The concavity of modern submarine canyon longitudinal profiles

Submarine canyons incise into continental shelves and slopes, and are important conduits for the 20 transport of sediment, nutrients, organic carbon and pollutants from continents to oceans. 21 Submarine canyons bear morphological similarities to subaerial valleys, such as their longitudinal 22 (long) profiles. Long profiles record the interaction between erosion and uplift, making their shape, 23 or concavity, a record of the environmental and tectonic processes that canyons are subject to. 24 The processes that govern concavity of subaerial valleys and rivers are well-documented on a 25 global-scale, however, the processes that control submarine canyon concavity are less well 26 constrained. We address this problem by utilizing existing geomorphological, tectonic and climatic 27 datasets to measure the long profiles and quantify the concavities of 555 modern submarine 28 canyons. Key results show that: 1) the dominant control on submarine canyon concavity is 29 tectonics, with passive margins hosting the most concave-up profiles, and forearcs hosting the 30 least concave-up profiles; 2) present-day canyon position affects canyon concavity, with river- 31 associated canyons showing greater morphological variance than canyons currently dissociated 32 from rivers; and 3) canyons subject to major Quaternary glacial runoff show increased concavity, 33 suggesting onshore climate affects canyon concavity through sediment supply variation. These 34 results show that tectonic and climatic processes are recorded in the morphologies of submarine 35 canyons on a global-scale, and that many canyons have been slow to respond to sea-level rise since 36 the Last Glacial Maximum. by this study and the correction applied to them to remove terrace deposition and irregular mapping. The original normalized concavity index (NCI) and the corrected NCI are shown.


Longitudinal profiles and the normalised concavity index (NCI)
137 Long-profiles were extracted from each canyon by sampling the depth of the canyon trace over formulae (Vincenty, 1975). This resulted in differences in measured lengths between Harris and 142 Whiteway (2011), who used a different method, and this study (Fig. S2). In order to mitigate against 143 the potential for profile smoothing by mapping across lower-resolution sections of the ETOPO1  the assumption that the intra-canyon deposition was too severe to allow for a reliable concavity criteria used for these omissions is strict, but aims to greatly improve the reliability of the results.

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The corrected, uncorrected and omitted profiles and their concavities of all 5849 canyons have 159 also been recorded ( Fig. S3; supplementary data).

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The concavity of each profile is represented by the normalized concavity index (NCI), which 161 measures the elevation difference between a straight line fitted between the most upstream and

Underlying controls
Following the methods used to assess the global controls on subaerial concavities (Chen et al., with a number of different geomorphological, climatic and tectonic datasets ( Fig. 2A). Canyon

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The impact of tectonics on concavity was assessed through grouping of canyons by the basin-type 181 in which they are located (Nyberg et al., 2018), and pairing them with onshore seismicity (peak-   concavity and onshore temperature (Fig. S4).

Other factors 247
When concavity is compared against other indices, statistically significant correlations are rare, and 248 only observed between concavity and minimum canyon slope on a margin-scale (Fig. 8). This 249 relationship is not preserved on smaller-scales, such as across UTM zones (Fig. S4) influence on submarine concavity morphology on a global-or continental-scale (Fig. S4). When 253 river-associated canyons are isolated, relatively strong positive correlations are documented 254 between dendricity and concavity (Fig. S4). ratio between seafloor deformation and downslope current capacity, and 2) the ratio between 259 sedimentation and downslope current capacity. Canyons become more concave when downslope 260 currents have greater capacity to erode and/or transport sediment downslope, and become less 261 concave when currents have insufficient capacity to erode or transport sediment downslope.

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A weak positive correlation also exists between NCI and onshore seismicity, i.e., canyons become 285 less concave with increasing onshore seismicity (Fig. 8). The opposing trend is documented in 286 subaerial river profiles, with increasing tectonic activity resulting in a global trend toward increasing 287 concavity as headwaters are uplifted and steepened (Seybold et al., 2020). This discrepancy may be 288 attributed to the greater degree of uplift in the uplands of tectonically-active subaerial environments compared with adjacent submarine environments, which is demonstrated by 290 calculating the elevation of a long profile as a function of uplift gradient (Eq. 1; Fig. 1A). When 291 the uplift gradient is varied from upstream-focused (> 0) to downstream-focused (< 0) the profiles 292 become increasingly more convex (Fig. 1A), with NCI values that are equivalent to the median of 293 forearc canyons when the downstream uplift gradient is around 80% of its maximum in the 294 example profile (Fig. 1B). This indicates that submarine canyons formed on convergent margins 295 and adjacent to seismically-active margins are subject to uplift primarily in their downstream 296 reaches, i.e., on the slope (Fig. 10). The increased concavity seen in canyons associated with islands   (Fig. 10). This may also contribute to the decreased concavity seen on convergent 306 margins, with large volumes of sediment derived from uplifting hinterlands primarily deposited on 307 the shelf and slope during the present-day highstand (Fig. 10). This is supported by a further 308 decrease in concavity when forearc basins are associated with rivers (Fig. 7B), which deliver vast 309 quantities of coarse sediment to oceans (e.g. Milliman and Syvitski, 1992). During highstand these 310 coarse grains will be more difficult to transport down-canyon and along-slope from the river- 10). This process is also suggested by the negative correlation between concavity and onshore 314 seismicity, relief, and suspended sediment load (Fig. 8), as high supplies of coarse-grained sediment 315 are expected from steep, tectonically-active hinterlands. These coarse-grained flows are also likely 316 to be more erosive, however this erosion must be concentrated on the lower-gradient shelf during 317 highstand, resulting in decreased concavity (Fig. 10). The weak negative correlation between 318 concavity and sinuosity at the margin-scale may also reflect sediment supply, with canyons 319 presently subject to high sediment supply and more frequent flows more likely to be sinuous than 320 those presently stranded at the shelf-break far from sediment sources.

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The impact of rivers on concavity may be reduced, or reversed, on passive margins due to the 322 longer subaerial transport distances and finer grain-sizes delivered to most passive margins and