2.5. Discussion
2.5.4. Predicted vs observed 18 O shell
2.5.4. Predicted vs. observed 18O shell
Salinity in the Pambula Lake estuary is variable and influenced by non-systematic patterns induced by rainfall events and evaporation. The high rainfall events of February and June 2007, and the consequent influx of fresh water with a more negative 18O value results in the prediction of low 18Oshell values of –8.5 and –6 ‰ V-PDB. It is presumed that the low salinity coincident with these extreme rain events was such that the oysters ceased to precipitate calcium carbonate at these times. Accordingly, these negative 18Oshell values are not recorded in the O.
angasi isotopic profiles, and are indicated by the dashed lines in the Pambula Lake predicted 18Oshell profile in Figure 2.4. This is supported by the shell extension rates (Figure 3), which show reduced growth rates for February (the
actual low salinity event) and March (the recovery period) of approximately two thirds the average growth rate for the entire experiment period. It follows that the oysters may have ceased calcite precipitation during this event, and so the shells do not record the predicted extreme 18Oshell values.
In the absence of observable growth structures that provide a temporal context for shell precipitation and that can be matched to the predicted 18Oshell profile, the predicted and observed O. angasi 18Oshell profiles were compared using AnalySeries software (Version 2.4.0.2). This program facilitated the comparison of the predicted and observed curves via manual determination of multiple discrete tie-points, and scaling of the x-axis (distance along the shell growth axis, i.e. growth rate). This accounts for the variable growth rate common to oysters and evident in the O. angasi growth records. As it is assumed that O. angasi will cease to precipitate any calcite during low salinity (and correspondingly negative 18Owater) events, a threshold of -1.87 ‰ V-PDB, the minimum value observed in the Pambula Lake O. angasi, was assigned for the Pambula Lake predicted 18Oshell (Figure 4).
Comparisons of the predicted 18Oshell and the analysed 18Oshell for the
O. angasi from each location are shown in Figure 2.6. Overall, the 18Oshell of the Pambula Lake O. angasi shows good agreement with the predicted 18Oshell profile. However, two sharp peaks of relatively high 18O that are not seen in the measured O. angasi18Oshell profiles occur in the predicted 18Oshell profile in mid- November 2006 and mid-December 2006. These correlate to sharp drops in temperature of ~1 week in duration. It appears that the sampled O. angasi18Oshell record is of insufficient resolution to record these brief temperature excursions, with the isotopic record being averaged over the time period that the sample
resolution would result in greater fidelity between the predicted and observed 18Oshell records.
Figure 2.6. Comparisons of predicted and observed 18O
shell for Pambula Lake and Little
Swanport O. angasi. These records correspond to the portion of shell growth that occurred during
the experiment period, i.e. the most recent year of growth only. Pambula Lake predicted 18O shell
was assigned a threshold value of −1.87 ‰ V-PDB. While overall the O. angasi18O
shell records
from both locations show good agreement with the predicted 18O
shell profiles, there are some
It is also apparent that the Pambula Lake O. angasi possibly do not record values greater than ~0.4 ‰ V-PDB in their 18Oshell. This could be explained if shell growth ceases or is greatly reduced at lower winter temperatures. However, this does not seem likely, as although the shell extension rates (Figure 2.3) for the 2006 winter period are lower than the average growth rate, there is no evidence for a complete cessation of growth. Moreover, the longer-lived O. angasi from the Little Swanport site consistently record 18Oshell values more positive than 0.4 ‰ V-PDB, (up to 1.87 ‰ V-PDB) and thus continue to grow at considerably lower temperatures.
Given the spatial extent between the two sites, it is possible that there exists some genetic variability between the two populations of O. angasi, which may exert some influence upon the 18Oshell, as discussed in Wanamaker et al. (2007). Evidence for genetic introgression with the New Zealand flat oyster,
Ostrea chilensis (=Tiostrea chilensis = Tiostrea lutaria = Ostrea lutaria) has been found among Tasmanian O. angasi (W. O’Connor, personal communication 2012). Significant similarities with O. edulis have been reported for O. angasi
populations in Albany, Western Australia (Morton et al., 2003). It has also been found that individual cohorts of O. angasi juveniles do not necessarily represent the genetic variability of the entire population (W. O’Connor, personal communication 2012). However, further rigorous genetic and geochemical studies would be required to 1) confirm the potential effects of genetic variability on variations between O. angasi populations, and 2) reasonably attribute differences in the 18Oshell of the O. angasi from different geographical locations to any such genetic variability.
