Megathrust earthquake drives drastic organic carbon supply to the hadal trench

The giant 2011 Tohoku-oki earthquake has been inferred to remobilise fine-grained, young surface sediment enriched in organic matter from the slope into the >7 km deep Japan Trench. Yet, this hypothesis and assessment of its significance for the carbon cycle has been hindered by limited data density and resolution in the hadal zone. Here we combine new high-resolution bathymetry data with sub-bottom profiler images and sediment cores taken during 2012–2016 in order to map for the first time the spatial extent of the earthquake-triggered event deposit along the hadal Japan Trench. We quantify a sediment volume of ~0.2 km3 deposited from spatially-widespread remobilisation of young surficial seafloor slope sediments triggered by the 2011 earthquake and its aftershock sequence. The mapped volume and organic carbon content in sediment cores encompassing the 2011 event reveals that this single tectonic event delivered >1 Tg of organic carbon to the hadal trench. This carbon supply is comparable to high carbon fluxes described for other Earth system processes, shedding new light on the impact of large earthquakes on long-term carbon cycling in the deep-sea.

Scientific RepoRts | (2019) 9:1553 | https://doi.org/10.1038/s41598-019-38834-x event deposits over the complete along-strike extent of the hadal trench has hampered quantification of sediment volume and OC content, and hence the assessment of its role and significance on the sediment budget and carbon cycle.
Here, we present newly-acquired high-resolution bathymetry data of the entire trench axis, along with >800 km of high-resolution subbottom profiler (SBP) data acquired during 2012-2016 along the Japan Trench between 36°N and 39.5°N (i.e., the extent roughly corresponding to the large coseismic slip of the 2011 earthquake 21,22 (Fig. 1a)). These data allow, for the first time, imaging and spatial mapping of the full extent of 2011 event deposit along the entire Japan Trench. When combined with radionuclide dating and total organic carbon (TOC) measurements on sediment cores, we estimate the total volume of fine-grained surficial sediment remobilised by the 2011 earthquake and the mass of associated OC deposited in the trench.

The 2011 Event Deposit in the Hadal Trench
Our high-resolution bathymetric data clearly image the N-S to NNW-SSE trending horst-and-graben structures formed by flexural bending of the subducting Pacific plate. This results in relatively rough trench-floor morphology with 0.5-15 km long and 0.5-5 km wide isolated trench-fill and graben-fill basins, with different along-strike connectivity (Figs 1, 2, Supplementary Figs 1-12). The trench-floor along its southernmost segment is as deep as 8,030 m, while water depth in the central trench ranges between 7,400 m and 7,700 m. Trench-basins in the southern Japan Trench tend to be comparatively larger than those in the central trench. In particular, the trench around 38°N is narrow and only comprises very small basins with limited lateral connectivity as a consequence of local slumps and trench-floor deformation by coseismic slip-to-the trench 12,15 .
High-resolution SBP data across flat trench-basins image distinct, acoustically-transparent bodies with ponding geometries (Fig. 1), that occur as the uppermost acoustic mappable unit immediately below the seafloor reflector (see "Methods"), suggesting very recent event-deposition of homogeneous, fine-grained sediments (c.f. ref. 23 ). The acoustic facies of these depositional bodies are distinct from chaotic reflection patterns in areas of rough seafloor where local deep-seated slumps have transported older sedimentary units into trench basins 15,24 . The interpretation of the recent deposition of fine-grained remobilised sediment in SBP data is validated by published core data around 38°N (ref. 17,20 ) and our new cores in the south (core GeoB21804; positive evidence) and north (core GeoB21812; absence of evidence) (Fig. 3). The cores document homogeneous diatomaceous mud deposits overlying a thin oxidized layer, implying very recent burial of the oxidised water-sediment interface by the mass deposition. Short-lived radionuclide data 4,17 demonstrate that these deposits must be related to recent resedimentation events linked to the 2011 earthquake ( Fig. 3). High excess (unsupported) 210 Pb activity throughout most of the thick event deposit cored in the southern-most trench-fill basin (core GeoB21804; Fig. 3 Table 3) now allows along-strike correlation of the 2011-event deposit of the entire trench. Notably, this finding also implies that the source of the sediment mass comprising the acoustically-transparent bodies (i) must result from earthquake-triggered remobilisation of surface-most slope sediments (i.e., the upper few centimetres of surficial sediment remobilised over large areas of the seafloor (c.f. ref. 18,25 ) and (ii) does not comprise significant amounts of older sediment derived from locally remobilised by earthquake-triggered slumps 15,24 .

