Next stop: Napier

16 June:  It is 14:00 and we have begun our steam back to port. The seismic gear is packed away, the heat flow probe is on the deck, and the winch wire rinsed clean of seawater.  All in all, this has been a very successful cruise. Large amounts of data were collected and will all prove to be invaluable for the continued study of the Hikurangi margin. We will be pulling in to port at 08:00 tomorrow – weary from travel, rough seas, and hard work – but feeling accomplished.

Anne asked me yesterday if I would come back to sea, given the opportunity. Not a moment passed before I responded with a resounding ‘Yes!’, and we began to converse on the challenges and rewards of field science on the ocean. I will look forward to that opportunity, should it come my way, and will not forget my experience here on the Roger Revelle.

A few pictures from final heat probe recovery, along with one of the more stunning sunsets:

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Waiting for the probe to surface. The guys are all around 6 feet tall, and the ship has a 12-foot freeboard…. We’re looking at a rather impressive swell here. Sporty!

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I was glad I was waiting in the hangar during this recovery.

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It was quite a dramatic recovery. No crewmen, scientists, or heat flow probes were injured.

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I will miss my nightly sunset viewing. This one offers a great view of the North Island, too.

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Marine Mammal Observer responsibilities

Patti Haase, Bridget Watts, and Julia O’Hern

“Shut down, shut down, shut down!” is the most important call
we make over the radio to the lab. When animals like the New
Zealand fur seal pop up off the side of the ship, our job is
to visually monitor and implement mitigation procedures to
minimize the possible effects on marine mammals and protected
species. The scientists in the lab reply “We are shut down,”
and we enter the time and animal sighting information in our
database.

The three of us (Patti, Bridget, and Julia, aka PB&J) maintain
a visual watch around the ship for any marine mammals or
protected species that come within 400 to 600 meters of the
seismic source. This “buffer zone” varies with water depth due
to the characteristics of sound propagation. If a fur seal,
whale, or group of dolphins is about to approach within 100 meters
of the seismic gear (into the “exclusion zone”), we make the
shut-down call. We are required to do this work by the National
Marine Fisheries Service, which is in charge of implementing the
provisions of the Marine Mammal Protection Act of 1972. We are
a US ship funded by the National Science Foundation and are
therefore bound by that law even though we are working offshore
of New Zealand.

We use 7 x 50 binoculars and 25 x 150 fixed-mount binoculars
(“Big Eyes”) to continually scan around the ship. So far on this
cruise we’ve seen an impressive number of New Zealand fur seals,
several pods of pilot whales, several distant baleen whales, and
a brief look at a beaked whale, also too wily to identify (they
are known to avoid ships and are notoriously difficult to
identify at sea). Interestingly, the pilot whales were in the
northern study area while almost all of the fur seals have been
seen in the southern study area. We also got a notable look at a
pod of killer whales, although they too remained at a good
distance from our ship.

When the science team is using the heat probe (the seismic
source and the heat probe aren’t deployed together), we are
also watching for marine mammals. We collect these data to
compare sighting rates between seismic and non-seismic operations,
although our sample sizes are likely too small to make statistical
conclusions about the comparison. It also keeps us busy, and in
the fresh air, which we like!

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New Zealand Fur Seal (photo by Bridget Watts)

Bridget-pilot whales

Pilot whales (photo by Bridget Watts)

Science in real time – pictures of seismic and heat flow from the deck.

 

11 June:

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Jay and Andrew guiding in the seismic line with a spectacular backdrop.

 

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Jay and Drew with the line.

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Removing one of the birds.

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The air guns being brought up on deck.

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Co-chief Anne hosing down the seismic line.

12 June:

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Current conditions – on our way out to recover the heat flow probe.

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Rachel in her element.

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Drew and Lee in less than ideal conditions, recovering the probe.

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Lee and Drew, fishing for the probe.

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Recovering the probe takes up to six people and has a lot of moving parts – literally and figuratively.

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Safe on the deck.

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Very unstable weather, as shown by the clouds.

 

10 June: Today marks the sixth day of our seismic reflection survey.

Dylan Baker & Andrew Gorman, University of Otago

In a marine seismic reflection survey, an airgun is used to generate seismic waves that propagate outwards into the ocean and subsurface. Seismic waves travel until they encounter a medium with different seismic impedance (seismic impedance is the product of density and seismic velocity). At these boundaries, some wave energy is transmitted and some wave energy is reflected. Waves which have been reflected towards the surface can be recorded by the long streamer that is being towed behind the ship. The streamer contains hydrophones that convert pressure changes caused by reflected seismic waves into an electrical pulse. By determining where each wave has been reflected from, a picture of the subsurface can be developed.

The last two and a half days of our seismic reflection survey have targeted various gas hydrate systems along the southern coast of the North Island. Gas hydrate is an ice-like substance which is made up of a central gas molecule trapped in a cage of hydrogen bonded water molecules. Gas hydrate is stable at low temperatures and high pressures within the gas hydrate stability zone (GHSZ). Gas hydrate can be inferred in seismic data primarily from a bottom simulating reflection (BSR). BSRs are an anomaly mostly caused by gas that accumulates beneath the GHSZ causing a contrast in seismic impedance. It is important to study gas hydrates as they play a significant role in hydrocarbon storage, climate change, and geohazard generation. Our work in this region is focusing on constraining the extent and dynamics of three previously identified gas hydrate anomalies using the high-resolution GI-gun seismic system on board the Roger Revelle.

