Abstract
Offshore aquifer research is an emerging field that is becoming increasingly important as population growth and climate change put pressure on coastal water resources. One of the largest ...reserves, globally, of offshore freshened groundwater (OFG) was recently identified off the South Island of New Zealand. This has highlighted the potential for OFG elsewhere in New Zealand. This study aims to: (1) screen for New Zealand coastal aquifers most likely to contain OFG and, (2) document evidence for OFG in New Zealand. An OFG-likelihood rating scheme was developed as part of the study. An application of the rating scheme used survey responses from regional councils responsible for groundwater management, in combination with national and regional-scale technical documents. The rating scheme was found to be a simple and transparent first-pass approach for highlighting areas where OFG is more or less likely at the national scale. Results are presented in a map showing the likelihood of OFG around the New Zealand coastline. Regions with aquifers where OFG likelihood is high include Greater Wellington, Canterbury, Tasman, Hawkes Bay and Marlborough. Aquifers in these regions are associated with major fluvial depositional systems, including glacial outwash gravels. Despite high dependence on groundwater in these regions and extensive groundwater extraction near the coast, there are no major reported incidences of seawater intrusion, suggesting offshore groundwater may be augmenting onshore extraction.
Résumé
La recherche d’un aquifère en mer est un sujet émergent qui devient de plus en plus important tant la croissance de la population et le changement climatique font pression sur les ressources en eau côtières. L’une des réserves les plus importantes au monde d’eaux souterraines douces en mer (ESDM) a été identifiée récemment au large de l’Ile du Sud, en Nouvelle Zélande. Ceci a mis en évidence le potentiel en ESDM ailleurs en Nouvelle Zélande. La présente étude vise à: (1) sélectionner les aquifères côtiers néo-zélandais les plus susceptibles de contenir des ESDM et (2) documenter les preuves d’ESDM en Nouvelle Zélande. Un système d’évaluation de la probabilité d’ESDM a été développé dans le cadre de l’étude Une application du système de notation a utilisé les réponses aux enquêtes émanant des conseils régionaux responsables de la gestion des eaux souterraines, corrélativement aux documents techniques d’échelle nationale et régionale. Le système de notation s’est avéré être une approche de première analyse simple et transparente pour mettre en évidence les zones où les ESDM existent plus ou moins probablement à l’échelle nationale. Les résultats sont présentés sur une carte montrant la probabilité d’ESDM le long de la ligne de côte néo-zélandaise. Les régions dans lesquelles les aquifères présentent une forte probabilité d’ESDM comprennent Greater Welligton, Canterbury, Tasman, Hawkes Bay et Marlborough. Les aquifères de ces régions sont associés à des systèmes de dépôts fluviatiles majeurs, incluant des graviers d’épandage glaciaire. En dépit de la forte dépendance de ces régions vis-à-vis des eaux souterraines et de l’exploitation considérable de celles-ci près de la côte, il n’y a pas d’incidences majeures signalées d’intrusion d’eau de mer, ce qui suggère que les ESDM peuvent permettre d’augmenter l’exploitation à terre.
Resumen
La investigación de los acuíferos submarinos es un campo emergente que adquiere cada vez más importancia a medida que el crecimiento demográfico y el cambio climático ejercen presión sobre los recursos hídricos costeros. Recientemente se ha identificado una de las mayores reservas de agua subterránea dulce de la costa (OFG) frente a la Isla Sur de Nueva Zelanda. Esto ha puesto de manifiesto el potencial de OFG en otros lugares de Nueva Zelanda. Este estudio tiene como objetivo: (1) detectar los acuíferos costeros de Nueva Zelanda con mayor probabilidad de contener OFG y (2) documentar las pruebas de OFG en Nueva Zelanda. Como parte del estudio se desarrolló un esquema de clasificación de la probabilidad de OFG. En la aplicación del esquema de clasificación se utilizaron las respuestas a una encuesta de los consejos regionales responsables de la gestión de las aguas subterráneas, en combinación con documentos técnicos a escala nacional y regional. El esquema de clasificación resultó ser un enfoque sencillo y transparente en una primera aproximación para destacar las zonas en las que la OFG es más o menos probable a escala nacional. Los resultados se presentan en un mapa que muestra la probabilidad de OFG alrededor de la costa de Nueva Zelanda. Las regiones con acuíferos donde la probabilidad de OFG es alta incluyen el Gran Wellington, Canterbury, Tasman, Hawkes Bay y Marlborough. Los acuíferos de estas regiones están asociados a los principales sistemas de deposición fluvial, incluidas las gravas de afloramiento glaciar. A pesar de la elevada dependencia de las aguas subterráneas en estas regiones y de la amplia extracción que se realiza cerca de la costa, no se han registrado incidencias importantes de intrusión de agua de mar, lo que sugiere que las aguas subterráneas en el mar pueden estar reforzando la extracción en tierra.
