× Climate proxies are indirect indicators of past climate states. They allow us to draw conclusions about temperature, precipitation, and vegetation conditions in the absence of direct measurements.

 × It seems obvious that the percentage of a pollen type in a sediment layer correlates linearly with the abundance of the respective plant species. However, it is a bit more complicated.

The deposit of pollen in sediments (abundance and spatial distribution) depends on factors such as pollen weight, capacity for dispersal and distribution mechanism, and therefore depends on the plant species. For example, pollen from wind-pollinated plants (e.g. birch or pine) is deposited in a sediment in disproportionately high amounts, whereas insect-pollinated species (many herbs) are underrepresented.

As a consequence, the pollen percentage in a sediment does not represent the actual abundance of the respective plant species.

In a pollen spectrum, the percentage of pollen is shown which has to sum up to 100%. If a species is overrepresented (for example, birch due to high pollen production), the share of the other species is biased downward – even if their pollen production remains the same. This is referred to as the Fagerlind effect. The relation between pollen share and vegetation density can be distorted, which leads to a biased representation of the real plant density.

The problem is mitigated by:

• Using absolute pollen numbers (pollen flux densities) in order to circumvent the percentage dependency distortion
• Applying correction factors such like extended R values (ERV) in order to compensate for species-dependent different pollen production

Cf. Bradley, Raymond S. (2015): Paleoclimatology: Reconstructing Climates of the Quaternary, 3rd Edition, Academic Press, San Diego, CA, ISBN: 978-0-12-386913-5, p. 412.

 × Cf. Rösch, Manfred (2012): Vegetation und Waldnutzung im Nordschwarzwald während sechs Jahrtausenden anhand von Profundalkernen aus dem Herrenwieser See. In: *Standort Wald* 47, p. 43–64

https://epic.awi.de/id/eprint/36566/24/Roesch_2012.pdf

 × Dendrochronlogy is derived from Greek: dendron (for tree) and kronos (for time).

 × The chronology reaching furthest into the past that was created with this method is the Hohenheimer Jahrringkalender. It covers the time from 10.461 BC until today. This chronology os a project of the Institute for Botany at the University Hohenheim, Stuttgart.

https://www.scinexx.de/dossierartikel/der-aelteste-kalender-der-welt

For a detailed introduction into the topic, cf. Bradley, Raymond S. (2015): Paleoclimatology: Reconstructing Climates of the Quaternary, 3rd Edition, Academic Press, Oxford, Amsterdam, Waltham MA, San Diego, CA, ISBN: 978-0-12-386913-5, chapter 13.

 × Radioactive decay is a property of matter that is, to a large extent, independent of chemical processes and physical processes (heating, etc.). The energies involved in changes of the atomic nucleus are about 1 million times higher than the energies that play a role in chemical reactions and about 10 000 to 100 000 times higher than the energies that bind electrons to the atomic nucleus. The latter, which is orders of magnitude smaller than the atom (electron shell), therefore, is to a large extent decoupled from chemical processes. For that reason, systems with radionuclides (unstable, that means: radioactive nuclei) are suitable to be used as geological clocks.

 × In order to understand what an isotope is, let's take a short look at the atomic structure of matter. Atoms of the same 'sort' belong to the same chemical element, for example, carbon, sulphur, oxygen or nitrogen. An element has specific material properties (for example, the melting point) and chemical properties (the ability to enter into a chemical compound with another element). On an atomic scale, the chemical properties are bonding properties. Atoms can connect with other atoms (of the same or different element) and, that way, form larger bodies: molecules.

Molecules have completely different properties than their basic products. Therefore: Although only 94 elements exist in nature, the world contains many more different substances. For example: Two oxygen atoms can connect with one carbon atom to form a carbon dioxide molecule. At room temperature, oxygen is a colorless gas that humans need to breathe, and carbon is a solid substance. Carbon dioxide, however, at room temperature is gaseous.

Atoms are composed of a compact nucleus (that contains positively charged protons and electrically neutral neutrons) and a shell that contains the (negatively charged) electrons. Why such a structure is stable can only be understood when this system is treated in terms of quantum mechanics (which is outside the scope of this article).

