SEA Currents: Robert C. Seamans
July 30, 2014
Finding Winslow Reef
In the vicinity of Winslow Reef,
Force 2-3 from ENE
Motorsailing under main, staysails and the jib.
07:50 We’ve been sailing in a large circle overnight, waiting for the daylight to begin our approach to Winslow Reef. The reason for this wait is that Winslow is one of those rare unmapped places of our planet, and so we have no good charts to rely on in the absence of daylight. To fix this situation a big part of todays mission is to use our onboard CHIRP sonar system to produce some accurate soundings of this large series of subsea peaks that may or may not pierce the surface of the sea. With the sun sufficiently high in the sky and the CHIRP pinging away we begin our first survey line toward a seamount some 8 nautical miles from what we think is the shallowest point of the reef.
09:20 We found the top of the seamount at 948 meters deep according to the CHIRP. Perhaps you are wondering exactly how we knew where to look for this seamount, given that no reliable charts exist. I should clarify here that a map of sorts does indeed exist of the seafloor of PIPA, indeed the whole Pacific Ocean. This map is made using information from a very sensitive satellite capable of measuring variation in the sea surface height down to centimeter (about half an inch) resolution. The rest is down to Newtonian physics: large features on the seafloor, seamounts and such, will increase the gravitational pull exerted on waters above and produce a slight dip in the sea surface. You can measure this dip and calculate the size and position of the subsea feature necessary to produce it. These satellite-derived seafloor topography maps are what we use to plan our mapping mission today. So how did our 948m measurement compare?
Turns out the satellite map was showing 851 meter depth, so it seems to have been off by about 100 meters or a little more than 10% - pretty good accuracy for the deep ocean!
11:10 We’re now creeping toward one of the two peaks that according to the satellite map might reach the surface. At depth of 700 meters, we stop and deploy the CTD rosette water sampler and tow the neuston net. This is at the same depth where we’ve conducted our other island stations, so I hope this an equivalent and comparable location. Interesting to think that if this reef was an island, it would be only mile or so distant while now we only see the unbroken horizon. The instruments on the rosette pick up clear signs of the interaction of ocean currents and the seafloor - the phytoplankton peaks both at depth and at the surface.
14:00 The station came and went, and we’ve been approaching what we thought would be the peak of this coral reef capped undersea mountain. The CHIRP shows 450 meters and the bottom starts sharply falling away from us. Nowhere around is there breaking water or changes in the color of the sea, both telltales of shallow water - we have a lookout up the mast to watch for these signs. Oh well, when you explore I guess you never know what you’ll not find No matter, there is another pinnacle to the west of us, and we change our course that way.
17:15 With a setting sun behind us, we’re now west of the reported position of the shallowest part of the Winslow Reef. In case you’re curious about the name of this reef, it dates back to the first report in 1851 by one Captain Winslow of the whaling ship Phoenix. The current British Admiralty Pilot relates that at the time the reef was reported to be extending 1 mile in a NW/SE direction, 71/2 cables wide, with two pointed rocks awash. The next report dates to 1944 and mentions a minimum depth of 11 meters but no rocks awash. So we creep cautiously forward with the CHIRP pinging and I’m about to head up the foremast to act as an aloft lookout.
18:20 Twelve hundred meters Eleven hundred meters Thousand meters Matt in the lab calls out CHIRP depths over the radio. Up aloft I see large flocks of birds feeding on schools of jumping fish that are trying to escape their aquatic predators, the pursuing tuna leaping high out of the water in chase of their prey and turning the sea white with splashes. A young masked booby circles the ship and I can hear the sound of its wings beating in the gentle evening breeze as it accelerates past my post in the foretopmast. Gradually a line in the water, ripples of waves and current emerge to show the outline of the reef ahead. We send our small boat out to verify what the CHIRP has indicated and lower our anchor in 49 of water onto a bed of coral sand and rubble. We are anchored on Winslow Reef! Tomorrow morning well go explore a little, and expect to have the first pictures of this reef that, as far as we know, has never been visited by anyone before us!
21:20 The small boat is splashing at the end of its painter behind the ship. Illuminated by the beam of the aft life raft floodlight Cam and Tane are busy sampling sharks again, making the best of this unexpected opportunity. The first catch, a small grouper, is used for bait, the smaller fish traded for bigger ones. Four Grey Reef Sharks are caught and sampled with a curious audience at the taffrail getting their first close look at the details of the Tweedles work. The water in the beam of light teems with fish; a small but ambitious flying fish leaps out of the water for the brief safety of its flight but is caught short as it collides with Tanes face. To the laughter of the onlookers, Tane collects the fish from the bottom of the boat, kisses it and throws it back into the dark sea. Looking away from the light I see the reflection of the Milky Way off the water on the dark side of the boat, reminding me to look up at the brilliant night sky. We are so very lucky to be here tonight, lucky to have this gentle, settled weather. Lucky for this unique, unforgettable anchorage in the middle of the Pacific Ocean with nothing but the horizon in sight!