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Representative SEM's of the nacreous microstructure of Baculites compressus analyzed for isotopes. The shell samples show excellent preservation, with Preservation Index 4-5 (Cochran and others, 2010). (A) AMNH 108142, non-seep site, AMNH loc. 3415. (B) AMNH 64427, seep site, AMNH loc. 3528. (C) AMNH 82654, seep site, AMNH loc. 3528. (D) AMNH 65596, seep site, AMNH loc. 3528. (E) AMNH 82655, seep site, AMNH loc. 3528. (F) AMNH 64526, seep site, AMNH loc. 3545.
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Ammonites, as well as other fauna, were common in methane seeps of the Late Cretaceous Western Interior Seaway (WIS) of North America. Biogeochemical processes at the seeps, in particular the anaerobic oxidation of methane, produced a dissolved inorganic carbon reservoir with a low 13C, manifested in the carbon isotope
composition of the inorganic...
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... even at this stage of preservation, the carbonate remains fundamentally aragonite, and Coch- ran and others (2010) have discussed the mechanisms responsible. Examples of preservation of the samples studied are shown in figure 6. The PI values of every sample are listed in tables A1-A7. ...
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... at 20m using a secondary electron detector. The shell surfaces were viewed and photographed at 2000X and shell cross sections were viewed and photographed at 5000 and 15000X. The state of preservation of each sample was scored according to the Preservation Index of Cochran and others (2010). The SEM preservation index ranged from 2.5 to 4.5 ( fig. 6). We only included samples with PI 3 in our study to assure that, in principle, the shell material preserved the original isotope signature. In AMNH 66330, the state of preservation was evaluated by Fatherree and others (1998) who noted "unaltered" nacreous tablets but did not assign a PI value. Based on their illustration, the PI 4 ...
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... be expected from alteration, that is, a positive correlation, as documented for WIS ammonites by Cochran and others (2010). This reflects both the well-preserved nature of the shell material selected for this study (figs. 4 and 5) and also the rigorous sample screening by examination of shell microstruc- ture by SEM on every sample analyzed ( fig. ...
Citations
... Previous studies of Western Interior Seaway skeletal biogenic carbonates have mostly used δ 18 O measurements to reconstruct paleotemperatures (Cochran et al., 2003;He et al., 2005;Landman et al., 2018a;Rowe et al., 2020;Tourtelot and Rye, 1969;Witts et al., 2020;Wright, 1987). However, the calculation to reconstruct temperature requires an assumed δ 18 O value for the water (δ 18 O seawater ) value in which the carbonate formed. ...
... This biozone was dated to ca. 73.8 Ma by Landman et al. (2018b). Abundant methane cold seeps occurred in the Western Interior Seaway during this time interval (Gao et al., 2021;Landman et al., 2018a;Rowe et al., 2020), and samples were selected from both a known methane seep and non-seep locality for comparison. All B. compressus zone samples are from offshore localities 150-250 km east of the Western Interior Seaway shoreline based on the reconstructions of Cobban et al. (1994). ...
... The low δ 13 C value of the sparry calcite is best explained by oxidation of organic matter or formation near a methane seep where the δ 13 C value could still be impacted. None of the δ 13 C values are indicative of direct formation within methane seeps where anerobic oxidation of methane results in δ 13 C values as low as −40‰ (Gao et al., 2021;Landman et al., 2018a;Rowe et al., 2020). Late Cretaceous ammonoids frequently have δ 13 C values that are at least 4‰ lower than those of other carbonate-secreting species found in the same location regardless of proximity to known methane seeps (Landman et al., 2018a;Landman and Klofak, 2012;Tobin and Ward, 2015). ...
