Could land‐based early photosynthesizing ecosystems have bioengineered the planet in mid‐Palaeozoic times?

Abstract

The Ordovician and Silurian periods were times of major geological activity as regards palaeogeography, volcanism and climate change, the last of these evidenced by a series of cooling episodes and glaciations that climaxed in the Hirnantian (Late Ordovician). The presence of cryptospores in the Darriwilian (Middle Ordovician) marked the advent of higher plants on land. A critical survey of direct (mega-and microfossils) and some indirect evidence in succeeding rocks indicates the presence of algae, Bacteria, Cyanobacteria, Fungi, probable lichens, cryptophytes and basal tracheophytes. Similar associations of photosynthesizers and decomposers occur today in cryptogamic covers (CCs), for example biological crusts, except that bryophytes replace cryptophytes (basal embryophytes) and tracheophytes are absent. Thus, extant CCs, which make significant contributions today to global carbon and nitrogen fixation and prevention of erosion, provide an excellent analogue for the impacts of early land vegetation on both lithosphere and atmosphere. As a prerequisite to assessing impacts in Ordovician-Silurian times, with particular consideration of parameters used by climate modellers, the effects of a number of abiotic factors on the growth and survival of extant cryptogamic ground covers and their environmental impacts are reviewed. Factors include photosynthetically active radiation, ultraviolet radiation, temperature, water, oxygen, carbon dioxide, nitrogen, phosphorus, iron, surface roughness and albedo. A survey of the nature and extent of weathering facilitated by such vegetation concludes that it was limited based on depth of weathering when compared with that from rooted tracheophytes today, with minor effects on carbon dioxide drawdown. As global net productivity from Ordovician-Silurian CCs was very probably lower than today, and while the small fraction of intractable material in their organic carbon would have resulted in a more rapid turnover of terrestrial biomass, we conclude that there was decreased possibility of long-term organic carbon burial. Hence, there would have been very limited increase in atmospheric oxygen and decrease in carbon dioxide resulting from carbon burial.

Earliest land plants created modern levels of atmospheric oxygen

“…The presence of these rock-weathering capabilities in two early diverging lineages (mosses and liverworts) suggests it is an ancestral trait. It has been argued (21,33) that such large measured local effects would not have scaled up to significant global effects, because of low global NPP (33) and a limited depth of influence in the soil (21). However, we estimate much higher global NPP (Fig.…”

“…S1), ice sheet cover (Fig. S2), and O 2 (Table S2), but is consistently higher than the 4.3 GtC·y −1 (7% of today) estimated elsewhere (33). In the original COPSE model (10), predicted NPP only reaches ∼5% of today's value in the Late Ordovician and Silurian, but when we assume a stronger Late Ordovician phase of land colonization by nonvascular plants (following ref.…”

“…In this new steady state, oxidative weathering was increased (becoming less sensitive to variations in O 2 ) and new fire-mediated negative feedbacks on O 2 were instigated that have played a key role in stabilizing atmospheric O 2 concentration up to the present day (22,43). For the earliest land plants to be responsible for such a major mid-Paleozoic oxygenation event requires that they were much more productive and globally extensive than has been previously assumed (7,10,33). This hypothesis makes testable predictions with regard to effects on other biogeochemical cycles, notably sulfur; if it stands up to further scrutiny, we can then infer that the earliest land plants created a stable oxygen-rich atmosphere that was necessary for the subsequent evolution of large, mobile, intelligent animals with a high respiratory oxygen demand, including ourselves.…”

Biosilicification Drives a Decline of Dissolved Si in the Oceans through Geologic Time

“…On land the uptake of DSi and deposition by many vascular plants provides ecological, physiological, or structural benefits (Epstein, 1999), creating an active terrestrial Si cycle (Conley, 2002). The origin of terrestrial plants and the spread of rooted vascular plants to upland areas during the Devonian Period most likely had an important effect on many Earth processes (Lenton et al, 2012; though see Edwards et al, 2015). The activities of plants should increase the efficiency of continental weathering (Berner, 1997) through stimulated dissolution of bedrock (Schwartzman and Volk, 1989), thus enhancing the removal of atmospheric CO 2 (Berner, 1990) until a new steady state is reached in weathering.…”

Possible patterns of marine primary productivity during the Great Ordovician Biodiversification Event

“…In addition, the spatial cover of this primitive vegetation is difficult to estimate for the remainder of the period (Edwards et al . ; Porada et al . ).…”