High Spatial Resolution Ion Microprobe Research Articles

U–Pb zircon eruption age of the Old Crow tephra and review of extant age constraints

Eruption of the Old Crow tephra deposited ~200 km 3 of volcanic ash throughout Alaska and the northwestern Yukon (eastern Beringia), providing an isochronous marker across the region on a scale unique in the Pleistocene. The Old Crow tephra represents a critical temporal piercing point used extensively to link geographically disparate stratigraphic sections and the paleo-environmental records they contain. Although the canonical age of the Old Crow suggests eruption during the transition between the glacial and interglacial periods of Marine Isotope Stages (MIS) 5 and 6 at ~125 ka, recent U–Th–Pb and (U–Th)/He zircon dating of the tephra suggests eruption at ~200 ka, within MIS 7. If accurate, this revised eruption age begets significant change to existing models describing the geologic and biotic evolution of Beringia in the Pleistocene. Thus, confidently knowing the age of the tephra is critical to its time-stratigraphic utility and for past and future work in the region where the tephra has been found. With this contribution, we review existing Old Crow age constraints and present an eruption age for the tephra determined via high spatial resolution ion microprobe U–Pb surface analysis on zircon crystals isolated from source-proximal (<500 km from plausible source) pumiceous pyroclasts of the tephra. By dating only glass-mantled crystals isolated from discrete pumice clasts, we limit the potential for sample contamination from exotic crystals and resulting age bias. The young population of dates from this dataset corroborate previous radiometric dates and confirm Old Crow eruption within late MIS 7 at 207 ± 13 ka.

A new U-Pb age for shock-recrystallised zircon from the Lappajärvi impact crater, Finland, and implications for the accurate dating of impact events

Accurate and precise dating of terrestrial impact craters is a critical requirement for correlating impacts with events such as mass extinctions. A number of isotopic systems have been applied to impact chronology but it is important to understand what an age actually represents and, thus, if it accurately represents the ‘true’ impact age and is suitable for use in correlation. Here we report imaging, microstructural characterisation and high spatial resolution ion microprobe U-Pb analysis of shocked zircon from the approximately 23 km-in-diameter Lappajärvi impact structure, Finland, for which a well-established 40Ar/39Ar framework exists. Microstructural analysis identified two distinct styles of shock recrystallisation: (i) granular zircon that displays multiple domains of similarly oriented neoblasts, some of which are interpreted to be the product of reversion from the high-pressure ZrSiO4 polymorph, reidite, and (ii) granular zircon composed entirely of similarly oriented neoblasts. Only the former gave concordant U-Pb data interpreted to record the age of the impact. The U-Pb ‘concordia age’ reported here, 77.85 ± 0.78 Ma (1.0%; MSWD = 0.60; probability = 0.87; n = 8; 2σ; full external uncertainty), is resolvable from the previously published ‘best estimate’ 40Ar/39Ar age for impact melt rock (76.20 ± 0.29 Ma) and 40Ar/39Ar K-feldspar ages as young as 75.11 ± 0.36 Ma, and is therefore interpreted to more accurately reflect the age of the impact event. The resolvable disparity between the zircon U-Pb and the 40Ar/39Ar data indicates that even the oldest statistically robust 40Ar/39Ar ages obtained at medium- and large-sized impact craters may not accurately record the timing of an impact event at a kyr level. The offset between the U-Pb and 40Ar/39Ar data is interpreted to be, at least in part, a result of the zircon data recording a higher isotopic closure temperature, and the younger 40Ar/39Ar ages recording the progressive cooling of different domains of the impact structure. The Lappajärvi impact structure is the first Phanerozoic impact structure dated by U-Pb analysis of shock-recrystallised zircon to better than, or equal to, 1.0% uncertainty. This further demonstrates that well-characterised granular zircon grains are likely to have wide utility in the accurate and precise dating of terrestrial impact events.

A single-cell view of ammonium assimilation in coral–dinoflagellate symbiosis

Assimilation of inorganic nitrogen from nutrient-poor tropical seas is an essential challenge for the endosymbiosis between reef-building corals and dinoflagellates. Despite the clear evidence that reef-building corals can use ammonium as inorganic nitrogen source, the dynamics and precise roles of host and symbionts in this fundamental process remain unclear. Here, we combine high spatial resolution ion microprobe imaging (NanoSIMS) and pulse-chase isotopic labeling in order to track the dynamics of ammonium incorporation within the intact symbiosis between the reef-building coral Acropora aspera and its dinoflagellate symbionts. We demonstrate that both dinoflagellate and animal cells have the capacity to rapidly fix nitrogen from seawater enriched in ammonium (in less than one hour). Further, by establishing the relative strengths of the capability to assimilate nitrogen for each cell compartment, we infer that dinoflagellate symbionts can fix 14 to 23 times more nitrogen than their coral host cells in response to a sudden pulse of ammonium-enriched seawater. Given the importance of nitrogen in cell maintenance, growth and functioning, the capability to fix ammonium from seawater into the symbiotic system may be a key component of coral nutrition. Interestingly, this metabolic response appears to be triggered rapidly by episodic nitrogen availability. The methods and results presented in this study open up for the exploration of dynamics and spatial patterns associated with metabolic activities and nutritional interactions in a multitude of organisms that live in symbiotic relationships.

