Ramped pyrolysis 14C age spectra of riverine particulate organic matter through a record hydrograph – the Great Atchafalaya Flood of 2011

Abstract:

Episodic flood events can have major effects on biogeochemistry of estuary and deltaic systems. In 2011, the Great Flood event that transpired in the Missouri-Mississippi River watershed resulted in likely the highest discharge of water and sediments at the Atchafalaya outflow in several thousand years (since the last time the mainstem discharge flowed through this area). We sampled the event hydrograph starting near the peak and measuring through the falling leg of the flood event. Here we describe particulate organic carbon (POC) age spectra from these samples referenced to selected samples from a >60 month time series of samples from several sites along the Atchafalaya outlet and building submarine delta. Similar to past work on high-water events, age spectra become older during the flood event. However, this flood event did not return ages as old as previous events analyzed, suggesting a provenance control on age spectra. Age spectra showed relatively youngest ages during the return to baseline conditions immediately following the flood hydrograph and then returned to intermediate ages afterwards. Our response to the event missed the early rising limb of the event, when sediment transport was highest, however the peak and falling limb of the Atchafalaya hydrograph were well-represented in our time series. We hypothesize that during the initial pulse of sediment, age spectra may have been older, but subsequent discharge at near bank-full and over bank flow at some river reaches carried more recent POC than initial erosion of bank deposits during the rising limb of the flood. Still, the age spectra observed in an extreme event do not approach the breadth of the spectra observed in more erosive and steep mountainous river systems.