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

Abstract

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 6180 data between Ammonia batavus and Quinqueloculina seminuluin 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 6180 and f X13C sampled in transects across the Celtic Sea front from mixed through frontal to stratified water masses. 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 6180 palaeodata from the Celtic Sea as a palaeostratification record and demonstrates that 6180 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.

Tidal evolution of the northwest European shelf seas from the Last Glacial Maximum to the present

“…This study verified the nonisostatically corrected M 2 model predictions for tidal mixing front change in the Celtic Sea by Austin [1991]. A study of the distribution of living benthic foraminiferal assemblages from across the Celtic Sea frontal region [ Scott et al , 2003] supports the interpretation of the core assemblages as driven by stratification, and parallel studies on the stable isotopic content of live benthic foraminifera [ Scourse et al , 2004] also support the hypothesis that these are controlled by bottom water temperature and are therefore linked to stratification. Scourse et al [2002], based on a suite of AMS 14 C dates, revised the timing of stratification to between 8990 and 8440 cal yr BP (8720 ± 2 σ ).…”

Tidal evolution of the northwest European shelf seas from the Last Glacial Maximum to the present

“…This study verified the nonisostatically corrected M 2 model predictions for tidal mixing front change in the Celtic Sea by Austin [1991]. A study of the distribution of living benthic foraminiferal assemblages from across the Celtic Sea frontal region [ Scott et al , 2003] supports the interpretation of the core assemblages as driven by stratification, and parallel studies on the stable isotopic content of live benthic foraminifera [ Scourse et al , 2004] also support the hypothesis that these are controlled by bottom water temperature and are therefore linked to stratification. Scourse et al [2002], based on a suite of AMS 14 C dates, revised the timing of stratification to between 8990 and 8440 cal yr BP (8720 ± 2 σ ).…”

Mid-latitude shelf seas: a NW European perspective on the seasonal dynamics of temperature, salinity and oxygen isotopes

“…In this study, the most sensitive regions appear to be the coastal waters of the southern North Sea, from the German Bight (Scherule and Hebbeln, 2003) towards the Baltic (Frö hlich et al, 1988) and along the west Norwegian coast (Mikalsen and Sejrup, 2000). European coastal waters least likely to be affected by marked salinity:d 18 O water relationships are those to the west, where the freshwater end-member (c) is least depleted (Austin and Inall, 2002;Owen et al, 2002;Scourse et al, 2004).…”

“…As one might expect from the GNIP data (Figure 1), estimates of the freshwater end-member (c) vary greatly throughout European and northern waters (Table 1), with the least fractionated values typically reported from the west coast regions, eg, (/6.0 Scottish west coast (Austin and Inall, 2002); (/7.42 Irish/Celtic Seas (Owen et al, 2002;Scourse et al, 2004). Values of c around the North Sea are variable, ranging from (/8.91 in the Baltic Sea (Frö hlich et al, 1988), (/9.36 in the German Bight (Scheurle and Hebbeln, 2003) and (/14.55 in the central North Sea (Ganssen in Witbaard et al, 1994).…”

Comparison of two methods to identify live benthic foraminifera: A test between Rose Bengal and CellTracker Green with implications for stable isotope paleoreconstructions

“…The slight offsets in Cibicidoides sp. and S. reticulata δ 18 O from that of bottom water could be explained by changes in the water's δ 18 O signature because of seasonality, as observed in some paleoceanographic studies [e.g., Austin and Scourse , 1997; Scourse et al , 2004]. Seasonality is an unlikely explanation in our study, however, because all foraminifers analyzed for isotopes were collected from depths >600 m, which exceeds the depth of the mixed zone and thus is expected to have insignificant seasonal temperature variations.…”

Tidal evolution of the northwest European shelf seas from the Last Glacial Maximum to the present

“…This study verified the nonisostatically corrected M 2 model predictions for tidal mixing front change in the Celtic Sea by Austin [1991]. A study of the distribution of living benthic foraminiferal assemblages from across the Celtic Sea frontal region [ Scott et al , 2003] supports the interpretation of the core assemblages as driven by stratification, and parallel studies on the stable isotopic content of live benthic foraminifera [ Scourse et al , 2004] also support the hypothesis that these are controlled by bottom water temperature and are therefore linked to stratification. Scourse et al [2002], based on a suite of AMS 14 C dates, revised the timing of stratification to between 8990 and 8440 cal yr BP (8720 ± 2 σ ).…”

