CL
Climate: Past, Present & Future

Proxy of the Month

Magnetic minerals: storytellers of environmental and climatic conditions

Magnetic minerals: storytellers of environmental and climatic conditions
Name of proxy

Environmental Magnetism (also known as enviromagnetics)

Type of record

Environment and climate proxy

Paleoenvironment

Sedimentary environments (for the most part)

Period of time investigated

Present times to millions of years (depending on the preservation conditions)

How does it work?

Magnetism is a physical property that results from the behaviour of elementary particles in any substance. Depending on the chemical composition and distribution of elements within the material, different kinds of magnetism may result. In environmental magnetism mainly the strong magnetic iron minerals in samples are analyses to gain information on environmental processes and climatic conditions. These samples frequently originate from hard or soft rocks from land, caves, lakes, rivers, or oceans. However, the method can be used to monitor environmental pollution in dust, water, or sediments as well [1]. 

Figure 1. Simplified representation of the methodological approach.

The presence or absence of certain minerals in a sample and its properties (e.g. their physical appearance) are typical of specific environmental and climatic conditions (Fig. 1). This is the basic assumption of environmental magnetism. The minerals can be detected by modern equipment even when they are only present in trace amounts. The identification of the minerals is performed by a number of experimental procedures, which all focus on monitoring changes in magnetic properties while subjecting the sample material to different magnetic fields or temperatures. The resulting measurement signal always shows the behaviour of all magnetic components in the sample material. This signal can already be used as proxy for environmental changes and climate conditions. However, only successively performed data analyses allow to distinguish different kinds of magnetic particles by varying magnetic properties. To fully understand the palaeoclimatic and palaeoenvironmental information of the collected data, one needs the information on the components and must understand the processes that form, transform, or break down magnetic minerals. If magnetic minerals are extracted from sample material, they can be subjected to optical or chemical analysis. Thereby, information on the physical appearance of individual grains and their exact chemical composition can supplement the magnetic data.

What are the key findings that have been done using this proxy?

Magnetic analyses were used to unveil environmental conditions in numerous studies. A famous example is the analysis of air-blown sediments (loess) from China [2]. The study of a thick sequence of more than a 100 m shows an alternation of high and low magnetisation values, which correspond to colour changes from brownish to yellowish, respectively (Fig. 2). The brownish sediments were formed during moist and warm conditions, whereas the yellowish loess deposits were accumulated during cold and dry periods. The variation in magnetic properties results from the different processes associated to the formation of minerals in soils, which only take place in warm and moist climates. The occurrence of these newly formed minerals can be monitored by the magnetic susceptibility, which is a measure of a material’s ability to be magnetized. Thereby, the magnetic susceptibility of the Chinese loess is a palaeoclimatic proxy for variations in temperature and rainfall. This information was used for the reconstructions of the past atmospheric circulation pattern and the evolution of the Asian monsoon.

Figure 2: Illustration of the change of a bulk sediment property, using the example of susceptibility. No real data is shown.

In another example, environmental magnetism was applied to sediment cores from the Heidelberg Basin in Germany [3]. Because of the complex genesis of these fluvial deposits, the sediments are composed of a number of different magnetic minerals, which are all telling parts of the story of this region. To identify the different minerals, their individual magnetic signals were extracted from the overall magnetic signal by different very specific and time consuming analytical methods. Additionally, the physical conditions of the magnetic minerals were determined (e.g. grain shape). The combination of all results revealed the lower half of the investigated sediment cores to be deposited under Mediterranean climate conditions in which the groundwater table fluctuated, while the upper part was formed under cooler climates and stable groundwater conditions. Geological archives of the evolution of the Rhine River are rare and most methods fail to disclose details on the past climate conditions.  Here, environmental magnetism provides valuable information on the hydrological regime and the climatic conditions.

Taken together, environmental magnetism is a non-destructive method that is applicable in a number of geological settings. The strengths of the method are manifold. In some settings well constraint information can be gained by fast and non-destructive measurements (example one). In other geological settings information on climatic and environmental conditions is unveiled, when other methods fail to contribute any result (example two).

This post has been reviewed by the editorial board

References

[1] EVANS, M. E. & HELLER, F. (2003) Environmental Magnetism - Principles and Applications of Enviromagnetics, San Diego, Academic Press.

[2] HELLER, F. & TUNG‐SHENG, L. (1986) Palaeoclimatic and sedimentary history from magnetic susceptibility of loess in China. Geophysical Research Letters, 13, 1169-1172.

