Geochemical constraints on the origin of the pore fluids and gas hydrate distribution at Atwater Valley and Keathley Canyon, northern Gulf of Mexico

Marine and Petroleum Geology 25 (2008) 860–872

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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo

Geochemical constraints on the origin of the pore fluids and gas hydrate distribution at Atwater Valley and Keathley Canyon, northern Gulf of Mexico Miriam Kastner a, *, George Claypool b, Gretchen Robertson a a b

Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093, USA 8910 West 2nd Avenue, Lake wood, CO 80226, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 June 2007 Received in revised form 17 October 2007 Accepted 31 January 2008

Pore fluids from Atwater Valley (AT 13/14) and Keathley Canyon (KC 151) in the northern Gulf of Mexico are surprisingly similar with respect to ionic concentrations and oxygen and strontium isotope values, as well as hydrocarbon geochemistry, suggesting that these widely separated localities share common deep subsurface fluid origins. Seafloor mounds with focused fluid migration pathways and inferred near-seafloor gas hydrates characterize the AT 13/14 region, whereas the KC 151 region has a bottom simulating reflector (BSR) at w310 mbsf, which is rather uncommon in the Gulf of Mexico (GOM). At these sites seafloor gas hydrates were not observed but the sediment surface in the vicinity and particularly at the mounds is populated with chemosynthetic communities that are commonly associated with seafloor gas emission. The geochemical results, together with the pressure core data, suggest that at the AT region methane hydrate mostly occurs in near-surface sediments at mounds, consistent with focused migration pathways. In the KC region methane hydrate mostly occurs deeper in the section, in highly fractured silty-clayey sediments from w220 to 300 mbsf. The pore fluids at the AT mounds and KC 151 are characterized by higher than seawater salinity. The more saline pore fluids at the AT mound and at KC151 sites, located w350 km apart, are almost chemically indistinct. Ionic ratios indicate that this distinct high salinity fluid is not from in situ salt dome halite dissolution. Rather, this fluid is a subsurface brine derived from Jurassic or Cenozoic evaporite formation, modified by fluid-sediment reactions, and migrated to the two sites analyzed. Despite porewater salinities elevated above that of seawater, the sediment temperatures are within the range of methane hydrate stability for each of the sites. Based on Cl dilutions the maximum gas hydrate pore volume occupancy at the AT mound sites would be w9%. At KC, Cl concentrations in pressure cores imply that in situ hydrate is unevenly distributed, with pore volume occupancy of 1–12%. Significant variations in sulfate gradients were observed, with the sulfate-to-methane transition zone (SMTZ) at or near the seafloor at the AT mound sites. At AT 13#2 the well-defined SMTZ is at w8 mbsf, and at KC 151#3 it is at w9 mbsf. There is no coincidence between the steepness of the sulfate gradients and the presence or depth of a BSR, suggesting that the SMTZ interfaces are measuring different aspects of the subsurface methane hydrology. At both AT and KC the d13C-DIC values clearly indicate that anaerobic oxidation of methane (AOM) is the dominant reaction responsible for sulfate reduction and the increased alkalinities observed. The most negative d13C-DIC values obtained are 46.3& and 49.6& at the SMTZs at AT 13#2 and KC 151#3, respectively. Ó 2008 Published by Elsevier Ltd.

Keywords: Pore fluids Gas hydrates Seawater Brine Methane-sulfate transition zone Gulf of Mexico

1. Introduction The northern Gulf of Mexico (GOM) continental slope is characterized by intense faulting due to subsurface salt movement and voluminous sediment deposition. These faults are mainly located on the margins of intraslope basins and provide pathways for migration of gas-charged fluids to the seafloor from deeper sediments. * Corresponding author. E-mail address: [email protected] (M. Kastner). 0264-8172/$ – see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.marpetgeo.2008.01.022

The impact of fluid expulsion is displayed in a variety of features (mud volcanoes, gas seeps, mounded carbonates, chemosynthetic communities, gas hydrates) that rim the northwest and northern GOM in water depths ranging from w500 to >2500 m (Roberts et al., 2001). The data reported here are part of a larger project aimed at developing a better understanding of gas hydrate distribution in the northern GOM. Gas hydrates (ice-like, crystalline inclusion compounds of water and natural gas) are believed to have implications with respect to seafloor geo-hazards, such as tsunamis, natural gas resources, and long-term climate variation. In

M. Kastner et al. / Marine and Petroleum Geology 25 (2008) 860–872

particular, there is some concern for slope stability and environmental consequences to petroleum industry infrastructure in future deepwater developments if gas hydrate that strengthens sediments undergoes dissociation. Investigations of gas seeps and related phenomena in the northern GOM have relied mainly on shallow (<10 m) piston cores, and manned submersible observations to characterize near-surface gas hydrates. Seep fluids, venting gas, gas hydrates, and authigenic carbonates are mostly related to focused fluid flow, and the methane (and in some cases petroleum hydrocarbons) supplied is of both biogenic and thermogenic origin (e.g., Brooks et al., 1994; MacDonald et al., 1994; Roberts and Carney, 1997; Milkov and Sassen, 2001; Sassen et al., 2001; Sassen et al., 2002). These nearseafloor gas hydrates are not necessarily associated with a bottom simulating reflector (BSR). Methane plumes from some of these vents are well-imaged in the water column and may reach the atmosphere (MacDonald et al., 2002; Leifer and MacDonald, 2003; Solomon et al., 2008). More recent studies have emphasized factors (high salinity, elevated heat flow, lack of petroleum hydrocarbons) that may limit the distribution of gas hydrates away from faults or other fluid conduits (Paull et al., 2005; Ruppel et al., 2005). Despite this past research, relatively little is known about the subsurface plumbing system and the distribution of gas hydrates in the subsurface. Deep formation waters, mainly produced from petroleum reservoirs, have been shown to have significant variability in salinity and ionic composition (e.g., Land et al., 1988; Land and Macpherson, 1992; Hanor and McIntosh, 2007). In addition to temperature, pressure, concentration and composition of hydrocarbon gases, the pore fluid chemistry, particularly salinity, also influence the stability field and the occurrence and spatial distribution of gas hydrates (i.e. Englesoz and Bishnoi, 1998; Handa, 1990; Dickens and Quinby-Hunt, 1997). Brines migrating from

861

depth may also be accompanied by hydrocarbons of thermogeneic or microbial origin. The elevated salinities of most of the formation waters in the GOM are ultimately related to Middle Jurassic Louann salt and associated brines (Kharaka and Hanor, 2004; Hanor and McIntosh, 2007, and references therein). Mass balance calculations indicate overprinting of the original chemistry by diagenesis (e.g., Land, 1995). Geochemical analyses provide critical information on the origin of the pore fluids at the two drill sites. The chloride ion is conservative at low to moderate temperatures and can help detect fluidsediment processes such as gas hydrate dissociation or mineral hydration-dehydration reactions. Chloride in combination with other ions can help distinguish between salt dissolution and the influence of residual formation brines on the in situ pore fluid chemistry. Key objectives of the geochemical program on the 2005 DOE-Chevron Joint Industry Project (JIP) drilling cruise were to document the pore fluid chemical and isotopic compositions at the AT 13/14 and KC 151 drill sites in order to determine: (1) the depth, spatial distribution, and concentration of gas hydrates and their relation to lithology and other physical properties; (2) the nature of the microbial reactions responsible for gas generation in situ and methane fluxes at the sulfate–methane interface; and (3) the subsurface hydrology and the pore fluid chemistry, which influences the gas hydrate distribution, concentration, and potential dissociation. 2. Sample locations and geologic setting Based on seismic surveys (Hutchinson and Hart, 2004; Snyder et al., 2004), two areas were selected for drilling and coring: Atwater Valley 13/14 (AT 13/14) and Keathley Canyon 151 (KC 151), both at w1300 m water depth (Fig. 1). These localities provided the

Fig. 1. Map showing the location of Atwater Valley and Keathley Canyon sites cored in the northern Gulf of Mexico (modified from Milkov and Sassen, 2001). Gas hydrate sites and petroleum occurrences are also shown.

