Santa Monica Basin Research Articles

Sedimentary coprostanol as an index of sewage addition in Santa Monica basin, southern California

Sediment cores from Santa Monica Basin and effluent from two major municipal wastewater dischargers in southern California were analyzed for sterols. Specifically the fecal sterols, coprostanols (coprostanol and epicoprostanol), were quantitated to determine the degree of sewage addition to the sediment. Although coprostanols are distributed throughout the Santa Monica Basin sediments in association with fine particles, some stations contain elevated levels, either due to their proximity to the outfalls or because of preferential advection of fine-grained sediments into their location where anoxicity aids in better preservation. The progressive seaward decline of coprostanols relative to total sterols from the outfalls represents dilution of sewage by biogenic sterols. The ratios of coprostanols to dinosterol appears to be a better indicator of the degree of sewage addition. A rapid increase in content of coprostanols from about 1935 coincides with the start of offshore wastewater discharge by JWPCP, the Los Angeles County Sanitation Districts on Palos Verdes Shelf. It is estimated that wastewater treatment plants release into southern California Bight 260 metric tons/yr of fecal sterols and 5 {times} 10{sup 4} metric tons/yr of sewage carbon.

Triterpenols from sediments of Santa Monica Basin, Southern California Bight, U.S.A.

The triterpenol geochemistry of the Santa Monica Basin from the Southern California Borderland, off the U.S.A., is described from the study of two sets of trap deployments, five box cores (≈30 cm) and a hydroplastic core (≈1 m). The biogenic sources and diagenetic stability of the triterpenols are discussed. The 17β(H), 21β(H)-hopanols (22R isomer) occur in the carbon number range from 30 to 32 and their abundance is nearly uniform in the shallow sediment sections. However, the three hopanols follow the order of abundance, C32 > C31 ≅ C30, in deeper sections. Their concentrations spans from trace levels to 156 μg/g organic carbon (<15 ng to 7 μg/g dry sediment). Tetrahymanol (gammaceran-3β-ol) has been identified in all the samples except in one set of trap particles collected at 100 m water depth, from trace level (<1 μg) to 215 μg/g organic carbon (<20 ng to 9 μg/g dry sediment). Diplopterol is also detected in trace amounts in some samples. The triterpenols in the trap material generally increase with the water column depth and decrease with the subbottom depth in the sediment cores. The extended hopanols are either degradation products of polyhydroxybacteriohopanes or are biosynthesized by bacteria. Tetrahymanol is probably the only suggested biological precursor of gammacerane (the reduced counterpart of tetrahymanol), which has been recognized in numerous crude oils and lithified sediments. Although it has been reported earlier from Green River Shale and from a residual Pleistocene lake sediment, tetrahymanol has so far been positively identified from recent marine sediments only in two recent studies. The decreasing content of tetrahymanol in sedimentary depth profiles in the Santa Monica Basin would favor an origin for this compound in the water column or at the sediment surface. The ubiquitous occurrence of this compound throughout the study area suggests that this triterpenol most probably originates from primitive organisms (protozoa, bacteria?), hitherto not identified or, more likely, not yet analyzed for their lipid composition.

Tetrahymanol: Its widespread occurrence and geochemical significance

The occurrence of tetrahymanol (gammaceran-3β-ol) in sediments from various marine depositional environments such as the Santa Monica Basin and Palos Verdes shelf (southern California Bight), the Santa Barbara Basin (off the northern California Borderland), the Atlantic shelf, slope, and rise, and the Antarctic region, as well as in bacterial/algal mats from the Santa Barbara Basin and Baja California, together with its detection in sediments from the Peru upwelling region (ODP Leg 112) and Baffin Bay (ODP Leg 105) suggests that tetrahymanol occurs ubiquitously in marine samples. Tetrahymanol is the only known likely biological precursor of gammacerane, which is found in many petroleums and shales. The common occurrence of tetrahymanol in marine environments implies that primitive organisms similar to Tetrahymena, or organisms other than Tetrahymena (other protozoa, bacteria?), are also likely to contain this compound. Its isomer, diplopterol, has also been detected in several sediment sections.

