Abstract
Abstract Persistently low atmospheric oxygen requires that net organic carbon burial was muted through much of Earth’s middle age. In order to achieve global mass balance with respect to O2, recent models have suggested that redox-dependent mechanisms, such as Fe(II)-phosphate precipitation, limited phosphate availability in dominantly anoxic and ferruginous oceans, in turn limiting net primary production, and therefore organic carbon burial. Nevertheless, observational constraints on phosphorus cycling in ferruginous Proterozoic systems are rare, leaving these models largely untested. Here, we present high-resolution petrographic and mineralogical data showing that the 1.3 Ga Sherwin Ironstone (Roper Group, Australia) was dominated by syndepositional precipitation of the Fe(II)-silicate minerals greenalite and berthierine, interlaminated with abundant authigenic calcium fluorapatite (CFA). Set in a quantitative geochemical framework, these data reveal that elevated marine SiO2(aq) concentrations facilitated extensive Fe(II)-silicate production, leaving CFA, rather than Fe(II)-phosphate, as the principal inorganic phosphorous sink in shallow-water Roper Group sediments. More broadly, the physical and chemical factors that triggered Fe(II)-silicate and CFA burial in the Roper Seaway highlight semi-restricted basins as important loci of phosphorus removal from the mid-Proterozoic ocean.
Highlights
Low atmospheric oxygen is considered a defining characteristic of the Precambrian Earth, and recent models have highlighted the biogeochemical implications of a stable low-oxygen world
Together, petrographic and mineralogical constraints indicate that calcium fluorapatite (CFA) was deposited with Fe(II)-silicates of the Sherwin Ironstone at or just below the seafloor (Figs. 3A, 3C, and 3D; Fig. DR6)
These data do not rule out the possibility that some of the PO4 in CFA minerals was derived through dissolution of Feoxides, they support the inference that the high aqueous PO4 concentrations necessary for CFA nucleation extended above the sediment-water interface
Summary
Low atmospheric oxygen is considered a defining characteristic of the Precambrian Earth, and recent models have highlighted the biogeochemical implications of a stable low-oxygen world. Low [P] in many mid-Proterozoic shales has been interpreted to reflect P limitation through Fe(II)-phosphate precipitation in ferruginous systems (Derry, 2015), P scavenging through green rust precipitation (Zegeye et al, 2012), and adsorption and /or coprecipitation with Fe(III)-oxides (Jones et al, 2015). These mechanisms receive support from environmental analogues and geochemical models, they have not been tested against detailed mineralogical and sedimentological data from Proterozoic rocks. Subsamples of CITATION: Johnson, B.R., et al, 2020, Phosphorus burial in ferruginous SiO2-rich Mesoproterozoic sediments: Geology, v. 48, p. 92–96, https://doi.org/10.1130/ G46824.1
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have