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M. Kastner et al. / Marine and Petroleum Geology 25 (2008) 860–872

opportunity to study different modes of gas hydrate occurrence. At AT 13/14 there is evidence of fluid venting and mounds that suggest gas hydrates near the seafloor (Hart et al., 2008; Wood et al., 2008). At KC 151 there is a clear BSR (Hutchinson et al., 2008), which is rare in the Gulf of Mexico and which is consistent with the presence of gas hydrate at greater burial depth. The Atwater Valley sites are located in the Mississippi Canyon in sediments that show seismic evidence of a series of depositional and erosional events. The original plan was to drill (a) twinned shallow (<30 m) cored holes on the top of the mound (ATM1 at 27 56011.5900 N, 89160 46.100 W and ATM2 at 27 560 11.5900 N, 89160 46.900 W), (b) twinned deeper holes for logging-while drilling and coring, at a reference site (cored hole was AT 13#2 at 27 560 49.7100 N, 89170 21.3100 N) and (c) twinned holes on the flank of the mound. Unfortunately, only the LWD hole (AT14#1) was drilled on the flank of the mound. The cored hole planned as a twin of AT14#1 was not drilled due to time constraints (Claypool, 2006). The five sites drilled in Atwater Valley provide a SE-NW transect along a seismic line connecting two mounds (mound D and mound F in the site planning documents). The mound cored at ATM1 and ATM2 contains a shallow high-amplitude seismic feature at a depth of w33 mbsf. The seismic event was interpreted as a shoaling of the base of gas hydrate stability caused by increased heat flow and high salinity fluids migrating beneath the mound (Wood et al., 2008). At Keathley Canyon drilling included twinned LWD and coring holes at the site where a regional BSR was best developed (cored hole was KC 151#3 at 26 490 22.6800 N, 92 590 11.940 W). The Keathley Canyon site is located west of a seismic feature that probably represents a salt-cored ridge and a large normal fault. The fault dips to the west while the BSR inferred as the base of gas hydrate stability increases in depth to the west, paralleling the seafloor (Hutchinson et al., 2008). Beneath the BSR a second high-amplitude zone is interpreted as a gas-charged sand-rich layer. Drilling was terminated just beneath the BSR and prior to penetration of the deeper high-amplitude feature. The Keathley Canyon holes penetrated layered, continuous sediments, which contrasts with the more seismically chaotic sediments at the Atwater Valley location (Claypool, 2006). Both the Atwater Valley and the Keathley Canyon locations appeared to contain abundant gas of deeper origin. Gas samples collected during the cruise from cores at both localities were analyzed in shipboard and shore-based laboratories, and the results are reported in a separate study (Lorenson et al., 2008). The gas at both localities was apparently microbial methane (96 to 99.9%), with minor amounts of CO2 and traces (up to a few hundreds of ppm relative to methane) of C2–C6 hydrocarbons. Methane d13CPDB values in both locations were fairly uniform, with most samples in the range of 76 to 72&. 3. Methods The main components analyzed in the pore fluids were pH, salinity, Cl, alkalinity, sulfate, sulfide, Ca2þ, Mg2þ, Sr2þ, Kþ, Naþ, Liþ, and silica concentrations, d13C of dissolved inorganic carbon (DIC), d18O values, and 87Sr/86Sr ratios. Bromide, iodide, and Ba2þ concentrations were determined by G. Snyder at Rice University. Samples for pore fluids were chosen from Fugro Hydraulic Piston Corer (FHPC) or Fugro Corer (FC) cores. The FHPC was mainly used in softer sediments at shallower depths, and the FC in deeper, stiffer sediments. The cores were divided into 1-m long sections, and samples taken routinely from each section in core 1, from 3–4 sections in cores 2 and 3, from 2 sections in core 4 and one per section from each of the deeper cores. In cores in which gas hydrate was inferred to be present from infrared images of the cores,

additional samples were identified and selected. After careful cleaning to remove outside layers of sediment possibly contaminated with seawater used in drilling fluids, the sediment sample was placed in titanium squeezers and subjected to pressures ranging from 800 to 40,000 lbs, using a Carver ‘‘Auto’’ Series automatic hydraulic press. In addition, pressure cores (Fugro Pressure Corer, FPC and Hyace Rotary Corer, HRC) were sub-sampled after they were degassed for methane concentration and handled following the procedure for the FHPC or FC samples. This sub-sampling was guided by X-rays of the cores taken before and after degassing, which indicated the spatial distribution of voids interpreted as loci of in situ hydrate concentration. The pore fluids derived from squeezing were filtered and analyzed shipboard for salinity by refractometry, and for the transient species alkalinity and sulfate, by the Gran-titration and spectrophotometry, respectively. The upper sections of the cores were analyzed for sulfate concentrations to determine the sulfate gradient and depth of the sulfate-to-methane interface or transition. Sulfide was captured by precipitation with CdNO3 for shore-based analysis. Following the alkalinity and sulfate analyses, the remaining volume of the squeezed pore fluids was subdivided for shore-based analyses of major and trace element concentrations and for d13C-DIC, d18O, 87Sr/86Sr ratios. Precisions for salinity, alkalinity, sulfate, major and minor element concentration data ranged between 1–3%.

4. Results of pore fluid chemistry 4.1. Atwater Valley 13/14 sites At AT13 #2 nine FHPC cores were recovered and sampled for pore fluid chemistry from the seafloor to 159 mbsf. Note that the interval between 48 and 118 mbsf was washed down without coring. The sediment consists of a soft, fine-grained and homogeneous green-gray clayey-silt. After core 1H, the next few cores had gas voids (visible separations within the core liner), but the deeper cores did not have obvious visible gas voids. Extensive dissolved gas expansion and separation features were, however, observed in all sediment samples removed from the cores and squeezed for pore fluid, even in the cores without gas voids. In addition, gas bubbles were evident in most pore fluid samples, and some of the pore fluids had a characteristic yellowish color, from dissolved organic matter and/or dissolved Fe. A strong odor of H2S accompanied some of the yellowish pore fluids. Alkalinity increases while sulfate decreases in the first six samples from core AT13 #2-1H (Table 1a), with inversely complementary concave upward profiles (Fig. 2a) down to a depth of 6.0 m beneath the seafloor (mbsf). A maximum in alkalinity (19.7 mM) and a minimum in meaningful measured sulfate (3.3 mM) occurs in the first sample from core AT13 #2-2H at a depth of 8.0 mbsf. The next sample at a depth of 10.0 mbsf shows a decrease in alkalinity (to 11.9 mM) and sulfate concentration effectively absent (0.3 mM). Although not well-defined by closely spaced samples, we infer the depth of the sulfate-to-methane transition to be at a depth of about 8 mbsf, which also is supported by headspace methane analyses (Lorenson et al., 2008). Below the maximum at 8 mbsf, alkalinity gradually decreases down-hole to 8.43 mM at 44.5 mbsf (Table 1a), while sulfate remains at effective zero. The d13C-DIC values, presented in Table 2 and shown in Fig. 2b and c, range from 24.5 to 46.3& (PDB) in the uppermost 8 m of the sediment section, then decrease regularly to near 0& or slightly positive values down to depths of 44.5 mbsf. This pattern mirrors the alkalinity versus depth profile (Fig. 2c) and is consistent with anaerobic oxidation of methane being the predominant reaction coupled with sulfate reduction at this site.

