U-series isochron dating of immature and mature calcretes as a basis for constructing Quaternary landform chronologies for the Sorbas basin, southeast Spain

Quaternary Research 64 (2005) 100 – 111 www.elsevier.com/locate/yqres

U-series isochron dating of immature and mature calcretes as a basis for constructing Quaternary landform chronologies for the Sorbas basin, southeast Spain Ian Candy a,*, Stuart Black b, Bruce W. Sellwood c a

Department of Geography, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK b Department of Archaeology, University of Reading, Reading, Berks RG6 6AB, UK c Department of Geography, University of Reading, Reading, Berks RG6 6AB, UK Received 22 December 2003

Abstract Immature and mature calcretes from an alluvial terrace sequence in the Sorbas basin, southeast Spain, were dated by the U-series isochron technique. The immature horizons consistently produced statistically reliable ages of high precision. The mature horizons typically produced statistically unreliable ages but, because of linear trends in the dataset and low errors associated with each data point, it was still possible to place a best-fit isochron through the dataset to produce an age with low associated uncertainties. It is, however, only possible to prove that these statistically unreliable ages have geochronological significance if multiple isochron ages are produced for a single site, and if these multiple ages are stratigraphically consistent. The geochronological significance of such ages can be further proven if at least one of the multiple ages is statistically reliable. By using this technique to date calcretes that have formed during terrace aggradation and at the terrace surface after terrace abandonment it is possible not only to date the timing of terrace aggradation but also to constrain the age at which the river switched from aggradation to incision. This approach, therefore, constrains the timing of changes in fluvial processes more reliably than any currently used geochronological procedure and is appropriate for dating terrace sequences in dryland regions worldwide, wherever calcrete horizons are present. D 2005 University of Washington. All rights reserved. Keywords: U-series; Isochron; Calcrete; Alluvial terrace sequence; Southeast Spain

Introduction Advances in the U-series isochron techniques are allowing a more diverse range of detritally contaminated carbonates to be reliably dated (e.g., Schwarcz and Latham, 1989; Bischoff and Fitzpatrick, 1991; Luo and Ku, 1991; Ludwig and Titterington, 1994). The isochron technique is the most appropriate U-series method for dating surficial carbonates (i.e., calcrete, travertine, aeolianite) because it corrects for detrital contamination. In addition, statistical summaries of the fit of the isochron to the dataset indicate

* Corresponding author. Fax: +44 1509 223930. E-mail address: [email protected] (I. Candy).

whether the carbonate has remained closed to losses and gains in U/Th and, therefore, whether the derived date is reliable (Bischoff and Fitzpatrick, 1991; Ludwig and Titterington, 1994). Calcretes are abundantly represented in Quaternary sediment and landform sequences within semi-arid/arid regions, so the dating of such horizons provides an important basis for the construction of highresolution Quaternary chronologies (Goudie, 1983; Candy et al., 2003). The main limitation of the application of isochron techniques to calcrete dating is the necessity of analysing sub-samples containing coeval authigenic carbonate fractions (Bischoff and Fitzpatrick, 1991). In appropriate climates calcretes form progressively over 100s to 100,000s of years and may, therefore, contain carbonate

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I. Candy et al. / Quaternary Research 64 (2005) 100 – 111

phases of a wide range of ages (Ku et al., 1979; Lucchitta et al., 2000; Pustovoytov, 2003). Random sampling of material from calcretes will, therefore, result in sub-samples that contain a mixture of carbonate phases. This may result in the production of isochron ages that are averages of the range of carbonate phases present. In this paper, we investigate the problems of extracting coeval sub-samples from calcrete horizons by dating both immature, nodular horizons and mature, hardpan horizons (stages II and IV/V of horizon development, respectively: Machette, 1985) by the isochron technique. The studied horizons were taken from an alluvial terrace sequence within the Sorbas basin, southeast Spain (Fig. 1). Our results suggest that the production of statistically reliable, high precision U-series ages from immature calcretes is routinely possible. This is because such horizons contain a highly restricted range of carbonate ages and this makes the sampling of coeval material within the analytical precision of the TIMS/MC-ICP-MS technique relatively straightforward. Dating mature horizons by this technique is more problematic. Mature calcretes form over a longer time period and, therefore, contain carbonate phases with a wide range of ages. This study suggests that to date such horizons reliably it is necessary either to carry out detailed subsampling procedures to extract coeval sub-samples (Candy et al., 2004a) or to produce multiple isochron dates from the same stratigraphic horizon. Multiple isochron dates can,

