Triassic Monsoonal Climate and its Signature in Ladinian-Carnian Carbonate Platforms (Southern Alps, Italy)

Abstract

ABSTRACT Paleoclimatic general circulation models suggest the existence of a monsoonal climate during the Permo-Triassic over wide parts of the megacontinent Pangea and its adjacent oceans. This paper discusses how Ladinian-Carnian sedimentary successions outcropping in the Southern Alps record the signature of this climate. Sedimentological associations of tepees capped by terra-rossa paleokarst, braided fluvial sediments capped by caliche soils, and evaporite beds alternating with clay-rich delta deposits all indicate that net precipitation values changed substantially over short geological time scales. Early diagenetic features including episodes of dissolution in stratigraphies of meteoric calcite cements, corrosion and hematitization of siliciclastic detritus prior to deposition, early eu edral tectosilicate cementation, and dolomitization from evaporation-concentrated seawater record frequently changing paleohydrological conditions. Sedimentological and early diagenetic data recording highly variable, seemingly conflicting paleoclimate information can best be attributed to fluctuations in net precipitation intensities controlled by monsoonal climate, ranging from seasons to 106 yr. d18O values in early meteoric cements (-5.2 to -6.4) reflect the presence of strongly depleted meteoric waters, which are not compatible with the Southern Alps paleolatitude or paleotopography in the Triassic and can be directly related to precipitation intensities associated with a monsoonal climate. Strong precipitation could have also resulted in decreased surface-water salinities and depleted d18O in surface waters. d18O time series from marine rocks and early meteoric cements indicate a trend from Middle Triassic values, generally deplet d with respect to the expected marine signature, to less depleted Late Triassic values. This is interpreted to represent an unusually wet episode (high net precipitation), transitionally grading in the Late Triassic into a relatively add period (or low net precipitation). Despite the fact that several factors controlled the intensity of monsoonal precipitation, the effects of its variations through time are evident at different time scales in the Middle and Upper Triassic record of the Southern Alps.