Obliquity-modulated East Asian monsoon variability recorded coherently in fine-grained red clays and black shales across a land–sea moisture gradient

Long-period obliquity modulation can drive low-frequency hydroclimate variability by changing meridional insolation gradients and influencing the position and intensity of the East Asian monsoon rainband. What is less clear is how consistently such low-frequency signals are captured across different fine-grained depositional settings. Here we compare two end-member archives along the East Asian continental foreland: (i) dust-derived aeolian red clays in semi-arid regions near desert source areas, and (ii) organic-rich black shales deposited in more moisture-proximal settings, from lakes to shallow seas. The depositional processes are different, but both archives often damp higher-frequency noise and preserve long-period orbital pacing, which makes them useful for evaluating the timing and spatial pattern of monsoon-related hydroclimate change.

In the northern late Eocene red-clay succession from the eastern Mongolian Plateau, rock-magnetic and geochemical proxies show pronounced orbital-scale variability between ~48 and 36 Ma. Obliquity-paced modulation is clear, and wetter intervals cluster around high-obliquity nodes. The site sits in a continental-foreland position where moisture delivery from the south and southeast is likely sensitive to north–south shifts of the monsoon rainband and associated subtropical circulation changes.

For a deeper-time shale endmember, we examine Middle Triassic black-shale successions formed during warming and broader Earth-system reorganization. At that time, the North China and South China blocks lay along the eastern margin of Pangea, facing the ocean and remaining sensitive to changes in moisture supply. Geochemical series and magnetic susceptibility from lacustrine to shallow-marine settings show strong obliquity modulation, expressed as a ~1.2 Myr envelope with embedded ~173 kyr variability, together with a 405 kyr band. These patterns suggest that the ~1.2 Myr and ~173 kyr obliquity components can organize rainfall variability in both continental and marine fine-grained archives, and that this behavior extends back to at least ~250 Ma.

Next, using published evidence for cyclicity in Eocene shales from the East China Sea region, we will test whether similar long-period obliquity bands occur in Eocene marine black shales and whether their phases match those in the terrestrial red-clay record. Comparing red clays and black shales as products of a land–sea moisture gradient, using environmental magnetism together with cyclostratigraphy, offers a direct way to connect shale formation and monsoon climate forcing.