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A paleoecological study on the demise
of a pine forest in Den Treek, Central
Netherlands
Figure 1: Preserved tree on Den Treek property (Cultural Heritage Agency Amersfoort)
Author: Marc Duvivié
Location: Amsterdam
Date: June 21, 2017
Supervisors: Dr. B. van Geel & Dr. K. Rijsdijk
Bachelor Thesis
2 Abstract
In Den Treek property in the Province of Utrecht, Central Netherlands, a 13.000 years old, well preserved pine forest has been discovered, dating from the transition of the Bølling-Allerød interstadial to the Younger Dryas stadial. The demise of the forest occurred during an abrupt and intense cooling of the climate. A plausible cause for this cooling could be the abrupt supply of continental melt water from North-America to the Atlantic ocean, causing a stagnation of the thermohaline circulation. Another cause, although less likely, could be an extra-terrestrial impact releasing particles in the atmosphere and consequently blocking light and heat from the sun. Detailed studies of the forest can deliver valuable scientific information about causes and effects of climatic shifts on ecosystems. The present study is mainly focused on the analysis of microfossils, which explains how the climatic shift changed the pine forest ecosystem. Studies on the organic matter and charcoal content in the sediment cores are conducted and play a supportive role in the analysis. At the transition the total amount of microfossils decreased significantly, especially Pinus pollen, due to the colder conditions, while the pollen of other species which are well adapted to colder conditions, like Betula nana, became more dominant. It can be concluded that the climatic shift had a dramatic effect on the pine forest, drastically changing the species composition. The trees were preserved after a short phase of peat growth and subsequent soil erosion in the region, leading to cover sand deposition at the sampling site.
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Table of contents
Introduction ... 4
Methods and Data ... 6
Methods ... 6
Data ... 7
Results …….………..7
Discussion ………..9
Conclusion .………..………11
Acknowledgements ….……….12
References .……….………13
Appendices ……..………15
4 Introduction
A well-preserved pine forest has been discovered in Den Treek property in the province of Utrecht, Central Netherlands. The pine trunks date from the transition period of the Bølling-Allerød interstadial to the Younger Dryas stadial, around 12,900 years ago. This period is characterized by an abrupt and intense cooling of the climate.The sudden cooling in Europe caused the demise of forests. The mean temperature in July during the Younger Dryas declined below 10 degrees Celsius, which is the minimum temperature on which the trees can survive (Birks et al., 2005).
The cooling is most likely caused by the sudden supply of continental meltwater from North America to the North-Atlantic ocean causing a stagnation of the thermohaline circulation (Carlson, 2010; Broecker et al., 2010). According to this hypothesis the water on the surface of the ocean became too fresh to sink. The sudden supply of meltwater was caused by a dramatic overflow of Lake Agassiz, the biggest lake in the world at that time. Geomorphic and sedimentological evidence, like canyons, has been found dating from the
beginning of the Younger Dryas to support this hypothesis (Broecker, 2006). Fresh water is less dense than the salt water beneath it and therefore it will not sink and so-called deep water formation decreases. As a result the heat transport from the lower latitudes to the northern higher latitudes stagnated (Carlson, 2010). Furthermore, an extensive winter sea ice cover was formed on the North Atlantic ocean, reflecting the sunlight and as a result decreasing the temperatures (albedo effect). The North-Atlantic sea ice cover also prevented the release of heat from the ocean to the atmosphere. In addition, the wind currents changed from a western path to a more northern path bringing cold air from higher latitudes , further decreasing the
temperatures in Europe (Broecker, 2010).
Another possible cause for the sudden cooling, although less likely, could be an extra-terrestrial impact (Firestone et al., 2007; Kennett et al., 2009). A meteorite or comet, whether it was an impact or an explosion above the surface, may have had a significant impact on the climate, causing abrupt cooling. The particles released in the air after an impact would have blocked the sunlight and prevented the warmth from reaching the earth, causing a sudden temperature drop. Evidence of impacts coinciding with the transition period have been found in sediment cores. At many sites charcoal has been found in sediment cores dating from the transition period. Some studies see this as evidence of a meteorite impact, because after such an impact large-scale forest fires occur (Firestone et al., 2007). Other evidence found in sediment samples, such as iridium or nanodiamonds, could also indicate or provide evidence for an extra-terrestrial impact.
