title: Modeling ocean temperature and ocean circulation in the past using Pa/Th
Creators: name: Scheen, Jeemijn
Publisher: name: Universität Bern
Year: 2025
Subjects: subject: 530 Physics
subject: 550 Earth sciences &amp; geology

description: This PhD thesis aims to enhance the understanding of past ocean circulation and temperature by using the Bern3D model, an ocean-atmosphere-tracer climate model of reduced complexity. Human-induced climate change affects ocean circulation, including the Atlantic Meridional Overturning Circulation (AMOC), which is crucial for European climate. While the AMOC is predicted to weaken under future climate change, the rate of this response is uncertain. A better understanding of the AMOC in the past will help to improve climate projections for the future.
In short, this thesis is about deep ocean temperature in the last millennium (Chapter 2) and the strength of ocean circulation in the last ice age (Chapters 3 and 4). The introduction (Chapter 1) lays the groundwork for the following chapters by summarizing background knowledge about past and present ocean circulation, the Bern3D model, and tracers in the ocean. In Chapter 2, historic deep ocean temperatures are simulated for the last millennium and their interaction with circulation changes is explored. Chapter 3 develops the interpretation of the protactinium-thorium ratio (Pa/Th) in the Atlantic Ocean, which is a useful tool to reconstruct the strength of past ocean circulation, particularly the AMOC. Chapter 4 examines model results for Pa/Th in the Pacific Ocean. Finally, Chapter 5 presents a brief outlook on possible directions for future research.
In more detail, Chapter 2 treats two periods of climate change in a simplified way: the Little Ice Age, which started in the medieval period (1200-1750 CE; see Sect. 1.3), and human-caused industrial warming (from 1750 CE onwards). We investigate howthese climate variations in the atmosphere lead to changes in temperature/heat in the ocean interior. This is done with simulations of the Bern3D model in its ocean-only setting (i.e., not coupled to an atmosphere), such that we can force the model at the boundary with given sea surface temperature variations for 1200-2000 CE.We address the following scientific questions: To which oceans and depths does the cold or heat go and how long does this take? How does a small change in ocean temperature influence the ocean currents? Does this change in ocean currents influence where the cold or heat travels? Is the deep ocean already warming up everywhere under anthropogenic climate change or are there locations that are not warming?
In Chapter 3, we go further back in time to the last ice age (ca. 115,000 to 12,000 years ago). We focus on the Last Glacial Maximum (ca. 23,000 to 18,000 years ago; see Sect. 1.3), when the Northern Hemispheric ice sheets came furthest south, covering entire Canada and reaching northern Germany. Here we focus on the following question: How was the ocean circulation in this cold climate? It was certainly different from today given the largely changed conditions: colder temperatures, large amounts of sea ice in regions where nowadays surface water sinks to the bottom, and different atmospheric circulation patterns because of large ‘mountain ranges’ of ice. This is an intricate question, so we narrow it down to the large-scale 3-dimensional ocean circulation in the Atlantic, the AMOC, which is important for the climate in Europe (see Sect. 1.2). The strength of the AMOC can be reconstructed with the protactinium-thorium (Pa-Th) proxy (see Sect. 1.4). This reconstruction method uses the rare metals protactinium (Pa) and thorium (Th), which occur naturally in seawater in dissolved form. Both of them bind to particles, which are formed in the surface ocean and sink to the ocean floor. Since thorium is more particle reactive, the majority binds to these particles and sinks along to the sediment. On the other hand, the amount of protactinium that reaches the sediment depends on the strength of the ocean currents. The stronger the meridional (north-south) currents are, the more protactinium they transport away with them. Consequently, the Pa/Th ratio found in ocean sediment can be used as indicator of the AMOC strength at the time in the past when the sediment was formed. Here, Th is used to normalize the signal such that it does not depend on the amount of particles. In this thesis, we use modelling to improve the interpretation of Pa/Th sediment measurements. We simulate the cycle of Pa and Th with the Bern3D model for different AMOC strengths and we compare our model results with sediment core data. We address a number of questions: Can we learn from the model about Pa/Th in regions of the Atlantic Ocean where no sediment cores are available? Is Pa/Th expected to be particularly sensitive to AMOC strength in certain regions or depths? Can we explain the unexpected measurements in parts of the northern North Atlantic: why is the Pa/Th signal there opposite to usual? Can we contribute to quantifying the strength of the AMOC at the Last Glacial Maximum?
In Chapter 4, we explore how the Pa/Th proxy could be used in the Pacific Ocean with the Bern3D model. We analyze the simulations from Chapter 3 now for the Pacific Ocean, in addition to a few new simulations. It is not clear whether Pa/Th can be used to reconstruct circulation strength here, because the Pacific has a much weaker Meridional Overturning Circulation (PMOC) than the Atlantic (AMOC). Instead, in the Pacific Ocean, Pa/Th is often used to reconstruct the amount of particles in the surface ocean in the past. Relevant questions are: Is it also possible to find this particle signal in the Pa/Th ratios simulated with the Bern3D model? How much of the particle signal depends on regional variations in particle concentrations, or in other words, how would Pa/Th change if the particle concentrations would simply be equal throughout the Pacific surface ocean? Regarding circulation strength, can we still expect to find a detectable PMOC signal in Pacific Pa/Th, or not?
The thesis concludes with an Outlook (Chapter 5), in which we discuss interesting directions for possible future work that follows up on Chapters 2, 3 and 4.