Fossiliferous carbonate concretions are commonly found in sediments deposited in the Late Cretaceous Western Interior Seaway. Although concretions are diagenetic features, well-preserved fossils from within them have been instrumental in reconstructing the temperature and δ18O value of Western Interior Seaway seawater, which is essential for accurate reconstruction of Late Cretaceous climate. Here, we constrain formation conditions of Late Campanian and early Maastrichtian carbonate concretions by combining triple oxygen isotope measurements with carbonate clumped isotope paleothermometry on different carbonate phases within the concretions. We measured both fossil skeletal aragonite and sparry calcite infill from cracks and within macrofossil voids to evaluate differences between “primary” and “altered” geochemical signals. Based on the two temperature-sensitive isotope systems of the primary fossil shell aragonite, the temperature of the Western Interior Seaway was between 20 °C and 40 °C and was likely thermally stratified during the Campanian. The reconstructed δ18Oseawater values of ∼−1‰ for Campanian Western Interior Seaway waters are similar to those expected for the open ocean during greenhouse climates, while the Maastrichtian Western Interior Seaway may have been more restricted, with a δ18Oseawater value of ∼2‰, which reflects more evaporative conditions. We reconstructed the diagenetic history of the sparry infill and altered fossils using a fluid-rock mixing model. Alteration temperature, alteration fluid δ18O value, and the initial formation temperature were calculated by applying the fluid-rock mixing model to a particle swarm optimization algorithm. We found a different range of initial formation temperatures between the Campanian (25−38 °C) and Maastrichtian (9−28 °C). We also found that alteration in the presence of light meteoric fluids (δ18O ≈ −10‰) is required to explain both the sparry infill and the altered fossil isotopic values. Based on our results, both lithification and alteration of the carbonates occurred soon after burial, and light meteoric fluids support prior findings that high-topographic relief existed on the western margin of the Western Interior Seaway during the Late Cretaceous. As one of the first studies to apply these techniques in concert and across multiple mineralogical phases within samples, our results provide important constraints on paleoenvironmental conditions in an enigmatic ocean system and will improve interpretations of the overall health of ecosystems leading into the end-Cretaceous mass extinction.
... The specimen studied herein (ALMNH:Paleo:6522) did not have sufficient sedimentary or concretionary matrix surrounding the specimen, so the matrix attached to two other decapods from the same locality and carbonate deposit, both benthic decapods (ALMNH:Paleo:20359, Bournelyreidus oaheensis; ALMNH:Paleo:20360a, Protocallianassa cheyennensis), were analyzed at the UC Santa Cruz Stable Isotope Laboratory (California, USA) using the Thermo Fisher Scientific Kiel IV-MAT 253 device. The methodology is described in detail in Landman et al. (2018b). figure 1). ...
... The geochemistry of cephalopod mollusks has been used to quantify paleoclimate [1,2] and investigate paleobiology [3][4][5][6][7][8][9] throughout their long fossil record [10]. Modern Nautilus is a model organism for the isotope geochemistry of extinct cephalopods, including the ammonoids and nautiloids, because it is the only remaining externally shelled cephalopod (Fig 1). ...
... Standard measurements suggest an instrumental precision of ±5% (RSD, relative standard deviation) for each element (Ca~0. 5 x 1000. Analyses of the OSIL SW standard yielded a reproducibility of ±0.04‰ (2σ SD ) during the analytical sessions including these samples. ...
Cephalopod carbonate geochemistry underpins studies ranging from Phanerozoic, global-scale change to outcrop-scale paleoecological reconstructions. Interpreting these data hinges on assumed similarity to model organisms, such as Nautilus, and generalization from other molluscan biomineralization processes. Aquarium rearing and capture of wild Nautilus suggest shell carbonate precipitates quickly (35 μm/day) in oxygen isotope equilibrium with seawater. Other components of Nautilus shell chemistry are less well-studied but have potential to serve as proxies for paleobiology and paleoceanography. To calibrate the geochemical response of cephalopod δ¹⁵Norg, δ¹³Corg, δ¹³Ccarb, δ¹⁸Ocarb, and δ44/40Cacarb to modern anthropogenic environmental change, we analyzed modern, historical, and subfossil Nautilus macromphalus from New Caledonia. Samples span initial human habitation, colonialization, and industrial pCO2 increase. This sampling strategy is advantageous because it avoids the shock response that can affect geochemical change in aquarium experiments. Given the range of living depths and more complex ecology of Nautilus, however, some anthropogenic signals, such as ocean acidification, may not have propagated to their living depths. Our data suggest some environmental changes are more easily preserved than others given variability in cephalopod average living depth. Calculation of the percent respired carbon incorporated into the shell using δ¹³Corg, δ¹³Ccarb, and Suess-effect corrected δ¹³CDIC suggests an increase in the last 130 years that may have been caused by increasing carbon dioxide concentration or decreasing oxygen concentration at the depths these individuals inhabited. This pattern is consistent with increasing atmospheric CO2 and/or eutrophication offshore of New Caledonia. We find that δ44/40Ca remains stable across the last 130 years. The subfossil shell from a cenote may exhibit early δ44/40Ca diagenesis. Questions remain about the proportion of dietary vs ambient seawater calcium incorporation into the Nautilus shell. Values of δ¹⁵N do not indicate trophic level change in the last 130 years, and the subfossil shell may show diagenetic alteration of δ¹⁵N toward lower values. Future work using historical collections of Sepia and Spirula may provide additional calibration of fossil cephalopod geochemistry.
