Terrestrial microbialites provide constraints on the mesoproterozoic atmosphere

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 pCO2 or more modest pCO2 with higher methane concentrations. This paper reports on the δ13C of organic matter in 1.1 Ga stromatolites from the Copper Harbor Conglomerate (CHC) of the Mesoproterozoic Midcontinent Rift (North America) and δ18O and Δ47 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 Δ47 data record moderate alteration, and therefore, the occluded organic matter was unlikely to have experienced significant thermal alteration after deposition. The δ13C values of ancient mat organic matter and inorganic carbonate show isotope discrimination (εp) 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 pCO2. This result agrees with Atmosphere‐Ocean Global Circulation Model reconstructions for Mesoproterozoic climate using 5–10 times present atmospheric levels pCO2 and pCH4 of >28 ppmv. However, given marine modelling constraints of CH4 production that suggest pCH4 was below 10 ppm, this creates a methane paradox. Either an additional source of CH4 (e.g. from terrestrial ecosystems) or another greenhouse gas, such as N2O, would have been necessary to maintain equable conditions in the Mesoproterozoic.This paper, entitled ‘Terrestrial Microbialites Provide Constraints on the Mesoproterozoic Atmosphere’ offers a new look at 1.1 billion‐year‐old stromatolites in the Mesoproterozoic Midcontinent Rift in Michigan to provide new constraints on pCO2 during the long period of earth’s history that is colloquially known as the ‘boring billion’. New clumped isotope temperature measurements are provided to constrain upper temperature bounds for microbial carbonate formation temperature and post‐depositional thermal alteration, as well as organic and inorganic carbon isotope data that are used to constrain carbon isotope discrimination that is regulated by atmospheric carbon dioxide concentrations.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154283/1/dep279_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154283/2/dep279.pd

Quaternary Glacial-Interglacial Climate Cycles in Hawaii

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/148629/1/Sheldon_2006_JGeol-Hawaiian_glacial-interglacial_paleoclimate.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/148629/2/Sheldon_2006_JGeol-Appendix_A.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/148629/3/Sheldon_2006_JGeol-Appendix_B.pd

Using carbon isotope equilibrium to screen pedogenic carbonate oxygen isotopes: implications for paleoaltimetry and paleotectonic studies

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/148628/1/Sheldon_2018_Geofluids-improving_paleoaltimetry_with_C_isotopes.pd

Regional Paleoprecipitation Records from the Late Eocene and Oligocene of North America

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/148635/1/Sheldon_and_Retallack_2004_JGeol-EOT_climate_spatial_gradients.pd

Evidence for an early sagebrush ecosystem in the latest Eocene of Montana

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/148619/1/Sheldon_and_Hamer_2010_JGeol-EOT_at_Pipestone.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/148619/2/Sheldon_and_Hamer_2010_JGeol-supplemental_data.pd

Interpreting an Archaean Paleoenvironment Through 3D Imagery of Microbialites

While stromatolites, and to a lesser extent thrombolites, have been extensively studied in order to unravel Precambrian (\u3e539 Ma) biological evolution, studies of clastic-dominated microbially induced sedimentary structures (MISS) are relatively scarce. The lack of a consolidated record of clastic microbialites creates questions about how much (and what) information on depositional and taphonomic settings can be gleaned from these fossils. We used μCT scanning, a non-destructive X-ray-based 3D imaging method, to reconstruct morphologies of ancient MISS and mat textures in two previously described coastal Archaean samples from the ~3.48 Ga Dresser Formation, Pilbara, Western Australia. The aim of this study was to test the ability of μCT scanning to visualize and make 3D measurements that can be used to interpret the biotic–environmental interactions. Fossil MISS including mat laminae with carpet-like textures in one sample and mat rip-up chips in the second sample were investigated. Compiled δ¹³C and δ³⁴S analyses of specimens from the Dresser Fm. are consistent with a taxonomically diverse community that could be capable of forming such MISS. 3D measurements of fossil microbial mat chips indicate significant biostabilization and suggest formation in flow velocities \u3e25 cm s−¹. Given the stratigraphic location of these chips in a low-flow lagoonal layer, we conclude that these chips formed due to tidal influence, as these assumed velocities are consistent with recent modeling of Archaean tides. The success of μCT scanning in documenting these microbialite features validates this technique both as a first step analysis for rare samples prior to the use of more destructive techniques and as a valuable tool for gaining insight into microbialite taphonomy

Contrasting geochemical signatures on land from the Middle and Late Permian extinction events

The end of the Palaeozoic is marked by two mass‐extinction events during the Middle Permian (Capitanian) and the Late Permian (Changhsingian). Given similarities between the two events in geochemical signatures, such as large magnitude negative δ 13 C anomalies, sedimentological signatures such as claystone breccias, and the approximate contemporaneous emplacement of large igneous provinces, many authors have sought a common causal mechanism. Here, a new high‐resolution continental record of the Capitanian event from Portal Mountain, Antarctica, is compared with previously published Changhsingian records of geochemical signatures of weathering intensity and palaeoclimatic change. Geochemical means of discriminating sedimentary provenance (Ti/Al, U/Th and La/Ce ratios) all indicate a common provenance for the Portal Mountain sediments and associated palaeosols, so changes spanning the Capitanian extinction represent changes in weathering intensity rather than sediment source. Proxies for weathering intensity chemical index of alteration, ∆ W and rare earth element accumulation all decline across the Capitanian extinction event at Portal Mountain, which is in contrast to the increased weathering recorded globally at the Late Permian extinction. Furthermore, palaeoclimatic proxies are consistent with unchanging or cooler climatic conditions throughout the Capitanian event, which contrasts with Changhsingian records that all indicate a significant syn‐extinction and post‐extinction series of greenhouse warming events. Although both the Capitanian and Changhsingian event records indicate significant redox shifts, palaeosol geochemistry of the Changhsingian event indicates more reducing conditions, whereas the new Capitanian record of reduced trace metal abundances (Cr, Cu, Ni and Ce) indicates more oxidizing conditions. Taken together, the differences in weathering intensity, redox and the lack of evidence for significant climatic change in the new record suggest that the Capitanian mass extinction was not triggered by dyke injection of coal‐beds, as in the Changhsingian extinction, and may instead have been triggered directly by the Emeishan large igneous province or by the interaction of Emeishan basalts with platform carbonates.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108696/1/sed12117.pd

Multiproxy evidence for tectonic control on the expansion of C4 grasses in northwest Argentina

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/148593/1/Cotton_et_al_2014_EPSL-late_Miocene_grasslands_in_Argentina.pd

Global aridity during the Early Miocene? A Terrestrial Paleoclimate Record from the Ebro Basin, Spain

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/148622/1/Hamer_et_al_2007_J_Geology-Miocene_climate.pd

Geochemical climofunctions from North America soils and application to paleosols across the Eocene-Oligocene boundary in Oregon

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/148640/1/Sheldon_et_al_2002_JGeol-new_paleoclimate_proxies_and_applications_to_the_EOT.pd