Over the next year Larry and I, with the collaboration of Mike Whiticar, were able
to establish that there were four types of methane present in Mono Lake (Oremland
et al. 1987). First, there was currently formed biological methane present in its
bottom sediments that diffused out to the water column and was oxidized at the
pycnocline/oxycline by methanotrophs (Joye et al. 1999), resulting in the residual,
un-oxidized methane in the surface water being enriched in
13 C. There was still a
sufficient abundance of dissolved methane in the surface water to achieve a measurable outward flux to the atmosphere (Miller and Oremland 1988). The vents and
seeps, however, came in two categories: ancient biogenic methane and thermogenic
methane. The former were enriched in
12 C and had little co-associated ethane and
propane while the latter were enriched in
13 C and had appreciable ethane and
propane. The thermogenic gases were only found in proximity to the hot spring,
while the ancient biogenic seeps occurred all around the lake. The clincher was
determining that all of the seeps and vents were radiocarbon (
14 C) dead, indicating
they were older than ~20,000 years, while the methane dissolved in the anoxic
monimolimnion was radiocarbon active, actually somewhat “future dated” stemming from
14 C-fallout from open air atomic testing conducted decades earlier. We
concluded that Mono Lake was underlain by a natural gas deposit formed much
earlier in its existence, a fact that was later confirmed by our colleague, Dr. Angela
Jayko. Angela conducted extensive bottom sound surveys of the lake and determined about half of its underlying deep benthos were gas-charged (Jayko et al.
2013). In addition, the major gas vents we found on the lake’s surface appeared to
originate from cracks and fault lines detected in the geologic structure beneath the
bottom sediments.
The collection of sufficient vent gases to conduct their radiocarbon analysis did
not present a problem as there was a surfeit of supply and it was just a matter of
waiting for collection bottles to be filled (~700 mL) with enough methane. We did
this in the days before radio mass-spectrometry was widely available, so we had to
collect a considerable amount of sample in order to achieve enough material be
“counted” using older, slower methodology. The dissolved methane in the bottom
water presented more of a challenge, but Larry came up with an ingenious solution.
He continuously pumped water up from beneath the pycnocline into a floating
cylindrical plastic chamber, where it was equilibrated with its helium headspace.
The helium itself was pump recirculated from a reservoir contained in a floating air
mattress and served to bubble strip the methane out of the bottom water. We kept the
mattress tethered to the Revenge, but kept a close eye on it lest the wind would pick
up and carry it aloft into the distant reaches of the Great Basin Desert. Indeed, we
also had to keep it filled to slightly below full capacity, as our return trip home would
carry us up and over Tioga Pass, where the volume would increase due to the drop in
pressure from driving from Lee Vining (altitude ¼ 6781 feet) up to Tioga pass
(9943 feet) and bursting open the air mattress. You got to think of this kind of stuff
when you’re running the show, and it’s not a topic covered in the literature.
You also get to worry a lot, as in the time Larry and Mike were operating out on
the lake when a powerful thunderstorm rolled in suddenly, as can occur when
summer monsoonal moisture comes up from the Gulf of California. For about 2 h,
31 Salty, Alkali-Laced Tales (Mostly True) from the Great Basin Desert, California. . .
671
to establish that there were four types of methane present in Mono Lake (Oremland
et al. 1987). First, there was currently formed biological methane present in its
bottom sediments that diffused out to the water column and was oxidized at the
pycnocline/oxycline by methanotrophs (Joye et al. 1999), resulting in the residual,
un-oxidized methane in the surface water being enriched in
13 C. There was still a
sufficient abundance of dissolved methane in the surface water to achieve a measurable outward flux to the atmosphere (Miller and Oremland 1988). The vents and
seeps, however, came in two categories: ancient biogenic methane and thermogenic
methane. The former were enriched in
12 C and had little co-associated ethane and
propane while the latter were enriched in
13 C and had appreciable ethane and
propane. The thermogenic gases were only found in proximity to the hot spring,
while the ancient biogenic seeps occurred all around the lake. The clincher was
determining that all of the seeps and vents were radiocarbon (
14 C) dead, indicating
they were older than ~20,000 years, while the methane dissolved in the anoxic
monimolimnion was radiocarbon active, actually somewhat “future dated” stemming from
14 C-fallout from open air atomic testing conducted decades earlier. We
concluded that Mono Lake was underlain by a natural gas deposit formed much
earlier in its existence, a fact that was later confirmed by our colleague, Dr. Angela
Jayko. Angela conducted extensive bottom sound surveys of the lake and determined about half of its underlying deep benthos were gas-charged (Jayko et al.
2013). In addition, the major gas vents we found on the lake’s surface appeared to
originate from cracks and fault lines detected in the geologic structure beneath the
bottom sediments.
The collection of sufficient vent gases to conduct their radiocarbon analysis did
not present a problem as there was a surfeit of supply and it was just a matter of
waiting for collection bottles to be filled (~700 mL) with enough methane. We did
this in the days before radio mass-spectrometry was widely available, so we had to
collect a considerable amount of sample in order to achieve enough material be
“counted” using older, slower methodology. The dissolved methane in the bottom
water presented more of a challenge, but Larry came up with an ingenious solution.
He continuously pumped water up from beneath the pycnocline into a floating
cylindrical plastic chamber, where it was equilibrated with its helium headspace.
The helium itself was pump recirculated from a reservoir contained in a floating air
mattress and served to bubble strip the methane out of the bottom water. We kept the
mattress tethered to the Revenge, but kept a close eye on it lest the wind would pick
up and carry it aloft into the distant reaches of the Great Basin Desert. Indeed, we
also had to keep it filled to slightly below full capacity, as our return trip home would
carry us up and over Tioga Pass, where the volume would increase due to the drop in
pressure from driving from Lee Vining (altitude ¼ 6781 feet) up to Tioga pass
(9943 feet) and bursting open the air mattress. You got to think of this kind of stuff
when you’re running the show, and it’s not a topic covered in the literature.
You also get to worry a lot, as in the time Larry and Mike were operating out on
the lake when a powerful thunderstorm rolled in suddenly, as can occur when
summer monsoonal moisture comes up from the Gulf of California. For about 2 h,
31 Salty, Alkali-Laced Tales (Mostly True) from the Great Basin Desert, California. . .
671