There is still a lot of debate among scientist about the possible cause of the climate cooling event. Often evidence of studies cannot be reproduced by more recent studies. The extra-terrestrial cause hypothesis has been contested by most recent studies and is therefore less likely to be true (van Hoesel et al., 2011).
According to van der Hammen and van Geel (2008) and Marlon et al. (2008), large-scale forest fires were mainly caused by climate change resulting in the death of trees, contradicting the hypothesis of an impact (Firestone, 2007; Kennett, 2009). The dead wood formed fuel for large-scale forest fires, as indicated by charcoal found in sediment cores. Furthermore, according to van Hoesel (2011), the evidence provided by Firestone (2007) were not reproducible as the evidence, such as the nanodiamonds, proofed to be dating from a period later than the transition period. The last word on the cause of the Younger Dryas event is definitely not yet spoken.
Prior to the Younger Dryas event the landscape in the Netherlands was mostly stable with abundant
vegetation cover. The world-wide cooling of the climate resulted in more dynamic and barren landscapes in the Northern Hemisphere, with open sandy soils where aeolian processes such as wind erosion became more important (van Geel et al., 1989). Quick sedimentation of sand caused by wind erosion reduced the vegetation
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growth because plants did not have the time to grow. The pine trunks fell down in a peat layer and subsequently cover sand was deposited on top of the peat. This explains why the pine forest has been
preserved so well. As a result of the barren landscapes, more dust was released into the atmosphere (Cronin, 1999). The Ca2+ (ppb) dust particles as measured in Greenland ice cores started to increase significantly during
the transition period (Fig. 2). Therefore Ca2+ (ppb) is a proxy of climate change. Another proxy for climate
change is the oxygen isotope ratio. 18O has two neutrons more than 16O and is therefore heavier and
evaporates slightly less readily. During glacial periods, when the water temperatures of the oceans were colder, relatively more 16O was evaporated and released from the ocean into the atmosphere in comparison
to the heavier 18O. The water molecules (H
2O) in the atmosphere contain the 16O and 18O isotopes. These
water molecules condensate at higher latitudes where it is colder and precipitate on the ice sheets. Ice cores containing a high amount of 16O give an indication that the cores originate from a colder period.
Figure 2: Ca2+ and oxygen isotope ratios as proxies of climate change. When the oxygen isotope ratio becomes more negative and
the Ca2+ increases, the temperature decreases. In this figure GI-1 (Greenland Interstadial-1) is the Bølling-Allerødand GS-1
(Greenland Stadial-1) is the Younger Dryas. At the transition a clear increase in Ca2+ and decrease in the oxygen isotope ratio is
visible, indicating a decrease of the temperatures. After Rasmussen et al. (2014).
The discovery of the sand-covered pine forest of Den Treek is a rare occasion. Only a few comparable forest deposits have been found across Europe and therefore the Den Treek site can deliver valuable scientific information about the cause and effects of the climate shift at the start of the Younger Dryas. There is
currently a gap in the dendrochronological master curve around the transition period. A dendrochronological study of the pine trunks could bridge this gap with additional radiocarbon dating.
The Treek study concerns a relevant topic in today’s society as climate change is a major topic world-wide. The world is currently dealing with the effects of global warming. However, a climatic cooling event like the
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one that happened during the start of the Younger Dryas could occur again. Perhaps global warming could even induce this event because of melting of continental ice, like at the end of the Bølling-Allerød interstadial. The aim of present study is to describe how the pine forest in Den Treek property came to a sudden demise with the help of a detailed microfossil analysis. This will be done by conducting a microscopic study and analysis of pollen and spores from a series of samples from the later Bølling-Allerød interstadial to the early part of the Younger Dryas stadial. The primary question this study will attempt to answer is: What were the effects of the abrupt and intense natural cooling on the pine forest ecosystem in Den Treek property? The microfossil analysis may help to specify the changing species composition at the sampling site during the transition period, and to find any further significant effects of the abrupt climate shift.