摘要
近海含水层研究是一个新兴领域，随着人口增长和气候变化对沿海水资源造成压力，其变得越来越重要。最近在新西兰南岛附近发现了全球最大的近海新鲜地下水 (OFG) 储量之一, 这凸显了 OFG 在新西兰其他地方的潜力。本研究研究目的在于:(1)筛选最可能含有OFG的新西兰沿海含水层，以及(2) 新西兰OFG的文件证据。作为研究的一部分，本次开发了一个 OFG 可能性分级方案。分级方案的应用使用了负责地下水管理的区域委员会的调查答复，并结合了国家和区域尺度的技术文件。本次分级方案是一种简单、透明的一次通过方法, 用于评估在全国范围内或多或少可能出现 OFG 的区域。 结果显示在一张地图中，表明新西兰海岸线周围发生 OFG 的可能性。OFG 可能性高的含水层区域包括大惠灵顿、坎特伯雷、塔斯曼、霍克斯湾和马尔堡。这些地区的含水层与主要的河流沉积系统有关，包括冰川外流砾石。尽管这些地区高度依赖地下水并且在海岸附近广泛开采地下水，但没有重大海水入侵事件的报道，这表明近海地下水可能会增加陆上开采。
Resumo
A pesquisa de aquíferos no mar é um campo emergente que está se tornando cada vez mais importante à medida que o crescimento populacional e as mudanças climáticas pressionam os recursos hídricos costeiros. Uma das maiores reservas, globalmente, de águas subterrâneas doces no mar (ASDM) foi recentemente identificada na Ilha Sul da Nova Zelândia. Isso destacou o potencial para ASDM em outros lugares da Nova Zelândia. Este estudo visa: (1) rastrear aquíferos costeiros da Nova Zelândia com maior probabilidade de conter ASDM e, (2) documentar evidências de ASDM na Nova Zelândia. Um esquema de classificação de probabilidade de ASDM foi desenvolvido como parte do estudo. Uma aplicação do esquema de classificação usou respostas de pesquisas de conselhos regionais responsáveis pela gestão de águas subterrâneas, em combinação com documentos técnicos em escala nacional e regional. O esquema de classificação foi considerado uma abordagem simples e transparente de primeira passagem para destacar áreas onde ASDM é mais ou menos provável na escala nacional. Os resultados são apresentados em um mapa mostrando a probabilidade de ASDM ao redor da costa da Nova Zelândia. Regiões com aquíferos onde a probabilidade de ASDM é alta incluem Greater Wellington, Canterbury, Tasman, Hawkes Bay e Marlborough. Os aquíferos nestas regiões estão associados aos principais sistemas deposicionais fluviais, incluindo cascalhos glaciais. Apesar da alta dependência das águas subterrâneas nessas regiões e da extensa extração de águas subterrâneas perto da costa, não há grandes incidências relatadas de intrusão de água do mar, sugerindo que as águas subterrâneas offshore podem estar aumentando a extração em terra.
Although offshore freshened groundwater (OFG) systems have been documented in numerous continental margins worldwide, their geometry, controls and emplacement dynamics remain poorly constrained. Here ...we integrate controlled-source electromagnetic, seismic reflection and borehole data with hydrological modelling to quantitatively characterise a previously unknown OFG system near Canterbury, New Zealand. The OFG system consists of one main, and two smaller, low salinity groundwater bodies. The main body extends up to 60 km from the coast and a seawater depth of 110 m. We attribute along-shelf variability in salinity to permeability heterogeneity due to permeable conduits and normal faults, and to recharge from rivers during sea level lowstands. A meteoric origin of the OFG and active groundwater migration from onshore are inferred. However, modelling results suggest that the majority of the OFG was emplaced via topographically-driven flow during sea level lowstands in the last 300 ka. Global volumetric estimates of OFG will be significantly revised if active margins, with steep coastal topographies like the Canterbury margin, are considered.
The imbricated frontal wedge of the central Hikurangi subduction margin is characteristic of wide (ca. 150 km), poorly drained and over pressured, low taper (∼
4°) thrust systems associated with a ...relatively smooth subducting plate, a thick trench sedimentary sequence (∼
3–4 km), weak basal décollement, and moderate convergence rate (∼
40 mm/yr). New seismic reflection and multibeam bathymetric data are used to interpret the regional tectonic structures, and to establish the geological framework for gas hydrates and fluid seeps. We discuss the stratigraphy of the subducting and accreting sequences, characterize stratigraphically the location of the interplate décollement, and describe the deformation of the upper plate thrust wedge together with its cover sequence of Miocene to Recent shelf and slope basin sediments. We identify approximately the contact between an inner foundation of deforming Late Cretaceous and Paleogene rocks, in which widespread out-of-sequence thrusting occurs, and a 65–70 km-wide outer wedge of late Cenozoic accreted turbidites. Although part of a seamount ridge is presently subducting beneath the deformation front at the widest part of the margin, the morphology of the accretionary wedge indicates that frontal accretion there has been largely uninhibited for at least 1–2 Myr. This differs from the offshore Hawkes Bay sector of the margin to the north where a substantial seamount with up to 3 km of relief has been subducted beneath the lower margin, resulting in uplift and complex deformation of the lower slope, and a narrow (10–20 km) active frontal wedge.
