During Expedition 374, the moonpool was fully opened to lower the subsea camera system to the sea floor.
I have always heard about Archimedes’ principle and I know that it explains why heavy ships like the JOIDES Resolution (145 m long and 21 m wide) can float on the water without sinking. But, I didn’t think that it could be compatible with the presence of a hole like the moonpool in the bottom of the ship!
When I saw that opening the moonpool in the ship didn’t change anything, I was very confused.
“What? There is a hole in the ship and she doesn’t sink? How is it possible?”
Video music by Mark Teckenbrock. Video by Kim Kenny.
And again, I had to ask for some help from a physicist.
First of all, we must understand Archimedes’ principle. The reason why ships don’t sink is known as Archimedean buoyant force.
Archimedes’ principle states that the upward buoyant force that is applied on a body immersed in the water is equal to the weight of the body. The ship, because of her weight, tends to sink by gravity. A consequence of Archimedes’ principle is that the water under the ship applies a counter pressure on it. This counter pressure is the same amount as the pressure from the weight of the ship but it is in the opposite direction and prevents the ship from sinking!
Physicists say that the ship is in a hydrostatic balance state (it means that she floats….)
But why doesn’t the opening of the moonpool fill the ship with water?
The moonpool is an opening that is 6.7 m in diameter in the center of the ship that gives access to the water below. We opened it during Expedition 374 because we wanted to lower the camera system.
And…it surprised me (it wasn’t the first time I saw the opening of the moonpool, but it surprises me each time….), the level of the water doesn’t change!
Let’s imagine the same phenomena at a smaller scale! This is a little experiment to try at home. When you put a plastic straw in a glass, the water in your glass will rise in the straw but only until its top! The water will stop when the pressure inside the straw and at the surface of the water in your glass is the same! It’s a question of equilibrium!
It’s the same thing when you open the moonpool on the JR, but at a bigger scale!
When you open the moonpool, the water will start to enter it because it is under pressure under the ship. But as soon as the water reaches the same pressure (atmospheric pressure) that the water around the ship is at, the water will stop rising.
And don’t be afraid: no risk of sinking because the sides of the moon pool inside the ship extend up well above the waterline. Also, the moon pool doors are closed whenever the ship is moving or in rough seas…
Maybe next time we will open the moon pool a seal will come and say “hello”! 😊
Un matin nous partons, le cerveau plein de flamme,
Le cœur gros de rancune et de désirs amers,
Et nous allons, suivant le rythme de la lame,
Berçant notre infini sur le fini des mers.
Mais les vrais voyageurs sont ceux-là seuls qui partent
Pour partir, cœurs légers, semblables aux ballons,
De leur fatalité jamais ils ne s’écartent,
Et sans savoir pourquoi, disent toujours : Allons !
Amer savoir, celui qu’on tire du voyage !
Le monde, monotone et petit, aujourd’hui,
Hier, demain, toujours, nous fait voir notre image :
Une oasis d’horreur dans un désert d’ennui !
Does the grey stuff in the bowl look like common mud to you? Maybe for us but definitely not for our paleontologists! This is a very special part of the core, delivered quickly to their lab soon after the core reached the core deck. These sediments were in the core catcher, at the bottom of the core. They are the oldest sediments of each core that we recover and our paleontologists come at each “core on deck” call on the catwalk to receive their special gift. They must quickly determine the age of the sample, and relay that to the science team.
Tracking the age of sediments using microfossils?
Paleontologists will determine the age of sediments by using the microfossil assemblages contained within them. In order for this approach to work, the fossils used must be widespread geographically and the sediments must contain a lot of different species.
Let’s try to do the job of one of them, Giuseppe Cortese, our radiolarian micropaleontologist onboard during Expedition 374. In this hard mud, Giuseppe will find tiny glass skeletons of radiolarians and use the species found in the core catcher to give an estimation of the age of the sediments.
But, before putting ourselves in his shoes, we have to keep in mind that species are not unchanging. They appear, evolve and disappear. Some of them evolved slowly and have existed for long time periods. Therefore, these fossils don’t provide a very good estimate of the age of the sediment. Short-lived species make our micropaleontology team happy.
Giuseppe is looking for particular species which give very specific constraints on the ages. Take a look at some of the species that he found in a core catcher:
So now, it’s your time to play. By using this partial Radiolarian zonation below, you should be able to estimate an age for that core.
Need some clues?
The fact that we find the species Antarctissa cylindrica in the sediment tells us that the age of that sample is older than 0.64 Ma as this species went extinct at 0.64 Ma and therefore should not be found in sediment that is younger than that.
The presence of Helotholus vema gives is more helpful because this species only lived between 4.59 and 2.40 Ma.
The first appearance of Cyladophora davisiana is at 2.61 Ma. As we find this species in the same sediment sample as Helotholus vema, this means that we can narrow our time interval down to 2.61 to 2.40 Ma (around the end of the Pliocene to the early Pleistocene).
