BIOTIC INFLUENCES ON ‘ABIOTIC’ PALEOPROTEROZOIC STROMATOLITES, BIWABIK IRON-FORMATION, MINNESOTA

Many stromatolites from the late Archean—Paleoproterozoic are composed of sea floor carbonate precipitates. The abundant precipitates have been linked to high carbonate concentrations in the early pre-Cambrian oceans. The presence of iron in anoxic pre-Cambrian oceans has also been implicated in precipitation. However, the role of microbes in forming precipitate stromatolites has not been elucidated. Indeed, many pre-Cambrian precipitate stromatolites are labeled as ‘abiogenic' constructions.

Two distinct types of stromatolites are found in the Paleoproterozoic Animikie Basin: domes with indistinct, mm-scale laminae and cm-wide columns with sharply distinct, µm thick laminae. Facies analysis suggests that domal stromatolites formed in the peritidal zone (Gunflint Iron-Fm., e.g., Schreiber beach). The columnar stromatolites are more enigmatic. Previous interpretations as abiogenic sinter deposits (Gunflint –Iron Fm., Mink Mt. and Biwabik Iron-Fm.) were based on comparison with modern Yellowstone sinter, the thinness of the laminae, and the paucity of microfossils.

New data on the macro- and microstructure and broad distribution of the ‘abiogenic' stromatolites invalidate a hot spring setting. These ‘abiotic' stromatolites are characterized by discrete, alternating dark laminae (mean 8 µm), and thicker, light laminae (mean 28 µm). The dark laminae consist of iron oxides and silica cement, enriched in carbon and sulfur. The lighter bands are replacement chalcedony and preserve two distinct textures: mechanically deposited, cross-laminated mud and ghost botryoidal fans. Based on stratigraphy and sedimentology, the stromatolites are interpreted to have formed in a storm-influenced, subtidal zone. We interpret the laminae as having formed when iron bacterial mats lowered local iron concentrations. Because iron is a strong calcite inhibitor, a decrease in iron concentration could have induced formation of the calcite precipitates. Formation of botryoidal fans rather than micrite would be expected with a reduced but still influential concentration of iron. In other stromatolites, bacterial mats sealed over channels filled with loose sediment. The ‘abiogenic' Animikie stromatolites are biogenic and record a mode of accretion via a previously underappreciated Paleoproterozoic ecosystem.