and Supplementary
We spatially mapped the extensive, acoustically-transparent 2011-event deposits, as imaged by SBP in 19 basins in the southern (36.0°-37.5°N: Basins S1-S13) and central (38.0°-39.5°N: Basins C01-C06) Japan Trench (Fig. 2, Supplementary Figs S1-S20, Supplementary Table 1). The thickness of the 2011 event deposit varies along the trench axis, being generally thicker in the southern part (e.g., 4.9 (+0.7/−0.1) m in basin S06 and 4.6 (+0.7/−0.1) m in basin S10 (Fig. 2e)), while the event deposit is generally thinner in the central Japan Trench (Fig. 2b). The areal extent of the 2011 event deposit in the southernmost basin (S01; 29.4 (±0.5) km 2 , Supplementary Table 1) is largest among all studied basins. Based on SBP data, we quantify the total areal extent and volume of the 2011 event deposits within the Japan Trench to be at least 125.7 (+4.3/−4.2) km 2 and 0.187 (+0.045/−0.018) km 3 , respectively ( Fig. 4a and Supplementary Table 1). These are considered conservative estimates because SBP data do not image deposits in small depositional basins, such as in the narrow trench axis around 38°N. Such small-scale seafloor roughness is beyond the horizontal resolution of the SBP (~80 m in 8 km water depth). While depositional basins can only be identified by the bathymetry data and be validated by sediment cores, we estimate that additional event-deposit volume missed by SBP data amounts to less than 0.007 km 3 (Supplementary Table 1).
Limited vertical and horizontal SBP resolution, however, may only partly explain our counter-intuitive findings of limited amount of remobilised surficial sediments deposited in the small trench-basins around 37°30′N-38°40′N (Fig. 2c). This area experienced largest-ever-recorded coseismic horizontal displacement and vertical uplift 11,12,21 which, intuitively, would be expected to deliver large quantities of remobilised surficial sediment to the trench axis (c.f. ref. 18,25 ). However, our findings for this part of the trench around 38°N are consistent with results from several cores that documented the 2011 event deposit to be only 5 to 40 cm in thickness 4,9,15,20 (Fig. 3). We attribute the limited event deposition in this area to the elevated position of this narrow part of the trench axis compared to its northern and southern counterparts and to the lack of large depositional basins. Gravity-focusing of fine-grained remobilised sediments thus results in an absence of large-scale deposition in the central part of the trench axis while leading to thicker deposits in the respective terminal basins, particularly towards the deeper, southern basins, where our data reveals maximum thickness of the 2011 event deposits of up to 4.9 m.