[3] SCHEIDT, S., EGLI, R., FREDERICHS, T., HAMBACH, U. & ROLF, C. (2017) A mineral magnetic characterization of the Plio-Pleistocene fluvial infill of the Heidelberg Basin (Germany). Geophysical Journal International, 210, 743-764.

How earthworms can help us understand past climates?

How earthworms can help us understand past climates?
Name of proxy

Earthworm calcite granules (ECG)

Type of record

Paleotemperature and paleoprecipitation reconstruction; radiocarbon dating

Paleoenvironment

Continental environments – loess/paleosol sequences

Period of time investigated

Mostly Last full Glacial cycle – from 112,000-15,000 years Before Present (BP) (or older depending on the preservation of the granules).

How does it work?

Earthworms are commonly found living in soil and feeding on organic matter at the soil surface. In carbonate soil, some of them secrete small granules (0.1 to 2 mm) within 20 cm of the soil surface (Fig. 1). These granules, composed of crystalline calcite, are formed in the calciferous glands of the common earthworm species Lumbricus (Fig. 1).

Figure 1. Formation and structure of earthworm calcite granules: A) Schema of the calciferous glands of Lumbricus terrestris (Canti, 1998; Darwin, 1881), B) Scanning Electron Microscopy of a fossil granule, modified from CoDEM/BATLAB C) Distribution of granules through present day experimental soil (Canti and Piearce, 2003), D) Thin section of a fossil granule (photo P. Antoine).

Fossil earthworm calcite granules (ECG) are common in various carbonate-rich Quaternary deposits and have been identified in loess-paleosol sequences in western Europe. Aeolian loess (i.e. accumulation of silt size sediment formed by the deposition of wind-blown dust) preserves evidence for climatic fluctuations in the past: generally, primary loess representing periglacial conditions coeval with expanded ice sheets alternates with tundra permafrost horizons and arctic soils representing milder climates.

Over the last glacial cycle (between 112-15 ky BP), the climate of the Earth varied on millennial timescales between cold (stadial) and temperate (interstadial) periods. This climate variability is reflected in the character of the loess sediment. These short-term climatic changes had a strong influence on landscapes, ecosystems, including human beings. However, loess sediment analysis only give us information on the relative changes of climates. We lack quantitative temperature and precipitation data to precisely reconstruct past conditions.

The granules of earthworms living in past loess environments provide a quantitative tool. The granule concentrations correlate with the nature of the loess sediment; paleosols preserve the highest concentrations while primary loess the lowest. These observations highlight a rapid response of the earthworm population to climatic variations suggesting milder climatic conditions during the formation of paleosol. ECG can be considered as a new paleoenvironmental proxy, capable of detecting rapid climatic events within the Last Glacial loess sequence. Furthermore, the chemistry of these ancient earthworms’ diets can be calibrated to the temperature and precipitation of the climate prevailing at the time.

What are the key findings that have been done using this proxy?

We developed a new method to calculate past temperatures and precipitations based on oxygen and carbon stable isotope compositions of earthworm granules from loess at the Nussloch site in the Rhine Valley, Germany. Our results provide the first quantitative past climate data from loess sediments.

Figure 2: First quantification of paleoclimate data in a loess sequence: Comparison between radiocarbon dating (Moine et al., 2017), granule concentration (Prud’homme et al., 2018b) and quantitative paleoclimate parameters (Tair and MAP, Prud’homme et al., 2016, 2018a) of the Nussloch loess sequence (Antoine et al., 2009) with the δ18O of Greenland ice core (NGRIP, Rasmussen et al., 2014).

Figure 2 shows the results stable isotope geochemistry of earthworm granules from selected strata within the loess sequence at Nussloch. Temperatures for the warmest months were estimated between 10 to 12°C and the mean annual precipitation was estimated between 250 and 400 mm during the formation of palaeosols. Our results suggest that the climate at Nussloch during the temperate periods (interstadials) were most likely subarctic with cool summers and very cold winters.

Earthworm granules can also be directly dated by radiocarbon methods (Moine et al., 2017). Since the nature of loess sediments reflects climatic variations over short (millennial) timescales, the lack of precise chronologies in loess can be a problem when trying to correlate with global climatic events. Our new approach, combining precise radiocarbon dating with quantitative climate reconstruction, represents a major advance for understanding climate in terrestrial regions.

