USING SEDIMENTARY HG AS A VOLCANIC PROXY: TESTING THE RELATIONSHIP BETWEEN THE MIOCENE CLIMATE OPTIMUM AND THE COLUMBIA RIVER FLOOD BASALTS

The Miocene Climate Optimum (MCO) was a period of elevated global temperatures that disrupted the long-term trend of Cenozoic cooling. MCO warming, which lasted from ~16.7-14 Ma, is often attributed to greenhouse warming from carbon dioxide released by the Columbia River Flood Basalt (CRB) large igneous province in the North American Pacific Northwest. Despite recent advances in high-precision dating of the CRB, the relative chronology of the eruptions and the warming are not fully constrained; the Steens Basalt, the oldest known CRB lava flow, was emplaced at 16.9 Ma, 100 kyr years after the MCO began. Here we use mercury concentration in marine sedimentary rocks as a proxy for CRB volcanism in order to develop an integrated record of CRB emplacement and MCO warming. We test whether cryptic degassing from the CRB magmas could have caused large scale global warming before the first lavas reached the surface. We report Hg normalized to organic carbon and sulfur in the El Capitan beach section of the Monterey Formation, a highly biogenic unit deposited in an upwelling zone with high levels of organic carbon burial. The El Capitan section and correlative sections have good chronostratigraphic control through foramanifera biostratigrpahy, magnetic reversal stratigraphy, and isotope chemostratigrapy. High levels of Hg found in rocks deposited before the Steens eruption would be consistent with the hypothesis that CRB activity triggered the MCO. A more precise chronology of this crucial time period will increase understanding of the relationship between large igneous provinces and Earth system disruptions like mass extinctions and ocean anoxic events.