Abstract
Hydrothermal fluids circulate through the upper portion of the oceanic crust. Isotopic analyses suggest that chemosynthetic microbial communities in the crust synthesize dissolved organic carbon in hydrothermal ridge-flank fluids.
Highlights
Hydrothermal fluids circulate through extensive areas of the upper oceanic crust
We report 14C and δ13C measurements of dissolved organic carbon in ridge-flank and on-axis hydrothermal fluids sampled from the Juan de Fuca Ridge
We suggest that ridge-flank circulation may support an indigenous biosphere extensive enough to export substantial fixed carbon, with distinct isotopic and probably compositional character, to the overlying ocean
Summary
Most hydrothermal circulation occurs on ridge flanks[1,2], where low-temperature fluids flow through porous basalts These fluids contain variable levels of dissolved organic carbon, but the source and composition of this carbon are uncertain. DOC probably represents most organic matter carried by crustal fluids and subsequently vented to the overlying ocean, with its chemical and isotopic composition providing an integrated view of sources and processes remote from local venting sites. We measured coupled stable carbon (δ13C) and radiocarbon ( 14C) isotopic compositions of ultrafiltered DOC (UDOC) isolated from both ridge-flank and selected on-axis low-temperature fluid sources on the JDFR system (Fig. 1; Supplementary Methods). The offset between on-axis and ridge-flank values is consistent with total DOC concentrations[6], and with the large background seawater admixture expected at on-axis lower-temperature sites (Supplementary Methods). Background seawater UDOC properties, including overall UDOC recovery (6.8 μM), and isotopic values (δ13C, −21.1 ; 14C, −443 ), very closely matched values expected for total DOC from
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