Earthquake-enhanced dissolved carbon cycles in ultra-deep ocean sediments

Domain: earth_science
Published: 2023-09-11
Editorial outcome: minor_revision

Abstract
Hadal trenches are unique geological and ecological systems located along subduction zones. Earthquake-triggered turbidites act as efficient transport pathways of organic carbon (OC), yet remineralization and transformation of OC in these systems are not comprehensively understood. Here we measure concentrations and stable- and radiocarbon isotope signatures of dissolved organic and inorganic carbon (DOC, DIC) in the subsurface sediment interstitial water along the Japan Trench axis collected during the IODP Expedition 386. We find accumulation and aging of DOC and DIC in the subsurface sediments, which we interpret as enhanced production of labile dissolved carbon owing to earthquake-triggered turbidites, which supports intensive microbial methanogenesis in the trench sediments. The residual dissolved carbon accumulates in deep subsurface sediments and may continue to fuel the deep biosphere. Tectonic events can therefore enhance carbon accumulation and stimulate carbon transformation in plate convergent trench systems, which may accelerate carbon export into the subduction zones.

Introduction
Hadal trenches form due to downward bending of subducting ocean crust along subduction zones and represent the least-known ultra-deep oceanic environment1. Earthquakes, the frequent tectonic events at convergent plate boundaries, trigger redistribution of sediments that contain organic carbon (OC)2–7 from the continental and trench slopes8 into the trench axis. Sediments are efficiently transported into and focused at the trenches due to their V-shaped morphology, resulting in rapid accumulation of sedimentary organic carbon (SOC) in the trenches that is ~70 times higher than the global deep sea average2,6,9. Active OC cycling in hadal trench sediments may be triggered by the high-volume material inputs and intensive microbial activities10–12. SOC is remineralized to dissolved organic and inorganic carbon (DOC, DIC) in the sediment interstitial water (IW). In addition, the sulfate-methane transition zone (SMTZ) constrains the preservation and production of biogenic methane13–15, serving as an OC diagenetic front in marine sediments16. The production and consumption of dissolved carbon in the hadal trench sediments are key processes in the deep subseafloor carbon cycle, supporting deep biosphere metabolisms. As carbon in trench sediments eventually enters the subduction zone system during plate convergence1, the transformations of SOC, DOC, DIC, and methane may influence the carbon exchange among the Earth’s surface biosphere, the lithosphere, and even the mantle17.

In the Japan Trench, pioneering studies documented SOC delivery to the trench by historical subduction zone mega-earthquakes2,6,18 with various magnitudes and recurrences on centennial-millennial time scales4, however, their impacts on the cycling of the dissolved carbon in trench sediments remain largely unexplored. Here we present the IW DOC and DIC concentrations, carbon isotopic profiles (δ13C and 14C ages) and headspace methane concentrations in combination of typical IW geochemical parameters from 12 sites along the Japan Trench axis (water depth >7.5 km, Fig. 1a) collected during the International Ocean Discovery Program (IODP) Expedition 38619. Using this deep subsurface dataset from a hadal trench, we attempt to elucidate the accumulation and transformation of dissolved carbon in the hadal trenches, and to decipher the roles of tectonic events in the carbon cycles of these ultra-deep-water environments connected to subduction zones.

Results
Labile dissolved carbon accumulation in trench sediments
The Japan Trench receives pelagic background SOC descended from the water column and earthquake-remobilized SOC with various provenances (land, continental shelf, slope, and trench slope, etc.) and isotopic signatures2,8. Previous studies found the pervasive existence of earthquake-generated turbidite deposits along the Japan Trench3,6,8. At Sites M0081, M0083, and M0084, the 14C and 13C discrepancies between different carbon pools (Supplementary Fig. 1) exclude the possibility that the majority of the dissolved carbon in Japan Trench subsurface sediments are sourced from the exchange with bottom water or remineralization of pelagic background SOC. Instead, the dissolved carbon is mainly produced from remineralization of earthquake-introduced, likely marine-sourced SOC with various 14C ages and chemical recalcitrance (Supplementary Text 1). Additionally, the high IW alkalinities along the trench axis (Fig. 1b) suggest that earthquake-triggered SOC accumulation2 is followed by intensive SOC remineralization. The surficial (above 0.35 meter below the seafloor, mbsf) IW DOC concentrations (Fig. 2) in the Japan Trench are ~6 times higher than those in other oceanic trenches20 and increase with depth, reaching maximum concentrations (Fig. 2) that are comparable to the coastal21 and continental shelf22 areas, highlighting a dynamic carbon cycle and huge dissolved carbon storage in the hadal trenches enhanced by earthquakes.

In many marine environments, most of the labile OC is selectively remineralized to DIC while the refractory fraction remains within the DOC pool20,23,24, potentially leading to older DO14C ages than DI14C ages23,25,26. However, our data in the Japan Trench exhibit similar DO14C and DI14C ages (Fig. 3) that increase almost linearly with depth (Fig. 2). Essentially, this suggests a relatively fast equilibration of dissolved carbon pools supported by rapid IW diffusion27 before their burial into the deeper subsurface. Additionally, no significant 14C discrepancy between the DOC and DIC pools in the hadal sediments means that at least part of the labile DOC is preserved, and go through aging in the trench sediments rather than being intensively oxidized or diffused overwhelmingly into the overlying seawater, in sharp contrast to other oceanic settings where mostly refractory DOC is preserved23,24,28. While the labile DOC in shallow subsurface sediments typically acts as a carbon source to the atmosphere due to microbial carbon turnover, those in the deeper-subsurface anoxic sediments tend to be chemically recalcitrant and elude aerobic oxidation29. In the Japan Trench, however, the instantaneous deposition of up to several meter thick event layers4,30 driven by earthquakes prevents the exposure to oxidation and exchange with ocean water for most of the sediment9,12, resulting in a chemically-active dissolved carbon sink in trench sediments.

