Uncovering a landscape buried by the super-eruption of Toba, 74,000 years ago: A multi-proxy environmental reconstruction of landscape heterogeneity in the Jurreru valley, south India more

Quaternary International xxx (2012) 1e13 Contents lists available at SciVerse ScienceDirect Quaternary International journal homepage: www.elsevier.com/locate/quaint Uncovering a landscape buried by the super-eruption of Toba, 74,000 years ago: A multi-proxy environmental reconstruction of landscape heterogeneity in the Jurreru Valley, south India J. Blinkhorn a, *, A.G. Parker b, P. Ditchfield a, M. Haslam a, M. Petraglia a a b Research Laboratory for Archaeology and the History of Art, School of Archaeology, University of Oxford, Oxford, UK Department of Anthropology and Geography, Oxford Brookes University, UK a r t i c l e i n f o Article history: Available online xxx a b s t r a c t The Youngest Toba Tuff (YTT) eruption w74 ka is the largest volcanic event to occur during the last two million years. This paper presents a high resolution landscape reconstruction for the Jurreru Valley, south India, immediately prior to this eruption. Primary ash fall deposits have sealed the pre-Toba surface of the Jurreru Valley, and subsequent deposition of remobilised ash has helped preserve this horizon. These primary YTT deposits provide an isochron that allows for the study of palaeoenvironmental conditions across a 25,000 m2 area in the Jurreru Valley, permitting the reconstruction of the pre-Toba landscape. Sixty sites with exposed primary ash deposits have been recorded as part of a Total Station survey, twelve of which have been subject to detailed stratigraphical study. This has enabled a reconstruction of the topography of the buried surface in the Jurreru Valley. Stable isotope and phytolith analyses are used to explore diversity across this buried landscape, indicating that the ratio of C3 to C4 plants varies with regard to changes in topographic height in the landscape of up to w5 m. High levels of spatial heterogeneity within these proxy data are indicated by this study, highlighting the risks of extrapolating regional palaeoenvironmental sequences from vertically sampled sedimentary sections, which may well reflect highly localised influences of topography and geomorphology. Ó 2011 Elsevier Ltd and INQUA. All rights reserved. 1. Introduction The eruption of Toba, Indonesia, 74,000 years ago is the largest volcanic event of the last two million years. Ash fall from this eruption (Youngest Toba Tuff [YTT]) covered w40,000,000 km2 of South and South-East Asia (Rose and Chesner, 1987; Chesner and Rose, 1991; Chesner et al., 1991). Much research has focused upon the destructive impact upon palaeoenvironmental conditions following the eruption, with debate surrounding the severity of climatic change (e.g. Rampino and Self, 1992; Oppenheimer, 2002; Williams, in press) and its impact upon hominin populations (Ambrose, 1998; Gathorne-Hardy and Harcourt-Smith, 2003; Petraglia et al., 2007; Williams et al., 2009; Haslam and Petraglia, 2010). YTT deposits are found in a number of river valley locations in South Asia (Basu et al., 1987; Acharyya and Basu, 1993; Westgate et al., 1998; Williams et al., 2009; Jones, 2010). Extensive YTT deposits are preserved in the Jurreru Valley, south India (Rao and Rao, 1992; Petraglia et al., 2007, in this issue) (Fig. 1). Primary * Corresponding author. E-mail address: james.blinkhorn@rlaha.ox.ac.uk (J. Blinkhorn). 1040-6182/$ e see front matter Ó 2011 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2011.12.008 air fall ash deposits, resulting from direct deposition from the Toba ash cloud, have been identified in the Jurreru Valley (Matthews et al., in this issue; Petraglia et al., 2007; see Fig. 2a). Subsequent deposition of remobilised ash on top of this has prevented significant taphonomic disturbance of this buried surface. These primary air fall deposits have therefore helped to preserve the preToba landscape in the Jurreru Valley. Primary YTT deposits act as an isochron, as well as a clear stratigraphic marker across this landscape. This isochron deposit allows for direct comparisons between sites within the Jurreru Valley and provides the means to investigate palaeoenvironmental proxy data with a uniquely high level of spatial resolution. This paper describes the results of stratigraphic logging of the interface between YTT and underlying, pre-Toba deposits, situating these observations within the context of a topographic survey of the buried surface, across w25,000 m2. It then explores the results of analyses of stable carbon and oxygen isotopes of pedogenic carbonate nodules from 12 sites and phytoliths from 11 sites, all recovered from 10 cm beneath the contact between pre-Toba sediments and the YTT horizons. This synchronic palaeoenvironmental study enables the investigation of spatial variation in the stable isotope and phytolith data, leading to Please cite this article in press as: Blinkhorn, J., et al., Uncovering a landscape buried by the super-eruption of Toba, 74,000 years ago: A multiproxy environmental reconstruction of landscape heterogeneity in the Jurreru Valley, south India, Quaternary International (2012), doi:10.1016/ j.quaint.2011.12.008 2 J. Blinkhorn et al. / Quaternary International xxx (2012) 1e13 Fig. 1. a) The location of the Jurreru Valley within India; b) DEM of the modern Jurreru Valley showing the location of the study area (Topographic height relative to local TBM e datum ¼ 100.00 m); c) map locating individual sites within study area and types of data available from each (scale in meters). a richer environmental reconstruction than has previously been possible in the Jurreru valley. This study highlights the potential for misleading interpretations as a result of extrapolating diachronic palaeoenvironmental patterns from single sites to regional sequences, due to local variation in ecological conditions, resulting from the location of sites within a heterogeneous landscape. 