Sequestration of Zn into mixed pyrite-zinc sulfide framboids: A key to Zn cycling in the ocean?

Si-Yu Hu,Katy Evans,Kirsten Rempel,Paul Guagliardo,Matt Kilburn,Dave Craw,Kliti Grice,Jeffrey Dick

doi:10.1016/j.gca.2018.08.039

Abstract

Zinc (Zn) is an important micronutrient in the ocean, and fixation of Zn into organic, trace element-rich sediments is an important contributor to Zn cycling in the ocean. Framboidal sulfides are considered to be the major host for Zn in such settings. The sequestration of Zn into framboids via biotic or abiotic processes is not fully understood, which presents difficulties for interpretation of Zn isotope values in sediments. In this work, we describe a novel type of framboid with mixed pyrite and zinc sulfide (sphalerite or wurtzite) microcrystals from meta-pelites of the Otago Schist, New Zealand. A combination of optical microscopy, scanning electron microscopy (SEM) and nanoscale secondary ion mass spectrometry (NanoSIMS) were utilized to assess the association between Zn, pyrite and organic matter in framboids. The distribution of Zn in framboids is variable. Most pyrite microcrystals include minor amounts of Zn. Trace Zn is also observed to co-locate with organic matter, which occurs on the boundaries of pyrite microcrystals. Finally, Zn is found as single zinc sulfide microcrystals or zinc sulfide rims around pyrite microcrystals within individual framboids. These textures have not been recorded before, to our knowledge. The sequence of events that sequesters Zn into framboids may affect Zn isotope fractionation from seawater to continental margin sediments.

Highlights

Framboids are spheroidal aggregates consisting of multiple microcrystals of sulfide minerals and are commonly observed in ancient sedimentary rocks and modern sediments (Love, 1971; MacLean et al, 2008; Gregory et al, 2015; Hu et al, 2016)
The observations presented in this study suggest that fixation of Zn in framboids is the result of a combination of three processes: (1) zinc sulfide precipitation during BSR activity; (2) assimilation of Zn absorbed onto OMfr (ZnS-OMfr association); and (3) direct incorporation into pyrite
NanoSIMS analysis revealed four types of microcrystals hosted by polyframboids in a sulphur-rich pelitic sample from the Otago Schist, including three types not previously described

Summary

Introduction

Framboids are spheroidal aggregates consisting of multiple microcrystals of sulfide minerals and are commonly observed in ancient sedimentary rocks and modern sediments (Love, 1971; MacLean et al, 2008; Gregory et al, 2015; Hu et al, 2016). Organic matter has been observed to envelop both individual pyrite microcrystals and the complete framboid in modern sediments (MacLean et al, 2008) and ancient sedimentary rock (Wacey et al, 2015), and it has been demonstrated that the formation of framboids is associated with sulfate-reducing microorganism activities (Machel, 2001; Schoonen, 2004). The degradation of microorganisms or microbial by-products results in the common occurrence of CMfr in the framboidal structure (e.g. Large et al, 2001)

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