Emission of Methane and Heavier Alkanes from the La Brea Tar Pits

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Journal of Geophysical Research: Atmospheres

RESEARCH ARTICLE Emission of Methane and Heavier Alkanes From 10.1002/2017JD027675 the La Brea Tar Pits Seepage Area, Los Angeles Key Points: G. Etiope1 , L. A. Doezema2 , and C. Pacheco2 • Considerable methane emission from the La Brea park in Los Angeles 1Istituto Nazionale di Geoﬁsica e Vulcanologia, Sezione Roma 2, Roma, Italy, and Faculty of Environmental Science and • Molecular alteration during seepage 2 must be considered in atmospheric Engineering, Babes-Bolyai University, Cluj-Napoca, Romania, Chemistry and Biochemistry, Loyola Marymount University, hydrocarbon emission modeling Los Angeles, CA, USA • A fault seems to act as a main pathway of gas migration from the Salt Lake oil reservoir to the surface Abstract Natural hydrocarbon (oil and gas) seeps are widespread in Los Angeles, California, due to gas migration, along faults, from numerous subsurface petroleum ﬁelds. These seeps may represent important

Supporting Information: natural contributors of methane (CH4) and heavier alkanes (C2-C4) to the atmosphere, in addition to • Supporting Information S1 anthropogenic fossil fuel and biogenic sources. We measured the CH4 flux by closed-chamber method from the La Brea Tar Pits park (0.1 km2), one of the largest seepage sites in Los Angeles. The gas seepage occurs Correspondence to: throughout the park, not only from visible oil-asphalt seeps but also diffusely from the soil, affecting grass G. Etiope, À1 [email protected] physiology. About 500 kg CH4 d is emitted from the park, especially along a belt of enhanced degassing that corresponds to the 6th Street Fault. Additional emissions are from bubble plumes in the lake within À the park (order of 102–103 kg d 1) and at the intersection of Wilshire Boulevard and Curson Avenue Citation: > À1 fl Etiope, G., Doezema, L. A., & Pacheco, C. ( 130 kg d ), along the same fault. The investigated area has the highest natural gas ux measured thus far (2017). Emission of methane and for any onshore seepage zone in the USA. Gas migration, oil biodegradation, and secondary methanogenesis heavier alkanes from the La Brea Tar Pits altered the molecular composition of the original gas accumulated in the Salt Lake Oil Field (>300 m deep), seepage area, Los Angeles. Journal of Geophysical Research: Atmospheres, 122. leading to high C1/C2+ and i-butane/n-butane ratios. These molecular alterations can be important tracers of https://doi.org/10.1002/2017JD027675 natural seepage and should be considered in the atmospheric modeling of the relative contribution of fossil fuel (anthropogenic fugitive emission and natural geologic sources) versus biogenic sources of methane, Received 30 AUG 2017 on local and global scales. Accepted 22 OCT 2017 Accepted article online 26 OCT 2017 Plain Language Summary The hydrocarbon seepage from subsurface petroleum (gas and oil) accumulations is a major natural source of atmospheric methane (CH4), ethane (C2H6), and propane (C3H8), which are powerful greenhouse gases and/or photochemical pollutants. We measured the natural gas flux from one of the largest seepage sites in California, the La Brea Tar Pits park in Los Angeles. The gas emission occurs throughout the park, not only from visible oil-asphalt seeps but also diffusely from the soil, affecting grass physiology. The investigated area has the highest natural gas flux measured thus far for any onshore seepage zone in the USA. Gas at the surface has a specific molecular composition, modified during migration from the original composition in the subsurface reservoir, which can help, through specific atmospheric modeling, to distinguish anthropogenic fugitive emission, natural geologic sources, and biogenic sources of methane, on local and global scales.

1. Introduction The hydrocarbon seepage from subsurface petroleum (gas and oil) accumulations is a major natural source of

atmospheric methane (CH4), ethane (C2H6), and propane (C3H8), which are powerful greenhouse gases and/or photochemical pollutants (e.g., Ciais et al., 2013; Etiope, 2015; Etiope et al., 2008; Etiope & Klusman, 2002; Nicewonger et al., 2016). Hydrocarbon seepage may also represent a geologic hazard, especially in urban or industrialized areas, due to the explosive properties of the gas, as documented in Los Angeles and some European towns (e.g., Baciu et al., 2008; Chilingar & Endres, 2005; Etiope et al., 2006). Global

geologic CH4 and C2H6 emission estimates from gas seepage in sedimentary, petroleum-bearing basins À and geothermal areas, based on bottom-up and top-down methods, are in the order of 50–70 Tg y 1 and À 2–4Tgy 1, respectively (Etiope, 2012; Etiope & Ciccioli, 2009; Etiope et al., 2008; Nicewonger et al., 2016;

Schwietzke et al., 2016). These values represent roughly half of the global CH4 and C2H6 emissions due to À1 industrial fossil fuel emissions, which are approximately 100–150 Tg CH4 yr (Schwietzke et al., 2016) and À1 6–8TgC2H6 yr (Nicewonger et al., 2016). In addition to the diffuse microseepage (considered as an areal ©2017. American Geophysical Union. seepage source) that is widespread in sedimentary basins, several thousand macroseeps (gas seeps, oil seeps, All Rights Reserved. and mud volcanoes) are documented on land (CGG, 2015; Clarke & Cleverly, 1991; Etiope, 2015; Link, 1952).

