Abstract
This thesis explores the ancient sedimentary archive of Earth history and examines enigmatic
climatic transitions that may have shaped the evolution of animal life. Much of our
understanding of the climatic history of Earth is based on chemical and isotopic measurements
of ancient sediments. One of the main limitations of this archive, however, is the
susceptibility of sediments to diagenesis that can alter the primary chemical signals. For
example, the concentration of iron (Fe) in specific mineral phases is used to infer oxygen
concentrations in the ancient water column. Chapter 2 of this thesis demonstrates that
modern weathering processes remobilize reactive Fe in outcrop samples which leads to a
loss of specific minerals phases (pyrite) and a gain of others (Fe-oxides). These results
highlight the need for caution when using Fe-speciation to catalog past environment redox
change from purely outcrop based records.
Despite the prevalence of diagenesis in sedimentary rocks there are currently few
robust geochemical tools capable of providing quantitative information on the extent of
alteration from the primary signal. In order to fill this gap, Chapter 3 presents a numerical
model of marine carbonate diagenesis that tracks dissolution of primary carbonates and
re-precipitation of secondary minerals. The model is ground-truthed using measurements
of calcium, magnesium, carbon, and oxygen isotopes in carbonate sediments from the
modern Bahamas platform. This model can be used as a ‘looking glass’ to see through
diagenesis and provide more robust estimates for past seawater chemistry.
Ancient carbonate rocks with extreme negative carbon isotopes are found worldwide
bracketing the Marinoan glaciation (∼635 Ma). There is no scientific consensus as to
whether these excursions originate from a primary perturbation in the carbon cycle or
from diagenetic alterations. Chapter 4 merges new measurements of calcium, magnesium,
and strontium isotopes in these sediments with the diagenetic model developed in Chapter
3 to offer new insights into the potential origin of these extreme isotope anomalies.
climatic transitions that may have shaped the evolution of animal life. Much of our
understanding of the climatic history of Earth is based on chemical and isotopic measurements
of ancient sediments. One of the main limitations of this archive, however, is the
susceptibility of sediments to diagenesis that can alter the primary chemical signals. For
example, the concentration of iron (Fe) in specific mineral phases is used to infer oxygen
concentrations in the ancient water column. Chapter 2 of this thesis demonstrates that
modern weathering processes remobilize reactive Fe in outcrop samples which leads to a
loss of specific minerals phases (pyrite) and a gain of others (Fe-oxides). These results
highlight the need for caution when using Fe-speciation to catalog past environment redox
change from purely outcrop based records.
Despite the prevalence of diagenesis in sedimentary rocks there are currently few
robust geochemical tools capable of providing quantitative information on the extent of
alteration from the primary signal. In order to fill this gap, Chapter 3 presents a numerical
model of marine carbonate diagenesis that tracks dissolution of primary carbonates and
re-precipitation of secondary minerals. The model is ground-truthed using measurements
of calcium, magnesium, carbon, and oxygen isotopes in carbonate sediments from the
modern Bahamas platform. This model can be used as a ‘looking glass’ to see through
diagenesis and provide more robust estimates for past seawater chemistry.
Ancient carbonate rocks with extreme negative carbon isotopes are found worldwide
bracketing the Marinoan glaciation (∼635 Ma). There is no scientific consensus as to
whether these excursions originate from a primary perturbation in the carbon cycle or
from diagenetic alterations. Chapter 4 merges new measurements of calcium, magnesium,
and strontium isotopes in these sediments with the diagenetic model developed in Chapter
3 to offer new insights into the potential origin of these extreme isotope anomalies.
| Original language | English |
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| Publisher | Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen |
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| Number of pages | 180 |
| Publication status | Published - Aug 2016 |
