Sedimentation on river floodplains is a complex process that involves overbank flooding, crevasse splaying, and river avulsion. The resulting floodplain stratigraphy often exhibits floodplain aggradation cycles with alternating fine-grained overbank flooding deposits that underwent significant petrogenesis, and coarser-grained, avulsion-belt deposits largely devoid of pedogenic impact. These cycles are linked to lateral migration and avulsion of channels driven by internal dynamics, external factors, or a combination of both. To better understand the spatial and vertical variability of such floodplain aggradation cycles, we map these in three dimensions using a photogrammetric model of the lower Eocene Willwood Formation in the northern Bighorn Basin, Wyoming, USA. This allows identifying 44 floodplain aggradation cycles in ∼300 m of strata with an average thickness of 6.8 m and a standard deviation of 2.0 m. All the cycles are traceable over the entire model, pointing to their spatial consistency over the 10 km2 study area. At the same time, rapid lateral thickness changes of the floodplain aggradation cycles occur with changes up to 4 m over a lateral distance of 400 m. Variogram analyses of both field and numerical-model results reveal stronger consistency of floodplain aggradation cycle thicknesses along the paleoflow direction compared to perpendicular to paleoflow. Strong compensational stacking occurs at the vertical scale of 2–3 floodplain aggradation cycles (14–20 m), while full compensational stacking occurs at larger scales of more than six floodplain aggradation cycles (>41 m). The lateral and vertical thickness variability of the floodplain aggradation cycles, as well as their compensational stacking behavior, are interpreted to be dominantly driven by autogenic processes such as crevasse splaying and avulsing that preferentially fill topographic lows. External climate forcing may have interacted with these autogenic processes, producing the laterally persistent and vertically repetitive floodplain aggradation cycles. The spatial variability of floodplain aggradation cycles demonstrated in this study highlights again the need for three-dimensional data collection in alluvial floodplain settings rather than depending on one-dimensional records.

Alluvial stratigraphy builds up over geologic time under the complex interplay of external climatic and tectonic forces and internal stochastic processes. This complexity makes it challenging to attribute alluvial stratigraphic changes to specific factors. Geological records indicate pronounced and persistent climatic changes during the Phanerozoic, while the effects of these changes on alluvial stratigraphy remain insufficiently documented. We provide evidence for 405 k.y. long-eccentricity climate forcing of alluvial stratigraphy in the lower Eocene Willwood Formation of the Bighorn Basin, Wyoming (USA). Two ∼90-m-thick intervals, characterized by a relative paucity of sand, dominance of sinuous-river channels, and floodplain sediments with better-developed paleosols, coincide with eccentricity maxima as determined through integrated stratigraphic methods. These intervals are interspersed with three contrasting intervals, marked by relatively high sand content, prevalent braided-river channels, and less-developed paleosols, corresponding to eccentricity minima. A comprehensive genetic model that integrates climate, source-to-sink system, and alluvial dynamics to explain these findings remains to be elucidated. Given the consistent presence of the 405 k.y. eccentricity cycle throughout Earth’s history, it is plausible to infer that its influence may be discernible across a wide array of alluvial stratigraphic records.

In this Article, a processing error led to the wrong versions of Fig. 3 and Extended Data Fig. 4 being published. Figure 3e did not include the entirety of the eastern Africa soil carbonate δ13C database as compiled by ref. 13. Fig. 3 of the original Article has been corrected, and Fig. 1 of this Amendment shows the original and corrected Fig. 3 side by side, for transparency. In the Methods section of the original Article, there are further details about how this record has been produced. The last paragraph of the Methods has been corrected; the original text was: “On the basis of ref. 13, time series of δ13C values from soil carbonate were combined for the Omo-Turkana Basin and the southern Kenyan-Tanzanian sites using their medians, and interquartile ranges using six-data-point bins.” Furthermore, the original version of Extended Data Fig. 4 did not display data from eastern African hominin site Afar; the figure and caption have been updated accordingly, and the original and corrected versions are shown as Fig. 2 to this Amendment. These changes do not alter any inferences drawn from the data. These errors have been corrected in the online version of the Article.

The middle Eocene Dongying sag in the Bohai Bay Basin of China has an estimated shale oil resource of approximately 1.1 billion t (8.06 billion bbl); flows of shale oil have been produced in the succession from tens of wells, where the daily production of a single well generally varies between 10 and 100 t (73.3–733 bbl). Therein, the mudrock successions composed of meter-scale mudstone–limestone couplets are the most important shale oil-producing layers. The controls on the deposition of the meter-scale mudstone–limestone couplets, however, remain enigmatic, constraining the analysis of lithofacies and, therefore, sweet spot distributions. Here, we analyze three continuously cored organic-rich successions of mudstone–limestone couplets (371 m [1217 ft] in total) in the middle Eocene Dongying sag, accompanied by decimeter- to meter-scale sampling and testing of mineralogy, organic geochemistry, and paleontology of the rocks. Our integrated cyclostratigraphic analysis shows that the observed mudstone–limestone couplets occur at periods that coincide with Milankovitch periodicities; 21-k.y. precession cycles are the main driver of the meter-scale mudstone–limestone couplets, with additional imprints of 41-k.y. obliquity cycles. Specifically, precession minima are associated with high summer insolation and consequently high summer monsoonal precipitation, which increased river discharge and terrigenous input to the basin, resulting in the deposition of siliciclastic-rich mudstones. In the study, low summer insolation during precession maxima led to decreased summer monsoonal precipitation, lower river discharge and terrigenous input, and increased lake water salinity, resulting in the deposition of authigenic lime mudstones. The shale reservoir quality kept pace with the orbital climate changes; compared with lime mudstones deposited during precession maxima, mudstones deposited during precession minima had higher total organic carbon, porosity, and oil content, but lower brittleness.

Today, the eastern African hydroclimate is tightly linked to fluctuations in the zonal atmospheric Walker circulation^1,2. A growing body of evidence indicates that this circulation shaped hydroclimatic conditions in the Indian Ocean region also on much longer, glacial–interglacial timescales^3–5, following the development of Pacific Walker circulation around 2.2–2.0 million years ago (Ma)^6,7. However, continuous long-term records to determine the timing and mechanisms of Pacific-influenced climate transitions in the Indian Ocean have been unavailable. Here we present a seven-million-year-long record of wind-driven circulation of the tropical Indian Ocean, as recorded in Mozambique Channel Throughflow (MCT) flow-speed variations. We show that the MCT flow speed was relatively weak and steady until 2.1 ± 0.1 Ma, when it began to increase, coincident with the intensification of the Pacific Walker circulation^6,7. Strong increases during glacial periods, which reached maxima after the Mid-Pleistocene Transition (0.9–0.64 Ma; ref. ⁸), were punctuated by weak flow speeds during interglacial periods. We provide a mechanism explaining that increasing MCT flow speeds reflect synchronous development of the Indo-Pacific Walker cells that promote aridification in Africa. Our results suggest that after about 2.1 Ma, the increasing aridification is punctuated by pronounced humid interglacial periods. This record will facilitate testing of hypotheses of climate–environmental drivers for hominin evolution and dispersal.