In verbal clauses, Samoan (Polynesian, Oceanic, Austronesian) exhibits an ergative case alignment, in which transitive subjects are marked by ergative case, while intransitive subjects and objects by absolutive case. In a particular type of nominalization, however, namely zero-nominalization, intransitive subjects show a split-marking: unergative subjects receive alienable possessive a-case, unaccusative subjects pattern with objects receiving inalienable possessive o-case, while ergative subjects maintain ergative (MH = Mosel & Hovdhaugen 1992, Mosel 1992). As a result, Samoan zero-nominalizations exhibit a cross-linguistically yet relatively rare pattern of tri-partite/inactive alignment which distinguishes three types of subjects, i.e. transitive, unergative, unaccusative (cf. Comrie 2013). We derive the observed case alignment in Samoan zero-nominalization from two independent syntactic factors: (i) split-possessive case that marks an (in)alienability contrasts on the possessor (MH 1992, cf. Walworth 2021, Chung 1972), and (ii) syntactic ergativity that arises from the prepositional nature of transitive subjects and structural absolutive case (Hopperdietzel 2020, Polinsky 2016, Alexiadou 2001 for nominalization). We show that in the absence of absolutive case in zero-nominalizations, the nominalizer n embeds a verbal projection up to the Voice level. As Voice-under-n cannot introduce a DP-argument itself (Alexiadou 2020, Iminashi 2014, Bruening 2013), unergative DP-subjects must be introduced in the nominal domain in a position where alienable a-case is assigned. In contrast, vP-internal unaccusative subjects and objects of transitive verbs are merged in the verbal domain and receive structural o-case in the absence of T. Ergative PP-subjects of transitive verbs instead occur in nominalizations as they are analogous to by-phrases in e.g. English, Romance and Greek nominalizations (Alexiadou 2001). Our analysis represents the first formal investigation of the syntactic properties of Samoan normalizations, but also has cross-linguistic implications on (i) split-intransitivity (ii) syntactic ergativity, and (iii) (in)alienable case assignment in the nominal domain.