Late Ordovician Glaciation Initiated by Early Land Plant Evolution and Punctuated by Greenhouse Mass Extinctions

Abstract

Late Ordovician (Hirnantian) glaciation, indicated by periglacial paleosols, tillites, and glacial pavements in Saharan Africa, has been attributed to advances in weathering and carbon sequestration due to evolution of early land plants, in the same way that Devonian-Permian glaciation has been attributed to evolution of forests and Quaternary glaciations to evolution of grasslands. Two problems for carbon cycle explanations of this glaciation are an Hirnantian CO 2 greenhouse estimated at 4035 ppmv from mass balance models and possibly related Hirnantian mass extinctions. This estimate of high CO 2 is a 10-m.yr. average, here evaluated against high-resolution sequences of paleosols that give estimates not only of CO 2 but also of paleoclimate and vegetation through the Late Ordovician. Katian paleosols of the eastern United States show increases in depth and in degree of carbon consumption and storage comparable with those preceding glaciation of the Devonian-Permian and Quaternary. Early thalloid land plants with rhizoids may have drawn down atmospheric CO 2 to only 166 5 83 ppmv during the Hirnantian, as estimated here from a pedogenic CO 2 paleobarometer. Ordovician paleosols also reveal a short-term greenhouse spike to as much as 4670 ppmv immediately before Hirnantian glaciation. This greenhouse spike may be due to thermogenic methane release from a large igneous province and is comparable with greenhouse spikes during other mass extinctions, such as those of the Late Permian and Late Jurassic.

Ordovician land plants and fungi from Douglas Dam, Tennessee

“…indet.) are known from Hirnantian, glacial deposits of the El Golea Member, Tamadjert Formation, near Zmeïlet Barka, Algeria (Arbey & Koeniguer, 1979;LeHeron & Craig, 2008), and from the Hirnantian, Juniata Formation at Beans Gap, Tennessee (Retallack, 2015a). Other remains comparable with Radix (Fritsch, 1908) are known from the early Katian, Gull River Formation near Ingham Mills, New York (Argast, 1992).…”

“…There is also evidence from palynology for Middle Ordovician glomalean fungi (Redecker et al, 2000(Redecker et al, , 2002, and other Glomeromycota may be represented by Ediacaran acritarchs (Retallack, 2015b), and Proterozoic megafossils (Retallack et al, 2013a(Retallack et al, , 2013b(Retallack et al, , 2018. Evidence of Cambrian and Ordovician land plants also comes from carbon isotopic composition of carbonaceous debris and sediments (Tomescu & Rothwell, 2006;Tomescu et al, 2009), and from chemical and textural profile differentiation of clayey Cambrian and Ordovician paleosols (Retallack, 2008(Retallack, , 2009(Retallack, , 2015a. Despite this circumstantial evidence for Ordovician vegetation, the macroscopic appearance, diversity and taxonomic affinities of Ordovician megafossils remained elusive until now.…”

“…No capsules are known for Akdalophyton, and its leaves are narrow with rounded apices, unlike Dollyphyton. Close helical arrangement of leaves differentiates Dollyphyton from other Ordovician leafy axes such as Hepaticaephyton (Kozlowski & Greguss, 1959), Musciphyton (Greguss, 1962), and un-named forms (Retallack, 2001b(Retallack, , 2015a, which are comparable with living soil-interstitial, leafy-liverworts such as Cephaloziella (Schuster, 1956;Newsham, 2010). Sphagnalean leaf fragments of Ordovician age have more hyaline cells (7-8) in each file than seen in Dollyphyton (Cardona-Correa et al, 2016).…”

“…It is also possible that single species of Prototaxites included hyphae of Basidiomycota, Ascomycota, and Glomeromycota, like modern multispecies lichens (Hawksworth, 1988). One reason to doubt that Prototaxites honeggeri is a record of Basidiomycota and Ascomycota, is the current lack of independent evidence from spores or mesofossils of those dikaryan clades before the Devonian (Retallack, 2015a).…”

“…L ATE Silurian is still regarded as a benchmark for the first appearance of land plants and consequent landscape modernization (McMahon & Davies, 2018), despite mounting circumstantial evidence to the contrary (Wellman et al, 2003;Tomescu et al, 2009;Retallack, 2015a;Steemans & Wellman, 2018;Morris et al, 2018). Because previously described Ordovician plant megafossils have been regarded as uninformative (Pant & Bhowmik, 1998;, the evolutionary origin of land plants remains obscure (Morris et al, 2018), embryophyte molecular clocks are insecure (Puttick et al, 2018), and the role of plants in global change is unclear (Porada et al, 2016).…”