Microfossils: for the 19th or 21st century?

Microfossils: the word alone is enough to send many people to sleep. Who would study them? Aren’t these today what they were in the 19^th century: a nice eccentric hobby for rich men with lots of time on their hands? And of no practical use?

A page from one of the reports from the Challenger expedition (1872-76), depicting sea floor sediments which contain many foraminifera. The report on foraminifera is still used as reference material; as recent as in 1994, another edition was published.

 

Microfossils have taught us much of what we know about the world we live in and how it changes. Consider, for instance, an archaeological excavation, and how pollen tell us of the vegetation at the time the site was occupied. But microfossils can do much more: an example that can illustrate the amazing amounts of information they contain is provided by foraminifera. Foraminifera are a group of unicellular organisms that live in the sea. Some species live floating around in the water column (planktonic foraminifera) and some live on or in the sediments on the sea floor (benthic foraminifera). They build exoskeletons (also called tests or shells), made of either calcite or material they find around (a bit like some fly larvae; some have been coerced into making their protection from gold; see pictures here). Their name means “Bearers of holes”, as their calcite skeleton tends to show pores. They are found almost everywhere in the world ocean. There are thousands of species of foraminifera, and they all have their own living preferences. When they die, their bodies decay rapidly, but their skeletons can be preserved for hundreds of millions of years. And the physical and chemical composition of the (calcitic) tests reflects the sea water they were formed in. So if you stick a core into the sediments on the sea floor, you’ll have a column of little measuring devices in neat chronological order. 

 

A Melonis barleeanum, clearly showing the pores that gave this group of organisms their name

 

So how do these tiny snail-like things record anything? The distribution of the various species already provides a wealth of information. If from the bottom of the core up you first find cold water species, and then increasing numbers of warm water species, most of which first being those that prefer fresh organic material as food, and then later species who don’t mind their food having gone off massively, you already know quite something about the changes that happened where you took your core. And some species have very convenient properties: for example, Globigerinoides ruber generally forms a bog-standard white calcitic tests, but in warm water (>~25°C) it can make pink ones too. And Neogloboquadrina pachyderma can coil in two directions; in water between freezing point and ~5°C it coils almost exclusively anticlockwise, and above ~10°C it coils largely clockwise. So one glance through the microscope can sometimes tell you the approximate temperature of the sea water the creatures lived in. 

 Globigerinoides ruber in both the white and the pink version. White: picture by C. de Vargas. Pink: from foraminifera.eu

The amount of information you can tap into increases greatly if you study the calcite the tests are made of. The oxygen in the CaCO₃ provides a lot of clues: Oxygen has several stable isotopes, of which the most common is ¹⁶O at ~99.7% of all oxygen on Earth. Another stable oxygen isotope that is often used in palaeoclimate studies is ¹⁸O. These isotopes have slightly different physical properties. As ¹⁸O is heavier, water molecules containing it tend to evaporate from liquid water with a bit more difficulty, and when they do, they rain out of the atmosphere a little bit easier. It also leads to ¹⁶O preferentially evaporating from the ocean and precipitating on the continents, which means the high latitude ice sheets are relatively enriched in ¹⁶O. Measuring the ¹⁸O/¹⁶O (δ¹⁸O) ratio in a series of foraminifera that span hundreds of thousands of years will thus reveal the familiar see-saw pattern of glacials and interglacials, which is also found in the CO₂ and Deuterium records from ice cores. In this indirect way, ¹⁸O can be used to date records based on foraminifera tests. 

Some glacial-interglacial cycles. The Epica and Vostok graphs show temperature reconstructions based on Deuterium measurements from two ice cores, and the bottom graph is a reconstruction of global ice volume based on ¹⁸O records of benthic foraminifera. Vostok record from Petit et al., Nature 1999. Epica record: EPICA project members, Nature 2004. Ice volume record: Lisiecki and Raymo, Paleoceanography 2005.

More information is stored in the chemical composition of the calcite; it does not consist of pure CaCO₃, as some of the Ca²⁺ atoms are replaced by other bivalent cations, such as Mg²⁺ atoms. As incorporating atoms of the “wrong” size in a crystal lattice requires extra energy, this occurs relatively often in warm environments, due to the ample availability of thermal energy. The ratio of Mg to Ca in a calcitic shell therefore is a measure for past sea water temperatures.

