25th International Meeting on Organic Geochemistry IMOG 2011

P-182
The influence of hydrogen on organic matter cycling at the lost
city hydrothermal field
Susan Lang 1 , Gretchen Früh-Green 1 , Stefano Bernasconi 1 , Marvin Lilley 2
1 ETH Zürich, Zürich, Switzerland, 2 University of Washington, Seattle, United States of America
(corresponding author:susanqlang@gmail.com)
Fluids from the Lost City Hydrothermal Field are rich
in hydrogen and methane, with high pHs (9 – 11), as
a result of serpentinization reactions at moderate
temperatures of approximately 120-200°C [1].
Carbonate chimneys in the middle of the field vent
fluids that are warmer (up to 91�C) and have higher
hydrogen concentrations (up to 14 mmol/kg) than
those on the periphery of the field (as low as 47�C,
1.3 mmol/kg) [2]. The decreases of hydrogen
concentrations have been attributed to sulfate
reduction. To gain insight into organic matter cycling
under these different chemical conditions, the organic
matter incorporated into high-hydrogen chimneys was
compared to that in low-hydrogen and extinct
chimneys that were no longer venting fluids. Chimney
samples were decarbonated and the residual was
analyzed for total organic carbon, total organic
nitrogen, stable isotopic composition ( 13 C, 15 N), and
radiocarbon content (Δ 14 C).
Residuals from actively venting chimneys had C/N
ratios similar to that of fresh microbial biomass, and
low � 15 N values that could be attributed to either
nitrogen fixation or assimilation of seawater nitrate.
No significant variations between chimneys hosting
low- or high- hydrogen fluids were found in C/N or
� 15 N values. Residuals from extinct structures had, on
average, higher C/N ratios and higher � 15 N values
than those of actively venting structures. While it may
not be the sole processes, these trends are consistent
with diagenesis.
In general, chimneys hosting high hydrogen fluids
were extremely low in total biomass. That biomass
was significantly more enriched in 13 C than typical
marine organic matter, as has been observed in other
studies [1,3]. Comparatively, chimneys hosting low
hydrogen fluids had higher biomass and were more
depleted in 13 C. Initial results also indicate differences
in the radiocarbon content of the biomass between
high- and low- hydrogen chimneys, suggesting there
is variability in the amount of mantle carbon
incorporated into microbial cellular components.
Further radiocarbon analyses are underway and will
help to constrain the processes that result in these
distinct signatures.
References:
[1] Kelley et al., (2005) Science 307:1428-1434.
[2] Proskurowski et al., (2006) Chemical Geology
229(4) 331-343.
[3] Bradley et al., (2009) Geochim. Cosmochim. Acta
73(1): 102-118
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