Evidence in Support of Carnian Triassic Climate Change

Late Triassic compositional changes of aeolian dusts in the pelagic Panthalassa: Response to the continental climatic change

Authors:

Nakada et al

Abstract:

The Carnian pluvial event (CPE) is considered as the drastic environmental change that occurred in the western Tethys during the Carnian (Late Triassic). Although some studies have suggested that the CPE was a global event, no studies have referred to pelagic environment related to the CPE. To reveal the pelagic environment during the Carnian, X-ray absorption fine structure (XAFS) analysis was used to determine the iron (Fe) species in chert samples collected from the Ladinian–Carnian strata in the Tamba–Mino–Ashio Belt, Japan. The X-ray absorption near-edge structure (XANES) analysis shows that all the red cherts contain hematite with a combination of clay minerals, such as illite, chlorite, and smectite. The existence of these minerals is identified using extended XAFS (EXAFS). The XANES analysis of the green and the white cherts does not show any hematite signatures, while the EXAFS analysis indicated increasing amounts of chlorite.

The stratigraphic change in the Fe-bearing compositions observed in this study can be divided into three stages: (i) a stage where a relatively stable mineral composition (chlorite + illite + hematite) is observed in cherts deposited during the early Carnian (Julian), (ii) the abrupt change in mineral composition (lack of chlorite and appearance of smectite) observed in the middle area in the studied section and corresponding to the middle part of Carnian (late Julian–early Tuvalian), and (iii) the recovery of the previous stable mineral composition during the late Carnian (Tuvalian). These variations in the mineralogical composition of pelagic cherts can be interpreted to mean that the continental climate conditions changed during the Carnian, and different clay fractions were delivered to the pelagic region. These data, combined with the results of previous studies performed around the continental realm, can be interpreted as follows: (i) there was a relatively stable dry climate before the CPE, (ii) there were increasing amounts of precipitation owing to the CPE, and (iii) the climate returned to dry conditions similar to those prior to the CPE.