Hoffman Paul F
School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8P 5C2, Canada.
Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138.
Proc Natl Acad Sci U S A. 2025 May 20;122(20):e2414059122. doi: 10.1073/pnas.2414059122. Epub 2025 May 5.
Geological observations informed by climate dynamics imply that the oceans were 99.9% covered by light-blocking ice shelves during two discrete, self-reversing Snowball Earth epochs spanning a combined 60 to 70 Myr of the Cryogenian Period (720 to 635 Ma). The timescale for initial ice advances across the tropical oceans is ~300 y in an ice-atmosphere-ocean general circulation model in Cryogenian paleogeography. Areas of optically thin oceanic ice are usually invoked to account for fossil marine phototrophs, including macroscopic multicellular eukaryotes, before and after each Snowball, but different taxa. Ecosystem relocation is a scenario that does not require thin marine ice. Assume that long before Cryogenian Snowballs, diverse supra- and periglacial biomes were established in polar-alpine regions. When the Snowball onsets occurred, those biomes migrated in step with their ice margins to the equatorial zone of net sublimation. There, they prospered and evolved, their habitat areas expanded, and the cruelty of winter reduced. Nutrients were supplied by dust (loess) derived from cozonal ablative lands where surface winds were strong. When each Snowball 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 while others restocked the oceans. This ecosystem relocation scenario makes testable predictions. The lineages required for post-Cryogenian biotic radiations should be present in modern polar-alpine biomes. Legacies of polar-alpine ancestry should be found in the genomes of living organisms. Examples of such tests are highlighted herein.
基于气候动力学的地质观测表明，在元古宙晚期（7.2亿至6.35亿年前）长达6000万至7000万年的两个离散且自我反转的“雪球地球”时期，海洋99.9%被遮光冰架覆盖。在元古宙古地理条件下的冰 - 大气 - 海洋环流模型中，热带海洋首次出现冰进的时间尺度约为300年。在每次“雪球”前后都存在光学薄的海洋冰区域，通常以此来解释包括宏观多细胞真核生物在内的化石海洋光合生物，但生物分类不同。生态系统迁移是一种不需要薄海冰的设想。假设在元古宙“雪球”事件之前很久，极地 - 高山地区就已形成了多样的超冰缘和冰缘生物群落。当“雪球”开始时，这些生物群落随着冰缘迁移到净升华的赤道区域。在那里，它们繁荣发展、进化，栖息地面积扩大，冬季的严酷程度降低。营养物质由来自共带剥蚀陆地的沙尘（黄土）提供，那里地表风力强劲。当每个“雪球”最终结束时，这些生物群落大多被以融水为主且迅速变暖的、营养丰富但生物匮乏的海洋覆盖。一些生物分类群回到山顶，而其他生物则重新在海洋中繁衍。这种生态系统迁移设想做出了可检验的预测。元古宙之后生物辐射所需的谱系应该存在于现代极地 - 高山生物群落中。在生物体的基因组中应该能找到极地 - 高山祖先的遗迹。本文重点介绍了此类测试的实例。
