Fay, CA; (2014) Mid-Cretaceous pCO₂, carbon-cycling and the rise of the flowering plants. Doctoral thesis , UCL (University College London).

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Abstract

The mid-Cretaceous (Aptian–Cenomanian) climate was characterised by steadily increasing temperatures likely driven by high atmospheric CO2. The climate system was dynamic: throughout this interval there were several dramatic carbon cycle perturbations (of 1–2 Myrs duration) due to initiation of marine anoxia (OAEs) resulting in burial of organic carbon. However, pCO2 values and trends are generally poorly constrained for much of this time interval. During the mid-Cretaceous, angiosperms (flowering plants) underwent a rapid poleward diversification and radiation; by the Cenomanian they comprised around 70% of floras. However, hypotheses detailing the competitive replacement of incumbent floras by advantageous angiosperm adaptations do not fully explain the timing and nature of early angiosperm evolution. This thesis provides a record of Albian–Cenomanian carbon cycling and explores the role of climate change and pCO2 decline (CO2 starvation hypothesis) as forcing factors on angiosperm radiation. This is achieved using fossil material from the Nuussuaq Peninsula, West Greenland. Carbon isotope stratigraphy constrains the stratigraphic age (Middle Albian-Cenomanian) and identifies two intervals of carbon cycle disturbance. Macerated leaf cuticle and palynological studies reveal detailed floral assemblages (in which angiosperms, including Eudicots, were poorly represented but present throughout) and unprecedented ecological information. New pCO2 estimates for the Middle Albian are generated from stomatal density measurements, which, integrated with other similar datasets, suggest average pCO2 in the Aptian-Early Cenomanian of 575 ppm with a decline of ~150 ppm in the Middle Albian. The subsequent rise in pCO2 through to the Late Albian coincides with a 30 % increase in angiosperm abundance and increased global temperatures; strongly suggesting the role of climate on angiosperm radiation. However, comparisons of vein density, stomatal conductance, stomatal density and pore length between fossil and extant angiosperms reveals angiosperms already possessed advantageous adaptations expected from a low pCO2 climate by the mid-Albian.

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