Possible Tsunami-Induced Sediment Transport From Coral Reef to Deep Sea Through Submarine Canyons on the Southern Ryukyu Forearc, Japan

Abstract

Submarine canyons are efficient sediment transport pathways from shallow marine areas to deep sea. Along active margins, large tsunamis are a trigger for sediment transport to deep sea. However, sediment transport through submarine canyons by such extreme wave events in the carbonate depositional environments has not been fully understood. Large tsunamis have repeatedly struck the coral reef islands of the southern Ryukyu Islands and have transported large boulders composed of coral from the reef to shore. In this study, we examined sediment cores collected near the mouths of submarine canyons and basin floor on the southern Ryukyu arc’s forearc. The presence of coarse calciturbidites containing coral, molluskan fragments, and coral reef benthic foraminifera was limited on the submarine fan formed at the mouth of the reef-connected and shelf-incised submarine canyon. In cores collected near the mouth of shelf-incised submarine canyons with no reef connection and slope-confined canyons, no coarse calciturbidite is observed. Few calciturbidites were found in more downslope cores, implying that most calcareous grains derived from shallow marine areas were deposited on this fan. Depositional intervals of turbidites were calculated to be several hundred–a few thousand years, which agree with the recurrence intervals of large tsunamis estimated from onshore tsunami deposits, based on radiocarbon dates in hemipelagic mud intervals. No temporal change in the depositional intervals of calciturbidites in the cores from the submarine fan since the last glacial maximum. Therefore, the tsunamis may be an important mechanism for surface sediment reworking in coral reefs, and shelf-incised and reef-connected submarine canyons can play an important role in the efficient transport of shallow marine calcareous grains to the deep sea.

Deep‐water sedimentary systems and tectono–sedimentary interactions on the oblique convergent margin in Antarctica

“…However, sedimentary features on the forearc of island–arc systems all seem to be influenced by coarse‐grained sediments. Tectonic collision and influxes of volcanic clastics trigger canyon formation on the Mariana and Tonga margins during the arc rifting (Clift et al ., 1998; Stern & Smoot, 1998; Wright et al ., 2000), while submarine canyons and fans on the southern Ryukyu margin are sourced by coral reef islands (Ikehara et al ., 2022). On the South Shetland margin, ice‐rafted debris (Hall & Perry, 2004; Wang et al ., 2014) along with volcanic clastics are transported to the continental slope.…”

“…This zone marks the lateral end of marginal subduction and the termination of the forearc basin (Jabaloy et al, 2003). Tectonic compression gradually ceased westward to the Antarctic Peninsula Pacific margin (Jabaloy et al, 2003;Berrocoso et al, 2016), resulting in the smoothest and relatively gentle slope morphology in the westernmost part of the South Shetland slope (Fig. 8A).…”

“…The downslope elongated mounded drift is located at the transition zone from passive (the Antarctic Peninsula Pacific margin) to active (the South Shetland margin) margins (Jabaloy et al, 2003;Fig. 3).…”

“…The last ridge-trench collision occurred at C3A (ca 6.5 Ma; Larter & Barker, 1991). Subduction of the former Phoenix Plate ceased around anomaly C2A (ca 3.3 Ma; Eagles, 2004), leaving a transition zone from passive to active margins at the HFZ region (Jabaloy et al, 2003).…”

“…1) is the only area in polar regions where subduction still takes place today (Berrocoso et al, 2016). This oblique convergent margin belongs to the forearc area of an island arc system where earthquakes, volcanism and faulting are currently active (Livermore et al, 2000;Jabaloy et al, 2003;Galindo-Zaldı ´var et al, 2004). The grounded Antarctic Peninsula Ice Sheet extended to the present-day shelf break during the past glacial periods (Simms et al, 2011;Davies et al, 2012), enabling the downslope transport of sediments on the margin (Maldonado et al, 1994).…”