Methods and Data
Methods
Firstly, literature research has been done to provide the required knowledge and to get known with the subject. Secondly, a microscopic study on pollen and spores was conducted to specify the changing species composition and to get an idea about the effects of climate change on the Treek forest.
From a particular sample series (sample box Treek-71), 30 microscopic slides were prepared. Each microscopic slide corresponds with a certain depth level and thus with a particular time period. Pollen were identified and analysed. The various pollen types were recorded for each microscopic slide. Tablets containing 18583
Lycopodium spores were added to the subsamples to give an indication about the pollen concentration. The
pollen concentration was calculated by multiplying the total lycopodium spores per tablet (18583) with the recorded amount of pollen, and subsequently divided by the recorded number of Lycopodium spores. The outcome of this sum was then divided by 0.3, as each slide contained 0.3 cc of sediment. Some slides were rich in pollen while the pollen concentration of other slides were rather low. If a slide had a pollen count of less than 250 (excluding the added Lycopodium spores), these slides were not analysed because such samples would hamper statistically sound conclusions. The recordings were converted to percentages by dividing the recordings of each pollen of a sample by the total amount of pollen (excluding Lycopodium) of the same sample. The pollen concentrations and percentages were visualized in a microfossil diagram. Such a diagram shows how the populations of taxa changed during the studied time interval, while changing accumulation rates may also strongly influence the pollen concentrations. The percentages of the different microfossil species are on the X-axis and the depth on the Y-axis. The total concentrations of microfossils (excluding the added Lycopodium spores) are also shown in the diagram to visualize when the total amount of species started decreasing. The microfossil diagram has been created with the help of the C2 program.
The microfossil study will be compared with a loss-on-ignition (LOI) study performed by W.Z. Hoek (Utrecht University). Loss-on-ignition results may show how the vegetation and the local sedimentation changed from a stable situation with abundant organic matter in the soil to an unstable phase with increased quantities of inorganic matter. For LOI measurements sediment samples are heated for 4 hours at a temperature of 550 degrees Celsius. The sample is weighted before the burning and after the burning. The organic matter burns away and the inorganic matter remains. Therefore the weight difference is the burned organic matter. The higher the weight difference the more organic matter was present in the deposit. The organic matter content has been visualized in a diagram (Fig. 5, available from W.Z. Hoek), showing the organic matter weight
percentages.
Furthermore, a microscopic study on charcoal has been conducted. Charcoal particles could give an indication about the occurrence of forest fires and perhaps about the question when the pine trees died. The charcoal
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has not been counted like the microfossils but semi-quantitative estimations were made. The amount of charcoal was estimated in five different categories, from none to many particles. Based on this charcoal study perhaps no firm conclusion can be made, because the method is not according to the conventions for charcoal studies, but the Treek charcoal record plays as a supportive role, while this project is mainly focused on pollen and spores.
Data
Data that have been used are the pollen recordings for site Treek-69 (van Geel, in prep.), and the organic matter content diagram from W.Z. Hoek (in prep.). The pollen recordings from Treek-69 are rough data
(pollen and spore records in numbers). To create a microfossil diagram the recordings need to be converted to percentages.
Results
Based on the microfossil data of core Treek-71 some major changes in the species composition are visible (Fig. 3). In the lower depths, up to 235 mm, the species composition and the pollen concentration remain
somewhat stable (Fig. 3., zone a). Pinus and Betula are the dominant species at these depths. What stands out in this zone is a local peak of Salix at a depth of 260 mm. At a depth of 235 mm the concentration of pollen, especially Pinus pollen (at a depth of 230 mm), starts decreasing (Fig. 3, zone b). The pollen concentration declines even further at higher depths up to 120 mm (Fig. 3, zone c). What stands out in the upper zone of the core is the changing pollen composition. Some species become more dominant in this zone, such as Betula nana, Artemisia and Ericales, while especially Pinus almost completely disappears.
