Publication Date

2014-07-22

Availability

Embargoed

Embargo Period

2016-07-21

Degree Name

Master of Science (MS)

Department

Marine Geology and Geophysics (Marine)

Date of Defense

2014-05-29

First Committee Member

R. Pamela Reid

Second Committee Member

Gregor P. Eberli

Third Committee Member

Peter K. Swart

Fourth Committee Member

James S. Klaus

Abstract

Stromatolites are lithified, laminated organosedimentary structures that dominate the fossil record for 80% of Earth History. Stromatolites form through two processes: the trapping and binding of detrital grains by microbial mats and precipitation of calcium carbonate within microbial mats. Stromatolites from Hamelin Pool, Shark Bay, Western Australia provide a unique opportunity to study depositional processes within the long-lived stromatolite ecosystem. This thesis focuses on investigating the applicability of various techniques for dating modern stromatolites and determining the role that stromatolites play as sediment sinks and sediment sources. Shark Bay stromatolites were dated using Uranium-Thorium geochronometry and Accelerator Mass Spectrometry (AMS) 14C-dating techniques to date the stromatolite samples. The Uranium-Thorium technique yielded some inaccurate dates because stromatolite heads have the potential to accumulate detrital thorium that is indistinguishable from the daughter thorium produced by the uranium. Only areas with high amounts of micritic precipitate produced realistic dates. In contrast, AMS Carbon-14 dating appeared to yield realistic dates. The small sample size (1.5-2mg) of the AMS technique allowed us to target precipitated calcium carbonate within the stromatolites, thereby minimizing contamination from detrital grains. The ages range from modern to approximately 1000 years before present and are in agreement with previous studies. Thus, AMS 14C-dating of small amounts of precipitated calcium carbonate is considered an ideal technique for dating Shark Bay stromatolites. Point counting revealed that the stromatolites of Hamelin Pool are important sediment sinks that generally reflect the sediment composition of adjacent sediment at the time of accretion. However, the material trapped by the microbial mat that builds the stromatolite structure is less diverse than the material found in the depositional environment. Preferential exclusion of larger grains during trapping and binding by microbial mats is likely the result of decreased availability of the larger components in the water column. In addition, peloids are more abundant in the stromatolites than in the adjacent sediment, possibly reflecting in situ precipitation within the microbial mats. Irregular micritic grains produced in gelatinous microbial mats are an important sediment component in the depositional environment of Hamelin Pool, comprising as much as 55-60% of some sediment samples. These irregular micritic grains appear to be erosional fragments of precipitates that formed within gelatinous microbial mats. The abundance of irregular micritic grains that microbial mats are an important source of sediment in Hamelin Pool.

Keywords

stromatolite: carbon-14 dating: uranium-thorium geochronometry: sediment sinks: micrite precipitation

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