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therefore, be used to validate the results, on the basis of stratigraphical consistency of the dates derived. This study concludes by presenting a calcrete chronology based on U-series dates for the terrace units of the Sorbas basin, southeast Spain. The terrace chronology is based on the dating of calcretes that formed as the terrace sediments aggraded and from calcretes that cap the terrace surfaces. This approach allows the timing of terrace aggradation to be dated. It also permits the timing of the switch from aggradation to incision to be constrained (Candy et al., 2003, 2004b). Application of this procedure to the dating of terrace units, therefore, permits the timing of changes in fluvial processes to be more reliably constrained than any other currently employed geochronological technique (e.g., Fuller et al., 1998; Macklin et al., 2002; Sharp et al., 2003). It is, therefore, applicable to landform sequences in dryland regions worldwide, provided there are calcretes present.

Background The production of isochrons to date detritally contaminated carbonates has been discussed in detail elsewhere (Schwarcz and Latham, 1989; Bischoff and Fitzpatrick, 1991; Luo and Ku, 1991; Ludwig and Titterington, 1994). The technique involves the extraction of multiple subsamples from a single horizon, the dissolution of each

Figure 1. Location of study catchment in regional context (inset A) and the location of terrace sediments that have been sampled for dating (B). Grid references of samples sites are given in Table 1.

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sample using a total sample dissolution or TSD (by HNO3 – HClO4 – HF mixture) and the analysis of the U/Th isotopic ratios of each sub-sample analyte (Bischoff and Fitzpatrick, 1991; Ludwig and Titterington, 1994). Plotting these isotopic ratios, generally normalised to either 232Th or 238 U, on an X – Y – Z plot produces an isochron surface, the slope and shape of which can be used to correct for detrital contamination. The reliability of a derived isochron age can be established statistically using the probability of fit and the mean sum of weighted deviations, or MSWD (Ludwig and Titterington, 1994). In cases where an isochron can be reliably fitted to the data set (probability of fit = ca. 1, MSWD = <1), it can be assumed that the uncertainties associated with the age are a function of the analytical uncertainties. Where the probability of fit is poor (probability of fit = 0, MSWD = >1) analytical uncertainties alone cannot explain the uncertainties associated with the age, which must, therefore, result from external factors such as diagenetic alteration. To minimise the effects of diagenesis in densely cemented carbonates careful sampling is necessary, firstly to avoid weathered material. In addition, detailed analysis of carbonate micro-morphology need to be carried out, to identify micro-scale diagenetic features such as carbonate dissolution, cement overprinting and neomorphism. Samples screened through such a procedure should consequently produce statistically reliable ages. It is noticeable, however, that a large number of previously published U-series isochron ages have probability of fit values of around 0 and MSWD values >1 and, therefore, are not statistically reliable. In the pioneering work of Kelly et al. (2000) on the U-series dating of a Spanish alluvial terrace sequence only four of the 20 published isochron ages showed probability of fit values around 1 and MSWD values <1. Rowe and Maher (2000) produced U-series isochron ages for two nodular pedogenic calcretes formed within a Chinese loess sequence and, although one of these had a relatively high

probability of fit (0.78), the second had a noticeably low probability of fit (0.14). The large number of ages derived from Quaternary calcretes with MSWD values >1 probably reflects the problems associated with extracting coeval sub-samples from individual horizons. The isochron technique assumes that the authigenic carbonate fraction within each subsample is coeval (Bischoff and Fitzpatrick, 1991). Such an assumption is not a problem when carbonates have a welldefined internal stratigraphy, (e.g., speleothems, corals and travertines). Calcretes, however, may contain no morphological or micro-morphological evidence to indicate where coeval cement phases may occur. Equally as calcretes may form over long periods of time (100 –1000s of years for immature calcretes (Pustovoytov, 2003) and 10,000 to 250,000 years for mature calcretes (Machette, 1985; Lucchitta et al., 2000; Candy et al., 2004a), it is likely that randomly selected sub-samples may contain carbonate with a wide range of ages (depending upon rates of calcrete formation and the maturity of the horizon). Sub-samples extracted from any calcrete horizon would, therefore, be expected to contain a mixture of carbonate ages. However, this mix of ages may not occur in the same proportion in each sub-sample. In such cases, the authigenic carbonate in each sub-sample will not be coeval. Consequently, there will be a poor probability of fit between the best fit isochron and the scatter of data points. As each subsample will frequently contain mixtures of the same range of carbonate ages, but in differing proportions, it is likely that there will be a linear trend in the dataset that is age related and through which an isochron may be fitted. If highprecision instrumentation is used (i.e., TIMS or MC-ICPMS) then the uncertainties associated with each data point may be so low that an age with low associated uncertainties may be derived from such an isochron. However, the probability of fit and MSWD statistics shows that the derived age does not meet the criteria laid out above.

