Abstract Detail


Boyce, C. Kevin [1], Ibarra, Daniel E. [2], Nelsen, Matthew [3], DAntonio, Michael [4].

Weathering, nutrient fluxes, and the long-term consequences of coal for the evolution of nitrogen-based symbioses.

Phosphorous and nitrogen are most likely to be limiting nutrients in terrestrial ecosystems. Since the later Mesozoic, the many parallel evolutions of nitrogen-based symbiotic associations—from root nodules to some lichens to ectomycorrhizal associations to termite mounds—suggest nitrogen limitation may have become more widespread over time and, by comparison, that the relative availability of phosphorous has increased. Nitrogen fixation draws from the atmosphere, but phosphorous is ultimately sourced from the chemical weathering of silicate rocks. Land plants play a key role in mediating weathering processes and land plant evolution has frequently been entertained as a driver of increased weathering rates, however the potential for weathering rates to change is highly constrained: silicate weathering is also a principal sink of CO2 and mass balance in Earth’s exogenic carbon cycle must be maintained over geologic timescales. If anything, Paleozoic plant evolution would have decreased globally integrated silicate weathering fluxes due to their concomitant increase in organic carbon burial. Both rock weathering and coal burial are carbon outputs that together must balance inputs to the system, e.g. volcanic outgassing of CO2 and subaerial organic matter oxidation. If inputs are constant, then an increase in coal production requires a decrease in rock weathering. Nonetheless, the advent of coal production represents a unique and unidirectional event in Earth history that we propose fundamentally changed the carbon cycle with major implications for nutrient fluxes over the Phanerozoic. Prior to the mid-Paleozoic, terrestrial sediments would have contained little organic matter, but the potential holding capacity of terrestrial rocks would have been progressively reached with the late Paleozoic peak of coal deposition. An equilibrium would then have been approached over the course of a continental rock cycle of 100 to 150 million years; coal production continues, but organic matter deposition would largely be offset by organic erosion since the mid-Mesozoic. This Phanerozoic trend in coal deposition and erosion—the one-time occurrence of a previously empty reservoir being progressively filled—would have strong implications for silicate weathering: vascular plant evolution would have initially suppressed weathering rates, but the later Mesozoic would then have seen the reestablishment of higher weathering rates and, thus, higher phosphorous fluxes last seen in the early Paleozoic. We therefore suggest that much of the convergent evolution of nitrogen-based symbioses and the Cretaceous flourishing of terrestrial life may have, at least in part, been a long-term consequence of Carboniferous and Permian coal deposition.

1 - Stanford University, Geological Sciences, 450 Jane Stanford Way, Bldg. 320, Stanford, CA, 94305, United States
2 - University of California, Earth and Planetary Science, 307 McCone Hall, Berkeley, CA, 94720-4767, USA
3 - Field Museum, Negaunee Integrative Research Center - Department Of Insects, 1400 S. Lake Shore Drive, Chicago, IL, 60605, United States
4 - Stanford University, Geological Sciences, 450 Jane Stanford Way, Building 320, Stanford, CA, 94305, United States

none specified

Presentation Type: Oral Paper
Session: PAL5, Paleobotany III: Patterns and trends
Location: Virtual/Virtual
Date: Tuesday, July 28th, 2020
Time: 4:00 PM
Number: PAL5005
Abstract ID:479
Candidate for Awards:None

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