Proposal for International Agreement on Ca Notation Resulting from Discussions at Workshops on Stable Isotope Measurements Held in Davos (Goldschmidt 2002) and Nice (EGS-AGU-EUG 2003)

A proposal is made to standardise the reporting of Ca isotope data to the δ⁴⁴Ca/⁴⁰Ca notation (or δ⁴⁴Ca/42Ca) and to adopt NIST SRM 915a as the reference standard.     

Transport of U in the Opalinus Clay on centimetre to decimetre scales

The Opalinus Clay formation in North Switzerland is a potential host rock for a deep underground radioactive waste repository. The distribution of ²³⁸U, ²³⁴U and ²³⁰Th was studied in rock samples of the Opalinus Clay from an exploratory borehole at Benken (Canton of Zurich) using MC-ICP-MS. The aim was to assess the in situ, long-term migration behaviour of ²³⁴U in this rock. Very low hydraulic conductivities of the Opalinus Clay, reducing potential of the pore water and its chemical equilibrium with the host rock are expected to render both ²³⁸U and ²³⁰Th immobile. If U is heterogeneously distributed in the Opalinus Clay, gradients in the supply of ²³⁴U from the rock matrix to the pore water by the decay of ²³⁸U will be established. Diffusive redistribution separates ²³⁴U from its immobile parent ²³⁸U resulting in bulk rock ²³⁴U/²³⁸U activity disequilibria. These may provide a means of estimating the mobility of ²³⁴U in the rock if the diffusion rate of ²³⁴U is significant compared to its decay rate. Sampling was carried out on two scales. Drilling of cm-spaced samples from the drill-core was done to study mobility over short distances and elucidate possible small-scale lithological control. Homogenized 25-cm-long portions of a 2-m-long drill-core section were prepared to provide information on transport over a longer distance. Variations in U and/or Th content on the cm-scale between clays and carbonate-sandy layers are revealed by β-scanning, which shows that the (dominant) clay is richer in both elements.     

High-resolution isotope records of early Holocene rapid climate change from two coeval stalagmites of Katerloch Cave,Austria

Two coeval stalagmites from Katerloch Cave show pronounced intervals of low δ¹⁸O values around 8.2, 9.1, and 10.0 kyr (all ages are reported before the year 2000 AD) and represent an inorganic U–Th dated climate archive from the southeast of the European Alps, a region where only very few well-dated climate records exist. The O isotope curves, providing near-annual resolution, allow a direct comparison to the Greenland ice core records, as temperature was the primary factor controlling the O isotopic composition of Katerloch speleothems.The 8.2 kyr climate anomaly lasted about one century, from 8196 to 8100 yr, with a maximum amplitude of 1.1‰ at 8175 yr. The event is characterized by a rapid onset and a more gradual demise and U–Th data as well as annual lamina counting support a rapid climate change towards cooler conditions within 10–20 yr. There is no strong evidence that the 8.2 kyr anomaly was superimposed on a pronounced longer-term cooling episode, nor do the new data support two separate cooling events within the 8.2 kyr event as reported by other studies. Our record also shows a distinct climate anomaly around 9.1 kyr, which lasted 70–110 yr and showed a maximum amplitude of 1.0‰, i.e. it had a similar duration and amplitude as the (central) 8.2 kyr event. Compared to the 8.2 kyr event, the 9.1 kyr anomaly shows a more symmetrical structure, but onset and demise still occurred within a few decades only. The different progression of the 8.2 (asymmetrical) and 9.1 kyr anomaly (symmetrical) suggests a fundamental difference in the trigger and/or the response of the climate system. Moreover, both stalagmites show evidence of a climate anomaly around 10.0 kyr, which was of comparable magnitude to the two subsequent events.Using a well constrained modern calibration between air temperature and δ¹⁸O of precipitation for the study area and cave monitoring data (confirming speleothem deposition in Katerloch reflecting cave air temperature), a maximum cooling by ca 3 °C can be inferred at 8.2 and 9.1 kyr, which is similar to other estimates, e.g., from Lake Ammersee north of the Alps. The O isotopic composition of meteoric precipitation, however, is a complex tracer of the hydrological cycle and these temperature estimates do not take into account additional effects such as changes in the source area or synoptic shifts. Apart from that, the relative thickness of the seasonally controlled lamina types in the Katerloch stalagmites remains rather constant across the intervals comprising the isotopic anomalies, i.e. the stalagmite petrography argues against major shifts in seasonality during the early Holocene climate excursions.     

Lead and strontium isotopes in Cretaceous kimberlites and mantle-derived xenoliths from Southern Africa

Concentrations of lead, uranium and thorium and isotopic compositions of lead are reported for twelve Cretaceous kimberlites and five Cretaceous nucleated autoliths. The samples are from Lesotho and from the area around Kimberley (Cape Province, South Africa). In the case of the autoliths potassium, rubidium and strontium concentrations and⁸⁷Sr/⁸⁶Sr ratios were also measured.Work reported on clinopyroxenes from mantle-derived xenoliths in kimberlites includes lead isotopes for twelve samples and strontium isotopes for nine of these.The autoliths have initial⁸⁷Sr/⁸⁶Sr ratios between 0.7035 and 0.7095. A large spread in initial lead isotope ratios (²⁰⁶Pb/²⁰⁴Pb: 17.6–20;²⁰⁸Pb/²⁰⁴Pb: 37.7–39.5) was found in the matrix kimberlites and autoliths. In the²⁰⁷Pb/²⁰⁴Pb vs.²⁰⁶Pb/²⁰⁴Pb plot, the initial lead isotope ratios of the kimberlite and autolith samples roughly define a slope of 0.10, corresponding to an age of 1575 m.y. With respect to the spread of initial ratios as well as with respect to this slope, the kimberlite and autolith lead isotopic pattern is comparable to patterns obtained from carbonatites and ocean island volcanics.The xenoliths studied include coarse-granular and porphyroclastic material from the Kimberley area and coarse-granular samples from Lesotho. Their⁸⁷Sr/⁸⁶Sr ratios are generally between 0.704 and 0.706 but a value of 0.713 was found in one sample. They show a surprisingly large spread in lead isotope ratios (²⁰⁶Pb/²⁰⁴Pb: 17.5–20;²⁰⁸Pb/²⁰⁴Pb: 37.3–39.4).The isotopic patterns of the xenolithic material and of the kimberlites and autoliths are considered to provide a strong indication that the upper mantle beneath Southern Africa is isotopically heterogeneous on a regional scale.