91B0FBB4-04A9-D5D7-16F0F3976AA697ED
C9A22247-E776-B892-2D807E7555171534
12/12 1600 hrs.
65 45 S
60 23 W
4 degrees C
Light winds, slight overcast

Around 2 a.m. this morning, we started our subsurface sampling, following some basic mapping of the sea floor topography to locate sampling sites.

The sea beam system on the ship consists of an array of  60 transmitters and receivers mounted in the form of a cross on the bottom of the ship. Transmitters send out a sound pulse and receiving units receive the reflected return from the bottom. Travel time for the pulse from transmission to return is fed into a computer. The computer factors in data on salinity and temperature as well to calculate the speed of sound for the given water conditions and then calculates a depth to the sea floor. In deep water the pulse rate is slower than in shallow water because the units have to wait longer for the return signal to reach them. As the ship travels along a computer generated topographic map of the sea floor slowly takes shape on a monitor. Depending on sea and ice conditions this data can have a lot of extraneous noise in it and so each file, one is saved every hour, must be inspected and edited. This is the famous ping editing which any alumni of these research cruises can tell you all about and which the current students are learning to do very quickly. Using the map on the monitor sample locations are chosen and physical sampling of the sea floor begins.

Water depths in this area are around 650 meters. Instruments are lowered at speeds around 30 meters per minute, so a round trip takes roughly 45 minutes. Combine this with prep time and each instrument package involves an hour or so if everything goes smoothly. Currently we are using 3 or four instruments at each station so 4-5 hours on station is not unusual. When things are not smooth this can easily approach 6-8 hours. With todays nice weather and being parked right along the Larsen Ice Shelf front being outside on the back deck for these operations is good duty.

Cheers,

Dave


Emily Backman and Kate McMullen examine a successful Kasten core operation. The mud covered barrel contains sea floor sediment from 600+ meters below the surface and may represent 12,000 years of sediment accumulation. This particular core filled about half of the 3 meter Kasten core length. (Click image for larger version.)

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