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A 200-year environmental record of a low-oxygen fjord, Sweden, elucidated by benthic foraminifera, sediment characteristics and hydrographic data

Journal article
Authors Helena L. Filipsson
Kjell Nordberg
Published in Journal of Foraminiferal Research
Volume 34
Issue 4
Pages 277-293
ISSN 0096-1191
Publication year 2004
Published at Department of Earth Sciences
Pages 277-293
Language en
Links <Go to ISI>://000225870900003
Keywords swedish west-coast, koljo-fjord, carbonate dissolution, north-sea, havstens-fjord, gullmar fjord, skagerrak, climate, diversity, survival
Subject categories Earth and Related Environmental Sciences

Abstract

In this study, we have detailed the benthic environment of a sill fjord on the Swedish west coast, which is characterized by brackish conditions, intermittent water exchanges, and periodic low oxygen. We have used three important tools-benthic foraminifera, sediment characteristics, and historical records of hydrographic data. Periods of low oxygen have resulted in sequences of laminated sediments in the fjord. The foraminiferal fauna present in the fjord is of low diversity and low abundance, mostly Elphidium species-an assemblage that has dominated the foraminiferal fauna for at least 170 years. During this time period, however, there have been events when normal marine species have been introduced, or foraminiferal abundance and faunal diversity have suddenly increased. These events are linked to short sequences of homogenous sediments within the laminated sequences that, in turn, suggest more oxic conditions. Since the late 1940s, we have a more-or-less continuous salinity record from the fjord, the product of an instrumental monitoring program. This record has been used, together with the foraminiferal distribution and sediment characteristics, to reconstruct the salinity variations that occurred prior to the instrumental monitoring program. The salinity in the fjord has alternated between periods of low salinity (26-28.5%), which correspond with non-laminated sediments, and periods of higher salinity (>28.5%), corresponding with laminated sequences. Despite a general low abundance of foraminiferal fauna, there was a significantly higher diversity during deposition of laminated sequences than non-laminated sequences. In the deepest part of the record, corresponding to the late eighteenth and early nineteenth century, a different fauna occurs, where species in addition to Elphidium occur, suggesting generally higher salinities (29-31).

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