Preferential dissolution of carbonate shells driven by petroleum seep activity in the Gulf of Mexico

Earth and Planetary Science Letters 248 (2006) 227 – 243 www.elsevier.com/locate/epsl

Preferential dissolution of carbonate shells driven by petroleum seep activity in the Gulf of Mexico Wei-Jun Cai a,⁎, Feizhou Chen a , Eric N. Powell b , Sally E. Walker c , Karla M. Parsons-Hubbard d , George M. Staff e , Yongchen Wang a , Kathryn A. Ashton-Alcox b , W. Russell Callender f , Carlton E. Brett g a

Department of Marine Sciences, University of Georgia, Athens, GA 30602, USA Haskin Shellfish Research Lab, Rutgers University, Port Norris, NJ 08349, USA c Department of Geology, University of Georgia, Athens, GA 30602, USA d Department of Geology, Oberlin College, Oberlin, OH 44074, USA e Geology Department, Austin Community College, Austin, Texas 78758, USA National Oceanic and Atmospheric Administration, National Ocean Service, Center for Coastal Monitoring and Assessment, Silver Spring MD 20910, USA g Department of Geology, University of Cincinnati, Cincinnati, OH 45221, USA b

f

Received 8 July 2005; received in revised form 14 May 2006; accepted 20 May 2006 Available online 30 June 2006 Editor: H. Elderfield

Abstract Authigenic carbonates are common at deep-sea petroleum seeps as a result of excess bicarbonate production during microbial degradation of hydrocarbons coupled to sulfate reduction. Consequently, these seep environments are supersaturated with respect to carbonates. This finding conflicts with taphonomic data that dissolution is the most pervasive mode of shell alteration at seeps. We provide here the first study linking the preservational process with the chemical characterization of the taphonomically-active zone at petroleum seep sites. This characterization is made possible using fine-scale porewater carbonate chemistry data and skeletal material deployed for 8 years at petroleum seep sites in the northwestern Gulf of Mexico. Microelectrode measurements of pH and pCO2 identify a very restricted zone of CaCO3 undersaturation immediately below the sediment–water interface in otherwise supersaturated environments (i.e., sandwiched between supersaturated bottom seawater and sediment porewater). This zonation characterizes the taphonomically-active zone, and is a result of a highly compressed redox front between acid-generating aerobic oxidation of reduced chemical species including hydrocarbons, H2S, and planktonic-and-terrestrial organic carbon and base-generating sulfate reduction coupled to CH4 oxidation. Porewater geochemistry is spatially variable at seep sites, and produces variable shell-dissolution signatures. Shells deployed at seep sites have moderate to severe dissolution that is consistent with a much higher flux of total dissolved inorganic carbon (DIC) from the porewater to the bottom water. Therefore, a mosaic of preservational conditions is directly related to the spatially and chemically varying taphonomically-active zones found at seep sites. These findings support the variability of carbonate preservation reported for globally-distributed Phanerozoic fossil seeps and the view that data from field taphonomy can significantly upgrade and validate carbonate destruction rates used in geochemical and climatic models. Carbon mass-balance analyses also lead to an important conclusion that carbonate dissolution forms a very important mechanism for benthic carbon

⁎ Corresponding author. Tel.: +1 706 542 1285. E-mail address: [email protected] (W.-J. Cai). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.05.020

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recycling, possibly accounting for 50% of the benthic DIC flux to the bottom water in northern Gulf of Mexico petroleum seep sediments. © 2006 Elsevier B.V. All rights reserved. Keywords: carbonate preservation/dissolution; petroleum seeps; porewater chemistry; microelectrodes; taphonomy

1. Introduction Deep-water petroleum seeps are important repositories for gas hydrates and are also important in global climate studies [1,2]. Geochemical processes at seeps are driven by microbially-mediated oxidation of hydrocarbons such as CH4 [3–7]. While the modern death assemblages of a few petroleum seeps have been characterized as taphonomically active [8–11], little is known about the geochemical milieu associated with the preservation of these marine shelly deposits [12]. Fossiliferous hydrocarbon seeps have an ancient history, dating from the Devonian [13]. Carbonates from fossil seeps yield a diagenetic spectrum related to the evolution of fluids stemming from reduction/oxidation chemical sequences [14]. Authigenic carbonates produced under these conditions [14] indicate an environment that is supersaturated with respect to carbonates that would enhance preservation of the megainvertebrate communities associated with them. However, biogenic carbonates, including fossiliferous material, have variable preservation from calcitic/aragonitic preservation of vestimentiferan worm tubes [14] to pyritization and/or shell corrosion due to sulfuric acid that can leach out of passive seep margin systems [15]. Based on our study, the preservation potential is variable at seep sites based on the patchy nature of the geochemical system. Thus, there exists an apparent paradox: seeps have both corrosive and supersaturated conditions that greatly affect carbonate dissolution and authigenic production. The question is: How does this paradox of diametrically different chemical systems occur in seemingly close proximity at seep sites? And, how does this complexity affect carbonate preservation and ultimately what is preserved in the fossil record? Geochemically-active petroleum seeps offer an excellent opportunity to compare the microbially-driven chemical system with the carbonate dissolution/preservation process. While a taphonomically-active zone (TAZ) associated with carbonate destruction is known to exist in shallow-marine terrigeneous clastic sediments [16] and was linked to porewater undersaturation at pH minimum zones [17,18], such a zone has not been characterized for petroleum seep sites. One unique feature of the cold

seep system is that the oxic–anoxic boundary is highly compressed to a few mm near the sediment–water interface at active sites; this would hamper studies via traditional porewater sectioning methods. One of the objectives of this study was to directly link long-term (8 years) taphonomic experiments using carbonate shells deployed at the sediment–water interface with microelectrode-millimeter-scale porewater chemical characterization within the framework of the Shelfand-Slope Experimental Taphonomy Initiative (SSETI) [19,20]. Using microelectrode measurements of O2, pH, and pCO2 in association with the skeletal geochemical experiments, we examined redox cycling and calcium carbonate dissolution/precipitation status at two petroleum seep sites in the Gulf of Mexico to understand how the fine-scale geochemical milieu affects carbonate dissolution/preservation at these geologically important sites. This research is the first report of fine scale pH, pCO2 and carbonate saturation state in petroleum seep sediments and their geochemical/paleontological implications. 2. Site description, experimental design, and methods Four sets of shelled-carbonate experiments containing up to twenty shells each of bivalve and gastropod species were deployed via submersible at two sites (site 2 and site 4) in Green Canyon lease block 234 (GC234: 27°44.72′N, 91°20.53′W) in the Gulf of Mexico in 1993 (Fig. 1 and Table 1). The geological and physical environment, living communities, and depth assemblages characteristic of petroleum seeps at GC234 are described in [9–11,21–23]. Briefly, Gulf of Mexico gas hydrates are rich in methane (about 40%), ethane, propane, iso-butane and butane (Sassen et al. [2]). Sediments in the active seep areas have elevated concentrations of hydrocarbons and oils as well as high H2S concentrations [2,7,24]. Site 2 is a methane seep (548 m water depth) with microbial mats, large and small tubeworms, mussels and other unidentified benthic organisms (Fig. 1). While abundant mats of Beggiatoa were most common in areas of active seepage at GC234 [7], it appears from core top photos (core 2A) taken at site 2 that the filamentous bacteria mats we

