Stable isotope geochemistry of pore waters and marine sediments from the New Jersey shelf: Methane formation and fluid origin

In 2009 Integrated Ocean Drilling Program (IODP) Expedition 313 drilled three sites into the passive continental margin on the New Jersey shelf. Sites M0027A, M0028A, and M0029A were drilled 45–67 km offshore (Fig. 1) at a water depth of ∼35 m from a mission-specific platform. The aim was to complete the existing data sets of earlier onshore and offshore drilling campaigns. The new results of this expedition will provide the basis for the construction of a transect of the upper slope and continental shelf of New Jersey. The depths reached ranged from 631 to 755 m below seafloor (mbsf), and there was excellent core recovery of the target intervals (Mountain et al., 2010). The principal objective of the expedition was to determine the timing and magnitude of past global sea-level changes during the early to middle Miocene and their relation to the architecture of sedimentary sequences. For a correlation of seismic sequence boundaries with the actual core lithology and sediment ages, extensive petrophysical and logging data were collected and good geochronologic control of the sediments was established. These analyses were accompanied by a detailed study of the interstitial water chemistry.

This study focuses on the results and interpretation of stable isotope geochemical analyses carried out on pore fluids, sediments, and gas that were sampled from the three cores of IODP Expedition 313. The isotope data discussed in this manuscript are available in the Supplemental File¹. The specific objectives of this study are (1) to identify the sources of the observed fresh-water–salt-water stratification in the sediments, and (2) to better understand the processes related to carbon cycling on a passive continental margin. The latter includes the processes of organic matter diagenesis, evolution of methane from buried sediments, and the origin of authigenic carbonates.

To address these objectives, stable isotope analysis of pore fluids and sediment is an ideal tool that has been used with great success in numerous studies. The stable isotope composition of oxygen and hydrogen in the water molecule is a valuable natural tracer for the determination of genesis and mixing of waters of different origins. This information has been used in various meteoric (Bowen and Wilkinson, 2002; Clark and Fritz, 1997; Dutton et al., 2005; Epstein and Mayeda, 1953; Kendall and Coplen, 2001; Kendall and McDonnell, 1998; Rozanski et al., 1993; Schulte et al., 2011) and marine (Bigg and Rohling, 2000; Craig and Gordon, 1965; Fairbanks et al., 1992; Khim and Krantz, 1996) studies to characterize and determine water sources. For example, the isotope compositions of precipitation and surface water have been monitored for many decades on a global scale by the International Atomic Energy Agency (IAEA) in Vienna (Global Network of Isotopes in Precipitation, GNIP; IAEA/WMO, 2006). In addition, the isotope composition of the oceans is documented in the global seawater oxygen-18 database (Schmidt et al., 1999), starting with isotope data from ca. A.D. 1950.

Stable carbon isotope ratios of the various carbon species of dissolved inorganic carbon (DIC), organic matter (C_org) and inorganic carbonates (C_carb) are used to obtain insight into the global carbon cycle (Berner, 1999; Emerson and Hedges, 2008; Holser, 1997; Kump and Arthur, 1999) and marine pore-water chemistry (Malone et al., 2002; Walter et al., 2007). Continental shelves are also potential sites for the formation of methane, a major greenhouse gas that can be released into the atmosphere during sea-level changes (Dickens, 2011; Kennett et al., 2000). This makes carbon isotopes an important tool for global carbon cycling and related marine-atmospheric interactions.

Sulfur isotope ratios of dissolved sulfate in pore fluids of marine sediments are indicative for sulfate sulfur sources and the activity of microbes from the deep biosphere (Böttcher et al., 1999, 1998, 2006; Wortmann et al., 2001). Fresh water may contain sulfate from multiple sources, but groundwater is typically dominated by the interaction with aquifer minerals, sulfur isotopes being an important tool for source characterization (e.g., Böttcher, 1999; Clark and Fritz, 1997).

The combination of isotope analyses of all phases (interstitial water, sediment, and gas) with pore-water chemistry is expected to provide a better understanding of processes in the sediment, and will help identify the origin of fluids under the New Jersey shelf.

The New Jersey Atlantic shelf and New Jersey coastal plain have been the locations of several drilling campaigns (Fig. 1), including Deep Sea Drilling Project (DSDP) Leg 95, Ocean Drilling Program (ODP) Legs 150, 150X, 174A, and 174AX (Austin et al., 1998; Browning et al., 2008; Miller et al., 1994a, 1994c, 1998a, 1998b), and the Atlantic Margin Coring Project (AMCOR) (Hathaway et al., 1979; for a more complete list, see Miller et al., 1994b; Christie-Blick et al., 1998; Mountain et al., 2010).

The target sequences during IODP Expedition 313 were early to middle Miocene sediments that were cored at Sites M0027A, M0028A, and M0029A (Fig. 2). The Oligocene-Miocene boundary was only drilled at M0027A, where late Eocene to early Oligocene sediments were encountered at the bottom. Sediments here cover a time interval of the past 34 m.y. The other two sites mainly drilled through sediments of Miocene age. The lithology of the cored sediments ranged from clay to sand with local lithification (Fig. 2). Different lithological units (I–VIII) were defined from the core descriptions and correlated with the seismic sequence boundaries identified from seismic profiles of the R/V Oceanus cruise 270 line 529 in 1995. Figure 3 shows the two-dimensional lateral structure of the lithological units across the New Jersey shelf. (For an overview of the sequence stratigraphy of the New Jersey shelf, see Monteverde et al., 2008; Miller et al., 2013, and references therein.)

The Cretaceous and Cenozoic geological strata and the related groundwater aquifers of New Jersey generally dip to the southeast below the New Jersey coastal plain and the Atlantic shelf (dePaul et al., 2009; Owens et al., 1999). The onshore outcrop of sediments strikes from the southwest to northeast, roughly following the shoreline of New Jersey. Along the outcrops, meteoric waters recharge the upper unconfined aquifer and a number of confined aquifers along the outcrop. The sandy aquifers are separated by confining units composed of silt- to clay-sized sediment. The main aquifers that are related to Miocene and younger sediments are the upper unconfined Kirkwood-Cohansey aquifer system and the Atlantic City 800-foot sand aquifer. Below them, the Piney Point and Englishtown aquifers belong to Eocene–Oligocene sediments and Upper Cretaceous units, respectively. However, it is difficult to follow the aquifer systems under the seafloor and relate them to lithological units in offshore wells. Therefore, the units defined at the IODP Expedition 313 sites cannot be directly connected with the known onshore geological and hydrogeological units (for hydrogeological details of New Jersey, see Pope and Gordon, 1999; dePaul et al., 2009).

