 0.15–0.2 m. Altogether this gives a range of 100–175 years for terrace abandonment. Note that in pit M1 the ages estimated for the top-most samples are not in stratigraphic order, giving ∼800 yr (Fig. 2e). This might indicate that at the latest stage in terrace maintenance, soil with a substantial residual OSL signal was added to the top of the tread. We used the high resolution topographic profiles of the terraces obtained from the photogrammetry survey together with field documentation, to estimate the volume of soil that was trapped behind terrace walls and the fraction that is now missing (Fig. S4; location of the profile is shown as a black line marked M3 on Fig. S2b). The position of the terrace walls was reconstructed from the lines of fallen stones located at the base of terrace risers (Fig. 1d). As the pits were excavated down to bedrock, the height of the terrace and the missing wall were estimated by the vertical distance from bedrock to tread remnants. After reconstructing the top of the wall (with an appropriate up-slope tilt), the original width of the cultivated tread was restored. Thus the total volume of the intact terrace infill was established. When imposing the total volume of the original terrace on the current topographic profile, the area of the missing, eroded soil can be calculated. Fig. S4 presents such an example of terrace soil content (for pit M3), including the reconstructed upper profile, the measured eroded profile, and observed bedrock profile (upper, middle and lower profiles, respectively). Calculations for the three excavated terraces on current steep slope provide soil loss values of ∼30% for terraces M1 and M3, and ∼45% for M2 (with estimated uncertainty of ∼10%). This is a maximum value as it does not take into account soil redeposited down-slope in the small aprons observed, which now cover the bottom of each terrace riser. The slope of the terrace risers, 65–70%, supports these relatively low soil losses: when terrace walls existed, the slope of the risers was ∼90%, as observed in standing terraces. The reduction in slope after the walls collapsed to 65–70% indicates a loss of 25–30%.
Thus, after abandonment terrace walls do undergo rapid degradation and there is some soil loss, but overall most of the soil is retained on the slopes as a stable cover. In the case of the Judean Highlands terraces, it seems that the transient erosional phase lasted less than 100 years and from then onwards the slopes are stabile. Soil erosion in this area is not as severe as has previously been assumed, and the lack of older terrace ages cannot be explained by erosion. Degradation of terraces from stepped to smooth slopes is a diffusionlike process, similar to fault scarp degradation (Enzel et al., 1996). After a riser forms, erosion starts rapidly and material is transported from the top of an upper tread to the inner edge of a tread below it, smoothing the profile. As slope moderates, the rate of transport slows down until equilibrium is reached, with no erosion of the stable slope. Future work will use modelling to calculate diffusion rates from degraded terraces of known age and use these rates for estimating the time of terrace abandonment from topographic profile alone. Acknowledgements This research was supported by an Israel Science Foundation grant (Grant No. 1691/13) awarded to Y.G. and N.P. Field work was conducted under license G-80/2015 from The Israel Antiquities Authority. We wish to thank the following persons for their assistance in the field and lab: Nitsan Ben-Melech, Boaz Gross, Helena Roth, Maya Hadash, Roni Avidav, Nitsan Shalom, Shirad Galmor, Danilo Giordano, Yael Yakobi and Ayelet Feldman. We thank Erez Biton, director of Terrascanlabs (www.tmt.co.il) for providing the photogrammetry data, and an anonymous reviewer for constructive comments. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.quageo.2018.04.001. References Ackermann, O., Maeir, A.M., Bruins, H.J., 2004. Unique human-made catenary changes and their effect on soil and vegetation in the semi-arid Mediterranean zone: a case study on Sarcopoterium spinosum distribution near Tell es-Safi/Gath, Israel. Catena 57, 309–330. Ackermann, O., Zhevelev, H.M., Svoray, T., 2013. Sarcopoterium spinosum from mosaic structure to matrix structure: impact of calcrete (Nari) on vegetation in a Mediterranean semi-arid landscape. Catena 101, 79–91. Ackermann, O., Weiss, E., Zhevelev, H., Maeir, A., Frumin, S., Horwitz, K.L., 2015. Key points in the paleo-anthropocene period in Israel: past human activity as the designer of the present-day landscape. The geography network. Hebrew e Journal for Geographical Research 8, 61–74. Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, NewYork. Arnaez, J., Lana-Renault, N., Lasanta, T., Ruiz-Flano, P., Castroviejo, J., 2015. Effects of farming terraces on hydrological and geomorphological processes: a review. Catena 128, 122–134. Avni, Y., 2005. Gully incision as a key factor in desertification in an arid environment, the Negev highlands, Israel. Catena 63 (2), 185–220. Bateman, M.D., Stein, S., Ashurst, R.A., Selby, K., 2015. Instant luminescence Chronologies? High resolution luminescence profiles using a portable luminescence reader. Quat. Geochronol. 30, 141–146. Bevan, A., Conolly, J., Colledge, S., Frederick, C., Palmer, C., Siddall, R., Stellatou, A., 2013. The long-term ecology of Agricultural terraces and enclosed fields from Antikythera, Greece. Hum. Ecol. 41, 255–272. Burbidge, C.I., Duller, G.A.T., Roberts, H.M., 2006. De determination for young samples using the standardised OSL response of coarse-grain quartz. Radiat. Meas. 41, 278–288. Burbidge, C.I., Sanderson, D.C.W., Housley, R.A., Jones, P.A., 2007. Survey of Palaeolithic sites by luminescence profiling, a case study from Eastern Europe. Quat. Geochronol. 2, 296–302. Certini, G., Scalenghe, R., 2011. Anthropogenic soils are the golden spikes for the Anthropocene. Holocene 21, 1269–1274. Davidovich, U., Porat, N., Gadot, Y., Avni, Y., Lipschits, O., 2012. Archaeological investigations and OSL dating of terraces at Ramat Rahel, Israel. J. Field Archaeol. 37, 192–208. de Geus, C.H.J., 1975. The importance of archaeological research into the Palestinian agricultural terraces, with an excursus on the Hebrew word gbî. Palest. Explor. Q. 107, 65–74. Durcan, J.A., Roberts, H.M., Duller, G.A.T., Alizai, A.H., 2010. Testing the use of rangefinder OSL dating to inform field sampling and laboratory processing strategies. Quat.
4. Conclusions This study shows that correlating a limited number of conventional full OSL dating with a large number of rapid PR measurements on small bulk samples is a powerful tool. Here it was used to construct a detailed chronology of agricultural terrace building and the history of soil accumulation, use and abandonment of terraces on the western slope of Nahal Shmuel in the Judean Highlands. In mid-Holocene the slopes were covered with natural soils. Cultivation and soil manipulation by man started in the study area during Roman times and the first phase lasted perhaps up to the early Byzantine period. That phase is represented by a thin layer in the pits and also in the colluvial aprons which could be the remains of eroded Roman-Byzantine terraces. The major episode of terrace construction began ∼800 years ago, in agreement with finds elsewhere in the Judean Highlands (Gadot et al., 2016a, 2016b), and continued uninterrupted until 175-100 years ago. It seems that terraces were maintained throughout that time as the estimated ages cover the entire period and decrease very gradually upsection. 175-100 years ago the terraces were abandoned and no longer maintained. Terrace walls crumbled, followed by soil erosion, where up to 45% of the soil originally behind the stone walls was lost downhill (but is perhaps still partially stored on the slope). However, the slope stabilized rapidly by lichen and vegetation, and is currently not undergoing much if any erosion. Possibly, the fenced plot, still operational today, was built to the north as an alternative. 5
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