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Fig. 1. Locality of the Green Canyon lease block 234. Site 2 is depicted and is an active petroleum seep occupied by bacterial mats, chemosynthetic tubeworms, clams, and mussels. Site 4, not depicted, is a reference site without chemosynthetic fauna and limited petroleum seepage.

sampled were likely Flexibacteria (S.B. Joye, pers. com., 2005). Site 4, located 203 m away from site 2 and upslope at 522 m water depth, has no obvious seep activity. Additional information on the sites was provided in [25]. The SSETI experimental design is described in detail in [19,20] and summarized briefly here. Each experimental array consisted of a series of 1-cm-mesh bags attached to a 1.5-m PVC pole. To each pole was added a 12-kg weight to counter a 25-cm square float made of 6-mm-thick sheet polypropylene. The float rose about 0.5 m above the array and served to mark the location of the experiment even when buried. The PVC pole was unattached to the bottom, anchored only by the 12-kg weight, and so was free to move, given sufficient current action. Two of the mesh bags on each pole contained molluscan shells, typically five individuals of five different species, each species compartmentalized from the others by plastic cable ties. Molluscan species deployed at GC234 included the ocean quahog Arctica islandica, the blue mussel Mytilus edulis, the lucinid Codakia orbicularis, the scallop Argopecten irradians, the conch Strombus luhuanus, and the turritellid, Turritella terebra. These shells were chosen for their range of mineral composition and microstructure (Table 1).

Experiments from sites 2 and 4 were recovered 8 years later in 2001, and the skeletal material was subjected to taphonomic analysis according to methods described in [16,20]. Dissolution indices were calculated separately for the inner and outer shell surfaces for bivalves and the spire and body whorl for gastropods to account for the differences in skeletal mineralogy and microstructure in these shell areas. Dissolution was assessed on eight sample shell areas on the inner and outer surface for each bivalve and five surface areas for each gastropod. A dissolution index for each shell was obtained as the area-weighted average of the shell areas. This index varies from 0, a pristine shell, to 3, a shell characterized by extreme corrosion including the loss of sculpture and eroded holes in shell. This analytical method is described in detail in [20,26,27]. Data were analyzed by an ANOVA using location and, where appropriate, species, as main effects. The dissolution index describes a doubly truncated distribution analogous to data on proportions, as values can vary only from 0 to 3. Such data are frequently strongly nonnormally distributed, and this was true for most species and site data subsets from GC234 (Shapiro-Wilk test; [28]). Consequently, the data were arcsine square-root transformed after standardization to a range of 0 to 1 [29]. A posteriori Least Square Means Tests were used

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Table 1 Average dissolution index after 8 years for species deployed at site 2 (the petroleum seep site) and site 4 (the reference site) Mollusk species

Total shell

Inner shell/ body whorl

Outer shell/ spire

Mineralogy and microstructure

Comments

Site 2 Site 4 Site 2 Site 4 Site 2 Site 4 Arctica islandica 1.01

1.15

0.16

0.86

1.86

1.44

Mytilus edulis

2.47

1.41

2.63

1.48

2.31

1.33

Codakia orbicularis

1.07

0.51

0.54

0.29

1.60

0.74

Strombus luhuanus

2.81

2.81

2.82

2.82

2.70

2.70

Argopecten irradians

1.87

0.53

1.63

0.32

2.11

0.73

Turritella terebra 2.50

1.17

2.47

1.19

2.50

1.17

Site 2: chalky, moderate–heavy dissolution, sculpture gone Site 4: minor–moderate dissolution Calcitic P outer layer; middle to inner Site 2: chalky, moderate–heavy layers of aragonitic nacre. dissolution Site 4: minor dissolution Aragonitic: outer layer CP, middle CL, Site 2: chalky, minor–moderate inner CCL dissolution Site 4: chalky, minor dissolution Aragonitic: three CL layers Site 2: chalky, heavy dissolution Site 4: chalky, moderate–heavy dissolution, microbial crusts Calcitic CF outer layer; middle aragonitic Site 2: minor–moderate dissolution CL; inner layer aragonitic CL; calcitic F Site 4: minor dissolution (near umbonal area) Aragonite: three CL layers Site 2: chalky, sculpture gone Site 4: chalky, minor pitting; microbial crusts Aragonitic: outer layer, H; middle layer, prismatic myostracum; outer layer, H.

Dissolution index ranges from 0 to 3 (see text). 0: undissolved; 1: surface chalky, minor pitting; 2: surface pitting moderate to heavy, surface soft, sculpture enhanced; and 3: surface sculpture gone, deeply dissolved, holes may be present. Microstructure key: CL, cross lamellar; CCL, complexcrossed lamellar; F, foliated; P, prismatic; CP, composite prismatic; H, homogeneous; minor dissolution: shell <25% dissolution; moderate dissolution, <50% dissolution; heavy dissolution, >50% <90% cover on shell). n = 10 for M. edulis, C. orbicularis, T. terebra, and S. luhuanus and 5 for A. islandica and A. irradians per site.

to identify sources of significance within the significant ANOVAs. For porewater geochemistry, sediment push cores were taken adjacent to the buried shells during dives with the Johnson-Sea-Link submersible at the time when shell bag array rods were recovered. We sampled three types of sediment push cores: (1) microbial matassociated sediments at or adjacent to the mat-covered area from site 2 (core 2A; dive number: 4339); (2) sediments slightly away from the thick mats at site 2 (core 2B for microelectrode work and similar cores for porewater processing) with bacterial filaments and small biogenic tubes in the sediment surface (dive number: 4338); (3) sediments with no apparent microbial mats from reference site 4 (dive number: 4340). Sediments collected here may represent a range of seep activities from active (site 2 core 2A), to moderately active (site 2 core 2B) to low seepage area (site 4). These push cores were returned to the surface for microelectrode studies and other porewater analyses. Note that, due to the limited number of cores, sediments from dive number 4339 were used only for microelectrode measurements and no porewater was obtained. Stable Clark-type O2 microelectrodes (tip dia. ∼ 50 μm), glass pH microelectrodes (tip dia.

