CHANGES IN THE SULFUR CYCLE AROUND THE 2.63 GA GIANT IMPACT RECORDED IN THE MONTEVILLE FORMATION, SOUTH AFRICA

A sufficiently large bolide impact will create a geologically instantaneous distal spherule layer that can be correlated in time-equivalent sediments across distinct environments. Both carbonates and clastics contain spherules created by a 2.63 Ga giant impact that appears to have changed feedbacks between Earth and life. Outcrops associated with the impact on the Monteville Farm, Northern Cape, South Africa, contain high volumes of oxidized sulfide minerals in beds that are laterally consistent over the 40 meters of cliff face. Oxidized sulfide textures are now preserved as iron-rich nodules, sheets, tubes, and laminae. Rolled laminae were cohesive but softly deformable when deposited, suggesting these textures are microbial in origin. Breakage of these structures and laminae deformed around vertically oriented sheets suggest that many pyritized features lithified before the sediment compacted. Thus, sulfide distribution likely reflects ocean and pore water chemistry, and the types of sulfidized features are different before and after the impact. Eight out of nine shales in the seventeen meters below the spherule layer contain nodular pyrite, while all but one or two shales within 67 meters above the spherule layer are nodule-free. Additionally, the pyritic microbial facies are less abundant above the impact. These differences suggest ocean and pore water chemistry changed in the aftermath of the impact. These results point to the important role for external forces in controlling the sulfur cycle on broad scales and to a potential role for sulfur-cycling microbes in controlling the distribution of sulfide preserved in rocks.