Five areas with multiple fluid seep sites, referred to informally as Wairarapa, Uruti Ridge, Omakere Ridge, Rock Garden, and Builders Pencil, typically lie in 700–1200 m water depth on the crests of thrust-faulted, anticlinal ridges along the mid-slope. Uruti Ridge sites also lie in close proximity to the eastern end of a major strike-slip fault. Rock Garden sites lie directly above a subducting seamount. Structural permeability is inferred to be important at all levels of the thrust system. There is a clear relationship between the seeps and major seaward-vergent thrust faults, near the outer edge of the deforming Cretaceous and Paleogene inner foundation rocks. This indicates that thrust faults are primary fluid conduits and that poor permeability of the Cretaceous and Paleogene inner foundation focuses fluid flow to its outer edge. The sources of fluids expelling at active seep sites along the middle slope may include the inner parts of the thrust wedge and subducting sediments below the décollement. Within anticlinal ridges beneath the active seep sites there is a conspicuous break in the bottom simulating reflector (BSR), and commonly a seismically-resolvable shallow fault network through which fluids and gas percolate to the seafloor. No active fluid venting has yet been recognized over the frontal accretionary wedge, but the presence of a widespread BSR, an extensive protothrust zone (>
200 km by 20 km) in the Hikurangi Trough, and two unconfirmed sites of possible previous fluid expulsion, suggest that the frontal wedge could be actively dewatering. There are presently no constraints on the relative fluid flux between the frontal wedge and the active mid-slope fluid seeps.
Offshore aquifer research is an emerging field that is becoming increasingly important as population growth and climate change put pressure on coastal water resources. One of the largest reserves, ...globally, of offshore freshened groundwater (OFG) was recently identified off the South Island of New Zealand. This has highlighted the potential for OFG elsewhere in New Zealand. This study aims to: (1) screen for New Zealand coastal aquifers most likely to contain OFG and, (2) document evidence for OFG in New Zealand. An OFG-likelihood rating scheme was developed as part of the study. An application of the rating scheme used survey responses from regional councils responsible for groundwater management, in combination with national and regional-scale technical documents. The rating scheme was found to be a simple and transparent first-pass approach for highlighting areas where OFG is more or less likely at the national scale. Results are presented in a map showing the likelihood of OFG around the New Zealand coastline. Regions with aquifers where OFG likelihood is high include Greater Wellington, Canterbury, Tasman, Hawkes Bay and Marlborough. Aquifers in these regions are associated with major fluvial depositional systems, including glacial outwash gravels. Despite high dependence on groundwater in these regions and extensive groundwater extraction near the coast, there are no major reported incidences of seawater intrusion, suggesting offshore groundwater may be augmenting onshore extraction.
The Tuaheni Landslide Complex (TLC) on the Hikurangi Margin is subject to reactivation, yet the timing of slide emplacement remains unknown. Here we modeled pore‐water data collected from the TLC ...during the International Ocean Discovery Program (IODP) Expedition 372 in 2017/2018 using a transient‐state reaction‐transport modeling approach. Our simulations reveal that the TLC was formed by two separate depositional events and that the most recent one occurred 12.5 ± 2.5 ka as a coherent ~40 m sediment block that carried its initial pore‐water signature. In addition, we show that the rapid burial of pore‐water SO42− in the pre‐slide sediments consumed an additional ~5.6 × 109 mole of CH4 over the entire Tuaheni South following the most recent depositional event. These findings provide significant insights into the nature and timing of the TLC and highlight the role of slope failure on subsurface methane cycling on millennial timescales.
Plain Language Summary
One remarkable feature of the Tuaheni Landslide Complex (TLC) is the geomorphic signature of ongoing post‐slide deformation of the slide debris, analogous to slow‐moving terrestrial earthflows and rock glaciers. To reconstruct the temporal history of the TLC and evaluate its potential effect on subsurface methane cycling, we modeled the pore‐water data collected from the TLC using a nonsteady‐state model. We demonstrate that this landslide complex was formed by two separate depositional events and that the most recent one occurred 12.5 ± 2.5 ka as a coherent ~40 m sediment block. In addition, modeled response of the methane cycling rates to the slide mass emplacement shows that the rapid burial of pore‐water SO42− in the pre‐slide sediments into the newly developed methanic zone consumed an additional ~70 mol m−2 of CH4 over the 12.5 kyr that followed the most recent depositional event. This is the first study that provides the age constraint of the TLC and highlights the role of slope failure on subsurface methane cycling on millennial timescales.