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Flux measurements of several hundred seeps worldwide allowed a first assessment of the macroseepage emission factors, a fundamental parameter to derive bottom-up global emission estimates (Etiope, 2015). Extending the seepage flux data set is essential to improve the emission factors and consequent global estimates. In this work we report the results of a gas seepage flux survey carried out in one of the most active hydrocarbon seeps in North America, the La Brea Tar Pits, in Los Angeles. The La Brea Tar Pits, located in an urban park called Hancock Park, are famous for the numerous oil and bitumen seeps, and the rich discoveries of Pleistocene plant and animal fossils (Stock, 1965). But Hancock Park is also characterized by intense degassing, which is visible as bubbling in oil or asphalt seeps and in the Lake Pit, and is suspected (but never investigated) to occur throughout the grassy ground of the park. Gas leaks also occur in streets adjacent to the park, such as at the intersection between Wilshire Boulevard and Curson Avenue. Previous gas studies in the La Brea area addressed, about 25 years ago, the molecular and isotopic compositions of the gas (Jeffrey et al., 1991). More recently, Weber et al. (2017) measured the gas flux from some oil seeps in the park. Farrell et al. (2013) performed a mobile survey with air sampling and gas chromatographic analyses from a recreational vehicle driven around the La Brea neighborhood. They

detected up to 48 ppmv of CH4 in the air, suggesting significant gas seepage throughout the area. CH4 sources at and around the La Brea park were also reported by Thorpe et al. (2013) using the Airborne Visible/Infrared Imaging Spectrometer. In May 2017, we performed gas flux measurements using the closed chamber technique throughout the park, with the objective of assessing the total emissions from the ground to the atmosphere of methane, ethane and propane, and the spatial distribution of the seepage. Methane emission was investigated for both oil-asphalt seeps (macroseepage) and the diffuse degassing from the soil (which is called “miniseepage”; the term “microseepage,” not applicable in our case, refers to diffuse exhalations in a sedimentary basin that are not related to a macroseepage zone; Etiope, 2015; Etiope et al., 2011). New molecular composition analyses of the gas have been carried out to assess, by comparison with those of Jeffrey et al. (1991): (a) possible temporal changes in the gas composition; (b) the methane/(ethane + propane) ratio in the seeping gas, which, due to molecular fractionation and biodegradation and secondary methanogenesis during upward migration, may be different from the ratio in the original reservoir gas; and (c) the isobutane and normal- butane concentrations, whose original relative composition can be altered due to biodegradation along the seepage system. Understanding the compositional ratios between several alkanes, and their modification during seepage, can help in modeling and estimating the relative contribution of fugitive (anthropogenic) fossil fuel leakage versus natural gas seepage on local and global scales (e.g., Hopkins et al., 2016; Nicewonger et al., 2016).

2. Description of the Seepage Area The city of Los Angeles contains numerous natural hydrocarbon seeps due to proliﬁc petroleum ﬁelds and active tectonics, where faults and fractured rocks form preferential conduits for gas and oil migration to the surface (e.g., Chang et al., 2014; Chilingar & Endres, 2005; Gurevich et al., 1993). Hancock Park and the associated Page Museum, where the La Brea Tar Pits are located, is about 0.1 km2 in area (roughly 0.3 km by 0.3 km; Figure 1), in the Fairfax District. Approximately 100 visible seeps (bitumen and oil leaks) occur throughout the park, although these are temporal in nature (as also observed by Weber et al., 2017), so new seeps are always forming and existing seeps sometimes cease. Major degassing is observable in the Lake Pit, at the southern side of the park, approximately 3,000 m2 in area and 5 m deep, where more than a hundred bubble plumes, with different size and bubble burst frequency, were observed during our survey (Figure 2a). Weaker gas bubbling occurs in some oil seeps emerging from the soil in the park (Figures 2b and 2c). Gardens and soil covered areas often show lack of grass, which is a suspected indication of oxygen deple-

tion in the soil (as observed, e.g., in CO2-rich mofettes; Rennert & Pfanz, 2016) due to enhanced hydrocarbon concentrations (Figure 2d).

The seeping gas is thermogenic, primarily composed of CH4 (~80–84 vol%), stemming from sand reservoirs (Upper Miocene to Pliocene Repetto and Puente Formations) of the Salt Lake Oil Field, located approximately from 300 to 1,000 m below the surface (Lang & Dreessen, 1975). The compositions of the seeping and reservoir gases are reported in Table 1. From the reservoirs, the gas naturally migrates to the surface likely through the 6th Street Fault, along the southern border of the Salt Lake Oil Field anticline (Wright, 1987) (Figure 1).

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Figure 1. Location and geologic setting of the La Brea Tar Pits redrawn from Antandrus (2009) using additional information from Samuelian (1990).

Figure 2. (a) The Lake pit with bubbling plumes. (b) The high degassing zone within the pits 3-4-61-67. (c) A gas bubble in one of the biggest oil seeps. (d) The closed chamber with portable sensors measuring gas ﬂux from soil. Note the variability of the grass color, which reﬂects the intensity of the methane seepage (see text for details).

