Sediment Traps and Plankton Tows

A very pretty Cocodrie sunset with the R/V Pelican.

Better late than never I suppose, but I wanted to document our short research cruise aboard the R/V Point Sur that happened early last month. It was another routine sediment trap operation, but this one really stood out because we happened to see some really cool critters at sea!

As usual, the northern Gulf of Mexico sediment trap crew (Eric Tappa from the University of South Carolina, Julie Richey and Caitlin Reynolds from the USGS, myself, and another helping hand from Michael Lis, an undergraduate student from UT Austin working with me) made our way down to LUMCON at Cocodrie, Louisiana and boarded the R/V Pelican to start our transit to the sediment trap mooring site. However, things did not go as planned (as is common in the field) and our overnight transit was halted short, as we had already turned back towards port after leaving it only some hours before! It turned out that there were some serious engine problems on the ship which entailed something we could not afford - delays. Luckily, as we were sulking around LUMCON taking an in-depth look at their impressive library, the R/V Point Sur came to our rescue and their crew graciously agreed to charter our operation. 

Relief as the sediment trap comes up to the surface. Picture credit: Michael Lis.

Relief as the sediment trap comes up to the surface. Picture credit: Michael Lis.

This time, apart from sediment trap redeployment operations, we had an additional exciting task: using the plankton tow. A plankton tow is a vital tool used to sample plankton that floats freely in the surface ocean. It is, in essence, a giant net with a fine mesh that we can trawl around and look at what comes up. We intended to sample some plankton at the sediment trap site. In any case, here’s a picture collage of some of the amazing creatures we were lucky enough to catch in our plankton tow:

I'm really excited about the upcoming science from this research trip. Hopefully, we can find some interesting results! Until my next voyage, I will certainly miss being out at sea!

Our Recent Paper in Four Tweets

As promised, this is the tl;dr version of my previous post, where I have tried to reduce our open access paper into four tweet-length snippets per sub-heading. Here goes:

The History: A particular plankton shell used to reconstruct climate is purported to have 2 morphs that live in different depths of the ocean.

The Importance: If true, previous studies that attempt to quantify past oceanic climates from non-selective morphs of that plankton species are biased.

The Study: We analyzed pairs of extreme & intermediate morphs, & with a model, found that all morphs live in the top (<30 m) part of the ocean. 

The Implications: We conclude that morph-based uncertainty in this species when used for studying ancient (Holocene) climates is little-to-none.

Back at sea!

The Trap (Picture courtesy Eric Tappa)

The Trap (Picture courtesy Eric Tappa)

Last week I was out in the Gulf of Mexico aboard the R/V Pelican for a short little research cruise. Our main intent was to find and redeploy a long-running sediment trap. A sediment trap is an instrument used in oceanographic studies to "trap" sediment formed in the column of water above it. They are extremely useful in quantifying fluxes of marine sediment and in constraining the variability in the production of sediment over time. Mainly, we were interested in quantifying the flux of planktic foraminifera: which foram species grow throughout the year; which species prefer warmer/cooler waters; how accurately their shell chemistry reflect environmental conditions (temperature, salinity) etc. In essence, we are trying to ground-truth the variability we observe in the chemistry of the forams preserved in marine sediment cores to reconstruct ancient water conditions (down-core variability). We can use the chemistry of the shells obtained from the sediment trap and utilize known, instrumental temperature and salinity conditions to build transfer functions for ancient, downcore chemical variations in the shells. Remember, these planktic forams live in the upper column of the ocean and build their shell with chemistry dependent on the environmental conditions during which they grew. After they die, the shells fall down towards the seafloor. Our sediment trap catches these shells and preserves them in cups. The trap is programmed to automatically close a cup every 7 or 14 days and subsequently, open a new one. As the cups get filled over a couple of months, we need to go out to sea, retrieve the trap, put in new cups, perform routine maintenance and redeploy the instrument.

My journey started with a flight to St. Petersburg, Florida. Our lab collaborates extensively with the USGS Coastal and Marine Science Center located in St. Pete. Here, I was invited to give a talk on my master's work on single forams by Julie Richey, who studied the Little Ice Age and Medieval Climate Anomaly in the Gulf of Mexico for her PhD work, and now overlooks the center's paleoceanography program. St. Pete is a cool little town and I greatly enjoyed chatting with the folks at USF and USGS. After packing all the equipment and material needed for our research cruise, thanks to the meticulous work of Caitlin Reynolds (a USGS co-author on my AGU presentation who has made the sediment trap "her baby"), we were off to New Orleans, Louisiana - a ~11 hr drive!

We stayed overnight at NOLA and picked up more material for the cruise from Brad Rosenheim's lab at Tulane University. Brad's recent Master's graduate, Matt Pendergraft (who has an excellent paper and video abstract out), would join us for the cruise. Next, we had to drive to LUMCON (Louisiana Universities Marine Consortium) at Cocodrie, LA with all our equipment to set sail on the Pelican.

The R/V Pelican is a ~120 ft. boat with a wide A-Frame capable of multiple oceanographic instrumentation. The crew are an excellent bunch who were very knowledgable about our scientific operation and included a great cook (always good for morale out at sea). At Cocodrie, we were joined by Eric Tappa, a research associate and sediment trap expert from the University of South Carolina. He brought two USC students, Natalie Umling and Jessica Holm, along for this cruise (more hands the better!)

The Crew (Picture courtesy Eric Tappa)

The Crew (Picture courtesy Eric Tappa)

At around 7PM on Thursday, the 21st of November, we were off! It took around 12 hrs for us to get to the sediment trap site. Fortunately the weather was great and the seas were calm. After we reached the vicinity of where the trap was deployed last (thanks to GPS) we sent out an acoustic ping to make sure it was nearby. Thankfully, we "heard" the sed. trap ping back. The sediment trap is maintained at a depth of ~700 m by two strategically chosen buoys that give it buoyancy and an anchor that holds it down. The anchor is attached to the sediment trap via an acoustic release. At the site, we send out a signal to the release to detach itself from the anchor, thereby enabling the buoys to push the trap to the surface ocean.

Seeing the buoys surface is a big relief! The sediment trap setup has survived for six months without going awry! Next, we pick up the sediment trap, install new cups, perform maintenance, redeploy it with a new anchor, and hope that it survives until we're back.

While we were out there, Julie and I wanted to get some core-top material (the topmost portion of the sea-floor). Core-tops are another means through which paleoceanographers can ground-truth down-core variability. For this operation, we turned to a multicorer (here's a neat underwater video). After getting successful core recovery (a total of 4 casts), we had to extrude and sub-sample all the core material at 0.5cm/sample (conventional sampling resolution). Mind you, there were 8 multicores per cast, each at ~45cm which equates to a lot of extruding!

The Cores (Picture courtesy Eric Tappa)

The Cores (Picture courtesy Eric Tappa)

The journey back to Cocodrie was largely uneventful and much to our liking, the seas stayed calm. It was almost a year since I had been out to sea and going back only reminded me how much I like it out there!