A Tale of Two Theses: High-Resolution Reconstructions of

Atmospheric CO2 in Deep Time Using Plant Stomata

Garland Upchurch, Texas State University

M.S. Students: Jonathan Richey and Joseph Milligan Thesis advisers: Garland Upchurch and Dana Royer Co-authors on papers listed later Introduction

• Paleobotany provides important information on paleoclimate. – Physical climate

– Past CO2 • Deep time studies typically have coarse stratigraphic resolution – Macrofossils – Sporadic occurrence – Long-term trends

• Today—plant fossils can provide data on CO2 in deep time with high stratigraphic resolution – Carbon cycle perturbations and extinctions of the deep geologic past. Plants and CO2 • Leaf function is an adaptive compromise. – Maximize carbon gain – Minimizing water loss

• Diffusion, CO2 and H2O – Stomata – Open and close • Today’s atmosphere

Cross section of a leaf showing the – 100 moles H2O: 1 mole CO2 movement of CO2, H2O & O2 – More favorable ratio under high CO2 • Reduce conductanceReduce water loss yet gain carbon • Long term: Changing number and dimensions of stomata Stomatal Methods for Paleo-CO2

• Stomatal Index – Woodward (1987) – Reduction in stomatal number on leaf with elevated CO2 – Species specific • Living fossils

– Levels out at high CO2

• Mechanistic stomatal model – Franks et al. (2014) – Applicable to all fossil species – Measure structural and chemical features on fossil – Calculate physiological parameters Stomatal Index for Ginkgo biloba. Solid – Estimate paleo-CO2 circles are historic records, open circles

are plants grown under elevated CO2. The Franks et al. Mechanistic Model

• Based on photosynthesis model of Farquhar, von Caemmerer, and co-workers • Measure stomatal size, density, fraction of leaf surface

– Calculate gc(tot) • Measure δ13C – Calculate Δleaf

– Calculate Ci/Ca

• Calculate Ca – Assumptions

• Assimilation rate: An • Temperature of photosynthesis – 19–26°C • Mean error ~28% – Extant plants – Royer et al. (2019, Clim. Past. 15: 795– 809) The Two Theses

• M.S. Thesis 1: Jonathan Richey, Texas State University – Supervisor: Garland Upchurch – Cretaceous: Albian-Cenomanian boundary • Ocean Anoxic Event 1d

• CO2 levels during major carbon cycle perturbation • Both Stomatal Index and Mechanistic Stomatal Model – Richey et al., 2018, Earth and Planetary Science Letters 491: 172–182.

• M.S. Thesis 2: Joseph Milligan, Wesleyan University – Supervisor: Dana Royer – Cretaceous-Paleogene boundary

• Evaluate evidence for major CO2 increase following Chicxulub Impact • Mechanistic Stomatal Model (vs. earlier Stomatal Index estimates) – Milligan et al., 2019, Geophysical Research Letters 46: 3462–3472. Thesis 1: Richey et al. (2018) Rose Creek Pit: Albian-Cenomanian Boundary

• Near Fairbury, NE • Leaf macroflora – Fine venation – Anatomy of epidermis • Cuticle • Leaf debris – Identified by cuticular anatomy – 15-cm intervals • High-resolution stratigraphy – Palynology – Carbon isotope stratigraphy – Sequence stratigraphy • Methods – Stomatal Index • Published calibrations • Extant Lauraceae – Mechanistic Stomatal Model Pandemophyllum: Lauraceae

• Venation HB • Oil cells • Epidermis – Hair base structure • (HB) – Stomata (gas-exchange HB pores) • Scale-shaped thickenings (arrows)— SC look like hard taco shells turned to side SC • Paired specialized cells (SC) Hiatus in section: Paleosol horizon: 300–500 kyr Correlates with eustatic drop in sea level at the Albian- Cenomanian boundary Marine Terrestrial: Rose Creek

Positive phase of isotopic excursion missing: 300–500 kyr Rose Creek: Important Results

• Both Stomatal Index and Mechanistic Stomatal Model indicate

increased CO2 during the early part of OAE1d, followed by a decrease.

– Mechanistic model gives higher CO2 estimates than Stomatal Index. – Stomatal index: 326–602 ppm, rising to 512–752 ppm • Calibration dependent – Mechanistic model: 464–805 ppm, rising to 1032–1598 ppm • Dependent on input parameters

• CO2 increases after the onset of the negative phase of carbon isotopic excursion and during return to pre-excursion values. • Similarity between OAE1d, OAE 1a, and Toarcian Ocean Anoxic Event – Common underlying cause Thesis 2: Milligan et al. (2019)

Plants used in this study. Living Ginkgo (top left) Fossil Ginkgo stomata (bottom left) Fossil fern leaflet (right) and its stomata (far right) Cretaceous-Paleogene Boundary: The Problem

• Previous Stomatal Index work – Beerling et al. (2002)

– Major increase in CO2 immediately after K-Pg event • 350–500 ppm before • >2300 ppm immediately after – Two taxa

• Ginkgo for long-term CO2 trends

• Fossil fern for CO2 shortly after bolide impact • Fossil fern had two issues. – Relationship with living ferns not close • Aff. Stenochlaena = affinity with, but not the living genus • Living Stenochlaena used to develop Stomatal Index curve – Fossil fern had Stomatal Index outside the calibration curve

• High pulse of CO2 • Earliest Paleocene values poorly constrained

• Recent modeling of impact indicates much lower CO2 input to atmosphere than earlier impact models. Study Design

• Reinvestigate taxa using the Mechanistic Stomatal Model – Leaf conductance estimated from existing slides of cuticle – Measurements of carbon isotopic composition of leaf materials – Growth chamber work, extant Stenochlaena (and other species) – Different assumptions 13 regarding An and δ C of atmosphere • Isotopic excursion just above K-Pg boundary Aff. Stenochlaena leaflet (left) and stomata (right) Results: Mechanistic Model

• Higher CO2 before and after K- Pg boundary event – ~625 ppm – Congruent with published study, Castle Rock Rainforest • Kowalczyk et al. (2018, Paleoceanography and Paleoclimatology 33: 1427– 1438.)

• Significantly lower CO2 increase due to the K-Pg impact – ~875 ppm: ~ 250 ppm increase – Significantly lower estimate of

CO2 released by bolide impact – In line with recent impact modeling Summary and Conclusions

• Terrestrial plants can provide reconstructions of paleo-CO2 in deep time with high temporal resolution. – Macrofossils – Dispersed leaf cuticles • Orders of magnitude more common than leaf macrofossils – High-resolution collecting and stratigraphic data • Mechanistic stomatal model of Franks et al. (2014) is applicable to a wide variety of plants. – Eliminates problem of changes in communities – Microstratigraphic sampling • Plant fossils and stomatal studies should be an integral part of future studies of carbon cycle perturbations and extinction boundaries.