LATE QUATERNARY ENVIRONMENTAL CHANGE FROM LAKE OMAPERE, NORTHLAND, NEW ZEALAND

Lake Omapere is a large, shallow (2 m), eutrophic lake in northern New Zealand (35° S, 174° E) from which we obtained a discontinuous record of environmental change since ~80 cal ka by analysis of a 7-m core. Ages were obtained via tephrochronology, paleomagnetism and ¹⁴C plus climato- and palynostratigraphy. Three of 14 tephras provided markers for correlating the record with other sequences and with marine isotope stages: T-13 (~74 cal ka) was deposited near MIS 5a-4 boundary, Rotoehu (Re) (~55 cal ka) was deposited early in MIS 3, and Kaharoa (Ka) (AD 1314 ± 12) marks late MIS 1. The initial (alkaline) lake formed ~80 cal ka, inundating peat and forest trees. It filled rapidly to levels ~1-2 m above that at present. The subsequent phase of variable but generally falling levels and increasing dystrophy may have been climatically controlled. The lake became swampy/dry early in MIS 3, soon after Re fell. Non-deposition (or non-preservation) characterizes most of MIS 3, all of MIS 2, and probably most of MIS 1 until formation of the modern lake ~700 cal yr ago, or soon after, as indicated by the presence of Ka near the top of the core. Ka also provided a maximum date for Polynesian settlement, shown palynologically by initial forest clearance soon after its deposition. Except for the human era, forest appears to have been continuously dominant near L. Omapere. Nothofagus (probably N. truncata) (beech) was much more common in Northland during the Last Glacial (LG) and its relative abundance vs. Agathis (kauri) pollen provides a better indicator of cooler vs. warmer intervals in the Quaternary than the ratio of tree to non-tree pollen. Moisture balance was probably more critical than temperature in controlling vegetation composition and distribution, particularly during MIS 2. Several tree species (Halocarpus bidwillii, H. biformis, Phyllocladus alpinus) occurred ~2° latitude farther north and at much lower altitudes than their current limits during cooler or drier phases of the LG, and a temperature depression of 4^o C at various times in Northland during the LG is inferred from these range expansions. But the persistence of widespread forest cover suggests that L. Pleistocene climates of Northland were less severe than elsewhere in New Zealand, supporting evidence for a strong latitudinal temperature gradient across N.Z. during MIS 4-1.