Kohout et al. (1977) and the drilling activities of the AMCOR project (Hathaway et al., 1979) discovered fresh water beneath the continental shelf down to a depth of several hundred meters below seafloor. The geometry was constructed by Hathaway et al. (1979) as relatively fresh groundwater that forms a flat-lying lens that extends 100–130 km offshore. The sharp upper boundary of the fresh-water plume is characterized by steeply increasing chloride concentration values that finally reach the chloride concentration of seawater. However, the AMCOR boreholes discovered increased salinities, as high as 1150 mmol/L, in wells near or on the shelf margin. This has been attributed to upward molecular diffusion of salt from deeply buried evaporitic strata.

The basic conceptual models that have been proposed for the low-salinity plume below the Atlantic shelf are summarized in Figure 4. Hathaway et al. (1979) proposed that the low fresh-water lens can be the result of submarine groundwater discharge (SGD) of mainland aquifers reaching under the shelf, or it may represent remnants of groundwater that infiltrated the sediments during the sea-level lowstand of the Last Glacial Maximum (LGM), when the shelf sediments were exposed. Younger sediments trapped the fresh water during the subsequent sea-level rise. Hathaway et al. (1979) concluded that the latter explanation is more likely, and that the fresh water is relict Pleistocene water. Meisler et al. (1984) suggested that the continental shelf is not in equilibrium with today’s sea level and that this phenomenon may have existed since the late Miocene. They interpreted the observed low-salinity plume as the result of advancing and retracting fresh-water fronts in response to numerous eustatic sea-level changes.

Hydrogeological models evaluated the role of onshore infiltration of subglacial recharge from the Laurentide ice sheet during the LGM (Cohen et al., 2010; Person et al., 2003, 2007). These studies suggest that the fresh water under the New Jersey shelf was emplaced by a combination of meteoric recharge during sea-level lowstands in combination with highly elevated (4–10 times modern values) onshore infiltration of glacial meltwaters by sub–ice sheet recharge and proglacial lakes. Cohen et al. (2010) also emphasized the role of submarine groundwater discharge from aquifers that crop out into submarine canyons near the continental slope.

The stable isotope geochemical data will be used to evaluate the proposed geometry of the fresh-water plume and to test the plausibility of the proposed models.

The principle water chemistry and methods were summarized in Mountain et al. (2010; for details on these procedures, refer to the methods chapter therein). Methods of stable isotope analyses are summarized briefly here.

Water samples were filled into 1.5 mL standard vials with cap. For the analysis of δ¹³C_DIC, 1 mL of filtered pore water was poisoned with HgCl₂ to prevent microbial activity and stored cold until analysis. Water samples were analyzed for δ¹⁸O and δ²H by an isotope ratio infrared spectroscopy analyzer, based on wavelength-scanned cavity ring-down spectroscopy (Picarro Inc. L 1102−i). Four sequential injections of each sample were measured, and raw data were corrected for sample-to-sample memory and excluded if necessary (van Geldern and Barth, 2012). The reported value is the mean value. The data were corrected for instrumental drift during the run and normalized to the Vienna Standard Mean Ocean Water (VSMOW)/Standard Light Antarctic Precipitation (SLAP) scale by assigning values of 0‰ and –55.5‰ (δ¹⁸O)/0‰ and –427.5‰ (δ²H) to VSMOW2 and SLAP2, respectively. For normalization, two laboratory standards that were calibrated directly against VSMOW2 and SLAP2 were measured in each run. External reproducibility, defined as standard deviation of a control standard during all runs, was better than 0.10‰ and 1.0‰ (1σ) for δ¹⁸O and δ²H, respectively.

Water samples were analyzed for δ¹³C_DIC using an automated equilibration unit (Gasbench 2, Thermo Finnigan) in continuous flow mode coupled to a Thermo Finnigan Delta plus XP isotope ratio mass spectrometer (IRMS). The data sets were corrected for instrumental drift during the run and normalized to the Vienna Peedee Belemnite (VPDB) scale by assigning a value of +1.95‰ and –46.6‰ to NBS 19 and LSVEC, respectively. External reproducibility was better than 0.15‰ (1σ) for δ¹³C_DIC.

For stable isotope analysis of organic matter, 0.5 mg to 19 mg (depending on C_org content) were weighed into silver cups, decalcified by 1N HCl and dried for 12 h at 60 °C. Organic matter was analyzed for δ¹³C using a Thermo Flash elemental analyzer (Series 1112) at 1020 °C in continuous flow mode coupled to a Thermo Scientific Delta plus IRMS. All samples were measured at least in duplicate, and the reported value is the mean value. The values are reported versus VPDB. Reproducibility determined by a laboratory control standard was better than 0.2‰ (1σ) for δ¹³C_org.

Carbonate measurements were conducted using a Thermo Finnigan Gasbench II for CO₂ processing and subsequently measured with a Thermo Finnigan Delta XP IRMS. Samples were reacted with 100% phosphoric acid at 70 °C for 12 h. Carbon and oxygen isotope compositions were measured and normalized against international standards (NBS-19 and LSVEC). Standard deviation for repeated measurements of a laboratory reference calcite (IAEA-CO1) is 0.12‰ for δ¹³C_carb and 0.22‰ for δ¹⁸O_carb. The millimeter-sized concretionary carbonates were powdered, homogenized, and analyzed as bulk samples. Samples were not pretreated to remove potential organic material due to the risk of alteration of the original isotope ratios. No corrections were made for oxygen isotope values for different phosphoric acid fractionation factors between the different carbonate phases.