Although the IW DOC is derived from earthquake-induced SOC remineralization, our results show that the relatively constant production of DOC may not be completely regulated by SOC content (Supplementary Fig. 2). It is possible that only the most labile fraction of the SOC is remineralized into DOC, supported by the positive δ13C evidence (Fig. 2). Despite processes such as remineralization and methanogenesis that consume IW DOC, we still find sufficient residual DOC that accumulates and ages with depth (Fig. 2), which 14 remains available for biogeochemical processes in the deeper subsurface sediments. In addition, the linearly-increasing 14C ages of the residual DOC indicate the time since sediment deposition and may constrain the chronological models for trench sediments devoid of pelagic carbonates. This implies a steady state DOC turnover in a non-steady state oceanic sedimentary setting that is irregularly disturbed by turbidite deposition. The dissolved carbon pools in the trench sediments thus provide potential information about the hadal sedimentation and diagenesis processes.

Enhanced methanogenesis in hadal zones
The large amount of dissolved carbon may fuel microbial metabolisms in the deep subsurface sediments31,32. Below the SMTZ, the methane concentrations along the Japan Trench (up to 7.35 mM, Fig. 1b) and the high ratios of methane to ethane and propane (C1/(C2+C3) > 1000, Fig. 1b) imply intensive microbial methanogenesis33. The abundant DIC in the deep subsurface sediments accumulated from intensive OC remineralization has the potential to promote methanogenesis through CO2 reduction. In turn, methane production and oxidation exert significant control on the DIC pools below and above the SMTZ, respectively (Supplementary Fig. 3 and Supplementary Text 1). The positive δ13C values in the deep subsurface sediments (Fig. 2) indicate a carbon isotope fractionation during microbial methanogenesis, where the residual DIC becomes more 13C-enriched33,34. Up to 24%–38% of the DIC is estimated to be utilized during methane production in the Japan Trench (Supplementary Fig. 4, see Method for calculation), highlighting intensive methanogenesis processes fueled by the accumulated DIC in the trench sediments.

Low concentration and short residence time of dissolved substrates may result in relatively less significant methanogenesis through fermentation in marine environments33,35. However, below 30 mbsf, the DOC is younger than DIC in the Japan Trench (Fig. 3), likely suggesting that a newly-added DOC pool is more bioavailable to heterotrophic organisms36. The labile DOC accumulation implies the possibility of intensified methanogenesis through fermentation37 in hadal trenches along subduction zones. The thick earthquake-triggered turbidite deposit results in less permeability38 and prevents oxygen and sulfate supplies to the sediments below it, leading to intensive OC remineralization12 and elevated SMTZ, thereby enabling substrate availability for in-situ microbial fermentation. Enhanced sediment compaction by repeated deposition of thick turbidites30 further contributes to these processes. The earthquake-triggered labile DOC production and preservation thus fodder microbial methanogenesis in the deep subsurface sediments.

With increasing time after deposition and repeated occurrences of large earthquakes, the IW volume is reduced due to sediment compaction and earthquake-induced dewatering and consolidation39, resulting in relatively constant DOC concentration below 20 mbsf (Fig. 2). The maximum concentration at 15~20 mbsf (Fig. 2) may indicate a compensation depth of the labile DOC, below which the DOC is consumed faster than it is produced and condensed, and may be continuously converted to methane by the microbial communities in deeper subsurface sediments40. The cascading effects of earthquake-induced processes including voluminous SOC deposition, rapid sediment burial, and sediment compaction consequently stimulate microbial methanogenesis via multiple metabolic pathways in the trench sediments (Fig. 4a).

Linking between shallow and deep carbon cycles
The OC preservation regulated by the penetration of electron acceptors (oxygen, sulfate, etc.) is usually correlated with water depth in deep-sea environments41. The OC preservation and turnover in the hadal trenches, instead, are regulated by earthquake disturbances. The earthquake-triggered process cascade prolongs the residence time of labile carbon, which would have been otherwise consumed and released into the bottom water on the timescale of centuries to millennia, and thereby promotes carbon aging and subduction in the trench sediments (Fig. 4a).

The labile DOC pools serve as food sources for microorganisms in the trench sediments and have the potential to fuel the microbial-mediated carbon transformation in the deep biosphere42,43 (Fig. 4b). Volcanic ashes, biosiliceous-rich sediments, detrital silicate minerals and the abundant alkalinity along the Japan Trench (Fig. 1b) are suggestive of a contribution from marine silicate weathering, which provides the alkalinity and cations needed for authigenic carbonate precipitation44 (Supplementary Fig. 5). This deeper-subsurface carbonate formation serves as a ‘carbon sink swap’ from OC to methane, carbonates and eventually to graphite as more stable forms in the subduction zone system45,46. Although substantial storage of methane has been reported in the continental shelf and cold seep sediments47, its presence in trench sediments is not thoroughly evaluated. A comparison of the methane data from the IODP database (Supplementary Fig. 6) suggests that the methane storage in the Japan Trench may be comparable to typical methane-rich seafloor areas. More methane supply from the deep subsurface would support stronger methane oxidation in the SMTZ and reduce the sulfate available for OC remineralization, thereby promoting deep carbon burial.

