Micro-scale geochemical and crystallographic analysis of Buccinum undatum statoliths supports an annual periodicity of growth ring deposition

The whelk Buccinum undatum is commercially important in the North Atlantic. However, monitoring the ontogenetic age and growth of populations has been problematic for fisheries scientists owing to the lack of a robust age determination method. We confirmed the annual periodicity of growth rings present in calcified statoliths located in the foot of field-collected and laboratory reared whelks using microscale measurements of trace element geochemistry. Using Secondary Ion Mass Spectrometry (SIMS), annual trace element profiles were quantified at 2 μm resolution in statoliths removed from whelks collected alive from three locations spanning the length of the UK; the Shetland Isles (North), the Menai Strait, North Wales (Mid) and Jersey (South). Clear cycles in the Mg/Ca ratio were apparent with minimum values corresponding with the visible dark statolith rings and comparatively higher ratios displayed in the first year of growth. Statoliths from one and two-year-old laboratory reared whelks of known age and life history contained one and two Mg/Ca cycles respectively and demonstrated that the statolith growth ring is formed during winter (February and March). Cycles of Na/Ca were found to be anti-correlated to Mg/Ca cycles, whilst ratios of Sr/Ca were inconsistent and showed an apparent ontogenetic increase, suggesting strong physiological control. Variability in elemental data will likely limit the usefulness of these structures as environmental recorders. The results obtained using SIMS for trace element analysis of statoliths confirms the robustness of the statolith rings in estimating whelk age. μXRD at 2 μm spatial resolution demonstrated the statoliths were wholly aragonitic and thus trace element variation was not the result of possible differences in CaCO3 polymorph within the statolith. Changing XRD patterns along with SEM imaging also reveal an ‘hourglass’ microstructure within each statolith. The validation of the annual periodicity of statolith growth rings now provides a robust and novel age determination technique that will lead to improved management of B. undatum stocks

Listening In on the Past: What Can Otolith δ18O Values Really Tell Us about the Environmental History of Fishes?

Oxygen isotope ratios from fish otoliths are used to discriminate marine stocks and reconstruct past climate, assuming that variations in otolith δ18O values closely reflect differences in temperature history of fish when accounting for salinity induced variability in water δ18O. To investigate this, we exploited the environmental and migratory data gathered from a decade using archival tags to study the behaviour of adult plaice (Pleuronectes platessa L.) in the North Sea. Based on the tag-derived monthly distributions of the fish and corresponding temperature and salinity estimates modelled across three consecutive years, we first predicted annual otolith δ18O values for three geographically discrete offshore sub-stocks, using three alternative plausible scenarios for otolith growth. Comparison of predicted vs. measured annual δ18O values demonstrated >96% correct prediction of sub-stock membership, irrespective of the otolith growth scenario. Pronounced inter-stock differences in δ18O values, notably in summer, provide a robust marker for reconstructing broad-scale plaice distribution in the North Sea. However, although largely congruent, measured and predicted annual δ18O values of did not fully match. Small, but consistent, offsets were also observed between individual high-resolution otolith δ18O values measured during tag recording time and corresponding δ18O predictions using concomitant tag-recorded temperatures and location-specific salinity estimates. The nature of the shifts differed among sub-stocks, suggesting specific vital effects linked to variation in physiological response to temperature. Therefore, although otolith δ18O in free-ranging fish largely reflects environmental temperature and salinity, we counsel prudence when interpreting otolith δ18O data for stock discrimination or temperature reconstruction until the mechanisms underpinning otolith δ18O signature acquisition, and associated variation, are clarified

Evaluation of the effects of composition on instrumental mass fractionation during SIMS oxygen isotope analyses of glasses

Significant instrumental mass fractionation (IMF) occurs during measurements of oxygen isotope ratios in magmatic glasses by SIMS. In order to characterise and correct for this fractionation, we measured oxygen isotope ratios in a range of international and internal glass standards ranging in composition from basalt (47 wt.% SiO2) to rhyolite (72 wt.% SiO2) and with known major element compositions. Oxygen isotope ratios were determined by laser fluorination at SUERC, East Kilbride, or taken from previously published values. A total of 1105 δ18O measurements were made over nine sessions on a Cameca IMS-1270 ion microprobe at the University of Edinburgh. SIMS measurements on glass standards had external precision better than ± 0.36‰ (1σ), and the reference material analysed alongside the unknown samples, USGS synthetic glass GSA-1G, had an average external precision of ± 0.14‰. The selected standards are thus sufficiently homogeneous in δ18O to be suitable calibration standards. In terms of δ18O, the SIMS measurements show that, within a single session, IMF may vary by up to 4.7‰ from one glass standard to another. IMF is strongly correlated with SiO2 and CaO. A least squares regression calculation was used to explore potential univariate and multivariate correction schemes. For each correction scheme, the correction coefficients determined for each session were then used to calculate the IMF and correct the measured isotopic ratio of each glass standard. A univariate correction scheme using only SiO2 to correct for IMF reproduced 75% of the glass standards to within ± 0.2‰ of their true δ18O, and 95% to within ± 0.4‰. Bivariate correction schemes using SiO2–CaO and FeO–CaO produced similar results, but did not significantly improve on the SiO2 correction. The correction schemes were applied to δ18O measurements made on melt inclusions and glasses from the Askja volcanic system, North Iceland. The uni- and bivariate correction schemes tested produced δ18O values within the published range for Icelandic basalts. We recommend a simple correction scheme based on the SiO2 content of appropriate standards, which should span a suitable compositional range from basalt to rhyolite

