Abstract Submarine canyons are known to force ocean mesoscale circulation and local hydrodynamics. Alternate up- and down-canyon near-bottom flows have been widely documented along the upper reaches, connecting the canyon heads with the contiguous outer shelves and vice versa. Nonetheless, we still miss clear evidence of bedform fields expressing these complex patterns. In this study, through a multi-scale analysis in both space and time, we document rare asymmetric bedforms, up to 880 m long and 10 m high, developing within a depth range of 168-220 m at the head of the Whittard Canyon (NE Atlantic). One field of well-developed sandwaves has an atypical up-slope asymmetry, with the steeper slope facing the shallower regions of the shelf, and contrasting with surrounding down-slope sandwaves facing the canyon. The bedforms are interpreted to represent both up-slope and down-slope bottom currents connecting the upper reaches of the canyon to the outer shelf on the southern Celtic Margin, in the Bay of Biscay. The sandwaves were surveyed with shipboard Multibeam bathymetry (5 m grid cell resolution), AUV sidescan sonar (0.15 m grid cell resolution) and ROV footage, and sampled with three ROV-mounted vibro-cores and two box-cores. Sidescan sonar mosaics groundtruthed by sediment samples and ROV footage show with unprecedented detail spectacular trains of fresh overprinting megaripples, previously undocumented sand peaks and bowl-shaped depressions on the crests of the tallest sandwaves. Differences in sedimentary settings and benthic habitats indicate that these features are currently active in particularly dynamic areas, allowing for very slow migration of sandwaves. Numerical modelling together with concurrent hydrographic observations suggest large-amplitude semi-diurnal internal tides, possibly transitioning to asymmetric internal bores, as the main mechanism maintaining the mapped up-slope sandwaves. This work highlights the importance of uncommon sediment dynamics in canyon head environments and adds insight to the traditional notions of gravity-driven processes, being dominant in these environments, envisaging implications for improving geo-hazard assessment of mobile substrates and quantification of offshore sediment and carbon fluxes.

中文翻译：

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海底峡谷（东北大西洋）顶部的双向床型场

摘要 众所周知，海底峡谷会推动海洋中尺度环流和局部流体动力学。上游和下游峡谷附近的交替流动已被广泛记录在案，将峡谷头与相邻的外大陆架连接起来，反之亦然。尽管如此，我们仍然错过了表达这些复杂模式的床型场的明确证据。在这项研究中，通过多尺度空间和时间分析，我们记录了罕见的不对称床型，长达 880 m 和 10 m 高，在 Whittard 峡谷顶部 168-220 m 的深度范围内发育（东北大西洋）。一个发育良好的沙波场具有非典型的上坡不对称性，较陡的坡面向陆架较浅的区域，与周围面向峡谷的下坡沙波形成对比。床型被解释为代表连接峡谷上游和比斯开湾凯尔特边缘南部外大陆架的上坡和下坡底流。沙波通过舰载多波束测深（5 m 网格单元分辨率）、AUV 侧扫声纳（0.15 m 网格单元分辨率）和 ROV 镜头进行调查，并使用三个 ROV 安装的振动核心和两个箱形核心进行采样。沉积物样本和 ROV 镜头证实的侧扫声纳马赛克以前所未有的细节展示了壮观的新叠印巨型波纹、先前未记录的沙峰和最高沙波波峰上的碗状凹陷。沉积环境和底栖栖息地的差异表明这些特征目前在特别活跃的地区活跃，允许沙波非常缓慢的迁移。数值模拟与同时进行的水文观测表明，大振幅半日内潮汐可能会转变为不对称的内井，这是维持绘制的上坡沙波的主要机制。这项工作强调了峡谷头环境中不常见的沉积物动力学的重要性，并增加了对重力驱动过程的传统概念的洞察力，在这些环境中占主导地位，设想对改善移动底物地质灾害评估和近海沉积物和碳量化的影响流动。作为维持映射的上坡沙波的主要机制。这项工作强调了峡谷头环境中不常见的沉积物动力学的重要性，并增加了对重力驱动过程的传统概念的洞察力，在这些环境中占主导地位，设想对改善移动底物地质灾害评估和近海沉积物和碳量化的影响流动。作为维持映射的上坡沙波的主要机制。这项工作强调了峡谷头环境中不常见的沉积物动力学的重要性，并增加了对重力驱动过程的传统概念的洞察力，在这些环境中占主导地位，设想对改善移动底物地质灾害评估和近海沉积物和碳量化的影响流动。