Hydrothermal alteration of organic matter in sediments of the Northeastern Pacific Ocean: Part 2. Escanaba Trough, Gorda Ridge

Applied Geochemistry 17 (2002) 1467–1494 www.elsevier.com/locate/apgeochem

Hydrothermal alteration of organic matter in sediments of the Northeastern Pacific Ocean: Part 2. Escanaba Trough, Gorda Ridge Ahmed I. Rushdi1, Bernd R.T. Simoneit* Environmental and Petroleum Geochemistry Group, College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA Received 25 February 2000; accepted 18 June 2001

Abstract Samples from the Ocean Drilling Program Leg 169 in Escanaba Trough, Gorda Ridge, NE Pacific Ocean, were analyzed to study maturation by accelerated diagenesis and/or by catagenesis of the sedimentary organic matter to hydrothermal petroleum. At Site 1038 the hydrothermal petroleums have migrated after generation to shallower horizons. The n-alkane maturation was indicated by the strong even C number preference (i.e., CPI < 1.0) as in the case of Middle Valley. All samples contained admixed organic matter of terrigenous and marine components as indicated by the distributions of the biomarkers. The biological precursors were catagenetically altered to their equivalent mature compounds. The presence of high molecular weight PAHs in some sediment sections at Site 1038 reflected the high temperature alteration and reworking of organic matter into mature hydrothermal petroleum. At Site 1037, the reference hole in the Escanaba Trough, the n-alkanes with a strong predominance of odd C number homologs reflected immature non-marine lipid components essentially throughout the hole. Maturation of organic matter was only observed below 450 mbsf with n-alkanes showing a CPI <1.0. The strong even C number predominance in those intervals was attributed to initial maturation by high heat flow during the early rifting process. # 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction As mentioned in the previous paper (Rushdi and Simoneit, 2002, this issue), hydrothermal activity at tectonic spreading centers has a significant effect on suspended and sedimentary detrital organic matter, where the sedimentary organic matter is rapidly pyrolyzed forming petroleum like products (Simoneit, 1985, 1992a). These hydrothermal pyrolysates range from trace levels found in regions with low sediment deposition to seabed condensates and asphaltic petroleum * Corresponding author. Tel.: +1-541-737-2155; fax: +1541-737-2064. E-mail address: [email protected] (B.R.T. Simoneit). 1 On leave from Department of Oceanography, College of Sciences, Sana’a University, Sana’a, Republic of Yemen.

encountered in locations with significant marine sediments such as Guaymas Basin, Gulf of California (Simoneit, 1982a,b, 1984a,b, 1985; Simoneit and Lonsdale, 1982; Simoneit et al., 1988), and Escanaba Trough and Middle Valley in the northeastern Pacific (Kvenvolden et al., 1986; 1988; Kvenvolden and Simoneit, 1990; Simoneit, 1994; Simoneit et al., 1992). Thus, hydrothermal vent systems passing fluids through sediments are considered as natural laboratories to study active petroleum generation, with concomitant expulsion and migration of the products (Didyk and Simoneit, 1989; 1990; Simoneit, 1992a,b; 1993; 1994; Kvenvolden et al., 1986; 1988). The Ocean Drilling Program (ODP), Leg 169, drilled Escanaba Trough in the northeastern Pacific Ocean, an active sediment covered rift system. The samples described in this paper were recovered on this drilling leg and are used to investigate the chemical alteration

0883-2927/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(02)00113-0

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of sedimentary organic matter into petroleum-like products, their expulsion and migration by comparison with a reference core. The conversion of organic biomarker precursors to products under hydrothermal conditions in this rift system is also examined. 1.1. Geological setting The Escanaba Trough rift system is located on the Gorda Ridge in the northeastern Pacific Ocean (Fig. 1) and is expressed as seafloor mineral deposits with massive sulfides. The individual deposits in this area are relatively surficial when compared to those of Middle Valley. Lithological units and alteration facies of the ODP drill cores are shown in Fig. 2. The mineral compositions of the deposits in Escanaba Trough indicate a dominantly sedimentary source for the metals in the sulfides (Zierenberg et al., 1993). The Escanaba Trough is located at the southern end of the Gorda Ridge and north of the Mendocino Fracture Zone (Fig. 1A). The water depth is about 3.2 km and the rift sediment thickness is about 500 m. The northern end of the axial valley is only a few kilometers wide, but widens to about 18 km at the southern end. The spreading rate is about 24 mm/a and the valley has a slow spreading ridge morphology (Atwater and Mudie, 1973). The steep flanks of the valley rise 900– 1500 m above the valley floor. Escanaba Trough is divided into two 80–100 km long segments separated by a 5 km right-lateral offset at 41
080 N. At the south end the sediment thickness exceeds 900 m. 1.1.1. Site 1037 Site 1037 is located east of the spreading axis and is considered the reference hole (Fig. 1B). The sediments are composed of turbidite sequences and are characterized by a ‘‘transparent’’ layer present in all seismic profiles across the trough (Morton and Fox, 1994; Davis and Becker, 1994). This 50 m thick layer occurs at approximately 100 meters below sea floor (mbsf) and consists of mud or fine sand turbidites (Fig. 2). There is no mineralogical evidence that high temperature hydrothermal fluids circulated within the sedimentary sequence. 1.1.2. Site 1038 Site 1038 is located on and in the vicinity of the Central Hill (Fig. 1C). It includes 9 holes (1038A–1038I) in the massive sulfide deposits and the sediments that cover the north and east flanks of the uplifted hill (Fig. 2). A continuous outcrop of massive sulfide with few sediment-covered areas is reported for this area (Fouquet et al., 1998). The Central Valley is about 1 km in diameter and 60 m deep, and is interpreted as a cylindrical block of sediments bordered by curvilinear normal faults. High-temperature hydrothermal fluid discharges were

observed at temperatures ranging between 108 and 217
C (Fouquet et al., 1998).

2. Experimental methods About 3 cm3 of each wet sediment was taken by minicore and extracted aboard ship with methanol and n-hexane (1:2, 3 ml each) by shaking, subsequent centrifugation (2000 rpm), and removal of the hexane supernatant. The supernatant hexane extract was pipetted into a second vial and evaporated under nitrogen blow-down at 30
C to a volume of 10–40 ml. Initial screening of each sediment extract was carried out aboard ship by GC and selected samples were analyzed by GC–MS. The extracted sediments were dried and weighed for total extract quantitation. Generally, a 1 ml aliquot from a measured total volume was injected into the gas chromatograph (GC) or GC–mass spectrometer (MS) using normal protocol as detailed below. Total extracts were also derivatized with silylating reagent to enhance the response for acids and alcohols in the GC–MS analysis by heating an aliquot with BSTFA at 70
C. The shipboard GC was a Hewlett-Packard Model 5890A, fitted with a 30 m0.25 mm capillary column coated with DB-1 (0.25 mm film thickness). The temperature was programmed as isothermal for 3 min at 30
C, 10
C/min to 220
C, 4
C/min to 300
C, and isothermal for 15 min, with the injector at 250
C, flame ionization detector (FID) at 300
C, and He as the carrier gas. Analyses by GC–MS were carried out using a Hewlett-Packard Model 5973 MSD quadrupole mass spectrometer operated at 70 eV over the mass range 50–650 da. The GC (HP Model 6890) was fitted with a 30 m0.25 mm capillary column coated either with DB-5 (0.25 mm film thickness) or with DB-1 (0.3 mm thickness). The GC oven temperature was programmed from 65
C ( hold for 2 min), to 310
C at 4
C/min, then isothermal for 30 min, with the injector at 290
C, FID at 325
C, and He as the carrier gas. The GC–MS data were acquired and processed with a Hewlett-Packard Chemstation data system. All compound identifications are based on comparisons with authentic standards, their GC retention time, literature mass spectra and interpretation of mass spectrometric fragmentation patterns.