M. Kastner et al. / Marine and Petroleum Geology 25 (2008) 860–872

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Table 1 Major and minor solutes chemistry of pore fluids at (a) Atwater Valley 13#2, (b) the two mound sites at Atwater Valley, and (c) Keathley Canyon 151#3 Hole Core-section (cm interval)

Depth (mbsf)

(a) Hole AT13 #2 pore fluid chemistry AT13 #2 1H-1 (85–100) 1.0 1H-2 (85–100) 2.0 1H-3 (85–100) 3.0 1H-4 (85–100) 4.0 1H-5 (85–100) 5.0 1H-6 (85–100) 6.0 2H-1 (85–100) 8.0 2H-3 (85–100) 10.0 2H-5 (85–100) 12.3 2H-7 (85–100) 14.3 4H-2 (85–100) 20.2 4H-5 (0–17) 22.4 6H-1 (85–100) 29.6 6H-4 (83–100) 32.6 8H-2 (84–100) 41.6 8H-5 (78–100) 44.5 9H-3 (0–20) 120.4 9H-6 (80–100) 124.2 11H-3 (80–100) 129.7 11H-5 (80–100) 131.7 13H-2 (80–100) 143.0 13H-5 (80–100) 148.5 14H-1 (80–100) 158.8

Salinity

Cl (mM)

Alk (mM)

SO2 4 (mM)

Caþ2 (mM)

Mgþ2 (mM)

34.5 34.5 34.0 34.0 34.0 33.0 33.0 32.0 32.0 32.0 32.0 32.0 33.5 33.5 36.5 37.5 31.5 31.0 30.5 31.0 32.5 32.5 33.0

555 555 547 554 555 559 565 562 563 562 562 565 584 587 639 651 561 557 554 560 562 560 574

5.39 6.70 7.36 9.64 8.54 19.47 19.71 11.88 9.43 9.37 9.70 8.82 8.85 9.28 8.74 8.43 11.11 9.38 7.52 6.93 8.81 9.23 8.43

25.28 23.59 23.36 19.34 20.99 9.81 3.25 0.30 0.20 0.18 0.88 0.95 0.00 0.85 0.08 0.00 0.10 0.00 0.05 0.84 0.68 0.00 0.03

10.61 10.91 9.88 9.81 9.92 8.17 8.05 7.18 6.50 6.33 6.72 6.54 6.29 6.80 7.90 8.40 7.90 9.00 8.57 8.59 8.73 8.24 9.04

52.67 51.96 50.60 50.99 51.44 47.53 51.33 53.10 49.98 50.87 52.35 51.37 56.10 53.90 61.07 66.21 50.17 50.23 48.61 50.76 53.99 52.02 54.00

971 971

8.43 8.09

0.00 0.00

33.75 33.89

919 930

6.82 7.82

0.00 0.00

Srþ2 (mM)

Kþ (mM)

Naþ (mM)

11.84 11.54 11.32 10.66 10.68 9.48 7.99 6.45 5.85 5.60 5.15 5.96 3.97 5.37 6.19 5.29 4.76 4.84 5.05 4.71 5.40 5.78 4.71

495 495 475 490 493 455 475 476 449 461 452 464 465 454 497 501 441 444 447 446 457 443 446

63.54 63.20

6.12 5.69

791 789

376 386

30.46 32.92

56.48 61.48

5.42 5.70

731 776

274 320

88.0

104.6 104.6 115.9 135.2 126.5

123.7 136.9

Liþ (mM)

20.4

10.2 10.4 10.4 11.6 12.6

11.8 11.4

H4SiO4 (mM)

296 316 232 308 318 346 474 408 318 276 404 281 400 370 366 352 561 487 433 401 483 357 443

(b) Holes ATM 1&2 pore fluid chemistry ATM1 1H-1 (85–100) 1.0 56.0 1H-2 (85–100) 2.0 56.0 2H-2 (40–50) 8.8 54.0 2H-3 (8–20) 9.5 53.5 2H-4 (80–100) 11.2 52.0 2H-6 (0–20) 12.4 51.5 2H-7 (20–30) 13.6 52.0 2H-9 (0–20) 15.4 55.0 2H-9 (20–40) 15.6 55.5 2H-9 (50–70) 15.9 55.0 5H-2 (80–100) 20.8 56.0 5H-5 (80–100) 23.8 56.0 5H-7 (80–100) 25.8 55.0

961

7.39

0.00

33.60

62.13

5.67

782

276

962 962 949

6.57 6.70 6.79

0.00 0.00 0.00

33.02 33.15 32.53

61.23 61.27 60.83

5.84 5.90 5.79

781 780 771

212 192 198

ATM2 1H-1 (75–100) 1H-3 (0–25) 2H-4 (0–20) 2H-5 (80–100) 3H-4 (0–26) 3H-4 (26–37) 3H-7 (0–27) 3H-7 (27–38)

966 965 959 961 966 904 958 906

7.96 8.04 6.12 6.75 7.04 7.04 6.58 6.07

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

33.80 34.27 34.61 34.99 34.07 31.28 33.52 31.40

63.76 64.87 61.84 62.97 64.07 58.22 63.02 59.08

6.64 6.20 6.15 6.15 6.06 5.53 6.25 6.12

805 804 789 791 801 733 791 760

556 562 560 561 570 576 582 636 651 657 677 687 698 706 744 757 761 770 777 784

3.01 2.87 2.86 3.84 4.55 6.73 10.57 16.06 14.89 13.42 12.29 10.82 9.63 8.53 5.09 4.49 3.69 3.44 3.20 3.08

27.90 26.60 26.30 26.30 25.60 22.74 16.52 0.30 0.00 0.48 0.00 0.00 0.00 0.00 0.30 0.00 0.23 0.23 0.20 0.35

10.14 9.98 10.02 10.09 10.76 9.39 8.32 8.12 7.97 9.59 9.89 10.38 10.89 11.54 13.05 13.21 13.69 13.62 14.05 13.63

52.91 52.79 53.03 53.91 53.59 53.24 53.81 57.37 60.01 63.02 62.80 64.58 64.43 62.89 68.41 67.88 69.83 67.80 68.41 66.30

11.86 11.47 11.11 11.02 11.36 11.29 10.43 8.40 9.06 8.32 8.36 7.64 7.68 7.06 6.12 6.61 6.56 6.33 5.44 5.67

491 488 489 493 491 496 493 497 522 529 532 537 542 530 575 590 610 603 621 614

0.9 2.2 11.0 12.8 20.5 20.7 23.5 23.7

56.0 55.5 55.0 55.0 55.5 51.0 55.0 54.0

(c) Keathley Canyon Hole 151#3 pore fluid chemistry KC151 #3 1H-1 (85–100) 1.0 35.0 1H-2 (85–100) 2.0 35.0 1H-3 (85–100) 3.0 35.0 1H-4 (85–100) 4.0 35.0 1H-5 (85–100) 5.0 35.0 1H-6 (85–100) 6.0 35.3 1H-7 (85–100) 7.0 35.5 2H-1 (85–100) 10.4 38.0 2H-2 (85–100) 11.4 38.0 2H-3 (85–100) 12.4 38.5 2H-4 (85–100) 13.4 39.5 2H-5 (85–100) 14.4 40.0 2H-6 (85–100) 15.4 40.0 2H-7 (85–100) 16.4 40.5 3H-1 (82–100) 19.5 42.5 3H-3 (77–100) 21.5 44.0 3H-5 (83–100) 23.5 44.5 3H-7 (77–100) 25.5 44.5 4H-2 (80–100) 29.6 44.5 4H-5 (80–100) 32.6 45.0

