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Climate: Past, Present & Future

Climate of the Past

Habits in numerical model construction

Habits in numerical model construction

 

Numerical models are omnipresent in climate research. Constructed to understand the past, to forecast future climate and to gain new knowledge on natural processes and interactions, they enable the simulation of experiments at otherwise unreachable time and spatial scales. These instruments have long been considered to be fed – let even determined – by either theories or observations alone. But are they really? Sociological factors are at play too. It is precisely these influences that the present blog entry attempts at presenting through the review of cornerstone sociological studies and new anthropological insights on decision-making of model builders.

 

What we here define as numerical models are computer simulations of processes occurring within a system. Global Circulation Models (GCMs), the climate models used to simulate the Earth’s climate and its response to changes of greenhouse gas concentrations in the atmosphere, are certainly the most widely known representatives of this category of scientific instruments. However, numerical models of all types abound in research fields associated to the study of climate change. Some require to be run on national supercomputing centres, while others run on a laptop. Some might be the product of entire research teams, while others result from individual endeavour. Some simulate climate processes at a global scale, while others focus on gas bubbles in a cubic meter of peat. The term “numerical model” employed hereafter embraces this diversity.

Models have long been considered by philosophers to be logically deduced from theory and observations alone. In the late 1990s, Morgan and Morrison (1999) initiated a new turn by insisting on the partial autonomy of models from theory and observations, arising from the diversity of their ingredients. It is this partial autonomy, the authors claim, which enables us to gain knowledge through models about theories and the world. Simultaneously, several studies of the domain of Science and Technology Studies (STS) attempted to address the actual activity of modelling through immersion within climate research institutes, hereby unveiling complex interactions between actors, institutions and stake-holders.

 

Why does it matter?

Model outputs are widely used as a basis for decision-making, at local up to international level. In a context of political and public defiance, much attention has been devoted to increasing the credibility of numerical models. Model intercomparison projects, uncertainty assessments and “best practice” guidelines have flourished. Yet improving modelling activity also necessarily goes through understanding and analysing current practices. Empirical studies addressing actual practices are a requisite to improve reflexivity – and thereby control – over implicit mechanisms likely to impact model outputs. The question ought hence to be how models are actually constructed, before addressing how they should be.

 

What we already know…

Current practices can be analysed at different levels, from individual to institutional. Most empirical sociological studies have so far investigated the interface between decision-making spheres and modelling, as well as institutional and disciplinary cultures of modelling. Shackley (1999) notably identified different “epistemic lifestyles” within global circulation modelling, which appeared to be influenced among others by the role, location, objectives and funding of the organisation within which the modelling project was conducted. He further highlighted with fellow researchers the influence of the modeller’s own perception of the policy process on his or her modelling practices (Shackley et al., 1999). Similar assertions have been made in neighbouring fields (such as hydrology, land use change and integrative assessment modelling), but rarely based on empirical material.

 

 … and what we know less

 The majority of existing studies considered the epistemic stance of modellers – their perception of the modelling activity, of its objectives and of the modellers’ own roles, as well as the modellers’ positioning with respect to particular issues encountered in climate modelling. However, how modellers really make choices during the construction of their model – and which factors influence these – has barely been examined. This is exactly what we aimed to scrutinize through our study presented at the EGU General assembly 2018.

 

Decisions in model construction

Studying decision-making within the process of model construction implies to assume that choices have to be made. As straightforward as this claim might seem, the very existence of choices has been largely absent from philosophical reflexion upon modelling during the 20th century and remains yet to be granted appropriate attention. Modellers however make, sometimes in an iterative manner, a plethora of choices during the model building activity. The temporal and spatial scales need to be selected and along with them the natural processes at play, their interactions, their representation through physical equations or parameterization, their numerical implementation, the source of data, the hardware and software at use, etc. (Babel et al., 2019).

The following video summarizes the rationale behind our research and the approach we used.