History of metal pollution in the Southern California Bight: an update

Box cores collected in 1985 and 1986 along an offshore transect in the Santa Monica Basin, CA, were analyzed for organic carbon, calcium carbonate, U-series radionuclides, and a suite of major and minor elements. Downcore profiles of metals and organic carbon reflect anthropogenic influence and diagenetic processes. The deep basin cores show pronounced subsurface maxima in Pb, Zn, Cr, and organic carbon during the time interval 1960-1970. Increases from the base of the core to this time horizon are consistent with increasing anthropogenic inputs to the Santa Monica Basin. The near-surface decreases in heavy-metal accumulation reflect recent improvements in waste water treatment. Sediment compositions cannot be fully explained by simple mixtures of sewage particles and natural sediment because of factors such as diagenesis of sewage during transport and sedimentation, additional metal sources, and variations in surface water productivity. The yellow-brown surface layer in deep basin cores is relatively enriched in Fe, Co, Cu, and P. This layer forms by reduction of Fe followed by upward diffusion and precipitation as amorphous oxyhydroxides; P, Cu, and CO, released during early diagenesis, are associated with this phase.

Role of plankton in the carbon and nitrogen budgets of Santa Monica Basin, California

Role of plankton in the carbon and nitrogen budgets of Santa Monica Basin, California

Cobalt and copper distributions in the waters of Santa Monica Basin, California

The trace metals cobalt and copper are removed from the oceans interior by scavenging on to particle surfaces1, but the mechanisms for removal of these two metals are probably quite different. Cobalt appears to be scavenged by manganese oxide particles1,2, whereas organic compounds are the main carrier phase for copper3,4. Remobilization of these metals in marine sediments therefore proceeds by different pathways. The differences in the pathways of remobilization are accentuated in oxygen-deficient environments: manganese oxide reduction is accelerated at low oxygen levels5 and organic carbon is preserved6. Cobalt fluxes from sediments underlying oxygen-deficient waters should be enhanced and copper fluxes reduced. We report here measurements of the cobalt and copper distributions in the waters of an oxygen-deficient marine basin in the Southern California Bight. Cobalt concentrations near the bottom are raised four times above the background level, whereas copper concentrations show no increase. These measurements confirm features of existing models for the oceanic cycles of these metals.

Major role of bacteria in biogeochemical fluxes in the ocean's interior

Spatial and temporal patterns in the flux of sinking organic matter are central to the understanding of elemental dynamics and food-web energetics in the global ocean1–3. Heterotrophic bacteria have been shown to play a part in the decomposition of large, rapidly sinking organic particles within and below the euphotic zone4–8. These previous studies suggest that decomposition by attached bacteria can explain only a trivial fraction of the observed decrease in the flux of organic matter with increasing depth. We report here that free-living bacteria, rather than the particle-feeding zooplankton, are the principal mediators of particle decomposition in the central north Pacific gyre and the eutrophic Santa Monica basin. We suggest that bacterial growth in the mesopelagial gives rise to the large-scale production of fine (0.3–0.6 μm), non-sinking particles at the expense of large, rapidly sinking particles. Our results have implications for models of biogeochemical dynamics of organic particles and surface-reactive materials such as radionu-clides in the ocean's interior3,9.

History of Contemporary Anaerobic Conditions in Santa Monica Basin, California Continental Borderland: ABSTRACT

Box cores from the basin plain of Santa Monica basin, California borderland, in the presently anoxic basin central plain, indicate that this condition is a very recent event in the basin's sedimentary history. From radiometric dating and varve counts, it has been determined that the initiation of anoxic conditions commenced about 180 years ago and increase to the present. The rate of growth appears to have declined in the past few decades. The center of deepest thickness of laminated undisturbed sediments is offset to the northwest from the actual bathymetric deepest point, which suggests that the condition is related to deep-basin circulation. The period since about 1800 coincides with the development of western settlement and population growth in southern California, beginning with the mission period and the establishment of the early pueblos. Rapid expansion of the zone during the 19th and early 20th centuries appears to parallel the growth of land development. There does not seem to be a climatic correlation and the only exceptional event at about the time of initiation of anoxic conditions is the Mission Quake of 1811. This event triggered large turbidity-current flows, but such large influxes of sediment and contained organic matter would not upsetmore » the oxygen demand balance for more than a few years given normal mixing and overturn of the deep basin waters. The continued growth of the zone with time suggests a continuing perturbation of the system and is most easily correlated with the anthropogenic effect of coastal land development and resulting introduction of oxygen-demanding wastes to the basin floor.« less

Profiles of dissolved and particulate thorium isotopes in the water column of coastal Southern California