2. Methods 2.1. Stratigraphy Detailed stratigraphic recording and sampling of the pre-Toba landscape was undertaken at twelve sites in the Jurreru Valley (see Fig. 1c). Sections through the YTT deposits that had been exposed through quarrying activities were cleaned. Sediments were recorded, described and sampled at 0.1 m intervals. Here, the focus is on sediment descriptions for the YTT horizon and the deposits immediately underlying them, referred to as Stratum C and D respectively (following Haslam et al., 2010). 2.2. Topography A total of sixty measurements were made using a Total Station (Zeiss Elta R 55) at the contact between Strata C and D, recording the site location in three dimensions with an accuracy of 5 mm. These measurements included the twelve sites which have seen detailed stratigraphic recording and sampling (see Fig. 1c). An established benchmark on the south side of the central barrage of the Jurreru Dam was used (datum ¼ 100.00 m), enabling the integration of the results of this survey with other mapping projects. These survey data were analysed in Surfer 8 and a 3D surface and contour map has been interpolated (66 columns by 57 rows, resulting in 10 m2 squares). The topographic survey took advantage of areas where modern mining revealed sections Please cite this article in press as: Blinkhorn, J., et al., Uncovering a landscape buried by the super-eruption of Toba, 74,000 years ago: A multiproxy environmental reconstruction of landscape heterogeneity in the Jurreru Valley, south India, Quaternary International (2012), doi:10.1016/ j.quaint.2011.12.008 J. Blinkhorn et al. / Quaternary International xxx (2012) 1e13 3 Fig. 2. a) Close up of contract between Strata D and primary ash fall (Stratum C), and between primary ash fall and reworked ash deposits at JV1 (Stratum C)(Photo: A. Durant); b) Contact between Stratum D and Stratum C deposits at JV1 (Photo: A. Durant); c) Laterally extensive YTT deposits (Stratum C) at JV13, with overlying Stratum B and underlying Stratum D deposits (Photo: A. Durant); d) Laterally extensive YTT deposits (Stratum C) at JV10, with overlying Stratum B and underlying Stratum D deposits (Photo: A. Durant); e) Thick, reworked ash deposits (Stratum C) at JV1 displaying lithified clay horizons, related to cyclical sequence of deposition, fining and desiccation (Photo: A. Durant); f) Pedogenised Stratum D deposits overlain by ash rich silt, containing pure ash pods (Photo: M. Haslam). through the YTT ash deposits. In some cases excavation was undertaken to ensure the contact between Strata C and D was exposed. Due to the variable distribution of modern mining activity, it was not possible to achieve an even spread of sites in the Jurreru Valley. This resulted in some variation in the accuracy of the interpolated surfaces. 2.3. Stable isotopes Sixty-seven pedogenic carbonate samples, comprised of nodules and rhizoliths from Stratum D, 10 cm beneath the contact with overlying YTT (Stratum C) deposits, were analysed from eleven newly sampled sites (see Fig. 1c). These samples were rinsed in Please cite this article in press as: Blinkhorn, J., et al., Uncovering a landscape buried by the super-eruption of Toba, 74,000 years ago: A multiproxy environmental reconstruction of landscape heterogeneity in the Jurreru Valley, south India, Quaternary International (2012), doi:10.1016/ j.quaint.2011.12.008 4 J. Blinkhorn et al. / Quaternary International xxx (2012) 1e13 ethanol to remove any adhering sediments prior to crushing in an agate pestle and mortar, and subsequently dried at 40  C. Oxygen and carbon stable isotopic results were obtained using a VG Isogas Prism II mass spectrometer with an on-line VG Isocarb common acid bath preparation system. Each sample was reacted with purified phosphoric acid (H3PO4) at 90  C with the liberated carbon dioxide cryogenically distilled prior to admission to the mass spectrometer. Both oxygen and carbon isotopic ratios are reported relative to the VPDB international standard. Calibration was against the in-house NOCZ Carrara Marble standard with a reproducibility of better than 0.2%. Stable isotope ratios are expressed using the notation as difference in parts per thousand (permil, &) relative to the PDB standard, calculated as d& ¼ ([Rsample/Rstandard] À 1) Â 1000, where R ¼ 13C/12C or 18O/16O. 2.4. Phytoliths Samples from eleven sites were prepared for phytolith analysis. They were recovered from Stratum D, 10 cm below the contact with overlying YTT (Stratum C) deposits (see Fig. 1c). Ten grams of sediment per sample were first sieved through a 2 mm sieve in order to remove the coarse fraction prior to phytolith extraction. Carbonates were removed using 10% HCl, followed by the removal of organics using 10% H2O2 in a heated sand bath (Ishida et al., 2003). This stage was followed by deflocculation using 50 ml of 2% Calgon and 250 ml of distilled water. The samples were then shaken continuously for 30 min before being passed through a 212 mm sieve and subjected to gravimetric sedimentation and separation. This was followed by heavy liquid separation using sodium polytungstate (2.35 s.g.). Clays and fine silts less than 5 mm in size were removed using the vacuum filtration method of Theunissen (1994). Samples were mounted onto microscope slides using Canada Balsam and identified at 400Â magnification using a Nikon Eclipse E400 light microscope. Phytoliths were counted, measured and classified according to shape and size. The size of the phytoliths was measured using a micrometer eyepiece and graticule and classified into short and long cells sensu Piperno (1988). Phytoliths were compared with keys including Piperno (1988), Cummings (1992), Mulholland and Rapp (1992), Rosen (1993) and Ball (2002), as well as reference material held by A.P for identification. The counting method adopted was based on those outlined by Runge (1999) and Pearsall (2000), whilst the classification used implemented a scheme from Pearsall (2000), which uses a modified version of the Twiss et al. (1969) system. Pooid (C3) morphotypes included round, oblong, square/rectangular, roundtrapezoid and rondels. Panicoid (C4) forms included bilobate, polylobate and crossbody morphotypes whilst Chloridoid (C4) comprised saddles. It should be noted that depending on environment, some morphological forms (e.g. rondels) may derive either from Pooideae or Chloridoideae grasses (e.g. Barboni et al., 1999), which can affect the interpretation of C3 to C4 ratios. 