2.5.5. 18O
shell-temperature reconstructions
2.5.5.1. Pambula Lake
To assess the accuracy of a temperature record obtained from O. angasi 18Oshell, paleotemperature Equations 2.1 and 2.3 were used to reconstruct temperature records from the 18Oshell. As it was not possible to ascribe a temporal context to the oyster 18Oshell from structural analysis, we were unable to constrain variations in 18Owater, so average values for 18Owater of 0.77‰ (V-SMOW) were used for both Pambula Lake and Little Swanport. This unavoidably introduces an error into the temperature records, as the 18Owater is clearly affected by sporadic influxes of fresh water in both estuaries. However, any application of the 18Oshell temperature proxy in a paleoclimate context requires this assumption, as exact values for paleo-18Owater will not be known (Brigaud et al., 2008; Freitas et al., 2012).
Figure 2.7 shows comparisons of the temperature record constructed from the Pambula Lake O. angasi shells using the two paleotemperature equations described earlier with the instrumental water temperature recorded during the experiment period.
The precision of the 18O measurements in this study, (i.e. better than 0.1 ‰ V-PDB) equates to a maximum temperature uncertainty at 18Oshell of −1.87 ‰ of 0.5 ºC for both Equation 2.1 and 2.3. This uncertainty decreases slightly at the highest reported 18Oshell of 1.87 ‰ to 0.4ºC for each equation.
The Pambula Lake O. angasi show a sharp peak in summer temperature, rather than the comparatively sustained period of ~23ºC in the instrumental temperature record. This is attributable to the growth cessation that resulted from the high rainfall event of February 2007.
Figure 2.7. Comparison of observed temperatures recorded during the in situ field culture
experiments at Pambula Lake with O. angasi18O
shell-reconstructed temperatures using the
equations of Epstein et al. (1953) and Kim & O’Neil (1997), after Wanamaker et al. (2006). The
O. angasi18O
shell-reconstructed temperatures tend to overestimate winter temperatures and show
a distinct peak in summer temperature rather than the comparatively sustained temperatures in the observed temperature record.
It is also notable that the 18Oshell-temperature profiles constructed from both calibration equations over-estimate calcification temperatures during each winter of the experiment period (Table 2.4 Figure 2.7). As previously discussed, it is unlikely that this is caused by a growth cessation at low temperature during the winter, as shell measurements indicate continued growth during winter, and lower temperatures than those experienced at Pambula Lake are recorded in the shells from Little Swanport. A possible explanation for the discrepancy is the use of a single average value for 18Owater (0.77 ‰ V-SMOW) for the entire temperature
during the first winter of the experiment period would have driven the Pambula Lake 18Owater to more negative values than the average 18Owater value of 0.77 ‰ V-SMOW. More negative 18Owater values during this winter period would cause reconstructed temperatures to appear warmer than true calcification temperatures.
The range of 18Owater values required to match shell calcification temperatures with instrumental temperatures of 12–14ºC during the 2006 winter period is −0.75−0.45 ‰ V-SMOW, which is equivalent to a salinity of ~25.5–27 (from Equation 2.4). These salinity values are plausible given the amount of rainfall received during this period, and fall within the tolerance range for O. angasi and so are unlikely to have caused a low-salinity growth cessation. Furthermore, despite the relatively short residence time of water in Pambula Lake of ~1.5 days, the prolonged nature of this rainfall event, where four out of the six days between 25 February and 2 March 2007 received appreciable rainfall, would have resulted in a persistently lower salinity within the lake.
The winter of 2007 experienced even higher rainfall, low salinity and 18Owater below the threshold value of 1.87 ‰ V-SMOW. Hence, the failure of the Pambula Lake O. angasi to record the low winter temperatures of this time could result from a low-salinity-induced growth cessation potentially compounded by the error introduced by using an average 18Owater value in the temperature calculation.
