Department of Earth and Planetary Sciences/GEOTOP, McGill University, Montréal, Québec, Canada.
Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, Ontario, Canada.
Geobiology. 2024 May-Jun;22(3):e12598. doi: 10.1111/gbi.12598.
Tonian (ca. 1000-720 Ma) marine environments are hypothesised to have experienced major redox changes coinciding with the evolution and diversification of multicellular eukaryotes. In particular, the earliest Tonian stratigraphic record features the colonisation of benthic habitats by multicellular macroscopic algae, which would have been powerful ecosystem engineers that contributed to the oxygenation of the oceans and the reorganisation of biogeochemical cycles. However, the paleoredox context of this expansion of macroalgal habitats in Tonian nearshore marine environments remains uncertain due to limited well-preserved fossils and stratigraphy. As such, the interdependent relationship between early complex life and ocean redox state is unclear. An assemblage of macrofossils including the chlorophyte macroalga Archaeochaeta guncho was recently discovered in the lower Mackenzie Mountains Supergroup in Yukon (Canada), which archives marine sedimentation from ca. 950-775 Ma, permitting investigation into environmental evolution coincident with eukaryotic ecosystem evolution and expansion. Here we present multi-proxy geochemical data from the lower Mackenzie Mountains Supergroup to constrain the paleoredox environment within which these large benthic macroalgae thrived. Two transects show evidence for basin-wide anoxic (ferruginous) oceanic conditions (i.e., high Fe/Fe, low Fe/Fe), with muted redox-sensitive trace metal enrichments and possible seasonal variability. However, the weathering of sulfide minerals in the studied samples may obscure geochemical signatures of euxinic conditions. These results suggest that macroalgae colonized shallow environments in an ocean that remained dominantly anoxic with limited evidence for oxygenation until ca. 850 Ma. Collectively, these geochemical results provide novel insights into the environmental conditions surrounding the evolution and expansion of benthic macroalgae and the eventual dominance of oxygenated oceanic conditions required for the later emergence of animals.
太古宙（约 10 亿至 720 百万年前）的海洋环境被假设经历了重大的氧化还原变化，与多细胞真核生物的演化和多样化相吻合。特别是，最早的太古宙地层记录显示了多细胞宏观藻类对海底生境的殖民化，这些藻类将是强大的生态系统工程师，它们促进了海洋的氧化和生物地球化学循环的重组。然而，由于有限的保存完好的化石和地层，太古宙近岸海洋环境中宏观藻类栖息地扩张的古氧化还原背景仍然不确定。因此，早期复杂生命与海洋氧化还原状态之间的相互依存关系尚不清楚。最近在加拿大育空地区的下麦肯齐山脉超群中发现了包括绿藻宏观藻类 Archaeochaeta guncho 在内的宏体化石组合，该地层记录了约 950-775 百万年前的海洋沉积，允许对与真核生物生态系统演化和扩张同时发生的环境演化进行研究。在这里，我们展示了下麦肯齐山脉超群的多组地质化学数据，以限制这些大型底栖宏藻类繁盛的古氧化还原环境。两条测线显示出盆地范围缺氧（高铁）海洋条件的证据（即高 Fe/Fe，低 Fe/Fe），具有微弱的氧化还原敏感痕量金属富集和可能的季节性变化。然而，研究样本中硫化物矿物的风化可能掩盖了缺氧条件的地球化学特征。这些结果表明，藻类在海洋中主要处于缺氧状态的浅海环境中殖民化，直到约 850 百万年前，才有有限的证据表明氧气增加。总的来说，这些地球化学结果为围绕底栖宏藻类的演化和扩张以及后来需要有氧海洋条件才能出现动物的最终优势的环境条件提供了新的见解。
