Innovative Tracers of the Cenozoic Carbon Cycle

PI : Laurie Reisberg

Participants : Nathalie Vigier (LOV/CNRS-UPMC, Villefranche-sur-Mer) ; Yves Godderis (GET/CNRS-Université Paul Sabatier, Toulouse) ; Guillaume Caro (CRPG) ; Mathis Neimard (CRPG)

At million-year timescales, large scale geodynamic processes control atmospheric contents of CO2 and other greenhouse gases, which in turn regulate the Earth’s surface temperature. However, controversy remains concerning the relative importance of the main regulatory processes : 1) CO2 release from the mantle at mid-ocean ridges and 2) CO2 removal by alteration of silicate rocks and/or organic carbon burial resulting from orogenesis. These issues can be addressed by studying marine isotopic records in seafloor sediments. Nevertheless, the two most widely used radiogenic marine tracers, 87Sr and 187Os, provide only partial responses to these questions, as both are potentially influenced by rare but highly radiogenic lithologies. New approaches, integrating results from several isotopic systems, are needed to evaluate the effect on atmospheric CO2 levels caused by alteration of the lithologically complex continental crust. The first goal of the INTOCC project is to add to the marine isotopic toolbox by improving our knowledge of the exogenic cycles of three tracers. Two of these, 186Os and 40Ca, are radiogenic, and thus strongly constrain the sources of the elemental flux to seawater, especially when used in conjunction with the traditional tracers 87Sr and 187Os. Current results suggest that seawater has retained an unradiogenic Ca composition over Earth’s history (Caro et al., 2010), suggesting a dominant mantle source of this element critical to the carbon cycle. Nevertheless, to quantitatively constrain the relative inputs of Ca to the ocean, the average 40Ca abundance of the continental crust must be determined, and one important aim of the our study will be to provide this information.

Because of the extremely low abundance of 186Os, data from marine and crustal sediments are almost non-existent, with the notable exception of those from McDaniel et al. (2004), which suggest an unradiogenic 186Os composition for seawater, in stark contrast with the radiogenic nature of marine 187Os. Understanding its implications of this intriguing observation, which first must be confirmed, requires knowlege of the exogenic cycle of 186Os, which is currently almost entirely lacking. Use of this tracer will require analytical development to allow 186Os analyses on tiny quantities of Os (<5ng). Towards this goal we have installed amplifiers with 1013 ohm resistors on the CRPG Triton mass spectrometer which will allow precise Os isotopic analyses at low signal intensities. The techniques developed will be used to study authigenic marine sediments, as well as crustal rocks and sediments that provide Os to the oceans.

The third tracer used in the INTOCC project, δ7Li, is a stable isotope tracer ideally suited to studying variations in continental weathering processes and intensity. In this case, one of the most important challenges is establishing a precise marine record of the Cenozoic Era known to be free of the vital effects that potentially bias Li isotopic results from biogenic sediments. This will be done at the Laboratoire Océanographique de Villefranche (LOV) through the analysis of authigenic smectites, which can be used to reliably reconstruct seawater δ7Li compositions (Vigier et al., 2014).

Brahmaputra River (photo M. Paul)

The overiding objective of the INTOCC project is to integrate the new data obtained, together with existing data, to enhance our understanding of the geochemical and climatic evolution of the Earth’s surface during the Cenozoic Era. More specifically, the new marine records for 40Ca, 186Os and δ7Li, coupled with the well-established 87Sr and 187Os records, will be incorporated into a quantitative numerical model (GEOCLIM), developed at the GET laboratory of the University Paul Sabatier of Toulouse. This is the only model that takes into account spatial distributions of different lithologies, soil thickness and vegetation and their impact on weathering and erosion rates. All of these factors will influence the isotopic signatures of authigenic marine sediments, so these records provide an essential test of the validity of the model and allow its refinement.