Moreover, the radiocarbon chronology facilitates precise correlation between terrestrial sequences and ice core records. This is fundamental for understanding teleconnections between mid- and high-latitude climate changes, as well as the spatial and temporal impact on prehistoric populations in Europe.

This post has been reviewed by the editor.

References

Antoine, P., Rousseau, D.D., Moine, O., Kunesch, S., Hatté, C., Lang, A., Tissoux, H. & Zöller, L. (2009) Rapid and cyclic aeolian deposition during the Last Glacial in European loess: a high-resolution record from Nussloch, Germany. Quaternary Science Reviews 28, 2955–2973.

Canti, M.G. (1998) Origin of calcium carbonate granules found in buried soils and Quaternary deposits. Boreas 27, 275–288.

Canti, M.G. & Piearce, T.G. (2003) Morphology and dynamics of calcium carbonate granules produced by different earthworm species. Pedobiologia 47, 511–521.

Darwin, C. (1881) The Formation of Vegetable Mould through the action of Worms, with Observations on their Habits. Murray, London.

Moine, O., Antoine, P., Hatté, C., Landais, A., Mathieu, J., Prud’homme, C. & Rousseau, D.D. (2017) The impact of Last Glacial climate variability in west-European loess revealed by radiocarbon dating of fossil earthworm granules. Proceedings of the National Academy of Sciences of the United States of America, 1–6.

Prud’homme, C., Lécuyer, C., Antoine, P., Moine, O., Hatté, C., Fourel, F., Martineau, F. & Rousseau, D.D. (2016) Palaeotemperature reconstruction during the Last Glacial from δ18O of earthworm calcite granules from Nussloch loess sequence, Germany. Earth and Planetary Science Letters 442, 13–20.

Prud’homme, C., Lécuyer, C., Antoine, P., Moine, O., Hatté, C., Fourel, F., Amiot, R., Martineau, F. & Rousseau, D.D. (2018) δ13C signal of earthworm calcite granules: a new proxy for palaeoprecipitation reconstructions during the Last Glacial in Western Europe. Quaternary Science Reviews 179, 158–166.

Prud’homme, C., Moine, O., Mathieu, J., Saulnier-Copard, S. & Antoine, P. High-resolution quantification of earthworm calcite granules from western European loess sequences reveals stadial–interstadial climatic variability during the Last Glacial. Boreas. Accepted 1st October 2018

Rasmussen, S.O., Bigler, M., Blockley, S.P., Blunier, T., Buchardt, S.L., Clausen, H.B., Cvijanovic, I., Dahl-Jensen, D., Johnsen, S.J., Fischer, H., Gkinis, V., Guillevic, M., Hoek, W.Z., Lowe, J.J., Pedro, J.B., 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. Quaternary Science Reviews 106, 14–28.

Levoglucosan, the witness of past fires

Levoglucosan, the witness of past fires
Name of proxy

Levoglucosan

Type of record

Biomass burning

Paleoenvironment

Lake and marine sediments and ice cores

Period of time investigated

Present to approximately 130,000 years ago

How does it work?

Levoglucosan is a molecule that is exclusively formed during the combustion of vegetation at low-temperature. It is therefore considered to be a source-specific tracer for biomass burning. During these fire events, levoglucosan is emitted into the atmosphere and can be transported over hundreds of kilometres. The extent of its atmospheric degradation is currently under debate, however, several studies have demonstrated that levoglucosan remains stable in the atmosphere for several days under most atmospheric conditions. It has been extensively used as a tracer for biomass burning in aerosols in numerous air-quality studies. Levoglucosan can provide information on the occurrence and origin of biomass burning, given that the source area of the levoglucosan is known. So far, levoglucosan is usually interpreted in terms of increased or decreased occurrence of biomass burning.

Figure 1: Illustration of levoglucosan transport from the land to the bottom of the ocean or lakes.

Recently, the use of levoglucosan as a biomass burning proxy in geological archives has gained increasing interest. Indeed, levoglucosan has been analysed in lake sediments (Battistel et al., 2017), in marine sediments (Lopes dos Santos et al., 2013) and in ice cores (You et al., 2016). It can be transported to these environments by atmospheric transport and by rivers (Fig. 1), where the biomass burning history of the source area is preserved. Seminal advances in the development of this proxy have occurred e.g. on the effect of transport and deposition on the levoglucosan. These molecules are likely transported to the ocean floor attached to marine biogenic particles in the water and do not substantially degrade during settling in the water column (Schreuder et al., 2018). However, they seem to be partially degraded in the top layer of the sediment and therefore changes in preservation conditions over time might influence the levoglucosan record. Other factors that might also influence levoglucosan accumulation are changes in wind strength and direction. This can result in decreasing or increasing transport of levoglucosan to the specific environment where the cores/samples are taken. This illustrates the importance to constrain the factors influencing the levoglucosan record in the context of a multi proxy approach.