Table 2.4. Temperatures reconstructed from Pambula Lake O. angasi 18Oshell, using the equations of Epstein et al. (1953) and Kim & O’Neil (1997), after Wanamaker et al. (2006), compared to the instrumental temperature recorded over the experiment periods (July 2006–June 2007). The offset between the reconstructed and instrumental temperature is shown in square brackets.
Growth period Pambula Lake temperature PLOa410 Epstein et al. (1953) PLOa410 Kim & O'Neil (1997), after Wanamaker et al. (2006) PLOa421 Epstein et al. (1953) PLOa421 Kim & O'Neil (1997), after Wanamaker et al. (2006) PLOa440 Epstein et al. (1953) PLOa440 Kim & O'Neil (1997), after Wanamaker et al. (2006) PLOa472 Epstein et al. (1953) PLOa472 Kim & O'Neil (1997), after Wanamaker et al. (2006) 2006 winter 11.6 18.3 [6.7] 18.2 [6.6] 17.3 [5.7] 17.1 [5.5] 17.6 [6.0] 17.4 [5.8] 17.7 [6.1] 17.6 [6.0] 2006-07 summer 24.7 27.5 [2.8] 26.6 [1.9] 25.7 [1.0] 25.1 [0.4] 25.2 [0.5] 24.7 [0] 26.2 [1.5] 25.5 [0.8] 2007 winter 11.9 19.6 [7.7] 19.5 [7.6] 17.0 [5.1] 16.8 [4.9] 17.2 [5.3] 17.0 [5.1] 16.4 [4.5] 16.2 [4.3] 2006-07 annual mean 17.5 21.4 [3.9] 21.1 [3.6] 20.5 [3] 20.3 [2.8] 20.3 [2.8] 20.3 [2.8] 20.1 [2.6] 19.8 [2.3]
Table 2.5. Temperatures reconstructed from Little Swanport O. angasi 18Oshell, using the equations of Epstein et al. (1953) and Kim & O’Neil (1997), after Wanamaker et al. (2006), compared to the instrumental temperature recorded over the experiment periods (September 2006–July 2007). Historical temperatures were calculated from the relationship between the Little Swanport water temperature and air temperature at the Orford Bureau of Meteorology field station. Also shown is the instrumental temperature record from the Maria Island Integrated Marine Observing System-Australian National Moorings Network National Reference Station, ~30km from the Little Swanport estuary. The offset between the reconstructed and instrumental temperature is shown in square brackets. Growth period Maria Island temperature Little Swanport (Orford) temperature LSOa754 Epstein et al. (1953) LSOa754 Kim & O'Neil (1997), after Wanamaker et al. (2006) LSOa756 Epstein et al. (1953) LSOa756 Kim & O'Neil (1997), after Wanamaker et al. (2006) LSOa793 Epstein et al. (1953) LSOa793 Kim & O'Neil (1997), after Wanamaker et al. (2006) 1999 winter 12.8 11.3 15.0 [3.7] 14.7 [3.4] 1999-00 summer 17.8 19.4 21.3 [1.9] 21.1 [1.8] 2000 winter 12.9 11.0 14.3 [3.3] 13.9 [2.9] 2000-01 summer 18.9 20.1 21.0 [0.8] 20.8 [0.7] 2001 winter 13.3 11.7 13.4 [1.7] 12.9 [1.2] 2001-02 summer 17.5 17.5 16.7 [-0.8] 16.5 [-1.0] 2002 winter 13.0 11.6 13.1 [1.5] 12.5 [1.0] 10.9 [-0.7] 9.9 [-1.7] 2002-03 summer 18.6 19.6 16.9 [-2.7] 16.7 [-2.9] 19.9 [0.3] 19.8 [0.1] 2003 winter 13.0 10.9 13.5 [2.6] 13.0 [2.1] 14.1 [3.1] 13.6 [2.7] 2003-04 summer 17.4 18.1 19.0 [0.9] 18.9 [0.8] 20.1 [2.0] 19.9 [1.9]
2004 winter 12.0 9.9 13.8 [3.8] 13.3 [3.4] 13.2 [3.3] 12.7 [2.8] 2004-05 summer 16.7 18.1 18.2 [0.1] 18.0 [0] 18.3 [0.3] 18.2 [0.2] 18.6 [0.5] 18.5 [0.4] 2005 winter 12.2 11.4 13.8 [2.4] 13.3 [1.9] 12.4 [1.1] 11.8 [0.4] 12.5 [1.2] 11.9 [0.5] 2005-06 summer 16.1 18.5 19.2 [0.7] 19.1 [0.5] 18.0 [-0.5] 17.8 [-0.7] 21.1 [2.6] 20.9 [2.4] 2006 winter 12.4 10.5 14.7 [4.1] 14.3 [3.8] 12.8 [2.3] 12.2 [1.7] 13.5 [3.0] 13.0 [2.5] 2006-07 summer 17.8 19.2 18.4 [-0.7] 18.3 [-0.9] 19.2 [0] 19.0 [-0.1] 20.9 [1.8] 20.8 [1.6] 2007 winter 13.7 9.9 17.3 [7.3] 17.1 [7.1] 11.7 [1.7] 10.9 [0.9] 11.5 [1.5] 10.6 [0.7] 1999-00 annual mean 15.0 15.0 17.4 [2.4] 17.2 [2.2] 2000-01 annual mean 15.9 15.1 18.0 [2.8] 17.7 [2.6] 2001-02 annual mean 15.6 14.7 14.8 [0] 14.4 [-0.3] 2002-03 annual mean 15.4 15.0 14.9 [-0.1] 14.6 [-0.4] 16.6 [1.5] 16.3 [1.2] 2003-04 annual mean 14.7 14.4 16.0 [1.7] 15.8 [1.4] 16.3 [1.9] 16.0 [1.7] 2004-05 annual mean 13.5 14.4 16.0 [1.6] 15.7 [1.3] 15.1 [0.7] 14.8 [0.4] 2005-06 annual mean 13.8 14.6 16.5 [1.9] 16.3 [1.6] 16.5 [1.9] 16.2 [1.6] 17.3 [2.7] 17.1 [2.4] 2006-07 annual mean 15.0 14.8 16.5 [1.8] 16.3 [1.5] 15.0 [0.2] 14.7 [-0.1] 17.1 [2.3] 16.8 [2.0]
2.5.5.2. Little Swanport
Comparisons of the Little Swanport O. angasi 18Oshell temperature reconstructions with recorded temperatures are shown in Figure 2.8. The greater sampling resolution achieved in the fast-growing earlier years compared to later years of the Little Swanport O. angasi shells growth is clearly evident. After the first two years of shell growth, the seasonal records appear to be truncated during the summer in the older shells. The early years of growth in shells LSOa754 and LSOa756 show seasonal amplitudes equivalent to that observed in the younger shell LSOa793.
Water temperature data for the Little Swanport estuary encompassing the entirety of the older Little Swanport O. angasi growth were not available. An estimate of water temperatures was calculated via comparison of air temperature records from the Orford Bureau of Meteorology field station (located close to the Little Swanport site) and the water temperature in the Little Swanport estuary over the monitoring period. Assuming a constant relationship between these datasets (i.e. an offset of ~2ºC) water temperature records were constructed for the Little Swanport estuary over the last eight years for the purpose of comparison with the earlier years of growth in shells LSOa754 and LSOa756 (Table 2.5, Figure 2.8). Also shown are water temperatures obtained from the Integrated Marine Observing System-Australian National Moorings Network National Reference Station at Maria Island (IMOS platform code: NRSMAI; http://www.imos.org.au/html), which is located ~30 km from the Little Swanport estuary. Temperature at this marine location undergoes a smaller degree of temperature fluctuation than in an estuarine setting. Furthermore, the available temperature record from Maria Island is of low resolution, with the number of data points for individual years ranging between three and eight. The uncertainties
and limitations associated with these two comparative datasets preclude an in- depth interpretation, but still enable a basic comparison of the environmental temperature with the O. angasi18Oshell records.
The Little Swanport O. angasi 18Oshell-reconstructed temperatures agree well with the instrumental temperatures over the experiment period, although summer maxima are underestimated in shells LSOa754 and LSOa756 (Table 2.5, Figure 2.8). This is most likely due to the lower sampling resolution resulting from the reduced growth rate of the older shells. The consequent time-averaging of the record results in dampened peak summer temperatures. Despite this underestimation of summer temperature in the experiment period, the majority of the other summer maxima in shells LSOa754 and LSOa756 overestimate the summer maxima of the instrumental temperature record.
