LIMITATIONS ON HOW LAND PLANT EVOLUTION COULD HAVE IMPACTED MARINE EXTINCTION AND DIVERSIFICATION VIA WEATHERING AND NUTRIENT FLUXES

The chemical weathering of silicate rocks is the primary sink of atmospheric CO₂ over geologic timescales and is the ultimate source of phosphorous and other key nutrients to the biosphere. Land plants play a key role in mediating weathering processes and land plant evolution has frequently been entertained as a driver of environmental and biotic evolution on a global scale including in the marine realm. For example, the evolution of bryophytic land plants, of vascular plant trees, and of flowering plants have all been implicated as potential causes of marine invertebrate diversification and/or mass extinction via their impact on nutrient delivery to the oceans or on atmospheric CO₂ and climate. However, the basic requirement of maintaining mass balance in Earth’s exogenic carbon cycle over geologic timescales is frequently violated in the paleontological literature surrounding these ideas. Land plants could not have promoted marine diversity with a permanent increase in nutrient delivery to the oceans. If anything, land plant evolution would have decreased silicate weathering due to their concomitant increase in organic carbon burial. The only route to a long-term increase in weathering-derived nutrient delivery would be via an increase in volcanic outgassing of CO₂ to the system. Land plants could have created shorter term perturbations to the carbon cycle that could have contributed to mass extinctions, but here too mass balance must be considered: maximum impact should be expressed on the order of 10⁵ years and full resolution of the perturbation and return to steady state should occur on the order of 10⁶ years. Thus, many of the events in land plant evolution that have received attention in this context—e.g. the first bryophytes or the first trees—can be excluded from consideration as having been too distant in time from some of the events they were purported to have caused.