Choosing how to represent a natural process

We decided to focus on one particular type of choice: the representation of natural processes through equations (time transfer functions) and their numerical implementation. Even when modellers have selected to simulate a particular natural process (evapotranspiration, for example), several representations of one and the same process can generally be contemplated (Guillemot, 2010). We then asked ourselves on which basis modellers choose one representation and not another. We expected mostly technical aspects to come forefront, such as the required data or software and hardware limitations. These indeed take a prominent place in specialized literature.

 

Interviewing modellers

We adopted a well-established methodology from social sciences based on semi-directed interviews, which were conducted with researchers who developed a model from its earliest stage on. The interviewees were not aware of the exact subject of the interview. Prior to the interviews, we identified in the literature accompanying the presentation of the models one or several processes for which no justification was given on the reasons leading to the choice of the employed representation. After introductive, general questions on the modelling project, the researchers were invited to explain the use of this particular representation.

All interviews were recorded and transcribed. The interviewees came from five universities or research institutes located in four different countries in Europe and Northern America. All but one were senior scientists. A diversity was sought among the types of models (from highly complex ones to models openly described as simplistic) and scientific disciplines, ranging from ecology to geochemistry. With the exception of astrophysics, which we included in order to test a hypothesis not detailed in the present blog entry, all models were devoted to research questions associated to climate change. A total of 14 interviews were conducted.

 

The role of actors – or what we did not expect

As stated above, we expected the modellers to justify the choice of representations with mostly technical constraints. They did not. Rather, the narratives granted particular emphasis to actors – colleagues, professors, PhD directors – who belonged to the modeller’s network during the construction of his or her model. Many of the interviewees had started building their model (which they nowadays continue developing) as doctorate students. The use of a certain process representation was often explained as having been transferred by the (PhD) research director or colleagues. Two decades later, the representation was still part of the model – and modelling practices of these actors played a paramount role in the modellers’ justification of its use, even in competitive and controversy-laden contexts (Babel et al., 2019).

 

From transfer to habit

Many of the interviewees were surprised to be asked about an equation or a numerical scheme they did not perceive to be a distinctive, novel feature of their model. Even if other alternatives to the process representation existed in all the analysed cases, they did not necessarily consider to have made a choice. The choice had often been made by others at the very beginning of their career and transferred to them by their PhD directors or colleagues. They incorporated it in their own practices, a process one of the interviewees described as a “natural evolution”.

(…) during my PhD, my PhD director was only working with [this process representation]. And so I was educated with it. And so I couldn’t imagine doing something else (…) And so after my thesis, I naturally evolved with this approach because it was what I knew. It was a natural evolution, it is as… yes, when we can speak a language, we evolve with this language. So here it is a bit the same, I knew how to speak [this process representation] and so I naturally kept on evolving with this approach. But it is true that… yes, it is the main reason, I believe “  (interview quoted in Babel et al., 2019).

The natural evolution this interviewee referred to can be equated to a path dependence. The modeller developed skills and expertise through the repeated use of the representation, which rendered its implementation increasingly evident in the course of his career. We employed the sociological concept of habit, notably analysed by Latour (2013) to describe the progressive incorporation of choices becoming self-evident practices.

Figure 1. Illustration of the transfer of natural processes representations and incorporation within modelling practices.

 

Habits are required – but self-reflexivity over them too

While the term has often a negative overtone in everyday language, habits can be considered as deeply necessary. As stated by Latour (2013), these smooth out the course of actions: a modeller who would constantly re-consider, on a daily basis, the use of a programming language, a database or a certain variable would lose herself in perpetual decision-making requiring both attention and time. By repeating actions without engaging on new paths – by evolving with the same language, as the interviewee quoted above put it – we gain in efficiency and expertise. Yet, a danger is looming: that of falling into automatisms, losing sight and control over the initial crossroad and hence the ability to reverse, whenever necessary, our paths of actions. Questioning modelling habits and tracking them back to their roots – both on an individual and a collective basis – appears an unavoidable step to gain a better understanding over existing modelling practices.

