8th International Nannoplankton Association Conference
Ben Walsworth-Bell, Paul R. Bown, Graham P. Weedon:
Introduction The application of calcareous nannofossils within the field of palaeoceanography is well-established, but less is known about the value of nannofossils in cyclostratigraphy. The aims of this study are to investigate: (1) the value of nannofossils as cyclostratigraphic tools; (2) nannofloral evidence for links between climatic cycles and rhythmic sedimentation in the Lias; and (3) ecological preferences of the ancestral nannoplankton.The Belemnite Marls The hemipelagic Belemnite Marls Formation (Dorset, S UK) was deposited in an epicontinental sea during the Early Pliensbachian. These calcareous mudstones consist principally of micritic carbonate (partially of nannoplanktonic origin), clay (terrestrial), and organic matter (chiefly marine). The sequence is characterised by light and dark marl bedding couplets at the decimetre scale, reflecting primary variations in the contribution of the above components to sedimentation. Cyclostratigraphic analysis (Weedon & Jenkyns, 1999) has revealed that these couplets represent orbital-climatic forcing, recording the precession cycle of the Milankovitch spectrum (average period in the Early Jurassic ca.20ky). Methods An approximately 6m interval (Beds 110a-114: Lang et al., 1928) was selected for high-resolution study, on the grounds of (a) a reasonably constant sedimentation rate, and (b) a clear inverse relationship between wt%CaCO3 and wt%TOC (both of which may be useful palaeoceanographic proxies). Samples were collected at 3cm intervals, and quantitative nannofloral data were generated using standard methods (Bown & Young, 1998). Spectral analysis was applied to these time series to test for the regular cyclicity diagnostic of orbital-climatic forcing. Cross-spectral analysis allowed relationships between datasets to be examined. The spectral methods employed are those of Weedon & Jenkyns (1999). Results The nannofloras recovered are typical of the Pliensbachian of the Boreal Realm. Consistently good preservation allows assemblage variation to be considered in terms of primary palaeoenvironmental signals. Two nannofossil time series (%Crepidolithus crassus and %Mitrolithus elegans) are discussed here, together with corresponding geochemical data. The %C. crassus and %M. elegans power spectra, together with those for wt%CaCO3 and wt%TOC, contain significant spectral peaks at a frequency of 2.67cycles/m (wavelength = 0.38m). Both these species and the geochemical data thus display regular cyclicity at the scale of the bedding couplets. The wt%CaCO3 and wt%TOC time series are significantly coherent (have correlated amplitude variations) at the frequency of these couplet cycles. The phase difference is, within error, indistinguishable from -180°, and thus these parameters vary inversely. The %C. crassus and wt%CaCO3 data are also coherent at the frequency of the 0.38m cycles, but here the phase difference is indistinguishable from 0°: these two parameters are in phase (have a positive relationship). %M. elegans is coherent, and in phase, with wt%TOC at the same frequency (and thus in anti-phase with %C. crassus and wt%CaCO3). Discussion & conclusions Time-series analysis of nannofloral data from the Belemnite Marls reveals the presence of regular cycles which are in phase with lithological alternations in the Milankovitch spectrum. Nannoplankton ecology is closely related to surface-water conditions, and relationships between assemblage composition and lithology may be considered in these terms. Species-diversity analysis suggests that the light (carbonate-rich) marls, which contain low-diversity assemblages (numerically dominated by C. crassus), record eutrophic surface-water conditions. These were probably deposited during periods of enhanced vertical mixing of the water-column. The dark (clay and organic carbon-rich) marls yield high-diversity assemblages. These most likely represent reduced surface-water fertility, due to water-column stratification. It is therefore likely that the rhythmic sedimentation in the Belemnite Marls records periodic variations in marine circulation, driven by orbital-climatic forcing (precession). A similar interpretation has been advanced for the Toarcian-Aalenian Fiuminuta section (central Italy: Mattioli, 1997). The dominance of C. crassus in the light marls suggests that this was an opportunistic species during this interval in the Boreal Realm (this taxon represents nearly 50% of all coccoliths recorded in this study). M. elegans, in phase with organic carbon, may have exhibited the preference for stratified surface-waters typical of most (specialist) nannoplankton. Figure 1: Top: Power spectra of selected Belemnite Marls nannofloral and geochemical time series. BW = bandwidth; CL = confidence level. Middle: Coherency spectra for the time series represented by the power spectra above. Bottom: Phase-difference spectra of the time series represented by the power spectra above. Phase difference values (+ symbols) are only plotted for frequencies where coherency is significant. The uncertainty of the phase estimates is indicated using vertical bars (±95% confidence intervals) at the frequencies of coherence maxima References Bown, P.R. (Ed.) 1998. Calcareous Nannofossil Biostratigraphy. Chapman & Hall. Bown, P.R. & Young, J.R. 1998. Techniques. In: P.R. Bown (Ed.). Calcareous Nannofossil Biostratigraphy. Chapman & Hall. Lang, W.D., Spath, L.F., Cox, L.R. & Muir-Wood, H.M. 1928. The Belemnite Marls of Charmouth, a series in the Lower Lias of the Dorset Coast. Quarterly Journal of the Geological Society, London, 84: 179-257. Mattioli, E. 1997. Nannoplankton productivity and diagenesis in the rhythmically-bedded Toarcian-Aalenian Fiuminuta section (Umbria-Marche Apennines, central Italy). Palaeogeography, Palaeoclimatology, Palaeoecology, 130: 113-33. Weedon, G.P. & Jenkyns, H.C. 1999. Cyclostratigraphy and the Early Jurassic time scale: data from the Belemnite Marls, Dorset, southern England. Geological Society of America Bulletin, 111(12): 1823-40.
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