Complex sentence: Since cosmogenic sources of 129 I produce very tiny quantities of it--too small to affect atomic weight measurements, iodine is thus also a mononuclidic element--one that is found in nature essentially as a single nuclide.
Split into simpler sentences: Since cosmogenic sources of 129 I produce very tiny quantities of it--too small to affect atomic weight measurements, iodine is thus also a mononuclic element. By definition, this is an element which is found in nature essentially as a single nuclide.