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2 A precise mapping of this fault was possible thanks to data from more than 500 wells (Lang &

C-CO Dreessen, 1975; Samuelian, 1990). 13 δ Gas also seeps at several points just outside the park. A particularly important case is the

4 intense gas leak at the intersection of Wilshire Boulevard and Curson Avenue (Figure 3), where 181 15.6 178183 25.9 25.4 in 1999 city authorities had to develop a permanent ventilation system. The reason for this leak H-CH À À À 2

δ is not clear. Chilingar and Endres (2005) mentioned the possible presence of an abandoned oil well, but the oil well database of the Department of Conservation’s Division of Oil, Gas, and 4 Geothermal Resources of California does not report any well at Wilshire Boulevard and 38.9 42.5 42.4 42.4 C-CH ﬁ À À À À Curson Avenue. More recently, Los Angeles city of cials considered the gas leak a natural see- 13 δ page phenomenon (e.g., Blevins, 2013). Furthermore, additional gas seepage has been

2 observed at other locations above the Salt Lake Oil Field. Most notably, a department store N exploded in 1985 due to a buildup of methane seepage at the corner of 3rd Street and Ogden Drive (about 0.5 km north of the tar pits). This event led to revised building codes, which 2

CO now require methane mitigation systems in neighborhood buildings in order to vent seeping gases (Chilingar & Endres, 2005). 5 0.001 0.001 n-C < < 3. Methodology 3.1. Gas Sampling and Analysis 5 ushing) and analyzed by GC (not corrected for air contamination); ﬂ -C i Molecular composition of the gas exhaling at the La Brea park was analyzed at three sites: (a) from a bubbling oil seep (point no. 198; southeast corner of the park, Figure 3); (b) from the

4 grassy ground inside the circular sidewalk near the central area of the park (point no. 160; cor- -C 0.1 1.1 n responding to Figure 2d; location shown in Figure 3); and (c) from the gas leak at the Wilshire- < Curson intersection. Gas was sampled using an inverted funnel from bubbling seep no. 198

4 and from a 10 L closed chamber on the degassing soil and at the Wilshire-Curson leak. -C i Funnel and chamber were ﬂushed by using a three-way valve, tubing, and a 50 mL syringe

) to reduce air contamination. 3

+C The molecular analysis of samples no. 198, no. 160, and the Wilshire-Curson leak was 2 performed using a Fourier Transform Infrared spectrometer (FTIR DX-4030, Gasmet, Finland) /(C ushing (no. 160 and Wilshire-Curson corner) or through inverted funnel (no. 198) and analyzed by FTIR (not 1 Molecular composition represents an average of 102 samples, and isotopic ratio represents an average of 2 ﬂ d C with a standard spectra library to rapidly and simultaneously determine 10 gases (CH4,CO2,

8 CO, C2H4,C2H6,C3H8, n-C4H10, i-C4H10, n-C5H12, and i-C5H12) with accuracy around 10% and , C-VPDB; H-VSMOW) Seeping at La Brea Tar Pits and in the Underlying Salt Lake Oil Field Reservoir H

3 – ‰ typical detection limits of 2 3 ppmv. ( C 2 For comparison, high precision gas chromatographic (GC) analyses of methane, ethane, 6 H

2 propane, i-butane, n-butane, i-pentane, and n-pentane were also carried out for two locations. and CO C 4 For point no. 160 (inside the central circular walkway) this comparison utilized 2 L canister samples connected to a 125 L closed-chamber where gas accumulated, without ﬂushing with 4 8.93 0.112 0.096 44 0.016atmospheric 0.022 0.007 0.002 air. A 2 L canister was also collected approximately 10 cm above one of the vents 68.5 0.175.078.0 0.02 0.419.63 1.3 0.179.9 0.153 0.584.5 523 0.067 0.72 0.69 150 0.18 0.013 0.17 89 43 0.002 0.017 Nd 89 0.021 0.02 0.001 98 0.002 Nd 0.004 0.023 0.03 0.021 0.001 0.002 0.002 11.4 0.008 n.d. 12.2 0.004 0.006 n.d. Tr. 13.5 n.d. 15.6 3.5 14.8 Tr. at the Wilshire-Curson intersection. Canisters were analyzed at Loyola Marymount University

using a Bruker 430 GC equipped with a ﬂame ionization detector (FID). C2–C5 alkanes were analyzed using a Varian 3400 GC-FID that utilized cryogenic concentration for samples. Analytical

(ppmv) 6377 51 0.6 124 1.2 details were reported in Weber et al. (2017). a b d , b 3.2. Gas Flux Measurements e e Collected in 2 L canister after accumulation in 125 L chamber (without atmospheric air Accumulated in 10 L chamber after atmospheric air b a We performed direct measurements of the ﬂux of methane and carbon dioxide (CO2) from the ground using the closed-chamber method and portable sensors and estimated the emission of a ethane and propane on the basis of their relative concentration to CH4 in the seeping gas. a c

Methane and CO2 ﬂuxes were measured at 214 points throughout Hancock Park, distributed 2