As an alternative to this sediment-process based explanation we also consider that the mainshock (M w 9.0-9.1) of the 2011 earthquake resulted in larger ground accelerations of 1,000-2,000 cm/s 2 than expected in Ibaraki Prefecture, south of the Tohoku coast [26][27][28] (Fig. 4). Such strong ground motions could have also triggered large amounts of surficial sediment remobilisation along the southern continental margin slopes. In addition, the largest aftershock (M w 7.9) of the 2011 Tohoku-oki earthquake occurred offshore Ibaraki Prefecture approximately 30 minutes after the mainshock 29   area. We thus infer that the widespread distribution of the 2011 event deposit over the southern Japan Trench could also be linked to significant remobilisation of surficial sediment offshore Ibaraki Prefecture, triggered by the 2011 mainshock and/or the largest aftershock, and subsequent funnelling and focusing of the muddy density  16,30 ). However, higher TOC contents of sediments from the southern trench (core GeoB21804) than those from the central trench (cores GeoB16431 and GeoB21823) may reflect the aforementioned hypothesis of massive delivery of remobilised surficial seafloor sediment into the southern trench via the Nakaminato submarine canyon. The TOC data and mapped volume of the 2011 event deposit (see "The 2011 Event Deposit in the Hadal Trench") allow constraints to be placed the total mass of OC within the 2011 event deposits over the entire studied Japan Trench (see "Methods"). Our results indicate that at least 1.73 (+0.96/−0.72) Tg (10 12 g) of surficial seafloor sediment OC was remobilised from a large swath of the margin and delivered to the trench by a single tectonic event (i.e., the 2011 Tohoku-oki earthquake and its aftershock sequence). Historical records (e.g., AD 1454 Kyotoku and AD 869 Jogan earthquakes) and turbidites and tsunami deposits during the last 4,000 years suggest that such giant earthquakes occur at an interval of 500-900 years 31,32 . Similar to the 2011 event deposits, these older event deposits also contain relatively young remobilised sediments ascertained by only a few thousand years of offset between background and event deposits 9 , suggesting the occurrence of comparable remobilisation and deposition processes. Thus, event-deposits in trench fill-basins of the Japan Trench act as an OC sink during interseismic periods. Given that sediment cores suggest accumulation of potentially even thicker event deposits in the central Japan Trench following historic earthquakes 9,17 , we estimate that, on average, on the order of 10 0 -10 1 Tg OC remobilised from surficial slope seafloor sediments is supplied to the trench per giant seismic event. Since interseismic sedimentation rates and carbon fluxes are lower by about two orders of magnitude 9,17 , the mechanism of megathrust earthquake-driven carbon supply to the hadal trench described here accounts for the majority of OC burial and, through eventual underthrusting, carbon sequestration on geological time-scales.

Relevance to Carbon Cycle in the Deep Sea
The fate of organic matter in the deep ocean is a key factor for understanding of the global carbon cycle 7,33 . Global estimates for the amount and accumulation rate of OC in surface marine sediments are 150 Pg (10 15 g) 33 and 170 Tg C per year 34 , respectively. Worldwide fluvial delivery of particulate and dissolved OC from the land to the ocean is estimated to be as high as 0.2-0.5 Pg C per year [35][36][37] . The Himalayan erosional system alone delivers ~4 Tg C per year of the relatively recent terrestrial OC to the Bengal fan via the Ganges-Brahmaputra rivers systems 38 . However, OC fluxes to the majority of the seafloor are small, averaging ~1 gCm −2 yr −1 (gram of carbon per m 2 per year) 39 . The major fraction of OC delivered from the continents to the ocean and produced in the ocean that survives remineralisation is buried on the continental margins, with <5% accumulating in abyssal ocean settings 33,34 . Processes that transfer carbon from the margin to the abyssal ocean may therefore exert a strong influence on deep ocean carbon budgets and associated biogeochemical cycles. On the Hikurangi margin, 7 Tg C was transferred by large-scale sediment transport of ~0.9 km 3 through submarine canyons to the deep ocean following the 2016 Kaikōura earthquake, whose recurrence interval is estimated to be ~150 years 40 . In comparison to these events and processes, the total mass of OC delivered to the Japan Trench (>1 Tg C) by a single tectonic event (here reconstructed for the 2011 Tohoku-oki earthquake and its aftershock sequences) represents an important contribution to marine sediments of hadal zones that may have broader-scale significance. Notably, the OC concentration per unit area within the 2011 event deposit (4.1 (+3.1/−2.2) to 31.7 (+17.5/−13.8) kgCm −2 ; ave., 12 kgCm −2 ) is an order of magnitude higher than the global average value of OC buried in surface marine sediments (0.4 kgCm −2 ; cf. 33 ). The hadal trenches previously studied, including Izu-Bonin (Ogasawara), Mariana, New Britain, Tonga, and Peru-Chile Trenches, have also been recognised as potential depocentres for sediments enriched in organic material (surface sediment OC concentrations range between 0.7-9.2 kg Cm −2 (Supplementary Table 2; refs 5,41-43 ). However, our newly reported estimates on OC contents of Japan Trench sediments related to the most recent earthquake-triggered remobilisation event rank up to a factor of 2 higher in comparison with corresponding data from other hadal trenches.
