Monsoon Rectification of Orbital Forcing near Pangean Equator

Monsoonal Evaporite varves from the equator of Pangea offer evidence of a tropical source for the enigmatic ~100,000 yr climate cycle. Details preserved within varves show that a seasonal hiatus in chemical precipitation of calcium sulfate that produced a varve also functioned as a nonlinear amplifier of effects of insolation on surface temperature. The hiatus is traced to a strong maritime monsoon, which, working in opposition with the continental monsoon, permitted surface temperature over land to react to an amplitude modulation of insolation over the course of 4 to 5, 23 ka precession cycles, and originates in changes of Earth’s elliptical orbit. This monsoonal process, also known as “clipping” or rectifying, allowed expression of a strong ~100 ka orbital cycle in surface temperature at the Pangean equator (35% of total variance). The climatic rectifying mechanism is traced to Pangea’s asymmetrical distribution relative to the equator, which placed continental and maritime monsoons in strong opposition near the equator. Efficiency of the maritime monsoon as an amplifier of ~100 ka forcing resides in the ocean’s large heat capacity, but the important contribution from Pangean geography suggests that a ~100 ka climate oscillation is generic to the monsoon, which may bear on the source of Late Pleistocene glacial-interglacial climate variation.