THE EARLY JURASSIC HG CYCLE: LARGE IGNEOUS PROVINCES, OCEANIC ANOXIC EVENTS, AND ENVIRONMENTAL CHANGE

Large Igneous Province (LIP) eruptions have been shown to be roughly coeval with severe environmental perturbations including climate change, mass extinctions, and oceanic anoxic events. However, many of the proposed disruptive effects of LIP eruptions occurred on shorter or similar timescales as the highest attainable precision on radiometric dates on LIP lava flows themselves. Volcanoes are among the main sources of natural mercury (Hg), and sedimentary Hg concentration data have been interpreted to reflect LIP activity. Thus, the Hg-proxy might fill the gap left by radiometric LIP dating methods, as Hg concentration data are obtained at the same resolution and from the same sedimentary sections as other paleoenvironmental proxies. While Hg concentration data are now widely used as a qualitative proxy for LIP activity, there remain key unresolved questions: whether periods of environmental change without any known LIP influence also cause perturbations to the Hg cycle and resulting fluctuations in sedimentary Hg, and how the Hg-cycle varied over geological timescales independently of volcanic activity.

We present an unprecedently long (~10 Myr) and high-resolution (~5–200 kyr) Hg record through the Lower Jurassic from the Mochras Farm (Llanbedr) Borehole, Wales. The core spans one of the most dynamic intervals in Earth history, including the recovery from the Triassic–Jurassic (T–J) mass extinction, the Sinemurian–Pliensbachian (S–P) negative carbon-isotope excursion (CIE), the Toarcian Oceanic Anoxic Event (T-OAE), and significant (>1 Myr) periods of relative environmental stability. The Mochras core is geochemically and paleontologically well studied, and the timing of key events is constrained via ammonite biostratigraphy, astrochronology, and δ¹³C. We propose a robust framework with which to evaluate Hg datasets and use it to quantitatively compare the post-T–J, S–P, T-OAE, and background intervals. We find that increases in TOC-corrected Hg deposition associated with the end-Pliensbachian event began simultaneously with the start of the δ¹³C perturbation, and large peaks in Hg occurred during the LIP-associated T-OAE (corroborating earlier studies). By contrast, the S–P CIE has no high-amplitude Hg peaks yet shows some perturbation to the Hg cycle relative to background intervals.