How efficient is organic carbon burial in deep-water sediments after the extreme glacial lake outburst flood in Elliott Creek (BC, Canada)

Previous work suggests that marine fjords are responsible for burying globally significant amounts of organic carbon (OC). Understanding the burial of terrestrial OC in fjords is important on a global scale, as previous work estimated that just the fine-grained fjord sediments are responsible for burying ~11% of global organic carbon flux into the oceans. More recently it was shown that coarse-grained sediment deposited by small rivers floods will significantly increase that amount of OC burial in fjords. Exceptionally large and infrequent floods, like glacial lake outburst floods (GLOFs), are likely to transport and bury larger amounts OC into coarse-grained fjord deposits, however their role remains poorly constrained due to their infrequent nature. Yet, constraining the full OC burial budget in marine sediments is important as these sediments are the second largest sink of atmospheric CO2 and thus contributes to long-term regulation of climate.

An exceptional GLOF occurred on 28th Nov 2020 at Elliot Creek (BC, Canada) that now provides us a unique opportunity to study the OC burial efficiency of such extreme events. Reconstruction from the measured 4.9 Mw seismic activity that coincided with the event indicates the original landslide dislodged 30 million tonnes of sediment and triggered a ~100m high displacement-wave in the glacial lake. This wave in turn broke through the lake's moraine-dam, releasing large amounts of water, which powerfully scoured Elliott Creek before entering into the fjord. The event posed a hazard to marine traffic and coastal structures and destroyed significant salmon, wildlife and forest resources of Homalco First Nations.
The Elliot Creek event moved large volumes of sediment and OC, which are ultimately deposited in the fjord. Fortuitously, Bute Inlet was previously the site of an extremely detailed time series of seabed surveys, as it was mapped ten times in between 2008 up to 2020. Recent work by our group at Bute Inlet has shown how turbidity currents transport young terrestrial OC from the river mouth down to the deeper fjord waters. The extensive background data on the bathymetry, the current dynamics and the geochemical signatures of the OC in the fjord under normal conditions now enable a unique opportunity to study the OC burial of large GLOFs.

This proposal aims to test two hypotheses, which are: 1) large GLOFs can transport vast amounts of sediment and OC to the distal lobe and (2) young terrestrial organic matter can be efficiently buried in these deep water fjord sediment. Here, efficiency refers to direct transport from source to sink, as opposed the normal conditions in which OC only reaches this sink through many staggered smaller transport events. The opportunity is unique due to the combination of an exceptional flood and world-leading baseline data of Bute Inlet, having been repeatedly mapped, monitored and sampled over the last decade. Additionally, this opportunity is time limited, as normal turbidity currents activity returns this spring as the snow start to melt. These activities will start to rework, blur and bury the signature of this GLOF.

The hypotheses will be tested by mapping the GLOF-related deposits throughout the fjord and characterising their OC components. Field data collection will be a combination of new bathymetric survey and sediment sampling, which can be compared to pre-event surveys and cores. Sediment and organic carbon characterisation will include grain-size analysis, XRD, total organic carbon, C-N-S, stable carbon isotope and radiocarbon composition as well as ramped pyrolysis-oxidation analysis. Combination of all routine organic geochemistry (TOC, CNS, d13C and 14C) and textural data will be used for initial fingerprinting of OC (marine or terrestrial source). Further separation of OC by RPO coupled with d13C and 14C measurements will detail the OC source, as previously demonstrated by our group.