The earthquake-enhanced dissolved carbon storage in the trench sediments may eventually contribute to carbon subduction1,48. Through sediment deposition and compaction, earthquake-triggered turbidites instantaneously increase the depth of the carbon residing in the sediments, leading to intensified carbon cycling in deeper subsurface depth than expected. The dissolved and sedimentary carbon may enter the subduction zones without becoming chemically recalcitrant for utilization by the deep biosphere12. The pore fluid containing dissolved carbon enters the crustal and mantle parts of the subducting slab and is transformed into solid phases, forming various carbon reservoirs (carbonates49, sediment-derived fluid50, graphite46,51 etc., Fig. 4b) at forearc depth52,53. Earthquakes therefore accelerate and magnify the processes in the subduction zones (Fig. 4b). In the meantime, spatial heterogeneity of our IW geochemical results exists along the trench. Considering the global variations of sedimentation environments, SOC characteristics, and earthquake frequencies in the trench systems11,54,55, we suggest that the spatial and temporal dynamics of dissolved carbon reservoirs will be further amplified by irregularity of earthquake occurrence and difference in their magnitudes in trenches globally, and will therefore cause critical and variable impacts on the global carbon cycle.

Our presented data and results, acquired through advanced drilling and radiocarbon measurement techniques, offer a comprehensive perspective of the hadal trench carbon cycle in the deepest and least explored environments on our planet, where geological events such as great subduction zone earthquakes may serve as key modulators. The contribution from our newly-discovered carbon reservoirs in the Japan Trench sediments provides essential, detailed information regarding how intensive earthquake-induced SOC accumulation leads to enhanced microbial activities in the deep biosphere. The occurrence of the dissolved carbon compensation depth indicates intensive carbon maturation and alteration in the trench subsurface sediments. These processes occur at the inlet to the subduction zone systems, where fresh materials are irregularly introduced by geological events. The cascade of the earthquake-triggered physical and chemical processes accelerates the transformation between carbon in different forms (i.e., ‘carbon sink swap’) at the shallowest setting of the subduction zone.

Methods
Research materials
The Japan Trench is located between the Kuril-Kamchatka Trench and the Izu-Bonin Trench. It is formed by the subduction of the Pacific Plate beneath the Okhotsk Plate with a convergence rate of 8.0–8.6 cm/yr (ref. 56). It is 611 km long and has a surface area of 37,854 km2 (ref. 57). Normal fault grabens resulting from the flexural bending of the slightly obliquely subducting Pacific Plate oceanic crust form numerous isolated basins that act as terminal sinks for trench-fill sedimentation along the axis of the Japan Trench30,58. Sediment cores were collected with up to 40 m long giant piston corers (GPC) on R/V Kaimei during the International Ocean Discovery Program (IODP) expedition 386 in 2021 (Fig. 1a & Supplementary Table 1).

Sample collection
Interstitial water (IW) was collected onboard R/V Kaimei immediately on recovery using Rhizone samplers (CSS 19.21.23F; Rhizosphere Research Products, Netherlands). Rhizone samplers consisted of the microporous tube (2.5 mm diameter, 5 cm length) that was supported by a nylon wire/PEEK wire and connected to PVC tubing with a female Luer lock connector, that can be attached to a syringe. Before use, Rhizone samplers were soaked in purified lab water for approximately 60 minutes. A 3.8 mm hole was drilled into the plastic core liner using the drill bit, and the Rhizone filament was inserted into the sediments. Negative pressure was applied by attaching a 24 mL plastic pulled-back syringe to the Rhizone sampler. The sediment cores remained capped throughout this process to minimize ambient gas penetration. First 0.1 mL (1–2 drops) were discarded because it would include purified lab water, and then Rhizone samplers were reassembled and remained in the cores for a maximum 6 h. The IW was split into sub-samples for alkalinity (3–4 mL) and sulfate (3 mL) measurements following IODP standards19, and for dissolved organic carbon (DOC, 8 mL) and inorganic carbon (DIC, 2 mL) measurements. Split samples for alkalinity and sulfate measurements were stored at 4°C. Sample splits were added with 50 µL 10% HCl and stored at –20°C for DOC measurement, or were added with 10 µL saturated HgCl2 and stored at 4°C for DIC measurement.

Bottom water (BW) was collected from the top of the pilot cores using a BW sampling system consisting of a Duran bottle and vacuum pump unit connected by tubes. Negative pressure was applied and BW samples were sucked into the Duran bottle through the tube placed in the overlying water of the trigger core. After filtering using a Rhizon sampler, BW samples were split for DOC (1 L) and DIC (5 mL) measurements. Sample splits were added with 1 mL 10% HCl and stored at –20°C for DOC measurement, or were added with 10 µL saturated HgCl2 and stored at 4°C for DIC measurement.

Headspace samples were taken directly from the base of each core section after core cutting. A total volume of 5 cc (using 2.5 cc cut tip syringe twice) was collected and each 2.5 cc sample was then added into 20 mL crimp vial with 5 mL of 1 M NaOH and stored upside down and cooled at 4°C.

Geochemical analysis
IW and BW geochemical analysis was carried out during the onshore and offshore phases of the IODP Expedition 386 following standard IODP procedure19.

Water splits were analyzed onboard for alkalinity immediately following extraction with an autotitrator (Metrohm 888 Titrando). An aliquot of 1–3 mL water split-adjusted to 3 mL with 0.7 M KCl solution was titrated with nominal 0.1 M HCl at 25°C. A 100 mM Na2CO3 solution was used for the calibration of the acid each week. Quality checks were conducted using a 50 mM solution of NaHCO3 and 0.1 M HCl once per day.