 

Collectives may reinforce path dependence

The modellers interviewed during our study displayed striking consciousness of their process representation being often particular to a certain collective (a “school” or “field”) they nowadays identified with. By transferring them with a process representation, their director or colleague had also anchored them within a network: that of scientists using the same representation. This anchoring, which was often unconscious at the beginning of their career, could act as reinforcing the path dependence. Changing of process representation would not only often necessitate considerable effort and time to reach the same level of efficiency and expertise gained over the years, but also imply to turn away from a collective within which the modellers had established themselves (Babel et al., 2019).

 

A word of caution

Our study does not describe modellers as being determined as habits. Rather, we aimed at shedding light on inter-individual and collective influences within the modelling process often disregarded in field-specific literature. We assume habits to play a role among other factors. The fact that these other (computational, cost-related) factors were rarely mentioned by the modeller during the interviews could be explained by them being perceived as evident or self-speaking; additional studies would be required to explore the intertwinement of other triggers of model decision with inter-individual and collective influences.

Finally, this study was based on a limited number of interviews: we did not seek for exhaustivity or generalizations, but for case studies enabling a first glance on rarely studied processes.

This post has been edited by Janina Bösken and Carole Nehme.

REFERENCES
Babel, L., Vinck, D., Karssenberg, D. (2019). Decision-making in model construction: unveiling habits. Environmental Modelling and Software, 120, in press. DOI: https://doi.org/10.1016/j.envsoft.2019.07.015

Guillemot, H. (2010). Connections between simulations and observation in climate computer modeling. Scientist’s practices and “bottom-up epistemology” lessons. Stud. Hist. Philos. Sci. Part B Stud. Hist. Philos. Mod. Phys., Special Issue: Modelling and Simulation in the Atmospheric and Climate Sciences 41, 242–252.

Latour, B. (2013). An Inquiry into Modes of Existence. An Anthropology of the Moderns. Harvard University Press, Cambridge, Massachusetts.

Morgan, M.S., Morrison, M. (1999). Models as Mediators: Perspective On Natural and Social Science. Cambridge University Press, Cambridge.

Shackley, S. (2001). Epistemic Lifestyles in Climate Change Modelling, in: Edwards, P.N. (Ed.), Changing the Atmosphere: Expert Knowledge and Environmental Governance. MIT Press, Cambridge, Massachusetts.

Shackley, S., Risbey, J., Stone, P., Wynne, B. (1999). Adjusting to Policy Expectations in Climate Change Modeling. Clim. Change 43, 413–454.

 



			

		

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.

How glowing sediment can help to decipher the Earth’s past climate !

How glowing sediment can help to decipher the Earth’s past climate !

The last 2.5 Million years of the Earth’s history (termed Quaternary) are characterised by climatic cycles oscillating between warm (interglacial) and cold (glacial) periods. To be able to fully understand and interpret past climate variations the development of accurate and precise chronological techniques is crucial. Optically stimulated luminescence (OSL) dating is a strong geochronological tool that can be used to date across a wide time range, from the modern days to a few hundred thousand years ago. It has been used to date sediments in nearly all parts of the world. The event that is being dated is the last time the sediment has been exposed to daylight, which means that the luminescence age is directly related to the time of sediment deposition.

How does OSL work?