∼ 100 μm), and pCO 2 microelectrodes (tip dia. ∼ 200 μm) were used for shipboard profiling. Precisions of pH and pCO2 measurements were 0.02 units and 3– 5%, respectively [30–32]. All measurements were carried out on board ship in a Faraday cage and cores were kept at near-bottom water temperature before and during profiling. pH microelectrodes were calibrated with pH 4 and pH 7 buffers and left in the overlying water until the reading was stable (∼ 30 min) together with a standard Orion Ross pH combination electrode which was calibrated in the same buffers. The pH value in the overlying water as measured by the large commercial electrode was used as the reference point for the microelectrode. Microelectrode readings in the overlying water before and after a sediment profile were used as an indicator of whether an electrode had drifted. pCO2 microelectrodes were calibrated immediately before use in a sealed cell by adding known amounts of NaHCO3 + Na2CO3 solution (with known total CO2 concentration measured against a reference standard before use) sequentially to 0.1 N HCl solution using a precision digital syringe pump. Such a calibration method is convenient to use on board ship and was checked against standard gases at our home laboratory [31]. After pH and pCO2 microelectrode measurements

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were completed, the core-tubing was drilled through (with a hand-held power drill) at 1- to 2-cm intervals and porewater pH was measured sequentially from the core top to the bottom by punch-in using a 6-mm diameter combination mini-pH electrode (Orion Ross electrode #8103BN). Profiles measured with the 6-mm commercial pH electrode agreed with that of the pH microelectrode reasonably well except in two cases. First, in the top 2 cm, the mini-electrode could not provide the fine spatial resolution of the microelectrode. Second, disagreement sometimes occurred in a certain area or depth as a result of sediment heterogeneity (i.e., in the case of GC234 Site 2, Core 2B). An alternative in the second case is that a microelectrode functioned well in the surface sediment, but was damaged at depth, and thus drifted [30]. We do not believe that microelectrode drift was a problem because the same readings were collected for the repeating profile, and readings in the overlying water before and after a profile indicated no electrode drift (Fig. 2B). Within 2 h after recovery, sediment cores were sectioned and porewaters were immediately expressed under N2 pressure (same device as in [7]). Sediments were sectioned at 1-cm intervals. Thus the 0.5-cm value is an average for the top 1 cm of sediment, the 1.5-cm value is an average for the 1- to 2-cm section, and so on.

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Porewaters were preserved with HgCl2 and analyzed for total dissolved inorganic carbon (DIC), total alkalinity (TAlk), and other porewater chemistry. Analytical precisions for DIC and TAlk were 0.2% [33]. Porewater Ca2+ samples were not preserved (but were kept cold under 5 °C) and were measured by EGTA titration shortly after the cruise with an analytical precision of 0.2% (as described in [34]). Porewater samples were analyzed for dissolved sulfide and sulfate by the colorimetric methylene-blue method [35] and the classic gravimetric method as described in [34], respectively. 3. Results and discussion 3.1. Carbonate shell preservational signatures Carbonate shells deployed for 8 years at site 2 were chalky with moderate to heavy dissolution (Fig. 3, Table 1). In contrast, shells deployed at site 4 were characterized by only minor dissolution, a slight chalky appearance, with microbial epibionts (Fig. 3, Table 1). Taphonomic condition was significantly influenced by molluscan species (ANOVA, P < 0.0001) and by site (ANOVA, P = 0.0001). Overall, shells deployed at site 2 were significantly more dissolved than shells deployed at site 4. The site-by-species interaction term was also

Fig. 2. Porewater pH profiles and the performance of the microelectrodes. The microelectrode profiling started and finalized at − 2 mm depth (i.e., the overlying water), demonstrating the stability of the electrode. The ending reading is given as at − 3 mm in panel A and − 2 mm in panel B. In panel B, a second profiling was executed after the first one. Repeatable results were obtained. For comparison and evaluation of the performance of the microelectrodes, porewater pH data were also collected with a commercial 6-mm diameter mini-electrode (Ross Orion) by punch-in of the electrode from the side of the core tubing.

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Fig. 3. Comparison of shell condition between site 2, active seep (left column), and site 4, reference site (right column). A., M. edulis exterior (site 2), B., M. edulis exterior (site 4). C., M. edulis interior (site 2). D., M. edulis interior (site 4), E., S. luhuanus, (site 2), F., S. luhuanus (site 4). G., T. terebra (site 2). H., T. terebra (site 4).

significant (ANOVA, P = 0.007), indicating that the effect of site was not uniform across all species. The tendency for S. luhuanus to be heavily dissolved at both sites was the primary determining factor. Shell types had the following dissolution cascade, from the most taphonomically affected to the least: Strombus >Mytilus > Turritella >Arctica > Argopecten> Codakia. The dissolution rankings of Mytilus and Codakia from this study agree with a previous report on comparative taphonomy of those two species [8]. The

highest-ranked species, Strombus, differed significantly from the other five (a posteriori LS-means test, P < 0.0001 in each case). T. terebra differed significantly from four of the other five taxa, A. irradians (P = 0.012), S. luhuanus and C. orbicularis (P < 0.0001), and A. islandica (P = 0.0048) (all LS-means tests). The lowest ranked species, C. orbicularis, differed significantly from three other taxa, M. edulis, S luhuanus, and T. terebra (all P < 0.0001). The other three taxa likewise differed significantly from most others. Thus considerable

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variability existed in the response of species to taphonomic attack. For this reason, and because the species-by-site interaction term was significant, further analyses considered species individually. M. edulis shells were significantly better preserved at site 4 (ANOVA, P = 0.04). This difference was established by a significantly higher dissolution score on the inner valve at site 2 (P = 0.036). C. orbicularis shells likewise were significantly better preserved at site 4 (ANOVA, P = 0.04). In this case, the difference was determined primarily by a significantly higher dissolution score on the outer valve at site 2 (P = 0.049). A. irradians shells were also significantly better preserved at site 4 (ANOVA, P = 0.026). In this case dissolution scores were significantly higher on both valves at site 2 (inner, P = 0.028; outer, P = 0.042). T. terebra shells diverged most significantly between sites (ANOVA, P = 0.0003). Both the spire and body whorl were significantly more dissolved at site 2 (spire and body whorl, P = 0.0006). The preservational state of the remaining species and shell areas did not differ significantly between sites. However, dissolution indices ranked higher at site 2 in all but one comparison (six species × two major shell areas with two ties), a directionality not anticipated to occur by chance (Binomial test, P < 0.01). Both sites were taphonomically-active sites, but shell degradation was proceeding at a more rapid rate at the active-seep site (site 2). Comparison with control shells reaffirms the view that site 2 was more taphonomically active. Five of six species differed significantly from controls: Strombus had enhanced dissolution of the body whorl (P < 0.0001), spire (P = 0.0002), and the average dissolution index was higher (P < 0.0001); Mytilus had enhanced dissolution of the inner shell area, P < 0.0001, and outer shell area, P < 0.0001, and the average dissolution index was higher, P < 0.0001); Turritella had enhanced dissolution on the spire and body whorl (P < 0.0001) and the average dissolution index was higher (P < 0.0001); Argopecten had significantly higher dissolution scores for the inner shell area (P < 0.0001) and the average dissolution index was lower (P = 0.007), but the outer shell surface did differ significantly. Codakia also was more dissolved (P < 0.0001), and this difference was apparent on both the inner (P = 0.01) and outer (P < 0.0001) shell surfaces. In contrast, only two species differed from controls at site 4. M. edulis and S. luhuanus, and both species differed from controls on both shell areas (P < 0.0002). Thus, comparison to controls confirms the trend towards increased shell degradation at site 2. These differences reflect the distribution of particular mineralogy and

