iMonsoon Playlist

It’s been about 15 days since my last blog post and we have already finished drilling five different holes at two different sites, despite downtime. We’ve seen a lot of core! As I may have mentioned, I’m on the PM-to-AM shift, so I get off my shift at midnight. Though we (lightly) listen to music during our shift, most of my music listening happens off my shift; either in the gym or on the deck (outside). Here’s a (mostly complete) list of albums I’ve been listening to thus far:

  • Mastodon – Once More Round the Sun (most played by far! Which was JUST now promptly deleted thanks to Spotify Offline lasting only for 30 days! #%@&*@#*)
  • Godflesh – A World Lit Only by Fire
  • Yob – Clearing the Path to Ascend
  • Electric Wizard – Time To Die
  • Devin Townsend Band – Accelerated Evolution
  • Aphex Twin – Syro
  • The War on Drugs – Lost in the Dream
  • Crustation – Bloom
  • Bethlehem – Dark Metal
  • Three Mile Pilot – Another Desert, Another Sea
  • Isis – Oceanic
  • Pelican – Australasia
  • The Jesus Lizard – Goat
  • Nick Cave and the Bad Seeds – Let Love In
  • Polvo – Exploded Drawing
  • Nitroseed – Molt
  • Anathema – A Fine Day to Exit
  • Miles Davis – A Kind of Blue
  • Built To Spill – Keep it like a Secret
  • Samael – Passage
  • The Obsessed – Lunar Womb
  • Gorillaz – Plastic Beach
  • Nightingale – I

iMonsoon: Life of a Sedimentologist

SedTThe main job of the JOIDES Resolution (or JR for short) is to drill beneath the seafloor and to collect intact sediment cores. Once it gets started, the JR does a really, really efficient job of retrieving core. Every fifteen minutes or so (depending on the water depth at a location, the type of drill bit used, and the characteristics of the material being drilled), one can hear ‘Core on Deck!’ This chant is frequent enough to develop a Pavlov-like reflex.

‘Core flow’ is a JR term to describe the journey of a piece of mud from its inception at the drill rig to its resting place in a U-tube in the basement refrigerator. The scientists at the foremost part of the core flow are those in charge of measuring physical properties on the whole-round core (imagine a clear, plastic cylinder filled with sediments and rocks).

Next, the technicians onboard (who brought out the core from the drill rig via the ‘catwalk’ in the first place) proceed to split the whole-round core into two halves: the working half and the archive half. Many scientists now descend upon the working half, carefully sampling the mud for various chemical and physical measurements. It’s quite a spectacle – especially when we’ve hit a transition or a well-known boundary! What about the archive half? Well, this is where the sedimentologists come into the picture.

The main job of the sedimentologists (8 of us in total on this expedition, with one soon to join) is to describe, characterize, and make detailed reports about the contents of the mud. We are also responsible for walking the archive halves through the SHIL and SHMSL: two fancy imaging instruments that can take high-resolution photographs, and make color-based and magnetic susceptibility measurements which become important for the stratigraphic correlators onboard.

Once these scans are finished, the fun begins. Using tried and tested, yet basic, tools (see picture) we try and characterize the makeup of the mud. We document the colors using Munsell charts, note the texture of the sediments using the spatula, and then try and see interesting features using our hand-lens. It is also our job to document how the drilling process might have disturbed the recovered cores. Another vital aspect of the description process is making smear slides, where a small amount of sediment is taken on a glass slide for observation under a powerful microscope. This can be really handy for distinguishing the amounts of clay, silt, sand, and even identifying minerals or volcanic ash!

After the first five cores or so, all of us sedimentologists (4 in the day shift) became cogs of a bigger, well-oiled machine. Mind you, there were ~50 cores in the first hole, each composed of 4-7 1.5 m sections (!) – so we see a LOT of core, and during most parts, it can be run-of-the-mill. However, when something exciting does pop up (which can happen quite frequently at times), we usually all gather around the description table, huddle together, take notes and photographs, and have lively conversations and debates, and ultimately marvel at how we can catch glimpses of a world that was millions of years younger…

iMonsoon

353 Sci party contact sheet

Ahoy! I am typing this blogpost aboard the JOIDES Resolution, the flagship of the International Ocean Discovery Program, in the middle of the Indian Ocean! Last night, we reached Ninetyeast Ridge (most-creatively named!), the first drilling site of our research expedition. Currently we are waiting for the drillers to make sure all the equipment is calibrated and ready-to-go. All of us on the day shift and a few from the night shift who are awake are excited and eager to see the first cores come up on deck.

The main goal of Expedition 353: Indian Monsoon Rainfall (or iMonsoon for short) is to understand how the Indian monsoon evolved over the last several tens of millions of years. How is this achieved? By retrieving land-based sediments that made their way to the seafloor through the numerous rivers that flow into Bay of Bengal and/or through wind-based transport. These sediments house the shells of oceanic critters that were living in the past (like foraminifera, diatoms, radiolarians etc.) One of the major factors that influences the chemistry of these sediments and the numerous fossils that are preserved in them is rainfall over the Indian Subcontinent.

