Composition and distribution of sediments and benthic foraminifera in a submerged caldera after 30 years of volcanic quiescence

Abstract

Abstract Deception Island (62°59′S, 60°34′W) is an active volcano with a submerged caldera (Port Foster) located northwest of the Antarctic Peninsula. Surveys following the most recent eruption of the volcano in 1970 showed that direct deposition of coarse pyroclastics into Port Foster decimated the benthic communities. The objective of this study is to examine how the submarine sediment distributions and the composition, distribution and abundance of benthic foraminiferal communities in Port Foster have changed after 30 years of volcanic quiescence. Grab, core and settling particulate matter were collected on a series of five cruises during 1999–2000 to monitor seasonal changes in the ecosystem of Port Foster. The caldera floor is covered with a layer at least 15 cm thick of yellowish-brown volacaniclastic sandy mud composed primarily of basaltic-andesitic lithic fragments and volcanic glass with mean and median grain diameters of 26±10 and 32±12 μm, respectively. We suggest that these sandy muds were deposited since the last volcanic eruption in 1970, yielding a minimum sediment accumulation rate of at least 5 mm/year. Settling particulate matter during the fall, winter and spring of 1999 and 2000 were finer grained than the seafloor samples, with mean and median grain diameters of 11±2 and 13±3 μm, respectively. Except for a slight coarsening of the mean grain diameter in the late spring, we observed no temporal changes in either the granulometry or the mineralogy of the settling particulate matter. The concentration of biogenous sediment in Port Foster is relatively low due to dilution by a high volume of lithogenic material being deposited in the bay. The abundance (from foraminifera numbers) and diversity of benthic foraminifera within Port Foster is spatially variable and patchy. Although previous studies of the benthic foraminiferal community following the 1967–1970 eruptions showed an increase in abundance (foraminifera number) of Port Foster's benthic foraminifera community from 1972 to 1976, our data show that these increases did not continue beyond the mid-1970s. The most notable similarities between the mid-1970s and 2000 assemblages are the predominance of Miliammina arenacea and Trochammina malovensis in Port Foster. Since the 1970s, there has been an increase in importance of Globocassidulina (the most abundant genera), a decrease in the relative abundance of Stainforthia fusiformis and Nonionella bradii, and the introduction of Reophax dentaliniformis, and Cassidulinoides parkerianus, which were not abundant in either Port Foster or on the outer slopes surrounding the flanks Deception Island during the 1970s. The later arrival of R. dentaliniformis to Port Foster suggests that it is not a pioneer colonizer following disturbance in all environments.