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microstructure within the shell for the different species (Table 1). 3.2. Porewater geochemical processes of acid-base, redox, dissolution and precipitation The variations of the O2, pH and pCO2 profiles are closely related to their localities around the seep (Figs. 3, 4 and 5). These profiles from different locations provide dramatic contrasts in their oxygen penetration depths and in their pH minimum–maximum changes. At or next to the mat-covered area (core 2A), oxygen is depleted largely within 0.2–0.4 cm of the sediment surface. However, a small but detectable amount of oxygen exists throughout perhaps the entire layer of the mat (∼ 1 cm, Fig. 4). In core 2B, which is slightly away from the mat area, oxygen penetration depth increases to an unusually high value of more than 3.5 cm. Note that even at the CH4-free reference site (site 4), oxygen penetration depth is only 1.2 cm. At normal continental shelf and slope sediments of this depth, oxygen penetration depths rarely exceed 1.5 cm [36,37]. Zimmermann et al. [38] also reported a deep oxygen penetration depth (∼ 10 cm) at a muddy seep site and

Fig. 4. Oxygen profiles from bacterial mat-covered core 2A from site 2 (red triangle, and green diamond), core 2B taken adjacent to the bacterial mats (blue circle) and a core taken at the reference site 4 (brown cross).

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Fig. 5. pH and pCO2 profiles from bacterial mat-covered core 2A from site 2, core 2B taken adjacent to the bacterial mat and a core taken at the reference site 4. In core 2A, a profile was collected directly in a thick mat area (marked as “at mats”) and another one was collected in an area not covered by thick mats (marked as “next to mats”).

shallow oxygen penetration depth (∼ 1 cm) at a nearby non-seep site. The abundant benthic organisms including small tubeworms and the mobile infaunal clams in these cores at the seep site may contribute to such unusually deep oxygen penetration. Large tubeworms specific to seep environments may also pump oxygen into sediments [39]. In addition, physical disturbance may also contribute to deep O2 penetration [38]. In contrast, while benthic organisms also exist in core 2A, at or next to the mats, a very high level of CH4 (increases from 0 to 1000 μM in top 6 cm, [7]) and H2S (increases from 0 to 6 μM in top 2 cm of sediment in a core similar to core 2B in this work; from a few tens of μM to few mM in active seep sites, [7,24]) depletes O2 quickly to near zero at 0.3-cm depth. Benthic (diffusive) O2 flux calculated from the two microelectrode profiles collected at the mat-covered core 2A is extremely high (15.2– 16.6 mmol m− 2 d− 1). Benthic O2 flux calculated for core 2B is moderately high (6.2 mmol m− 2 d− 1). Flux for the reference site (site 4) is relatively low (3.6 mmol m− 2 d− 1) (Table 2). In the mat-covered area, porewater pH decreases sharply from the overlying water value of ∼ 7.9 to a minimum of 7.4 at 0.3-cm depth before quickly increasing to a very high pH maximum of 8.5 at 2-cm depth (pH profile of core 2A “at mats” in Fig. 5A). Below this broader pH peak, pH decreases again to

about 7.5 at depths of 5 to 10 cm. The deviations of pH minimum and maximum from the overlying water value are smaller for the profile collected in the same core but not directly in the thick-mat area (pH profile of core 2A “next to mats” in Fig. 5A). For the core away from the mats but generally at the seep site (core 2B), while the tendency of pH to decrease and increase is not greatly different from those at or next to the mats, the rate of pH decrease is much smaller and the minimum occurs at deeper depth (0.7 cm). However, below the pH minimum, the pH increase is very small, remaining nearly constant at a low pH value for a 2-cm interval to a depth of 3.5 cm. Below 3.5 cm, pH increases again to about 7.9. The absence of a sharp pH increase below its minimum is well correlated with a deep oxygen penetration depth (3.5 cm) in this core (core 2B). Site 4 serves as a reference site and has much smaller pH changes in either direction (i.e., the minimum and maximum) over the top 10 cm of the sediment column. The profiles of log pCO2 vary somewhat in mirror image with the pH profiles both vertically and horizontally, i.e., in general log pCO2 decreases (or increases) when pH increases (or decreases) (Fig. 5A and B). However, some significant differences exist. For example, a broad log pCO2 maximum corresponds to the very sharp pH minimum in the mat-covered site. The further increase in pCO2 below the pH minimum is a

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Table 2 Estimates of diffusive flux of various species Species

Din situ (109 m2 s− 1)

Depth zone (cm)

Porosity

Gradient (μM cm− 1)

Flux (mmol m− 2 d− 1)

Seep site (site 2) O2 CH4 (hydrocarbons) HS− DIC DIC TAlk TAlk Ca2+ Ca2+

1.60 1.15 1.53 0.836 0.836 0.836 0.836 0.575 0.575

0 0–1.5 0.5–1.5 0–0.5 1.5–2.5 0–0.5 1.5–2.5 0–1.5 1.5–2.5

0.90 0.75 0.70 0.80 0.67 0.80 0.67 0.80 0.67

− 500 to − 1333 167 6–60 2053 240 2193 217 596 − 470

6.22–16.6 (12.7) − 1.24 (− 3.1?) − 0.06 to − 0.6 − 11.9 − 1.15 − 12.7 − 1.16 − 2.22 1.57

Reference site (site 4) O2 DIC Ca2+ Ca2+

1.60 0.836 0.575 0.575

0 0–0.5 0–0.5 0.5–1.5

0.94 0.80 0.80 0.67

− 160 564 220 − 80

3.38 − 3.26 − 0.82 0.27

Positive flux is defined as downwards into the sediment. Diffusive coefficients at 25 °C and infinite-dilution state are taken from Boudeau [58]. Viscosity equations from the same reference are used to adjust the diffusion coefficients to values at in situ temperature (12 °C), salinity (35) and pressure (54 atm). The same viscosity data are applied to the equations and data given in Li and Gregory [59]. Agreements on calculated diffusion coefficients are generally within 5%. The diffusion coefficient for HCO−3 is used for DIC (or TAlk) as HCO−3 is over 90% of the DIC (or TAlk) in the porewater. Porosity (cm3 dry sediment vs cm3 wet sediment) was measured except that at the sediment–water interface, which was interpolated between 0.5 cm and overlying water (which equals 1 by definition). Flux is calculated as, Flux = − Porosity ⁎ Din situ ⁎ Gradient. For O2 gradient at the sediment–water interface, a 2nd-order polynomial model as presented in [37] is used to fit the data. For the seep site (site 2), the high end of the O2 flux is from the mat-covered area and the low end is the non-mat area. DIC and Ca2+ data are only available from the non-mat area. Regression line of data at 0, 0.5 and 1.5 was used to interpolate the Ca2+ gradient between 0 and 0.5 cm at site 2. For all other flux calculation, concentrations at the two end points of the indicated depth range are used. CH4 gradient and the high end HS− gradient are estimated from [7].