Water splits were measured for sulfate on a Metrohm 882 Compact IC ion chromatograph at the MARUM. A 40-fold dilution of IAPSO seawater and standards prepared from commercial single anion standards was used for calibration. Analytical precisions were ±0.95% for sulfate.

Gas concentrations in headspace samples were measured between September and November 2021 onboard of D/V Chikyu. The samples were vigorously shaken by hand for 2 minutes, then shaken by a lab shaker for 1 h and left for 23 h at room temperature. Each vial was placed in an Agilent 7697A headspace sampler, and was heated to 70°C for 30 minutes. An aliquot of the headspace gas was then automatically injected (split mode) into an Agilent 7890B gas chromatography equipped with a packed column (HP PLOT-Q) and linked to a flame ionization detector. The carrier gas helium flow rate was 10 cm³/min. The oven temperature was programmed to start at 60°C and was ramped to 150°C at a rate of 10°C/min. The instrument was calibrated for the chromatographic response with commercial standards (Types VIII–XII, GL Sciences, Japan). Quantification of hydrocarbon gases was achieved by comparison of its chromatographic response with a three-point calibration curve.

DOC and DIC concentration and isotopic analysis
Holes M0081D, M0083D, and M0084D were chosen to represent the DOC and DIC signatures of the corresponding sites (Supplementary Table 1). All DOC and DIC samples were analyzed at the National Ocean Science Accelerator Mass Spectrometer (NOSAMS) facility at Woods Hole Oceanographic Institution for concentration, 13C and 14C analyses. Due to the pandemic, water splits were stored at the Kochi Institute for Core Sample Research of the Japan Agency for Marine-Earth Science and Technology for 6 months before being shipped to NOSAMS. The DIC concentration was likely to be affected by some carbonate precipitation during storage given the extreme IW alkalinities of up to 100 mM. The DOC and DIC measurement procedures were described in detail in refs. 59,60. Briefly, BW samples were filtered through pre-combusted 0.45 μm GF/F films before measurement. A quartz reactor prefilled with Milli-Q water was irradiated with UV for 1.5 h to reduce the potential contribution of extraneous carbon. Approximately 2 mL water sample and 0.4 g 33% ultra-pure hydrochloric acid were then added to the reactor. DOC samples were irradiated with UV for 3 hr. The generated CO2 was purged by ultra-high purity helium (UHP He) at 120 mL/min for 66 min, and was cryogenically trapped, purified, manometrically quantified, and collected by flame sealing in a 6.4 mm OD Pyrex glass tube. DIC was also extracted on the DOC line by replacing the quartz reactor with a gas-washing device equipped with an injection port. The device was added with Milli-Q water and H3PO4 and then cleaned with He before sample injection. The generated CO2 was purged by UHP He at 120 mL/min and collected by a CO2 trap.

δ13C were measured by a VG Optima or VG prism Isotope Ratio Mass Spectrometer at NOSAMS. δ13C values were reported in ‰ relative to the VPDB standard. The typical precision of δ13C measurement was between 0.1‰–0.23‰. 14C were measured by a continuous-flow accelerator mass spectrometer (CFAMS) system constructed around an accelerator (National Electrostatics Corporation, Middleton, WI, model 1.5SDH-1) at NOSAMS61,62. AMS uncertainty was calculated as the larger of either the statistical uncertainty using the total number of 14C counts measured for the target, or an uncertainty calculated from the reproducibility of multiple measurements of the target, both propagated with uncertainties from the normalizing standards and blank subtraction. Δ14C values were reported as 14C ages63. For the IW samples, the 14C measurement errors were within 0.34%–2.03% and thus didn’t impact data robustness.

DIC conversion estimation
During methanogenesis via CO2 reduction in the marine environment, DIC is utilized by methanogens with a kinetic isotope effect that discriminates 13C. The fractionation factor of DIC to methane (εc) is between –50 to –90‰ in natural observations33. Due to the lack of δ13C methane data, we use both reported εc values33 as upper and lower limits to estimate the amount of DIC that is converted to methane. A Rayleigh distillation function can be used40 to describe the 13C fractionation during methanogenesis in sediment IW:

δ13CDIC,t = δ13CDIC,0 + εc ln fresidual (1)

Where δ13CDIC,0 and δ13CDIC,t are the isotopic compositions of the initial and residual DIC pools at a certain time, respectively, and fresidual represents the amount of residual DIC expressed as a fraction (f ranges from 0 to 1). The fraction of DIC that has been utilized during methanogenesis, fconverted, can be calculated as:

fconverted = 1 − fresidual (2)

