Hydrothermal fluids circulate through extensive areas of the upper oceanic crust. Most hydrothermal circulation occurs on ridge flanks1, 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. Here, we report Δ14C and δ13C measurements of dissolved organic carbon in ridge-flank and on-axis hydrothermal fluids sampled from the Juan de Fuca Ridge. Dissolved organic carbon from two independent ridge-flank sites was characterized by low δ13C and Δ14C values. The δ13C values ranged from −26 to −35‰, and were consistent with a chemoautotrophic origin. The 14C ages of the dissolved organic carbon ranged from 11,800 to 14,400 years before present, revealing that the carbon was around three times older than dissolved organics in the deep ocean. The Δ14C values of the ridge-flank dissolved organic matter also corresponded closely to those of dissolved inorganic carbon in the same fluid samples. Taken together, the data suggest that chemosynthetic crustal microbial communities synthesize dissolved organic carbon from inorganic carbon in ridge-flank fluids. 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.

Hydrothermal fluids circulate through extensive areas of the upper oceanic crust. Most hydrothermal circulation occurs on ridge flanks 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. Here, we report Δ14 C and δ13 C measurements of dissolved organic carbon in ridge-flank and on-axis hydrothermal fluids sampled from the Juan de Fuca Ridge. Dissolved organic carbon from two independent ridge-flank sites was characterized by low δ13 C and Δ14 C values. The δ13 C values ranged from -26 to -35‰, and were consistent with a chemoautotrophic origin. The 14 C ages of the dissolved organic carbon ranged from 11,800 to 14,400 years before present, revealing that the carbon was around three times older than dissolved organics in the deep ocean. The Δ14 C values of the ridge-flank dissolved organic matter also corresponded closely to those of dissolved inorganic carbon in the same fluid samples. Taken together, the data suggest that chemosynthetic crustal microbial communities synthesize dissolved organic carbon from inorganic carbon in ridge-flank fluids. 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. © 2011 Macmillan Publishers Limited. All rights reserved.

Marine dissolved organic carbon (DOC) is a major global carbon reservoir, yet its cycling remains poorly understood. Previous work suggests that DOC molecular size and chemical composition can significantly affect its bioavailability. Thus, DOC size and composition may control DOC cycling and radiocarbon age (via Δ14C). Here we show that DOC molecular size is correlated to DOC Δ14C in the Pacific Ocean. Our results, based on a series of increasing molecular size fractions from three depths in the Pacific, show increasing DOC Δ14C with increasing molecular size. We use a size-age distribution model to predict the DOC and Δ14C of ultrafiltered DOC. The model predicts both large and small surface DOC with high Δ14C and a narrow range (200–500 Da) of low Δ14C DOC. Deep model offsets suggest different size distributions and/or Δ14C sources at 670–915 m. Our results suggest that molecular size and composition are linked to DOC reactivity and storage in the ocean.

. Radiocarbon (∆14C) measurements of monthly samples from a Galapagos surface coral are among the first data sets from the new Keck Carbon Cycle Accelerator Mass Spectrometry laboratory at the University of California, Irvine. An average ∆14C value of –62‰ is obtained for 144 measurements of samples from monthly coral bands that lived from about AD 1760–1771 (±6 yr). High ∆14C values were found during January through March, when upwelling was weak or absent at the Galapagos Islands. Low ∆14C values were obtained mid-year during strong upwelling. The average seasonal variability of ∆14C was 15–25‰, which is greater than that at other tropical and subtropical locations in the Pacific Ocean because of intense seasonal upwelling at this site. Periods of sustained high ∆14C values were found during 1762–1763 and 1766. A spectral analysis revealed that the spectral density for the ∆14C data displays most of its variance at the 5-yr cycle, which is reflective of El Niño periodicity during the 20th century.