result of a sharp DIC increase (an increase of 1.5 mM in the top 1.5 cm of sediment) in seep sediments as porewater CO2 concentration is linked to DIC and [H+] DIC⁎½Hþ 2 as ½CO2  ¼ ½Hþ 2 þ½Hþ K þK K , where K1 and K2 are the 1 1 2 dissociate constants of carbonic acid [40]. We use the constants of Mehrbach et al. [41] for all equilibrium calculations. Similar pH minimum and maximum changes and the mirror image changes in log pCO2 with the pH profiles from GC234 site 2 were also observed at a nearby Garden Bank block 425 site (GB425: 27°33.56′N, 92°32.34′W) during the same cruise and at a Bush Hill site (27°78′N, 91°51′W) during another cruise in June 2002 (W.-J. Cai unpublished data). Thus such sharp pH decreases and subsequent increases are common features of petroleum seep sediments although they have not been reported before. It should also be pointed out that the exact shapes and locations of the oxygen gradients, the oxygen penetration depths, and the pH and pCO2 minimum–maximum changes vary not only between seep and non-seep sites, but also vary considerably within a seep site and among seep sites in the Gulf of Mexico. For example, deep O2 penetration accompanied by an extended zone of low-pH (site 2, core 2B in Fig. 4 and Fig. 5A) was not observed at the

nearby GB425 site and this may correlate with an absence of tubeworms at the sediment surface of this other locale (W-J Cai unpublished data). Other porewater chemicals also differ between site 2 and site 4. Total dissolved inorganic carbon (DIC) and total alkalinity (TAlk) profiles show a greater accumulation of metabolic products (Fig. 6A), and hence, a higher microbial oxidation rate at site 2 than at site 4. The large increase in DIC and TAlk could cause carbonate precipitation at deep depths. Increased H2S and decreased sulfate concentrations at depth also indicate a high rate of sulfate reduction at site 2, but an insignificant rate at site 4 (Fig. 6B) (also see [24]). Again, it should be noted that these porewater profiles were collected in cores similar to core 2B and microbial activities there must be less intensive as compared to core 2A where only microelectrode profiles were collected. The vertical changes in pH and pCO2 profiles are consistent with microbial oxidation of planktonic organic matter, hydrocarbons such as CH4, and other reduced species such as H2S as well as carbonate dissolution and precipitation. Porewater pH may decrease either as a result of CO2 addition from aerobic oxidation of planktonic or terrestrial organic matter or

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Fig. 6. Profiles of porewater DIC, TAlk, and Ca2+ (A) and sulfate and sulfide (B) in sediments from the seep site (site 2, filled symbols) and the reference site (site 4, open symbols).

hydrocarbons such as CH4 according to the following diagenetic pathways [42,43]: ðCH2 OÞ106 ðNH3 Þ16 ðH3 PO4 Þ þ 138O2 ⇒106CO2 þ 16HNO3 þ H3 PO4

ð1Þ

CH4 þ 2O2 ⇒CO2 þ 2H2 O

ð2Þ

Redfield CNP ratio is assumed for the organic matter in Eq. (1). Following this diagenetic pathway, total alkalinity generation rate vs that of DIC, ΔTAlk/ΔDIC, is − 0.17. The production of a weak acid CO2 will cause a decrease in pH. In deep sea sediments, the rate of organic matter oxidation is slow and the pH decrease is generally small. The pH change is particularly small when CaCO3 dissolution neutralizes part of the diagenetically produced CO2 [44–46]. In coastal marine sediments, in addition to the CO2 produced from organic matter oxidation, an even more important factor that causes an often very low porewater pH value is the oxidation of reduced porewater species that have diffused upward from deep depths [17,30,32,47]. However, it appears that algal or terrestrial organic matter inputs are insignificant relative to hydrocarbons at the Gulf of Mexico seep sites [24]. One observation that strongly supports this conclusion is that PO43− and NH4+ concentrations are very low in porewater (∼ 10 and 0 μM respectively at 10 cm, [7] and Cai unpublished data) resulting in unusually high C/P and C/N ratios in porewater. We measured DIC/DIN ratios

at depths below 5 cm as high as 45–480 (data not reported; DIN: total dissolved inorganic nitrogen) suggesting no plankton organic carbon oxidation (with expected DIC / DIN = 6.6) below the very surface layer in our cores. However, we cannot rule out that oxidation of marine and terrestrial organic matter makes an important contribution toward driving the pH decrease in the top few millimeters of the sediment. The aerobic oxidation of 1 mol of CH4 will increase 1 mol of DIC but cause no change in TAlk (i.e., ΔTAlk / ΔDIC = 0). A nearly perfect positive linear TAlk to DIC relationship (slope = 1.26; r2 = 0.998) in our data (Fig. 7)

Fig. 7. TAlk to DIC relationships. Filled circles represent the seep site (site 2) and open circles represent the reference site (site 4). Dotted line is the linear regression line of all the seep-site data from bottom water to the 16-cm depth.

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appears at odds with such a mechanism of aerobic oxidation of hydrocarbons and/or planktonic organic matter. However, simple mass balance calculations suggest that this mechanism is consistent with our data for several reasons. First, the overall TAlk to DIC correlation is likely a combined result of CaCO3 dissolution promoted by aerobic oxidation of CH4, H2S and planktonic organic matter in surface sediments (ΔTAlk / ΔDIC ≈ 1) as well as SO42− reduction coupled to hydrocarbon oxidation (ΔTAlk / ΔDIC ≈ 2), and other diagenetic processes at deep depths. Second, at the DIC and TAlk values in the surface sediment (∼ 3.3 mM), equilibrium calculations suggest that a relative increase of 100, 200, and 300 μM in DIC vs. TAlk (or a similar decrease in TAlk relative to DIC) will cause a pH decrease of 0.18, 0.40 and 0.63 units respectively. Increasing the DIC values in the first two depths (i.e., 0.5 and 1.5 cm) by 100–300 μM will not alter the TAlk to DIC plot noticeably (R2 actually improves from 0.998 to 0.999) although it will cause a significant decrease in pH. Using a CH4 gradient measured at the GC234 seep site by Joye et al. [7], we calculate a CH4 flux towards the surface sediment at 1.2 mmol m− 2 d− 1 (Table 2). If all CH4 is oxidized to CO2 over a distance of 1 cm immediately below the sediment–water interface, then the DIC increase will be around 120 μM d− 1 (i.e., 1.2 mmol m− 2 d− 1 divided by 0.01 m). Within a diffusion time scale of 12 h (time = distance2 / 2D where D is the diffusion coefficient) for HCO3−, the DIC accumulation is around 60 μM. As CH4 content is only about 40% of the total hydrocarbons in the Gulf of Mexico [2], a reasonable DIC increase of 140 μM from hydrocarbon oxidation may be expected. This CO2 generation rate is close to what is required by the equilibrium calculation to cause a significant pH decrease. Carbon and oxygen mass balance analysis in Section 3.3 suggests that the amount of CO2 generated in the surface sediment by the oxidation of hydrocarbons is about 2.5 times that of planktonic organic carbon. Thus oxidation of planktonic organic carbon can provide an additional 29% of CO2 for driving the pH decrease. We therefore suggest that the oxidation of hydrocarbons plays an important role in the rapid pH decrease of 0.5 units over the top 1 cm of sediments in the Northern Gulf of Mexico seep areas. Another important factor that can decrease porewater pH is the proton release from the oxidation of reduced species that have diffused upward from deep depths, such as H2S (i.e., H2S + 2O2 ⇒ SO42− + 2H+), NH4+, Fe2+ and Mn2+ [42,43]. We now estimate whether these reactions are sufficient to cause the surface porewater pH decrease to the magnitude observed. We have