... As a result, the DIC reservoir from which they formed reflects a mixture of methane-derived DIC and overlying water DIC. Landman et al. (2018) used the δ 13 C in well-preserved shells of seep and nonseep ammonites to estimate that the δ 13 C of the DIC in the overlying water was +2.7‰ at non-seep sites but was several permil lower, ~ −0.7‰, at seep sites. Additional sources of 13 C-enriched DIC include those linked to methanogenesis. ...
... Virtually all studies of ancient seeps use the δ 13 C of the carbonates to validate that the site was in fact a seep environment. Additional information on the seep environment may be gained through isotope analyses of the shells of mollusks and other carbonatesecreting fauna found at seeps (Lietard and Pierre 2009;Landman et al. 2012Landman et al. , 2018Cochran et al. 2015). For modern seeps, this is straightforward in that the shells are well-preserved (Lietard and Pierre 2009). ...
... Poor preservation of fossil shell material at ancient methane seeps commonly precludes meaningful O and C isotope measurements. However, Landman et al. (2012Landman et al. ( , 2018, Cochran et al. (2015), and Rowe et al. (2020) reported results on wellpreserved shell material from methane seep deposits in the Upper Cretaceous (Campanian) Pierre Shale of South Dakota, USA (see Landman et al. this volume). Preservation was rigorously assessed using scanning electron microscopy (SEM) according to the Preservation Index scale of Cochran et al. (2010), and only the best-preserved shell material was used. ...
A fundamental geochemical process operating at methane seeps is the anaerobic oxidation of methane (AOM) by which methane is oxidized and sulfate is reduced. This process takes place in the sulfate-methane transition zone (SMTZ), generally located below the sediment-water interface. Methane has a low δ13C signature, and this is transferred to the dissolved inorganic carbon (DIC) reservoir during AOM. The increase in alkalinity from AOM can cause various carbonate minerals to precipitate with low δ13C. Indeed, very low δ13C values of ancient calcium carbonates are taken as prima facie evidence that the carbonates formed in a cold hydrocarbon seep environment. Isotope systems applied to ancient hydrocarbon seeps include those of carbon, oxygen, strontium, neodymium, and sulfur. These provide information on carbon source, carbonate formation temperature, the involvement of deep-sourced fluids, and fluid pathways in transferring methane to the SMTZ. Variations of rare earth elements (REEs) provide clues to the environmental conditions under which seep carbonates formed, with implications for the precipitation depth and flow regime. Other trace elements (Fe, Mn, Sr, Mg) in seep carbonates reflect mineralogical differences, and redox-sensitive trace elements (Mo, U, Cd, Sb, As) provide constraints on fluid flux and the dynamics of redox conditions.KeywordsAnaerobic oxidation of methane (AOM)Organoclastic sulfate reductionSulfate-methane transition zoneMethanotrophyAnaerobic methanotrophic archaea (ANME)Iron cyclingSulfur cyclingCarbon isotopesOxygen isotopesStrontium isotopesNeodymium isotopesTrace elementsSulfur isotopesCarbonate-clumped isotopesRare earth elements
... A change of habitat throughout ontogeny was suggested on the basis of results of oxygen stable isotope analysis of ammonite septa and shell walls (Fatherree et al., 1998;Lukeneder et al., 2010;Zakharov et al., 2011;Lukeneder, 2015;Stevens et al., 2015). However, most studies into d 18 O, d 13 C in ammonite shells and ammonite habitat involved Late Cretaceous material (e.g., Landman et al., 2018;Linzmeier et al., 2018;Witts et al., 2020). ...
