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 euhedral 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 10 6 yr. delta 18 O values in early meteoric cements (-5.2 to -6.4 per thousand ) 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 delta 18 O in surface waters. delta 18 O time series from marine rocks and early meteoric cements indicate a trend from Middle Triassic values, generally depleted 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.

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