For gas sampling, a plug of sediment weighing 8.5 g was injected into a 20 mL glass vial (prefilled with 10 mL of a solution of 0.5 mol/L NaCl and 0.1 mol/L HgCl₂) for shore-based determination of the concentration of dissolved methane and its stable carbon isotope composition in the headspace. Analyses of gaseous samples were done by a Varian CP-3800 gas chromatograph with an injection volume of 250 μL. Carbon isotope analyses used a Finnigan MAT 252 IRMS with a gas chromatographic inlet system for the analysis of a mixture of gases (permanent gas and hydrocarbons C1 to C3).

The dissolved sulfate from acidified pore-water samples was precipitated quantitatively as anhydrous barium sulfate by the addition of an aqueous (5%) BaCl₂ solution. Barium sulfate precipitates were washed with deionized water and dried at 70 °C. For stable sulfur isotope (³⁴S/³²S) measurements on BaSO₄, sample powders were placed in tin cups with p.a. (pro analysi) grade V₂O₅ as a catalyst, and combusted in a Thermo Scientific Flash 2000 elemental analyzer coupled to a Thermo Finnigan 253 IRMS. Silver sulfide isotope reference materials (IAEA S-1, IAEA S-2, and IAEA S-3) were used to normalize raw values to the Vienna Canyon Diablo Troilite (VCDT) scale (Mann et al., 2009). Replicate measurements agreed with better than ±0.3‰.

Mineralogy of carbonate concretions was determined by X-ray diffraction (XRD) in a Siemens D5000 powder diffractometer in Bragg-Brentano geometry (CuKα). Quantitative evaluation of phases identified in the diffractogram was carried out by Rietveld refinement. Formation temperatures for calcite were calculated with the equation of O’Neil et al. (1969) considering the modifications by Friedmann and O’Neil (1977).

IODP Expedition 313 discovered for the first time a clear, multilayered structure of alternating fresh-water–salt-water intervals (Mountain et al., 2010). This distinct succession is indicated by variable chloride concentrations that change between seawater and fresh-water signatures with depth at Site M0027A (Fig. 5). At Site M0029A salinities increase above seawater values in the lower core parts. The lowest observed chloride concentrations are 41 mmol/L at Site M0027A and 19 mmol/L at Site M0029A. In contrast, analyzed seawater has a value of 515 mmol/L, and uppermost interstitial waters (down to 5 mbsf) at M0027A show concentrations of 524 mmol/L. The chloride profiles also show that changes from fresh water to salt water occur at relatively sharp boundaries in the upper parts of the cores. Mixing here occurs within short intervals, i.e., within 10–20 m at M0027A down to 415 mbsf. The picture is different for the lower core parts, where a gradual increase toward higher chloride concentrations is observed at both sites. This increase starts at ∼415 mbsf at M0027A and ∼340 mbsf at M0029A. However, the starting situation is different at both locations. Whereas the gradual increase starts with fresh water in Hole M0027A, it begins with brackish to salt water at Site M0029A. Highest concentrations are observed in the lower part of core M0029A with concentrations to 994 mmol/L, which is nearly twice the concentration of seawater. The chloride concentrations tend to reach a maximum in the lowermost part with no further increase. A maximum of 453 mmol/L at the bottom is reached at M0027A.

The fresh-water–salt-water succession indicated by the chloride concentration profile is mirrored by the hydrogen and oxygen stable isotope composition of pore fluids (Fig. 5); δ¹⁸O and δ²H show a virtually 1:1 correlation. Therefore, only oxygen stable isotope values are discussed and shown here. Salt-water intervals with high chloride concentrations correlate with higher δ¹⁸O values, whereas fresh-water intervals correspond to lower oxygen isotope values. Seawater at the drilling site had a mean value of –0.8‰ (δ¹⁸O) and –7.2 (δ²H), close to the analyzed pore-water values in the uppermost part at Site M0027A. In fresh-water intervals the minimum δ¹⁸O values of –6.9‰ (M0027A) and –6.6‰ (M0029A) correspond to minimum chloride concentrations. However, high chloride concentrations are accompanied by water stable isotope values close or identical to those of seawater. The gradual increase of chloride concentrations in the lower core section of M0027A also shows a clear correlation with δ¹⁸O values, although the increase relative to chloride is smaller than in the upper part of the core (i.e., the slope of the chloride-δ¹⁸O correlation changes). A value of –4.1‰ is reached at the bottom. In contrast, at Site M0029A a stable δ¹⁸O value of –0.9‰ at ∼340 mbsf is observed. Despite further increasing chloride concentrations there is no further increase in δ¹⁸O values above –0.8‰.

The comparison of the identified fresh-water–salt-water intervals with lithology (Fig. 2) reveals that fresh water characterized by low salinities and low δ¹⁸O values corresponds to fine-grained silt and clay sediments (i.e., 180–350 mbsf) at Site M0027A. In contrast, the salt-water intervals with higher δ¹⁸O values correlate with coarse-grained lithology (i.e., 350–410 mbsf). Identical correlations are observed for the upper parts of Sites M0028A and M0029A (Fig. 6). In contrast, the high salinities observed at Site M0029A below 340 mbsf do not correlate with lithology. The high-salinity fluid is present in both fine-grained and sandy lithologies.

Shipboard seawater at Site M0027A has a δ³⁴S value of +20.8‰ (Table 1), close to the considered average seawater value of +21.0‰ (Böttcher et al., 2007). The samples between 170 and 215 mbsf show increasing δ³⁴S values while salinity in this interval decreases, indicating a change from salt water to fresh water. The highest sulfur isotope ratios with a maximum of +27.4‰ are observed in samples from the large fresh-water interval that starts at ∼170 mbsf. Samples from the lower core between 360 and 410 mbsf plot in a salt-water zone and have δ³⁴S values slightly below the seawater value. The lowermost sample at 506 mbsf, taken from the interval with increasing chloride concentrations, has a value of +21.2‰, close to that of seawater.