It remains to be determined whether the remobilized fresh OC delivered following tectonic events to hadal-trench systems is reactive and subject to microbial degradation 44 and benthic recycling, or whether it is subducted to contribute to the deep carbon cycle, possibly eventually influencing CO 2 degassing through the proximal volcanic activity 8 . Nevertheless, our new high-resolution hydroacoustic measurements coupled with sedimentological and geochemical data allow a first-ever trench-wide quantification of megathrust earthquake-induced sediment and OC translocation, and virtually instantaneous supply of >1 Tg OC to the hadal zone. These fluxes are comparable with those described for other Earth system processes, implying potential global significance. Taken in the context of estimates of carbon burial in other trench systems, our findings highlight the importance of tectonic events for carbon cycling in hadal trenches of plate subduction margins.

Methods
Bathymetric data. Bathymetric data were acquired by a 12 kHz frequency KONGSBERG EM122 system equipped on R/V Sonne during SO251-1 cruise in October 2016 (ref. 45 ), which has 432 beams with a transducer of 0.5 (transmission) by 1 (receiving) degrees, generating a footprint of 60-70 meters along track by 130-140 meters across-track in the trench axis, and has a dual swath (multiping) function. Such small transducer configuration, which equals small footprints, together with higher beam counts and dual ping system makes spatial resolution and signal-to-noise ratio of the bathymetric data resulting from this latest state-of-the-art acquisition system uniquely capable of identifying even small depositional basins within the structurally complex and deep-water trench-floor system (Figs 1, 2, Supplementary Figs 1-12). The PARASOUND echosounder emits two high frequencies of 18 kHz and 22 kHz, and non-linear interference of the high frequencies produces a secondary frequency of about 4 kHz. The TOPAS system uses a primary frequency of 15-21 kHz, and a secondary frequency of 0.5-6.0 kHz. Frequency filtering was done through low-pass bandpass at 6 kHz for PARASOUND data 45 and high-and low-pass filters at 2 and 7 kHz for TOPAS data. The SBP taken by PARASOUND records mostly at 30-70 m per shot point (SP), while that taken by TOPAS system at 10-40 m per SP. The theoretical vertical resolutions of all SBP data are 10-20 cm, which enabled identifying the decimetre to meter scale event deposit layers (Fig. 1, Supplementary Figs 1-12). For better visual continuity of the reflections, the trace envelope of the SBP data is shown. We postprocessed the data for cases of noisy SBP data due to adverse weather conditions or interference with the ship's multibeam pings (see Supplementary Information). Radionuclide dating, total organic carbon (TOC) content, and dry density measurements of sediment cores. To correlate acoustically-transparent bodies from SBP data to the event deposit triggered by the 2011 earthquake, and to measure carbon content and estimate carbon mass flux to the hadal trench, we use existing core data from cores MR12-E01-PL03 (ref. 17 ) and GeoB16431 (ref. 9 ), and add new data from the cores GeoB21804 and GeoB21823, retrieved during R/V Sonne cruise SO251-1 in 2016 (ref. 45 ).

