A late Holocene record of vegetation change from Whanganui Inlet and Mangarakau Swamp northwest Nelson

Abstract

Whanganui Inlet and Mangarakau Swamp lie on the upper west coast of the South Island, an area which has had little uplift or incision during the Holocene. The inlet was flooded ~8,000 years ago as sea-level rose, since this time it has been accumulating sediment. Mangarakau Swamp lies adjacent to the inlet's southern margins and consists largely of fine peat. This research aims to extract a record of late Holocene vegetation change from both Mangarakau Swamp and Whanganui Inlet by analysing the fossil pollen assemblages. These sites also provide a unique opportunity to study two distinct pollen records preserved in different depositional environments, terrestrial peat and estuarine mud. To this end five vibracores were extracted from Whanganui Inlet and two 'D' cores from Mangarakau Swamp. Five cores from the inlet and two from the swamp were constrained chronologically using radiocarbon dates and pollen. A maximum age of 7,700 years CalBP was obtained for inlet cores and 6,500 CalBP for swamp cores. Swamp cores are potentially as old as ~10,000 however, no datable material was preserved at the base of the oldest peat core so this is inferred. As the inlet flooded, sediment accumulated rapidly at a rate of 10.8mmyr-1 until 7,350 years CalBP after which sedimentation is negligible. An increase in sedimentation over the last hundred years to 5.16 mmyr-1 is attributed to forest clearance associated with European arrival in the area. In Mangarakau Swamp peat accumulated much more consistently, prior to European arrival at an average rate of ~1 mm yr-1. Over the last hundred years accumulation increased to 1.42 mm yr-1. Three vibracores and two peat cores were analysed for pollen. In estuarine cores three pollen zones were recognised, whilst in swamp cores four were identified. Zone one of the swamp occurs only in peat core one, it depicts a shrubland dominated by Myrsine, Coprosma and Nothofagus menziesii. This zone also suggests that a lake was present in part of the area now covered by Mangarakau Swamp. Zone two of the swamp cores equates with zone one from Whanganui Inlet. These zones describes a forest dominated by Dacrydium cupressinum, with Cyathea, and Ascarina lucida. On the swamp Dacrycarpus dacrydioides, Kunzea ericoides and Letospermum scoparium grew. Zone two from the inlet and three from the swamp are defined by the spread of fuscaspora type trees, this coincides with a decline in Dacrydium cupressinum, Cyathea and Ascarina lucida possibly signalling a deterioration in climate. The final zone records the presence of introduced taxa (notably Pinus) and identifies the modern period of sedimentation, this coincides with a decline in forest taxa identifying forest clearance in the area. Maori had no measurable effect on the vegetation. Pollen assemblages differ markedly between the two sites. Local signatures of vegetation are overwhelmed in estuarine cores as the pollen is largely regional sourced from the inlet's river catchment. An over-representation of Cyathea spores in estuarine samples implies that water is the main transport vector for pollen preserved in the inlet. Conversely, swamp samples record a strong locally sourced pollen component from wetland taxa.