Elsevier

Marine Geology

Volume 272, Issues 1–4, 15 July 2010, Pages 127-140
Marine Geology

Testing proposed mechanisms for seafloor weakening at the top of gas hydrate stability on an uplifted submarine ridge (Rock Garden), New Zealand

https://doi.org/10.1016/j.margeo.2009.10.008Get rights and content

Abstract

We evaluate different hypotheses concerning the formation of a peculiar, flat-topped ridge at Rock Garden, offshore of the North Island of New Zealand. The coincidence of the ridge bathymetry with the depth at which gas hydrate stability intersects the seafloor has been previously used to propose that processes at the top of gas hydrate stability may cause seafloor erosion, giving rise to the flat ridge morphology. Two mechanisms that lead to increased fluid pressure (and sediment weakening) have previously been proposed: (1) periodic formation (association) and dissociation of gas hydrates during seafloor temperature fluctuations; and (2) dissociation of gas hydrates at the base of gas hydrate stability during ridge uplift. We use numerical models to test these hypotheses, as well as to evaluate whether the ridge morphology can develop by tectonic deformation during subduction of a seamount, without any involvement from gas hydrates. We apply a commonly-used 1D approach to model gas hydrate formation and dissociation, and develop a 2D mechanical model to evaluate tectonic deformation. Our results indicate that: (1) Tectonics (subduction of a seamount) may cause a temporary flat ridge morphology to develop, but this evolves over time and is unlikely to provide the main explanation for the ridge morphology; (2) Where high methane flux overwhelms the anaerobic oxidation of methane via sulphate reduction near the seafloor, short-period temperature fluctuations (but on timescales of years, not months as proposed originally) in the bottom water can lead to periodic association and dissociation of a small percentage of gas hydrate in the top of the sediment column. However, the effect of this on sediment strength is likely to be small, as evidenced by the negligible change in computed effective pressure; (3) The most likely mechanism to cause sediment weakening, leading to seafloor erosion, results from the interaction of gas hydrate stability with tectonic uplift of the ridge, provided bulk permeability strongly decreases with increasing hydrate content. Rather than overpressure developing from dissociation of hydrates at the base of gas hydrate stability (as previously thought), we found that the weakening is caused by focusing of gas hydrate formation at shallow sediment levels. This creates large fluid pressures and can lead to negative effective pressures near the seafloor, reducing the sediment strength.

Introduction

Formation (association) of methane gas hydrate beneath the seafloor is limited by the range in temperature and pressure at which gas hydrate is stable, and also by the available supply of methane and water (e.g., Rempel and Buffett, 1997, Sloan, 1998, Xu and Ruppel, 1999). Over time, gas hydrate zones can shrink or expand as local pressure, temperature, and methane supply changes (e.g., Sultan et al., 2004). Such variations are thought to have significant effects on physical properties of sediment. For example, overpressure generated from volume expansion during dissociation of hydrate to gas and water may lead to sediment weakening at the base of the gas hydrate stability zone and possibly trigger submarine slope failure (Mienert et al., 1998, Mienert et al., 2005, Xu and Germanovich, 2006). Conversely, several laboratory studies have shown that the presence of gas hydrate in the pore space increases sediment strength (Yun et al., 2007, Winters et al., 2007).

In this paper, we investigate a possible link between gas hydrate dissociation and sediment strength along the Hikurangi Margin of New Zealand (Fig. 1). The central part of the Hikurangi Margin of New Zealand is characterised by a series of long (< 100 km), sub-parallel accretionary ridges (Barnes and Mercier de Lépinay, 1997). Along part of this margin, in contrast to the usual ridge morphology, some ridges have a several-kilometre wide flat top. The observation of pinch-outs of bottom simulating reflectors (BSRs) at the edges of the flat-topped ridges led Pecher et al. (2005) to suggest a mechanism of gas-hydrate-related seafloor erosion. Two processes were proposed (Fig. 2): (1) During ridge uplift, an upward migrating base of gas hydrate stability (BGHS) with respect to the seafloor may lead to gas hydrate dissociation, overpressure, seafloor weakening, and ultimately, slope failure. After sliding, water depth, and hence hydrate stability, increase again and the process may repeat itself during continued uplift. Evidence from bathymetric and seismic data supports the presence of small slides on the edges of the plateau; (2) Repeated dissociation and formation of gas hydrates on the ridge crests caused by water temperature fluctuations may contribute to seafloor weakening. During warm-water periods, gas hydrates can dissociate, leading to net pore-volume expansion, whereas pore volume can contract during gas hydrate formation in cold-water periods. It was hypothesized that repeated pore-volume contraction and expansion could cause weakening of the seafloor. Weakened sediments would then slide down the steep ridge flanks and/or be eroded by water currents. It was known that water temperatures in the study area varied by at least ± 0.9 °C. Hence, the top of the gas hydrate stability in the ocean was predicted to repeatedly move up and down by ± 35 m. In other words, the seafloor around the BSR pinch-outs was predicted to repeatedly enter and leave the gas hydrate stability field, so that any gas hydrate close to the seafloor would repeatedly form and dissociate. Meso-scale (100–200-day) variations of the Wairarapa Eddy were suggested to be the most likely cause for the temperature changes.

As pointed out by Pecher et al. (2005), several parts of the proposed gas-hydrate-related weakening mechanisms need to be verified. Dissociation of gas hydrates at the BGHS requires that the ridges need to remain within the gas hydrate stability field (for the average bottom-water temperature), which seems to contradict the presence of BSR pinch-outs. For repeated dissociation of gas hydrates to cause weakening, the thermal signal needs to reach the depths of gas-hydrate-bearing layers, which may not reach the seafloor owing to oxidation at the sulphate–methane interface (Borowski et al., 1996).