3. Results and discussion 3.1. Bitumen data The bitumen parameters and biomarker data are presented as trends based on the alterations observed in maturation of sedimentary organic matter (Hunt, 1996;

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Fig. 1. Location maps of Escanaba Trough showing: (A) Escanaba Trough in the system of spreading axes and transform faults in the NE Pacific, (B) the Central Hill area in Escanaba Trough and ODP Site 1037, and (C) the Central Hill area with the holes of ODP Site 1038.

1470 A.I. Rushdi, B.R.T. Simoneit / Applied Geochemistry 17 (2002) 1467–1494 Fig. 2. Correlation diagrams showing lithologic units and sediment alteration facies at the Central Hill (Site 1038) and reference hole (Site 1037) drill sites of Escanaba Trough (Sediment alteration facies are as follows: a=unaltered, b=carbonate cements and nodules, c=clay-altered, noncalcareous, and d=noncalcareous).

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Tissot and Welte, 1984). The parameter trends versus depth are avoided due to extensive lateral fluid intrusion causing local alteration of organic matter. Therefore, the data herein are discussed in two parts: First, the trends of the bitumen parameters are generalized for each site; and second, the precursor–product relationships of biomarker compounds are summarized versus maturity. 3.1.1. Site 1038 The total C for sediments from Site 1038 ranges from 0.5 to 1.6% and TOC is low, ranging from 0.0 to 1.1% (Fouquet et al., 1998). Black soot is present in the altered and lithified sediments at 8.5, 34, 0.5, 1.4, 33, 11, 22, 0.0, and 105 mbsf for Holes 1038A, 1038B, 1038C, 1038D, 1038E, 1038F, 1038G, 1038H, and 1038I, respectively (Fig. 2) and represents in part the kerogen C residue after in situ organic matter alteration. The bitumen yields range from 15.3 to 9308 mg/g dry weight of sediment with an average of 855  2372 mg/g (Fig. 3). Most sedimentary organic matter at this site has been altered to hydrothermal petroleum and migrated into discrete horizons at shallow depths. The majority of the resolved compounds are polynuclear aromatic hydrocarbons (PAHs) (e.g., Fig. 4A, B, C, and F), unresolved complex mixture (UCM of branched and cyclic compounds) (e.g., Fig. 4A, B, D–F) or aliphatic hydrothermal petroleum (e.g., Fig. 4D and E). PAHs are found in all samples with various concentrations. Based on their compositions, these in situ oils and solid tars represent the seared bitumen residues after extensive flushing by high temperature fluids at about 250– 350
C (Simoneit, 1984a,b; Kawka and Simoneit, 1990; Simoneit and Fetzer, 1996). Although these samples do not have the same overall compositions as the hydrothermal petroleums analyzed previously from the seabed of Escanaba Trough, they have similar PAH contents (Kvenvolden et al., 1986; Kvenvolden and Simoneit, 1990). These petroleums were injected as veins into sulfide minerals of this hydrothermal system, which in turn collapsed into talus and subsequently weathered leaving behind the petroleum tar (Simoneit et al., 1992). The samples from Hole 1038F, are mixtures of PAH and aliphatic hydrocarbons with slight odd to even C number preferences > C22 (CPI=0.9–1.8) (e.g., Fig. 4C). This reflects a minor terrigenous source component of wax from higher plants (Simoneit, 1977, 1978) (Table 1). The samples of hydrothermal petroleums from Hole 1038G have the same compositions as those reported by Kvenvolden et al. (1986, 1994). The catagenetic stage of organic matter alteration is evident for the examples shown in Fig. 4C, D, and E which indicates that hydrocarbons have been cracked to the low molecular weight homologs and the more water soluble PAH and alkyl–PAH compounds are superimposed on the UCM. Full maturation of in situ bitumen is observed at

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depths of 20 and 30 mbsf in Holes 1038F and 1038G, respectively. This maturity continues with depth for Hole 1038G, based on the CPI values of about unity (Fig. 5 and Table 1). The alkanes of some samples in Holes 1038G and 1038H have slight even to odd C number preferences > C26 (i.e., CPI <1.0) (e.g., Fig. 4D and E). This is also observed for other hydrothermal petroleum samples from Escanaba Trough and Middle Valley (Simoneit, 1994; Kvenvolden et al., 1994; Rushdi and Simoneit, 2002) and is probably related to the source of the organic matter or its catagenesis. The higher CPI values and their variability in shallow sediments indicate an admixture of terrestrial and marine organic matter with probably different maturities. The Pr/Ph ratio shows low and variable values in most intervals of these cores (Table 1). The low Pr/Ph values reflect full organic matter maturation (Simoneit, 1981), or result from differing organic matter sources (Didyk et al., 1978), or both. The hydrothermally-altered sedimentary organic matter in Holes 1038B, 1038E and 1038H has migrated to within the first meter of the seabed by fluid flow. This is confirmed by the presence of the aromatic components in the hydrothermal petroleum at depths of 1–2 mbsf in these holes. The sedimentary bitumen in Hole 1038F is fully mature and resides within the upper 20 mbsf, while in Hole 1038G it has completed maturation at 30 mbsf and remained in situ. Organic matter maturation with depth in Hole 1038I occurred in situ without migration and is complete at about 160 mbsf. This indicates high heat flow, but not high fluid flow to reach the upper sections of the holes. 3.1.2. Site 1037 (reference hole): The total organic C content of sediments from Site 1037 is low, ranging from 0.0 to 1.1% and shows no major trends except for an initial sharp decrease with depth (Fouquet et al., 1998). The bitumen yields are low and range from 1.2 to 15.7 mg/g dry weight of sediment with an average of 5.6  6 mg/g (Fig. 3). Extracts from near surface sediments were yellow and became colorless in deeper sections. Only the 4 deepest samples showed fluorescence. Black soot also occurs in the deeper lithified sediments below 400 mbsf and represents the kerogen C residue after in situ generation of bitumen. Most of the bitumen components increase in maturity but are hydrothermally unaltered to a depth of 400 mbsf. The C number maximum (Cmax) is found mainly at 29 or 31 (Table 1, e.g., Fig. 6Fig. 6B), typical for immature hydrocarbons with an origin from terrestrial higher plant waxes (Simoneit, 1977, 1978). The nalkanes > C24 have a strong predominance of odd carbon number homologs (CPI > 1.0), which reflects immature non-marine lipid sources (Figs. 5 and 6A–E). The bitumens in the upper sediments of Hole 1037A and 1037B have alkane distributions and relative amounts of

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Fig. 3. Yields of total bitumen extracts (mg/g) for sediments from Holes 1038C, 1038G, 1038H, 1038I, and 1037B.

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Fig. 4. Gas chromatograms of total extracts of representative samples from Site 1038: (A) 1038B-1R-1, 0.5 (overmature); (B) 1038B1R-1, 51–52 (overmature); (C) 1038F-2R-1, 100–101 (mature/immature mixture); (D) 1038G-3H-3, 47–48 (mature); (E) 1038G-3H-6, 104–105 (mature); (F) 1038G-3H-9, 20–22 (mature); (Numbers refer to carbon chain length of n-alkanes, UCM=unresolved complex mixture, N=naphthalene, MN=methylnaphthalenes, P=phenanthrene, Pr=pristane, Ph=phytane, Py=pyrene, BaA=benz[a]anthracene, MP=methylphenanthrenes, Inpy=indeno[1,2,3-cd]pyrene, Bbf=benzo[b]fluoranthene).

