Journal
GEOCHIMICA ET COSMOCHIMICA ACTA
Volume 251, Issue -, Pages 56-72Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2019.02.019
Keywords
Rare earth elements and yttrium; Calcium phosphates; Early diagenetic alteration; Coupled substitution; CCZ
Categories
Funding
- German Federal Ministry of Education and Research [03F0707F+G]
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Rare earth elements and yttrium (REY) are often used as proxies for (paleo)environmental conditions and for the reconstruction of element sources and transport pathways. Many geological systems are well described with respect to the behavior of REY but deep-sea sediments with their manifold processes impacting the sediment during early diagenesis leave some questions about the origin and development of the shale-normalized REY (REYSN) patterns unanswered. Here we report REY data for sediment solid phase and pore water from the upper 10 m of deep-sea sediments from the Clarion Clipperton Zone (CCZ) in the central equatorial Pacific. The solid-phase REY profiles show highest concentrations at depth below 5-8 m. The REYSN patterns show an enrichment in middle REY (MREY) (La-SN/Gd-SN between 0.35 and 0.60; Gd-SN/Yb-SN between 1.19 and 1.47) and either no or negative Ce-SN and Y-SN anomalies (i.e. chondritic to sub-chondritic Y/Ho ratios between 24.7 and 28.7). Based on correlation analyses of bulk sediment element concentrations and sequential extractions, we suggest that a Ca phosphate phase controls the distribution and the patterns of REY in these silty clay pelagic sediments rich in siliceous ooze. The MREY enrichment develops at the sediment-water interface and intensifies systematically with depth. The negative Ce-SN anomaly intensifies with depth possibly because Ce is mostly bound to Mn- and Fe-(oxyhydr)oxides. Therefore, Ce concentrations remain relatively constant throughout the sediment core, while its trivalent REY neighbors are mostly hosted by the Ca phosphate phase that continuously incorporates REY from ambient pore waters. The non-redox-sensitive trivalent REY concentrations increase with depth, producing or enhancing a negative Ce-SN anomaly through coupled substitution of REY 3+ and Na+ for Ca2+. The solid-phase REYSN pattern is therefore determined by the pore-water REY sN pattern and not suitable for paleoceanographic interpretation. The similarity of the pore-water and solid-phase REYSN patterns suggests, however, that only minor fractionation occurs during REY incorporation into the Ca phosphate crystal structure. (C) 2019 Elsevier Ltd. All rights reserved.
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