Casting About

Hello Fellow MIST-ery Men, Women and Children,

We are still steaming towards Taiwan at a steady 10 knots. This has been a long transit (9 days so far), so we have been taking turns presenting our research every night. We have a variety of research disciplines represented on this cruise, and it has been very interesting to hear about the work that each person is doing and how it relates to our cruise’s research aims. Last night Jensen Jacob from the Earth Observatory of Singapore (EOS) presented some of his work on mapping the tectonics of the ocean basins near Indonesia and Australia.

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 Jensen Jacob presents his work on tectonic mapping

  As promised in the last blog post, I thought I would chat a little about the CTD data that we acquire. The suite of data collected from a CTD/Rosette ‘cast’ usually consists of two parts: the data acquired in real time by the CTD instrument, and the data we acquire later by analyzing the bottle samples that are collected as part of the cast.  I thought it might be interesting to go over what we saw for our CTD casts for the real-time data first.

As soon as the CTD enters the water, it begins to take measurements of conductivity (which is converted to salinity) and temperature. As it moves down the water column, it continues to take measurements, all the while recording the pressure (which can be converted to depth).  The CTD continues to record data all the way down until the deepest sample for the rosette, at which point it begins its long ascent back to the surface. If you look closely at the data below, you can see that there are actually two vertical tracks for each measurement that closely mirror each other (red is temperature, yellow is salinity, green is oxygen) – these two tracks represent the instrument’s travels down and back up the water column. You can imagine that this is a useful tool to have in real time, especially if we are trying to sample specific features in the water column (i.e. a salinity maximum or an oxygen minimum). The numbered horizontal red lines in the picture below are the depths at which we took samples by firing the bottles (as I talked about last time).

 CTD Image

Real time data from a CTD Cast

  The first obvious feature of the water column can be seen between 0 – 70 meters (roughly) depth. Look at the temperature measurements (in red). The temperature of the surface ocean barely changes over the first 70 meters, as compared with the deeper parts of the ocean. This is the Mixed Layer. This surface layer is well-mixed, meaning that the temperature and salinity properties of the surface waters should be roughly the same throughout this relatively small upper layer. The mixed layer is also well-equilabrated with the atmosphere, meaning that oxygen levels are close to full saturation for these waters.

 Mixed Layer

Same data – The mixed layer is circled in grey

  What creates the mixed layer? In a general sense, the mixed layer is created by a combination of wind (similar to how you would be able to mix a cup of coffee by blowing across the top)  and surface cooling (cooling causes the water at the surface to become more dense and sink, which mixes the surface layer). By contrast, the mixed layer can be ‘unmixed’ and made shallower by either warming or precipitation at the surface (both of which increase the surface buoyancy and in turn make mixing less likely). This means that the depth that mixed layer extends to is constantly changing with time and location. In general, the mixed layer does not extend much deeper than 100-150 meters, and is usually deepest at the end of winter (when strong cooling and increased wind mixing are occurring). There are locations, however, where vigorous cooling at the surface can cause local deepening of the mixed layer of up to 1000 meters!

As we move down on our CTD cast, we see an area where the temperature (in red) is changing rapidly. The region which has a high temperature gradient in the vertical is called the thermocline. The thermocline we observed on this cruise is fairly shallow, as thermoclines in low and middle latitudes can range between 200 and 1000 meters depth.  In polar latitudes, the thermocline can be non existent, as surface cooling reduces the temperature gradient to zero. In those locations, the pycnocline (density gradient) is maintained almost solely by the halocline (salinity gradient).

 Thermocline

Same data again – the thermocline is within the grey box

  Why is the depth of the thermocline important? Im sure that Riley can explain in much more detail in a separate post, but one of the main aspects of his work involves characterizing the history of the thermocline changes in the Indian Ocean over the 19th and 20th centuries using various paleo-tracers. By taking water samples along gradients in salinity and temperature on our cruise, Riley can characterize the relationship of salinity, temperature, and oxygen-18/oxygen-16 ratios (a paleo-tracer) in the modern ocean.  This is extremely helpful for his work, as he will be able to use these relationships to extract information about the last 150 years of Indian Ocean temperature variability from records of oxygen 18 that he has measured in calcareous fossil sponge skeletons. Eventually he will develop this into a record of upper ocean warming in the Indian Ocean over this time period.

 

From the CTD data, you can see that the oxygen concentrations stay high at the surface, and begin to decrease further down the water column. As I mentioned before, you would expect the surface water to be saturated with oxygen because it is well-mixed and in equilibrium with the atmosphere. As you move down the water column, however, you might expect to see higher concentrations of dissolved oxygen, as colder waters should hold more oxygen. Instead you see the opposite. So whats going on?!?

 Oxygen Minimum

Same data again – The oxygen minimum is circled

  The answer is reminerilization. Reminerilization is the respiration of organic matter in the water column by bacteria. Respiration uses oxygen and breaks up the organic matter (dead plankton and fecal matter) into nitrate and phosphate (oceanographers commonly refer to these as nutrients). Given that there is no source of oxygen below the mixed layer, oxygen decreases with depth because it is being used up. As you might expect, nutrients are low at the surface (where they are consumed by photosynthetic organisms) and increase with depth as reminerilization releases them back into the water column. Below I put a couple of measurements from the CLIVAR I09 cruise – you can see how nitrate and phosphate increase further down in the water column as oxygen is consumed.

 Nutrients

Distribution of phosphate and nitrate vertically in the water column

  Deeper in the water column the distribution of chemical properties (salinity, oxygen, and nutrient values are examples) are governed  by circulation. As such, it is possible to discern a water mass (a body of water that has its properties set by a unique process and is circulated and mixed throughout the ocean) on the basis of its chemical composition. This is a second of our CTD sampling aims – the identification of the modern distribution of oxygen-18/oxygen-16 and carbon-13/carbon-12 ratios in the water column. Understanding this distribution gives us a basis for comparison for our reconstructions of past currents and circulation (i.e. the ‘paleo’ water column). Below is a section of phosphate measurements in the Indian Ocean along ~7°S – and you can see the division around 1.8 km depth between the Deep and Intermediate water masses.

Phosphate Section

 

As you can see, it is possible to get a lot of information from a CTD/rosette cast… and it gives you an idea why it has been such a useful tool for so long!

 

Thats all for now – the weather has started to pick up a bit in the last few days!

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Sunset from 10 days ago

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Sunset yesterday

As always, thanks for reading,

–Alan