IAP-24-092

Quantifying variations in Southern Ocean primary productivity over the last 2000 years from Sulfur Isotopes in Antarctic Ice cores

Biological and physical processes in the Southern Ocean play a fundamental role in the global carbon cycle. The waters upwelled around Antarctica are rich in carbon and nutrients, and in the pre-industrial it is generally thought that this region was a source of CO2 to the atmosphere (e.g. Sigman et al., 2010). Today, in part due to increased atmospheric CO2 levels, this region has been generally acting as a sink for CO2. However, there is a delicate and changing balance between source and sink in the Southern Ocean, and this balance can be influenced by changes in biological productivity or the impact of sea ice influencing both the amount of air-sea gas exchange and the large-scale circulation pattern from buoyancy forcing (e.g. Takahashi et al., 2012; Gruber et al., 2019). Unfortunately the lack of prolonged observational data in this region means we have a limited understanding of how these crucial processes can change and their patterns of variability on longer than decadal timescales (Fay et al. 2014; McKinley et al. 2016, 2017). This PhD project will fill a crucial part of that knowledge gap by reconstructing changes in biological productivity in the Antarctic zone of the Southern Ocean over centennial to millennial timescales using sulfur isotopes in ice cores. Sulfur isotopes in ice cores reflect the relative proportions of different sources of sulfate to the ice sheet, namely sulfate oxidized from marine biological production of dimethysulfide, oxidized volcanic sulfur gases, and sulfate from sea salt and terrestrial sources. These sources can be disentangled because of their distinct isotopic signatures, and thus we can quantify the amount of biologically produced sulfate in Antarctic ice (e.g. Jongebloed et al., 2023).
This PhD project has two major aims: (1) calibration of the sulfur isotope proxy from snow and firn samples over recent time periods where satellite observations of ocean productivity and volcanic gas emissions are available and (2) reconstruction of centennial to millennial changes in biological productivity over the last 2000 years.

This project will take advantage of recent advances in analytical capabilities to measure sulfur isotopes in high resolution (sub-seasonal) over recent time periods in Antarctic ice cores (e.g. Burke et al., 2019, 2023). Using multi-collector inductively coupled plasma mass spectrometry at the St Andrews Isotope Geochemistry (STAiG) labs, we can now measure sulfur isotopes on samples with as little as 2 nmol of sulfur. This reduction in sample size requirements means that for the first time we can resolve changes in sulfur sources to ice sheets on subseasonal timescales. The project will leverage a wealth of existing Antarctic ice cores spanning the Common Era, allowing for considerations of spatial variability in sulfur transport and depsosition as well.
The student will also benefit from a research placement at BAS to help with the continuous flow analysis of new ice cores drilled in upcoming seasons. Although the project is not reliant on new ice core samples, there will be an exciting opportunity to work with a new ice core , the REWIND core, to be drilled on the Antarctic Peninsula during 2024-2026. Additionally, the student will actively participate in large-scale international ice core drilling initiatives, including the EU-funded Beyond EPICA project, which seeks to extract the oldest ice core ever drilled.

Year 1

Training in ice core analysis and sample preparation, including working in a -25°C cold laboratory and a class-100 cleanroom. The first year will incorporate instrument specific training on an ion chromatograph to measure sulfate concentration, the prepFAST automated column chemistry instrument to isolate the sulfate fraction in ice, and a multi-collector inductively coupled mass spectrometer (MC-ICP-MS) to measure sulfur isotopes. By the end of year 1, the student will be a competent MC-ICP-MS user capable of measuring high resolution sulfur isotopes in ice cores. The student will also start analysis of recent Antarctic snow and firn samples from a suite of sites on the Antarctic peninsula to calibrate the sulfur isotope proxy with satellite observations.

Year 2

During year 2, the student will continue calibrating the sulfur isotope proxy against satellite data and will investigate the spatial variability of sulfur transport and deposition. The student will be encouraged to write up the calibration and validation study during this time. The student will also have the opportunity to support the continuous flow analysis (CFA) on the REWIND ice core (during year 2 & 3) helping with continuous melting and analysis of ice, data quality control and interpretation of records. In the event of unforeseen delays to the REWIND drilling, the student will have the option to work on one of the many existing Antarctic ice cores in the BAS archive for their research.

Year 3

Starting in Year 2 and finishing in Year 3, the student will generate longer term records of sulfur isotopes. High priority time intervals of interest may be the last 200 years, or the Little Ice Age, though exact intervals of study can in part be guided by the interests of the student.

Year 3.5

The final 6-months will be dedicated to summarising the results and writing up the final thesis (or publications).

In-house training for ice core specific sampling and analysis.
Instrument specific training for MC-ICP-MS, IC, and prepFAST
Training in data processing
Training in Matlab/Python to support data processing
Opportunities to participate in international ice core analysis campaigns, including the Beyond EPICA Oldest Ice project.
Opportunities to undertake lab visits and exchanges with UK and international collaborators.