Values corrected for air contamination. over an area of 69,400 m (Figure 3). These samples included 26 points over visible oil and e bitumen seeps and 188 points that were on soil or grassy areas, either surrounding or far from the seeps. Three additional ﬂux measurements were performed in correspondence with

tr., traces; n.d., not detected. the Wilshire/Curson gas leak, from manhole covers and sidewalk cracks, a few meters outside the park (Figure 3). It is recognized that pavement can focus leakage toward existing cracks Collected in 2 L canister and analyzed by GC (not corrected for air contamination); Sample 1,060 by Jeffrey et al. (1991) Note. Sample 1,061 by Jeffrey etSalt al. Lake (1991) Reservoir, Puente F. (Jeffrey et al., 1991) 80.5 1.59 0.60 37 0.23 0.38 0.17 0.07 14.4 0.07 samples; and Sample CH Average oil seep by Weber et al, (2017) Bubbling Oil Seep (no 198) Circular sidewalk degassing soil (no. 160) Wilshire-Curson corner Wilshire-Curson corner corrected for air contamination); Circular sidewalk degassing soil (no. 160) c Table 1 Molecular Composition (vol %) of the Gas and Isotopic Ratios of CH and openings.

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Figure 3. Distribution of the gas ﬂux measurement points in correspondence with oil-asphalt seeps and over the soil.

Flux measurements were performed using widely tested instrumentation (e.g., Etiope, Christodoulou, et al., 2013; Etiope, Drobniak, et al., 2013), based on an aluminum, 10 L closed chamber (Figure 2d; West Systems

srl, Italy) connected to portable CH4 and CO2 sensors: a Tunable Diode Laser Absorption Spectroscopy CH4 detector (accuracy 0.1 ppmv, lower detection limit 0.1 ppmv) and a double beam infrared CO2 sensor (Licor; accuracy 2%, repeatability ±5 ppmv). The data are transmitted via Bluetooth communication to an Android smartphone. The gas flux is calculated through a linear regression of the gas concentration buildup (ppmv/s) in the chamber; for high fluxes, where the flux curve tends to become convex due to advection decrease as the gas pressure within the chamber increases, the first linear part of the curve is considered. À2 À1 À2 À1 Fluxes are expressed as mg CH4 m d and g CO2 m d , analogous with previous works on gas seepage and biologic fluxes from soils. Carbon dioxide, a primary greenhouse gas, generally is not a major component of the petroleum seepage

(CO2 is more important in volcanoes and geothermal manifestations). However, the Salt Lake Oil Field gas has about 15 vol % CO2 (Table 1). Carbon dioxide fluxes from the ground are expected to be comparable, and mixed, with CO2 from soil respiration, except in high flux gas vents, where the CH4/CO2 ratio may be similar to the reservoir gas. Simultaneous CH4 and CO2 measurements in hydrocarbon seeps are additionally useful to verify that the chamber is correctly positioned on the soil surface, without leakages; this reduces considerably the uncertainty of the flux measurement. The accuracy of the flux measurement indicated by laboratory calibration tests is ±10% (www.westsystems.com), and field tests performed on various seepage sites indicated reproducibility <20% (Etiope, unpublished data). In general, due to perturbations of the pressure gradient in the soil, accumulation chambers may underestimate the gas flux by 0 to 25% (Evans et al., 2001).

The total CH4 output from diffuse degassing (“miniseepage,” as described in section 1) was estimated using a “Natural Neighbor” spatial interpolation between individual gas measurements (Sibson, 1981). This method of spatial interpolation provides the best contour estimates for sampling of irregularly spaced points

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avoiding the assignment of flux values to sectors that were not actually sampled. Total gas output from visible seeps/vents was calculated by summing the emission measured in each vent. Within the Pit 3-4-61-67 2 enclosure (600 m ; Figure 2b, location indicated in Figure 3) the diffuse CH4 flux values were very high, reach- À À ing on the order of 105 mg m 2 d 1, distinctly higher than those measured outside the enclosure. The gas output from this area was calculated (by Natural Neighbor interpolation) independently to avoid overesti- mated extrapolations for the surrounding area outside of the enclosure. Accordingly, total gas output from the La Brea park is the sum of the emission from (a) all vents, (b) the diffuse miniseepage, and (c) the high degassing Pit 3-4-61-67. Gas flux from the numerous bubble plumes in the Lake Pit (Figure 2a) was not measured, but the order of magnitude of the gas output was roughly estimated on the basis of bubble plume number, sizes, and bursting frequency, which were observed by video recording (up to 10 min long, for a total recording of 14 min). Bubble plume features were compared with experimental flux data and images of bubble plumes obtained in other seepage areas (Etiope, Christodoulou, et al., 2013 and unpublished data archive), in conditions similar

to those of the Lake Pit (water depth of 7 m and CH4 concentration in bubbles of ~80 vol %). These measurements allowed us to determine a ﬂux of methane, in terms of order of magnitude, as a function of the size and frequency of individual bubble trains, which was consistent with theoretical models (Etiope et al., 2004). The emission of heavier alkanes, ethane, and propane was estimated considering their molecular ratio to

methane (C2/C1 and C3/C1) observed in the oil seeps and in the gas exhaling from the soil, assuming that the relative composition of the gases in oil seeps and soil degassing does not signiﬁcantly change over the seepage area.

