Quaternary pollen records of environmental and vegetation change from Lakes Kai Iwi and Kanono, two Northland dune lakes.

Abstract

Lake sediments from two Northland dune lakes, Lakes Kanono and Kai Iwi, were investigated to reconstruct Quaternary records of environmental and vegetation change. Lake Kanono resides at the southern end of the Pouto Peninsula and Lake Kai Iwi is located 25 km north-west of Dargaville. Composite sediment cores from these two dune lakes were analysed to determine a record of environmental and vegetation change proximal to each lake and the wider Northland region, alongside their associated drivers of environmental and ecological change. Currently a sparse record of Quaternary environmental and vegetation change pre-LGM exists in the Northland region. A multi-proxy analysis was carried out on the Lake Kanono and Kai Iwi lake sediments using paleoecology, elemental geochemistry and index properties. Pollen analysis was conducted for both lake records, to construct a composite chronology of vegetation and environmental change constrained using tephrochronology, pollen-inferred stratigraphic correlations, AMS 14C ages and 210Pb for the late Holocene to recent. Where available, organic matter content, sedimentation rates and sediment elemental geochemistry (ITRAX) was used to determine ecological and physical processes acting proximal to and within the lake. Specifically, environmental and vegetation change in Northland is tracked through the appearance and disappearance of Agathis australis (kauri) and Nothofagus spp (beech) trees in the Lake Kanono and Kai Iwi pollen records. The Lake Kanono composite core record is a well-constrained record of environmental and vegetation change, that has produced a discontinuous pollen record predominantly spanning the Holocene period, chronologically constrained back to ca. 5000 cal. yr BP. A fragment of the pollen record covers from ca. 48 kyr to 45 kyr, respectively during the Aurora Interstadial until deposition of the Rotoehu tephra (ca. 45 kyr). Following tephra deposition the core record is inferred to partially span the MIS 3 -2 hiatus. Both Polynesian and European land cover modifications are recorded proximal to Lake Kanono, respectively occurring after ca. 1315 AD coinciding with the rise of Pteridium and around 1886 correlated to large sedimentary influxes. The Lake Kai Iwi composite core is a long lake record of sedimentation producing a pollen record that discontinuously extends as far back as MIS 5 (118 – 75 kyr). The lower pollen zones (KI5, KI6 and KI7; 5.5 – 9.85 m-depth) in the Lake Kai Iwi pollen record are proposed to extend discontinuously from MIS 5a to MIS 5e. Tephrochronology aided in constraining the top 4.5 metres of the Lake Kai Iwi core history through three identified tephra layers: Hauparu (ca. 36.5 kyr), Rotoehu (ca. 45 kyr) and a Mayor Island (ca. 79.5 kyr). Human land cover modification within the proximal environment is observed within the top 13 cm of the Lake Kai Iwi pollen record, constrained by 210Pb to the timing of European occupation.