230Th and 231Pa – tracers for particle fluxes and deep water circulation in the Central Arctic Ocean

The currently ongoing climate change can be ascribed without exaggerating as one the most important topics and threats for humanity. Understanding its mechanisms and consequences is an important step to target and manage arising problems, which accompany a rapidly warming climate. The oceans play a crucial role in the Earth’s climate controlling and response mechanism. Its physical and biological carbon pump systems are crucial regulators for the atmospheric CO2 content. Biological primary production is an important part of the Oceans CO2 uptake capability. Some trace elements are important (micro-) nutrients in oceanic primary production. Therefore it is important to investigate and understand the reaction of those elements to changing environmental conditions. Particle fluxes and ocean circulation contribute to their distribution in the water column. 230Th and 231Pa are suitable tracers for both particle fluxes and deep water circulation. Their well-known sources and production ratio, as well as their fractionation by particle fluxes and deep water circulation, enables their use as tracers. Their water column distribution serves as an indicator for recent environmental changes, while their sedimentary 231Pa/230Th activity ratio is used as a paleoceanographic tool. Therefore 230Th and 231Pa are standard parameters of GEOTRACES, an international programme with the goal to improve understanding of the cycling of trace elements and their isotopes in the Ocean. Different areas of the World’s Ocean react in different velocities and intensities to climate change. The Arctic Ocean is the most sensitive one to climate change. Climate change related consequences are already visible, e.g. the retreat and thinning of sea ice. Other consequences, like increasing particle fluxes and changing particle composition, as well as potentially changing circulation patterns and ventilation times are less obvious. It is expected that climate change will cause significant changes on the Arctic Oceans’ primary production and particle input. The consequences of these changes are not well understood and known. Therefore it is important to investigate changes in particle fluxes and composition, already in an initial stage of these changes. 230Th and 231Pa are valuable tools to gain insights into changes, which will potentially influence the global climate in the near future. In order to derive information about trace element cycling from 230Th and 231Pa in a changing Arctic Ocean, it is therefore crucial to investigate and understand the processes that control the distribution and concentrations of 230Th and 231Pa in the Arctic Ocean. To achieve this, a time series of this tracer pair, consisting of data from 1991, 2007 and 2015, was created to investigate the temporal development of 230Th and 231Pa over the past three decades. This new time series revealed quite variable 230Th and 231Pa inventories, indicating changing removal processes, caused by changing environmental conditions. This thesis consists of three first author manuscripts that are either published or in preparation for submission to international peer-reviewed journals. Additionally, two co-author manuscripts, published in international peer-reviewed journals, are part of this thesis. This section assigns the role of each manuscript, presented in this thesis, in the context of the general introduction. Changes in scavenging behaviour of 230Th need not necessarily have to be related to a changing climate. Hydrothermal activity and submarine volcanic eruptions at the ultra-slow spreading Gakkel Ridge caused a significant reduction of dissolved 230Th in only eight years in the deep Nansen Basin, contributing to sporadically increased removal and sedimentation rates of 230Th. The role of hydrothermal activity in the variation of scavenging behaviour of 230Th in the Eurasian Basin is described in CHAPTER 2 (Valk et al., 2018). Changing environmental conditions caused a significant decrease of dissolved 230Th concentrations in the entire Eurasian Basin between 2007 and 2015. Those changes include elevated particle fluxes at the shelves and margins of the deep basins, specifically the Barents Sea shelf and the Nansen Basin margin. Those increased particle fluxes caused increased scavenging removal of 230Th and to a minor degree of 231Pa, indicating an increasing sink for particle reactive trace elements. Increased scavenging removal of 230Th at the Barents Sea shelf and at the margins of the Nansen Basin caused a drastic decrease of dissolved 230Th in the central Amundsen Basin (CHAPTER 3, Valk et al., 2020). This highlights the increasing importance of shelf-basin interactions in the Arctic Ocean, due to rapidly increasing particle fluxes at the shelves and margins. Even before particle fluxes within the central basins increase, climate change already causes notable changes in trace element distributions and probably in their export to the North Atlantic. This is important for the nutrient availability in the North Atlantic as well as the paleoceanographic application of the 231Pa/230Th sedimentary activity ratio. Distribution and concentrations of dissolved 230Th and 231Pa can change significantly within less than ten years in the central Arctic Ocean. In CHAPTER 4 (Valk et al., in preparation) new budgets for dissolved 230Th and 231Pa, based on a water column data from 1991 over 2007 to 2015, as well as box models, are presented to illustrate scavenging removal and sedimentation patterns for these tracers. The model results are discussed in the context of boundary scavenging of 230Th and 231Pa in the Eurasian Basin and their export to the GIN Seas (Greenland, Iceland and Norwegian Seas). Consequences of the identified removal processes of 230Th and 231Pa from CHAPTER 1 and CHAPTER 2 on the sedimentary 231Pa/230Th activity ratios in the Eurasian Basin, its margins and the GIN Seas are discussed in the context of the paleoceanographic use of these tracers. It is an open question whether 230Th and 231Pa are subject to boundary scavenging in the Arctic Ocean. The study presented in CHAPTER 5 (Gdaniec et al., 2020) investigates the influence of boundary scavenging and shelf-basin interactions on the observed distribution of 230Th and 231Pa in the Arctic Ocean. A modelling approach adapted from Roy-Barman (2009) is used to constrain the scavenging behaviour of 230Th and 231Pa between the Arctic margin and the inner ocean. This study links very well to CHAPTER 3 and CHAPTER 4, giving detailed insight in particulate and dissolved radionuclide data from 2015. The study presented in CHAPTER 6 (Grenier et al., 2019) investigates the temporal 230Th and 231Pa developments in the Amerasian Basin of the Arctic Ocean, based on 230Th and 231Pa time series and therefore links well to CHAPTER 2, CHAPTER 3 and CHAPTER 4 which focus on the Eurasian Basin. The Amerasian Basin time series reveal a large scale decrease in dissolved 230Th and 231Pa concentrations, indicating intensification of scavenging removal, especially in coastal areas. This study illustrates how dissolved 230Th and 231Pa combined with εNd, can give insights into changes in particle fluxes, as well as into the evolution of ocean circulation and mixing. Thus the research presented in this thesis contributes to the understanding of trace element cycling and the tracer application of 230Th and 231Pa in the Arctic Ocean under changing environmental conditions