The first appearance of Desmospyris spongiosa is less well known (around 2.46 Ma), which is why it isn’t on the zonation chart above, but the approximate age of its evolution is coherent with our previous estimation.
Have you noticed that radiolarians are not alone in the slides? Some diatoms also hide in this sample!
Now, it’s time for David Harwood, our night shift diatom specialist, to play.
Will you be able to estimate the age of the sample using the fossil diatoms that he has found?
A few clues
Thalassiosira inura lived from 4.9 Ma to 2 Ma. David also found Thalassiosora vulnifica in this sample. This species allows us to limit the time interval of the sample to between 3.2 and 2.2 Ma.
Actinocyclus maccollumii provides an even better constraint on the age because its first appearance is at 2.8 Ma and its last appearance at 2.4 Ma, and because Actinocylcus fasciculatus first appeared (evolved) at 2.7 Ma, David can estimate a time range for this sample of 2.7 to 2.4 Ma.
So, according to Giusseppe and David, whose data agree, our core catcher sample is estimated to be 2.6-2.4 Ma, which means at the end of the Pliocene to early Pleistocene.
Why use so many species? Why is one short lived species not enough?
This is mainly because one species is not a good indicator of a former ecosystem. A single species could be reworked from older sediments?
Targeting the age of the cores is not the only skill of our paleontogists on board. They are also able to estimate former oceanic temperatures, for example, or whether the oceans were cold enough to freeze. Stay tuned if you want to know more about this topic!
Ice shelves are floating tongues of ice that extend from grounded glaciers on land. The place where the ice sheets touch the ocean floor is called the grounding line. The grounding line is the border between the floating ice shelf and the land-based ice sheet.
Ice shelves surround 75% of Antarctica’s coastline and they can be up to 2000 m thick. The Ross ice shelf is the largest one, and is a floating piece of ice that is the size of France.
Ice shelves gain mass from ice flowing into them from glaciers on land, from snow accumulation at their surface, and from the freezing of marine ice (sea water) to their undersides.
Ice shelves lose ice by intermittently calving (breaking off) of large icebergs and by melting from below (from relatively warm ocean currents) and from above (from warm air temperatures).
Why ice shelves are important?
Ice shelves are essential in the stability of the ice sheet because they act as buttress. By creating friction as their base, they hold back the glaciers that feed them and slowing the flow of ice to the ocean.
Today, many ice shelves around Antarctica are thinning and shrinking. During February 2002, 3250 km2 of Larsen B ice shelf (NW of the Weddell sea, west Antarctica) were lost by calving icebergs. It was the largest collapse of an ice shelf ever observed. You can see it here:
Long-term changing in environmental changes can break up the equilibrium between gain and loss in the ice shelf and lead to its collapse and consequently, to the destabilization of the ice sheet.
Before collapse, ice shelves first undergo a long period of thinning. Ice shelves thin at their base when an increase of incursion of relatively warm waters (Circumpolar Deep Water) is transported to the ice shelf base by ocean currents. The increase of temperature also causes meltwater at the surface. That water percolates through the ice shelf crevasses and because water is denser than ice, the meltwater forces it way down and acts as a wedge to push the crevasses apart far enough apart to form an iceberg. When this melt is widespread on an ice shelf, it can lead to a domino effect where many crevasses are filling with water at once and many icebergs calve very rapid – as happened in the Larsen Ice Shelf.
When the ice shelf begins to retreat, the butressing force that used to slow the glacier flow are not efficient anymore and the glacier speeds up on its way to the sea and its front calves rapidly. There is a positive feedback loop which increases the collapse of the ice sheet.
When icebergs melt, cold water is released into the ocean which has a significant impact on the formation of sea ice around Antarctica during winter. It can be surprising but an increase of amount of sea ice during the winter can indicate the vulnerability an ice shelf, or other important environmental changes in Antarctica .
ANDRILL core records collected a decade ago, reveal that the Ross sea Ice Shelf has already collapsed in the past, especially during the Pliocene (5-2.5 million years ago). During this time, the atmospheric CO2 concentration was close to the current level (~400ppm).
Expedition 374 aims to identify these previous ice sheet collapse events and link them to oceanic temperatures during these times with the aim to better understand how they collapsed (especially what is the threshold in oceanic temperatures which induces the collapse).
Having a better understanding of the past will help to know how our future will be.
They are fossilized remains of tiny organisms such as algae. The fossils are formed from the hard parts of the organisms. We call these hard parts a test, which is really just a tiny shell. They are generally between 0.001 mm and 1 mm in size so that their study requires the use of the microscope. To illustrate how tiny they are, 4 species of radiolarians are compared to the thickness of a strand of hair in the picture below.
When we drill into sediments deposited on the seafloor, the deeper we drill the older the sediments are. We can use the fossils to tell us the age because different species have appeared, evolved and become extinct during the past. The main goal of our micropaleontology team is to find and identify these fossils to determine the age of the sediments.
