Environmental Filtering of Volcanic Mercury Signals and Its Implications for Organic Matter Enrichment in the Lower Silurian Longmaxi Formation, South China

Following the end of the Hirnantian glaciation, organic matter (OM) rich black shales were widely deposited during the Rhuddanian, and volcanism has commonly been invoked as an important external driver of OM enrichment during this interval. Mercury (Hg), as a widely used proxy for volcanic activity, has therefore been extensively employed to trace volcanic influence. However, in black shale depositional systems, Hg is strongly influenced by host phase control and environmental filtering, particularly through coupled enrichment with OM and sulfides, such that sedimentary Hg records do not directly reflect volcanic input intensity. As a result, sedimentary Hg anomalies may record either excess volcanogenic Hg loading or enrichment driven solely by host phase control, leading to conflicting interpretations of the relationship between volcanism and OM enrichment. Establishing a robust quantitative metric that can effectively remove host phase effects and identify excess volcanogenic Hg input is therefore a critical prerequisite for reassessing the relationship between volcanism and OM enrichment in black shales. To address this problem, we investigated the lower Longmaxi Formation in South China using new Hg and total sulful(TS) data from the CN and YH cores, which represent contrasting depositional settings, together with compiled published datasets from other Lower Silurian black shale sections. We first identified that sulfides were the dominant host phase of Hg during this interval. We therefore adapted the residual Hg(HgR) framework of Fendley et al. (2024) by prioritizing TS as the first order host phase correction.The results show that HgR differs markedly from raw Hg and conventional normalized indices, indicating substantial reconstruction of the stratigraphic signal after host phase correction. Comparison of HgR and TOC stratigraphic trends indicates that they are not consistently coupled, and that their relationship varies with redox conditions and depositional position among sections.Volcanic Hg signals therefore show environmentally dependent and stratigraphically variable relationships with OM enrichment after host phase correction. These results indicate that environmental filtering is the key mechanism governing how volcanic Hg signals are preserved and expressed in the sedimentary record. Volcanic forcing enters the system as external Hg loading, but its stratigraphic expression is subsequently modulated by host phase partitioning, sedimentary preservation, and the conditions governing OM accumulation. Accordingly, the effect of a given volcanic input on OM enrichment may be amplified, attenuated, or decoupled under different depositional conditions. Coupling between volcanic Hg signals and OM enrichment is thus conditional rather than intrinsic. This framework improves interpretation of Hg proxies in black shales and refines how volcanism–OM coupling should be evaluated in deep time marine systems.