OSL dating is based on the ability of minerals to store energy (Preusser et al., 2008). Luckily quartz and feldspar, the two most common minerals in the Earth’s crust have this ability. They work like small batteries, which get charged when the sediment is buried (Fig. 1 C, Duller, 2008). This is due to radiation from naturally occurring radioactive material (uranium, thorium and potassium) in the surrounding sediment, and from cosmic rays for samples closer to the surface. Like a battery, the quartz and feldspar grains have a finite capacity for storing energy. Once completely charged, the battery-like grain is considered as being saturated. This upper age limit of OSL dating depends on the ability of the grain to store the energy and on the rate at which the grain is charged (i.e. the dose rate, derived from uranium, thorium, potassium and cosmic radiation). If the surrounding material is very radioactive, the dose rate is very high, which means that the grain saturates rather quickly (Fig. 1C), but if the dose rate is low, the battery-like grain charges more slowly and OSL can be used to date geomorphological processes further back in time. Exposure to natural sunlight will remove charge from the grain. This bleaching or resetting occurs e.g. during sediment transport and deposition (Fig. 1A, D). Subsequently, the sediment can be buried again and the grain will get charged (Fig. 1D) until it is saturated, or until the sediment is transported and re-deposited, or is sampled (Fig. 1F). Once sampled in opaque tubes (Fig. 2 A), so that no daylight can affect the amount of energy stored in the grains, the sample is ready to be processed in the laboratory.

Figure 1: . Accumulation and release of charge within mineral grains (modified from Duller, 2008). (A) During transport of sediment the accumulated charge gets released due to energy provided by natural sunlight; (B) At deposition, the charge is fully released and the “battery” is emptied; (C) Charge can accumulate within the grain during burial due to natural ionising radiation; (D) and (E) The process described before can happen multiple times over geological times scales, depending on the environment and the geological processes; (F) When a sample is taken in opaque tubes or using cores the stored energy at that particular time gets sampled and can later be released in the laboratory to obtain a luminescence age.

What happens in the laboratory?

Sample preparation involves multiple steps to isolate the right minerals and grain size for dating. A key step is the decision on the mineral that will be used for dating. Whilst quartz gets reset more quickly when exposed to sunlight, feldspar has the potential of dating events further back in time.

The energy stored in the mineral grains cannot only be released by natural sunlight, but also in the laboratory using a defined wavelength under controlled conditions (Fig. 2B). During this process, the grain emits light, which is collected. This emitted light gives information on the amount of stored energy (Duller et al., 2008; Preusser et al., 2008). When comparing this light output, from a natural radiation dose received, to a light output generated by a known laboratory dose, the so-called laboratory equivalent dose is obtained. This equivalent dose (in Gy), divided by the natural dose rate (in Gy/1,000 years) will give the OSL age in thousand years (Duller et al., 2008; Preusser et al., 2008). OSL dating can be done using different grain sizes of sediment, either mounted as patches of grains on aluminium or steel discs (Fig. 2E) or as single grains, brushed into very small holes on a disc (Fig. 2C, D). As an alternative to grains, slices of rock can be used (e.g. Sohbati et al., 2011; Jenkins et al., 2018). Important is a representative number of sub-samples, which will be analysed using statistical means to get a valid age (Galbraith et al., 1999; Galbraith and Roberts, 2012).

Figure 2: (A) Sampling of sediment in an opaque tube for OSL dating; (B) Luminescence instruments (here Risø Readers) used to date the samples. The picture also shows the photographic read light conditions under which samples are prepared and measured to avoid resetting of the battery-like grains; (C) Sample carousel with single grain discs; (D) Close-up of a single grain discs containing grains in 100 holes (300 µm in diameter); (E) Steel discs containing fine silt size material (4-11 µm). Credit photos: (A) H. M. Roberts, (B)-(D) S. Riedesel, (E) A. M. Zander.

Where can and has OSL dating been applied? –Some examples from past climate research

Luminescence dating can be a valuable geochronological tool in very different climatic parts of the Earth: the terrestrial systems with loess and lake records for example, the glacial land system, with a focus on ice marginal archives and the deep marine archives with long sedimentary records.

