Rare Earth Elements geochemistry of Lake Baikal sediment: its implication for geochemical response 5 to climate change during the Last Glacial/Interglacial transition,

Abstract Sediments deposited on the bottom of Lake Baikal have contributed to the understanding of a long-term environmental history of continents. Rare earth elements (REEs) along with major elements and loss on ignition (LOI) of Baikal sediments were determined with the aim of evaluating their suitability for a new paleoenvironmental proxy. Our interest is concentrated on paleoenvironmental change during the Last Glacial/Interglacial transition (LGIT). Chondrite-normalized REE patterns for Baikal sediments show a similar variation to those for typical upper continental crustal materials. Three parameters of (La/Yb) n (n: chondrite-normalized value) ratio, SREE/TiO 2 and Eu anomaly were used to express detailed characteristics of Baikal sediments. Depth profile of (La/Yb) n ratio shows abrupt change, whose timing corresponds to the beginning of climatic warming inferred from the profiles of SiO 2 /TiO 2 and LOI. In addition, (La/Yb) n ratio, SREE/TiO 2 and the degree of Eu anomaly correlate with each other. This suggests that inflow process of particulate materials into the lake may have changed during the LGIT. The analytical results of this study lead to the conclusion that REE is a useful paleoenvironmental proxy in the Baikal region.

A puzzle of Gd-break and tetrad effect of aqueous lanthanide(III)-EDTA complex formation: Different Racah parameters between two lanthanide-EDTA complex series with distinct hydration states

The logarithmic formation constants (logK) for aqueous lanthanide(III)-EDTA complexes show a Gd-break and subtle tetrad effect, but the ΔSr and ΔHr data indicate step-like changes in the middle Ln. UV-Vis spectra of Eu3+ for Eu-EDTA solution suggest the hydration change occurring across the middle Ln-EDTA series. This is supported by the recent studies of luminescence kinetics on the inner-sphere waters of Eu-EDTA and Tb-EDTA, and by the lanthanide-induced 17O NMR shifts of Ln-EDTA. Such spectroscopic studies allow to re-examine the long-lasting controversy of the Gd-break and tetrad effect of logK(Ln-EDTA). Here is proposed a thermodynamic model to elucidate it from the viewpoints: i) hydration changes of Ln-EDTA and light Ln3+ (aq) series, ii) the nephelauxetic effect due to the coordination change of Ln3+ of Ln-EDTA, and iii) the improved equation of Jφrgensen theory applicable to ΔHr and ΔGr. This model satisfies the spectroscopic constraints on the hydration states of Ln-EDTA series: Three and two water molecules exist in the inner spheres of light Ln(La~Nd)-EDTA and heavy Ln(Dy~Lu)-EDTA series, respectively. Each middle Ln(Sm~Tb)-EDTA is a mixture of the two hydrate species, and their abundances are given by the equilibrium of hydration change reaction: [Ln ⋅ EDTA⋅ (H2O)3 ]− (aq ) = [Ln ⋅ EDTA⋅ (H2O)2 ]− (aq ) + H2O(l ). The ΔHr, ΔSr and ΔGr data for Ln-EDTA formation, when corrected for hydration changes in Ln-EDTA and light Ln3+ (aq) series, can be fitted by the improved equation of Jφrgensen theory, and the thermodynamic parameters for the formation of two Ln-EDTA series from octahydrate Ln3+ (aq) have been evaluated. The hydration change of middle Ln-EDTA series explains the apparent Gd-break of logK(Ln-EDTA). The nephelauxetic effect by the change in coordination number for Ln3+ (ΔCN) is evident in ΔH values for the three reactions: (a) [Ln ⋅ EDTA⋅ (H2O)3 ]− (aq ) formation from Ln(oct, aq ) 3+ (ΔCN=+1), (b) [Ln ⋅ EDTA⋅ (H2O)2 ]− (aq ) formation from Ln(oct, aq) 3+ (ΔCN=0), and (c) the hydration change reaction of Ln-EDTA given as (b)-(a) (ΔCN=−1). The relative Racah E1 parameter values for Nd3+, ΔE1(Nd3+), from the tetrad effects of ΔH for (a), (b) and (c), are +(39±14) cm-1, −(19±7) cm-1, and −(58±18) cm-1, respectively. The ΔHr values for (a) with ΔCN=+1 show a large convex tetrad effect, and the ΔSr values also exhibit a similar tetrad effect. Therefore, their tetrad effects are significantly cancelled in ΔGr. This is a reason why the subtle tetrad-like variation of logK(Ln-EDTA) is asymmetrical between light and heavy Ln, along with the hydration change effects of middle Ln-EDTA and light Ln3+ (aq) series cancelled partly within the light Ln