Key Points
The Tuaheni Landslide Complex was formed by two separate depositional events
The most recent depositional event occurred 12.5 ± 2.5 ka as a coherent ~40 m sediment block
Burial of sulfate in the pre‐slide sediments consumed an additional ~5.6 × 109 mol of methane over the entire Tuaheni South
Morphological and seismic data from a submarine landslide complex east of New Zealand indicate flow‐like deformation within gas hydrate‐bearing sediment. This “creeping” deformation occurs ...immediately downslope of where the base of gas hydrate stability reaches the seafloor, suggesting involvement of gas hydrates. We present evidence that, contrary to conventional views, gas hydrates can directly destabilize the seafloor. Three mechanisms could explain how the shallow gas hydrate system could control these landslides. (1) Gas hydrate dissociation could result in excess pore pressure within the upper reaches of the landslide. (2) Overpressure below low‐permeability gas hydrate‐bearing sediments could cause hydrofracturing in the gas hydrate zone valving excess pore pressure into the landslide body. (3) Gas hydrate‐bearing sediment could exhibit time‐dependent plastic deformation enabling glacial‐style deformation. We favor the final hypothesis that the landslides are actually creeping seafloor glaciers. The viability of rheologically controlled deformation of a hydrate sediment mix is supported by recent laboratory observations of time‐dependent deformation behavior of gas hydrate‐bearing sands. The controlling hydrate is likely to be strongly dependent on formation controls and intersediment hydrate morphology. Our results constitute a paradigm shift for evaluating the effect of gas hydrates on seafloor strength which, given the widespread occurrence of gas hydrates in the submarine environment, may require a reevaluation of slope stability following future climate‐forced variation in bottom‐water temperature.
Key Points
Low‐velocity active landslides are proposed to occur on the seafloor
Gas hydrates provide a perturbation mechanism for ongoing landslide mobility
We propose an active, mixed hydrate‐sediment seafloor glacier
Upper‐plate normal faults are a widespread structural element in erosive plate margins. Increasing coverage of marine geophysical data has proven that similar features also exist in accretionary ...margins where horizontal compression usually results in folding and thrust faulting. There is a general lack of understanding of the role and importance of normal faulting for the structural and tectonic evolution of accretionary margins. Here we use high‐resolution 2‐D and 3‐D seismic reflection data and derived seismic attributes to map and analyze upper‐plate normal faulting in the marine forearc of the accretionary Hikurangi margin, New Zealand. We document extension of the marine forearc over a wide area along the upper continental slope. The seismically imaged normal faults show low vertical displacements, high dip angles, a preference for landward dip, and often en echelon patterns. We evaluate different processes, which may cause the observed extension, including (1) stress change during the earthquake cycle, (2) regional or local uplift and decoupling of shallow strata from compression at depth, and (3) rotation of crustal blocks and resulting differential stresses at the block boundaries. The results suggest that normal faults play an important role in the structural and tectonic evolution of accretionary margins, including the northern Hikurangi forearc.
Key Points
High‐resolution 3‐D seismic data indicate widespread normal faulting on the upper slope of the northeastern Hikurangi margin
The normal faults show two major strike directions, primarily landward dip, low vertical displacements, and steep dip angles
Extension may be controlled by regional uplift or/and extensional strain due to rotation of tectonic blocks around nearby poles
Although the global flux of sediment and carbon from land to the coastal ocean is well known, the volume of material that reaches the deep ocean-the ultimate sink-and the mechanisms by which it is ...transferred are poorly documented. Using a globally unique data set of repeat seafloor measurements and samples, we show that the moment magnitude (
) 7.8 November 2016 Kaikōura earthquake (New Zealand) triggered widespread landslides in a submarine canyon, causing a powerful "canyon flushing" event and turbidity current that traveled >680 km along one of the world's longest deep-sea channels. These observations provide the first quantification of seafloor landscape change and large-scale sediment transport associated with an earthquake-triggered full canyon flushing event. The calculated interevent time of ~140 years indicates a canyon incision rate of 40 mm year
, substantially higher than that of most terrestrial rivers, while synchronously transferring large volumes of sediment 850 metric megatons (Mt) and organic carbon (7 Mt) to the deep ocean. These observations demonstrate that earthquake-triggered canyon flushing is a primary driver of submarine canyon development and material transfer from active continental margins to the deep ocean.
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