3. Results 3.1. Stratigraphy At ten of the twelve sites recorded, sedimentary profiles indicate considerable homogeneity in both the nature and succession of the latest Stratum D deposits, and the overlying primary YTT ash fall horizon at the base of Stratum C deposits. These include the sites of JV1, JV7, JV13 and JV22, which have been identified as primary ash fall following grain size and chemical analyses by Matthews et al. (in this issue) (see Fig. 2a). Stratum D deposits (see Fig. 2bed) are typically a red to yellowish-red massive clay with silt, which coarsens downwards to include fine sand. The upper 0.1 m appears cemented, with occasional rhizoliths and concretions within upper 0.5 m. The upper surface of these deposits has an iron-red colouration and includes black specks of manganese oxide. Their characterisation as wetland deposits are based upon evidence for low carbonate levels and decreased magnetic susceptibility compared to higher deposits (Haslam et al., 2010). However the lack of distinct laminations, the preservation of rhizoliths and presence of tephra filled burrows suggests any standing body of water may have been shallow, seasonal and bioturbated (Jones, 2010). Directly overlying these deposits in Stratum C, a pure 4e5 cm thick white very fine ash layer occurs with soft sediment deformation structures, which appears to correspond to primary ash fall onto saturated sediments (see Fig. 2b). Above this, a cyclical sequence of up to 6 reworked white to light grey very fine ash/fine volcanic ash is present, each capped by lithified clay layers 2e10 cm thick (see Fig. 2e). Some of these fine layers have mud-cracks indicative of desiccation. Including the primary ash horizons, Stratum C deposits range in thickness between 10 cm at JV10 and 2.3 m at JV1. These are laterally extensive deposits, which can easily be followed across the modern landscape due to exposure relating to mining activities (see Fig. 2d and e). The overlying Stratum B deposits comprise mottled yellowish-red silts between 0.35 and 1.3 m thick with a conformable contact with Stratum C. This suggests the YTT deposits have largely been preserved in situ, with little or no impact of post-depositional processes. Inspection of JWP 22 and the Dry Well suggest similar silty clay deposits occurring in Stratum D as elsewhere in the valley. However, these sediments vary in colour, appearing as yellowish brown deposits, and have marked characteristics of illuviation, indicating presence of a mature soil profile (Haslam et al., in this issue). A wavy but sharp contact with the overlying 10e20 cm thick ash rich silt is observed, which contains pure ash pods and is marked by the lowest mean grain size throughout the soil profile (Haslam et al., in this issue) (Fig. 2f). Evidence of the abundant plant fossil record associated with the YTT horizon has been recorded. A number of in situ fossilised tree trunks have been recorded at JV7, JV32 and JV21 (Fig. 3)(see also Jones, 2010). Fossilisation appears to have been enabled by the rapid deposition of YTT ash. Although the branching structure of these tree fossils can be seen in Stratum C (Fig. 3b), no fossils are preserved in the sediments overlying the tephra. A number of these fossils are rooted into the sediments below the YTT horizon, suggesting they were alive at the time of the Toba ash fall in the Jurreru Valley. In addition to this, excavations at JV7 undertaken to record the sedimentary profile revealed numerous fossil leaf casts occurring at the interface between the primary ash fall layer and overlying reworked tephra deposits. The occurrence of leaf fall on top of the primary ash has been linked to a potential defoliation event, related to the toxic impact of the ashfall upon the trees (Oppenheimer, 2011). The broad morphology of these samples indicate both Palmate and Pinnate species were present (Fuller pers. comm. 2009), although further sampling is required to refine these identifications. 3.2. Topography Fig. 4 illustrates the results of the topographic survey of the contact between Strata C and D, revealing the land surface of the Jurreru Valley that was buried by the YTT ash fall, over an area of w25,000 m2. A maximum of 4.91 m of relief variation is observed between the 60 sites surveyed, with altitudes ranging from 82.97 m to 87.88 m with respect to datum. Overall, a W-E trend can be observed in the pre-Toba topography, in rough alignment with the modern Jurreru Valley axis and surface, with sites located further Please cite this article in press as: Blinkhorn, J., et al., Uncovering a landscape buried by the super-eruption of Toba, 74,000 years ago: A multiproxy environmental reconstruction of landscape heterogeneity in the Jurreru Valley, south India, Quaternary International (2012), doi:10.1016/ j.quaint.2011.12.008 J. Blinkhorn et al. / Quaternary International xxx (2012) 1e13 5 Fig. 3. a) Fossilised tree trunk in situ in YTT deposits (Stratum C) (Scale bar ¼ 50 cm)(Photo: A. Durant); b) Fossilised tree trunk exhibiting branching structure through YTT deposits (Stratum C) (Scale bar ¼ 10 cm)(Photo: M. Haslam). upstream in the valley returning larger z values. The highest ground recorded for this palaeosurface is located in the SW corner of the survey area, toward Kuppakonda, a hill in the centre of the Jurreru Valley, which matches the observed modern topography. However, a palaeorelief feature absent from the modern topography is a pronounced depression in the SE corner of the survey area, identified by numerous data points. This depression, which occupies almost half the surveyed area, provided greater topographic relief than is observed in the modern Jurreru Valley land surface. This survey has, therefore, identified a more varied palaeotopography than could be inferred from the modern surface alone, providing the means to explore the impact of local topography upon pedogenic carbonates and phytolith assemblages. 