The drilling itself was an epic of endurance. Equipment was mobilized roughly 4,400 kilometers from Mariana in southeastern Brazil to the municipality of Rodrigues Alves, arriving in May 2023. Using a wireline diamond coring rig, the team began coring on 16 June 2023 with PQ tools producing cores 8.3 centimeters in diameter, progressively narrowing to HQ tools as depth increased. The original target was 2,000 meters, deep enough to reach the base of the Cenozoic fill, but the operation was halted at 923 meters after a cascade of setbacks. Alternating layers of unconsolidated sand and strongly cemented mudstone repeatedly defeated the engineers: casing had to be extended far deeper than planned at an added cost of about 150,000 US dollars, a casing string was dropped into the hole and had to be fished out, and at 750 meters the drill string became stuck by differential pressure and was ultimately cemented in place. When the borehole blocked at roughly 752 meters and two sidetrack attempts failed, the project’s executive committee chose to abandon the hole in December 2023 to preserve funds for a second site in the Marajó Basin. Even so, the team achieved 93 percent core recovery, an exceptional figure under such conditions.

What the core revealed surprised the sedimentologists. Apart from the uppermost 12 meters, the entire sequence belongs to a single lithostratigraphic unit, the Solimões Formation, dominated by feldspar-rich sands of Andean origin. The sediments record four facies associations: fining-upward channel and bar deposits up to 25 meters thick, cross-laminated fine sandstones, inclined heterolithic successions of sand and mud, and laminated claystones laid down on floodplains. Together they paint a picture of a vast, dynamic river system with migrating channels, exposed floodplains, and recurring soil formation, punctuated by paleosols bearing carbonate concretions and, in the deepest levels, gypsum crystals. Notably, the core contains no marine or brackish palynomorphs, weighing against long-standing hypotheses of marine transgressions or mega-lakes in the western Amazon during this interval and supporting instead a fluvial world akin to today’s great rivers and floodplain lakes.

The provenance story is unambiguous. Detrital zircon uranium-lead geochronology on sandstones, analyzed at rates of 300 grains per hour at the University of Arizona LaserChron Center, shows multi-modal age populations dominated by Permo-Triassic grains, with maximum depositional ages increasing steadily down-core: 5.8 plus or minus 0.21 million years at 178 meters, 7.8 at 300 meters, 8.6 at 497 meters, and 11.8 plus or minus 0.4 million years near the base at 863 meters. Heavy mineral assemblages rich in augite, hypersthene, and hornblende mirror those of the modern Amazon River channels, pointing to basic igneous rocks in the Andes as the dominant sediment source. Neodymium and strontium isotopes plot squarely within the field of eastern tropical Andean sediments, matching the suspended loads of the Solimões and Madeira rivers, the Amazon’s two great Andean tributaries. Feldspar-rich sands with low luminescence sensitivity quartz confirm immature, rapidly eroding Andean sources throughout most of the drilled section.

The pollen record may prove the core’s greatest gift to science. Of more than 1,800 palynomorph species identified so far, the assemblage differs markedly from Early to Middle Miocene floras recovered elsewhere in the western Amazon, such as at Contamana in Peru and in the Solimões Basin. Key biostratigraphic markers, including the first appearance datum of Cyatheacidites annulatus at 512 meters and the last appearance of Grimsdalea magnaclavata at 154 meters, indicate the upper 512 meters is no older than about 6 million years. Many of the dominant floral elements of the famous Pebas wetland system are rare or absent, suggesting the core samples a post-Pebas ecosystem that has rarely been studied because its sediments are seldom preserved. The flora is dominated by angiosperms and is highly diverse, with samples yielding up to 183 taxa in a single count of 745 grains, many represented by only one or two individuals, a pattern strikingly reminiscent of modern rainforest. Grasses, cycads, and conifers remain consistently low, offering no evidence of extensive savannas or climates significantly drier than today’s.

The uppermost 12 meters tell a different tale. Resting on a major erosional unconformity, these quartz-rich, mature sands contrast sharply with the feldspar-dominated sediments below. Luminescence dating, with quartz optically stimulated luminescence and feldspar post-infrared infrared stimulated luminescence protocols, indicates the sediments are older than roughly 200,000 years, likely older than 250,000 years, and the shift in composition suggests a fundamental reorganization of drainage: possibly the moment the Acre region was uplifted and disconnected from Andean sediment supply, giving rise to the modern watersheds of the Juruá and Purus rivers with headwaters in the elevated Fitzcarrald Arch.

