Benthic Foraminifera in Two Stressed Environments of the Northern Gulf of Mexico.

Abstract

This is a study of Foraminifera from two stressed environments of the northern Gulf of Mexico: one of coastal seasonal hypoxia, the other of hydrocarbon seeps in bathyal depths---oxygen depletion being the common stressor. Many species of benthic Foraminifera survive the extreme oxygen depletion of Louisiana shelf waters in spring and summer. The dominant species are infaunal and have an adaptive tolerance to near anoxia or anoxia. A census of foraminiferal species present in dated sediment core samples reveals significant changes in the composition of the Louisiana-shelf benthic foraminiferal community in the past century. Agglutinated and porcelaneous orders living in water depths between 27 and 60 m suffered a noticeable decline or even disappeared during this time. The declining trend of agglutinated and porcelaneous taxa agrees with the rising trend of the Ammonia-Elphidium index that was previously noticed in cores from <30-m depths. These trends may be explained by an increase in the severity of seasonal hypoxia in Louisiana coastal waters. Current data indicate that this hypoxia, related to eutrophication and water stratification, worsened in the past century, even near the outer edge of the present-day zone of oxygen depletion in spring and summer. Communities of living benthic Foraminifera were found in association with the chemolithotrophic, sulfide oxidizing bacterium Beggiatoa in hydrocarbon seeps of the northern Gulf of Mexico. Populations of foraminiferal species survive under Beggiatoa mats to a substrate depth of 4--5 cm where anoxia, H2S, and gaseous and/or liquid petroleum are major environmental stressors. The few commonly occurring live species found in surface and subsurface sediments at cold seeps can survive limited amounts of both oil and H2S but none of the species apparently benefits from these compounds. Stable carbon isotope composition of calcitic foraminiferal tests from a 24-cm long core shows a progressively decreasing trend with time, which match well the trend of assemblage density, percentage of total carbon, and increased coarse sediment fraction. These findings reflect temporal variations in seep activity and show that delta 13C values of foraminiferal tests are good indicators of hydrocarbon releases, and can be used in their historical reconstructions.