Rare earth elements and neodymium isotopes in sedimentary organic matter

Abstract

International audienceWe report rare earth element (REE) and neodymium (Nd) isotope data for the organic fraction of sediments collected from various depositional environments, i.e. rivers (n=25), estuaries (n=18), open-ocean settings (n=15), and cold seeps (n=12). Sedimentary organic matter (SOM) was extracted using a mixed hydrogen peroxide/nitric acid solution (20%-H2O2-0.02M-HNO3), after removal of carbonate and oxy-hydroxide phases with dilute hydrochloric acid (0.25M-HCl). A series of experimental tests indicate that extraction of sedimentary organic compounds using H2O2 may be complicated occasionally by partial dissolution of sulphide minerals and residual carbonates. However, this contamination is expected to be minor for REE because measured concentrations in H2O2 leachates are about two-orders of magnitude higher than in the above mentioned phases.The mean REE concentrations determined in the H2O2 leachates for samples from rivers, estuaries, coastal seas and open-ocean settings yield relatively similar levels, with σREE=109±86ppm (mean±s; n=58). The organic fractions leached from cold seep sediments display even higher concentration levels (285±150ppm; mean±s; n=12). The H2O2 leachates for most sediments exhibit remarkably similar shale-normalized REE patterns, all characterized by a mid-REE enrichment compared to the other REE. This suggests that the distribution of REE in leached sedimentary organic phases is controlled primarily by biogeochemical processes, rather than by the composition of the source from which they derive (e.g. pore, river or sea-water).The Nd isotopic compositions for organic phases leached from river sediments are very similar to those for the corresponding detrital fractions. In contrast, the SOM extracted from marine sediments display εNd values that typically range between the εNd signatures for terrestrial organic matter (inferred from the analysis of the sedimentary detrital fractions) and marine organic matter (inferred from the analysis of local surface seawater). A notable exception is the case of organic matter (OM) fractions leached from cold seep sediment samples, which sometimes exhibit εNd values markedly different from both terrigenous and surface seawater signatures. This suggests that a significant fraction of organic compounds in these sediments may be derived from chemosynthetic processes, recycling pore water REE characterized by a distinct isotopic composition.Overall, our results confirm that organic matter probably plays an important role in the oceanic REE budget, through direct scavenging and remineralization within the water column. Both the high REE abundances and the shape of shale-normalized patterns for leached SOM also suggest that OM degradation in sub-surface marine sediments during early diagenesis could control, to a large extent, the distribution of REE in pore waters. Benthic fluxes of organic-bound REE could hence substantially contribute to the exchange processes between particulates and seawater that take place at ocean margins. Neodymium ...

The Impact of Benthic Processes on Rare Earth Element and Neodymium Isotope Distributions in the Oceans

“…Finally, regardless of the impact on the water column, we emphasize that all of the potential "top down" mechanisms for remobilizing surface REEs at depth would be greatly enhanced within seafloor sediments (Freslon et al, 2014); i.e., manifest as a pore water flux. In summary, while scavenging certainly happens on particles, we argue that the REE mass transfer is likely relatively small and subsequent desorption at depth in the water column is unlikely.…”

The impact of sedimentary coatings on the diagenetic Nd flux

“…Frank, 2002;Goldstein and Hemming, 2003). The neodymium isotope value is reported as ε Nd , defined as [( 143 Nd sample / 144 Nd Sample )/( 143 Nd CHUR / 144 Nd CHUR ) − 1] × 10 4 where CHUR is the Chondritic Uniform Reserve, used as an average earth value ( 143 Nd/ 144 Nd = 0.512638; Jacobsen and Wasserburg, 1980).…”

REY and Trace Element Chemistry of Fluorite from Post-Variscan Hydrothermal Veins in Paleozoic Units of the North German Basin

“…However, a primary volcanogenic fluid source can be discarded for both the ABB and LSB [2,12,18,93]. The flat REY CN signature [95,96] and the overall low concentrations are strong indicators that basinal brines derived from marine water represent the mineralizing fluid in the LSB. A comparison of a selection of whole rock volcanic REY CN patterns with median ABB fluorite signatures is shown in Figure 6.…”