MICROBIALLY MEDIATED PRECIPITATION OF CARBONATES AND EXCEPTIONAL PRESERVATION OF FOSSILS IN THE KIRKPATRICK BASALT (JURASSIC) OF ANTARCTICA

The Kirkpatrick Basalt (Jurassic; Transantarctic Mountains, Antarctica) comprises a series of continental flood basalts, cataclastic rocks, paleosols, and sparse sedimentary interbeds. Two dominant types of sedimentary interbeds of aqueous origin are represented in the Kirkpatrick Basalt: 1, unusual carbonate beds occurring in intimate association with pillow basalts, that were evidently deposited in small, possibly sulfidic thermal pools; and 2, fine-grained, thinly laminated siliciclastic beds that were probably deposited in small alkaline pools.

The carbonate beds contain weak laminae that are subhorizontal to concentric around basalt pillows. These carbonates lack macroscopic body fossils. Backscatter SEM-EDS analysis reveals minute, irregular, organic morphologies preserved as carbon. Morphologies observed are consistent with those recorded from modern thermoacidophils (Archaea: Crenarchaeota). Organic structures are rimmed by thin silica-rich carbonate halos, and surrounding carbonate mud lacks needles or other indicators of precipitation by macroscopic organisms. This suggests that thermophilic microbes mediated carbonate and perhaps silica precipitation in pools closely associated with hydrothermal vents and basaltic extrusion.

The siliciclastic beds are horizontally laminated but contain synsedimentary deformation structures associated with igneous extrusion. Macrofossils present include abundant conchostracan arthropods. Less common fossils include ostracodes, insects, and plants. Animal body fossils are preserved by coatings of calcium phosphate or silica. Minute strands and round bodies suggest that fungal hyphae and bacteria were present on non-biomineralizing animals and that microbially mediated precipitation was a key step in fossilization in this environment. The sicliciclastic interbeds lack thermophiles; such pools are inferred to have been too cool to support thermophilic prokaryotes.