Correction: Evidence for gas hydrate-filled fractures forming at the sulfate-methane transition zone, Hikurangi subduction margin, New Zealand
Year: 2025
Type: erratum
Abstract: 1 IntroductionThe formation of gas hydrate in marine sediments resultsfrom a complex interplay among microbial activity, thermodynamicconditions, fluid migration, and sediment properties. Gas hydrateis an ice-like solid composed of H20 and CH4 that forms underhigh-pressure and low-temperature conditions typically foundwithin marine sediments on continental margins (Kvenvolden,1993). Methane within gas hydrate systems can originate fromtwo different sources: 1) biogenic methane produced by microbialdegradation of organic matter (Dickens, 2001), or 2) thermogenicmethane migrating from mature source rocks (Milkov, 2011). Mosthydrate systems, especially systems with low hydrate saturation,are likely sourced via microbial methanogenesis (You et al., 2019).For hydrate formation to occur, methane concentration mustexceed the solubility threshold within the gas hydrate stabilityzone (GHSZ) (Davie and Buffett, 2001).In general, gas hydrate does not appear above the sulfatemethanetransition zone (SMTZ), where upward-diffusingmethane meets downward-diffusing sulfate. This interface fuelsanaerobic oxidation of methane (AOM) by microbial communities(Boetius et al., 2000; Egger et al., 2018). Below the SMTZ,methane can be generated and accumulated in pore waterseither through local microbial methanogenesis or by methaneadvection. On continental slopes, the SMTZ is located at shallowdepths–typically between 5 and 50 m below the seafloor (mbsf)– and in most cases within the GHSZ. Thus, this zone representsa potential site for early-stage hydrate formation when methaneconcentration exceeds solubility (Bowles et al., 2014; Malinvernoand Pohlman, 2011).1.1 Gas hydrate in fracturesIn coarse sands or silts, gas hydrate typically forms withinprimary pore spaces (Waite et al., 2009). In contrast, in clayrichsediments, gas hydrate forms in secondary porosity asnodules, veins or fractures, with fractures being the mostcommonly observed morphology (Weinberger and Brown, 2006;Cook et al., 2014; Daigle and Dugan, 2010). Hydrate-filledfractures have been observed at both gas venting sites, whereactive fluid advection occurs (Kim et al., 2011), and at nonventingsites lacking evident fluid or gas advection (Cook andGoldberg, 2008; Cook et al., 2014).Oti et al. (2019) identified and described relic gas hydratefilledfractures using X-ray computed tomography (X-CT) scansof sediment cores from a non-venting site in the Gulf of Mexico.X-CT is a non-destructive imaging technique that reconstructsthree-dimensional images of a sediment core’s internal structurebased on variations in X-ray attenuation. In their data, Oti et al.(2019) observed that relic gas hydrate-filled fractures appeared aslow-density, planar features dipping at ∼40° to near-vertical, withwispy or diffuse boundaries. These relic fractures are distinguishablefrom sharp, horizontal, drilling and coring-induced fractures. Inaddition, the relics of gas hydrate filled-fractures often exhibitbranching or bifurcating patterns and tend to occur perpendicularto bedding planes. The fractures described by Oti et al. (2019)were located within a known zone of gas hydrate-filled fracturesfirst described by Cook et al. (2008) using logging-while drillingresistivity images.Oti et al. (2019) hypothesized that these fractures are the resultof gas hydrate initially forming within burrow networks, sourced byin situ microbial methanogenesis. Continued methanogenesis thensustained gas hydrate growth, which in turns generated sufficientstress to propagate fractures that are oriented approximatelyvertically, which is the direction of the maximum stress.1.2 Difficulty of studying gas hydrate indeep-marine environmentsStudying gas hydrate in deep-marine environments remainsa challenge due to both the logistical difficulty and high costof accessing these settings, as well as the difficulty of preservinggas hydrate samples in their natural state. Most hydrate-bearingsediments lie hundreds of meters below the seafloor, where highpressureand low-temperature conditions maintain hydrate stability.However, during core recovery, the rapid decrease in pressure andtemperature often causes gas hydrate to dissociate, leading to partialor complete dissociation before they can be analyzed (Weinbergerand Brown, 2006).Downhole resistivity logs are sensitive to the presence ofgas hydrate (Waite et al., 2009), and when resistivity is coupledwith