Figure 3: Microfossil percentage diagram of Treek-71.
Like Treek 71, Treek 69 also displays a changing species composition (Fig. 4). The lower depths of Treek-69 are characterized by a low pollen concentration, with a relatively low Pinus percentage (Fig. 4, zone a). The
dominant species in these depths are Betula, Poaceae, Cyperaceae and Selaginella selaginoides. This is followed by an increase in Pinus and thereby an increase of the pollen concentration at a depth of 230 mm (Fig. 4, zone b). At a depth of 220 to 160 mm the species composition is somewhat stable, there are some fluctuations but no significant changes (Fig. 4, zone c). At a depth of 160 mm there is a significant decrease of
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the pollen concentration is very low and Pinus pollen grains mostly disappear (Fig. 4, zone e). In this zone
Betula nana, Artemisia, Poaceae, Cyperaceae and Ericales become more dominant.
Figure 4: Microfossil percentage diagram of Treek-69. Data obtained from B. van Geel (in prep.).
The organic matter content in sediment core Treek-71 starts decreasing significantly from a depth of 200 mm. At a depth of 200 mm the organic matter content is almost 60% and decreases to almost 0% at a depth of 120 mm (Fig 4). The organic matter content in sediment core Treek-69 starts decreasing at a depth of 140 mm.
Figure 5: Percentage loss-on-ignition (4hrs at 550 °C) of Treek-69 and Treek-71 (from W.Z. Hoek, in prep.).
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mm the charcoals starts decreasing until there is almost no charcoal left at a depth of 215 mm. At a depth of 190 mm there is suddenly a lot of charcoal present in the sediment again (Fig. 6).
Figure 6: Charcoal diagram for core Treek-71. Categories from none to many charcoal particles in the microfossil slides.
Discussion
In the Treek-71 core, at the lower depths up to 235 mm, the species composition and pollen concentration remain stable (Fig. 3, zone a). The local peak of Salix (willows) is what stands out in the lower zone. Salix could indicate a initially wet environment at the particular depth. A climatic shift would change the species
composition significantly, therefore, in the lower zone of core Treek-71, the Younger Dryas cooling event probably did not occur yet as the situation is stable. Therefore, the lower zone of core Treek-71 probably corresponds with the Bølling-Allerød interstadial, characterized by a stable landscape with abundant vegetation, especially pine trees.
At a depth of 235 mm the pollen concentration and especially Pinus (from a depth of 230 mm) in the Treek-71 core starts decreasing significantly (Fig. 3, zone b). It is plausible that the transition period and start of the Younger Dryas cooling event occurred at this depth. The disappearance of the (pine) trees could indicate a cooling of the climate as trees die due to the cooler temperatures. The significant decrease of the pollen concentration further reinforces this hypothesis.
At a depth of 185 mm and higher (Fig. 3, zone c) there is almost no Pinus left and the pollen concentration is significantly lower compared to the lower depths. The changed species composition and reduced pollen concentration in this upper zone of core Treek-71 indicates a cold climate and a bare dynamic landscape with almost no vegetation and sandy sediment on the sampling site. Wind erosion starts to play a role and quick sand deposition reduces the time for vegetation to grow. The pollen records display that in the upper zone shrubs like the pioneers Betula nana, Artemisia and Ericales become more dominant, and are the first to colonize this changed dynamic landscape. These taxa are better adapted to the changed environmental conditions, in contrast to Pinus which mostly disappears due to the colder conditions. Based on these results, at the higher depths the climate has probably fully transitioned from the Bølling-Allerød interstadial to the Younger Dryas stadial.