We use the linearly-extrapolated δ13CDIC at 0 mbsf (–12.43‰, –14.00‰ and –13.58‰ for Sites M0081, M0083 and M0084, respectively) as δ13CDIC,0. The fraction of residual DIC can thus be calculated according to Eq. (1) by using the δ13CDIC at the given depth as illustrated in Supplementary Fig. 4. Calculation results of the deepest IW sample of each Hole are presented in Supplementary Table 2.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This is a well-written and interesting manuscript providing new insights on the digenesis and burial of organic carbon in the Japan trench. Hadal trenches are apparently unproportionally important for marine (and global?) element cycling due to; material focusing, seismic driven mass wasting and potentially translocation of material by tsunamis. But our current understanding on the processes governing transport, deposition and mineralization at these extreme depths are still rudimentary – but research interests are increasing. The study is timely, and the Japan trench is a very interesting case study for the biogeochemical function of hadal trench systems.\nThe data set is impressive, and the main conclusions are well substantiated. The finding complements other recent studies of hadal trench system, but also provide original and novel findings. Personally, I value the strong evidence of active early diagenesis (methanogenesis) in deep subsurface settings drive by relatively labile DOC. At times I feel the authors over sell some of their findings – or at least they should acknowledge other recent advances that also demonstrate i) intensified burial of Org C in hadal trench system and ii) subsurface burial of labile organics in hadal sediment fueling intensified microbial activity. I understand that the hadal research is advancing rapidly, and that recent publications may not find their way into manuscripts that are being developed concurrently. But fortunately, the authors still have a chance to do that.\nIn conclusion, recommend the manuscript is accepted with relatively minor revision. Most importantly the authors should place there study in the context of other recent and highly relevant publications.\nMore specific comments are listed below.\nAbstract:\nLine 79; I suggest changing “most mysterious” to “unique”.\nLine 86-89 & 90-93; I acknowledge the novel contribution of this high-quality study (See above/below). But conceptually, the fact that earthquake-triggered turbidity in hadal trenches result in subsurface layers of labile organic material and intensified microbial activity – and intensified burial and subduction of organic material has been documented and discussed in recent publications. For example;\nZabel et al. (2022) High carbon mineralization rates in subseafloor Hadal sediments – result of frequent mass wasting. Geochem, Geophy, Geosystem 23. Where some of the evidence originates from gravity cores that have been recovered from the Japan trench and include the SMTZ. I am particular surprised that this paper isn’t considered in this submission.\nOguri et al (2022) Sediment accumulation and carbon burial in four hadal trench systems. JGR\nBiogeosciences 127. The manuscript, target mores sites along different trench systems, it is estimated that carbon burial rates in hadal trenches are 70 times higher than the global average for the deep ocean.\nSchauberger et al (2021) Spatial variability of prokaryotic and viral abundance in the Kermadec and Atacama trench regions. Limnol Oceanogr 66: 2095-2109.\nNone of these publications are acknowledged or cited in the current manuscript., but I find that the current submission should place their findings in the context of these recent advances.\nLine 154-159; This point has been presented and discussed in two recent papers of Zabel et al 2022 & Oguri et al 2022. Referencing and relating to these studies would substantiate your claims.\nLine 194-202; Can these claims be substantiated (no references are included, but they are presented as facts), or is these hypotheses? Please be specific.\nLine 211-213; Evidence in the form of sequencing data (barcoding or metagenomics) or specific biomarkers for methanogens would have strengthen the proposed concept significantly. I don’t know if there are any initiatives pursuing this for the recovered samples\nLine 220; I suggest changing “atmosphere” to “bottom water”. I acknowledge that “quickly” may mean different things for different people. But “quickly is unspecific and venting of deep oceanic waters is on time scales of centuries to millennia.\nLine 227-231; Are there any hardcore evidence that authentic carbonate precipitation is occurring in hadal sediments? Or is this another hypothesis/claim? Please be specific.\nLine 236 ; A reference here would be appropriate\nLine 254-256; that is not correct – there have been more “coordinated international research efforts that perform high spatiotemporal resolution investigations and sampling of a hadal oceanic trench” For instance, the cruises SO261 and TAN1711 – that explored the deposition dynamic, carbon burial rates and carbon mineralization rates as well as microbial communities structures in sediment along the axes of the Atacama Trench and the Kermadec trench and relating condition to adjacent abyssal refences sites – a long list of papers from these expeditions have been published and more are in the pipeline. I fully acknowledge that this was not the impressively long cores that you obtain with the I ODP efforts but “only” gravity cores, multiples cores and box cores. But the spatiotemporal variability along the trench axes and linkage to mass wasting ws a core focus of these expeditions.\nI suggest eliminating this sentence.", "reviewer_2": "Review for Bao\nI really appreciate to read a follow up study of the paper by Bao et al. On Hadal zone from a few years ago. It gives a sense of active progress on a key issue. That of turbidities and their role in transport (most?) organic carbon to trench.\nThe introduction is very clear, the goal is clearly stated.\nI felt that the text is very interesting, but it reads sometimes like a review. You may spend a bit more time on your figures, explain very clearly what are your data, and do not mix so much your own work with that of your colleagues. This would bring a bit more clarity and focus to the main text.\nI was fascinated by the process oriented study in hadal zone. The topic is very important, and the authors may be the only one focusing on this. The article reorts a DOC compensation depth in the Japan hadal zone, they have evidences for carbon sink swaps (from organic carbon to carbonate) which is all very fundamental and important.