accurate in situ porosity or bulk density measurements,hydrate saturation can be reliably calculated. However, in mostscientific drilling holes, the upper 50 to 100 mbsf of the hole areheld open using drilling pipe, preventing the collection of welllogging data in this interval. As a result, this unlogged intervallikely means that many shallow gas hydrate accumulations arenot detected.1.3 Study areaThe Hikurangi subduction zone, located off the northeasterncoast of New Zealand (Figure 1A), is a tectonically active regionalong the Pacific-Australian plate boundary, characterized byfrequent seismic activity and recurrent sedimentary masstransportprocesses (Wallace et al., 2009; Barnes et al., 2020;Strachan et al., 2022). The region also hosts widespread gashydrate systems, which may play a role in slope stability, eitherby enhancing sediment strength through cementation or bypreconditioning slopes for failure when gas hydrate dissociate(Burton et al., 2020; Kroeger et al., 2019; Plaza-Faverola et al., 2012;Pecher et al., 2019; Claussmann et al., 2023).A key feature of this margin is the Tuaheni Landslide Complex(TLC), located on the upper slope near Poverty Bay (Figures 1B, C).The TLC is one of the most extensive and well-documentedsubmarine landslide systems offshore New Zealand, comprising aseries of mass-transport deposits that exhibit morphological andgeophysical evidence of past - and potentially ongoing - slope failure(Mountjoy et al., 2014; Couvin et al., 2020).IODP Site U1517 intersects the TLC and lies within thegas hydrate stability zone (GHSZ). The GHSZ defines thedepth range within which gas hydrate is thermodynamicallystable; at Site U1517, this zone extends from 650 m below seaFrontiers in Earth Science 02 frontiersin.orgBrunet et al. 10.3389/feart.2025.1630118FIGURE 1(A) Study area location in the Hikurangi margin, north-east of North Island of New Zealand; (B) Bathymetric map showing North and South Tuahenilocation with thrust faults offshore and drill site U1517 from IODP Expedition 372 (November 2017 – January 2018, Pecher et al., 2019); (C) Zoom onthe two South Tuaheni lobes and their rough surficial morphology, displaying uncommon extensional deformation (slight white) downstreamdisplaying uncommon extensional deformation (slight gray) downstream and compressional deformation (slight gray) upstream. Drill site U1517 islocated into the extensional deformation zone. Gas flares detected on the edges of Tuaheni South Deposit during the RV Tangaroa cruise TAN1404(Gross et al., 2018); (D) Seismic line crossing the Tuaheni South Deposit from NW to SE, illustrating both the compressional and extensionaldeformations zones, location of drill site U1517, the intra-debris reflector from Gross et al., 2018 (dashed purple line), base of the Tuaheni South Deposit(turquoise blue line) and the Basal Gas Hydrate Surface (BGHS; dashed black line). Gas-charged strata are identified below the BGHS so as seafloorundulations at deposits’s surface; (E) Zoom on the U1517 drill site which cored key structural interfaces. Bathymetric (C) and seismic (D, E) illustrationsmodified from Mountjoy et al. (2014).level to ∼162 m below seafloor, as constrained by geophysicaland geochemical data (Figure 1D; Mountjoy et al., 2014;Gross et al., 2018; Pecher et al., 2019).In this study, we combine geochemical and X-CT datasetsfrom sediment cores collected during the IODP Expedition372 to investigate gas hydrate occurrence in the TLC offshoreNew Zealand (Figure 1E). We report, for the first-time, evidence ofgas hydrate-filled fractures at very shallow depths within the SMTZ,and we discuss this unique observation regarding (1) the potentialfor similar fractures in other gas hydrate systems worldwide, (2) theanalytical implications for future marine gas hydrate investigations,and (3) the possible consequences for climate-sensitive regions.2 Materials and methods2.1 Sediment cores from IODP expedition372IODP Expedition 372 Site U1517 was designed to log andsample through both the Tuaheni Landslide Complex and the gashydrate stability zone (Figure 1; Pecher et al., 2019; Wallace et al.,2019). This study utilizes data from two holes drilled at thesite. Hole U1517-A was drilled in 725 m of water depth usinglogging-while-drilling (LWD) tools to a depth of 205 mbsf. InHole U1517-C, 188.5 m of sediments were recovered using theFrontiers in Earth Science 03 frontiersin.orgBrunet et al. 10.3389/feart.2025.1630118advanced piston corer (APC) and half-length APC (HLAPC).These two holes crossed the entire thickness of the TLC, from theseafloor