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: The internal microstructure of a ferromanganese nodule (#2392, from 154°37.52′W, 9°37.56′N, at water depth 5194 m) was examined in detail on polished sections, and radiometrically dated (230Thex/232Th) along a high-resolution (0.1 mm) depth profile (0–1.3 mm), spanning approximately 271 ka. The fabric shows typical stromatolithic structure and exhibits four orders of basic cyclic growth pattern, namely laminae bands, laminae zones, laminae groups and laminae pairs having average thicknesses of 402–454, 185–206, 58–67 and 15–18 μm, respectively. A profile from the depth of 200–2661 μm was selected to obtain the geochemical series using line-scanning electron microprobe analyses, which provide a record of paleoceanographic oscillations during the growth period. Power spectral analysis of the geochemical series for Al, Mn, Fe and Fe/Mn from the depth of 200–1220 μm, where no obvious discontinuity could be observed, display conspicuous cyclicities. The cycles of laminae zones, laminae groups and laminae pairs are reflected in the spectral patterns as well. The significant spectral peaks are located at 186, 108, 66 μm. Together with the cycle of laminae bands, the ratios of these cyclicities are close to those of Milankovitch orbital cycles. Through tuning to orbital cycles, a net growth rate of 4.5 mm/Ma is derived for the profile, which is in perfect agreement with the growth rate of 4.6 mm/Ma determined by 230Thex/232Th dating. Therefore, the rhythmic growth of ferromanganese nodules appears definitely associated with Milankovitch cycles, and the growth cyclicities may offer a new tool for estimating growth rates of ferromanganese nodules and paleoenvironmental reconstruction at substage resolution when supported by radiometric dating.