To get an almost complete picture of the carbon cycle, the isotope composition of the various carbon reservoirs was analyzed instead of focusing only on one or two carbon phases. The δ¹³C_DIC of the dissolved inorganic carbon was analyzed from the pore-fluid samples, whereas δ¹³C_org and δ¹³C_carb represent the solid organic and inorganic reservoir and were analyzed from sediment samples and carbonate concretions, respectively. Concentrations of methane (CH₄) and the corresponding carbon stable isotope ratio (δ¹³C_meth) were analyzed from the gaseous phase. The profiles for all three sites are shown in Figure 6. Additional water chemistry data and total organic carbon (TOC) contents from Mountain et al. (2010) are shown in Figure 7. The chloride profiles are also shown to allow for the identification of fresh water and salt water intervals.

The δ¹³C_DIC of seawater sampled during the expedition was –3.1‰. This value is close to the δ¹³C_DIC value of –3.4‰ in the uppermost fluids at Site M0027A. At 20 mbsf, the DIC carbon isotope values drop to a minimum of –10.2‰. Further down the core, the isotope values generally increase with increasing depth. However, above 310 mbsf the δ¹³C_DIC values rise and fall between –1‰ and –10‰, before a more gradual and continuous increase to values of ∼–1‰ in the lower units (∼610 mbsf). In general, there is no significant correlation between the chloride concentration indicating fresh-water–salt-water transitions and δ¹³C_DIC values. Nevertheless, the variations between 50 and 310 mbsf may be partly related to the fresh-water–salt-water transitions in this interval, although there is no consistent correlation, as lower chloride values do not always correspond to lower δ¹³C_DIC values.

The upper 200 mbsf at Site M0028A were not cored. At 220 mbsf the δ¹³C_DIC profile starts at values of ∼–10‰, gradually increases by ∼5‰ toward 275 mbsf, and remains ∼–5‰ over the rest of the core. The only exception is a positive excursion to +6.5‰ at 407 mbsf.

At Site M0029A the δ¹³C_DIC values are relatively constant at ∼–8‰ followed by a positive excursion to values near 0‰, and a subsequent decrease down to a minimum isotope ratio of –13‰ at 345 mbsf. From there, values increase parallel to the chloride concentration and reach relatively constant values of ∼–4‰ at 470 mbsf. In the lowermost part, below 700 mbsf, the δ¹³C_DIC values increase to a maximum of +3‰.

Carbon isotope ratios of marine organic matter range between –22‰ and –28‰ at all three sites. The record is sparser than the pore-fluid profile due to the siliciclastic lithology of the sediments. TOC content in the sediment is typically <4%, whereas the higher concentrations are observed in the lower parts of the cores (Fig. 7). At Site M0027A a drop from –26‰ to –28‰ in the upper 30 mbsf can be observed and isotope ratios remain at low values until 200 mbsf. Below that depth no clear trend is apparent and δ¹³C_org values range between –22‰ and –27‰. At Site M0028A an increase from –27‰ to –24‰ between 200 and 300 mbsf can be noticed. Values stay at ∼–24‰ to 600 mbsf with a subsequent drop to –26‰ below 610 mbsf. The δ¹³C_org data from M0029A are sparse, but values are within the same range observed at the other two drill sites.

The methane concentration profile at Site M0027A can be divided into three parts. There is virtually no methane present until 230 mbsf, between 230 and 400 mbsf concentrations to 1000 ppm occur in some horizons, and below 400 mbsf significant methane concentrations (to 8850 ppm) exist (Fig. 6). At Site M0028A methane is present below 250 mbsf. Below 600 mbsf, higher concentrations with as much as 11000 ppm are observed. Site M0029A shows no methane above 360 mbsf. Below that depth, high CH₄ concentrations with a maximum of 20000 ppm are present. The stable carbon isotope compositions of methane (δ¹³C_meth) of all three drilling sites range between –73‰ and –87‰.

A total of 11 authigenic carbonate concretions and nodules with different mineralogy were sampled from the cores. The carbonates were analyzed for their mineralogy by XRD, and subsequently for stable isotopes. Most nodules were composed of only one carbonate mineral, while in some concretions a second, minor phase is present. The phases identified are siderite (n = 9), ankerite (n = 1), calcite (n = 4), calcite plus minor amounts of siderite (n = 4), and calcite plus minor amounts of dolomite (n = 2) (Fig. 6). The δ¹³C_carb values of the carbonates show a wide range from –54.4‰ to +8.3‰. This very large range narrows if carbonates are grouped by their mineralogy (Fig. 8). Siderite reveals the most scatter with δ¹³C_carb values ranging from –21.8‰ to +8.3‰. Ankerite also has relatively high values in this context with –4.7‰. Pure calcite concretions range between –23.3‰ and –14.0‰, whereas the calcite concretions with minor amounts siderite have δ¹³C_carb values between –23.2‰ and –23.3‰. The most extreme δ¹³C_carb values, –54.4‰ and –49.5‰, are observed in the calcite concretions that contain dolomite.

In contrast to the very wide range of carbon isotope compositions, the variation of the oxygen isotope values is much smaller (Fig. 8). Siderites range from +1.4‰ to +4.8‰, calcite ranges from +1.5‰ to +2.2‰, calcite plus siderite concretions ranges from +0.4‰ to +1.9‰, and the two dolomite containing concretions show values of +2.2‰ and +2.9‰.

The presence of fresh water below the Atlantic continental shelf is known from earlier drilling campaigns (Hathaway et al., 1979), and internal layering was noticed by Malone et al. (2002). Prior studies related fresh-water intervals to high-porosity, sand-rich horizons (Person et al., 2003). This led to the assumption that fresh waters would correspond to sandy intervals that are recharged by onshore meteoric water, whereas salt-water intervals, if present at all, would be most likely stored in the fine-grained sediments of the confining units. This contradicts new findings of IODP Expedition 313. Fresh-water intervals clearly correspond to fine-grained, silty, and clay-rich sediments, whereas salt water mainly occurs within coarse-grained, sandy intervals (J. Lofi and colleagues, 2012, personal commun.).

