Banded iron formations as critical paleomarine proxies

• Author / Creator

  Robbins, Leslie J

• Banded iron formations (BIF) are a key chemical sedimentary record for examining the evolution of the early oceans, atmosphere, and biosphere. While the classic model suggests that BIF were deposited by either anoxygenic photoferrotrophs or oxygen producing cyanobacteria, recent studies have invoked an alternative depositional model reliant on the precipitation of a ferrous silicate precursor and the subsequent post- depositional oxidation of BIF by groundwater. Inspired by this suggestion, this work evaluates the proposed depositional model of BIF, the reliability of trace element signatures in these chemical sediments as ancient marine geochemical proxies, and aspects of ancient marine chemistry that may affect the interpretation of the paleo-seawater record. Hydrogeological and stratigraphic constraints from a simplistic box model indicate that the post-depositional oxidation of a ferrous silicate precursor phase to BIF, such as greenalite, is highly unlikely except under highly non-actualistic constraints such as grossly elevated oxygen levels in groundwater, untenable hydraulic gradients, and permeabilities inconsistent with the lithology of BIF. Therefore, an iron oxyhydroxide precursor phase is favoured and strongly suggests the influence of the biosphere in oxidizing ferrous iron, leading to BIF deposition. A statistical assessment of the trace element chemistry of BIF generated through complementary analytical techniques, namely bulk digestion high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) and laser ablation ICP-MS (LA-HR- ICP-MS), indicate that significant differences can exist in signals measured in BIF by these two methods. Bulk digestion analyses tend to produce more muted geochemical trends and may significantly mask certain geochemical signals that can be ascertained using an in-situ analysis of individual mineral grains by LA-HR- ICP-MS. In order to fully capture the wealth of information recorded by BIF, an approach that combines the two methodologies likely represents the best practice in the continued use of BIF as Archean to Paleoproterozoic marine archives. Finally, marine pH plays a fundamental role in controlling a number of processes that would have been operative in the ancient oceans including the authigenic precipitation of mineral phases, the surface reactivity of particle phases such as the iron oxyhydroxide precursor to BIF, and the speciation of trace elements in seawater. Thermodynamic calculations and flux estimates indicate that carbonate buffering in the early oceans should have been established within 0.1 to 10 million years, suggesting a relatively restricted range of pH conditions that would were favourable for iron oxyhydroxides to quantitatively, and reliably, capture the composition of the ancient water column.

• Subjects / Keywords

  • Precambrian
  • banded iron formations
  • diagenesis
  • trace metals

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R30R9MK8S

• License

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