Jörg Bollmann

North Pacific seasonality and the glaciation of North America 2.7 million years ago

In the context of gradual Cenozoic cooling, the timing of the onset of significant Northern Hemisphere glaciation 2.7 million years ago is consistent with Milankovitchs orbital theory, which posited that ice sheets grow when polar summertime insolation and temperature are low. However, the role of moisture supply in the initiation of large Northern Hemisphere ice sheets has remained unclear. The subarctic Pacific Ocean represents a significant source of water vapour to boreal North America, but it has been largely overlooked in efforts to explain Northern Hemisphere glaciation. Here we present alkenone unsaturation ratios and diatom oxygen isotope ratios from a sediment core in the western subarctic Pacific Ocean, indicating that 2.7 million years ago late-summer sea surface temperatures in this ocean region rose in response to an increase in stratification. At the same time, winter sea surface temperatures cooled, winter floating ice became more abundant and global climate descended into glacial conditions. We suggest that the observed summer warming extended into the autumn, providing water vapour to northern North America, where it precipitated and accumulated as snow, and thus allowed the initiation of Northern Hemisphere glaciation.

Morphology and biogeography of Gephyrocapsa coccoliths in Holocene sediments

Morphological analysis of 70 globally distributed Holocene sediment assemblages of the calcareous marine phytoplankton genus Gephyrocapsa (Prymnesiophyta) displays significant correlations between the measured morphological parameters and environmental gradients. By extrapolating these essential findings, paleoecological and evolutionary interpretations can be obtained for the Pleistocene period. Measurements of various morphometric parameters of 3600 coccoliths showed that the morphologically most distinct features arc bridge angle and placolith length. Bridge angle ranged from 120 ° to 3 ° and placolith length from 6.2 μm to 0.9 μm. The combination of the mean bridge angle and the mean length of each sediment assemblage (Holocene) leads to six morphological groups which characterise different environments and geographic areas: Equatorial associations have a mean bridge angle larger than 56 ° and a mean size between 3.1 μm and 3.9 μm. The temperature range of these associations is 25 °C to 29.5 °C mean sea surface temperature (MSST). Abbreviation GE: Gephyrocapsa Equatorial. Subtropical central water-mass associations have a mean bridge angle between 27 ° and 56 ° and a mean length larger than 3.1 μm. These associations are only found in oligotrophic central gyre regions with a MSST between 22 °C and 25 °C. Abbreviation GO: Gephyrocapsa Oligotrophic. Transitional associations have a mean bridge angle between 27 ° and 56 ° and a mean length between 2.4 μm and 3.1 μm. These associations are only found in areas with a mean SST between 19 °C and 20 °C. Abbreviation GT: Gephyrocapsa Transitional. Transitional to subarctic cold associations have a mean bridge angle less than 27 ° and a mean length larger than 2.4 μm. These associations are found in moderately productive regions with a mean SST lower than 21 °C. Abbreviation GC: Gephyrocapsa Cold. Moderately productive temperate associations have a mean bridge angle larger than 56 ° and a mean length larger than 3.9 μm. These associations are dominant in upwelling regions, but also occur in non-upwelling areas. Their temperature range is 18 °C to 23 °C. The relative abundance of these associations within the Gephyrocapsa complex in individual samples decreases with increasing distance from the coast. Abbreviation: GL Gephyrocapsa Larger. Associations consisting of small placoliths with a mean bridge angle between 20 ° and 50 ° and a mean length less than 2.4 μm show no well defined environmental or geographic preference. Such forms seem to be frequent near islands and coasts (neritic) in tropical to subtropical regions. Abbreviation: GM: Gephyrocapsa Minute. The observed morphological discontinuities along the environmental gradients are suggestive of distinct and possibly genetically isolated species populations. In individual assemblages, however, considerable morphological variability is observed and characteristic morphological traits show gradual change along environmental gradients. These features could be expected, if a single, globally homogeneous genotypic species Gephyrocapsa oceanica should show high phenotypic plasticity. Future morphological analysis of plankton samples from various areas and taken at various seasons are expected to confirm the conclusion of environmental dependence of Gephyrocapsa morphology drawn from this analysis.

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.

Biogeography of selected Holocene coccoliths in the Atlantic Ocean

In this chapter we present a revision of the biogeographical distribution of five coccolithophorid species (Coccolithus pelagicus, Calcidiscus leptoporus, Heli-cosphaera carteri, Syracosphaera pulchra and Umbilicosphaera sibogae) and the genus Gephyrocapsa in the Atlantic Ocean. The mapping is based on surface sediment samples. Each of the taxa considered here constitutes an unambiguous morphological group ideal for rapid low taxonomic resolution analysis of assemblages, which is a tempting strategy for ecological and paleoecological analysis of assemblages. However, in each case recent research has indicated that these broad taxa are in fact composed of several discrete species, or sub-species. The clearest example is C. pelagicus, with discrete morphotypes in sub-Arctic and temperate upwelling areas. For Gephyrocapsa and Umbilicosphaera the separation is less obvious but still unambiguous. Species separation is manifestly essential to understanding the biogeography of these taxa. For H. carteri and S. pulchra the mapped distributions are relatively straightforward and we do not yet know how they relate to the recently proven genotypic variation within the taxa.

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.

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.

Species level variation in coccolithophores

Coccolithophores are an ideal test group for investigating fine-scale differentiation within the phytoplankton since their taxonomy is rather well-documented and their biomineralised periplasts – the coccoliths – provide a rich suite of qualitative and quantitative morphological characters and a uniquely extensive fossil record. In addition, extant coccolithophore species can be grown in culture and hence are available for studies of morphological variability under controlled conditions, molecular genetic studies and cytological research.

Determination of absolute coccolith abundances in deep-sea sediments by spiking with microbeads and spraying (SMS-method)

A quick new method is described for the quantification of absolute nannofossil proportions in deep-sea sediments. This method (SMS) is the combination of Spiking a sample with Microbeads and Spraying it on a cover slide. It is suitable for scanning electron microscope (SEM) analyses and for light microscope (LM) analyses. Repeated preparation and counting of the same sample (30 times) revealed a standard deviation of10.5%. The application of tracer microbeads with different diameters and densities revealed no statistically significant differences between counts. The SMS-method yielded coccolith numbers that are statistically not significantly different from values obtained from the filtration-method. However, coccolith counts obtained by the random settling method are three times higher than the values obtained by the SMS- and the filtration-method.

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.