The properties of the electronic shell determine the bonding properties of the atom. Therefore, the chemical element is determined by the number of electrons. This number equals the number of protons in the nucleus – otherwise, the atom wouldn't be electrically neutral. An atom of the element carbon has 6 protons (in the nucleus) and 6 electrons (in the shell). An element can have different 'variants' that differ in the number of neutrons. These variants are referred to as isotopes.

The word isotope is derived from Greek: isos means equal and topos means location. Isotopes are 'at the same location' within the periodic table of elements. Different isotopes of the same element differ in the structure of the atomic nucleus (they have a slightly different weight), but behave chemically largely the same.

The most abundant carbon isotope contains 6 protons and 6 neutrons, and has in total 12 nucleons. Therefore it is indicated by the following notation ${}^{12}\mathrm{C}$. To a small amount, in nature there is also the isotope ${}^{13}\mathrm{C}$, which has 7 instead of 6 neutrons (in total, 13 nucleons). Chemically, it behaves largely identically to ${}^{12}\mathrm{C}$, meaning it connects in the same way with oxygen to form carbon dioxide. The restrictive modifier 'largely' is added because different isotopes of the same element slightly differ in their weight (due to the different numbers of neutrons), by which certain chemical and physical processes go along with isotope fractionation. That means, considering chemical processes: The chemical reaction is the same but might happen at a different rate – depending on the involved isotope.

Isotope fractionation can also happen with physical processes. For example, the oxygen isotope ${}^{18}\mathrm{O}$ has a slightly higher weight than the isotope ${}^{16}\mathrm{O}$. Therefore, evaporation of water molecules containing ${}^{18}\mathrm{O}$ happens at a lower rate than of water molecules that contain ${}^{16}\mathrm{O}$. This is an important fact when looking in more detail into the topic of climate proxies in a later blogpost.

Cf. Lunine, Jonathan I. (2013): Earth: Evolution of a Habitable World, 2nd edition, Cambridge University Press, ISBN: 978-0521615198, p. 55

The following figure shows a small section of the periodic table with the elements carbon (C), nitrogen (N), and oxygen (O).

The figure also shows the isotopes of these three elements that exist in nature. The colors indicate that isotopes of the same element have the same chemical properties.

Two isotopes in the picture, ${}^{14}\mathrm{C}$ and ${}^{13}\mathrm{N}$, are unstable and decay over time into isotopes of other elements.

 × There are stable and unstable isotopes. Unstable isotopes are radioactive, that means, the atomic nucleus decays over time. In this process, the isotop transmutes into another isotope (in most cases, of another element).

The carbon isotope ${}^{14}\mathrm{C}$ has 8 neutrons and (like all carbon isotopes) 6 protons – that means, in total 14 nucleons. ${}^{14}\mathrm{C}$ is radioactive, which means its atomic nuclei decay with a certain probability and transmute into the nucleus of another element (in this case, the nitrogen isotope ${}^{14}\mathrm{N}$).

For a certain atomic nucleus, we cannot determine exactly when it decays. This can happen within a fraction of a second or take thousands or millions of years. However, for a large number of atoms, we can determine the time it takes for a certain amount to decay. The half-life is the time it takes for half of the nuclei to decay. The half-life is a property of the isotope. It depends on the isotope and can range from a fraction of a second to billions of years – depending on the isotope. This property is also independent of environmental factors such as pressure, temperature, or the chemical environment (i.e., the molecule in which the atom is incorporated).

Because of this, radionuclides are good candidates for use in geologic clocks. A clock, in general, is a system in which a certain change occurs at a well-known rate – in this case, the radioactive decay of one isotope into another. The isotope ${}^{14}\mathrm{C}$ has a half-life of about 5700 years and is suitable to be used as a geological clock for material with ages of up to tens of thousands of years.

Other radioactive isotopes with much longer half-lives – such as, for example ${}^{238}\mathrm{U}$ (half-life: 4.47 billion years) or ${}^{40}\mathrm{K}$ (half-life: 1.25 billion years) – can be used to date rock that is millions or billions of years old.

 × Through the nuclear reaction ${}^{14}\mathrm{N} + n \rightarrow {}^{14}\mathrm{C} + p$ (or shortly: $^{14}\mathrm{N}(n,p)^{14}\mathrm{C}$) a ${}^{14}\mathrm{N}$ nucleus is transmutated by a neutron (from cosmic radiation) into ${}^{14}\mathrm{C}$ by emitting one proton.