\nThe issue I see is that some may say ''we already know carbon matures and produces a bit of methane''. And I would agree that the way the text is written does not really answer this potential criticism, or at least does not make clear enough why such criticism does not affect the work. What you did here is not just show the OC transforms and procudes CH4. You should highlight the combination of tools, the focus on a very hard to study field site, that your have actually advanced our understanding of OC maturation in this very specific location. You should answer those points right away, be clear about your contribution.\nThe conclusion is dispointing. It is full of ''platitudes'' and generalities unrelated to your work and not sexy at all (to me), they actually undermine your work greatly. What's sexy in your work is the novelties, the creative concepts blend with solid data, the amount of work your did. You should conclude your work with a big highlight on some of the key findings you made, and trust that we understand this is important. We do not need generalities to feel secure. Make us insecure instead, and show us that you are pushing forward in directions that no one is even looking at...\nI think the study would benefit from a bit more work on the structure: your data first, and then a bit more discussion. Here you tried to merge everything to an extreme degree and you lost clarity in my opinion, it dilutes your contribution.\nI'd be happy to check a final revised version.\n''supporting the deep biosphere metabolites'' does not really make sense. Metabolisms maybe.\nL207: ''compensatio deth of labile DOC'' I find this notion intriguing.\nL215 – 222: very interesting!\nL227: do you mean authigenic?\nL227-229: for a ''carbon sink swap'' the other way around, check out Galvez et al 2012: carbonate to graphite.\nL260-269: very dispointed by this: this sounds like this could have been written elsewhere. I felt throughout that your research strength is that it focuses on a the detailed processes happening in the hadal zone, the inlet to the subduction system. Even though everything you mention is correct, I would not conclude this work by those generalities. Conclude by stressing again your contribution. Mention again the notion of a DOC compensation depth, the notion that carbon sink swap may exist as shallow as on the seafloor and pore water. Those notions and the data that you provide to support them are truly innovative, finish with that."}, "author_response": "We deeply appreciate the constructive comments from the Reviewers concerning our\nmanuscript entitled “Earthquake-enhanced dissolved carbon cycles in ultra-deep ocean\nsediments”. These comments are very helpful for improving our manuscript. In this point-by-\npoint Response to Reviewers we have addressed all the comments and suggestions. The\nresponses to the Reviewers’ comments are listed below, with comments listed first in blue italics,\nfollowed by our response in black. We are pleased to provide the revised manuscript and hope\nboth Reviewers are satisfied with our responses.\nComments from Reviewer #1\nThis is a well-written and interesting manuscript providing new insights on the digenesis and\nburial of organic carbon in the Japan trench. Hadal trenches are apparently unproportionally\nimportant for marine (and global?) element cycling due to; material focusing, seismic driven\nmass wasting and potentially translocation of material by tsunamis. But our current\nunderstanding on the processes governing transport, deposition and mineralization at these\nextreme depths are still rudimentary – but research interests are increasing. The study is timely,\nand the Japan trench is a very interesting case study for the biogeochemical function of hadal\ntrench systems.\nResponse: We are grateful for your efforts on reviewing our manuscript and precisely\nhighlighting the context of our study.\nThe data set is impressive, and the main conclusions are well substantiated. The finding\ncomplements other recent studies of hadal trench system, but also provide original and novel\nfindings. Personally, I value the strong evidence of active early diagenesis (methanogenesis) in\ndeep subsurface settings drive by relatively labile DOC.\nResponse: Thank you for your positive comments.\nAt times I feel the authors over sell some of their findings – or at least they should acknowledge\nother recent advances that also demonstrate i) intensified burial of Org C in hadal trench system\nand ii) subsurface burial of labile organics in hadal sediment fueling intensified microbial\nactivity. I understand that the hadal research is advancing rapidly, and that recent publications\nmay not find their way into manuscripts that are being developed concurrently. But fortunately,\nthe authors still have a chance to do that.\nIn conclusion, recommend the manuscript is accepted with relatively minor revision. Most\nimportantly the authors should place your study in the context of other recent and highly\nrelevant publications.\nResponse: Thank you for the positive feedback. Below we have addressed your detailed\ncomments and specified several places where we added relevant papers in the revised\nmanuscript.\nMore specific comments are listed below.\nAbstract:\nLine 79; I suggest changing “most mysterious” to “unique”.\nResponse: This is changed in the revised manuscript.\nLine 86-89 & 90-93; I acknowledge the novel contribution of this high-quality study (See\nabove/below). But conceptually, the fact that earthquake-triggered turbidity in hadal trenches\nresult in subsurface layers of labile organic material and intensified microbial activity – and\nintensified burial and subduction of organic material has been documented and discussed in\nrecent publications. For example;\nZabel et al. (2022) High carbon mineralization rates in subseafloor Hadal sediments – result\nof frequent mass wasting. Geochem, Geophy, Geosystem 23. Where some of the evidence\noriginates from gravity cores that have been recovered from the Japan trench and include the\nSMTZ. I am particular surprised that this paper ins’ t considered in this submission.\nOguri et al (2022) Sediment accumulation and carbon burial in four hadal trench systems. JGR\nBiogeosciences 127. The manuscript, target mores sites along different trench systems, it is\nestimated that carbon burial rates in hadal trenches are 70 times higher than the global average\nfor the deep ocean.\nSchauberger et al (2021) Spatial variability of prokaryotic and viral abundance in the\nKermadec and Atacama trench regions. Limnol Oceanogr 66: 2095-2109.\nNone of these publications are acknowledged or cited in the current manuscript., but I find that\nthe current submission should place their findings in the context of these recent advances.\nResponse: Thank you for your suggestions. We specifically value the work by Zabel et al. (2022)\nand Oguri et al. (2022). The three papers are now cited in the revised manuscript.