A GEOCHEMICAL INVESTIGATION OF ENVIRONMENTAL RESPONSE TO AFRO-ARABIAN LARGE IGNEOUS PROVINCE VOLCANISM USING OLIGOCENE CARBONATE SEDIMENTS FROM OCEAN DRILLING PROGRAM LEG 115 SITE 709, INDIAN OCEAN

Major and trace element contents in the carbonate phase of Oligocene sediments from ODP Leg 115 Site 709 in the Indian Ocean were measured with the aim of investigating potential environmental responses to the extensive volcanism associated with the c. 30 Ma Afro-Arabian large igneous province (in Yemen and Ethiopia). Many large igneous provinces appear to coincide with episodes of global environmental change, as recorded in faunal extinction events, anomalies in the global carbon cycle, and changes in ocean water composition. A major issue with linking volcanic events to environmental change is determining synchronicity between volcanic and sedimentary records. The studied Site 709 sediments contain silicic tephra layers that can be correlated to specific onshore eruptions of known age from the Afro-Arabian province 2800 km to the NW, and provide a definitive record of the timing of volcanism in this core. Two stratigraphic groups can be identified from an abrupt change in the compositions of carbonate at a depth of 243.40 ± 0.75 mbsf. These groups are differentiated by a sudden increase of concentration of manganese and rare earth elements. This boundary has an inferred age of 30.38 ± 0.18 Ma, estimated from the ⁴⁰Ar/³⁹Ar age for the onshore ignimbrite correlated to the deepest tephra layer age and an average sedimentation rate, and is consistent with the known timing for the onset of Afro-Arabian volcanism, at 30.3 ± 0.2 Ma. The processes that produced this compositional shift in carbonate composition are being evaluated. Carbon and oxygen isotope analyses are in progress to see if the compositional change is linked to global perturbations in the carbon cycle and temperatures.