Pleistocene to Holocene offshore tephrostratigraphy of highly explosive eruptions from the southwestern Cape Verde Archipelago

Thirteen sediment gravity cores from the seafloor around the southern Cape Verdean islands Fogo and Brava as well as the Cadamosto seamount recovered 43 mafic and 5 phonolitic Pleistocene to Holocene primary ash layers. Twelve of these layers could be identified in several cores; they evidently cover areas of at least 6200-17,650km2 corresponding to minimum tephra volumes of ~1km3 (Volcanic Explosivity Index (VEI) 5), and thus are attributed to eruptions of sub-Plinian to Plinian dimensions. Provenance analyses based on geochemical compositions and geological evidence link the mafic tephras to eruptions on Fogo island. The detailed foraminiferal δ18O stratigraphy of one core yields a high-resolution age scale that can be applied to other cores via correlated ash beds and background sediment intervals. Sedimentation rates of the background sediment then constrain ages of other ash beds in the cores. The resulting temporal distribution of the marine tephras indicates that relatively large magnitude, highly explosive eruptions occurred about every 3000years on Fogo during the last 150kyrs; during the past 30kyrs the rate has increased to one in 2000years. In addition, we identified a 117kyr old thick turbidite sequence containing both mafic and phonolitic ash. We interpret this as the deposit of density currents generated by the Monte Amarelo flank collapse on Fogo and its associated tsunami.On the other hand, only a single widespread phonolitic tephra layer (145. kyrs old) could be correlated to Brava island indicating that highly explosive activity, producing deposits spread widely beyond the island's shores, did not occur anymore over the last 145. kyrs. No equivalents of the caldera filling ignimbrites on Brava could be found in the cores and therefore we infer that the caldera formed earlier than the 155. ka reached by coring. Two widespread phonolitic tephra layers, 17. ka and 40. ka old, are correlated to the Cadamosto seamount and thus derived from eruptions that occurred at >. 1380. m water depths where thermal granulation producing blocky glass shards was a major fragmentation process. We interpret that these eruptions produced huge submarine volcanic ash-water plumes that spread widely across the seafloor.In conclusion, our marine tephrostratigraphy provides the first evidence of frequent highly explosive volcanic eruptions at the southwestern part of the Cape Verdes during the past 155. kyrs. A somewhat surprising result is that such large eruptions occurred much more frequently from mafic alkalic magmas at Fogo than from highly evolved phonolitic magmas on Brava.