This thesis presents the most focussed investigation on the Quaternary of Chatham Island to date. A range of techniques have been applied to the island’s Quaternary deposits to constrain their age and to determine their relationship to Quaternary climatic fluctuations. These techniques include tephrochronology, palynology, stratigraphy and geomorphology.
Some of the major findings of the project include:
• Recognition and identification of the 345 ka Rangitawa Tephra on Chatham Island. The Tephra provides a significant marker horizon within Quaternary sequences on Chatham Island, in addition to the previously recognised 27.1 ka Kawakawa Tephra
• Definition of new Quaternary formations and members, resulting in a more comprehensive Quaternary stratigraphy for Chatham Island.
• Obtaining a late Castlecliffian paleovegetation record for Chatham Island, a time period from which few records exist on mainland New Zealand
• Recognition of marine terraces at various heights on Chatham Island, and applying their heights and interpreted ages to generate the first rates of uplift and deformation for the island
Certain logistical and environmental factors have limited the conclusiveness of some findings in this project; some of which have been touched upon in the previous chapters. These include:
• A lack of control on the ages of deposits and surfaces apart from the two rhyolitic tephra beds – 27.1 ka Kawakawa and 345 ka Rangitawa Tephras.
• A paucity of good exposures spanning extended periods of time. The combination of low uplift rates, repeated eustatic sea level rises, and a dominantly terrestrial depositional environment has resulted in poor preservation potential for long sequences on Chatham Island.
• Young (<125ka) peat and sand obscures much of the older deposits and land form features (e.g. strandlines).
• Poor understanding of the characteristics and history of tectonics and uplift of the Chatham Islands area.
Although this work is the first that deals specifically with Chatham Island Quaternary geology, the thesis essentially presents only a relatively broad overview, with many aspects requiring further and more detailed study. Research investigations that would make significant contributions to a fuller and more comprehensive knowledge of Quaternary history and environmental change on Chatham Island are described in the following text.
A drilling/coring programme dedicated to Quaternary stratigraphy.
Although drilling projects have been undertaken on the island previously, (e.g. Fletcher Challenge, Soil Bureau), these were largely for the purposes of economic reconnaissance (Chapter 2). The stratigraphy, in terms of the age of sediments and relationship to Quaternary climatic fluctuations, and OIS chronology were not investigated in any detail. Much of the information and data produced remains largely unpublished and the original cores have now been discarded, precluding any renewed investigation. A new drilling and coring expedition with particular focus on Quaternary sediments and stratigraphy is highly desirable. This would involve obtaining drill cores from strategic locations, particularly from cover-bed sequences of marine terraces, representative locations on the southern table lands, Te Whanga Lagoon, and from near shore areas of the sea bed around Chatham Island, i.e. the continental shelf.
Numerical age dating of Chatham sediments
As discussed previously, the conclusions of this work are significantly limited by a lack of numerical dating and solid age control. Obtaining numerical ages on key deposits, particularly those directly overlying wave-cut platforms, would confirm the ages of these surfaces and correlations with OIS chronology. The most suitable dating techniques for use on Chatham Island sediments (older than 60,000 yrs) would be luminescence dating and possibly amino-acid racemisation dating. Luminescence dating could be applied to date the quartzose sands that typically overly wave-cut surfaces on Chatham Island. This has been applied elsewhere with some success, although there is still some doubt over the reliability of OSl/TL dating. Amino-acid racemisation (AAR) dating could be applied to plant remains within the older peat deposits that are older than the maximum age range of radiocarbon dating. The maximum age limit for AAR varies depending on the type of material, and ambient temperatures under which the material has existed, but it has been used to date plant materials in excess of 190 ka (Pillans 1981). Thus, AAR would be useful in pin- pointing the timing of shifts in vegetation composition represented by fossil pollen within some pre-Last Interglacial peat sequences.
Further paleovegetational work
High resolution sampling of peat sections from a wider range of locations across Chatham Island would produce a more detailed picture of vegetation dynamics over the Quaternary and would identify other changes occurring in response to global climate fluctuations that may have previously been overlooked due to large sampling intervals. A dedicated study into long-distance pollen rain on Chatham Island is necessary to gain an enhanced understanding of the significance of this fraction of the pollen record. In combination with high-resolution sampling described above, the variations in levels and species composition within fossil pollen sequences could be examined for short (100 yr) periods. A comprehensive modern pollen rain study carried out over several years is necessary to examine the seasonal variations in levels and species composition of long-distance pollen; identify primary source areas and investigate if the location of source areas changes seasonally; and also to examine variations associated with ENSO patterns i.e. El –Niño vs. La Niña years.
To briefly summarise, Chatham Island has probably been a relatively low-lying landmass for most of its sub-aerial existence. Sedimentation has followed a general pattern governed largely by eustatic sea level change associated with global climatic fluctuations of the Quaternary. Low uplift rates and a terrestrial depositional environment result in low preservation potential for Quaternary sediments on Chatham Island, which in turn has resulted in a relatively poor Quaternary record on the Island.
To conclude, Chatham Island is a most unique environment to work in, and offers so much, not only for the geologist, but also for ecologists, botanists, entomologists, anthropologists, to name but a few. I hope that the information presented within this thesis will stimulate other scientists to make the pilgrimage out to the raw and stunning environment that is Chathams, to witness first hand the myriad of possibilities for research projects in all disciplines of natural science.