C.A. Nigrini

Code numbers for Cenozoic low latitude radiolarian biostratigraphic zones and GPTS conversion tables

Abstract Code numbers for the radiolarian zonation for the tropical Pacific, Indian and Atlantic oceans (RP1–RP22 for the Paleogene and RN1–RN17 for the Neogene) are standardized and introduced. Thirty-nine zones are recognized from the Lower Paleocene to the Holocene. Revision of previously separate upper Neogene zonations for the Pacific and Indian oceans, in which the same zonal names were applied to different stratigraphic intervals, resulted in a single tropical zonation. We change four upper Neogene Indian Ocean zones to subzones and refer to them simply by their code numbers (RN12b = Pterocanium prismatium, RN12a = Anthocyrtidium jenghisi, RN11b = Stichocorys peregrina, RN11a = Phormostichoartus fistula). We introduce the name Lychnodictyum audax Interval Zone (RN11) for the interval stratigraphically equivalent to the combined Stichocorys peregrina and Phormostichoartus fistula zones in the Indian Ocean, and to the Anthocyrtidium jenghisi Zone in the Pacific Ocean. Two Paleocene zones, Bekoma bidartensis (RP7) and B. campechensis (RP6), and two subzones, Stylotrochus nitidus-Pterocodon (?) poculum (RP6c) and Orbula discipulus (RP6b), are emended. Each zone is formally defined and we include a stratigraphically ordered list of radiolarian events falling within each zone. Mean numerical ages for zonal boundary events were culled from previous literature and converted to the geomagnetic polarity time scale (GPTS). References to the original description and concept herein applied of all included taxa, and tables which allow for accurate conversion in simple increments between the different published GPTSs are included in this paper.

Pliocene-Pleistocene radiolarian events and magnetostratigraphic calibrations for the tropical Indian Ocean

Johnson, D.A., Schneider, D.A., Nigrini, C.A., Canlet, J.P. and Kent, D.V., 1989. Pliocene-Pleistocene radiolarien events and magnetostratigraphic calibrations for the tropical Indian Ocean. Micropaleontol., 14: 33-64. A composite of four piston cores from the Central Indian Basin and adjacent Ninetyeast Ridge has yielded a continuous magnetobiostratigraphic reference section for most of the Pliocene and the Pleistocene (0.0-4.5 Ma). We identified thirty-three radiolarian events (first- or last-occurrences), and precisely correlated each event to the Neogene geomagnetic polarity time scale. Thirteen of these events are based on revised taxonomic studies of the genera Anthocyrtidium and Pterocorys. Some events show significant departures from synchroneity: five of the radiolarian first-appearances and seven of the last-appearances are time-transgressive by 0.4 m.y. or greater. We here propose a revised, eleven-fold radiolarian zonation for the Pliocene-Pleistocene, using zonal boundaries defined by events which are easily recognized and are demonstrably synchronous in the tropical Indo-Paciflc. The sequence of faunal and floral events reported in this paper will allow high-resolution biostratigraphic correlations within the tropical Indian Ocean; however, the same sequence of events is not necessarily applicable to other tropical or extratropical regions.

Late Neogene radiolarian assemblages characteristic of Indo-Pacific areas of upwelling

Comparison of radiolarian faunas in Late Neogene sediments from the Peru margin, the Oman margin and from beneath the Somalian gyres allowed definition of an assemblage characteristic of areas of upwelling. We describe the stratigraphic and geographic ranges of 12 previously studied and 10 new or newly described taxa. These forms fall into 3 categories: (1) endemic upwelling (14 species); (2) displaced temperate (3 species); and (3) enhanced tropical (5 species). Not all elements of the assemblage are present, or in like abundances, in each upwelling area studied, but there is sufficient matching to allow identification of the assemblage. Eight long ranging species exhibit diachronous first appearances up to 7 my older in the Oman material than in the Peru material.

