Effects of organic acids and dissolved oxygen on apatite and chalcopyrite dissolution: Implications for using elements as organomarkers and oxymarkers

Abstract

We have previously suggested that mobility patterns of P and Y in paleosols could serve as “organomarkers” to denote the presence of organic ligands secreted by terrestrial organisms on early Earth. In addition, our data indicate that Cu depletion may provide a viable oxymarker for determining the presence of atmospheric oxygen during soil weathering processes. In this research, we continue pursuing the potential for utilizing these elements as markers by investigating dissolution of Durango apatite (Ca 5(PO 4) 2.82(F,Cl,OH) 1.54) and Messina chalcopyrite (CuFeS 2) reacted under batch conditions in the presence and absence of two aliphatic and aromatic organic acids under oxic and anoxic conditions. In general, results show that organic acids enhance element release from apatite (Ca, P and Y) and chalcopyrite (Cu, Fe, and Y), and increasing organic acid concentrations from 1 to 10 mM results in greater dissolution. The aliphatic organic acid citrate enhances mineral dissolution to the greatest extent and dissolution in the presence of aromatic salicylate or absence of ligand is lowest. Release of Ca, P, and Y from apatite was not impacted by dissolved O 2(g) while release of Cu from chalcopyrite was impacted. Aqueous Cu concentrations at the end of batch experiments with chalcopyrite are four orders of magnitude greater under oxic conditions, whereas Fe concentrations are substantially higher under anoxic conditions. These data support the hypothesis that release of P and Y from apatite is enhanced by organic acids and that Cu release is impacted significantly by dissolved O 2(g) and, to a lesser extent, organic acids. Thus, it seems plausible that geochemical and mineralogical signatures of P, Y, and Cu may have utility for distinguishing the presence of terrestrial organisms and atmospheric conditions during soil weathering processes on early Earth.