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Table 1 Various parameters for the solvent-soluble organic matter in sediments from ODP Leg 169 in Escanaba Trough Core, section, interval (cm)

Depth (mbsf)

Bitumen yielda

n-Alkanes Cmaxb

169–1037A1H-4, 135–140

5.88

169–1037B1H-5, 15–16 2H-2, 40–41 2H-6, 63–64 3H-4, 140–150 6H-3, 140–150 7H-3, 120–125 7H-3, 120–125 9H-3, 140–150 13H-2, 140–150 14H-4, 140–150 17X-3, 140–150 20X-3, 140–150 22X-2, 140–150 25X-5, 140–150 30X-3, 140–150 33X-3, 140–150 35X-3, 140–150 36X-1, 140–150 37X-4, 140–150 40X-3, 140–150 43X-3, 140–150 45X-3, 140–150 48X-3, 140–150 50X-3, 140–150 53X-1, 140–150 55X-cc, 15–20 56R-1, 30–37

6.15 8.50 14.74 22.05 49.05 58.33 58.33 77.55 114.05 126.55 152.55 182.05 195.15 228.45 273.45 302.15 321.35 327.95 342.05 369.45 398.35 417.65 446.45 465.75 491.60 501.15 505.00

169–1038A2R-1, 3–4 8R-1, 10–11 11R-cc, 3–4 12R-cc, 3–4

8.53 66.71 104.80 114.40

Range

CPIc

Pr/Ph

11,18,27

10–35

2.85

1.4

29 29,27 29,27 31 29 29 31 29 29,12 29,31 29 31,29 11,29 31 29 31,29 31 31 29 29 29 29 29,11 10,25 11,27 11,25 11,26

15–35 10–35 10–35 10–35 10–35 10–35 10–35 10–35 10–35 10–35 10–35 10–35 10–35 10–35 15–35 10–35 10–35 10–35 10–35 10–35 10–35 10–35 < 10–35 < 10–35 < 10–35 < 10–35 < 10–35

4.04 4.10 3.66 3.14 3.93 3.98 2.74 2.76 2.83 3.97 4.52 5.62 3.68 5.08 4.52 4.53 4.38 4.6 3.5 3.49 3.41 4.57 2.25 0.93 1.23 1.51 0.89

2.2 1.80 1.7 2 1.7 2.4 2.3 1.9 1.5 1.8 1.9 2.5 2.4 2.7 2.0 1.2 1.6 1.8 1.7 1.7 1.5 2.0 2.5 1.1 1.3 1.1 0.6

27 UCM 25 24

15–35

1.19

0.71

10–35 15–35

0.89 0.84

4.8 1.20

15.3

Py Py Py Py Py N,P,25

19–35

0.84

1.8 2.2 3.2 1.2 3.8 2.6

1.8 4.3 3.3 2.8 15.7 3.2 5.6 4.5 5.2 6.5 4.2 3.5 2.4 4.8 4.0 33.8 8.4

169–1038B1R-1, 0–10 1R-1, 43–44 1R-1, 51–52 1R-1, 117–120 1R-cc, 3–5 5R-1, 19–29

0.05 0.44 0.52 1.18 6.8 34.14

169–1038C1R-1, 30–31 1R-1, 50–65 3R-1, 85–95 4R-1, 51–61

0.30 0.53 23.50 32.76

258.0 336.0 245.3 65.7

Py Py P,28 N,28

16–35

0.97

0.67

1.41

74.9

P,24

15–35

0.86

0.46

169–1038D1R-1, 136–146

25.0 128.0

(continued on next page)

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A.I. Rushdi, B.R.T. Simoneit / Applied Geochemistry 17 (2002) 1467–1494 Table 1 (continued) Core, section, interval (cm)

Depth (mbsf)

Bitumen yielda

n-Alkanes Cmaxb

169–1038E2R-1, pc2 2R-1, 100–101 4R-2, 45–55

13.4 14.35 33.00

169–1038F2R-1, 100–101 3R-1, 140–150 4R-cc, 3–4

10.65 22.05 38.3

169–1038G3H-1, 8–9 3H-2, 80–81 3H-2, 142–150 3H-3, 47–48 3H-3, 48–66 3H-3, 104–105 3H-4, 95–96 3H-5, 140–150 3H-6, 56–57 3H-6, 59–60 3H-6, 105–106 3H-6, 136–137 3H-6, 140–141 3H-7, 7–8 3H-8, 54–55 3H-9, 20–22 4H-1, 54–55 4H-2, 81–82 5H-2, 20–40 7X-2, 140–150

22.08 24.31 24.96 25.48 25.57 26.05 27.46 29.45 30.06 30.10 30.56 30.87 30.91 31.08 31.25 31.31 32.04 33.82 42.80 63.45

169–1038H1X-1, 3–4 1X-1, 10–11 1X-1, 140–150 2X-1, 140–150 4X-3, 140–150

0.03 0.11 1.45 13.35 34.25

169–1038I1X-1, 140–150 1X-3, 140–150 2X-2, 140–150 3H-2, 140–150 4H-3, 140–150 5H-3, 140–150 9X-2, 140–150 11X-3, 140–150 17X-1, 119–129

1.45 4.45 12.25 20.25 31.25 40.75 77.35 98.05 152.54

a

106.6

Range

P UCM 29,Pr,N

CPIc

Pr/Ph

1.61

3.7

N,P,27 N,P,11 16,Py

10–35 10–35 10–35

1.79 0.9 1.03

2.4 1.4 0.7

86.5 160.8 114.0 508.3 895.0 86.9 95.1 178.1 137.3 146.0 8510.0 8650.0 9308.0 270.8 86.5 278.1 233.6 268.7 146.0 37.9

11,27 P,20 N,P,27 11,16 11,17 N,P,23 14,27 N,P,22 P,22,27 23 8,20 9,20 8,20 11,20 9,N 11,N 11,N 11,N 22 11,Py,25

10–35 10–35 10–35 10–35 10–35 10–35 10–35 10–35 15–35 13–35 8–35 8–35 8–35 8–35 8–33 8–33 8–33

1.54 1.52 1.96 0.79 0.73 1.59 1.47 1.24 1.37 1.09 0.72 0.77 0.75 0.77

1.6 2.5 3.8 1.4 1.4 3.7 1.2 1.6 1.1 1.2 1.7 1.6 1.7 1.8

19–35 10–35

0.93 0.91

1.1

317.4 263.1

Py,22 BaA,Py,26 9,24 Py,23 26

16–35 15–35 9–35 18–35 16–35

0.87 0.93 0.83 0.94 0.89

0.8

Pr,21 27 29,27 27 29,27 27 N,Pr,29 N,25 N,22

14–33 15–35 15–35 11–35 10–35 16–35 10–35 8–28 11–19

1.25 3 2.88 3.11 2.4 3.83 2.0 1.2 1.02

2.3 2.3 2.0 2.3 2.5 1.4 6.1 1.8 1.1

105.6 66.2

98.2 74.5 32.1 56.0 17.0

0.8

Determined as mg/g dry weight of sediment. Major homologs are listed in decreasing order of intesity (Pr=pristane, Ph=phytane, N=naphthalene, P=phenanthrene, Py=pyrene, BaA=benz[a]anthracene). c CPI=
(C27+C29+C31+C33+C35)/
(C26+C28+C30+C32+C34). b

