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Arctic dinoflagellate migrations mark the strongest Oligocene glaciations
Stefaan Van Simaeys*
Historical Geology, University of Leuven, Redingenstraat 16, B-3000 Leuven, Belgium
Department of Palaeoecology, Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4,
3584 CD Utrecht, Netherlands
Jo ̈rg Pross
Institute of Geology and Paleontology, J.W. Goethe University Frankfurt, Senckenberganlage 32-34, D-60054 Frankfurt, Germany Graham L. Williams
Geological Survey of Canada (Atlantic), Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth NS B2Y 4A2, Canada James C. Zachos
University of California–Santa Cruz, Earth Science Department, Santa Cruz, California 95064, USA
Here we report on mid-Oligocene globally synchronous Arctic dinoflagellate migra-
tion events, calibrated against chron C9n. We show that sudden appearances and marked abundance increases of the Arctic taxon Svalbardella at lower and middle latitudes coin- cide with the Oi-2b benthic 18O glacial episode, dated as ca. 27.1 Ma. These unprece- dented migrations are taken to indicate anomalously strong surface-water cooling during Oi-2b time, in turn associated with strong concomitant Antarctic ice-sheet growth and sea-level lowering. We estimate the duration of these unique Svalbardella migrations and the associated episode of profound cooling as 500 k.y. Our records suggest a close link between this distinct Oligocene glaciation episode, strong sea-level fall, and the classic lower-upper Oligocene, or Rupelian-Chattian, boundary, dating this boundary as ca. 27.1 Ma.
Keywords: Oligocene, dinoflagellate cysts, migration, global atmospheric cooling, sea-level change, Rupelian-Chattian boundary.
Although the nature of and mechanisms underlying the transition from the Eocene greenhouse world to the icehouse conditions of the early Oligocene have received wide attention (e.g., Prothero et al., 2003, and references therein), detailed climatic and oceanographic studies of the remainder of the Oligocene are relatively scarce. It has become generally known that by the onset of the Oligocene, progres- sive global cooling resulted in the development of a significant Ant- arctic cryosphere (e.g., Zachos et al., 2001; DeConto and Pollard, 2003). Furthermore, several global increases in deep-sea benthic fora- miniferal 18O records were identified throughout the Oligocene and Miocene, with the most characteristic 18O maxima labeled as Oi and Mi events, glaciations, or zones (e.g., Miller et al., 1991, 1998; Zachos et al., 1993, 1994, 1996, 2001). The earliest Oligocene is characterized by a major 18O maximum calibrated against chron C13n ca. 33.5 Ma, generally referred to as the Oi-1 glaciation (e.g., Miller et al., 1991). Miller and coworkers identified other 18O maxima in the Oligocene, labeled as Oi-1b, Oi-2, Oi-2a, Oi-2b, and Oi-2c, as well as a still- unnamed event or zone (e.g., Miller et al., 1991; Pekar et al., 2002). These early records indicate that Oligocene climates were highly var- iable. The first high-resolution Oligocene planktic and benthic 18O and 13C records from equatorial Pacific successions have been pub- lished (Ocean Drilling Program [ODP] Leg 199; Wade and Pa ̈like, 2004). Throughout the interval spanning magnetochrons C9n to C11n.2n, three distinct episodes during which isotope values display above-average values ( 2.90‰) have been correlated to Oi-2b, Oi-2a,
and Oi-2*, respectively. Furthermore, Wade and Pa ̈like (2004) pro- posed that glaciations during the Oligocene were primarily driven by eccentricity cycles, but with enhanced probability of glaciations during intervals of low-obliquity amplitude variations.
In an effort to better understand the dynamics of these mid- Oligocene glaciations, we analyzed several chronostratigraphically well calibrated, continuous mid-Oligocene sections from central Italy, off- shore Tasmania (ODP Site 1168), and the North Sea Basin containing remains of typical surface-dwelling organisms, dinoflagellates (Fig. 1). Dinoflagellates are single-celled, predominantly marine phytoplankton that typically occur as motile cells in surface waters (Fensome et al., 1996). As part of their life cycle, many dinoflagellates produce pre-
Figure 1. Site locations of sections included in this study, plotted on mid-Oligocene paleogeographic reconstruction. (1) North Sea Ba- sin. (2) Tethyan sections. (3) Ocean Drilling Program Site 1168.
2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or email@example.com.
Geology; September 2005; v. 33; no. 9; p. 709–712; doi: 10.1130/G21634.1; 2 figures. 709
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