Publication Date

2019-04-09

Availability

Open access

Embargo Period

2019-04-09

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Marine Geology and Geophysics (Marine)

Date of Defense

2017-07-21

First Committee Member

Peter K. Swart

Second Committee Member

R. Pamela Reid

Third Committee Member

Gregor P. Eberli

Abstract

Hamelin Pool, the eastern embayment in Western Australia’s Shark Bay, hosts the world’s largest assemblage of actively growing marine stromatolites. In 1996, UNESCO named Shark Bay a World Heritage site. Consequently, Hamelin Pool is strictly protected and only limited research has been approved in the area. In this thesis, I have investigated some of the stable isotopic and geochemical signatures of Hamelin Pool basinal waters and sediments collected during 2013 and 2014. Prior investigations show the pool is hypersaline, with salinity values in the southern waters measuring nearly double that of sea water. Hamelin Pool’s salinity distribution is partially mixed, with increasing values from north to south. Similar to the salinity distribution, Hamelin Pool’s average δ18O value measured enriched at +3.95‰, with maximum value of +5.27‰ at the southern end of and minimum value of +3.16‰ at the northern end. The mean δ2H values measured +22.9‰ with maxima at +26.73‰ and minima of +14.32‰, again increasing from north to south. Sill growth and restriction directly impacts the water chemistry. Modeling results show that nearly 50% of the basinal water evaporated each year for ~700 yrs to reach its current δ18O and δ2H values. Modeling suggests impacts from Sea Level Rise within a 70yr time period. The Sr, Mg, Ca and Cl ratios indicate that Hamelin Pool basinal waters are evolved seawater. Heightened Sr/Ca ratios in basinal waters suggest calcite precipitation reactions occur in Shark Bay prior to reaching Hamelin Pool. The distribution of Sr/Ca ratios mirror the salinity and stable isotopic values, which implies a high residence time of water in the southern end of the pool. Sediment minerology is predominantly (96%) aragonite with residual amounts of High-Mg Calcite. To complement the water samples, basinal sediments were also analyzed for their inorganic δ13C and δ18O values. The δ13C values ranged +6.18 to +2.83‰, and the δ18O values ranged +4.07 to +2.17‰. The values increase from north to south, further supporting high residence times in the southern end of Hamelin Pool. Finally, organic matter present in the sediments was also analyzed for its δ15N, δ13C values and C:N ratios. The organic δ15N mean value measured +0.77‰ with maxima at +9.06‰ and minima of -4.28‰. The organic δ13C mean value measured -15.38‰ with maxima at -8.95 and minima of -21.58‰. The atomic C:N ratios of the organic matter ranged from a 1.5:0.35 to 0.43:0.02 with an average of 10.1. The organic matter appeared to be sourced from a mixture of seagrass & microbial mat decay, with an enriched source of 13C. This enrichment can be explained by high residence times of the restricted water body. The geochemical properties measured create a baseline for Hamelin Pool basinal water chemistry to be evaluated over time. The predictions made in this thesis may help in the understanding of the magnitude and pace of chemical changes in the modern environment, which may stress the growth of marine stromatolites.

Keywords

Hamelin Pool; Geochemistry; Stable Isotopes; Trace Metals; Carbonate Sedimentology; Stromatolites

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