Terrestrial archives – loess and lakes

Loess is silt-size (4-63 μm) sediment transported by wind (aeolian transport), which has been exposed to sufficient daylight to fully reset the stored luminescence signal. This makes it favourable for luminescence dating. The Chinese Loess Plateau (CLP) is the Earth’s most important terrestrial climate archive. Changes in the accumulation of loess and/or the occurrence of soil horizons within the loess sequences, give information on changes in past climate (e.g. temperature, wind direction and intensity, precipitation). For a long time it has been considered as a continuous past climate archive (Liu and Ding, 1998). High-resolution OSL dating at different sites across the CLP gave new insights. It showed that the loess record is neither homogenous nor continuous (Stevens et al., 2007; Stevens et al., 2018). Unconformities could be detected and related to erosional processes, disturbances or diagenetic modifications (Roberts et al., 2001; Stevens et al., 2007; Buylaert et al., 2008). The application of OSL dating to loess has also helped to gain knowledge on e.g. variations in wind directions in the past (e.g. the East Asian Monsoon behaviour; Stevens et al., 2006; Kang et al., 2018).

Lake sediments also provide long records of past climate changes. Lamb et al. (2018) established a chronology based on a combination of OSL and radiocarbon dates for the past 150,000 years of Lake Tana in Ethiopia. This chronology helped to infer time spans of favourable climatic conditions for early human migrations. Another example is the late Quaternary chronology of the Xingkai Lake in northeast Asia by Long et al. (2015). It spans the past 130,000 years and shows how important independent age control is when performing geochronological research. The combination of OSL and radiocarbon ages highlights the potential of OSL to date events beyond the age range of radiocarbon (ca. 45,000 years, Walker, 2005).

Remnants of former lakes can also be used as archives, e.g. the beach ridges, marking former shorelines of palaeo-lake systems in the present day Kalahari Desert, bear witness to a wetter climate in the past (Burrough et al., 2009). OSL has been the key tool to establish a 280,000 year chronology of these palaeo-lake high-stands, giving insights into late Quaternary changes between arid and humid phases in southern Africa (Burrough et al., 2009).

The deep sea – Potential and challenges of these long records using OSL

In marine sediments, OSL dating can be used in addition to radiocarbon dating, or to date beyond the range of the latter (Stokes et al., 2003; Olley et al., 2004, Armitage and Pinder, 2017), or where insufficient biogenic carbonate is available. As radiocarbon dating of material in marine sediments can suffer from a reservoir age induced by old carbon in marine water, OSL dating can be a useful alternative (Olley et al., 2004). However, OSL dating of marine sediments can be challenging, since transport and deposition of the sediment under water complicates the bleaching of the sediment grains (Olley et al., 2004, Armitage and Pinder, 2017). Nevertheless, successful OSL research has been conducted in deep-sea environments. For example, Sugisaki et al. (2010) were able to establish an OSL-chronology of sediments from the Okhotsk Sea, including the last glacial-interglacial transition by covering a time span from 140,000 to 15,000 years.

Glacial land forms – Ice marginal features: What can they tell us about glacial advance and retreat?

OSL dating can be used in the cryosphere, e.g. to date relics of past glaciations. Smedley et al. (2016) used single grains of feldspar to date glacial advances during the last glacial maximum (LGM) in Patagonia. While their results for glacial advances at the onset of and during the LGM correlate with other studies in South America, the final glacial advance in their study area at ~15,000 years is later than elsewhere, which may hint towards local topographic and regional climatic factors (precipitation), controlling glacial responses, or preservation issues elsewhere.

The recent development of dating cobbles in glacial contexts not only enables the age determination of glacial features, such as glaciofluvial sediments, it also gives further insights into the transport history of the cobbles prior to their burial (Freiesleben et al., 2015; Jenkins et al., 2018). Cobbles can record the deposition event in a similar way to the one described above for much finer grained sediment. The great advantage of cobbles is their potential to record multiple exposure events (Freiesleben et al., 2015). In the case of a cobble from the Isle of Man, it potentially records the advance and retreat of the Irish Sea Ice Stream (Jenkins et al., 2018).

The examples presented here show the wide applicability of OSL dating to directly date the last exposure of sediment to daylight in various environmental settings. OSL techniques are able to date geological events beyond the age range of radiocarbon and recent developments improve the reliability of OSL dating in geomorphological settings, where resetting of the stored charge might otherwise be challenging.

This post has been edited by the editorial board.

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