3.3. Stable isotopes The results of the stable isotope analysis upon pedogenic carbonate samples from 10 cm below the Stratum CeD contact from the Jurreru Valley are presented in Table 1 and Fig. 5. The mean and standard deviation of stable isotope results are presented in Table 2 and Fig. 4. Dating the formation of pedogenic carbonates can be problematic, depending upon the stability and development of the landform in which they develop (Deutz et al., 2002; Breecker et al., 2009). In the Jurreru Valley, the sediments from which pedogenic carbonates have been recovered overlie a horizon dated to 77 Æ 6 ka (Petraglia et al., 2007). This suggests that the study of stable isotopes is based pedogenic carbonates that have formed within sediments deposited less than 3 ka before the Toba eruption. 3.3.1. Carbon isotopes In the Jurreru Valley, stable carbon isotope results range from À8.376& to À2.123&. JV1 exhibits the lowest absolute (À8.376&) and mean (À6.850&) d13Cpd values, while JV10 exhibits the highest absolute (À2.123&) and mean (À3.811&) d13Cpd values. Pedogenic carbonate formed in a pure C3 habitat, typically trees and shrubs, has d13C values of À9& to À12& whereas under a pure C4 habitat, typically tropical grasses, pedogenic carbonates have d13C values of 1&e3& (Cerling, 1999). As calcitic precipitation is seasonal, the contribution of different vegetation to soil CO2, which is recorded in pedogenic carbonates, may vary dependent upon seasonal abundance of certain plant types. Pedogenic carbonates form during periods of heightened aridity, suggesting evidence for arid tolerant C4 vegetation may be preferentially recorded, although the impact of this is limited in mixed C3eC4 habitats (Breecker et al., 2009). These results indicate that mean C4 grass coverage ranged between 30% (JV1) to 55% (JV10). This range of C4 coverage overlaps with a wide range of modern habitat types, including riparian to grassy woodlands and acacia savannah (Sikes et al., 1999; Cerling et al., 2010). Semi-arid landscapes are often relatively open, despite the dominance of C3 vegetation (Gichohi et al., 1996; Swift et al., 1996). These results indicate a variable, but predominately open grassy landscape with some tree cover, matching a range of modern savannah habitats. 3.3.2. Oxygen isotopes In the majority of circumstances, the stable oxygen isotope ratio of pedogenic carbonate is in equilibrium with that of soil water, which is related to meteoric water (Breecker et al., 2009). The monsoonal regime and various groundwater inputs may complicate this scenario in the Jurreru Valley. At shallow depths from the surface (<30 cm), evaporation becomes an increasingly dominant process for water loss, leading to the enrichment of soil water, and resultant carbonates, in 18O (Breecker et al., 2009). As the majority of stable oxygen isotope values from the Jurreru Valley are derived from samples taken from 10 cm beneath a palaeosurface, it is probable that evaporation has had a significant impact upon the results. However, considerable covariance (r2 ¼ 0.49) is observed between stable oxygen and carbon isotope results from the Jurreru Valley suggest that surface evaporation is unlikely to be the only factor affecting the diversity of oxygen isotope values from the palaeosurface (see Fig. 5c). Moreover, this covariance provides an indication that variability in the stable isotope data is the result of spatial variation between the sampled sites. One site, JV10, shows a marked departure from the overall trend of covariance, suggesting conditions of pedogenic carbonate formation may have been considerably different to the rest of the sampled sites. Please cite this article in press as: Blinkhorn, J., et al., Uncovering a landscape buried by the super-eruption of Toba, 74,000 years ago: A multiproxy environmental reconstruction of landscape heterogeneity in the Jurreru Valley, south India, Quaternary International (2012), doi:10.1016/ j.quaint.2011.12.008 6 J. Blinkhorn et al. / Quaternary International xxx (2012) 1e13 Fig. 4. a) Interpolated contour map illustrating the topography of the buried pre-Toba surface in the Jurreru Valley (contour lines ¼ 20 cm; scale in meters); b) Interpolated 3D surface (with vertical exageration) illustrating topography of the buried pre-Toba surface in the Jurreru Valley (contour lines ¼ 20 cm; scale in meters). 3.4. Phytoliths Ten of the eleven samples contain phytoliths and preservation was generally good, with the exception of JV1, where there was no phytolith preservation. The number of phytoliths ranged between 135 and 480 per sample. Overall, the phytolith results indicate a mosaic of vegetation types with distinct differences observable between sites (Fig. 6). Pooid types account for between w5% and w22% of the phytolith record, with panicoid types accounting for w5% to w20% and chloridoid types comprising 0%e15%. Other grasses range between w3% and 10% of the assemblage, suggesting an overall range of grass composition of w20% to w66%. Evidence for tree cover is provided by ligneous dicotyledonous types, which range from w2% to w16%, and circular crenate forms, which are characteristic of palms, with values between 0.28% and 1.21%. D/P ratios, which provide an index of tree cover density (Alexandre et al., 1997), range from w0.025 to w0.35, with an average of w0.17. This suggests that woodland coverage was highly variable across the Jurreru landscape, ranging from extremely low tree cover to w1/3 coverage, but on average providing roughly 1/5 of the vegetative cover. It should be noted that phytolith analysis may under-estimate the total proportions of tree cover as trees and shrubs tend to produce less biogenic silica than grasses (Hodson et al., 2005, 2008). The Iph index, based on the proportions of mesic and xeric grasses (Diester-Haas et al., 1973), ranges from 0% to w43%. Studies beyond South Asia indicate that short grasslands Please cite this article in press as: Blinkhorn, J., et al., Uncovering a landscape buried by the super-eruption of Toba, 74,000 years ago: A multiproxy environmental reconstruction of landscape heterogeneity in the Jurreru Valley, south India, Quaternary International (2012), doi:10.1016/ j.quaint.2011.12.008 J. Blinkhorn et al. / Quaternary International xxx (2012) 1e13 7 Table 1 Stable carbon and oxygen isotope data from 10 cm below Stratum CeD contact in Jurreru Valley. Site JV1 JV1 JV1 JV1 JV1 JV1 JWP3 JWP3 JV14 JV14 JV14 JV14 JV14 JV14 JV7 JV7 JV7 JV7 JV7 JV7 JV13 JV13 JV13 JV13 JV13 JV28 JV28 JV28 JV28 JV28 JV28 JV22 JV22 JV22 JV22 JV22 