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Evolution of Late Cretaceous Radiolaria - in relation with the Caribbean Large Igneous Province and carbon isotope shifts

Published regional Late Cretaceous radiolarian biozonations provide rather conflicting ranges of individual radiolarian species, making precise age calls difficult. Principal reasons may be: 1. radiolarian preservation becomes very spotty in post- Cenomanian sediments and is restricted to Oceanic Anoxic Events (AOE) and deeper, or upwelling settings espcaping the dominant pelagic marl/carbonate sedimentation. Hence, radiolarians rarely co-occur with calcareous microfossils that allow for biochronologic calibration. 2. The "semantics" of Late Cretaceous radiolarian species has gradually widened, resulting in long ranges of loosely defined groups. 3. Paleogeographic and paleoenvironmental factors cause truncated ranges of many species, especially of low latitude forms in high latitude assemblages. In cotrast, a major radiolarian faunal turnover was proposed across the Cenomanian/Turonian Boundary Event (CTBE, OAE2, Bonarelli-level), marked by a major positive carbon-isorope shift (+2-3‰ ∂ 13C, Jarvis et al. 2006 ; Fig.1). About 40% of radiolarian species disappear, and about 35% new species appear (Musavu- Moussavou et al. 2007). At present, the only compilation on post-CTBE radiolarian faunal turnover is that of O'Dogherty et al. (2009). It shows an important generic radiolarian turnover also at other Late Cretaceous boundaries. Their informal time subdivisions (53-65) can be matched well with carbon-isotopic events (Fig. 1). The CTBE has been usually correlated with the formation of the Caribbean Large Igneous Province (CLIP). According to Kerr (1998) the CTBE, then dated around 90 Ma, was caused by the formation of oceanic plateaus, including the CLIP. A more recent compilation of Ar/Ar ages of the CLIP (Fig.1) shows indeed, that most ages of the CLIP cluster around 90 Ma, while the recent ICS chart pins the C/T-boundary at 93.9 Ma. While the CLIP only starts around 94 Ma, the Agulhas Plateau, NE-Georgia Rise, Maud Rise, Hess Rise and the Central Kerguelen Plateau formed between 100-94 Ma, just before the CTBE. Together, these plateaus produced about 9 times more magma volume than the CLIP and are, hence, better candidates for causing the CTBE. Radiolarian assemblages interbedded with, and overlying basalt flows of the main CLIP phase, can be dated as Coniacian to/or Santonian, based on the current, status of Late Cretaceous radiolarian biochronology (Baumgartner et al. 2008) This age corresponds well with the youngest Ar/Ar ages of the main CLIP phase. We work on a systematic revision of radiolarian genera and species, using many DSDP, ODP and land samples from the Tethyan and Pacific realms, with the objective to elaborate a new global, low-latitude radiolarian Late Cretaceous biochronology. According to our data, Coniacian-Santonian samples have only a minority of species in common with the assemblages from the lower Turonian Scaglia Bianca Limestone of Umbria-Marche. Radiolarian faunal turnover may be very important during the Late Turonian/Santonian, marked by a minor positive carbon isotope shift (+1 ‰ ∂ 13C, Fig.1), but continuous radiolarian-bearing sections are difficult to find for this interval. Radiolarian faunal turnover may be even more important during the Campanian-Maastrichtian time interval, as shown in Fig. 1. Oceanic anoxic events, if related to super greenhouse climate, should favour radiolarian productivity through intensified continental weathering and nutrient input to the ocean. Anoxia of bottom waters clearly enhances radiolarian preservation. Good examples are radiolarian-rich cherts marking OAEs in otherwise calcareous pelagic series. Hence, peri-tropical radiolariates of the Coniacian-Santonian may be related to the isotopically poorly defined "OAE3", whereas the Campanian to middle Eocene radiolarite occurrences of S-Central- and N-South-America could be related with upwelling along the peri-tropical W-coasts of the Americas. Besides the chert-rich OAEs, well-preserved and abundant radiolarians are restricted to settings, such as: 1. areas of coastal or equatorial upwelling (e.g. fore-arc areas around the Pacific), 2. hemipelagic to "flysch" basins in the Tethyan realm, 3. in the open ocean, below a deepening, global CCD. Radiolarian preservation seems to be enhanced in glass-rich, tuffaceous, volcanogenic sediments.

Late Cretaceous, Radiolaria, Caribbean Large Igneous Province, carbon isotopes

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