Terrestrial microbialites provide constraints on the mesoproterozoic atmosphere

Abstract

Palaeoclimate data indicate that Earth surface temperatures have remained largely temperate for the past 3.5 Byr despite significantly lower solar luminosity over this time relative to the present day. There is evidence for episodic early and late Proterozoic glaciation, but little evidence of glaciation in the intervening billion years. A prolonged equable Mesoproterozoic Earth requires elevated greenhouse gas concentrations. Two endmember scenarios have been proposed for maintaining global warmth. These include extremely high pCO 2 or more modest pCO 2 with higher methane concentrations. This paper reports on the δ 13 C of organic matter in 1.1 Ga stromatolites from the Copper Harbor Conglomerate (CHC) of the Mesoproterozoic Midcontinent Rift (North America) and δ 18 O and Δ 47 temperatures of inorganic stromatolite carbonate to constrain formation and burial conditions and the magnitude of ancient carbon isotope discrimination. CHC sediments have never been heated above ~125-155°C, providing a novel geochemical archive of the ancient environment. Stromatolite Δ 47 data record moderate alteration, and therefore, the occluded organic matter was unlikely to have experienced significant thermal alteration after deposition. The δ 13 C values of ancient mat organic matter and inorganic carbonate show isotope discrimination (ε p ) values ~15.5-18.5‰, similar to modern microbial mats formed in equilibrium with low concentrations of dissolved inorganic carbon.In combination, these data are consistent with a temperate climate Mesoproterozoic biosphere supported by relatively modest pCO 2 . This result agrees with Atmosphere-Ocean Global Circulation Model reconstructions for Mesoproterozoic climate using 5-10 times present atmospheric levels pCO 2 and pCH 4 of >28 ppmv. However, given marine modelling constraints of CH 4 production that suggest pCH 4 was below 10 ppm, this creates a methane paradox. Either an additional source of CH 4 (e.g. from terrestrial ecosystems) or another greenhouse gas, such as N 2 O, would have been necessary to maintain equable conditions in the Mesoproterozoic. K E Y W O R D Sclumped Isotopes, Mesoproterozoic, mid-continent rift system, pCO 2 , stromatolites

The Nonesuch Formation Lagerstätte: a rare window into freshwater life one billion years ago

“…Both may be considered as Lagerstätten, the Torridonian because of exceptional preservation in phosphate nodules (Wacey et al 2014(Wacey et al , 2019 and the Nonesuch Lagerstätte because of the exceptional quality of the preserved palynomorphs. The latter is a function of the clay-rich nature of the host sediment that has experienced a maximum temperature of only 125-150°C (Gallagher et al 2017;Hren and Sheldon 2019). The Nonesuch Lagerstätte allows us to address important evolutionary questions with respect to eukaryotes during an extended period of time when diversification within major clades appears to have been in stasis (Knoll et al 2006;Knoll 2015).…”