C1C8 hydrocarbons in sediments from Guaymas Basin, Gulf of California—Comparison to Peru Margin, Japan Trench and California Borderlands

Org. Geochem. Vol. 12, No. 2, pp. 171-194, 1988 Printed in Great Britain. All rights reserved

0146-6380/88 $3.00+0.00 Copyright © 1988 PergamonPress pie

Ct-Cg hydrocarbons in sediments from Guaymas Basin, Gulf of California---Comparison to Peru Margin, Japan Trench and California Borderlands JEAN K. WHELAN1, BERND R. T. SIMONEIT2 and MARTHA E. TARAFA~ ~Department of Chemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A. 2Petroleum Research Group, College of Oceanography, Oregon State University, Corvallis, OR 97331, U.S.A. (Received 20 May 1985; accepted 24 November 1987)

Almtraet--Surface seafloor sediments, hydrothermal vent samples, and Deep Sea Drilling Project sediments (Hole 481 A) from the Guaymas Basin were examined for C 1 ~ hydrocarbons. The proportions of various classes of compounds were examined and compared to those from other geographic areas (Peru upwelling region and Japan Trench) to gain insight into the relative importance of thermal generation, migration and biodegradation. Concentrations of C2--C7 hydrocarbons were about I0-10,000 times higher in geothermally warm (estimated to have been exposed to maximum temperatures in the range of 30-150°C) Guaymas Basin sediments in comparison to the low concentrations (0.1-10 ppb per compound) typical of geothermally cold (maximum thermal exposure less than 20°C) seafloor and DSDP diatomaceous sediments. However, one sediment sample from DSDP Site 477, estimated to have been exposed to temperatures of 300°C or higher in the past, showed only a limited hydrocarbon composition, consisting of C ~ 3 alkanes and aromatic hydrocarbons only. Alkene/alkane ratios of 0. I or greater were typical of both geothermally cold sediments and also of very hydrocarbon-rich Alvin samples recovered from the seafloor. Because little or no alkene was generally detected in buried sediments exposed to geothermal temperatures greater than 30°C, it is suggested that the alkenes are produced by biogenic processes. Normal alkanes predominated over cyclic and branched structures in geothermally cooler (<20°C) sediments, with the proportion of cyclic and branched compounds increasing in hotter sediments. Concentrations of gem-dimethyl and aromatic compounds generally remained approximately constant or increased slightly with temperature in comparison with geothermally cold shallow sediments. Similarities in compositions of branched and cyclic compounds were observed in some pairs of bitumen-rich Guaymas seafloor samples recovered from different areas, suggesting common mechanisms of light hydrocarbon generation and/or migration. Localized increases in ratios of specific cycloalkane ratios were observed adjacent to sill intrusions. Key words: gas, light hydrocarbon, alkene, hydrothermal vent, Guaymas, Peru, Japan Trench

INTRODUCTION The purpose of this work is to present initial results on the distribution of CI--Cs hydrocarbons in sediments from the hydrothermally active Guaymas Basin area of the Gulf of California. Relative amounts and distributions of these compounds in samples taken by the submersible Alvin and by the Deep Sea Drilling Project (DSDP) Leg 64 (Whelan and Hunt, 1982), are compared with similar organic-rich sediments from the nearby California Borderland area subjected to normal or somewhat elevated geothermal gradients (~40-100°C/km). Comparisons are also made to geothermally cold (<20°C) shallow seafloor sediments of the Peru Shelf and subducting Japan Trench (DSDP), which are similar to those of Guaymas Basin in their contents of high proportions of marine diatomaceous constituents (Henrichs, 1980; Honza et al. 1980; Curray et al., 1982). The hydrocarbon formation and migration processes in the Guaymas Basin area are unusual as compared to normal petroleum generation (cata-

genesis). Two basic types of heating processes have been proposed for these young organic-rich sediments: one results from intrusion of magma into the sediments causing very localized heating effects; the other results from heating via a shallow magma chamber and involves upward percolation of hot water causing high temperatures over a broad area (Kastner, 1982). Both processes are different from conventional geothermal heating involved in petroleum generation where the gradient generally increases in a gradual and regular way with increasing sediment depth (Tissot and Welte, 1984; Hunt, 1979). In the Guaymas Basin, sill intrusion tends to cause only localized hydrocarbon generation and migration away from the sill [typically effective over distances of a few meters at most, similar to processes described by Simoneit et al. (1981) and Peters et aL (1979, 1983)]. In contrast, the upward percolation of hydrothermal fluids through a fairly thick sedimentary section can subject sediments over a wide area to temperatures of 200 and 250°C or higher (Lonsdale and Becker, 1985), well above the maximum of about

171 O.G. 12/2--F

172

JEAN K. Wr~LANet al.

150°C, generally considered to be the end of normal catagenic liquid petroleum generation (Hunt, 1979; Tissot and Welte, 1984). The upward permeating water can also carry some of the light hydrocarbons thermally generated within the sediments into the cooler overlying sediments and water column where they may be biodegraded. Because the Guaymas hydrocarbon generation processes are unusual when compared to normal petroleum generation and migration, this paper is, of necessity, limited to presenting the initial examples of relative amounts and compositions of C1-Cs hydrocarbons which occur in the various areas of the basin. Possible alternative production/destruction processes consistent with the observed hydrocarbon distributions are discussed and compared to those observed in other areas. Analysis of additional samples in future work will be required to better distinguish these processes.

inside, on a vibrating paint-shaking machine. After the sample is dispersed, the vessel is placed in a water bath at 90°C for 0.5 h. Aliquots of the headspace gas are withdrawn and analyzed by gas chromatography, following the procedure of Whelan (1984). A packed column (Spherosil+OV-101 on Anakrom AS) was used for complete resolution of the C~--C5 hydrocarbons. A capillary column (Hexadecane-hexadecene-kel F) was used for the complete resolution of all the C6--C7 and most of the C8 hydrocarbons. All peak areas on chromatograms were measured by an electronic integrator (Columbia Scientific Industries Supergrator III). The method was standardized by analysis of a known mixture of pure hydrocarbons. The maturity of the organic matter in some samples with respect to petroleum generation was measured by pyrolysis (Huc and Hunt, 1980). Briefly, the samples are heated at 30°C per min in a helium stream and total hydrocarbons are monitored as a General Area function of temperature. Hydrocarbons are evolved Tectonic activity in the Gulf of California area has in two peaks: P, at 150-250°C measures generated resulted in extremely diverse thermal conditions petroleum in the sample and P2 at 400-550°C reprewithin the various sedimentary sequences, especially sents hydrocarbons cracked from the organic matrix. in Guaymas Basin (Curray et al., 1979, 1982; Einsele P2 is related to the petroleum generating potential of et al., 1980). The structural geology of the Guaymas the sediment if it were subjected to further heating Basin is determined by two short axes of bottom (burial). The production index (PI) is the ratio of expansion (rifts) and is confined by two long trans- sorbed generated hydrocarbons (P~) to the total form faults (Fig. 1) (Lonsdale and Becker, 1985). The petroleum generating potential of the sediment basin occupies an active rift with a typical oceanic (P~ + P2). The results for the PI are summarized in crustal structure (Curray et al., 1982). Its heat flow is Table 1 along with amounts of total extractable uneven, maximizing at about 30/zeal cm -2 sbitumen and various hydrocarbon parameters. (1.2 W m -2) (Lonsdale and Becker, 1985). The basin RESULTS AND DISCUSSION has a rather high rate of sediment accumulation (2.Tm/103yr), with considerable biological pro(A ) Geological Setting ductivity in the overlying water column. The rapid burial prevents extensive aerobic biological oxidation The first indication of sedimentary material with of the organic matter. possible petroleum components was observed in a The sediment depth in the rifts is 300-500 m. This gravity core from the northern rift of the Guaymas region is of unique interest because it offers an oppor- Basin collected in 1972 by the SIO Hypogene tunity to study the effect of increased temperatures on Expedition (Site 30G, Fig. 1 and 2a; 27°23.0'N, immature organic matter (Einsele et aL, 1980). 111°26.9'W; 1968m water depth; Simoneit et al., 1979; Goidhaber, 1974; Kalil, 1976). The core sections from subbottom depths greater than about EXPERIMENTAL 2.8 m had a strong petroliferous odor and contained The samples for this work were sealed either in elevated levels of CI--C8 hydrocarbons (Simoneit et cans or bags (Kapak plastic) onboard the ocean- al., 1979). ographic vessel and then stored frozen until analysis. The Deep Sea Drilling Project (DSDP-IPOD) Cz-C8 hydrocarbons were analyzed in the wet drilled Site 481 (27°15.2'N, 111°30.46'W; 1998m sediment by a headspace (sorbed gas) technique water depth; Curray et al., 1982) about 14-15 km (Whelan, 1984). A frozen sample of 4-10g of wet southwest of Site 30G (Figs 1 and 2a) is also located sediment is introduced into a small stainless steel in the northern rift. A similar petroliferous odor vessel fitted with two stainless steel balls of different was encountered at a subbottom depth of about sizes. The vessel is then filled two-thirds full with 150o170 m between the two upper sills of Hole 481A water, through which helium has been bubbled, and (Curray et al., 1982; Simoneit, 1982). High levels of closed tightly using a silicone rubber seal in a glove Cr-Cs hydrocarbons were confirmed in these sedibag filled with helium. The vessel is equipped with a ments (Whelan and Hunt, 1982). septum nut on the side, through which gas samples Dredging of the seabed on a deep-tow and heatflow can be withdrawn. The sample, with water and survey cruise (SIO) in 1980 recovered mineral mound helium, is broken up by shaking with the steel balls material cemented with petroleum (cf. Fig. 2b, dredge

173

C:-C s hydrocarbons in sediments from Guaymas Basin 112eW

tlI*W

27"58'N,

I

115°W

Diego

b e°

0

o

to 20

26"35'h 30°N

30*

liO*

Guoymas

Cabo San Lucas

?/' \ %

'" 'i(..!:.

::

23"

105"W

Fig. 1. Map of transform faults, gravity core site (30G), and DSDP sites in Gulf of California (Guaymas Basin).

site 7D~ Simoneit and Lonsdalel 1982). These samples again had the typical petroliferous odor and conrained high concentrations of CI--Cs hydrocarbons (Simoneit and Lonsdale, 1982). Also, piston cores from the northern rift (Sites 9, 13 and 15, Fig. 2a) had a petroliferous odor starting at shallow and continuing into deeper and variable intervals below the sediment-water interface (Simoneit, 1984a). The diving program with the D.S.R.V. Alvin in 1982 in the south rift area recovered numerous petroliferous samples, most with the characteristic petroliferous odor (Simoneit and Kawka, 1987; Si-

moneit 1984a, 1985a, b). The sample designations, descriptions and bulk analytical results are summarized in Table I. Data from gravity or piston core, dredge and DSDP samples recovered from the same area are also shown for comparison.

