Paleoclimate variations in Western Amazon based on Lago Airo (Brazil) diatoms from the last 13,300 years cal BP

The study of paleolimnological and paleoclimatic variations in the Amazon from the last 13,300 cal yr BP is essential for interpreting the climatic and environmental history of this region, as it is one of the largest and most diverse biomes on the planet. Diatom assembly (density and relative abundance), sedimentological, bulk, and isotopic organic geochemistry analyses were obtained from a 146 cm-long core collected in a marginal lake nowadays disconnected from Rio Negro (Lago Airo; 00º19’37.225’’S, 66º08’33.266’’W). Seven organic matter samples were dated by using AMS radiocarbon determination. The 73 species found mostly comprised acidophilic species. We discerned four main phases: Phase IV, from ca. 13,400 cal yr BP to ca 11,800 cal yr BP: diatoms density was relatively low (i.e., ranging from 88 to 37.5 x 10³ valves/g) with mainly periphytic taxa. Sediments are comprised of coarser grains (sand > 99%). Geochemical data shows a TOC average of 2.2%, the C/N ratio is higher (average = 52.1), and chlorophyll derivatives are lower (average = 5.6 SPDU). 15N values show a source related to nitrogen atmospheric fixation metabolism, probably due to the extremely oligotrophic nature of the system. Our results suggest that Phase IV represented the Lago Airo as a lotic-like ecosystem with connections with the Rio Negro, characterized by high energy and thereby low diatom sedimentation. Phase III: 11800 to 8800 cal yr BP: The diatom community kept characteristics of low density with the domain of Eunotia spp (E. hirudo, E. bilunaris, and E. floweri). This phase exhibits a significant increase in clay content of around an order of magnitude, rising from 0.256% in phase IV to 2.05% in phase III. Although the absolute values are still low, they still constitute a sandy-textured sediment. TOC values increased compared to the previous phase, reaching 2.39%, with an increase in the C/N ratio denoting the transport of detrital organic matter from the basins. Phase II: 8800 to 4100 cal yr BP: The diatom community keep characteristics of phase III. This phase exhibits a decrease in clay content to 0.299% with a progressive increase in TOC, compared to the previous phase, reaching 3.47%. This increase in C/N ratio indicates the transport of detrital organic matter from the basins. On the onset of phase I, the Rio Negro became disconnected from Lago Airo, which is suggested by the predominance of silt, associated with low hydrodynamics, and an increase in diatom valve density. Phase I: 4100 to actual: This phase shows an increased diatom valves density (between 30.9 x 104 and 14.1 x 106), composed mainly by planktonic taxa. Silt became dominant (average = 59.1; clay = 10.2) simultaneously with. Higher silt contribution with a pronounced decrease in C/N ratios (average = 34.9) is indicative of declining influence of the Rio Negro over Lago Airo and an increase in autochthonous production, represented by the increase in TOC, changing from 3.47 in phase II to 25.0%, and chlorophyll derivatives (average = 12.7 SPDU). Our results suggest an increase in the mean water level in Lago Airo (characterized by higher valves/g, planktonic taxa dominance - Aulacoseira spp. - A. distans and A. granulata, mainly, and δ15N values increase), which was probably the result of higher precipitation regimes in the South American Monsoon System.