Mid Paleocene fossil floras and climate from western Scotland

Fossil plants of Paleocene age (62 to 59.7 Ma) are preserved within lava sequences in the Hebrides and Northern Ireland. These lavas, collectively referred to as the British Tertiary Volcanic Group (BTVP), were formed due to rifting in the North Atlantic. The fossil floras represent forests that grew between the lavas in periods of quiescence, during which fluvio-lacustrine environments developed on the lava surfaces. The fossil plants provide information on the composition of the Paleocene vegetation and the climate during that interval. New collections of fossil plants have been made from the Isle of Skye and other collections from Ardtun on Mull were studied. The Allt Mor locality on Skye (60.16 ± 0.45 Ma) has provided the majority of the specimens for this study and represents a pond deposit where broadleaved angiosperm and conifer leaves accumulated. Fourteen angiosperm morphotypes have been identified, many of which show similarities to modern families, including the Platanaceae, Cercidiphyllaceae, Betulaceae, Fagaceae, Juglandaceae, Vitaceae, Cornaceae and Trochodendraceae. Conifer fossils include three ovulate cone types and eight shoot morphotypes attributed to the Cupressaceae and Pinaceae. The Allt Mor assemblage indicates that mixed coniferous forests developed within a palaeovalley, with the vegetation fringing the valley sides and colonising the floodplains of the valley floor. The vegetation of Allt Mor was dominated by climax conifers similar to Metasequoia, Sequoia, Chamaecyparis and Thuja. Broadleaved angiosperms such as Platanites, Trochodendroides, Corylites and "Platycarya cf" dominated the riparian margins and formed an understory of trees and shrubs in the conifer forests. Comparison with Paleocene floras from other Northern Hemisphere localities indicates that the floras of Skye are most similar to those of the Arctic, particularly those of Svalbard, Greenland and Canada, and were thus part of the Arcto-Tertiary floras of the northern high latitudes. Palaeoclimate estimates derived from the fossil angiosperm leaves indicate that the climate of Skye had a mean annual temperature (MAT) of -5°C to 9°C, was seasonal with warm summers (warm month mean 19.0°C to 25.8°C) and cool to cold winters (cold month mean -3.0°C to 4.2°C). Comparisons with the flora of Ardtun on Mull (60.5 ± 0.3 Ma) reveal that, although these floras share many of the same taxa, the flora is different, with angiosperms dominant, and the conifers that are common in the Allt Mor assemblage are rare or absent. The Ardtun flora also contains thermophilic plants such as Camptodromites major, C. multinervatus and Amentotaxus gladifolia, which are not present on Skye. Palaeoclimatedata from these fossil plants indicate significant cooling during the mid-Paleocene, confirmed by other marine and terrestrial proxies, marking a cool climate phase in the middle of the otherwise globally warm Paleocene climate.