This is the first time that a clear stratification with distinct fresh-water and salt-water intervals is observed below the New Jersey Atlantic shelf. Before, the fresh-water plume was regarded as a nearly homogeneous lens with a sharp upper boundary (Hathaway et al., 1979). The results of the chloride concentration profiles and the stable water isotope values clearly indicate the presence of alternating fresh-water–salt-water intervals in the upper parts of the cores at Sites M0027A (above 415 mbsf) and M0029A (above 340 mbsf) (Fig. 5). Fresh-water intervals are also present at Site M0028A (Fig. 6), although the boundaries are less pronounced here. A lateral correlation of the fresh-water intervals between the drilling sites is not possible due to the variable thickness and structure of the sediments (Fig. 3).

The general hydrogeology of New Jersey, with a southeastward dip of the aquifers below the seafloor (see Fig. 4), suggests that the water may originate from onshore meteoric recharge. Oxygen and hydrogen stable isotope values of precipitation usually form a linear relationship that is known as the meteoric water line and can be explained by the Rayleigh distillation model (Craig, 1961; Dansgaard, 1964). The linear regression through the global data set of δ¹⁸O and δ²H precipitation values defines the so-called global meteoric water line (GMWL) with a slop of 8 (Craig, 1961; Rozanski et al., 1993). The local meteoric water line (LMWL) of New Jersey was defined by Kendall and Coplen (2001) and is based on river-water analyses (Fig. 9A). Surface-water δ¹⁸O values in the eastern United States represent the local precipitation (Dutton et al., 2005). The mean annual values are calculated with –7.0‰ (δ¹⁸O) and –42‰ (δ²H). Groundwater stable isotope analyses of different New Jersey aquifers (Lower Kirkwood, Piney Point, and Englishtown) also show values similar to modern meteoric recharge (Brown, 2005) (Fig. 9A).

Pore-water stable isotope values (δ¹⁸O and δ²H) from M0027A and M0029A indicate mixing between two sources (Fig. 9B). The end members defined by this mixing line have δ¹⁸O and δ²H values of –7.0‰ and –41‰ for fresh water, and –0.8‰ and –6‰ for salt water. This strongly suggests that onshore meteoric recharge and seawater are the two sources for fresh and salt water in the marine sediments.

The δ¹⁸O and δ²H values of the fresh water found below the New Jersey shelf clearly indicate its meteoric origin. Together with the hydrogeologic situation (aquifers dip below the seafloor), the results indicate that meteoric waters recharge the fresh-water intervals via onshore outcrops. The isotope ratios are similar to modern meteoric recharge of the New Jersey mainland and may point to a modern age of the waters. The data show no stable isotope geochemistry evidence for older emplacement by vertical meteoric recharge during Pleistocene sea-level lowstands and onshore subglacial recharge from the Laurentide ice sheet during the last glacial maximum, as proposed by Person et al. (2003) and Cohen et al. (2010). Pleistocene meltwater and precipitation from the LGM would be characterized by a shift along the GMWL to significantly lower isotope values, i.e., δ¹⁸O values of –20‰ and below (Clark and Fritz, 1997; Mook, 2000) (see Fig. 9B). Such an origin is not confirmed by the isotope values from the fresh water below the New Jersey Atlantic shelf.

However, stable isotopes do not yield information about absolute ages as does, e.g., radiocarbon dating (cf. Clark and Fritz, 1997). No absolute water ages for pore-water samples from IODP Expedition 313 are available. Radiocarbon water ages for New Jersey groundwater analyzed between 1984 and 1986 were reported by McAuley et al. (2001). Four wells that tap the Atlantic City 800-foot sand aquifer along a transect from inland New Jersey to wells located 3 and 8.5 km offshore and southeast from Atlantic City were sampled (Fig. 1). The results showed youngest radiocarbon ages of 2070 B.P. (¹⁴C years before present; present is defined as A.D. 1950) for the onshore well at Egg Harbor City close to the outcrop of the aquifer. Radiocarbon ages increased toward the coastline to 17,950 B.P. at Pleasantville and yield ages older than 22,000 B.P. in the two offshore wells. This information seems to contradict a modern recharge interpretation of the stable isotope data from IODP Expedition 313 wells that are located 45–67 km offshore. However, no information is available on potential correction models used in the calculation of radiocarbon ages. The available hydrochemical analyses of the radiocarbon dated wells from McAuley et al. (2001) and Barton et al. (1993) show much higher HCO₃⁻ contents in samples of older radiocarbon ages than in those with younger ages. This suggests that the original percent modern carbon (pMC) may have changed after water infiltration. If it is not considered in corrections, this leads to a significant overestimation of ages (cf. Clark and Fritz, 1997).

From the Illinois Basin, located in the midcontinent of the United States, water bodies with unusually low isotope values within Pleistocene groundwater aquifers are reported (e.g., Schlegel et al., 2011; Siegel, 1991). Groundwater here is typically characterized by depleted isotope values with δ¹⁸O values as low as –18‰ (Siegel, 1991), and is interpreted to be remnant Pleistocene ice sheet meltwater. However, some water masses within these aquifers show unusual high values (>–10‰) that seem to be inconsistent with a glacial origin. Siegel (1991) described such a water body from Iowa that is located within a confined Cambrian–Ordovician groundwater aquifer that otherwise shows depleted values of –11‰ to –18‰, consistent with meltwater origin. The water body was dated as Pleistocene in age by radiocarbon and yielded high stable isotope values that are comparable to modern recharge at Iowa. Siegel (1991) concluded that the water was recharged during a mild climate at the end of the last glaciation with isotope precipitation values similar to that of modern rain. In a study from the Illinois Basin, Schlegel et al. (2011) argued that δ¹⁸O values of Pleistocene paleoprecipitation of the region were close to modern values and ranged between –5‰ and –10‰. They suggested that the more positive than expected δ¹⁸O values in an Upper Devonian aquifer are the result of primary recharge by precipitation and only minor contributions from meltwater to the groundwater body.

The fresh water below the Atlantic shelf was attributed by Meisler et al. (1984) to be as old as Miocene. The global climate of the Miocene is generally characterized as warm and humid with a global warming event, the Middle Miocene Climatic Optimum between 17 and 15 Ma, followed by major and permanent cooling (see Zachos et al., 2001). This may have enabled conditions in the past comparable to modern climate with identical precipitation isotope values. However, such in an interpretation remains highly speculative.

