Environmental changes in mountain environments during Holocene. A record from organic proxies in stalagmites.

International audienceStalagmites are well established as accurate archives for paleoclimate and paleoenvironmental records, with inorganic proxies being widely used. Organic proxies are a growing field, providing access to information stored in a separate carbon pool, and relating to microbial activity, vegetation cover, and temperature. However, the key factors controlling these proxies are not yet well defined. δ13C analysis of speleothem calcite (effectively the dissolved CO2 pool) is a routinely used technique, but the controls are not as well defined as for δ18O, with the proxy record potentially affected by vegetation change, soil conditions or microbial activity. Parallel measurement of δ13C of the preserved organic matter has been suggested as a method of resolving this signal, with an inverse relationship between the two records previously being suggested as a proxy for microbial control. However, this technique is in its infancy in speleothems, and requires testing in a greater range of contexts. This study uses a multi-proxy approach, combining δ13C analysis of calcite and the preserved organic matter, with a highly novel laser fluorescence technique to investigate climatic and land-use driven environmental variations during the Holocene period. Four stalagmites were sampled in the same room of the Garde-Cavale karst system (1300m a.s.l.), in the Bauges massif, in the northern Prealps. These samples cover the last 12 ka. δ13C analysis of soluble acid bulk organic matter was performed via LC-IRMS at La Trobe University and the University of Orléans. δ13C of the calcite was performed at University of Melbourne, Australia. Organic matter fluorescence was measured at the University of Savoie, France. This fluorescence technique provides a high resolution record (100 µm step along the growth axis), and is a non-destructive method. The three proxies show different trends during the Holocene period, although major environmental events are marked by changes in all records. Further work is needed to establish whether the differences in the proxy records are due to differing environmental controls, or whether there are methodological factors affecting the record. In particular, the organic δ13C proxy is currently under further investigation to test for sensitivity to fluctuations in instrumental parameters and matrix effects. However, in the long term, this multi-proxy approach, combining organic and inorganic techniques has great promise in resolving our understanding of environmental variations