Longer Snowball Earth deglaciation could have driven multiple phases of sea level rise and fall
Global palaeogeography at 600 Ma, during the Cryogenian, showing the distribution of Earth's continents and the maximum extent of major ice sheets prior to deglaciation. There is an additional focus on the palaeogeography and ice sheets of Africa's Kalahari Craton. Credit: Morris et al., 2024.
Snowball Earth defines periods of our planet's history when ice spanned the globe, even reaching the equator. The planetary-scale freeze is thought to have been driven by ice sheet expansion triggering a climatic tipping point that led to runaway ice-albedo feedbacks: the ice sheets reflected incoming solar radiation back to space, causing climate cooling and continued ice sheet formation.
The Cryogenian period (~720–635 million years ago, Ma) is defined by a pair of these Snowball Earth events; the longer glaciation (Sturtian) lasted for ~57 million years, followed by a brief time of interglacial melting, before a second shorter glaciation (Marinoan) lasting ~15 million years.
Exiting such an extended period of extreme conditions represented a major transition for Earth's climate, with a relatively rapid shift to a greenhouse climate, intense continental weathering and a general rise in global sea level as continental ice sheets melted. These had consequences for the functioning of terrestrial and marine realms, as well as the organisms inhabiting them.
The Cryogenian period is believed to have played a significant role in the emergence of complex, multicellular life, with animal and algal-based ecosystems beginning to appear once the ice sheets retreated.
New research, published in Earth and Planetary Science Letters, has focused on the marine impacts by determining the effect of Marinoan deglaciation ~635 Ma on global sea level.
In previous investigations of the rock record from the Naukluft Mountains of Namibia, researchers have identified two distinct intervals of water depth increase and decrease in rocks associated with the Marinoan deglaciation.
Dr. Freya Morris, of California Institute of Technology, and colleagues analyzed glaciogenic deposits and their overlying cap carbonates (primarily dolomite) to interpret the interplay of global sea level fluctuations with local effects of sedimentation, tectonic uplift or subsidence, and glacial isostatic adjustment. The latter references the uplift of land and crustal deformation in response to the loss of ice sheet mass loading as they melt.
"The deglaciation of the Marinoan Snowball Earth represents one of the most dramatic episodes of climate change in the history of the world," Dr. Morris says. "This modeling work explores the surprising ways in which the duration of the ice sheets melting can impact the resulting sea level changes along continental margins.
"The research was inspired by the fieldwork my colleagues and I conducted in the Naukluft Mountains of Namibia. There we interpreted a relatively complex pattern of water depth changes in the rock record associated with the Snowball Earth deglaciation and, as a result, we turned to sea level modeling to explore possible mechanisms and explanations for our geologic observations."
To consider possible explanations, the research team modeled relative sea level changes following continuous Marinoan deglaciation of different durations, finding a distinct pattern as melting progresses over longer timescales. Shorter deglaciation events lasting 2,000 years led solely to sea level rise, or one phase of sea level rise and fall. However, as melting occurred on timescales an order of magnitude higher (~10–30,000 years), two distinct phases of sea level rise and fall occurred.
Longer Snowball Earth deglaciation could have driven multiple phases of sea level rise and fall
Relative sea level, global mean sea level, direct gravitational attraction between ice sheets and the oceans, and crustal deformation with increasing distance from the continental interior of the Kalahari Craton. Credit: Morris et al., 2024.
Relative to the start of Snowball Earth deglaciation when the model used a global mean (eustatic) sea level of 800 m, global sea levels fell by 880 m over continental interiors where ice sheets previously prevailed, but rose by 800 m in the oceans at the farthest distance from ice sheet margins.
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Between these regions, the continental margin experienced the more complex scenario of multiple sea level rise and fall cycles, which may be comparable to the geologic observations made in the Naukluft Mountains succession where water depths are interpreted to fluctuate at the scale of tens of meters.
Dr. Morris acknowledges that this scale is "as precise as we can confidently be given the limitations of interpreting water depth changes in deformed rocks over half a billion years old."
This more unusual pattern may be explained by a balance between eustatic sea level rise and changing the influence of glacial isostatic adjustment, Dr. Morris explains.
"One of the primary components of glacial isostatic adjustment is crustal deformation. The mass of large ice sheets can significantly depress the underlying crust, while raising the crust surrounding the ice sheet into a 'peripheral bulge.' When the ice sheet melts, the formerly depressed crust that was under the ice sheet 'rebounds' upward, while the peripheral bulge subsides.
"In addition to this crustal deformation, the mass of ice sheets gravitationally attracts the oceans to the ice sheets. When the ice sheets melt, this gravitational attraction dissipates, resulting in a local recession of the oceans. The predicted relative sea level patterns result from the competing influences of the global mean sea level rise, crustal deformation and gravitational attraction across the deglaciation.
"The longer deglaciations (10–30,000 years) produce more complex relative sea level patterns than short deglaciations (2,000 years), because the rate of global mean seal level rise across the deglaciation is of similar magnitude to the impact on sea level by glacial isostatic adjustment. As a result, shifts in the balance between these forces can result in measurable fluctuations in sea level along continental margins."
Multiple viable depositional models have been proposed previously to account for the geologic interpretations made of the cap carbonate succession in the Naukluft Mountains of Namibia, but this work considers an additional explanation.
If the deglaciation of Snowball Earth occurred over a longer duration of ~10–30,000 years, it is possible that the sea level fluctuations recorded in the Naukluft Mountains (and potentially other cap carbonate successions around the world) may have been primarily driven by the competing balance of global mean sea level rise and glacial isostatic adjustment.
The intriguing possibilities explored by this work highlight the importance for future research into the Marinoan Snowball Earth deglaciation, with regard to both further geologic observations and climate modeling.
More information: Freya K. Morris et al, Melting the Marinoan Snowball Earth: The impact of deglaciation duration on the sea-level history of continental margins, Earth and Planetary Science Letters (2024). DOI: 10.1016/j.epsl.2024.119132.
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