Analyses of δ¹⁸O and δ¹³C of original shells of late Valanginian (verrucosum to triptychoides ammonite zones) ammonites preserved in clay at localities in central Poland, and of some Berriasian samples from well cores have been carried out for the first time. Analyses of carbon stable isotope composition have revealed diverse values, representing vital effects rather than recording the palaeoenvironment. In turn, analyses of oxygen stable isotope composition in samples from the verrucosum and triptychoides ammonite zones clearly indicates the connection between particular species and stable isotope ratios, and thus palaeotemperature. The interpretation of ammonite palaeoecology in the present paper is based on estimated palaeotemperatures and supports other interpretations of ammonite behaviour, mainly based on functional morphology of shells. There are two main groups of ammonites associated with lower and higher temperatures. The first group of δ¹⁸ O values indicates lower palaeotemperatures. Genera analysed include Bochianites, Dichotomites and Prodichotomites; a nekto-benthic (demersal) habitat is suggested for these. The second group, characterised by higher palaeotemperatures, includes Saynoceras, Valanginites and Olcostephanus, with a suggested nektonic (or planktic) habitat (free-floating), Neohoploceras and Neocomites with a suggested nektonic habitat (free-swimming) and a seasonal migration from warmer (Tethys) to cooler waters of the Polish Basin or lived in the Polish Basin for several summer and winter seasons. Co-occurring bivalves, echinoid spines and scaphopods were analysed for comparison with results obtained from ammonites for a better overview of the palaeoenvironment. The estimated palaeotemperatures for the Polish Basin are between 12−13°C (at the sea floor) to 21−24°C (in the upper water column), while for the Tethys or summer surface waters of the Polish basin, these were in the range of 26−30°C.
... Another approach to throw light on the habitat and life history of these animals is the analysis of stable isotopes of carbon and oxygen preserved in the shell. The isotope ratios 18 O/ 16 O and 13 C/ 12 C, expressed as delta values (δ 18 O, δ 13 C) relative to a standard (PeeDee Belemnite, PDB), can provide information on the temperature of the water [5][6][7], and on the isotope composition of carbon in the dissolved inorganic carbon reservoir (δ 13 C) in which the shell formed [8,9]. ...
... Because direct observations of these animals are impossible, the use of oxygen and carbon isotopes offer important insights, provided that the shells are well enough preserved and have not suffered diagenetic alteration [10]. Combined with clues from facies distribution, faunal association, and fossil preservation, analysis of the oxygen and carbon isotope composition of the shells can shed light on their habitat and rate of the growth of these extinct organisms [11][12][13][14][15]. ...
... The lower calculated values of δ 13 C DIC at this site are consistent with the influence of the anaerobic oxidation of methane with low δ 13 C impacting the DIC reservoir in near-surface sediments. The presence of seep fluids in the immediate overlying water column at the site is supported by 87 Sr/ 86 Sr ratios in the shells of ammonites and nautilids that differ from coeval seawater values [52], as well as by patterns of δ 13 C in the shells of B. compressus and other ammonites [13,53]. Future work is needed to determine whether the δ 13 C of fossil nautilid shells can reasonably be used as a proxy for δ 13 C of paleo-DIC. ...