\nOguri et al. (2022) as ref. 4 at Lines 85, 106 and 166;\nZabel et al. (2022) as ref. 14 at Lines 108, 166 and 206;\nSchauberger et al (2021) as ref. 33 at Line 185.\nLine 154-159; This point has been presented and discussed in two recent papers of Zabel et al\n2022 & Oguri et al 2022. Referencing and relating to these studies would substantiate your\nclaims.\nResponse: Thank you. These two references are added at several relevant places in the\nmanuscript.\nLine 194-202; Can these claims be substantiated (no references are included, but they are\npresented as facts), or is these hypotheses? Please be specific.\nResponse: These texts are added with references at where we find relevant at Lines 198-210,\nwhere we attempted to hold the relevant researches together to provide a framework for our\nstory, which is that earthquake-triggered turbidites enhance methanogenesis via various\nchemical (dissolved carbon supplement) and physical (sediment compaction) mechanisms.\nLine 211-213; Evidence in the form of sequencing data (barcoding or metagenomics) or specific\nbiomarkers for methanogens would have strengthen the proposed concept significantly. I don’t\nknow if there are any initiatives pursuing this for the recovered samples\nResponse: This topic will be pursued by our colleagues from IODP Exp. 386.\nLine 220; I suggest changing “atmosphere” to “bottom water”. I acknowledge that “quickly”\nmay mean different things for different people. But “quickly” is unspecific and venting of deep\noceanic waters is on time scales of centuries to millennia.\nResponse: This suggestion is appreciated. This is changed in the revised manuscript at Lines\n227-230:\n“The earthquake-triggered process cascade prolongs the residence time of labile carbon14,\nwhich would have been otherwise consumed and released into the bottom water on time\nscales of centuries to millennia, and thereby promotes carbon aging and subduction in the\ntrench sediments (Fig. 4a).”\nLine 227-231; Are there any hardcore evidence that authentic carbonate precipitation is\noccurring in hadal sediments? Or is this another hypothesis/claim? Please be specific.\nResponse: Yes, we have added photos of the authigenic carbonate precipitation in the Japan\nTrench sediments as Supplementary Fig. 5:\nSupplementary Fig. 5. The observed authigenic carbonates (ikaites) in Sites M0084 (a & b),\nM0088 (c & d), and M0087 (e).\nLine 236; A reference here would be appropriate\nResponse: We have now referenced (Plank & Manning, 2019) and (Debret et al., 2022) at Lines\n247-248, where the seawater-derived, OC-containing fluids in the subduction zones are\ndiscussed.\nLine 254-256; that is not correct – there have been more “coordinated international research\nefforts that perform high spatiotemporal resolution investigations and sampling of a hadal\noceanic trench” For instance, the cruises SO261 and TAN1711 –that explored the deposition\ndynamic, carbon burial rates and carbon mineralization rates as well as microbial communities\nstructures in sediment along the axes of the Atacama Trench and the Kermadec trench and\nrelating condition to adjacent abyssal refences sites – a long list of papers from these\nexpeditions have been published and more are in the pipeline. I fully acknowledge that this was\nnot the impressively long cores that you obtain with the IODP efforts but “only” gravity cores,\nmultiples cores and box cores. But the spatiotemporal variability along the trench axes and\nlinkage to mass wasting were a core focus of these expeditions.\nI suggest eliminating this sentence.\nResponse: Thank you. This sentence is removed in the revised version.\nComments from Reviewer #2\nReview for Bao\nI really appreciate to read a follow up study of the paper by Bao et al. On Hadal zone from a\nfew years ago. It gives a sense of active progress on a key issue. That of turbidities and their\nrole in transport (most?) organic carbon to trench.\nThe introduction is very clear, the goal is clearly stated.\nResponse: We appreciate your continuous attention for our research and your positive feedback.\nI felt that the text is very interesting, but it reads sometimes like a review. You may spend a bit\nmore time on your figures, explain very clearly what are your data, and do not mix so much\nyour own work with that of your colleagues. This would bring a bit more clarity and focus to\nthe main text.\nResponse: Thank you for your suggestion. We have now made minor modifications and added\nreferences where we find appropriate throughout the manuscript.\nI was fascinated by the process-oriented study in hadal zone. The topic is very important, and\nthe authors may be the only one focusing on this. The article reports a DOC compensation\ndepth in the Japan hadal zone, they have evidences for carbon sink swaps (from organic carbon\nto carbonate) which is all very fundamental and important.\nResponse: Your positive feedback is appreciated.\nThe issue I see is that some may say ''we already know carbon matures and produces a bit of\nmethane''. And I would agree that the way the text is written does not really answer this potential\ncriticism, or at least does not make clear enough why such criticism does not affect the work.\nWhat you did here is not just show the OC transforms and produces CH4. You should highlight\nthe combination of tools, the focus on a very hard to study field site, that you have actually\nadvanced our understanding of OC maturation in this very specific location. You should answer\nthose points right away, be clear about your contribution.\nResponse: Thank you very much for helping us to clarify the focus of this manuscript. Several\nmodifications were made regarding your suggestion:\n Highlighting the technical advancement of this study at Lines 264-268:\n“Our presented data and results, acquired through advanced drilling and radiocarbon\nmeasurement techniques, offer a comprehensive perspective of the hadal trench carbon\ncycle in the deepest and least explored environments on our planet, where geological events\nsuch as great subduction zone earthquakes may serve as key modulators.”\n Stating that our study shed light on a detailed process in the hadal trenches at Lines 268-\n271:\n“The contribution from our newly-discovered carbon reservoirs in the Japan Trench\nsediments provide essential, detailed information regarding how intensive earthquake-\ninduced SOC accumulation leads to enhanced microbial activities in the deep biosphere.”\n Mentioning the significance of the DOC compensation depth and carbon sink swap\noccurring above the subduction zones at Lines 271-277:\n“The occurrence of the dissolved carbon compensation depth indicates intensive carbon\nmaturation and alteration in the trench subsurface sediments. These processes occur at the\ninlet to the subduction zone systems, where fresh materials are irregularly introduced by\ngeological events. The cascade of the earthquake-triggered physical and chemical\nprocesses accelerates the transformation between carbon in different forms (i.e., ‘carbon\nsink swap’) at the shallowest setting of the subduction zone systems.”