25th International Meeting on Organic Geochemistry IMOG 2011

P-439
Origin of alkane hydrocarbons in the hydrothermal deposits of
the Mid-Atlantic Ridge and East Pacific Rise
Natalia Shulga 1 , Valerij Peresypkin 2
1 P.P. Shirshov Institute of Oceanology, Moscow, Russian Federation, 2 P.P. Shirshov Institute of Oceanology,
Moscow, Russian Federation (corresponding author:nash.ocean@gmail.com)
The purpose of this abstract is to present results of
initial investigation related to n-alkanes distribution in
various regions of the oceanic rift systems. Alkane
hydrocarbons are type of biomarkers show little or no
change in structure from their parent organic
molecules in living organisms, bacteria or plants.
They are chemically stable and, hence, retain primary
biological and geological information.
The samples for research were collected from
numerous expeditions by using deep manned
submersibles Mir-1 and Mir-2.
Molecular and group composition of n-alkanes were
analyzed in sulphide and carbonate deposits of
hydrothermal fields from the Mid-Atlantic Ridge and
East Pacific Rise, related to the process of
serpentinization of ultrabasic rocks in the oceanic
core (Rainbow, Lost City); basaltic volcanism (Broken
Spur, Lucky Strike, TAG, Snake Pit, 9°50‘N) and oilsaturated
ores from Guaymas Basin. All solvent
extracts from samples have been studied by gas
chromatography/ mass-spectrometry method.
Total n-alkanes concentrations in the samples are low
and vary widely from 0,01 to 1,82 µg/g of dry weight
of ore deposits, but this values incommensurable low
in comparison with concentrations of hydrothermal
sediments. These differences can be explained by
organic matter formation features of both in
hydrothermal deposits and sediments (pressure,
temperature, different mineralogical composition etc.).
It was shown a presence of n-alkanes C12-C35 in most
explored samples of hydrothermal deposits. The
major amount of n-alkanes from hydrothermal
deposits has a biologic origin (transformation of
chemoautotrophic organisms), except samples from
Guaymas Basin, where instant thermolytic generation
of hydrocarbons (so-called ‘‘hydrothermal petroleum‘‘)
has been observed [1].
The n-alkane distributions in the hydrothermal
formations from Rainbow, 9°50‘N East Pacific Rise
and Guaymas Basin has comparable concentrations
of even/odd homologies (CPI=0,90÷1) while the
values of C23-C35 range from 11,7 to 29,6 (% relat.).
This distribution can be interpreted as abiogenic
thermocatalitic oil, since it difficult to supply highmolecule
weight odd n-alkanes derived from
terrestrial plants to these sediment-free hydrothermal
systems of the ocean. Along with this, one may
assume that in the hydrothermal systems with no
sedimentary layer, oil hydrocarbons are also formed
owing to the Fisher-Tropsch type reactions [2].
Fig.1. Relative distribution of n-alkanes in the
samples from the A) Rainbow (4396-1), B) 9°50‘N
East Pacific Rise (4658-3), C) Guaymas Basin (4713-
3).
References
[1] Simoneit, B.R.T. Appl. Geochem., No. 6, 49-56
(1985).
[2] McCollom, T.M., Seewald, J.S. Chem. Rev.
107(2), 382-401 (2007).
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