Table 1 Location and calcrete morphology of each dated site Terrace Unit

Grid Reference (GPS)

Material dated

A terrace

N.037.006.297/W.002.007.376

B terrace

N.037.006.241/W.002.007.325

C terrace

N.037.005.783/W.002.006.309

D1 terrace (D1a)

N.037.005.891/W.002.005.521

D1 terrace (D1b)

N.037.005.991/W.002.005.567

D2 terrace (D2a)

N.037.006.159/W.002.005.521

D2 terrace (D2a)

N.037.005.784/W.002.005.619

D3 terrace

N.037.005.994/W.002.006.146

Laminar pedogenic calcrete from a stage V calcrete profile capping A the A terrace surface Material from hardpan and laminar calcrete within a stage V pedogenic calcrete capping the B terrace surface Pedogenic hardpan calcrete and travertine capping C terrace, rhizogenic calcretes within terrace sediments Calcrete nodules from a stage II pedogenic calcrete capping the D1 terrace surface Exhumed groundwater calcretes (dates taken from Kelly et al., 2000) Calcrete nodules from a stage II pedogenic calcrete capping D2 terrace surface Calcrete nodules from a stage II pedogenic calcrete capping the D2 terrace surface and gypsum crystals within D2 terrace sediments Densely cemented vadose (non-pedogenic) carbonate at base of terrace gravels

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Recent studies have also shown that pedogenic carbonate may be dated by the U-series technique without the necessity of isochron construction if the U/Th isotopic ratios within the detrital phase are estimated (Ludwig and Paces, 2002; Sharp et al., 2003). Although this technique has much potential, it is unlikely to be appropriate for dating the majority of calcretes because it is only applicable to pedogenic carbonates with low levels of detrital contamination (i.e., <10 ppb conc. of 232Th and >25 of 230Th/232Th). None of the calcretes dated in this study could be dated by this technique because they contain large amounts of detrital contamination (>1 ppm conc. of 232 Th and <1 of 230 Th/232Th: Ivanovich et al., 1992), which is typical of many calcretes. Also, by comparison with the isochron technique, the assumption of detrital U/Th ratios, and the application of this to single samples, makes it difficult to assess whether the effect of the sub-sampling of multiple carbonate phases or the extent of diagenetic U/Th alteration has been significant. Although this technique may be ideal in some circumstances, it does not have the widespread applicability of the isochron technique.

Method In the present study, calcretes from an alluvial terrace sequence within the Sorbas basin, southeast Spain, have been sampled and dated (Fig. 1, Tables 1, 2, and Supplementary Table 1). The sequence consists of six terrace units given the nomenclature A, B, C, D1, D2 and D3 in age descending order. This locality was considered ideal for this study for two main reasons. Firstly, the Sorbas basin contains a well-mapped terrace chronology so a relative stratigraphy exists with which all calcrete ages can be compared (Harvey, 1987; Harvey and Wells, 1987;

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Harvey et al., 1995; Mather and Harvey, 1995). Secondly, as the terrace sequence spans a significant part of the Middle and Late Quaternary (Mather and Harvey, 1995; Kelly et al., 2000), the different terrace units are capped by pedogenic calcretes of a range of maturities from immature (stages I and II: Machette, 1985) nodular horizons to more mature (stage IV: Machette, 1985) hardpan horizons to supermature horizons (stage V) comprising hardpan horizons overlain by thick laminar crusts (Harvey et al., 1995; Candy et al., 2003). The Sorbas basin, therefore, offers the opportunity to study the problems of sub-sampling coeval material from a range of calcrete types (Table 1). Pedogenic horizons over a range of morphologies were sampled from relatively immature horizons containing discrete carbonate nodules 1 – 2 cm in diameter, to mature hardpan horizons overlain by relatively thick (3 cm) laminar calcretes. Before horizons were selected for dating, each sample was analysed by optical microscopy, XRD and XRF to establish their suitability for dating. Material was deemed unsuitable if there was evidence for carbonate dissolution (and, therefore, differential U/Th leaching), neomorphism (detected by pseudospar or aggrading pseudospar cements) and infilling of porous areas with a secondary cement. In the laboratory, the surface of the samples was cleaned to remove weathering rinds. Initially, isochrons were constructed using ‘‘bulk’’ samples. These are samples taken from calcrete horizons without any attempt to sub-sample coeval cement phases. In nodular horizons, a single cleaned nodule was used for each sub-sample, whereas for more mature calcretes (hardpan horizons and root mats) wellcemented calcrete samples were split and then individual fragments were used as sub-samples. Although it is unreasonable to expect randomly sampled fragments from mature calcretes to represent coeval sub-samples, this approach is, in itself, a useful test of the isochron technique.

