Melting ice sheets & the mystery of rapid historical sea level rise

This blog was written for us by Louise Guillaume, a first class MSci Geology graduate from Imperial College London. Louise has particular interests in volcanology, geochemistry, science communication and the interaction of social and physical sciences. In this piece she describes her recent research on ice-sheet dynamics and considers what it can tell us about sea level rise in the geological past.

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When ice-sheets separate…

Towards the end of the Last Glacial Period, 14,650 years ago, one of the most rapid sea-level rises of recent geological time occurred. This event, known to scientists as Meltwater Pulse 1a (MWP-1a), produced a global sea-level rise of around 16 meters in just 350 years. But, where most of this meltwater came from is still a matter of dispute.

One suggestion is that the separation of the Laurentide and Cordilleran ice sheets of North America caused the generation of meltwater to accelerate between the ice masses (Gregoire et al., 2012). Until know, it has been hard to constrain the timing of this separation, and therefore difficult know whether it coincided with the meltwater pulse.

Whilst the timing remains poorly understood, the geographical location of the separation is well known. Our work used cosmogenic isotope exposure age dating to determine points of separation along the suture line between ice sheets using samples from glacial erratic boulders, in what is today Western Canada. From this work, we were able to infer whether the timing of the suture was coincidental with the Meltwater Pulse 1a.

Dating ice sheet separation with cosmogenic isotopes

Cosmic rays enter the Earth’s atmosphere from space. They interact with atoms in the atmosphere and in rocks on the Earth’s surface and cause spallation (the break-up of atomic nuclei into new elements). One rare isotope that is made in this way is beryllium-10, which originates from the spallation of oxygen or nitrogen. The longer a surface is exposed to the atmosphere, the more ¹⁰Be is made from these reactions.

Cosmogenic isotopes act as a clock, recording how long a surface has been exposed on Earth’s surface (or unburied by other surface material). By counting the number of ¹⁰Be atoms trapped in quartz crystals in glacial rocks, we can calculate how long it has been since they were released from the ice that initally shielded them. To do this calculation, corrections must be made based on the latitude and elevation of each sample, as well as any local shielding the boulder might have had, such as vegetation cover.

This dating method relies on the assumption that rocks on the surface are exposed to consistent levels of cosmic rays from the atmosphere. Rocks can inherit more ¹⁰Be than expected during multiple or previous periods of exposure which can result in an older estimation of age. The opposite can be true if they are shielded by post-glacial features such as lakes or till which reduced exposure to cosmic rays and diminish the estimation of age. To reduce uncertainties like this, several samples in this study were collected at each locality and an average exposure age calculated after removing outliers.

Rock samples collected in the field were taken to a lab where we isolated the quartz and from this extracted Beryllium. A particularly sensitive mass spectrometer (Accelerator Mass Spectrometer) can count the ratio of ¹⁰Be to the common ⁹Be isotope, which can be then translated to an exposure age.

Geological and Anthropological Implications

Our results show that separation occurred from 115.4 ka -13.8 ka. This timing is coincident with, but extends beyond the observed period of sea-level rise already well known. Thus, the mystery of MWP-1a cannot be considered fully solved. It is likely that there were several sources for this notable meltwater signal, with the North American ice sheet separation making up just part of the contribution.

Another interesting consequence of the timing is the implication for the arrival of early humans onto the North American continent. The use of an ice-free corridor between the Laurentide and Cordilleran ice masses in northernmost America has been suggested as a viable route for this first migration. However, archaeological evidence has shown that humans were present in North America at the Cooper’s Ferry site as early as 16 ka (Davis et al., 2019).  Our dating of the final separation suggests these first humans could not have arrived via such a corridor, as it would still be inaccessible for another two thousand years. This suggests early migration most likely happened by other routes, such as via the Pacific coast.

References

Davis, L. G., Madsen, D. B., Becerra-Valdivia, L., Higham, T., Sisson, D. A., Skinner, S. M., Stueber, D., Nyers, A. J., Keen-Zebert, A., Neudorf, C., Cheyney, M., Izuho, M., Iizuka, F., Burns, S. R., Epps, C. W., Willis, S. C., and Buvit, I., 2019, Late Upper Paleolithic occupation at Cooper’s Ferry, Idaho, USA, ~16,000 years ago: Science, v. 365, no. 6456, p. 891-897.

Dyke, A. S., 2004, An outline of North American deglaciation with emphasis on central and northern Canada: Quaternary Glaciations – Extent and Chronology, v. 2, p. 373-424.

Gregoire, L. J., Payne, A. J., and Valdes, P. J., 2012, Deglacial rapid sea level rises caused by ice-sheet saddle collapses: Nature, v. 487, no. 7406, p. 219-222.

von Blanckenburg, F., and Willenbring, J., 2014, Cosmogenic Nuclides: Dates and Rates of Earth-Surface Change: Elements,v. 10 , no. 5, p. 342.

This research was conducted by Louise Guillaume, Imperial College London, in collaboration with Dr. Dylan Rood, Dr. Anders Carlson, and Dr. Alberto Reyes.