Influence of CLIMAte on landscape and EROsion The Himalaya/Ganga plain case study

PI : Jérôme Lavé

CRPG participants : Pierre-Henri Blard, Julien Charreau, Christian France-lanord, Florian Gallo, Maarten Lupker, Guillaume Morin, Raphael Pik, Nicolas Puchol

Understanding how climate changes affect the earth surface is one of the most challenging issues in earth sciences, and have implications at all time scales : during or after Paleosozoic ice-age or the Eocene greenhouse epoch ; during orogen lifetime through its role in the retroactions between tectonic, climate, erosion and topographic construction ; at the scale of the Quaternary in response to glacial/interglacial cycles ; or in the recent past and future in response to anthropogenic perturbations. Through mostly variations in precipitation and the temperature, the climate directly controls the vegetation cover, the weathering processes and soil production, but also the snow cover, the hydrology and all the chain of sediment transport between topographic highs and foreland basin or offshore fans.

Over the last few decades, numerous studies have addressed the question of the climate changes in sedimentary archives, whatever they are drill cores in deposits on offshore margins, or in sections across folds affecting foreland basin (e.g. Clift et al., 2008). Despite using different geochemical proxies, none of these studies tried to interpret these signals in a quantitative way by taking into account the erosion and weathering processes, and the filter imposed by the "fluvial conveyor" during transport from source to sink. Even a record as simple as paleoerosion rates of a range relies only on sediment budget in peripheral basins with sometimes large discrepancies between studies (e.g. Métivier, 1999a&b ; Clift, 2006) around the Indo-Asia collision zone, which prevents for example to properly address a major question such as the first order effect of the Plio-Pleistocene climatic transition.

Fig. : Diagram of a mountain range, of its foreland alluvial basin, and of the final sediment deposit in the offshore fan. Physical erosion and chemical weathering on the hillslopes of the mountain range are sensitive to climatic variations in particular to rainfall variations. Associated changes in sediment flux and characteristics are expected to be expressed in subsiding areas, both in the proximal and distal part of the alluvial plain (see location of drilling to recover information from sedimentary archives). However, a full understanding of the expression of any climatic change in those archives requires to know how the hillslope react to climatic variations and how sediment transfer in the mountain range and in the alluvial plain transforms this initial signal.

In this project, we are proposing two complementary approaches to tackle these problems : (1) the study of the present day mechanisms of erosion and the influence of climatic forcing on them, (2) and the investigation of their expressions in sediment archives at two major climatic transition. (1) We will first study in details several watersheds of the central Himalaya and track the influence of the climate on the hillslopes processes in particular through the use of different geochemical markers of erosion and weathering. Indeed, understanding precisely the full range of present day controls and forcing of erosion is a fundamental pre-requisite to be able to track correctly erosion/climate interactions in the past. We will pay then a particular attention to the sediment transfer process (sorting and alteration of the sediment nature) from the moment they leave a range to their final deposition, in order to quantify precisely the way a sediment is transformed before its record in geological archives. This is also a fundamental aspect to decipher before interpreting successfully sedimentary archives in which the history of the Earth Surface is recorded. Finally, such study will be complemented by numerical modelling of weathering, erosion and fluvial transport in order to reproduce present geochemical and geomorphologic observables and further draw how climatic change might be recorded in the sediment message at the outlet of a range or in the more distal offsore basins (Figure). (2) In the light of the so-obtained erosional "transfer function", we will first look at the last glacial interglacial transition that occurred in the Late Pleistocene which represents the youngest major climatic change that affected the Earth surface. This question will rely on the analysis of geochemical (including cosmogenic nuclides analyses) elements in continuous sediment cores of proximal and distal depositional areas to unravel the erosional response of landscape to Late Quaternary climatic changes. Our project will also focus on another key period, which is the Plio- Pleistocene transition, when the Quaternary glaciations emplaced after the relatively warm period of the late Miocene-Pliocene .

In doing so, we hope to better understand how much for example the Quaternary cycles could have increase the erosion of the landscape and renewed topographic relief (Zhang et al. 2001) as well as to propose a framework for further studies on the effect of global warming scenarios on erosion and on fertile sediments delivery or shortage to cultivated basins.