Hans R. Thierstein

Calcareous nannofossil zonation of the Jurassic-Cretaceous boundary interval and correlation with the geomagnetic polarity timescale

Abstract Calcareous nannofossil stratigraphy has been investigated in six European land sections and at two Deep Sea Drilling Project Sites (391C and 534A) ranging from the Upper Kimmeridgian to the Lower Valanginian. Most of the sections contain a continuous record of the Jurassic-Cretaceous boundary interval, a time of rapid increase in nannofossil abundance and diversity and allow development of a revised nannofossil zonation scheme for this period. Numerous nannofossil lineages have been studied in detail and the results help to increase potential stratigraphic resolution in this interval. The proposed scheme consists of six zones which are units recognizable in all sections regardless of nannofossil preservation and eleven subzones determinable in all but the most poorly preserved material. Thirty-eight additional biohorizons are proposed based on the stratigraphy of the two DSDP sites. These can be accurately established only in moderately well-preserved material but spotty occurrences of these taxa in other sections may be stratigraphically useful. Most of the sections studied have been investigated magnetostratigraphically and the results of this study allow correlation between nannofossil zonation and the geomagnetic polarity time scale. Magnetostratigraphy indicates that many nannofossil events can be quite precisely determined and are not significantly time-transgressive. Finally, the magnetostratigraphic definitions of two stage boundaries are refined. The Kimmeridgian-Tithonian and Berriasian-Valanginian boundaries are placed within Chrons CM22n and CM15n respectively. Two new genera, Faviconus and Umbria , and seven new species are described: Umbria granulosa, Rhagodiscus nebulosus, Cretarhabdus octofenestratus, Faviconus multicolumnatus, Nannoconus infans, Nannoconus compressus and Nannoconus wintereri . Eight subspecies and three taxonomic emendations are also described.

Coccolithophore ecology at the HOT station ALOHA, Hawaii

Cell densities of total coccolithophores and dominant taxa were determined in 183 samples from the upper 200 m of the water column at about monthly intervals between January 1994 and August 1996 at the HOT station ALOHA, Hawaii. High cell densities were observed twice a year, in March (up to 41×103 cells l−1) and in September/October (up to 52×103 cells l−1). In the intervening months, cell densities were extremely low (0–20×103 cells l−1), reflecting a strong seasonality. The main production of coccolithophores took place in the middle photic zone between 50 and 100 m water depth. In total 125 coccolithophore species were identified but only five constituted on average more than 30% of the community: Emiliania huxleyi, Umbellosphaera irregularis, U. tenuis, Florisphaera profunda and Gephyrocapsa ericsonii. The generally low, but seasonally dynamic coccolithophore cell density variability is compared with in situ measurements of environmental parameters. Correlation analyses between cell density variability of the dominant taxa and potentially controlling environmental parameters show significant correlation coefficients when the data set was separated into upper and lower photic zone. Cell densities of all dominant taxa are most highly correlated with temperature variability. U. irregularis is positively correlated in the upper photic zone, whereas E. huxleyi and G. ericsonii are negatively correlated. In the lower photic zone, F. profunda cell densities are positively correlated with light, which corresponds to the maximum bottom-up control (i.e. by physical forcing) of any species encountered. The surprisingly low correlations of cell densities with nitrate and phosphate may be caused by insufficient sampling resolution, nutrient levels close to detection limits, or both.

Global dominance of Gephyrocapsa coccoliths in the Late Pleistocene: Selective dissolution, evolution, or global environmental change?

Individual species of marine calcareous phytoplankton are known to occur in blooms in todays ocean and to have dominated fossil assemblages. Interest in the study of marine phytoplankton-ocean-climate interactions has increased because of the potential influence of phytoplankton species like Emiliania huxleyi on global climate and on the global carbon cycle. There is evidence that Gephyrocapsa caribbeanica, which is closely related to E. huxleyi, was globally dominant in the late Pleistocene (480–262 ka). Morphological analyses of Gephyrocapsa coccoliths reveal that only two of six Holocene morphological associations occurred during this time interval. We examine three potential causes for the dominance: preservation, environment, and evolutionary adaptation. We conclude that evolutionary adaptation was the likely process responsible for Gephyrocapsa dominance, although there are indications in the mid-Brunhes for warmer climates than today.

