Obliquity-paced SE Asian monsoon variability during the Middle Miocene

Abstract:

Middle Miocene climate had boundary conditions different from the Pleistocene but similar to those predicted for Earth’s future including substantial Antarctic ice cover without permanent northern hemisphere ice sheets. Under these boundary conditions, comparatively little is known about monsoon variability. Here we show using terrestrial sediments in the Tianshui Basin, Gansu, China that East Asian monsoon variability during a portion of the Middle Miocene (~13.9 – 13.4 Ma) was obliquity-paced. The sediments, part of the Yanwan section, consist of siltstones strongly modified by pedogenesis, such that primary sedimentary structures are largely absent. 20 cm thick, well-cemented CaCO₃ cliff-forming horizons containing root-pore cements and clay nodules are inter-bedded at regular ~1m intervals with slope-forming siltstones containing clay films, well-preserved roots up to 2 cm in diameter, and variable abundances of 0.1–2cm diameter CaCO₃ nodules. Stronger pedogenesis in the well-cemented horizons typify soil K horizons that develop in seasonal climates when sedimentation is slower, whereas the weaker pedogenesis in the siltstones occurs when sedimentation is faster. Thus this cyclic stratigraphy was likely generated by changes in sedimentation rate, which governed the intensity of pedogenesis and was likely controlled by variability of the winter monsoon (if the sediments are aeolian) or the summer monsoon (if fluvial). Obliquity (41 kyr) pacing of the inferred monsoon variability is concluded from counting the cycles in a 12m subsection dated by magnetostratigraphy. Obliquity–paced (41kyr) monsoon variability was likely controlled by insolation-driven changes in the meridional temperature gradient and may relate to southern hemisphere ice volume.