Formation, redistribution, and stabilization of framboidal pyrite in shallowly buried deep-sea sediments

Kiichiro Kawamura 

Framboidal pyrite is widespread in anoxic marine sediments, but the processes that control the redistribution and preservation of pyrite aggregates after their formation remain poorly constrained. Here, I examine the microfabric and spatial organization of framboidal pyrite in shallowly buried deep-sea sediments from the Sagami Trough and the Sea of Japan using complementary freeze-dried and resin-embedded specimens. Framboidal pyrite occurs preferentially within the cavities of benthic and planktonic microfossils, where aggregates are concentrated along the lower sides of the cavities and form distinct geopetal fabrics. Loosely aggregated framboids composed of sub-spherical microcrystals occur together with densely packed framboids, indicating differences in aggregate packing and crystal development. Filamentous material less than 1 μm in thickness occurs around and between framboids within microfossil cavities, whereas isolated framboids in the surrounding sediment matrix generally lack comparable material. The preferential concentration of framboids along the lower sides of cavities is consistent with gravitational redistribution of mobile pyrite aggregates after their formation, followed by physical stabilization within confined pore spaces. The spatial association between filamentous material and pyrite further suggests that such material may have contributed to the retention of pyrite aggregates, although its composition and origin cannot be determined from the present observations. These observations indicate that the spatial organization of framboidal pyrite in marine sediments can record not only mineral formation but also subsequent physical redistribution and stabilization within sedimentary microenvironments.