4. Results 4.1. Molecular Composition of the Gas As expected, the seeping gas is dominated by methane (Jeffrey et al., 1991), with concentrations observed in

the collecting chamber or funnel (not corrected for air contamination as O2 was not analyzed) up to 78 vol % (Table 1). Carbon dioxide is the second most abundant gas, exceeding 10 vol %, as reported in previous

analyses (Jeffrey et al., 1991). The CH4/CO2 ratio in the gas seeping in the park is around 6, similar to the ratio at the Wilshire-Curson leak (5.8) and in the underground reservoir (5.6; Jeffrey et al., 1991). The C1/(C2 +C3) ratio was much higher in one of the degassing soil samples (523) compared to the bubbling seep (150) and especially compared to the Wilshire-Curson leak (~45), which exhibited a ratio close to that of the reservoir (37). The isobutane/normal-butane volumetric ratio of the gas seeping in the park (>1) is also different from the original gas in the reservoir (<1), while the ratio in the Wilshire-Curson leak is similar to the reservoir gas.

4.2. Methane Flux The gas flux measurements exhibited a widespread emission of methane throughout the park. Methane À À fluxes from oil-bitumen and bubbling seeps range from 6,900 to approximately 53,800,000 mg m 2 d 1 À À (supporting information S1), with the highest values (orders of 106–107 mg m 2 d 1) corresponding to À À seeps with visible gas bubbling. Methane flux from the grassy soil ranges from À3mgm 2 d 1 (a normal À À value in dry soil in temperate climates) to more than 9 × 106 mg m 2 d 1. Interestingly, the highest flux values were systematically observed in correspondence with grass characterized by a dull green color (Figure 2d); medium-high values were detected where grass was yellowish or not growing and, lower values where the grass had a more vivid green color. These are likely indications of plant stress and response to high hydrocarbon content in the soil, coupled with low oxygen. The reason why yellowish

grass, or its absence, exhibited lower CH4 flux than the dull green grass is unknown; we suspect that gas seepage changes its position over time (channeled gas flow may migrate horizontally), and yellowish grass represents the site where gas seepage was very high until recently; the current higher flux is starting to affect the grass producing a dull green color, which will become yellowish later. Specific biochemical

analyses in the future may reveal the mechanism. Very high CH4 ﬂuxes were concentrated within the Tar À À Pits 3-4-61-67 area (Figure 2b), ranging from 882 to 789,000 mg m 2 d 1, over several small seeps and fractured soil. In this area a vigorous bubbling seep was responsible for the highest absolute gas À À ﬂux (53,800,000 mg m 2 d 1).

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À À Figure 4. Contour map of methane seepage in the La Brea park (iso-ﬂux lines in mg m 2 d 1; methane ﬂux is shown in different colors and its intensity increases from white, yellow, green, and orange to dark pink) and photos of shattered pavement, due to gas seepage, outside the southeastern corner of the park and at the Wilshire-Curson intersection.

As outlined in the section 3, total gas output from the La Brea park is calculated as the sum of the emission À À from (a) all vents (96 kg d 1), (b) the diffuse miniseepage (177 kg d 1, covering 69,400 m2 of the park, esti- À mated via Natural Neighbor interpolation), and (c) the high degassing Pit 3-4-61-67 (220 kg d 1 over 500 m2, via Natural Neighbor interpolation and adding separately the most intense seep, so as not to distort À À neighboring points of measurement). Total methane emission from the park is 493 kg d 1 (180 t yr 1). The uncertainty of the natural neighbor interpolation estimate of the diffuse emission, assessed by twofold À cross-validation technique (e.g., Ciotoli et al., 2015), is within ±40 kg d 1. About 30 major bubble plumes, with different size and bursting frequency, could be detected in the La Brea Lake through video recording. Additionally, ~ 70–80 small and episodic bubble trains, not recorded by the camera, were observed near the banks of the lake. Three plumes were continuously active over 10 min of video recording, with a diameter of about 0.5–1 m. By comparison with experimental data, as described in

section 3, we estimated that these gas plumes may emit individually an amount of CH4 in the order of À1 10–100 kg CH4 d . Other plumes were episodic, with diameters from about 5 to 30 cm, and bubbling events lasting from a few seconds to 1 min. A group of 10 more active plumes may emit, individually, about 1–10 kg À1 CH4 d . Seventeen other minor plumes (recorded by the camera), and those (>70) observed at the lake À1 banks, could emit orders of 0.1–1kgCH4 d . The order of magnitude of the total CH4 output from the bub- À bling lake may then range from 102 to 103 kg d 1. However, these are first-order semiquantitative estimates awaiting verification by direct flux measurements. The spatial distribution of the methane seepage (Figure 4) clearly shows a linear trend crossing diagonally the park from NW to SE and a second N-S trend on the eastern boundary of the park. The bubbling plumes in the lake are in positions consistent with the NW-SE seepage lineament. À À Carbon dioxide fluxes from the seeps range from 16 to 9,400 g m 2 d 1 (supporting information). Carbon dioxide flux from the soil was, for most sites, within the range of normal soil respiration (typically À2 À1 À2 À1 10–50 g m d ), and it exceeds 100 g m d only in association to the highest CH4 fluxes. Because CO2 in the seeping gas does not exceed 15 vol % (Table 1), in locations where the seepage is low the biologic soil respiration may prevail over the geologic CO2 input, making it difficult to distinguish between the two CO2 sources. In these cases, the CH4/CO2 mol/mol ratio in the flux data is much lower (often <1) than the original CH4/CO2 ratio of the seeping gas (which is around 5–6), due to the excess biologic CO2. At high seepage

