Glaucony authigenesis, maturity and alteration in the Weddell Sea: An indicator of paleoenvironmental conditions before the onset of Antarctic glaciation.

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+.

Amorphous Ca-phosphate precursors for Ca-carbonate biominerals mediated by Chromohalobacter marismortui.

Although diverse microbial metabolisms are known to induce the precipitation of carbonate minerals, the mechanisms involved in the bacterial mediation, in particular nucleation, are still debated. The study of aragonite precipitation by Chromohalobacter marismortui during the early stages (3-7 days) of culture experiments, and its relation to bacterial metabolic pathways, shows that: (1) carbonate nucleation occurs after precipitation of an amorphous Ca phosphate precursor phase on bacterial cell surfaces and/or embedded in bacterial films; (2) precipitation of this precursor phase results from local high concentrations of PO(4)(3-) and Ca(2+) binding around bacterial cell envelopes; and (3) crystalline nanoparticles, a few hundred nanometres in diametre, form after dissolution of precursor phosphate globules, and later aggregate, allowing the accretion of aragonite bioliths.