As mentioned previously, this truncation appears to occur consistently in the summer time of shells LSOa754 and LSOa756 after the first two years of growth, while winter 18Oshell values remain consistent throughout the shell (albeit with an overestimation of winter minima by ~3ºC). A number of studies discuss the effect of ontogeny upon bivalve 18Oshell (Krantz et al., 1987; Harrington, 1989; Krantz et al., 1989; Kennedy et al., 2001; Freitas et al., 2005; Schöne, 2008), describing the attenuation of 18Oshell amplitude as the animal ages. Freitas et al., (2005), in their study of the fan mussel, Pinna nobilis, describe this phenomenon as mainly affecting the most positive (winter) 18Oshell values. Goodwin et al. (2003) discuss 18Oshell isotopic amplitude attenuation in Chione
cortezi as being the product of reduced growth in terms of both shorter growth periods and also a decreased volume of carbonate precipitation, whereby C. cortezi exhibits an ongoing reduction in the amplitude of winter 18Oshell peaks,
whereas summer peaks remain comparatively constant. A similar, if reversed, pattern is seen in the Little Swanport O. angasi.
Figure 2.8. Comparison of Little Swanport O. angasi18O
shell-reconstructed temperatures with
water temperatures. Water temperatures beyond the most recent year of growth were calculated via a comparison of Little Swanport temperatures over the field culture experiment period with air temperature records from Orford and water temperature records from Maria Island. The enlarged panels show comparison of 18O
shell-reconstructed temperatures for the latest year of growth with
the observed temperature during the Little Swanport in situ field culture experiments. All three
shells show good agreement between the recorded and 18O
shell-reconstructed temperatures, in
It has been suggested that bivalve 18Oshell may be disrupted due to biologically-induced reductions in shell growth rates or growth cessations, related to the onset of sexual maturity and gametogenesis (Romanek et al., 1987; Surge et al., 2001). However, unlike other bivalves, the sexual maturity, fecundity and reproductive cycle of O. angasi are variable and not restricted to certain times of the year. Furthermore, sexual maturity can be reached as early as eight months of age (S. O’Connor, personal communication, 2012). Hence, this is not a likely explanation for the underestimation of summer temperatures after the first two years of growth in the Little Swanport shells.
Other authors have attributed disequilibrium precipitation in biogenic carbonates resulting from kinetic fractionation effects (McConnaughey, 1989a; 1989b), where higher growth rates result in a greater incorporation of 16O, pushing the 18Oshell to more negative values and the temperature record to warmer temperatures. Owen et al. (2002a) reported a significant kinetic fractionation effect upon the 18Oshell of Pecten maximus, whereby higher growth rates in the early years of growth (combined with a growth cessation at low temperatures) resulted in a more negative median 18Oshell value that overestimated actual median temperature by ~1.5ºC. A similar offset is seen in the Little Swanport O. angasi, where annual mean temperatures for the first two years of growth are overestimated in shells LSOa754 and LSOa793 by >2ºC and by nearly 2ºC in LSOa793 (Table 2.5). Further long-term calibration experiments would be required to confirm the existence of a kinetic fractionation effect in O. angasi.
Shells LSOa754 and LSOa756 both underestimate summer maxima in the summers of 2001–02 and 2002–03. This corresponds to a time of extreme drought
region was considerably below average (Australian Government Bureau of Meteorology, Little Swanport (Wineglass Cottage) field station, data from 1995– 2011; http://www.bom.gov.au/climate/data). High rates of evaporation in the Little Swanport estuary may have resulted in more positive 18Owater, which is then reflected in the underestimated 18Oshell-derived temperatures.
The O. angasi from each location give consistently overestimated mean annual temperatures, by up to nearly 4ºC for the Pambula Lake (Table 2.5) and by up to ~3ºC for the Little Swanport. This could be in part due to the variable growth rates throughout the year biasing the average temperature towards warmer temperatures. These temperature offsets are similar to those reported for other oyster studies: C. virginica can overestimate temperature by up to 4 ºC (Surge et al., 2001); C. gigas by ~5 ºC (Lartaud et al., 2010; Ullman et al., 2010).