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Table 2 À Total Emission of Methane, Ethane, and Propane (kg d 1) From the La Brea Park Area

Seepage type or sector (measured area) CH4 C2H6 C3H8 Diffuse degassing (69,400 m2) 177 0.5 0.14 Pits 3-4-61-67 (500 m2) 220 2.20 0.8 Oil-gas seeps (28) 96 1 0.3 Total emission from the park ground 493 3.7 1.2 First-order emission estimate from the lake (3,000 m2)102–103 100–101 100–101 Wilshire-Curson corner leak (3 m2) >130 4 2.3 Total output (order of magnitude) 103 >10 >5

points, the CH4/CO2 ratio ranges from 3 to 9, which is consistent with the molecular ratio of the seeping gas (Table 1). This is an indication of the good quality of the ﬂux data.

4.3. Ethane and Propane Fluxes

Knowing the CH4 ﬂux and the C1/C2 and C1/C3 molecular ratio observed in the seeping gas (C1/C2: 685 and C1/C3: 3425 for soil degassing; 187 and 750 for bubbling seeps; 60 and 156 for the Wilshire-Curson leak; Table 1), and considering the molecular mass correction, the total emission of ethane and propane from À the park was estimated in the order of 4 and 1 kg d 1, respectively (Table 2). These estimates assume that the relative composition of the gas does not signiﬁcantly change over the seepage area. Our partial measure-

ments of the Wilshire-Curson leak, performed at only 3 points, showed C2-C3 emissions equivalent to those of the entire park. The uncertainty of these emission estimates derives from the methane ﬂux uncertainty (discussed above).

5. Discussion 5.1. Postgenetic Alteration of Gas Composition: Molecular Fractionation and Biodegradation

Jeffrey et al. (1991) showed that La Brea seeping CH4 and CO2 has the same isotopic composition as the reser- 13 voir gas (thermogenic CH4 signature and positive δ C-CO2 values, suggesting oil biodegradation; Table 1) but a slightly different concentration of several alkanes. This suggests that there is no isotopic fractionation during gas migration to the surface, as typically observed in many seeps worldwide (e.g., Baciu et al., 2017;

Etiope et al., 2009). Instead, the C1/(C2 +C3) ratio of the reservoir is 37 and increases to 98 in a seep (Jeffrey et al., 1991). We have observed similar values and trends in our seeping gas data. The Wilshire-

Curson leak displays a C1/(C2 +C3) ratio (~43) very close to the ratio of the Salt Lake Oil Field reservoir (Table 1). The ratio increases in the bubbling seep (150) and even more in one of the degassing soil samples

(523). Basically, the C1/(C2 +C3) ratio increases as the gas ﬂux decreases, which is typical of gas seeps (Etiope et al., 2009, 2011). This phenomenon can be attributed to molecular fractionation during the advective gas migration from the reservoir to the surface. The process is a type of distillation, or differential segregation of light hydrocarbon molecules as a function of their adsorption and solubility properties, so that seeping

gas is dryer (higher C1/C2+ ratio) than the original reservoir gas (e.g., Brown, 2014; Etiope et al., 2009; Waseda & Iwano, 2008). Therefore, the lower the ﬂux, the higher the residence time in the migration pathway system, and the higher the fractionation.

It is likely, however, that the C1/C2+ ratio also increases due to addition of secondary microbial methane, fol- 13 lowing oil biodegradation: the reservoir is biodegraded (δ C-CO2: +15.6‰; Table 1) but additional biodegradation and methanogenesis may take place along the seepage systems. This is suggested by the higher δ13C-

CO2 value in the seeping gas, which is up to +25.9‰ (Jeffrey et al., 1991) and, in particular, by the ratio between isobutane and normal butane. Isobutane to n-butane ratios (iC4/nC4) in nonbiodegraded reservoirs are typically around 0.1–1.5 (e.g., Larter & di Primio, 2005; Pallasser, 2000), and the ratio generally increases

with the maturity of the source rock (e.g., Prinzhofer et al., 2000). The iC4/nC4 ratio may, however, increase substantially, exceeding 5–10, due to preferential microbial degradation of n-alkanes versus i-alkanes (Pallasser, 2000). The ratios measured by us in La Brea are 17 in the bubbling oil seep (no. 198) and from 6 to 250 in the degassing soil site (no. 160). These elevated ratios are consistent with those previously observed in seeps by Jeffrey et al. (1991) and Weber et al. (2017). The ratio is much lower (<1) in the reservoir and in the

