Erosion-climate interactions: Isotopic constraints from the sedimentary record – ECLIP-SED

The ECLIP-SED project featured a suite of novel geochemical tracers having previously demonstrated great promises for reconstructing continental chemical weathering. Amongst these proxies, the combined analysis of radiogenic isotope ratios of hafnium (176Hf/177Hf) and neodymium (143Nd/144Nd) in the fine-grained fraction of silicilastic sediments or sedimentary rocks can be used to reconstruct the intensity of chemical weathering in the geological past, without being affected by potential source effect resulting from changes in sediment provenance. Another major interest of Hf isotopes is that they are highly sensitive to the dissolution of apatite and other phosphate minerals on continents. Upon chemical weathering, these minerals typically release a highly radiogenic fraction of dissolved Hf (characterized by high 176Hf/177Hf ratios) that can be subsequently incorporated in secondary clays formed in soils. Consequently, the application of Hf isotope ratios to fine-grained marine sediments can provide indirect information on phosphorus cycling in surface environments and the export of this key bio-essential element to the ocean. Finally, the ECLIP-SED project also focused on the application of lithium isotopes to clay fractions (d7Li), a proxy for silicate weathering processes, and the distribution of rare earth elements in the labile Fe oxyhydroxide phases of sediments ; a new approach that offers great potential for identifying the occurrence of pyrite oxidation in river catchments.

An important part of the project focused on the analysis of an exceptional marine sediment record recovered from the Amazon Fan. As part of a co-supervised PhD project between IFREMER / Geo-Ocean, the ‘Géosciences Environnement Toulouse’ (GET) laboratory, and the University of Brasilia (UnB), this work aimed at reconstructing the evolution of chemical weathering in the Amazon Basin and its linkage to the Andean uplift and Cenozoic climate change. The ECLIP-SED project also included the investigation of a 8-Ma drill core from the northwestern Australian margin in order to reconstruct the evolution of chemical weathering linked to the emergence of southeast Asian islands following arc-continent collision between Australia and the Banda volcanic arc during the Neogene. Finally, an unique suite of black shales covering the last 2.7 billion years has been analysed during the project, with the aim to discuss past relationships between continental weathering and the evolution of the Earth’s atmosphere since the Archean.

The results acquired during the course of ECLIP-SED project provides new insights on the role played by continental weathering in driving global climate change over geological times. The application of Nd and Hf isotopes to black shales over the past 2.7 Ga showed that continental weathering co-varied with atmospheric oxygen levels since the late Archean.

The geochemical investigation of IODP Site U1482 recovered offshore western Australia showed that mafic rock weathering accelerated over the past 4 Ma, following the emergence of southeast Asian islands. Combined with geochemical modeling of the global seawater strontium isotope curve, these findings provided direct support to previous hypotheses suggesting that enhanced silicate weathering driven by the obduction of ophiolic rocks during the Banda arc-continent collision possibly played a key role in driving global cooling at that time, via intense atmospheric CO2 consumption, ultimately leading to the onset of permanent ice caps in the northern hemisphere.

Finally, the results obtained on the BP-3 sediment record from the Amazon Fan allowed us to reconstruct the evolution of sediment routing across the Amazon basin over the last 20 Ma, showing that a permanent transcontinental Amazon River flowing from the Andes to the Atlantic Ocean was established from 12.0 ± 1.2 Ma. The novel geochemical approach used in this project for discriminating the nature of weathered rocks on continents was successfully applied to the Amazon Basin, providing the first separate records of silicate versus black shale weathering through time. A major finding of this work was to document a gradual shift towards enhanced silicate weathering during the late Pliocene, coincident with global climate cooling. Preliminary interpretation of these results suggests that enhanced chemical weathering at that time was driven by increased physical erosion in Andean regions, caused by the expansion of glaciers and/or wetter hydroclimate conditions, and the subsequent development of floodplains downstream acting as ‘silicate weathering reactors’.

The ECLIP-SED project demonstrated the utility of studied geochemical proxies for identifying the nature of weathered rocks on continents and for reconstructing the separate evolution of silicate versus sedimentary rock weathering in marine sediment records. While our approach remains qualitative and does not allow for direct quantification of past chemical weathering fluxes, the increase in physical erosion rates observed in studied records from Southeast Asia and the Amazon Basin over the last few million years appears to have been clearly accompanied by an intensification of silicate rock weathering, most likely enhanced by prevailing humid tropical conditions in these regions. These findings add weight to the hypothesis that chemical weathering of silicate rocks likely played a major role in driving long-term climate cooling during the Neogene. The novel geochemical approach developed in the ECLIP-SED project could be applied to other major orogenic events in the geological past, in order to further test the importance of the chemical weathering feedback operating in low-latitude regions during past glaciations (e.g., Late Ordovicien).

Chemical weathering plays an important role in sequestering atmospheric CO2, but its potential influence on global climate over geological time remains debated. To some extent, this uncertainty arises from the difficulty in separating the respective contribution of igneous/metamorphic and sedimentary rocks to total weathering rates during mountain erosion; two types of rocks having different impact on the long-term carbon cycle. Here, we propose a novel approach for identifying the origin of weathered rocks on continents and its evolution through time, based on the combined analysis of iron oxide and detrital silicate fractions in the sediment record. After a first phase of proxy development (Nd, Hf, Fe, Si & Li isotopes), using a collection of modern river sediment samples, our new approach will be used to reconstruct, for the first time, the weathering history of the Amazon Basin for the last 30 Ma and its relationships to the Andean uplift and global climate.