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an unresolved complex mixture (UCM) of branched and cyclic hydrocarbons typical of mature bitumens (e.g., Fig. 6A, Fouquet et al., 1998). Redeposition from eroded older formations by the rapid turbidite sedimentation may explain their occurrence (Kvenvolden et al., 1994). The maturity of the organic matter below 400 mbsf increases dramatically with depth (Fig. 4G and H) and indicates in situ alteration based on the uniform and low concentrations. The CPI values indicate accelerated maturation below 450 mbsf (Fig. 5). The strong even C number predominance in some intervals with CPI < 1.0 may be related to autochthonous marine sources of the organic matter. CPI values < 1.0 have been reported for sediments from the Escanaba Trough (Kvenvolden et al., 1994), Middle Valley (Simoneit, 1994; Rushdi and Simoneit, 2002) and for other geographical regions (Simoneit, 1977; Grimalt and Albaiges, 1987). Therefore, the variability of the CPI values for bitumens in

shallow intervals of Site 1037 indicates an admixture of terrigenous (turbidite deposition) and minor marine organic matter and the different maturities of the source inputs. The Pr/Ph ratios vary with depth (Table 1) and approach low values at intermittent intervals in shallower horizons and at depth, which may reflect the source imprint of the primary organic matter in shallow horizons and full maturation at depth. 3.2. Temperature regime and petroleum formation processes In the Escanaba Trough (Site 1038) the geothermal gradient is 2.0
C/m (Fouquet et al., 1998). Thus petroleum hydrocarbons ( >C24) should survive to a depth of about 50 mbsf, where the in situ temperature is about 100
C. The measured temperature at 56.8 mbsf is 115.9
C in Hole 1038I (Fouquet et al., 1998) and trace amounts of n-alkanes are observed in this core (Table 2). The low temperature of 50
C for the beginning of the conventional oil generation window, with peak generation occurring at about 80–100
C (Hunt, 1996; Tissot and Welte, 1984) indicate that temperatures for Site 1038 appear to be high enough to generate the observed amounts by hydrothermal petroleum in situ. Migration and deposition of additional bitumen into these intervals is also likely to have occurred, particularly into zones where hydrothermal flow may have been channelled due to capping by carbonate precipitation (Davis et al., 1992). Bitumen concentrations in the intervals below the hydrothermal petroleum zones are low, indicating any bitumen which had formed there, had already migrated either upward and/or laterally with hydrothermal fluid flow. The temperature gradient at Site 1037 is found to be linear with depth (Fouquet et al., 1998), with an estimated temperature of 87
C at the bottom of Hole 1037B. Accordingly, the peak of the conventional oil generation window (at 80–100
C, Hunt, 1996; Tissot and Welte, 1984) would be found between 475 and 596 mbsf and the beginning of oil generation may start at about 294 mbsf (at  50
C). 3.3. Biomarkers A full range of biomarkers, from precursors to products is found in these bitumen series. The effects of hydrothermal alteration on the indigenous sedimentary organic matter, as well as its expulsion, migration, and mixing of migrated hydrothermal petroleum with immature sedimentary lipids are shown by these samples.

Fig. 5. Carbon Preference Index (CPI) for n-alkanes (range C26–C35) in the sedimentary bitumens of drill holes in Escanaba Trough: Holes 1037B, 1038G, 1038H, and 1038I.

3.3.1. Immature precursors The biomarkers of samples from the upper sections of all holes are immature. Normal maturation is observed versus depth, except in samples where mature or over-

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mature hydrothermal bitumen or hot fluid intruded the section. The presence of various biomarkers for selected samples and key maturity parameters are summarized in Table 2.

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A preliminary search was made for potential precursors responsible for the enhancement of the even-Cchain length n-alkanes in the mature oils. As discussed earlier, the n-alkanols and/or n-alkanoic acids in the

Fig. 6. Gas chromatograms of total extracts of representative samples from Site 1037: (A) 1037A-1H-4, 140–150 (immature/mature mixture); (B) 1037B-20X-3, 140–145 (immature); (C) 1037B-22X-2, 140–150 (immature/mature mixture): (D) 1037B-43X-3, 140–150 (immature); (E) 1037B-48X-3, 140–150 (immature); and (F) 1037B-56R-1, 30–37 (mature). Numbers and abbreviations as in Fig. 4.

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A.I. Rushdi, B.R.T. Simoneit / Applied Geochemistry 17 (2002) 1467–1494

extractable lipids or bound in the organic detritus (e.g., as part of membrane residues) could be these precursors. Both n-alkanoic acids and n-alkanols are found in these samples (Fig. 7 and Table 3). The n-alkanols range from C18 to C30 with a Cmax at 22, 24 or 26 and a strong predominance of even C homologs (i.e., CPI ranges from 9.6 to 18.3), which indicates that they are of a vascular plant origin. The n-alkanols are the major homologous compounds in the derivatized total extract of shallow samples from Site 1038 and to considerable depths in Site 1037. A typical ratio of a maximum nalkanol content versus the n-alkanes from higher plant waxes is for example  5.0 for C26H53OH/C29H60 (Table 3), which confirms the major excess of alkanols over alkanes. Thus during hydrothermal alteration the alkanols would undergo dehydration and reduction yielding even chain alkanes, which become superimposed on the odd chain alkanes resulting in a mixture with CPI < 1.0 as observed in the mature sample of Site 1038. In fact, the n-alkanols disappear in the thermallymatured samples and only n-alkanoic acids prevail. The n-alkanoic acids with a strong even C number predominance (CPI range between 1.8 and 25.0, Table 3) are found in all samples, indicating a biotic origin. They have a Cmax at 16, 24, and 26 from an admixture of algal and vascular plant lipids. The presence of nonanoic acid (C9) in most samples is interpreted as an indicator for the oxidation of elaidic and oleic acids in sedimentary lipids.

Aliphatic ketones are present in sediments of the immature sections of Sites 1037 and 1038. They consist of n-alkan-2-ones, ranging from 4C16 to C33, with Cmax typically at 29 and strong odd carbon number predominances (e.g. Fig. 8). The n-alkanones are indicators for a terrigenous input, because they are found in terrestrial soils, sediments and vegetative detritus and may form by microbial processes from the n-alkanes of plant wax (e.g. Morrison and Bick, 1966; Oros et al., 2002). A minor amount of isoprenoidal ketones (mainly 6,10,14trimethylpentadecan-2-one and traces of 6,10-dimethylundencan-2-one) is also present in these samples (cf., Fig. 8). The isoprenoidal ketones are interpreted to derive from both bacterial and photochemical oxidation of phytol, the isoprenoidyl side chain of chlorophyll a (Brooks and Maxwell, 1974; Rontani and Giusti, 1988). The n-alkan-2-ones and isoprenoidal ketones decrease in concentration and ultimately disappear with increasing burial and higher thermal stress. Sterols are found in samples from shallow core sections of Site 1038 and Throughout Site 1037 and range from C27 to C29 (Figs. 7 and 9). The general sterol distribution for the samples analyzed shows cholesterol (I, R=H, all chemical structures cited are shown in Appendix A) dominant or equal to b-sitosterol (I, R=bC2H5), with 24 methylcholesta-5,22-dien-3-ol (brassicasterol, II, R=aCH3) and dinosterol (III) as minor components. Cholesterol, dinosterol and brassicasterol are interpreted to be from marine sources

Table 2 Occurrence and maturity parameters of biomarkers Core, Section, Interval (cm)

Depth (mbsf)

Steranes

Hopanes

27R/29Ra

S/(S+R)29b

S/(S+R)27Dc

30H/29Hd

S/(S+R)31e

S/(S+R)32f

169–1038E2R-1, PC2

13.4

0.49

0.23

0.59

0.57

0.50

0.34

169–1038G3H-1, 8–9 3H-2, 80–81 3H-3, 48–66 3H-9, 20–22

22.08 24.31 25.57 31.31

0.63 0.69 0.93 1.20

0.32 0.23 0.29 0.35

– 0.64 0.57 0.57

1.05 1.07 1.31 1.44

– 0.45 0.51 0.60

– 0.39 0.41 0.57

169–1038I1X-1, 140–150 1X-3, 140–150 2X-2, 140–150 3H-2, 140–150

1.45 4.45 12.25 20.54

– – 2.44 2.54

– – 0.44 0.58

– – 0.41 0.50

1.19 1.61 1.83 1.46

0.61 0.92 0.61 0.25

0.76 0.60 0.58 0.59

Not detected. a C27 Sterane, 20Raaa/C29 Sterane, 20Raaa. b (20S)/(20S+20R) for C29 diasterane. c (20S)/(20S+20R) for C27 sterane d C30hopane/C29hopane. e (22S)/(22S+22R) for C31 hopane. f (22S)/(22S+22R) for C32 hopane.

