Open AccessTerrestrial microbialites provide constraints on the mesoproterozoic atmosphere
Author(s) -
Hren Michael T.,
Sheldon Nathan D.
Publication year - 2020
Publication title -
the depositional record
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 3
ISSN - 2055-4877
DOI - 10.1002/dep2.79
Subject(s) - stromatolite , proterozoic , biosphere , snowball earth , geology , carbonate , earth science , atmosphere (unit) , glacial period , hydrosphere , organic matter , paleoclimatology , temperate climate , precambrian , geologic record , rift , geochemistry , paleontology , climate change , oceanography , chemistry , ecology , physics , organic chemistry , structural basin , biology , tectonics , thermodynamics
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 p CO 2 or more modest p CO 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 p CO 2 . This result agrees with Atmosphere‐Ocean Global Circulation Model reconstructions for Mesoproterozoic climate using 5–10 times present atmospheric levels p CO 2 and p CH 4 of >28 ppmv. However, given marine modelling constraints of CH 4 production that suggest p CH 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.