Stratigraphy, Sedimentology and Palaeontology

Stratigraphy, Sedimentology and Palaeontology

Glacial grooves from the Laurentide Ice Sheet (Québec, Canada)

These impressive glacial grooves observed along the North Shore of the St. Lawrence Estuary (Québec, Canada) were carved into the crystalline bedrock by the Laurentide Ice Sheet.
The grooves mark the basement of a complex sedimentary system known as the Tadoussac Delta, lying at the mouth of the Saguenay Fjord and intimately tied to the Late Pleistocene-Early Holocene deglaciation of the area.
The exact glacial dynamics that created these straight regular marks has still to be constrained. For more information on the formative processes and the general context, keep an eye on a forthcoming paper by Lajeunesse et al…
These grooves were visited this summer during a fantastic field trip of the ISC2018: “Landforms, sedimentary facies and stratigraphic architecture of a deglacial, forced-regressive context: the Québec North Shore” by Pierre Dietrich, Patrick Lajeunesse and Jean-François Ghienne.

The microworld of the past

In my last blog, I described the diverse world of pollen and how palynology – the study pollen, is used in geosciences. Today, I turn to another microcosmos: that of finest layers deposited at the bottom of a lake.

A large majority of geoscientists would tell you the best part of their job is field work. Despite sometimes harsh weather conditions, long hikes in wind, rain or merciless sun, many geologists enjoy the outdoors and look forward to a great rock exposure. While I very much enjoy field work myself, some of my current research brings me to the core lab at best. Here, at the University of Aberdeen, we have access to a 400 m long core of rock, which was drilled 10 years ago with the help of the Open University and University of Kiev in the Ukraine – a core drilled in the centre of the so-called Boltysh meteorite impact crater.

Part of the core recovered by drilling ancient lake sediments in the Boltysh meteorite impact crater, Ukraine.

The Boltysh crater formed about 65 My ago, which brings it in the same age range as the more prominent Chicxulub impact, which by some scientists is considered as the cause for the extinction of dinosaurs. The Boltysh crater, however, probably formed a few thousand years prior to the Chicxulub impact, and is of much smaller size. What makes this crater so interesting for us is that shortly after its formation, a lake began to form at the crater bottom, slowly filling up the entire crater with finely layered lake sediments over a period of nearly 1 million years. Importantly, these sediments cover the first 500.000 years of the earliest Palaeogene age (=Danian), which is known for its unstable climatic conditions. Understanding how climate and environment changed in the past is a crucial step in better understanding modern climate change and its short- and long-term impact on Earth.

Modern lakes typically produce very fine layers of debris at the bottom, often less than 1 mm thick. These layers, sometimes referred to as varves, may vary from year to year, and even from season to season, similar to concentric tree rings.  Ancient lakes and their deposits show very similar lamination of sediment. That way, both modern and ancient lake sediments may give detailed insights into how environment and climate changed over a few years or decades – and this even from many million years ago!

Slide scanner image of very fine lamination formed during deposition of sediments at lake bottom, due to seasonal changes in sediment supply and composition (Boltysh meteorite impact crater).

The core recovered from the Boltysh meteorite impact crater reveals lamination as fine as 0.08 mm. Individual layers – or laminae, therefore need to be examined with a high-resolution microscope or core scanner to distinguish slight changes in mineral, chemical and grain size composition, as well as fossil flora and fauna content. These information are used to reconstruct how environment and ultimately climate changed through geologically short periods of time. Although 65 million years old, this core contains valuable information on how Earth responded to climate change in the past – which helps to better predict changes of our environment in modern and future times.

Plant fossil (here part of a conifer), Boltysh meteorite impact crater. Core width c. 8 cm.

Slimy Landscapes

When we think about how our landscapes evolve and change we probably tend to go straight for the big stuff, things like the climate, tectonics or geology, and in my previous blog I looked at how rainfall contributes to changing the planet’s surface. However, it is not just the big stuff which controls these changes, smaller things do as well, for example changes in land cover (ie, from grasses to trees, or the other way round) results in changes in the patterns of erosion and deposition, not just in the areas where the land cover changed but also further along a system.

There are also several examples of how animals can influence the processes driving landscape changes too. In Yellowstone Park, USA, the hydrological conditions in the area changed when the main predator, the grey wolf, disappeared from the area causing a knock-on effect on grazers removing sapling trees which in turn stops woodland reaching maturity resulting in a change in land cover. To try and reverse this the grey wolf was reintroduced but so far has not been successful returning the system to its former state. Recently it has been shown how spawning Pacific salmon can make localised changes to river channels and increase the ability of flows to erode the river, impacting the whole river profile.

But let’s get even smaller, right down to the smallest living things on Earth. Surely things like bacteria cannot influence how landscapes evolve over long periods of time? Well, think again. Serena Teasdale is an Early Career Researcher who works with me at the University of Hull, and she is researching how bacteria can change the erosion rates of shorelines.

