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Gas and Oil Seepage and Hydrothermal Venting in the Ocean Bottom
— Detection by Fluorescence


Jean K. Whelan, Nathan Mah, and Greg Eischeid, Departement of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole MA 02543

Robert Chen and Xuchen Wang, Department of Environmental Sciences, University of Massachusetts, Boston, MA 02125

Harry Roberts and Paul Aharon, Center for Coastal Studies and Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803

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Methods Results Discussion Conclusions Figures and Tables References

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Introduction

An abundance of evidence suggests continuous or episodic upward movement of fluids from deeper sediments into surface sediments and ocean bottom waters (Whelan, 1997). These seepages may have been volumetrically underestimated in the past, both in oil and gas productive areas and in hydrothermal vent areas (Table 1) because they often occur through small seemingly unimportant localized cracks in the seafloor. However, even if these venting features are small, the volume of expelled fluid can be important: even a small fracture can deliver orders of magnitude more fluid than ordinary compaction and diffusion processes (e.g. Hunt, 1996) Here we present initial results on successful deployment of CTD fluorescence for continuous detection of bottom venting fluids both in a known oil and gas seep area (Green Canyon in the offshore Louisiana Gulf of Mexico, Figures 1a and 1b) and in a hydrothermal vent area (Guaymas Basin, Gulf of California, Figures 1c and 1d ). Our initial results confirm the very localized and possibly episodic nature of these venting features which would not have been detected without a continuous localized detection technique (Figure 6).


Methods


Results

Louisiana Gulf Coast upper continental shelf Green Canyon area
The offshore Louisiana Gulf Coast area studied from the manned submersible, the Johnson Sealink in August of 1997 is shown in Figures 1a and 1b. Widespread natural oil and gas seepage has been documented over the entire study area as reviewed in Roberts and Carney (1997). The oil and gas venting commonly occur through observable faults and fractures in the seafloor.


Here we show data from three dives:

Dive 2893 was in 1400 feet of water located in Green Canyon Block 184 (Figure 1b) where oil seepage was visible from the submersible as small oil bubbles discharging from bottom sediments. Because of calm sea conditions, upward moving oil drops were also visible from the ship, appearing episodically as small slicks dispersing on the sea surface over the entire area. Large communities of tubeworms and other organisms were pervasive over the bottom at this site.

CTD fluorescence (Figure 2A) shows a small but distinct signal near the bottom which we attribute to oil seepage accompanied by a slight increase in DOC (Figure 3). GCMS of a methylene chloride extract (Figure 4) shows a homologous series of n-alkanes typical of oil.

Dive 2894 Bush Hill (Figure 2B) had free gas bubbles, some 3 feet or more in diameter, disrupted bottom sediments around the submersible and stirred large chunks of bottom sediments and thioplocca mat throughout the water column. Previous work showed this gas to be biogenic methane delivered to the seafloor through a fault from Pleistocene gas reservoir. The near-bottom CTD fluorescence signal for this site was very strong throughout the gassy region. This signal disappears within about 50m of the bottom, probably due to bubble dissolution during upward ascent. The fluorescence signal is strongest during the last half of the dive, where the largest bubbles were also visible. Bubbles were not visually evident in the early part of the dive, although visibility was poor because of the particles in the water column. No water was collected so we don't know whether the fluorescence signal is entirely due to light reflection/refraction of the bubbles or whether there is also some contribution from bottom pore-water humic substances, as well.

Dive 2896 was in an area of no visual gas or oil bubbling and minimal bottom biology. No CTD fluorescence was evident in bottom waters (Figure 2C).

Dive 2900 was in Green Canyon Block 232 in 1867 feet of water. The initial landing site was on a gas hydrate mound covered with tubeworms. Numerous patches of white and a few of red Beggiatoa were present. Gas bubbles and expelled yellowish chunks of hydrate were visible. The small gas bubbles may be responsible for the somewhat noisy baseline observed in the CTD fluorescence profile near the bottom (not shown) and for the increases in DOC within 200 feet of the bottom (Figure 3). Trace amount of n-alkanes are present at this site (Figure 4).

Dive 2901 was at Green Canyon Block 152 in 1500 feet of water was not visually very interesting. It does show a slightly noisy CTD baseline near the seafloor, similar to that observed for Site 2893, suggesting oil seepage. DOC increases near the bottom (Figure 3). However, GCMS shows little or no n-alkane signal at this site.

