The 36C1 ages of the brines in the Magadi-Natron basin, East Africa

Abstract

The depression in the East African Rift which includes both Lake Magadi and Lake Natron forms a closed basin within which almost all the dissolved chloride originates in precipitation, since there is no important source of very ancient sedimentary chloride. This provides an ideal setting for the evaluation of the 36Cl methodology as a geochemical and hydrological tracer. The main source of recent water, as represented by the most dilute samples measured, is characterized by a 36C1/C1 ratio of 2.5 × 10 −14, in agreement with the calculated value expected in precipitation. Surface evaporation increases the chlorinity of the local freshwater inflow by about a factor of 110 without changing the isotopic ratio, indicating that little chloride enters the system in the form of sediment leachate. A second type of brine found in the basin occurs in a hot deep groundwater reservoir and is characterized by lower 36C1/C1 ratios (<1.2 × 10 -14). By comparing this value with the2.5× 10 −14 in recent recharge, one obtains an approximate salt accumulation age of 760 Ka which is consistent with the time of the first appearance of the lake. These older brines also have lower 18O and 2H values which indicate that they were recharged during a climatically different era. The 36C1/C1 ratios in the inflowing waters and in the accumulated brine, together with the known age of the Lake Magadi basin, may be used to estimate the importance of the hypogene and epigene, as opposed to the meteoric, mode of 36C1 production. Such a calculation shows that the hypogene and epigene processes together contribute less than 6% of the total 36C1 present in the lake.