(B) Summary of Processes Affecting C : C s Hydrocarbons in Sediments Processes which can affect hydrocarbon distributions in all sediments include changes in input (via factors such as transport by bottom currents or slumping; deposition of marine and/or terrestrial

30G (2.51-2.58 m)

30G (2.59-2.65 m)

30G (2.94-2.99 m)

3043 (3.31-3.37 m)

30G (3.38-3.45 m)

7D-3B

7D-5A

481 A-4-2, 110--125 cm 481A-8-2, 115-125cm 481A-10-2, 137-150em 481A-12-2, 120-135 cm

Number

1

2

3

4

5

6

7

8

481A-22-.4, 122-150cm

481A-24-5, 121-131 cm

481A-26-5, 120-131 em

481A-30-5, 117-122em

12a

13

14

15

11a

10

9

Sample designation

2.0

1.4

1.8

1.7

0.8

1.3

2.0

2.1

(%)

Total organic carbon

324.7

286.7

267.7

247.7a

149.2a

130.4

111.2

73.1

3.41

3.35

2.95

2.62

2.55

(in)

Subbottom depth

Diatom. silty clay

Diatom. silty clay

Diatom. silty clay

Diatom. clay, carbonate

Silty clay, carbonate

Diatom. clay

Diatom. clay

Diatom. clay

Hydrothermal min. claystone Hydrothermal min. daystone

Unconsolidated sediment (diatom ooze) Unconsolidated sediment (diatom ooze) Unconsolidated sediment (diatom ooze) Unconsolidated sediment (diatom ooze) Unconsolidated sediment (diatom ooze)

Sample description

41

17

48

ND

ND

Dredge (1980) 15070 2,1

5360

17060

7970

ND

1.2

2.2

2.0

2.2

9.5

12.7

iCs/nC 5

ND

ND

ND

ND

ND

ND

ND

ND

1080

140

200

1210

3040

980

1240

80

5

15

23

18

0.7

7

28

17

4.3

5.6

4.6

4.1

9.7

1.9

3.1

9.2

Deep Sea Drilling Project (1979) Leg 64

90O00

32000

60

ND

300

1.4

1660

ND

CI (C 2 + C3)

ND

Total C2-C 6 hydrocarbons (ng/g)

Gravity Core (1972) 2130 149

Total bitumen (pg/g)

ND

0

0

0

0

0

0

0

0

670

ND

302

ND

1i 4

ND

Total C4-C s alkenes

Table 1. Sample designations, description and analytical results

Whelan and Hunt (1982)

<0.01

<0.01

0.46

<0.01

0.12

Whelan and Hunt (1982)

Whelan and Hunt (1982)

Whelan and Hunt (1982)

Whelan and Hunt (1982)

Whelan and Hunt (1982)

0.015 Whelan and Hunt (1982)

<0.01

Whelan and Hunt (1982)

--

ND ND

Simoneit et al. (1979)

Simoneit et al. (1979)

--

<0.1

References Simoneit et al. (1979)

ND

ND

ND

ND

ND

ND

(PI + P2)

PV= PI

0.56

ND

0.046

ND

5.6

ND

C4~:~s alkenes/ YC4-Csn-alkanes

t~t

Z

.gL

C~-Cs hydrocarbons in sediments from Guaymas Basin

particles; and riverine or atmospheric input), generation via either diagenetic or catagenetic processes, migration, and biodegradation. The sediment input processes for the Guaymas Basin area are very similar to those prevailing in marine diatomaceous organicrich sediments from a number of other areas, including the Peru shelf and slope, off-shore Baja California and the Japan Trench (Henrichs, 1980; Honza et aL, 1980; Curray et al., 1982). However, the hydrocarbon generation and migration processes in Guaymas Basin are considerably different compared to these other areas or to conventional petroleum basins, because of sporadic magmatic and hydrothermal heating. Some general characteristics of light hydrocarbon profiles as they are normally influenced by generation, migration, and biodegradation processes are described below in sections corresponding to different classes of compounds. The distributions observed in the Guaymas Basin area are then described and, by comparison with other areas, an attempt is made to constrain the processes which appear to be responsible for the Guaymas distributions.

.8-~.~.~ o E

~ E

E

oo

o~

175

o

E

m.

(C) Total Concentrations of C FCs Hydrocarbons ee~

General

~

m

"d

E

'r"

0

0

o

0

0

0

0

0

0

0

0

~

e~

<

II

A minimum of about 10-80 ppb (ng hydrocarbon per g dry weight sediment) concentrations of C~-C8 hydrocarbons can be found in almost all marine sediments which have not been exposed to geothermal temperatures greater than 30°C as shown for examples from the Peru Shelf, the Japan Trench, and California in Tables 2-5 (Whelan and Hunt, 1983; Whe!an, 1984). Methane concentrations of several orders of magnitude higher are typical of sediments exposed to anoxic methanogenic microorganisms as discussed more fully in Section F. The C2-Cs hydrocarbon distributions in geothermally cold sediments tend to be fairly specific, generally with 0.1-10 ppb each of only a few compounds (generally less than 10) present. The specific compounds vary from one area to another, but a hydrocarbon group tends to occur consistently within any one shallow sediment core and often throughout a particular area. For example, Peru shelf and slope cores BC7 (water depth 87 m), BC4 and BC5 (water depth greater than 400 m) all show very similar concentrations and distributions of various classes of light hydrocarbons (Table 2, Whelan and Hunt, 1983). Selectivity for particular structures in specific areas is very general for most compound classes, as will become clear from examples below. Some of the C5--C8 light hydrocarbons have been detected in macroalgae (Whelan et aL, 1982) and have also been produced in amounts similar to those in sediments via laboratory microbial degradation of terpenoids (Hunt et al., 1980b). The hydrocarbon concentrations and distributions from either of these sources and in most recent sediments are very different from those observed from typical thermal

176

J~N K. WHELANet al. 32'W I

1ll*50'w

;~'W

(o

III'26'W

20'N

22'W

,

(b)

27* 22'N

,/

24'W

04'Nl-i

_

02'N

/ 0c 18'N

¢

o

,

I km

g

ii. I. •

+

-7 I

I6'N

~

y

I

..~ PATCHES OF

- - HYDROTHERMAL SINTER AND MOUNDS I

I

Fig. 2. Locations of hydrothermal vent, piston core, dredge and DSDP sites (Guaymas Basin): (a) North Rift; (b) South Rift. generation processes. Thermal generation typically causes much higher concentrations of individual C2-C8 alkanes (generally by at least one to three orders of magnitude) and much more complex hydrocarbon mixtures (Whelan, 1984; Hunt, 1985). These observations, together with the ubiquity of lesser amounts of specific light hydrocarbon structures in geothermally cold sediments, have led to the conclusion that the low concentrations of light hydrocarbons present in immature (<20°C) fine-grained sediments are generally formed from a combination of in situ microbiological and low-temperature chemical processes (Whelan, 1984). In fine-grained sediments, the geothermal gradient (rather than age or depth) seems to be the most important factor in changing the light hydrocarbon amounts and distributions assuming that sediment type and general levels of organic carbon remain roughly constant. For example, at DSDP Site 467 located off the California coast north of Los Angeles, individual C2-Cs compounds occur in concentrations of 0-3ng/g at 19.5m subbottom and increase to about 10--500ng/g between 233 and 558m subbottom (Table 2, Whelan and Hunt, 1981). In contrast, Site 440 in the Japan Trench, which penetrates slightly older and deeper (maximum depth 780 m subbottom) sediments, exhibits concentrations of individual Cz--Cs alkanes in the range of 0-3.6 ng/g with most compounds being in the range of 0.2-0.5 ng/g. Sediments in both the Japan Trench and offshore California are generally diatomaceous and generally

contain similar amounts of organic carbon (0.5-2.5%). The major difference is that the Japan Trench sediments have been exposed to a lower geothermal gradient (24°C/kin for Sites 434 and 440, and 32°C/kin for Site 438 and 36°C/kin for Site 439, Langseth and Burch, 1982) compared to a minimum of 63°C/kin for offshore California Site 467 and 70°C/kin for Site 471 (Yeats et al., 1981, as summarized in Table 5). Guaymas Basin

Guaymas Basin contains a high percentage of organic-rich diatomaceous sediments, similar to those described for the other areas in Tables 2-5. In the Guaymas area, the general concentrations of individual light hydrocarbons are high--levels at DSDP Site 481A (Figs 3-7) are similar to those observed in the geothermally warmer sediments in Table 2. The surface samples collected from the rift areas by the Alvin are even richer than the Site 481A samples--often by several orders of magnitude (Table 1 and Figs 3-7). Such a large hydrocarbon increase is typical of sediments undergoing active petroleum generation or accumulation (Hunt, 1979; Tissot and Welte, 1984). The general richness of the Alvin samples is even more impressive considering that these samples were observed to lose significant quantities of gas, including hydrocarbons smaller than Ct0 in being brought to the surface. Thus, the high concentrations in the Alvin samples are minimum values.

C ~ s hydrocarbons in sediments from Guaymas Basin

177

Table 2. C4-C 7 hydrocarbons in seafloor and DSDP sediments---total amounts (in ng hydrocarbon per g dry weight sediment) and composition (normal ffi straight chain; bra = branched; cyclo = cycloalkane; gem = two methyls attached to quaternary carbon; and aro = aromatic) Total C 4 ~ 7 alkanes (%) Sample

Total organic carbon (%)

Number

Depth

samples

Normal

Bra

Gem

Aro

Sum C4.~ 7 (ng/g)

4 9 5

18 7 3

28 17 8

83 73 16

4 8 6

7 6 8

31 21 22

48 32 61

19

41

20

53 72 7 25 94 52

30 30 4 15 7 7

40 20 7 0.9

9 13 5 24

Cyclo

Peru---anoxic sediment-water interface SC6 SC12 SC12

4.2-7.9 7.2-9.3 3.5-4.1

0057 cm 0012 cm 21-36cm

BC7 BC4 BC5

2.4-4.0 4.3-5.3 3.6-5.3

0024 cm 009 cm 0-20 cm

436

0.34).8

61-284 cm

13 5 3

41 59 77

15 21 39

Peru---oxic sediment-water interface 8 5 8

48 57 60

21 14 10

Japan Trench----oxic (Mn nodules and fish teeth present) 5

3

19

18

Japan Trench--mixed--diatomaceous material--probably deposited at oxic sediment-water interface 438 438 439 440 440 440

