The Geological History and Ecosystem Dynamics of an Exceptionally Well-Preserved in situ Carboniferous Flora in Brymbo, Wrexham, North Wales.

Abstract

This research project provides a detailed analysis of the Brymbo fossil forest (Wrexham, North Wales), an exceptionally preserved Carboniferous flora from the middle Duckmantian substage, approximately 314 million years ago. The study employs a multi-disciplinary approach including paleoecological, sedimentological, geochemical and isotopic techniques to advance our understanding of Carboniferous coal-forming environments. The quality of preservation of plant remains at Brymbo has been attributed to rapid burial during flooding, which deposited large quantities of sediment and organic debris. Fossils are preserved as thin carbon films within siderite concretions and as infill casts and compressions. Early precipitation of these minerals, possibly mediated by bacterial biofilms, was crucial for preservation. Geochemical analysis of siderite indicates two cementation phases with distinct magnesium contents, shedding light on the site’s complex pore fluid evolution and diagenetic history. Isotopic data shows a trend potentially explained by evolving surface water conditions during the precipitation of one chemically distinct siderite throughout the entire concretion. However, once combined with evidence from other techniques, this trend was found to be due to a mixing effect from sampling of the two siderite phases, which themselves are both zoned on a microscopic level. Once the end member siderite phases were isolated via extrapolation, I found that the early forming siderite isotopes recorded a purely meteoric water source and precipitation temperatures around 30°C. The later phase was found to have formed much deeper (~1km) and therefore not useful for environmental purposes. These findings, despite highlighting the complexity of studying these minerals, suggest the potential of siderite concretions in paleoenvironmental studies and propose the need for thorough, multiple method investigations in understanding siderite mineralisation and its uses in similar sites. Isotopic analysis of organic carbon from plant fossils used a novel extraction method involving acid dissolution and density separation. This study integrated geochemical data with morphological analysis to explore the ecological strategies of Carboniferous plants under environmental stresses. The results showed enriched δ13C values in lycopods, equisetidae, and ferns, indicating these plants experienced some water stress. Particularly, lycophytes displayed broad isotopic variations, suggesting adaptive responses to fluctuating water availability. This adaptation, previously unreported, is crucial for understanding Brymbo’s dynamic paleoecology. Water stress in a ‘typical’ Westphalian coal forming forest such as Brymbo is a phenomenon not previously reported and has significant implications for palaeoecology reconstructions going forward. This isotopic insight not only illuminates the paleoenvironmental conditions at Brymbo but also expands our knowledge of plant adaptation to water stress during the Carboniferous. The implications for understanding ecological dynamics in similar fossil forests globally are significant, as this emphasizes the role of isotopic data in complementing traditional paleobotanical studies for a nuanced view of ancient plant ecologies and their climate responses. The decline of arborescent lycopods at the Westphalian-Stephanian boundary was examined, suggesting climatic changes may have impacted reproductive success more than growth. Despite their adaptability, they could not compete with the other plant groups and their initial reduction in numbers increased canopy closure from rapidly proliferating ferns and Equisetidae, possibly reducing regional rainfall, and contributing to the arborescent lycopods’ further decline and eventual extinction. Experimental studies on extant horsetails, analogues for Carboniferous calamites, demonstrated that variation in water availability could result in a spectrum of isotopic values within an individual taxon inside a single plant community, mirroring Brymbo's fossil data. This supports the use of isotopic analysis in paleoecological research and contributes to our understanding of ancient plants’ interactions with their environments. The findings are crucial for reconstructing past ecological and environmental conditions and have relevance for current issues of climate change and ecological resilience.

Date of Award	4 Jul 2025
Original language	English
Awarding Institution	Bangor University
Sponsors	Stori Brymbo & Kess II East
Supervisor	Alexander Papadopulos (Supervisor), Simon Creer (Supervisor) & Barry Lomax (Supervisor)