Lower and middle Jurassic woods of the Cleveland Basin (North Yorkshire), England

Abstract

ABSTRACT. Allochthonous Lower and Middle Jurassic (Pliensbachian-Bathonian) permineralized and charcoalified woods from the Cleveland Basin (North Yorkshire, England) are assigned to five genera of gymnospermous wood (Cupressinoxylon spp., Taxodioxylon spp., Cedroxylon spp., Xenoxylon and Araucarioxylon) and two species (Xenoxylon phyllocladoides and Araucarioxylon lindlei). Cell structure is commonly well preserved, particularly in calcified and charcoalified samples, and several features of the growth rings have palaeoclimatological significance. These include distinct growth rings, commonly narrow latewood, low to moderate annual sensitivity and mean sensitivity values, and presence of false rings. Overall, these characteristics reflect fairly consistent growth, both during the growing season and from year to year. Temporally, there is a transition towards narrower rings of more variable width, proportionally wider latewood, and more abundant false rings in Bathonian woods. Water supply is interpreted as the primary limiting control on growth and was probably more restricted in the Bathonian. This is suggested to be a consequence of relatively low sea-level and, more importantly, a genuinely drier climate at this time, manifested as an accentuated annual dry season. In all, the increase in seasonality during the Early and Mid Jurassic in North Yorkshire is compatible with the effects of a developing Mediterranean-type climate. G R O W T H -R I N G analysis has proved to be a powerful tool in the field of palaeoclimatology (Creber 1977;Creber and Chaloner 1984; Francis 1984 Francis , 1986, particularly when considering that wood (secondary xylem) yields one of the few accurate and high-resolution records of climatic variability on land. Several features of wood anatomy yield climate data, the most fundamental of which is the presence of growth rings, which are invariably indicative of seasonal growth and usually of a temperate climate (Fritts 1976;Creber 1977;Creber and Chaloner 1984, 1985). The relationship between modern tree rings and the meteorological record provides the basis for studying palaeoclimates using fossil wood (Fritts 1976). This approach has proved successful in the interpretation of past climates (e.g. Jefferson 1982; Francis 1984 Francis , 1986Spicer and Parrish 1990) and forms the basis to this study. The object of this paper is therefore to report a detailed record of Lower and Middle Jurassic (Pliensbachian-Bathonian) palaeoclimate for the Cleveland Basin (which today North Yorkshire, England represents) through growth-ring analysis. This analysis is carried out within the context of gymnospermous wood taxonomy. In the absence of a change in latitude for Britain during this period (Ziegler et al. 1993), temporal variations apparent in the growth characteristics may be related to palaeoclimatic changes. This study succeeds in refining the commonly held perception of a warm, wet and subtropical climate during the Mid Jurassic in the Cleveland Basin (e.g. Alexander 1992; Rawson and Wrigh...

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