0.5-1 0.4--0.7 0.4-0.7 0.6-1.5 0.54).7 0.6-1.2

37-233 m 332-997 m 893-1093 m 42-165 m 250~57 m 504-619 m

467 468 469 471

2.4 0.41-3.1 0.44-1.1 0.85-1.04

19.5 m 8-407 m 10o388 m 22-410 m

4 10 4 3 5 3

18 9 50 16 1 15

17 7 21 41 1 18

3 4 4 10 I 5

9 8 18 8 3 10

Offshore California and Baja (cold)---Sum C~-C 7 <25ng/g 1 5 7 3

40 27 23 21

14 36 36 14

1 17 30 52

4 0.4 4 13

5 11 13

6 8 6

27 27 36

12 12 25

54

2

11

79

Offshore Baja and Guaymas (coM) 474 & 476 478 479

2.3-3.9 1.7-3.6 2.9-3.5

3.6-58 m 1-109 m

5 5 5

33 24 13

29 30 32

Guaymas Slope (warmer) 479

3.3

314m

1

6

28

Offshore California and Baja (warmer)---Sum C~-C 7 > lOOng/g 467 467 471

2.2-8.6 2.7-4.8 0.65-1.0

233-588 m 690-967 m 593~98 m

5 3 3

478 479 479

0.7-3.6 2.3-3.0 1.4-4.5

123-248 m 153-267 m 360~36 m

3 3 3

477 477 477

2.3-3.0 1.0 0.95

11-50 m 122-126m 154m

4 2 I

17 19 9

56 26 30

18 18 61

1 5 0.2

7 33 0.1

1739 1000 11781

42 18 44

21 59 45

0 4 1

19 12 4

102 115 415

52 16 2

13 9 0

7 0.1 0

5 45 96

16 274 196

Guaymas Basin and Slope (warmer) 17 7 6

Site 477 (warmer)

These sporadic occurrences of high levels of complex light hydrocarbon mixtures are probably due to intermittent magmatic heating which influences both hydrocarbon generation and migration throughout this area, as described in more detail for specific classes of compounds below. For example, Alvin samples 1172-1 and 1177-3 are of comparable richness and are also very similar in overall C6-C7 composition as shown in Figs 3-7. In contrast, some of the near seafloor samples from core 30G contain less hydrocarbons than the Alvin samples, but still show elevated concentrations when compared to the richest Site 481 DSDP samples from the same general area (i.e. 481A-12-2 and 481A-30-5, 150 m and 325 m subbottom, respectively). Both of these 481A samples show elevated light hydrocarbon concentrations when compared to typical seabed sediments (Table 2) or to other Site 481A samples. They were recovered

23 29 2

from the interval immediately above two magmatic sills (172-204 m and 330--336 m, respectively) which intruded into the sediment (Curray et al., 1982) and probably simultaneously generated the higher C6--C7 concentrations by localized heating (Table 1 and Fig. 3b). A water sample collected by the Alvin in the rift area, 1172-IA, although not directly comparable with sediment samples, showed a large amount of a complex hydrocarbon mixture (Fig. 3) consistent with the presence of thermally generated hydrocarbons.

(D ) Proportions of normal to branched hydrocarbons General In normal catagenetic petroleum generation in the temperature range of about 50-200°C in fine-grained sediments, the ratio of normal to branched and cyclic

178

JEAN K. WHELAN et al. Table 3. C4-C7 alkanes and alkenes in representative seafloor and DSDP sediments Sum C4-C 7

Depth

Sample

Number samples

Alkenes (ng/g)

Range

Range

Alkenes/ alkanes

83 73 16

30-187 49-127 11-52

0.09 1.27 13.31

48 32 61

10-48 18-72 10-194

0,18 0,14 0,11

(ng/g)

Peru---anoxic sediment-water interface SC6 SC 12 SC12

0-57 cm 0-12 cm 21-36 cm

13 5 3

BC7 SC4 SC5

0-24 cm 0-9 c'm 0-20 cm

8 5 8

436

61-284 m

7.2 166 213

1.3-21 0-427 40-415

Peru----oxic sediment-water interface 8.5 4.6 6.9

0.03-16 1.6-11 0.30-44

Japan-Trench--oxic (Mn nodules and fish teeth present) 5

51

1.7-123

20

0.1--40

2,55

1~68 2.6-48 5--11 1.6-18 0.2-3.5 4-13

1.23 0.93 7.50 5.27 0.29 2.29

9 13 5 2d

1.7-33 2.4-8.8 21-50

0.03 0.00 0.00 0.00

12 12 25

5.1-25 7-25 4.1-37

0.11 0.34 0.02

Japan-Trench---mixed--diatomaceous--probable oxic sedimentwater interface at time of deposition 438 438 439 440 440 440

37-223 m 332-997 m 893-1093 m 42-165 m 250--457 m 504---619m

4 10 4 3 5 3

467 468 469 471

19.5 m 8-407 m 10-388 m 22-4 10 m

I 5 7 3

474 & 476 478 479

12-59m 3.6-58 m 1-109 m

5 5 5

479

314 m

l

37 28 30 79 2 16

6-.49 0.75-70 10--81 1-123 0.02-4 6-32

30 30 4 15 7 7

Offshore California and Baja (cold, Sum C4-C 7 <25 ng/g) 0.3 0 0 0

Offshore Baja and Guaymas 1.3 4.1 0.6

0.1-1.4 0.9-12 0-2.9

Guaymas Slope (warmer) 2

74

0.03

Offshore California and Baja (warmer, Sum C~-C z > lOOng/g) 467 467 471

233-588 m 690-967 m 593-698 m

5 3 3

478 479 479

123-249 m 153-267 m 360-436 m

3 3 3

477 477 477

11-50 m 122-126 m 154 m

0 0.1 0

04).2

1739 1000 11791

145--3435 27-2291 4500--23570

0.00 0.00 0.00

28-199 72-124 319-478

0.01 O.12 0.00

6.6-32 196-344

0.07 0.00 0.00

Guaymas Basin and Slope 0.9 14 I

0.5-1.6 3-34 0-1.5

102 115 415

Guaymas--Site 477 (bottom hole temp. > 300°C) 4 2 1

1.2 0.4 0

Table 4. C4-C7 alkanes and alkenes in Peru shallow sediments recovered from anoxic bottom water Depth

Alkenes

Core

(crn)

SC6

0-3 3-6 6-9 9-12 12-15 15-18 18-21 21-27 27-33 33-39 39-45 45-51 51-57

SC12

0-2 2-4

(ng/g)

C4-C7 alkanes (ng/g)

Alkenes/ alkanes

5.3 7 5.5 4.6 7.3 19 2.9 7.8 21 1.5 1.3 12.9 6.1

147 120 63 32 61 60 67 187 153 90 65 82 30

0.04 0.06 0.09 0.14 0.12 0.32 0.04 0.04 0.14 0.02 0.02 0.16 0.20

4.6 175

127 56

0.04 3.12

4--6

0

6-9 9-12 12-15 15-18 18-21 21-24 24-30 30-36

427 225 415 379 93 100 251 40

68 62 49 52 36 20 9.2 29 11

0.00 6.89 4.59 7.98 10.53 4.65 10.87 8.66 3.64

0-4.7 0-1.3

16 274 196

C~C7 structures of the same carbon number generally increases with increasing depth (maturity) (Philippi, 1975; Thompson, 1979 and references cited; Hunt et al., 1980; Hunt, 1985; Whelan et al., 1986). This behavior has been shown to be relatively independent of geographic area and kerogen type for the C7 hydrocarbons (Thompson, 1979). At lower geothermal temperatures, iso/n-alkane ratios for C4, C5 and C6 generally all show subsurface maxima which occur at roughly the beginning of the petroleum generation (catagenetic) window (Hunt et al., 1980, 1985; Whelan et al., 1986). Thus, the straight-chain isomer is favored both in colder (less than 20°C) sediments and in hotter (greater than 100°C) sediments. The gem-dimethyl structures (i.e. compounds having two methyl groups bonded to the same quaternary carbon) show a similar bimodal distribution with a maximum in concentration occurring sporadically in surface sediments and a second deeper maximum near the base of the oil generation window

C : C s hydrocarbons in sediments from Guaymas Basin (at about 130°C--below the maximum for the straight chain isomers, Hunt et al., 1980, 1985; Whelan, 1984; Whelan et al., 1986). In geothermally cold immature sediments, specific compounds are thought to be formed by a combination of biological and low temperature (<20°C) processes as discussed in the previous section. These processes generally favor the straight chain isomer as shown for a number of shallow cores from the Peru shelf/slope area in Table 2. In the rapidly-depositing, organic-rich, fine-grained, diatomaceous sediments of the Peru upwelling region, these hydrocarbons are almost certainly formed by in situ low temperature (<20°C) processes (Whelan and Hunt, 1983). Table 2 also shows light hydrocarbon distributions for geothermally cold diatomaceous sediments recovered from the much greater subbottom depths of the Japan Trench. The proportions of branched, cyclic, and gem-dimethyl compounds are roughly equal to those of Peru. However, the Japan sediments generally exhibit a lower proportion of n-alkanes and higher proportions of aromatic structures in comparison with the Peru seabed sediments. Organic carbon levels are generally lower in the Japan Trench sediments (about 0.5-1% compared to 2-12% off Peru). In addition, there is evidence for a high degree of fracturing in most of the Japan Trench sediments (Arthur et al., 1980), so that their organic matter may have been exposed to percolating oxygenated waters (and partial biodegradation?) after initial burial. Support for this hypothesis is the increased freshening with depth observed for pore waters from Sites 438 and 439 (von Huene et al., 1980).

Guaymas Basin and offshore California and Baja The sediments showing low concentrations of C4--C7 hydrocarbons (<25 ng/g) from geothermally colder intervals discussed above (e.g. DSDP Sites 474 and 476, and shallower sections of Sites 468, 469 and 471, Table 2) all contain proportionately more of the straight chain and less of the cyclic structures with concentrations typical of seafloor sediments. The deeper and geothermally warmer sections of the offshore California and the Guaymas Basin sediments (including Site 478, 120-248m; Site 479, 150--440m; Site 467, 233-970m, and Site 471, 593-700 m in Table 2, and some sections of Site 481, Fig. 3), which show higher concentrations of C:C7 hydrocarbons (greater than 100ng/g), also exhibit lower proportions of both normal and gem-dimethyl compounds compared to branched and cyclic structures. The most extreme example occurs in Site 471, 593-700m, from offshore California (Baja) where total levels of C4-C7 hydrocarbons increase to about 12,000 ng/g and branched plus cyclic alkanes account for 91% of the compounds. Note that this site also has a very high geothermal gradient (Table 5; in the range of 70-154°C/km, Yeats et al., 1981). Alternatively, there is evidence that this material may represent migrated thermally-generated petroleum

179

(Simoneit et al., 1980; Rullkrtter et al., 1980), possibly originating from a deep hot source near an intruded diabase sill at 823 m. There is evidence of intense local heating just above the sill with altered metalliferous sediments and upward migration of these light hydrocarbons away from the intrusion. In addition, Yeats and Haq (1981) point out that Site 471 was in a favorable updip position to accumulate gases migrating along thin turbidite sand layers characteristic of this middle Miocene fan sequence. The Guaymas Basin samples having highly elevated concentrations of C4~:~7compounds (i.e. 30G, 331cm; 1172-4; 1177-3; and 1172-1) also generally have a predominance of cyclic and branched structures (Fig. 3). Elevated concentrations of normal alkanes appear in Alvin Guaymas Basin seafloor sediment samples 1172-4 and dredge 7D-3B, shown in Fig. 3a. The higher proportion of n-alkanes in sediment 1172-4 and dredge sample 7D-3B in comparison with other Guaymas samples suggests that the associated bitumens have experienced higher geothermal temperatures than most other Guaymas or California borderland samples examined in this work. An estimation of the approximate degree of maturation represented by these compositional patterns can be made by noting that the iCs/nC5 maximum occurs at a present day maximum geothermal exposure of about 90-100°C in the Texas Gulf Coast COST wells, consistent with patterns observed in a number of other wells (Hunt et al., 1980a). More recent data shows the peak in both iC4/nC4 and iCs/nC5 at a vitrinite reflectance value of about 0.6% in the Alaskan North Slope Ikpikpuk well (Whelan et al., 1986). The age of this section is about 150-200 Myr so that this vitrinite reflectance value would also correspond to a maximum geothermal exposure of about 70-80°C, assuming a response similar to that for the Jurassic sample shown in Hunt (1979, p. 347). Very rapid heating, as occurs in Guaymas Basin, would tend to shift maximum iCs/nC5 ratios to higher temperatures than observed in either of the cases cited above where considerably longer heating times were available.

