Quantifying uncertainties in marine volcanic ash layer records from ocean drilling cores

Ocean drilling provides a global record of deposits throughout the oceans. An international collaborative ocean research programme was established in 1966, and the International Ocean Discovery Program (IODP) is the most recent manifestation of this. During each expedition a large amount of data are generated and collected by the shipboard science team. It could be suggested that due to the different staff and objectives during each expedition there would be significant variations in the consistency of data recorded in the shipboard visual core descriptions (VCDs). Use of shipboard VCD data in a global study of volcanism through time required the ground truthing of VCD data, to assess the consistency of ash layer reporting and to identify the amount of under/over recording of volcanic ash layers in cores. Approximately 1400 ash layers in DSDP, ODP, IODP and JAMSTEC cores were examined and it was found that on average 70-75% of recorded volcanic ash layers were present as described, an average of 17-20% were over recorded and 10-15% were under recorded. A number of factors could contribute to this variability, such as VCD format, lack of time for shipboard sampling of every ash layer, significant ash layer colour changes since time of coring, or differences in VCD recording and volcanic ash layer identification and description schemes between expeditions. These findings are important and will allow greater confidence in further studies based on data compiled from shipboard VCDs.