G02_07

Seawater dissolved organic carbon is rapidly removed in ultramafic hydrothermal systems and replaced by ¹⁴C-free labile organics

Lang S^1,2,  Benitez-Nelson B², Vincent M², Mau A², Simpson A³, Kock F³, Lysak D³, Soong R³

¹Woods Hole Oceanographic Institution, Woods Hole, United States, ²University of South Carolina, Columbia, United States, ³University of Toronto, Tronto, Canada

Large volumes of water have passed through the hot, rocky subseafloor throughout Earth’s history. This circulation of fluids through oceanic rocks is sufficiently large to impact the cycling of marine dissolved organic carbon (DOC) and to sequester oceanic carbon in the subseafloor over geologic timescales. While the fate of DOC in numerous mafic systems has been examined, there have been no previous reports on the less studied but still abundant ultramafic systems. We analyzed the concentration and composition of DOC from two systems hosted on ultramafic rocks, the Lost City hydrothermal field (30°N, Mid-Atlantic Ridge) and the Von Damm hydrothermal system (Mid-Cayman Rise). We show that per liter of seawater, more DOC is removed and at an >700 time faster rate than in mafic ridge flank systems. Simultaneously, labile ¹⁴C-free organics are exported from the system in concentrations up to 20 times higher than deep seawater. Early in Earth’s history, similar sequestration could have served to concentrate organic molecules for further prebiotic geochemical reactions, in the lead up to early life.