 × Through the nnuclear reaction ${}^{14}\mathrm{C} \rightarrow {}^{14}\mathrm{N} + e^- + \bar{\nu}_e$. This is also called a beta minus decay, where a neutron in the parent nucleus is transmutated to a proton.

 × For the discovery and development of this method, the American chemist Willard Libby (1908 – 1980) was granted the Nobel Prize for Chemistry in the year 1960. His Nobel lecture contains further details about the history of this discovery and the method.

https://www.nobelprize.org/uploads/2018/06/libby-lecture.pdf

To determine the amount of ${}^{14}\mathrm{C}$ still present in the dated material, there are different technologies. Libby directly measured the activity of ${}^{14}\mathrm{C}$ with a Geiger-Müller tube. Because of the low activity, for this method a lot of material is required, in order to get a measurable activity still for samples that are several thousands of years old. In addition, the tube needs to be shielded very well against other sources of natural radiation (for example, cosmic radiation).

A much more precise method to determine the amount of parent isotope in a sample is mass spectrometry. Using this method, isotope ratios can be measured directly.

 × The production of ${}^{14}\mathrm{C}$ requires further clarification:

The charged particles of cosmic radiation (primarily protons) generate secondary neutrons in the upper atmosphere through collisions with air molecules. These neutrons react with ${}^{14}\mathrm{N}$ to produce ${}^{14}\mathrm{C}$.

The intensity of cosmic radiation – and thus the number of available secondary neutrons – depends on several external factors.

The solar wind is a stream of particles ejected from the Sun. Since it contains charged particles (protons and electrons), it carries the interplanetary magnetic field with it. Cosmic radiation also contains charged particles that interact with the interplanetary magnetic field and are deflected by it. High solar activity leads to a stronger solar wind, which in turn increases the deflection of charged particles from cosmic radiation. This means less cosmic radiation reaches Earth. Therefore, during elevated solar activity, fewer secondary neutrons are available, causing the production rate of ${}^{14}\mathrm{C}$ to decrease.

Solar activity fluctuates in a cycle of approximately 11 years. A maximum, such as the one expected for 2024 – 2025, also results in a higher intensity of auroras. This 11-year cycle (also known as the Schwabe cycle) is superimposed by fluctuations in solar activity on larger timescales. Solar activity also varies with a period of 180 to 210 years (Suess or de Vries cycle) – a cycle that effectively modulates the 11-year cycle.

Changes in the Earth's magnetic field also influence the production rate of ${}^{14}\mathrm{C}$, as the Earth's magnetic field likewise shields against cosmic radiation.

 × There are also factors that have a direct influence on the concentration of ${}^{14}\mathrm{C}$ in the atmosphere.

Volcanic eruptions can have a direct influence on the atmospheric ${}^{14}\mathrm{C}$ concentration. The carbon dioxide released during a volcanic eruption comes from geological reservoirs in the Earth's crust or the Earth's mantle and, therefore, is free of ${}^{14}\mathrm{C}$. This is due to the fact that this carbon reservoir was separated for a long time from the atmospheric carbon reservoir (where continuously new ${}^{14}\mathrm{C}$ is produced) and, therefore, has already decayed.

A similar effect has the burning of fossil resources. The carbon emitted here is also nearly free of ${}^{14}\mathrm{C}$, and carbon dioxide from fossil fuels therefore dilutes the atmospheric ${}^{14}\mathrm{C}$ concentration.

Another anthropogenic, external factor is the influence on neutron flux caused by atmospheric nuclear weapons tests. These tests significantly increased the production rate and the atmospheric ${}^{14}\mathrm{C}$ concentration, creating what is known as the „bomb pulse“. While the vast majority of atmospheric tests were conducted by the United States, the Soviet Union, and the United Kingdom in the 1950s and early 1960s—and were banned for these nations by the 1963 Partial Test Ban Treaty—France continued atmospheric testing until 1974, and China conducted its last atmospheric test on October 16, 1980.

