Two Examples from the Late Cretaceous of Western North America

Paleobotanical Estimates of Climate in Deep Time: Two Examples From the Late Cretaceous of Western North America Garland Upchurch1, Ann Marie Prue1, Joan Parrott1, Emilio Estrada-Ruiz2, and Dori Contreras3 1 Texas State University, San Marcos, TX 2 National Polytechnic Institute of Mexico, Mexico City, Mexico 3 University of California, Berkeley, CA Introduction • Paleobotany provides important information on paleoclimate. – Shallow geologic time • Closest living relatives of fossil plant species • Paleoclimate inferred from modern species distributions – Deep geologic time • Relationships with extant species and genera more distant. • Adaptive characteristics of plants – Physiognomy • Paleoclimate inferred from relation between plant adaptive characteristics and modern climate Physiognomy • Physiognomy: Adaptive features of plant structure – Environment: Evolved multiple times – Life form (PFT) – Organs – Tissue structure Tropical leaf (left) vs. • Leaves of dicot flowering plants temperate zone leaf – Margin, shape, size • Woods – Presence/absence of annual rings – Dicot woods: Amount of soft tissue, water conduit diameter, other features Tropical (balsa) wood Physiognomic Methods • Leaves – Simple Linear Regression • Leaf Margin Analysis (LMA) – Mean Annual Temperature (MAT) • Leaf Area Analysis (LAA) Left to right: – Mean Annual Precipitation (MAP) Tropical, desert, and – Multivariate ordination temperate leaves • Climate Leaf Analysis Multivariate Program (CLAMP) • Mean annual and seasonal temperatures, precipitation – Multiple regression • Digital Leaf Physiognomy (DiLP) • Mean Annual Temperature, Precipitation • Woods – Multiple regression • Mean annual and seasonal temperature, precipitation Study Sites • Campanian Stage • Two Medicine Formation – Montana – ~79.6 ma Two Medicine – One horizon Formation • Jose Creek Member, McRae Formation – New Mexico – 76.1 to >72.5 ma McRae Formation – Leaves • One horizon – Woods • Formation thickness Two Medicine Formation • M.S. Thesis, Ann Marie Prue – Texas State University, 2018 • Assemblage of 31 dicot leaf species • Palm leaf fossils – MAT >13°C, CMMT >5°C • Calcrete higher in formation – Seasonal precipitation – MAP <1 m (?) Two Medicine Temperatures • Leaf Margin Analysis: Too cold for palms – MAT 7–12°C • DiLP: Highest temperature, good for palms – MAT = 19°C – Much warmer than LMA – Comparable to certain younger floras, WY and MT • CLAMP: Mixed – Different calibrations, highest values for Asia – MAT = 12–15°C – WMMT = 20–23°C – CMMT = 2–7°C Two Medicine Precipitation • Leaf Area Analysis: Lowest MAP – 140–160 cm • DiLP: Highest MAP – 240 cm • CLAMP: No MAP, just seasonal values – 3 wet months: 55–69 cm – 3 dry months: 9–23 cm • Wet/dry = ~4 – Growing season: 78–135 cm – Reinforces evidence for monsoon inferred from model and isotopic data (Fricke et al., 2010, EPSL) Jose Creek Member, McRae Formation • Work published and in progress – Joan Parrott, Emilio Estrada-Ruiz, Jacqueline Scherer, Dori Contreras • Leaf macrofossils – One volcanic ash bed • Dicot woods – Entire thickness of Jose Creek Member • Palm and cycad leaves, palm stems – MAT >13°C, CMMT >5°C • Non-calcareous paleosols – MAP = 100–150 cm (Buck and Mack, 1995, Cretaceous Research) Cycad leaf Jose Creek Temperatures • Leaf Margin Analysis – MAT 21–22°C – Sample sizes: 42 spp. and 150 spp. • CLAMP (preliminary, no DiLP yet) – MAT = 12°C • Dicot wood anatomy – MAT = 24°C (30 spp.) and 27°C (38 spp.) – CMMT = 15°C (30 spp.) and 23°C (38 spp.) – Diversity of species with high volume of soft tissue Dicot wood with wide rays (soft tissue) Jose Creek Precipitation • Leaf Area Analysis, various regressions – MAP = 60–150 cm • Dicot wood anatomy – MAP = 350 cm – Upper limit • Palms, gingers – Year-round precipitation – Well drained environments • No groundwater • No standing water – In line with fossil soils Palm leaf Summary and Conclusions • No one method works all the time. – LMA: MAT too cold for Two Medicine Formation – CLAMP: MAT too cold for Jose Creek Member • CLAMP potentially valuable method for detecting seasonality. – Reinforces evidence for Late Cretaceous monsoon, Western North America • DiLP gives high estimates of MAT. – Assemblages north of 50°N – Perhaps best method for MAT but: • Extremely time-consuming: 1 month per assemblage • Diverse calibrations and plant proxies need to be used. – Congruence between proxies and calibrations • Error – Compare output of Earth System Models.

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