Reply to: Evidence confirms an anthropogenic origin of the Amazon dark lands

Lombardo et al. argue that, if our hypothesis is correct, ADEs should be continuous rather than unequal. However, alluvial deposition can be an uneven process and the distribution of large and small ADE patches can be regionally predicted based on fluvial geomorphology. For example, 89% of all known ADEs were predictively mapped using elevation, distance to bluff, and geologic provenance as primary predictors (with a 6.5% false negative rate and a false positives of 4.7%).ten. The predicted areas include small and large patches of ADEs, up to several square kilometres, and indicate that ADEs cover approximately 154,000 km2 mainly in central and western Amazonia. This may seem like a very large area (>3% of the Amazon basin), but it is only a fraction of the projections found in some of the most cited anthropic theories.11. Assuming the same excess fertility observed at our site, the creation of these ADEs would have required a prohibitive amount of biomass burning, in areas 800 to 1680 times larger (Fig. 1), which is incompatible with the deposition small-scale centralized system proposed by Lombardo et al. In this regional scenario, it is still unclear how many Amazons would have been needed to create the already mapped ADEs.

Lombardo et al. center their opinion on settlements in other parts of the Amazon basin, in different socio-ecological and geomorphological contexts, and where the data we have developed are not available for comparison. Their account confuses the Brazilian plain with other regions, such as the Llanos de Moxos and other systems in the Bolivian-Peruvian foreland basins, where older archaeological sites are found. Their comments on the mineral composition of ADEs seem to contradict recent findings (made by some of their co-authors)12 which show that some oxides found at our ADE site have “no relation to anthropogenic activity” because “their sources are attributed to the weathering of micas, feldspars, mafic minerals (pyroxene) and sodium plagioclase” which are not found locally. To explain the inconsistency between these results and the current theory of ADE formation, Macedo et al. argue that “sediment deposition in floodplain soils” that “is unrelated to human occupation” should be considered. This suggestion is consistent with our data which indicates the deposition of exogenous materials at the site before the invention of agriculture in the central Amazon.

Our study area is on a tertiary terrace, and we recognize in our paper that it is above the modern flood height of 100 years for Manaus. However, significant Pleistocene and Holocene tectonic activity and river aggradation/degradation have clearly affected flood heights over time. A complex neotectonic history affected terrace elevations, nutrient deposition and remobilization, and flood heights and worsening, resulting in higher base levels that today were several meters above the waters flood over the past millennia.13,14,15. In addition, the rivers transported and dispersed sediments from the Andes to the lowlands, which were unevenly remobilized and redeposited, from the floodplains several times between 20 and 5 thousand years ago.16,17,18. Such mineral inputs by past avulsion events may have occurred earlier in the Quaternary and remain as relict soil where it has not subsequently eroded.19. The older weathered sediments on the upper terraces bordering the river bear no resemblance to recent alluvium, and the distribution of elements and their assemblages at our site are consistent with alluvial deposits at other sites. This process is explained in studies cited by Lombardo et al. (eg, Pupim et al.), who note several periods of river aggradation, which support our hypothesis.

As explained in our original paper, our data does not rule out a more recent human effect on the local landscape. The wisdom of indigenous peoples, manifested in the spreading of waste on agricultural sites (since at least the late Holocene), may have further enriched ADEs or countered their natural degradation. Recent studies12, 16, 17, which are later than the studies of Lombardo et al. cite to argue against geogenic influence, reveal dynamic neotectonic history, and support our hypothesis. Thus, the extent to which other ADE sites arise from depositional processes should be investigated based on evidence beyond that presented by Lombardo et al.

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