Autoxidation as a major player in the fate of terrestrial particulate organic matter in seawater

Abstract

The Rhône River plays a major role in the Mediterranean Sea, being both its main freshwater source and its major particulate matter provider. This survey of the fate of terrestrial particulate organic matter (POM) was conducted along the salinity gradient of the Rhône River plume, between 2012 and 2014. It revealed that autoxidation acts rapidly and intensely upon the POM's arrival at sea, with α‐amyrin and β‐amyrin autoxidation rates going from 12.9 ± 2.9% to 45.0 ± 6.4% and 10.7 ± 4.0% to 50.3 ± 4.4%, respectively, between fresh water (salinity 0) and seawater (salinity 38). These compounds, being unambiguous markers of the terrestrial origin of POM, allow us to unequivocally characterize the POM as terrestrial. While it was originally believed that a desorption of redox‐active trace metal ions was the favoring factor that kick‐started this intense autoxidation, this study evidences no trace metal desorption in the Rhône River mixing zone and hence no correlation between high autoxidation rates and the presence of trace metal ions. Autoxidation rates however were very well correlated with salinity levels within the river plume, with r2 reaching 0.801, 0.962, and 0.943 for sitosterol, α‐amyrin, and β‐amyrin, respectively, in November 2014.

Lipoxygenase-induced autoxidative degradation of terrestrial particulate organic matter in estuaries: A widespread process enhanced at high and low latitude

“…Cleavage was attributed to some redox-active metal ions released from suspended particulate matter (SPM) in the mixing zone of riverine water and marine water. Using new lipid tracers specific to the degradation of terrestrial higher plants at the mouth of the Rhône River (Galeron et al, 2016a,b;Rontani et al, 2015), we confirmed the role played by autoxidation in the degradation of terrestrial POM (TPOM) in estuaries (Galeron et al, 2017). As in the Beaufort Sea (Rontani et al, 2014a), autoxidation rate and salinity level appeared to correlate well within the river plume in the Mediterranean Sea (Galeron et al, 2017).…”

“…1). Sampling and hydrographic conditions have been described by Galeron et al (2017). High frequency monitoring of the nutrients and of the PM input from the Rhône to the Mediterranean has been carried out since 2010 in the framework of the national program MOOSE (Mediterranean Ocean Observing System for the Environment).…”

“…SPM samples from the Mackenzie River were collected in Tsiigehtchic (67°27′7′′N, 133°45′32′′W) in June 2011 ( Fig. 1), using the same protocol as that used for the Rhône plume SPM collection (Galeron et al, 2017). Sediment material was collected at station M434 near to the river mouth (70°10′12′′N, 133°35′24′′W) as part of the ArcticNet and IPY-CFL system studies on board the CCGS Amundsen in 2008.…”

“…In order to determine the degradation kinetics of the Rhône SPM upon reaching seawater, we incubated SPM from the river at different salinity values. Rhône water was collected on November 18, 2014, during a flood (water flow 3700 m 3 /s, SPM 217.1 mg/l, POC 119.01 μM) and seawater (salinity 38 g/kg) during the MORTIMER sampling campaign of November 12, 2014 (Galeron et al, 2017). Dilution was carried out with 1 l glass flasks using Rhône water and filtered 0.7 µm seawater in order to reach salinity of 0 (Rhône water only), 15 and 25 g/kg.…”

Electron ionization mass spectrometry fragmentation and multiple reaction monitoring quantification of autoxidation products of α‐ and β‐amyrins in natural samples

“…The low autoxidation percentages of amyrins measured in the samples investigated (Table ) give good support to these previous observations. Interestingly, the autoxidation percentages (Table ) also appeared to be considerably lower in the Têt River than in the closer Rhône River . These differences were attributed to the size of the particles analysed (>63 μm in the Têt River and > 0.7 μm in the Rhône River).…”