30.6 A Tale of Hydrocarbon-Rich Soda Lakes
I was first drawn to the study meromictic soda lakes because their anoxic water
columns and high productivity which resulted in the accumulation of organic matter
in their sediments, the first step in the process of petroleum formation. Fortunately,
there were two lakes located relatively nearby, in the Great Basin Desert of the
western USA: Big Soda Lake, Nevada, and Mono Lake, California. These seemed
good field locations to pursue the biogenic side of light hydrocarbon production.
Both also had sources of geothermal heat flowing into their basins. Big Soda Lake
occupied an entire volcanic crater that erupted in the Pleistocene, while the much
larger Mono Lake is surrounded (and penetrated) by volcanoes, volcanic features,
and heat flow (e.g., hot springs).
The lakes did not disappoint, as each sported not only dissolved methane but also
appreciable levels of ethane, propane, and butanes in their highly sulfidic anoxic
bottom waters and sediments. Big Soda Lake sediments yielded strong signals for
biogenic sources, for at a sediment depth of 1.75 m, methane was detected at
345 μmoles/kg. The methane had a “light” (e.g.,
12 C-enriched relative to
13 C) stable
carbon isotopic composition value [“б
13 C-CH 4 ”] of À74 per mil, along with a clear
presence of ethane (0.18 μmoles/kg), propane (0.066 μmoles/kg), and butanes
(collectively 0.02 μmoles/kg). The CH 4 /(C 2 H 6 + C 3 H 8 ) ratio at this depth was
1390, another strong biogenic indicator. All four alkanes were present in the anoxic
monimolimnion and CH 4 /(C 2 H 6 + C 3 H 8 ) ranged between 161–333, depending on
depth (Oremland and Des Marais 1983) likely reflecting biological activity which
included methanogenesis and methane-oxidation in both the sediments and water
column (Oremland et al. 1982a; Iversen et al. 1987). Similar results were obtained in
Mono Lake, with extracted pelagic sediment CH 4 concentrations reaching saturation
(~3 mM) by 0.6 m depth, and sediment CH 4 /(C 2 H 6 + C 3 H 8 ) ratios running between
600 and 750 over the length of the 1.1 m core (Oremland et al., 1987). Sediment and
anoxic water column stable carbon isotopic values (“б
13 C-CH 4 ”) mostly ranged
between À70 and –85 per mil, again strongly enriched in
12 C relative to
13 C and
clearly indicative of biological sources.
Yet because these lakes were located in regions of active heat flow, a thermogenic
component to the source of C 2 + gases was also a possibility. It was time to
investigate what possible metabolic precursors of alkylated-S-CoM compounds
could serve to promote C 2 + hydrocarbon production in sediments from these and
other locations.
30 BUBBLES in the MUD: A Reminiscence and Perspective
645
I was first drawn to the study meromictic soda lakes because their anoxic water
columns and high productivity which resulted in the accumulation of organic matter
in their sediments, the first step in the process of petroleum formation. Fortunately,
there were two lakes located relatively nearby, in the Great Basin Desert of the
western USA: Big Soda Lake, Nevada, and Mono Lake, California. These seemed
good field locations to pursue the biogenic side of light hydrocarbon production.
Both also had sources of geothermal heat flowing into their basins. Big Soda Lake
occupied an entire volcanic crater that erupted in the Pleistocene, while the much
larger Mono Lake is surrounded (and penetrated) by volcanoes, volcanic features,
and heat flow (e.g., hot springs).
The lakes did not disappoint, as each sported not only dissolved methane but also
appreciable levels of ethane, propane, and butanes in their highly sulfidic anoxic
bottom waters and sediments. Big Soda Lake sediments yielded strong signals for
biogenic sources, for at a sediment depth of 1.75 m, methane was detected at
345 μmoles/kg. The methane had a “light” (e.g.,
12 C-enriched relative to
13 C) stable
carbon isotopic composition value [“б
13 C-CH 4 ”] of À74 per mil, along with a clear
presence of ethane (0.18 μmoles/kg), propane (0.066 μmoles/kg), and butanes
(collectively 0.02 μmoles/kg). The CH 4 /(C 2 H 6 + C 3 H 8 ) ratio at this depth was
1390, another strong biogenic indicator. All four alkanes were present in the anoxic
monimolimnion and CH 4 /(C 2 H 6 + C 3 H 8 ) ranged between 161–333, depending on
depth (Oremland and Des Marais 1983) likely reflecting biological activity which
included methanogenesis and methane-oxidation in both the sediments and water
column (Oremland et al. 1982a; Iversen et al. 1987). Similar results were obtained in
Mono Lake, with extracted pelagic sediment CH 4 concentrations reaching saturation
(~3 mM) by 0.6 m depth, and sediment CH 4 /(C 2 H 6 + C 3 H 8 ) ratios running between
600 and 750 over the length of the 1.1 m core (Oremland et al., 1987). Sediment and
anoxic water column stable carbon isotopic values (“б
13 C-CH 4 ”) mostly ranged
between À70 and –85 per mil, again strongly enriched in
12 C relative to
13 C and
clearly indicative of biological sources.
Yet because these lakes were located in regions of active heat flow, a thermogenic
component to the source of C 2 + gases was also a possibility. It was time to
investigate what possible metabolic precursors of alkylated-S-CoM compounds
could serve to promote C 2 + hydrocarbon production in sediments from these and
other locations.
30 BUBBLES in the MUD: A Reminiscence and Perspective
645