Yanni Gunnell

Denudation history of the continental margin of western peninsular India since the early Mesozoic - reconciling apatite fission-track data with geomorphology

A comprehensive apatite fission-track (AFT) study of the passive margin of western peninsular India between 12° and 16°N is used to reconstruct the denudation chronology of the continental hinterland. In common with other rifted margins, the morphology is characterised by a low-lying coastal plateau separated from an elevated inland plateau by an erosionally controlled escarpment (Western Ghats). We modelled the fission track data using the commonly adopted annealing algorithm of Laslett et al. [Chem. Geol. 65 (1987) 1–13]. Using the default parametrisation (i.e. an initial track length of 16.3 μm), it was found that during the Mesozoic denudation rates remained extremely low, increasing sporadically when erosion peaked at 130 Ma (rifting with Antarctica) and 80 Ma (rifting with Madagascar). Denudation rates rose considerably during the Cenozoic, reaching maxima of ca 120 m/Myr. Such values are, however, considered as major overestimates and the effects of the Seychelles rifting at 65 Ma remain suspiciously unrecorded. We explored the consequences of changing the initial track length in this model to a value of 14.5 μm. In practice, this reduces the rapid Cenozoic denudation artefact, model peak rates during the Mesozoic are much more variable, and during the Cenozoic reach values an order of magnitude lower than with the original initial track length. The response to the Seychelles rifting event is almost immediate. Just as previous model calibrations in AFT analysis have been relatively empirical, this revised approach does not provide insights into the physical mechanisms of low-temperature annealing. However, it is shown to agree much better with independently established geomorphic, cosmogenic, stratigraphic and tectono-magmatic evidence in this and other stable shield regions in terms of both the timing and the magnitude of geological events, and the geomorphic response of the landscape to them.

Early Pleistocene Presence of Acheulian Hominins in South India

Dates from a site in southeast India imply an early migration of Homo through Eurasia about 1.1 to 1.5 million years ago. South Asia is rich in Lower Paleolithic Acheulian sites. These have been attributed to the Middle Pleistocene on the basis of a small number of dates, with a few older but disputed age estimates. Here, we report new ages from the excavated site of Attirampakkam, where paleomagnetic measurements and direct 26Al/10Be burial dating of stone artifacts now position the earliest Acheulian levels as no younger than 1.07 million years ago (Ma), with a pooled average age of 1.51 ± 0.07 Ma. These results reveal that, during the Early Pleistocene, India was already occupied by hominins fully conversant with an Acheulian technology including handaxes and cleavers among other artifacts. This implies that a spread of bifacial technologies across Asia occurred earlier than previously accepted.

Relief and climate in South Asia: the influence of the western ghats on the current climate pattern of peninsular India

The 1500-km-long Western Ghats mountain barrier of peninsular India interacts with the southwest monsoon in a manner which bears heavily on the exceptionally varied climate pattern of the Deccan. Karnataka Province alone concentrates five of the six major climate types of the entire Indian Union. This review explores the interactions between atmospheric structure over South Asia and relief and discusses the efficiency with which the passive margin uplift, second only to the Himalayan barrier, acts as a climatic gatekeeper to the subcontinent. Particular attention is given to rainfall patterns and regimes. These are revealed by a variety of statistical classification and mapping techniques, and the analysis is guided by the steep environmental gradient observed on the immediate backslope of the Ghats, where annual totals can drop from 6000 to 600 mm in ca. 80 km. This is strongly reflected in the landform, soil, vegetation and cropping patterns and raises the question of the relationship between the uplift history of the mountain barrier at geological time-scales, the history of the South Asian monsoon circulation and the stability and diversity of the climatic pattern as seen today. The tightly arranged suite of bioclimatic regions also provides a unique geographical backdrop to the agricultural diversity of South India, rarely found on such a scale in other monsoon contexts of the Tropics.

Chapter 11 - Recent Advances in Research on Quaternary Glaciations in the Pyrenees

In the Pyrenees, new optically stimulated luminescence, 14C and 10Be ages, allows a more detailed image of the Pleistocene glaciations. MIS 6 moraines are dated on Ariege and Gallego. For the last glacial cycle, maximum ice extent occurred early, during MIS 4. A powerful advance during the global last glacial maximum is known on the Mediterranean side only. Deglaciation was very rapid after 20 ka.

Small, isolated glacial catchments as priority targets for cosmogenic surface exposure dating of Pleistocene climate fluctuations, southeastern Pyrenees

A new 10 Be data set from a small glacial catchment in the Pyrenees confirms almost full glacier extent during Marine Isotope Stage (MIS) 2, extends the oldest cosmogenic exposure ages of moraines in this mountain range to MIS 5, and constrains paleoclimate reconstructions in the Mediterranean region. It also illustrates the advantageous conditions for the long-term preservation of terminal moraines provided by small, low-gradient glacier catchments that did not merge with compound ice masses even during glacial maxima. These small, gently sloping, and isolated glacier catchments are proposed as priority targets for terrestrial cosmogenic nuclide dating of peak Pleistocene glacier advances, providing scope for greater accuracy in interregional paleoclimatic correlations.

