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.

How to differentiate between G. ruber and G. sacculifer

Comparison between G. ruber and G. sacculifer

Comparison between G. ruber and G. sacculifer

Globigerinoides ruber and Globigerinoides sacculifer (yup, that's a mouthful) are two types of planktic foraminifera (unicellular microplankton that float near the ocean surface) that are commonly used for paleoclimate reconstructions from deep-sea sediment cores. They live for about 2-4 weeks at the sea surface and create a shell in the process, which then falls to the sea floor after they die. Subsequently, the shells get covered with sediment that houses future generations of microplankton shells. Pesky paleoceanographers such as myself then retrieve sediment cores from the sea floor and try to reconstruct ancient climates by, amongst other means, analyzing various chemicals in the shells of the foraminifera (or "bugs" as we affectionately call them).

One of the most important steps in producing a paleoclimate record using foraminifera is to identify and pick particular species of interest from a washed sediment sample. Planktic foraminifera live at the top of the water column, so their shells can be used to reconstruct water conditions (like temperature, salinity) at the sea surface. Shells of benthic species, which live at the bottom of the water column several thousands of meters below the sea-surface, can be used to reconstruct bottom water conditions. So, apart from identifying foraminifera from all the other microfossils under the microscope, distinguishing between planktic and benthic species is the first order step of picking forams.

There are also many differences between different types of planktic (or benthic) species. For example, there are some planktic species that live or migrate through the thermocline as opposed to the mixed layer - both of which have their own particular physical properties; some species only grow in the winter whereas some bugs prefer to thrive in warmer waters. Further, different species have different metabolic pathways through which they process trace metals and stable isotopes from the seawater, which ultimately results in a different chemical signature in their shell. Therefore, unique species of foraminifera have unique applications in reconstructing various aspects of past climates. Unknowingly mixing different bugs can yield a chemical signal that has no accompanying physical basis. So rule number one: don't mix different species! 

G. ruber and G. sacculifer (rubers and sacs for short) are both common planktic species found throughout most tropical and sub-tropical waters. Both of them dwell in the top ~50 m of the water column though the sacs appear to migrate to greater depths (e.g. here and here). Both species belong to the same genus, have an algal symiont, possess spines and both have shells that are trochospiral in nature. Specifically, the three chambered sac (aka Globigerinoides trilobus) is very similar to the white ruber - the four-chambered sac has a relatively giant sack as its fourth chamber. To the casual reader, suffice it to say that both of these bugs look very similar to an inexperienced eye.

For the beginner bug-pickers though, here are some under-the-microscope pointers in distinguishing between G. ruber and G. sacculifer:

  1. If you have any doubt at all when specifically picking either a ruber or a sac, zoom in and take a better look!
  2. An easy tell - look at the texture and structure of the bug: sacs have a characteristic honey-comb surface (the pores on the shell look like benzene molecules!) whereas rubers' honey comb structure is less pronounced and more lobey/smaller. If it isn't clear, changing the position of the light might help. To me, the sacs look much more sugary smooth than the rubers.
  3. The primary aperture (or central hole) of the sacs can vary from an oval to a split whereas the rubers' tend to be more rounded. Further, the rubers' primary aperture lies over its umblical suture, which may not necessarily be the case with the sacs.
  4. The suture on the spiral side of the rubers is more prominent than with the sacs.
  5. If your sediment sample contains pink rubers along with the white rubers, compare the bug in question with a pink ruber and see whether they're the same species!
  6. Of course, if all else fails and you are still in doubt - move on to the next bug! Don't pick a questionable bug for chemical analysis!

The advanced bug-pickers - please feel free to provide tips of your own!