Spatial heterogeneity of land cover response to climatic change in the Nilgiri highlands (southern India) since the Last Glacial Maximum

25 pagesFourteen hillslope soil profiles were sampled under natural vegetation (i.e., grassland or forest) and plantations in the Nilgiri highlands, southern India. Delta 13C ratios were measured at different depths and 14C ages determined for six profiles. In these highland soils where the turnover rate of organic matter is extremely low, the Δ13C ratios of entire soil profiles have recorded signatures of past land cover. By correlating the data with results previously obtained from peat bogs and with knowledge concerning the history of human settlement, we distinguish three contrasting trajectories of palaeoenvironmental history and landscape change since the Last Glacial Maximum. In the central Nilgiris, between 18 and 10 ka BP, forest expansion occurred due to the conjunction of a wetter climate (the maximum of southwest monsoon-related humidity occurring at ca. 11 ka BP) and higher temperatures; since 10 ka BP, the reversal towards grassland vegetation is attributed to drier conditions. In the western Nilgiris, where strong southwest monsoon winds permanently restrict forest patches to sheltered valley sites, steady but limited expansion of forest from 18 ka BP to the present is recorded and attributed to rising temperatures. The southern and eastern Nilgiris, where the northeast monsoon contributes 20% of the annual rainfall, are the less sensitive to fluctuations in the southwest monsoon. In these areas, rapid and extensive expansion of forest occurred mainly as a consequence of higher temperatures from 18 ka BP to the present. Massive deforestation by Badaga cultivators and Europeans planters followed after the 16th century AD. As a result, and in contrast with the western Nilgiris where the land cover mosaic has remained remarkably stable in the last 18 ka BP, the current landscape differs sharply from the land cover pattern detected by the soil record

Regional Soil Patterns as Indicators of Late Cenozoic Change in the Critical Zone: A Baseline Synthesis for the Landscapes of Peninsular India

Regolith across the South Indian shield has not previously been mapped. Here we provide a diagnosis of directional and lasting climate change from humid to semi-arid since the late Cenozoic based on evidence provided by mosaics of 1) residual, 2) colluvial and 3) alluvial soils across 700,000 km2 of southern peninsular India. Results are inferred from a systematic geomorphological and palaeoenvironmental interpretation of 1:250,000 scale legacy soil maps at order to subgroup level, complemented by field surveys and controls of soil parent material—i.e., regolith. The inventory highlights two generations of residual soils: 1) deep Lixisols, hosting low-activity clays and large iron hydroxide concentrations indicative of humid conditions in the geological past; and 2) shallow Luvisols containing high-activity clays and large stocks of exchangeable bases, indicative of drier conditions compatible with the modern climate. Where still present, the relict Lixisol inliers straddle drainage divides and are in the final stages of being thinned or fully stripped by headward stream erosion. They are being replaced by the Luvisols over shallow weathering fronts. Colluvial and alluvial soils, including widespread Vertisols, are used as tools for detecting and mapping different generations of Quaternary flood deposits: fluvial terraces, coastal fan-deltas, and shallow upland palaeolakes. In a region mostly devoid of carbonate rock outcrops, the widely distributed pool of soils hosting abundant accumulations of pedogenic CaCO3 also reveals the magnitude of silicate bedrock weathering as a process for generating secondary calcium carbonate in the rock cycle, thereby highlighting an under-appreciated contribution to inorganic carbon sequestration in the global carbon cycle. The results and maps produced provide exploration tools for future, more systematic and coordinated investigations of the nature and chronology of Quaternary deposits in peninsular India. This includes assessing their potential for hosting different generations of prehistoric archaeological remains

Large-scale rock slope failures in the eastern Pyrenees: identifying a sparse but significant population in paraglacial and parafluvial contexts

This is the accepted version of the following article: [Jarman, D., Calvet, M., Corominas, J., Delmas, M. and Gunnell, Y. (2014), Large-Scale Rock Slope Failures in the Eastern Pyrenees: Identifying a Sparse But Significant Population in Paraglacial and Parafluvial Contexts. Geografiska Annaler: Series A, Physical Geography, 96: 357–391. doi: 10.1111/geoa.12060], which has been published in final form at http://onlinelibrary.wiley.com/doi/10.1111/geoa.12060/fullThis first overview of large-scale rock slope failure (RSF) in the Pyrenees addresses the eastern third of the range. Around 30 principal RSFs >0.25 km2 and 20 lesser or uncertain cases have been identified from remote imagery and groundtruthing. Compared with other European mountain ranges, RSF incidence is relatively sparse, displays no obvious regional trend or spatial clustering, and occurs across diverse landscape types, if mainly on metamorphic rocks. A transition is observed from paraglacial RSFs in formerly-glaciated valleys to what are here termed ‘parafluvial’ RSFs, within wholly or mainly fluvial valleys but where slope failure is not directly provoked by or linked to river erosion. RSFs are particularly found in three topographic settings: (i) at cirque and trough-head thresholds (transition zones of elevated instability between cirque and main glaciated trough walls); (ii) near the upper or outer periphery of the ice field, where glacial adaptation of fluvial valleys is incomplete; and (iii) in fluvial valleys beyond glacial limits where incision is locally intense. RSF is absent from the range divide, from within cirques, and from most main valleys. In the montane areas, RSF is strongly associated with vestiges of preglacial summit surfaces, confirming that plateau ridges are less stable than sharpened crests and horns. RSF is contributing significantly to the progressive destruction of this paleic relief. The overall sparsity of RSF indicates insufficient rock mass stresses, including rebound after concentrated bedrock erosion. This may reflect a relatively weak imprint of glacial erosion, including breaching, in a context of relatively low mean rates of neotectonic uplift, possibly signalling overall that eastern Pyrenees landscapes are close to dynamic equilibrium.Peer ReviewedPostprint (author’s final draft

Oblique rifting and segmentation of the NE Gulf of Aden passive margin

The Gulf of Aden is a young, obliquely opening, oceanic basin where tectonic structures can easily be followed and correlated from the passive margins to the active mid-oceanic ridge. It is an ideal laboratory for studies of continental lithosphere breakup from rifting to spreading. The northeastern margin of the Gulf of Aden offers the opportunity to study on land the deformation associated with oblique rifting over a wide area encompassing two segments of the passive margin, on either side of the Socotra fracture zone, exhibiting distinct morphologic, stratigraphic, and structural features. The western segment is characterized by an elevated rift shoulder and large grabens filled with thick synrift series, whereas the eastern segment exhibits low elevation and is devoid of major extensional structures and typical synrift deposits. Though the morphostructural features of the margin segments are different, the stress field analysis provides coherent results all along the margin. Four directions of extension have been recognized and are considered to be representative of two tensional stress fields with permutations of the horizontal principal stresses s2 and s3. The two dominant directions of extension, N150 E and N20 E, are perpendicular to the mean trend of the Gulf of Aden (N75 E) and parallel to its opening direction (N20 E-N30 E), respectively. Unlike another study in the western part of the gulf, our data suggest that the N150 E extension stage is older than the N20 E extension stage. These conflicting chronologies, which are nowhere unambiguously established, suggest that the two extensions coexisted during the rifting. On-land data are compared with offshore data and are interpreted with reference to oblique rifting. The passive margin segmentation represents a local accommodation of the extensional deformation in a homogeneous regional stress field, which reveals the asymmetry of the rifting process. The first-order segmentation of the Sheba Ridge is inherited from the prior segmentation of the passive margin