Table 2 Data derived from ISOPLOT for isochrons constructed for each terrace unit Terrace Material

Authigenic (234U/238U)

D3 D2a D2b D2b D1a C C C C B

1.100 1.084 1.070 1.052 1.036 1.088 1.010 1.100 1.046 1.034

B B A

Carbonate cement at terrace base Nodules from Urra Nodules from above gypsum gorge Gypsum within D2 sediments Nodules at terrace surface Hardpan capping terrace surface Travertine capping terrace surface Calcified root mat in C sediments Calcified root mat in C sediments Laminar crust overlying terrace capping hardpan Carbonate from cemented pores of terrace capping hardpan Clast coatings from terrace capping hardpan Laminar crust overlying terrace capping hardpan

T T T T T T T T T T

0.031 0.017 0.025 0.038 0.013 0.013 0.023 0.026 0.023 0.038

Authigenic (230Th/238U) 0.085 T 0.092 T 0.091 T 0.117 T 0.110 T 0.519 T 0.515 T 0.564 T 0.537 T 0.669 T

0.002 0.005 0.007 0.005 0.003 0.006 0.014 0.028 0.013 0.042

Initial (234U/238U) 1.103 1.087 1.072 1.054 1.037 1.107 1.122 1.125 1.058 1.047

T T T T T T T T T T

0.031 0.018 0.026 0.039 0.013 0.016 0.040 0.031 0.029 0.053

Initial (230Th/238U) 0.177 0.287 0.191 0.155 0.232 0.056 0.210 0.199 0.032 0.098

T T T T T T T T T T

MSWD Probability No. of Age of fit samples (103 yr)

0.004 18 0.006 0.54 0.005 0.28 0.007 37 0.003 0.25 0.002 5.5 0.060 16 0.019 3.1 0.006 9.4 0.025 4

0 0.93 0.99 0 1 0 0 0 0 0

10 10 12 6 14 12 8 10 12 12

8.69 T 9.66 T 9.67 T 12.80 T 12.14 T 69.80 T 67.90 T 77.1 T 77.7 T 112 T

0.46 0.53 0.82 1.10 0.37 4.70 4.7 4.4 4.4 15*

1.002 T 0.015

0.760 T 0.014 1.003 T 0.024 0.056 T 0.006

0.117

1

10

155 T 9*

1.015 T 0.011

0.866 T 0.010 1.026 T 0.020 0.011 T 0.005

3.6

0

8

207 T 11*

1.009 T 0.009

0.950 T 0.010 1.022 T 0.020 0.002 T 0.008

0.77

0.72

10

304 T 26

All errors are 2 sigma except those marked * where the errors are 1 sigma.

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Sampling techniques are, however, also proposed that may allow coeval sub-samples to be collected (Candy et al., 2004a). This was done by drilling material out of a hardpan horizon at various locations using a 2-mm diamond-bit drill. The theory and application of this approach is described below. 0.5 g of each sub-sample was dissolved using a HNO3 – HClO4 – HF mixture. The abundant 232Th and 238U isotopes were analysed on a Perkin-Elmer Elan 6000 ICP-MS whilst the less abundant 230Th and 234U isotopes were analysed on a VG 336 TIMS (Candy et al., 2004a provide a discussion of this method). The abundance of the 230Th and 234U isotopes were obtained in dynamic mode by analysing 230 Th/232Th and 234U/238U ratios. The A-THO standard reference was run in conjunction with these samples and the results obtained were in agreement with previously published results. Calcrete ages were produced using 3-D isochrons (232Th/238U – 230Th/238U – 234U/238U) that were constructed by using the ISOPLOT program (v. 2.49 Ludwig, 2001). This approach uses the intercept of the best-fit isochron to calculate the 232Th-free, and, therefore, detritus-free, ratios of 230Th/238U and 234U/238U that are required for the age calculations. ISOPLOT calculates: (1) the detritally corrected age of the sample and the uncertainties associated with each age, and (2) the MSWD and probability of fit statistics which are necessary to assess the validity of the calcrete age.