Profiles of 234Th, 232Th, and 230Th in the Santa Monica Basin were measured. 234Th in the upper part of the water column suggests that the residence time of dissolved 234Th, with respect to adsorption onto and removal by particles, increases with depth, from 3–4 weeks at the surface to > 5 months below the euphotic zone. The partition of 234Th between dissolved and particulate forms points to a residence time of 4–7 days for particulate 234Th in the surface euphotic zone. Rapid removal of particulate 234Th from the surface water precludes the possibility of an adsorption-desorption equilibrium state between dissolved and particulate 234Th. Based on the 230Th data, the overall residence time of dissolved Th in the water column is estimated at 5–6 years. Both dissolved 230Th and 232Th profiles exhibit higher concentrations at the surface than immediately underneath, suggesting a terrestrial input for them. From the surface to the bottom of the water column, the concentration of dissolved 232Th decreases by a factor of 3–4, and the dissolved 230Th/ 232Th activity ratio increases by a factor of about five (from 2–3 to 9–12). This reflects the surface source of the terrestrial end-member and its removal at depth. On the average about 6% of the 234Th, 25% of the 230Th, and 50% of the 232Th were found in the particulate form, which is as expected and relates to their respective sources, to the difference in radioactive decay half-life, and to the kinetics of the scavenging process.

Budgets and behaviors of uranium and thorium series isotopes in Santa Monica Basin sediments

Nine natural decay-series isotopes were measured in six box cores collected from a transect across the Santa Monica Basin. The 210Pb-derived sedimentation rate decreases from ~80 mg/cm 2-yr at the slope to ~20 mg/cm 2-yr in the deep central basin. Sediment mixing prevails in sites underlying oxic waters, but is subdued in the anoxic deep basin below the sill depth. Uranium contents in sediments are controlled by levels of authigenic U, which are higher in the more reduced condition in the deep basin. Most of the authigenic U results from precipitation within the sediments. The 232Th- 228Th disequilibrium in sediments indicates that 228Ra is lost from the sediments from a depth of ~ 10 cm upward. Modelling the distribution of excess 228Th and 234Th in the surficial layers of the deep basin sediments results in a mean sediment mixing coefficient of 0.2 cm 2/yr and a sedimentation rate close to that based on 210Pb. There is no evidence of changing sedimentation rate in the central basin during the past century. Fluxes of excess 210Pb, 230Th and 231Pa to the central Santa Monica Basin sediments are much higher than what can be predicted from local supply. Advective input of open ocean waters coupled with enhanced scavenging of these reactive nuclides at the ocean margin is considered to be the primary cause.

Proteolytic activity in coastal oceanic waters: depth distribution and relationship to bacterial populations

Proteolytic activity and the role of bacteria in protein degradation in seawater were investigated during 2 cruises to the Santa Monica Basin. The rate of hydrolysis of L-leucyl-P-naphthylaniine was used as a measure of proteolytic activity, and was compared with bacterial numbers, production, and growth rates throughout the water column (905 m max. depth). Peak proteolytic activities in the upper 100 m of the water column were more than 10 times those of deeper waters, though the natural substrate concentrations (combined hydrolyzable amlno acids) in the Santa Monica Basin have prev~ously been found to vary by only 3 to 4-fold Sediment porewaters were assayed during one cruise and found to have 20 to 30 times more proteolytic activity than the peak water column samples from the same cruise, though the origin of this activity could not be determined. Most of the proteolytic activity in the water column was associated with particles the size of bacteria (0.2 to 0.8 pm) and, overall, total proteolytic activity was highly correlated with all bacterial variables tested. However, the average activities per cell were not related to average cell sizes, suggesting that proteolytic activity is not s~mply a function of bacterial biomass. In samples from one of the crulses, the average activities per cell covarled with bacterial growth rates (thymidme incorporation per cell), but this was not true for samples taken during the second cruise. Laboratory expenments using 'seawater cultures' indicated that neither changes in cell size nor growth rate alone could account for changes in the amount of proteolytic enzyme expressed by marine bacteria. These results suggest that proteolytic activity in the water column is primarily associated with bacteria and is likely to be controlled by a number of factors related to the bacterioplankton, including biomass and growth rates.