Dynamics of sheet nacre formation in bivalves

Formation of nacre (mother-of-pearl) is a biomineralization process of fundamental scientific as well as industrial importance. However, the dynamics of the formation process is still not understood. Here, we use scanning electron microscopy and high spatial resolution ion microprobe depth-profiling to image the full three-dimensional distribution of organic materials around individual tablets in the top-most layer of forming nacre in bivalves. Nacre formation proceeds by lateral, symmetric growth of individual tablets mediated by a growth-ring rich in organics, in which aragonite crystallizes from amorphous precursors. The pivotal role in nacre formation played by the growth-ring structure documented in this study adds further complexity to a highly dynamical biomineralization process.

High precision, high accuracy measurement of oxygen isotopes in a large lunar zircon by SIMS

Ninety-seven high precision, high spatial-resolution ion microprobe analyses of oxygen isotopes in a single, large zircon crystal from a lunar soil sample yield a surprising and unexpectedly large range in δ 18O SMOW from 4.5 to 9.2‰. Detailed consideration of sample mounting, polishing, and sample-standard orientation suggests that this variation was caused by analytical artefact(s) degrading the accuracy of δ 18O values. Careful control of these parameters effectively eliminated the variation and the two most reliable sets of analyses, obtained when the sample and standard were mounted within 2 mm in the centre of the mount, yield a combined weighted average δ 18O SMOW value of 5.70 ± 0.07‰ (95% confidence). This value is indistinguishable from bulk rock analyses of low-Ti lunar basalts and suggests that, at least under anhydrous conditions, there is minimal fractionation accompanying zircon crystallisation.

Extinct manganese-53 in carbonates from the Orgueil meteorite

The presence of the radionuclide 53Mn in the early Solar System is well established. Manganese-53 decays by e-capture (half-life 3.7Ma) and can be detected as isotopic overabundances of its daughter 53Cr in meteorites. It is produced in Type II supernovae but not in AGB stars. Production by bombardment of Fe nuclei with energetic particles from the young Sun is a distinct possibility. The study of the Mn–Cr isotope system in meteorites has provided a wealth of useful information on early Solar System history. Evidence for the former presence of 53Mn has been found in a variety of Solar System objects, including carbonates. High spatial resolution ion microprobe analyses of individual carbonate grains from the Orgueil meteorite have shown linear correlations between 53Cr excesses and Mn/Cr ratios in individual grains, indicative of in situ decay of radioactive 53Mn. The well-defined isochrons have chronological significance, with absolute ages between 4561 and 4565Ma. The carbonate data imply extensive aqueous activity on the Orgueil parent body over a period of several million years, starting ∼3Ma after Solar System formation that most likely was the result of impact heating and latent heat from the decay of radioactive 26Al and 60Fe.

High spatial resolution ion microprobe measurements refine chronology of carbonate formation in Orgueil

Abstract— Aqueous activity on meteorite parent bodies is indicated by the presence of carbonates. High spatial resolution ion microprobe analyses of nine individual carbonate grains (four dolomites, five breunnerites) from the Orgueil meteorite reveal linear correlations between 53Cr excesses and Mn/Cr ratios in all grains, indicative of in situ decay of radioactive 53Mn (half‐life 3.7 million years). The well‐defined isochrons appear to have chronological significance. If this is the case, then 53Mn/55Mn ratios between 2.1 and 4.7 × 10−6 are inferred for the time of carbonate formation and absolute ages of between 4561 and 4565 Ma are calculated (systematic uncertainty of ±2 Ma). Dolomites tend to have formed slightly earlier than the breunnerites. Our data imply extensive aqueous activity on the Orgueil parent body over a period of several million years, starting ∼3–4 Myr after formation of the solar system, that most likely was the result of impact heating and latent heat from the decay of radioactive 26Al and 60Fe.

