UPPER JURASSIC PALEOCLIMATE OF PORTUGAL RECONSTRUCTED FROM PALEOSOLS

The Upper Jurassic Alcobaça and Lourinhã formations, exposed along the western coast of Portugal, present over 400m of terrestrial strata and contain an abundance of paleosols. The most common types of paleosols observed within these fluvio-deltaic deposits are Calcisols, calcic Vertisols, and Protosols. Calcic and vertic features dominate the lower portions of the coastal section, whereas thick sequences of poorly-developed Protosols are more abundant near the top. This marked shift in paleosol features is mirrored by a distinctive facies change, in which coarser grained sandstones and conglomerates increase slightly in frequency and channel bodies become thicker and more laterally extensive in the upper half of the section. Preliminary isotopic analysis of pedogenic carbonates provides a range of d¹³C values from -7.4‰ to -6.4‰ PDB, corresponding to an estimated paleoatmospheric pCO₂ level on the order of 1400ppm, somewhat lower than most estimates of Jurassic CO₂ concentrations. d¹³C values derived from organic samples vary between -22.5‰ and -21.4‰ PDB and show a consistent trend of enrichment in ¹³C.

The presence of abundant pedogenic carbonates and vertic features throughout much of the measured section implies semi-arid and seasonal climatic conditions at the time the Alcobaça and Lourinhã formations were deposited. The observed shift from well-developed to poorly-developed soils, along with the increase in the frequency of sandstone occurrence, suggests that the landscape was becoming less stable and rates of deposition were increasing. This intensification of fluvial influence may be attributed to lowering of base-level, greater precipitation, or a combination of both factors. Paleoclimatic conditions in the Late Jurassic of Portugal are strikingly similar to those inferred for the Upper Jurassic Morrison Formation of North America. In addition to climatic similarities, Portugal and North America also exhibit a significant resemblance in Late Jurassic terrestrial faunal assemblages.