A.I. Rushdi, B.R.T. Simoneit / Applied Geochemistry 17 (2002) 1467–1494 1479

Fig. 7. Representative mass fragmentograms of m/z 75 for n-alkanols and n-alkanoic acids in silylated total (BSTFA) extracts of selected samples: (A) 1037B-14H-4, 140–150; (B) 1037B-22X-2, 135–146; (C) 1038I-1X-1, 140–150; (D) 1038F-2R-1, 100–110; (E) 1037B-56R-1, 30–37; and (F) 1038G-3H-4, 95–96.

1480

A.I. Rushdi, B.R.T. Simoneit / Applied Geochemistry 17 (2002) 1467–1494

Table 3 Occurrence of n-alkanoic acids and n-alkanols Core, section, interval (cm)

Depth (mbsf)

n-Alkanoic acids

n-Alkanols

Cmax

Range

CPIa

C9 C16 þC18

Cmax

Range




26ol b 29HC

CPIc

169–1037A1H-4, 135–140

5.88

16

8–28

25.0

0.67

24

18.30

169–1037B1H-5, 15–16 3H-4, 140–150 6H-3, 140–150 7H-3, 120–125 14H-4, 140–150 17X-3, 140–150 20X-3, 140–150 22X-2, 135–146 25X-5, 140–150 33X-3, 140–150 35X-3, 140–150 36X-1, 140–150 43X-3, 140–150 56X-1, 30–37

6.15 22.05 49.05 58.33 126.55 152.55 182.05 195.15 228.45 302.15 321.35 327.95 398.35 505.00

16 16 16 16 16 9 16 16 9 16 16 16 16 16

8–20 8–20 9–20 8–30 8–26 8–18 8–20 8–20 8–20 8–20 8–20 8–20 8–20 8–20

13.0 17.6 10.5 11.6 25.3 13.1 12.0 16.3 16.0 13.0 5.2 12.8 12.3

0.11 0.11 0.10 0.36 0.67 1.13 0.38 0.46 0.91 0.42 0.21 0.41 0.42 0.29

– 26 22 22 26 26 26 22,24 22 22,26 18 18 18 18

– 18–30 18–18 18–30 18–30 18–28 18–30 18–30 18–28 18–30 18–26 18–26 18–26 18

2.3 2.6 4.5 1.4 0.19 0.05 0.03 1.2

12.7 E 12.2 10.1 E 16.3 18.3 9.6 10.6 E E E E

169–1038A2R-1, 3–4

8.53

16

16

E

–

18

18–26

4.5

E

169–1038B1R-1, 117–120 5R-1, 19–29

1.18 34.14

9,16 16

9–20 9–20

E E

– 0.48

– –

– –

169–1038C1R-1, 30–31 1R-1, 50–65 4R-1, 51–61

0.30 0.53 32.76

16 9,16 16

8–20 9–20 9–20

9.5 5.0 16.3

0.50 0.75 0.15

18 18 –

18 18 –

169–1038D1R-1, 136–146

1.41

9

9–20

8.0

1.88

–

–

169–1038E2R-1, pc2 4R-2, 45–55

13.4 33.00

16 9

9–22 8–20

22.4 13

0.64 1.56

– 18

– 18

169–1038F2R-1, 100–101 3R-1, 140–150 4R-cc, 3–4

10.65 22.05 38.3

16 16 16

8–28 9–20 9–20

5.2 9.8 7.2

0.45 0.40 0.49

– – –

– – –

169–1038G3H-1, 8–9 3H-2, 80–81 3H-3, 48–66 3H-4, 95–96 3H-5, 140–150 3H-6, 59–60 3H-7, 7–8 3H-8, 54–55 3H-9, 20–22 7X-2, 140–150

22.08 24.31 25.57 27.46 29.45 30.06 31.08 31.25 31.31 63.45

16 16 26 22 16 16 9 16 18 16

8–28 9–20 8–30 9–28 8–20 8–28 9–16 16 8–18 8–20

3.5 13.3 5.2 1.8 8.7 2.7 E – E 11.0

0.07 0.07 0.30 0.10 0.26 0.09 3 – 0.14 0.40

– – – – – – – – – –

– – – – – – – – – –

10.2

14.2 9.4 5.0 7.8

(continued on next page)

1481

A.I. Rushdi, B.R.T. Simoneit / Applied Geochemistry 17 (2002) 1467–1494 Table 3 (continued) Core, section, interval (cm)

Depth (mbsf)

n-Alkanoic acids

n-Alkanols

Cmax

Range

CPIa

C9 C16 þC18

Cmax

Range

169–1038H2X-1, 3–

0.03

18

8–20

19.7

0.47

18

18

169–1038I1X-1, 140–150 5H-3, 140–150 9X-2, 140–150

1.45 40.75 77.35

16 16 16

8–20 8–20 8–20

20.8 8.9 16.8

0.67 0.35 0.33

22 18 18

20–26 18 18




26ol b 29HC

CPIc

13.2

Not detected; E=only even carbon numbered homologs. a b c

14 þ...þC18 CPI ¼
C
C15 þ...þC19 : 26ol ¼ C26 alcohol : 29HC C29 alkane 18 þ...þC28 CPI ¼ SC SC17 þ...þC27 :

(Volkman, 1986). The dominance of b-sitosterol in the other samples is interpreted to originate mainly from terrigenous sources. Alterations of sterols by accelerated diagenesis due to thermal stress is observed to yield stenones (IV) and stanones (V) with the same range from C27 to C29 ( C26 is not detectable and C30 is present as a trace component) (e.g., Fig. 9). The C28 stenones, 24-methylcholesta4,22-dien-3-one and 24-methylcholest-22-en-3-one, are not detectable in most samples although their precursor

Fig. 8. Representative mass fragmentograms (m/z 58 and 59) for the aliphatic ketones in bitumen extracts of sediments from Leg 169: (A) 1037B-8H-4, 140–145 and (B) 1038I-3H-2, 140– 150 (numbers refer to chain length of n-alkan-2-ones, 18ip=6,10,14-trimethyl-pentadecan-2-one).

brassicasterol (II) is found. Stanols are detectable in some samples as trace components. This suggests that products from dehydration reactions of sterols, as were described in sediments from Bransfield Strait (Brault and Simoneit, 1988) are not observed in this sample set. Triterpenoid markers from terrestrial sources are detectable in samples from the upper core sections of Hole 1038 and throughout Hole 1037. The dominant biological precusores, a-amyrin (VI, R=bOH) and bamyrin (VII, R=bOH) (Fig. 10), from higher vascular plants are present (Brassell, et al., 1983; Simoneit, 1986). a-Amyrone (urs-12-en-3-one,VI, R=O) and b-amyrone (olean-12-en-3-one, VII, R=O), and lesser amounts of olean-12-ene (VIII) and urs-12-ene (VI, R=H), the thermally altered derivatives, are also found (Fig. 10). Some samples also contain urs-20-ene, an isomer of urs12-ene, and taraxera-2,14-dien-3-ol (IX) probably also derived from terrestrial sources. The immature organic matter of both sites contains additional terrigenous biomarkers. The tracers generally specific for an origin from conifer (gymnosperm) vegetation consist of dehydroabietic acid (X) and n-nonacosan-10-ol ( C29H59OH) and are indicated in Fig. 11. Dehydroabietic acid is the major biomarker derived from conifer resin acids and has been indentified previously in sediments near this area (Simoneit, 1977). The mass spectrum of n-nonacosan-10-ol trimethlsilylether is given in Fig. 11B and the characteristic fragments are interpreted on the structure shown. This compound is a tracer for epicuticular waxes from mainly gymnosperms (Tulloch, 1976; Oros et al., 2002; Schulten et al., 1986) and some angiosperms (e.g., Gu¨lz et al., 1992). A group of tetracyclic aromatic hydrocarbons derived from the amyrin triterpenoids by oxidation and loss of ring-A is also present (cf. Fig. 11A). The mass spectra of two examples are shown in Fig. 11C and D and these C22H28 compounds comprise des-A-26,27-dinoroleana-5,7,9,11,13-

1482 A.I. Rushdi, B.R.T. Simoneit / Applied Geochemistry 17 (2002) 1467–1494 Fig. 9. Representative mass fragmentograms for the diagenetic steroids in bitumen extracts of sediments from Leg 169: (A) 1037B-23H-4, 145 and (B) 1038I-3H-2, 140–150 (m/z 124=stenones, m/z 231=stanones; C=cholesterol; S=stigmasterol).

