Transit time distributions in Lake Issyk‐Kul

Abstract

Measurements of sulfur hexafluoride (SF 6 ) and chlorofluorocarbons (CFCs) are used to constrain the timescales for deep‐water renewal in Lake Issyk‐Kul. As these tracers have different tropospheric histories their combination provides more transport information than one tracer alone. In particular, from these measurements the mean, Γ, and standard deviation, σ, of the distributions of transit times since water made last contact with the surface can be tightly constrained. Γ is older than the age determined from SF 6 and younger than the ages from the CFCs, and increases from around 4 yrs at 200 m to around 10.5 yrs at the deepest location (655 m). σ also increases with depth and equals around 0.7 to 0.8 Γ, which corresponds to large ranges of transit times, and implies mixing processes play a major role in the transport. The approach used can also be applied to similar tracer measurements in the oceans and groundwaters to constrain transport in these geophysical systems.