Budgets and behaviors of uranium and thorium series isotopes in Santa Monica Basin sediments

Budgets and behaviors of uranium and thorium series isotopes in Santa Monica Basin sediments

A deliberate tracer experiment in Santa Monica Basin

The mixing of heat and chemical constituents across density strata in the ocean plays a major role in ocean circulation and in the response of the climate to thermal and chemical perturbations. We are developing a tracer technique which, in conjunction with fluid dynamical measurements, promises to improve our understanding and our estimates of such diapycnal mixing. In a prototype tracer experiment in Santa Monica Basin, 50km west of Los Angeles, two tracers, sulphur hexafluoride (SF6) and per-fluorodecalin (PFD), have been injected on an isopycnal surface near the centre of the basin. After 50 days, the tracers had mixed along the isopycnal surface to nearly every part of the basin, although relatively little had penetrated to the basin walls. The diapycnal spreading of the tracer distributions during this first stage of the experiment yielded an apparent eddy diffusivity of 0.33±0.08 cm2s−1 at the ambient density gradient of 4.0±0.5×10−9g cm−4.

Euxinic Biofacies in Anoxic Basins: San Pedro and Santa Barbara Basins, California Continental Borderland: ABSTRACT

As part of an ongoing study of anoxic California continental borderland basins, the relationships between dissolved oxygen content, sediment fabric, biogenic structure distribution and size trends, and species richness in the San Pedro and Santa Barbara basins were examined. Results of these analyses are essentially identical to those of a previous study of the adjacent Santa Monica basin. The relationships observed in these basins will be useful as criteria for the reconstruction of ancient anoxic basins, the strata of which have high potential as hydrocarbon source beds. Six sediment fabric types, subjectively classified on the basis of degree of preservation of primary sedimentary structures versus degree of destruction by biogenic activity, were observed in box core x-radiographs. These are distributed concentrically around the centers of the basins with a progressive trend toward increased preservation of primary structures with depth and decreased oxygen content. Intermediate sediment fabric types are formed by the short-term fluctuations of the position of the dysaerobic-anaerobic boundary in the water column. Box core x-radiograph analysis of biogenic structures indicates that most burrow types occur over a wide range of depth and dissolved oxygen content. However, burrow diameter analysis indicates that burrow size decreases with a decrease in dissolved oxygen content. Bottom photograph analysis of surface biogenic structures indicates that aerobic and upper dysaerobic environments are characterized by abundant tracks and trails, lower dysaerobic environments are dominated by burrow openings, and anaerobic sea floor lacks any visible biogenic structures. Size trends of burrow openings indicate a decrease in size with decreased oxygen. However, organism-sediment interactions and environmental energy also exert a control on burrow-opening size. Bottom photograph analysis of visible organisms shows that aerobic and upper dysaerobic environments are dominated by urchins, while lower dysaerobic environments are characterized by holothurians, polychaetes, small arthropods, and the small gastropod, Mitrella permodesta. Anaerobic surface environments are essentially devoid of visible macrobenthic organisms. Although these trends reflect changes in dissolved oxygen content, evidence suggests that grain size also exhibits control on organism distribution. Species richness data from the upper portion of box cores indicates that diversity of macrobenthic organisms generally decreases with decreased oxygen content. The greatest decrease in diversity is coincident with the shelf break rather than the aerobic-dysaerobic boundary (1.0 mL/L dissolved oxygen). A loss of macrobenthic organisms that significantly affect sediment fabric occurs at the dysaerobic-anaerobic boundary. Although observations made in these modern basins support parts of previously developed biofacies models designed for use in reconstruction of ancient anoxic basins, several aspects of these models now appear to be invalid. In particular, there appears to be no definitive change in any of the studied parameters at the aerobic-dysaerobic boundary. The use of sediment fabric, biogenic structure trends, and fossil evidence may aid in the reconstruction of ancient basins providing that controlling factors other than oxygen are thoroughly considered. End_of_Article - Last_Page 544------------

Processes and Rates of Recent (Holocene) Basin Filling in an Active Transform Margin: Santa Monica Basin, California Continental Borderland