Guaymas Basin
Guaymas Basin is a sediment covered hydrothermal ridge in the Gulf of California (Figure 1c). Magma drives waters contained in sediments upward into the overlying water column. The hydrothermal heating also produces in situ oil and gas generation in the organic-rich sediments. Waters from the vents are somewhat enriched in DOC (62 to 69 µM in comparison to normal open ocean mid-water sea water values of about 45 µM).

The venting fluids also show ubiquitous fluorescence, as shown by the relative strength of fluorescence on continuous tracks for Jason on the first day of dives when the whole southern rift area (Figure 5) was surveyed. Comparison of the fluorescence maxima with the cruise event log clearly shows the association between CTD fluorescence and the hydrothermal plume. This result was unexpected, but turned out to be quite useful in locating the plume when it was not visually present.

Generally, the DOC (Seatech) and aromatic hydrocarbon (Chelsea) fluorometers showed maxima at the same location, with the Seatech DOC signal being generally stronger (Figures 5 and 6). However, in a few locations, the Chelsea gave a stronger signal possibly indicative of the presence of hydrothermal petroleum.

Maxima in fluorescence, reflecting maxima in venting fluids, are very localized, often occurring on less than a one meter scale (Figure 6). These features could have been easily missed without the continuous CTD fluorescence profiles.


Discussion

Green Canyon
In Green Canyon, all CTD fluorescence was produced by gas and oil seepage. In Dive 2893 to an oil seep site, the small fluorescence anomolies are associated with seeping oil. The presence of oil dispersed in the water column is also shown by a gas chromatograph of an organic extract of bottom water from the same site which shows the homologous n-alkanes typical of unbiodegraded oil. These peaks are much weaker for water collected at Dive Site 2901 (Figure 4).

The very high CTD fluorescent spikes observed for Green Canyon Dive site 2894 are most likely caused largely by light refraction from the large gas bubbles which were visually observed to be present. This hypothesis is supported by the decrease and then disappearance of CTD fluorescence during the ascent through the water column (Figure 2B). Methane has a solubility in water of about 3%, so that methane bubbles should disappear relatively quickly as they ascend through the water column. Part of the Dive 2894 fluorescence could be derived from bottom sediments and the DOM associated with pore waters being flung upward into the water column by the ejecting gas. No gas bubbles were seen during the first half of the dive, although the waters were so cloudy that small bubbles would not have been distinguishable from particles. Previously, particles alone have not been found to be a source of fluorescence; however, we have not previously encountered the abundance of particles observed here. Unfortunately, no water samples were collected from this site so that we were unable to carry out chemical analyses to distinguish these possibilities.

Guaymas
CTD fluorescence from the unmanned submersible Jason is clearly tracking the hydrothermal plume (Figures 5 and 6). The substance responsible for the fluorescence could be DOM or petroleum pushed out of the sediment pores and up into the water column by the venting fluid. Alternatively, fluorescing biota associated with the chemoautotrophic ecosystem in the plumes could also be responsible.

The most important information provided by the Guaymas CTD fluorescence data is that plume fluorescence is often intense, but is also very localized, even on distance scales of less than a meter (Figure 6). Any discrete sampling program, even on a very fine grid, would have missed the fluids corresponding with the fluorescent maxima. Also, even through Jason tracks are crossing each other on a scale of a meter or less, the CTD signal is sometimes stronger on one crossing than another. This could be due to depth variations of the cruise track. Alternatively, fluorescence and the plume could also be changing temporally, even within the short time span of a dive.


Conclusions

In the offshore Gulf Coast oil and gas seep area: 1) CTD fluorescence was ubiquitous at all oil and gas seep sites;
2) No fluorescence was observed in areas of no seepage;
3) Free gas bubbles produce a very strong CTD fluorescence signal.
In the Guaymas Basin hydrothermal area: 4) Strong CTD fluorescent signals were found to be diagnostic of the plume.
5) Different types of fluorescent organic matter were detected using two fluorometers operating at different wavelengths. In most cases, the signal from the longer wavelength Seatech DOM fluorometer was stronger. In a few cases, the signal from the shorter wavelength Chelsea "aromatic" fluorometer was stronger, suggesting a stronger influence from hydrothermally produced petroleum.
6) Large fluorescence variations occur on very short distance scales (less than 1m) and also possibly on short time scales (less than an hour). Fluorescence maxima appear to correspond to plume maxima and probably would not have been detected by a descrete sampling program.