\n Raising further questions and outlooks at Lines 277-281:\n“Meanwhile, open questions remain regarding whether, and how much of, the carbon in\nthe trench sediments is subducted in its various forms. We suggest that the earthquake-\nenhanced dissolved carbon reservoir in the trenches will constitute a potential source for\nthe deep carbon reservoir in the lithosphere or mantle.”\nThe conclusion is disappointing. It is full of ''platitudes'' and generalities unrelated to your work\nand not sexy at all (to me), they actually undermine your work greatly. What's sexy in your work\nis the novelties, the creative concepts blend with solid data, the amount of work you did. You\nshould conclude your work with a big highlight on some of the key findings you made, and trust\nthat we understand this is important. We do not need generalities to feel secure. Make us\ninsecure instead, and show us that you are pushing forward in directions that no one is even\nlooking at...\nI think the study would benefit from a bit more work on the structure: your data first, and then\na bit more discussion. Here you tried to merge everything to an extreme degree and you lost\nclarity in my opinion, it dilutes your contribution.\nI'd be happy to check a final revised version.\nBest\nMatthieu Galvez\nResponse: We delightfully accept your suggestion. The conclusion part is now substantially\nrevised by putting more emphases on our data and discovery at Lines 264-281, with detailed\nexplanations above.\n''supporting the deep biosphere metabolites'' does not really make sense. Metabolisms maybe.\nResponse: Thank you for the correction. Changed to metabolisms.\nL207: ''compensation depth of labile DOC'' I find this notion intriguing.\nResponse: Thank you. We will do more works in the future regarding how these DOC are\nproduced and utilized in the deep subsurface.\nL215 – 222: very interesting!\nResponse: Thank you.\nL227: do you mean authigenic?\nResponse: Yes. This is now corrected.\nL227-229: for a ''carbon sink swap'' the other way around, check out Galvez et al 2012:\ncarbonate to graphite.\nResponse: Thank you. We have stated that the carbon may be eventually transformed to\ngraphite at Lines 239 and 255, where this paper is referenced as ref. 47:\n47. Galvez, M. E. et al. Micro- and nano-textural evidence of Ti(-Ca-Fe) mobility during fluid-\nrock interactions in carbonaceous lawsonite-bearing rocks from New Zealand. Contrib. to\nMineral. Petrol. 164, 895–914 (2012).\nL260-269: very disappointed by this: this sounds like this could have been written elsewhere. I\nfelt throughout that your research strength is that it focuses on a the detailed processes\nhappening in the hadal zone, the inlet to the subduction system. Even though everything you\nmention is correct, I would not conclude this work by those generalities. Conclude by stressing\nagain your contribution. Mention again the notion of a DOC compensation depth, the notion\nthat carbon sink swap may exist as shallow as on the seafloor and pore water. Those notions\nand the data that you provide to support them are truly innovative, finish with that.\nResponse: Thank you for the encouragement. We have now revised the conclusion part by\nputting more emphases on our contribution at Lines 264-281:\n“Our presented data and results, acquired through advanced drilling and radiocarbon\nmeasurement techniques, offer a comprehensive perspective of the hadal trench carbon\ncycle in the deepest and least explored environments on our planet, where geological events\nsuch as great subduction zone earthquakes may serve as key modulators. The contribution\nfrom our newly-discovered carbon reservoirs in the Japan Trench sediments provide\nessential, detailed information regarding how intensive earthquake-induced SOC\naccumulation leads to enhanced microbial activities in the deep biosphere. The occurrence\nof the dissolved carbon compensation depth indicates intensive carbon maturation and\nalteration in the trench subsurface sediments. These processes occur at the inlet to the\nsubduction zone systems, where fresh materials are irregularly introduced by geological\nevents. The cascade of the earthquake-triggered physical and chemical processes\naccelerates the transformation between carbon in different forms (i.e., ‘carbon sink swap’)\nat the shallowest setting of the subduction zone systems. Meanwhile, open questions remain\nregarding whether, and how much of, the carbon in the trench sediments is subducted in its\nvarious forms. We suggest that the earthquake-enhanced dissolved carbon reservoir in the\ntrenches will constitute a potential source for the deep carbon reservoir in the lithosphere\nor mantle.”"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have satisfyingly address all my concerns, I encourage acceptance of the manuscript for publication", "reviewer_2": "Dear authors and editors,\nI have checked your revision after my previous review. I felt my comments and suggestions have been considered. I will be pleased to see this paper published as it brings valuable data to the community, and it will also stimulate the community toward a direction of research which remains underrepresented (oceanographic approaches to the subduction carbon cycle, the inlet of the system)."}, "author_response": "We appreciate the feedbacks from the Reviewers concerning our manuscript\n“Earthquake-enhanced dissolved carbon cycles in ultra-deep ocean sediments”. The\nresponses to the Reviewers’ comments are listed below, with comments listed first in\nblue italics, followed by our response in black. We are pleased that both Reviewers are\nsatisfied with our revised manuscript.\nComments from Reviewer #1\nThe authors have satisfyingly address all my concerns, I encourage acceptance of the\nmanuscript for publication\nResponse: We thank you again for your efforts reviewing our manuscript.\nComments from Reviewer #2\nDear authors and editors,\nI have checked your revision after my previous review. I felt my comments and\nsuggestions have been considered. I will be pleased to see this paper published as it\nbrings valuable data to the community, and it will also stimulate the community toward\na direction of research which remains underrepresented (oceanographic approaches to\nthe subduction carbon cycle, the inlet of the system).\nBest\nMatthieu\nResponse: Thank you. Your comments have helped us to substantially improve our\nmanuscript."}]

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