What are the key findings that have be done using this proxy ?

So far, levoglucosan studies have mainly focused on reconstructing fire history of the last few centuries and of the Holocene (e.g. Shanahan et al., 2016; Zennaro et al., 2014), but it has also been detected in sediments up to 130 kyrs (Lopes dos Santos et al., 2013). It shows that levoglucosan has the potential to reconstruct fire history on short time scales (i.e. yrs to kyrs) as well as on long time scales (i.e. kyrs to myrs). The fire biomarker is frequently used together with proxies for vegetation and climate to get a better understanding of the interactions between fire, humans, vegetation and climate. For example, Lopes dos Santos et al. (2013) used the levoglucosan proxy in a marine sediment core offshore Australia to reconstruct past levels of biomass burning on the Australian continent over the last 130 kyrs. They also studied biomarkers for vegetation composition and archived information on human arrival and extinction of animals heavier than 40 kg (megafauna). They found out that around 44-42 kyrs, vegetation change was the consequence of the extinction of megafaunal browsers and led to the build-up of fire-prone vegetation in the Australian landscape, as illustrated in figure 2.

Figure 2: Interactions between fire and the environment in Australia around 44-42 kyears.

                                                                                          This article has been edited by the editorial board

 

References

            Battistel D., Argiriadis E., Kehrwald N., Spigariol M., Russell J.M. and Barbante C. (2017) Fire and human record at Lake Victoria, East Africa, during the Early Iron Age: Did humans or climate cause massive ecosystem changes? The Holocene 27, 997-1007.

Lopes dos Santos R.A., De Deckker P., Hopmans E.C., Magee J.W., Mets A., Damsté J.S.S. and Schouten S. (2013) Abrupt vegetation change after the Late Quaternary megafaunal extinction in southeastern Australia. Nature Geoscience 6, 627-631.

Schreuder L.T., Hopmans E.C., Stuut J.-B.W., Damsté J.S.S. and Schouten S. (2018) Transport and deposition of the fire biomarker levoglucosan across the tropical North Atlantic Ocean. Geochimica et Cosmochimica Acta.

Shanahan T.M., Hughen K.A., McKay N.P., Overpeck J.T., Scholz C.A., Gosling W.D., Miller C.S., Peck J.A., King J.W. and Heil C.W. (2016) CO2 and fire influence tropical ecosystem stability in response to climate change. Scientific reports 6, 29587.

You C., Xu C., Xu B., Zhao H. and Song L. (2016) Levoglucosan evidence for biomass burning records over Tibetan glaciers. Environmental Pollution 216, 173-181.

Zennaro P., Kehrwald N., McConnell J.R., Schüpbach S., Maselli O.J., Marlon J., Vallelonga P., Leuenberger D., Zangrando R. and Spolaor A. (2014) Fire in ice: two millennia of boreal forest fire history from the Greenland NEEM ice core. Climate of the Past 10, 1905-1924.

Pollen, more than forests’ story-tellers

Pollen, more than forests’ story-tellers
Name of proxy

Sporomorphs (pollen grains and fern spores)

Type of record

Biostratigraphy and Geochronology markers, Vegetation dynamics

Paleoenvironment

Terrestrial environment

Period of time investigated

Present to 360 million years

How does it work?

The sporomorphs (pollen grains and fern spores) are cells produced by plants involved in the reproduction. They are microscopic (less than a fifth of a millimeter) and contain a molecule called sporopollenin in their cell wall, which is very resistant to degradation. The sporopollenin molecule allows sporomorphs to be preserved in sedimentary archives such as lake sediment or peat deposits.

These reproductive structures appeared during the Paleozoic (570 million years ago) but the first spores looked rather similar and were indistinguishable among species. Later speciation of plants promoted the diversification of the reproductive cells between species and brought the opportunity to relate the fossil sporomorphs found in the sedimentary archives to the parental plant that produced them.

