Chemosynthetic origin of 14C-depleted dissolved organic matter in a ridge-flank hydrothermal system

Abstract

Circulation of hydrothermal fluids in upper oceanic crust may support one of the most extensive, but least understood, of the Earth's biogeochemical systems. However, little is known about non-living organic matter carried in crustal hydrothermal fluids or its possible impact on the carbon cycle of the overlying ocean. Elevated concentrations of particulate organic carbon with distinct composition are present near venting sites 3 , but particulate organic carbon composes only a small fraction of organic matter in most natural waters, with dissolved organic carbon (DOC) making up most of it 4 . Particulate organic carbon is typically both more labile and surface-active than DOC, so that particle compositions tend to reflect local conditions and relatively short-timescale processes 5 . 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.

Pacific carbon cycling constrained by organic matter size, age and composition relationships

“…Multiple alternate deep DOC sources have been proposed, which might alter DOC Δ 14 C values at depth, including hydrothermal DOC, chemosynthesis, and particle solubilization [Hansman et al, 2009;McCarthy et al, 2011;Smith et al, 1992]. If any of these processes were in fact significant, this would result in source heterogeneity consistent with the mismatch between our model results in surface versus deep ocean.…”

Chemical Characterization and Cycling of Dissolved Organic Matter

“…Global DOC losses in hydrothermal systems are low, <0.0002 PgC year −1 , but measurements are few and this may be an underestimate. The composition and lability of the ~20 μM DOC that exits with hydrothermal fluids has not been studied, but radiocarbon values of the HMWDOC fraction range from −772‰ to −835‰, significantly depleted relative to overlying seawater (McCarthy et al, 2010). Other processes such as microbial oxidation and removal by selfassembling organic gels may be active and need further study.…”

Data report: cultivation of microorganisms from basaltic rock and sediment cores from the North Pond on the western flank of the Mid-Atlantic Ridge, IODP Expedition 336

“…The circulation of seawater through the upper ocean crust can therefore energize potential endolithic communities in which chemolithoautoData report: cultivation of microorganisms from basaltic rock and sediment cores from the North Pond on the western flank of the Mid-Atlantic Ridge, IODP Expedition 336 1 trophs utilizing Fe(II), sulfur, and hydrogen play a role as primary producers. This type of crustal biosphere is most likely to exist on young ridge flanks (Fisher et al, 2003;Lever et al, 2013;McCarthy et al, 2011). The North Pond site located on the western flank of the Mid-Atlantic Ridge is one of ideal study sites to examine the ridge flank crustal biosphere (see the "Expedition 336 summary" chapter [Expedition 336 Scientists, 2012a]).…”