The middle of the Atlantic

Today is our 18th day at sea.  We have sailed more than 3,000 km from our first mooring not far from the coast of Africa and there is another 3,000 km before we reach our last site, just a few miles from the Bahamas.

I have been reading about the Challenger Expedition of 1872-1876.  Our 6 weeks at sea may seem like a long time to be away but the 19th century pioneers of oceanography endured three-and-half-years on HMS Challenger.   In the first trans-Atlantic leg of their expedition, like us, they left Tenerife in February and headed towards America.   As they went they stopped to lower wires to measure the ocean depth, and trawl for samples on the seafloor.  It was the first systematic survey of the deep ocean.     They found the water depth increased as they sailed west from the Canaries, but then in the middle of the ocean their soundings became shallower.    Today this is known as the mid-Atlantic ridge and that is where the RRS James Cook is now.   We have completed our service of the moorings on the east side of the ridge and later today we will commence work on the west side.    The mid-Atlantic ridge was a great discovery that ultimately led to the understanding of plate-tectonics and the movement of the continents.

The scientists on HMS Challenger observed that the sediments retrieved from the seafloor changed as the depth changed.   On the slopes around the Canary islands it was a white ooze. As the depth increased the colour became grey and then below about 4,000 m the sediment was a red clay.    Over the mid-Atlantic ridge grey ooze was found again and the same pattern was seen in the western basin of the Atlantic too.  In fact the Challenger expedition found the same pattern around the world and in the Atlantic the transition to red clays occurred deeper than elsewhere.     Today we know that the acidity of the ocean increases with depth and the transition to red clays occurs at the calcite compensation depth below which the tiny calcium carbonate shells that form the ooze are dissolved by the higher acidity.   As the oceans become more acidic due to increasing CO2 from anthropogenic emissions, the calcite compensation depth is shallowing.  This brings us back to our work tomorrow.

The project ‘ABC Fluxes’ (www.rapid.ac.uk/abc) aims to use measurements from the RAPID array to measure changes in the transport and accumulation of carbon in the Atlantic.   Tomorrow we will be retrieving the second of our moorings equipped with new biogeochemical sensors; including a water sampler that has been collecting samples since it was first deployed 18 months ago.   You can read more about this in a blog Pete posted during our last RAPID cruise (http://rapidchallenge2015.blogspot.co.uk/2015/11/biogeochemical-sensors-added-to-rapid.html).  

Painting of the HMS Challenger by William Frederick Mitchell. From Wikimedia Commons

Before - the biogeochemical sampler and sensor package waiting to be deployed at site EBI

After - Pete and Darren inspect the recovered package after 18 months in the ocean