Clumped isotope determination of deglacial changes in Atlantic Water (AW) temperature entering the Arctic

Project description
There is tantalizing evidence that the deep Arctic Ocean may have warmed during the last glaciation (Cronin et al., 2012) despite the fact that global mean surface temperature and global ocean temperature (Bereiter et al., 2018) declined significantly. Indeed, the northern North Atlantic, which forms a tightly coupled circulation system with the Arctic (the Arctic Mediterranean circulation), was one of the regions that cooled most during the last glaciation (Tierney et al., 2020). The cause of Arctic Warming in the face of large scale cooling and cryospheric expansion remains a paradox. Deepening of the Arctic halocline, or a reduction in the contribution of cold shelf water to the interior Arctic due to lowered sea level have been proposed as potential explanations. However, the role of incoming Atlantic Water, which plays a critical role in feeding the interior Arctic today, has not been considered.

Emerging clumped isotope results from the Nansen Legacy project suggest that near surface waters along the flow path of Atlantic Water entering the Arctic may have increased in temperature prior to the Holocene. However, it is not clear to what extent these waters represent the temperature of Atlantic Water more generally. If correct, the observation that Arctic feed waters warmed may not only resolve the paradox of why the Arctic warmed in the face of global cooling but also challenge a number of prevailing paradigms about how sea ice, heat transport, and deep ocean circulation are related.

The working Hypothesis: How does deglacial changes in Atlantic Water temperature entering the Arctic modulate deep Arctic properties and Atlantic Overturning circulation.

To test this hypothesis the project will utilize sediments from the Nansen Legacy project (e.g. Stations 14 and 15) to reconstruct past changes in the bottom water temperatures at the lower boundary of Atlantic Water entering the Arctic. The student will constrain past changes in bottom water temperature using clumped isotope thermometry on enthic foraminifera. The work involves lab work at EARTHLAB, microscope work, and sample preparation and analysis in FARLAB at UiB.

References:
Bereiter, B., Shackleton, S., Baggenstos, D. et al. Mean global ocean temperatures during the last glacial transition. Nature 553, 39–44 (2018). https://doi.org/10.1038/nature25152
Cronin, T., Dwyer, G., Farmer, J. et al. Deep Arctic Ocean warming during the last glacial cycle. Nature Geosci 5, 631–634 (2012). https://doi.org/10.1038/ngeo1557
Tierney, J.E., Zhu, J., King, J. et al. Glacial cooling and climate sensitivity revisited. Nature 584, 569–573 (2020). https://doi.org/10.1038/s41586-020-2617-x