While we waiting for our very first core, I thought I would introduce you to some of the labs we have on the ship. One of most favorite labs is the sedimentology lab. This is where the core is described and scanned with a photospectrometer and is high resolution pictures are taken. The last ocean drilling expedition I was on back in 2000, I was a sedimentologist and thoroughly enjoyed being able to see the core.
My lab is the paleontology lab, the place that we study microfossils, the shells made by one-celled organisms.
On the left side is where the palynologist (looks for pollen). You might wonder why we should study pollen while in the middle of the ocean? Well, pollen grains are often blown by the wind and can be carried far out to sea where they will fall and then get buried in the ocean sediments for us to recover much later. The pollen can tell us about what plants were living on land during any time period and since plants are sensitive to climate changes, these little pollen grains can tell us a great deal about how climate on the land changes. Behind that station is Kathy and Matsao, who study diatoms. Diatoms make their own food, like plants and are one-celled creatures that can live almost anywhere in the ocean or fresh water bodies within the photic zone (the region in the water column that light can penetrate). They are quite small, being usually less than one tenth of a millimeter in size. What we look at are their tiny shells they make out of silica (glass or quartz is made of silica) and so their shells are transparent and glass like. They are excellent for telling us about the paleo-environment as well as telling us the age of the sediment they were deposited in.
On the right is yours truly, with my microscope behind me. I will be looking at foraminifers through a high powered bi-ocular microscope.
These are also one-celled creatures that look like little amoebas, but make beautiful little shells out of calcium carbonate, the same material that seashells are made of. They do not make their own food but consume other organisms (so they are more like us). They can live anywhere in the water column from floating near the surface (called planktonic foraminifers) or on or just below the sea bottom (called benthic foraminifers). The planktonic species typically exist over relatively short time intervals so identifying the individual species can provide us with relatively precise age dates. We can do this as we look at samples through the core going up section and identify when we see the first occurrence and last occurrence. The lowest occurrence that we see a species is when it originated and the highest occurrence we see a species in the core is when it became extinct. Since the ages of originations and extinction has already been determined, I can place tight age constraints on when the sediments were deposited at any time interval.
In contrast, the foraminifers that live on the sea bottom or within the sediments, called benthic foraminifers, are extremely sensitive to environmental changes. This includes nutrients in the water, oxygen levels, sediment type, which are all related to water depths. Therefore, in the shallow waters, benthic foraminiferal are extremely good at constraining water depths. For the shallow water sites at Wilkes Land, the benthic foraminifers will be able to provide accurate water depth changes, which are related to the size of the ice sheet on the Antarctic continent. For example, as the ice sheet grows, water will be sequestered in the ice, thereby reducing the water in the ocean, which results in sea level to fall. In contrast, when the ice melts, the water will return to the ocean, resulting in sea level to rise. At the deeper sites, the benthic foraminifers will be able to provide evidence for changes in the deep water circulation patterns as each water mass that moves as great currents along the bottom of the ocean basins will have different oxygen and nutrient levels, which the benthic foraminifers are sensitive to. For example, different species will inhabit the ocean bottom depending on the oxygen and nutrient levels at the sea bottom.
Some very cute foraminfers that live on the sea bottom.
However, the most important aspect of foraminifers is that we can do chemistry on their shells. For example, the stable isotope ratios of oxygen (oxygen-16 and oxygen-18) in the shells of foraminifers are related to the temperature that the calcite is formed in (water temperature) as well as the isotopic value of the seawater. In fact, the most complete and continuous record of overall climate change for the last 100 million years are the oxygen isotope records from benthic foraminifers (See figure below). The isotopic value of the seawater is a function of how much evaporation and precipitation has occurred for that water mass and also how much ice is locked up at the poles. How can the oxygen isotopes from some little tiny shell made by a one-celled creature living on the bottom of the ocean tell us about how big the glacier was at the pole, you might be wondering?
This figure shows on the general climate patterns of the last 70 million years. Overall the warmest period was between 55 and 52 million years ago, when tropical conditions reached the Arctic Circle and Antarctica was believed to be ice free. A slow cooling is indicated by the increasing oxygen isotopc values from 50 to 34 million years ago, when an abrupt increase occurs at 34 million years ago, which is due to the expansion of the ice sheets in Antarctica from very small to at least as large as today. The extremely high value of today shows that the last two million years has been the coldest the Earth has seen in over 100 million years. However, as we put more and more CO2 into the atmosphere, we may return to former hothouse world conditions.
Foraminifers make their shells out of calcium carbonate, in which the chemical formula is Ca CO3. The O3 means that there are three oxygen atoms in each calcium carbonate molecule. This is important, as there are two stable isotopes of oxygen, with an atomic weight of 16 and one with 18. The difference between the two is that the oxygen 18 has two additional neutrons. This is important as when water evaporates from the ocean, the oxygen-16 preferentially evaporate more easily. So as water evaporates, water molecules are enriched in oxygen 16 leaving the oxygen 18 behind. Additionally, when the water condenses out of clouds as rain or snow, the oxygen 18 rain/ snow out more readily, leaving the clouds even further enriched in oxygen 16 (this make the oxygen isotope value more negative). By the time the moisture reaches the high latitudes, they have become many times more enriched in oxygen 16 than the seawater they were originally evaporated from. So when the snow falls at high latitudes forming large ice sheets, the snow is highly enriched in oxygen 16 often 20 to 50 times more than sea water (so the oxygen isotopic value of the ice becomes very negative, see figure below). When the ice sheets grow they sequester highly ice highly enriched in oxygen 16. This results in the oceans to become more enriched in oxygen 18 (making the oxygen isotopic vlaue of the water more positive).
By putting the shells of the foraminifers into a mass spectrometer, we can determine the ratio of oxygen 16 to oxygen 18 and thereby place constrains on the size of the ice sheet. However, it is not quite that easy as the ratio of oxygen 16 to 18 is also affected by the water temperature that the shell forms. As the waters warm, more of the oxygen 16 are incorporated in the shells (making the isotopic value lower). So there is always the problem of having two variables and only one equation (one type of data). The good side to this is as the climate cools, generally water temperatures cool as well as ice sheets become larger.
Additionally, there is a relatively new method that can estimate water temperatures by determining the ratio of two elements, magnesium and calcium obtained from the shells of foraminifers. While this new methods is not perfect (e.g., problems with diagensis and dissolution of the shells, estimating past concentrations of magnesium and calcium in the oceans), it has provided bottom water temperatures so we could make better estimates of ice volume changes going back tens of millions of years ago.