Argument structure and agency in Bemba passives

Bemba employs two passive constructions: an older one with verbal extension -w- and a more recent construction involving the class 2 subject marker ba-. We argue that ba- is ambiguous between an ordinary, referential class 2 marker, and an underspecified passive marker, and is disambiguated by the overt encoding of a class 2 subject, or an oblique semantic agent phrase. Under the passive interpretation, the semantic patient displays both subject-like and object-like properties, posing a problem for the analysis of argument structure in these constructions, and of subjects and objects in Bantu. In contrast, the -w- passive extension is increasingly used in contexts where the agent cannot be expressed, but also in combination with the neutro-passive extension -ik-, that is, with predicates with reduced valency, where it licenses the expression of an agent oblique phrase. We argue that the ba- passive is used in more typical passive contexts, while the -w- passive becomes increasingly restricted to more marginal grammatical contexts. The paper shows that both passive constructions are taking part in a wider grammaticalization process, in which two main functions of the passive, change of argument structure and encoding of agency, are becoming dissociated