Investigating environmental response to Afro-Arabian flood basalt volcanism with elemental analysis of Oligocene carbonate sediments from ODP Leg 115 Site 709, Indian Ocean.

摘要

The goal of this project is to investigate potential environmental responses to the extensive volcanism associated with the Afro-Arabian Large Igneous Province, by analyzing major and trace element contents in the carbonate phase of Oligocene sediments from Ocean Drilling Program Leg 115 Site 709. These sediments were deposited above the carbonate compensation depth and span the main eruption interval of the Afro-Arabian Province at ~30 Ma based on the presence of correlated volcanic ash layers. Large Igneous Provinces erupt large magma volumes (>0.1 million km3) over a short time period (<1 million years). Due to their size, many large igneous provinces 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 sediments of this study contain tephra layers that can correlated to specific onshore eruptions from the Afro-Arabian large igneous province, and provide a definitive record of the timing of volcanism. Major and trace element analysis of the sediments show five groups of elements that share similar chemical behaviors. The sediments can be divided into three stratigraphic groups with different compositions. The boundary between Group 2 (the middle group) and Group 3 (the shallow group) occurs at a depth of 243 ± 0.75 meters below sea floor, and is marked by a sudden increase in manganese and rare earth elements. Its inferred age of 30.45 ± 0.10 Ma is consistent with the onset of Afro-Arabian volcanism. The boundary between Group 1, the deepest stratigraphic group, and Group 2 may occur at a depth of 258.21 ± 0.75 meters below sea floor, with an inferred age of 32.31 ± 0.10 Ma, defined by a gradual decrease in manganese and rare earth elements. This chemical change might correlate with the timing of the Eocene-Oligocene Oi-1 global cooling event, however additional analyses of δ18O and δ 13C stable isotopes are necessary to fully resolve any connection.