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Rhone River Flood Deposits in Lake Le Bourget
Reference
Chapron, E., Arnaud, F., Noel, H., Revel, M., Desmet, M. and Perdereau, L. 2005. Rohne River flood deposits in Lake Le Bourget: a proxy for Holocene environmental changes in the NW Alps, France. Boreas 34: 404-416.

What was done
Noting that "millennial-scale Holocene climate fluctuations have been documented by lake level fluctuations, archaeological and palynological records for many small lakes in the Jura Mountains and several larger peri-alpine lakes," the authors sought to learn more about the pervasive climatic oscillation behind this phenomenon by documenting the Holocene evolution of Rhone River clastic sediment supply in Lake Le Bourget via sub-bottom seismic profiling and multidisciplinary analysis of well-dated sediment cores.

What was learned
Chapron et al. report that "up to five 'Little Ice Age- like' Holocene cold periods developing enhanced Rhone River flooding activity in Lake Le Bourget [were] documented at c. 7200, 5200, 2800, 1600 and 200 cal. yr BP," and that "these abrupt climate changes were associated in the NW Alps with Mont Blanc glacier advances, enhanced glaciofluvial regimes and high lake levels." They also report that "correlations with European lake level fluctuations and winter precipitation regimes inferred from glacier fluctuations in western Norway suggest that these five Holocene cooling events at 45°N were associated with enhanced westerlies, possibly resulting from a persistent negative mode of the North Atlantic Oscillation."

What it means
Situated between these Little Ice Age-like periods would have been Current Warm Period-like conditions. The most recent of these prior warm regimes (the Medieval Warm Period) would thus have been centered somewhere in the vicinity of AD 1100, while the next one back in time (the Roman Warm Period) would have been centered somewhere in the vicinity of 200 BC, which matches well with what we know about these warm regimes from many other studies (see Medieval Warm Period and Roman Warm Period in our Subject Index). In addition, since something other than an increase in the atmosphere's CO2 concentration was obviously responsible for the establishment of these prior Current Warm Period-like regimes, it is reasonable to assume that another increase in that same "something" - and not the coincidental rise in the air's CO2 content - was likely responsible for ushering in the Current Warm Period.

Reviewed 20 September 2006