In addition to Rock Garden, two ridges further to the north — the western-most and possibly, the eastern-most ridges of Ritchie Banks — display a similar flattened ridge top flanked by BSR pinch-outs. Hence, erosion at these depths must be linked to a regional mechanism. Regional uplift, sub-aerial and/or wave erosion, followed by subsidence to current depths, was considered unlikely by Pecher et al. (2005) because of the smooth flanks of another ridge that crosses the ∼ 600 m water depth. If this mechanism were responsible for ridge morphology, regional uplift should have exposed this ridge to sub-aerial erosion, and hence a change in slope dip would be expected. Likewise, current erosion above ∼ 600 m water depth (as an alternative regional erosion mechanism) would also affect this ridge, leading to a change in slope dip which is not observed. This suggests that although currents may ultimately be responsible for erosion at Rock Garden, their efficacy must be enhanced by some process that weakens near-seafloor sediments.

Rock Garden is thought to be uplifted by a subducting seamount (Henrys et al., 2006, Barnes et al., 2010). Recently, the shape of this seamount has been constrained more accurately. Depth conversions using wide-angle velocity data indicate the top of the seamount to be ∼ 5 km beneath the seafloor and stand approximately 3 km above the subducting Hikurangi Plateau (Fig. 3B). This enables us to test whether Rock Garden ridge morphology results from the subducting seamount and not from the interaction with gas hydrates in the sediment at all. We note that it would be a coincidence if this mechanism also applies to shape the western-most Ritchie Banks ridge.

In this paper, we use numerical models to evaluate the proposed mechanisms for creation of the flat-topped ridges of Rock Garden. A one-dimensional model of fluid flow and transport of heat, methane, and salt in a porous medium is used (Xu, 2004) to simulate the effects of uplift and of a small-scale temperature fluctuation at the seafloor on gas hydrate formation, dissolution, and dissociation. We constrain uplift rates using a 2D mechanical model. These models, together with a review of tectonic and methane constraints from data collected on and around Rock Garden and Ritchie Ridge, are used to evaluate whether either of the following hypotheses is a plausible explanation for the ridge morphology:

Hypothesis 1

A consequence of tectonics only (subduction of a seamount or lower plate high), so that the correspondence between a flat bathymetry and the predicted depth of BSR pinch-outs is purely coincidental;

Hypothesis 2

Caused by mechanical strength changes owing to interaction of sediment with gas hydrates, either by:

  • a.

    Repeated formation and dissociation of hydrates near the seafloor as a result of temperature changes; or

  • b.

    Dissociation of gas hydrates at the base of the gas hydrate stability zone resulting from tectonic uplift.

For Hypothesis 2, we assume that once sediment is mechanically weakened, it is then eroded by bottom currents and/or internal waves, creating flat-topped ridges by submarine erosion (e.g., Cacchione et al., 2002).

Section snippets

Data constraints

Along the Hikurangi subduction margin, North Island New Zealand, the oblique subduction of Pacific oceanic lithosphere beneath continental Australian plate lithosphere has created a series of uplifted anticlines within the offshore accretionary wedge (Fig. 1, Fig. 3; Davey et al., 1986, Barnes and Mercier de Lépinay, 1997, Henrys et al., 2003a). While the accretionary morphology along the southern part of this margin is thought to be entirely controlled by thrusting, seamount subduction is

Hypothesis 1: is ridge morphology caused by tectonics?

The ridge at Rock Garden is flatter than similar structures along the Hikurangi Margin (Barnes et al., 2010 — this issue). Subduction of seamounts at other convergent margins causes a wide variety of deformation styles, depending on the material composition of the over-riding plate (Lallemand et al., 1989, von Huene et al., 1997, Dominguez et al., 1998, Kobayashi et al., 1998, Park et al., 1999). Seamount subduction has affected the style of deformation at the trench front and within the

Hypothesis 2: is ridge morphology controlled by sediment strength changes from gas hydrate evolution?

To investigate whether mechanical weakening of sediment can be caused by changes in gas hydrate stability, we have conducted numerical experiments using a one-dimensional, three-phase finite-difference code that predicts fluid flow, transport of heat, methane, and salt in a porous medium (Fig. 5; see Xu, 2004 for details). We considered model behaviour under a range of conditions, starting from the simplest — a uniform depth, seafloor temperature, and methane flux — and progressing to a more

Effect of seafloor temperature fluctuations

The models testing Hypothesis 2a (sediment weakening due to thermal cycling at the seafloor) show how temperature perturbations can cause periodic formation and dissociation of gas hydrates when a high flux of methane can be generated by dissociation of deeper hydrate layers. Similarly, Sultan et al. (2004) investigated the effect of a 230-day-period temperature variation on gas hydrate distribution near the surface for the Congo continental slope. They found that the transient thermal signal

Conclusions

Several hypotheses for the formation of a flat-topped ridge near the intersection depth of the gas hydrate stability zone and the seafloor, offshore North Island, New Zealand have been tested using 2D mechanical and 1D gas hydrate modelling. Preliminary conclusions indicate that:

  • -

    Tectonic uplift above a seamount can sometimes cause flat ridge morphology, but this is a transient process. Such an effect may enhance the flat-shaped nature of Rock Garden but is unlikely to be the dominant cause for

Acknowledgements

Funding for this study was provided by a Royal Society of New Zealand Marsden Grant, contract number GNS0403, and Foundation for Research Science and Technology Contract C05X0703. Jens Greinert thanks the European Union for financial support and the possibility to work at GNS Science and RCMG via a Marie Curie grant (MOIF-CT-2005-007436). We thank Gareth Crutchley and two anonymous reviewers for their constructive comments on the manuscript, and Steve Chiswell for advice on ocean currents at

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