(E) Alkenes General Previous work in many laboratories has shown that alkenes are not produced during normal petroleum generating processes (Hunt, 1979; Tissot and Welte, 1984). C4--C8 alkenes are also generally not found in more than trace amounts in immature DSDP sediments recovered from depths of 10 m or greater (Whelan, 1984). Some of the alkenes, such as ethylene, have known biogenic sources (Gerarde, 1962; Stotzky and Schenck, 1976). Ethylene has also been detected in hydrothermal vent waters from the East Pacific Rise but not in gas pockets from Guaymas DSDP Core 477 by Welhan and Lupton

0-24 crn 0-9 crn 0-2 em 2-20 cm

61-284 m

37-223 m 332-997 m 893--i 093 m 42-165 m 250-457 m 504-619 m

19.5 m 8-407 m 10-388 m 22-124 m

BC7 BC4 BC5 BC5

436

438 438 439 440 440 440

467 468 469 471

126 96 96 220 220 220

123--248 m 153--267 m 360--436 m

11-50m 122-126m 154m

478 479 479 Site 477 477 477 477

63* 63* 70

233--588 m 690-967 m 593--698 m

467 467 471

96

314 m

114 and > 7 0 126 96

63" ND ND 70

32 32 36 24 24 24

ND

ND ND ND ND

ND ND ND ND

Geothermal gradient (°C/km) C2

.

.

.

.

.

~

C3

n C4

7.2 27 4.4 2.8

0.3 3.3 3.2 4.6

.

.

1 1.6 1.2 1.5

0.75 0.2 0 0

0.44 0 0 0

0 28 1,8 3.2

n C5

4

0,12

0.06

0

0

japan-Trench--oxic (Mn nodules and fish teeth present)

0,71 1 1,4 1.1

Peru---oxic sediment-water interface

8 18 4.7 4

.

.

.

.

0.38

8.5 16 113 6,6

28 38 26 2.8

n C6

ng hydroearbon/g dry wt sediment

Peru--~noxic sediment-water interface

7.3 8.6 31 18

46 35 14 23

CI

.

4 2 1

3 3 3

5 3 3

1

5 5 5

1 5 7 2

4 10 4 3 5 3

2.8 1.7 9,5 6 1 2

2.4 0.65 2 0.88 0.29 0.7

3 0,25 0.66 0.77 0 0,11

2 0.57 1,8 0,34 0 0.07

1.9 2.4 0.26 1.8

0.5 0.38 0 0

1.9 1.3 4.2

64

16

i .6

0.67 0.05 0,72

Guaymas Slope (warmer)

1.4 1.8 10

Offshore Baja and Guaymas (cold)

3. i 2 0,65 2

1.1

0.5 0 0,93

0 0,4 0 0

419 428 522

2951 1010 963

2008 52 31

698 106 78

189 59 262

7 188 1046

12 10 71

3.4 t92 140

13 6.8 53

1,7 51 3.2

4.2 0.47 7,3

GuaymasBasm and SIope (warmer)

220 40 17

0.88 5.8 0

4.7 3.4 8.6

73 45 410

Offshore California and Baja (warmer--Sum C~C 7 > lOOng/g)

2085

154 16 1851

29 13 9 22

Offshore California and Baja (coM--Sum C~-C7 <25ng/g)

636 483 39 586 413 617

0.7 6.2 0

2.8 2,8 5.2

19 14 202

1.2

1.2 0.96 0,62

0.7 0.78 0.38 0.25

1.2 0.52 0.16 0.04 0 0.48

Japan-Trench--mixed---diatomaceous material--probable oxic sediment-water interface at time of deposition

5

8 5 2 6

13 1 4 3

Number of samples

0.53 15 0

2.3 1.5 2.7

iI 8.6 117

0.84

0.6 1.9 0.7

0.4 1.1 0,79 2

1.4 0,82 0.05 0.18 0.29 0.25

0.36

1.1 3.7 12 0.69

2,8 27 2.4 2,4

n C~

=

Table 5. Concentration of straight chain C , - n C s alkanes in representative seafloor and D S D P sediments

479

474 & 476 12-59m 478 3.6-58 m 479 1-109m

0-57 em 0-2 cm 2-12 em 21-36

Depth

SC6 SC!2 SC!2 SCi2

Sample

_ :

0 0.5 0

0.17 1.3 2,6

8.7 1.2 45

1.6

0,06 0.22 0

0 1,1 0.33 0.75

ND ND ND ND ND ND

ND

ND ND ND ND

ND ND ND ND

nCs

60 2.3 0.5

246 101 14

9.1 1.3 1.8

33

110.00 8.89 185.10

9.35 6.50 13,85 11.00

227 284 4.1 98 413 308

10 8.6 22 16

5,8 1.9 3.0 5.8

C~/C2

C~-C s hydrocarbons in sediments from G u a y m a s Basin

e q N

~ .-. ~ ,~.

i.~

~.

-q,~

m

m

m

@ ^ L~

l

,m,

---~,

_<

t'~

~ ~ ~, ~ ,

i

181

(1987). They propose that the compound may be produced by the extensive biota at the East Pacific Rise and is not a primary vent product. It has also been shown that low molecular weight alkenes tend to concentrate at the sediment-water interface in areas containing oxic bottom waters such as the Gulf of Maine (Whelan, 1984; Scranton and Whelan, 1987) and the Arabian Sea (as summarized in Whelan, 1984). It was also found that these compounds concentrated at pore water microbial redox boundaries in Peru shelf and slope shallow cores (Whelan and Hunt, 1983). For example, sharp maxima in alkene profiles generally occur at the sediment water interface and in deeper sections at about the depth where pore water nitrate concentrations dropped to less than 3 #M. Table 3 shows concentrations of alkenes in comparison with C4-C7 alkanes for the same typical marine surface and DSDP sediments described previously. The ratio of alkenes to alkanes is typically greater than 0.1 in all Peru seafloor sediments~ including those recovered below both oxic and anoxic bottom waters. Much higher ratios occur in SC12 (Table 4), recovered from the edge of the anoxic bottom water zone. The highest ratios occur in the deepest SC12 samples below about 12 cm where alkene concentrations remain about the same, but the C4-C7 alkane concentrations have generally decreased. However, total C4-C7 alkanes remain the same or increase slightly in SC6, while both total concentrations of alkenes and the ratio of alkenes to alkanes decreased in SC6 as compared to SC12. SC6 was recovered from the center of the anoxic bottom water zone. These data taken together have been interpreted to mean that the alkenes, which occur widely in geothermally cold recent sediments, generally have a biological source (Whelan, 1984). Based on the relation of sediment light hydrocarbon profiles and the associated pore water nutrients in both Peru (Whelan and Hunt, 1983) and the Gulf of Maine (Scranton and Whelan, 1987), it was further concluded that these low molecular weight atkenes are diagnostic of aerobic or suboxic (rather than anoxic) microbiological sediment processes. It was mentioned previously that the major difference between the two anoxic Peru cores SC6 and SCt2 is that the latter was recovered from the edge and the former from the center of the current anoxic bottom water zone. Therefore, because the anoxic zone moves to some extent over time, the sporadic high alkene concentrations in SC12 (as compared to SC6, Table 4) may be the result of periodic exposure of sedimentary organic matter to oxic conditions at or near the sediment-water interface. This scenario suggests that the high alkene content in SC12 may be the result of partial biodegradation of the sedimentary organic during periods when bottom waters were oxic to some extent. Such aerobic generation of low molecular weight alkenes would also explain why

182

JEAN K. WI-I~LANet al.

3b GUAYMAS

BASIN

C6-C7

-

481 A-311-'5

L4le .NIl ,2N -IN

481A

.4E .U .2g .111

325M

3a GUAYMAS BASIN- C6-C7

!

241 lm

"!

810 212 11;0

448

287M

[]

401^-24-$ 268M

I

.15 .1@ .S

48|A-~'~-4 , 1 | 4 248M '78 []

315 2111

i

I !i

'~'"

!

tl 77-3

..1 1 7 N

1175~.1

~

248O11

481A-12-2 150M D

I

I

CLASS

481A-~2 111M 481A-4-2 73M

i

g CO,POUNO

481^-1f~2 130M

COMPOUNC}

In

D

.lg8 '75 .58 .25 '9 "8 .4 "2

CLASS

Fig. 3. Composition of headspace C6-C7 hydrocarbons as compound types ("cyclo"= cycloalkanes; "BrAlk" = branched alkanes; "nAlk" = normal (straight chain) alkanes; "ARO" = aromatic compounds; "GEMDM" = gem-dimethylalkanes). Subbottom depths are indicated. these compounds tend to collect near the sediment-water interface in areas where sedimentary organic matter is exposed to oxic bottom waters. Shallow samples from DSDP Holes 474 and 476 (recovered from less than 60m subbottom) are probably the most representative geothermally cold (maximum geothermal exposure <20°C; Curray

et al., 1982) California (Baja) area cores shown in Table 3. Predominantly diatomaceous sediments from both sites were deposited in oxic bottom waters (Curray et al., 1982). Concentrations of alkenes and alkanes are somewhat lower than for the oxic Peru cores. However, the ratio of alkenes to alkanes are almost identical to values for the oxic Peru cores.

4b

4a GUAYMAS

BASIN

Ip

lJ

3~tC[]

30G

-

-IN

I

GUAYMAS

•

30G289CId

BASIN

DSDP

482^-38-5 325M []

~

;a 481A-2~-5

287M

"48

I

n

o

,o:,

.

"28 m

"N

481A-24-5 268 M

.888

m

.

.

,

i,iin m

•

J tt72-t4 (Liquid)

481A-22-4 248M

'4N C] ~.

~T2-4

tt77-3

n

In

a

[]

R

m

,INg m

.