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The science team deploying the seismic line.

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Andrew and Dylan (advisor and Master’s student, respectively) pause for a photo during deployment.

Cook Strait

7 June:  All is well on the Revelle and today marks the start of the second high resolution seismic survey of the cruise. We have traveled to the southern reach of the North Island and are now collecting data from southern end of the Hikurangi margin. In contrast to the northern area, more sediment is available on the incoming plate to form the accretionary wedge. The main purpose of the seismic survey in this area is to collect a grid of lines across interesting features seen in single profiles in order to understand their structure in three dimensions. This region is characterized by the presence of spectacular reflection patterns indicative of the presence of methane hydrates. Stay tuned for reflection images of both field areas!

Guess what day it is!!!

2 June: Greetings from the Revelle! Today marks the half way point in the STINGS cruise and the science is flowing smoothly. We are currently measuring heat flow again after a successful seismic survey. The weather set us back 3+ days, but now the swells are low (<3 m) and the spirits are high again. There is more weather en route after a few days of calm seas, but it is not forecast to last. It does make deploying and recovering the heat probe a bit tricky, but we’ve got the process dialed in, thanks to our incredible ship crew and techs. We have a couple more heat flow transects in the area before heading to the southern coast of the North Island, where we will finish up more heat flow along with another seismic survey.

In my down time I have been reading about the geologic history of this area. It has become very clear to me why the work we are doing here is so important.  The following is a very brief synopsis of what I’ve learned through the literature:

The Hikurangi Plateau is one piece of what is thought to be a massive large igneous province (3 – 5 million km3 of lava) consisting of the Hikurangi, Manihiki, and Ontong Java plateaus. There is much uncertainty about when the plateau formed and how (one hypothesis is a combination of a mantle plume and rifting). The three masses of flood basalt rifted apart ~120 Ma (Ontong Java first, followed by the separation of the other two), sending the Hikurangi southward toward the Chatham Rise margin of the Gondwana supercontinent. The collision occurred ~105 Ma, but the young, buoyant Hikurangi Plateau did not effectively subduct under the Chatham Rise, causing a slowing and eventual cessation of subduction. A fresh round of volcanism began ~99 Ma throughout the Hikurangi Plateau and Chatham Rise, producing a number of seamount ridges and guyots, lasting approximately 15 Ma. Spreading started again ~85 Ma, marking the beginning of the opening of the Southern Ocean. This reconstruction has been inferred by modeling, deep crustal reflection, dredging, and one drill hole. The real mystery at hand currently is what happens during the subduction of a large igneous province – this area is one of the only places on earth (if not the only) where this is occurring (now that the Hikurangi old and cold, it is subducting beneath the North Island of New Zealand). Our work here on STINGS is imperative to understanding the tectonic processes here, and our seismic and heat flow data will assist in the proposed drilling that will take place next year. We are all very excited to be a part of this cruise!

-Nicole

 

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Simplified schematic of mid-late Cretaceous plate motion of the Ontong Java-Manihiki-Hikurangi Plateau (figure from Davy et al., 2008).

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Bathymetric map of the SW Pacific showing Ontong Java, Manihiki, and Hikurangi Plateaus (figure from Hoernle et al., 2010).

 

Another day, another heat flow measurement…

Rob Harris, Oregon State University

 The weather has cleared with incredibly calm seas given our location in the Southern Ocean.  We hope this good weather will continue for the remainder of our research cruise and the next cruise where seafloor instruments that have been deployed over the last year are scheduled to be recovered. 

We have successfully completed the seismic survey of the northern area and are now in the midst of a final scramble to finish our heat flow transects in this area before moving to the southern area. A figure of our current transect is shown below and as I write this we are working our way from west to east (going downhill) and have finished about 10 measurements.  We expect this transect to take about 48 hours to complete. 

 While the probe is on the seafloor it sends pings though the water column that allows us to assess its performance.  We monitor the probe tilt to make sure it has not fallen over, and the top, middle, and bottom thermistor.  Monitoring the thermistors allows us to determine whether the probe is fully inserted into the seafloor and gives a qualitative indication of the thermal gradient.  So far it’s all good.  Data for quantitative analysis is stored in the data logger that we only get access once the probe is recovered and the data is downloaded.

This is an exciting transect because we cross several interesting targets.  These targets include the location of a proposed scientific drill hole and faults near the deformation front.  The measurements across the fault zones will tell us whether significant quantities of fluid are moving up to the seafloor along these faults. We are also crossing two basement highs that are buried in sediment just east of the deformation front.  Significant variation in heat flow across these features will tell us about the magnitude of fluid flow through these features and conversely, a lack of variation will indicate that fluids are not advecting heat.  In both of these cases we are using heat as a tracer for fluid flow.  Measurements in front of the deformation front will be used to estimate the thermal structure of this subduction zone. 

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