A global palaeoclimatic reconstruction for the Valanginian based on clay mineralogical and geochemical data

High-resolution clay mineral and delta O-18(carb) analyses have been performed on three sections of Valanginian age (Early Cretaceous), from northwestern (similar to 20-30 degrees N) and southern (similar to 53 degrees S) Tethyan realms. The data have been integrated in a large set of published mineralogical (clays), and geochemical (delta O-18(carb), delta O-18(bivalve), delta O-18(lenticulina), delta O-18(belemnite), and TEX86) data from 17 sections, situated between similar to 45 degrees N and similar to 53 degrees S. Based on this data set, we provide new insights into Valanginian climate conditions from a wide range of palaeolatitudes. We highlight climate trends within specific areas and identify main climate belts. In the Proto-North Atlantic realm (similar to 15-17 degrees N) large amounts of smectite together with the occurrence of palygorskite testify for a semi-arid climate belt throughout the Valanginian. Significant variations in kaolinite content in the northwestern Tethyan realm (similar to 20 to 30 degrees N) suggest that this realm has been more sensitive to changes in hydrological cycling and subsequent erosion and runoff processes during the early to early-late Valanginian transition interval. This time interval, that is coincident with the pronounced positive and globally recognized carbon isotope excursion (CIE; the Weissert episode), documents the wet conditions of a humid subtropical climate belt. Temperature ranges derived from delta O-18 records suggest climate warming by about 3-5 +/- 0.9 degrees C in the northern hemisphere, during the Weissert episode. This is followed by a global cooling of about ca. 1-5 +/- 0.9 degrees C in both hemispheres, during the late Valanginian. We postulate that these patterns were due to interplays between tectonic and orbital factors, which have controlled the distribution of regional palaeoclimate belts during the Valanginian. The semi-arid belt expressed in the Proto North Atlantic realm has probably been induced by its specific palaeogeographic configuration and the existence of isolated basins, which were each characterized by strong and continuous thermohaline circulation. An eccentricity paced monsoon-like system might have played a significant role in maintaining the subtropical belt around the northwestern Tethyan realm, up to the mid latitudes (similar to 35-45 degrees N). Superimposed on these regional processes, a widespread magmatic pulse in Parana-Etendeka volcanic activity might have stimulated greenhouse conditions, impacting marine biota and favouring the development of a dense vegetation cover on the continent. An increase in sedimentary burial of C-13 depleted organic carbon on the continents may have contributed to the progressive global positive carbon isotope excursion. In the medium term, it has also probably led to a significant decrease in atmospheric CO2, as testified by the global cooling recorded during the late Valanginian in both hemispheres and the probable concomitant extension of polar ice caps.