A.I. Rushdi, B.R.T. Simoneit / Applied Geochemistry 17 (2002) 1467–1494

pentaene (XI), des-A-26,27-dinorursana-5,7,9,11,13pentaene (XII) and des-A-26,27-dinorlupana-5,7,9,11,13pentaene (XIII). These compounds occur mainly in the very immature, shallow sediments of both sites and have been identified in other recent sediments (e.g., Trendel et al., 1989; Ten Haven et al., 1992; Simoneit, 1998), which supports an origin from microbial alteration of the terrestrial precursor triterpenoids. Various hopanoid precursors are present in samples from shallow core sections of all holes in Escanaba Trough. Diploptene (XIV) occurs in the upper sections and undergoes diagenesis via 17b(H)-hop-21-ene to hop-17(21)-ene (XV) versus depth (Fig. 12), however, the diagenetic isomers are found at trace levels only. This is the same trend of accelerated thermal diagenesis as was described for surficial sediments and immature bitumen in Guaymas Basin (Simoneit et al., 1979, 1984; Simoneit and Philp, 1982) and for Bransfield Strait sediments (Brault and Simoneit, 1988). Oxygenated species, such as 17b(H),21b(H)-hopan-29-ol (XVI) and 17b(H)-22,29,30-trisnorhopan-21-one (XVII), and other triterpenes (e.g., friedel-4(23)-ene and lup-29-ene) are found in the reference Hole 1037 (Fig. 12). 3.3.2. Sterane maturation Sterane hydrocarbons, are used as markers for oilsource rock correlations, maturity comparisons and source differences among various samples (Mackenzie et al., 1982; Seifert and Moldowan, 1978). Fig. 13 shows examples of the sterane maturation in the samples from

1483

Escanaba Trough and various parameters are listed in Table 2. In all selected samples the C29 steranes are dominant with significant C27 steranes, and the C28 steranes are minor components. The ratio of C27(20Raaa)/ C29(20Raaa) ranges from 0.49 to 2.54 (Table 2) and a value of <1 indicates a stronger input of C29 steroid precursors from terrestrial sources (Volkman, 1986). All samples show low amounts of C27–C29 steranes with the 5a(H),14a(H),17a(H)-20R (XVIII) and minor amounts of the thermally less stable 5b(H),14a(H),17a(H)20R configurations (Kawka and Simoneit, 1987). Samples 1038G-3H-2, 80–81 (24.31 mbsf) (Fig. 13) reflect the onset of sterane epimerization at C-20 as a result of thermal maturation (Seifert and Moldowan, 1978; Mackenzie et al., 1980). The ratio of sterane epimerization at C-20, ð20SÞ=ð20S þ 20RÞ for C29, increases from 0.23 to 0.58 for these selected samples. For the samples from Hole 1038I full maturity is found at a depth of 12.25 mbsf with a value of 0.44, while for a sample from Hole 1038G full maturity is found at a depth of > 24.0 mbsf with a value of 0.23. The further increase in maturity results in additional isomerization to the 5a(H),14b(H),17b(H)-20R and 20Ssteranes (XIX, e.g., Fig. 13), and the appearance of diasteranes [XX, 13b(H),17a(H)-20R]. The epimerization parameter at C-20 for C27 the diasteranes, ð20SÞ=ð20S þ 20RÞ ranges between 0.41 and 0.64 for these samples (Table 2). The equilibrium ratio is between 0.52 and 0.55 for typically mature sediment samples (Rushdi and Simoneit, 2002; Seifert and Moldowan, 1978). These sterane compositions are similar to those reported for other samples from Escanaba Trough, Middle Valley and Guaymas Basin ( Rushdi and Simoneit, 2002; Simoneit et al., 1992; Simoneit, 1994; Kawka and Simoneit, 1987; Kvenvolden and Simoneit, 1990).

Fig. 10. Typical mass fragmentograms of m/z 218 for the terrestrial triterpenoid biomarkers in bitumen extracts of Leg 169 sediments: (A) 1037B-22X-2, 135–146; and (B) 1037B-23H-4, 145.

3.3.3. Triterpane maturation The maturation of microbial hopanes from immature precursors proceeds from the 17b(H), 21b(H)-hopanes (XXI) and moretanes [17b(H), 21a(H)-hopanes, XXII], to the 17a(H), 21b(H)-hopanes (XXIII). For this series, the configuration of the biological precursors is 17b(H), 21b(H) with the R epimer at C-22 for the extended homologs > C31 (Ensminger et al., 1974, 1977). During hydrothermal maturation the precursors also convert to the more stable 17a(H), 21b(H) configuration, with both S and R epimers at C-22. The equilibrium ratio for C-22 epimerization, ð22SÞ=ð22S þ 22RÞ

1484 A.I. Rushdi, B.R.T. Simoneit / Applied Geochemistry 17 (2002) 1467–1494 Fig. 11. Representative total ion current trace showing other terrestrial biomarkers and some of their mass spectra: (A) Partial TIC trace for 1037B-3H-4, 140–150 (as TMS derivatives), (B) mass spectrum of n-nonacosan-10-ol trimethylsilyl ether, (C) mass spectrum of des-A-26,27-dinoroleana-5,7,9,11,13-pentaene, and (D) mass spectrum of des-A-26,27dinorlupana-5,7,9,11,13-pentaene.

A.I. Rushdi, B.R.T. Simoneit / Applied Geochemistry 17 (2002) 1467–1494 Fig. 12. Representative mass fragmentograms of m/z 191 for the triterpenoid hydrocarbons in bitumen extracts of Leg 169 sediments: (A) 1038I-1X-1, 140–150; (B) 1038I-2X-2, 140–150; (C) 1038G-3H-2, 80–81; and (D) 1038G-3H-9, 20–22 (numbers refers to the carbon skeleton, a=17a(H),21b(H)- and ba=17b(H),21a(H)- configurations of the hopane biomarkers). 1485

1486

Table 4 Occurrence and various ratios of polynuclear aromatic hydrocarbons (PAHs) Core, section, interval (cm)

Depth (mbsf)

