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Stable isotopic analyses of modern benthic foraminifera from seasonally stratified shelf seas: disequilibria and the 'seasonal effect'

School of Ocean Sciences, University of Wales (Bangor), Menai Bridge, Anglesey LL59 5AB, UK; j.scourse{at}bangor.ac.uk

H. Kennedy

G. A. Scott

School of Ocean Sciences, University of Wales (Bangor), Menai Bridge, Anglesey LL59 5AB, UK

W. E.N. Austin

School of Geography and Geosciences, University of St Andrews, Irvine Building, North Street, St Andrews, Fife KY16 9AL, UK

Previously published stable isotopic data on benthic foraminiferal species from a Holocene sequence in the Celtic Sea have been interpreted in terms of the progressive replacement of a tidally mixed by a stratified water mass. Offsets in the ¹⁸O data between Ammonia batavus and Quinqueloculina seminulum were attributed to a ‘seasonal effect’ in which these two species were hypothesized to have calcified at different times of the year. The aims of this study were to test the hypotheses (1) that benthic foraminiferal stable isotope records from across the Celtic Sea front reflect seasonal stratification and (2) that offsets in the oxygen isotope record between different species are the result of the postulated seasonal effect. Hypothesis 1 was tested through investigation of live and dead benthic foraminiferal and bottom-water ¹⁸O and ¹³C sampled in transects across the Celtic Sea front from mixed through frontal to stratified water masses. Measurements of bottom-water salinity enabled a mixing-line equation to be developed for this area enabling quantitative reconstructions of bottom-water temperature from the isotopic data. Samples from stratified settings are characterized by heavier ¹⁸O_foram and lighter ¹³C_foram values than the mixed samples. Offsets in ¹⁸O_foram between A. batavus and Q. seminum support the notion of the seasonal effect. A. batavus produces values close to equilibrium while Q. seminulum overestimates temperature by up to 2°C and this might explain some of the offset observed between the two species observed in the palaeodata. Comparison of the ¹⁸O_foarm data with measured seasonal temperature cycles from mixed and stratified localities in the Celtic Sea demonstrates that, while most foraminifera calcify during the summer months, different species calcify at, or are preserved from, different times within this warm part of the seasonal cycle; Q. seminulum calcifies during September when peak bottom-water temperatures occur, while A. batavus calcifies during September in stratified localities, but during spring or early summer in mixed localities. This study confirms the interpretation of the ¹⁸O palaeodata from the Celtic Sea as a palaeostratification record and demonstrates that ¹⁸O data from shelf-sea cores can be used to supplement benthic foraminiferal assemblages as a tool for reconstructing the long-term dynamics of seasonal stratification.

Key Words: Celtic Sea • seasonal stratification • benthic foraminifera • stable isotopes • isotopic disequilibria • shelf sea • palaeoceanography

The Holocene, Vol. 14, No. 5, 747-758 (2004)
DOI: 10.1191/0959683604hl753rp


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