An integrated terrestrial paleoenvironmental record from the Mid-Pleistocene transition, eastern North Island, New Zealand

A tephra-bearing lake sequence from near Ormond, New Zealand, provided the opportunity to conduct a multi-proxy paleoenvironmental analysis within the Mid-Pleistocene time period. A 10.5-m-thick section was measured and analysed for pollen, spores, diatoms, macrofossils, and tephra geochemistry. Palynological assemblages in the lower 4 m of the section indicate an ameliorating temperate climate and increased humidity. The upper 6.5 m is dominated by diatomite that did not yield sufficient palynomorphs for study. The source vegetation was distal lowland mixed broadleaf podocarp forest, swamp forest from ca. 2.5-4.0 m, and proximal scrubland with sporadic forest encroachment. Abundant freshwater algae in the samples suggest that any brackish influence in the lower part of the section was minor. Marine dinoflagellates found in the basal pollen samples could be mostly recycled. Diatomite samples were overwhelmingly dominated by a freshwater but mildly salt-tolerant diatom. We interpret the section as representing a shallow coastal lowland lake that intermittently had access to the sea. The chronology of the section is based on a single glass-ITPFT age of 0.62±0.09 Ma from a tephra interbed (T8) in the lower part of the section. In addition, the lowermost tephra in the sequence (T1) is geochemically correlated to AT-485, preserved in the marine record east of New Zealand, with an astronomically tuned age of 0.7163 Ma. This chronology places the basal portion of the Ormond section within the Brunhes normal Chron with sequence deposition occurring during either the OIS 18/17 or 16/15 transitions. However, paleomagnetic data indicate a clear reverse polarity from an equivalent stratigraphic position and at this stage it is difficult to reconcile this age discrepancy.