Impact of glacial-interglacial cycles on denudation rates in Western Mediterranean – EroMed

Scientific rationale: Since the Earth surface is up to now the only viable place for humanity, it is critical to predict how it will respond to future climatic variations. This requires a good understanding of how past climate changes have affected continental surfaces. One of the key factors controlling the surface dynamic and the long-term climate is denudation (or erosion), i.e. the sum of physical erosion and chemical alteration. However, the impact of climatic variations on denudation during geological time remains poorly understood and controversial (Herman and Champagnac, 2016 vs Willenbring and Jerolmack, 2016). This is particularly critical regarding the impact of glacial-interglacial cycles on denudation. These climatic cycles affected the Earth with a rhythmicity of 100 kyr and temperature amplitudes of ca. 8°C (with significant regional disparities) during the last million of year. According to certain authors (e.g. Molnar, 2004; Herman et al., 2013), the apparition of these glacial fluctuations induced a major increase of denudation worldwide, while other scientists (e.g. Willenbring and von Blanckenburg, 2010) argue that these climatic changes had no effect on denudation. This controversy is in part due to methodological biases and to the fact that tectonics may hide the pure climate forcing on long time scales.
To overcome this debate, there is a need of high-resolution data based on unequivocal proxies of denudation, applied to archives that precisely document the effect of glacial-interglacial cycles, independently from the tectonic forcing. In recent years, our CRPG research group has been a pioneer in showing that cosmogenic nuclides (e.g. 10Be) can be used as powerful geochemical tools to quantitatively reconstruct paleo-denudation rates over geological time scales.

Goals of the project: In this project, we will study the impact of glacial-interglacial variations on erosion. To this end, we will quantify paleo-denudation rates using in situ10Be, a robust denudation proxy that is not affected by the same bias as other methods. Our effort will focus both on large glacial-interglacial cycles (100 kyr periods) and both on short-term millennial climatic fluctuations (Dansgaard-Oeschger events), using well-dated marine sedimentary records in contexts that have undergone major and variable changes in glacial extents. Over this time scale, the impact of uplift may safely be assumed as negligible. Secondary questions will also be addressed: what is the seasonal effect on the cosmogenic 10Be concentrations? What is the impact of the watershed size on the sediment transfer time? Does the sediment granulometry have a predictable effect on the cosmogenic nuclides concentrations in quartz?

Material and methods: We will apply this approach on modern and ancient sediments of 3 Mediterranean river watersheds having different sizes, elevations and climatic characteristics: the Rhône (over a 0-25 ka period), the Var (0-70 ka) and the Golo (0-500 ka). Our approach will combine several complementary methods: (i) field geology to collect samples in modern watersheds, (ii) GIS (Geographical Information Systems) analysis of watershed, (iii) rock source tracking (e.g. Nd isotopes), iv) marine sedimentology, v) analysis of in situ cosmogenic 10Be, 26Al (and 14C) in quartz of cored sediments, vi) numerical modelling to constrain and classify the effects of several key parameters: equilibrium line altitude of glaciers, temperature, the amount and time repartition of rainfall, geology substratum, vegetation.

Societal impacts of future results: Our ability of understanding the soil resilience to external climatic changes is crucial to anticipate the impact of global warming on the stability of the Critical Zone. This knowledge may have major implications on risk evaluation, urbanization and agricultural policies.