Why are we using microfossils?
Microfossils are especially valuable for determining the relative ages of marine rock layers for several reasons:
Their remains are frequently found in sediments.
They have been around for millions of years.
They show fairly continuous evolutionary development, so different species are found at different times.
They are abundant, widespread, and found in nearly all marine environments.
Finally, they are small and easy to collect, even from deep drill holes.
On board, our micropaleontologists are specialized in a different microfossil groups. Let me introduce these groups to you!
During Expedition 374, we have specialists for:
Radiolarians: These fossils are the glassy skeletal remains of a radiolarian, which is a single-celled animal-like organism. Their skeletons tend to have arm-like extensions that resemble spikes, which are used both to increase surface area for buoyancy and to capture prey. All radiolarians are planktonic (floating in the water), and move around by coasting along ocean currents. They are a very ancient group (going back to the Cambrian Period, 541–485 million years ago) and so they are very useful to date sediments.
Diatoms: These are protists (single-celled organisms) with a test (shell) made out of silica (glass), but they are usually much smaller that radiolarians. One of the main differences between diatoms and radiolarians is that diatoms are photosynthetic and consequently are restricted to the photic zone (water depths less than 100 m depending on clarity or the water). Both benthic and planktonic forms exist. Planktonic forms are free floating (like the radiolarians), whereas benthic forms live attached to something, such as the seafloor, kelp, etc.
Diatoms may occur in such large numbers and be well preserved enough to form sediments composed almost entirely of diatom tests: these sediments are called diatomites.
Foraminifers. These single-celled organisms have skeletons made of calcium carbonate (the mineral calcite) or particles that they find in the ocean and stick together to form a shell (called agglutinated). Most species have tests composed of multiple chambers that increase in size during growth. Most of the species live on or in the sand, mud, rocks, and plants at the bottom of the ocean (benthic) and the remainder are planktonic (living in the water column).
Dinoflagellates are mostly single-celled, although they sometimes lived together in chains. Their size is between ~20 and 200 μm. Their name derives from the Greek (in greek, “dino” mean whirling) as this the type of movement is characteristic of these organisms:
Some species are photosynthetic, others are exclusively heterotrophic and some can be both. As a consequence, they are prominent members of both the phytoplankton and the zooplankton of marine and freshwater ecosystems and are important in ice communities from Antarctica.
So, our micropaleontologists on board are the timelords of the ship (together with our paleomagnetists!). If you want to discover a few of their secrets, stay tuned for the next episode!
The Antarctic ice sheet is the main polar ice cap of the Earth and covers about 98% of the continent. About 61% of all the fresh water on Earth is held in this ice sheet which covers almost 14 million square kilometers.
However, the ice sheet which covers West Antarctica does not have the same behavior as the one which lies on East Antarctica.
This picture shows that the average ice thickness is different across the continent, with a much thinner ice sheet on West Antarctica (around 2 km) than on East Antarctica (around 3 km). East and West Antarctica are separated by the high Transantarctic Mountains (around 2000m high).
There is another major difference between these two ice sheets: the East Antarctic Ice Sheet is grounded largely above sea level, whereas the West Antarctic Ice Sheet is mostly grounded well below sea level. The WAIS is called a “marine ice sheet” and is therefore very sensitive to sea level rise and changes in ocean heat (and because water is more efficient than air at melting ice).
There is evidence that the WAIS has known events of collapse in the past when temperatures were higher than present (Pliocene and Miocene). ANDRILL core records show brief intervals of collapse during the Pliocene. But the records of the Miocene were less clear and thanks to Expedition 374, we will probably get more information of how the WAIS has behaved during the Miocene.
A total melting of the WAIS could trigger a rise in sea level of about 5 m compared to the actual sea level. But the isostatic rebound after the melting of the ice sheet could make up in part for that rise of sea level. Another crucial parameter to consider is the thermal expansion of ocean during warm periods, which tends to increase sea levels…
Past variations of the extension of the WAIS have to be linked to former sea levels to better understand what will happen in the future.
We nowadays have proof that some of the glaciers of the WAIS are thinning (the Pine Island Glacier for example). You can visualize the actual ice flow in the Antarctic ice sheet on this numerical model:
The white dots show how particles move with the ice, which are initially randomly distributed over the ice surface. The colors show the flow speed.
Global sea levels are currently rising at an average rate of 1.8 mm per year since 1961, and 3.1 mm per year since 1993. The main contributions for this rise are from melting glaciers and ice caps and thermal expansion of the ocean.
There are so many parameters to consider, and many ice sheet processes that are poorly understood, that it is difficult to predict how much the sea level will rise in the future. But the amount of ice that terminates in the ocean is definitely a key factor in the potential risk of future rapid retreat of the Antarctic ice sheet.