20 / 2025-03-25 03:59:04

Ecosystem relocation on Snowball Earth: polar-alpine ancestry of the extant surface biosphere?

Snowball Earth, polar-alpine microbial ecology, green plant evolution, Prochlorococcus gene reduction, ancestral-state reconstruction, phytoplankton evolution

Theme 3: Earth-Life Co-evolution Through Time > Session 3b: Evolution of the Earth-Life System During the Neoproterozoic-Paleozoic Transition

Geological observations informed by numerical climate models imply that the oceans were 99.9% covered by light-blocking ice shelves (‘sea glaciers’) during two discrete, self-reversing, Snowball Earth epochs spanning a combined 60−70 Myr of the Cryogenian Period (720−635 Ma). Optically-thin marine ice, occupying 0.1% of global ocean area in equatorial restricted embayments surrounded by lowlands, has been invoked to account for the occurrence of fossil marine phototrophs, including macroscopic multicellular eukaryotes (of different taxa) before and after each Snowball Earth epoch. However, a Snowball ocean would be prohibitively cold, 267−269.3K under thin ice, due to 100−60 ppt salinity. Ecosystem relocation is a scenario not dependent on thin marine ice. Assume that long before the Cryogenian Period, diverse supra- and periglacial biomes (e.g., cryoconite holes and ponds on ablative glacial ice; perennially ice-covered, salt-stratified, dry-valley lakes) were established in polar−alpine regions. At Cryogenian glacial onsets, those biomes would have migrated in step with their ice margins to the equatorial zone of net sublimation on the new Snowball Earth. There they prospered and evolved, their habitat areas vastly expanded and the cruelty of winter diminished under equatorial seasonality. Nutrients were supplied by dust (loess) derived from cozonal ablative lands, where surface winds were strong. When each Snowball Earth epoch finally ended, those biomes were mostly inundated by the meltwater-dominated and rapidly-warming lid of a nutrient-rich but depauperate ocean. Some taxa returned to the mountaintops and polar regions, while others restocked the oceans. This scenario is consistent with (I) the freshwater ancestry of modern marine primary producers inferred from molecular phylogenetic ancestral state reconstruction (1−2), (II) the deep phylogenetic split of all green plants between Streptophyta (e.g., evolved from freshwater cryoconite biomes) and Chlorophyta (e.g., from saline deep-water biomes of dry-valley ice-covered lakes), viewed as selective survivorship consilient with their contrasting salinity preferences and photo-respiration mechanisms (3), and (III) dated Cryogenian gene reduction in the picocyanobacterium Prochlorococcus, including loss of a kai gene that regulates circadian rhythm where the diurnal cycle changes seasonally, thereby restricting its meridional range to the tropical ocean (4). Ecosystem relocation, in altitude and latitude, is a natural response to global climatic change. Extreme relocation implies that the extant surface biosphere evolved from a polar−alpine subset of pre-Cryogenian biotic diversity. Accordingly, most if not all pre-Cryogenian marine fossils represent stem groups, although many have crown sister groups depending on the pre-Cryogenian history of polar−alpine ecosystem colonization.

Geological observations informed by numerical climate models imply that the oceans were 99.9% covered by light-blocking ice shelves (‘sea glaciers’) during two discrete, self-reversing, Snowball Earth epochs spanning a combined 60−70 Myr of the Cryogenian Period (720−635 Ma). Optically-thin marine ice, occupying 0.1% of global ocean area in equatorial restricted embayments surrounded by lowlands, has been invoked to account for the occurrence of fossil marine phototrophs, including macroscopic multicellular eukaryotes (of different taxa) before and after each Snowball Earth epoch. However, a Snowball ocean would be prohibitively cold, 267−269.3K under thin ice, due to 100−60 ppt salinity. Ecosystem relocation is a scenario not dependent on thin marine ice. Assume that long before the Cryogenian Period, diverse supra- and periglacial biomes (e.g., cryoconite holes and ponds on ablative glacial ice; perennially ice-covered, salt-stratified, dry-valley lakes) were established in polar−alpine regions. At Cryogenian glacial onsets, those biomes would have migrated in step with their ice margins to the equatorial zone of net sublimation on the new Snowball Earth. There they prospered and evolved, their habitat areas vastly expanded and the cruelty of winter diminished under equatorial seasonality. Nutrients were supplied by dust (loess) derived from cozonal ablative lands, where surface winds were strong. When each Snowball Earth epoch finally ended, those biomes were mostly inundated by the meltwater-dominated and rapidly-warming lid of a nutrient-rich but depauperate ocean. Some taxa returned to the mountaintops and polar regions, while others restocked the oceans. This scenario is consistent with (I) the freshwater ancestry of modern marine primary producers inferred from molecular phylogenetic ancestral state reconstruction (1−2), (II) the deep phylogenetic split of all green plants between Streptophyta (e.g., evolved from freshwater cryoconite biomes) and Chlorophyta (e.g., from saline deep-water biomes of dry-valley ice-covered lakes), viewed as selective survivorship consilient with their contrasting salinity preferences and photo-respiration mechanisms (3), and (III) dated Cryogenian gene reduction in the picocyanobacterium Prochlorococcus, including loss of a kai gene that regulates circadian rhythm where the diurnal cycle changes seasonally, thereby restricting its meridional range to the tropical ocean (4). Ecosystem relocation, in altitude and latitude, is a natural response to global climatic change. Extreme relocation implies that the extant surface biosphere evolved from a polar−alpine subset of pre-Cryogenian biotic diversity. Accordingly, most if not all pre-Cryogenian marine fossils represent stem groups, although many have crown sister groups depending on the pre-Cryogenian history of polar−alpine ecosystem colonization.