JV22 JV32 JV32 JV32 JV32 JV32 JV32 JV16 JV16 JV16 JV16 JV16 JV16 JV10 JV10 JV10 JV10 JV10 JV10 Dry Well Dry Well Dry Well Dry Well Dry Well Dry Well JWP22 JWP22 JWP22 JWP22 JWP22 JWP22 Sample code JWP 138 18-1-1 A JWP 138 18-1-1 B JWP 138 18-1-1 C JWP 138 18-1-1 D JWP 138 18-1-1 E JWP 138 18-1-1 F JWP-3-01S-SCJ-B* JWP-3-01S-SCJ-A* JV-14-3 A JV-14-3 B JV-14-3 C JV-14-3 D JV-14-3 E JV-14-3 F 24-1-A3 A 24-1-A3 B 24-1-A3 C 24-1-A3 D 24-1-A3 E 24-1-A3 F JV-13-PS1 A JV-13-PS1 B JV-13-PS1 C JV-13-PS1 E JV-13-PS1 F JV-28-1 A JV-28-1 B JV-28-1 C JV-28-1 D JV-28-1 E JV-28-1 F JV-22-1 A JV-22-1 B JV-22-1 C JV-22-1 D JV-22-1 E JV-22-1 F JV-32-1 A JV-32-1 B JV-32-1 C JV-32-1 D JV-32-1 E JV-32-1 F 21-1G-A2 A 21-1G-A2 B 21-1G-A2 C 21-1G-A2 D 21-1G-A2 E 21-1G-A2 F 21-1-1A2 A 21-1-1A2 B 21-1-1A2 C 21-1D-A2 D 21-1D-A2 E 21-1D-A2 F DW5 A DW5 B DW5 C DW5 D DW5 E DW5 F Loc22 SS19 A Loc22 SS19 B Loc22 SS19 C Loc22 SS19 D Loc22 SS19 E Loc22 SS19 F Deptha 82.867 82.867 82.867 82.867 82.867 82.867 83.449 83.449 83.471 83.471 83.471 83.471 83.471 83.471 83.823 83.823 83.823 83.823 83.823 83.823 84.506 84.506 84.506 84.506 84.506 84.620 84.620 84.620 84.620 84.620 84.620 85.258 85.258 85.258 85.258 85.258 85.258 85.308 85.308 85.308 85.308 85.308 85.308 85.340 85.340 85.340 85.340 85.340 85.340 86.127 86.127 86.127 86.127 86.127 86.127 87.381 87.381 87.381 87.381 87.381 87.381 87.560 87.560 87.560 87.560 87.560 87.560 d13C&pd À7.995 À8.376 À5.763 À8.045 À3.992 À6.929 À4.59 À4.27 À7.360 À7.019 À4.482 À4.801 À3.070 À4.965 À6.267 À5.090 À5.571 À4.115 À5.140 À4.973 À7.271 À6.015 À5.967 À5.172 À6.337 À4.559 À4.592 À3.999 À4.310 À4.517 À4.342 À4.386 À4.507 À4.421 À4.376 À4.358 À4.620 À3.898 À3.966 À4.356 À3.816 À3.944 À4.091 À4.171 À4.227 À4.247 À4.746 À4.445 À3.950 À5.371 À2.634 À3.461 À3.529 À2.123 À5.751 À5.807 À6.315 À6.322 À4.882 À5.758 À5.544 À6.301 À6.460 À7.832 À5.938 À6.159 À6.974 d18O&pd À4.353 À4.315 À2.125 À4.580 À1.561 À4.071 À1.00 À0.72 À3.573 À3.444 À0.805 À0.405 À1.995 À1.721 À1.877 À1.076 À1.288 À1.087 À1.260 À1.211 À3.067 À1.792 À1.867 À1.519 À2.095 À0.617 À0.321 À0.299 À0.160 À0.334 À0.403 À0.970 À1.114 À0.934 À1.140 À1.087 À1.252 À0.260 À0.393 À0.417 À0.441 À0.362 À0.399 À0.817 À0.822 À0.737 À1.686 À1.026 À0.862 À2.256 À2.925 À2.628 À1.825 À3.099 À2.196 À2.971 À2.946 À3.817 À2.939 À1.918 À1.793 À4.116 À2.327 À4.977 À2.934 À2.181 À2.681 can typically be differentiated from tall grasslands by values raging above 30% (Alexandre et al., 1997) to 45% (Kurmann, 1985; Fredlund and Tieszen, 1994), although this is complicated by the presence of pooid types (Bremond et al., 2008). In the Jurreru Valley, Iph values at four sites (JWP 22; JV10, JV22 and JV32) are between 30 and 43%, suggesting a higher participation of short C4 grasses and indicating stronger aridity, compared to the remaining sites, which appear dominated by tall C4 types, associated with more humid conditions. The Ic index, based upon proportions of C3 and C4 grasses (Twiss, 1987), ranges from w18% to w70%, displaying considerable heterogeneity in the make-up of grass coverage across the Jurreru valley. Four samples (JV7, JV13, JV14 and JV28) exhibit an Ic value above 50%, indicating the dominance of C3 grasses at these sites. 4. Discussion 4.1. Spatial variability in geomorphic setting The results of this landscape reconstruction provide support for the suggestion that a wetland environment existed in the Jurreru Valley immediately pre-Toba. A localised depositional centre is observed in the SE corner of the surveyed area, for which the site of JV1 appears to be the lowest surveyed point (see Fig. 4). In contrast to this, JWP 22 and the Dry Well provide the highest surveyed points within the study area in the SW corner (see Fig. 4). The occurrence of pedogenised silty clays on the highest ground of this reconstructed pre-Toba landscape is consistent with their position on the margin of a wetland environment. Wetland clay deposits identified across the rest of the survey area are found below this topographic high, providing further support for the presence of a body of standing water in the Jurreru Valley prior to the eruption of Toba. This reconstruction is also supported by the presence of soft sediment deformation features in the primary ash horizon across most of the studied area, implying that Stratum D deposits were waterlogged at the time of Toba ash fall. Stratigraphic evidence at site JV10 reveals the highest occurrence of the primary ash fall layer, recorded at 86.23 m. This suggests most of the sediments within the survey area were saturated at the time of YTT ash fall in the Jurreru Valley. Three further sites (JV16, JV22 and JV32) preserve primary ash fall layers on relatively high ground in the survey area, between 85.36 m and 85.44 m, which supports the suggestion that the majority of the survey area was saturated with water during the YTT primary ash fall event. Given this evidence for a submerged landscape in the Jurreru Valley immediately prior to the eruption of Toba, the locations of tree fossils may assist with the characterisation of this wetland environment. Tree fossils rooted into pre-Toba sediments occur both on higher ground, at JV32 (85.41 m) and JV21 (86.21 m), and lower ground, at JV7 (83.92 m), all of which appear below the potential high water mark of 86.23 m. This suggests either these trees, which included palms, were adapted to swamp or mangrovelike conditions, or that they were able to withstand seasonal inundation associated with monsoonal rainfall. 