Nathan Sheldon

University of Michigan | U-M · Department of Earth and Environmental Sciences

The chemical compositions of paleosols are frequently used to reconstruct paleoclimatic and paleoenvironmental conditions using proxy relationships derived from modern soils. However, through time, some of the proxy relationships have been applied to paleosol types that were not part of the original training data sets or they have analyzed followin...

Although continental weathering intensity has been invoked as a primary control on biogeochemistry, tectonics, and the carbon cycle throughout geologic history, it remains poorly quantified over Earth’s history. As a direct product of continental weathering, paleosols (fossil soils) offer unique insight into past weathering intensity, but they rema...

As atmospheric carbon dioxide (CO2) and temperatures increase with modern climate change, ancient hothouse periods become a focal point for understanding ecosystem function under similar conditions. The early Eocene exhibited high temperatures, high CO2 levels, and similar tectonic plate configuration as today, so it has been invoked as an analog t...

Paleosols represent fossil records of paleolandscape processes, paleobiotic interactions with the land surface, and paleoclimate. Paleosol-based reconstructions have figured prominently in the study of significant changes in global climate and terrestrial life, with one of the more highly studied examples being the end-Permian extinction (EPE). The...

Paleosols represent fossil records of paleolandscape processes, paleobiotic interactions with the land surface, and paleoclimate. Paleosol-based reconstructions have figured prominently in the study of significant changes in global climate and terrestrial life, with one of the more highly studied examples being the end-Permian extinction (EPE). The...

Carbon isotope ecology can be used as a measure of plant water stress. Plant water stress is complicated: multiple factors (e.g., evapotranspiration rates, temperature, precipitation, soil texture, and available water) control the magnitude of the isotopic expression. Often, large ocean–atmosphere pressure systems result in the convergence of const...

Paleosols (fossil soils) are valuable records of terrestrial climate and environments, and paleosol-based proxies are commonly used to reconstruct past climates and ecosystems. Results from relatively small outcrops or transects or from single vertical sections are frequently scaled up to represent basin-scale processes and conditions, and reconstr...

The sedimentary pyrite sulfur isotope (δ34S) record is an archive of ancient microbial sulfur cycling and environmental conditions. Interpretations of pyrite δ34S signatures in sediments deposited in microbial mat ecosystems are based on studies of modern microbial mat porewater sulfide δ34S geochemistry. Pyrite δ34S values often capture δ34S signa...

As increasing atmospheric carbon dioxide (CO2) and temperatures accompany modern climate change, ancient hothouse periods become a focal point for understanding ecosystem function under similar conditions. The early Eocene exhibited high temperatures, high CO2 levels, and similar tectonic plate configuration to today, so it has been invoked as an a...

Anthropogenic climate change has significant impacts at the ecosystem scale including widespread drought, flooding, and other natural disasters related to precipitation extremes. To contextualize modern climate change, scientists often look to ancient climate changes, such as shifts in ancient precipitation ranges. Previous studies have used fossil...

Paleosols formed in direct contact with the Earth's atmosphere, so they can record the composition of the atmosphere through weathering processes and products. Herein we critically review a variety of different approaches for reconstructing atmospheric O2 and CO2 over the past three billion years. Paleosols indicate relatively low CO2 over that tim...

Phosphorus (P) is an essential limiting nutrient in marine and terrestrial ecosystems. Understanding the natural and anthropogenic influence on P concentration in soils is critical for predicting how its distribution in soils may shift as climate changes. While it is known that P is sourced from bedrock weathering, relationships between weathering,...

Sample information, bulk geochemical data, Fe extraction data for the non-USGS soils used in Dzombak & Sheldon (2020) Soil Systems.

(1) Plant carbon isotope discrimination is complex, and could be driven by climate, evolution, and/or edaphic factors. We tested the climate drivers of carbon isotope discrimination in modern and historical plant chemistry, and focus in particular on the relationship between rising [CO2] over Industrialization and carbon isotope discrimination. (2)...

The question of what caused the mass extinction at 66 Ma persists. Using fossil soils within the Deccan Traps in India, we reconstructed climate on land during volcanism and across the Cretaceous-Paleogene boundary (66Ma), and found no significant climate perturbations during that period. We conclude that although volcanism likely contributed to lo...

Large igneous provinces (LIPs) have been temporally correlated to mass extinctions throughout the Phanerozoic, including the emplacement of the Deccan Volcanic Province (DVP; 66.3–65.6 Ma) in western and central India, which has been invoked as either a cause or exacerbating factor in the Cretaceous-Paleogene (K-Pg) extinction. However, relatively...

Constraints on Precambrian ocean chemistry are dependent upon sediment geochemistry. However, diagenesis and metamorphism can destroy primary biosignatures, making it difficult to consider biology when interpreting geochemical data. Modern analogues for ancient ecosystems can be useful tools for identifying how sediment geochemistry records an acti...

