Evolutionary rise of deep-rooting forests and enhanced chemical weathering: Quantitative investigations into the current paradigm

The co-evolution and geographical spread of trees and deep-rooting systems is widely proposed to represent the 'Devonian engine' of global change that drove the weathering of soil minerals and biogeochemical cycling of elements to exert a major influence on the Earth's atmospheric CO2 history. If correct, this paradigm suggests the evolutionary appearance of forested ecosystems through the Devonian (418-360 Myr ago) constitutes the single most important biotic feedback on the geochemical carbon cycle to emerge during the entire 540 Myr duration of the Phanaerozoic. Crucially, no link has yet been established between the evolutionary advance of trees and their geochemical impacts on palaeosols. Direct evidence that one has affected the other is still awaited, largely because of the lack of cross-disciplinary investigations to date.

Our proposal addresses this high level 'earth system science' challenge. The overarching objective is to provide a mechanistic understanding of how the evolutionary rise of deep-rotting forests intensified weathering and pedogenesis that constitute the primary biotic feedbacks on the long-term C-cycle. Our central hypothesis is that the evolutionary advance of trees left geochemical effects detectable in palaeosols as forested ecosystems increased the quantity and depth of chemical energy transported into the soil through roots, mycorrhizal fungi and litter. This intensified soil acidification, increased the strength of isotopic and elemental enrichment in surface soil horizons, enhanced the weathering of Ca-Si and Ca-P minerals, and the formation of pedogenic clays, leading to long-term sequestration of atmospheric CO2 through the formation of marine carbonates with the liberated terrestrial Ca.

We will investigate this research hypothesis by obtaining and analysing well-preserved palaeosol profiles from a time sequence of localities in the eastern North America through the critical Silurian-Devonian interval that represents Earth's transition to a forested planet. These palaeosol sequences will then be subjected to targeted geochemical, clay mineralogical and palaeontological analyses. This will allow, for the first time, the rooting structures of mixed and monospecific Mid-Devonian forests to be directly linked to palaeosol weathering profiles obtained by drilling replicate unweathered profiles. Weathering by these forests will be compared with the 'control case' - weathering by pre-forest, early vascular land plants with diminutive/shallow rooting systems from Silurian and lower Devonian localities. These sites afford us the previously unexploited ability to characterize the evolution of plant-root-soil relationships during the critical Silurian-Devonian interval, whilst at the same time controlling for the effects of palaeogeography and provenance on palaeosol development. Applying geochemical analyses targeted at elements and isotopes that are strongly concentrated by trees at the surface of contemporary soils, and which show major changes in abundance through mineral weathering under forests, provides a powerful new strategy to resolve and reconstruct the intensity and depth of weathering and pedogenesis at different stages in the evolution of forested ecosystems.

The project is tightly focused on "improving current knowledge of the interaction between the evolution of life and the Earth", which represents one of the three high level challenges within NERC's Earth System Science Theme.

Our proposed research project addresses this long-standing 'big science' question. We anticipate that our focused multidisciplinary project will, therefore, be the subject of considerable interest not only to a broad spectrum of the scientific community but also to the 'next generation' of researchers in Schools and Universities. Engaging younger generations is especially important as the Earth sciences receive very limited coverage in schools.

Deliverables.
The main deliverables of our project are open-access public archives of data and fossil plant/palaeosol materials, underpinned by methodological and data standards. These are likely to have impacts in both the academic and industrial sectors. A further major deliverable will be significant outreach activities aimed at inspiring and attracting young people into interdisciplinary scientific careers by showing how such activities shed new light on Earth's dynamic history.

Beneficiaries and specific users of this research.
Beneficiaries of the research will include a cross-disciplinary range of scientists from the international academic community, government funded research agencies, and stakeholders. These groups include geochemists, plant physiologists, mycologists, palaeontologists, Earth system modellers, and palaeoceanographers. Our new findings will benefit those sectors involved in the deep-time modelling of geochemical cycles, atmospheric composition and climate. Modellers will benefit from the first quantitative estimates of plant weathering during this critical period to better constrain our understanding of Earth's CO2 history. This diverse array of scientists will benefit from the provision of new data in a field that is traditionally been based on theoretical research rather than driven by empirical research findings.

Impacts will be achieved through :

(1) Dedicated Website. (2) Outreach activities in schools. (3) Outreach activities to the general public and (4) Outreach activities to graduate students in
i) Summer School. ii) MSc course. (5) Industry forum. We will demonstrate the potential applications of our data and materials archives to those involved in the hydrocarbon exploration and development industry.

See Pathways to Impact Plan.