ABSTRACT Santa Monica Basin, an inner basin of the California Continental Borderland, is a deep margin basin presently receiving sediments under high sea-level conditions. Since the slowing of the last transgression, the basin has been receiving primarily suspended load contributions with rare turbidites except in the most headward parts of the two principal canyons. One turbidite, found throughout the basin, was laid down about 200 years BP, and was probably deposited by a turbidity current generated by mass movement of slope sediments in the slope apron adjacent to the major active canyons. Similar basin-wide events have occurred throughout Holocene time at intervals of about 500-1000 years and these represent the effects of big earthquakes, major floods, or other exceptional happenings not commonly recorded in historic time. Except for these exceptional events, the basin has received fine clastic sedimentation derived from the mainland rivers and transported by nepheloid plumes near bottom and in the surface mixed layer. Biogenic carbonate and organic carbon input from areas of high productivity fed by upwelling is masked by the fine detrital influx over the major fans by dilution from the rain of particulates from the major surface and near-bottom turbid plumes that pass over the basin center. A maxima of carbonate and organic content occurs in the basin floor sediments most distant from the combined diluting effects of these two major detrital inputs. The basin, slope-fan, and shelf-bank top sediments are differentiated on the basis of mean particle size and skewness. Changes in sand and silt content are related to the pattern of the prevailing surface and near-bottom water circulation with their reservoirs of fine suspended particulates. The principal process removing the fine particles from the surface plumes is biological filtering and pelleting. Sedimentation rates of major sediment components show that near-bottom nepheloid plumes are trapped by the closed deep basin, and their load settles out as the turbid waters circle over the central basin floor. Sedimentation rates vary from 40-80 cm/1000 years. Average sediment delivery is about 106 tons, but exceptional floods can deliver several-fold more sediment at 10-30 year intervals.

Comparison of Sand-Layer Geometry on Flat Floors of 10 Modern Depositional Basins

A comparative study of 10 deep turbidite basins indicates that sand-layer thickness, frequency, and continuity can be related to their basin geometries, tectonics, and source areas. This study is based on piston-core data from the flat floors of four oceanic-crust abyssal plains (Sohm, Hatteras, Blake-Bahama, and Silver), two Bahama-Plateau reentrant basins (Columbus basin and Tongue of the Ocean), three subduction zone--island arc basins (Hispaniola-Caicos, Navidad, and St. Croix basins), and one continental-borderland basin (Santa Monica basin). With increasing tectonic activity, sand layers on flat basin floors tend to be thinner and more frequent. Except in elongated narrow basins such as Navidad, source-area size is directly related to proximal sand-layer thickness. Continuity of individual sand layers is a function of drainage-basin size and depositional-basin shape. For example, on the Hatteras Abyssal Plain long-distance continuity (up to 500 km) of individual sand layers is achieved because most of the turbidites are large and are introduced only at the north upstream end. In the small horseshoe-shaped Columbus basin, sand-layer continuity is limited because flows are small and are introduced from three sides. In most basins, sands thin and the percentage of sand layers in the sediment column decreases away from sour e areas. However, in basins that are small relative to the typical turbidity-current size (e.g., Hispaniola-Caicos basin), the differences between basin-edge and basin-center sand layers are slight.

Organic chemical characterization of sediment-trap particulates from San Nicolas, Santa Barbara, Santa Monica and San Pedro Basins, California

Particulate matter was collected during September–October, 1977, in particle traps suspended 30–60 m above the floor of San Nicolas, Santa Barbara, Santa Monica and San Pedro Basins, off the coast of southern California. The trap particulates were analyzed for C 15–C 35 hydrocarbons using gas chromatography (GC) and GC-mass spectrometry. Kerogens and humic acids were characterized by H/C, N/C, δ 13 C, δ 15 N and δ 34 S ratios, and by electron-spin resonance. Hydrocarbons arising from fresh and weathered petroleum, marine autochthonous and terrestrial sources were identified. The rates of petroleum deposition during the collection period followed the order: San Nicolas Basin < Santa Barbara Basin ~ Santa Monica Basin < San Pedro Basin, with the largest amount of weathered petroleum being deposited in San Pedro Basin. The rates of petroleum deposition are correlated more strongly with human activities such as shipping, and the discharge of municipal and industrial wastes, than with natural submarine oil seepage. Analyses of kerogens and humic acids indicate that the majority of the organic matter in the trap particulates is of marine origin. The water column overlying Santa Barbara Basin appears to have the highest marine productivity of the four basins studied.