ETIOPE ET AL. GAS SEEPAGE AT LA BREA TAR PITS 8 Journal of Geophysical Research: Atmospheres 10.1002/2017JD027675

gas of the Wilshire-Curson leak, which, due to the intense and rapid ﬂow, does not exhibit alteration or fractionation from the reservoir (as

also indicated by the C1/C2+ ratio; Table 1). The fact that the reservoir 13 appears to be biodegraded (positive δ C-CO2), but without signiﬁcant n-C4 consumption, may suggest that oil biodegradation in the reservoir did not perturb the iC4/nC4 ratio. Alternatively, the biodegradation was simultaneous to a progressive accumulation (from deeper reservoirs or

source rocks) of wet gas with an unaltered iC4/nC4 ratio. Combining 13 δ C-CO2 and iC4/nC4, it is clear that seeping gas is more biodegraded than the reservoir gas. Though Weber et al. (2017) suggested that the biological degradation is occurring in the wet asphalt, the increased ratio at the site of soil degassing no. 160 (a “dry” surface vent with no associated asphalt) suggests that soil microbes are perhaps an addi-

tional, or more likely, cause of nC4 consumption. 5.2. Seepage Distribution The NW-SE seepage lineament (Figures 4 and 5) has the same orienta- Figure 5. The 3-D plot of methane emission in the La Brea park with approximate position of the 6th Street Fault, based on detailed geologic map after tion and approximate position of the 6th Street Fault (Figure 1). The Wright (1987). Wilshire-Curson leak is also positioned in line with the seepage lineament and the fault. These data support the hypothesis that the 6th Street Fault is actually the main preferential pathway of migration of gas from the Salt Lake Oil Field to the surface. Our spatial survey suggests however the possible presence of a secondary N-S pathway, transversal to and intercepting the 6th Street Fault near the southeastern corner of the park, at roughly the Wilshire- Curson intersection. The strong emission of the Wilshire-Curson leak could then be explained by the occurrence of a very permeable gas migration pathway, an enhanced fracture zone, created by the intersection of two faults. Large seeps are in fact frequently observed at the crossing of two or more faults (e.g., Etiope, 2015). The possible existence of a N-S fault should be examined reviewing available borehole and geophysical prospection data. However, as discussed above, we cannot exclude that the Wilshire-Curson gas is leaking from an abandoned, yet unknown (non-inventoried) well, as some authors hypothesized (Chilingar & Endres, 2005).

5.3. Significance of Methane and C2+ Emissions À À Individual methane fluxes measured as high as 9.0 × 106 mg m 2 d 1 for miniseepage measurements made À À over soil/grass and 5.4 × 107 mg m 2 d 1 for visible oil/asphalt seeps. These values are similar to fluxes À À observed in/around other seeps (typically in the order of 105–107 mg m 2 d 1; e.g., Etiope et al., 2011; Etiope, 2015). The fluxes measured in this study from visible seeps are more than an order of magnitude higher than those reported by Weber et al. (2017), which were considered as the lower limit of emissions because of sealing problems with the flux chamber. Thus, the results of this study seem to confirm that the Weber et al. (2017) study underestimated emissions from visible seeps. À1 The estimated overall CH4 output from the La Brea park (including the measured emission of 493 kg d and À the estimated order of magnitude of emission from the lake, 102–103 kg d 1) is comparable with the emission of large gas seeps and mud volcanoes studied in Europe and Asia (Etiope, 2015, and references therein).

If the Wilshire-Curson leak can be considered a natural seep, then the overall natural CH4 emission from the À La Brea area may exceed 103 kg d 1. Regardless, the gas emissions from the ground within the park À (~180 t yr 1) is the highest measured so far in onshore seeps in the USA. Smaller seepage fluxes were reported by Duffy et al. (2007) just to the west in Ventura County, California, Walter Anthony et al. (2012) in Alaska, LTE (LT Environmental, Inc.) (2007) in Colorado, and Etiope, Drobniak, et al. (2013) in New York State. A technical report by Exploration Technologies, Inc. (2001) reported methane flux data from the Playa Vista seepage zone in Los Angeles; applying the Natural Neighbor interpolation method on the flux À data indicated in the report, we estimated a total output of about 160 kg d 1, much lower than at La Brea. À2 À1 The La Brea CH4 emission factor (total emission divided by area) is on the order of 7,000 t km yr (considering park and lake emissions; Table 2). This is higher than the global average emission factor estimated for À À mud volcanoes (about 3,100 t km 2 yr 1; Etiope, 2015) and similar, in terms of order of magnitude, to the

ETIOPE ET AL. GAS SEEPAGE AT LA BREA TAR PITS 9 Journal of Geophysical Research: Atmospheres 10.1002/2017JD027675

emission factor of the offshore Coal Oil Point seepage zone, near Santa À1 Barbara (California), which releases about 30,000 t CH4 yr over a relatively large area, ~ 3 km2 (Hornaﬁus, et al., 1999).

Through a mobile survey, Farrell et al. (2013) estimated a CH4 output of about “1/6 of a kiloton per day” for the entire La Brea neighborhood. By À comparison with our estimated order of magnitude of 103 kg d 1 for the park only, the Farrell et al. (2013) data would suggest that consider-

able CH4 emissions occur in the area around the park. It is likely that signiﬁcant gas leaks, similar to the one partially measured by us at the Wilshire-Curson intersection, occur elsewhere at one or more locations in the neighborhood. Ethane and propane emissions, estimated in total at least 10 and À 5kgd 1, respectively, are not especially large when compared to seeps measured in other regions. Much smaller, individual vents but with

higher relative C2+ content release similar total C2-C3 amounts (e.g., Etiope, Drobniak, et al., 2013). Clearly, the molecular fractionation and

biodegradation during the seepage, leading to high C1/C2+, makes the La Brea park seepage a less signiﬁcant source of heavier alkanes. The Wilshire-Curson leak, where gas is not fractionated during migration, is a relatively larger ethane-propane emitter. A more complete and exhaustive set of ﬂux measurements should be performed at this

site to better evaluate the overall C1-C3 output.

