Paleoenvironmental reconstruction of a well-preserved Stage 7 forest sequence catastrophically buried by basaltic eruptive deposits, northern New Zealand

Forest remnants, buried by catastrophically emplaced volcanic deposits, are identified on the shores of the Manukau Harbour, in northern New Zealand. Carbonaceous muds associated with the fossil forest were investigated through a combination of stratigraphy, chronology and palaeoecological proxies. The fossil forest was destroyed and buried by phreatomagmatic explosions from the adjacent Maungataketake Volcano. Small trees (<1 m diameter) in position of growth are observed extending 1–2 m upwards into the overlying volcanic deposits. The lowermost part of the phreatomagmatic succession contains well-preserved leaf fossils and small fallen logs and branches, many of which were incorporated into the phreatomagmatic succession during the initial eruptive phase. Optically stimulated luminescence dating of the phreatomagmatic succession together with palaeoecological evidence for interglacial climate suggests deposition in late Marine Isotope Stage (MIS) 7. This extends the known age of volcanism in the Auckland Volcanic Field by up to 40 ka.Beetle and pollen assemblages indicate a northern conifer forest growing adjacent to a wetland on or near a coastal plain. The pollen record shows forest compositional changes, in particular a period of Agathis australis—dominance between two phases of Dacrydium cupressinum dominance. Concomitant developments in the wetland flora may have been linked to changes in groundwater conditions as sea level fluctuated across the coastal plain. Despite overall similarity to the modern flora and fauna, both beetle and pollen assemblages include elements that are found today at higher elevations in the region, indicating that slightly cooler climate conditions existed during late MIS 7 compared to present. We estimate this temperature depression at less than 1 °C relative to the present, broadly consistent with the late MIS 7 marine (ODP-1123) record located east of New Zealand. This study clearly demonstrates the complementarity of coleopteran and palynological analyses and the utility of a multi-proxy approach to reconstructing terrestrial environmental change.