Sedimentation of Pliocene Sandstones in Santa Barbara Channel, California: ABSTRACT

Pliocene rocks in the Ventura basin, including the part under the Santa Barbara Channel, provide an excellent area to study a strongly deformed but essentially intact turbidite basin. Conglomeratic beds, containing typical turbidites, are present along the northern margin of the basin. Isopach, sand-isolith, and sand-percentage maps of the Pliocene Repetto and Pico Formations in the eastwest Central basin show thick deposits bounded on both sides by thinner deposits in the Rincon trend on the north and the Montalvo trend on the south. Deposition was controlled by partly effective fault barriers on the north and south margins of the Central basin. Sand-isolith and sand-percentage maps indicate local increases in sand in the north on the Rincon trend suggesting the presence of ancient subsea fans in the Repetto and lower Pico. The overall decrease in sand on the west denotes a major influx of sand from the east down the Central basin axis. The first influx of sand from the southern margin of the basin is found on the sand-percentage map of the middle Pico A. Deposit on of sand was also affected by topographic highs associated with growing faults and anticlines. Distribution, textural properties, primary structures, and microfauna of sandstones in the Ventura basin are comparable to the modern deposits in the nearby Santa Monica basin. Stratigraphic maps reveal Pliocene subsea fans, and observations of outcrops and cores show the same type of beds as described in the Santa Monica basin. Hence, sedimentation was controlled by sand deposition in and at the foot of submarine canyons in the form of subsea fans. End_of_Article - Last_Page 335------------

Foraminiferal Biofacies, Santa Monica Bay, California: ABSTRACT

Santa Monica Bay embraces an area extending more than 45 km. between Point Dume on the north and Palos Verdes Point on the south, and from the shoreline out into Santa Monica basin. Principal features are the mainland shelf (0 to 80 meters), the basin slope (80 to about 900 meters), and the basin floor which extends down to about 938 meters. The mainland shelf is a truncated positive feature separating the Santa Monica basin on the west from the filled Los Angeles basin on the east. Exposed Miocene-Pleistocene rocks on the shelf suggest this feature was formed during the Quaternary. Relict upper Pleistocene-Holocene sediments and faunas provide evidence of an episodic transgression of sea-level across the shelf. Analysis of 85 bottom samples indicates that Recent sediments become finer offshore with the exception of relict grains, and nitrogen values generally are higher in the deeper, fine-grained sediments. Echinoderms and mollusks dominate the megafauna near the edge of the shelf. General foraminiferal trends are: foraminiferal abundance increases offshore; live specimens are most abundant in the central shelf area; living species are most abundant in the central shelf area and in the bathyal zone; higher live/dead ratios occur on the central and outer shelf in areas of slow deposition; arenaceous types dominate dead populations on the central shelf; and live hyaline types are more than 8 times as abundant as live arenaceous forms in most areas. Inshore foraminiferal groups are characterized by species of Rotorbinella, Discorbis, and Elphidium. Open shelf biofacies are characterized by Buliminella elegantissima, Eggerella advena, Bulimina marginata denudata, and Trochammina pacifica. Species of Nonionella, normally abundant, are relatively rare, possibly because of ocean pollution. A Bolivina acuminata group of species is characteristic of the shelf edge and the upper basin slope. A deeper water Bolivina argentea basin facies appears at about 400 meters and is dominant on the floor of Santa Monica basin. End_of_Article - Last_Page 1079------------

Pennsylvania's ancient sea

Sources of methane to sedimentary environments are commonly identified and quantified using the stable isotopic compositions of methane. The methane “clumped-isotope geothermometer”, based on the measurement of multiply substituted methane isotopologues (13CH3D and 12CH2D2), shows promise in adding new constraints to the sources and formational environments of both biogenic and thermogenic methane. However, questions remain about how this geothermometer behaves in systems with mixtures of biogenic and thermogenic gases and different biogenic environments. We have applied the methane clumped-isotope thermometer to a mixed biogenic–thermogenic system (Antrim Shale, USA) and to biogenic gas from gas seeps (Santa Barbara and Santa Monica Basin, USA), a pond on the Caltech campus, and methanogens grown in pure culture. We demonstrate that clumped-isotope based temperatures add new quantitative constraints to the relative amounts of biogenic vs. thermogenic gases in the Antrim Shale indicating a larger proportion (∼50%) of thermogenic gas in the system than previously thought. Additionally, we find that the clumped-isotope temperature of biogenic methane appears related to the environmental settings in which the gas forms. In systems where methane generation rates appear to be slow (e.g., the Antrim Shale and gas seeps), microbial methane forms in or near both internal isotopic equilibrium and hydrogen-isotope equilibrium with environmental waters. In systems where methane forms rapidly, microbial methane is neither in internal isotopic equilibrium nor hydrogen-isotope equilibrium with environmental waters. A quantitative model of microbial methanogenesis that incorporates isotopes is proposed to explain these results.