OAE2 Research Papers - Academia.edu (original) (raw)

From thinly laminated marlstones of the Hesseltal
Formation, representing the Late Cenomanian Oceanic
Anoxic Event (OAE) 2, at Lengerich in the Teutoburger
Wald (Westfalen, north-west Germany), 17 sediment-compacted
baculitid ammonites with carbonised and partially
phosphatised soft parts are recorded. Some preserve remains
of the buccal mass, including jaws (occasionally articulated)
and radulae, as well as of the cephalic cartilage, such as eye
capsules. Such have not yet been recorded previously for the
order Ammonoidea. In addition, originally unmineralised
parts found preserved in these specimens include extensive
portions of the digestive tract, the siphonal tube, false colour
patterns (megastriae), as well as traces of what would appear
to be the oviduct. At the same levels, patches with numerous
isolated horny upper and rarer lower jaws as well as radulae
occur; these may represent regurgitates or faeces of larger
predators. The cephalopod remains described were deposited
in an epicontinental setting, possibly at palaeodepths between
200 and 600 m. In this particular Late Cretaceous fossil
Lagersta¨tte, upper jaws and anaptychi of ammonites rank
among the commonest fossils.

- by
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- Cretaceous, Ammonoidea, OAE2, Exceptional Preservation

Open marine sediments deposited during the CenomanianeTuronian transition are well exposed in the Spanish Ba~ nos de la Hedionda section (Betic Cordillera, South Iberian Palaeomargin). Analysis of fora-miniferal assemblages and geochemical proxies allow inferences on the impact of the Oceanic Anoxic Event 2 (OAE2) in this area of the western Tethys. Three main intervals have been identified corresponding to different lithological units and biozones. (1) The top of the Capas Blancas Member (Rotali-pora cushmani Biozone) represents the pre-extinction phase with diverse foraminiferal assemblages and well developed water-column tiering, well-oxygenated, oligotrophic deep-waters and oxygenated to poorly oxygenated, mesotrophic surface-waters. Foraminiferal opportunist species point to a minor event with dysoxic conditions preceding the OAE2. (2) The black radiolaritic shales (Whiteinella archaeocretacea Biozone) consist of a foraminiferal-barren interval, except for the lowermost centimetres where planktic surface-dweller opportunists are common. Redox sensitive elements (Cr/Al, V/Al, U/Th, Mo EF , Mo aut , U EF and U aut) and increased TOC values reflect oxygen depleted conditions related to the OAE2. The increase in P/Ti values at the base of this stratigraphic interval indicates an abrupt increase in productivity. High concentrations of radiolarians are congruent with high surface productivity probably related to changes in oceanic circulation and enhanced upwelling currents, as well as subsequent shallowing of the oxygen-minimum zone. The increase in Mo EF and Mo aut towards the top of the black radiolaritic shales indicates temporal euxinic conditions. (3) A slow, bottom-up recovery of foraminiferal assemblages is inferred at the base of the Boquer on Member (Helvetoglobotruncana helvetica Biozone), with seafloor recolonization by benthic foraminifera being recorded previous to the water column colonization by planktic forms, mainly by intermediate-dwellers typical of mesotrophic waters. The subsequent proliferation of surface-dweller opportunists and deep-dweller opportunists adapted to mesotrophic to eutrophic conditions, and the decrease in planktic foraminiferal diversity, may indicate the persistence of poorly oxygenated conditions in the water column towards the lower-middle part of the H. helvetica Biozone.

- by Eustoquio Molina
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- Planktonic Foraminifera, Benthic foraminifera, Oceanic Anoxic Events, Cretaceous

Herein we report morphologically and histologically diagnostic trionychid specimens from the Barremian– Aptian (129.4–113.0 Ma) of Japan. One specimen (FPDM-V9487) is an associated skeleton that consists of a scapula, a humerus, an ischium, and hypoplastra. The limb and girdle bones are similar in morphology to those of modern trionychids. The hypoplastron also resembles that of modern trionychids; however, it is distinctive in that it completely lacks callosities. The other three new specimens are fragmentary costals that show a reduction in the shell bones and an absence of scute sulci. Two of the newly reported costals and FPDM-V0127 were examined histologically. Bone fiber bundles organized in a plywood-like structure, which is unique to trionychids, were identified. Fossil occurrence data indicate that morphologically and histologically typical trionychids already inhabited the coastal region of Asia (e.g., western Japan) as early as the Aptian. In contrast, the Hauterivian–Aptian stem trionychid Kappachelys okurai from Japan does not show a plywood-like shell microstructure, suggesting that K. okurai could be the earliest-branching taxon of known stem trionychids or a carettochelyid. Our paleobiogeographic compilation suggests that the spread of wetlands in the Northern Hemisphere and the high global temperature during the middle Cretaceous (Aptian–Turonian, 125.0–89.8 Ma) might have contributed to the dispersal of trionychids from Asia, allowing for the establishment of this modern freshwater reptilian fauna.

- by Yasuhisa Nakajima and +1
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- Paleontology, Vertebrate Paleontology, Cretaceous life, Dinosaur Paleontology

Although ocean circulation plays a vital role in the climate system, its response to major carbon-cycle perturbations during the mid-Cretaceous, including mid-Cenomanian event I (MCE I) and the Cenomanian-Turonian oceanic anoxic event (OAE 2), is poorly constrained. Here we present Nd isotope evidence for episodic increases in the influence of boreal seawater in the European epicontinental sea during MCE I. The start of this circulation reorganization lagged the onset of the δ13C positive excursion defining MCE I. This sequence of change is similar to that observed during OAE 2 in the same area, showing a consistent response of regional circulation to changes in the global carbon cycle. Brief intervals of invasion of boreal fauna to mid-latitude seas, two during MCE I and one during OAE 2 (Plenus cold event), all started after the influence of boreal seawater was enhanced, implying a slower biological response to climate cooling rather than passive transport of fauna by boreal waters. The lack of an Nd isotope positive excursion in our record across MCE I supports a volcanic origin for prominent increases in seawater Nd isotope values found in the European epicontinental sea and the tropical Atlantic during OAE 2. The observed tight circulation-carbon cycle coupling may help the upper ocean replenish nutrients from deep waters and/or volcanic sources, providing a critical feedback allowing continuation of MCE I and OAE 2 over long durations.

- by HANNI Idriss
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- Evolutionary Biology, Paleontology, Evolution, Algeria