| 1. |
- Kiel, Steffen, et al.
(författare)
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A thyasirid-dominated methane-seep deposit from Montañita, southwestern Ecuador, from the Oligocene-Miocene boundary
- 2021
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Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - Amsterdam : Elsevier. - 0031-0182 .- 1872-616X. ; 575
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Tidskriftsartikel (refereegranskat)abstract
- A mass occurrence of the thyasirid bivalve Thyasira montanita in a limestone bed, exposed at Punta Montañita on the northern side of the Santa Elena peninsula in southeastern Ecuador, is here identified as an ancient methane-seep deposit. The massive to nodular limestone shows carbonate phases and microfabrics typical of seep limestones, such as banded and botryoidal cements. Its δ13 Ccarbonate values, as low as −52.6‰, suggest a biogenic methane source. Furthermore, the carbonate microfabrics and biomarkers typical of ANME-2 archaea indicate high seepage rates. Tubular fossils previously interpreted as worm tubes show cone-shaped projections and an internal structure consistent with that of crustacean cuticle and are here interpreted as crustacean fragments. Seep deposits with mass occurrences of thyasirid bivalves have so far only been reported from the Cretaceous, making the Montañita seep deposit the first such example from the Cenozoic. The faunal and biomarker inventories of this deposit indicate deposition on an open shelf; the oysters on the upper surface of the limestone are likely secondary encrusters that arrived only after the emergence of this deposit in shallower water.
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| 2. |
- Kiel, S., et al.
(författare)
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Brachiopods in early Mesozoic cryptic habitats: Continuous colonization, rapid adaptation, and wide geographic distribution
- 2021
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Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - Amsterdam : Elsevier. - 0031-0182 .- 1872-616X. ; 583, s. 110668-110668
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Tidskriftsartikel (refereegranskat)abstract
- Late Triassic and early Jurassic dikes and fissures in the Dachstein Limestone in the Northern Calcareous Alps harbor mass occurrences of the rhynchonellide brachiopods Sulcirostra juvavica and Halorella amphitoma. To test recent hypotheses about their paleoecology, we characterized these habitats using petrography, carbon stable isotopes, and trace element patterns, and found no evidence for hydrocarbon seepage or hydrothermal venting. Thus the brachiopods lived under normal-marine conditions, in darkness and absence of local photosynthetic primary production, hence relying on the supply of limited and presumably small-sized food washed into the dikes and fissures. Because Halorella and Sulcirostra occur in dikes and fissures since the beginning of their stratigraphic ranges, these rhynchonellides are not relics of formerly widely distributed taxa, but instead are two genera that rapidly adapted to these habitats. Both Halorella and Sulcirostra occur also in late Triassic and early Jurassic deep-water settings such as deep-marine sills and hydrocarbon seeps, indicating that close phylogenetic relationships between submarine cave faunas and deep-sea faunas, as seen today, existed also in the early Mesozoic, albeit among very different taxa. Another analogy to the modern cave fauna is the wide but disjunct geographic distribution of Sulcirostra and Halorella, both found throughout the Tethys and Panthalassa oceans. Our findings support the view that submarine cave habitats were continuously colonized by new taxa throughout Earth’s history.
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| 3. |
- Zwicker, Jennifer, et al.
(författare)
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Mineral authigenesis within chemosynthetic microbial mats: Coated grain formation and phosphogenesis at a Cretaceous hydrocarbon seep, New Zealand
- 2021
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Ingår i: The Depositional Record. - : John Wiley & Sons. - 2055-4877. ; 7:2, s. 294-310
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Tidskriftsartikel (refereegranskat)abstract
- Marine hydrocarbon seeps are sites of chemosynthetic microbial activity and authigenic carbonate formation. Seep limestones are typified by a range of geochemical signatures of microbial hydrocarbon oxidation, but only few seep deposits reveal mesofabrics that can be regarded as evidence of microbial activity. A Cretaceous methane-seep limestone from Waipiro Bay, New Zealand, exhibits a fabric composed of cryptocrystalline carbonate fluorapatite between carbonate coated grains and spheroidal calcite. To understand the unusual Waipiro deposit, a paragenetic sequence has been derived for coated grains, spheroidal calcite and carbonate fluorapatite using thin section petrography, scanning electron microscopy, Fourier-transform infrared spectroscopy and stable isotope geochemistry. The formation of 13C-depleted coated grains (δ13C values as low as -15.8‰ Vienna-Pee Dee Belemnite) and spheroidal calcite (δ13C values as low as -21.3‰) was favoured by hydrocarbon oxidation. Fibrous banded and botryoidal cement, a typical early diagenetic phase of hydrocarbon-seep deposits, features δ13C values as low as -22.9‰. Coated grains and spheroidal calcite grew by displacive growth in a gel-like medium, probably a microbial mat. Phosphorus is a mobile element and marine pore waters are typically undersaturated with respect to carbonate fluorapatite. Specific conditions are consequently required to retain sufficient concentrations to precipitate carbonate fluorapatite. Possible sources of phosphorus for the formation of the 13C-depleted Waipiro carbonate fluorapatite (mean δ13C value of -15.4‰) include (a) the oxidation of sedimentary organic matter by organoclastic sulphate reduction, (b) the degradation of the microbial mat itself and (c) the active release of polyphosphate by sulphide-oxidizing bacteria. This study suggests that the formation of the Cretaceous Waipiro seep limestone involved an interplay of biogeochemical processes including sulphate-driven anaerobic oxidation of methane, organoclastic sulphate reduction and possibly nitrate-dependent sulphide oxidation. It further demonstrates that coated grains resembling oncoids are not restricted to shallow water, photosynthesis dependent ecosystems.
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