Variability of Pliocene nannoplankton populations

• Main Author: Gibbs, Samantha Jane
• Published: University of Cambridge 2003
• Subjects: 560
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.599385

Here I focus on the construction of high-resolution nannofossil records, in addition to carbonate preservation, isotope, and other environmental proxy records, for the equatorial Atlantic ODP Sites 662, 926 and 929, and the sub-equatorial Atlantic Site 659. This integrated study demonstrates the detailed character of he nannofossil populations (overall abundance, evolution and morphometric patterns). It also allows an assessment to be made of the stratigraphic and evolutionary importance of nannoplankton events during the mid Pliocene from 3.45-3.95 Ma. The mid Pliocene is demonstrated here to be an interval of subtle reorganisation characterised by the last occurrences of <i>Sphenolithus </i>sp. and <i>Reticulofenestra pseudoumbilicus. </i>The mid Pliocene lies in a wider interval of progressive extinctions of warm water, oligotrophic taxa associated with climatic deterioration from the Late Pliocene to Recent. Nannofossil sediment assemblages are controlled by both the initial production of the nannoplankton in the surface waters and dissolution (in the water column and pre- and post-burial). By the comparison of synchronous nannofossil and non-nannofossil records from Ceara Rise Sites 929 (4397 m water depth) with Site 926 (3598 m water depth), an ecologically dominated assemblage can be clearly distinguished from one that has experienced significant subsequent taphonomic overprinting by dissolution. The latter is differentiated by the number of co-varying relationships, the taxa that form those co-varying relationships, the coupling of dissolution indices with each other and with the co-varying nannofossil abundances, and the loss or reduction of dissolution-susceptible taxa. In non-dissolution influenced populations, two main signals are found to shape the downcore variability in nannofossil assemblages. The main signal is an evolutionary trend. To a lesser extent, the signal is dominated by strong cyclic abundance changes responding to orbitally forced climate changes. Only some of the variance observed in nannofossil abundance patterns can be explained by direct linear climatic forcing. This is to be expected given that in studies of seasonal coccolithophorid assemblages, only part of the variation observed can be explained by simple correlation with abiotic processes.