Land plant transformation of the terrestrial biosphere

5.10. Land plant transformation of the terrestrial biosphere#

Professor: William Macmahon (Department of Earth Sciences)

Learning objectives:

  • To understand how land plant evolution influenced Earth’s climate and oxygenation state

  • To understand how weathering intensity changed in synchrony with evolving land plants

  • To understand the sedimentological impact of evolving land plants

The Paleozoic greening of the continents#

  • First roots: Lower Devonian

  • First wood: Lower Devonian

  • First trees: Middle Devonian

  • First forests: Middle Devonian

  • Important expansion and diversification of ‘the critical zone’ (Part 2)

  • The critical zone is the living boundary layer where rock, soil, water, air, and living organisms interact. These complex interactions regulate the natural habitat and determine the availability of life-sustaining resources.

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Fig. 5.45 History of plant colonisation of the continents. From Strullu‐Derrien et al. (2018).#

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Fig. 5.46 The critical zone. Courtesy of Catalina-Jemez, Critical Zone Observatory.#

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Fig. 5.47 Plants now dominate terrestrial biomass. Consequently, surface processes on Earth are always impacted by vegetation, either directly or indirectly. From Bar-on et al. (2018).#

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Fig. 5.48 Vegetation controls parameters that influence river channel patterns and therefore sedimentary facies. From Davies & Gibling (2010).#

Pre-Vegetation Earth Alluvium:#

  • 1000’s metres of sands

  • Almost no mud

  • Architectural elements = dominantly tabular sandstones

  • Laterally mobile channels \(\pm\) high aspect ratios

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Fig. 5.49 A schematic of a planet’s sedimentary landscape without life. From Davies et al. (2011).#

Increase in Lateral Accretion Sets following the evolution of land plants

  • Stabilized by roots?

  • Stabilized by above-ground plant effects (‘baffling’)?

  • More mud?

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Fig. 5.50 Channel deposits in the field. From McMahon & Davies (2018).#

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Fig. 5.51 The geological expression of plants. From Davies & Gibling (2010).#

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Fig. 5.52 The proportion of mud in sedimentary successions over time. From McMahon & Davies (2018).#

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Fig. 5.53 A schematic of a planet’s sedimentary landscape shaped by life. From Davies et al. (2011).#

Unvegetated Martian meanders#

  • Lower acceleration due to gravity reduces the flow resistance to maintain a single threaded river(?)

  • More cohesive mud in hydrologically closed impact crater basins(?)

  • Scale differences(?)

  • Incorrect interpretations of pre-vegetation Earth alluvium(?)

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Fig. 5.54 Martian meanders. From Matsubara et al. (2015).#

Plants intensify chemical weathering#

Plants alter the terrestrial silicate weathering feedback – one of the dominant cycles which regulates temperature on Earth. The basic idea of this feedback is that any increases in CO\(_2\) lead to increased surface temperatures, acceleration of the hydrological cycle, and larger fluxes of silicate chemical weathering, which in turn lead to greater CO\(_2\) removal from the ocean-atmosphere system through continental weathering.

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Fig. 5.55 Carbon cycle schematic. McMahon (unpublished).#

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Fig. 5.56 The local effect of plants on the carbon cycle. McMahon (unpublished).#

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