Rare Earth Elements in early-diagenetic foraminifer 'coatings': Pore-water controls and potential palaeoceanographic applications
File(s)
Accepted version
Author(s)
Type
Journal Article
Abstract
Rare Earth Element (REE) distributions in the ocean bear the fingerprints of several key environmental processes, including vertical particle/organic carbon fluxes, water column/pore-water oxygenation and ocean transports. The use of ‘fossil’ REE analyses in the service of palaeoceanography as redox, water transport or nutrient cycling ‘proxies’ has long been a tantalizing possibility. Here we demonstrate the application of a novel laser-ablation microanalysis approach for the rapid and accurate measurement of the REE composition of early diagenetic ‘coatings’ on fossil foraminifera. By applying this new method to a range of core-top and multi-core samples, we show that ‘authigenic’ REE enrichments on planktonic foraminifer surfaces (REEfs) reflect a primary seawater signature that becomes overprinted during sediment burial due to early diagenetic processes that control the flux of REEs to pore-fluids. Thus ‘light’ REEs (LREEs), and eventually ‘middle’ REEs (MREEs) are generally enriched in foraminifer 'coatings' relative to seawater, while Ce-anomalies (Ce/Ce*) recorded in surface sediments are typically more positive than local seawater values and are further ‘eroded’ during burial with the onset of anoxic conditions in the sediment. Similar patterns have previously been observed in pore-fluid measurements. Indeed, we show that Mn and Fe concentrations measured in foraminifer ‘coatings’ track the availability of these elements in pore-water, indicating that they are not associated with a secondary oxide phase. We propose that these elements, along with REEs are instead adsorbed directly from pore-fluids. In contrast, U in authigenic coatings tracks the removal of this element from solution under sub-oxic conditions, supporting the use of U/Ca in foraminifer coatings as a redox proxy. Although our results confirm a significant early diagenetic influence on REEfs, we also demonstrate the potential utility of ‘Ce-enrichment’ relative to expected seawater values as a palaeo-oxygenation proxy. We support this proposal with down-core measurements of U/Ca and Ce-enrichment spanning the last deglaciation in the sub-Antarctic Atlantic, as well as a global array of LGM measurements, which are found to co-vary with parallel estimates of radiocarbon ventilation age, consistent with a widespread drop in oxygen suggesting an increase in the efficiency of the biological pump. Our results suggest that laser-ablation REEfs measurements may hold some promise for palaeoceanographic reconstruction, in particular through a shift in emphasis away from the reconstruction of primary seawater signals to the analysis of diagenetic impacts that are sensitive to changing hydrographic conditions.
Date Issued
2019-01-15
Date Acceptance
2018-10-27
Citation
Geochimica et Cosmochimica Acta, 2019, 245, pp.118-132
ISSN
0016-7037
Publisher
Elsevier
Start Page
118
End Page
132
Journal / Book Title
Geochimica et Cosmochimica Acta
Volume
245
Copyright Statement
© 2018 Elsevier Ltd. All rights reserved. This manuscript is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International Licence http://creativecommons.org/licenses/by-nc-nd/4.0/
Sponsor
Natural Environment Research Council [2006-2012]
Natural Environment Research Council (NERC)
Identifier
http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000453771600007&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=1ba7043ffcc86c417c072aa74d649202
Grant Number
NE/M017826/1
NE/M017826/2
Subjects
Science & Technology
Physical Sciences
Geochemistry & Geophysics
Rare Earth Elements (REE)
Oxygenation
Laser-ablation
SOUTHERN-OCEAN
ISOTOPIC COMPOSITION
NEODYMIUM ISOTOPES
NATURAL-WATERS
ICP-MS
ATLANTIC
SEAWATER
COMPLEXATION
ASSOCIATION
VENTILATION
Publication Status
Published
Date Publish Online
2018-11-03