When did early plants transform the Earth system?
Supervisors
Main supervisor: Doctor Paul Kenrick (Natural History Museum)
Co-supervisor: Professor Tim Lenton (University of Exeter)
Project enquiries - Email: [email protected] Contact number: +44 (0) 02079425586
Host Institution: Natural History Museum
Project description
The evolution of life has transformed the Earth system, altering the composition of the
atmosphere and oceans, the climate, and the land surface. A key change occurred when plants
and their associated fungi colonised the land surface, during the Paleozoic Era. However, the
timing of land colonisation and its Earth system effects remain uncertain. The textbook view
is that during the Devonian Period (416-360 Myr ago), the evolution of deep-rooting trees
greatly accelerated the weathering of silicate rocks, removing carbon dioxide (CO2) from the
atmosphere and cooling the Earth into the Permo-Carboniferous glaciations. However, the
first plants colonised the land surface long before this, in the Mid-Late Ordovician (470-445
Myr ago), and possibly much earlier (Kenrick et al. 2012). These first plants were nonvascular bryophytes whose extant descendants are mosses, liverworts and hornworts. Existing
models have assumed they had a negligible effect on weathering. However,
experiments have shown that a modern moss can greatly accelerate silicate weathering
(Lenton et al. 2012; Fig.1). Hence the first plants could have lowered atmospheric CO2,
cooling the planet and helping explain the Late Ordovician glaciations (Fig.2,3), which have
long puzzled geologists.
This PhD project will explore how and when the first land plants transformed the carbon
cycle, testing the hypothesis that they caused Ordovician-Silurian global change. You will
start by updating the timeline of land colonisation, bringing together the latest evidence from
molecular clocks, fossils and ancient soils, including compiling existing data on soil profiles
depths over time. Then you will have the opportunity to conduct experiments to quantify the
effect of bryophytes and their associated fungi on weathering, and to make in situ
field measurements of their effects on weathering in Iceland. Chemical analysis will be
supported by the Natural History Museum’s outstanding Imaging and Analysis Centre.
Finally, you will integrate what you have learned into an improved model of the effects of
land colonisation on the Earth system. This will include (i) modelling the effect of early
plants and mycorrhizal fungi on soil profile, chemistry and weathering; (ii) modelling the
global extent of early plant cover and its effects on weathering based on paleo-climate
simulations; (iii) modelling the consequences for global biogeochemical cycles and climate.
We seek a highly motivated candidate wanting to gain an Earth system understanding of the
interactions between plant and fungal biology, soil formation, global biogeochemical cycles
and climate. Candidates should have a relevant degree and necessary skills.
Kenrick, P., et al., A timeline for terrestrialization: consequences for the carbon cycle in the
Palaeozoic. Phil
Trans. B 367 (1588), 519-536 (2012).
Lenton, T.M., et al., First plants cooled the Ordovician. Nature Geoscience 5 (2), 86-89
(2012).
Figure 1: Microcosm experiments used to
quantify the effect of bryophytes on
weathering: (a) moss growing
on granite, (b) control
Figure 2: Global changes during the Ordovician period, which may be explained by the
weathering effects of early land plants.
Figure 3: Model predicted Ordovician
variations in (a) atmospheric CO2, (b)
global temperature, and (c) isotopic
composition of marine carbonates, for:
baseline model with Ordovician
geological forcing and changing solar
luminosity only (red dotted line),
including measured enhancement of
silicate weathering by bryophytes (blue
dashed line), and adding transient
enhancement of phosphorus weathering
by early plants(green solid line).
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