Boron isotopes indicate a possibility of subglacial geochemical cycles

Snowball events are one of the most drastic episodes of climate change in Earth’s history. Its impact is considered to propagate every aspect of the planet, from atmospheric and oceanic compositions to biological evolution. Among the known three snowball events, the second (i.e., Sturtian) and third (i.e., Marinoan) glaciations occurred between 720 Myrs ago and 635 Myrs ago. Despite the relatively short time difference in these events, those durations differed by a factor of four to fifteen. This difference might indicate a diversity of various snowball events, but we still don’t know what caused it.

Recent studies revealed that boron isotopes show different behavior between the Sturtian and Marinoan events (e.g., Kasemann et al., 2010). After the Sturtian glaciation, the boron isotope ratio (δ¹¹B) was almost constant, while after the Marinoan glaciation, δ¹¹B had a negative excursion. δ¹¹B is often used as a paleo pH index. Hence, the δ¹¹B negative excursion after the Marinoan glaciation was considered to imply a temporal decrease in ocean pH owing to a sudden dissolution of atmospheric CO₂ into the ocean after deglaciation of global ice (e.g., Kasemann et al., 2010). However, the atmosphere and the ocean would be in an equilibrium state in terms of gas exchange (e.g., Le Hir, 2008), and therefore, the scenario of sudden CO₂ dissolution after deglaciation is debatable.

In this study, we constructed a model for the boron cycle considering the modern boron cycle. We simulated the evolution of the oceanic boron reservoir and its boron isotope ratios during and after a global glaciation.

The model reproduced a negative excursion in δ¹¹B after deglaciation. This results from two assumptions. The first assumption is that continental weathering, which is a major source for oceanic boron, would cease under a global glaciation. The second assumption is that sinks of oceanic boron favor light boron (i.e., ¹⁰B) compared to heavy boron (i.e., ¹¹B), causing δ¹¹B to be larger in the ocean than in both sources and sinks. The cessation of continental weathering reduces the boron reservoir size during a global glaciation. The deglaciation resumes continental weathering, introducing light boron into the ocean. A heavy B in the ocean is diluted by light boron in the source (continental weathering), leading to a temporal decrease in δ¹¹B.

Thus, the negative excursion of δ¹¹B can be explained by a cessation of continental weathering during a snowball event. On the other hand, if continental weathering could be active under the glacier, the negative excursion of δ¹¹B could be suppressed. As introduced above, δ¹¹B was almost constant after the Sturtian glaciation, while δ¹¹B shows a negative excursion after the Marinoan glaciation. This difference in δ¹¹B behavior might be explained by the difference in the boron cycle caused by the activity of syn-glacial continental weathering. Furthermore, as the continental weathering is an important sink of atmospheric CO₂, the potential difference in syn-glacial weathering might contribute to the known difference in the duration of the Neoproterozoic snowball events.