A profound change has come over most of us aboard since the last update - we’ve crossed the Antarctic Circle. All of a sudden, temperatures dropped off in both the ocean and air, and – and this was a big hint – huge icebergs started to appear. After many years of doing this kind of work, I still take a breath when I seen the first few large bergs (and really, these aren’t that big – less than 1 km long). I have to say, on this voyage, with a number of people aboard for the first time – their delight and wide-eyed wonder at the sights (there’s been sea ice and a lot of whales too) is an absolute pleasure.
From here we are moving onto the Continental Shelf, where the ocean becomes shallower from around 3500 m to less than 1000 m, and on to a rather long list of tasks.
Much of these tasks involve our CTD and considering that January 22 was CTD Appreciation Day, I decided to dedicate much of this update to this incredibly useful piece of kit! This might seem a little niche but CTDs are a mainstay of oceanographic instrumentation and have evolved to be critical pieces of kit for almost all ocean research.
A CTD – conductivity temperature depth – profiler package is a suite of ocean instruments measuring far more than temperature and conductivity (which gives salinity). Oxygen, light, turbidity, and other data if you have the sensors, are all possible. Decades of development have enabled these sensors to be highly accurate over most ocean conditions. For example, the temperature sensors are accurate to within a few millidegrees! It’s a struggle to even find an independent sensor good enough to check that level of accuracy. And yet it can make a difference for how we categorise deep ocean waters, especially as they change with climate.
In addition, the instruments are topped by a carousel of water sample tubes that snap shut at desired depths. This water provides raw material for a whole slew of science from eDNA through to measuring oxygen levels.
So, a “CTD cast” involves careful pre-deployment checks, then the whole assembly is carefully winched over the side, followed by the slow lowering of the package – sometimes for many hours. All the while data come up the wire and we watch the ocean structure unfold before us. The instrument has a seabed detector, so we know not to crash into the seafloor. As we haul the package back in, we stop at preselected depths to close a sample bottle. Finally, it all comes aboard thanks to the deck crew. It’s impressive to watch this team, many with decades of experience of getting delicate science gear in and out of the ocean, in all weather conditions.
When the instrument comes on board, and once the Ship’s deck crew have safely secured everything, it looks like a Formula One pitstop as the team gather round to collect samples and check electronics. My co-Voyage Lead Dr Denise Fernandez is first in the queue to grab small vials of seawater (the same size as for a blood sample at the Doctors) that will be processed to quantify the level and origin of oxygen in the seawater.
The samples are being collected for UK colleague Dr Kathryn Gunn (University of Southampton) who is leading the large international project “FRESH” that seeks to map out the freshwater budget of the Ross Sea. We know the amount of freshwater diluted in the ocean water in the region is important for regional and global circulation, as well as ecology and sea ice formation. However, if we are going to identify how this might change in a warming world we need to know where the freshwater has actually come from – glaciers, sea ice or even submarine groundwater.
FRESH is one of a number of international connections happening onboard, reflecting New Zealand’s strong presence in the highly collaborative Antarctic Oceanographic research community. This only happens with continuous long-term funding, such as through the Antarctic Science Platform and NIWA's Strategic Science Investment Funds, that allows relationships to develop and for succession plans to be achievable
With the many uses of the seawater comes a challenge of course, as each CTD deployment brings back only a certain amount of water, and only from certain depths. One of Denise’s (many) key roles on the voyage is to manage the sampling from the CTD rosette so that nothing is wasted and we hit all our target samples.
Given the central place the CTD holds in the voyage, there’s a lot of pressure for the CTD to operate in good working order under sometimes difficult circumstances. On this trip, Dr. Jasmin McInerney leads the CTD controls with support from technical experts Nick Eton and Jacob Hall. It requires a significant amount of experience and expertise to keep this kit running - especially in Antarctic waters where even the seawater is often at subzero temperatures. We'll collect around 50 CTD casts on this voyage and the data will eventually, after much checking, find their way into various open databases.
Craig Stevens & Denise Fernandez