“Often when we consider the role of biology, our initial assumption is that of vegetation.  However, studies have begun to highlight the fundamental importance of EPS [technical way of saying slime, more on this later – Chris] on driving sediment behaviour, notably stabilising sediment and enhancing the erosion threshold.  This occurs as the EPS matrix, secreted by microorganisms, forms bridging structures between sediment particles which prevent them from moving freely” she tells me.

Serena has developed a really neat demonstration to show the impact this slime can have on the resilience of sediment – she uses sandcastles. It’s simple but highly effective. One sandcastle is made using normal sand and a second has been mixed with xanthan gum, a proxy for this slime and it is commonly used as a binding agent in fat free mayonnaise. She pours water from a watering can over both and times how long it takes for them to fall apart – it is clear that the one full of slime takes much longer to collapse.

This slime is actually called Extracellular Polymeric Substances (EPS) and whilst this might sound like something made up for the Ghostbusters’ franchise it exerts huge influence over landforms. Physical experiments have shown that mixing sand and mud with different volumes of the xanthan gum proxy, and you only need a tiny amount, can shift river bed landforms from dunes, to ripples, to flat beds. It means our current models of interpreting past environments from buried bed forms might be wrong as these have not considered the effects of the bacterial slime.

The slime therefore can impact changes in sand castles and on river beds, but what about the evolution of landscapes? I spoke to another Early Career colleague, Josh Johnson, about his research –

“The extra stickiness causes deltas to form with fewer distributaries but these are larger and more stable.”

Comparisons of two deltas – both the same discharge but the right has low cohesion (stickiness) and the left high.

Josh has created an interactive display using animations of his modelling to show the impacts of river discharges and ‘stickiness’ on the developing shape and channel networks on deltas. In it you can choose low, medium and high levels of both and it shows you the result. Above we can see a low stickiness and high stickiness delta for medium discharges.

So next time you are out in the pristine landscape and considering just how it came to be, don’t just dwell on the big stuff, it might be all because of bacteria snot. Isn’t nature beautiful?

You wouldn’t go in the early basement during the upper afternoon, don’t you?

I remember it perfectly. It was 13 years ago, while writing my first manuscript, I was first confronted with that thing that challenges a lot of junior stratigraphers, especially when they are not a native English: Geochronology vs. Chronostratigraphy! Or to simplify, how to properly distinguish time and time-rock units in your writings.

Several papers have been published on this subject, out of which I would recommend the recent Zalasiewicz et al. (2013). But, although these papers do provide a very clear scientific explanation on this subject, I always remember myself 13 years ago and how difficult it was for me back in those days to understand these concepts!

So, is it that difficult? Or is there an easy way to spot the light at the end of the tunnel? Well, if you have spotted the oddness in the title of this blog, the good news is that you have already made 3/4 of the way. Of course, the trick is in the use of early/late vs. lower/upper, that is to say on the distinction between time (geochronology) and space (chronostratigraphy), respectively.

Let’s have a little test to check that. Which of the following five sentences are wrong?
1. The late Bajocian is 500m-thick in this region.
2. The upper Bajocian can be correlated throughout this region.
3. The lower Bajocian has experienced environmental changes.
4. A carbon cycle perturbation occurred during the early Bajocian.
5. The lower Bajocian carbon isotope excursion.

Without having too much suspense: it’s the sentences 1 and 3 that are wrong.
In sentence #1, the term “late” is used whereas the sentence makes reference to time-rock unit, i.e. chronostratigraphy. Here, Bajocian refers to the thickness of the sedimentary sequence, so one should use the term “upper”. However, if you insist on using “late”, then you should write the sentence as following: “The thickness of the sedimentary succession dated from the late Bajocian is 500m”.
In sentence #3, we have the opposite case, i.e. the use of “lower” while referring to time, i.e. geochronology. It is as odd as “having a meeting in the upper afternoon”.

There you go, you have done 3/4 of the way. What about the last quarter? The answer is in sentence #5. It is indeed correct, and refers to the carbon isotope excursion you have measured in the Lower Bajocian, i.e. in the section you have worked on. But writing “the early Bajocian carbon isotope excursion” is also correct, but this time it refers to the carbon cycle perturbation that has occurred during the early Bajocian. You know… the one that is recorded in the lower Bajocian! But never say that it is recorded in the early Bajocian, that would be wrong.

Zalasiewicz, J., Cita, M.B., Hilgen, F.J., Pratt, B.R., Strasser, A., Thierry, J. and Weissert, H. (2013) Chronostratigraphy and geochronology: A proposed realignment. GSA Today 23, 4–8.

Author: Dr. Stéphane Bodin, University of Aarhus