TY - BOOK

T1 - Refining the Neoproterozoic and Early Paleozoic record of carbon cycling and seawater chemistry using quantitative geochemical models of redox dynamics and carbonate diagenesis

AU - Ahm, Anne-Sofie Crüger

N1 - Ph.d.-grad opnået ved mundtligt forsvar 8. november 2016

PY - 2016/8

Y1 - 2016/8

N2 - This thesis explores the ancient sedimentary archive of Earth history and examines enigmaticclimatic transitions that may have shaped the evolution of animal life. Much of ourunderstanding of the climatic history of Earth is based on chemical and isotopic measurementsof ancient sediments. One of the main limitations of this archive, however, is thesusceptibility of sediments to diagenesis that can alter the primary chemical signals. Forexample, the concentration of iron (Fe) in specific mineral phases is used to infer oxygenconcentrations in the ancient water column. Chapter 2 of this thesis demonstrates thatmodern weathering processes remobilize reactive Fe in outcrop samples which leads to aloss of specific minerals phases (pyrite) and a gain of others (Fe-oxides). These resultshighlight the need for caution when using Fe-speciation to catalog past environment redoxchange from purely outcrop based records.Despite the prevalence of diagenesis in sedimentary rocks there are currently fewrobust geochemical tools capable of providing quantitative information on the extent ofalteration from the primary signal. In order to fill this gap, Chapter 3 presents a numericalmodel of marine carbonate diagenesis that tracks dissolution of primary carbonates andre-precipitation of secondary minerals. The model is ground-truthed using measurementsof calcium, magnesium, carbon, and oxygen isotopes in carbonate sediments from themodern Bahamas platform. This model can be used as a ‘looking glass’ to see throughdiagenesis and provide more robust estimates for past seawater chemistry.Ancient carbonate rocks with extreme negative carbon isotopes are found worldwidebracketing the Marinoan glaciation (∼635 Ma). There is no scientific consensus as towhether these excursions originate from a primary perturbation in the carbon cycle orfrom diagenetic alterations. Chapter 4 merges new measurements of calcium, magnesium,and strontium isotopes in these sediments with the diagenetic model developed in Chapter3 to offer new insights into the potential origin of these extreme isotope anomalies.

AB - This thesis explores the ancient sedimentary archive of Earth history and examines enigmaticclimatic transitions that may have shaped the evolution of animal life. Much of ourunderstanding of the climatic history of Earth is based on chemical and isotopic measurementsof ancient sediments. One of the main limitations of this archive, however, is thesusceptibility of sediments to diagenesis that can alter the primary chemical signals. Forexample, the concentration of iron (Fe) in specific mineral phases is used to infer oxygenconcentrations in the ancient water column. Chapter 2 of this thesis demonstrates thatmodern weathering processes remobilize reactive Fe in outcrop samples which leads to aloss of specific minerals phases (pyrite) and a gain of others (Fe-oxides). These resultshighlight the need for caution when using Fe-speciation to catalog past environment redoxchange from purely outcrop based records.Despite the prevalence of diagenesis in sedimentary rocks there are currently fewrobust geochemical tools capable of providing quantitative information on the extent ofalteration from the primary signal. In order to fill this gap, Chapter 3 presents a numericalmodel of marine carbonate diagenesis that tracks dissolution of primary carbonates andre-precipitation of secondary minerals. The model is ground-truthed using measurementsof calcium, magnesium, carbon, and oxygen isotopes in carbonate sediments from themodern Bahamas platform. This model can be used as a ‘looking glass’ to see throughdiagenesis and provide more robust estimates for past seawater chemistry.Ancient carbonate rocks with extreme negative carbon isotopes are found worldwidebracketing the Marinoan glaciation (∼635 Ma). There is no scientific consensus as towhether these excursions originate from a primary perturbation in the carbon cycle orfrom diagenetic alterations. Chapter 4 merges new measurements of calcium, magnesium,and strontium isotopes in these sediments with the diagenetic model developed in Chapter3 to offer new insights into the potential origin of these extreme isotope anomalies.

UR - https://soeg.kb.dk/permalink/45KBDK_KGL/fbp0ps/alma99122244511505763

M3 - Ph.D. thesis

BT - Refining the Neoproterozoic and Early Paleozoic record of carbon cycling and seawater chemistry using quantitative geochemical models of redox dynamics and carbonate diagenesis

PB - Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen

ER -