Deciphering paleohydrology from the petrography and microstratigraphy of stalagmites (MIS 5 – Holocene, Moulis Cave, Ariège, S. France)

Abstract

A stalagmite from Moulis Cave in the northern Pyrenean piedmont in SW France provides a discontinuous record between the last interglacial and the Holocene. The integrated study of petrography, microstratigraphy and internal architecture of the stalagmite documents paleohydrological history of the drip site during speleothem growth, including infiltration conditions and calcite saturation state of the water feeding the stalagmite. Following diagenetic screening which demonstrates the excellent state of preservation of the speleothem, a set of petrographical, mineralogical and geochemical analytical techniques was used to characterize: 1. the successive growth units and their bounding surfaces, including facies and nature of processes occurring at the level of each discontinuity, and 2. the morphological changes of the stalagmite apex and stacking pattern.Observations reveal that each bounding surface results from a particular group of events, the nature and effects of which are petrographically clearly identifiable. During the interstadials 5b-5a of the last interglacial, the stalagmite developed at a relatively moderate growth rate and under stable drip rate conditions, except at the onset of the 5b cooling. The highest growth rates are recorded during the Bølling-Allerød to Younger Dryas, and the Early-Middle Holocene. During the Younger Dryas, a cold and humid climate is suggested on the northern piedmont of the Pyrenean range while temperate and humid conditions prevailed during the Early Holocene. The Middle Holocene was still humid North of the Pyrenees, except during the cold and dry event at ∼6.2 kyrs. Finally, during the last growth phase of the stalagmite, the growth rate slowed down drastically in relation to increasing frequency of high infiltration events, possible floodings and a decreasing saturation of the drip water relatively to calcite.