From the introduction: "Paumarí, a Brazilian language belonging to the Arawaken family, has a split case-marking system of a type which, so far as I am aware, has not been previously reported. The ergative system occurs in clauses having the basic word order pattern, or in clauses where a significant part of the basic pattern is preserved. The nominative-accusative system occurs in clauses where other word orders are used. For the purpose of this paper, by "basic word order" I mean the pragmatically least marked and statistically most frequent order.
"Dixon (1979, 79-80) explains all split case systems in semantic terms, and discusses three basic types of conditioning factor: the semantic content of verbs, the semantic content of NP's, and the tense/aspect choice. He goes so far as to explain the "grammatically conditioned split" that has been reported for some languages, i.e. where the morphologiccal marking differs between main and subordinate clauses, as being primarily conditioned by the semantics of the subordinate clause. In Paumarí, however, there does not seem to be any way of explaining the phenomenon except in terms of a grammatically conditioned split, this being strictly along the lines of word order patterns, although it seems more appropriate to describe it as two coexisting systems rather than a a single, split system. The semantic distinctions that have been proposed to account for split systems are not relevant, since the same range of semantic phenomena occurs in both the ergative and accusative systems.
"In this paper I first describe the dual case-marking system in Paumarí (s.1), and then discuss some implications it has for a theory of transitivity such as that proposed by Hopper and Thompson (1980) (s. 2). In the course of this discussion, I draw attention to a likely functional explanation for the existence of the two systems and for the word order patterns with which they correlate."
Derbyshire, Desmond C.
"Ergativity and transitivity in Paumarí,"
Work Papers of the Summer Institute of Linguistics, University of North Dakota Session: Vol. 27, Article 2.
Available at: https://commons.und.edu/sil-work-papers/vol27/iss1/2