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Palaeoclimate of Late Jurassic of Portugal



From: Ben Creisler
bcreisler@gmail.com

A recent online paper:

MYERS, T. S., TABOR, N. J., JACOBS, L. L. and MATEUS, O. (2012)
Palaeoclimate of the Late Jurassic of Portugal: comparison with the
Western United States.
Sedimentology (advance online publication)
doi: 10.1111/j.1365-3091.2012.01322.x
http://onlinelibrary.wiley.com/doi/10.1111/j.1365-3091.2012.01322.x/abstract



Investigation of the palaeoclimatic conditions associated with Upper
Jurassic strata in Portugal and comparison with published
palaeoclimate reconstructions of the Upper Jurassic Morrison Formation
in western North America provide important insights into the
conditions that allowed two of the richest terrestrial faunas of this
period to flourish. Geochemical analyses and observations of palaeosol
morphology in the informally named Upper Jurassic Lourinhã formation
of western Portugal indicate warm and wet palaeoclimatic conditions
with strongly seasonal precipitation patterns. Palaeosol profiles are
dominated by carbonate accumulations and abundant shrink-swell
(vertic) features that are both indicative of seasonal variation in
moisture availability. The δ18OSMOW and δDSMOW values of
phyllosilicates sampled from palaeosol profiles range from +22·4‰ to
+22·7‰ and −53·0‰ to −37·3‰, respectively. These isotope values
correspond to temperatures of formation between 32°C and 39°C ± 3°,
with an average of 36°C, which suggest surface temperatures between
27°C and 34°C (average 31°C). On average, these surface temperature
estimates are 1°C higher than the highest summer temperatures modelled
for Late Jurassic Iberia using general circulation models. Elemental
analysis of matrix material from palaeosol B-horizons provides proxy
(chemical index of alteration minus potassium) estimates of mean
annual precipitation ranging from 766 to 1394 mm/year, with an average
of approximately 1100 mm/year. Palaeoclimatic conditions during
deposition of the Lourinhã formation are broadly similar to those
inferred for the Morrison Formation, except somewhat wetter. Seasonal
variation in moisture availability does not seem to have negatively
impacted the ability of these environments to support rich and
relatively abundant faunas. The similar climate between these two Late
Jurassic terrestrial ecosystems is probably one of the factors which
explains the similarity of their vertebrate faunas.