These measurements can be performed on many tests at the same time; this will provide a measure of average conditions. They can also be measured individually, which gives information about only the lifespan of the individual organism. And it is even possible to blast the tests with laser beams, and measure the CaCO₃ that then evaporates. (A nice picture and explanation can be found here.) That will give the evolution of the temperature of the water the creature has lived in through the course of its life. 

 

The sort of machine with which one determines the Mg/Ca ratio in foraminifera tests

Given how specific one can target what to measure, a wide range of oceanic conditions can be reconstructed. For instance, a researcher who wants to reconstruct the stratification of the ocean could measure the properties of the tests of a shallow dwelling species and a deeper living species. When the signals resemble each other, the water masses were thoroughly mixed. Large differences point to well-developed stratification. 

You want to know how strong the circumpolar current, which flows all around Antarctica, and keeps the continent thermally insulated, has been through time? Drill a core where it currently flows, one core at its furthest reach, and one core just outside it reach. And then check the amount of polar and temperate species in all three through time.

 The Antarctic Circumpolar Current

Do you want to know how strong the Gulf Stream was? This current brings warm, salty water to the North Atlantic, where it cools down, and sinks to the bottom. Drill a core somewhere in its path, and check the temperatures as documented in both the surface-dwelling and bottom-dwelling foraminifera. If the difference is large, the Gulf Stream was likely strong. 

Do you want to know how strong the Indian monsoon has been through time? The summer and winter monsoon seasons both have a cooling effect on the sea surface temperatures; the stronger the monsoon, the colder the sea water gets. And each monsoon season has its own distinctive foraminifera assemblages, so these temperatures can be reconstructed independently. If you measure single specimens, you will also catch the few individuals that live in the very warm water between the monsoon seasons.

All of these examples of course come with their caveats. The reconstructions you can make of these features are not perfect, but if you corroborate them with other measurements you can paint a very detailed picture of how countless many aspects of how the world works. And if you know what it does, you may be able to find out why it does it, and that may tell you what to expect for the future. So even though staring down a microscope staring at small things that look a bit like miniature cauliflower might feel very 19^th century, it might actually be part of cutting-edge modern research!

 Picture: Edal Anton Lefterov

Mass extinctions

Ask a person to name a mass extinction event, and chances are very high they’ll come up with the event that eradicated the dinosaurs. Whether the cause was a meteorite impact or not; 65 million years ago, in what is commonly known as the K-T event, a large group of charismatic megafauna vanished. And not just them; an estimated 76% of species (at maximum) got extinct. And that number earns it a place among the “Big Five” extinction events. 

 

T. rex; probably the most famous victim of the K-T event

Why am I writing about mass extinctions now? About three weeks ago, mass extinction expert Richard Twitchett held his inaugural lecture at Plymouth University. He gave us some insight into how studying the predicaments of ancient ecosystems during mass extinction events can give us some insight into what the modern ecosystems will face in the future. (Do read an article that appeared in the Age on Richard’s work here.)

Biodiversity and extinction since the Cambrian

The K-T event is the most high-profile mass extinction event, but is it also the biggest? The most relevant? The first it certainly isn’t: that honour befalls what’s often called the late Permian ent, which happened ~250 million years ago. No less than 90-95% of species is estimated to have gone extinct. Why? Nobody knows for sure, but many fingers point at epic scale volcanism in Siberia. Fissure eruptions, so big the lavas covered an area the size of Europe, spit out large amounts of greenhouse gases, raising global temperatures. Then add continental weathering, melting of gas hydrates, nutrients ending up in the ocean, ocean warming, and reduced ocean circulation, and you have a maelstrom of interconnected environmental disasters, and, le voila: a mass extinction. 

 The extent of the Siberian Traps; the lavas that erupted ~250 million years ago

So does that mean the late Permian extinction even is the most relevant? That question isn’t really answerable. But I offer another candidate. What about the 6^th big one? Look out of the window. Right there, an extinction event is taking place. Think of the dodo, the great auk, the Tasmanian tiger, the golden toad, the quagga, the Cape lion. And for every species of charismatic megafauna there are numerous more obscure species that have vanished, many of which have never even been festooned with a common name. Spare a thought for the Lake Pedder earthworm, the American Chestnut Moth, Blackburns weevil, the Passenger Pigeon and its companion, the Passenger Pigeon Mite; the Hawaii Chaff Flower, the arcuate pearly mussel, the Rubious Cave Amphipod… Only in hindsight will we know if this is the start of a mass extinction event.

