15:45 〜 16:00
[MIS19-07] Neoproterosoic boron isotopes imply the variation in syn-glacial interaction between the ocean and continent
★Invited Papers
Neoproterozoic Earth experienced two Snowball Earth events, namely Sturtian and Marinoan glaciations. While these two events occurred within a relatively short period (<~ 100 Myr), their durations differed by a factor of 4 to 15; the Sturtian and Marinoan glaciations lasted 60 Myrs and 4 to 15 Myrs, respectively (e.g., Hoffman et al., 2017). The difference would result from the diversity in snowball events, such as varieties in threshold CO2 level for deglaciation, CO2 degassing rate, and/or CO2 uptake via syn-glacial weathering. Nonetheless, the actual cause is still uncertain.
The boron isotope ratio (δ11B) could be a key to understanding this difference, given its common usage as an indicator of paleo pH and, therefore, atmospheric CO2 level. Previous papers observed constant δ11B value after the Sturtian glaciation and negative excursion in δ11B after the Marinoan glaciation (e.g., Kasemann et al., 2005). This negative excursion in δ11B implied ocean acidification, likely resulting from the dissolution of massive CO2 into the ocean after deglaciation (e.g., Kasemann et al., 2005). However, gas exchange between the atmosphere and ocean during global glaciation could potentially acidify the ocean before the deglaciation (Le Hir et al., 2008). Moreover, the constant δ11B value after the Sturtian glaciation, implying stable ocean pH and moderate CO2 threshold level, appears inconsistent with the longer duration of the Sturtian glaciation.
This study aims to investigate the oceanic boron cycle associated with a snowball event to re-interpret the boron isotope data. We constructed a boron cycle model coupled with a carbon cycle model, simulating the change in the oceanic boron concentration, atmospheric CO2 level, surface temperature, and ocean pH during and after a global glaciation.
During a global glaciation, continental weathering would shrink due to the lack of liquid water on the continent (e.g., Kirschvink, 1992). Since continental weathering is a major source of oceanic boron (e.g., Lemarchand et al., 2002), this reduction decreases the reservoir size of oceanic boron. After deglaciation, the resumption of the weathering leads to an increase of the boron in the ocean. Additionally, given that known sinks of oceanic boron favor a lighter isotope (e.g., Lemarchand et al., 2002), δ11B tends to be heavier in the ocean than in the sources and sinks. Hence, the increase in the oceanic boron is associated with the dilution of heavier δ11B in the ocean by lighter δ11B from the source (i.e., continental weathering). Therefore, the perturbation by a snowball event would inherently cause a negative excursion in δ11B even without a change in ocean pH.
The negative excursion in δ11B due to the snowball perturbation depends on the decrease in oceanic boron under global glaciation. Consequently, the longer global glaciation (i.e., a prolonged reduction in continental weathering) leads to a more significant negative excursion in δ11B afterward. Conversely, active syn-glacial continental weathering mitigates the reduction in oceanic boron during a global glaciation, thus preventing a negative excursion in δ11B.
The negative excursion in δ11B resulting from the reduction in continental weathering aligns with the Marinoan δ11B data. On the other hand, the longer duration and constant δ11B value of the Sturtian snowball event might indicate active syn-glacial weathering during the glaciation. If variations existed in syn-glacial weathering between the Sturtian and Marinoan glaciations, it would also explain the difference in their durations. Thus, the Neoproterozoic boron data could imply the variation in the interaction between the continent and ocean through weathering under a globally ice-covered state.
The boron isotope ratio (δ11B) could be a key to understanding this difference, given its common usage as an indicator of paleo pH and, therefore, atmospheric CO2 level. Previous papers observed constant δ11B value after the Sturtian glaciation and negative excursion in δ11B after the Marinoan glaciation (e.g., Kasemann et al., 2005). This negative excursion in δ11B implied ocean acidification, likely resulting from the dissolution of massive CO2 into the ocean after deglaciation (e.g., Kasemann et al., 2005). However, gas exchange between the atmosphere and ocean during global glaciation could potentially acidify the ocean before the deglaciation (Le Hir et al., 2008). Moreover, the constant δ11B value after the Sturtian glaciation, implying stable ocean pH and moderate CO2 threshold level, appears inconsistent with the longer duration of the Sturtian glaciation.
This study aims to investigate the oceanic boron cycle associated with a snowball event to re-interpret the boron isotope data. We constructed a boron cycle model coupled with a carbon cycle model, simulating the change in the oceanic boron concentration, atmospheric CO2 level, surface temperature, and ocean pH during and after a global glaciation.
During a global glaciation, continental weathering would shrink due to the lack of liquid water on the continent (e.g., Kirschvink, 1992). Since continental weathering is a major source of oceanic boron (e.g., Lemarchand et al., 2002), this reduction decreases the reservoir size of oceanic boron. After deglaciation, the resumption of the weathering leads to an increase of the boron in the ocean. Additionally, given that known sinks of oceanic boron favor a lighter isotope (e.g., Lemarchand et al., 2002), δ11B tends to be heavier in the ocean than in the sources and sinks. Hence, the increase in the oceanic boron is associated with the dilution of heavier δ11B in the ocean by lighter δ11B from the source (i.e., continental weathering). Therefore, the perturbation by a snowball event would inherently cause a negative excursion in δ11B even without a change in ocean pH.
The negative excursion in δ11B due to the snowball perturbation depends on the decrease in oceanic boron under global glaciation. Consequently, the longer global glaciation (i.e., a prolonged reduction in continental weathering) leads to a more significant negative excursion in δ11B afterward. Conversely, active syn-glacial continental weathering mitigates the reduction in oceanic boron during a global glaciation, thus preventing a negative excursion in δ11B.
The negative excursion in δ11B resulting from the reduction in continental weathering aligns with the Marinoan δ11B data. On the other hand, the longer duration and constant δ11B value of the Sturtian snowball event might indicate active syn-glacial weathering during the glaciation. If variations existed in syn-glacial weathering between the Sturtian and Marinoan glaciations, it would also explain the difference in their durations. Thus, the Neoproterozoic boron data could imply the variation in the interaction between the continent and ocean through weathering under a globally ice-covered state.