Palaeoecology of a Devonian Flood Plain

The Devonian is generally known as the age of fishes, of which various types were proliferating in forms that have long been extinct but were all part of the increasing population of the seas and oceans. On land, plants had been around since the Ordovician period some 90 million years before, but these were relatively small and low growing. These early plants were related to mosses and liverworts – they had no circulatory system, no penetrating root system and were not even developed into separate organs such as stems, roots and leaves. They existed on the surface of the earth limited to areas where moisture was plentiful – they relied on the production of spores to reproduce. Spores are different to seeds in that they do not require fertilisation to grow, but develop into small organisms in a separate generation which then either produce eggs within relatively open organs, or sperm cells that are free swimming. Hence the need for damp environments to enable the fertilisation within this gametophyte generation. The limited root system also meant the plant needed ample moisture to survive.

But as the Devonian period progressed plants developed and evolved. Ferns spread, also requiring moisture and also propagating by spores. The ferns were slightly larger, reaching maybe a metre in height and thereby starting to influence microclimates by more profuse foliage. Still lacking root systems their rhizomes were shallow and had little effect on the ground surface. Fern fronds, such as we are familiar with, did not develop until later, most ferns having frond like organs on the end of stems with no side shoots. However, the middle Devonian saw the fairly rapid development of plants – the development of a tree like habit (arborescence), more extensive and deeper rooting systems with wood like formation in the stems and roots enabled plants to grow taller, side shoots and branching meant a proliferation of foliage and the change from reliance on spores to development of seeds with internal pollination type fertilisation. These changes meant that plants became larger, more stable, less reliant on damp environments, less vulnerable to environmental change, and that they could spread across the land surface, which they did. This explosion of plant growth was significant, it was massive, and it wrought changes that affected life across the globe, prompted global extinctions, transgressions of the sea across the land, and set up a chain of events that is so familiar to our modern day situation of climate change, although reversed.

The atmosphere of the early Devonian was of a very high CO2 content and a rising oxygen content. The oxygen in the atmosphere was a result of the primitive plant life contributing what at first was a poisonous and useless gas – but life forms soon started to employ this constantly increasing level of oxygen – by the Devonian period it was as fundamental to life as it is now. The high carbon dioxide levels were caused by the amount breathed out by the planet in all the volcanic degassing that had occurred and yet with no plant life to speak of to draw it down in photosynthesis and low levels – if any – of chemical weathering. Temperatures globally were high with a high sea surface temperature and no ice at the poles.

The proliferation of plants changed all this.

The sediment that was washing into the Munster Basin was largely a production of physical runoff of seasonal or periodic rain storms transporting the sediment from mountains to plains. The rocks of the mountains were eroded and weathered physically by wind, temperature changes, rain, and gravity. The primitive low growing plants in damp areas contributed little to stabilising the ground surface or contributing organic matter. As plant growth developed several processes took place – root systems anchored sediment within the network of roots. Organic acids secreted by plants, by the increasing amount of organic litter – leaf litter – and the bacteria and fungi that decomposed it permeated the sediment. Soils started to form in greater depths and widely spreading areas, soils with organic acids, supporting life, retaining moisture, providing a new habitat for fungi and bacteria. These acids contributed to the breakdown of minerals such as calcium silicates and magnesium silicates, releasing elements for nutrition of plant life and causing resistant minerals to breakdown into soluble compounds that washed away. Chemical breakdown of rock surfaces became significant contributing an increasingly important amount to the weathering. Such chemical reactions caused increasing draw down of carbon dioxide from the atmosphere – the levels dropped in the second half of the Devonian period from 10 to 20 times above our pre-industrial level of 270 ppm to levels similar to what we are familiar with. The planet cooled – a reverse greenhouse effect.

The increased weathering initially, at the high CO2 levels, gave rise to massive increase in mineral breakdown with an increase in sediment flow. This is probably why the Munster Basin was inundated by such vast amounts of sediment and became the deepest subsiding sedimentary basin in Europe, reaching 7km depth of sediment in the most active part under what is now the Kenmare river, north Beara. Vast loads of sediment washed down by run off incorporating the increasing amount of organic detritus and acids. This was a global change and the influx of such large amounts of nutrients to the oceans caused changes that ocean life was not prepared for. Between 10 and 20 extinction events occurred in the marine environments of the late Devonian, most probably due to the eutrophication of the waters, by oxygen starvation and the creation of anoxic black sludge beds.

But the land was greening. Vegetation expanded out from damp and moist channels and sheltered low spots. Now plants with the new developments could colonise higher drier ground, could expand in forests across the plains reaching heights of 30 to 40 metres, producing vast amounts of leaf litter and forming deep organic soils. Micro climates formed beneath the forest canopies and on a far wider scale the climate changed as forests breathed out moisture, absorbed oxygen and held the earth tightly within its roots.

Towards the end of the Devonian and as the Carboniferous period started the now much lower carbon dioxide levels meant that chemical weathering had slowed down. The amount of sediment released was reduced as a consequence and sediment flow into the Munster Basin slowed. But the subsidence continued. With deposition no longer keeping pace with subsidence, the land surface dropped. The sea to the south gradually flowed in across the land, encroaching further north over the years, changing the surface of the deep terrestrial sedimentary basin into a shallow shelf sea.

NEXT - Formations of West Cork - different sediments became the various formations of rock that we can see today in West Cork, and these enable us to understand the changing environments of deposition.