Enigmatic High Palaeolatitude Forests of Gondwanaland - a Case Study from the New Zealand Jurassic

Abstract

Middle Jurassic fossil forest deposits from New Zealand, at a palaeolatitude of ~75-78° South, provide evidence for a diverse vegetation association dominated by Filicopsida and Gymnospermopsida plants. Macrofloral and microfloral studies at present day Kawhia Harbour (North Island) and in the Curio Bay region (South Island) also recognise the preserved remains of Chlorophyta, Bryophyta and Lycopsida and interpret vegetation palaeo-profiles from the fossil flora assemblages. Fossil wood analysis identifies distinct, newly described taxa: Araucarioxylon sp. A, Araucarioxylon sp., Protocupressinoxylon sp. A, Podocarpoxylon sp. A and Protopodocarpoxylon sp.. Facies analysis implies vegetation growth in riverine depositional settings located on merged alluvial fan-toes along the Murihiku basin margin of the south-eastern Gondwanaland coast. Evidence from palaeosols and in-growth-position fossilised tree stumps suggest major floods inundated and destroyed successive forest generations at a minimum recurrence interval of at least 100-150 years. Interpretations from the distribution and structure of the in-growth-position fossilised tree stumps imply the growth of forests during the Middle Jurassic of varying, but generally high, densities with a distinct vertical structure of undergrowth, canopy and emergent layers. Analysis of fossil wood growth ring widths implies the trees grew rapidly when environmental conditions were favourable and that the forest productivity levels were comparable to those of modern tropical woodlands. Growth ring sequences studied imply dominantly uniform growing conditions, year to year, with variations possibly reflecting fluctuations in water supply. Rarely, variation in cell widths within individual growth rings suggests growth was disrupted during the first half of certain growing seasons, perhaps also due to changes in water supply or the occurrence of severe storms. Empirical solar radiation data from 75° North applied to the Middle Jurassic palaeoenvironment and comparison with late-early Permian high palaeolatitude wood from Allan Hills in Antarctica suggests the trees were highly adapted to the near-polar light regime. Wood production was maximised even within the shaded canopy. Climatic parameters interpreted from the sediments and fossil flora suggest a near-tropical, but seasonal, palaeoclimate with wet and warm summers and arid, cool winters. These criteria compare well to simulated Jurassic Global Climate Models for the southeastern margin of Gondwanaland implying model reliability for the prediction of future global temperature trends.