438.3

445.3 443.2

90.5

139.7

208.1

94.7

91.1 92.9

27.7

23.5

14.3

263 271 167 179 263 278 169 137

142 140 150 174 160 202 262 546 446 517 430 514 437 490 405 234 263 187 261 119

(continued on next page)

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Table 1 (continued ) Hole Core-section (cm interval)

Depth (mbsf)

Salinity

Cl (mM)

4H-7 (80–100) 5H-3 (78–100) 5H-6 (82–100) 6C-2 (80–100) 8C-2 (80–100) 8C-3 (80–100) 10C-2 (75–100) 12C-2 (0–25) 12C-3 (75–100) 14C-2 (80–100) 14C-3 (80–100) 15C-2 (39–64) 15C-3 (75–100) 17H-2 (75–100) 17H-5 (0–25) 19H-2 (57–65) 19H-6 (65–100) 20H-2 (75–100) 20H-6 (75–100) 21H-1 (71–100) 22C-2 (75–100) 24C-1 (76–100) 25C-1 (75–100)

34.6 40.1 43.1 101.9 216.6 217.6 225.0 231.3 232.0 243.9 244.9 253.6 255.0 257.9 260.2 276.4 280.5 294.8 298.8 312.4 332.3 370.3 379.5

45.0 45.0 45.0 36.0 50.0 50.0 50.0 51.0 50.0 50.0 51.0 52.0 52.0 52.5 51.0 52.0 50.0 52.0 53.5 54.0 51.2 55.0 54.0

784 782 780 628 863 857 868 878 858 864 869 890 898 906 892 889 890 902 914 924 908 934 930

Alk (mM)

SO2 4 (mM)

Caþ2 (mM)

Mgþ2 (mM)

Srþ2 (mM)

Kþ (mM)

3.06 3.35 3.20 7.50 6.10 5.12 5.51 5.22 4.34 4.87 4.06 3.85 3.29 4.73 4.12 2.84 3.50 2.48 2.34 2.53 3.16 3.89 3.43

0.20 0.38 0.18 0.17 0.00 0.33 0.10 0.08 0.43 0.53 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.05 0.00 0.00 0.00 0.00

13.54 14.31 13.52 9.34 22.14 21.05 22.65 21.76 21.14 20.62 21.33 20.59 20.84 21.10 20.02 21.74 20.36 19.87 19.11 18.47 19.14 21.95 21.81

65.12 64.55 65.43 46.47 79.05 76.14 79.11 80.47 77.88 77.75 79.90 80.68 82.41 82.40 81.01 81.97 82.37 83.79 85.02 82.00 85.07 89.93 87.19

203.7

5.82 5.82 5.27 5.79 6.16 6.12 5.52 6.10 5.75 5.24 5.34 5.84 6.14 5.07 5.53 5.17 5.01 4.69 5.24 5.80 3.87 3.60 4.00

At AT13 #2 near the seafloor, salinity is 34.5, close to modern seawater salinity. Between 10 and 22.4 mbsf, a distinct zone of low salinity (32.0) is observed (Table 1a; Fig. 3). Below w30 mbsf salinity increases to higher than seawater value (37.5) at 44–45 mbsf, where continuous coring was suspended. The minimum salinity zone at 10–22.4 mbsf coincides with the coldest interval imaged by IR scanning. This degree of dilution suggests a gas hydrate concentration of w4% of pore space, which subsequently dissociated during core retrieval. This level of gas hydrate occurrence also is supported by the elevated d18O values in this depth interval, presented in Table 2. The sharp rise in salinity between w23 and 44 mbsf, from 32 to 37.5, suggests the presence of a saline formation fluid at greater depth. Although chloride concentrations do not mirror the shallow salinity minimum in the 10–22.4 mbsf depth interval (Table 1a; Fig. 3b), there is less of an increase than would be expected from the elevated Cl values (584–651 mM) at depths of 30–44.5 mbsf. The less radiogenic than seawater 87Sr/86Sr ratios,

138.6

215.0

271.1

305.7

Naþ (mM)

Liþ (mM)

H4SiO4 (mM)

615 624 629 503 690 681 706 723 700 687 69 713 724 704 698 688 674 699 711 720 712 716 735

14.1

109 87 183 113 349 307 405 293 251 298 291 237 215 331 209 191 143 135 77 51 209 131 125

8.0

36.0

32.2

27.4

presented in Table 3, also reflect the influence of the deeper saline formation fluid. Deeper in the section, a second low salinity minimum of 30.5 occurs at w130 mbsf in more sandy sediments. Infrared images were not particularly anomalous, and the oxygen isotope values do not support the presence and dissociation of gas hydrate in this depth interval. Lateral fluid flow along a more permeable sand-rich horizon may be responsible for this lower salinity zone. Two mound sites (ATM1 and ATM2) were drilled w125 m apart and are located about 1.7 km southeast of AT 13#2. Two ROV push cores also were recovered in the vicinity of the mounds. At ATM1 and ATM2, three FHPC cores were recovered and sampled for pore fluid chemistry from the seafloor to 25.9 and 23.8 mbsf, respectively. In addition, at ATM1, Fugro Pressure Cores were obtained between FHPC cores 1, 2, and 5. The sediment recovered in the cores is a dark-gray silty-clay that is soft, moussey, and somewhat soupy, textures that are produced when decomposing gas

Fig. 2. Alkalinity and sulfate concentrations, and d13C-DIC depth profiles at site AT13-2.

M. Kastner et al. / Marine and Petroleum Geology 25 (2008) 860–872 Table 2 Pore fluid d18O and d13C-DIC values at Atwater Valley 13#2 and mound sites and at Keathley Canyon 151 sites Hole core-section (cm interval)

Depth (mbsf)

d18OPDB

AT13 #2 1H-1 (85–100) 1H-3 (85–100) 1H-4 (85–100) 1H-5 (85–100) 1H-6 (85–100) 2H-1 (85–100) 2H-3 (85–100) 2H-5 (85–100) 4H-2 (85–100) 4H-5 (0–17) 6H-4 (83–100) 8H-5 (78–100) 9H-6 (80–100) 11H-3 (80–100) 13H-2 (80–100) 14H-1 (80–100)

1.0 3.0 4.0 5.0 6.0 8.0 10.0 12.3 20.2 22.4 32.6 44.5 124.6 129.7 143.0 158.8

0.23

KC151 #3 1H-1 (85–100) 1H-3 (85–100) 1H-5 (85–100) 1H-6 (85–100) 1H-7 (85–100) 2H-1 (85–100) 2H-2 (85–100) 2H-3 (85–100) 2H-5 (85–100) 2H-7 (85–100) 3H-1 (82–100) 3H-3 (77–100) 4H-2 (80–100) 5H-6 (82–100) 6C-2 (80–100) 8C-2 (80–100) 8C-3 (80–100) 14C-2 (80–100) 17H-2 (75–100) 20H-2 (75–100) 25C-1 (75–100)

1.0 3.0 5.0 6.0 7.0 10.4 11.4 12.4 14.4 16.4 19.5 21.5 29.6 43.1 101.9 216.6 217.6 243.9 257.9 294.8 379.5

ATM1 1H-1 (85–100) 1H-2 (85–100) 2H-4 (80–100) 2H-6 (0–20) 2H-9 (0–20) 5H-2 (80–100) 5H-5 (80–100) 5H-7 (80–100)

