Just a brief follow-up regarding Bronkhorst's work, since I am one of the individuals whose work Bronkhorst cites as significantly misunderstanding the import of the "early" Upanisadic formulations of karma (see, The Vedic Origins of Karma, 1989, a revision of my dissertation).
Bronkhorst's work views the Indian texts as well as the assorted strands of Buddhism, Jainism, and Vedic thought with a deep chronological (a term that occurs frequently in the work) bias. Personally, I find many of his arguments compelling, and I consider his work ground-breaking (this is a notoriously difficult realm to penetrate; Witzel, too, has made significant strides here).
My argument, however, was made on a quite different basis. Following the type of work that I (as a graduate student) believed Heesterman was engaged in, I sought out conceptual (not chronological) origins; that is, I wanted to see what Vedic concepts could be "unearthed" within the representation of karma in the Upanishads; in other words, to get a sense of the Vedic framework--which is stated in fairly clear terms throughout the Satapatha Brahmana in particular (as discussed in my work)--on which karma hangs in the Upanishads (in this, there are some implicit chronological assumptions of "early and late"). As I view the project now, I recall some Durkheim-ian influences as I considered the Upanishads, and their doctrines, as having to be meaningful within a "collective consciousness" of Indian thought that encompassed the Vedic period broadly (again, not as a matter of chronology, but as a matter of discourse).
with regards,
Herman Tull