Therefore, the results can be interpreted in two alternate ways. (1) The fresh water at Sites M0027A to M0029A is connected to onshore aquifers and is recharged relatively quickly by modern meteoric fresh water. (2) The fresh water is older than Pleistocene and was recharged before the LGM at climatic conditions comparable to modern. The isotope composition of meteoric recharge at this time was identical to the modern New Jersey LMWL.

Stable isotope data of the interstitial water indicate seawater with a δ¹⁸O value of –0.8‰ to –1‰ as the source of the salty end member. This corresponds to observed values of modern Atlantic shelf seawater. Analyses of this study showed a value of –0.8‰, which is good agreement with values between –0.87‰ to –1.53‰ reported by Khim and Krantz (1996) for the region. In contrast to meteoric precipitation, seawater isotope ratios are relatively constant, spatially and temporally. Except for the northern polar regions, the δ¹⁸O of modern oceans typically range between –1.5‰ and +1.5‰, whereas the values correlate to salinities that represent the fresh-water influx (i.e., influx of ¹⁸O-depleted water from large river systems or glaciers; Bigg and Rohling, 2000; Fairbanks et al., 1992; Rohling and Bigg, 1998). During glacial times, the global average δ¹⁸O of seawater increases by +0.8‰ to +1.0‰ (Schrag et al., 2002, 1996). This is due to the preferential incorporation of ¹⁶O into ice and the relative enrichment of the remaining ocean water with respect to ¹⁸O. The δ²H values will be shifted simultaneously, but ∼8 times larger than δ¹⁸O (Schrag et al., 2002). This defines a slope of ∼8 and is identical to the slope of the GMWL. Cooler climate will shift the isotope composition of the seawater end member parallel to the GMWL toward higher values (i.e., to the upper right in Fig. 9B). In contrast, warmer climates with smaller or no polar ice will result in a lower global seawater isotope value (Savin, 1977; Shackleton and Kennett, 1975). In both scenarios, the seawater value would not plot at the observed end-member value of the pore-water isotope data from the New Jersey shelf in Figure 9B. Glacial seawater would be shifted to higher isotope values, whereas seawater from warm periods would be shifted to lower ratios. However, the geochemical results of the pore fluids are in very good agreement with modern values. Therefore, the most obvious and simple explanation is preferred here, i.e., the salt water of the sandy intervals in the upper core parts represents modern seawater.

A possible explanation for this unexpected finding could be infiltration or diffusion of seawater into the more permeable layers via lateral connections to the seafloor and/or the shelf margin. However, actual pathways are difficult to identify from geochemical data alone. The data from this study may help to improve the hydrogeological models of the New Jersey shelf in the future. This may help to better understand the hydraulic conditions below the seafloor.

In the lower parts of the cores at Sites M0027A (below 415 mbsf) and M0029A (below 340 mbsf), the chloride concentrations start to increase gradually with depth instead of showing sharp transitions, as observed higher in the core. At Site M0029A the chloride concentrations increase to values of ∼950 mmol/L, approximately twice the salinity of the modern shelf seawater. The combination of chloride concentrations and stable isotope data indicates a mixing between seawater and meteoric waters at both drilling sites for fluids from upper core parts (Fig. 10). This observation is similar to the results from stable isotope data alone (Fig. 9B). The combination of δ¹⁸O values with pore-water chemistry reveals an additional third fluid source in the lower parts of the cores. The gradual mixing with saline waters starts with fresh water at Site M0027A and salt water at Site M0029A. This enables the study of two independent mixtures with the third source. A brine with chloride concentrations of ∼950 mmol/L can be identified from the intersection of the mixing lines in Figure 10. The δ¹⁸O values of the brine are ∼–1‰, almost identical to modern Atlantic shelf seawater. Therefore, the brine cannot be detected in a cross-plot of water stable isotopes (δ¹⁸O versus δ²H; Fig. 9B). Mixing between brine and fresh water will plot along an identical mixing line defined by seawater and meteoric water, because the stable isotope composition of the brine is similar to that of seawater.

Pore-water fluids with increased salinities were also observed in the Deep Sea Drilling Project (Manheim and Hall, 1976) and from OPD Leg 150 at the continental slope (Miller et al., 1994c). Manheim and Hall (1976) attributed the brines to Jurassic evaporites in the deeper subsurface. Chemical parameters at Site M0029A also point to evaporites as a source for the brine. For example, lithium concentrations increase parallel to the chloride concentrations in the lower parts of cores at Sites M0028A and M0029A (Fig. 7). Lithium concentrations increase as much as 20 times above that of seawater and may indicate evaporites (Schreiber and El Tabakh, 2000; Warren, 2006). The brine may ascend from evaporites at depth locally along faults observed on seismic profiles close to Site M0029A (Fig. 3).

Sulfur isotopes provide another possibility to distinguish between seawater and evaporitic brines. The stable sulfur isotope composition allows for a characterization of the sulfur sources and possible microbial overprints. The two major sources for dissolved sulfate are modern seawater, with an isotope composition of ∼+21‰ (e.g., Böttcher et al., 2007), and Jurassic evaporites, with δ³⁴S values between +12‰ and +19‰ (Strauss, 1997; Williford et al., 2009), that were dissolved by the brine found at depth. The sulfur isotope results obtained for the selected pore waters are in agreement with these sources. In the sediment section under the influence of fresh waters, the isotope composition increases above the modern seawater sulfate value (Table 1). This shift toward heavier isotope values between 195 and 200 mbsf is probably due to the superimposition of the primary isotope composition by microbial sulfate reduction. That process has frequently been demonstrated in ODP pore waters to be accompanied by an enrichment of the heavy isotope in the residual sulfate (Böttcher et al., 1999, 1998, 2006; Wortmann et al., 2001) that is due to the preferential reduction of ³²SO₄^2– compared to ³⁴SO₄^2– (Hartmann and Nielsen, 2012; Kaplan and Rittenberg, 1964; Sim et al., 2011). The observed shifts are consistent with net sulfate reduction as deduced from a balance of the pore-water sulfate to chloride ratios. Therefore, the sulfur isotope data are in accordance with the interpretations derived from the other geochemical and stable isotope geochemical data.