Synchronous and time-transgressive Neogene radiolarian datum levels in the Equatorial Indian and Pacific Oceans

Abstract Fifty radiolarian events of early Pleistocene and Neogene age were identified in an E-W transect of equatorial DSDP sites, extending from the Gulf of Panama to the western Pacific and eastern Indian Oceans. Our objective was to document the degree of synchroneity or time-transgressiveness of stratigraphically-useful datum levels from this geologic time interval. We restricted our study to low latitudes within which morphological variations of individual taxa are minimal, the total assemblage diversity remains high, and stratigraphic continuity is well-documented by an independent set of criteria. Each of the five sites chosen (503, 573, 289/586, 214) was calibrated to an “absolute” time scale, using a multiple of planktonic foraminiferal, nannofossil, and diatom datum levels which have been independently correlated to the paleomagnetic polarity time scale in piston core material. With these correlations we have assigned “absolute” ages to each radiolarian event, with a precision of 0.1–0.2 m.y. and an accuracy of 0.2–0.4 m.y. On this basis we have classified each of the events as either: (a) synchronous (range of ages 1.0 m.y.); and (c) not resolvable (range of ages 0.4–1.0 m.y.). Our results show that, among the synchronous datum levels, a large majority (15 out of 19) are last occurrences. Among those events which are clearly time-transgressive, most are first appearances (10 out of 13). In many instances taxa appear to evolve first in the Indian Ocean, and subsequently in the western and eastern Pacific Ocean. This pattern is particularly unexpected in view of the strong east-to-west zonal flow in equatorial latitudes. Three of the time-transgressive events have been used to define zonal boundaries: the first appearances of Spongaster pentas, Diartus hughesi , and D. petterssoni . Our results suggest that biostratigraphic non-synchroneity may be substantial (i.e., greater than 1 m.y.) within a given latitudinal zone; one would expect this effect to be even more pronounced across oceanographic and climatic gradients. We anticipate that the extent of diachroneity may be comparable for diatom, foraminiferal, and nannofossil datum levels as well. If this proves true, global “time scales” may need to be re-formulated on the basis of a smaller number of demonstrably synchronous events.

Variation of South Somalian upwelling during the last 160 ka: radiolarian and foraminifera records in core MD 85674

Abstract Indicators of upwelling activity and surface-water productivity for the last 160 ka have been studied in the ‘Marion Dufresne’ core MD 85674 taken off Somalia (3°11, 2 N-50°26, 3 E; 4875 m depth). Quantitative changes in the abundances of radiolarian species which are restricted to upwelling areas (A. murrayana, C. irregularis, D. infabricatus, L. nigriniae, P. caryoforma, P. crustula, and P. minythorax) were used to monitor the variation of vertical advection of deep water. These changes are compared with those recorded by the stable carbon isotopes of a foraminiferal thermocline dweller, N. dutertrei, and with quantitative variations of some planktonic foraminifers (N. dutertrei, G. bulloides, G. menardii, G. sacculifer and G. glutinata). Taken together, our data indicate that, under the south Somalian gyre, upwelling activity was maximal during transition between isotope stages 6 and 5, isotope stage 3, and transition between isotope stages 2 and 1 (respectively at about 130 ka, 65 to 25 ka, and 15 to 10 ka). These data also suggest that, at least during the last 60 ka, periods of increased activity in the Somalian, Arabian and Peruvian upwelling systems were synchronous.

Diversity of radiolarian families through time

The examination of radiolarian biodiversity at the family level through Phanerozoic time reveals some general trends known in other groups of organisms, especially among plankton, while some other trends seem to be quite peculiar. The Permian /Triassic crisis that is one of the most important in the evolution of marine organisms, is marked in radiolarian assemblages by the extinction of two orders (Albaillellaria and Latentifistularia) towards the end of the Permian, and mostly by the tremendous diversification of Spumellaria and Nassellaria in the early-mid Triassic. Radiolarian diversity increased from Cambrian to Jurassic, remained quite stable during the Cretaceous and has decreased slightly since then.

