Paleoenvironmental changes in a lower Rio Negro floodplain lake (Anavilhanas Archipelago,Brazil) during the past 4550 years

Abstract

This study investigates Holocene climate variability and its influence on organic carbon accumulation patterns in the Anavilhanas Archipelago, Rio Negro, Brazilian Amazon. We analyzed a 404 cm sediment core from a floodplain lake to reconstruct paleoenvironmental changes and carbon storage dynamics over the last 4550 cal yr BP. The Middle to Late-Holocene transition (4550–4270 cal yr BP) was characterized by significant hydrological changes. Low chlorophyll derivative values during this period reflect elevated water levels and variable discharge regimes, consistent with a braided channel system exhibiting high hydrodynamic energy and unstable fluvial architecture. These conditions prevented the establishment of stable lentic environments, resulting in highly variable deposition of organic compounds. Between 4270 and 3865 cal yr BP, sedimentary and organic proxies showed pronounced variability, indicating shifts in precipitation patterns and fluvial geomorphology. Organic matter exhibited high C/N ratios and δ¹³C values, indicating a predominantly allochthonous origin, likely from vascular plant material with C4 signatures. Carbon accumulation rates peaked at 196 g m⁻² yr⁻¹ around 4050 cal yr BP. The period 3865–3200 cal yr BP marked the establishment of arboreal vegetation alongside increased lacustrine productivity with enhanced algal contributions, signaling a transition to more humid conditions. Maximum primary productivity occurred between 3200 and 2100 cal yr BP, coinciding with the development of the current levee-lake morphology, with carbon accumulation averaging 18 g m⁻² yr⁻¹. From 2100 to 760 cal yr BP, sediment records indicate an intensified dryness compared to preceding and subsequent periods, with reduced carbon accumulation rate of only ~15 g m⁻² yr⁻¹. The most recent 760 years, particularly the last two centuries, show significantly increased carbon fluxes (38.1 g m⁻² yr⁻¹) associated with wetter conditions. Our results demonstrate that Late-Holocene hydroclimatic variations, including increased humidity and seasonal variability, exerted fundamental control on sedimentary dynamics, lacustrine productivity, and carbon storage in the Rio Negro floodplain system.