ALBERT

Description: Here, we present a new technique for the direct measurement of 44Ca/40Ca isotope ratios on a Multicollector Inductively Coupled Plasma Mass Spectrometer (MC–ICP–MS, AXIOM) using the “cool plasma” technique. By reducing the plasma energy to about 400 W, the isobaric effect resulting from 40Ar+ can be significantly reduced, enabling the simultaneous and precise measurement of 44Ca and 40Ca beam intensities in different Faraday cups. In contrast to the TIMS technique requiring a 43Ca/48Ca double spike, the isotope measurements on MC–ICP–MS can be performed by bracketing standards. We express the calcium isotope variation relative to NIST SRM 915a (δ44/40Ca [‰]=[((44Ca/40Ca)sample/(44Ca/40Ca)NIST SRM 915a)−1]*1000). Isobaric effects of 24Mg16O+ and 23Na16OH+ interfering with 40Ca and 26Mg16OH2+ with 44Ca can be neglected by measuring calcium isotopes near the low-mass edge of the peaks. No influence of 87Sr2+ monitored on 43.5 atomic mass units (amu) was found. Repeated measurements of two Johnson Matthey CaCO3 standards (lot No. 4064 and lot No. 9912) revealed values of about −11.29 (‰ SRM 915a) and 0.57 (‰ SRM 915a). These values are in accordance with previous values published by Russell et al. [Geochim. Cosmochim. Acta 42 (1978) 1075], Heuser et al. [Int. J. Mass Spectrom. 220 (2002) 385], Hippler et al. [Geostand. Newsl. 27 (2003) 267] and Schmitt et al. [Geochim. Cosmochim. Acta 67 (2003) 2607]. Repeated measurement of the NIST SRM 915a CaCO3 standard showed that the variance of a single δ44/40Ca measurement is about 0.14‰ RSD being comparable with TIMS. MC–ICP–MS-based δ44/40Ca values measured on inorganically precipitated aragonite samples are indistinguishable from earlier measurements based on TIMS, confirming the positive correlation of δ44/40Ca and temperature. MC–ICP–MS-based δ44/40Ca measurements on cultured Orbulina universa showed a slope of about 0.026‰/°C being similar to the TIMS-based δ44/40Ca measurements showing a slope of about 0.019‰/°C. The large offset of about 5‰ between the two techniques is shown to be caused by a “matrix” effect, indicating that any δ44/40Ca measurements on MC–ICP–MS are sensitively controlled by the Ca concentration and the acidity of the solution. Our study demonstrates the possibility to measure the whole dispersion of calcium isotopes with MC–ICP–MS, showing that 40Ca can be used for normalization of 44Ca.

Description: We analyzed 238U, 234U, 232Th, 230Th, and 226Ra by thermal ionization mass spectrometry (TIMS) and Ba by inductively coupled plasma optical emission spectrometry (ICP-OES) on eight Mn/Fe crusts from the Mecklenburg Bay (SW Baltic) and on one from the Bothnian Bay (N Baltic) to test the 226Raex/Ba ratio as potential geochronometer. 226Raex/Ba ratios decrease as a function of depth within the concretions in all analyzed profiles. Calculated diffusion coefficients are relatively low (∼9 · 10−7 cm2/yr for Ra and 5 · 10−7 cm2/yr for Ba) and suggest that diffusion is negligible for the Ra and Ba record. In addition, 226Raex/Ba ages are consistent and independent from the growth rate and growth direction within a crust. Thus, the decline in 226Raex/Ba ratio is most likely due to radioactive decay of 226Raex, although the influence of varying oxic conditions has still to be evaluated. 226Raex/Ba growth rates range from 0.021 to 0.0017 mm/yr and tend to be lower than those calculated and based on stratigraphic methods (1 to 0.013 mm/yr). 226Raex/Ba ages of concretions from shallow water environment (20 m depth, Mecklenburg Bay/SW Baltic) cover a time interval from 990 ± 140 yr to 4310 ± 310 yr BP corresponding to the stabilization of the sea level close to the present position about 5500 to 4500 yr ago. One sample from greater depth (70 m, Bothnian Bay-/N Baltic) showed a higher 226Raex/Ba age of 6460 ± 520 yr BP

Description: Results are presented for the first in-depth investigation of Tl isotope variations in marine materials. The Tl isotopic measurements were conducted by multiple collector-inductively coupled plasma mass spectrometry for a comprehensive suite of hydrogenetic ferromanganese crusts, diagenetic Fe–Mn nodules, hydrothermal manganese deposits and seawater samples. The natural variability of Tl isotope compositions in these samples exceeds the analytical reproducibility (±0.05‰) by more than a factor of 40. Hydrogenetic Fe–Mn crusts have ϵ205Tl of +10 to +14, whereas seawater is characterized by values as low as −8 (ϵ205Tl represents the deviation of the 205Tl/203Tl ratio of a sample from the NIST SRM 997 Tl isotope standard in parts per 104). This ∼2‰ difference in isotope composition is thought to result from the isotope fractionation that accompanies the adsorption of Tl onto ferromanganese particles. An equilibrium fractionation factor of α∼1.0021 is calculated for this process. Ferromanganese nodules and hydrothermal manganese deposits have variable Tl isotope compositions that range between the values obtained for seawater and hydrogenetic Fe–Mn crusts. The variability in ϵ205Tl in diagenetic nodules appears to be caused by the adsorption of Tl from pore fluids, which act as a closed-system reservoir with a Tl isotope composition that is inferred to be similar to seawater. Nodules with ϵ205Tl values similar to seawater are found if the scavenging of Tl is nearly quantitative. Hydrothermal manganese deposits display a positive correlation between ϵ205Tl and Mn/Fe. This trend is thought to be due to the derivation of Tl from distinct hydrothermal sources. Deposits with low Mn/Fe ratios and low ϵ205Tl are produced by the adsorption of Tl from fluids that are sampled close to hydrothermal sources. Such fluids have low Mn/Fe ratios and relatively high temperatures, such that only minor isotope fractionation occurs during adsorption. Hydrothermal manganese deposits with high Mn/Fe and high ϵ205Tl are generated by scavenging of Tl from colder, more distal hydrothermal fluids. Under such conditions, adsorption is associated with significant isotope fractionation, and this produces deposits with higher ϵ205Tl values coupled with high Mn/Fe.