Annual monsoon rains recorded by Jurassic dunes

Abstract

Pangaea, the largest landmass in the Earth's history, was nearly bisected by the Equator during the late Palaeozoic and early Mesozoic eras. Modelling experiments and stratigraphic studies have suggested that the supercontinent generated a monsoonal atmospheric circulation that led to extreme seasonality, but direct evidence for annual rainfall periodicity has been lacking. In the Mesozoic era, about 190 million years ago, thick deposits of wind-blown sand accumulated in dunes of a vast, low-latitude desert at Pangaea's western margin. These deposits are now situated in the southwestern USA. Here we analyse slump masses in the annual depositional cycles within these deposits, which have been described for some outcrops of the Navajo Sandstone. Twenty-four slumps, which were generated by heavy rainfall, appear within one interval representing 36 years of dune migration. We interpret the positions of 20 of these masses to indicate slumping during summer monsoon rains, with the other four having been the result of winter storms. The slumped lee faces of these Jurassic dunes therefore represent a prehistoric record of yearly rain events.

Inconsistencies Between Pangean Reconstructions and Basic Climate Controls

“…Furthermore, every model configuration yielded the necessary countervailing southeasterly winds during SH winter, which are consistent with the key characteristics of the southernmost paleodunes. In addition, every simulation exhibits seasonally dry periods for the region of the Colorado Plateau, which is consistent with the rock record (12). Finally, although the atmospheric concentration of CO 2 for the Early Jurassic is uncertain, the model results obtained to date show that the winds are insensitive to a range of CO 2 values from the presentday concentration (355 parts per million) to eight times that amount.…”

“…During the entire interval, the dominant winds in the north came from the northeast, curving to become northwesterly over the southern portion of the outcrops. Jurassic dunes in the south were seasonally drenched with rain, whereas weaker southeasterly winds reversed the dry-season slip faces (12).…”

The global monsoon across timescales: coherent variability of regional monsoons

“…In the southwestern United States, wind-deposited sands accumulated to a thickness of 2500 m during the 160 million years that Pangaea straddled the Equator. These strata are the thickest and most widespread eolian dune deposits known from the entire global sedimentary record (Loope et al, 2001), and are accompanied by loess accumulation in semi-arid regions (Soreghan et al, 2008). The Pangaea mega-monsoon is likely to have experienced significant variations in its intensity due to orbital forcing, as manifested in the orbital cycles in the Triassic playas and lakes.…”

Sheeting joints and polygonal patterns in the Navajo Sandstone, southern Utah: Controlled by rock fabric, tectonic joints, buckling, and gullying

“…In comparison to typical Navajo strata, the Navajo at our Coyote Buttes and Buckskin Gulch study areas (Fig. 1) contains thicker and more numerous grain-flow (avalanche) cross-strata (Loope et al, 2001). The rocks at these sites are also coarser and more friable than at other Navajo outcrops.…”