1.0 2.0 11.2 12.4 15.4 20.8 23.8 25.8

ATM2 1H-1 (75–100) 1H-3 (0–25) 2H-5 (80–100) 3H-4 (0–26) 3H-7 (0–27)

0.9 2.2 12.8 20.5 23.5

d13CPDB (DIC) 24.5 30.0

0.30

0.55 0.46

34.5 46.3 31.1 7.2 0.5 2.8

0.35 0.11 0.07

1.7 0.6 6.9

0.13 0.21 0.23

0.17

0.04

0.74 0.11

8.6 8.4 12.5 21.4 33.4 43.5 49.6 43.4 39.5 27.8 17.6 18.7 3.6 3.2 2.7 6.2 5.4

1.14 1.04 1.04 1.24

1.31 1.27 1.21 1.29

1.8 0.5 1.8 3.6 3.8 3.3 3.6 4.6

1.39 3.4 4.2 4.7 2.8 3.8 2.9

hydrate destroys soil fabric (Francisca et al., 2005). In addition to the pore fluid core sampling scheme followed at AT 1 3#2, based on infrared imaging, extra samples of the coldest and adjacent intervals were obtained as well for pore fluid analyses. Gas bubbles were evident in each pore fluid sample. At both mound sites, the shallowest samples (recovered at 0.9 mbsf) had sulfate concentrations that were already below detection, while alkalinity concentrations were moderately elevated and rather constant (between 6.1 and 8.4 mM) throughout the depth intervals cored (Table 1b). Residual dissolved methane was present in all cores sampled at the mound sites (Lorenson et al., 2008) indicating that the sulfate-to-methane transition is near or just below the seafloor. In a ROV push core taken at the ATM1

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mound site, the shallowest sample (0–0.1 mbsf) immediately beneath the seafloor had elevated alkalinity (16.4 mM) and depleted sulfate concentration (4.74 mM). A push core sample at 0.20–0.35 mbsf had a sulfate concentration of 0.21 mM, suggesting that the sulfate-to-methane transition zone is at a depth of w0.4– 0.6 mbsf. The salinity and chloride concentration values at the AT 13#2 reference site and at the AT mound sites are distinct. At AT13 #2 salinities are close to modern seawater value, with variation between 30.5 at 129.7 mbsf and 37.5 at 44.5 mbsf (Table 1, Fig. 3). At the adjacent two mound and push core sites, however, salinities and chloride concentrations are significantly higher, at about 1.7 times modern seawater value, having salinity of 56 and Cl concentration of w970 mM at and immediately below the sediment– water interface. At both mound sites the background salinity value is 56, as documented at ATM1 in FHPC cores 1H and 5H and in ATM2 core 1H (Table 1, Fig. 3). In core 2H at ATM1 and 3H at ATM2 salinity varies from between 55.5 to 51.5 and 51.0, respectively. These minimum salinity values coincide with the coldest intervals imaged with the infrared camera. The positive correlation between the lowest salinity values and coldest areas in the cores suggests the former presence of gas hydrate, at concentration of about 4–5% of pore space. The mounds cored at ATM1 and ATM2 contain a gascharged saline fluid. This sulfate-free and moderate alkalinity saline fluid is distinctly different from modern seawater. This fluid is highly enriched in Ca2þ, Mg2þ, Sr2þ, Naþ, Liþ, and silica, has a higher Caþ2/Mgþ2 ratio, and a more negative d18O value, and the 87Sr/86Sr ratio is somewhat less radiogenic than modern seawater, as shown in Tables 1–3. The Br concentrations are considerably higher as well (G. Snyder, personal communication). 4.2. Keathley Canyon 151 site At KC151 #3 nine FHPC cores and nine FC cores were recovered and sampled for pore fluids from the seafloor to 380 mbsf, about the depth of the BSR. The interval between 45 and 100 mbsf was drilled without coring (Claypool, 2006). While the FHPC cores below 1H all had gas voids, the deeper FC cores did not exhibit ‘obvious’ gas voids. This was probably due to gas loss associated with the hammer coring technique used in FC cores. Alkalinity increased slowly (from 3 to 6.7 Mm) over the uppermost 6 mbsf, then more rapidly (to 10.6 mM) at the base of core 1H at 7 mbsf. The first sample in core 2H at 10.4 mbsf contained the maximum measured alkalinity at 16.1 mM. The sulfate profile showed degrees of depletion that were more or less proportional to the alkalinity increase. At 10.4 mbsf the sulfate concentration was essentially zero, indicating that the sulfate-to-methane transition zone is at a shallower depth not sampled. Examination of the core logging and imaging for evidence of the onset of gas expansion cracks indicates that this transition occurs within the 2.3 m coring gap between cores 1H and 2H. The sulfate-to-methane transition is at a projected depth of 8 mbsf, based on the sulfate and alkalinity gradients. The alkalinity and sulfate data for the uppermost 44 m of sediment in hole KC 151 #3 are listed in Table 1c and shown in Fig. 4. Alkalinity in hole KC151 #3 decreases rapidly beneath the maximum of 16.1 mM at 10.3 mbsf to w3 mM at 44 mbsf; it then increases to 7.5 mM in the single sample at 101.2 mbsf, before diminishing regularly from 6 mM at 217 mbsf to 2.5–3.9 mM in the deepest sample of cored sediments at 380 mbsf. In this interval of low alkalinity, carbonate appears in sediment cores as disseminated flecks to thin beds. Sulfate was absent from the pore fluid in all cores below a depth of w10–11 mbsf, except in cores 12C-3 and 14C-2, where the FC cores may have contained a small amount of seawater contamination. The d13C-DIC pore fluid data at this site, presented in Table 2, show highly negative values, with a minimum

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Fig. 3. (a) Salinity and (b) chloride concentrations plotted versus depth of burial at Atwater Valley 13-14 and Keathley Canyon 151 sites cored.

of 49.6& at 10.3 mbsf, the depth of maximum alkalinity, suggesting intense anaerobic oxidation of methane at this site. A sample of authigenic carbonate recovered from core 2H at w11 mbsf has a d13C value of 54.3& (PDB), which also indicates that methane oxidation is occurring at this site. Salinity shows the only significant variation with depth, as illustrated in Fig. 3a. Salinity increases rapidly from a seawater value (35) near the seafloor to about 44 at a depth of 24 mbsf. The salinity then appears to increase fairly linearly with depth to 55 at 380 mbsf, with the exception of the low salinity (36) in a sandy silt layer at w100 mbsf that appears on seismic records to intersect the seafloor. The variation in the salinity profile is small down to a depth of w250 mbsf and then shows increasing scatter from 250 to 380 mbsf. The chloride concentration–depth profile, shown in Fig. 3b, mimics the salinity profile and shows a similar scatter in Table 3 Pore fluid strontium concentrations and 87Sr/86Sr ratios, at Atwater Valley 13#2 and one mound site ATM2, and at Keathley Canyon 151 sites Hole core-section (cm interval)

Depth (mbsf)

Srþ2 (mM)

87

Sr/86Sr

AT 13 #2 1H-2 (85–100) 2H-7 (85–100) 4H-5 (0–17) 6H-4 (83–100) 8H-5 (78–100) 9H-6 (80–100) 13H-2 (80–100) 14H-1 (80–100)

2.0 14.3 22.4 32.6 44.5 124.2 143.0 158.8

88.0 104.6 104.6 115.9 135.2 126.5 123.7 136.9

0.709174 0.708971 0.708990 0.708982 0.708974 0.708920 0.708908 0.708832

KC151 #3 1H-2 (85–100) 2H-1 (85–100) 3H-7 (77–100) 4H-7 (80–100) 6C-2 (80–100) 12C-2 (0–25) 20H-6 (75–100) 24C-1 (76–100)

