Ethan G. Hyland

A new paleoprecipitation proxy based on soil magnetic properties: Implications for expanding paleoclimate reconstructions

Description of precipitation patterns and changes in the hydrological cycle during periods of past global change is crucial for providing an understanding of terrestrial climate systems and for predicting impacts of future climate change such as shifting water availability. While a number of proxies and climofunctions exist for reconstructing paleoprecipitation using paleosols, all of the available tools for reconstructing paleoprecipitation are either limited to certain precipitation ranges (effective only for low-precipitation regimes; e.g., depth to Bk, chemical index of alteration [CIA-K]), or are relevant only to a limited range of paleosols (single-pedotype relationships; e.g., calcium-magnesium index [CALMAG]). Here, we measure the acquisition of isothermal remanent magnetization in B horizons of modern soils to quantify the ratio of pedogenic magnetic minerals goethite and hematite, and we use the relationship between these soil magnetic properties and measured climatic variables at each soil site to derive a new quantitative proxy for precipitation. By compiling both literature-derived and measured goethite-hematite (G/H) ratios and mean annual precipitation estimates for a global suite of modern soils ( n = 70), we describe a strong linear relationship ( R 2 = 0.96) between the G/H ratios of soil B horizons and mean annual precipitation that can be used to estimate paleoprecipitation values for a wide range of climatic regimes (100–3300 mm yr –1 ) and soil types (Inceptisols, Alfisols, Ultisols, Oxisols, Mollisols, Aridisols, Spodosols). We tested the new climofunction using paleosols from the early Eocene of Wyoming, which show that estimates based on G/H ratios compare favorably to and expand upon previously published estimates based on paleosol data.

Terrestrial paleoenvironmental reconstructions indicate transient peak warming during the early Eocene climatic optimum

Major changes in climate and ecology occurred during the early Eocene climatic optimum, sometime between 52 and 50 Ma. Recent work suggests that the timing and duration of the event are characterized by different responses in the marine and terrestrial realms, and that traditional causal mechanisms may not adequately explain such differences. We applied high-resolution paleopedology, geochemical analysis, and phytolith biostratigraphy techniques to paleosol suites within the well-described Wind River Formation of western Wyoming, USA. This multiproxy record indicates a short (<1 m.y.) peak period of carbon isotopic enrichment (up to 2‰ higher) and elevated pCO2, high temperatures (up to 8 °C higher), increased precipitation (up to 500 mm yr –1 higher), and shifts in fl oral composition (up to 10%). Terrestrial climatic and ecological changes of this kind during the early Eocene climatic optimum are consistent with changes in contemporaneous records that have been ascribed to high atmospheric pCO2, but a transient peak interval suggests that the cause of high atmospheric pCO2 during the early Eocene was likely not increased volcanism or decreased silicate weathering, which operate on longer timescales. Instead, terrestrial records from across western North America agree that early Eocene climatic optimum changes may have been caused by other sources, such as a combination of increased ventilation of oceanic carbon and increased petroleum generation in sedimentary basins. The climatic and environmental changes exhibited by this and other North American terrestrial records also defi ne a pattern of regional response that is relevant for understanding the impacts of global climate change events.

Constraining the early Eocene climatic optimum: A terrestrial interhemispheric comparison

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 traditional causal mechanisms may not adequately explain the dynamics of the early Eocene climatic optimum. Here, we present a new high-resolution multiproxy record of terrestrial climatic and environmental conditions during the early Eocene climatic optimum from the Southern Hemisphere and compare this reconstruction to other marine and terrestrial records. Similar to Northern Hemisphere terrestrial records, there is a transient peak period of atmospheric carbon isotope enrichment as well as increased temperatures and precipitation, which indicate that terrestrial environmental responses to the early Eocene climatic optimum were broadly consistent in temperate settings worldwide. This global consistency in terrestrial records demonstrates differences in peak warming time scales and carbon isotope responses between marine and terrestrial systems, which further constrain potential causes for the early Eocene climatic optimum to multiple-system or nontraditional mechanisms and highlight the importance of paired records for understanding past climate.

Phytoliths as Tracers of Recent Environmental Change

Phytoliths are more widespread, accessible, and characteristic of a local area than other terrestrial vegetation proxies. Despite work on recent soil and Cenozoic paleosol phytolith assemblages, environmental applications lag significantly behind their potential in terms of temporal resolution. Modern soil phytolith assemblages, aboveground vegetation, and soil features from Inceptisols with known vegetation and environmental histories were sampled in order to develop methods for describing rapid environmental change events at a high temporal resolution. Samples included agricultural fields, fluvial meanders, and wildfire sites. In each case, soil phytolith assemblages were unrepresentative of current vegetation but were characteristic of the known environmental history. As a result, rapid changes in land use or environment are identifiable in phytolith assemblages; agricultural sites can be identified by Ap horizons and grass phytoliths, fluvial meanders by weakly developed soils with channel features and spatial phytolith gradients, and wildfire sites by charcoal bodies and bimodal phytolith assemblages. These sites also provide rates of change, specific to each type of environmental change. An ecosystem experiencing wildfires changes assemblages rapidly (1–2 % per year), while change resulting from channel migration occurs slightly slower (~ 0.5 % per year), and that from field abandonment occurs significantly slower (< 0.25 % per year). These methods can be applied to paleovegetation reconstructions, providing additional environmental information and higher-resolution vegetation interpretations. Along with more work on spatial and depth-profile sampling, these results will allow high temporal resolution for environmental and vegetation change records both in the modern and throughout the Cenozoic era.