Conference talk from GSA - North Central Section meeting 2020

Plants are direct recorders of the environmental conditions in which they were growing through their carbon assimilation. Both δ13Cleaf (measured C isotopic composition of the leaf) and Δleaf (isotopic discrimination from δ13Catm by the leaf) values have been proposed to reflect a variety of environmental conditions including MAT, MAP, latitude, el...

Paleosols comprise an important archive of continental paleoclimate information throughout geologic history, but resolving temperature seasonality poses a challenge to the application of paleosol-derived temperature proxies. Not only does seasonality exert a strong control on the biosphere, but it can also obfuscate accurate interpretation of proxy...

Atmospheric oxygen levels control the oxidative side of key biogeochemical cycles and place limits on the development of high‐energy metabolisms. Understanding Earth's oxygenation is thus critical to developing a clearer picture of Earth's long‐term evolution. However, there is currently vigorous debate about even basic aspects of the timing and pa...

Carbon isotope values of leaves (d 13 C leaf) from meta-analyses and growth chamber studies of C 3 plants have been used to propose generalized relationships between d 13 C leaf and climate variables such as mean annual precipitation (MAP), atmospheric concentration of carbon dioxide ([CO 2 ]), and other climate variables. These generalized relatio...

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 prolonge...

While marine sediments have been used to constrain a history of redox chemistry throughout the Precambrian, far fewer data have been generated from lakes. With major biological innovations thought to have occurred in Proterozoic lakes, understanding their chemistry is critical for understanding the evolution of eukaryotic life. We use sediment geoc...

It has been argued that phosphorus supply to the ocean (via continental weathering) has been a primary control on marine productivity and oxygen production 1,2 , and that during the late Precambrian, the terrestrial biosphere expanded and increased oxygen through intense weathering and nutrient delivery to shallow marine systems 2,3. Here, we compi...

Proterozoic oceans are characterized by a thin veneer of oxygenated surface waters underlain by anoxic or euxinic deeper waters, and it has been hypothesized that metabolically flexible microbes such as cyanobacteria would have thrived under these conditions. However, relatively little is known about the relationship between water chemistry, microb...

https://deepblue.lib.umich.edu/bitstream/2027.42/148595/1/Tessin_et_al_2019_Geology-Mo_speciation_as_an_important_tool_for_understanding_redox.pdf

Stable isotope compositions of pedogenic carbonates ( δ¹³ C carb , δ¹⁸ O carb ) are widely used in paleoenvironmental and paleoaltimetry studies. At the same time, both in vertical stratigraphic sections and in horizontal transects of single paleosols, significant variability in δ¹⁸ O carb values is observed well in excess of what could reasonably...

Molybdenum (Mo) geochemistry is widely used to reconstruct ocean oxygenation throughout Earth's history. However, gaps in our fundamental knowledge of Mo burial within sediments hinder the utility of Mo as a proxy for paleo-redox reconstruction. To improve our understanding of Mo burial pathways and sedimentary Mo speciation, we present combined ge...

The sedimentary record of the Late Cretaceous North American Western Interior Seaway (WIS) is characterized by periods of enhanced organic carbon burial, including an interval that has been identified as Oceanic Anoxic Event (OAE) 3. The timing and duration of this event has proved difficult to constrain, calling into question whether a widespread...

Paleogene greenhouse climate equability has long been a paradox in paleoclimate research. However, recent developments in proxy and modeling methods have suggested that strong seasonality may be a feature of at least some greenhouse Earth periods. Here we present the first multi-proxy record of seasonal temperatures during the Paleogene from paleof...

C4 grasslands are a major global ecosystem with an important role as the primary source of food resources and agricultural land for the planet. Despite this, the causes and timeline of their expansion are still not fully known and appear to be variable in different parts of the world. By combining phytolith and stable isotope methods, we produce ro...

I analyzed our expanded soil dataset (about 140 soils, B horizons) and a set of climatic/environmental variables using Principle Components Analysis in order to see (a) which Fe pools are most strongly associated with each other, and (b) which, if any, climatic variables were associated with each Fe pool. While I had expected to find strong associa...

The oxygenation of the atmosphere — one of the most fundamental transformations in Earth’s history — dramatically altered the chemical composition of the oceans and provides a compelling example of how life can reshape planetary surface environments. Furthermore, it is commonly proposed that surface oxygen levels played a key role in controlling th...

