Pore water composition of volcanogenic sediments from across the Central American Subduction Zone

Abstract

Subduction Zones are among the most dynamic tectonic environments on Earth. Thus, the Central American Suduction Zone became the focus of research cruise Meteor 66 since here the sediment cover is strongly influenced by the highly explosive volcanism along the Central American Volcanic Arc. The main focus of this study is to analyse the influence of alteration reactions on the chemical composition of pore waters extracted from volcanogenic surface sediments. During research cruise RV Meteor 66, a program of sampling and analysis of pore waters from retrieved sediment cores was carried out. Alkalinity, sulphate, calcium, magnesium, potassiumand silica concentration-depth profiles of deep-sea sediment coresweremeasured to define the ash alteration reactions that occur in surface sediments. The concentrationprofiles ofmajor cations showed no clear effects of ash alteration, which is in contrast to investigations performed on deep sediment cores (N100 m depth) during the Deep Sea (DSDP) and Ocean Drilling (ODP) Programs. However, no significant difference in dissolved Ca and Mg could be observed between the ash-bearing cores and the ash-free reference core, confirming that the distribution of alkaline earth elements in the surface sediments investigated in our study is not significantly affected by ash alteration. The missing signal of ash alteration is probably caused by low reaction rates and the high background concentration of major dissolved ions in the seawaterderived pore waters. Apart from the generally low reactivity of the investigated of the investigated sediments, two interesting observations could be made: (1) Potassium pore water concentrations are enriched in background sediments and depleted in ash-bearing sediment layers; (2) Ash alteration seems to affect dissolved silica contents in pore waters, causing concentrations about two times higher than in the ash-free reference core.