Holocene river behaviour in New Zealand: response to regional centennial-scale climate forcing

This paper applies meta-analysis techniques to a database of 401 14C-dated Holocene fluvial units in New Zealand. We use the database to produce a probability-based reconstruction of Holocene river behaviour at a national and regional scale. Records of river activity in northern (North Island) and southern (South Island) New Zealand are compared with independent hydro-climate proxy records that reflect regional, tropical and polar influences on Southern Hemisphere climate. During the Holocene, 12 multi-centennial length episodes of river activity and flooding were identified in the North Island, and in the South Island record 11 periods exceed the mean relative probability of activity. These records show that episodes of river activity have exhibited a predominantly out-of-phase relationship, suggesting the relationship between ENSO and SAM, and the relative dominance of the two modes, may be influencing Holocene river activity in New Zealand. The emerging pattern in the South Island Holocene fluvial record is one of increased river activity in response to enhanced westerly atmospheric circulation associated with a predominance of trough regime synoptic type (negative SAM-like circulation). In the North Island episodes of river activity are driven by increased meridional atmospheric circulation associated with blocking regime synoptic conditions (La Niña-like and positive SAM-like circulation). Analysis of floodplain sedimentation rates shows a rapid increase after ∼500 cal yr BP following the arrival of humans and the beginning of widespread deforestation. Regional climate complexity in New Zealand presents opportunities for palaeoclimate reconstruction, with the New Zealand fluvial 14C-database ideally placed to fill geographical gaps in the long-term hydrological record.