Climatic implications of Ginkgoites waarrensis Douglas emend. from the south polar Tupuangi flora, Late Cretaceous (Cenomanian), Chatham Islands

Abstract

Abstract The flora of the Cenomanian–Turonian (ca. 96–90 Ma) Tupuangi Formation, Chatham Islands, New Zealand, was inhabiting a region well within the south polar circle (~ 70–80° S) during the early Late Cretaceous, an interval characterised by extreme global greenhouse conditions. The Tupuangi flora offers a unique perspective into an ecological and environmental setting which has no extant analogue, whilst providing proxies of polar palaeoclimatic conditions during a phase of extreme global warming. Ginkgoites waarrensis Douglas, 1965 (emended herein), a species known previously from a single occurrence in Australia, is an abundant element of the Tupuangi flora. Forty-five leaf samples from three localities are reported, and a systematic treatment of this species revealed a wide morphological range. In contrast to the exclusively riparian niche of more recent members of Ginkgoales, associated sedimentological and palaeoecological data suggest that this species had an ecological preference for regularly disturbed, coastal deltaic settings. Herein, we review the geographic and stratigraphic distributions of Cretaceous Gondwanan ginkgoalean leaf taxa. An increasing diversity of this group from the Early Cretaceous to the early Late Cretaceous supports a broader trend of floral provincialisation throughout this interval, most likely driven by concurrent global transgression and active tectonic extension across southern Gondwana. Carbon dioxide has been inferred as a primary proximate cause of the mid-Cretaceous global greenhouse. The leaf cuticles of Ginkgoites waarrensis were utilised to approximate atmospheric carbon dioxide (pCO2) during the Cenomanian. Stomatal index (SI) data were collected from ten specimens, and the stomatal ratio method yielded a semi-quantitative pCO2 estimate of 1150–1350 ppmv, which is consistent with modelled and proxy estimates of the Cenomanian. The present study explores the inherent limitations of the transfer function method for estimating CO2 when applied to taxa with very low SI values, such as G. waarrensis. In addition to pCO2, temperature and irradiance are identified as environmental variables which may have systematically promoted the low SI of G. waarrensis, but their combined influence is likely mitigated by the relatively high temperature of this region during the mid-Cretaceous and the high summer insolation at polar latitudes.