Clear et al.: Validation of annual increments in otoliths of Thunnus maccoyii 



33 



4095 



Ca 



3 

 O ^ 



O 



before Sr band (background Sr level) 

 O 



5110 



B 



5110 



Hgure 5 



Examples of EDS spectra from a sectioned SET otolith showing peaks due 

 to background levels of Sr (A) and enhanced Sr levels associated with the 

 strontium mark (B). Spectra similar to that shown in Figure 6B were used 

 to positively identify the location of strontium marks in sections smd etched 

 whole otoliths (see text for further details). 



preceding the mark (e.g. Fig. 5A) from one acquired 

 on the bright band (Fig 5B), which demonstrated that 

 strontium levels were enhanced and calcium was re- 

 duced in the bright band; however, no increase in chlo- 

 rine was apparent. A quantitative EDS linescan across 

 a bright band in one specimen, SBT OB 96, sectioned 

 in oblique LS along the PR axis and analyzed along 

 the direction of maximum observed growth, revealed 

 Isackgroimd" levels of 0.1% to 0.25% Sr by weight up 

 to 5 microns before the band and a measured peak 

 of 7.1% Sr on the band, falling to 3.5% (50% of peak 

 level) 6 pm after the start of the band, and 0.7% (10% 

 of peak level) approximately 15 pm after the start 

 of the band. There is some indication of continuing 

 sHghtly elevated Sr levels out to aroimd 50 pm beyond 

 the band, although visibility of these levels is at the 

 Umits of the EDS technique (Fig. 6). 



Accompanying the measured maximum 7.1% 

 increase in Sr level in the bright band is a fall in 

 measured Ca concentration from 39% to 40% before the 

 band to a minimum of 35.5% on the band — a decrease 



of 3.5-4.5% in absolute value or 10% relative value. 

 Within the limits of accuracy of the EDS technique, 

 this decrease in calcium concentration supports the 

 theory that Ca atoms are being replaced by Sr atoms 

 in the atomic structure on a 1:1 basis, each Sr atom 

 being approximately twice as heavy as a Ca atom. 

 Calculation of the increase in mean atomic number of 

 the specimen resulting from a 7% increase in Sr and 

 a 3.5% decrease in Ca gives a value of approximately 

 104 for the Sr-enriched zone. This value compares with 

 100 for the tmaltered CaCOg — a difference resolvable 

 with backscattered electron imaging on the SEM 

 on a suitably polished and coated specimen. The 

 extent of the visible bright band in this specimen 

 (OB 96) coincided with measured Sr levels in the 

 range of 5—7%; thus it is possible that elevated Sr 

 concentrations in the range of 0.5—5% may not be 

 detectable by backscattered imaging although they 

 should still be detectable by EDS. The EDS system is 

 also essential for testing the identity of weak bright 

 bands in sectioned specimens when it is not clear from 



