ORBITALLY FORCED VARIABILITY IN MERCURY DEPOSITION? VARIABLE HG SIGNATURES AND COMPLEX CONTROLS IN A LOWER JURASSIC SECTION (PLIENSBACHIAN, UK)

Mercury (Hg) concentration in sedimentary rocks has been increasingly used as a proxy for volcanic activity in the geological past. Increases in Hg concentration around the world have been found to be coeval with emplacement of known Large Igneous Provinces (LIPs). However, like in the modern, temporal and spatial variability in Hg deposition may also occur over geological timescales in the absence of LIP events. This may occur in response to orbital forcing, via enhanced emissions from terrestrial Hg reservoirs (permafrost, peat, soils), or weathering and erosion under varying hydrological regimes. However, these processes remain largely unconstrained due to a lack of high-resolution data from non-LIP time periods. These data will also test the uniqueness of the Hg signals seen during LIP emplacement.

We address this critical knowledge gap by creating a high stratigraphic resolution (3 cm, ~2 kya) Hg concentration record spanning ~1.5 Myr (24 m) of the Lower Jurassic Belemnite Marls, Dorset, UK: a sequence of orbitally (precession-scale) forced marl-limestone couplets deposited in a shallow-marine epicontinental basin, during a time period without known LIP influence.

We use this record to interrogate the deep time Hg cycle, and in particular its relationship to organic carbon (TOC) and CaCO₃ content. We show that the lower Pliensbachian (jamesonii Ammonite Zone) sedimentary Hg record is not linked to apparent precession scale (~20 kyr) forcing on the local depositional environment, unlike variations in TOC and CaCO₃ content. The one exception is a 5 m interval of the Hg record which has significant power at the same frequency as TOC, indicating orbitally induced TOC variation significantly affected Hg drawdown here. Furthermore, we quantitatively assess the relationships between Hg, TOC, and CaCO₃ content using time series analysis techniques, e.g., causality analysis, and compare this with expectations from our understanding of the modern Hg and carbon cycles.

Our results indicate that variability in lower Jurassic Hg records are not controlled solely by TOC deposition and preservation (as often assumed). Instead, even background (non-LIP) Hg concentrations are strongly influenced by a range of as-yet poorly characterized environmental and diagenetic processes.