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When comparing the results of core Treek-71 with core Treek-69, similar trends are visible. There are however some major differences between these cores. In core Treek-69 the lower depths, up to 230 mm (Fig. 4, zone a), are characterized by a relatively low Pinus percentage and pollen concentration, similar to the upper zone of core Treek-71 (Fig. 3, zone c). Betula nana, Poaceae, Cyperaceae and Selaginella selagnoides are more dominant at these depths, indicating a colder climate. At the depths above, up to 220 mm (Fig. 4, zone b), the pollen recordings display a significant increase in Pinus and a small increase of the pollen concentration. Pollen of Betula nana, Poaceae, Cyperaceae and spores of Selaginella selaginoides mostly disappear. This zone could indicate the transition from a stadial to the Bølling-Allerød interstadial and the beginning phase of a forming pine forest. The zone above, from a depth of 220 up to 160 mm, displays a more stable situation (Fig. 4, zone c). The Pinus and pollen concentrations fluctuate but remain around the same value. This zone is comparable with the lower depths of Treek-71 (Fig. 3, zone a), indicating a decent possibility that this zone corresponds with the Pinus-phase of the Bølling-Allerød interstadial as the stable and relatively warm
conditions of this period are suitable for vegetation growth. At the depths from 160 to 150 mm (Fig. 4, zone d) the Pinus pollen and pollen concentrations decline, comparable to the middle depths of Treek-71 (Fig. 3, zone b), indicating the start of the transition period. However, the decline of pollen in core Treek-69 is much more abrupt and intense. The upper depths of core Treek-69 (Fig. 4, zone e) are comparable with the upper depths of core Treek-71 (Fig. 3, zone c), indicating the Younger Dryas stadial and the end of the transition period. Based on the results it could be concluded that the depths between both cores are different and do not correspond. It seems that Treek-69 contains a longer time span compared to Treek-71 as the recordings of Treek-69 display part of the ‘pre-Pinus phase’ of the Bølling-Allerød interstadial. This is partly the case because Treek-69 has a slightly higher range, from 105 to 170 mm, in contrast to the range of Treek-71, from 120 to 175 mm. However, a more viable reason could be the local environmental conditions causing differences in the accumulation rate of sediment between both cores. A slower accumulation rate means that the particular depths contain a longer time span. Therefore, core Treek-69 probably had local conditions in which the accumulation rate of sediment was slower than in core Treek-71. To further support this hypothesis the demise of Pinus in Treek-69 is more abrupt and intense compared to Treek-71, but the length of the transition period for both cores should be the same. The faster accumulation in Treek-69 explains why the demise is more abrupt, as the depths probably contain a shorter time span compared to Treek-71 (Fig. 4). As a results the pollen recordings might show a more abrupt and intense increase or decrease in certain pollen or concentrations than really is the case, due to the accumulation rate differences.
The microfossil recordings of Treek-69 and Treek-71 are compared with the organic matter content study by Hoek (Fig. 5) and display the same trends. The pollen concentration and organic matter content in the soil are related, because a high amount of pollen indicates abundant vegetation and therefore a high organic matter content. The depths with a low organic matter content contain a high amount of inorganic material, mainly sand, which supports the hypothesis that the higher depths correspond with the Younger Dryas stadial (Fig. 5). The lower depths of core Treek-71 display a more stable situation (Fig. 5), further supporting the hypothesis that these depths correspond with the Bølling-Allerød interstadial. At a depth of 200 mm the organic matter starts decreasing and at a depth of 140 mm there is almost no organic matter left, indicating the transition period.
The organic matter contents of core Treek-69 also display the same trend as its microfossil recordings counterpart. The lower depths contain low organic matter, indicating a stadial before the Bølling-Allerød interstadial (Fig. 5). The organic matter starts increasing at a depth of 180 mm which corresponds to the transition of the stadial to the Bølling-Allerød. From the depths of 170 to 140 mm the organic matter remains stable and high indicating that these depths correspond with the Bølling-Allerød. The organic matter
decreases significantly at a depth of 140 mm and at a depth of 120 mm there is almost no organic matter left, displaying the Younger Dryas cooling event.