Isochron dating of immature calcretes Three immature, nodular calcrete horizons were dated from the D2 (2 horizons) and D1 (1 horizon) terrace surfaces. The derived isochrons are shown in Figure 2. The data derived from the isochrons and used to construct these ages are given in Table 2. All of these isochrons produce high-precision ages with uncertainties of around 5– 9% of the derived age. The statistics associated with these ages indicate that the uncertainties associated with each age are a function of the analytical uncertainties rather than any external factors (i.e., non-coeval sub-samples). Within the precision of the analytical technique, every sub-sample, for each of the three isochrons, appears to be coeval and this suggests that these three horizons formed within the uncertainties associated with the age. It is possible that the apparently coeval nature of the subsamples in each isochron could be a function of each subsample containing an identical mixture of carbonate phases of a very wide range of ages. If so, it could imply that the horizons formed over a very long period of time. This is considered unlikely, however, because of the consistency of the two ages derived from different horizons at the D2 terrace surface, which give ages of 9660 T 530 and 9670 T 820 yr. We consider it highly unlikely that random subsampling of material from calcrete horizons containing a

Figure 2. U-series isochrons for nodular calcretes capping the D1 and D2 terraces. (a) D1 terrace (D1a)-12,140 T 370 yr. (b) D2 terrace (D2a)— 9600 T 530 yr. (c) D2 terrace (D2b)—9670 T 820 yr. Only the 232 Th/238U – 230Th/238U plot of a 3D isochron is shown in each case. Plots, ages and derived uncertainties were derived from the ISOPLOT program (Ludwig, 2001). The stratigraphic setting of these deposits is shown in Figure 4. In each case, data-point error ellipses are two sigma.

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wide range of carbonate phases would produce two identical ages at geographically different but stratigraphically similar levels. We, therefore, conclude that these nodular horizons formed very rapidly, within 700 yr (D1 horizon) and within 1000 –1600 yr (D2 horizons).

Isochron dating of mature calcrete horizons A hardpan calcrete capping the C terrace was randomly sub-sampled and analysed to construct an isochron (Fig. 3a and Table 2 and Supplementary Table 1). It can be seen that the derived isochron is a best-fit line that passes through a linear dataset. However, not all of the data points plot on this best-fit line. The derived age (69,800 T 2200 yr) may, therefore, result from a

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mixture of carbonate phases, as reflected in the statistics associated with the isochron (probability of fit = 0, MSWD = 5.5). The morphology and micro-morphology of this horizon indicates that it is densely cemented, with no evidence for any diagenesis and, therefore, is ideal for dating purposes. We suggest that the high MSWD value derived from this isochron is a function of the authigenic carbonate fractions found in each sub-sample not being coeval, rather than because of diagenetic U/Th alteration. The linear trend in the dataset is likely to be a function of the sub-samples containing mixtures, in differing proportions, of the same range of carbonate ages. It is, therefore likely that this isochron does contain some age information. That is, it may reflect an average age but on its own it is not possible to attach any geochronological significance to the derived date.

Figure 3. U-series isochrons for the C terrace. (a) Terrace-capping hardpan calcrete—69,800 T 2200 yr. (b) Calcified root mats: rhizogenic calcrete—77,700 T 4400 yr. (c) Calcified root mats: rhizogenic calcrete—77,100 T 6300 yr. (d) Terrace-capping travertine horizon—67,900 T 4700 yr. In each case, only the 232 Th/238U – 230Th/238U plot of a 3D isochron is shown. Plots, ages and uncertainties were derived from the ISOPLOT program (Ludwig, 2001). The stratigraphic setting of these deposits is shown in Figure 4. In each case, data-point error ellipses are two sigma.