_ tt72-1

n

[]

.

m

-

481^-8-2 (1ttM) ~

Fig. 4. Composition of headspace gas--straight chain alkanes.

B

.158 qfJe .5B .150

1 5 g ~. IN D .58 \

58

481~-11F-2 (t30M)

m

,48RI

'IN

481A"-I2-2 t49ld

.l,,m .l|a .58

ISII IU M N N

Cc-C s hydrocarbons in sediments from Guaymas Basin

183

5b GUAYMAS

BASIN

OSDP

4BIA'3g"5 5a

GUAYHAS BASIN

III

N m im

~

_

.

481A-28-5

El

481^-24-5

'144 "llle .72 .29

"15 ,Ill '5 "12 "8

.

pj

El

"4

. 481^-22-4

1

1•

7

2

.

1

A

~

--

R

n

--

n

[]

--

PJ

m

.

481^-12-2

ge )-

"48 >,211 n,, Q ,525 t~ .35g

M

"175

o

481A-Ill-2

Z

' IBI- 15111

zm

"Se

481^-8-2

me 41IN 2ram

"388

"2H 'lg 481^-4-2

3~g

15ira

'2B '15 .IB

CZ ~

COMPOUND

_

COMPOUND

Fig. 5. Composition of headspace gas--branched alkanes ( " i C 4 " = isobutane; " i C 5 " = isopcntane; "23DMB" = 2,3-dimethylbutane; "2MP" and " 3 M P " = 2and 3-methylpcntane; " 2 4 D M P " = 2,4-dimethylpentane; " 2 M H " = 2-methylhexane; "23DMP" = 2,3-dimethylpcntane; " 3 M H " = 3-methylhexane; "3EP" = 3-ethylpentane; "25DMH" and 24DMH" = 2,5and 2,4-dimethylhexane; "234TMP" = 2,3,4-trimethylpentane).

6b

6a

GUAYMAS

BASIN

GUAYMAS BASIN OSDP 481A-~-5

I

N

mN 11

,_

_

H

H

li

.,,. .

H

t172-tA (Liquid)

n

J

o.

44 22

::

'";

.N >" ,mz Q~

88 60

4B|^-2~5

481^-24-5

I

,

nl

8 8

4 2 L

.

3

481A-22-4 -ZO

"°

Hn

. 481^-12-2

-5

-411 ,29

t177-3

]1

i

~-.

_,i !i I

481A-IJ-2

-Bg .BID -411

481^-8-2

-

Fig. 6. Composition o f headspace gas--cycloalkanes ( " C P " =cyclopentane; " C H " =cyclohexane; " M C P " = methylcyclopentane; " M C H " = methyicyclohexane; " I C 3 D M C P , 1T3DMCP, IT2DMCP and I C 2 D M C P " = dimethylcyclopcntanes; " C " = cis and "T" = trans methyl groups; " E C P " = ethylcyclopontane; "1C2T3T" ffi 1-cis-2-trans-3-trimethylcyclopentane).

-611

-411 • 211

Q 0

184

JEAN K. WHELAN et al.

7b

GUAYMAS BASIN

I

7a GUAYMAS

BASIN-

GEM D I M E i

n

I

,

831CM

DSDP

,75 .-.a

-

GEM D I M E

[

" " " 41J1A-28-5

l

25904

48|A-24-5

,75 ,u .25

7D-3B

~.Ts ,5 .25

,25

I

fl

481A-22-4

,21D .15

.75 .S .25 3 2 !

::' ~, R

1172-tA (Liquid)

.I n

I

,

2 l 4111A-1|-2

1t7Z-4

.

o

.IIU

n ~t77-3

.

481^-12-2

I

.1411

ft t172-1

.4211 .2118 .14i

COMPOUNO

I

lIB >r~ Q % 7

1.5

[]

481A'-8-2

II

m

481A-4-2

' COMPOUND

Fig. 7. Composition of headspace gas--gem-dimethyl alkanes ("neoC5" = neopentane; "22DMB" = 2,2-dimethylbutane; "22DMP and 33DMP" = 2,2- and 3,3-dimethylpentane; "11DMCP" = I,l-dimethylcyclopentane; "224TMP" = 2,2,4-trimethylpentane). Most of the other California and Guaymas DSDP sites shown in Table 3 (including those listed under "cold") have experienced a slightly greater degree of thermal stress, as calculated from geothermal gradients in Table 5, and are more typical of DSDP sediments from other areas which generally contain little or no aikenes (Whelan, 1984). In the deeper California sections shown in Table 3, the small amounts of alkenes generally either remain constant or decrease--often to zero--while the alkanes increase exponentially in the same intervals as sediments are exposed to the higher geothermal stress. It was mentioned previously that alkenes are generally not observed in DSDP sediments recovered from subbottom depths greater than about l0 m (Whelan, 1984; Hunt, 1985). DSDP sediments in the Japan Trench are the exception (Table 3; Whelan and Hunt, 1980). Abnormally high alkene/alkane ratios were found throughout the geothermally cold (see Table 5) Japan Trench sediments. For example, sediments from Site 436 were almost certainly deposited under oxic bottom water conditions as indicated by the presence of manganese nodules and fish teeth throughout these cores (Langseth et al., 1980). Thus the high ratio of alkenes to alkanes could also have been produced by aerobic microbial degradation of sedimentary organic matter at or near the sediment-water interface. In addition, Sites 438, 439, and 440 from the opposite trench wall, which also generally show high ratios of alkene and alkane, were probably also deposited under oxic bottom water conditions as evidenced by the common occurrence

of pyritized burrows (Langseth et al., 1980). There was also extensive fracturing of sediments at these sites (Arthur et al., 1980), as well as strong freshening of pore waters with depth at Sites 438 and 439 so that some sections may have been exposed to oxygenated water (and possibly also aerobic bacteria) after burial. The fracturing was caused by tectonic stresses which occur throughout this plate subduction zone. In some cases, such as at Site 438, high levels of both alkenes and neo-pentane (a gem-dimethyl alkan¢ as discussed earlier) were found to be associated with zones showing rehealed sediment fractures (Whelan and Hunt, 1980). These results suggest two patterns which can be used to evaluate the sources of low molecular weight alkenes in Guaymas Basin samples:

(1) Small amounts (less than 100 ng/g) of alkenes are often produced in geothermally cold sediments. Concentrations decrease both absolutely and in proportion to alkanes as sediments experience even mild geothermal heating. (2) One source of these alkenes appears to be formation via partial aerobic degradation of sedimentary organic matter, generally at or near the sediment-water interface, or in association with micro and macrofauna as may be occurring near the East Pacific Rise vents (Welhan and Lupton, 1987). Alkenes can remain at depth as relics of oxic processes which occurred at or near the sediment-water inter-

C ~ s hydrocarbons in sediments from Guaymas Basin face in sediments which are not exposed to geothermal temperatures higher than about 20°C. Guaymas Basin

The Guaymas Basin sediments which have experienced the highest degree of hydrothermal heating (greater than 300°C) in the rifts, namely those from deeper sections of Site 477, also show little or no alkene (Table 3). Similar results were obtained for the DSDP gas pocket C,--C5 hydrocarbons from Site 477 by Welhan and Lupton (1987). Table 1 shows total levels of C4--Cs alkenes and the ratio of alkenes to alkanes for other Guaymas DSDP and Alvin samples. The highest sediment ratio occurs in the shallow part of gravity core 30G which, based on total C2-Cs alkane concentrations and distributions, as discussed above, and the biomarker data (Simoneit et al., 1979), is the section most likely to have been influenced by partially biodegraded hydrocarbons (cf. next part). The alkene to alkane ratio of 5.6 is about the same as that found in the upper sections of one very organic-rich Peru gravity core recovered from the edge of the anoxic bottom water zone (SC12, Tables 3 and 4) as described previously. The alkene to alkane ratio then decreases to lower values in deeper parts of Guaymas gravity core 30G. Even though concentrations of C4--Cs alkenes increase with depth in this interval, the total saturated Ca~8 alkanes increase even faster so that the aikene to alkane ratio undergoes an overal decrease. Comparison of Tables 1 and 3 also shows that total alkene levels are generally higher for the deepest sections of core 30G than for any samples shown in Table 3. In the Alvin samples, the total alkenes increase to very high concentrations (Table 1)--about 2 orders of magnitude higher than for any other samples in Tables 1 or 3. However, the total alkanes also increase by a comparable amount in the same interval so that the alkene to alkane ratios remain in the range of 0.3-1, as observed for shallower organic-rich sections of Peru core SC12 and for most of the Japan Trench sediments. This pattern, including the ratio of alkenes to alkanes, reminiscent of oxic marine sediments as shown in Table 3, suggests that the aikenes arise from partial aerobic biodegradation of the petrogenic alkanes or asphaltenes which result from exposure of organic-rich surface sediments to the high hydrothermal stress of magmatic processes. Thus, DSDP Core 481A as well as most other geothermally warm Guaymas Basin sediments (e.g. Site 477 and deeper sections of Sites 478 and 479), which were not exposed to oxic waters after catagenetic hydrocarbon formation show little or no alkene signal (Table 3). In the case of DSDP Core 481A, the high alkane levels observed in some sections, such as those at 149, 248, and 325 m subbottom (Table 1), were probably caused by localized heating of organic-rich sediments

185

by diorite intruding into the sediment after burial. The fact that no alkene is detected in the same sections suggests that either a similar heating process is not responsible for alkene production in the Alvin samples or that initially produced alkenes undergo rapid reduction within sediments not subsequently exposed to oxygen. Alternatively, it has been suggested that higher molecular weight alkenes in the Guaymas Basin samples are produced as part of an in situ pyrolysis process of n-alkanols (Simoneit and Kawka, 1987; Kawka and Simoneit, 1987). However, there is no analogy for such a process either in natural petroleum generation or in laboratory hydrous pyrolysis of sedimentary organic matter (Hunt, 1979; Tissot and Welte, 1984; Lewan, 1985), where no more than traces of alkenes can be detected. Magmatic or hydrothermal processes in Guaymas Basin can be considered as a comparable in situ hydrous pyrolysis process occurring under pressure in the presence of water. Therefore, by analogy, alkene production would not be expected. In addition, the higher molecular weight alkenes which are produced in the laboratory in "open-system" pyrolysis experiments are predominantly terminal (e.g. van de Meent et al., 1980; Simoneit and Philp, 1982; Simoneit et al., 1984; Whelan et al., 1983a) rather than "in-chain" as found among organic-rich hydrothermal deposits (Simoneit and Kawka, 1987). Thus, these alkene structural differences also suggest that if the Guaymas Basin alkenes are the result of a pyrolysis process, it must be of an unusual type. (F) n-Alkanes--Distributions of Individual C j--Cs Compounds General

Some typical distributions of individual straight chain C~-nC8 hydrocarbons are shown in Table 5 for the same set of seafloor and immature DSDP samples discussed previously. In DSDP sediments subjected to the influence of methanogenic bacteria, the concentrations of sorbed gases are typically C~ much greater than C: to nC8 as shown for Japan Trench sediments. The individual C2--C8hydrocarbons generally occur in approximately equal concentrations, often with a slight preference for a few specific compounds, such as nC6 and nC7 in the case of the Japan Trench Site 436 samples; ethane in Japan Trench Site 439 sediments; and nC6 in most of the Peru seafloor sediments (Table 5). As discussed above, this type of selectivity is typical of biological but not of catagenetic petroleum generation proceSseS.

