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The Franco-Bolivian Mission “Palaeoenvironment and Archaeology of the Guaquira –Tiwanaku River” tries to restore the evolution of the landscape in contact with one of the most emblematic cultures of the Andes. The Tiwanakota culture developed on the shores of Lake Titicaca at the end of the second millennium BC and disappears between 1100 and 1200 AD following a drought that would have affected the entire Altiplano. In this essay we present the methodology and the work in progress. It is designed to allow scholars to better understand the interactions between the Tiwanakota culture and the immediate environment surrounding the monumental archaeological site.

Magnetic susceptibility (MS) is currently used as a directly representative proxy for the study of climatic variations, and for cyclostratigraphic studies. It depends on the concentration of magnetic minerals in the rocks, but does not allow identifying the magnetic minerals. In the case of argillaceous sediments, the paramagnetism of clay particles often plays a major role in determining the magnitude of their magnetic susceptibility, while the presence of ferrimagnetic iron oxides or sulfides cannot be assessedusing susceptibility measurements alone. Among the different methods that can be used to detect ferrimagnetic particles magnetic viscosity (MV) characterizing the delay corresponding to the acquisition or loss of induced magnetization, has the same advantages as MS. Itsmeasurement is direct, rapid and has been proven to be very efficient in detecting the presence of secondary ferrimagnetic minerals in soils. A MV measurement technique was tested on cores taken from a borehole, in Callovian-Oxfordian formations in the eastern Paris Basin (France) (Fig. 1). Although the MV values are very small, they have cyclic variations of which strongest values are found at the base of the Lower Oxfordian. These values, when correlated to MS,Gamma Ray (GR) and a sequential interpretation of a borehole drilled close-by are found to be associated with the maximum clay fraction in the core samples, a MS maximum, and a major transgression event. Consequently, a significant increase in ferrimagnetic minerals can be associated with this event.