down to the base at approximately 70 mbsf, and intersectedthe bottom simulating reflection (BSR) at ∼162 mbsf. Highresolutioncore photographs from the Expedition were modifiedand enhanced applying histogram equalization in Corel PHOTOPAINT®software. This processing method improves the visibilityof color contrasts, thus helping in the identification of lithofacies,deformation structures and sedimentary features. It may alsoproduce slightly artificial colors. The processed images were used forvisual interpretation of the core sedimentology.2.2 Pore water geochemistry analysesDuring IODP Expedition 372, pore water geochemistryanalyses included chloride concentration (Cl−), sulfate (SO42−)and methane (CH4). Pore water was extracted from sedimentcores using Rhizon samplers or a hydraulic press for squeezing,depending on sediment compaction and water content (Manheim,1966; Dickens et al., 1995). Chloride concentrations weremeasured using ion chromatography, with deviations fromseawater values used as indicators of gas hydrate presence ordissociation (Egeberg and Dickens, 1999). Sulfate concentrationswere determined using spectrophotometric methods, allowingfor the identification of the sulfate-methane transition zone(SMTZ), which marks the depth at which microbial anaerobicoxidation of methane (AOM) occurs (Borowski et al., 1996;Treude et al., 2003). Methane concentrations were quantified usinggas chromatography (GC), where headspace gas samples weretaken immediately after core recovery to minimize methane loss(Claypool and Kvenvolden, 1983; Pimmel and Claypool, 2001).These geochemical measurements provided crucial insights intogas hydrate occurrence and associated biogeochemical processesalong the Hikurangi subduction margin (Crutchley et al., 2018;Pecher et al., 2019).2.3 X-ray computed tomography (X-CT)X-CT is a non-destructive imaging technique that generateshigh-resolution, three-dimensional representations of internalstructures based on the differential attenuation of X-raysby the materials of varying density and composition. Thismethod is particularly effective for sediment cores analysis, asit reveals micro-scale deformation structures, subtle densityvariations, and sedimentary features that are often invisible to thenaked eye (Cnudde and Boone, 2013).For this study, the archival half core from Hole U1517-Cwas scanned using a Neurologica CereTom system at Ohio StateUniversity, to image the sediment interval from 0 to 71 mbsf. Thescanner resolution is 0.5 × 0.5 × 0.625 mm. All scans were performedwith a peak energy of 100 keV, providing sufficient contrast toresolve density variations associated with subtle lithological changesand deformation features. On the selected display scale, highdensitymaterials appear white, while low-density materials suchas air appear dark, making density contrasts–particularly betweensediments and air-filled features–clearly visible.The X-CT data were processed and visualized using theopen-source software FIJI®. Pre-processing steps includedimage alignment, noise reduction, and contrast enhancement tooptimize the visibility of fine-scale features. Three-dimensionalreconstructions and orthogonal slices were generated to facilitatethe identification of fluid escape features and other sedimentarystructures.To estimate hydrate content, fracture volumes were quantified bysegmenting the fractures using a thresholding approach. The areaof the thresholded regions was divided by the total image area tocalculate the relative hydrate fraction. This ratio was then applied tothe total bulk volume of the core segment to estimate the absolutevolume of hydrate-filled fractures.3 Results3.1 Fluid geochemistry3.1.1 Gas hydrate occurrenceChloride concentration measurements at Site U1517 provideclear evidence of gas hydrate presence below 81 mbsf. During corerecovery, gas hydrate dissociates due to pressure and temperaturechanges, releasing freshwater that dilutes pore water chloride. AtSite U1517, a notable decrease in chloride concentrations between81 and 179 mbsf is interpreted as a signal of gas hydrate intervalsabove ∼162 mbsf and free gas below, consistent with the baseof the hydrate stability zone established as approximately 162mbsf (Figure 2A; Pecher et al., 2019). Gas hydrate saturation(Sh, expressed as a percentage of pore space; Figure 2B) wasestimated using the LWD data. Calculated saturation values rangefrom 2% to 68% between and mbsf, with gas in methane Site U1517, methane concentrations in headspace gas from from 0 to mbsf, methanogenesis shallow This interval to the where microbial