Paleogene greenhouse climate equability has long been a paradox in paleoclimate research. However, recent developments in proxy and modeling methods have suggested that strong seasonality may be a feature of at least some greenhouse periods. Here we present the first multi-proxy record of seasonal temperatures during the Paleogene from paleofloras,...

https://deepblue.lib.umich.edu/bitstream/2027.42/148644/1/Hyland_et_al_2018_CotP-Eocene_Climate_Equability.pdf

https://deepblue.lib.umich.edu/bitstream/2027.42/148645/1/Planavsky_et_al_2018_ETLS-low_O2_in_the_Proterozoic.pdf

Understanding the functional response of ecosystems to past global change is crucial to predicting performance in future environments. One sensitive and functionally significant attribute of grassland ecosystems is the percentage of species that use the C4 versus C3 photosynthetic pathway. Grasses using C3 and C4 pathways are expected to have diffe...

The Late Cretaceous sedimentary record of the North American Western Interior Seaway is characterized by cyclic deposition of organic carbon-rich sediments. One notable interval during the late Coniacian-Santonian is recorded by the Niobrara Formation. The organic carbon-rich interval within the Niobrara Formation has been identified as Oceanic Ano...

The mid-Miocene climatic optimum (MMCO; initial warming beginning ca. 18 Ma and with peak warming ca. 17–14.75 Ma) constitutes the Earth's most recent greenhouse episode, characterized by a transient shift to higher global atmospheric CO2 levels and warmer, possibly wetter, climatic conditions. Combined with the spread of grass-dominated habitats,...

The early–middle Miocene was an important transitional period in the evolution of Earth's biota and climate that has been poorly understood in North America due to a paucity of continuous, fossil-bearing rock records in this interval for which the ages have been robustly constrained. In the northern Rocky Mountains, United States, one site in parti...

Andrews et al. (2017) present a paleosol-based paleoclimate reconstruction of the Paleocene-Eocene Thermal Maximum from localities in northern Argentina. We suggest that the age model presented for these localities is incorrect based on recently published magnetostratigraphy from the area, and that the studied Maiz Gordo and Lumbrera Formations ins...

The Midcontinent Rift System (MRS) is a Late Mesoproterozoic (∼1.1Ga) sequence of volcanic and sedimentary rocks exposed in the Lake Superior Region of North America. The MRS continues to be the focus of much research due to its economic mineral deposits as well as its archive of Precambrian life and tectonic processes. In order to constrain the po...

The Proterozoic Eon (2500–542 Ma) appears to have been a warm period bookended by glaciations, despite a 5%–18% reduction in solar output compared to modern during this interval. Radiative-convective climate models suggest that glaciation could have been avoided if pCO2 were 30–300× preindustrial atmospheric levels (PIAL, 280 ppmv). Constraints fro...

https://deepblue.lib.umich.edu/bitstream/2027.42/148574/1/Harris_et_al_2017_Palaeo-3-ecosystem_shifts_in_the_lead_up_to_the_MMCO.pdf

https://deepblue.lib.umich.edu/bitstream/2027.42/148576/1/Harris_et_al_2017_GSA_Bulletin-revised_chronology_for_Railroad_Canyon.pdf

The early Eocene climatic optimum was a period of major climatic and environmental change that was caused by perturbations to the global carbon cycle. Recent work from terrestrial sections in the Northern Hemisphere demonstrates that the period was characterized by different responses in the terrestrial and marine realms, suggesting that traditiona...

Pedogenic carbonate is an important archive for paleoclimate, paleoecology, and paleoelevation studies. However, it can form under seasonal environmental conditions that differ significantly from the mean growing season environment or mean annual conditions, potentially complicating its use for proxy reconstructions. The observed seasonal temperatu...

The Western Interior Seaway (WIS) was a shallow and expansive body of water that covered the central United States during the Late Cretaceous. Attempts to reconstruct temperatures in the seaway using the oxygen isotopic composition of biogenic carbonates have suffered from uncertainty in the oxygen isotopic composition of seawater (delta O-18(w)) i...

The sedimentary record of the Coniacian–Santonian Oceanic Anoxic Event 3 (OAE 3) in the North American Western Interior Seaway is characterized by a prolonged period of enhanced organic carbon (OC) burial. This study investigates the role of Fe in enhancing organic matter preservation and maintaining elevated primary productivity to sustain black s...

For a large part of earth's history, cyanobacterial mats thrived in low-oxygen conditions, yet our understanding of their ecological functioning is limited. Extant cyanobacterial mats provide windows into the putative functioning of ancient ecosystems, and they continue to mediate biogeochemical transformations and nutrient transport across the sed...

Fluvial, alluvial, lacustrine, and debris flow sedimentary rocks from the Keweenaw Peninsula (Michigan, USA) are geochemically characterized to examine weathering processes and provenance. These sedimentary rocks result from the erosion, weathering, and redistribution of effusive and plutonic igneous lithologies during periods of volcanic quiescenc...

Terrestrial sediments are a significant archive of past conditions, and the critical zone in particular is important for understanding past and ongoing interactions between the atmosphere, biosphere and lithosphere. Understanding the details of a palaeolandscape or palaeocatena record can greatly improve palaeoenvironmental descriptions and palaeoc...