Foraminifera, climatic change, and pollution: a study of late Holocene sediments in Drammensfiord, southeast Norway

Abstract

Benthic foraminifera are shown to reflect both natural (including climatic) and human- induced environmental alterations over the last 1500 years in Drammensfjord, Norway. Today, Drammensfjord is a 124 m deep, partly anoxic, hyposaline basin (salinity <32%o) separated from the adjoining Oslo Fjord by a sill at 10 m water depth. It has been affected by human-induced organic pollution for more than a century. Based on major faunal trends, sediment characteristics and concentrations of plant fragments and pollen grains, four environmental zones are distinguished. The faunal composition of zones 1-3 (before AD 500 to about AD 1850) reflects the natural environmental development from a period with frequent deep water renewals and diverse, calcareous dominated assemblages, to periods with increasing river flow which impeded effective deep water renewals, and gave rise to agglutinated assemblages of low diversity. Increasing human-induced organic flux over the last 130-140 years has elevated the redox cline. The general faunal responses as represented by zone 4 are: 1) development from agglutinated assemblages of zone 3 via assemblages dominated by Spiroplectammina biformis or Stainforthia fusiformis to the present anoxic, afaunal conditions, and 2) strongly increasing foraminiferal abundance with maximum values just prior to the afaunal intervals. The approach has considerable potential for differentiating the effects of pollution from the natural 'background'.