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Dr Alan Channing

Hot spring environments and ecosystems through time
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Communities and palaeo-ecophysiology of fossil hot spring ecosystems through time

A three year NERC funded investigation of the ecology and ecophysiology of animals and plants preserved in fossil hot spring ecosystems.

Terrestrial hot springs have an important role in elucidating past records of life, in that they fossilize complete ecosystems in situ and to the cellular level. Unlike most other terrestrial clastic rocks e.g. those deposited in fluvial channels or lakes which often contain transported plant material, they thus provide direct evidence of the chemical and physical environments in which organisms lived.

Plantago maritima
growing and being fossilised by warm, salty and silica-rich water flowing from Geysir hot spring on Iceland.

The Lower Devonian Rhynie Chert of Aberdeenshire, Scotland is the oldest fossiliferous hot spring deposit yet found. It contains by far the most complete record of an early terrestrial ecosystem. The organisms preserved include microbes (such as fungi and cyanobacteria), plants and animals (spiders, crustaceans and myriopods). The chert provides crucial information on life cycles (e.g. plant reproduction and development) and interactions of elements of the biota (fungal/algal/bacterial symbioses, fungal degradation of plants and animal/plant food-webs).

A Rhynie arthropod surrounded by a meshwork of delicate microbial filaments  preserved between the stems of silicfied plants  stems.

Traditionally the plants and animals of the Rhynie Chert have been viewed as part of a normal dry-land (as opposed to wetland) terrestrial ecosystem. However, observations of modern hot spring systems reveal that geological and physicochemical properties exert a strong influence on ecosystem composition, diversity and fossilization. Sinter and travertine deposition occurs dominantly around boiling, alkaline springs, with waters rich in heavy metals and with brackish salinity. Plants and animals colonising these environments thus have to withstand extremes of pH, temperature, salinity and heavy metal concentrations.

Wetland forming at the margin of the sinter apron created by water flowing from Porkchop Geyser, Yellowstone National Park.
Flooding of the roots of pines near the spring has killed them and they have been replaced by salt and heavy metal tolerant wetland plants.

My investigations of modern hot springs and their ecosystems at Yellowstone National Park, Wyoming, USA, the Taupo Volcanic Zone, New Zealand, El Tatio, Chile and on Iceland have clarified that hot spring vegetation, and in particular that part of the vegetation that has a good to high chance of being preserved by permineralisation, is dominated by plants that are hydrophytic or flooding tolerant as well as salt, alkalinity and heavy metal tolerant. They are thus physiologically stressed. For example: at Yellowstone the vegetation is dominated by plants found more widely distributed in the region around saline-seeps; Iceland hot spring vegetation is dominated by coastal marsh or dune-slack plants; whilst at El Tatio, Chile the vegetation is dominated by species found in abundance in Andean salt lakes.

Geothermal wetland at Big Blue Hot Spring, Yellowstone NP. Mesophytic forest vegetation grows only on high ground beyond the wetland showing strong partitioning of the flora.

At the generic, and in some cases at the species level, modern hot spring plants have broad regional or even global distributions principally in stressed environments, but are out-competed in mesophytic environments. The same observations are true for other groups including diatoms, charophytes and crustaceans all of which are also typical of brackish waters associated with coastal marshes or ephemeral evaporation dominated inland water-bodies. Conversely, in modern hot spring basins “normal” mesophytes cannot colonise hot spring influenced substrates and are either completely absent or represented only by transported leaves and pollen. Thus in modern hot spring environments, there is a strong bias to the preservation of ecosystems with highly-adapted ecophysiologies that are atypical and therefore not representative of most components of the modern terrestrial biota.

The salt marsh plant Triglochin maritimum growing on the edge of the vent pool of Big Blue Hot Spring, Yellowstone NP.

Recent advances in our understanding of the geochemistry, sedimentology and stratigraphy of the Rhynie deposit have resulted in a reconsideration of depositional environments and preservation. The former is now viewed in the context of a wetland, cyclically influenced by inputs from geothermal and riverine sources; whilst taphonomic experiments have shown that the vascular plants were growing in low temperature wetlands. Such studies  lead to the hypothesis that the Rhynie Chert flora was also not typical of surrounding mesophytic vegetation. However elements of the flora may have been present elsewhere in stressed habitats on the Old Red Sandstone Continent, such that plants may have been pre-adapted to the much rarer hot spring wetlands. Testing of such hypotheses is hampered by, 1) the taphonomic bias introduced by the essentially parenchymatous construction of the Rhynie Chert plants, 2) the absence of non-hot spring assemblages from unequivocal coastal or playa lake settings and 3) with the exception of a lycophyte, the absence of close living relatives. Such limitations question the value of a uniquely preserved ecosystem to broader evolutionary and ecological studies and prompted me to search for more recent examples in the fossil record.

San Agustin, Santa Cruz Province, Argentina. A Jurassic hot spring complex with an entombed ecosystem.

In 2005 a collaborative effort by the hot spring research group of Cardiff University and exploration geologists and palaeobotanists from University National La Plata (UNLP), Argentina discovered richly fossiliferous Jurassic hot spring deposits within the vast Deseado Massif epithermal field of Santa Cruz Province, Patagonia. Based on this success I have developed links with exploration geologists and mining companies around the world who are using advanced exploration techniques to search for epithermal systems, the gold/silver/mercury bearing plumbing systems often preserved in the shallow subsurface below hot spring sinter and travertine deposits. This has resulted in the discovery of new major sinter and travertine deposits in Patagonia. The major thrusts of this project was to undertake detailed investigations of the geochemistry, sedimentology, palaeoenvironments and ecosystems contained in the Jurassic hot spring influenced environments of Patagonia, to investigate how Mesozoic fossilised hot spring ecosystems were adapted to their environment relative to plants preserved in “more normal” clastic and volcaniclastic deposits coeval with the hot spring environments and where vegetation contains gymnosperms, with some extant representatives.

San Agustin chert with abundant plant fossils.

The second aim of the project is to make initial exploratory palaeontological investigations of the three additional epithermal gold/silver provinces (in Nevada, USA; Heilongjiang Province, NE China and Massif Central, France) where hot spring deposits had been discovered by exploration geologists. Individually each of these offer the opportunity to validate our observations and findings in Patagonia and collectively the chance to study how hot spring ecosystems have developed through geological time and to identify if a set of similar ecophysiological and behavioural adaptations are present in widely divergent plant lineages.

A 350 million year old sinter apron surface with a silica encrusted stem of a Late Devonian lycopophyte, Drummond Basin, Queensland, Australia.

To date I have studied fossil hot spring deposits from the Late Devonian/Early Carboniferous hot spring environments of the Drummond Basin Australia, Late Carboniferous deposits of the Massif Central France, Jurassic of Santa Cruz province, Argentina and Tertiary to Quaternary deposits of the US, New Zealand and Iceland. This work has provided a wealth of new data on hot spring ecosystem and demonstrated that wetlands are by far the most important hot spring sub-environment for plant preservation and that wetland plants that have living relatives that are tolerant of salinity and metal stress dominate hot spring ecosystems at least as far back as the Mesozoic.

So how much evidence is there out there? and what does it tell us? The following pages give brief details of some of the hot spring deposits and entombed ecosystems that I have studied.