Size distribution of Holocene planktic foraminifer assemblages: biogeography, ecology and adaptation

The size of any organism is influenced by the surrounding ecological conditions. In this study, we investigate the effects of such factors on the size spectra of planktic foraminiferal assemblages from Holocene surface sediments. We analyzed assemblages from 69 Holocene samples, which cover the major physical and chemical gradients of the oceans. On a global scale, the range of sizes in assemblages triples from the poles to the tropics. This general temperature-related size increase is interrupted by smaller sizes at temperatures characteristic of the polar and subtropical fronts, at 2°C and 17°C, respectively, as well as in upwelling areas. On a regional scale, surface water stratification, seasonality and primary productivity are highly correlated with the size patterns. Such environmentally controlled size changes are not only characteristic for entire assemblage, but also for the dominant single species.

Morphologic variability of the coccolithophorid Calcidiscus leptoporus in the plankton, surface sediments and from the Early Pleistocene

Abstract On a global scale, morphological variability of the extant coccolithophorid Calcidiscus leptoporus (Murray and Blackman, 1898) Loeblich and Tappan was investigated in surface sediments and plankton samples and from an Early Pleistocene time-slice (1.8 Ma to 1.6 Ma). In the bivariate space coccolith diameter versus number of rays in the distal shield, Holocene samples follow a single, unimodal morphocline. Sample means of coccolith size and number of elements group in three clusters, I, II and III, which are of biogeographic significance. Clusters II and III coccoliths (mean coccolith size of 5.0 μm and 20.9 elements, and 6.6 μm and 25.6 elements, respectively) are found in a tropical belt extending from 11 °N to 17 °S with an annual minimum sea-surface temperature above 23.5 °C. Cluster I coccoliths (5.8 μm, 20.7 elements) are found in samples outside that belt. The distribution of coccoliths in the surface sediments is tentatively interpreted to be a result of mixing to a varying degree of at least three different morphotypes (‘small’, ‘intermediate’ and ‘large’), which were identified in the living plankton, and which are separated from each other at 5 μm and 8 μm mean coccolith diameter, respectively. A comparison of the surface sediments with the Early Pleistocene assemblages revealed that between 1.6 Ma and 1.8 Ma two morphoclines A and B existed, the first of which persisted until the Holocene in the form of C. leptoporus , while the second comprises only extinct morphotypes including Calcidiscus macintyrei as one end-member. During the Early Pleistocene morphocline A was more homogeneous and no clusters were evident. Morphocline B shows a clear bimodality with a separation of morphotypes at 9.5 μm. Our observations suggest that morphoclines are subsets within the total stratigraphical range of a single species, and represent the global variability of that species in a particular time interval. Morphotypes, which belong to a morphocline, represent the infra-specific variability of that species within the biogeographic and stratigraphic limits of that species.

Techniques for quantitative analyses of calcareous marine phytoplankton

Abstract This paper discusses the techniques used to sample and analyse living marine calcareous phytoplankton. The various methods are described and tested within several research projects aimed at the determination of coccolithophore cell densities in seawater. In addition, the potential advantages and drawbacks associated with the application of light and scanning electron microscopic techniques to the quantitative analysis of coccolithophores are discussed. Several tests have been carried out in order to quantify potential errors related to: (1) homogeneity of material distribution on filter membranes; (2) use of different microscopes (scanning electron microscope versus light microscope); (3) use of different filter membranes (cellulose mixed-ester membranes versus polycarbonate membranes); and (4) Utermohl settling versus filtration method. These tests revealed that major errors in cell density calculations could result from the uneven distribution of coccolithophore specimens on a filter membrane. The error resulting from the use of a light microscope arises from its low resolution, which restricts the identification of species, especially of small coccospheres. The use of different filter membranes does not show a statistically significant difference in cell density calculations, although polycarbonate membranes can be examined much more efficiently with the scanning electron microscopy than cellulose mixed-ester membranes. The Utermohl method, however, gives lower cell densities consistently (several times) than the filtration method.

