Matthew P. Garb

Methane seeps as ammonite habitats in the U.S. Western Interior Seaway revealed by isotopic analyses of well-preserved shell material

Methane seep deposits are common in the Upper Cretaceous Pierre Shale of the U.S. Western Interior. They contain a rich fauna including ammonites, bivalves, gastropods, sponges, corals, echinoids, crinoids, and fish. In an effort to understand the role of ammonites in these ecosystems, we examined a seep from the upper Campanian Didymoceras cheyennense Zone in Custer County, South Dakota, that contains molluscs with well-preserved shell material permitting isotopic analyses. Values of δ13C of the micritic limestone at the seep range from −46.94‰ to −11.49‰, confirming the influence of anaerobic oxidation of methane on the isotopic composition of the dissolved inorganic carbon reservoir. The ammonites also consistently display light values of δ13C ranging from −13.71‰ to 0.68‰. These values are generally lighter than those in nonseep specimens from age-equivalent rocks elsewhere in the basin (–1.75‰ to 3.42‰). In a single specimen of Baculites corrugatus from the seep, light δ13C values occur throughout ontogeny. These data suggest that ammonites incorporated isotopically light methane-derived carbon in their shells and lived near the vent fluids and methane-oxidizing bacteria. Both juvenile and adult specimens are present, implying that these ammonites spent their entire lives at the seep and formed an integral part of an interwoven community. The values of 87Sr/86Sr in the limestone and well-preserved fossils at the seep (0.707690–0.707728) are higher than that of the open ocean at this time (0.707659). These elevated values suggest that the seep fluids were imprinted with a radiogenic Sr signature, perhaps derived from equilibration with granitic deposits at depth during the initial uplift of the Black Hills.

Short-term survival of ammonites in New Jersey after the end-Cretaceous bolide impact

A section containing the Cretaceous/Paleogene (= Cretaceous/Tertiary) boundary in Monmouth County, New Jersey, preserves a record of ammonites extending from the end of the Cretaceous into possibly the beginning of the Danian. The section includes the upper part of the Tinton Formation and lower part of the Hornerstown Formation. The top of the Tinton Formation is represented by a richly fossiliferous unit (the Pinna Layer) that contains many bivalves in life position as well as ammonite jaws preserved inside body chambers. Ammonites include Pachydiscus (Neodesmoceras) mokotibensis, Sphenodiscus lobatus, Eubaculites carinatus, E. latecarinatus; Discoscaphites iris, D. sphaeroidalis; D. minardi, and D. jerseyensis. The Pinna Layer probably represents a relatively short interval of time lasting tens to hundreds of years; it is conformably overlain by the Burrowed Unit, which contains a single fragment of Discoscaphites sp. and several fragments of E. latecarinatus, as well as several isolated specimens of ammonite jaws including two of Eubaculites. Examination of the mode of preservation of the ammonites and jaws suggests that they were fossilized during deposition of the Burrowed Unit and were not reworked from older deposits. Based on the ammonites and dinoflagellates in the Pinna Layer and the Burrowed Unit, these strata traditionally would be assigned to the uppermost Maastrichtian, corresponding to calcareous nannofossil Subzone CC26b. However, a weak iridium anomaly (500–600 pg/g) is present at the base of the Pinna Layer, which presumably represents the record of the bolide impact. Correlation with the iridium layer at the Global Stratotype Section and Point at El Kef, Tunisia, would, therefore, imply that these assemblages are actually Danian, provided that the iridium anomaly is in place and the ammonites and dinoflagellates are not reworked. If the iridium anomaly is in place, or even if it has migrated downward from the top of the Pinna Layer, the ammonites would have survived the impact at this site for a brief interval of time lasting from a few days to hundreds of years.

Ammonite habitat revealed via isotopic composition and comparisons with co-occurring benthic and planktonic organisms

Significance Because ammonites are one of the most diverse, abundant, and well-preserved clades in the history of life, they are a mainstay in macroevolutionary and biodiversity studies; however, their ecologies are poorly understood, and it is unknown whether taxa lived near the sea surface or seafloor. This uncertainty undermines their use in paleoecological and paleoenvironmental reconstructions, which depend on knowledge of organisms’ depth preferences. Here, we use a rare co-occurrence of exquisitely well-preserved ammonites and planktonic and benthic organisms to constrain depth preferences of three common ammonite families by comparing the oxygen and carbon isotopic signatures of these taxa. The ammonites fall into two distinct depth habitats, enhancing the utility of these families for highly refined paleoecological and paleoclimatic studies. Ammonites are among the best-known fossils of the Phanerozoic, yet their habitat is poorly understood. Three common ammonite families (Baculitidae, Scaphitidae, and Sphenodiscidae) co-occur with well-preserved planktonic and benthic organisms at the type locality of the upper Maastrichtian Owl Creek Formation, offering an excellent opportunity to constrain their depth habitats through isotopic comparisons among taxa. Based on sedimentary evidence and the micro- and macrofauna at this site, we infer that the 9-m-thick sequence was deposited at a paleodepth of 70–150 m. Taxa present throughout the sequence include a diverse assemblage of ammonites, bivalves, and gastropods, abundant benthic foraminifera, and rare planktonic foraminifera. No stratigraphic trends are observed in the isotopic data of any taxon, and thus all of the data from each taxon are considered as replicates. Oxygen isotope-based temperature estimates from the baculites and scaphites overlap with those of the benthos and are distinct from those of the plankton. In contrast, sphenodiscid temperature estimates span a range that includes estimates of the planktonic foraminifera and of the warmer half of the benthic values. These results suggest baculites and scaphites lived close to the seafloor, whereas sphenodiscids sometimes inhabited the upper water column and/or lived closer to shore. In fact, the rarity and poorer preservation of the sphenodiscids relative to the baculites and scaphites suggests that the sphenodiscid shells may have only reached the Owl Creek locality by drifting seaward after death.

Isotope sclerochronology of ammonites (Baculites Compressus) from methane seep and non-seep sites in the Late Cretaceous Western Interior Seaway, USA: Implications for ammonite habitat and mode of life

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 δ13C, manifested in the carbon isotope composition of the inorganic calcium carbonate concretions associated with the seeps and recorded in well-preserved shells of ammonites documented at the sites. Detailed sclerochronological sampling of six well-preserved specimens of Baculites compressus collected at seep sites in the Pierre Shale of South Dakota reveals three patterns that can be explained by reference to two specimens of the same species collected at age-equivalent non-seep sites. Three of the specimens exhibit uniformly low values of δ13C that are significantly different (unpaired t-test, p < .0001) from similarly sized specimens of the same species collected at age-equivalent non-seep sites, suggesting that these ammonites lived at the seeps during the time interval over which the shell was secreted (adult portion of the shell). Two of the specimens collected from a seep site exhibit values of δ13C consistent with early ontogeny at a non-seep site followed by later ontogeny at a seep site. The values of δ18O of all the specimens reveal water temperatures of 16 to 28 °C. One small juvenile (15 mm long) collected at a seep site exhibits higher values of δ13C consistent with a non-seep environment, but values of δ18O that indicate very warm or slightly brackish water, suggesting that this animal lived in surface waters during its early ontogeny and died soon after arriving at the seep. Our results demonstrate that seep fluids affected the geochemistry of the water column above the seeps and that seeps provided habitats for ammonites in the WIS. Thus, although ammonites were mobile animals, they probably exploited a low-energy life style, remaining at the same site for extended periods of time.