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- Backman, Jan, et al.
(författare)
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Species-specific responses of late Miocene Discoaster spp. to enhanced biosilica productivity conditions in the equatorial Pacific and the Mediterranean
- 2013
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Ingår i: Geo-Marine Letters. - : Springer Science and Business Media LLC. - 0276-0460 .- 1432-1157. ; 33:4, s. 285-298
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Tidskriftsartikel (refereegranskat)abstract
- Census data of a major Cenozoic calcareous nannofossil genus (Discoaster) have been acquired from Site U1338, located near the Equator in the eastern Pacific Ocean and drilled in 2009 during Integrated Ocean Drilling Program (IODP) Expedition 321. The investigated 147.53 m thick upper Miocene sediment sequence is primarily composed of biogenic carbonate and biogenic silica. Diatom biostratigraphic data were used to develop a revised biomagnetostratigraphic age model, resulting in more variable late Miocene sedimentation rates. Carbonate content variations mainly reflect dilution by biogenic silica production, although intense carbonate dissolution affects a few shorter intervals. Abundance variations of discoasters show no distinct correlation with either carbonate or biosilica contents. The two dominant Discoaster taxa are D. brouweri and D. variabilis, except for a 12 m thick interval where D. bellus outnumbers the sum of all other discoasters by a factor of 4.6. Data presented indicate that first D. hamatus and then D. berggrenii both evolved from D. bellus. Three unusual morphotypes, here referred to as Discoaster A, B and C, increase in relative abundance during episodes of enhanced biosilica production in the upper half of the investigated sequence (Messinian). Strikingly similar morphotypes have been observed previously in Messinian age sediments from the Mediterranean, characterized by alternating deposition of biogenic carbonate and biosilica. This suggests a species-specific response among some of the late Miocene discoasters to broader oceanographic and climatic forcing that promoted episodes of enhanced deposition of biogenic silica.
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| 2. |
- Ciummelli, Marina, et al.
(författare)
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Biostratigraphy and evolution of Miocene Discoaster spp. from IODP Site U1338 in the equatorial Pacific Ocean
- 2017
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Ingår i: Journal of Micropalaeontology. - : Copernicus GmbH. - 0262-821X .- 2041-4978. ; 36, s. 137-152
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Tidskriftsartikel (refereegranskat)abstract
- Assemblages of upper lower through upper Miocene Discoaster spp. have been quantified from Integrated Ocean Drilling Program (IODP) Site U1338 in the eastern equatorial Pacific Ocean. These assemblages can be grouped into five broad morphological categories: six-rayed with bifurcated ray tips, six-rayed with large central areas, six-rayed with pointed ray tips, five-rayed with bifurcated ray tips and five-rayed with pointed ray tips. Discoaster deflandrei dominates the assemblages prior to 15.8 Ma. The decline in abundance of D. deflandrei close to the early-middle Miocene boundary occurs together with the evolution of the D. variabilis group, including D. signus and D. exilis. Six-rayed discoasters having large central areas become a prominent member of the assemblages for a 400 ka interval in the late middle Miocene. Five-and six-rayed forms having pointed tips become prominent in the early late Miocene and show a strong antiphasing relationship with the D. variabilis group. Discoaster bellus completely dominates the Discoaster assemblages for a 400 ka interval in the middle late Miocene. Abundances of all discoasters, or discoasters at the species level, show only (surprisingly) weak correlations to carbonate contents or oxygen and carbon isotopes of bulk sediment when calculated over the entire sample interval.
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