CHRIMP 2013

CHRIMP 2013- SO226 legs 1 and 2, January 7-March 1 2013

Scientific objectives

  • Investigate the mechanism(s) leading to the formation of the seafloor depressions (pockmarks) on the southern Chatham Rise, including their link to gas release and gas hydrate dissociation
  • Constrain timing and paleoceanographic conditions during formation of the seafloor depressions

Commentary
Cruise SO226-1, was the first of two back-to-back surveys (the CHRIMP project) undertaken by the German vessel R/V Sonne to explore the giant pockmarks discovered a few years ago on the Chatham Rise (Davy et al. 2010). The surveys form a major component of a Marsden project, led by Dr. Ingo Pecher (Auckland University and GNS Science), to investigate gas release from dissociating hydrates during glacial-interglacial cycles which may have led to the formation of the seafloor depressions on the Chatham Rise.

17 scientists joined the survey in the port of Auckland on 07th Jan 2013. The expedition, led by GEOMAR, Kiel, Germany was undertaken in co-operation with scientists from GNS Science, the University of Otago, NIWA, the University of Auckland and the University of Southampton. Two GNS scientists – Dr Bryan Davy and Dr Karsten Kroeger - joined the scientific team collecting and interpreting the seismic and multi-beam bathymetry data.

The Sonne is an old friend to many in the Marine Geosciences Department at GNS with frequent surveys in the New Zealand region bringing expertise and equipment New Zealand does not have. In this instance it was 19 ocean bottom seismometers (OBS’s), primarily for getting sediment velocity information and GEOMAR’s P-cable – an array of 11 short streamers towed in parallel to build up a high-resolution 3D seismic cube.

Survey areas 1-3 covered by SO226-1 – the two north-easternmost giant pockmarks in the inset image from area 1 were discovered on this leg. (Background bathymetry courtesy NIWA)

Survey areas 1-3 covered by SO226-1 – the two north-easternmost giant pockmarks in the inset image from area 1 were discovered on this leg. (Background bathymetry courtesy NIWA)

Dr Karsten Kroeger (GNS Science) approaches a line of OBS’s being prepared for deployment to the seafloor.

Dr Karsten Kroeger (GNS Science) approaches a line of OBS’s being prepared for deployment to the seafloor.

The survey discovered several large pockmarks including a giant (c. 11x6 km by 100 m deep) pockmark in water depths of about 1000 m. Such pockmarks are immense compared with pockmarks observed elsewhere in the world, being large enough to enclose lower Manhattan or the Wellington City urban area.

2D and 3D seismic reflection and Parasound sub-bottom profiling performed during SO226-1 has revealed subsurface structures characteristic of large scale gas-rich fluid migration about 500 km east of Christchurch.

A second voyage, SO226-2, with American scientists joining the team, to explore the pockmarks – sea floor craters – has however found no indication of the methane flux to the ocean as indicated by the first voyage. This result suggests that gas-charged fluid escape leading to the pockmark formation may have occurred in the past, but seafloor gas seeps are not currently active.

While the gas chemistry results suggest this system is no longer active, smaller, approximately 200 m wide and 10 m deep pockmarks on the seafloor suggest sporadic gas escape may be occurring, possibly only during glacial intervals every 20,000 or 100,000 years. On-going seismic interpretation and pore-water chemistry studies, to be undertaken by the international team of investigators, are expected to clarify the history of the enigmatic giant pockmarks and underlying sedimentary structures.

Figure 3. A tense moment on the rear-deck as the P-cable is hauled-in during rough weather following a break in one of the main tow-leads

Figure 3. A tense moment on the rear-deck as the P-cable is hauled-in during rough weather following a break in one of the main tow-leads

Collaborators

  • NIWA
  • IfM Geomar
  • GNS Science
  • University of Auckland
  • University of Otago
  • U.S. Naval Research Laboratory (NRL)
  • University of Southampton

Experiments

  • Multichannel seismic
  • P-cable high resolution 3D seismic
  • Multibeam bathymetry
  • Parasound sub-bottom profiling
  • Piston coring for pore-water chemistry, isotopes, and microbiology
  • Piston coring for paeloceanographic analyses
  • Water column acoustic (echosounder)