Mercury Isotopic Evidence that global carbon cycle disturbance decoupled from volcanism during the Oceanic Anoxic Event 1b

Oceanic Anoxic Event 1b (OAE1b) occurred near the Aptian–Albian boundary during the mid-Cretaceous and represents a unique long-lasting global perturbation of the carbon cycle, characterized by multiple black shale intervals and four distinct negative carbon isotope excursions (Jacob/113, Kilian, Urbino/Paquier, and Leenhardt events). Compared with other OAEs, OAE1b is notable for its prolonged duration (~4 Myr) and its subdivision into multiple sub-events. Despite extensive marine studies, its triggering mechanisms remain controversial, with proposed drivers including volcanism related to the Southern Kerguelen Plateau, enhanced ocean stratification, intensified monsoonal circulation, and methane hydrate dissociation. However, terrestrial environmental responses to OAE1b remain poorly constrained.

Here we present a high-resolution terrestrial record of OAE1b from the Songliao Basin, northeastern China, based on the ICDP SK-2 borehole. Integrated analyses of organic carbon isotopes (δ¹³C_org), mercury concentrations, mercury isotopes (Δ¹⁹⁹Hg), and major and trace elements, combined with an established astrochronological framework, allow identification of three OAE1b sub-events (Jacob, Kilian, and Paquier) in terrestrial deposits. For the first time, mercury isotope evidence reveals three episodes of globally significant volcanic activity occurring prior to the Jacob event, prior to the Kilian event, and following the Kilian event. These volcanic signals correlate well with records from other basins worldwide, indicating a global volcanic influence.

Notably, the temporal decoupling between volcanic pulses and OAE1b sub-events suggests that volcanism was unlikely the direct trigger of OAE1b. Instead, relatively weak and predominantly subaerial volcanism of the Southern Kerguelen Plateau may have exerted a longer-term climatic influence, promoting a transition from transient cooling to greenhouse conditions and enhancing continental weathering. This long-term forcing, superimposed on orbital-scale monsoon intensification and increased wildfire activity, likely enhanced primary productivity and organic carbon burial, ultimately contributing to the development of OAE1b.