Present, past and potential denudation rates : is there a link ? Tentative evidence from fission-track data, river sediment loads and terrain analysis in the South Indian shield

Abstract Ahnert [Ahnert, F., 1970. Functional relationships between denudation, relief and uplift in large mid-latitude drainage basins. Am. J. Sci. 268, 243–263] proposed that a denudation rate D could be predicted by the relation D =0.1535 h , where h is local relief, a substitute for mean slope. The present study seeks to explore the value of this simple equation as a normative geomorphological tool. In two successive and complementary stages, its principles are applied to the Western Dharwar Craton of South India. First, the robustness of local relief as a reliable substitute for mean slope is examined. Following the demonstration that the equation can successfully be used in the region of interest pending correction factors for rock resistance, a potential denudation map of the Western Dharwar Craton is established. The pattern of potential denudation values is compared with known measurements of both long-term (∼10 2 Ma) denudation rates inferred from apatite fission track data and modern (∼10 −5 Ma) denudation rates derived from available sediment load measurements in relevant river basins. Unexpectedly close similarities between long-term, short-term and potential denudation rates prompt a discussion on the merits of potential denudation as a possible standard in the appraisal of measured rates and as a tool for establishing permissible denudation rates when assessing the impact of man-induced erosion. A landscape-specific basal metabolic rate of erosion is defined as a new concept. The methodological pitfalls in extrapolating denudation rates from the measurement of sediment loads in rivers are reviewed and suggestions for tighter investigation are made.

Soils and climatic geomorphology on the Karnataka plateau, peninsular India

Abstract As a result of extensive field reconnaissance in southern India, a geographical traverse across the planation surface of the Karnataka uplands, which lie in the rainshadow of the Western Ghats, was selected to assess the consequences of a steep rainfall gradient on soil and landform patterns. The study of the traverse, described in detail, reveals a characteristic progression of soil type and wash divide form from the high-rainfall to the semi-arid end of the continuum and invites a critical reappraisal of classical notions in climatic geomorphology and the soil sciences. The assumption that alternations of wet and dry climates are necessary to achieve planation is given special consideration, and the often held belief that kaolinite minerals are diagnostic of humid palaeoclimates when found in a semi-arid climate is challenged. The findings for the Indian traverse are set against the better known data for West Africa, where the rainfall range between the Gulf of Guinea and the Sahel is comparable with that observed in the south-western Deccan. Differences outweighing similarities, attention is drawn to the possibility that tectonic regime, which also controls the degree of landscape stripping, is a major key to understanding the climatic geography of soils, the tread of planation surfaces and the shape of hillslopes from one region of the Tropics to another.

Spatial heterogeneity of land cover response to climatic change in the Nilgiri highlands (Southern India) since the last glacial maximum

Fourteen hillslope soil profiles were sampled under natural vegetation (ie, 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 through the conjunction of a wetter climate (the maximum of southwest monsoon-related humidity occurring at c. 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 sixteenth 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.

Radiometric ages of laterites and constraints on long-term denudation rates in West Africa

In stable cratonic regions, most tropical weathering mantles evolve over long time scales and record long-term environmental change. They may therefore also reflect tectonism and its denudation-related signals detected through apatite fission-track thermochronology, cosmogenic radionuclide dating, and the age bracketing of laterites by 4 0 Ar/ 3 9 Ar dating of potassium- and manganese-rich oxides. Based on an existing Cenozoic pedimentation model for the West African craton, this study uses the three combined radiometric methods to define rates of Cenozoic denudation. Denudation rates of <2 m/m.y. on the lateritic plateaus, in comparison to rates of 7-13 m/m.y. adjacent to them, fit ages of 45-50 Ma for late stages of bauxite development and 24-25 Ma for one phase of lateritization. Together, they support the theory implicit in the landscape model that depths of denudation in cratonic interiors are equal to, or not significantly greater than, existing elevation differences between lateritic landforms.

Passive margin uplifts and their influence on climatic change and weathering patterns of tropical shield regions

Abstract Large scale plateau uplift influences long-term atmospheric circulation patterns. Recent speculation on the role of passive margin mountains in initiating late Cenozoic glaciation around the North Atlantic adds to the debate on the connections between tectonics and climate patterns, and calls for broadening the scope of examining causes of climate change to include other existing passive margins. Three examples from the intertropical zone, the Western Ghats of India, the Borborema of northeast Brazil, and the West African Leo uplift are compared. Palaeoenvironmental and fission track constraints on amplitude, timing and wavelength of the shoulder uplift history for the Western Ghats reveal that high elevation margins can act as effective environmental filters, both as physical barriers to atmospheric circulation and as pace-makers for denudation rates, which in turn steer weathering processes towards producing specific soil and vegetation suites. Predicting interaction between climate and rift–flank tectonics is however shown to be model-dependent, in that assumptions on whether uplift is transient or permanent will influence the timescales and effectiveness of any consequent environmental change.