Investigation into the post-mortem transport of benthic foraminifera

Kelham, Angela

October 2011

Thesis or dissertation

© 2011 Angela Kelham. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.

Palaeoenvironmental reconstruction using foraminifera relies on the assumption that assemblages reflect the ecological conditions at the time of deposition. However, the distribution of taxa can be greatly affected by transport and reworking of tests. This is particularly important in high energy environments such as submarine canyon and fan systems, which are major pathways for sediment transported from the continental shelf to the abyssal plain. Traditionally, these assemblages have been abandoned as hopelessly taphonomically corrupted, but it is possible that these assemblages contain useful hydraulic information. This project aims to develop the fundamental concepts needed to extract this information, via a series of classical particle hydraulics experiments on empty tests in static water and unidirectional currents. Hyaline foraminifera have been selected for these experiments, as they are the most abundant tests found in shelf and upper-slope environments and consequently are most likely taxa to be entrained into gravity flows.

Static water experiments have shown that settling velocities are significantly different between taxa, meaning that assemblages are likely to fractionate according to species during transportation. Settling velocities range from 0.01 to 0.06 ms-1 with larger specimens falling faster than smaller ones. Elphidium crispum exhibited the fastest average settling velocity of 0.03 ms-1 while Planorbulina mediterranensis fell with the lowest average settling velocity of 0.01 ms-1. The occurrence of spatial separation of taxa within a single flow is directly tested using a flume where a spatially waning turbidity current is simulated by a saline density flow. Results show that the slowly settling tests such as P. mediterranensis and Cibicides lobatulus remain suspended in the current for longer, and are thus transported further than more rapidly settling taxa such as E. cripsum and Ammonia beccarii.

The experiments have shown that there are significant statistical differences in settling velocity of foraminiferal species and this does result in significantly distinct travelling distances between species in a turbidite. This information is related to the oceanic environment in the Gulf of Cadiz. The signal of fractionation is then identified in core data from Trinidad supplied by Ichron showing that useful assemblage data can be extracted to interpret the depositional environment.

Department of Geography, The University of Hull
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