High C~/(C2+C3) ratios (> 1000) are typical of organic rich sediments which have been influenced by methanogenic microorganisms (Claypool and Kvenvolden, 1983). In anoxic sulfate-free or low sulfate sediments beneath a water column of sufficient depth to inhibit bubble formation, the biogenic methane

186

JEANK. WHELAr~et aL

content can be as high as 33,000-1.3 million ppb (Claypool and Kvenvolden, 1983). In the case of sediment-sorbed gases, a C~/C2 ratio of 100 or higher is typical of sediments exposed to methanogens, as shown in Table 5 for Japan Trench sediments and as summarized in Whelan and Hunt (1983). The table also shows examples where the C~/(C2 + C3) ratios are generally low in the surface Peru sediments, probably because these samples are shallower than the methanogenic zone. Examples of typical sorbed gas C~/C2 ratios from geothermally cold areas are shown in Table 5 for the Peru sediments, Japan Trench Site 439, and the shallower sections of California and Guaymas shelf Sites 467, 468, 469, 471 and 478. Because the C2 and C3 concentrations remain low in all of these cases (generally less than 10ppb), low ratios of Cz/C2 together with low amounts of sorbed C~--C3 are thought to be typical of low temperature processes rather than catagenetic petroleum generation (Claypool and Kvenvolden, 1983; Whelan, 1984). It is possible that Peru sediments (as well as other sediments examined in this work) may have preferentially lost some of the sorbed methane initially present (in comparison to the heavier constituents) during sample collection and storage. This effect, which would be expected to be greater for unconsolidated shallow (Peru) sediments, than for the more lithified DSDP cores, has been shown experimentally and discussed previously (Whelan et al., 1984). Such a selective loss would explain the lower sorbed Cl concentrations observed for the Peru cores in comparison to higher DSDP Japan Trench and California borderland values. This effect may cause some decrease in the observed ratio, but is not believed to invalidate the data for qualitative purposes, because much larger sorbed methane concentrations have been observed in other unconsolidated seafloor sediments. For example, methane concentrations of at least 100 times higher than those observed in Peru were obtained for shallow cores from two other geographical areas--Walvis Bay off the Southwest African coast (Whelan et al., 1980) and the Pettaquamscutt River in Rhode Island (Whelan et al., 1983b). In both cases, sorbed C~--C3 concentrations were measured in sediments which had been recovered, stored, and analyzed under conditions comparable to those used for the Peru cores. It can be seen in Table 5 that concentrations of individual C2~s n-alkanes are about the same in both shallow and deep sediments not exposed to geothermal temperatures in excess of 20°C. Thus, the levels of individual C2~s n-alkanes are about the same in the Peru shallow cores, all of the Japan Trench DSDP sediments, and in the shallower (geothermally colder) sediments from the California borderland DSDP holes. These similarities in concentration are consistent with generation by similar biological, low temperature, chemical processes in all three cases. Microbiological methanogenesis is then

superimposed in sections where conditions appropriate to anaerobic methanogenic bacterial growth are present (i.e. adequate organic carbon and pore space; absence of pore water sulfate; etc.--see Rice and Claypool, 1981; Claypool and Kvenvolden, 1983). The overall result is a large increase in methane compared to the C2-Cs hydrocarbons in immature sediments where methanogenesis is important. Guaymas Basin and offshore California

As the sediments are exposed to higher thermal regimes, the amounts of C2-nCs generally increase by at least an order of magnitude when compared to the shallower sections of the same holes. For example, compare the deeper (360-436 m) with the shallower sections (1-109 m) of DSDP Hole 479 (Guaymas Slope, Table 5). The increase in geothermal heating required to cause this change is not great, i.e. the geothermal gradient at this site is 96°C/km (Curray et al., 1982, p. 439), so that the maximum temperature to which any of the sediments were exposed was about 42°C, less than 50°C, the temperature generally accepted as the minimum for the onset of petroleum generation (Hunt, 1979; Tissot and Welte, 1984). In the Guaymas Slope and offshore California sediments shown in Table 5, as well as in those from other areas with comparable organic carbon types and amounts, a similar small but significant elevation (about 10 times) of C2-C8 hydrocarbons is almost always observable at a sediment depth which would correspond to a geothermal temperature of about 30°C (Table 5; Whelan, 1984; Whelan and Hunt, 1983). This mild heating process does not seem to favor any individual carbon range--all of the C2 to n Cs concentrations seem to increase together compared to the scatter often observable in shallower sections of each hole, as can be seen for the California borderland sections labelled "warmer" in Table 5. This pattern is typical for early stages of catagenetic petroleum generation (Hunt, 1979, Tissot and Welte, 1984). The light n-alkane distributions in the shallowest sections of Guaymas Basin Site 477 (11-50 m) show high methane and low C2-Cs concentrations, patterns very similar to those observed for geothermally cold sediments discussed above and shown in Table 5. For example, the concentrations observed in this section, above a thick dolerite sill from 60 to 105 m, are very similar to those of the 37-223 m interval of Japan Trench DSDP Site 438. This evidence suggests that these shallow Site 477 samples have been subjected to minimal geothermal heating. The next two deeper Site 477 samples, from 122 to 126m and below the sill, show increased concentrations of the individual C2-Cs compounds-comparable to the highest levels occurring in the "warmer" offshore California samples (e.g. the 233-588 m section of Site 467). This pattern is consistent with thermal production in this section. The

Ciq2s hydrocarbons in sediments from Guaymas Basin similarity of the 122-126 m values to those of the deeper Site 467 samples suggests that the heating, whatever its source, has been relatively mild (less than 50°C, calculated from the measured geothermal gradient of 220°C/kin, Table 5). The pyrolysis production index, PI, values of about 0.2 in the site 477 (122-126m) samples are considerably higher than values of less than 0.1, which would be expected considering the low geothermal temperatures. This inconsistency together with elevated amounts of thermally produced bitumen in the same section (Simoneit and Philp, 1982; Simoneit et al., 1984) suggest that intruding magma favored thermal hydrocarbon production, but that subsequently the light hydrocarbons were partially swept out of the sample--possibly by upward circulating hydrothermal fluids. The deepest Site 477 sample at 154m shows elevated concentrations of C2 and C 3 but not of nC4-nC 8. In this case, the high levels of C2 and C 3, as well as aromatics (discussed in detail in a later section) and the absence of any other C5-C8 hydrocarbons is an unusual distribution compared to any other sample examined in our own work or reported in the literature. A downhole temperature measurement at 168 m subbottom showed a minimum temperature of 125°C and a maximum of past thermal exposure of > 200°C based on greenschist facies in the core (Curray et al., 1982, p. 235), so the absence of C5-C7 can be explained on the basis of in situ geothermal temperatures. It is also possible that this unusual mixture of light hydrocarbons is produced at much higher temperatures at depth and then migrates upward. This suggestion is consistent with data of Simoneit and Galimov (1984) and Welhan and Lupton (1987), which, based on isotopic evidence, shows that the low levels of methane present at this depth are thermally generated. The light n-alkanes at DSDP site 481 clearly show the thermal effects of the intruded sills (Fig. 4 and Table 5). The three shallowest samples from 6.6 to 73.1 m have concentrations and n-alkane distributions typical of geothermally cold marine sediments. The next three deeper samples from 111 to 149 m and at 247 m, just above a dolerite sill, display elevated levels of C 2 to n C6, consistent with a localized heating effect. The localized character of the thermal stress is also evident from the fact that the C2-nC 8 concentrations decrease from 268 to 287 m and then show another increase in the 325 m sample just above a second sill. Organic carbon values within the same interval decrease on approaching the sill (Curray et al., 1982, p. 272). It should also be noted that the total amounts of the sorbed C2-n Cs alkanes for Site 481 samples above the sills are abnormally high considering just geothermal gradients (i.e. downhole temperature estimated to be less than 25°C for all three samples above the 170-200 m sill based on the measured geothermal gradient of 129°C/km, Table 5). Geochemical evidence of similar localized inO.G 1 2 / 2 ~

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trusive heating has been discussed for a number of other gas and pore water constituents in Curray et al. (1982, pp. 266-272) and for a different area by Simoneit et al. (1981) and Peters et al. (1979, 1983) for a magmatic sill intruded into organic-rich sediments of DSDP Site 368, off the Northwest African coast. The 259 cm section of gravity core 30G (Fig. 4), also taken from the Northern Rift area (Fig. 1), shows a relatively low C1 concentration and C2 to n Cs levels which are in the same range as observed for Peru seabed core SCI2 recovered under anoxic bottom water. Therefore, it is concluded that thermal effects are minimal for this section. In contrast, the 331 cm section shows high concentrations of nC4 to n C6, which are one to two orders of magnitude higher than any values shown in Table 5, typical of a thermal production source. Because all other organic geochemical indicators for this section are typically immature (Simoneit et al., 1979), it is concluded that nC4 to nC6 migrated into this section. Dredge sample 7D-3B, from the northern section of the Southern Rift (Fig. 2) shows a similar but smaller elevation of C2-nC7 alkanes. However, the extractable bitumen for 7D-3B was also abnormally high for this seabed sediment and the n-alkanes have a CPI of approximately one, typical of mature petroleum (Kawka and Simoneit, 1987). Thus, the distributions of C2-n C7 hydrocarbons in this sample are consistent with those of the C~u+) fraction in indicating a thermal origin. The distributions of C~ to nCs alkanes for the Alvin samples recovered from the Southern Rift area (Fig. 2) are also shown in Fig. 4. All of these samples were very unconsolidated as compared to cores and evolution of gas bubbles was observed visually during sample recovery. The resulting gas losses, which would be expected to affect the lightest hydrocarbons preferentially, mean that all of these samples are probably depleted with respect to C~ and C2 compared to the cores. However, qualitative comparisons between the Alvin samples can still be made by assuming that all have been affected about equally by this degassing. The concentrations in the 1172 and 1177 samples are generally very high (compared to the values in Table 5). In particular, sample 1172-4 exhibits low proportions of CI and C2 compared to C3 to nCs, typical of thermal (catagenetic petroleum) hydrocarbon generation. Sample 1177-3 has lower concentrations of light hydrocarbons with a predominance of CI, followed by nCs, while both samples 1172-1 and I 177-3 show high levels of methane compared to C2-nCs components. The high proportion of methane, in spite of sample recovery degassing, suggests that all three samples have a significant contribution of microbiological methane. The concentrations of C~ to n Cg constituents, which are higher than any of the values shown in Table 5, are typical of thermal hydrocarbon generation, even though they are lower

