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SUMMARY:Teaching a new trick to old data: reassessing the Neogene history 
 of C4 grasses in the Great Plains of North America - David Fox\, Universit
 y of Minnesota
DTSTART:20231031T120000Z
DTEND:20231031T130000Z
UID:TALK206590@talks.cam.ac.uk
CONTACT:Dr Rachael Rhodes
DESCRIPTION:Modern grasslands are responsible for a disproportionate fract
 ion of terrestrial primary productivity relative to their areal extent\, a
 nd the origin of these ecosystems and their associated mammalian faunas ov
 er the Cenozoic has been the focus of extensive research. Multiple lines o
 f evidence indicate the origin of the modern grassland ecosystems in the G
 reat Plains of North America proceeded in two stages. Plant microfossils (
 phytoliths) and fossil mammalian herbivore teeth indicate that open\, gras
 s-dominated habitats first appear in the Early Miocene and included grasse
 s using the C3 photosynthetic pathway\, with C4 grasses emerging later in 
 the Miocene to Pliocene. Stable carbon isotope ratios (δ13C values) from 
 tooth enamel of large-bodied mammalian herbivores largely support this mod
 el\, with the first C4 consumers first evident by 6.6 Ma in Texas and then
  at ca. 6.25 Ma in Nebraska. However\, pedogenic carbonate δ13C values ar
 e consistent with C4 grasses being present since at least 23 Ma\, broadly 
 coincident with the first phytolith evidence for open habitats. Key to the
  different interpretations of the carbon isotope records are the assumed e
 ndmember δ13C values for C3 and C4 plants in the past and whether and how
  measured δ13C values are interpreted as fraction C4 biomass (fC4). Here 
 we reinterpret published carbon isotope records from the Neogene and Quate
 rnary of Nebraska using a global compilation of more than 5\,000 modern pl
 ant δ13C values and a Monte Carlo method for estimating fC4 from measured
  δ13C values. This approach applies linear mixing that accounts for natur
 al variation in plant endmembers and uncertainties in the δ13C of atmosph
 eric CO2 in the past and the fractionations associated with pedogenic carb
 onate and bioapatite. For a measured δ13C value\, the MC method generates
  a distribution of 10\,000 fC4 values. Estimation of fC4 from δ13C values
  of modern mammals from tropical forests and Paleogene fossil mammals for 
 which C4 grasses were not available indicate that median fC4 values are go
 od predictors of C4 in diet\, but the 5th percentile of the distributions 
 allows for statistical assessment of the C4 component. We focus on the rec
 ord from Nebraska for its long duration and to minimize the role of latitu
 dinal diachroneity in increased abundance of C4 grasses. Our reanalysis br
 ings the paleosol and mammalian isotopic records into agreement\, provides
  statistical support for the original interpretation of the Miocene carbon
 ate record from the Great Plains\, and indicates that mammals in the regio
 n were consuming C4 grasses during the Early Miocene\, much earlier than h
 eretofore recognized and possibly before anywhere else on Earth.
LOCATION:Department of Earth Sciences\, Tilley Lecture Theatre
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