Sympatric speciation and phyletic change in Globorotalia truncatulinoides

Speciation processes are only rarely studied with fossil materials, even though in principle hypotheses of speciation patterns are most directly testable in the fossil record. We quantitatively document in two widely separated South Pacific DSDP holes the mid-Pliocene speciation of the planktonic foraminifer Globorotalia truncatulinoides. Speciation, with continuous geographic co-oc- currence of ancestor and descendant forms, occurred simultaneously at both localities over a period of -500,000 years. This suggests a sympatric speciation process that involved a large, geographically extensive population. Globorotalia truncatulinoides underwent its most rapid and extensive evolu- tionary change between -2.8 and 2.5 Ma. This time interval corresponds to the development of northern hemisphere glaciation, suggesting that climate-controlled paleoceanographic change may have played a significant role in the evolution of G. truncatulinoides.

DISINTEGRATION OF AGGREGATES AND COCCOSPHERES IN SEDIMENT TRAP SAMPLES

Abstract A method for non-destructive disaggregation of coccoliths contained in fecal pellets, marine snow and on coccospheres is described. It consists of repeated chemical oxidation of organic material with (NaClO and H2O2) combined with brief periods of ultrasonification.

Plio-Pleistocene calcareous plankton biochronology

Abstract A reliability assessment of 20 commonly employed Late Pliocene and Pleistocene calcareous plankton biostratigraphic datums published for 30 DSDP holes has revealed a subset of ten which appear to be reasonably isochronous and consistently recognizable by a large number of biostratigraphers. The assessment is based on an analysis of datum rank order reliability, and is supplemented by an examination of the absolute age estimates for the same 20 datums which were derived using the paleomagnetic stratigraphy presented for 19 of the 30 holes. The results indicate only four of the initial eleven datums based on planktonic foraminifers can be considered reliable, compared to seven reliable calcareous nannofossil datums of an initial nine. Compared to calcareous nannofossils, planktonic foraminifera thus show greater provincialism and more fluid ecophenotypic and evolutionary changes, implying that they are more interesting subjects for evolutionary studies but relatively less suitable tools for biostratigraphy. An important datum rejected by this assessment is the first appearance datum (FAD) of the planktonic foraminifer Globorotalia truncatulinoides , once considered a reliable indicator of the Plio-Pleistocene boundary. While the FAD for this species occurs at its commonly accepted level of approximately 1.9 Ma in the equatorial Pacific and throughout the temperate and tropical Atlantic, its age in the Indo-Pacific between about 15°S and 40°S is strongly diachronous. The earliest FAD is dated at a maximum of 2.6–2.7 Ma between about 20°S and 35°S in the Indo-Pacific. A global extremum in the average size of Holocene populations has been previously recognized within this same latitudinal zone.

How synchronous are neogene marine plankton events

An electronic supplement of this material may be obtained on adiskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN toAGUs FTP account using ANONYMOUS as the username andGUEST as the password. Go to the right directory by typing CDAPEND. Type LS to see what files are available. Type GET and thename of the file to get it. Finally, type EXIT to leave the system.)(Paper 94PA01456, How synchronous are Neogene marine planktonevents?, by C. Spencer-Cervato, H. R. Thierstein, D. B. Lazarus, andJ-P Beckmann). Diskette may be ordered from American GeophysicalUnion, 2000 Florida Avenue, N.W., Washington, DC 20009;