Modern nautilids (Nautilus and Allonautilus) have often been studied by paleontologists to better understand the anatomy and ecology of fossil relatives. Because direct observations of these animals are difficult, the analysis of light stable isotopes (C, O) preserved in their shells has been employed to reveal their habitat and life history. We aim to (1) reconstruct the habitat depth of Nautilus macromphalus and (2) decipher the fraction of metabolic carbon in its shell by analyzing oxygen and carbon isotopes (δ¹⁸O, δ¹³C) in the septa of two specimens in combination with analyses of water samples from the area. Additionally, we investigate whether morphological changes during ontogeny are reflected in the isotopic values of the shells. Results reveal that the patterns of change of δ¹⁸O and δ¹³C in the septa of N. macromphalus pre- and post-hatching are consistent with previous studies. Values of δ¹⁸Owater range from 0.7 to 1.4‰ (VSMOW), with a maximum value coincident with a salinity maximum at ~150 m. We use the temperature and δ¹⁸Owater profiles to calculate equilibrium values of δ¹⁸Oaragonite with depth. Comparing these values with the measured δ¹⁸O of the septa shows that the habitat depth of N. macromphalus is ~140 m pre-hatching and ~370 m post-hatching. Using δ¹³C of shell carbonate and published data on metabolic carbon, the fraction of metabolic carbon is reconstructed as ~21% and 14% pre- and post-hatching, respectively. The reconstructed depth pre-hatching is slightly shallower than in N. pompilius from the Philippines and Fiji, but the post-hatching depth is similar. However, it is important to emphasize that these estimates represent average over time and space because nautilus is a mobile animal. Lastly, the changes in morphological parameters and the changes in δ¹³C and δ¹⁸O during ontogeny do not coincide except at hatching and at the onset of maturity.
... Isotopic analysis of SACs revealed depleted δ 13 C values (−33.11‰ to −36.03‰; Table S1 in the Supplemental Material 1 ). These are consistent with values from other WIS seep deposits (Kauffman et al., 1996;Cochran et al., 2015;Landman et al., 2018b) and indicative of the effect of anaerobic methane oxidation on the dissolved inorganic carbon reservoir from which the carbonates precipitated. Fossils are common, especially the chemosymbiotic bivalve Nymphalucina (Figs. 2 and 3). ...
... Ammonites were more abundant at the seeps. Studies suggest WIS seeps were a preferred habitat for planktivorous ammonites, probably attracted to the abundant and persistent food source sustained by seep emissions (Landman et al., 2012(Landman et al., , 2018bCochran et al., 2015;Rowe et al., 2020). We suggest the nature and persistence of the chemosynthetic food web at our studied seep provided a refuge following a substantial ash fall, whereas surrounding non-seep communities fared worse, showing little evidence of recovery in our studied section. ...
Methane seeps host rich biotic communities, forming patchy yet highly productive ecosystems across the global ocean. Persistent hydrocarbon emissions fuel chemosynthetic food webs at seeps. Methane seeps were abundant in the Western Interior Seaway of North America during the Late Cretaceous. This area also experienced intermittent ash falls, which negatively impacted the marine fauna. We propose that methane seeps acted as refugia during these environmental perturbations. We report a laterally continuous bentonite within the upper Campanian Baculites compressus Zone of the Pierre Shale in southwestern South Dakota (USA) that fortuitously cuts across a methane seep deposit. We compare the macroinvertebrate record below and above the bentonite at seep and non-seep sites. Our results reveal that the paleocommunity (measured by abundance and diversity) was largely unaffected by the ash fall at the seep site, whereas it was significantly altered at the non-seep site. Thus, methane seeps in the Western Interior Seaway may have provided refuges or served as oases in the aftermath of severe environmental perturbations.
... Orthoconic ammonoids, in contrast, are generally found in neritic and epicontinental settings (Kennedy & Cobban, 1976;Wright, Callomon & Howarth, 1996). The depth range of baculitid ammonoids was around 50-100 m based on isotopic analyses of well-preserved shell material (Fatherree, Harries & Quinn, 1998;Lukeneder et al., 2010;Henderson & Price, 2012;Lukeneder, 2015;Sessa et al., 2015;Landman et al., 2018;Hoffmann et al., 2021). A demersal life habit is generally inferred from these analyses due to their isotopic similarity with the benthos (Landman et al., 2018;Ferguson et al., 2019;Hoffmann et al., 2021). ...
