THE UNEXPECTED 1870s/1880s RESPONSE TO THE MID-CENOMANIAN EVENT: AN INVESTIGATION IN THE IONA-1 CORE

The emplacement of either the Caribbean Large Igneous Province (CLIP) or High Arctic Large Igneous Province (HALIP) is implicated in the triggering of the CenomanianTuronian Ocean Anoxic Event 2 (OAE-2; ca 94 - 95 Ma). Evidence for a similar initiation mechanism for the Mid-Cenomanian Event (MCE; ca 96.5 Ma), a precursor to OAE-2 during which comparable environmental conditions persisted, is contradictory or absent. In this study, a reconstruction of mid-Cenomanian seawater 187Os/188Os from the Iona-1 core, SW Texas, the first for the Cretaceous Western Interior Seaway, tests the role of LIP activity in triggering the MCE. The absence of a prolonged unradiogenic Os-isotope excursion (low 187Os/188Os) during the MCE interval argues against CLIP involvement in the event's initiation. Instead, the coincidence of a muted unradiogenic Os-isotope excursion and elevated Hg concentrations with published 40Ar39Ar ages of 96.4 Ma of basalts from Ellesmere Island, Canada, suggests HALIP-related volcanic activity may have precipitated the MCE. In addition, the correlation of a pronounced radiogenic Os-isotope excursion (high 187Os/188Os) between Iona-1 and previous measurements from the English Chalk, Eastbourne, England, provides evidence for elevated pCO2, an accelerated global hydrologic cycle and increased continental weathering rates -- conditions known to exist during OAE-2 but previously only postulated for the MCE.