sulfate the approximately mbsf, methane concentrations to at mbsf, microbial methanogenesis (Figure This depth the location of the base the zone, where sulfate and to (Borowski et al., Dickens, is to that methane measurements of the in situ methane as during core recovery, and hydrate dissociation the methane X-CT evidence of gas was core which in Earth Science frontiersin.orgBrunet et al. 10.3389/feart.2025.1630118FIGURE Chloride at Site U1517 and Gas hydrate dissociation is by low chloride values to and are used to estimate (B) of gas hydrate saturation from chloride. (C) and methane in the upper m of The Methane is by a dashed line and is of where sulfate is and methane is with depth et al., X-CT to sedimentary structures that are not through to mbsf to X-CT data a of structures (Figures Most of these or diffuse selected low-density materials as air appear dark, while highdensitymaterials appear The structures in from 0.5 to 5 between and between and total fractures were or the to making likely that not structures structures are most with the of the zone fractures (Figure a of structures with consistent of hydrate-filled fractures Oti et al.,2019). These fractures are from which typically appear as fractures (Figure and are in blue in Figure Hydrate filled-fractures forming at Site U1517, the gas hydrate stability zone extends 650 m below to m as constrained by and geochemical et al., 2014; Gross et al., 2018; Pecher et al., 2019). InHole U1517-C, we identified potential relic of gas hydrate and mbsf, well within the gas zone (Figure These fractures are observed in sediments (Figures and exhibit consistent with features described fractures et al., 2019). These planar features by diffuse edges and relative to the sediment as are open and on the the observed fractures, we estimate that the fractures of the bulk which is to saturation of a porosity of Site U1517, the SMTZ was identified at approximately et al., 2019; Methane can this where sulfate is and is gas hydrate formation (Borowski et al., we not in Earth Science frontiersin.orgBrunet et al. 10.3389/feart.2025.1630118FIGURE of relics of gas hydrate filled-fractures by from as and in is a of line line and X-ray cores and depth below seafloor are methanogenesis from sediments typically range from to Egger et al., 2018). of of to saturation and for the of local methane as particularly near the We this hydrate formation may from the of methane via fractures and In the of these and the SMTZ that hydrate in situ during gas hydrate recovered from the the morphology of fractures is similar to that of gas fractures other It is likely that the gas hydrate core we these low-density structures as hydrate-filled fractures based on their planar morphology, and occurrence near the we features may potentially from other formation within for to and propagate fractures et the between biogenic and when observed with X-CT We are of that structures in these and be the of these to other to other where hydrate been the fractures at Site U1517 are in and occur at fractures to in described relic hydrate-filled fractures the entire that the of not have been during et al., gas hydrate filled-fractures identified in LWD data the and the to by resistivity tools Cook et al., These often hydrate filled-fractures for of meters as they the In contrast, the observed in core likely not in Earth Science frontiersin.orgBrunet et al. 10.3389/feart.2025.1630118FIGURE of the core to of X-CT image slices from core low-density features in and features in The and bottom bottom image display of the of the gas fractures are in based on diffuse low-density and fractures et al., while coring-induced fractures are in are in The Methane at mbsf at site U1517 et al., is by a dashed in resistivity logs or resistivity images the can these features and the sediment filled-fractures at active or are typically at depths at Site U1517 The in Earth Science frontiersin.orgBrunet et al. 10.3389/feart.2025.1630118FIGURE of relic gas fractures from X-CT scans in the is from The coring-induced fracture is in consistent with Figure The features of gas as described by Oti et al. and diffuse X-CT at also in Earth Science frontiersin.orgBrunet et al. 10.3389/feart.2025.1630118FIGURE structural and geochemical of cores down to mbsf. structures are also on core compressional porosity bulk density and are In (LWD) data from Site as and and bulk density for filled-fractures were identified in logging-while drillingresistivity images at mbsf offshore (Cook and while most of fractures are and mbsf (Cook et al., 2014; Oti