Sedimentary trace element sinks in a tropical upwelling system
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The accumulation of trace elements in sediments from highly productive continental margins may depend on the affinity of these elements for organic matter and their degrees of further incorporation into pyrite (FeS2). We tested the hypothesis that the relative contributions of these geochemical phases play a substantial role as trace element (As, Cd, Cr, Cu, Mn, Ni, and Zn) sinks in the highly bioturbated sediments from the tropical upwelling system off Cabo Frio, southeastern Brazil.
Materials and methods
Four sediment cores sampled across the Cabo Frio continental shelf were submitted to a sequential extraction procedure performed to separate three different operationally defined fractions, i.e., the geochemical phases soluble in 1 M HCl (considered as the “reactive” fraction), concentrated H2SO4 (considered as the organic matter-bound phase), and concentrated HNO3 (considered as the pyrite-bound phase). The trace metal incorporation into pyrite was assessed by estimating the degree of trace metal pyritization (DTMP), while the pyrite sulfur stable isotope signatures (δ 34SPyr) were used as proxies for sulfur redox cycling intensity.
Results and discussion
Relative contributions of trace element fixation by organic matter and pyrite were positively correlated for Mn, Cr, and Ni on one hand, and negative correlated for Cu, on the other hand. The positive correlations imply in synergistic roles of these geochemical phases in determining the trace elements sedimentary sinks, while the negative relationship found for Cu reflects differences in the predominant retention mechanisms along with sediment burial. The δ34SPyr signatures were negatively correlated with DTMP values of As, Cd, and Mn, suggesting a diminishing effect of the sulfur redox cycling on trace elements pyritization. These δ 34SPyr signatures were not correlated with DTMP values of Cr, Cu, and Ni, which were dominantly associated with the high organic matter contents found in this upwelling system.
The role of pelagic organic matter scavenging of metals and later fueling of benthic microbial sulfate reduction and pyrite accumulation were evidenced as highly variable across the Cabo Frio shelf sediments. Differences in the organic matter accumulation in response to upwelling-enhanced primary productivity and in the intensity of bioturbation-driven sulfur redox cycling help to explain the spatial variability in the biogeochemical processes affecting the sedimentary trace metal sinks.
KeywordsDegree of pyritization Organic matter binding Redox dynamics Stable sulfur isotopes Trace elements Upwelling system
This work was funded by the Geochemistry Network of PETROBRAS/CENPES and the Brazilian National Petroleum and the Biofuels Agency (ANP). MM and RD received grants from Rio de Janeiro State Research Foundation (FAPERJ) and Brazilian Ministry of Education (CAPES). MEB received support from Leibniz IOW, and wishes to thank I. Scherff and P. Escher (IOW) for analytical support, and the late J.W. Morse for inspiring discussions.
- Albuquerque ALS, Belém AL, Zuluaga FJB, Cordeiro LGM, Mendoza U, Knoppers BA, Gurgel MHC, Meyers PA, Capilla R (2014) Particle fluxes and bulk geochemical characterization of the Cabo Frio upwelling system in southeastern Brazil: sediment trap experiments between spring 2010 and summer 2012. An Acad Bras Cienc 86:601–620CrossRefGoogle Scholar
- Böttcher ME, Hespenheide B, Llobet-Brossa E, Beardsley C, Larsen O, Schramm A, Wieland A, Böttcher G, Berninger U-G, Amann R (2000) The biogeochemistry, stable isotope geochemistry, and microbial community structure of a temperate intertidal mudflat: an integrated study. Cont Shelf Res 20:1749–1769CrossRefGoogle Scholar
- Huerta-Diaz MA, Delgadillo-Hinojosa F, Otero X, Segovia-Zavala J, Martin Hernandez-Ayon J, Galindo-Bect M, Amaro-Franco E (2011) Iron and trace metals in microbial mats and underlying sediments: results from Guerrero Negro Saltern, Baja California Sur, Mexico. Aquat Geochem 17:603–628CrossRefGoogle Scholar
- Morse JW (1994b) Release of toxic metals via oxidation of authigenic pyrite in resuspended sediments. In: Alpers CN, Blowes DW (eds) Environmental geochemistry of sulfide oxidation. American Chemical Society, Washington, DC, pp 289–297Google Scholar
- Neumann T, Scholz F, Kramar U, Ostermaier M, Rausch N, Berner Z (2013) Arsenic in framboidal pyrite from recent sediments of a shallow water lagoon of the Baltic Sea. Sedimentology 60:1389–1414Google Scholar
- Noël V, Marchand C, Juillot F, Ona-Nguema G, Viollier E, Marakovic G, Olivi L, Delbes L, Gelebart F, Morin G (2014) EXAFS analysis of iron cycling in mangrove sediments downstream a lateritized ultramafic watershed (Vavouto Bay, New Caledonia). Geochim Cosmochim Acta 136:211–228CrossRefGoogle Scholar
- Soliman MF, El Goresy A (2012) Framboidal and idiomorphic pyrite in the upper Maastrichtian sedimentary rocks at Gabal Oweina, Nile Valley, Egypt: formation processes, oxidation products and genetic implications to the origin of framboidal pyrite. Geochim Cosmochim Acta 90:195–220CrossRefGoogle Scholar
- Wu Z, Ren D, Zhou H, Gao H, Li J (2016) Sulfate reduction and formation of iron sulfide minerals in nearshore sediments from Qi'ao island, Pearl River estuary, southern China. Quatern Int. doi: 10.1016/j.quaint.2016.06.003