Ratio of Na to K in feldspars as a predictable index to trace the source of sediment: Based on the mass-balance model combined with feldspar weathering kinetics and a case study

The Na/K ratio in feldspars in sediment has been proven to be a stable and predictable index. The relationship between the sediment and its source in terms of the Na/K ratio in feldspars can be defined using the massbalance model combined with the feldspar weathering kinetics as K-1 = gamma (*) K-0 gamma = (1 - (t/t(Na-L))(0.39)/1 - (t/t(K-L))(0.39)) where K-0 and K-1 are the Na/K ratios in feldspars in the parent rocks and sediment, respectively; t represents the time interval; gamma is the ratio of K-1 to K-0 ; and t(Na-L) and t(K-L) are the lifetimes of Na-containing and K-containing feldspars, respectively. According to this model, the change in the Na/K ratio in feldspars as a function of time for different combinations of feldspars can be predicted. Further, t(Na-L) and t(K-L) can be replaced by the lifetimes of the Na-containing and K-containing feldspars with the shortest lifetimes, respectively, if there are other types of feldspars. The ratio depends on the time interval and feldspar lifetime. The Na/K ratio in feldspars changes by < 5% over nearly 10(4) years for nine selected combinations of Na-containing and K-containing feldspars (microcline-labradorite, microcline-sanidine, microcline-K-feldspar, albite-labradorite, albite-sanidine, albite-K-feldspar, oligoclase-labradorite, oligoclase-sanidine, oligoclase-K-feldspar), although similar to 20% of these feldspars is lost over 10(4) years. A case study of a lake affected by the slow lake filling between the 1960s and 1980s showed that the Na/K ratio in feldspar is more sensitive than major elements to changes in the sediment source and more stable than trace metals, which are easily affected by human activities.