Showing papers by "Jan Kramers published in 2002"
TL;DR: The Scottish record is in line with previous studies of past atmospheric lead deposition, however, the Canadian deposit suggests that the wind derived, pre-industrial lead, is less radiogenic as previously implied using sediment archives.
95 citations
TL;DR: It is demonstrated that this poorer reproducibility of the Lu/Hf ratio is truly caused by sample heterogeneity; thus, results are geologically reasonable.
Abstract: A combined procedure for separating Lu, Hf, Sm, Nd, and rare earth elements (REEs) from a single sample digest is presented. The procedure consists of the following five steps: (1) sample dissolution via sodium peroxide sintering; (2) separation of the high field strength elements from the REEs and other matrix elements by a HF-free anion-exchange column procedure; (3) purification of Hf on a cation-exchange resin; (4) separation of REEs from other matrix elements by cation exchange; (5) Lu, Sm, and Nd separation from the other REEs by reversed-phase ion chromatography. Analytical reproducibilities of Sm-Nd and Lu-Hf isotope systematics are demonstrated for standard solutions and international rock reference materials. Results show overall good reproducibilities for Sm-Nd systematics independent of the rock type analyzed. For the Lu-Hf systematics, the reproducibility of the parent/daughter ratio is much better for JB-1 (basalt) than for two analyzed felsic crustal rocks (DR-N and an Archaean granitoid). It is demonstrated that this poorer reproducibility of the Lu/Hf ratio is truly caused by sample heterogeneity; thus, results are geologically reasonable.
54 citations
TL;DR: In this paper, the possible drawdown of a massive CO2 atmosphere in early Earth history is discussed using two working hypotheses: first, that this removal of CO2 from the atmosphere occurred mainly via silicate weathering; second, that crust-to-mantle recycling rates found from forward modelling of crust- mantle history can be used to estimate rates of this ancient silicate washing.
Abstract: Abstract The possible drawdown of a massive CO2 atmosphere in early Earth history is discussed using two working hypotheses: first, that this removal of CO2 from the atmosphere occurred mainly via silicate weathering; second, that crust-to-mantle recycling rates found from forward modelling of crust-mantle history can be used to estimate rates of this ancient silicate weathering. Previous U-Th-Pb and Sm-Nd forward modelling efforts are reviewed, from which it was concluded that an insignificant amount of continental crust existed at 4.4 Ga, i.e. so-called ‘no-growth models’ for the continental crust appear untenable. New modelling carried out is based on a crustal growth curve starting with zero mass at 4.2 Ga and reaching 75% of the present crust mass by 2 Ga. It concerns variations in crust-to-mantle recycling rates through geological time. Best fits to isotope data are obtained if it is assumed that erosion rates (mass removal per unit surface) were approximately constant from early Archaean time to the present. From the results it can be estimated that drawdown of a massive CO2 atmosphere by silicate weathering could have been completed by the end of Archaean time at the earliest, and about 1.5 Ga ago at the latest.
26 citations