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The major differences between the organic matter content and the pollen concentration is that the depths of the climatic shifts do not correspond. The decrease in organic matter stars at a higher depth and occurs therefore later in comparison to the microfossil recordings. This could be due to the good preservation of organic matter well after the vegetation dies, while the amount of pollen diminishes earlier. Furthermore, it is plausible that the organic matter temporarily increases when the pollen concentration decreases as
vegetation dies, as the above ground dead wood goes in to the soil increasing the organic matter in the sediment.
The amount of charcoal particles displays an increase at a depth of 190 mm in core Treek-71 (Fig. 6). At the same depth the organic matter decreases (Fig. 5). Perhaps there is a negative relation between the organic matter and amount of charcoal particles. As a result of forest fires the supply of carbon to the soil decreases as most carbon gets burned and is instead released to the atmosphere.
The results of the charcoal study can also give an indication on when the climatic shifts occurred. According to van Hoesel et al. (2011), a climatic shift is often accompanied by forest fires. At the lower depths of core Treek-71 the amount of charcoal remains stable up to a depth of 235 mm. There is abundant vegetation available to be burned. At a depth of 235, which corresponds with the depth on which the Pinus percentage decreases, the amount of charcoal starts decreasing to a level were almost no charcoal is left at a depth of 215 mm (Fig. 6). This is probably caused by the loss of vegetation which results in less biomass that can be burned. Subsequently, at a depth of 190 mm the charcoal starts increasing corresponding with the loss of organic matter. At this depth the climate changed and the environment was dryer, increasing the chance of forest fires. The remaining preserved dead wood formed fuel to this large-scale forest fires. At a depth of 180 mm the charcoal decreases probably because most of the wood is burned up.
The results of the present study answer the research question about the effects of the climate change during the transition period on the species composition. To give a more precise answer to the research question some research methods could be improved and further research is needed. However, due to the limited time that could be spent on present project the study is mainly focused on microfossils. An additional study on macrofossils could increase the quality and validity of the present study. The pollen record gives information about the regional vegetation because pollen can travel long distances by wind. Macrofossils give a more local indication of the vegetation as these fossils will not travel over long distances.
Furthermore, because this project is part of an interdisciplinary project, it could gain a lot of information from other disciplines working on this project. However, this study had to be finished early and therefore the results of other scientists, who did not finish, are not yet available. It would have been useful to obtain, for instance, the results of the dendrochronologist and the radiocarbon dating results of the pine trees. With such dating results the depths of the sediment core could be linked chronologically, and thus linked with the
Greenland ice core climate data. The dating results could also provide valuable information about the
accumulation rate of the sediment. This would certainly have contributed to the quality of the present study.
Conclusion
The abrupt and intense cooling of the climate during the transition period from the Bølling-Allerød interstadial to the Younger Dryas has caused a significant change and demise of the pine forest in Den Treek. This cooling event is either caused by an extra-terrestrial impact or due to the supply of cold meltwater from continental ice from North America to the Atlantic Ocean. Based on the results of the analysis on microfossils it can be concluded that the abrupt climate cooling has caused the demise of the pine forest in Den Treek. The
microfossil recordings and organic matter content (Fig. 3, 4 & 5) display a clear decrease in the amount of pine trees and the pollen concentration, and a changing species composition during the transition period. In the lower depths, corresponding to the Bølling-Allerød, the pine trees dominated. However in the higher depths
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the pine and birch trees steadily became less dominant. The ecosystem could not cope with the colder and drier conditions and changed from a stable forest with pine trees, to a dynamic open landscape where pioneers, like Betula nana, Ericales and Artemisia became more dominant.
Acknowledgements
This study is part of the interdisciplinary Treek project, supervised by the Cultural Heritage Agency in Amersfoort. Firstly, I would like to thank Dr. B. van Geel of the University of Amsterdam for supervising present project and providing the materials, literature and the microfossil recordings of Treek-69.