4.2. Spatial variation in stable isotopes Spatial variation can be observed in the stable isotope data from the Jurreru Valley with respect to both geomorphology and topography. Pedogenic carbonate samples from wetland clay deposits become increasingly enriched in 13C with increasing topographic height. This probably indicates that C3 vegetation was more dominant in the lower areas of this landscape, with increasing participation of C4 vegetation toward the higher ground. Among high ground deposits, carbon isotope values from pedogenised contexts are notably lower than those from waterlogged *Data from Haslam et al., 2010. a Depth relative to local benchmark (TBM ¼ 100.00 m). Please cite this article in press as: Blinkhorn, J., et al., Uncovering a landscape buried by the super-eruption of Toba, 74,000 years ago: A multiproxy environmental reconstruction of landscape heterogeneity in the Jurreru Valley, south India, Quaternary International (2012), doi:10.1016/ j.quaint.2011.12.008 8 J. Blinkhorn et al. / Quaternary International xxx (2012) 1e13 Fig. 5. a) Stable carbon isotope values plotted against topographic height, including diachronic data from JWP3 (data from Haslam et al., 2010); b) Stable oxygen isotope values plotted against topographic height, including diachronic data from JWP3 (data from Haslam et al., 2010); c) Stable carbon and oxygen isotope data, including diachronic data from JWP3 (data from Haslam et al., 2010). contexts. This indicates a pattern of increased dominance of C3 vegetation on the most waterlogged sediments and pedogenised deposits, with increased prevalence of C4 vegetation at the margins of the waterlogged zone (Fig. 7). Table 2 Mean and standard deviation of carbon and oxygen isotope results from the Jurreru Valley. Site JV1 JWP3 JV14 JV7 JV13 JV28 JV22 JV32 JV16 JV10 Dry Well JWP22 # of Mean d13Cpd& SD d13Cpd& Mean d18Opd& SD d18Opd& samples 6 2 6 6 5 6 6 6 6 6 5 6 À6.850 À4.430 À5.283 À5.193 À6.153 À4.386 À4.445 À4.012 À4.298 À3.811 À5.771 À6.610 1.550 0.160 1.483 0.648 0.678 0.203 0.092 0.175 0.247 1.331 0.490 0.632 À3.501 À0.86 À1.998 À1.3 À2.068 À0.356 À1.083 À0.379 À0.991 À2.488 À2.731 À3.203 1.193 0.14 1.188 0.270 0.532 0.138 0.106 0.058 0.323 0.440 0.691 1.011 A similar pattern can be seen for 18O values from pedogenic carbonates. Waterlogged sediments are increasingly enriched in 18 O with increasing altitude, whereas pedogenised sediments have consistently lower d18O values. This trend may reflect an increasing impact of evaporation on the isotopic composition of soil water with increasing topographic height. Longer periods of waterlogging may have blunted the effects of evaporation on topographic lows. A departure from this trend is present at one location (JV10), where stable oxygen isotope values are lower than those at sites at a similar topographic height, and closer to the values from the sites upon the highest ground (JWP22, Dry Well). If, as appears likely, the dominant control over oxygen isotope composition of pedogenic carbonates was the local variation in evaporation rates, these results may indicate similarities in soil moisture conditions at JV10, JWP22 and the Dry Well during carbonate precipitation. As previously noted, comparisons of stable isotope values from pedogenic carbonates have highlighted the site JV10 as departing from the general pattern of covariance. If vegetative cover had some control over the degree of evaporation, the highest d18O values, reflecting the highest rates of evaporation, should coincide with the Please cite this article in press as: Blinkhorn, J., et al., Uncovering a landscape buried by the super-eruption of Toba, 74,000 years ago: A multiproxy environmental reconstruction of landscape heterogeneity in the Jurreru Valley, south India, Quaternary International (2012), doi:10.1016/ j.quaint.2011.12.008 J. Blinkhorn et al. / Quaternary International xxx (2012) 1e13 9 Fig. 6. a) Summary phytolith diagram from Jurreru Valley showing key long and short morphotypes, and; b) key phytolith indices. highest d13C, reflecting a higher proportion of C4 vegetation associated with reduced canopy cover. Instead, at JV10 lower stable oxygen isotope values associated with the highest stable carbon isotope values are observed. Although the factors affecting pedogenic carbonate formation are complex, geomorphological and topographic evidence suggest that JV10 occupies a unique position within this landscape, potentially at the very edge of the waterlogged deposits. The location of the site may help explain the results of isotopic analysis, illustrating the importance of understanding the landscape setting of sampled sites for palaeoenvironmental analysis. 4.3. Diachronic and synchronic variation in stable isotopes This study highlights that the stable isotope composition of pedogenic carbonates from the buried surface of the Jurreru Valley Please cite this article in press as: Blinkhorn, J., et al., Uncovering a landscape buried by the super-eruption of Toba, 74,000 years ago: A multiproxy environmental reconstruction of landscape heterogeneity in the Jurreru Valley, south India, Quaternary International (2012), doi:10.1016/ j.quaint.2011.12.008 10 J. Blinkhorn et al. / Quaternary International xxx (2012) 1e13 Fig. 7. Contour map showing spatial variability within stable carbon isotope (d13Cpd) results (contour lines ¼ 0.2&) and sampled sites, overlain on 3D frame of buried surface. varies with respect to local geomorphological and topographic conditions. Given this evidence for synchronic variability across the Jurreru landscape, it may be fruitful to reconsider the results of an earlier diachronic stable isotope study from the site JWP3 (Haslam et al., 2010). In that study, a steady enrichment from the lowest sampled pre-Toba deposits (Stratum F; 2.85 m below YTT) to the uppermost pre-Toba deposits (Stratum D; 10 cm below YTT) was observed in both carbon and oxygen stable isotope values (see Fig. 6; Haslam et al., 2010). This was interpreted as indicating that the Jurreru ecosystem experienced a “distinct cooling/drying trend throughout the period leading to the Toba eruption” (Haslam et al., 2010: 3375) with an increasing proportion of C4 vegetation in the landscape, becoming more open through time. The covariance between stable carbon and oxygen isotope values observed within the sample from JWP3 is similar to that of the broader sample of this study, suggesting similar controls upon the formation of pedogenic carbonates in each case (see Fig. 5). A comparison of the range of d13Cpd values from both studies shows that the range of isotopic values (min¼ À8.376&; max ¼ À2.123&) across the isochronous surface immediately prior to the deposition of primary YTT ashfall (this study) exceeds that of the diachronic study at JWP3 (Max ¼ À8.37&; min ¼ À4.19&) (see Fig. 5). This may indicate that the steady increase of stable carbon isotope values in the 2.85 m of pre-Toba deposits studied at JWP3 may reflect local, microecological changes in the formation conditions of pedogenic carbonates rather than a wholesale increase in the proportion of C4 plants in the Jurreru Valley and the concomitant opening up of the ecosystem. JWP3 is located in a clear topographic depression, which is likely to have acted as a localised depositional centre immediately prior to the eruption of Toba. The enrichment of carbon isotope values in the profile at JWP3 may have resulted from increased sediment deposition within this depression, affecting the position of the site within its topographic context, which has in turn impacted the composition of local floral communities. This interpretation is supported by the d18O results from the diachronic study, suggesting increasing impact of evaporation through time, in a similar manner to the increase in topographic height seen in the synchronic study. 