So, these sediments hold the key to understanding past monsoon strength (or lack of it). The deeper we drill – the older the sediments. Why do we care about monsoons in the past? Well, we only have very brief instrumental measurements of how much it rained over India, perhaps, 100 years or so – a geological instant. The Indian monsoon has been around a lot longer than that and thus, to fully understand how the monsoon is capable of changing, we need to be really knowledgeable about its past.

The JOIDES Resolution is a very capable research vessel. There are 30 shipboard scientists aboard, each with a unique job assignment. In total, including the technical staff, drilling staff, and the crew, I think we are about ~100 people on the ship. The food is fantastic (lots of options), and the ship is equipped with a gym and movie room.

I am sailing as a sedimentologist on this expedition, which means I will be inspecting, characterizing, and describing all the cores that we collect. As I mentioned, we are eager to start seeing cores and getting a glimpse of what we will be working for the coming few years! I will try and update this spot periodically with our progress. Wish us luck!

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?

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.

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.

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.

Propagation of a Taxonomic Error

Taxonomic identification lies at the crux of foraminiferal paleoceanography. I have written before about the importance of properly identifying and reporting the species of foraminifera used for geochemical analysis in a study. This has implications not only for placing the extracted geochemical signals in a physical context with appropriate uncertainty bounds, but also for communicating the methodologies employed in the study for future replication and reproducibility purposes, a tenet of science.

Recently, Yi Ge Zhang and colleagues had an article in Science magazine about their work on a 12-million-year-old temperature reconstruction in the tropical Pacific Ocean. The study produces new, long and detailed tropical sea-surface temperature records using the TEX86 paleothermometer. Their results are remarkable as they show a sustained zonal tropical Pacific temperature gradient through the Pliocene, going against the paradigm of “permanent El Niño”. The Pliocene is a time period where the tropical Pacific gradient was thought to have collapsed, similar to what happens during a brief El Niño event. This inference comes from paleoceanographic records produced from the magnesium-to-calcium (Mg/Ca) ratios of planktic foraminifera. The authors make the case that this technique has limitations compared to TEX86 reconstructions on these time scales.

Discussing these previous studies that investigated Pliocene climate using planktic foraminifera, the authors write:

Published temperature records based on magnesium-to-calcium ratios (Mg/Ca) of the planktonic foraminifera Globoritalia sacculifer, from Ocean Drilling Program (ODP) site 806 (0°N, 159°E) (Fig. 1) (8), suggest that warm pool temperatures remained relatively constant as Earth cooled over the past 5 million years.

I was immediately struck by ‘Globoritalia’ – I had never heard of the genus, only Globorotalia. Furthermore, the only sacculifer species I was familiar with was Globigerinoides sacculifer. Being relatively inexperienced in non-Quaternary foraminifera, I assumed Globoritalia sacculifer was some kind of Pliocene planktic species. Skimming through foraminiferal identification books, I could not find any mention of the species. Google Scholar too yielded only five results for ‘Globoritalia sacculifer (including Zhang et al.) with all of them discussing the Pliocene mean state hypothesis. I was increasingly beginning to suspect that the species did not exist. The final nail came when I questioned my more experienced colleagues including foraminiferal expert Dick Poore, all of whom immediately recognized the error when I pointed it out: that it should be Globigerinoides sacculifer and not Globoritalia sacculifer (and Globorotalia not Globoritalia).

The origin of the taxonomic error seems to stem from another Science article, Permanent El Niño-Like Conditions During the Pliocene Warm Period by Michael Wara and colleagues, published in 2005. They write:

To track changes in the mean thermocline depth at the EEP site, we used ∆δ18O, the difference in δ18O between surface-dwelling Globoritalia sacculifer (without sac) and G. tumida (355 to 425 mm) (Fig. 2A), which occupies the base of the photic zone.

The error also seems to have propagated into a Journal of Climate article by Carl Wunsch (where Globigerinoides sacculifer is written correctly but Globorotalia tumida is written as Globoritalia tumida) and an article by Guodong Ji and others published in Geophysical Research Letters.

Though Globoritalia sacculifer was probably the result of an innocuous spelling oversight that somehow made its way past editing and though this error has no bearing on the scientific results contained therein, I find it very impressive that it has made its way into Science magazine, a highly prestigious journal, not once but twice over the last nine years.

Academia Bizarro: Apocalypse π

There’s a potential Ig-Nobel worthy paper on arXiv that was submitted earlier this month: A Ballistic Monte Carlo Approximation of π. The authors use a Mossberg 500 pump-action shotgun and an aluminium foil to calculate π with a Monte Carlo approximation. What is their motivation, you ask? From the introduction:

The ratio between a circle’s circumference and its diameter, named π, is a mathematical constant of crucial importance to science, yet most scientists rely on pre-computed approximations of π for their research. This is problematic, because scientific progress relies on information that will very likely disappear in case of a cataclysmic event, such as a zombie apocalypse. In such case, scientific progress might even stop entirely. This motivates the need for a robust, yet easily applicable method to estimate π.

How cool is that! Thanks to Gail Gutowski for the tip.