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Deriving geochronologically significant ages from mature calcrete horizons As shown above, the necessity of sub-sample coevality is a major limitation for the isochron dating of mature calcrete horizons. In this section, two procedures are presented for deriving geochronologically meaningful dates from mature calcrete horizons. The first involves producing multiple ages for a single sediment body. The second involves subsampling carbonate material of different ages from a single hardpan horizon (Candy et al., 2004a). Multiple-horizon dating Deposits of the C terrace contain calcretes that have formed within the sediments as the terrace has aggraded as well as calcretes that have developed at the C terrace surface after the river had abandoned that particular terrace level, as it switched to incision (Candy et al., 2003). Within the C terrace sediments, those calcretes that have formed during terrace aggradation are typically calcified root mats comprising morphologies and micromorphologies comparable to those described by Wright et al. (1995) and Candy (2002). As these calcified root mats occur directly below the terrace capping calcrete that has been isochron dated (Fig. 3a), this sequence is ideal for testing the geochronological significance of statistically unreliable isochron ages. Two phreatic calcified root mats were selected from within the terrace sediments for dating (Figs. 3b and c). These were considered the most appropriate for dating purposes as they are densely cemented, are vertically separated by only a few centimetres and are truncated by the same cut and fill event within the aggradational history of the terrace. Therefore, as well as being suitable for dating, these horizons are also in a strongly defined stratigraphical context with both one another and with the sedimentation history of the terrace. As well as the hardpan calcrete capping the C terrace, a terrace capping travertine was also dated (Fig. 3d). Both of the calcite-cemented root mats produced ages with high MSWD values (3.1 and 9.4). However, the derived ages were essentially identical (77,100 T 6300 and 77,700 T 4400 yr). The age derived from the travertine also had a high MSWD value (16) but the derived age (67,900 T 4700 yr) is consistent with that from the terrace-capping calcrete (69,800 T 4700 yr). Despite the fact that all the ages derived from the C terrace have MSWD values >1, indicating a mixing of carbonate phases, they are internally consistent with each other and consistent within the context of the history of terrace development. In other words, the calcretes that formed within the terrace sediments are older than those that cap the terrace surface (see the diagram of C terrace ages in Fig. 4). This stratigraphical consistency suggests that, despite the scatter on these isochrons, the linear trend in each dataset is age related and that the derived ages are average ages for

each horizon. We can, therefore, suggest that the C terrace sediments began to aggrade prior to 77,700 T 4400 yr, at around which time the calcified root mats developed. These carbonates were then truncated by a cut-and-fill event after which the river system continued to aggrade. The terrace surface was subsequently abandoned because of incision, after which the travertine and terrace-capping calcretes developed, indicating that the river had switched to incision more than 69,800 T 4400 yr ago. This interpretation of ages in this manner is only possible where: (a) multiple isochrons have been produced, (b) where the derived ages show an internal consistency and, hence, (c) where the isochrons and the ages derived from them can be shown to have geochronological significance. Hardpan calcrete sub-sampling Candy et al. (2004a) have shown that it is possible to estimate where the earliest and latest cements occur within a hardpan horizon despite the absence of corroborating micromorphological evidence. This estimation is based on the assumption that the morphological style of pedogenic calcrete development is well understood (Gile et al., 1966; Machette, 1985). In coarse pebble/cobble sediments, the initial development of calcretes consists of carbonate clast coatings developed predominantly on the underside of clasts. As such clast coatings grow progressively, they gradually coalesce until they form a plugged hardpan horizon over which a horizontal, laminar calcrete gradually develops. Candy et al. (2004a) have suggested that this pattern of development allows cements of different ages to be drilled out from hardpan horizons with the earliest hardpan cement occurring directly beneath the undersides of clasts and the latest hardpan cement occurring within the centre of carbonate-filled pores. The youngest cement of all should be present in the laminar crust capping the hardpan horizon. By drilling material out from these locations from a stage V calcrete that caps the B terrace Candy et al. (2004a) produced three separate isochrons. The ages derived from each sampling locality were in stratigraphical agreement with the accepted mechanism of peodgenic calcrete development. The carbonate from directly below the undersides of detrital clasts produced an age of 207,000 T 11,000 yr (MSWD = 3.6, probability of fit = 0), the carbonate from the centre of carbonate filled pores produced an age of 155,000 T 9000 yr (MSWD = 0.117, probability of fit = 1), whilst the carbonate from the laminar crust produced an age of 112,000 T 15,000 yr (MSWD = 4, probability of fit = 0). Of the three derived ages two have high MSWDs, although the values were relatively low (3.6 and 4). As has been suggested above for the multiple dating of a single terrace unit, the stratigraphical consistency (Fig. 4) of the ages suggests that these ages represent average ages. Candy et al. (2004a), therefore, considered it reasonable to suggest that the age of carbonate drilled out from directly beneath

Figure 4. U-series isochron dates derived from each of the six terrace units of the Sorbas basin. The gap in log D2 (ii) marked + represents a sequence of sediments of some 22 m in thickness that have been described by Candy et al. (2004b). The hardpan calcrete date from profile D1 (ii) is taken from three separate isochron ages published by Kelly et al. (2000).

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the undersides of clasts within the B terrace hardpan (207,000 T 11,000 yr) represented an approximation for the earliest cement present within this horizon and, therefore, the age prior to which this horizon began to develop.

