Marine Tephras offshore Nicaragua and Costa Rica – Determination of sedimentation rates, eruption volumes, marine stratigraphy

Abstract

Gravity cores were collected during SONNE Cruise SO173/3 and METEOR Cruise M54/3 offshore Nicaragua and Costa Rica on the lower continental slope and on the ocean plate, at distances of 150-350 km to the Central American Volcanic Arc (CAVA). Thirteen core sections up to eleven meters below seafloor contain a total of 77 primary ash layers, commonly with sharp contacts at the bottom and diffuse transitions to terrigenous and pelagic sediments at the top. Ash layer thickness ranges from 0.5 to 23 cm with typical grain sizes from medium silt to coarse sand. Electron microprobe analyses of 2000 glass shards yield compositions ranging from basalt to rhyolite. Felsic ashes can be divided into four compositional groups by means of silica and potassium. Correlations between the cores and well dated on-land tephras are constrained by petrographical and stratigraphical criteria, major element geochemistry of glasses and minerals, and trace element data from LA-ICPMS analyses. The marine tephra record provides important data for ongoing studies of CAVA volcanism: (a) re-evaluated erupted tephra volumes yield 2-3 times the values derived from on-land exposures limited by the coast 30 km from the arc; (b) stratigraphic correlations along the entire arc can be traced much more completely in the marine sediment cores than by limited exposures on land. Moreover, well correlated and dated marine ash layers help to reveal the forearc sedimentary history, e.g. by providing clastic sedimentation rates (Ø 8 cm/ka ocean plate; Ø 40 cm/ka slope) or dating slumping events.