2.0 10.4 25.5 34.6 101.9 231.3 298.8 370.3

90.5 139.7 208.1 203.7 138.6 215.0 271.1 305.7

0.709168 0.709163 0.709137 0.709165 0.709201 0.709085 0.708825 0.708742

ATM2 1H-1 (75–100) 3H-4 (0–26) 3H-7 (0–27)

0.9 20.5 23.5

438.3 445.3 443.2

0.708691 0.708715 0.708707

values from 250 to 380 mbsf. Some of the variations in salinity and chloride concentrations are most likely due to decomposition of gas hydrates, which were suggested to be concentrated in this depth interval based on the LWD resistivity logs (Lee and Collett, 2008) and supported by some of the pressure core degassing experiments. Following controlled degassing of the pressure cores, closely spaced sampling and analyses of Cl concentrations in several of the w1-m long pressure cores indicate that gas hydrate distribution must have been highly heterogeneous in these silt- and clay-rich sediments. X-ray imaging of pressure cores before and after degassing were used to guide sub-sampling of the 1-m long cores. Fig. 5 demonstrates the spatial distributions of salinity and Cl concentrations in three successful pressure cores from within the interval of elevated resistivity. Based on total methane obtained from the degassing experiments, the average pore volumes that have been occupied by gas hydrate in situ were w0.5%, w6%, and w1.5% in cores 11P, 13R, and 26R, respectively (Schultheiss et al., 2006). The observed variations in chloride concentrations correspond to gas hydrate occupancies from 1–12% of the pore volumes. The salinity and Cl concentration–depth profiles at KC are shown in Fig. 3a and b and are compared with those at the AT sites, including the AT mound sites. Interestingly, the salinity and Cl concentrations in the deepest pore fluids at Site KC151 are rather similar to the values at the Atwater Valley mound sites, situated over 350 km NW of Keathley Canyon. The oxygen and strontium isotope values of the sulfate-free and moderate alkalinity saline fluids at KC and AT mounds have similar values as well. As at the mound sites, this saline fluid is enriched in Ca2þ, Mg2þ, Sr2þ, Naþ, and silica, but not in Liþ; the KC 151 saline pore fluids have Liþ concentrations below or close of that of seawater. The Caþ2/Mgþ2 ratios (w0.25) are higher than the ratio in modern seawater (0.185) but not as high as at the AT mound sites, where the ratio is almost three times (w0.535) the modern seawater ratio. The Br concentrations are considerably higher as well (G. Snyder, personal communication). The solute concentrations and isotope data are presented in Tables 1–3. The significantly lower salinity and Cl concentration pore fluid at w100 mbsf (36 and 628 mM, respectively; Table 1) is situated in the sandy silt layer that may have some fluid connection with the seafloor. This is supported by the distinct d18O and 87Sr/86Sr values,

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Fig. 4. Alkalinity and sulfate concentrations, and d13C-DIC depth profiles, site KC151.

shown in Tables 2 and 3, respectively, that are closer to the respective bottom seawater values.

The GOM DOE-JIP cores provide a contrast between a diapiric mound site and two non-mound sites, one with a deep BSR.

5. Discussion

5.1. Sulfate, alkalinity, d13C-DIC, and methane fluxes

Formation of natural gas hydrate accumulations depends on an adequate supply of hydrocarbon gas. In the northern GOM, obvious gas sources are present as leakage from deeper oil and gas accumulations. These seeps are seismically imaged as gas chimneys, fault zones and mud volcanoes, and have been sampled in shallow coring and submarine operations (e.g., Brooks et al., 1994; MacDonald et al., 1994; Sager et al., 1999, 2003; Ruppel et al., 2005). Many of these seeps are the direct result of leakage from underlying petroleum accumulations (Sassen et al., 2001), but more commonly ‘‘high flux’’ seeps are composed of relatively pure methane of apparent microbial origin (Sassen et al., 2002). There remains some question of how common gas hydrate accumulations might be away from the obvious mounds or vent regions (Paull et al., 2005).

Dissolved sulfate is one of the main electron acceptors available for oxidation of organic matter in marine sediments, and is supplied mainly by diffusion from, or burial of, overlying seawater (neglecting here the re-oxidation of sulfide). Electrons for reduction of sulfate are supplied by oxidation of sedimentary organic matter, methane, or other petroleum hydrocarbons. At the GOM DOE-JIP sites there is no evidence for large quantities of petroleum (C6þ) hydrocarbons, beyond the traces of C2–C6 hydrocarbons in the analyzed gases (Lorenson et al., 2008). The relationship between the consumption of sulfate and the amount and carbon isotopic composition of DIC can indicate the nature of the oxidation process that fuels sulfate reduction. In addition, when the main electron source is a supply of methane from depth, then the apparent rate of

Fig. 5. Salinity and Cl concentrations versus depth in three pressure cores from the high resistivity zone at Keathley Canyon 151.

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Fig. 6. (a) Plot of alkalinity (cation-adjusted) added versus sulfate removed (relative to seawater concentrations). Diagonal lines indicate 1:1 and 1:2 ratios. The 1:1 ratio is consistent with methane oxidation being the electron-generating process that fuels sulfate reduction. The 1:2 ratio is consistent with organic matter oxidation linked to sulfate reduction. The data from AT13-2 and KC151-3 cores actually plot on the approximate slope of 1:1.3 ratio. (b) Alkalinity versus d13C-DIC. Mixing line is based on addition during sulfate reduction of DIC with d13C of 55& to initial 2.5 mM DIC with d13C of 0&. The added DIC is 70:30 mixture of 70& DIC from oxidation of methane and 20& DIC from oxidation of sedimentary organic matter. Analyses from beneath the sulfate reduction zone are not plotted.

sulfate reduction and the depth to the zone of sulfate exhaustion is proportional to the methane flux. Organic matter oxidation linked to sulfate reduction produces 2 moles of bicarbonate per mole sulfate reduced: 2CH2O þ SO2 4 / 2HCO 3 þ H2S. In areas of upward methane flux, anaerobic oxidation of methane (AOM) is coupled with sulfate reduction and produces 1 mole of bicarbonate per mole sulfate reduced: CH4 þ SO2 4 /  HCO 3 þ HS þ H2O. Accordingly, the proportions of sulfate reduced and bicarbonate produced provide evidence of the dominant oxidation process fueling sulfate reduction. In addition, the d13C of methane is 20–75& more negative than organic matter (depending on the sources) and is reflected in the d13C of the bicarbonate produced. Fig. 6a is a plot of cation-adjusted alkalinity (used as an indicator of bicarbonate concentration) added versus sulfate removed. Diagonal solid lines indicate the 1:1 and 1:2 SO2 4 :Alk ratios. After adjusting alkalinity for net cation depletion due to carbonate precipitation and brine influx, the data from AT13-2 and KC151-3 cores cluster at a slope of w1:1.3. This sulfur to carbon stoichiometry is consistent with sulfate being reduced by electrons derived w70% from methane oxidation and w30% from organic matter oxidation. The methane undergoing oxidation has d13C of about 70& (Lorenson et al., 2008), and the sedimentary organic matter in nearsurface sediments has d13C of about 20& (Goni et al., 1998). Oxidation of a 70:30 mixture produces DIC with d13C of about 55&. Fig. 6b is a plot of cation-adjusted alkalinity versus d13CDIC, and shows that the measured DIC is consistent with addition of bicarbonate with d13C of 55& delineated by the curved line. Fig. 6b shows slightly more methane oxidation at KC than at AT. The depth of the sulfate-to-methane transition is about 8 mbsf at AT13-2 and about 9 mbsf at KC151-3. At these depths, sulfate concentrations are at or approaching zero, and alkalinities are at their maximum values. Authigenic carbonate is present in the cores, with a nodule at 10.3 mbsf in KC151-3 having an extreme negative d13C value (54.3&), indicating that even more negative d13C-DIC probably is present at the sulfate–methane transition in the coring and sample gap. At depths below the sulfate–methane transition the alkalinity decreases and d13C-DIC increases due to the onset of methanogensis. Below depths of w20 mbsf at KC151-3, the alkalinity and d13C-DIC level off in the range of 5  2 mM and w0  3&, respectively. Methane d13C values show similar trends over a comparable depth interval (16–40 mbsf), increasing from 78 to 73& and remaining relatively constant in deeper parts of

