In a second related study using the IMS-1280 ion microprobe, we conducted a high resolution δ18O spot sample analysis of otoliths from seven Pacific cod from which bi-hourly temperature and depth records were stored from electronic archived tags. These Pacific cod were also analyzed for δ18O from micromilling and conventional continuous flow isotope ratio mass spectrometry techniques as part of another study to validate ageing using δ18O signatures in otoliths. In this analysis our goal was to 1) compare IMS-1280 mass spectrometry and micromilling/conventional ratio mass spectrometry for intra-otolith δ18O signatures; 2) confirm that otolith δ18O signatures can be used as a tool to validate ageing; and 3) calibrate the δ18O – temperature (in situ from electronic archive tags) relationship from all seven fish and develop a fractionation equation for aragonite in the Bering Sea. For example, Figure 4 shows sequence of δ18O measurements obtained using SIMS in a traverse section of a 6-year-old Pacific cod (ID 812, tagged and recaptured near Amak, Alaska). We obtained 87 spot samples of δ18O with very high analytical precision from the otolith core to the edge, which represents a sampling density approximately 2 to 3 times greater than micromilling/conventional mass spectrometry techniques. Measured peaks in the δ18O signature in this otolith, which corresponds to six seasonal temperature lows in the Bering Sea (assuming oxygen isotopes and temperature are inversely related), confirming the years of life for this Pacific cod.
Figure 4. Ion microprobe spot samples and measured sequence of secondary ion mass spectrometry stable oxygen isotopes δ18O (‰ VPDB, ±2 S.D.) made at WiscSIMS in a traverse section of a 6-year-old Pacific cod. Measured peaks in the oxygen isotope signature correspond to seasonal temperature lows in the Bering Sea and help researchers elucidate the age of fish (oxygen isotopes and temperature are inversely related-below).
The otoliths from archival-tagged Pacific cod afforded a rare opportunity to relate the δ18O measurements to in situ instrumental temperatures recorded during the time the animal was at liberty, and hence estimate the fractionation equation for Bering Sea aragonite. More importantly, it allows the equation to be estimated individually for each archival-tagged Pacific cod that was SIMS analyzed and lends insight to the degree to which such curves vary among individuals of the same species.
Figure 5. Sequence of ion microprobe spot samples measuring stable oxygen isotopes δ18O (‰ VPDB, ±2 S.D.) made at WiscSIMS from a traverse sectioned Pacific cod tagged with an electronic data logger (temperature and depth) and at liberty for 716 days. Spot samples 1-31 were sampled near the outer edge of the otolith and represented the aragonite material accreted during the period at liberty, and illustrates the strong inverse relationship between δ18O and in situ temperatures.
To achieve this goal, we identified the spot samples of measured δ18O values that were sampled near the outer edge of the otolith and represented the aragonite material accreted during the period at liberty. In the case of Pacific cod 1169, which was at liberty for 716 days and captured during late winter 2004, we isolated 31 spot samples between the outer edge and the last two annual growth zones. The δ18O samples measured from the edge of the otolith show a highly coherent relationship with archival tag temperatures since capture (Fig. 5), illustrating their inverse relationship and the fact that δ18O may be used as a proxy for temperature reconstruction. We then estimated the fractionation equation from all seven Pacific cod using a linear mixed effects model.
Figure 6. Estimated fractionation equation from otoliths (aragonite) extracted from 7 Pacific cod tagged with electronic data recorders (in situ temperature data). As expected, relationship between Pacific cod otolith aragonite (δ18O) and bottom temperature showed an inverse, statistically significant linear relationship (Fig.6; r=0.75, p<0.001).
As expected, the relationship between Pacific cod otolith aragonite (δ18O) and bottom temperature showed an inverse, statistically significant linear relationship (Fig.6; r=0.75, p<0.001). Mean δ18O declined with temperature at a rate of about 0.25‰/°C, which is approximately consistent with the expected temperature-driven variation in δ18O of aragonite of 0.2‰/ °C from North Atlantic waters. Here, as in the North Atlantic, a 5°C variation should produce a ~1o/oo change in the otolith’s
δ18O composition based on the equilibrium fractionation curves calculated for biogenic aragonite. We are not aware of fractionation curves for the North Pacific, so the reported fractionation relationship here is the first for aragonite δ18O of Pacific cod otoliths vs. temperature and shows the oxygen isotopic equilibrium with ambient sea water for different temperatures.