Cf. Graven, Heather and Keeling, Ralph F. and Rogelj, Joeri (2020): Changes to carbon isotopes in atmospheric CO2 over the industrial era and into the future, Global Biogeochemical Cycles, 34, e2019GB006170,

https://doi.org/10.1029/2019GB006170

As you know, the concentration of the the gas carbon dioxide continually increases since decades – as shown by the Keeling curve. Cf.

https://keelingcurve.ucsd.edu/

The next figure shows the development of the atmospheric concentration of the isotope ${}^{14}\mathrm{C}$ since 1850.

You can clearly see the peak, known as the „bomb pulse“, which was caused by above-ground nuclear tests. The most dramatic increase in ${}^{14}\mathrm{C}$ began in the 1950s as testing intensified, reaching its maximum around 1963–1965.

It is interesting to note that, apart from that peak, the ${}^{14}\mathrm{C}$ concentration has been continually decreasing for more than a century. This behavior can be explained by the fact that during this time, humans have been releasing carbon from old reservoirs into the atmosphere – namely, by burning fossil fuels.

The 'old' carbon released into the atmosphere had been decoupled from the atmospheric carbon reservoir for millions of years – because this carbon comes from plants that lived millions of years ago and were transformed into fossil fuels (coal, mineral oil) in the meantime. As discussed, this material no longer contains any measurable amount of ${}^{14}\mathrm{C}$ and, once released into the atmosphere, dilutes the atmospheric concentration of this isotope.

This is known as the ${}^{14}\mathrm{C}$ Suess effect, named after the chemist Hans Suess (1909 – 1993).

As more and more fossil carbon is released into the atmosphere, the concentration of ${}^{14}\mathrm{C}$ continually decreases, even though the absolute amount of carbon dioxide is constantly increasing.

Strictly speaking, carbon released during volcanic eruptions also originates from sources in which ${}^{14}\mathrm{C}$ has already decayed. However, volcanic eruptions are individual events that have not shown any systematic increase over the last century. For this reason, the decrease in atmospheric ${}^{14}\mathrm{C}$ concentration can only be explained by the anthropogenic release of carbon dioxide.

If you are looking for even more conclusive proof of the anthropogenic origin of increased carbon dioxide levels, you can examine the trend in atmospheric concentration of another carbon isotope, ${}^{13}\mathrm{C}$ (see also the study cited above). The concentration of this isotope has also been decreasing for many decades. This effect (the ${}^{13}\mathrm{C}$ Suess effect) can only be explained by isotope fractionation during photosynthesis – a process that slightly prefers ${}^{12}\mathrm{C}$. Consequently, fossil carbon dioxide (which ultimately originates from plants that lived millions of years ago) contains less ${}^{13}\mathrm{C}$ than atmospheric carbon dioxide. Volcanic carbon dioxide has not undergone such fractionation.

Therefore, the decrease in ${}^{13}\mathrm{C}$ is the strongest proof that the increased level of carbon dioxide in the atmosphere is caused by the burning of fossil fuels.

Returning to the radiocarbon method: All the discussed factors must be considered, and errors can be factored out using various methods. For example, dates obtained from ${}^{14}\mathrm{C}$ measurements can be calibrated using another dating method, such as dendrochronology. This is why studies using radiocarbon dating report different carbon ages: The conventional ${}^{14}\mathrm{C}$ age assumes that ${}^{14}\mathrm{C}$ production remained constant over the observation period. The calibrated carbon age is usually indicated as cal BP, where BP stands for 'Before Present'. In most cases, 'Present' refers to the year 1950, as the method was standardized in that year. Additionally, 1950 predates the systematic atmospheric nuclear testing that caused the bomb pulse.

 × An early study to date 'Ötzi' (using mass spectroscopy): Cf. Bonani, Georges and Ivy, Steven D. and Hajdas, Irka and Niklaus, Thomas R. and Suter, Martin (1994): AMS $^{14}$C Age Determinations of Tissue, Bone and Grass Samples from the Öttztal Ice Man}, Radiocarbon, Vol. 36, No. 2, 247–250, DOI: 10.1017/S0033822200040534, Cambridge University Press

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/91BC7943558A041387DEEDD6E4A82071/S0033822200040534a.pdf

Kutschera, Walter (2001), Radiocarbon Dating of the Iceman Ötzi with Accelerator Mass Spectrometry, VERA Laboratory, Institute for Isotope Research and Nuclear Physics, University of Vienna, Invited paper at the Workshop of the Nuclear Physics European Collaboration Committee (NuPECC) on Nuclear Science: Impact, Applications, Interactions, Dourdan, France, Special NuPECC Report (2001)

https://www2.chemistry.msu.edu/courses/cem485/lectures/icemanage.pdf

 × Thinking in terms of an open system (a living plant in exchange with the atmosphere) versus a closed system (a dead plant; exchange terminated) is very helpful when trying to understand how other geological clocks, based on the radioactive decay of other isotopes (for example, uranium), work.

