Changes in ocean geochemistry across the K-Pg boundary interval
Author
Summary, in English
The Cretaceous - Paleogene boundary (K-Pg, 66 Ma) was a time interval with dramatic changes in the Earth’s climate and the ocean environment. It was characterized by severe disruptions of marine biogeochemical cycles due to a meteorite impact during a time of active volcanism. The extinction of calcifiers, among others, influenced biogeochemical cycles and changed the conditions for carbonate deposition in the global ocean. Our understanding of how these cycles and the ocean geochemistry have changed through this time is still limited. Questions remain about the extent of these changes and their relationship to the biotic recovery in the surface and deep ocean. This thesis aims to provide more insights into the changes in the ocean geochemistry across the K-Pg boundary and the Paleocene (66 – 56 Ma) to improve our knowledge. A goal is to investigate how the marine silicon (Si) cycle has been changed and recovered during this time. This is important due to its link to carbon drawdown and climatic changes via silicate weathering and the biological pump. Additionally, a goal is to gain a better understanding of how ecosystems reacted and recovered in the aftermath of the crisis.
The Cerithium Limestone Member (Mb) was deposited in the earliest Paleocene in the aftermath of the crisis and provides an excellent opportunity to investigate the development of non-tropical carbonate depositional systems throughout this time interval. Paper I aims, therefore, to better constrain the genesis and paleoenvironmental conditions of the deposition of Cerithium Limestone Mb. For this study, a detailed analysis of the microfacies and microfossil content of the Cerithium Limestone in the Rødvig section of Stevns Klint succession in Denmark was made. Analysis of thin sections and SEM has revealed four different microfacies with varying amounts of bioclasts in two distinct stratigraphic parts. The lowermost part consists of a thin layer of a bryozoan-rich packstone, which is interpreted as reworked material from the crests of the underlying Maastrichtian mounds. In the upper part, the microfacies appear randomly distributed due to significant bioturbation. The Cerithium Limestone is interpreted as formed in a mainly low-energy, open-marine, outer ramp-like sedimentary environment. Cyanobacteria are likely producers of the micrite of the Cerithium Limestone, as indicated by the dominance of very fine crystals (1– 4 μm).
Silicon plays a crucial role in global biogeochemical cycles as an essential nutrient for many marine organisms due to its link to the biological pump. Since the onset of the Phanerozoic, Si cycling has been strongly impacted by the biosilicification of sponges and radiolaria. The stable silicon isotope composition (δ30Si) of siliceous marine diatoms, radiolaria and sponges can be used as a proxy for past changes in dissolved Si (dSi) concentrations and Si cycling processes. Paper II discusses the changes in the marine Si cycle across the K-Pg boundary with the main focus on the early-mid Paleocene. The study presents the results of δ30Si analyses of sponge spicules and radiolaria from sediment samples from DSDP Site 208 (Leg 21) in the Southwest Pacific and suggests that the deep Southwest Pacific was already dSi depleted at the base of the Paleogene (66 Ma).
In recent years, the Si isotope fractionation during radiolaria biomineralization has been a topic of interest in the scientific community. In this context, the determination of order-specific and taxa-specific fractionation factors of radiolaria is crucial for the accurate interpretation of the δ30Si record. Paper III investigates the δ30Si signatures of radiolaria from the orders Nassellaria and Spumellaria. Our results show that it is crucial to consider changes in d30Si of individual taxa (especially from the same order) by the interpretation of the d30Si record in the past. We advise caution when interpreting d30Si signatures from the taxa Spongodiscoidea due to the potential influence by deeper dwelling species on their d30Si record.
This thesis fills some of the existing knowledge gaps regarding the development of the Si cycle across the K-Pg boundary and in the early-mid Paleocene and provides new insight into the genesis and paleoenvironmental conditions of the deposition of Cerithium Limestone.
The Cerithium Limestone Member (Mb) was deposited in the earliest Paleocene in the aftermath of the crisis and provides an excellent opportunity to investigate the development of non-tropical carbonate depositional systems throughout this time interval. Paper I aims, therefore, to better constrain the genesis and paleoenvironmental conditions of the deposition of Cerithium Limestone Mb. For this study, a detailed analysis of the microfacies and microfossil content of the Cerithium Limestone in the Rødvig section of Stevns Klint succession in Denmark was made. Analysis of thin sections and SEM has revealed four different microfacies with varying amounts of bioclasts in two distinct stratigraphic parts. The lowermost part consists of a thin layer of a bryozoan-rich packstone, which is interpreted as reworked material from the crests of the underlying Maastrichtian mounds. In the upper part, the microfacies appear randomly distributed due to significant bioturbation. The Cerithium Limestone is interpreted as formed in a mainly low-energy, open-marine, outer ramp-like sedimentary environment. Cyanobacteria are likely producers of the micrite of the Cerithium Limestone, as indicated by the dominance of very fine crystals (1– 4 μm).
Silicon plays a crucial role in global biogeochemical cycles as an essential nutrient for many marine organisms due to its link to the biological pump. Since the onset of the Phanerozoic, Si cycling has been strongly impacted by the biosilicification of sponges and radiolaria. The stable silicon isotope composition (δ30Si) of siliceous marine diatoms, radiolaria and sponges can be used as a proxy for past changes in dissolved Si (dSi) concentrations and Si cycling processes. Paper II discusses the changes in the marine Si cycle across the K-Pg boundary with the main focus on the early-mid Paleocene. The study presents the results of δ30Si analyses of sponge spicules and radiolaria from sediment samples from DSDP Site 208 (Leg 21) in the Southwest Pacific and suggests that the deep Southwest Pacific was already dSi depleted at the base of the Paleogene (66 Ma).
In recent years, the Si isotope fractionation during radiolaria biomineralization has been a topic of interest in the scientific community. In this context, the determination of order-specific and taxa-specific fractionation factors of radiolaria is crucial for the accurate interpretation of the δ30Si record. Paper III investigates the δ30Si signatures of radiolaria from the orders Nassellaria and Spumellaria. Our results show that it is crucial to consider changes in d30Si of individual taxa (especially from the same order) by the interpretation of the d30Si record in the past. We advise caution when interpreting d30Si signatures from the taxa Spongodiscoidea due to the potential influence by deeper dwelling species on their d30Si record.
This thesis fills some of the existing knowledge gaps regarding the development of the Si cycle across the K-Pg boundary and in the early-mid Paleocene and provides new insight into the genesis and paleoenvironmental conditions of the deposition of Cerithium Limestone.
Department/s
Publishing year
2024
Language
English
Publication/Series
Litholund theses
Volume
40
Full text
Document type
Dissertation
Publisher
Media-Tryck, Lund University, Sweden
Topic
- Geochemistry
Keywords
- K-Pg boundary
- Paleocene
- stable silicon isotopes
- sponge spicules
- radiolaria
- Cerithium Limestone
Status
Published
Supervisor
- Daniel Conley
- Sylvain Richoz
- Katharine Hendry
ISBN/ISSN/Other
- ISSN: 1651-6648
- ISBN: 978-91-87847-80-6
- ISBN: 978-91-87847-81-3
Defence date
5 April 2024
Defence time
13:15
Defence place
Pangea, Geocentrum II, Department of Geology, Sölvegatan 12.
Opponent
- Nicolas Thibault (Dr.)