Palynological reconnaissance of Early Cretaceous to Holocene sediments, Chatham Islands, New Zealand

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Mildenhall, D.C. 1994 Palynological reconnaissance of Early Cretaceous to Holocene sediments, Chatham Islands, New Zealand. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences monograph 7; New Zealand Geological Survey paleontological bulletin 67 204 p. + 23 plates

Abstract: Pollen analyses of over 320 samples from mid-Cretaceous to Holocene of Chatham, Pitt and outlying islets has demonstrated a range of vegetational changes through time. Deltaic and shallow-water marine sediments of the Cretaceous Waihere Bay and Pitt Island groups contain abundant and diverse plant fossils derived from dense podocarp and araucarian forests. These are interspersed with occasional fernlands, especially at the base of the sequence where abundant fern and bryophyte spores indicate a swampy, moist environment, prior to invasion by forest. Angiosperm pollen appears just above the base of the sequence. Palynomorphs from these sediments are described and/or illustrated. One new species, Nyssapollenites chathamicus, is described. Because of the degree of recycling, including older Cretaceous into younger Cretaceous, no zonation is proposed, but the distribution of key taxa is compared to the zonal schemes proposed for Australia and New Zealand. Cenozoic plant fossil horizons are very infrequent, occurring only in the Paleocene to Early Eocene Tioriori Group, the Paleocene part of the Red Bluff Tuff, and the Pliocene to Early Pleistocene Mairangi Group. Many of the Cenozoic samples are contaminated by Pleistocene pollen. All extinct pollen types are illustrated. The Pleistocene vegetation prior to the Last Glaciation consisted of wetland, herbaceous vegetation dominated by sedges and rushes, alternating with dryland woody vegetation dominated by Dracophyllum, Myrsine, and various composites. The primarily Holocene and Last Glaciation Moorland Peat, a zonal blanket peat, covering most of Chatham Island, contains evidence of considerable climatic fluctuations, based on comparison with the distribution of current plant associations. Wet acid peat bogs dominated by rushes and Gleichenia represent high rainfall (high watertable) and cool conditions; wetland vegetation dominated by composites, grasses, sedges and Myrsine represents moist, cool, but drier conditions; and Dracophyllum forests represent moist (swampy), but drier and warmer soil conditions. On this basis high rainfall and cool temperatures existed prior to 50 000 years B.P., then temperatures rose and rainfall decreased until just prior to 33 000 years B.P. when a further cooling occurred. Cooler drier conditions existed from about 33 000 years B.P. until at about 14 000 years B.P. when rainfall abruptly increased and conditions appear to have reached their maximum cold. Conditions then fluctuated until about 2 700 years B.P. when the watertable lowered allowing the development of Dracophyllum forest which persisted until recent loss by human-induced fires. Much of the Holocene is missing from peats in the lower parts of Chatham Island. The prevalence of exotic podocarp pollen over Nothofagus fusca type pollen is indicative of warmer conditions, and variations in their ratio coincide with fluctuations in Chatham Island watertables. Fluctuations in the vegetation's response to changin g levels in the watertable permit a zonation to be developed into which randomly collected samples from undated exposures of the Moorland Peat can be placed. Thus, four biostratigraphic zones are proposed comprising the Restionaceae Zone, beyond radiocarbon dating; the Dracopyllum/Myrsine Zone, also beyond radiocarbon dating, but possibly as young as 50 000 years B.P.; the Compositae Zone, at 14 000 years B.P. to c. 50 000 years B.P.; and the Restionaceae/Dracophyllum Zone, younger than 14 000 years B.P. Most of the exotic pollen located in the Chatham Island peats are illustrated. A site at Te Pukaha, Chatham Island, is selected as the type locality for the Moorland Peat. (auth/NJT)

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