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than the maximum concentrations observed in 1172-4. However, this may be an artifact of sample recovery degassing. It is difficult to relate the n-alkane concentrations in liquid 1172-1A (water sample) to those of the sediment samples discussed above because the hydrocarbon values had to be normalized to total water weight rather than to dry sediment weight. The n-alkane concentrations are very high compared to values in Claypool and Kvenvolden (1983) and the distribution is very similar to that in sample 1172-1. In this case, the high proportion of methane in this sample may be due to preferential redistribution of this lightest alkane into the aqueous phase. (G) Aromatic Compounds General

Benzene and toluene, which are ubiquitous in surface and deeper sediments, have a number of potential sources which are similar to those of other C4-C8 alkanes as described above. They are also often observed not to covary with the saturated alkanes, both in surface sediments such as off Peru (Whelan and Hunt, 1983) Walvis Bay (Whelan et al., 1980) and the Pettaquamscutt River, Rhode Island (Whelan et al., 1983b), and in deeper more mature sediments undergoing catagenetic petroleum generation (Thompson, 1979). This observed lack of correlation with other C4--C8 hydrocarbons has several potential causes. Different sedimentary organic matter types probably differ in their ability to generate benzene during catagenesis. In addition, there are two major differences in the basic physical and chemical properties of saturated compared to the aromatic hydrocarbons which can also contribute to fractionation. The aromatic hydrocarbons are 104-105 times more water soluble than the C3-C8 alkanes at 25°C (McAuliffe, 1966). They also show greater binding to both polar liquids and solids compared to the saturated hydrocarbons, as shown by differences in octanol-water partition coefficients (Leo et al., 1971; Lyman et al., 1982) and relative polarities in liquid chromatography (Snyder, 1970). Therefore, even if alkanes and aromatics were initially produced together in a constant ratio, fractionation would be expected as the compounds moved away from the generation source. Evidence has been presented that such fractionation occurs in petroleum migration (Thompson, 1979; Leythaeuser et al., 1987). In addition, the aromatic hydrocarbons are products of combustion (and other high temperature disproportionation processes) and can have significant anthropogenic and aeolian sources (LaFlamme and Hites, 1978, 1979). The aromatic hydrocarbons are also favored during some types of secondary pyrolysis processes, such as thermal breakdown of kerogen in the presence of certain types of catalytically active mineral matrices (e.g. montmorillonite and illite; Larter, 1984 and references cited). The aromatics frequently show subsurface maxima

in recent sediments recovered from organic-rich anoxic sequences. For example, samples recovered from anoxic bottom waters of both Walvis Bay (Whelan et al., 1980) and the Peru Shelf (Whelan and Hunt, 1983) display this behavior. In both cases, the shapes of the aromatic hydrocarbon depth profiles are not typical of diffusion from a deeper source. The high deposition rates in these upwelling areas together with the fine-grained nature of the sediments makes diffusion from deeper underlying petroleum reservoirs unlikely. Pollution would also be expected to be minimal in these remote regions. Thus, in situ biological-low temperature (less than 20°C) chemical processes occurring in anoxic sediments have been proposed as the most likely cause of these particular aromatic hydrocarbon profiles (Wbelan et aL, 1980, 1984). Some possible low temperature chemical processes by which such generation might take place from naturally occurring precursors have been described (Whelan et al., 1980). Table 2 shows aromatic hydrocarbons as a proportion of total C4-C7 for the same reference sediments which were discussed earlier. This percentage remains roughly constant in all of the Peru cores including those recovered under both oxic and anoxic bottom waters, even though total C4-C8 are generally somewhat higher in sediments recovered from anoxic bottom waters. In contrast, the aromatic hydrocarbons in the geothermally cold Japan Trench sediments show much wider range (7-94%) and sometimes constitute a much higher percentage (by a factor of 2 or 3) of the total C4--C7signal even though total levels are generally lower than for Peru. Sediments recovered from the Japan Trench, Peru and the "cold" offshore California Baja, and Guaymas sediments shown in Table 2 are similar in being largely diatomaceous and geothermally cold. However, the Japan Trench sediments were highly fractured and rehealed--particularly at Site 440--due to the tectonic stress in this plate subduction zone (Arthur et aL, 1980). In addition, pore waters for Japan Trench Sites 438 and 439 showed some freshening with depth (Moore and Gieskes, 1980). Therefore, sedimentary organic matter may have been subjected to a higher degree of subsurface water movement than occurred in either the Peru or California cases. The enhanced aromatic to saturated C4--C7 hydrocarbon signal in the Japan Trench seems to be associated with sections that have experienced a greater degree of subsurface water percolation. Subsurface water movement could have affected aromatic/alkane ratios in two ways: (1) Water preferentially moved the aromatic hydrocarbons or, (2) subsurface water movement may have carried aerated waters and bacteria into the subsurface causing preferential biodegradation of the aliphatic structures. Aerobic biodegradation of hydrocarbons with preferential degradation of low molecular weight n-alkanes has been well documented (McKenna and Kallio, 1965; Ahearn and Meyers, 1973, among others).

C~-Cs hydrocarbons in sediments from Guaymas Basin Guaymas Basin and other California sediments The percentage of C4--C7 aromatics in the geothermally cold California Baja, and Guaymas sediments are generally the same or lower than those in the Peru sediments, even though the total C4-C7 concentrations are lower and comparable to those occurring in the Japan Trench sediments (Table 2). The percentage of aromatic hydrocarbons in geothermally "warmer" sections, as shown in Table 2, remain about the same or decrease slightly compared to the colder sections, even though the total levels of C4--C7 increase exponentially (by one or two orders of magnitude) within the same sections. In Guaymas Basin, for the geothermally hottest sediments encountered at DSDP Site 477, the total amounts of C4-C7 are roughly comparable to those in other geothermally "warmer" California and Guaymas samples from Sites 478 and 479 (Table 2). However, the proportion of aromatic hydrocarbons increases substantially in the two deepest sections of Site 477. In the deepest at 130 m, a totally unique C,-C 7 composition occurred which consisted of only the aromatics--benzene, toluene, and xylene--with the lightest hydrocarbons--methane, ethane, and propane (Whelan and Hunt, 1982). Two possible alternative processes might explain the Site 477, 130 m light hydrocarbon results: (1) The light and aromatic hydrocarbons are preferentially generated thermally at depth and are then carried upward with circulating hydrothermal waters. This scenario is consistent both with the lightest and aromatic hydrocarbons being those most likely to survive severe thermal conditions and also being the most water soluble of all CI-C7 structures. It is also consistent with compositional and isotopic measurements which show gas pocket methane in this section to be thermogenic (Galimov and Simoneit, 1982; Simoneit and Galimov, 1984; Welhan and Lupton, 1987). (2) The aromatic hydrocarbons were thermally generated and were then preferentially sorbed to residual carbon as upward percolating waters carry out less strongly sorbed species. In this case the light C r C 3 alkanes would represent the steady state residue of much larger concentrations of these compounds either generated in situ or brought up from depth. This process is not consistent with thermal desorption experiments carried out on the "spent" kerogen in this interval which did show the presence of C~-C3, but no aromatic hydrocarbons (Curray et al., 1982, p. 225). Gas (methane) formation is typical of the deepest, hottest petroleum generation processes which occur in the range of about 150-250°C (Hunt, 1979; Tissot and Welte, 1984). Downhole temperature measurements showed a temperature 125°C at 168 m and a

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geothermal gradient of 200°C/km or more. Thus, upward diffusion of thermally produced gas is consistent with the measured thermal regime. In addition, mineral alteration of sediments beneath the sill show exposure to temperatures of 300°C or higher in the past (Einsele et al., 1980). It is suggested that this peculiar hydrocarbon distribution is also a relic of past sediment exposure to these very high temperatures. Thus, a "high thermogenic" light hydrocarbon mixture trapped beneath the sill is consistent with the extreme alteration noted in other properties of the sedimentary organic matter, such as extremely high C/N ratios (Curray et al., 1982, p. 227) and low petroleum source rock potential, as shown by an abnormally low pyrolysis hydrogen index (Whelan and Hunt, 1982). At DSDP Site 481, which was exposed to episodic and localized intrusive heating, total concentrations of aromatic hydrocarbons are relatively constant (about 20-40 ng/g) below 73 m (Fig. 3). An increase, to 120 ng/g, is observed just above the upper sill at 248 m. However, in several samples, such as those at 130 and 325 m, the proportion of total C6--C7 hydrocarbons which the aromatics represent decreases because the total saturated C6-C 7 fraction increases by almost an order of magnitude. The proportion of aromatic hydrocarbons in all of the Alvin samples is low, even though total C2-C 8 hydrocarbon concentrations vary over a considerable range (Table 1 and Fig. 3). However, the concentrations of the aromatic hydrocarbons in these samples are the same as observed for the deeper Site 477 samples and somewhat higher than for the Site 481 samples. The high ratio of alkanes to aromatics in the Alvin samples suggests light hydrocarbon generation under relatively mild hydrothermal conditions-typical of the localized effects caused by magmatic intrusions (such as those encountered at DSDP Site 481) rather than the more severe type such as that which occurred beneath the sill at Site 477. (H) Compositions of Individual Isomeric Branched and Cyclic Alkanes General It was previously pointed out that the composition of individual C2 to Cs structures in immature surface and DSDP sediments tend to vary from one area to another, but often will remain the same within a specific region or lithologic section. In mature source rocks which are undergoing catagenetic petroleum generation, the general hydrocarbon composition will usually stay the same within a specific area and formation in the absence of extensive migration [see, for example, deep sections of DSDP Site 415, the Moroccan Basin (Whelan and Hunt, 1980), and detailed compositions of hotter sections of DSDP Holes 467 and 479 in Whelan and Hunt (1981, 1982) respectively], even though total amounts of compounds increase exponentially and certain isomeric