The project also probed the deep biosphere and the carbon cycle. Real-time monitoring of drilling mud gas using the ICDP Online Gas Analysis system detected methane-rich zones at 250 to 380 and 420 to 588 meters, with stable carbon isotope values of methane ranging from minus 35 to minus 25 per mil, hinting at a mix of microbial and thermogenic sources. Pore-water isotopes, extruded from whole-round core samples with a hydraulic press, show a lower deuterium excess than local rainwater, consistent with water-rock interaction over long residence times. Laboratory incubations of sediment from 140 and 502 meters produced carbon dioxide but no methane after 22 days, leaving open whether the methane measured during drilling derives from an active microbial system, a dormant ancient one, or gases migrating from depth. DNA extractions from 66 depths, despite nine different protocols, consistently fell below detection limits, underscoring the extremely low biomass of this deep subsurface habitat.

Chemical weathering proxies add another layer. X-ray fluorescence of 213 samples reveals that the iron-to-potassium ratio, a tracer of Andean versus lowland sources and of weathering intensity, averages 0.56, close to modern suspended sediments of the Solimões River. X-ray diffraction shows sandstones averaging 55 percent quartz and 22 percent feldspar, confirming their arkosic, mineralogically immature character. Natural gamma spikes at seven depths, most dramatically a sharp peak at 346 meters coinciding with a magnetic susceptibility maximum, are suspected to be altered volcanic ashes, which could eventually provide precise tephrochronological anchors for the sequence.

Why does this matter beyond Amazonia? The Late Miocene and Pliocene were pivotal on multiple fronts: most living Amazonian species emerged then, Andean uplift accelerated and reorganized drainages, and atmospheric carbon dioxide levels were similar to those of 2025 and the coming decades. A high-resolution record from this interval, in the heart of the world’s largest rainforest, offers a natural experiment in how tropical biodiversity responds to tectonic landscape change and greenhouse climates. The companion core from the Marajó Basin in eastern Amazonia, drilled in 2024, likely covers the same interval, promising a basin-wide comparison from the Andean foreland to the Atlantic margin.

The team also invested in the region’s people. Twelve educational activities, from workshops to hands-on sessions with cores, rocks, and river sands, reached 240 residents of Rodrigues Alves and surrounding communities, while an inventory of 11 geosites along the Juruá River, including the thrust-faulted landscapes of Serra do Divisor National Park, laid groundwork for geoconservation. A virtual tour built from 21 panoramic 360-degree images now traces a geocultural itinerary from the Divisor Range to the Croa River. For the scientists, however, the real treasure remains locked in the 860 meters of core now archived at the University of Minnesota, where 34 researchers from seven countries logged and sampled roughly 5,000 specimens for more than 30 types of analyses. As those analyses mature, the Acre core is poised to fill the single most conspicuous gap in the fossil record of the Amazon’s making.

Subject of Research: Sedimentology, geochronology, and paleoecology of a Trans-Amazon Drilling Project core from the Acre Basin, western Brazil

Article Title: Initial results from a Trans-Amazon Drilling Project core from the Acre Basin of Brazil

Article References: Fritz, S. C., Sawakuchi, A. O., Noren, A., Baker, P. A., Silva, C., Jaramillo, C., de Almeida, R. P., Janikian, L., Bezerra, I. S., Barbosa, M., Bertassoli, D., Blankenship, R., Chiessi, C. M., Feakins, S. J., Garcia, M. D. G., Gautheron, C., Grivna, B., Hartmann, G., Kunkel, C., … Zamudio, B. (2026). Initial results from a Trans-Amazon Drilling Project core from the Acre Basin of Brazil. Scientific Drilling, 35(1), 127-157. https://doi.org/10.5194/sd-35-127-2026

Image Credits: AI Generated

DOI: 10.5194/sd-35-127-2026

Keywords: Amazon rainforest, Trans-Amazon Drilling Project, Acre Basin, Solimões Formation, drill core, Miocene, Pliocene, paleoclimate, biodiversity, detrital zircon, palynology, Andean uplift