Geochemical signatures and mercury stable isotopes associated with sedimentary processes at the Amazon River mouth during the Quaternary

The Amazon River is the largest fluvial system on Earth in terms of water and sediment discharge, exporting approximately 1.2 million tons of sediment per year to the Atlantic Ocean [1]. This flux modulates sedimentary and biogeochemical processes along the equatorial Atlantic margin and Amazon River mouth, reflecting interactions between continental, oceanic, and atmospheric processes [2]. This study evaluates three sediment cores collected at AMARYLLIS-AMAGAS II cruise (2023) in the Amazon River mouth region along a shelf–slope gradient, at water depths of 70 m (outer shelf), 696 m (upper slope), and 1696 m (mid-slope). The cores were collected using a CASQ corer and reach lengths of up to 11 meters. A multi-proxy approach was applied, including total organic carbon (TOC), inorganic carbon, calcium carbonate (CaCO₃), major and trace elements, rare earth elements (REEs) normalized to PAAS, as well as total mercury (Hg) and its stable isotopes. Geochemical ratios such as Ca/Ti, Al/Ca, and Ti/Al were used to evaluate the balance between terrigenous and carbonate components. The results indicate significant geochemical variability along the bathymetric gradient. Overall, the cores display TOC values between 1.1 - 3.3%, inorganic carbon between 1.21 - 5.81%, and CaCO₃ contents ranging from 10 to 48%. The shelf core (70 m) shows the highest variability, with CaCO₃ between 15 - 30% and fluctuations in Ca/Ti, Al/Ca, and Ti/Al ratios, reflecting hydrodynamic influence and sediment reworking. The upper slope (696 m) exhibits intermediate behaviour, with more moderate CaCO₃ contents (10–15%), indicating mixing between shelf signals and sediment transfer to deeper ocean environments. In contrast, the deeper slope core (1696 m) records a more integrated signal of sediment export and oceanic deposition, with elevated CaCO₃ contents in the upper intervals (48%), Ti/Al ratios increasing with depth, and reduced carbonate contents (< 20%), indicating enhanced terrigenous input in deeper intervals. PAAS-normalized REE patterns were parallel across all cores, indicating a relatively constant continental source consistent with the upper continental crust. The records show light to moderate enrichment of light REEs relative to heavy REEs (La/Yb 1.1), no Ce anomalies and positive Eu anomalies. Mercury isotope data show δ²⁰²Hg values between −0.9 and −1.6‰, indicating mass-dependent fractionation (MDF) dominated by light isotopes associated with terrigenous input, whereas Δ¹⁹⁹Hg (−0.35 to 0.00‰) and Δ²⁰¹Hg (−0.30 to 0.00‰) values indicate the influence of photochemical processes in the water column, such as Hg(II) photoreduction and methylmercury photodemethylation. Overall, the records suggest changes in oceanic and atmospheric processes in the Amazon River mouth influenced the sediment transport and deposition along the Amazon margin during the Quaternary.

[1] D. Feng, et al., Nat Commun 16 (2025) 3148.

[2] C.A. Nittrouer, et al., Annu. Rev. Mar. Sci. 13 (2021) 501–536.