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ratios change in a consistent way as a function of temperature (maturation) (Thompson, 1979; Hunt, 1985; Mango, 1987). The relative concentrations of individual compounds generally become much less covariant and more erratic in sections influenced by diffusional migration processes (Thompson, 1979; Leythaeuser et al., 1979; Whelan et al., 1986). Guaymas Basin Concentration distributions of some individual branched and cyclic structures for the Guaymas Basin area are shown in Figs 5 and 6. Compound distributions vary considerably from one location to another. Because the sediments throughout the region are fairly similar (i.e. primarily diatomaceous and exhibiting similar pyrolysis GC patterns; Simoneit et al., 1984), the varying C6-C7 distributions suggest a significant influence of migrated hydrocarbons. An exception occurs for the branched compounds in DSDP Hole 481(Fig., 5) which, except for iC4, remain fairly constant throughout the entire hole. Both Figs 5 and 6 show very similar compositions for Alvin samples 1172-1 and 1177-3. Similarities can also be observed for Alvin sample 1172-4 and dredge sample 7D-3B even though the latter pair has minor but significant differences compared to the first pair. For example, 2-methylpentane (2MP) and 3-methylhexane (3MH) in Fig. 5a and methylcyclopentane (MCP) in Fig. 6a are all significantly higher in 1172-4 and dredge 7D-3B than in samples 1172-1 and 1177-3. These compositions suggest that the light hydrocarbons dissolved in heavy petroleum deposits in samples 1172-1 and 1177-3 either have similar hydrocarbon sources or have been formed/deposited by similar processes, since a significant fractionation between isomeric structures of the same general carbon number and structure would not be expected and has not been observed in most types of migration processes (Thompson, 1979, Hunt, 1985; Mango, 1987). Via analogous arguments, samples 1172-4 and 7D-3B are probably related in a similar way. However, all four samples came from different areas which are not in close proximity. Therefore, it is difficult to interpret the meaning of there similarities more precisely with the current limited data set. The compositions of branched compounds throughout DSDP 481 and gravity core 30G (taken from the same rift area) are very similar, with the exception of the shallowest sample (30G, 259 cm) and, to a lesser extent, the deepest, 481A-30-5 (Fig. 5). These similarities do not seem to depend on the total amounts of these compounds present in each section, which vary over roughly a two order of magnitude range. In contrast, sample 30G (259 cm) is unique, implying a different hydrocarbon source and consistent with the interpretation of a localized lateral migration process proposed earlier to explain

other differences from the 30G-331 cm and DSDP Site 481 samples. The individual cyclic structures (Fig. 6) show more variation within the same set of samples. At DSDP Site 481, because migrational and/or source changes are unlikely based on the branched alkane results discussed above, it is suggested that observed differences may be caused by changes in localized heating regimes. For example, samples 481A-12-2 and 481A-30-5, both just above intrusive sills, show higher ratios of cyclobexane (CH) to methylcyclopentane (MCP) and trans-l,2-dimethylcyclopentane (IT2DMCP) compared to the sum of other isomeric dimethylcyclopentanes. Both ratios may have responded slightly to the higher localized heating adjacent to and just above intrusive sills. A number of light hydrocarbon ratios have been used by various workers as maturation indicators (see, for example, Thompson, 1979; Philippi, 1975 and references cited). Ratios of DMCPs have not previously been suggested as maturation indicators--possibly because the capillary GC columns commonly used in many laboratories do not completely separate all of the structures from adjacent GC peaks so that it has been difficult to measure the ratios reliably. However, Mango (unpublished results) has found dimethylcyclopentane ratios to be excellent maturation indicators for a wide variety of oils and source rocks. (I) Migration vs Generation vs Biodegradation-Guaymas Basin Relationship of Light Hydrocarbons to Other Data A few generalizations can be made regarding the relative influences of migration vs biodegradation vs generation even with the current limited data set. All of the Alvin samples, consisting of heavy tarry materials and complex seabed solidified and reworked residues, come from different areas and are visually very different. These differences have also been demonstrated by solvent extraction (Kawka and Simoneit, 1987) and by pyrolysisq3C (Simoneit et al., 1984). However, in some cases, samples from different areas and containing different matrices (such as the various Alvin samples and the dredge sample shown in Table 1), contain similar suites of light hydrocarbons suggesting either that similar thermal generation mechanisms must be occurring or that migrated Cg-Cs hydrocarbons must be important in some of these samples. The distances over which these migrational processes might operate, which are difficult to assess without much finer scale sampling, probably differ considerably from sample to sample. However, the fact that two of the immature 30G samples, which are separated by a vertical distance of only 70 cm, show different suites of light hydrocarbons (Figs 3-6) suggest that light hydrocarbon migration in the sediments influenced by sill and dyke intrusions can be very localized and, in this case, occurs preferentially in a lateral rather than vertical

CI-C8 hydrocarbons in sediments from Guaymas Basin

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direction. Alternatively, these profiles could also be interior would become progressively more anaerobic caused by biodegradation occurring in uncon- and, therefore, less prone to aerobic biodegradation solidated surface sediments as hydrothermally gener- processes. Such progressive biodegradation would, ated hydrocarbons diffuse upward (Simoneit, 1984b). therefore, be expected to show a gradual decrease in The latter hypothesis is consistent with the fact that alkene/alkane ratios, an increase in the proportion of the branched, gem-dimethyl alkanes, and alkenes extractable n-alkanes and a decrease in the proporare more concentrated in the 259 on sample, com- tion of N, S and O rich heterocyclic bitumen (aspared to the 331 on sample (Fig. 3). Comparisons of phaltic) material in going from the outside to the relative biodegradability of various classes of petro- inside of a petroliferous asphaltic concretion. leum hydrocarbons have shown that if selectivity occurs--for example, when aerobic biodegradation is only partial--straight chain hydrocarbons are deCONCLUSIONS graded in preference to branched and cyclic alkanes (1) Concentrations and compositions of light (McKenna and Kallio, 1965). The Alvin samples contain unusually high amounts hydrocarbons are compared for sediments and of migrated hydrocarbons, which have deposited Alvin samples recovered from geothermally warm in the seafloor mounds (Simoneit and Lonsdale, (Guaymas and offshore California) and cold seabed 1982; Lonsdale, 1985; Lonsdale and Becker, 1985; (Peru) and DSDP (Japan Trench) sediments. Total Simoneit, 1983, 1984a, b, 1985a, b; Simoneit and concentrations of C2-C7 alkanes are low (typically 0.1 Kawka, 1987). Hydrocarbon migration in petroleum to 10 ppb of about 10 compounds) in geothermally source rocks can be detected using pyrolysis pro- cold Guaymas sediments, comparable to those decedures. Typically, production index (PI) values tected in most other seafloor and DSDP sediments greater than 0.1 are measured in mature petroleum not exposed to temperatures greater than ~20°C. source rocks, while very high values (greater than Both the complexity (i.e. number of different comabout 0.5 and higher, particularly for immature pounds represented) and the concentration of each sediments) are typical of migrated hydrocarbons. PI compound increase exponentially in Guaymas Basin values for the 481A samples are typically immature DSDP sediments and Alvin samples with temperature (less than 0.1 and in many cases less than 0.01, Table starting with mild (30°C) geothermal exposure and 1), except for the 481A-12-2 section just above the progressing through the catagenetic heating (50 to upper dolerite sill which shows a value of 0.12, > 150°C) range. The high concentrations of light characteristic of a sediment entering the petroleum hydrocarbons in the Guaymas Basin samples are generation zone (geothermal temperature greater consistent with the high geothermal gradients in the than 50°C). In contrast, the PI values of the Alvin area. Localized light hydrocarbon anomalies were sediment samples are all very high (greater than 0.9) also observed adjacent to magmatic intrusions. indicative of migrated petroleum. Thus, the light (2) The deepest section of Guaymas DSDP Site 477 hydrocarbon and pyrolysis data are most consistent examined (130m) contained a very unusual light with the DSDP Site 481A samples containing pre- hydrocarbon distribution consisting of methane, ethdominantly compounds either generated in situ or ane, propane and aromatic compounds only. It is migrated only very short distances (10's of cm) postulated that these compounds migrated upward away from intruding sills. In contrast, the high from deeper sections which were probably exposed to concentrations of light hydrocarbons together with maximum geothermal temperatures in excess of elevated amounts of Cls+ bitumen in the Alvin 300°C. The percentage of aromatic compounds in samples (Table 1) are consistent with a major contri- other Guaymas Basin C4427 hydrocarbon fractions is bution from migrated hydrothermally generated typically the same or lower than found in geohydrocarbons. thermally cold sediments. Arguments were presented previously that it is (3) Alkene/alkane ratios were 0.1 or greater in unlikely that significant proportions of the light geothermally cold seafloor sediments, including those alkenes found in most Guaymas Basin samples are recovered from an oxic sediment-water interface. derived directly from geothermal pyrolysis processes. Very high ratios (> 1) were observed for one Peru Based on the light hydrocarbon alkene to alkane core recovered from the edge of the anoxic bottom ratios, it is suggested that a significant fraction of the water zone. In the offshore California and Guaymas light Guaymas alkenes may be derived from partial shelf samples, this ratio approached zero in sediments biodegradation of the hydrothermally produced bitu- exposed to temperatures above 30°C. men. Aerobic biodegradation would be expected to (4) Alkene/alkane ratios were also high in a numbe fairly rapid on the surface of thermally-produced ber of Alvin samples collected from the seafloor in bitumens, such as those recovered by the Alvin. One Guaymas Basin. The very high total C2-C7 concenway to demonstrate such a process would be to trations in these samples are consistent only with a examine closely spaced samples progressing from thermogenic light hydrocarbon source. Because of exterior toward the interior sections of organic-rich associated compositional differences in the C,5+ alconcretions, since it would be expected that the kenes, which are different from those typically oh-

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served in pyrolysis processes, it is suggested that the light alkenes in these organic-rich sediments are produced by partial (aerobic?) biological hydrocarbon degradation processes. (5) Normal alkanes predominated in geothermally cooler (<20°C) seafloor sediments with the proportion of cyclic and branched compounds generally increasing with thermal stress. This result suggests that these sediments were not exposed to high enough time-temperature conditions (i.e. > 100°C) to cause a reversal of this trend as commonly observed in deeper hotter wells. (6) The concentrations of gem-dimethyl compounds, which tend to be favored in either geothermally cold (<20°C) or hot ( > about 120-150°C) regimes, remained relatively constant. However, their proportion decreased as total C2~:~7 concentrations and thermogenic exposure increased. (7) Methane, typically produced by methanogenic bacteria in organic-rich deep ocean sediments, predominated over C2-Cs hydrocarbons in the geothermally coldest sediments and decreased proportionately as C2--C7 increased exponentially in geothermally warmer samples. (8) Similarities in branched and cyclic structures were observed in some pairs of the Alvin and Guaymas Basin core samples which were recovered from non-adjacent areas. It is proposed that hydrocarbon generation and migration processes are related for these geographically non-proximal sample pairs. (9) Higher ratios of CH/MCP and IT2DMCP to other dimethylcyclopentane isomers were observed adjacent to and above intruding sills at DSDP Site 481 consistent with response of these ratios to localized heating. Acknowledgements--We thank the National Science Foundation, the Scripps Institution of Oceanography (crew of R. V. Melville)and the Deep Sea Drilling Project for access to various samples, Dr P. Lonsdale for dredge samples, Mr B. Lather, Mr C. Jackson and Ms C. Burton for technical assistance, and Dr John Farrington and an anonymous reviewer for helpful comments regarding the manuscript. Financial support from the National Science Foundation, Division of Ocean Sciences (Grants OCE81-18897 and OCE83-12036 to B.R.T.S., and Grant OCE83-00485 to Jean K. Whelan and John M. Hunt) and from the Department of Energy (Grant DE-FG02-86ERI3466 to Jean Whelan and John Farrington and Contract EG-770204392 to John M. Hunt) is gratefully acknowledged. This is Woods Hole Oceanographic Institution Contribution No. 6582. REFERENCES

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