CNRS scientists have developed a system for simultaneous continuous
measurement of laser fluorescence parameters in order to probe a stalagmite's
secrets and provide insights into environmental and climatic evolution
and rhythms in the Vercors massive. The researchers working on this
project come from various CNRS units: the "Enregistrement à
haute résolution de l'environnement dans les dépôts
laminés karstiques" (High Resolution Environmental Analysis
of Stratified Karst Deposits") group; the "Laboratoire de
physique des lasers, atomes et molécules" (Laboratory
for the Physics of Lasers, Atoms and Molecules); and the "Laboratoire
de géographie de l'Université de Savoie" (University
of Savoy Geography Laboratory)
A
stalagmite's growth is linked to its external environment. CO2-charged
water percolates into the ground and dissolves limestone. When the
water enters an underground void, a drop in CO2
pressure disturbs the equilibrium and precipitates carbonates, producing
calcite stalagmites and stalactites drop by drop, and trapping particles
from the external environment inside them. Growth is seasonal, dependent
on water volume and the number and type of particles, and as a result
the stalagmites develop a stratified structure that reflects their
environment rather like tree rings.
Non-destructive
laser-induced fluorescence is being used to interpret these strata.
Three parameters are studied: reflectance, which provides information
regarding the macroscopic optical properties of the calcite, showing
up particles due, for example, to land clearance; fluorescence, which
characterizes the organic matter trapped in the calcite and, being
seasonal, produces a high resolution geochronometer enabling the stalagmites
to be dated; and the maximum fluorescence wavelength, which characterizes
the size of the trapped organic matter, enabling local soil formation
to be studied. The team has developed a new fluorescence index, the
Laser Induced Fluorescence Index (LIFI), for deducing the maximum
wavelength at any point in a sample without measuring the whole emission
spectrum.
This
new tool paves the way for powerful mathematical tools to interpret
the records contained in stalagmites.
