Determining tephra fall deposit thickness in sedimentary record from magnetic susceptibility curve: Example of four Ethiopian tephras

Abstract

The ejecta volume associated to a volcanic eruption is commonly estimated by extrapolating the exponential thinning relations beyond the tephra fall deposit thickness preserved in the sedimentary record. The thickness of a tephra is usually determined visually, eventually including the counting of the volcanic grains to take into account the vertical dilution of volcanic material within the sedimentary column. An alternative method is presented. Most of tephra fall deposits preserved within marine sedimentary records are marked by magnetic susceptibility peaks, that stand out against the low signal usually observed in marine environment. The peak area is proportional to the thickness of the tephra and to the specific magnetic susceptibility of volcanic material χvol. Therefore integrating the surface below a gaussian fit of dry mass magnetic susceptibility corrected from the sedimentary background magnetic contribution gives the apparent tephra fall deposit thickness, knowing χvol. This method is applied to four Ethiopian traps tephra observed in four sites from Ocean Drilling Program (ODP) Leg 115, drilled in the southern Indian Ocean, 2600 km away from the Ethiopian traps. The characteristics of magnetic susceptibility peaks associated to the tephras vary from one site to one other, with lithology, sedimentation rate and bioturbation. Despite these different conditions, estimations of tephra thicknesses are similar in all sites, as expected for close sites. These tephras are interpreted as markers of four ignimbrite‐forming eruptions as important as the Toba eruption at 75 k.y., during a long‐lived eruptive phase.