The genus Pterocorys (Radiolaria) from the tropical late Neogene of the Indian and Pacific oceans

An extensive study has been made of the radiolarian genus Pterocorys Haeckel in Neogene (< 8 Ma) sediments from the tropical Indian and Pacific Oceans. The detailed morphologies of eight species are described along with their geographic and stratigraphic ranges. Morphometric analysis has suggested certain phylogenetic lineages which are described in detail. Since th merous transitional forms, and since several species are apparently delicate and, therefore, rare, only 2 stratigraphically useful and easily recognizable Neogene datum levels (F.A.D. of P. hertwigii and L.A.D. of P. campanula, both between 0.6 and 1.1 Ma) based on Pterocorys can be identified and paleomagnetically dated. Stratigraphic events associated with the genus Pterocorys and their absolute ages are presented in tabular form.

The Genus Anthocyrtidium (Radiolaria) from the Tropical Late Neogene of the Indian and Pacific Oceans

Extensive study of the radiolarian genus Anthocyrtidium Haeckel in Neogene (< 8 Ma) sediment from the tropical Indian and Pacific Oceans permits description of the detailed morphologies of ten species (two new) along with their geographic and stratigraphic ranges. Phylogenetic lineages may be proposed in some cases, but in general our studies have not revealed clear evolutionary trends. However, eight stratigraphically useful and easily recognizable Neogene datum levels based on Anthocyrtidium can be identified and paleomagnetically dated.

High resolution Pliocene-Pleistocene radiolarian stratigraphy of the tropical Indian Ocean

Abstract We refined the positions of 31 Plio-Pleistocene radiolarian datum levels by examining 3 paleomagnetically dated cores and comparing results with 4 previously studied cores from the Central Indian Basin. When not affected by hiatuses or drastic changes in rates of sedimentation, absolute ages of radiolarian events in multiple cores from this restricted geographic area are precise to within 0.05 million years. Some events are complicated by minor morphotypic changes at the beginning or end of ranges or within evolutionary lineages; species definitions have to be restricted accordingly. Further study of these variations within an accurate chronostratigraphic framework should permit determination of the rates of these evolutionary changes.

Radiolarian stratigraphy across the Oligocene/Miocene transition

Abstract Recently the International Union of Geological Sciences (Commission on Stratigraphy, Working Group on the Paleogene/Neogene Boundary) proposed that the Oligocene/Miocene boundary be placed at the base of Chron C6Cn2n at 23.8 Ma on the Cande and Kent (1992) magnetic time scale, where it is approximated by planktic foraminifera at the first occurrence of Globorotalia kugleri , and by calcareous nannofossils at the last occurrence of Sphenolithus ciperoensis and the first and last occurrences of Sphenolithus delphix and S. capricornutus . Herein we show that, in terms of radiolarians, the base of Chron C6Cn2n can be correlated with the upper part of the Lychnocanoma elongata Zone between the last occurrence of Artophormis gracilis (23.94 Ma) and the first occurrence of Cyrtocapsella tetrapera (23.69 Ma). Since the proposed stratotype at Lemme-Carrosio (Italy) does not contain radiolarians at the boundary, we re-examined 13 DSDP sites and established the stratigraphic sequence of 29 first and last radiolarian occurrences and one evolutionary transition across the boundary. Nine of these sites contain both calcareous and siliceous microfossils and thus allow for an integrated biostratigraphy. Paleomagnetic stratigraphy is not available for any of the DSDP cores examined. However, use of Hodell and Woodruffs (1994) strontium isotope curve from DSDP Site 289 has permitted calibration of several low latitude microfossil datum levels against the geomagnetic polarity scale. Two new species, Lychnocanoma apodora and Eucyrtidium plesiodiaphanes , are described.