Other isochron ages from the Sorbas basin Three other isochron ages were derived from secondary precipitates within Sorbas basin terrace sediments. These are important with regard to understanding the timing of landscape develop but are of little significance to understanding the problems of isochron dating calcretes. The materials dated were a thin (<2 cm) carbonate crust that formed within sediments of the D3 terrace at the erosional contact with underlying bedrock marls, a horizon of gypsum crystals within D2 terrace sediments and a laminar calcrete that caps a supermature calcrete formed at the surface of the A terrace. The ages derived from the D2 terrace gypsum and the D3 terrace carbonate, 12,800 T 1100 and 8690 T 460 yr, respectively, are discussed in Candy et al. (2004b), but are not discussed here as they are not relevant to the dating of immature and mature calcretes. The A terrace age (304,000 T 26,000 yr) was derived from dating a laminated calcrete cap that had a well-defined stratigraphy. Dating this horizon did not, therefore, present the problems associated with extracting coeval sub-samples that are discussed above, this is reflected in the good probability of fit statistics associated with this age (probability of fit = 0.72, MSWD = 0.72).

Discussion U-series isochron dating of Quaternary calcretes allows the production of ages that are corrected for detrital contamination. A statistical summary of the relationship between the best-fit isochron and the data set allows the reliability of the derived age to be assessed. Where statistically reliable ages are produced, the potential for the production of a high-resolution sediment/landform chronology exists. Where ages with high MSWD values are produced, they may still be suitable for constructing detailed chronologies, however, the derived ages must be treated with greater caution. The suitability of a calcrete horizon for isochron dating will depend upon the maturity of the calcrete horizon and the rate at which calcrete development occurs. In the Mediteranean region, for example, rates of calcrete development appear to be very rapid with this study indicating that immature stage II calcretes could form within 1000 years. Any sub-samples taken from such a horizon will, therefore, contain a very restricted range of carbonate ages. The restricted range of carbonate ages present in each subsample will, within the precision of the technique, mean that each sub-sample is coeval.

If a more mature horizon is dated then derived subsamples can potentially contain a wider range of carbonate ages, thus making the extraction of coeval sub-samples problematic. The most likely outcome of dating such horizons is the production of a data set characterised by a linear trend with a large degree of scatter about the best-fit isochron. Any age derived from such a horizon will have a low probability of fit and a MSWD value >1. As shown above many such ages are likely to be averages of the range of carbonate ages present in the sub-sample, although this can only be proven where: (1) multiple isochron ages are produced, and (2) these multiple ages are stratigraphically consistent with one another. The geochronological significance of the derived ages is proven where at least one of the multiple ages is proven to be statistically reliable. In such cases, the derived calcrete ages may be used for chronology construction. Despite the problems associated with the U-series isochron dating of Quaternary calcretes, the authors still consider this the most appropriate technique for the reliable dating of these horizons. This is for two main reasons, firstly the relationship between a best-fit isochron and the source data set allows comments to be made on the reliability of the derived age and on the affects of diagenesis and the sampling of material containing carbonate with a range of ages. This cannot be done when U-series dates are derived from single analyses even though diagenesis and the presence of multi-age carbonate will still be significant. Secondly, the isochron technique is applicable to calcrete horizons containing very high through to very low levels of detrital contamination as opposed to other U-series techniques that are only applicable to calcretes containing low levels of detrital contamination (Sharp et al., 2003). The U-series isochron dating of calcretes can be applied to all the terrace units within the Sorbas basin to produce a chronology for landform evolution in this region (Figs. 4 and 5). The derived ages for all of the terrace units are internally consistent, and consistent with the overall terrace stratigraphy. The chronology is different from many other terrace chronologies previously published as, with regard to the C, D1 and D2 terraces, calcretes that have formed as the terrace sediments have accumulated as well as those that have formed after the terrace surface has been abandoned have both been dated (Candy et al., 2003, 2004b). The intraterrace calcretes give an age at which the terrace sediments were aggrading, whilst by combining the age of the terrace capping calcrete and the intra-terrace calcrete, the timing of the switch from aggradation to incision can be constrained. The age of the A and B terraces are relatively poorly constrained. However, they provide minimum dates for the age of the terrace surfaces. The B terrace age (207,000 T 11,000 yr), as discussed, is a minimum age of the oldest cement fabrics present and, therefore, probably represents the initiation of soil development, consequently the B terrace age is probably a close approximation to the age of the terrace surface (Candy et al., 2004a). The A terrace age (304,000 T

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Figure 5. Evolution of the Sorbas basin terrace sequence over time. Error bars are shown unless they are smaller than the actual data points. (a) Evolution of the A, B, C and D1 terraces. (b) Evolution of the C, D1, D2 and D3 terraces. The date for the A terrace represents the age of a laminar calcrete overlying a hardpan calcrete. As such mature calcretes require at least 70,000 yr to form in the Sorbas basin (Candy et al., 2004a), the age of the A terrace is estimated to be at least 400,000 yr. In reality, the age of the A terrace is likely to be greatly in excess of this as the hardpan profile below the laminar crust is highly complex (Candy et al., 2003).