N

MN

169–1037B52X-3, 145

485.3

M

I

P

MP

Py

MPy

BA

MBA

T

–

T

–

–

–

BPy

InPy

MBPy

Bpe

MBPe

Co

–

–

–

–

–

–

P/MP

MPI 1a

MPI 2a

Fa/Py

Pe/BFa

BaP/BeP

–

–

–

–

–

–

0.05 0.52 1.18

– – –

– – –

– – T

– – T

– M M

– M M

T T T

M M I

M M I

I I T

I I I

M M M

M I M

M M M

– – 0.3

– – 1.4

– – 1.5

– 0.08 0.09

1.83 0.82 0.61

0.18 0.17 0.23

169–1038C1R-1, 30–31 1R-1, 50–65

0.03 0.53

– –

– –

– –

– –

M M

I I

T T

T T

M M

I I

T T

M M

I I

M M

– –

– –

– –

0.04 0.04

0.16 0.17

0.14 0.18

169–1038E2R-1, pc2

13.4

–

–

–

–

–

–

–

–

T

–

T

I

T

T

–

–

–

–

–

0.2

169–1038F2R-1, 100–101

10.65

M

M

T

T

T

–

–

–

–

–

–

–

–

–

1.81

0.48

0.51

0.09

–

–

169–1038G3H-2, 80–81 3H-4, 95–96 3H-5, 140–150 3H-6, 59–60 3H-7, 7–8 3H-9, 20–22 4H-5, 20–22

24.08 27.46 29.56 30.10 31.08 31.31 34.00

I I – T M I –

I M – – M T –

T M M M M M T

T I M M M M M

T T M M M M M

– – M M M M M

– – M M M M –

– – I M I I –

– – T I I I –

– – – T – – –

– – – I T T –

– – – M I – –

– – – I T – –

– – – I T – –

1.33 2.37 0.71 0.54 0.40 0.35 0.30

0.26 0.39 0.94 1.34 1.23 1.35 1.08

0.31 0.41 0.93 1.53 1.25 1.36 1.18

0.15 0.22 0.15 0.14 0.12 0.16 0.14

0.21 – 0.10 0.08 0.22 0.09 0.01

2.60 – 0.24 0.22 0.19 0.10 0.12

0.03

–

–

–

–

M

M

T

I

I

T

I

M

I

M

-

–

–

0.04

0.72

0.35

T – – –

T – – –

– – – –

T – – –

T – – –

– – – –

– – – –

– – – –

– – – –

0.05 – – –

– – 0.13 0.05

0.15 0.13 0.14 0.11

169–1038H1X-1, 2–3 169–1038I1X-1, 140–150 1X-3, 140–150 2X-2, 140–150 3H-2, 140–150

1.45 4.45 12.25 20.25

– – – –

– – – –

– – – –

– – – T

T – – –

T – – –

T T T T

– – – –

DBT=dibenzothiophene; N=naphthalene; MN=methylnaphthalenes; P=phenanthrene; MP=methylphenanthrenes; Fa=fluoranthene; Py=pyrene; MPy=methylpyrenes; BA=benzanthracene; MBA=methylbenzanthracenes; BFa=benzofluoranthenes; BPy=benzopyrenes; Pe=perylene; MBPy=methylbenzopyrenes; InPy=indeno[cd]pyrene; BPe=benzoperylene; MBPe=methylbenzoperylene; Co=coronene. – not detectable; T=traces; I=intermediate; M=major. a Methylphenanthrene indices (Radke and Welte, 1983).

A.I. Rushdi, B.R.T. Simoneit / Applied Geochemistry 17 (2002) 1467–1494

169–1038B1R-1, 0–10 1R-1, 52–53 1R-1, 117–120

A.I. Rushdi, B.R.T. Simoneit / Applied Geochemistry 17 (2002) 1467–1494

for the extended homologs > C31 is about 0.6 (Ensminger et al., 1974, 1977; Seifert and Moldowan, 1978). Examples of this series are illustrated in Fig. 12 and the maturity parameters are listed in Table 2. Traces of 17a(H),21b(H)-hopanes are detectable in the immature sections of Site 1038 and to lesser extent in Hole 1037B along with oxygenated and unsaturated precursors (e.g., Fig.12A and B). The homohopane epimer ratios in such sections are < 0.4, not all fully mature. No significant amounts of 17b(H),21b(H)- and only minor amounts of 17b(H),21a(H)-hopanes are present (Fig. 12). The mature 17a(H),21b(H)-hopanes are found in deeper sections as shown in samples 1038G-3H-2, 80–81 (24.31mbsf), and 1038G-3H-9, 20– 22 (31.31 mbsf) (Fig. 12C and D). They range from C27 to C34 and the dominant homologs are C29, C30 and C31. The homohopane maturity parameter varies from 0.25 to 0.92 for the C31 homologs and from 0.34 to 0.76 for the C32 homologs (Table 2). The 18a(H)- and 18b(H)oleananes are not detecteable in this sample set. These hopane maturation trends and distributions are similar to those described for sediments and hydrothermal petroleum from Guaymas Basin, Middle Valley and Escanaba Trough (Kawka and Simoneit, 1987, 1994: Kvenvolden and Simoneit, 1990; Simoneit et al., 1984; Rushdi and Simoneit, 2002). 3.3.4. Polynuclear aromatic hydrocarbons Selected polynuclear aromatic hydrocarbon (PAHs), their abundance and their various ratios in these samples are listed in Table 4. PAHs are trace components in the extracts from sediments of the reference Hole 1037B. The samples from Site 1038 are generally depleted with respect to low molecular weight PAHs and show some alkyl aromatic hydrocarbons (Table 4). Alkyl naphthalenes and phenanthrene/alkyl phenanthrenes are found at significant concentrations in samples 1038G-3H-4, 95–96, 1038G-3H-5, 140–150, 1038G-3H-6, 59–60, and 1038G-3H-7, 7–8, at minor abundances in samples 1038B-1R1, 117–121, 1038G-3H-2, 80–81, and 1038G3H-9, 20–22, and are almost nil in all other samples (e.g., Fig. 14A). The absence of low molecular weight PAHs in some samples may be due to their removal because of their greater water solubility (Kawka and Simoneit, 1990). Nevertheless, both the ratio of phenanthrene/methylphenanthrenes (P/MP) and the methylphenanthrene indices, MPI 1 and MPI 2, are calculated according to Radke and Welte (1983). The values obtained for samples where these components are detectable are in the ranges: P/MP =0.35–2.27, MPI 1=0.26–1.35 and MPI 2=0.31–1.53 (Table 4). Values for these indices of P/ MP=0.1, and MPI 1=0.4–1.5 are considered fully mature (Radke and Welte, 1983). Therefore, these few samples are assumed to be less mature or were altered by aqueous fluid washing based on their P/MP index

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and fully mature based on the MPI 1 and MPI 2 indices. As discussed above, lower molecular weight PAHs are more soluble and have been removed, thus P/MP indices may not be valid here. Similar distributions and ratios are also reported for other hydrothermal petroleums (Kawka and Simoneit, 1990; Kvenvolden and Simoneit, 1990). The higher molecular weight PAHs are comprised mainly of the m/z 276 series (indeno[cd]pyrene, benzo[ghi]perylene) and coronene with lesser amounts of the m/z 252 series (benzofluoranthenes, benzo[e]pyrene, benzo[a]pyrene) (e.g., Fig. 14B and D). Traces of PAHs with molecular weights of 302, 328, 352 da and higher are also present (Fig. 14C). Some of the very high molecular weight (HMW) PAHs have been identified in other samples from Middle Valley and Escanaba Trough (Simoneit and Fetzer, 1996). Samples 1038C-1R-1, 30–31 and 1038C-1R-1, 50–65 have the following HMW-PAH suite: m/z 302 dibenzo[bdef]chrysene plus other unknowns, m/z 328 benzo[b]picene and other unknowns and m/z 352 benzo[pqr]naphtho[8,1,2-bcd]perylene. The occurrences of compounds with a 5-membered alicyclic ring (e.g., benzofluoranthenes, indenopyrene) and other peri-condensed aromatic hydrocarbons, indicate that these PAHs are formed as a result of high-

Fig. 13. Representative mass fragmentograms of m/z 217 for steranes in bitumen extracts of sediments from Site 1038: (A) 1038G-3H-2, 80–81; and (B) 1038G-3H-9, 20–22 (numbers refer to carbon skeleton, aaa=5a(H),14a(H),17a(H)-, abb=5a(H), 14b(H),17b(H)-, baa=5b(H),14a(H),17a(H)-, baD=13b(H), 17a(H)-diasterane configurations, R and S are conformation at C-20).

1488 A.I. Rushdi, B.R.T. Simoneit / Applied Geochemistry 17 (2002) 1467–1494 Fig. 14. Features of the PAH distributions in samples from Site 1038: (A) 1038G-3H-4, 95–96, sum of m/z 128, 142, 156, 170, 178 and 192; (B) 1038C-1R-1, 30–31, sum of m/z 202, 228, 252, 276, 278 and 300; (C) 1038C-1R-1, 30–31, sum of m/z 302, 328, and 352; and (D) 1038B-1R-1, 0–10, sum of m/z 202, 228, 252, 276, and 300 (1=naphthalene; 2=methylnaphthalenes, 3=C2-naphthalenes, 4=C3-naphthalenes, 5=phenanthrene, 6=methylphenanthrenes, 7=C2-phenanthrenes, 8=dihydropyrene, 9=pyrene, 10=benzofluoranthenes, 11=benzo[a]pyrene, 12=benzo[e]pyrene, 13=perylene, 14=benzo[g]chrysene, 15=indeno[1,2,3-cd] pyrene, 16=benzo[ghi]perylene, 17=coronene).

