Into the mud!

MUD!!!   Sunday was our first day coring, and what a day!!!! Better than anyone could have asked for! Sunny, warm, and calm seas. After a bit of a rough day on the water Saturday, a welcomed change in the seas!

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Awaiting our mud!

For my project, we need many (many, many) cores and we also need to preserve the very upper layer of the sediments. An instrument called a “multi-core” can achieve both of these objectives. First, as the name suggests we can collect several cores on a single deployment (the multi-corer on the R/V Investigator can collect 6 cores!). Furthermore, the cores are slowly pressed into the sediments so on a good recovery you can get a well preserved interface with (clear!) bottom water overlying the sediments!

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Multi-corer launch using the A-frame!  The A-frame is extended to its full position to get the corer as far from the boat as possible

Very useful, the multi-corer is also a complex (and finicky!) piece of equipment! Like many bottom samplers, the multi-corer uses a spring/tension system. This means, that a spring-loaded mechanism holds the cores in the open position as long as there is tension in the cable. The idea is that the tension on the cable is only released when the corer hits the bottom. Great in theory, a little more complicated at sea… where there are waves… waves that can rapidly move a ship up and down, jerking or releasing tension on a cable. Essentially, this additional movement can trick the corer- it thinks it hit bottom when really the cable just momentarily slackened. But, no such misfortune!

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Success!  Once the corer is secured on deck we can start getting the cores off and resetting for the next deployment!

With a large portion of the science on board focused on life in the water column, pulling up mud was excitement!   So we had quite the audience in addition to the team needed to deploy and recover the core safely. Anticipation grew across the deck as scientists and crew watched the corer go out through the A-frame and disappear into the water. This particular site, near Port Hacking, was shallow (only about 100 m)- so the entire deployment was fairly quick (<10 min) even though it felt much much longer as we held our breathe! Success!!

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Smiles all around after a successful deployment! We couldn’t do it without these guys!

The first deployment came back with good penetration (the core tubes contained almost 40 cm of sediment!) and minimal disturbance (a little sediment had been kicked up into the over lying water but not ‘mud soup’ and the interface was flat). We carefully loaded the cores into the core cozies and carried them from the back deck into the constant temperature lab (the room is a giant walk in cooler, for this voyage it is being kept at a constant 4°C).

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One of 12 core cozies we use to keep the caps secure on the core while moving the core around the vessel.  Clean, secure, and easy to use!

We also sampled water from a niskin bottle (same water sampling device as on the CTD! Just a bit of a smaller version). This particular niskin bottle is mounted onto the multi-core in such a way that 1) we know we get bottom water (we can measure the distance from the top of the sediment in the core tube to the bottle, always < 50 cm) and 2) the bottle fires simultaneously with the trigger closing the cores (this prevents contamination of the bottom water as the corer can kick up sediment).

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Evaluating core quality upon recovery.  Great penetration (almost 40 cm!), some disturbance at the interface (closest core most disturbed) but overall clear overlying water!

The turnover went quickly, and it was straight back into the water for a second deployment! The second deployment went even better! The cores looked great. Into the constant temperature lab they went and it was time to get started! Port Hacking is a biologically interesting region with the influence of the East Australian current (warm waters flowing south along the east coast of Australia) so there were various requests for sediments both from scientists on the vessel and collaborators back on shore. We started off ‘easy’- sectioning a core between these multiple projects. Each request for a share of sediment had been accompanied by a quick summary of the research to be completed (the interesting part!) and the conditions that had to be met (the logistically important information: sample size, depth in core, exposure).   From this, we could design a sampling plan that met everyone’s needs! We had help to make things go smoothly, quickly, and accurately- with a note taker recording every step. Each fraction went into a container specific to the future analysis goals labelled with location, researcher, and depth in core. After we divvied up this core, we started the set up for my pore fluid collection.

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A platform secured around the top of the core extruder is the home to the first of two glove bags.  As long as the overlying water remains in the core tube, the sediment is ‘protected’ from gas exchange (we’re mainly concerned with oxygen!).  Once we start sectioning the core, the protection is lost- therefore we section in a nitrogen environment! 

We set up two glove bags- why glove bags? The chemistry of the sediments and the pore water is very sensitive to exposure to oxygen so we need to control the air the core contacts. At collection, the overlying water in the tube “protects” the sediment from gas exchange- at least in the near term. The ‘bag’ allows us to restrict the air touching the core to just inert nitrogen. And the ‘glove’ part allows us to manipulate objects inside the bag while remaining outside of the clean nitrogen environment. Our bags are set up to share a single nitrogen tank to minimize the amount of compressed gas stored inside the lab at any given time. We just clamp the line to one glove bag at a time so the bags can operate independently.

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To separate out the pore fluid, the sectioned sediments need to be centrifuged.  The challenge? Centrifuges like to operate on stable, level surfaces- two conditions not frequently met on board a ship!  The table holding the centrifuge acts as a gimbal, allowing the centrifuge to stay still while the boat moves around it!

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Glove bag number 2!  This is where we pull the pore water off the centrifuged sediments and filter it into sample containers.

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This ~1cm diameter hole was created by burrowing shrimp and extended over 19 cm down into the sediment column.  Bioturbation (mixing by little critters) can drastically alter the chemistry of the surrounding mud.

Being from a shallow site (~100 meters water depth) these cores were full of life!  Fortunately we’re sharing a ship with biologists who could help identify the critters!  These little guys cause a lot of disturbance in the sediment column through vertical mixing, and the chemistry in and around their burrows can vary greatly from the surrounding sediment.

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One of the perpetrators causing mixing in the sediment column- this little guy survived the centrifuge and was quickly trying to remix the pore fluid with the sediment! Made filtering a little bit more of a challenge!

The first core is always the slowest- training, getting used to the set up, finding where everything is packed.. and this was no exception. But we got into the rhythm and had lots of help along the way! It is amazing the difference a few extra sets of hands can make! That’s part of the community you build at sea- if you’re not busy with your own work, you hep as needed with others. You learn more and everyone can achieve more! It is also more fun to have people to talk to! 2 days later (with very little sleep!) the 7 cores we processed for core fluid are completed and we have a bit of down time to clean up and set up for the next adventure!

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Our amazing team pulling an all nighter to finish the cores from the first site.

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