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measured an increase in H2S concentration from 0 at 0.5 cm to 6 μM at 1.5 cm in a core similar to core 2B (Fig. 6B). We estimated a H2S flux of 0.06 mmol m− 2 d− 1 and a potential TAlk increase of only 3.5 μM if all H2S is oxidized in the top 1 cm using our H2S data (Table 2). However, an increase in H2S flux of 10 times or more will contribute significantly to a low alkalinity. [H2S] of a few ten μM to a few mM were reported at a few cm depth at similar sites [7,24]. Thus, at least directly under the mat-covered area or at active seep sites, the H2S oxidation can be responsible for a significant part of the sharp pH decrease. Another potentially important source of acid generation that we have not investigated experimentally is the oxidation of solid forms of reduced sulfides such as FeS and pyrite [17] as authigenic pyrites are common in seep areas [5]. Based on the fact that NH4+ concentration is almost zero below 5-cm depth, we assume that oxidation of NH4+ diffused upward from deep depths is insufficient to cause the pH decrease in the surface sediment. Finally, Fe2+ and Mn2+ concentrations in our cores are all within a few μM (data not reported) and thus their oxidation will likely not contribute to the alkalinity budget significantly. Low pH and high pCO2 in subsurface sediment can cause a strong carbonate undersaturation (Fig. 8) and thus CaCO3 dissolves (CaCO3 + H+ ⇒ Ca2+ + HCO3− and/or CaCO 3 + CO 2 + H 2 O ⇒ Ca 2+ + 2HCO 3− with ΔTAlk / ΔDIC = 2). Below the oxygen penetration depth, sulfate reduction coupled to CH4 oxidation can drive pH to very high values (CH4 + SO42− + 2H+⇒H2S + CO2 + H2O with ΔTAlk / ΔDIC = 2). At such high pH (8.5) and supersaturation as observed under the microbial mats in core 2A at 2-cm depth (Fig. 5A), carbonate precipitation (Ca 2+ + CO32− ⇒ CaCO3 with ΔTAlk / ΔDIC = 2) must occur at a high rate, which is consistent with the observations of significant carbonate build-up at seep sites [5,13]. Thus, the narrow zone of carbonate undersaturation immediately below the sediment–water interface is most likely a result of a highly compressed redox front between the aerobic oxidation of reduced chemicals (i.e., hydrocarbon, H2S and planktonic organic carbon) (decreasing pH) and anaerobic sulfate reduction coupled to methane oxidation (increasing pH). Below the pH maximum depth (2 cm), porewater pH was reduced to a lower value as a result of carbonate precipitation at deeper depths (Fig. 5A). Note that our sampling sites are not directly located at carbonate buildup sites (where we may expect more dramatic pH and pCO2 minimum and maximum changes). Also note that as pH microelectrode profiles and porewater concentrations of DIC, TAlk, Ca2+, and SO42− are not collected in

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Fig. 8. Calculated porewater ion molarity product (IMP) in sediments from the seep site (site 2, filled symbols) and the reference site (site 4, open symbols). Calcite and aragonite saturation states are also indicated. The temperature, salinity and pressure dependent equations given in UNESCO [60] and Mucci [61] were followed. However, a constant of − 0.02 is added to Mucci's equations to bring his pKsp (calc.) at 1 atm, 25 °C and salinity of 35 down from 6.37 to 6.35 (i.e., to be closer to other data listed in the UNESCO document as was the practice adopted in Cai et al. [45]). IMP is calculated from the finescale pH profile of core 2B and interpolated porewater DIC and Ca2+ data (from a core similar to core 2B) and corrected to in situ pressure and temperature. Saturation state was not calculated from pH and pCO2 due to the very sharp gradient and possible spatial mismatch of the two profiles (see discussion in Cai et al., [32]).

the same cores, their changes with depth do not need to be closely correlated to each other. While we did not measure methane concentration and sulfate reduction rates, the observed pattern in a pH and pCO2 minimum and maximum change can be explained readily by the processes described above and by high CH4 (and or other hydrocarbons) concentration and high sulfate reduction rates reported at these seep sites [7,24]. However, further research is needed to conclusively link the relationship between a highly compressed redox front and a pH/pCO2 minimum and maximum change to specific redox reactions. The porewater Ca2+ profile is a direct indicator of CaCO3 dissolution and precipitation. Below the sediment–water interface, [Ca2+] increases sharply at site 2 but only slightly at site 4 indicating more dissolution at site 2 than at site 4 at the depth where the pH minimum exists (Fig. 5A). The actualistic taphonomic study of buried shells also supports enhanced CaCO3 dissolution at site 2 versus site 4 (Table 1 and discussion in Section

3.1). [Ca2+] decreased at deeper depths at the seep site (site 2) but not at the reference site (site 4) indicating CaCO3 precipitation at high pH at the seep site. As a result of carbonate precipitation pH is lower at deep depths in seep sediments. Note that porewater was not extracted from the mat-covered core (core 2A) where we may expect a more dramatic porewater [Ca2+] change. The potential for carbonate dissolution and precipitation is further and best illustrated by the porewater carbonate saturation state. Calculated ion molarity product (IMP) profiles (Fig. 8) have a very narrow zone (at 0.3 to 2-cm depths) of aragonite undersaturation at both the active seep site and the reference site. This narrow subsurface zone of undersaturation is sandwiched between a supersaturated overlying bottom water and a highly supersaturated sediment porewater below a few cm depth. Once the shells pass through this narrow window, their chances of preservation will be much improved. This narrow undersaturation window should contribute to the shell dissolution observed in the actualistic taphonomic study (Table 1). Bio-irrigation of oxygen into the porewater may enhance the degree of undersaturation and widen the window. On the other hand, bioturbation can prolong the time a shell experiences this undersaturation window by making a shell repeatedly go through it. Such an alternation of porewater carbonate chemistry by bioturbation and irrigation was reported in nearshore shallow marine sediments [17,18], but has not been reported at a deeper continental slope petroleum seep site prior to this study. Below the ion molarity product (IMP) minimum (Fig. 8), porewater is highly supersaturated with respect to CaCO3 at the seep site but only slightly at the reference site. This supports the general notion that carbonate minerals are formed at methane seep sites [4,5]. 3.3. Carbon mass balances and their implications We calculated DIC, TAlk and Ca2+ fluxes for the seep site and the reference site (Table 2). In the seep site, DIC increases by 1 mM from the bottom water value of 2293 μM to 3349 μM in the top 1-cm section of sediment (thus 0.5 cm as its depth). TAlk increases similarly. Substantial fluxes of inorganic carbon species to the overlying water are expected for such large concentration gradients. DIC and TAlk gradients diminished greatly immediately below the surface 2 cm. For example, the gradient between 1.5 cm and 2.5 cm is less than 10% of that between 0 and 0.5 cm. Thus high rates of DIC and TAlk production are expected in the surface 2 cm of sediment due to the

