Paleohydrological and Paleolimnological variations in Western Amazon based on multi-proxy data from Lago Airo (Brazil) from the last 15,050 years cal BP

Abstract

Understanding paleolimnological dynamics in the Rio Negro floodplain provides a window into broader paleoclimatic variability and long-term environmental change across western Amazonia. Here, we analyzed diatom assemblages (relative abundance and valve density; valves g −1 ), grain size, bulk organic geochemistry (TOC and C/N), chlorophyll derivatives (SPDU), δ 13 C and δ 15 N in a 146 cm sediment core from Lago Airo, recovering the last 15,050 years, based on seven radiocarbon dates. A total of 73 diatom taxa were identified, dominated by acidophilic species, revealing four successive stratigraphic Units that document a long-term transition from a river-influenced and low-productive environment to a more internally driven lacustrine system. Unit IV (15,050–10,100 cal yr BP) is sand-dominated (> 99%) with low diatom valve density, low TOC, very high C/N, low chlorophyll derivatives, and low δ 15 N values, indicating an oligotrophic setting strongly influenced by fluvial inputs. The very high C/N ratios likely reflect the dominance of allochthonous, humic-rich organic matter (humic/fulvic acids) leached from surrounding soils and forest litter, which is carbon-rich and nitrogen-poor, simultaneously reflecting strong terrestrial inputs and low in-lake productivity. This condition persists into Unit III (10,110–8540 cal yr BP), with intermittent peaks in clay and TOC and/or in planktonic taxa that reflect short-lived lentic conditions and in Unit II (8540–3370 cal yr BP), except for a slight upward increase in TOC and silt. The higher relative contribution of benthic taxa in this Unit suggests shallower conditions, in line with the Middle Holocene dry phase reported for other Amazonian sites. As a result of lower water levels and increased hydrological variability, C/N ratios increased markedly, indicating a stronger terrestrial imprint on organic matter; in this context, the modest increase in TOC toward the top may partly reflect enhanced allochthonous inputs. A marked shift occurs in Unit I (since ~ 3370 cal yr BP), when silt becomes dominant and diatom valve density rises sharply with the expansion of planktonic taxa (notably Aulacoseira distans and A. granulata ). This interval is accompanied by higher TOC and chlorophyll derivatives, lower C/N, and higher δ 15 N, indicating reduced riverine influence and a stronger expression of lacustrine conditions with increased autochthonous production. Across the entire profile, δ 13 C varies only modestly, indicating a persistent dominance of C 3 -derived organic matter.

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