 

A great auk (picture: Mike Pennington)

Comparing modern extinction rates with fossil ones is fraught with difficulty. In the fossil record, you need to fossilise to stand and be counted. Hence the name, indeed. Nobody really knows how many non-fossilising species there have been. And we don’t know how many species there are today. How many have already gone extinct before ever having been discovered? And of many species that have been described, we don’t really know if they are still there or not. And then, of course, it’s not just the rate that counts; it’s also the duration of the event. And if you would assume that sufficient knowledge on taxonomy to be able to say anything useful about extinction rates starts with Linnaeus, that leaves us with less than 300 years of data. We do know that the rate of extinctions we experience now is estimated to be several orders of magnitude higher than the background extinction rate. And we know that if we go on like this, we’ll have made ourselves our very own mass extinction event in a few centuries to a few thousand years. If things get worse we might manage in even less time. Which would make it the fastest mass extinction event in the Earth’s history. Behold man, truly the master of creation!

Sea level project re-aligned

We’re midway. Are we on schedule? For 1.5 years, researchers all over the UK have been trying to get a grip on interglacial sea level changes, within the iGlass project (official link). We’d like to know if sea level during periods of low polar ice cover fluctuates in general, and if so, how much then. Sea level is currently rising; how much faster can we expect it to go? Geological data suggests the fastest rates of sea level occur when there is loads of ice; quite as one would expect. But just the fact we don’t expect rates of several metres per century doesn’t mean we can sit back and relax. So we do not. And the time had come to see how far we had come. 

All researchers involved in the project from the various institutes gathered in Southampton, where our coordinator was based. And in a meeting room with a view on the very sea, we brought each other up to date. There is one team trying to constrain interglacial sea level changes using stable isotopes in foraminifera from the Red Sea (how they manage that is a complicated story – that merits a blog post in itself). A team in Oxford was trying to use dripstone formations for this purpose (same there!). We had been coring around in Norfolk and Cambridgeshire, looking for marine microfossils in sediments, to do our bit. And all that adventure was complemented by the sturdy attempts of several teams of data gatherers and modellers: the former would compile all information already available in literature so as to not have to do double work. And the latter would try to understand the distribution of land ice at the time intervals for which we had sea level data. They would also try to get a grip on the depression and subsequent bouncing back up of the Earth’s crust due to fluctuations in that ice; you can use that to detect where large masses of ice have appeared and disappeared (related to the process described here), and thus point to a specific ice mass as a culprit if you find a big sea level change in your data. 

 

The view from our meeting room

So what, other than just being kept in the loop, is the use of getting together? Well, science isn’t a linear process. A project never works out exactly as it was set out in the beginning. And one needs to adapt to such changes. 

We were faced with several changes relating to the people involved: five of us would move to a different institute than we started out in at the beginning of the project. Two go abroad; out of reach of our UKfunding agency, so we needed replacements. One of us would even leave science altogether. 

More detailed issues that needed to be discussed were for instance which interglacial periods (there are many; for practical reasons, we limit ourselves to the last five) we will focus on, and which time intervals within these periods. The previous interglacial (~125.000 years ago) is a favourable one, for the simple reason it is the most recent, so it’s best documented in the sedimentary archive. Three interglacials back (~400.000 years ago) is a special one too; the Sun and the Earth were configured in practically the same way as they are now (quite unlike in the previous interglacial), and it was a very long one; we decided prioritise these two. And the time slices the modellers will target are the ones for which the data gatherers have found the most information. 

 The home base of the project: the National Oceanography Centre in Southampton

Another issue to be discussed was that we, the micropalaeontological team, had only budgeted for fieldwork in the UK, but we had found out about much more promising sediments in the USA. Should we shift some money around so we could chase these up? It was decided we would. An exciting prospect opened up! 

Two days of presentations, discussions, and a nice dinner at the end of the first day later, we all dispersed again. We were all singing from the same hymn sheet again, and the music on it has been brought up to date with where our data has taken us!