EFFECTS OF A CALDERA-FORMING ERUPTION ON THE FORAMINIFERAL COMMUNITY AT AXIAL SEAMOUNT, JUAN DE FUCA RIDGE

Caldera formation on submarine volcanoes has the potential to drastically alter the submarine ecosystem. Here we use foraminiferal records from sediment cores on Axial Seamount, Juan De Fuca Ridge, to evaluate the impact of deep marine caldera formation on planktic and benthic species. We approach this problem by examining the diversity and abundance of foraminifera deposited over the last 2300 cal. yr. BP, before, during and after a caldera-forming eruption. Caldera formation occurred ~1300-800 yr BP and is constrained by ¹⁴C age data and sedimentary lithofacies characteristics. Shannon diversity and Jaccard-Chao similarity indices were used to compare diversity and proportional abundance of foraminifera sampled from four cores near the caldera edge and one core ~3.2km east of the caldera.

Preliminary results show that prior to caldera formation, species richness ranged from 10 to 15. Hydrothermal sediments representing caldera formation typically contain no foraminifera, except two cores where diverse and abundant foraminifera assemblages were found with species abundances most similar to the diversity distribution of pre-caldera samples. Layers deposited after the caldera-forming eruption show an increase in species richness (20-65 species) and a relatively more even abundance distribution. Taxonomic overlap was high between pre- and post-eruptive facies suggesting little lasting change to the foraminifera community as a result of the caldera-forming event. Species composition and relative abundance are also consistent among comparable facies at core sites spread around the caldera perimeter and at ~3.2km distance from the caldera. Time-averaging of post-eruptive facies might have allowed the accumulation of rare taxa resulting in elevated species richness and more even abundance distributions compared to pre-eruptive sediment accumulations. Long-term shifts in sediment composition or changes to the surrounding marine environment over the ~2300 years of deposition represented by these samples might also result in the observed shifts in diversity.