Carbon stable isotope analyses of dissolved inorganic carbon, organic material, and carbonate concretions, together with concentration and isotope analyses of methane that was found in the lower parts of the cores, reveal insights into the carbon cycle and formation conditions of the authigenic nodules. Buried marine organic matter typically decays along a sequence of reactions with electron acceptors (Emerson and Hedges, 2008). In a simplified and idealized depth profile, the succession of reactions would be aerobic respiration (electron acceptor O₂), nitrate reduction (NO₃⁻), manganese reduction [Mn(IV)], iron reduction [(Fe(III)] and sulfate reduction (SO₄^2–). The acceptors become depleted while hydrogen (H₂) and carbon dioxide (CO₂) are enriched. At the last step of decay, when all other acceptors are exhausted, CO₂ reduction produces methane (CH₄), a process that is termed methanogenesis. In nearshore environments with sufficient organic flux to the sediment, O₂, NO₃⁻, and Mn(IV) reactions can be very thin or obscured. However, sulfate reduction indicated by decreasing SO₄^2– concentrations should be visible above the zone of methanogenensis.

At the IODP Expedition 313 drilling sites methane concentrations increase with depth, whereas the onset of higher CH₄ concentrations seems to be associated with the beginning of the mixing interval with the brine in M0027A and M0029A (Fig. 6). The δ¹³C_meth shows strongly ¹³C depleted values between –73‰ and –87‰ with a mean value of ∼–80‰. Such low values are typical for methane derived from the decay and fermentation of organic matter due to the large fractionation factor that is associated with this process (see Emerson and Hedges, 2008; Lansdown et al., 1992). At Site M0029A the onset of methane production correlates with a sharp drop of the sulfate concentration. Sulfate concentration below 360 mbsf is typically <3 mmol/L (Fig. 7).

Methanogenesis has been suspected at ODP Leg 174A Sites 1071 and 1072 based on the analysis of carbonate concretions (Malone et al., 2002), but could not be proven directly. The data of this study from the deep drillings of IODP Expedition 313 confirms by direct analyses that methane production occurs in the deeper subsurface. Authigenic calcite with minor amounts of dolomite from the lower part of Site M0028A has strongly depleted δ¹³C_carb values of ∼–50‰ (Fig. 6). This indicates the presence of a strongly ¹³C depleted carbon source, probably from CO₂ that was generated by anaerobic oxidation of ¹³C-depleted methane coupled with sulfate reduction (Claypool et al., 2004). Other calcite concretions are characterized by δ¹³C_carb values of ∼–20‰ (Fig. 8). In contrast to the strongly depleted carbonates, these less depleted values might also result from aerobic or anaerobic respiration processes and do not necessarily require the incorporation of methane-derived inorganic carbon into these authigenic carbonates.

Methane does not seem to reach the seafloor under present conditions. Geochemical data indicate that methane, a major greenhouse gas, is oxidized to CO₂ before it has the chance to vent out. The situation may have been different during Neogene sea-level fluctuations with direct export of methane to the atmosphere during sea-level lowstands.

The δ¹³C_DIC values typically range between –3‰ and –10‰ with local outliers of –13‰ and +7‰ in some single intervals (Fig. 6). In the upper parts of M0027A and M0029A, the δ¹³C_DIC value of the pore waters drops below the initial seawater value of ∼–3‰. This likely indicates organic matter decomposition into ¹²C-enriched CO₂. Oxidation of methane may also play a minor role here. These processes shift δ¹³C_DIC values to more negative numbers. During subsequent methane formation by CO₂ reduction in the deeper core, residual δ¹³C_DIC values are shifted back to more positive values due to the loss of the light ¹²C isotope into CH₄. The residual δ¹³C_DIC can finally reach positive values (Claypool and Kvenvolden, 1983). The δ¹³C_DIC values at M0029A show a gradual increase in δ¹³C_DIC values and finally reach positive numbers of ∼+3‰ in the lowermost units below 725 mbsf (Fig. 6).

Carbon isotope analyses of organic matter do not show such a continuous record as obtained for the other carbon species. That is mostly due to the siliciclastic lithology of the cored sediments that have low organic carbon contents in many intervals. Values of δ¹³C_org range between –22‰ and –28‰ and are somewhat lower than values from typical bulk marine sediments. Marine dissolved and particulate organic carbon was found to have δ¹³C_org values between –21‰ and –23‰ (Benner et al., 1997). The enriched values observed in the marine sediments from the New Jersey shelf indicate a mixture of marine and terrestrial organic matter, whereas the latter can be characterized by lower isotope values in case it originates from C₃ plant vegetation (Emerson and Hedges, 2008; Fry and Sherr, 1984; Onstad et al., 2000). For example, the suspended particulate organic matter from the Connecticut River north of New Jersey is characterized by a mean carbon isotope value of –26.4‰ (Onstad et al., 2000).

At Site M0027A the observed drop from –26‰ to –28‰ in the upper 30 mbsf may correspond to the drop of the δ¹³C_DIC in this interval. The same may be the case for the increase from –24‰ to –26‰ between 200 and 300 mbsf at Site M0028A. However, correlations between δ¹³C_org and δ¹³C_DIC are not always consistent, and variations in δ¹³C_org may also be simply related to changes in the bulk isotope composition of the buried primary organic material. Methane formation in the lower core sections does not seem to influence the δ¹³C_org values.

