David M. Kroeck, Micropalaeontologist/Palynologist

What is your favourite part about being a scientist and how did you get interested in  science in general? When I was young, I was, as many kids, particularly interested in dinosaurs and other  fossils. I liked nothing more than visiting a natural history museum marvelling at the  wonders of nature‘s past. And of course, I had a proper collection of dino toys. My primary  school teachers gifted me a small book about Earth history before I left, knowing very well  about this passion of mine. I suppose I just didn‘t grow out of this passion (Certain movies  by Spielberg might have played a part in it as well …). Thus, still aspiring to become a  palaeontologist, I registered in Bonn University for the geosciences Bachelors program in  2010, which I finished in 2013. I really enjoyed my studies there, so naturally I followed  up with the master‘s program that I finished in 2016. 

What interests me the most in sciences is the pursue of knowledge. To enhance our  knowledge by finding the natural coherence of things. Finding traces of what is yet hidden  in the dark, making hypotheses, searching for more clues, trying to see and understand  more and more. A great aspect in geosciences is field work. It is such a thrilling experience  to visit an outcrop and reconstruct the past, which is, for me, quite a lot like detective work.  Looking at all the little puzzle pieces of past ecosystems, such as fossils and  sedimentological features, then trying to put it all together into a bigger picture. Since I  was young I would read with excitement about the explorers of old times – Humboldt,  Darwin, Shackleton, Fawcett, and the like – dreaming of going on such expeditions myself  one day. Indeed, my studies brought me to many places, not seldom quite off of touristic  trails, and sometimes even a slight bit dangerous. It‘s as close to the travels of these past  explorers as I could have wished for. 

In laymen‘s terms, what do you do? My current research is focused on ancient marine organic-walled phytoplankton. Plankton  describes the organisms that float in the water column. Within the plankton we have  zooplankton and phytoplankton. The former are heterotroph, which means they need to  consume other organisms to gain energy, while the latter are autotroph, meaning they  obtain energy through photosynthesis, just like plants on land. In today‘s oceans we find  a variety of groups in the phytoplankton, such as diatoms, coccolithophores, green algae,  dinoflagellates and cyanobacteria. I am working on phytoplankton from the Palaeozoic, a  time interval dated roughly between 541 Million and 250 Million years ago. During this  time the phytoplankton was represented mostly by what we call acritarchs. So what are  acritarchs? I‘m not sure, actually. And that‘s why they are called acritarchs, as the name  means „uncertain origin“. We don‘t know the biological affinity of acritarchs, and they  surely belonged to a variety of groups, but most of them are interpreted to represent the  remains of phytoplanktic organisms, some of which might be related to today‘s dinoflagellates. 

So how can we study microscopic remains of organic-walled plankton that lived hundreds  of millions of years ago? Actually, these little things are quite resistant. In order to process  a rock sample for palynological analysis, we dissolve the rock in different acids. What  remains are organic-walled microfossils, so called palynomorphs, such as the acritarchs,  that we can study under a microscope. But what is so interesting in microscopic  organisms that were floating in the ancient seas? First, they help us to define the age of  sediment rocks. Many palynomorphs represent important index fossils, and thus, have a  stratigraphic value. Then, since phytoplankton is often bound to certain environmental  conditions, palynomorph assemblage analyses can help us reconstruct parameters, such  as water temperature, depth, or distance to land, during the time of the deposition of the  sediment: That is how the distribution of different taxa of phytoplankton can give us  valuable information about the palaeoenvironment. Another and major aspect of  phytoplankton is their photosynthetic activity. While often the continental forests are called  the „lungs of the Earth“, phytoplankton are responsible for 50–80 % of the production of  the oxygen in the atmosphere. Through their photosynthetic activity phytoplankton take  up great amounts of CO2 from the atmosphere. Large quantities of this carbon is then  stored in deeper parts of the ocean when phytoplankton die and sink to the seabed.  During the early Palaeozoic the importance of phytoplankton within the carbon cycle was  much bigger, since plants were yet to conquer the land. Another important aspect is the  fact that phytoplankton is at the base of marine food webs. For these reasons we assume  that changes in phytoplankton through time must have had an impact on both Earth‘s  climate and marine ecosystems. My studies aim to find correlations between biodiversity  changes of the phytoplankton and changes in different palaeo-environmental parameters,  such as temperature, atmospheric O2 and CO2 concentrations, sea level, and  palaeogeography.

Different acritarchs from the Ordovician of Columbia (from Kroeck, D. M., Pardo-Trujillo, A., Torres, A. P., Romero-Baéz, M., Servais, T., 2020. Peri-Gondwanan acritarchs from the Ordovician of the Llanos Orientales Basin, Colombia. Palynology 44, 419–432).

How does your research/goals/outreach contribute to the understanding of climate  change, evolution, paleontology, or to the betterment of society in general? While palaeontology is the study of past processes, it can be of great value for the present.  Awareness of climate change as a major global crisis has significantly increased in the  last decades. Its effects are already perceptible in many of the Earth’s ecosystems. It has  become an important task to estimate future consequences of the rapidly changing  climate. Palaeontological investigations provide an important tool for predicting processes  in changing environments by reconstructing past intercorrelations. Inversing the famous  quote of the Scottish geologist Sir Charles Lyell, “The present is the key to the past”, our  knowledge of processes in Earth history may help us to estimate future developments.  Several important extinction events are known, some of which are related to increases in  greenhouse gases. Thus, investigating biotic changes during these crucial time intervals  and comparing the results with recent developments is very important. I want to contribute  with my work to our understanding of today‘s profound changes in the biosphere caused  by human activities. 

If you are writing about your research: What are your data and how do you obtain your  data? In other words, is there a certain proxy you work with, a specific fossil group, preexisting datasets, etc.? During my Ph.D. project I mostly worked on a database of the Palaeozoic phytoplankton  comprising occurrence data from published literature including stratigraphic and  geographic information. We used this database to create diversity curves for the  Palaeozoic phytoplankton. But I also went on sampling trips myself, which is basically  taking rock samples from different stratigraphic layers. In the laboratory these samples  are being processed, generally by dissolving the rock in acids and sieving the residues.  Then palynological slides are being produced by distributing the sieved residues on glass  slides and embedding them in a clear medium. After, the samples are analysed under the  microscope. For some of my work I did morphometrics, which is measuring certain  parameters of microfossil specimens in larger population in order to statistically analyse  them. This can help assessing morphological variability and to review taxonomic  classifications. 

What advice do you have for aspiring scientists? Working in science can be frustrating at times. That‘s part of it, I suppose. Don‘t let it  discourage you. Follow your passion. Other than that, „Explore. Dream. Discover.“ – H.  Jackson Brown Jr.

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