... The depth range of baculitid ammonoids was around 50-100 m based on isotopic analyses of well-preserved shell material (Fatherree, Harries & Quinn, 1998;Lukeneder et al., 2010;Henderson & Price, 2012;Lukeneder, 2015;Sessa et al., 2015;Landman et al., 2018;Hoffmann et al., 2021). A demersal life habit is generally inferred from these analyses due to their isotopic similarity with the benthos (Landman et al., 2018;Ferguson et al., 2019;Hoffmann et al., 2021). Isotopic studies also suggest that some baculitids spent most of their lives at methane seeps, supporting a somewhat sedentary lifestyle (Landman et al., 2018;Rowe et al., 2020). ...
... A demersal life habit is generally inferred from these analyses due to their isotopic similarity with the benthos (Landman et al., 2018;Ferguson et al., 2019;Hoffmann et al., 2021). Isotopic studies also suggest that some baculitids spent most of their lives at methane seeps, supporting a somewhat sedentary lifestyle (Landman et al., 2018;Rowe et al., 2020). However, baculitid associations with streamlined midwater swimmers, and occurrences in deposits lacking demersal taxa, suggest that these species could cope with life higher in the water column as well (Tsujita & Westermann, 1998;Landman, Cobban & Larson, 2012). ...
Measuring locomotion tactics available to ancient sea animals can link functional morphology with evolution and ecology over geologic timescales. Externally-shelled cephalopods are particularly important for their central roles in marine trophic exchanges, but most fossil taxa lack sufficient modern analogues for comparison. In particular, phylogenetically diverse cephalopods produced orthoconic conchs (straight shells) repeatedly through time. Persistent re-evolution of this morphotype suggests that it possesses adaptive value. Practical lateral propulsion is ruled out as an adaptive driver among orthoconic cephalopods due to the stable, vertical orientations of taxa lacking sufficient counterweights. However, this constraint grants the possibility of rapid (or at least efficient) vertical propulsion. We experiment with this form of movement using 3D-printed models of Baculites compressus , weighted to mimic hydrostatic properties inferred by virtual models. Furthermore, model buoyancy was manipulated to impart simulated thrust within four independent scenarios ( Nautilus -like cruising thrust; a similar thrust scaled by the mantle cavity of Sepia ; sustained peak Nautilus -like thrust; and passive, slightly negative buoyancy). Each model was monitored underwater with two submerged cameras as they rose/fell over ~2 m, and their kinematics were computed with 3D motion tracking. Our results demonstrate that orthocones require very low input thrust for high output in movement and velocity. With Nautilus -like peak thrust, the model reaches velocities of 1.2 m/s (2.1 body lengths per second) within one second starting from a static initial condition. While cephalopods with orthoconic conchs likely assumed a variety of life habits, these experiments illuminate some first-order constraints. Low hydrodynamic drag inferred by vertical displacement suggests that vertical migration would incur very low metabolic cost. While these cephalopods likely assumed low energy lifestyles day-to-day, they may have had a fighting chance to escape from larger, faster predators by performing quick, upward dodges. The current experiments suggest that orthocones sacrifice horizontal mobility and maneuverability in exchange for highly streamlined, vertically-stable, upwardly-motile conchs.
... Some other, quite intriguing taxa are also represented in sclerochronological research. These include belemnites, an extinct order of cephalopods (phylum Mollusca) (Mettam et al., 2014;Wierzbowski et al., 2018) and ammonites (class Cephalopoda) (e.g., Sessa et al., 2015;Landman et al., 2018;Wierzbowski et al., 2018). These two taxa present important archives of paleodata. ...
