Paleowave

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.

Foraminiferal Morphotypes: Birds of a Feather?

G. ruber morphotypes (1) a and b: sensu lato; (2) c and d: sensu stricto. There are numerous intermediate transitional forms between these.
G. ruber morphotypes (1) a and b: sensu lato; (2) c and d: sensu stricto. There are numerous intermediate transitional forms between these.

We have a new open access paper (yes, anyone, including you, can access it!) out in Scientific Reports titled Globigerinoides ruber morphotypes in the Gulf of Mexico: A test of null hypothesis. Here is a breakdown of the paper:

The History

  • Globingerinoides ruber (G. ruber) is a rather famous planktic foraminfer (or foram for short), whose shell chemistry has been widely (and successfully) used to reconstruct ancient surface ocean parameters such as temperature and salinity. This foram lives in the upper ocean and creates a shell for its protection; the shell later sinks to the seafloor after its death.
  • G. ruber shells were first identified and reported by French naturalist Alcide d'Orbigny in 1839. Since then, several morphotypes of the species have been reported. These morphotypes have seemingly minor variations in their shell characteristics (e.g. smaller aperture hole, more arched chambers etc.)
  • In 2000, a core-top (or near-modern) study by Chinese paleoceanographer, Luejiang Wang (who tragically passed away drilling corals in the South China Sea), analyzed stable isotopes in the two principal morphotypes of G. ruber's white variety: sensu alto - sl & sensu stricto - ss (as he christened them).
  • The study seemed to indicate differences in the stable isotopic signatures of these morphotypes: ss seemed to have a warmer signature while sl was cooler.
  • Wang suggested that sl might live deeper than ss and is hence, cold-biased (the deeper you go in the ocean, the colder it gets!)
  • More recent studies seemed to find equivocal/ambiguous results for similar analyses i.e. some found significant differences but others didn't. However, nobody sought out to perform a comprehensive, controlled experiment specifically for G. ruber morphotypes.

The Importance

  • A lot of our knowledge about past climate change in the oceans comes from studies analyzing G. ruber shells.
  • If these studies did not selectively discriminate between the two morphotypes prior to analyses, the Wang, 2000 study and others suggest that these reconstructions could be biased as we would be averaging signals from two different depths. Thus, our quantitative understanding of climate change itself may be biased!
  • Furthermore, all our calibrations and verification exercises on G. ruber have been done on non-selective mixtures of these morphotypes.
  • It is logistically very difficult to observe these critters in the wild. Here is a nice video that details the challenging process of culturing forams.
  • It is NON-TRIVIAL to differentiate between these two morphotypes as there are numerous transitional shell forms between ss and sl. It is HIGHLY subjective! (one man's sensu stricto is another's sensu lato)
  • Genetic work shows that it is NON-TRIVIAL to select different genotypes based on the shell morphology alone.
  • As a birder, here is an analogy with birds: two birds that look very, very similar may, in reality, be different species and have completely different habitats and/or eating habits etc. If we are looking to gain information from the physiological chemistry of these birds (say, their feathers) to infer something about the environment they live in - it would be prudent NOT to mix samples of both the birds, correct?
  • But... it is impractical to perform pilot genetic studies on living forams in tandem with paleoceanographic reconstructions using foram shells.
  • So, how much would it matter if we did not perform genetic analyses accompanying paleoclimate reconstructions in the curious case of these two G. ruber morphotypes? Do they really live at different depths? How much does it matter if they did?

The Study

  • To shed some light on (some) of these important questions, we turned to the abundant resources that are available to us in the northern Gulf of Mexico. These include:
    1. A Sediment Trap: A device that collects foram shells before they hit the seafloor.
    2. Core-tops: The topmost portion of the seafloor, where recently dead foram shells accumulate.
    3. Downcore material: Cores spanning the last 4,000 years containing ancient foram shells.
  • We sat down and decided to chalk out a strategy to be consistent in how we selected the stereotypical ss and sl morphotype sample.
  • We decided to perform a geochemical test of "null hypothesis", where, along with the stereotypical ss and sl morphotypes, we analyzed samples of 'intermediate' morphotypes that had transitional shell characteristics to these extreme morphotypes:
    • If the geochemical variability between the sets of 'intermediate' morphotypes was consistently different from the ss-sl sets, then the shape of the shell dictates its stable isotope signature, and hence provides evidence for cold/warm biases.
    • On the contrary, if the 'intermediate' sets showed comparable variability to the ss-sl sets, then we cannot reject the null hypothesis that morphotypical variability has no effect on the stable isotope signature.

The Results

  • We found that the  ss-sl isotopic signatures for 37 sets from was statistically indistinguishable.
  • The 'intermediate' sets showed variability very similar to the offsets in the ss-sl pairs.
  • The sediment trap results indicated that the ss, sl, and intermediate morphotypes are good indicators of sea-surface conditions (and not deeper).
  • They also revealed no seasonal differences between these morphotypes (i.e. all morphotypes grow throughout the year)
  • Using a forward model and our observations, we found that both ss and sl morphotypes live and calcify in the upper ~35 m of the water column in the Gulf of Mexico.

The Implications

  • In the Gulf of Mexico, the uncertainty due to morphotypes in Holocene-based reconstructions is little-to-none.
  • G. ruber (at least in this part of the world) appears to calcify in the topmost portion of the surface ocean.
  • Not all previous reconstructions, calibrations, verification experiments that didn't discriminate between G. ruber morphotypes are wrong.

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!