Sea ice is a defining feature of the Southern Ocean, yet significant gaps remain in our understanding of how it forms, evolves and interacts with the broader climate system. My PhD research focused on the physical and biogeochemical properties of seasonal sea ice in the Antarctic Marginal Ice Zone (AMIZ), the region where the open ocean transitions into the ice-covered ocean. The AMIZ is therefore a dynamic boundary where new ice forms each winter and interacts with the ocean and atmosphere.
Traditionally, studies of Antarctic sea ice biogeochemistry have concentrated on summer conditions and older, consolidated pack ice. In contrast, winter measurements, particularly from the AMIZ, are scarce (Figure 1).
Figure 1. (a) Map of the Antarctic region showing the location of all sea ice cores analysed for biogeochemical properties, historic cores were compiled by Fripiat et al. (2017). (b) Zoom-in of my study region, with the cores collected over the course of my work highlighted to indicate how my samples fill a data gap in the Antarctic Marginal Ice Zone (AMIZ). Cores are categorised by season (colour) and ice type (symbol shape). The corresponding lines indicate the location of the AMIZ in winter (blue) and spring (purple). Note that the winter cores (blue symbols) from my work are the first samples collected in the AMIZ during winter; and my pancake ice cores (square symbol) and brash ice (circle symbol) are the first of their kind.
My thesis addresses that gap by presenting the first biogeochemical datasets for sea ice collected in the Atlantic AMIZ during winter and spring, including measurements from young pancake ice, consolidated first-year ice and brash ice (Figure 2). It also proposes a revised set of standard operating procedures for conducting interdisciplinary sampling in complex marginal ice conditions.
Figure 2. Locations and ice conditions for the stations presented in the thesis. The left panel shows the winter stations and right shows spring. These stations were sampled during the 2019 Southern oCean seAsonaL Experiment.
Measurements of sea-ice temperature, salinity, crystal structure, oxygen isotopes (δ18O), chlorophyll and nutrient concentrations were combined with model simulations to decipher the conditions under which the ice formed and grew, how these conditions influenced the subsequent biogeochemical environment and how the sea-ice properties evolved from winter to spring.
The findings highlight several key insights, mainly that sea ice growth in the AMIZ is not a simple, linear thickening process. Instead, storms and ocean turbulence drive repeated cycles of break-up, rafting and consolidation. These dynamic processes not only shape the physical structure of the ice but also influence its internal biogeochemical environment (Figure 3).
Figure 3. Schematic of a proposed rafting mechanism that results in the enhancement of nutrient and chlorophyll concentrations in first-year winter sea ice. This work is published in Audh et al. (2023) [https://doi.org/10.1029/2023JC019925].
Additionally, the results demonstrated that biological and chemical activity is already underway during winter, well before the more widely recognised spring bloom period. Nitrogen cycle processes, such as nitrate assimilation and nitrification, were observed in winter ice, suggesting that sea ice functions as an active biogeochemical reservoir throughout the growth season, even under limited light conditions.
These findings challenge long-standing assumptions about the seasonality of sea ice processes and underscore the need for integrated, multidisciplinary approaches to studying the AMIZ. Physical dynamics, biogeochemical cycling and ecosystem interactions are tightly interconnected, and existing models that treat these processes in isolation risk oversimplifying this complexity. A “whole-of-system” approach that links physics, chemistry, biology and modelling is essential for improving predictive capacity and for understanding the AMIZ in the context of a changing climate.
Overall, the work detailed in my thesis significantly advances the available data and knowledge base for the AMIZ, particularly related to the biogeochemistry of sea ice. By documenting the interplay between physical and biogeochemical processes, it lays a foundation for future research and highlights the importance of continued, collaborative efforts to better represent Antarctic sea ice in global climate models.
This research was supported by the National Research Foundation of South Africa through the South African National Antarctic Program (grant nos. 118745 and 129232). This work also received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 101003826 via project CRiceS (Climate-relevant interactions and feedbacks: the key role of sea ice and snow in the polar and global climate system) and the Whales and Climate Research Program (https://whalesandclimate.org). I also acknowledge the South African Department of Science, Technology and Innovation’s Biogeochemistry Research Infrastructure Platform and the South African Polar Research Infrastructure for research infrastructure support.
Dr. Riesna R. Audh graduated with her PhD thesis, titled Physical and biogeochemical properties of seasonal sea ice in the Atlantic sector of the Antarctic Marginal Ice Zone in 2025.
Riesna R. Audh completed her BSc and BSc (Hons) qualifications at the University of Cape Town (UCT). She began her MSc in 2018, which was upgraded to a PhD in 2019. Her PhD thesis, titled Physical and biogeochemical properties of seasonal sea ice in the Atlantic sector of the Antarctic Marginal Ice Zone, was supervised by Professor Marcello Vichi and Associate Professor Sarah Fawcett from the Oceanography Department at UCT. Riesna is the research coordinator for the South African Polar Research Infrastructure’s (SAPRI) Polar Lab Integrated Facility. SAPRI is hosted by the NRF-SAEON Egagasini Node in Cape Town.
This article was originally published on the South African Environmental Observation Network (NRF-SAEON) website authored by Dr. Riesna R. Audh from the South African Polar Research Infrastructure (SAPRI) (link).
