Early-Middle Devonian paleosols and palustrine beds of NW Canada in the context of land plant evolution and global spreads of anoxia

摘要

Dozens of subaerial exposure surfaces are assessed in cores from cyclic peritidal carbonates of the Emsian-Eifelian (～410-385 Ma) age. These surfaces range from incipient erosional surfaces to paleokarst profiles and thick (＞1 m) calcretic-clayey paleosols. Palustrine carbonates intervening at multiple levels in same strata are the earliest known occurrence of a typical palustrine fades in a coastal carbonate plain environment. None of paleokarst and paleosol profiles contain traces of vascular-plant root penetration, and only palustrine facies exhibit swarms of thin rhizoliths. These findings are within the context of Devonian paleosols on marine carbonate substrates where root traces and laminar calcretes are extremely rare (only 2 reports), and no instances of root penetrations are trackable from unconformities in pre-Givetian carbonate successions. The δ~(13)C and δ~(18)O signatures indicate variable diagenetic reset of isotopic composition. Modest δ~(13)C_(vpdb) offset towards lighter values is preserved in two thicker paleosols (-3.4 to -8.0‰ in calcretic matrix vs. -1.35 to -6.5‰ in parent limestone). Such offsets are very rare in pre-Late Devonian pedogenic carbonates, and their attribution to plant-derived CO_2 is dubious. It is inferred that the land surface in calcimagnesian landscapes remained a regolith or primary desert (i.e., was never colonized by tracheophytes) long after the spread of vascular plants in more favorable wetland settings. Furthermore, the advent of seed reproductive strategy in the latest Devonian and plant adaptations to aridic habitats manifesting in fossil floras only since Pennsylvanian indicate that much of land surface remained within the realm of primary desert long after the afforestation of wetlands. The direct (plant roots, shoots and spores) and indirect (sedimentary features) fossil records of land greening during the Paleozoic are not corroborating the hypothesis holding the primordial embryophytic cover on land accountable for the Late Ordovician atmospheric oxygenation-decarbonization event. It is further argued that the evolution of root systems and plant stature could hardly trigger anoxic events and biotic crises in the Devonian marine realm.