Rare earth element association with Foraminifera

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doi: 10.1016/j.gca.2012.07.009
Author(s): Roberts, Natalie L.; Piotrowski, Alexander M.; Elderfield, Henry; Eglinton, Timothy I.; Lomas, Michael W.
Author Affiliation(s): Primary:
University of Cambridge, Department of Earth Sciences, Cambridge, United Kingdom
Other:
Eidgenössische Technische Hochschule Zürich, Switzerland
Bermuda Institute of Ocean Sciences, Bermuda
Volume Title: Geochimica et Cosmochimica Acta
Source: Geochimica et Cosmochimica Acta, Vol.94, p.57-71. Publisher: Elsevier, New York, NY, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. Includes appendices. 78 refs.; illus., incl. 1 table, sketch map
Summary: Neodymium isotopes are becoming widely used as a palaeoceanographic tool for reconstructing the source and flow direction of water masses. A new method using planktonic foraminifera which have not been chemically cleaned has proven to be a promising means of avoiding contamination of the deep ocean palaeoceanographic signal by detrital material. However, the exact mechanism by which the Nd isotope signal from bottom waters becomes associated with planktonic foraminifera, the spatial distribution of rare earth element (REE) concentrations within the shell, and the possible mobility of REE ions during changing redox conditions, have not been fully investigated. Here we present REE concentration and Nd isotope data from mixed species of planktonic foraminifera taken from plankton tows, sediment traps and a sediment core from the NW Atlantic. We used multiple geochemical techniques to evaluate how, where and when REEs become associated with planktonic foraminifera as they settle through the water column, reside at the surface and are buried in the sediment. Analyses of foraminifera shells from plankton tows and sediment traps between 200 and 2938 m water depth indicate that only ∼20% of their associated Nd is biogenically incorporated into the calcite structure. The remaining 80% is associated with authigenic metal oxides and organic matter, which form in the water column, and remain extraneous to the carbonate structure. Remineralisation of these organic and authigenic phases releases ions back into solution and creates new binding sites, allowing the Nd isotope ratio to undergo partial equilibration with the ambient seawater, as the foraminifera fall through the water column. Analyses of fossil foraminifera shells from sediment cores show that their REE concentrations increase by up to 10-fold at the sediment-water interface, and acquire an isotopic signature of bottom water. Adsorption and complexation of REE3+ ions between the inner layers of calcite contributes significantly to elevated REE concentrations in foraminifera. The most likely source of REE ions at this stage of enrichment is from bottom waters and from the remineralisation of oxide phases which are in chemical equilibrium with the bottom waters. As planktonic foraminifera are buried below the sediment-water interface redox-sensitive ion concentrations are adjusted within the shells depending on the pore-water oxygen concentration. The concentration of ions which are passively redox sensitive, such as REE3+ ions, is also controlled to some extent by this process. We infer that (a) the Nd isotope signature of bottom water is preserved in planktonic foraminifera and (b) that it relies on the limited mobility of particle reactive REE3+ ions, aided in some environments by micron-scale precipitation of MnCO3. This study indicates that there may be sedimentary environments under which the bottom water Nd isotope signature is not preserved by planktonic foraminifera. Tests to validate other core sites must be carried out before downcore records can be used to interpret palaeoceanographic changes. Abstract Copyright (2012) Elsevier, B.V.
Year of Publication: 2012
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 10 Paleontology, Invertebrate; Absolute age; Atlantic Ocean; Bermuda Rise; Biochemistry; Blake-Bahama Outer Ridge; Calcite; Carbonates; Chemical composition; Cores; Crystal chemistry; Dates; Foraminifera; ICP mass spectra; Invertebrata; Isotope ratios; Isotopes; Leg 172; Marine sediments; Mass spectra; Metals; Microfossils; Mobilization; Nd-144/Nd-143; Neodymium; North Atlantic; Northwest Atlantic; ODP Site 1060; Ocean Drilling Program; Paleo-oceanography; Paleoenvironment; Planktonic taxa; Protista; Rare earths; Reconstruction; Sea water; Sediments; Shells; Sm/Nd; Spectra; Stable isotopes
Coordinates: N304535 N304535 W0742759 W0742759
N300000 N400000 W0550000 W0650000
Record ID: 2012095610
Copyright Information: GeoRef, Copyright 2017 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands