Temporal and Water Column Variability in Particulate Organic Carbon Composition on the Amazon River Main-stem

Abstract:

The Amazon River exports ~14 teragrams (0.014 gigatons) of particulate organic carbon (POC) to the Atlantic coast each year, ~15% of the global riverine carbon source to the oceans. Understanding the source and fate of this exported POC is complicated by (1) hydrodynamic sorting of suspended particles in the river cross-section, and (2) seasonality in discharge over the hydrological cycle. Here, we characterize suspended POC composition down the water column (surface-to-bed) and through time (rising discharge in April 2014, falling discharge in July 2014) to assess the extent of and mechanism underlying this variability. Depth-specific sampling of the river cross-section took place at Óbidos, the most downstream gauging station on the main-stem, and was coupled to an Acoustic Doppler Current Profiler to calculate export fluxes at both times. Between April and July, raw water discharge increased from 150,000 to 250,000 m³/s. Bulk compositional features (e.g., % OC, δ¹³C, C/N) varied with both hydrodynamic sorting and seasonality, while thermal stability, derived from ramped oxidation of the suspended sediments, did not differ with depth or season. Compound-specific δ ¹³C of extracted lipids varied seasonally, as well. We plan to supplement these preliminary data with measurements of POC ¹⁴C content across space and time. The observations thus far suggest that the variability in suspended POC composition with depth and season is dominated by physical changes in source. Moreover, the similarities in thermal stability suggest that POC reactivity, and relatedly, its fate downstream and ultimately in the coastal ocean, is relatively invariant across these variable sources.