Palaeoclimate of the Late Jurassic of Portugal: comparison with the Western United States

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 d 18 O SMOW and dD SMOW values of phyllosilicates sampled from palaeosol profiles range from +22AE4& to +22AE7& and )53AE0& to )37AE3&, 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.

Paleosols as Indicators of Paleoenvironment and Paleoclimate

“…However, there is a disconnect between reported oxygen and hydrogen isotope fractionation equations for phyllosilicates (Delgado & Reyes 1996, Sheppard & Gilg 1996 and those for paleosol clay assemblages, which are typically complex mixtures of kaolinite, smectite, illite, and mixed-layer illite-smectite (e.g., Rosenau & Tabor 2013). Application of experimentally derived fractionation factors for pure smectite to phyllosilicate assemblages that also include kaolinite and illite yields overestimates of actual crystallization temperatures (Myers et al 2012b). X-ray diffraction analysis of mineral composition, in conjunction with elemental analysis, allows calculation of unique fractionation factors for each clay assemblage (Rosenau & Tabor 2013).…”

“…This proxy is inappropriate for paleosols with near-surface carbonate accumulations, but has been used for paleosols in which calcic horizons occur at depths ≥50 cm below the profile surface (Sheldon & Retallack 2004). Attempts have been made to apply the CIA−K proxy to calcic paleosols by subtracting CaO derived from calcium carbonate from the total measured CaO (Myers et al 2012b). This correction counterintuitively produces higher MAP estimates for paleosols with greater amounts of bulk carbonate as a result of removal of base cations from clay exchange sites by acid attack (H + ), and its use is not recommended (Myers et al 2014).…”

The Lourinhã Formation: the Upper Jurassic to lower most Cretaceous of the Lusitanian Basin, Portugal – landscapes where dinosaurs walked

“…has a slightly higher average estimates for soil pCO2 when compared with the coeval Morrison Formation, implying for the Lourinhã Formation a deposition under somehow wetter conditions, higher productivity and an estimated greater faunal richness. After Myers et al, (2012b), the geochemistry and morphology of paleosols profiles show warm and wet paleoclimatic conditions with a strongly seasonal precipitation pattern, and the oxygen isotopes suggest surface temperatures reaching between 27º C and 34º C (average 31ºC) and a mean annual precipitation of 1100 mm/yr.…”

Integrated sedimentology and ichnology of Late Jurassic fluvial point‐bars – facies architecture and colonization styles (Lourinhã Formation, Lusitanian Basin, western Portugal)

“…This process is taking place under rather cooler conditions than those derived by Myers et al . (), notably under a warm temperate maritime climate with an average high summer temperature of 21·3°C and average low summer temperature of 16·5°C (source: http://www.en.climate-data.org).…”

“…In a recent study of Lourinhã Formation palaeosols in the Areia Branca area, Myers et al . () infer an average rainfall of 1100 mm year −1 as derived from elemental analysis. This annual rainfall figure certainly falls within the range suggested by Retallack () for the depth of the main calcic horizon (currently 60 cm), particularly if one allows for some erosion of the top of the palaeosol profile beneath channel‐fill 2.…”