Interpretation
The radioactivity of 210 Pb (with a half-life of 22.3 yr) from the core GeoB21804 was measured gamma spectrometry at the Rensellaer Polytechnique Institute (Fig. 3 and Supplementary Table 3). The excess 210 Pb was defined following the method described by Appleby et al. 46 .
Total organic carbon (TOC) content (wt%) of the deep-sea trench sediments was quantified by using the Elemental Analyser (EA; Elementar, Germany) at ETH Zürich, with a relative standard deviation (RSD) of 2% (ref. 47 ). For this purpose, about 15 mg of freeze-dried and powdered subsamples of the cores GeoB21804, GeoB16431, and GeoB21823 were weighed into pre-combusted Ag capsules. The Ag capsules containing the sediment were placed on a ceramic plate in a desiccator and fumigated for 72 h at 60 °C with ~30 ml of concentrated HCl (37%, metal-trace purity), placed in a pre-combusted petri dish at the bottom of the desiccator. The acidified subsamples were neutralised with ~20 g NaOH pellets for another 72 h at 60 °C. TOC contents are 2.16 (+0.26/−0.63) wt% and 1.57 (±0.18) wt% in the southern (from GeoB21804 core) and central Japan Trench (GeoB16431 and GeoB21823 cores), respectively (Fig. 3). Dry density of the 2011 event deposit was calculated from the ratio of dry mass and total volume of the sample taken from the cores GeoB21804 and GeoB16431. The dry mass was corrected for the mass of salt that remains within the pore space after drying in an oven at 100 °C for 24 hours. The total volume consists of the sum of the volume of water and the volume of grains. The volume of water was derived from the water content determined following DIN EN ISO 17892-1. The volume of grains was determined with a gas pycnometer (Pentapyc 5200e, Quantachrome Instruments), following DIN 66137-2 (Determination of solid state density-Part 2: Gaspycnometry, 2004). Dry densities of the 2011 event deposit from the cores GeoB21804 and GeoB21823 were ρ d,S = 330-430 kg/m 3 and ρ d,C = 790-950 kg/m 3 , respectively. The difference in dry density form the southern and central area may be explained by the fact that compaction of southern thick event deposits supplied through the proximal canyon has not been completed, as also indicated by very low undrained share strength values for the uppermost few meters of the cores 45 .
Mass of organic carbon in the 2011 event deposit. Masses of organic carbon in the southern (36.0°-37.5°N) and central Japan Trench (38.0°-39.5°N) are calculated using the measured TOC contents and dry densities from core GeoB21804 and those from the cores GeoB16431 and GeoB21823, respectively (see the previous Method subsection "Radionuclide dating, total organic carbon (TOC) content, and dry density measurements of sediment cores"). Mass of organic carbon mOC S,i (kg) at a basin i in the southern Japan Trench (i = S01, S02, …, S13; Supplementary where TOC S (wt%) is TOC content measured from the core GeoB21804, ρ d,S (kg/m 3 ) is the dry density measured from the core GeoB21804 (see the previous Method subsection "Radionuclide dating, total organic carbon (TOC) content, and dry density measurements of sediment cores"), and V S,i (m 3 ) is volume of the 2011 event deposit at the basin i in the southern Japan Trench obtained by bathymetry and SBP data (Supplementary  Table 1). Similarly, mass of organic carbon mOC C,i (kg) at a basin i in the central Japan Trench (i = C01, C02, …; Supplementary Table 1) is produced by: where TOC C (wt%) is TOC content measured from the cores GeoB16431 and GeoB21823, ρ d,C (kg/m 3 ) is the dry density measured from the core GeoB21823 (see the previous Method subsection "Radionuclide dating, total organic carbon (TOC) content, and dry density measurements of sediment cores"), and V C,i (m 3 ) is volume of the 2011 event deposit at the basin i in the central Japan Trench obtained by bathymetry and SBP data (Supplementary Table 1). The organic carbon masses mOC S,i and mOC C,i are computed taking into account the uncertainty in volume determination (Supplementary Table 1).