Furthermore, regardless of the specific system, the functioning of a geological clock is always based on the fact that, from a certain point in time, a system becomes separated from a reservoir (which, until that moment, had provided an influx of the parent isotope). As soon as the system is separated from the reservoir, the geological 'clock' begins to 'tick'.

 × Palynological studies involve a significant amount of tedious manual work. After preparing the sample chemically, sections of lake sediment are observed under a microscope, and the different plant pollen grains are counted.

 × Silva (Latin) means forest; nigra means black.

 × Also other authors come to similar conclusions about the vegetation development and the history of human influence in the Black Forest. Cf. the article by Burkhard Frenzel as of p. 14 in Lorenz, Sönke (2001): Der Nordschwarzwald: Von der Wildnis zur Wachstumsregion, Markstein Verlag, Filderstadt, ISBN: 9783935129015

On the development of the tree line in the Black Forest, cf. Lang, Gerhard (2006): Late-glacial fluctuations of timberline in the Black Forest (SW Germany): A revised approach to a climatic reconstruction, Vegetation History and Archaeobotany 15, 373–375, DOI: 10.1007/s00334-006-0048-8

https://boris.unibe.ch/102033/1/2006_VegetHistArchaeobot_15_373

 × Vgl. Lorenz, Sönke (2001): Der Nordschwarzwald: Von der Wildnis zur Wachstumsregion, Markstein Verlag, Filderstadt, ISBN: 9783935129015, p. 14.

 × There are pollen archives that go back much further into the past. For example, in the peat bog Le Grand Pile in the southern Vosges, France, a pollen archive was discovered that represents a time range that covers the complete last glacial period including parts of the preceding interglacial (Eem interglacial). We talk here about a time range of 130 000 years.

Cf. Helmens, Karin F. (2013): The Last Interglacial-Glacial cycle (MIS 5-2) re-examined based on long proxy records from central and northern Europe, Department of Physical Geography and Quaternary Geology, Stockholm University, Svensk Kärnbränslehantering AB TR-13-02

https://skb.com/publication/2677502/TR-13-02.pdf

 × See the comment of the word 'misse' above.

 × In earlier centuries, no machines were available to process felled timber on site or to transport it. Consequently, people relied on the water of rivers and streams to move logs from higher mountain locations down to the valleys. In certain places, the stream water – or, as in this case, that of a small lake – was dammed up and then briefly released, using the force of the water to propel the wood toward the valley.

On maps of the Black Forest, one often encounters place names containing the element Schwallung (from the German anschwellen, meaning 'to swell up' or 'to surge').

The text from Simplicissimus suggests that this practice was also carried out at the Mummelsee.

In an article on the village chronology of Oberachern (a village in the Black Forest), there are hints that the Mummelsee was once dammed by Daniel Kückh in the 1730s in order to allow the transport of wood on the river Acher.

Cf.

Heimat- und Verschönerungsverein Oberachern e.V.: Flößerei auf der Acher: Ein Stück Regionalgeschichte

Grimmelshausen lived from 1622 – 1676, that means, he could not have any knowledge on this. However, it is known that timber rafting was practiced from the early medieval times on.

The earliest sources on this business go back to the 14th century. For example, on the emblem of the town of Gernsbach (in the northern Black Forest), from which prints are available since 1394, there is a five-leaved rose of the Dukes of Eberstein and below it two raft hooks – the tool of the raftsmen.

Cf. Lorenz, Sönke (2001): Der Nordschwarzwald: Von der Wildnis zur Wachstumsregion, Markstein Verlag, Filderstadt, ISBN: 9783935129015, p. 9

This means that the Mummelsee played a role in the timber business already 100 years before the measures described for Daniel Kückh.

Consequently, the text in Simplicissimus already hints at anthropogenic changes to the landscape that ultimately led to an overexploitation of nature. This stands in contrast to the fact that the old trade of timber rafting is usually portrayed in an idealistic and romantic way.

