Application of the Helium Isotopic System to Accretion of Terrestrial and Extraterrestrial Dust through the Cenozoic
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CitationBhattacharya, Atreyee. 2012. Application of the Helium Isotopic System to Accretion of Terrestrial and Extraterrestrial Dust through the Cenozoic. Doctoral dissertation, Harvard University.
AbstractThe Helium isotopic system provides novel tools to probe the sedimentary record of the earth over the last few hundred million years. Radiogeneically derived \(^4He\) and solar wind implanted \(^3He\) is delivered to sediments by weathered continental material and interplanetary dust particles, respectively. The purpose of research presented in this thesis is to use \(^4He\) of terrestrial and \(^3He\) of extraterrestrial origins as tools to investigate the relationship between global climate and surface processes on earth. I measured \(^4He\) in annual growth bands in a Porites coral from the northern Red sea, lacustrine carbonate sediments from the Bahamas and marine sediments in the North Atlantic (Ocean Drilling Program site 1313). Terrestrially derived fraction of the measured \(^4He\) in the Red Sea Porites coral and carbonates of the Bahamas together provide accurate information about changes in dust export rates from North Africa over the last millennium; dust fluxes are intimately tied to droughts in North Africa at decadal to centennial time scales over the last millennium that in turn, appears to be modulated by sea surface temperatures (SSTs) in the Indian and the Atlantic Ocean. On the other hand, \(^4He\) content in marine sediments in the North Atlantic provide information about changes in sources of detrital over the last six million years that in turn appears to reflect changes in ocean circulation in the sub-polar north Atlantic. Accretion rates of solar wind implanted \(^3He\) provide independent constraints on time distribution and physical processes operational during the deposition of marine sedimentary sequences. I measured extraterrestrial \(^3He\) in sediments from the late Cretaceous through the early Danian at one site in the North Pacific (Ocean Drilling Program site 1209 on the Shatsky Rise) and in three sites from the South Atlantic (Deep Sea Drilling Projects sites 516F on Rio De Grande Rise and 528 on the Walvis Ridge). The high-resolution record of extraterrestrial \(^3He\) at the Shatsky Rise demonstrates that there was no increase in solar system dustiness associated with the meteorite impact thought to be responsible for the end cretaceous mass extinction. The assumption of constant delivery of extraterrestrial material therefore resulted in independent constraints on sediment accumulation rates and resultant age model through the early Danian, the latter marking the recovery of ocean system following the events of the end Cretaceous mass extinction. On the other hand, the extraterrestrial \(^3He\) derived mass accumulation rates in the south Atlantic sites demonstrate that carbonate-clay cyclicity in sediments—used by cyclostratigraphy to derive age models—are generated by different physical processes at different sites. It is therefore, crucial to understand processes underlying the carbonate-clay cyclicity before interpreting sediment couplets in terms of age model.
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