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

先前的研究表明，海洋峡湾负责掩埋全球大量的有机碳（OC）。了解峡湾中陆地OC的埋藏在全球范围内是很重要的，因为之前的研究估计，仅仅是细粒度的峡湾沉积物就将全球有机碳通量的11%埋进了海洋。最近的研究表明，小河流洪水沉积的粗粒度沉积物将显著增加峡湾中埋藏的有机碳量。异常大而不频繁的洪水，如冰湖溃决洪水（GLOFs），可能会将大量的OC运输和掩埋到粗粒度的峡湾沉积物中，但由于其不频繁的性质，它们的作用仍然很少受到限制。然而，限制海洋沉积物中全部的OC埋藏预算是重要的，因为这些沉积物是大气CO2的第二大汇，因此有助于气候的长期调节。2020年11月28日，加拿大不列颠哥伦比亚省Elliot Creek发生了一次特殊的GLOF，这为我们研究这种极端事件的OC埋葬效率提供了一个独特的机会。与该事件同时发生的测量到的4.9 Mw地震活动的重建表明，最初的滑坡移动了3000万吨沉积物，并在冰湖中引发了约100米高的位移波。这一波又冲破了湖中的冰碛坝，释放出大量的水，在进入峡湾之前猛烈地冲刷了艾略特溪。这一事件对海上交通和沿海建筑造成了危害，并摧毁了Homalco原住民的重要鲑鱼、野生动物和森林资源。埃利奥特溪事件移动了大量的沉积物和OC，最终沉积在峡湾。幸运的是，在2008年至2020年期间，butte Inlet曾经是一个非常详细的海底调查时间序列的地点，因为它被绘制了十次。我们小组最近在布特湾的工作表明，浊流是如何将年轻的陆地OC从河口输送到更深的峡湾水域的。广泛的水深背景数据、当前动态和正常条件下峡湾OC的地球化学特征，为研究大型GLOFs的OC埋藏提供了独特的机会。该提案旨在验证两个假设：1)大型GLOFs可以将大量的沉积物和OC运输到远叶；2)年轻的陆生有机物可以有效地埋在这些深水峡湾沉积物中。在这里，效率指的是从源到汇的直接传输，而不是通常情况下OC只能通过许多交错的较小传输事件到达该汇。由于在过去十年中反复绘制、监测和采样的布特湾的异常洪水和世界领先的基线数据相结合，这个机会是独一无二的。此外，这个机会是有限的，因为正常的浊流活动在今年春天随着雪开始融化而恢复。这些活动将开始重做，模糊和埋葬这个GLOF的签名。这些假设将通过绘制整个峡湾与glof有关的沉积物并描述其OC成分来进行测试。现场数据收集将结合新的水深测量和沉积物采样，可以与事前调查和岩心进行比较。沉积物和有机碳的表征将包括粒度分析、XRD、总有机碳、C-N-S、稳定碳同位素和放射性碳组成，以及斜坡热解氧化分析。结合常规有机地球化学（TOC、CNS、d13C和14C）和纹理数据，初步进行有机质（海相或陆相）指纹识别。通过RPO进一步分离OC，再加上d13C和14C的测量，将详细说明OC的来源，正如我们的团队之前所证明的那样。

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.