Furthermore, his knowledge on this subject has helped me a lot while performing present study. Secondly, I would like to thank Dr. W.Z. Hoek of the University of Utrecht for performing the loss-on-ignition
measurements and sharing the data. Lastly, I would like to thank Prof. Dr. J. Bazelmans for allowing me to attend the interdisciplinary meeting of Cultural Heritage Agency in Amersfoort and providing some valuable literature.
13 References
Beug, H.J. 2004. Leitfaden der Pollenbestimmung fur Mitteleuropa und angrenzende Gebiete. Publisher Verlag Friedrich Pfeil; 542 pp. 120 plates, 12 tables.
Birks, H.H., Kristensen, D.K., Dokken, T.M., et al. 2005. Exploratory comparisons of quantative temperature estimates over the last deglaciation in Norway and the Norwegian Sea. The Nordic Seas: An Integrated
Perspective Oceanography, Climatology, Biogeochemistry and Modelling, Geophysical Monograph Series; Vol. 158, pp. 341-355
Boyle, E.A. and Keigwin, L. 1987. North Atlantic Thermohaline circulation during the past 20,000 years linked to high-latitude surface temperature. Nature; Vol. 330, No. 6143, pp. 35-40.
Broecker, W.S., Denton, G.H., Edwards, R.L., et al. 2010. Putting the Younger Dryas cold event into context. Quartenary Science Review; Vol. 29, Issue 9, pp 1078-1081.
Broecker, W.S. 2006. Was the Younger Dryas triggered by a flood? Science; Vol. 312, pp. 1146-1148. Carlson, A.E. 2010. What caused the younger Dryas cold Event? Geology; Vol. 38, pp. 383-384.
Clarke, P.U., Pisias, N.G., Stocker, T.F. and Weaver, A.J. 2002. The role of thermohaline circulation in abrupt climate change. Nature; Vol. 415, pp. 863-869.
Cronin, T.M. 1999. Principles of Paleoclimatology. Columbia University Press.
van Geel, B., Coope, G.R. & van der Hammen, T. 1989. Paleoecology and stratigraphy of the Lateglacial type section at Usselo (the Netherlands). Review of Palaeobotany and Palynology 60; pp. 25-129.
Haynes Jr, C.V. 2008. Younger Dryas “black mats” and the Rancholabrean termination in North America. PNAS; Vol. 105 (18), pp. 6520-6525.
van der Hammen, T., & van Geel, B. (2008). Charcoal in soils of the Allerød-Younger Dryas transition were the result of natural fires and not necessarily the effect of an extra-terrestrial impact. Geologie & Mijnbouw; Vol. 87 (4), pp. 359-361.
Hoek, W.Z. 1997. Late-glacial and early Holocene climatic events and chronology of vegetation development in the Netherlands. Vegetation History and Archaeobotany 6; pp. 197-213.
Van Hoesel, A., Hoek, W.Z., Braadbaart, F., et al. 2011. Nanodiamonds and wildfire evidence in the Usselo horizon postdate the Allerød-Younger Dryas boundary. PNAS; Vol. 109, no. 20, pp. 7648-7653.
Kennett, D.J., Kennett, J.P., West, A., et al. 2009. Nanodiamonds in the Younger Dryas Boundary Sediment Layer. Science; Vol. 323, Issue 5910, pp. 94.
Marlon, J.R., Bartlein, P.J., Walsh, M.K., et al. 2008. Wildfire responses to abrupt climate change in North America. PNAS; Vol. 106, no 8, pp. 2519-2524.
Overholt, A.C. and Melott, A.L. 2013. Cosmogenic nuclide enhancement via deposition from long-period comets as a test of the Younger Dryas impact hypothesis. Earth and Planetary Science Letters; Volumes 377-378, pp. 55-61.
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Rasmussen, S.O., Bigler, M., Blokcley, S.P., et al. 2014. A stratigraphic framework for abrupt climatic changes during the last glacial period based on three synchronized Greenland ice-core records: refining and extending the INTIMATE event stratigraphy. Quarternary Science Reviews 106; pp. 14-28.
15 Appendices
Appendix A
16
Appendix B