Another hint at the use of the Mummelsee in historical times is provided by the geologist Prof. Dr. Dieter Ortlam.

• From the Oberachern village book ... it is recorded that in the extremely dry summers of 1471 and 1534, the Mummelsee was used as a natural barrier lake to augment the low flow of the Acher through the following measures: the natural outflow was lowered by breaching the end moraine of this tarn. This ensured a constant flow through the Seebach and the Acher, which kept the vitally important mill wheels in the Acher valley operating.*

German text (translated by me):

Aus der Oberacherner Dorfbuch (W. Teichmann 1934, Stadtarchiv Achern) sind so extrem trockene Sommer in den Jahren 1471 und 1534 überliefert, dass der Mummelsee sogar als natürlicher Stausee zur Niedrigwasser-Aufhöhung der Acher genutzt wurde, indem man seinen natürlichen Abfluss über die Endmoräne dieses würmeiszeitlichen Karsees aufgrabend stetig tieferlegte (Abb. 3 und 4). Auf diese Weise war ein konstanter Zufluss über (!)die Seebach (siehe Situationsplan der Sägemühle Bürck, Seebach, von 1841, Abb. 5) und die Acher gewährleistet, um die vielen lebenswichtigen Mühlräder des Achertales in Betrieb zu halten.

Cf.

Der Ursprung der Acher (Nord-Schwarzwald) anhand historischer Recherchen sowie die glaziale Genese des Ruhesteins (Schwarzwaldhochstrasse)

 × Cf. Lorenz, Sönke (2001): Der Nordschwarzwald: Von der Wildnis zur Wachstumsregion, Markstein Verlag, Filderstadt, ISBN: 9783935129015, p. 97:

Enormous, almost unimaginable quantities of wood that were required to cover the energy need were transported during these times on the Murg river. The extreme usage through timber chute ('Scheiterschläge') and Dutch hewing method ('Holländerhiebe') led to an unequalled devastation of the forest.

German text (translated by me):

Es waren gewaltige, fast unvorstellbare Holzmengen, die zur Deckung des Energiebedarfes damals auf der Murg transportiert wurden. Doch haben die extreme Nutzung durch Scheiterschläge und Holländerhiebe zu einer Waldverwüstung ohnegleichen geführt.

'Scheiterschläge' means felling of firewood and 'Holländerhiebe' refers to the felling of large, high-quality trees for shipbuilding in the Netherlands.

The history of timber rafting in the Back Forest is also described in: Reinbolz, Andreas und Ludemann, Thomas (2004):, Wald- und Forstgeschichte im Schwarzwald, Forstliche Versuchs- und Forschungsanstalt Baden-Württemberg, Waldwissen.net

https://www.waldwissen.net/de/lernen-und-vermitteln/forstgeschichte/wald-und-forstgeschichte-im-schwarzwald

 × German text (translated by me):

Auch zu den Seen hat sich das Verhältnis der Menschen gewandelt. Noch im 17. Jahrhundert wurde der herrliche Ellbachsee zur Gewinnung von einigen Ar schlechter Wiese abgelassen und der Wildsee im 18. Jahrhundert als Schwellweiher für die Flößerei verwendet (...), ebenso der Buhlbachsee (...), wahrscheinlich stammen auch die Staumauern des Huzenbacher und Glaswaldsees aus dieser Zeit. Erst in diesem Jahrhundert sah man die Seen nicht nur vom wirtschaftlichen Standpunkt aus an; am Wildsee hat man ihn sogar ganz zurückgestellt. Auch der Buhlbachsee soll wieder aufgestaut werden, um Fremde anzuziehen. Bei langsamem Aufstauen könnte man auch den Ellbachsee wieder herstellen, ohne die Flora zu vernichten. Dieser Prototyp eines Kars mit seiner wuchtigen Schönheit verdient den Naturschutz unbedingt.

Cf. Fezer, Fritz (1957): Eiszeitliche Erscheinungen im nördlichen Schwarzwald, Selbstverlag der Bundesanstalt für Landeskunde, p. 54.

https://e-docs.geo-leo.de/entities/publication/2b5f9138-0e6a-4154-96a5-bde0923a75e6