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Figures and Tables

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Figure 1: Locations of initial bottom CTD fluorescence measurements:
a) overview of location of Gulf Coast Green Canyon area;
b) close-up: location of Green Canyon manned submersible dive sites;
c) overview of location of Gulf of California Guaymas Basin hydrothermal venting area;
d) close-up: Guaymas Basin, location of Jason (unmanned submersible) cruise track
Figure 2: Green Canyon: CTD fluorescence data, Seatech DOC fluorometer. Fluorescence is plotted versus time on the left and versus depth on the right. A) Oil seep site - Dive 2893; B) Gas seep site - Dive 2894; C) Site of no gas or oil leakage, Dive Site 2896
Figure 3: Green Canyon: Dissolved organic carbon
Figure 4: Green Canyon water, GCMS total ion chromatogram (TIC) of methylene chloride extract in comparison to tetracosane recovery standard.
Figure 5: Guaymas Basin: CTD fluorescence data, Seatech (DOC) and Chelsea (aromatic hydrocarbon) fluorometers - overall.
Figure 6: Guaymas Basin comparative fluorometer measurements (dive of 4/20/98)
Table 1: Summary of global oil seepage data suggesting seepage to bottom sediments may have been underestimated in past. Note that total estimated oil seepage volumes from the two single events shown are approximately equal to the entire estimated world seepage for a single year. Also note the large petroleum sources potentially available to produce additional undetected seepage.


References
Hunt, J.M. 1996. Petroleum Geochemistry and Geology. Freeman, San Francisco, 2nd Ed., Chapter 9
Peltzer, E.T. and P.G. Brewer (1993) Some practical aspects of measuring DOC -- sampling artifacts and analytical problems with marine samples. Marine Chemistry, v.41, pp 243-252.
Roberts, H.H. and Carney, R. (1997) Evidence of episodic fluid, gas and sedimnet venting on the northern Gulf of Mexico continental slope. Economic Geology, v. 92, pp 863-879.
Whelan, J.K. (1997) The dynamic migration hypothesis. How fast are oil and gas leaking to the ocean floor and replenishing themselves in some reservoirs? Sea Technology, v.38, No. 9, pp 10-18.

Acknowledgements

Captain and crew of Edwin Link and Johnson Sea Link; Dana Yoerger and the Deep Submergence Group for "piggybacking" us on their Guaymas Jason dive; Al Bradley and Ellen Druffel for helpful discussions which got us to thinking about this work. Financial support from the Vetlesen Foundation through a grant to the Woods Hole Oceanographic Foundation and from the Department of Energy (grant no. DE-FG02-86ER13466 to Jean Whelan) is gratefully acknowledged.


Appendix:   Methods

CTD fluorescence
Initial measurements were carried out using a DOM fluorometer (Sea Tech) mounted about eight feet from the base of the manned submersible Johnson Sealink. Continuous measurements were made during 12 dives carried out over a six day period in August, 1997 (Figure 2). Fluorometer readings in Figure 2 are relative and were not calibrated to an external standard in this preliminary work. In the Sea Tech fluorometer, the emission and absorbance windows are mounted at 90 to each other and fluorescence is measured on in situ seawater present within a light path of about 1 cm2 outside of the pressure casing. Maximum excitation and emission wavelengths were 300-360nm and 410-480 nm, respectively.

For later measurements in the Guaymas Basin hydrothermal vent field from the unmanned submersible, Jason, a second lower wavelength fluorometer (Chelsea Instruments) was added, maximum excitation and emission wavelengths 226-252 nm and 347-373 nm, respectively.

The signal outputs of the Sea Tech and Chelsea fluorometer intensities were linear and logarithmic signals, respectively.

DOC measurements
The water for DOC measurements was collected in 100ml precombusted bottles following precautions described by Peltzer (JGOFS) and summarized in Peltzer and Brewer (1993). Prior to sample collection, the sample lines and collection devices were completely flushed with indigenous seawater to eliminate any "memory" of previous samples. As soon as possible after each collection, water samples with no visible particles were treated with phosphoric acid and either frozen or stored at 4C . Samples suspected of containing particles were filtered on precombusted glass fiber filters. Dissolved organic carbon measurements were carried out using the stripping-combustion method of Peltzer and Brewer (1993). Whenever possible, a larger 5-liter sample was also collected near the bottom and treated and stored in the same way as the smaller samples.
Water extraction; analysis of methylene chloride extract.

Water samples (500 ml) were spiked with a deuterated tetracosane recovery standard, extracted with methylene chloride, and the extract dried with anhydrous sodium sulfate. After concentration by rotary evaporation, the residual extract was analyzed on a Hewlett Packard 6890 Gas Chromatograph equipped with a model no. 5973 mass selective detector, a 50m DB1 capillary column, an auto sampler and a HP Chem Station data handling system.


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