A phytolith analysis-based paleoenvironmental interpretation of the Lower Taieri Plain, Otago, New Zealand

Abstract

A phytolith based late Quaternary vegetation record is presented from a 154.5m long sediment core from Lower Taieri Plain, Otago, New Zealand. Phytoliths were preserved discontinuously through the core but were preserved in Penultimate Glaciation (Marine Oxygen Isotope Stage (MIS) 6) aged sediments with good preservation in fine sediments from the Late Glacial/Holocene and Last Interglaciation (MIS 5). Stand alone phytolith based inferences for the Penultimate Glaciation and much of the Last Interglaciation were consistent with pollen based inferences. Phytoliths also provide the main record of vegetation change from 12 000 to 10 000 yr BP where few pollen grains were preserved. The Holocene phytolith record indicated 'cooler' climatic conditions dominated by chionochloid and chloridoid morphotypes characteristic of tussock grasslands of the cooler inland basins and upland areas of Otago, contrasting with the warm climate forest inferences of the pollen record. The contrasting mechanisms of phytolith deposition and taxonomic constraints are likely to contribute to these differing interpretations. By combining phytolith and pollen records in conjunction with paleoenvironmental inferences from diatom analysis of the same sediments, three interglacial sedimentary settings are distinguishable a) swamp forest (MIS 5e) b) freshwater lake (MIS 5c-a) and c) estuary (Holocene). The swamp forest is characterised by local phytolith deposition while the lake environment and estuarine environments are more complex, being influenced by the immediate vegetation and longer distance phytolith deposition. We suggest that the estuarine environment of the Holocene prevented phytoliths forest vegetation entering the record while the phytoliths transported from the cooler upland regions in water lain sediments were retained. We suggest phytolith analysis should mainly be used in a multiproxy approach, particularly for complex sedimentary settings. Phytoliths provide a means of assessing local environmental conditions including local vegetation signals and inferred sediment deposition patterns, not detected by other proxies. This work contributes significantly to our understanding of past vegetation change in Otago New Zealand.