ACCESS NUMBER: 30660-19946 

STUDY TITLE: The Fate and Effects of Barium and Radium-Rich Fluid Emissions from 

Hydrocarbon Seeps on the Benthic Habitats of the Gulf of Mexico Offshore Louisiana 

REPORT TITLE: Fate and Effects of Barium and Radium-Rich Fluid Emissions from 

Hydrocarbon Seeps on the Benthic Habitats of the Gulf of Mexico Offshore Louisiana 

CONTRACT NUMBER(S): 14-35-0001-30660-19946 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA(S): Central Gulf of Mexico 

FISCAL YEAR(S) OF PROJECT FUNDING: 1997 

COMPLETION DATE OF REPORT: December 2000 

CUMMULATIVE PROJECT COST: $270,410 

PROJECT MANAGER: P. Aharon 

AFFILIATION: Louisiana State University 

ADDRESS: Department of Geology and Geophysics, Baton Rouge, LA 70803 

PRINCIPAL INVESTIGATOR(S): P. Aharon, D. Van Gent, B. Fu, L. M. Scott. 

KEY WORDS: Central Gulf; Louisiana; deepwater; hydrocarbon seeps; fluid emissions; 

barium; radium; radioactivity; radiation; barites; chimneys; crusts; chemical fingerprints; 

age dating; radioisotopes; fluxes; benthic chemosynthetic fauna; methanotrophic 

mussels. 

BACKROUND: In addition to hydrocarbons, some of the seeps occurring at upper 

bathyal depths (510 to 657 m) in Garden Banks-382 (27o 37.77' N; 92o 28.08' W), 

Garden Banks-338 (27o37.79'N; 92o28.12'W) and Mississippi Canyon-929 (28o

01.46'N; 89o 43.63'W) are also issuing copious amounts of barium and radium-rich 

fluids. Upon their exit on the seafloor, these fluids are likely to act as the primary source 

for the extensive radium-rich barite deposits of varied morphological shapes 

documented on the seafloor. The impact of the excess barium and radium on the 

benthic habitats is unknown. The deepwater habitats along the slope are relatively 

pristine. However, as the oil and gas industry is moving its exploration and production 

main activities to deepwaters, their present pristine status is likely to change. It is 

therefore of importance to explore the natural processes occurring in deepwaters ahead 

of the anticipated anthropogenic complications. 

OBJECTIVES: The primary objectives of this study were to document products and 

processes related to naturally occurring barium and radium-rich fluid emissions from 
ACCESS NUMBER: 30660-19946 

hydrocarbon seeps, and assess their impact on the offshore habitats in deepwater Gulf 

of Mexico.

DESCRIPTION: The barite-bearing seeps have been documented and sampled during 

submersible dives. The sites are characterized by intense gaseous hydrocarbon and 

fluid mud venting occurring from numerous cone-shaped mud volcanoes. Subsurface 

seismic data indicate that shallow salt diapirs are present under the sites and seepage 

occurs through normal faults associated with the diapirs. Dense communities of 

methanotrophic mussels (Bathymodiolus sp.), neritid gastropods and galatheid crabs 

inhabit the flanks of these mud volcanoes. 

Specific determinations of barium, radium and selected chemical elements and their 

isotopes in pore fluids have allowed us to fingerprint the fluid emissions.Measurements 

of barium, radium and selected chemical elements and their isotopes in solid barites 

and determinations of radium and barium in both soft tissues and hard skeletons from 

methanotrophic mussels and heterotrophic fauna provided data relevant to mobility of 

the elements in the benthic habitats. Radium and barium in growth increments from the 

barite chimneys were used as an archived history and record of episodic venting whose 

chronology was derived from the radium-decay series isotopes. External gamma ray 

and internal alpha radiation (derived by ingestion of radioactive barite) exposure and 

dose to epifauna inhabiting the barite seeps, consisting of vagrant heterotrophs (crabs 

and starfishes) and sessile methanotrophic mussels, was estimated from measured 

radium-decay series specific concentrations. 