4.4. Spatial variation in phytoliths Considerable spatial heterogeneity is also observed in the phytolith results. These results suggest a mosaic of vegetation across the landscape, with an overall open character. Two complimentary trends are observed in the spatial patterning of % grass and D/P ratio: grasses comprise the majority of vegetation on the higher ground, with an increasing proportion of tree cover toward the lower ground. Ic ratios display a similar trend, with C4 grasses predominant on higher ground, and a growing C3 component at progressively lower ground (Fig. 8). High levels of C4 grasses are seen at JV32, JV10 and JWP22, matching lows in the D/P ratios. The Iph index also highlights the impact of topographic relief upon phytolith composition, generally indicating shorter, more xeric adapted C4 grass types on the higher ground, while tall C4 grasses appear to have been more prominent in lower topographic settings. The areas around JV32 and JV10 appear to constitute a significant, although localised, departure from this trend, with a greater proportion of short C4 types than other sites with similar topographic settings. Topography appears to have had a significant impact upon the vegetation of the pre-Toba Jurreru Valley, as suggested by the phytolith data. Given the nature of the sediments described at these sites, it appears that the hydrology of this landscape is likely to have played an important role in generating the observed floral mosaic. In particular, monsoonal rainfall is likely to have resulted in lower areas of this landscape becoming seasonally submerged. Samples from JV28, JV22 and JV32 all contained sedge phytoliths suggesting marginal or periodically inundated areas. The high proportions of tree cover, including the presence of palms, inferred for these areas are likely to have been dominated by varieties that can withstand seasonal inundation, and would benefit from increased access to groundwater supplies during the dry season. Similarly, the increased proportion of C3 grasses on the low ground may well relate to an increased requirement for water resources and/or capacity to survive the monsoon season in these environments. C4 grasses were better equipped to inhabit the drier, higher ground. However, the anomalous results from JV32 suggest that Please cite this article in press as: Blinkhorn, J., et al., Uncovering a landscape buried by the super-eruption of Toba, 74,000 years ago: A multiproxy environmental reconstruction of landscape heterogeneity in the Jurreru Valley, south India, Quaternary International (2012), doi:10.1016/ j.quaint.2011.12.008 J. Blinkhorn et al. / Quaternary International xxx (2012) 1e13 11 Fig. 8. Contour map showing spatial variability of phytolith Ic index (%)(contour lines ¼ 2%) and sampled sites, overlain on 3D frame of buried surface. topography, and its concomitant impacts upon hydrology, are not the only factor determining vegetative variability in this landscape. 4.5. Comparison of stable isotope and phytolith evidence for pre-Toba palaeoenvironments The comparison of the results of the spatial variability of stable isotopes and phytoliths within the pre-Toba Jurreru Valley permits a coherent reconstruction of palaeoenvironmental conditions. Both sets of data demonstrate marked variation with changing topographic position in the palaeolandscape, although geomorphic setting has a more pronounced impact on isotopes than on phytolith assemblages. Sites from the high ground in the reconstructed pre-Toba topography have yielded results indicative of more arid local conditions, and higher proportions of plant communities better adapted to these conditions, than low ground sites, where vegetation appears either more tolerant of or adapted to more humid conditions. Increased proportions of C4 plants (inferred from d13Cpd and Ic%) and grasses (inferred from %grass and D/P) also suggest better drained soils at higher sites than in the localised depositional centre, likely to have been submerged during the monsoon season, and provide better access to groundwater resources during the dry season. Fig. 9 shows that vegetation reconstructions from phytoliths (Ic) and stable carbon isotopes are in agreement for samples from the lowest parts of the basin. Such an agreement between isotope and phytolith-derived vegetation reconstructions is common in grassland dominated ecosystems (e.g. Parker et al., 2011). For the Jurreru landscape there is a divergence in values though for the rest of the samples. The proportions of C4 cover derived from phytolith Ic are higher than those derived from isotopes. However, the trends are generally the same. The Ic derived C4 values are derived from grasses alone and do not reflect the proportion of C3 derived from trees and shrubs. Thus the C4 values derived from phytoliths are most likely over-represented when compared to the stable carbon isotope data. Interestingly, both isotopic and phytolith of analyses have highlighted a couple of sites which do not appear to fit well with the general pattern. The sites of JV10 and JV32 indicate lower tree coverage (D/P), sharp evaporative gradient (d18Opd), elevated levels of C4 plants (d13Cpd and Ic), and high occurrence of short C4 grass forms (Iph) than would be anticipated by their topographic position alone. Concordance between a range of proxies suggests that this anomaly is more likely to represent