NOTE Toole and Nielsen: Microprobe precision associated with Sr Ca ratios 



427 



At least 40-sec counts would be necessary to detect 

 a 1°C change in temperature experienced by herring 

 (Townsend et al. 1989, Radtke et al. 1990) at all tem- 

 perature levels examined in those studies. Detection 

 of a 1°C temperature change in cod (Radtke 1984) and 

 Fundulus (Radtke 1989) would require much longer 

 counting times, beyond the range examined in this 

 experiment. 



This experiment documents the improvement in 

 precision which is possible when otoliths are analyzed 

 at longer counting times and higher beam power den- 

 sities. Neither treatment appeared to affect the level 

 of Sr/Ca accuracy under the range of conditions ex- 

 amined. Obvious burns on the otolith (Fig. 1) indicate 

 that beam damage did occur in all of our experimental 

 treatments, and we suspect that it also occurred in 

 other studies using similar analytical conditions. How- 

 ever, whatever effect this may have had on the ac- 

 curacy of the molecular weight percent concentrations 

 for Ca and Sr, the ratio of the two elements remained 

 constant, indicating no observable fractionation. 



One implication of these results is that Sr/Ca preci- 

 sion can be increased, with no apparent loss of ac- 

 curacy, when analyses are conducted for 40-sec rather 

 than 20-sec counting times, and at 5/.im rather than 

 10 (im beam sizes, at an accelerating voltage of 20 nA. 

 The 5pim beam allows greater temporal resolution, 

 which is helpful when matching the sample location to 

 structures such as daily growth increments. These may 

 be as small as 0.1-0.2^m, depending upon species, 

 growth rate, and age (Campagna and Neilson 1985). 



Because the level of precision may affect conclusions 

 of studies relating otolith Sr/Ca levels to environmen- 

 tal temperature, it is important to know the analytical 

 conditions under which each study is conducted. 

 Minimal information required includes beam current 

 and voltage, beam size, counting time for each element, 

 standards used, and precision of measurements. This 

 information has not been reported in sufficient detail 

 in some of the previous studies, making interpretation 

 difficult. The methods described in this experiment are 

 proposed as a means of defining measurement preci- 

 sion in future studies of Sr/Ca ratios in fish otoliths. 



Acknowledgments 



We are grateful to Douglas Markle for his advice and 

 support. Comments by John Kalish on an earlier draft 

 greatly improved the final version. This work was 

 funded in part by the Oregon Sea Grant Program, 

 Project No. NA85AA-D-SG095; Pacific Outer Con- 

 tinental Shelf Region of the Minerals Management Ser- 



vice, Department of Interior Contract No. 14-12-0001- 

 30429; and the Oregon State University Research Of- 

 fice. We also thank Capt. Terry Thompson of the FV 

 Olympic for donation of a portion of the ship time. 



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