High Resolution Dating

Carrer Integration Grant – Marie Curie

What is the challenge of Earth and Environmental sciences today? Since the revolution of the theory of plate tectonics, evolution of continents has been associated with the interactions of tectonic plates. More recently, the revolution in digital modeling has shifted the paradigm of plate tectonics by showing the importance of interactions between external processes (e.g. climate, oceanic currents, erosion, weathering…) and internal processes (e.g. geodynamics, dynamic topography…). On the one hand, climate models show that climate changes (global warming, CO2 levels, monsoons, aridification …) can be attributed to internal processes through changes in the distribution of reliefs, seas and continents. On the other hand, the models show that these geomorphological internal processes can be influenced in turn by climatic variations through the erosion of the continents.

What is needed now to test and calibrate the models, it is a methodology that allows to differentiate and quantify the respective contributions of internal and external processes on palaeoenvironmental variations observed during the regional and global changes.

The overall objective of this project is to build the facilities that will enable to date with the highest possible resolution the sedimentary archives of past climate and environmental changes using the most accurate dating techniques at hand today. The expected results will enable to constrain in time past environmental changes (warming, CO2 levels, aridification, monsoons, sea retreat, mountain building…) with sufficiently high resolution to allow correlating them to known variations of global climate available from marine records.

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A multi-disciplinary approach is tailored for the research questions and geologic settings to be investigated in this project. The aim is to constrain climatic and tectonic evolution independently during basin formation. Tectonic and paleo-environmental proxies are recorded within the basin strata that are calibrated to the astronomically tuned timescale using combined magnetostratigraphy and cyclostratigraphy. Tectonic events are recorded independently by thermochronologic analysis of the exhumation of neighbouring mountain ranges (Fig. 2).

Magnetostratigraphy and cyclo-stratigraphy is performed to obtain a chronostratigraphic framework with a resolution down to the precession cycle ( 20 kyr). A complete record of magnetic polarity zones (=chrons) throughout the sections is obtained by high-resolution sampling down to the shortest chron duration. This improves our ability to (1) determine a perfect correlation between our sections and the geomagnetic polarity time scale and (2) recognize astronomically driven cyclicity (eccentricity, obliquity and precession) expressed in the sediments, which improves correlation to the timescale and provides additional time resolution).

Along with the age of sediments, magnetostratigraphy also provides the timing and magnitude of tectonism through analysis of tectonic rotations, paleolatitude variations and changes in sediment accumulation rates. This can be associated to low temperature thermochronology (Apatite Fission Track and U-Th/He) for which the host institution provide excellent facilities and expertise.

In combination with the dating of sediments proposed in this project, thermochronology performed both on the basin-bounding mountain ranges and/or in the sediments drained from those ranges provide timing and rate of exhumation as well as lag time (=exhumation age – depositional age) related to basin formation during mountain uplift.

Paleoenvironmental proxies are jointly gathered for paleoclimatic reconstruction and astronomical calibration of observed cyclicity. Rock magnetic analysis are performed at using methodologies previously successfully applied to sedimentary archives. These results can be associated to other independent climate proxies in collaboration with various experts of the host and other institutes.

This is usually complemented by sedimentologic facies analysis, major element and stable isotope geochemistry, Scanning Electron Microscopy (SEM), colour reflectance and paleontologic analysis of macro- and microfossils, such as pollen, ostracods and/or foraminifera depending on environment and fossil content.

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The overall set objective of the original project are to build the facilities to enable dating with the highest possible resolution the sedimentary archives of past climate and environmental changes using the accurate dating techniques at hand today:

Objective 1. Build a magnetically-shielded room to host the automated system and improve the reliability of the results by eliminating background noise.

Objective 2. Build an automated sample-handler for in-line magnetic processing of long core as well as discrete samples for massive processing of the widest possible range of applications.

Objective 3. With the built facility, apply and develop state-of-the-art multiproxy paleomagnetic methods for dating and assessment of past environments. Integrate a wide range of research applications in collaboration with academic and industrial partners.

Mid-Term report summary

Objective 1 has been achieved. The results from this objective went far beyond our expectations. The ambient earth magnetic field of ca. 50,000 nano-Teslas (nT) as well as the shorter time scale natural and anthropogenic variations are now shielded in the room to reach values that are everywhere below 50 nT – except at the room entrance of course – well beyond our objective of reaching values as low as 200 nT (see Figure below).

Objective 2 has not been implemented yet. We greatly suffered from the accidental death of the engineer originally associated with the project that incurred significant delays.

Objective 3 has already been successfully and largely implemented despite the delay in the automated system. It follows of two main directions, (1) applications and (2) methodological developments. These are being undertaken within several ongoing research international projects with external funding.

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Fig. 3. Left: building of the magnetically-shielded room with graduate student Wei Yang (pekin-Rennes University) Setting of the outer shell made out of three layers of electric steel plates with specific magnetic susceptibility. 25 cm inside the outer shell, the inner shell was then built with two layers of electric steel. Right: This resulted in a 15 sqm room with a residual field below 50nT in which the magnetomether and other instruments were finally installed providing results of unexpected quality and accuracy as mesured by graduate student Tasmin Blayney (Lancaster University)

Final report summary

Beyond the evident transfer of knowledge to graduate students involved in the projects, the new laboratory hosted in the shielded room has become a very popular place to visit at the host institution. Both for undergraduate and graduate classes taken at the Rennes University but also for visiting researchers as well as high-school classes.