the effect of some localised, ancient phenomena rather than sampling errors or differential Fig. 9. Comparison of C4 percentages derived from stable carbon isotopes (d13Cpd), where À10.5& ¼ 100% C3 and þ2& ¼ 100% C4, and phytoliths (100-Ic%), plotted against topographic height. Please cite this article in press as: Blinkhorn, J., et al., Uncovering a landscape buried by the super-eruption of Toba, 74,000 years ago: A multiproxy environmental reconstruction of landscape heterogeneity in the Jurreru Valley, south India, Quaternary International (2012), doi:10.1016/ j.quaint.2011.12.008 12 J. Blinkhorn et al. / Quaternary International xxx (2012) 1e13 conditions of preservation. Since these sites are amongst the highest within the reconstructed wetland, some feature associated with the wetland palaeo-shoreline may have caused the anomalous results, although further specification is precluded without the availability of suitable modern analogies from South Asia. However, this once again highlights the importance of understanding the local context of sites sampled; although topography and geomorphology present clear trends within the palaeoenvironmental proxy data, without the landscape approach undertaken in the sampling, the results from JV10 and JV32 would not appear anomalous for their location, and misleading extrapolations may ensue. 5. Conclusion This study has highlighted the potential to illuminate remarkable heterogeneity within palaeoenvironmental conditions by sampling numerous sites at a localised, landscape scale. In the Jurreru Valley, geomorphological and topographic recording has provided strong evidence from which to infer the presence of a seasonally inundated landscape ranging from its margins, at JWP22, to a clear depositional centre, around JV1. Stable isotope and phytolith analyses have illustrated considerable levels of variation at a landscape scale, showing greater levels of palaeoenvironmental diversity than could have been predicted from the previous studies alone. Capitalising upon the clear stratigraphic and chronological marker provided by a horizon of primary YTT ash, it has been possible to show that topography appears to have had a significant impact upon palaeoenvironmental conditions, indicated by both stable isotope and phytolith analyses. Stable isotope values from pedogenic carbonates vary with respect to topographic location, and although some variation is observed due to geomorphological change, this is also likely to be the result of topographical differences and its impact upon palaeohydrology. Similarly, a range of phytolith indices show variation with respect to topographic height of the site sampled. Although there is some potential for non-local intrusion for both isotopic and phytolith inputs, particularly given the seasonal wetland setting that is inferred, covariance between both forms of palaeoenvironmental proxy suggests this had only a limited impact upon the results. Comparisons of the stable carbon isotope and phytolith data allow some conclusions regarding the plant communities extant in the Jurreru Valley immediately prior to eruption of Toba. This reconstruction suggests spatial variability within the pre-Toba vegetative communities, ranging from a mixture of C3 trees and tall grasses in regions susceptible to inundation during the monsoon to more open, short C4 grasslands upon the higher, drier ground. It is within this mosaic landscape that some of the earliest Middle Palaeolithic assemblages are known from South Asia, potentially relating to the arrival of modern humans (Petraglia et al., 2007, in this issue). Williams et al. (2009) suggest the replacement of dense C3 dominated forests by wooded grasslands in the Son Valley, northern India, marked a catastrophic global decline in environmental productivity, with significant consequences for a range of fauna including hominins. The results from the Jurreru Valley suggest Middle Palaeolithic hominin occupation of such wooded grasslands did occur in South Asia, and this is not incongruous with some degree of population continuity in South Asia before and after the eruption of Toba. Finally, Monger et al. (2009) have highlighted the importance of scale with regards to isotopic analysis of pedogenic carbonates, although their conclusions may be equally valid for other forms of palaeoenvironmental analysis. If, as shown, local geomorphology and topography can have significant impacts upon both the isotopic composition of pedogenic carbonates and phytoliths, it is crucial to understand these contextual factors for sites sampled for diachronic palaeoenvironmental studies in order to extrapolate changes in palaeoenvironmental conditions to a landscape or regional scale. Without these additional lines of evidence, patterns of palaeoenvironmental change may erroneously be inferred from diachronic stable isotope studies. In light of the heterogeneity exhibited at a small scale within the Jurreru Valley, sampling single sites for palaeoenvironmental analyses appears inadequate, and more spatially diverse sampling programs are likely to yield richer results at a landscape scale, and provide a firmer basis with which to investigate regional patterns. Acknowledgements We thank the villagers of Jwalapuram and all members of the Kurnool District Archaeological Project for their support, especially J. Bora, A. Durant, E. Gatti, A-M. Hart, R. Korisettar, J. Koshy, N. Matthews, C. Neudorf, C. Oppenheimer, and K. Price. J.B. thanks John Pouncett for training and support in undertaking the topographic mapping. We thank the Archaeological Survey of India for permitting this research, and the American Institute of Indian Studies for logistical support. Funding was awarded by the British Academy and Leverhulme Trust to M.P. References Acharyya, S.K., Basu, P.K., 1993. Toba ash on the Indian subcontinent and its implications for correlation of Late Pleistocene alluvium. Quaternary Research 40, 10e19. Alexandre, A., Meunier, J.D., Lezine, A.M., Vincens, A., Schwartz, D., 1997. Phytoliths: indicators of grassland dynamics during the late Holocene in intertropical Africa. Palaeogeography, Palaeoclimatology, Palaeoecology 136, 213e229. 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