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Abstract Magnetism, redness, and Fe oxides are indicators of pedoclimatic conditions. However, uncertainties with observing how Fe oxides form within soils has led to debates about relationships between magnetic mineral assemblages, temperature, and rainfall. To address these issues, Fe oxides from the equatorial tropics of Kenya were examined in Pliocene soils that developed under orbital forcing of the monsoon. Results demonstrate that with warm‐wet monsoons, ferrimagnetic production was increased and correlated with hematite concentrations, in accordance with expectations that ferrimagnetic and hematite minerals codevelop from amorphous Fe oxides. With cool‐dry monsoons, hematite concentrations increased but ferrimagnetic production decreased and decoupled from hematite development. These findings suggest that decreased rainfall rather than temperature change favored the dehydration step required to catalyze hematite enrichment within soils. This study explains Fe oxides origins under variable monsoonal climates and recognizes moisture changes in comparison to temperature as stronger controls on the production of soil‐formed hematite. 

Abstract The extinction of theParanthropus boiseiestimated to just before 1 Ma occurred when C₄grasslands dominated landscapes of the Eastern African Rift System (EARS).P. boiseihas been characterized as an herbivorous C₄specialist, and paradoxically, its demise coincided with habitats favorable to its dietary ecology. Here we report new pedogenic carbonate stable carbon (δ¹³C_PC) and oxygen (δ¹⁸O_PC) values (nodules = 53, analyses = 95) from an under-sampled interval (1.4–0.7 Ma) in the Turkana Basin (Kenya), one of the most fossiliferous locales ofP. boisei. We combined our new results with published δ¹³C_PCvalues from the EARS dated to 3–0 Ma, conducted time-series analysis of woody cover (ƒ_WC), and compared the EARS ƒ_WCtrends to regional and global paleo-environmental and -climatic datasets. Our results demonstrate that the long-term rise of C₄grasslands was punctuated by a transient but significant increase in C₃vegetation and warmer temperatures, coincident with the Mid-Pleistocene Transition (1.3–0.7 Ma) and implicating a short-term rise inpCO₂. The contraction of C₄grasslands escalated dietary competition amongst the abundant C₄-feeders, likely influencingP. boisei’s demise. 

Hematite is the most abundant surficial iron oxide on Earth resulting from near-surface processes that make it important for addressing numerous geologic problems. While red beds have proved to be excellent paleomagnetic recorders, the early diagenetic origin of hematite in these units is often questioned. Here, we validate pigmentary hematite (“pigmentite”) as a proxy indicator for the Late Triassic environment and its penecontemporaneous origin by analyzing spectrophotometric measurements of a 14.5-My–long red bed sequence in scientific drill core CPCP-PFNP13-1A of the Chinle Formation, Arizona. Pigmentite concentrations in the red beds track the evolving pattern of the Late Triassic monsoon and indicate a long-term rise in aridity beginning at ∼215 Ma followed by increased oscillatory climate change at ∼213 Ma. These monsoonal changes are attributed to the northward drift of the Colorado Plateau as part of Laurentia into the arid subtropics during a time of fluctuating CO₂. Our results refine the record of the Late Triassic monsoon and indicate significant changes in rainfall proximal to the Adamanian–Revueltian biotic transition that thus may have contributed to apparent faunal and floral events at 216 to 213 Ma. 

Abstract Pliocene Vertisols from the Turkana Basin of northwest Kenya (~4°N latitude) have been examined using isothermal remanent magnetization (IRM) experiments, susceptibility measurements, and diffuse reflectance spectroscopy. The complete vertical profile of each paleosol is almost intact, measuring >2 m in thickness and being strongly magnetic (IRM at 1.0T ranges from 4 to 25 × 10⁻³Am²/kg). Downprofile changes to the proxy indicators suggest uninterrupted pedodevelopment and a lack of stratigraphic inversions caused by argilli‐pedoturbation. Magnetic minerals consist of hematite to a lesser degree, and pedogenic ferrimagnets derived from moisture cycles that were controlled by monsoonal rainfall. Basal pedogenic zones of the paleosols are magnetically intense, and preserve the greatest slickenside development indicating pronounced seasonal wetting and drying. These observations indicate a deeper dry season water table as compared to poorly drained temperate/tropical Vertisols, which suffer reductive dissolution of Fe oxides in basal pedogenic horizons associated with weak magnetic intensities. Magnetic susceptibilities and the well‐represented presence of fine pedogenic ferrimagnets suggest that Pliocene rainfall was greater than the local modern rate of 200 mm/year and may have been within the range of 500–1,000 mm/year. The study highlights the need to expand the database on Vertisols Fe oxides and magnetism, especially considering the anomalously dry and bimodal rainfall seasonality of East Africa.