the core (Lorenson et al., 2008). This pattern is consistent with a zone of methanogenesis just beneath the sulfate–methane transition superimposed on an upward migrating supply of isotopically light methane (w70&) from depth. At the ATM1 and ATM2 sites on top of the mound, the sulfate– methane transition is in the uppermost 10 cm of the sediment section. A push core on the flank of the mound (LWD site AT14-1), 125 m from the top of the mound in the direction of AT13-2, showed that this transition zone is at w60 cm below the seafloor. With sulfate reduction confined to the uppermost 8 m at AT13-2, the thickness of the zone of sulfate reduction increases with increasing distance away from the mound sites. Assuming steady-state conditions, when AOM is the dominant oxidation process fueling sulfate reduction, the downward diffusive flux of sulfate can be estimated and used to indicate the magnitude of methane flux required to support the inferred rates of sulfate reduction by anaerobic oxidation of methane (Borowski et al., 1996). The results for the Atwater Valley sites are presented in Table 4, where sulfate flux is assumed equivalent to methane flux in molar units, with methane also expressed in volumetric units to better indicate the possible magnitude of gas flux at surface conditions. Volumes at the seafloor would be less than 1% of surface (1 atm) volumes. For comparison, a similar calculation based on measured sulfate reduction rates at the southern summit of Hydrate Ridge at the mound sites is equal to w2.5 times the flux at the AT mound sites. 5.2. Constraints on in situ gas hydrate Pore fluid geochemical results, together with the infrared images of cores and the methane volumes recovered from pressure cores, provide established inferences that are consistent with the presence of gas hydrates at each of the sites cored at Atwater Valley and Keathley Canyon, including sites with salinity/chloride concentrations that are 1.7 times modern seawater. Under these conditions and at the prevailing seafloor pressure (13.1 MPa) the equilibrium destabilization temperature decreases from 14.3 to 13.2  C (Sloan, 1998; CSMHYD program), and the required dissolved methane concentration decreases from 46 to 45.2 mM (Xu, 2002). Overall, the elevated salinity has relatively minor influence on methane hydrate stability. Elevated heat flow has been observed at the AT mound site (Coffin et al., 2005) and limits the thickness of the methane hydrate stability zone (Ruppel et al., 2005).

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Table 4 Calculated methane fluxes required supporting the inferred rates of sulfate reduction by anaerobic oxidation of methane at the Atwater Valley sites Site

Estimated porosity

Ds(SO4)a m2 year1

SMIb depth (cm)

SO4c gradient (mM m1)

Diffusive SO4 flux (mM m2 year1)

Equivalentd CH4 flux (std cm3 m2 d1)

ATM1 ATM2 AT14-1 AT13Hydrate Ridgee

0.85 0.85 0.80 0.75 0.85

1.22E-02 1.22E-02 1.10E-02 1.01E-02 1.22E-02

10 10 60 700 4

280.0 280.0 49.0 8.8 700.0

2901 2901 437 67 7252

178 178 27 4 445

a b c d e

Whole sediment diffusion coefficient. Sulfate–methane interface. Gradient estimated just above the SMI. Assuming all sulfate reduction due to anaerobic methane oxidation (calculations according to Niewohner, GCA 62, 455–564). Beggiatoa mat on top of Hydrate Ridge (Boetius et al., 2000, Nature 407, 623–625).

Although seafloor gas hydrates were not observed, ROV observations showed that the sediment surface in the vicinity of the mounds is covered with black mineral residue (iron monosulfide?) and populated with chemosynthetic communities, consistent with methane advection supporting intense sulfate reduction. Hart et al. (2008) provide an assessment of other seafloor indicators associated with fluid flux at the AT sites. In the Atwater Valley location, methane hydrate inferences are mostly concentrated in near-surface sediments at the mound sites, indicating focused migration pathways of methane-saturated fluids into the gas hydrate stability zone. However, even at the AT13-2 reference site small amounts of methane hydrate (<3–4% pore volume occupancy) is present in the uppermost w30 mbsf. The established indirect indicators of methane hydrate include (1) the presence of gas expansion and separation features, (2) mousse-like textures in the sediment cores, (3) gas bubbles, and (4) low salinity and chloride concentrations in the pore fluids that were coincident with the coldest intervals imaged by the infrared camera. In the KC region, methane hydrate is more pronounced in the deeper part of the sedimentary section in the highly fractured siltyclayey sediments in this area, primarily from w220 to 300 mbsf, as also suggested by the elevated resistivity values (Lee and Collett, 2008) and the work of Cook et al. (2008). Indeed, the significant fluctuations in the salinity and chloride concentrations from w200 mbsf to the bottom of the hole (Table 1 and Fig. 3) are most likely due to dissociation of gas hydrates. Pressure core degassing experiments produced enough gas to account for a maximum of 5– 6% pore volume occupancy by gas hydrate in the few cores that recovered sediment (Schultheiss et al., 2006). Following degassing of the pressure cores, high resolution sampling for salinity and Cl concentrations, guided by X-ray imaging of the 1-m long cores before and after degassing, indicates that in the silty-clay-rich sediments gas hydrate distribution is highly heterogeneous and locally ranges between 1 and 12% pore volume occupancy (Fig. 5). X-rays of the pressure cores reveal gas hydrate, occurrence in thin layers, small nodules, in veinlets (some at high angles), and in a disseminated mode (Schultheiss et al., 2006).

beyond that reached by JIP drilling is 0.5 mm year1, assuming an average temperature of 12  C and 50% porosity. An evaluation of the concentration–depth profiles of the other major solutes and stable isotopes of oxygen and strontium can provide insight into the origin of the saline fluid. Possible origins are (1) subsurface dissolution of evaporite minerals (e.g., the widespread halite (NaCl) in the adjacent salt diapers) or (2) a marine residual brine (Kharaka and Hanor, 2004) laterally transported from elsewhere (e.g., Middle Jurassic Louann or Cenozoic evaporites). The chemical evolution of modern seawater during progressive evaporation has been extensively studied (e.g., Usiglio, 1849; Braitsch, 1971; McCaffrey et al., 1987, and therein), and the relations of the Mesozoic and Cenozoic seawater, described by Lowenstein et al. (2003), to the GOM basinal brines, were recently synthesized by Hanor and McIntosh (2007). In summary, marine brine within the halite facies should be enriched in Cl, sulfate, Naþ, Kþ, Mgþ2, Liþ and depleted in Caþ2 and Srþ2. An exception is in organic matter-rich and oil-bearing basins where sulfate is microbially reduced, affecting Caþ2 and Srþ2 concentrations. In addition, Caþ2, Mgþ2, and sulfate concentrations fluctuated greatly during Mesozoic and Cenozoic evolution of seawater (Lowenstein et al., 2003). Fig. 7 is a plot of the excess Naþ and Cl concentrations in the pore fluids at KC 151. The slope lines of the Naþ/Cl ratio of halite (1:1) and of modern seawater (w0.85) are also shown. The KC151-3 data points clearly plot below both the 1:1 line and below the modern

5.3. The nature and origin of the basinal brine A distinctive feature of the hydrogeochemical regime at Keathley Canyon is the gradient of increasing salinity and chloride with depth (Fig. 3). The maximum measured values of 55 ppt salinity and w935 mM chloride at 370–380 mbsf correspond to a degree of seawater evaporation of w1.7 times. The chloride concentration– depth profile appears to be linear from w200 to 380 mbsf. A cored sandy silt layer at w100 mbsf that appears in seismic records to breach the seafloor disconnects the shallow and deeper sections of the depth profiles (for example, see Fig. 3). A rough estimate of the advection rate of the subsurface saline fluid originating at a depth

Fig. 7. Plots of Naþ in excess of seawater concentration versus chloride in excess of seawater concentration in pore fluids at Atwater Valley and Keathley Canyon sites cored. Lines of slope Naþ/Cl ratio of halite (1:1) and of modern seawater are also shown.