Figure 1. Plant communities are different depending on a wide range of environmental conditions. Above: Andean grasslands (páramo) in Ecuador. Below: Swamp forest in Orinoco Delta (Venezuela).

Plants are immobile organisms, and each species has its own tolerance range to the existing environmental conditions. The occurrence of certain plant communities in a specific environment depends on their different tolerance ranges. For instance, we do not observe today the same plants growing in the tropical rainforests of South America than in the polar tundra (Figure 1). Paleopalynology is the discipline that helps characterizing which plant species have occurred at a specific location during a particular time period. This provides information on the environmental conditions of the studied region. To identify the different species, palynologists have to analyze under the optical microscope the specific features of the sporomorphs’ cell walls. They look at e.g. the presence of spines or air sacs, or the number of apertures that the pollen grain has (Figure 2). These features are specific for each plant, which allows relating the pollen grain found in the sedimentary archive to the plant that produced it at the study location at a particular period of time.

Figure 2. Pollen grains have very different morphologies that allow identification of the plant that produce them. A: Byttneria asterotricha (Sterculiaceae); B: Triplaris americana (Polygonaceae); and C: Calyptranthes nervata (Myrtaceae). Bar scales in the pictures represent 25 micrometers.

What are the key findings made using this proxy?

Paleopalynology has a wide range of applications in geoscience. For instance, the presence of specific sporomorphs has been used as chronological markers to pinpoint several geological periods, especially in the far past biostratigraphy (million years ago)  (Salard-Cheboldaeff 1990).

In palaeoecology (the ecology of past ecosystems), the analyses of fossil sporomorphs help in specifying the dynamics of vegetation communities through time. This type of work started a century ago by Lennart van Post (1916) and provided the opportunity to study plants population and community natural trends within the appropriate temporal frame for long-lived species (i.e. tree species such as pines or oaks can live several centuries). Moreover, it provides a unique empirical evidence of the actual responses of vegetation to disturbances that occurred in the past, e.g. natural hazards, human populations land use and other anthropogenic impacts, or climatic shifts.

For instance, regarding past climates, paleopalynology allowed us to:

i)               understand the independent behavior of the species during glacial cycles (i.e., when a single species responded to changes, but the plant community as a unity did not respond) in forming new plant communities each time (Davis 1981; Williams and Jackson 2007);

ii)             map the re-colonization events and the assemblages formed during the last deglaciation until the vegetation communities we observe today (Giesecke et al. 2017).

In addition, in some characteristic environments, such as mountain regions, the occurrence and disappearance of specific species can allow the estimation of the temperature change with respect to present-day conditions. Another example has been developed in the last decade: the study of organic compounds contained in the sporopollenin of the sporomorphs’ walls. It has been identified as an accurate proxy that registers UV-B rays’ signals (Fraser et al. 2014). As UV-B rays’ are related to solar irradiation trends through time, reconstructing the organic compound variations in the sporomorphs’ walls allows reconstituting past solar irradiation trends in continuous archives such as lake and peat deposits.

This all shows that despite being a tiny structure, pollen grains are the story tellers of how the planet has been changing through history and can provide a wide range of outcomes essential for geosciences.

References

Davis, M.B. (1981). Quaternary history and the stability of forest communities. In: West, D.C., Shugart, H.H., Botkin D.B. (Eds.) Forest succession. New York, NY: Springer-Verlag.

Fraser, W.T., Lomax, B.H., Jardine, P.E., Gosling, W.D., Sephton, M.A. (2014). Pollen and spores as a passive monitor of ultraviolet radiation. Frontiers in Ecology & Evolution 2: 12.

Giesecke, T., Brewer, S., Finsinger, W., Leydet, M., Bradshaw, R.H.W. (2017). Patterns and dynamics of European vegetation change over the last 15,000 years. Journal of Biogeography 44: 1441-1456.

Salard-Cheboldaeff, M. (1990). Interptropical African palynostratigraphy from Cretaceous to late Quaternary times. Journal of African Earth Sciences 11: 1-24.

Von Post, L. (1916). Om skogsträdpollen i sydsvenska torfmosslagerföljder. Geol.   Fören. Stockh. Förhandlingar 38, 384–390.

Williams, J.W., Jackson, S.T. (2007). Novel climates, no-analog communities, and ecologica lsurprises. Frontiers in Ecology and the Environment 5: 475-482.

                                                                                                                           Edited by Célia Sapart and Carole Nehme