Tethyan radiolarians at the Cenomanian-Turonian Anoxic Event from the Apennines (Umbria-Marche) and the Outer Carpathians: palaeoecological and palaeoenvironmental implications
Marta Bąk
Studia Geologica Polonica, 134: 5-279.  PDF - Full-text Article

Abstract
The radiolarian fauna from deep-water settings in the Umbria- Marche and the Outer Carpathian basins of the Western Tethys is used for biostratigraphic purpose and interpretation of environmental changes during the late Cenomanian through the Early Turonian time interval. The frequency of 184 recognized species, thirteen of them newly described, has been processed and analyzed.
Three radiolarian biozones and one subzone are distinguished based on radiolarian datum events, correlated with the chronostratigraphic scale through the δ¹³C curve.
The whole radiolarian set has been subdivided into six super groups, including 25 groups related to specific water masses. The assembled species represented various feeding preferences and ecological strategies. An increase of radiolarian total number in the sediments related to the Bonarelli Level (BL) displays a positive correlation with an increase of phosphorus (P) content, and with a significant decrease in radiolarian diversity. Most of radiolarian species avoided levels with high P content, in contrast, some species as H. barbui and C. conara increased significantly in number of specimens. Diversified radiolarian assemblages appeared at levels, directly preceded by a notable P increase, marking a period when the water system was saturated in relation to nitrogen.
The radiolarian abundance in the sediments was strongly related to their preservation during sinking in the water column and at the water/sediment interface, increasing significantly at levels, marked by high pellet production. Thus, pelletization played an important role in the transport of radiolarian skeletons and their further preservation, irrespective to conditions of radiolarians growth.
Radiolarians experienced and responded to environmental changes during the 1.8 Ma around the Bonarelli Interval (BI). The Cenomanian-Turonian press extinction event – a period of unquestionable eutrophication represented by the BI, did not result in great radiolarian extinction and turnovers. The radiolarian radiation preceded the BI by over 330 ka. The extinction, directly connected with the OAE2 started ca. 240 ka before the end of the organic-rich sedimentation, coinciding with the onset of enhanced diatom frustules deposition, recorded in the siliceous part of the BI. Since this period, a step-wise radiolarian extinction continued through the Early Turonian. Many of the radiolarian species previously considered as terminating during the BI, in fact outlived up to “post-Bonarelli” times, having their last occurrence after the BI or even in the Early Turonian. In the case of the radiolarian fauna, the Bonarelli period caused the disappearance of many Lazarus taxa, which returned in almost their initial state during 940 ka after the BI.
The current radiolarian study sheds light on the biological effects of anoxic events, indicating the role of the mesopelagic zone and oceanic circulation in nutrient exchange, which modulates and controls the OAE2. The mesopelagic zone played an important role in P sequestration and was responsible for the release of the most of P, leading to enhanced eutrophication of water column.

Key words: Radiolarian ecology; stratigraphy; Cenomanian–Turonian press extinction; OAE2; eutrophication; Umbria-Marche; Outer Carpathians