Palaeozoic co-evolution of rivers and vegetation: a synthesis of current knowledge
Introduction
The term “evolution” has generally been applied to living organisms, following the work of Charles Darwin and the growth of evolutionary biology. In an essay written late in his life and based on a contemporary understanding of biology, the famous fluvial geomorphologist Luna B. Leopold (1915–2006) argued that the behaviour and morphology of rivers have parallels with biological systems (Leopold, 1994). He pointed out that although rivers as a whole show predictable responses to external factors, each river system is unique and its individual reaches have a natural variability. Thus, a river system is analogous to a species composed of a population of reaches, the variability of which reflects current processes and the system's geological history. The river system might evolve gradually or through abrupt events.
The parallel with biological systems as set out by Leopold largely represents the way in which organised systems respond to external stimuli. He envisaged river evolution mainly as a gradual response to changes in factors such as climate. Of course, in the absence of genetic material in fluvial systems, the parallel should not be taken too far. However, Leopold's essay implies that the term “evolution” may be applied legitimately to landforms and other essentially physical systems, raising some interesting questions. Might radical changes to the Earth system influence virtually all river “species” worldwide, promoting a higher order of river evolution? Individual rivers might presumably become “extinct”, but might entire styles of river behaviour and morphology also become extinct? In contrast to the irreversible nature of biological evolution, might “primitive” fluvial styles reappear if radical changes took place in the physical environment and the biosphere? And over what timescales might gradual or sudden river evolution take place?
Since the late 1960s, research by numerous authors has established that, as a consequence of the evolution of terrestrial vegetation, river systems as a whole evolved dramatically through the ∼240 million years of the Palaeozoic Era, especially during the ∼120 million years of the Devonian and Carboniferous periods (Schumm, 1968, Cotter, 1978, Went, 2005, Corenblit and Steiger, 2009, Davies and Gibling, 2010a, Davies and Gibling, 2010b, Davies and Gibling, 2013, Gibling and Davies, 2012). Among many effects, plants influenced rock and sediment weathering, the grain-size spectrum supplied to rivers, landscape stability, the roughness of sediment surfaces, and the capacity for sediment storage, as well as the composition of the ocean and atmosphere (Algeo and Scheckler, 1998, Berner, 2006, Davies and Gibling, 2010a).
We provide here a synthesis of current knowledge of the Palaeozoic evolution of river systems, first quantified by Cotter (1978). The account is based on a worldwide literature compilation of 330 alluvial formations of Cambrian to Pennsylvanian age, set out in three papers (Davies and Gibling, 2010a, Davies and Gibling, 2011, Davies and Gibling, 2013) and referred to below as “the compilation”. The reader is referred to these papers for approaches used and the limitations of the results. Although it is probably not possible to determine precisely when and where particular fluvial styles first appeared or disappeared, the geological record provides a broad representation of events in river evolution, which are set out following a summary of key events in the evolution of terrestrial vegetation (Fig. 1).
Section snippets
Palaeozoic vegetation
From the early Precambrian onwards, cryptobiotic films and crusts inhabited the terrestrial realm and were probably more widespread than records indicate (Horodyski and Knauth, 1994, Prave, 2002, Labandeira, 2005). Eukaryotic material from the Stoer Group and Diabaig Formation (Torridonian Sandstone) of Scotland indicates that, about one billion years ago, these organisms inhabited a range of freshwater settings (Strother et al., 2011). Although it is difficult to gauge their effect on
Cambrian to Silurian rivers: sheet-braided systems
Trunk river systems of this age are universally preserved as sandstone bodies composed of relatively thin sheets (Fig. 2A), prompting Cotter (1978) to term them “sheet-braided”. In one of the best exposed examples, the Cambrian Alderney Sandstone of the Channel Islands, the sheets are a few decimetres to metres thick and are predominantly trough cross-bedded with some accretionary bar forms but with minimal mudstone beds, fragments or matrix (Todd and Went, 1991, Davies et al., 2011, Went, 2013
Devonian to Mississippian rivers: the rise of channelled-braided and meandering systems
By the late Silurian, the internal architecture of many fluvial deposits had become more complex, with more lensoid units and an increased proportion of mudstone beds and clasts (Fig. 2B) – the “channelled-braided” systems described by Cotter (1978). The compilation contains few Silurian formations and it is uncertain when channelled-braided rivers became common, although they are certainly present within some early Old Red Sandstone-type successions, such as the Ludlovian Stubdal Formation of
Pennsylvanian rivers: the rise of anabranching systems
The Early Pennsylvanian marks the rise of two types of multi-channel system. In redbed formations laid down under conditions of seasonal flow and lowered water tables, relatively narrow channel-sandstone bodies encased in mudstone are prominent for the first time (Davies and Gibling, 2011) (Fig. 2D). The channel bodies have a low width:thickness ratio, typically less than about 30:1 (ribbons and narrow sheets: Gibling, 2006). Characterised by stable banks and predominant vertical accretion,
Discussion
Fig 1 suggests a causative link between the Palaeozoic rise of vegetation and the evolution of fluvial style. The evidence is circumstantial and depends on an understanding of the influence of vegetation in modern systems and analogue models (Gibling and Davies, 2012). Nevertheless, the stepwise and unidirectional Palaeozoic evolution of rivers cannot entirely be attributed to such extrabasinal factors as tectonism, climate and sea-level, all of which must have influenced rivers since the
Conclusions
The Palaeozoic evolution of vegetation forced a profound evolution of river systems and terrestrial landscapes over little more than 120 million years – a remarkably rapid development for one of the most profound events in Earth history. A range of new fluvial planforms, floodplain elements, and physical processes emerged as river systems co-evolved with plants and animals, and the generation of new ecospace may have been a major factor in promoting biological evolution. Entirely new classes of
Acknowledgments
We are grateful to the Geologists’ Association and The Devonshire Association for an invitation to present these results at a conference in Exeter, U.K. in 2012, and particularly thank David Bridgland, Jenny Bennett and Sarah Stafford for organising the conference. Two anonymous reviewers made suggestions that greatly improved the manuscript. The research was funded mainly from a Discovery Grant to MRG from the Natural Sciences and Engineering Research Council of Canada. We thank many
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Current address: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK.