Implementing a new method to measure δ³⁴SSO₄ in ice cores to assess sulfate sources in West Antarctica
Sulfate sources in Antarctica can reveal information about the interconnection of climate systems and past climate events. The major sulfate sources in Antarctica are sea salt, biogenic activity, and volcanic activity, though volcanic events have a limited ~1–2 year deposition period. Each source can be identified by its unique sulfur isotopic composition of sulfate (δ³⁴SSO₄). However, δ³⁴SSO₄ measurements in Antarctic ice cores are scarce and have poor temporal resolution due to the large sample volume required for isotopic analysis.
For this thesis, I established a new method to measure δ³⁴SSO₄ in ice cores via multicollector inductively coupled mass spectrometry (MC-ICP-MS) at the University of Cambridge. This technique requires < 30 nmol of sulfur compared to the ~1 µmol previously required for analysis with gas source mass spectrometry (GS-MS). Using this method, I produced the first seasonal record of δ³⁴SSO₄ in an ice core to reconstruct sub-annual changes in sulfate sources at Dyer Plateau in West Antarctica. I also confirmed the δ³⁴SSO₄ signature of sea salt from the sea ice surface and further constrained the sulfur isotopic composition of biogenic sulfate. However, I was unable to reconstruct short-term changes in sea ice extent, which was the original aim of this research, because of the presence of an additional unknown sulfate source. This source has a low δ³⁴SSO₄ signature and increased winter deposition, suggesting that it is likely of volcanic and/or stratospheric origin.
I then measured δ³⁴SSO₄ in two additional West Antarctic ice cores to explore potential spatial variability in the third sulfate source. Sherman Island and Skytrain Ice Rise ice cores both showed the same unknown sulfate source with a low sulfur isotopic composition that I had found in the Dyer Plateau ice core. A similar source has been reported for numerous ice cores in East and West Antarctica, but the source was ~3x greater in the West Antarctica study. My results were similar to the East Antarctica findings, suggesting that there is no clear distinction in the third sulfate source between East and West Antarctica, but instead significant variability on a smaller spatial scale.
Lastly, I considered long-term changes in sulfate sources in Antarctica. I found a significant increase in sea salt and biogenic sulfate emissions at Skytrain Ice Rise between the early and late Holocene, supporting the proposed retreat of the Ronne Ice Shelf ~8,000 years ago. I also measured δ³⁴SSO₄ in glacial and Holocene samples to explore the possibility of a large terrestrial sulfate source during the Last Glacial Maximum. Glacial δ³⁴SSO₄ values were 3–5‰ lower than in Holocene samples, which could be explained by a sulfate-rich terrestrial dust source that was ~50% of total sulfate. However, the origin of such a source with the required low δ³⁴SSO₄ signature is unclear.
This thesis highlights the importance of additional δ³⁴SSO₄ measurements in Antarctic ice cores. My results show that sulfur isotope ratios can be used to reconstruct past climate events, but background sulfate sources must be better characterized before we can use δ³⁴SSO₄ to reconstruct short-term climate processes.