et al., 2019; 2011). We that the fractures Site U1517 the of gas hydrate which their and shallow as to on the seafloor and depth fractures may and into (Figure investigate similar hydrate filled-fractures may or near the SMTZ in other we scans of sediment cores from the the not X-CT on sediment when they shallow other core scans that both the GHSZ and the of these were offshore site from Expedition et al., 2009), one site et al., one site from et al., and from Expedition et al., identified potential relic hydrate-filled fractures at sites, are located the and IODP Expedition Site provided the evidence for the presence of relic of gas These fractures were observed in core in and between and These fractures (Figure are at Site U1517, and and exhibit the diffuse edges of et al., 2019). in Earth Science frontiersin.orgBrunet et al. 1 The the depth and sulfate methane transition zone at with gas hydrate of gas or of SMTZ Gulf of Cook et al. Oti et al. et al. Gulf of Cook et al. and et Gulf of Cook et al. Site 5 Cook and Cook et and et al. Site Cook et al. Site Cook et al. and et Site U1517 Hikurangi This study and Pecher et al. Site This study and et al. showing hydrate fracture system formation at the sulfate-methane transition zone Below the by a the solubility of methane is not (B) the methane exceeds the solubility for hydrate to in forming in open pore fractures. (C) As the hydrate fracture fractures are the identified at Site U1517, located to the SMTZ fractures The fractures occur at m which is at approximately mbsf based water geochemistry et al., The occurrence fractures both the and New Zealand near the that hydrate in to this due to the of in Earth Science frontiersin.orgBrunet et al. 10.3389/feart.2025.1630118FIGURE of the core to of X-CT image slices from of the core low-density features in black and features in relics of gas hydrate-filled fractures are on identification diffuse low-density and fractures et al., 2019). The sulfate-methane at mbsf at site et al., is by a dashed for that hydrate-filled fractures at or near the SMTZ in methane is these fractures this of gas hydrate formation - particularly fractures forming to - have implications in as the where shallow gas are to dissociation due to in Earth Science frontiersin.orgBrunet et al. and data that in bottom water can to gas in shallow et al., and low saturation hydrate occurs pore of sediment and the may not a and in et al., 2019). methane forms a that through the pore the seafloor, most likely be by oxidation of within the sediment et al., 2018; In contrast, methane from hydrate-filled as a gas and may This gas through the potentially the In a methane fractures can the in a matter of et al., of methane into the presence and dissociation of hydrate-filled the of methane as a in may that and to that as for methane hydrate formation near the we this likely The relative of fracture may and as sediment methane interpretation the potentialfor to to shallow the in sedimentary the gashydrate in fractures and dissociation can and and geochemical in 2003). the dissociation or of hydrate-filled the of microbial and with potential consequences for microbial and biogeochemical et al., for future of shallow gas hydrate the and X-CT into sediment core in at and the sourced methane is and are the sedimentary evidence of past gas hydrate when gas hydrate samples are X-CT data in can sedimentary features as that are not through et al., fractures observed at Site below the threshold of (Figure hydrate-filled fracture systems - as offshore and in the Gulf of 1) - may through this study that fractures can to the seafloor In most the m below theseafloor are not by data the drill to the to We likely to the and of the occurrence and of gashydrate study relic gas hydrate-filled fractures shallow with the zone (SMTZ), based on of marine sediment cores from IODP Expedition372 offshore New These of hydrate at and These fractures may to in in depth as microbial methane dissociation or of hydrate fractures sediment fluid and during In the presence ofgas hydrate in methane to the seafloor especially regions the features sediment cores from Expedition in the shallow hydrate-filled fractures may be due tothe of X-CT imaging in shallow drilling interpretation by and is based as geochemical analysis, and to the hydrate of these also the relative of microbial and gas in shallow
Source: Frontiers in Earth Science
Language: English
Subfield: Environmental Chemistry
Field: Environmental Science
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $3,801
OpenAlex ID: W4413824513