Oxygen isotopes in melt inclusions and glasses from the Askja volcanic system, North Iceland

Primitive melt inclusions trapped during the earliest stages of fractional crystallisation are able to preserve oxygen isotope ratios inherited from mantle-derived melts. However, assimilation of low-δ18O hydrothermally altered crustal material and mixing with magmas held in shallow reservoirs may exert a strong control on the δ18O of melt inclusions trapped during later stages of crystallisation. Oxygen isotope ratios in olivine- and plagioclase-hosted melt inclusions and glasses from tephra samples collected from the Askja central volcano and Askja volcanic system indicate significant differences in the mechanisms of magma supply and storage between the northern and southern segments of the Askja volcanic system. Melt inclusions from the Holuhraun fissure eruption, ∼20 km south of Askja, mostly preserve δ18O signatures of +4.1‰ to +5.4‰, suggesting that this magma underwent minimal modification by magma mixing or crustal assimilation prior to its eruption. By contrast, melt inclusions and glasses from the Nýjahraun fissure eruption, ∼60 km north of Askja, have δ18O between +3.1‰ and +4.0‰. These relatively evolved melt inclusions (∼3.9–4.3 wt.% MgO) were probably trapped during late-stage fractional crystallisation in a shallow magma storage zone. Melt inclusions from two phreatomagmatic tuff sequences within the Askja caldera have δ18O between +2.1‰ and +5.2‰, and this variability cannot be explained by mixing with low-δ18O rhyolitic or andesitic contaminants in the upper crust. Instead, mixing of the ascending magmas with hydrated, low-δ18O basaltic magmas is invoked, thus acquiring a low δ18O signature with minimal modification to the magma’s bulk composition. Such magma bodies are likely to be found throughout the upper 11 km of the crust beneath Askja. Assimilation of low-δ18O meta-basalt in the upper crust is also likely to affect the δ18O of ascending magmas

Detrital zircon U-Pb-Hf and O isotope character of the Cahill Formation and Nourlangie Schist, Pine Creek Orogen: Implications for the tectonic correlation and evolution of the North Australian Craton

Detrital zircon age and Hf isotope patterns for the Cahill Formation and Nourlangie Schist are distinctly different from other Paleoproterozoic successions in the North Australian Craton. The Cahill Formation and Nourlangie Schist comprise the bulk of the Paleoproterozoic strata in the Nimbuwah Domain, the easternmost part of the Pine Creek Orogen on the northern margin of the North Australian Craton. They comprise micaceous and quartzofeldspathic schist, carbonaceous schist, calc-silicate rock, amphibolite and quartzite, deformed and metamorphosed during emplacement of the granitic to dioritic Nimbuwah Complex at 1867-1857Ma. The Cahill Formation hosts several world-class uranium deposits including Ranger, Jabiluka and Nabarlek. U-Pb SHRIMP and LA-SF-ICPMS detrital zircon spectra for four samples of the Cahill Formation and six samples of the Nourlangie Schist show a similar broad spectrum of ages mainly in the range 3300-1900Ma. A ubiquitous dominant peak at 2530-2470Ma matches the age of underlying Neoarchean basement, but is distinct in its dominantly mantle-like Hf and O zircon isotopic character, which shows closer similarity with possible source rocks from the Gawler Craton or alternatively from the Dharwar Craton. Common smaller age peaks occur at 2180Ma, 2080Ma and 2020Ma. The first two have no known magmatic age correlatives in the North Australian Craton. Zircons of the 2020Ma peak have distinctively unradiogenic Hf and elevated d18O, at variance with local rocks of this age but similar to detrital zircon of the same age from the Gawler Craton. In contrast to younger Proterozoic sedimentary successions within the Pine Creek Orogen, which contain ubiquitous ca. 1870Ma detritus, the detrital spectra for the Cahill Formation and Nourlangie Schist contain almost no ca. 1870Ma detritus. A maximum deposition age of ca. 1866Ma indicates deposition within error of intrusion of the Nimbuwah Complex. We propose that the Cahill Formation and Nourlangie Schist were deposited at a plate margin immediately prior to convergent tectonism. This resulted in their burial, deformation and amphibolite facies metamorphism during orogenesis associated with the Nimbuwah Event. These findings have implications for understanding the Paleoproterozoic evolution of the Pine Creek Orogen within the context of northern Australia. © 2014

Sub-stock specific vital effects?

<p>Measured <i>vs</i>. predicted intra-annual otolith δ¹⁸O values obtained for the 4 fish per sub-stock analysed with SIMS. In each case, intra-annual otolith δ¹⁸O measurements made during DST recording time were matched with best corresponding monthly δ¹⁸O values predicted from concomitant <i>in situ</i> tag temperature records using the equation from Kim et al. (2007). The grey area around the 1∶1 line represents the analytical error using SIMS. NB: the number of measurements differs between sub-stocks due to inter-individual differences in tag recording times. For all individuals, minimum predicted monthly δ¹⁸O values were consistently observed in the summer (in July, August or September) and maximum ones in the winter (in February or March).</p

Details of fish analysed for annual otolith δ¹⁸O signatures of sub-stocks A (NNS), B (ENS) and C (WNS).

<p>Bold indicates individuals selected for detailed analysis of intra-annual variations in δ¹⁸O. CNS: central North Sea; ENS: eastern North Sea; SNS: southern North Sea; EC: English Channel.</p><p>Details of fish analysed for annual otolith δ¹⁸O signatures of sub-stocks A (NNS), B (ENS) and C (WNS).</p