Mid-latitude shelf seas: a NW European perspective on the seasonal dynamics of temperature, salinity and oxygen isotopes

“…In this study, the most sensitive regions appear to be the coastal waters of the southern North Sea, from the German Bight (Scherule and Hebbeln, 2003) towards the Baltic (Frö hlich et al, 1988) and along the west Norwegian coast (Mikalsen and Sejrup, 2000). European coastal waters least likely to be affected by marked salinity:d 18 O water relationships are those to the west, where the freshwater end-member (c) is least depleted (Austin and Inall, 2002;Owen et al, 2002;Scourse et al, 2004).…”

“…As one might expect from the GNIP data (Figure 1), estimates of the freshwater end-member (c) vary greatly throughout European and northern waters (Table 1), with the least fractionated values typically reported from the west coast regions, eg, (/6.0 Scottish west coast (Austin and Inall, 2002); (/7.42 Irish/Celtic Seas (Owen et al, 2002;Scourse et al, 2004). Values of c around the North Sea are variable, ranging from (/8.91 in the Baltic Sea (Frö hlich et al, 1988), (/9.36 in the German Bight (Scheurle and Hebbeln, 2003) and (/14.55 in the central North Sea (Ganssen in Witbaard et al, 1994).…”

Comparison of two methods to identify live benthic foraminifera: A test between Rose Bengal and CellTracker Green with implications for stable isotope paleoreconstructions

“…The slight offsets in Cibicidoides sp. and S. reticulata δ 18 O from that of bottom water could be explained by changes in the water's δ 18 O signature because of seasonality, as observed in some paleoceanographic studies [e.g., Austin and Scourse , 1997; Scourse et al , 2004]. Seasonality is an unlikely explanation in our study, however, because all foraminifers analyzed for isotopes were collected from depths >600 m, which exceeds the depth of the mixed zone and thus is expected to have insignificant seasonal temperature variations.…”

Tidal evolution of the northwest European shelf seas from the Last Glacial Maximum to the present

“…This study verified the nonisostatically corrected M 2 model predictions for tidal mixing front change in the Celtic Sea by Austin [1991]. A study of the distribution of living benthic foraminiferal assemblages from across the Celtic Sea frontal region [ Scott et al , 2003] supports the interpretation of the core assemblages as driven by stratification, and parallel studies on the stable isotopic content of live benthic foraminifera [ Scourse et al , 2004] also support the hypothesis that these are controlled by bottom water temperature and are therefore linked to stratification. Scourse et al [2002], based on a suite of AMS 14 C dates, revised the timing of stratification to between 8990 and 8440 cal yr BP (8720 ± 2 σ ).…”

Mid-latitude shelf seas: a NW European perspective on the seasonal dynamics of temperature, salinity and oxygen isotopes

“…In this study, the most sensitive regions appear to be the coastal waters of the southern North Sea, from the German Bight (Scherule and Hebbeln, 2003) towards the Baltic (Frö hlich et al, 1988) and along the west Norwegian coast (Mikalsen and Sejrup, 2000). European coastal waters least likely to be affected by marked salinity:d 18 O water relationships are those to the west, where the freshwater end-member (c) is least depleted (Austin and Inall, 2002;Owen et al, 2002;Scourse et al, 2004).…”

“…As one might expect from the GNIP data (Figure 1), estimates of the freshwater end-member (c) vary greatly throughout European and northern waters (Table 1), with the least fractionated values typically reported from the west coast regions, eg, (/6.0 Scottish west coast (Austin and Inall, 2002); (/7.42 Irish/Celtic Seas (Owen et al, 2002;Scourse et al, 2004). Values of c around the North Sea are variable, ranging from (/8.91 in the Baltic Sea (Frö hlich et al, 1988), (/9.36 in the German Bight (Scheurle and Hebbeln, 2003) and (/14.55 in the central North Sea (Ganssen in Witbaard et al, 1994).…”

Comparison of two methods to identify live benthic foraminifera: A test between Rose Bengal and CellTracker Green with implications for stable isotope paleoreconstructions

“…The slight offsets in Cibicidoides sp. and S. reticulata δ 18 O from that of bottom water could be explained by changes in the water's δ 18 O signature because of seasonality, as observed in some paleoceanographic studies [e.g., Austin and Scourse , 1997; Scourse et al , 2004]. Seasonality is an unlikely explanation in our study, however, because all foraminifers analyzed for isotopes were collected from depths >600 m, which exceeds the depth of the mixed zone and thus is expected to have insignificant seasonal temperature variations.…”