Organic-rich Shales Reveal Local Controls That Enhanced Mercury Accumulation During a non-LIP Interval of the Miocene: Implications for the Mercury Paleoproxy

Sedimentary mercury (Hg) concentrations are traditionally used to track atmospheric Hg deposition, which is thought to be controlled by volcanic outgassing and potentially the emplacement of large igneous provinces (LIPs). Sedimentary Hg enrichments are subsequently inferred to represent ancient intervals of massive volcanism and are often used to link the destabilization of Earth’s environment to extinction. The biogeochemical cycling and controls on Hg sequestration in sediments, however, are both dynamic and complex, with wide spatiotemporal variability. To better elucidate the controls on Hg cycling, sediments from the Miocene Monterey Formation were studied in three separate sedimentary basins (San Joaquin Basin [SJB], Santa Barbara Basin [SBB], Santa Maria Basin [SMB]) from the eastern Pacific margin, representing approximately seven million years of quasi-contemporaneous deposition under predominantly reducing conditions during a non-LIP interval. Furthermore, the sites were located in close proximity to terrestrial volcanic centers. The modes of Hg delivery and sequestration were generally different in each of these basins. In the proximal SJB, Hg contents were related to aluminum and pyrite concentrations, which are proxies for aluminosilicate/detrital input and relative degrees of sulfate-reducing conditions in sediments and water column, respectively. In the more distal SMM and SBB, Hg contents were controlled by the amount of pyrite burial and organic matter concentrations, respectively. In the SBB, however, two clear populations of geochemical data suggest a shift in the contribution of Hg-enriched detrital materials to this basin controlling Hg delivery or variations in Hg scavenging efficiency. These multiproxy relationships make it clear that a range of geochemical proxies applied in multiple syndepositional settings with different environmental conditions should be used in tandem to distinguish local versus global controls on Hg deposition. We suggest that biogeochemical feedbacks during intervals of massive volcanism in the past, related to changes in weathering and erosion of soils on land and local redox, may be a critical if not dominant driver of sedimentary Hg enrichments. These observations highlight the importance of ruling out local to regional processes that can enrich sediments in Hg before ascribing massive volcanism as the foremost source of excess mercury in shale sequences.