Authigenic carbonates composed of calcite and mixtures of calcite and siderite show the smallest variations, whereas siderites showed the greatest range in δ¹³C_carb and δ¹⁸O_carb values (Fig. 8). This is consistent with data from authigenic carbonates for OPD Leg 174A (Malone et al., 2002). Pure calcite nodules and mixtures of calcite with other carbonate minerals tend to have less depleted carbon isotope ratio with a δ¹³C_carb mean value of –19‰ for calcite. The δ¹³C_carb values of most siderites are enriched over calcite with values as positive as +8.3‰. The systematic offset between these two phases may indicate that they formed at different depths in the sediment. Whereas the calcite could have formed from pore waters influenced by the decomposition of ¹³C-depleted organic matter in the upper part of the sediment column, siderite is more likely to form in the deeper iron reduction zone, where Fe²⁺ becomes available for siderite formation from the reduction of Fe³⁺. Pore fluids in this zone are increasingly enriched in δ¹³C values, as indicated by the generally increasing δ¹³C_DIC values. Two carbonate samples with low δ¹³C_carb values of ∼–50‰ from Site M0028A indicate the incorporation of carbon from the oxidation of methane, and therefore may have formed in the lower sulfate reduction zone, where methane ascended from the subjacent zone of methanogenensis.

The relation between temperature and the oxygen isotope composition of precipitated calcite is well established (e.g., O’Neil et al., 1969). Formation temperatures can be calculated from the δ¹⁸O_carb values of the calcite by assuming the oxygen isotope ratio of the solution from which the mineral has precipitated (Table 2). The calculations show that the influence of fresh water during carbonate formation can be ruled out. A water isotope ratio of –7‰ results in unrealistic temperatures below the freezing point. Assuming the observed δ¹⁸O shelf seawater value of ∼–0.8‰ results in temperatures of 3–6 °C at time of formation. Such low bottom-water temperatures are observed at the New Jersey shelf in late winter and spring (January to May), but increase to 15 °C and above in summer (Benway, 1986; Kohut et al., 2004). This discrepancy leads to two options: (1) the calcite precipitated at such cool temperatures, or (2) the δ¹⁸O value of the fluid was enriched with respect to ¹⁸O during formation.

For example, a shift of ∼0.8‰ as inferred for the LGM due to the buildup of ice (Schrag et al., 1996) would increase the calculated temperatures to values between 7 and 9 °C (Table 2). However, influx of ¹⁸O-depleted meltwater from the continental ice sheet may also shift the δ¹⁸O values of the nearshore seawater in the other direction during cooler times, a situation that is observed today near Greenland (Fairbanks et al., 1992). Another option would be the enrichment of the shallow shelf waters by evaporation (Mook, 2000) during warmer climates, as inferred for the Miocene. At drilling Sites M0027A and M0029A the pore-water values does not increase above –0.7‰ (Fig. 9). As outlined here, it may be the case that fossil paleoseawater is not present anymore. This will preclude any proper assumptions about fluid compositions at time of formation.

The geochemical data and interpretations presented in this study supply the missing link between existing onshore and offshore data and provide the basis for an integrated approach to construct a geochemical transect across the New Jersey shallow shelf. The results of IODP Expedition 313 presented here indicate a more complex geometry than previously assumed, with alternating fresh-water–salt-water intervals divided by relatively sharp boundaries in the upper part of the cores.

The combination of pore-water chemistry and stable isotope identified three fluid sources: (1) fresh water with low δ¹⁸O values of –7‰ that represents modern meteoric water; (2) salt water of modern marine origin with a δ¹⁸O values of ∼–0.8‰; and (3) brine with a δ¹⁸O value of –1‰, close to that of seawater.

The fresh water probably originates from onshore meteoric recharge through aquifer systems that crop out on the New Jersey mainland. The fresh-water stable isotope ratios are identical to modern precipitation in New Jersey. This indicates either a modern age, or that the water was recharged at a time with climatic and hydrologic conditions similar to modern conditions. Existing models for the Atlantic continental shelf may have to be refined with the new data of IODP Expedition 313 to properly explain the origin and spatial structure of the emplaced fresh-water resources within the shelf sediments. An origin from Pleistocene glacial meltwaters, as assumed by other studies, is not confirmed here. Salt water also represents modern isotope values, suggesting an infiltration along permeable, coarse-grained sandy units. The chemical composition of brine indicates an origin from evaporites that should be present in the underlying Jurassic sediments. A possible pathway for the brine might be ascension along faults that are observed on seismic profiles.

Stable carbon isotope and gas analyses could prove the existence of methane formation from buried marine organic matter at depths below 350 mbsf. Although not reaching the surface at present conditions, the venting out of variable fluxes of methane during Cenozoic sea-level changes is imaginable. The implication is that methane fluxes from passive continental margins may play an important role in the long-term global carbon cycle and need to be considered in global carbon models.

In summary, the detailed geochemical analyses of pore fluids and sediments for their stable isotope compositions from the three drill sites of IODP Expedition 313 shows the first chemical high-resolution picture and new insights into the buried sediments of the Atlantic shelf. The data enhance our knowledge about the relevant processes that occur in the subsurface, and have implications for offshore fresh-water resources and the role of passive continental margins in the long-term carbon cycle. Further work should integrate the data into global carbon cycle simulations and existing groundwater models of the Atlantic shelf. The fresh water below the shelf has long been regarded as a nonrenewable resource emplaced during the LGM. The new findings of this study may change this view.

We thank S. Meyer, S. Konrad, S. Krumm, and C. Weinzierl (Friedrich-Alexander-University Erlangen-Nuremberg) for their help with stable isotope and X-ray diffraction analyses, and A. Jäckel (Leibniz-Institute for Applied Geophysics, Hanover) for help with carbonate isotope analyses. We thank M. Kölling, L. Schnieders and colleagues from the University of Bremen and MARUM, Bremen, for sampling and analytical support for chemical analyses of pore waters. P. Escher (Leibniz Institute for Baltic Sea Research, Warnemünde) carried out sulfur isotope measurements. We thank S. Schlömer, C. Poggenburg, and D. Laszinski for methane analyses, and U. Berner and colleagues for additional total organic carbon analyses (Federal Institute for Geosciences and Natural Resources [BGR], Hanover). The Integrated Ocean Drilling Program (IODP) financed offshore and onshore activities of S. Stadler as a science party member, and provided supporting data. We also acknowledge drillers, platform staff, and scientists of IODP Expedition 313 for great collaboration. The thorough comments of two anonymous reviewers helped to improve the final version of this manuscript.