Over the past decade, sclerochronological research has continued to develop rapidly and is diversifying with respect to methods, taxa, geographic coverage as well as temporal depth. Chonologically aligned environmental records from bivalves, gastropods, coralline algae, corals, and many other periodically formed biogenic hard parts are integrated to build networks across broad spatial domains and trophic levels. Replication and exact dating ensure that environmental signals are fully preserved and facilitate the integration among chronologies as well as observational records of climatic and biological phenomena. The proliferation of chronologies promises to usher in a new era of synthesis that integrates tropical to polar environments and links with other high-resolution archives such as tree-ring chronologies to assess broad-scale couplings between the ocean and atmosphere across different latitudes. An increasing number of studies also applies sclerochronological methods to fossils from the more distant past and studies paleoclimate variability in deep time. At the same time, rapid advances are being made in developing, optimizing and validating proxies from isotopes, trace and minor elements, and ultrastructures (aka microstructures) of periodically growing skeletal hard parts to reveal new parameters of environmental variability from these exactly dated frameworks. Beyond the importance for paleoclimatology, information recorded in such archives is of increasing relevance to ecology and management to provide insights on life history, population connectivity, productivity, and disentangling the impacts of natural and anthropogenic environmental and climate change. Likewise, environmental information from archaeological samples are providing new insights into long-term interactions between climate variability and dynamics of past human societies. This introductory review paper provides insights into advances in the field of sclerochronology, with an emphasis on mollusks, including trends in the analysis of growth patterns, development and interpretation of proxies, diversity of taxa used in sclerochronological research, as well as the geographic and temporal coverage of sclerochronological research.
... Isotopic data from well-preserved shell material of fossil fauna preserved at WIS seeps also suggest that methane-rich fluids influenced the geochemistry of the water column, either directly or through incorporation into the rich food chain that likely surrounded the seeps (Landman et al. 2012a;Cochran et al. 2015;Landman et al. 2018a). The source and nature (biogenic vs. thermogenic origin) of the methane are still an area of debate; development of the seeps is interpreted to have coincided with ongoing Laramide tectonic activity, which may have tapped underlying organic-rich sediments (e.g. the Niobrara and Pierre Shale formations) and associated nutrient-rich brines (Metz 2010;Larson et al. 2014). ...
... Methane seep deposits from the WIS contain an extremely diverse assemblage of organisms, suggesting they hosted abundant and complex marine communities. These include ammonites, bivalves (especially dense aggregations of lucinids), which like their modern counterparts may have harboured chemosymbionts (Taylor & Glover 2010), gastropods, crabs, crinoids, brittle stars, echinoids, sponges, tube worms and microbial mats (Bishop & Williams 2000;Landman et al. 2012a;Larson et al. 2014;Meehan & Landman 2016;Hunter et al. 2016;Thuy et al. 2018;Blake et al. 2018;Meehan et al. 2018;Landman et al. 2018a;Laird & Belanger 2019). Species-richness is often very highmore than 30 species being reported from some seeps. ...
... Carbon isotope values from Hoploscaphites at the seep are depleted compared to those from Hoploscaphites at the non-seep site, which average −0.5‰ . Similar depleted values from ammonite shell material at other WIS seep deposits (Landman et al. 2012a(Landman et al. , 2018aRowe et al. 2020) are interpreted as primary, reflecting a methane-derived 12 C-enriched signal in the DIC reservoir above the seep. However, the values as low as −50‰ suggest that contamination with small pieces of the extremely isotopically depleted carbonate matrix must have occurred during sampling of this specimen (for an extended discussion about the risks of contamination, see Rowe et al. (2020)). ...
Methane seeps were a common feature in the Late Cretaceous Western Interior Seaway of the United States. We document the occurrence of methane seep deposits in the Pierre Shale on the Cedar Creek Anticline in east‐central Montana for the first time. The seep deposits occur in the lowermost part of the Baculites baculus Zone (the Endocostea typica Zone), corresponding to the lowermost Maastrichtian. They are therefore the youngest seeps yet described from the Western Interior Seaway. We conducted a detailed faunal analysis of a single seep deposit, together with geochemical investigation of both seep carbonates and molluscan shell material to determine palaeoenvironmental conditions. Oxygen isotope analysis of well‐preserved molluscan shell material reveals water temperatures of between 19 and 27°C, while depleted carbon isotope values of seep carbonates are indicative of the anaerobic oxidation of methane. The morphology of the seep deposit suggests a strong advective flux of methane to the sediment–water interface. Comparison to a nearby contemporaneous non‐seep site reveals that similar groups of organisms occur in both settings, albeit with varying relative abundances – the seep is numerically dominated by the lucinid bivalve Nymphalucina occidentalis. Substrate appears to be the major control on the diversity and palaeoecological composition of both seep and non‐seep sites.