Abstract
Three types of glaucony grains were identified in the late Eocene (~35.5–34.1 Ma) sediments from Ocean Drilling Program (ODP) Hole 696B in the northwestern Weddell Sea (Antarctica). The grains are K2O-rich (~7 wt%) and formed by smectite-poor interstratified ~10 Å glauconite-smectite with flaky/rosette-shaped surface nanostructures. Two glaucony types reflect an evolved (types 1 and 2 glaucony; less mature to mature) stage and long term glauconitization, attesting to the glaucony grains being formed in situ, whereas the third type (type 3 glaucony) shows evidences of alteration and reworking from nearby areas. Conditions for the glaucony authigenesis occurred in an open-shelf environment deeper than 50 m, under sub-oxic conditions near the sediment-water interface. These environmental conditions were triggered by low sedimentation rates and recurrent winnowing action by bottom-currents, leading to stratigraphic condensation. The condensed glaucony-bearing section provides an overview of continuous sea-level rise conditions pre-dating the onset of Antarctic glaciation during the Eocene-Oligocene transition. Sediment burial, drop of O2 levels, and ongoing reducing (postoxic to sulphidic) conditions at Hole 696B, resulting in iron-sulphide precipitation, were a key limiting factor for the glauconitization by sequestration of Fe2+.
Highlights
Glauconite is the iron-potassium hydrous phyllosilicate mineral typical of the glaucony green marine clay facies, which includes Fe-rich, 2:1 dioctahedral specimens with expandable layers of randomly interstratified glauconite-smectite[1]
Type 1 glaucony often preserve ill-defined globules and caterpillar nanostructures[31]. These nanostructures most resemble bacterial threads and coccoid-like forms (Fig. 2E–H; Supplementary Fig. S4D) similar to those described in other studies[11,32], but are comparable with those of silica microtexture surfaces[33]
Type 2 glaucony is made of rounded mammillated to lobate, dark green grains with smooth surfaces with flaky honeycombed nanostructure (Fig. 2A,I; Supplementary Fig. S4F) and often cracked at the margins (Supplementary Figs S4C, S5C)
Summary
Glauconite is the iron-potassium hydrous phyllosilicate mineral typical of the glaucony green marine clay facies, which includes Fe-rich, 2:1 dioctahedral specimens with expandable layers of randomly interstratified glauconite-smectite[1]. In this contribution we describe, for the first time in the Southern Ocean, a significant Cenozoic (late Eocene) glauconitization event in the vicinity of the SOM shelf (Site 69617; Fig. 1 and Supplementary Fig. S2A), utilizing textural, mineralogical and geochemical analyses Based on this multi-proxy approach, we reconstruct the paleoenvironmental conditions, including water depth changes, sediment-water interface (before burial) oxygenation conditions, and the influence of post-depositional alterations on authigenic mature glaucony.
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