Evaluation of micromilling/conventional isotope ratio mass spectrometry and secondary ion mass spectrometry of δ18 O values in fish otoliths for sclerochronology.

RATIONALE: Stable oxygen isotope ratios (δ18 O values) measured in fish otoliths can provide valuable detailed information on fish life history, fish age determination, and ocean thermography. Traditionally, otoliths are sampled by micromilling followed by isotope ratio mass spectrometry (IRMS), but direct analysis by secondary ion mass spectrometry (SIMS) is becoming more common. However, these two methods have not been compared to determine which, if either, is better for fish age validation studies. Hence, the goals were to: (1) determine if the δ18 O signatures from the two different methods are similar, (2) determine which method is better for fish age validation studies, and (3) examine biogeographic and migration history. METHODS: Both analytical techniques, micromilling/IRMS and SIMS, were used to measure δ18 O values in six Pacific cod (Gadus macrocephalus) otoliths. A series of measurements was made from the center of each otolith to its edge to develop a life-history δ18 O signature for each fish. RESULTS: The sampling resolution of SIMS analyses was 2-3 times greater than that obtained by micromilling/IRMS. We found an offset between SIMS and micromilling/IRMS δ18 O values, about 0.5‰ on average, with SIMS yielding lower values. However, the δ18 O patterns from both methods (i.e., the number of δ18 O maxima) correspond to the estimated age determined by otolith growth-zone counts, validating fish age determination methods. CONCLUSIONS: Both techniques resolved δ18 O life-history signatures and showed patterns consistent with seasonal variation in temperatures and changes due to fish migration. When otoliths are large, micromilling/IRMS can provide adequate resolution for fish age validation. However, SIMS is the better option if greater sampling resolution is required, such as when otoliths are small or specimens are longer lived and have compact growth zones.

Relationship between blood mercury levels and components of male song in Nelson's sparrows (Ammodramus nelsoni).

Mercury (Hg) adversely affects the health and behavior of exposed wildlife; however, behavioral effects remain largely unknown. Changes in avian singing behavior may affect a male's fitness because song reveals male quality and thus influences female mate choice and male territory-holding ability. Nelson's sparrows (Ammodramus nelsoni) live exclusively on salt marshes and risk high levels of Hg exposure and bioaccumulation. We recorded songs of male Nelson's sparrows at two locations with different Hg exposure to determine if total blood Hg concentration was related to song characteristics, as previously reported for other species. Males with higher blood Hg levels sang at higher maximum tonal frequency, but blood Hg and site location did not influence low tonal frequency and bout duration, contrary to predictions based on other species. Within the contaminated site, Hg levels were related to bouts per minute and gap duration, such that males at that site sang faster songs. Hg influences hormones and alters brain development, raising questions about specific effects on the brains and singing behavior of male Nelson's sparrows.

The role of Northeast Pacific meltwater events in deglacial climate change.

Columbia River megafloods occurred repeatedly during the last deglaciation, but the impacts of this fresh water on Pacific hydrography are largely unknown. To reconstruct changes in ocean circulation during this period, we used a numerical model to simulate the flow trajectory of Columbia River megafloods and compiled records of sea surface temperature, paleo-salinity, and deep-water radiocarbon from marine sediment cores in the Northeast Pacific. The North Pacific sea surface cooled and freshened during the early deglacial (19.0-16.5 ka) and Younger Dryas (12.9-11.7 ka) intervals, coincident with the appearance of subsurface water masses depleted in radiocarbon relative to the sea surface. We infer that Pacific meltwater fluxes contributed to net Northern Hemisphere cooling prior to North Atlantic Heinrich Events, and again during the Younger Dryas stadial. Abrupt warming in the Northeast Pacific similarly contributed to hemispheric warming during the Bølling and Holocene transitions. These findings underscore the importance of changes in North Pacific freshwater fluxes and circulation in deglacial climate events.