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oxidation of hydrocarbons and planktonic organic matter as well as the dissolution of carbonate minerals. An inorganic carbon budget analysis and a mechanistic illustration of DIC and TAlk production for the surface 2 cm of sediment are presented in Fig. 9. Carbonate dissolution rate is determined as one-half of the TAlk generation rate (or net flux change) in this zone (i.e., (12.67 − 1.06) / 2 = 5.81 mmol m− 2 d− 1) assuming SO42− reduction is insignificant in this zone [48]. Total Ca2+ production rate is the sum of Ca2+ flux to the overlying water and that to the deeper depths (i.e., 2.22 + 1.57 = 3.8 mmol m− 2 d− 1) and is less than that predicted from one-half of the TAlk fluxes. Thus we assume the rest of the carbonate dissolution is from Mg-carbonate (5.81 − 3.79 = 2.02 mmol m− 2 d− 1). This calculation leads to a Ca/Mg ratio of 1.9 or a carbonate composition of Ca0.65Mg0.35CO3. This is consistent with our measured solid phase Ca/Mg ratio of 2.0 in the top few cm of sediment at the seep site (solid samples were dissolved following the procedure in [49,50], then quantified by a Thermo Jarrell-Ash 965 Inductively Coupled Argon Plasma, ICAP; service provided by University of Georgia Analytical Center). Thus at the seep site, among the total of 11.9 mmol m− 2 d− 1 benthic DIC flux to the overlying water, 5.8 mmol m− 2 d− 1 is from carbonate dissolution (48%), perhaps 3.1 mmol m− 2 d− 1 is from hydrocarbon oxidation (26%), and about 1.2 mmol m− 2d− 1 is supplied from deeper depths via diffusion (i.e., SO42− reduction coupled to hydrocarbon oxidation less authigenic CaCO3 precipitation). Thus only about 11% of DIC benthic flux is supported by the oxidation of planktonic or terrestrial organic carbon (1.78 mmol m− 2 d− 1) near the sediment–water interface. Benthic DIC flux to the overlying water in the nonseep reference site (3.38 mmol m− 2 d− 1) is only about 30% that of the seep site. In great contrast, TAlk flux is nearly zero at the non-seep site as the TAlk values at 0.5 cm is the same as in the overlying water. While not

Fig. 9. A mechanistic illustration and a budget analysis for the surface 2 cm of sediment at the Gulf of Mexico seep site (in mmol m− 2 d− 1).

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extensively reported and certainly not explained fully before, a much smaller TAlk flux than that of DIC in non-seep terrigenous marine sediments has been seen in previous work. Aller [17] reported two such examples with a minimum porewater TAlk near the sediment– water interface (i.e., no TAlk flux to the overlying water or possibly an opposite flux towards the deeper sediments). Jahnke [51] reported a large DIC flux in a benthic chamber coupled with a nearly zero DIC gradient and a TAlk minimum across the sediment– water interface in a central California basin site. Zhao and Cai ([52]; W. Cai unpublished) have measured a benthic DIC flux that is several times larger than that of TAlk flux in sediments from Buzzards Bay, MA. Qualitatively speaking, this is a result of porewater acidification by the oxidation of reduced species (Fe2+, Mn2+, NH4+, HS−, etc.) that are diffusing upward from deeper depths [17,30,32,47]. A quantitative explanation involving full scale modeling of all acid-base and redox reactions is beyond the scope of this paper [43]. This issue of less TAlk flux than DIC flux to the bottom water in non-seep terrigenous sediments, though of great general importance to coastal ocean carbon recycling and ocean acidification, will have to be dealt with elsewhere. As we are not able to assess carbonate dissolution rate from TAlk flux change due to the noncarbonate related contribution to TAlk at this site, we seek to accomplish this task directly using the flux of Ca2+ and Mg2+. Ca2+ flux to the overlying water (0.82 mmol m− 2 d− 1) and total Ca2+ generation rate (0.82 + 0.27 = 1.1 mmol m− 2 d− 1) in the top 1-cm depth in the non-seep site are 40% and 30% of their respective values of the seep site (Table 2). The Ca-to-Mg ratio in solid phase is 2.5 as measured by ICAP method (Cai unpublished data) for the nonseep sediments, and thus by scaling to this ratio, the total Ca(1−x)MgxCO3 dissolution rate is 1.5 mmol m− 2 d− 1 (i.e., 1.1 + 1.1/2.5) and the flux of (Ca + Mg) to the overlying water is 1.2 mmol m− 2 d− 1 (i.e., 0.82 + 0.82/ 2.5). If all the DIC derived from carbonate dissolution in the surface sediment is diffused to the overlying water, then its contribution to the benthic DIC flux is 47% (i.e., 1.6/3.4) at the non-seep reference site (site 4). This high contribution of carbonate dissolution to benthic DIC flux should be viewed cautiously as we did not measure Mg2+ concentration in porewater and the true ΔCa2+/ ΔMg2+ ratio in porewater could be higher than the solid phase. The above carbon mass balance exercise leads to two important geochemical implications. First, the oxidation of CH4 and other hydrocarbons can be the dominant or at least an important mechanism for the sharp decrease

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in porewater pH and thus carbonate mineral dissolution in the northern Gulf of Mexico petroleum seep areas. Second, carbonate dissolution driven by CO2 and acid produced during the oxidation of hydrocarbons, dissolved or solid sulfides, and planktonic organic carbon forms a very important mechanism for benthic carbon recycling possibly accounting for 50% of the benthic DIC supply in northern Gulf of Mexico sediments. 3.4. Linkage between porewater geochemistry and carbonate shell preservation The dissolution at GC234 is in accordance with the earlier observations of a strong taphonomic bias at petroleum seeps [9–11,53]. A linkage between the taphonomically-active zone (TAZ), skeletal preservation, and carbonate saturation states has been suggested for shallow marine systems such as Long Island Sound [17,18] and carbonate banks [54] based on pH measurements in coarse-scale sectioned muds or centrifuged sediments, but not at deep-marine sites or petroleum seeps. Evidence of a relationship between porewater saturation state and long-term preservation potential is just becoming available [12]. Actualistic taphonomic experimentation allows us to link environmental controls directly to taphonomic responses and it demonstrates that taphonomic processes generally proceed at their highest rates at or just below the sediment–water interface. Powell et al. [20] observed after 2 years of shell deployment that burial most often slowed taphonomic attack at SSETI sites; but local conditions were strong modulators of the effectiveness of burial, probably by influencing the depth and intensity of the TAZ. The microelectrode work and other porewater chemistry provide a chemical background for such observations. While our microelectrode work and porewater chemistry data at the seep sites support the TAZ concept, we also emphasize that the TAZ, which was defined by taphonomic observation and not chemical characterizations, should be revised to include the time a shell is in contact with the carbonate undersaturated porewater and the degree of undersaturation. Although considerable within-site variability exists at site 2 in the geochemical composition, the geochemical distinctiveness between site 2 and site 4 is profound, as is the likelihood of preservation of skeletal carbonate. Our findings suggest several important outcomes related to shell mineralogy/microstructure of molluscan carbonate in relation to porewater geochemistry. First, similar mineralogies and microstructures of skeletal hard-parts may respond differently to porewater conditions, and this may be directly related to the size/

