Elemental release rates from dissolving basalt and granite with and without organic ligands

Abstract

Bacteria, fungi, lichen and plants all produce organic acids, which can strongly affect weathering by increasing the solubility and mobility of elements. Leaching by organic acids may therefore produce trace element signatures which could record the presence of life in the rock record from early Earth. To elucidate this effect, long term column experiments were performed with powdered granite and basalt with and without 0.01 M citrate at pH=6 for 45 weeks. Both granite (8.44 X 10-13,3.39 x 10-13) and basalt (2.94 X 10-14,6.47 x 10-14) dissolution rates mol (Ca, Mg) (m-2 s-1 respectively) were enhanced in the presence of citrate relative to the organic-free controls: granite (3.17 X 10 -14, 4.4 X 10-15) and basalt (1.01 X 10-14, 1.04 X 10-14). Enhanced release of individual elements in the presence of citrate was strongly correlated with the stability constant of the citrate-element complex. Elements which might be useful as biosignatures are those elements that showed a strong enrichment in the presence of citrate: Zr, Sc and Mn (basalt), V and Zn (granite), and Y, La, Ce, Th, Ti, Al, P, Pb, Ni and Fe (both basalt and granite). Release of these elements from the rock material in the columns is consistent with dissolution of apatite + Fe sulfides + Fe oxides + augite in basalt and apatite + sphene + hornblende in granite. Similar groups of elements have been reported to be enriched in organic-rich rivers, suggesting leaching of strongly-complexed elements could be useful as biosignatures and may have left mineralogical traces on early Earth.