26,000 yr) is derived from the dating of the laminar calcrete capping a stage V calcrete at the A terrace surface. The A terrace age, therefore, dates the youngest cement phase in the calcrete profile, grossly underestimating the age of the land surface. Candy et al. (2004a) have suggested that, within the Sorbas basin, stage V calcretes may require between 70,000 and 120,000 yr to form, implying that the actual age of the A terrace could be >>400,000 yr. The C through to D2 terraces are better constrained and represent changing fluvial processes over the past 80,000 yr. The timing of C terrace aggradation corresponds well with the beginning of the last glacial stage whilst the timing of D1 terrace aggradation corresponds well with the major climatic deterioration in marine oxygen isotopic stage 2 (incorporating the last glacial maximum or LGM). Episodes of fluvial aggradation during this period are common across southern and eastern Spain (Fuller et al., 1998; Rose et al., 1999; Rose and Meng, 1999). The timing of D2 terrace aggradation has previously been discussed by Candy et al. (2004b). Although the aggradation of this unit corresponds well with the timing of the Younger Dryas event, the

sedimentology of the D2 terrace shows that its accumulation is a result of local tectonism and/or diapirism/karst processes in the underlying gypsum bedrock (Mather et al., 1991; Harvey, 2001, in Mather et al., 2001) and its accumulation is, therefore, unrelated to climate change. The terrace chronology shows that the landforms of the Sorbas basin only record a very small part of Quaternary time, approximately the last 400,000 yr, rather than reflecting evolution over the majority of Quaternary time as has been previously suggested (Mather and Harvey, 1995; Harvey et al., 1995). Only two terrace units exist that record periods of fluvial aggradation from 400,000 to 80,000 yr ago, whilst four terrace units have accumulated over the last 80,000 yr. This difference in the geomorphic record of these two periods could reflect progressive changes in tectonic activity, styles of fluvial response to climate or landform preservation potential. However, the accelerated rates of fluvial incision after the abandonment of the C terrace are undoubtedly related to a major river capture event that occurred during this period (Harvey, 1987; Harvey and Wells, 1987; Stokes et al., 2002).

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By selecting calcretes that have different geochronological significances, with regard to terrace body evolution, and dating those calcretes by the U-series isochron technique, it is possible to tightly constrain the temporal changes in fluvial processes that terrace sequences record. Consequently, the Sorbas basin terrace record is better constrained than any previously published terrace chronology as previous studies have focused on dating either the age of terrace sediment aggradation (e.g., luminescence studies: Fuller et al., 1998; Macklin et al., 2002) or providing minimum ages for the terrace surface (e.g., calcrete dating studies: Kelly et al., 2000; Sharp et al., 2003), but never both of these factors. The procedure we present above is, therefore, a major advance in the study of Quaternary fluvial response and is appropriate for constructing terrace chronologies in dryland regions worldwide wherever calcretes are present.

Conclusions & U-series isochron dating allows reliable, high-resolution calcrete ages to be produced where immature calcretes that have formed relatively rapidly (100s to 1000s of years) exist. & The dating of mature calcretes, or calcretes that have formed over prolonged periods of time (10,000s to 100,000s of years), frequently produce isochrons with low probability of fit and high MSWD values that may represent an average age for the horizons in question. & Statistically unreliable isochron calcrete ages can be shown to have any geochronological significance where: (1) multiple ages are produced, (2) these multiple ages are stratigraphically consistent and (3) one of the ages is statistically reliable. & Provided the dating results are properly scrutinised, this technique can act as the basis for the construction of high-resolution sediment/landform chronologies. & By dating calcretes that have formed during terrace aggradation as well as calcretes that cap the terrace surface, it is possible to constrain the timing of changes in fluvial processes more reliably by the U-series isochron technique than by any other dating method currently applied to date terrace sequences.

Acknowledgments The authors thank Mr. John Jack, Mr. Mike Andrews and Dr. Lorna Simpson (PRIS, University of Reading, UK) for their help in the petrographic, X-ray and ICPMS analysis of Sorbas basin calcrete samples. Prof. Adrian Harvey (University of Liverpool, UK) and Dr. Dave Nash (University of Brighton, UK) are thanked for their detailed and incisive comments on this paper. This work was carried out when Ian Candy was in receipt of a

National Environmental Research Council PhD studentship (Grant no. GT4/99/236).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.yqres. 2005.05.002.

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