The variability of the C1/C2 and C1/C3 ratios, due to molecular fractionation during seepage, is an important process that should be taken into account in the attribution of gas sources (natural versus anthropogenic; fossil fuels versus biogenic), as performed by Peischl et al. (2013) and Figure 6. Isobutane versus normal-butane plot for La Brea seeping gas, ﬁ compared with anthropogenic fossil fuel data, petroleum reservoirs, and Hopkins et al. (2016) for the Los Angeles area. Our data con rm that nonbiodegraded seeps in Los Angeles, after Peischl et al. (2013). the C1/C2+ ratio of geologic seepage (a natural fossil fuel source) can be lower than that of leaks from boreholes and pipelines (i.e., fugitive emissions and anthropogenic fossil fuels) where gas composition is practically the same as the subsurface

reservoir. Seepage C1/C2+ ratios can even be similar to those of biogenic sources (>500) so that, in the measurements performed in the atmosphere, geologic seeps may be confounded for biogenic sources if the underground source is not known, as suggested by Hopkins et al. (2016). A similar discussion can be made concerning the use of isobutane to normal-butane ratio for source attribu-

tion (e.g., Peischl et al., 2013). Our data show that biodegraded seeps may release gas with a very high iC4/nC4 ratio (>6), higher than that of the natural gas distribution network or vehicles (trafﬁc), which are typically ≤1 (Peischl et al., 2013). We note that Peischl et al. (2013), in their Figure 6f, considered seeps from Los Angeles

(data from Jeffrey et al., 1991) releasing gas that is scarcely or nonbiodegraded, thus with iC4/nC4 ratios ≤1. We have redrawn Figure 6f of Peischl et al. (2013), adding all available La Brea data, including those of Jeffrey et al. (1991), Weber et al. (2017), and those from the present work (Figure 6). Clearly, seepage of bio-

degraded reservoir gas provides an iC4/nC4 signal in the atmosphere that is completely different from that of anthropogenic sources. Accordingly, the iC4/nC4 ratio can be used as a speciﬁc tracer of a natural source of fossil fuel (i.e., seepage), but geochemical analyses of both seeping and reservoir gas are necessary to evaluate if and where molecular alteration by biodegradation occurred. In Los Angeles, if biodegraded seepage of

gas, with high iC4/nC4 ratio, is occurring over a large enough area, it could account for some of the missing isobutane in the local budget estimates made by Peischl et al. (2013).

6. Conclusions Thermogenic methane and minor amounts of heavier alkanes (ethane, propane, and butane) are emitted from the La Brea Tar Pits from visible oil-asphalt and bubbling seeps and, diffusely, from the soil, throughout the entire area of the park. Gas seepage clearly affects grass physiology, as evidenced by changes in the plant

ETIOPE ET AL. GAS SEEPAGE AT LA BREA TAR PITS 10 Journal of Geophysical Research: Atmospheres 10.1002/2017JD027675

color and growing patterns in relation to the measured CH4 flux. This phenomenon would deserve a specific À biochemical study. We estimated a total methane emission from the ground of about 0.5 t d 1, of which ~35% is from soil degassing, ~20% from 26 active oil/asphalt seeps, and ~45% from a small area with a high degassing rate, the Pits 3-4-61-67 enclosure (including soil degassing and seeps). Additional significant emis- À sions, roughly estimated in the order of 102–103 kg d 1, are from bubble plumes in the lake located on the southern margin of the park. Another important gas emission occurs just outside the park, at the intersection À of Wilshire Boulevard and Curson Avenue (>130 kg d 1 measured at only 3 points). La Brea park represents the onshore seepage zone with the highest methane output measured in the USA, to this point in time. The higher gas fluxes are distributed along a NW-SE lineament that has the same orientation and position as the 6th Street Fault. As suggested by previous geologic surveys, this fault seems to act as a main pathway of gas migration from the Salt Lake oil reservoir (>300 m deep) to the surface. Our data suggest, however, that the gas seepage reaches beyond the boundaries of the park, and the total gas output we estimated is a fraction of the actual seepage in the La Brea-Hancock area. Further gas flux measurements should be carried out at the Wilshire-Curson intersection and in all streets surrounding the park. 13 The seeping gas shows geochemical features typical of oil biodegradation (positive δ C-CO2 values and high i-butane/n-butane ratio). The gas has also a higher C1/C2+ ratio compared to the reservoir gas, which can be due to molecular fractionation and secondary methanogenesis along the seepage system. These molecular alterations are important tracers of natural seepage and should be considered in future studies and models aimed at assessing the fossil fuel (anthropogenic versus natural) or biogenic origin of atmospheric methane over a given region.

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