Sedimentary hiatus causes abrupt decline and shifts in marine subsurface sediment microbial communities: a study from IODP Exp. 378 Site U1553 offshore southern New Zealand.

Hiatuses within marine sediment are discontinuities in stratigraphic sediment records, forming adjacent layers of different ages. To evaluate how such discontinuities relate to subsurface microbial distribution, we examined microbial cell abundance and 16S rRNA gene-based community structure along a sediment core from International Ocean Discovery Program Expedition 378 Site U1553 (offshore southern New Zealand). Cell abundances decreased logarithmically with depth but showed two step-like declines: a sharp drop across a major hiatus at ∼4 mbsf and a second decline at ∼21.5-25 mbsf that overlaps, within sampling resolution, a reported Mn-Fe transition in porewater chemistry. Across the major hiatus, community composition and alpha diversity shifted markedly, indicating a persistent imprint of depositional discontinuity on subseafloor communities. Within the interval where community data were obtained (1.5-21.5 mbsf), community variation was more strongly associated with sediment depth and age than with the measured porewater solutes. These observations highlight stratigraphic age discontinuities as important contextual features for interpreting subseafloor microbial biomass and community patterns.