Early Paleoproterozoic warming and oxidation of the Planet recorded by sedimentology and rare earth elements of two Huronian carbonate deposits

• 2.3 Ga Huronian carbonates record costal environments and their geochemistry. • Evidence of acidic weathering freeing Fe, V, U and Cr during Great Oxidation Event. • Particulate Fe hydroxide delivered middle rare earth elements (REEs) to the ocean. • This affected the chemistry of carbonates precipitated in subtidal and intertidal environments. • Preoxidized sabkha sediments were not enriched in middle REEs, V or Cr. One of the most important transitions the Earth experienced is the appearance of significant free oxygen in its atmosphere at approximately 2.4 Ga. This drove the Earth into a protracted series of glaciations well recorded in the Huronian Supergroup of central Canada. Greenhouse conditions developed as the Earth emerged from this event. Carbonate units in the 2.31 Ga Gordon Lake Formation and 2.17 Ga Kona Dolomite, of the Huronian Supergroup and Chocolay Group respectively, record deposition in sabkha to shallow nearshore environments. Nearshore seawater chemistry reflected by these early and middle Lomagundi Event carbonates differs from other time periods. All carbonates contain abundant hematite as particulate matter scattered throughout, as accumulations in depressions between small pseudocolumnar stromatolites, and forming layers separating them. Rare earth elements (REEs) in the carbonates have patterns that differ from those of other time periods. Sabkha carbonates have pronounced middle REE (MREE) and heavy REE (HREE) enrichments. Clays preferentially binding light REEs in the detritus during weathering on the craton probably led to the groundwater flow to the sabkha having those REE characteristics. Intertidal to subtidal carbonates contain rare earth element patterns with only excess middle REEs. Some of the iron oxyhydroxides deposited in the carbonates of the tidal flats and nearshore would have released their REEs, which were enriched in MREEs, during early diagenetic reduction by degrading organic matter. These REEs combined with those provided by seawater were then incorporated into the carbonate cements. The combination of abundant glacially created debris covering the craton and acidic drainage created the distinct REE characteristics in these carbonates.