RESULTS AND INTERPRETATION

The results of isotopic analysis are shown in Figure 4. ¹⁸O and ¹³C values are plotted on the y-axis (values decreasing upward, as conventional) against shell height from the origin of growth (except for specimen PB1, where the early part of the shell had been destroyed; in this specimen height is expressed relative to the ventral margin). A tabulated version of the full dataset, together with calculated temperatures, can be obtained as a downloadable Excel (v.5/95) file.

All shells analyzed show cyclical patterns of ¹⁸O, with values mostly ranging between approximately -0.5 and +2.5. This range, together with the shell-height separation of minima and maxima, is similar to that observed in modern indigenous A. opercularis from the southern North Sea (Figure 5), and, as in these, can be interpreted as the result of seasonal temperature variation (given equilibrium deposition, demonstrated in modern A. opercularis, shell isotopic composition is determined only by temperature and the isotopic composition of ambient water; the latter can be assumed to be constant—see above). Single-point large interruptions (in ¹⁸O and ¹³C) to the cyclical pattern—at 29 mm from the umbo in PB2, 10 mm in PB4 and 30 mm in SP1—probably constitute a "transient spike" (Krantz et al. 1988), or analytical artifact, rather than an environmental effect. Minor peaks and troughs in the profiles are almost certainly related to environmental influences; similar small fluctuations, greater than levels of analytical precision, were noted in modern shells and explained by reference to measured temperature fluctuations that probably relate to movements of fronts separating water bodies of differing compositions and temperatures (Hickson et al. 1999).

In detail, results from the Holocene shells differ from modern shells. If a ¹⁸O value of 0.79 (corresponding to a temperature of 13°, roughly halfway between typical extreme summer and winter temperatures for the southern North Sea; Figure 6) is taken to mark the transition between the warmest ("summer") and coolest ("winter") halves of the year, then the "summer" sectors (¹⁸O values <0.79; T >13°) are shorter in the Holocene shells than in the modern shells; the "winter" sectors (¹⁸O values >0.79; T <13°) are markedly longer in terms of shell height for the Holocene shells (Figure 4 and Figure 5; Hickson 1997). As well as exhibiting more extensive winter growth in comparison to modern shells, Holocene shells generally lack winter growth rings (marking growth interruptions) and yield larger extreme winter ¹⁸O values, corresponding to lower temperatures (Figure 4 and Figure 6). The temperatures represented by the largest positive ¹⁸O values of modern shells are several degrees above the coldest temperatures that the animals experienced (Figure 5 and Figure 6). Given the presence of strong growth rings in modern shells at positions corresponding to the largest positive ¹⁸O values (Figure 5; Hickson et al. 1999), this undoubtedly reflects growth cessation during the period of lowest temperatures. Greater overall winter growth and the general absence of growth rings suggest, by contrast, that the Holocene shells deposited shell material throughout the winter and that the (lower) minimum temperatures registered are representative of the coldest experienced. The slightly less extensive summer growth of Holocene shells is not matched by the presence of growth rings; hence, as in modern shells, it can be assumed that growth continued uninterrupted during the summer and that the highest temperatures registered are indicative of the warmest temperatures experienced.

Some ¹³C profiles show cyclicity (in phase with ¹⁸O), but this is not represented in every shell investigated, and variation in ¹³C (mainly between +0.5 and +1.5) is much less than for ¹⁸O. Carbon isotope variations are more difficult to interpret than oxygen; work on non-biogenic carbonates (Romanek et al. 1992) has indicated that temperature is not a primary control. Prior studies of A. opercularis have shown that external rather than metabolic sources of carbon are the most important (Hickson et al. 1999); therefore, the likely cause of carbon-isotopic variation in shells is change in the ¹³C of dissolved inorganic carbon (DIC), which varies in relation to photosynthesis and decomposition of organic carbon (Arthur et al. 1983). The patterns observed in A. opercularis shells (modern as well as Holocene) do not, however, conform to a model (Purton and Brasier 1997; figure 4a) proposed for a hydrographic setting such as that of the southern North Sea: shallow, well-mixed water unaffected by upwelling. Purton and Brasier's model predicts antiphase cyclical variations in ¹⁸O and ¹³C, such that when ¹⁸O values are at their highest (in winter/early spring), ¹³C values will be at their lowest, due to breakdown of ¹²C-rich organic matter; conversely, when ¹⁸O values are low due to higher temperatures, ¹³C values should be at their highest, reflecting uptake of ¹²C by phytoplankton. In A. opercularis, however, fluctuations are either clearly in-phase (e.g., Figure 4e and 4f) or carbon-isotope variation is not cyclical at all (e.g., Figure 4a). Perhaps this in some way relates to the pattern of variation in phytoplankton abundance in the southern North Sea, which differs from that of other temperate shelf seas: at the present time a spring "bloom" does occur, but the population crash that would normally follow this is not as marked as it is, for example, in the northern North Sea (Tett and Walne 1995). In addition, fluctuation in ¹³C _DIC might conceivably be dampened by decomposition (particularly in summer) of ¹²C-rich organic matter supplied by rivers entering the North Sea or introduced in the form of oil and gas seepages.

Although there appears to be no pattern (or straightforward explanation) of short-term variation in ¹³C (i.e., as represented within shells), there is a striking difference in the range and mean of ¹³C values from Holocene and modern shells. Most Holocene values are greater than +0.5 (mean +0.79; n = 209); modern values are nearly always less than +0.5 (mean -0.04; n = 197; sample consisting of all values given by Hickson 1997). The difference in means is statistically significant (F and t tests; = 0.05). Conceivably, this difference could be due to higher year-round productivity in the Holocene (leading to higher ¹³C _DIC). This would be consistent with evidence (see above) of more extensive winter growth in the Holocene (winter growth deceleration in modern A. opercularis may be partly due to reduced food supply rather than low temperature [Broom and Mason 1978]) and might also explain slightly less extensive summer growth in that phytoplankton levels above those of the modern North Sea during summer could cause "clogging" of the gills and slower growth (Chavaud et al. 1998). However, there is no independent evidence of higher Holocene productivity and intuitively one would expect modern productivity to be higher, given the vast present-day supply of nutrients to the North Sea from domestic and agricultural sources. By contrast, there is voluminous evidence that, as the result of greater combustion of fossil fuels, modern atmospheric CO₂ levels are higher than in the pre-industrial Holocene (Friedli et al. 1986), and that associated reductions in atmospheric ¹³C have been translated through air-sea CO₂ exchange into lower modern values of ¹³C for DIC and biogenic carbonate (Beveridge and Shackleton 1994; Böhm et al. 1996). Like values of ¹³C from modern A. opercularis, those from Pliocene examples are also lower than values from Holocene shells (Johnson et al. 2000). Given that independent evidence suggests that Pliocene atmospheric CO₂ levels were comparable to present (Kürschner et al. 1996), it seems reasonable to conclude that the high ¹³C values of Holocene A. opercularis shells are a reflection of low atmospheric CO₂.