SIGNIFICANT CONCLUSIONS: Barite (BaSO4) deposits consist of chimneys, crusts 

blanketing mud volcanoes, and unconsolidated sands infilling gulleys. The chimneys 

are dominated by the mineral celesto-barite (i.e., Sr-rich barite), whereas the crusts 

consist of both barite and calcite. The Gulf of Mexico barites are anomalously enriched 

in Sr, Ca and Ra by comparison with barites from normal marine settings elsewhere. 

Fluid fluxes from point sources in the Garden Banks and Mississippi Canyon, estimated 

on the basis of material mass balance, are 250 L/yr and 2920 L/yr, respectively. 

Assuming that the Ba-Ra-rich seeps cover about 0.1% of the Louisiana upper slope, the 

annual fluxes of Ba and Ra are estimated to be of the same order of magnitude as their 

annual discharges from the Mississippi River. The largest possible radiation dose to 

fauna from Ra-rich barites (0.0128 mGy/day) is substantially lower than the threshold of 

detrimental radiation effects for aquatic animals. However, the cumulative radiation 

dose and consequential genetic effects over a mean lifespan of 25 years for mussels 

could be substantial. 

STUDY RESULTS: Formation-water-derived fluids emerging on the seafloor are 

anomalously enriched in Ra and Ba relative to seawater by factors of up to 4.5x104 and 

15x104, respectively. Their migration time from the source to the seafloor is established 

to be < 20 years on the basis of the relatively short-lived 228Ra isotope content. Sulfur, 

oxygen and strontium isotope compositions indicate that the Gulf of Mexico seep barite 

deposits form by the mixing of sulfate-rich seawater with Ba-, Sr-, and Ca-rich formation 

fluids seeping with gaseous hydrocarbons, primarily thermogenic methane. The 
ACCESS NUMBER: 30660-19946 

chimneys are erected vertically above the seafloor whereas the crusts drape mud 

volcanoes and/or form pavements at the sediment/water interface. Both 226Ra and 

228Ra are "orphans" in the sense that their respective parents 238U and 232Th are 

practically absent in the fluids and barites. The "orphan" property of Ra affords 

radiometric dating of the barites by 210Pb/226Ra and 228Th/228Ra daughter/parent 

isotope pairs. The assays indicate that chimney ages range from 0.5 to 6.5 years 

whereas the crust ages range from 9.0 to 23.1 years. The growth rates of the chimneys 

vary from 4.4 cm/yr to 9.1 cm/yr. These results indicate that the Ba-Ra-rich fluid 

expulsion on the seafloor, triggering barite deposition, is a rapid and recent event. The 

uptake of Ba and Ra by the fauna inhabiting the barite-bearing seeps was found to be 

proportional to the Ba/Ra ratio in the pore fluids. Mussels harboring methanotrophic 

endosymbionts average 0.6 dpm/g and 0.5 dpm/g 226Ra for soft tissues and 

calcareous shells, respectively, which are significantly higher than those reported for 

shallow marine mussels. Vagrant, heterotrophic, fauna living around the seeps yield 

226Ra and Ba up to 10.7 dpm/g and 4319 ppm, respectively. The considerably higher 

levels of Ra and Ba in the heterotrophs are probably acquired through ingestion of 

barite particles. 

STUDY PRODUCTS: P. Aharon, D. Van Gent, B. Fu, L. M. Scott. 2001. Fate and 

Effects of Barium and Radium-Rich Fluid Emissions from Hydrocarbon Seeps on the 

Benthic Habitats of the Gulf of Mexico Offshore Louisiana. A final report by the 

Louisiana State University for the U. S. Department of the Interior, Mineral Management 

Service, Gulf of Mexico OCS Office, New Orleans, LA. Contract No. 14-35-0001-30660-

19946. 151 pp 

*P.I.’s affiliation may be different than that for Project Manager(s).