Deep sea sediment cores – they’re cold, they’re muddy, and they’re revealing 30,000 years of climate history – as PhD student Julia Gottschalk reports from her voyage aboard the James Cook research ship last summer.
Stepping out onto the deck, I see a bright-blue sea, glittering like silk as the sun begins to rise... and the arrival of the box corer...
The vast and enigmatic ocean covers about three quarters of the Earth, yet the information we have from its depths is comparable with the distribution and size of the holes on a golf course.
I am waking up at sea. It is 7.50am. In 10 minutes, my daily shift on our oceanographic expedition will start and I had better be on deck.
The James Cook, a British Royal Research Ship operated by the Natural Environment Research Council and the National Oceanographic Centre, left Southampton about three weeks ago. Aboard with me are 26 scientists from Cambridge, Zurich, Lisbon, Barcelona and Bremen, plus crew. After rough seas in the Bay of Biscay, an escort of dolphins and a stop in Spain to load geophysical equipment, we have made our way to our working area – the continental margin off Portugal.
It’s an extraordinary region for the study of past ocean chemistry and climate dynamics. Sea floor sediments capture the ocean’s history because the nature and the chemical composition of particles settling to the sea floor varied as the atmospheric and oceanic circulation changed over time.
The pioneering work of Cambridge palaeoclimatologist Professor Sir Nicholas Shackleton showed that these sediments record climate signals from the North Atlantic and the Southern Ocean. The discovery of interhemispherically linked climate changes recorded by tiny microfossils the size of sugar grains preserved in sea floor sediments at a single site was a milestone in the field.
I’m here as part of my PhD research with Dr Luke Skinner and Professor David Hodell, and our mission is to continue Shackleton’s legacy by studying climate history further back in time and in more detail.
The geophysical-acoustic imaging of Iberian margin sediment below the sea floor, one of the objectives of this voyage, was finished several days ago and sediment coring operations have started – or as our geophysicist colleagues call it – playing with mud!
It is very hard to get up from my berth. A quick look out of the porthole... it doesn’t look like a washing machine, it will be a calm day at sea. Making my way to the ship’s working deck and jumping into my already mud-covered overall, I discuss with Natalia from the ‘four-to-eight’ shift what I can expect during my shift.
Stepping out onto the deck, I see a bright-blue sea, glittering like silk as the sun begins to rise... and the arrival of the box corer, a 40 by 40 cm steel box that carves into the sediment to sample a chunk of sea floor mud and ideally some bottom water from just above the sea floor. As we look at the newly retrieved box core we are impressed by the chunk of original sea floor that rose from 4 km below the point we are standing now. Charly, my shift-mate, and I invent names for the tube worms and some gastropods we see, and then start to subsample and scrape off the surface of mud.
The topmost sediment reflects modern conditions and can therefore be used for calibrations. The sediments are extremely cold. It’s 2°C at that depth, quite a contrast to the air temperature of 23°C today.
We hand subsamples over to the micro-sensor measurement and water chemistry groups who work in specialised labs on the ship while we continue sampling for later analyses. Other scientists prepare new water sample measurements and add chemicals to the sampled sediments, the deck crew prepares the deployment of the next coring device, the bridge plans the route to the next station, the geophysicists analyse data, and the kitchen crew prepares another delicious meal… we are a floating science factory at 10°W, 37.5°N on the Atlantic Ocean.
The digital screen showing data from the winch indicates 10 minutes to the retrieval of the kasten corer. This long, steel container penetrates the sediment to much greater depths than the box corer. The deck crew lay the kasten corer horizontally down on the deck for us with a crane and after disassembling the 6-metre-long core barrel, bolt by bolt, we open the lid of the barrel – the moment of revelation. How much has been sampled? What does the sediment look like?
We get a beautiful view of 4 m of original Iberian margin sediments – about 30,000 years of climate history. The changes in colour document the oxygen-rich, bright sediment full of tiny fossils of marine plankton that thrived in the oceans of the past 10,000 years. We even find a fossil coral.
Sediments formed during the last ice age, when the oceans were much cooler, are usually much darker, which suggests that ocean circulation and ecosystems were very different. Everybody is excited about the new samples. Many hands are needed to process the bulky kasten core but this will yield enough study material for a range of different analyses.
By the end of our five-week-long cruise, we will have sampled 166 m of ocean sediment, analysed 1,000 water samples, seismically imaged the sea floor along a track 755 km long and studied 47,000 litres of ocean water. The ocean remains big and mysterious but we are getting closer to its secrets day by day.
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