In situ measurement of seasonal δ18O variations and analysis of isotopic trends in a modern speleothem from southwest Australia

We present a record of seasonal and inter-annual oxygen ( δ 18O) and carbon ( δ 13C) isotope ratios from an 81-year-old stalagmite from Moondyne Cave, southwest Australia. The growth history of stalagmite MND-S1 is known since it grew on a cave boardwalk that was installed in 1911 and removed in 1992. This stalagmite provides an excellent test of speleothem climate proxies because the regional climate is strongly seasonal (wet winter/dry summer) and has experienced a 200 mm (20%) reduction of mean rainfall since the mid-1960s, and a 0.8 °C temperature rise since ∼ 1953. Seasonal variations in calcite δ 18O were measured in situ by high spatial resolution ion microprobe, whilst inter-annual variations of δ 18O and δ 13C were measured by conventional gas-source mass spectrometry. Comparison of the speleothem stable isotopes and instrumental temperature records reveals that δ 18O variations are too large to be driven by temperature alone, and are in the opposite sense. However, daily rainfall δ 18O measurements show that the mean seasonal range in δ 18O of rainfall in southwest Australia is large (2‰) and inversely correlated with rainfall amount. A rainfall driver for the speleothem δ 18O is confirmed by the detection of seasonal shifts of 0.7–1.5‰ in speleothem δ 18O that track rainfall δ 18O, smoothed by storage in the overlying limestone. The seasonal range in speleothem δ 18O is larger than any inter-annual and decadal variation observed in the record. The prominent annual cycles in speleothem δ 18O revealed by ion microprobe analysis indicate that subtle changes in the frequency of intense winter rainfall events, or possibly also moisture sources, could produce significant changes in mean speleothem δ 18O. The ion microprobe results also raise the possibility that the masses of speleothem calcite deposited in winter and summer could vary as a function of the seasonal drip rate and carbonate saturation state of these waters. If this is the case, then small changes in the relative masses of calcite deposited in winter and summer could produce significant shifts in mean δ 18O and δ 13C that have a complex relation to climate. This finding should be generally applicable to the interpretation of long-term trends in speleothem geochemical records for shallow cave sites where seasonal variations in geochemical tracers are relatively large, including most of the sub-tropical monsoon belts and mid to high latitudes with distinct rainfall seasons.

Monthly Strontium/Calcium oscillations in symbiotic coral aragonite: Biological effects limiting the precision of the paleotemperature proxy

In thermodynamic equilibrium with sea water the Sr/Ca ratio of aragonite varies predictably with temperature and the Sr/Ca ratio in coral have thus become a frequently used proxy for past Sea Surface Temperature (SST). However, biological effects can offset the Sr/Ca ratio from its equilibrium value. We report high spatial resolution ion microprobe analyses of well defined skeletal elements in the reef‐building coral Porites lutea that reveal distinct monthly oscillations in the Sr/Ca ratio, with an amplitude in excess of ten percent. The extreme Sr/Ca variations, which we propose result from metabolic changes synchronous with the lunar cycle, introduce variability in Sr/Ca measurements based on conventional sampling techniques well beyond the analytical precision. These variations can limit the accuracy of Sr/Ca paleothermometry by conventional sampling techniques to about 2°C. Our results may help explain the notorious difficulties involved in obtaining an accurate and consistent calibration of the Sr/Ca vs. SST relationship.

A high spatial resolution δ18O profile of a speleothem using an ion-microprobe

The oxygen isotopic composition of a stalactite from the Soreq Cave, Israel has been investigated by in situ high spatial resolution ion microprobe analysis. The δ 18O record obtained on portions of the stalactite, which were deposited during the time interval of 18–14.3, 10.8–8.5 and 1.5–0.2 ka, has a temporal resolution ranging from ∼1 to 25 years (depending on the deposition rate during the period investigated). Comparison to results obtained by micro-drilling and phosphoric acid reaction gas-phase mass spectrometry indicates that the ion microprobe analyses are precise and accurate at the 0.4–0.5‰ level. The high resolution of the ion microprobe profile enables the detection of short duration excursions toward low δ 18O values during deglaciation (from 18 to 14.3 ka), providing further evidence indicating the instability of the glacial climate.

Recombinant RNA Research

In the last paragraph of Arthur L. Robinson's Research News article "High spatial resolution ion microprobe" (14 Sept., p. 1137), J. Ronald Hass' name was misspelled.

Erratum: High Spatial Resolution Ion Microprobe

In the last paragraph of Arthur L. Robinson's Research News article "High spatial resolution ion microprobe" (14 Sept., p. 1137), J. Ronald Hass' name was misspelled.

High Spatial Resolution Ion Microprobe

High Spatial Resolution Ion Microprobe