thickness of the hard-part. While the two gastropod species, Strombus and Turritella, are composed of similar aragonitic microstructure, and both were significantly degraded compared to controls, thinner-shelled Turritella showed significantly more dissolution only at site 2 whereas Strombus was strongly impacted at both sites. Second, although several of the bivalves had calcitic mineralogy, parts of their shells were affected differently by dissolution. For example, the outer shell surface of Mytilus was more heavily dissolved at both site 2 and site 4 than the inner shell areas. The outer shell layer of Mytilus is prismatic calcite. In contrast, dissolution indices averaged lower for Argopecten. Argopecten also has an external calcite layer, but unlike Mytilus, Argopecten has a cross-foliated microstructure that makes it potentially more resistant to dissolution. Third, the microstructure may be an important control of dissolution potential of shell aragonite under certain porewater conditions. Codakia is a case in point: although composed of aragonite, the outer layer is composite prismatic and the inner layer is complexcross lamellar. This species was the least dissolved of all deployed species. Of particular note is the limited effect of dissolution on the complex-cross lamellar aragonite of the inner shell surface in comparison to the aragonitic nacre of the inner valve of M. edulis. Thus, for taphonomic studies, a shell may be composed of more than one type of mineralogy and microstructure, and these can work in tandem to produce a collage of chemically-derived taphonomic signatures preserved on the shells. The complex interplay of species and environment documented in Table 1 is the result. Because continental-margin bottom water is generally supersaturated with calcium carbonate, it is the variability in microenvironments in surface sediments that dictate the fate of biogenic carbonate. While a subsurface pH minimum promotes CaCO3 dissolution, the subsequent pH maximum does the opposite. The time a shell is in contact with the undersaturated porewater in the TAZ and the degree of undersaturation must exert an important control on shell preservation. Therefore, the short-term effect of exposure to petroleum seep sediments on shell preservation may be quite different from the long-term effect depending on the size of a shell and specific local environmental conditions such as the sedimentation rate. For example, relatively small shells may be dissolved completely when they move through the undersaturated window; thus seep activities act as promoting dissolution. On the other hand, seep activities may promote preservation of larger shells if they survive the undersaturated window as porewater is highly supersaturated below that layer.

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Thus it may not be a surprise that shell assemblages with almost indistinguishable alteration in short-term burial may have the greatest differences in taphonomic alteration during long-term preservation. However, overall, our work suggests that active petroleum seep activities promote shell dissolution. Sedimentation rate at the seep sites may be only 0.01–0.06 cm yr−1 [8]. However, bioturbation, bottom currents and sediment degassing events could cover the shells with 0.5 to 3 cm of sediment in a few years [25]. Local burial rates at petroleum seeps can be even higher than this ([25]; E. Powell, personal observations of SSETI deployment). Even when a shell sits on the surface of the sediment or is only shallowly buried, the point of contact with the substrate could experience low pH and carbonate undersaturated porewater and thus be subject to dissolution. When shells are buried to a few cm depth, their exposed shell surface could still be located in the undersaturation zone and subject to dissolution. After rapid initial sediment coverage, different parts of the shells may experience undersaturated microenvironments for a prolonged time due to biological reworking and so undergo significant dissolution. Only when the whole shell has passed through the TAZ, can preservation be assured. Burial even a few millimeters below this undersaturated zone makes long-term preservation more likely. Thus the TAZ should primarily be viewed as a shallow subsurface phenomenon, and its vertical extent is controlled by the aerobic and anaerobic chemical processes that control pH and the saturation state of CaCO3. Finally, since CH4 seep-driven microbial mats are not a permanent feature, TAZs around seep areas must be highly dynamic both in space and time. Measurement of time-since-death of molluscan shells provides empirical evidence of the low probability of their long-term preservation in marine soft sediments [55–57]. In these sediments, the competing influences of dissolution and burial are considered key. We therefore expect a correlation between sediment chemistry, skeletal carbonate composition, and the long-term process of preservation. Shells deployed for 8 years at these seeps show evidence of dissolution consistent with expectations from porewater geochemical analyses although conclusions derived from such short-term field experiments may not be applied to the long-term preservation behavior of natural shells. The microscale spatial variability in sediment chemistry at site 2 suggests a spatially and temporally variable geochemical environment, an inference corroborated by other studies [1,10]. Site 2 also had a highly corrosive environment in comparison to site 4, a contrast also

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observed in the higher degree of dissolution on shells deployed at site 2. Fossil seeps are well known to vary in carbonate preservation [14]. Our work demonstrates the high spatial variability of microbially-mediated geochemical processes at petroleum seep sites. Thus, not only is a chemical-characterization of the taphonomically-active zone possible for seep sites, but also the taphonomic resolution of the vagaries of preservation may be elucidated at globally-extensive seep sites. 4. Conclusions Comparing the O2, pH, and pCO2 fine-scale profiles along with other porewater geochemistry parameters (including DIC, TAlk, and associated chemistry) in different types of sediments reveals patterns that reflect the patchy nature of seep-related biogeochemical processes and are strongly influenced by proximity to seepage and by benthic faunal activities. Generally, an active seep will cause a more compressed redox front with greater microbial respiration. Continuous accumulation of total alkalinity could cause authigenic carbonate precipitation at deeper depths. Profiles of fine-scale pH and pCO2 identify a very restricted CaCO3 undersaturated zone immediately below the sediment–water interface in an otherwise supersaturated environment. This zonation is most likely a result of a highly compressed redox front between acid-generating aerobic oxidation of reduced species and base-generating sulfate reduction, and characterizes the taphonomically-active zone (TAZ). The linkage of the TAZ concept to the carbonate undersaturated zone is confirmed here based on microelectrode measurements and experimental taphonomic studies. However, the TAZ concept must be interpreted in the context of the time a shell remains in contact with the carbonate undersaturated porewater and the degree of carbonate undersaturation. Both must exert an important control on shell preservation. Although carbonate shells deployed where seep influence is greater (site 2) were characterized by more dissolution, depending on how fast a shell is going through the undersaturated window seep influence can change from promoting dissolution to promoting preservation depending on the shell size and specific local environmental conditions. Thus, as a consequence, a high spatial variability of porewater geochemistry at the seep site can produce variable shell-dissolution signatures as a consequence. Simple carbon mass balance analysis shows that the oxidation of hydrocarbons can be the dominant or at least an important mechanism for the sharp decrease in

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