22 April 2001 - Back in the North Once More
RRS James Clark Ross Diary
Noon Position : 15° 45' North, 47° 00' West, 750 miles from Barbados.
Distance travelled since Grimsby: 31,870 nautical miles
Air temperature @ noon: 27.0 degrees Celsius
Sea temperature @ noon: 25.0 degrees Celsius
You find us this week back in the Northern Hemisphere, having crossed the line in the early hours of Monday morning, The week has been a busy one for all onboard with a certain amount of adjustments to peoples routines to accommodate the changes in personnel in Recife.
So apart from a slight diversion on Monday to welcome all those new people to the "joys" of crossing the line for the first time it has been science all the way.
Science All the way! (Unofficial sciency bit in the middle)
Our main survey area is on the Mid-Atlantic ridge 15 degrees North of the equator and as the route from Recife allowed us to follow the ridge with our old friend the swath bathymetry adding to the mapping of the areas passed over. The first "over the side" excitement came on Thursday evening and the hunt for "Popping" rocks en route. No we have not lost it completely here, but we performed our first rock dredge, on part of the seabed, at a site where a previous cruise had managed to recover rocks with gas bubbles trapped in them. So when they were brought to the surface and the pressure exerted on the rocks reduced, the trapped gas expands and goes "pop" and maybe even cracks the rocks. Unfortunately despite preparations to try and capture some of this gas, it wasn't to be. However some rock sample were obtained and it was a useful shake down for the equipment and personnel. Below is a picture of the dredge, more pictures of it in action and its contents in future weeks.
We hope when things calm down here in to a better routine our Principal Scientist will be able to fill you in with more details of the cruise, but unfortunately until then you have our lay-man's view.
So as we start our next week of Science we are towing TOBI (Towed Ocean Bottom Instrument) from Southampton Oceanography Centre (SOC). For our regular readers you will remember that this was used from the ship in the Arctic last summer. We have just deployed TOBI so hope to have some results of the survey to share with you in our next update.Southampton Oceanography Centre
Science All the way! (Official sciency bit in the middle)
By Chris MacLeod (Principal Scientist - JR63)
Aims of the cruise
The discovery of peridotites - rocks derived from the Earth's mantle - in some dredge hauls from mid-ocean ridge axes has long intrigued marine geologists. The observation cannot be reconciled with models derived from seismic refraction experiments, which predict that a uniform 6-7km thickness of ocean crust is produced at all but the very slowest spreading ridges. Exposure of peridotite on the seafloor could mean that the full thickness of ocean crust was never formed: one proposition is that the lower crust may not be a continuous layer of gabbro (coarse-grained equivalent of a basalt lava, cooled slowly within a magma chamber beneath the ridge axis), but a collage of small bodies of gabbro intruded into partially serpentinised (altered) mantle peridotite. An alternative, potentially complementary, mechanism for exhuming the deep ocean crust is that the crust has been removed by tectonic means (faulting). However, faults documented at spreading centres such as the Mid-Atlantic Ridge are not large enough to explain removal of the entire thickness of the crust.
In 1996 a British sidescan sonar survey of a portion of the Mid-Atlantic Ridge revealed that a number of flat-topped highs adjacent to the axis had spectacular parallel striations on their upper surfaces. The favoured interpretation of these structures is that they are the surface expressions of huge near-horizontal extensional faults (detachments) that cut through the entire crust, helping to accommodate separation of the plates at the ridge axis when magma supply from the mantle is low. The limited sampling carried out to date suggest that they are probably responsible for exhuming deep levels of the crust and the shallow mantle on the seafloor.
The main aims of RRS James Clark Ross cruise JR63 are to work out how these flat-topped corrugated massifs (often termed "megamullions"or "oceanic core complexes") form and, by mapping out and sampling the exposures of gabbro and peridotite, to gain a better understanding of how the lower ocean crust is put together. We hope to be able to do this by making a detailed geological study of several corrugated surfaces that are exposed near the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge.
We are using the TOBI deep-towed sidescan sonar together with the ship's swath bathymetry to determine where the corrugated surfaces occur on the seafloor and to map out the surrounding faults, after which we shall try to sample the exposed surfaces using the BRIDGE wireline corer. This is a rotary drill mounted in a tripod frame which is lowered to the seafloor on a cable and drills 1m-long orientated rock cores. We hope to "pogo" around with this drill on the corrugated outcrops, getting large numbers of cores of peridotite and gabbro, building up a geological map and a suite of samples that we will study back in the laboratory.
Crossing The Line
A most unusual occurrence - we had a total of 16 persons who had not crossed the equator before, or who we decided did not have the proof. Here follows a brief gallery of what the lucky people went through in the court of King Neptune.
Above left we have King Neptune and His Queen alias Kenny Weston (Steward) and James Kennedy (AB) in fully regalia. For those who know James he did enter into the part well, some suggesting too well, but got his just desserts at the end as it was his first time across the line as well, with so many first-timers some exceptions had to be made to etiquette this time. On the right we have our magistrate (David Gooberman, Second Officer) examining the appalling charges that the accused were charged with, these having to be passed by the Captain before sentence could be carried out and to see fair play all round.
Below we can see on the left a special inclusion for our Southampton readers with Lee Fowler preparing to take his medicine which he was expecting to taste horrid, but would the Doctor make any medicine taken unpleasant? We'll leave to our own experiences of Doctors' medicine to decide. Even the Principal Scientist was not excused this pleasure though we do believe he might have the last laugh with the cruise still to start properly, I wonder what jobs he can dream up?
Just to prove that we treat everyone equally and it's not just the Science Party that are first timers here we have Scott our illustrious Third Officer in the "hot" seat, though from the smile on his face he seems to be enjoying this all far too much.
Parting Shot (sorry)