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Fig. 8. Sodium (b) and potassium (c) concentration–depth profiles, the chloride depth profile (a) is also shown for comparison.

seawater line. A brine that formed from halite dissolution would plot on the 1:1 line, and a brine formed from evaporated seawater before halite precipitation would have the same Naþ/Cl ratio of seawater. Because Naþ and Cl behave differently within and beyond the halite facies, halite precipitation further decreases the Naþ/Cl ratio, as observed in Fig. 7; hence, the conclusion that the pore fluids at KC151-3 are influenced by advection of a residual brine from within or beyond halite precipitation. During transport, diagenetic reactions often modify the chemistry of brines. The solutes most influenced by diagenesis of carbonates or silicates are Caþ2, Mgþ2, (Srþ2), Kþ, and Naþ (e.g., Milliken, 1988; Aharon et al., 1992; Land and Macpherson, 1992). Other important characteristics of the subsurface brine, shown in Tables 1–3, and plotted in Figs. 8–11 are: Naþ concentrations increase with depth and its profile is similar to that of Cl, whereas Kþ concentrations decrease with depth (Fig. 8), possibly as a result of clay diagenesis. Caþ2 and Mgþ2 concentrations increase with increasing Cl concentrations, thus with depth, but not at a 1:1 ratio (Fig. 9); and Caþ2 increases more steeply than Mgþ2, as shown in Fig. 9, meaning that the Mgþ2/Caþ2 ratio is w3.7 (compared to 5.4 in modern seawater), most likely caused by dolomitization. The d18O values become more negative with depth, thus decreasing with increasing Cl concentrations (Figs. 10 and 11). The 87Sr/86Sr

ratios become less radiogenic with depth and therefore also with increasing Cl concentrations (Figs. 10 and 11). Both the d18O and 87 Sr/86Sr trends indicate mixing with an older brine having more negative oxygen isotopes than modern seawater. The 87Sr/86Sr ratio of the most saline sample at KC151-3 is 0.708742, a value approaching the somewhat less radiogenic value of 0.70838, of the Louann brine (Steuber et al., 1984). The Sr isotope value of the Middle Louann salt is more radiogenic than the Jurassic seawater Sr isotope values that range from 0.7068 to 0.7076, e.g., Burke et al., (1982) and Veizer (1989). This implies that the Middle Jurassic saline fluid strontium isotopes became modified (more radiogenic) by either mixing with continental fluids or by diagenesis of terrigeneous sediments. Clearly, diagenesis with the detrital terrigeneous sediments in the GOM, characterized by highly radiogenic 87Sr/86Sr, having an average value of 0.720 (Steuber et al., 1984; Land et al., 1988), has little influence on the basinal brine value, as also supported by the inverse relation between Cl concentrations and d18O values of the pore fluids (Fig. 11). Most note-worthy is the observation that, except for the Caþ2 and Mgþ2 concentrations, the chemical and isotopic (d18O and 87 Sr/86Sr) compositions of the saline pore fluids at the Atwater Valley mound sites are strikingly similar to those at KC 151-3 located w350 km NW (Fig. 1), as indicated in Tables 1–3 and Figs. 3, 7,

Fig. 9. Concentration cross-plots of (a) Caþ2 versus Cl, (b) Mgþ2 versus Cl, and (c) Mgþ2 versus Caþ2, at Atwater Valley 13-14 and Keathley Canyon 151 sites cored.

M. Kastner et al. / Marine and Petroleum Geology 25 (2008) 860–872

Fig. 10. Depth profiles of (a) d18O values, and (b)

87

871

Sr/86Sr ratios at Atwater Valley 13/14 and Keathley Canyon 151 sites cored.

and 8–11. The similarity suggests some common factors in the origin of the subsurface fluids at both Keathley Canyon and Atwater Valley. The methane carbon isotope data (Lorenson et al., 2008) and the pore fluid Br concentrations (G. Snyder, personal communication) are also consistent with this interpretation. Diagenesis, particularly dolomitization of CaCO3, is most likely responsible for (1) the higher concentration of Caþ2 (>3 times seawater) at the AT mound sites compared with the about twice seawater Caþ2 concentration at KC151; and (2) Mgþ2 concentrations of w1.2 seawater value at the AT mounds and w1.6 times seawater value at KC151, as shown in the cross-plots of Caþ2, Mgþ2, and Cl in Fig. 9a–c. Albitization of Ca plagioclases would only affect the Caþ2 but not the Mgþ2 concentrations. 6. Conclusions The pore fluid collected from cores recovered during 2005 DOEJIP drilling in the northern Gulf of Mexico provided information on site specific chemical and isotopic gradients and fluxes of solutes, on the relation to gas hydrate occurrence and distribution, and on the origin of formation fluids encountered in the cores. We infer the occurrence of in situ gas hydrates at all sites, including at the sites with high salinity and elevated porewater chloride concentration.

Fig. 11. Cross-plots of chloride versus (a) d18O values, and (b)

87

Accordingly, at the AT region the methane hydrate occurrence is mostly concentrated at mounds in near-surface sediments, indicating focused migration of hydrocarbons, whereas at the KC region methane hydrate is concentrated at greater depths, in highly fractured silty-clayey sediments. High resolution analyses of Cl concentrations in pressure cores imply that any gas hydrate present in situ is distributed very heterogeneously in the silt- and clay-rich sediments. The sulfate and alkalinity gradients mostly reflect the upward flux of methane. Sulfate is being reduced by electrons derived w70% from methane and w30% from organic matter oxidation, as indicated by the approximate 1:1.3 stoichiometric ratio of sulfate depletion relative to alkalinity addition, shown in Figs. 2, 4, and 6. Methane oxidation as the dominant process linked to sulfate reduction was confirmed by the d13C of DIC, with values as negative as 50& (PDB). Elevated pore fluid salinities (up to 56) were observed in cores from the Atwater Valley Mound sites and Keathley Canyon 151. Both areas are underlained by salt structures, but the Naþ/Cl ratios of the pore fluids indicate that the observed high salinities are not the product of simple dissolution of halite, but instead reflect the regional influence on the pore fluids of a diagenetically modified basinal brine.

Sr/86Sr ratios at Atwater Valley 13/14 and Keathley Canyon 151 sites cored.

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M. Kastner et al. / Marine and Petroleum Geology 25 (2008) 860–872

Acknowledgements We thank Jerry Dickens, an anonymous reviewer, and Dr. Carolyn Ruppel, the editor of this volume, for their valuable comments. This paper was prepared with the support of the U.S. Department of Energy, under Award No. DE-FC26-01NT41330. However, any opinions, findings conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE.

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