Quantifying paleorecharge in the Continental Intercalaire (CI) aquifer by a Monte-Carlo inversion approach of 36Cl/Cl data

- We investigate past recharge of the Continental Intercalaire aquifer.
- We develop an original inversed approach combining 36Cl data, piston flow model.
- A transient recharge function is required to simulate 36Cl in large aquifer.
- We quantitatively reconstruct variations of the recharge over the last 775 kyr.
- Computed recharges reveal that MIS 5 was wetter than the Holocene period.

Past variations of the recharge is estimated in the Atlas Mountains, the main recharge area of the Continental Intercalaire (CI) aquifer, one of the major Saharan aquifers, over the last 775 kyr. In this Mediterranean climatic context, continental archives generally record single humid events, grouped in hypothetical time interval and do not offer continuous chronicles of precipitation. We propose to use spatially distributed 36Cl/Cl data as a temporal constraint, to infer the past recharge in the Atlas area. Based on a simplified but robust climatic scenario, assuming a piston model, we apply a Markov Chain Monte Carlo (MCMC) inversion approach to attribute a specific recharge to the last nine interglacial periods and an undifferentiated recharge to the glacial periods. The interglacial recharge values vary from a few mm yr−1 to more than 60 mm yr−1. Glacial recharge is less than 1 mm yr−1. These values are then analyzed in terms of intensity and allow questioning some initial hypothesis, especially the generally accepted value of the initial 36Cl/Cl value (around 133×10-15atat-1). Our analysis suggests a higher value, around 175×10-15atat-1. This approach allows us to bring out paleoclimatic information retained in these continental archives. Especially, computed recharges provide reliable evidence that Marine Isotope Stage (MIS) 5 was more humid than the Holocene period in agreement with marine archives that document wet condition in the North Sahara during that period.