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temperature alteration (Simoneit and Lonsdale, 1982; Blumer, 1975; Scott, 1982; Simoneit, 1984a,b, 1994; Simoneit and Fetzer, 1996). It is observed that the PAH distributions present in immature to mature samples (cf. Fig. 14 and Table 4) decrease in the concentration of perylene against benzofluoranthenes (Pe/BFa=0.01 to 5.6, Table 4). This trend is also observed for sediments and hydrothermal petroleums from Guaymas Basin and Middle Valley (Baker and Louda, 1982; Simoneit and Philp, 1982; Simoneit et al., 1984; Kawka and Simoneit, 1990; Rushdi and Simoneit, 2002). Perylene is generated by diagenetic processes at depth, but it is not stable at catagenetic temperatures (Louda and Baker, 1984; Kawka and Simoneit, 1990). Fluoranthene and pyrene are found in variable amounts in these samples, generally with fluoranthene low or absent (Fa/Py range 0.03–0.98, Table 4). Some samples contain dihydropyrene (e.g., Fig. 14B) which may be a hydrogenation product from pyrene under the strong reducing conditions of the hydrothermal environment. Significant amounts of benzopyrenes are found in most samples, and the ratio of benzo[a]pyrene to benzo[e]pyrene (BaP/BeP, Table 4) has been used to gauge the extent of secondary oxidation of the PAH once formed. This is based on the reports that BaP is less stable than BeP in the oxidative atmospheric environment (Nielsen, 1984; Nielsen et al., 1984). At the seafloor and in sediments these compounds may be reactive to other oxidizing agents, but with similar reactivities. The BaP/BeP range for these samples is from < 0.08 to 2.6, where a typical precursor value is > 0.4 and those samples with a ratio < 0.4 have an oxidative or thermal loss of BaP.

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Trough than in Middle Valley. The thermal alteration of these sediments with low organic matter contents yielded hydrocarbons with CPI values 41.0 versus CPI > 1.0 for thermally unaltered sections (e.g., seabed to 450 mbsf for reference Site 1037). Catagenetic hydrothermal activity at Site 1038 has also altered the natural product precursors (sterols, triterpenoids, and hopanoids) to their geological derivative compounds (sterane and triterpane biomarkers). The occurrence of PAHs in these samples is typical for hydrothermal petroleum (Simoneit, 1984a; Kawka and Simoneit, 1990; Simoneit and Fetzer, 1996). The low molecular weight PAHs (naphthalene and phenanthrene series) are rarely found in deeper sediment sections, and phenanthrene was detected only in shallow sections of Site 1038. Both vertical and lateral flow of hot fluids and the higher solubility of the low molecular weight PAH compounds caused their removal from deeper sediment intervals. Higher molecular weight PAHs (e.g., benzopyrenes, benzoperylene, coronene and others) are the major compounds in most of these samples and were also detected in upper sediments at 0.05 mbsf in Hole 1038B and at 0.3 mbsf in Hole 1038C. The presence and compositions of the PAHs in these sediments represent high temperature alteration of organic matter and product migration with water washing. At Site 1037 traces of PAHs are detected in deeper sediments (> 450 mbsf), however, only low molecular weight PAHs (naphthalene and phenantharene) were found.

4. Conclusions 3.4. Implications Although the content of organic matter in Escanaba Trough sediments is low (TOC=0.0–1.1%, mean 0.34% at Site 1037 and 0.0–1.12%, mean 0.27% at Site 1038, Fouquet et al., 1998), catagenetic processes have also occurred due to hydrothermal activity. At Site 1038 accelerated catagenesis of immature organic matter by high temperature fluid flow produced hydrothermal petroleum. Both thermal alteration and migration are observed at Site 1038, whereas at Site 1037 thermal alteration only occurs in deeper sediments (> 450 mbsf). The diverse hydrocarbon signatures versus depth at Site 1038 indicate that migration of hydrothermal petroleums occurred laterally rather than vertically. The source of the organic matter in Escanaba Trough sediments is mainly allochthonous inputs from rapid turbidite deposition. In the reference hole, the terrestrial biomarkers and lipids are enriched compared to the marine derived components and the organic matter is immature, supporting rapid turbidite infilling of the trough. The terrigenous components are more concentrated in the unaltered sediments of Escanaba

The organic matter content of the sediments from Site 1038 in Escanaba Trough is low and heavily altered. Overmature bitumen intervals are found with different organic matter compositions such as mainly condensed PAH compounds. Hydrothermal petroleum occurs in the shallow sections of several holes. These petroleums are products of severe organic matter alteration and reworking, and have migrated into zones near the seafloor. The full range of maturities (catagenesis to metagenesis) is observed with compositions varying from aliphatic to heavy aromatic (enriched in PAH) mixtures. The regional organic matter maturation around Central Hill in Escanaba Trough has been rapid for all holes and the temperatures were higher for this area than in the Middle Valley system (Simoneit, 1994; Rushdi and Simoneit, 2002). The sedimentary organic matter in the holes on hydrothermal mounds has been altered by hot fluid and high heat flows and the products have moved into shallower and cooler zones near the seafloor. The pyrolized kerogen residue resides in situ as carbonized soot, which is found at depth in all holes. The organic matter in sediments of Hole 1038I, located between the

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hydrothermal mounds, has been matured in situ without extensive fluid migration to remove the bitumen products. The sedimentary organic matter in the reference Site 1037 is generally immature from the top to approximately 450 mbsf. This is based on the analysis of the gas and bitumen compositions. Most of the bitumens reflect the immature n-alkane signature typical of a minor marine and major terrestrial higher plant (Cmax at 29 or 31 and CPI > 2.5) inputs. There are a few horizons with more mature bitumen, which is interpreted to be derived from recycled older sedimentary detritus carried in by turbidites. The low extract yields throughout the hole indicate that there are no petroleum zones. Below 450 mbsf the organic matter is thermally altered as reflected by the mature n-alkane and biomarker compositions. The biomarker signatures show an enrichment of terrestrial component inputs to these locales, more than in Middle Valley. Biomarkers are found to occur as precursors, intermediates or fully mature derivative products. For example, sterols, stenones, stanones and steranes are found in samples from all sites. The terrigenous triterpenols (i.e., amyrins) which interconvert to triterpenones and triterpenes are also present. The degradation of hopanes > C31 to C29 and C27 hopanes, reflecting cracking reactions caused by high temperature ( 280–300
C), is observed for samples from Site 1038. The biomarker signatures in these sediments reflect catagenesis of the precursors with both lateral and vertical product migration. Flow of fluids through these sediments added minor

amounts of dissolved organic components (PAH) from deeper high temperature sections to the in situ compounds, resulting in mixtures of bitumens with various maturities. PAH compounds are present in varying amounts in samples from Site 1038 and reflect the high temperature interconversion and reworking of sedimentary organic matter into hydrothermal petroleum tar. The low molecular weight PAHs are scarcely found in samples from Site 1038, and mainly high molecular weight PAH compounds occur. This may be explained by the removal of the lighter PAHs, which are more soluble in hot fluid, from the sediment by the hydrothermal flow, leaving the heavy PAHs behind.

Acknowledgements Financial support from the National Science Foundation and JOI, Inc., U.S. Science Support Program of the Ocean Drilling Program is gratefully acknowledged. We thank Dr. R.N. Leif and Dr. D.R. Oros for their reviews of this paper. Disclaimer: The U.S. Science Program associated with the Ocean Drilling Program is sponsored by the National Science Foundation and the Joint Oceanographic Institutions, Inc. Any opinions, findings and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the National Science Foundation, the Joint Oceanographic Institutions, Inc., or Texas A&M University.

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Appendix. Chemical Structures Cited

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