# XLUM: an open data format for exchange and long-term preservation of luminescence data

Sebastian Kreutzer, Steve Grehl, Michael Höhne, Oliver Simmank, Kay

Dornich, Grzegorz Adamiec, Christoph Burow, Helen Roberts, Geoff Duller

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Sebastian Kreutzer, Steve Grehl, Michael Höhne, Oliver Simmank, Kay Dornich, et al.. XLUM: an open data format for exchange and long-term preservation of luminescence data. Geochronology, 2023, 5 (1), pp.271-284. 10.5194/gchron-5-271-2023. hal-04120907

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# XLUM: an open data format for exchange and long-term preservation of luminescence data

Sebastian Kreutzer<fnref n="1" />,<fnref n="2" />,ª, Steve Grehl<fnref n="3" />, Michael Höhne<fnref n="4" />, Oliver Simmank<fnref n="4" />, Kay Dornich<fnref n="4" />, Grzegorz Adamiec<fnref n="5" />, Christoph Burow<fnref n="6" />, Helen M. Roberts<fnref n="1" />, and Geoff A. T. Duller<fnref n="1" />

**Correspondence:** Sebastian Kreutzer (sebastian.kreutzer@uni-heidelberg.de)

Received: 26 November 2022 – Discussion started: 22 December 2022

Revised: 7 April 2023 – Accepted: 26 April 2023 – Published: 6 June 2023

**Abstract.** The concept of open data has become the modern science meme, and major funding bodies and publishers support open data. On a daily basis, however, the open data mandate frequently encounters technical obstacles, such as a lack of a suitable data format for data sharing and long-term data preservation. Such issues are often community-specific and best addressed through community-tailored solutions. In Quaternary sciences, luminescence dating is widely used for constraining the timing of event-based processes (e.g. sediment transport). Every luminescence dating study produces a vast body of primary data that usually remains inaccessible and incompatible with future studies or adjacent scientific disciplines. To facilitate data exchange and long-term data preservation (in short, open data) in luminescence dating studies, we propose a new XML-based structured data format called XLUM. The format applies a hierarchical data storage concept consisting of a root node (node 0), a sample (node 1), a sequence (node 2), a record (node 3), and a curve (node 4). The curve level holds information on the technical component (e.g. photomultiplier, thermocouple). A finite number of curves represent a record (e.g. an optically stimulated luminescence curve). Records are part of a sequence measured for a particular sample. This design concept allows the user to retain information on a technical component level from the measurement process. The additional storage of re-

lated metadata fosters future data mining projects on large datasets. The XML-based format is less memory-efficient than binary formats; however, its focus is data exchange, preservation, and hence XLUM long-term format stability by design. XLUM is inherently stable to future updates and backwards-compatible. We support XLUM through a new R package *xlum*, facilitating the conversion of different formats into the new XLUM format. XLUM is licensed under the MIT licence and hence available for free to be used in open- and closed-source commercial and non-commercial software and research projects.

## 1 Introduction

Wilkinson et al. (2016) proposed four key principles for scientific data management towards open science: findability, accessibility, interoperability, and reusability – the FAIR guidelines. Since then, major funding bodies (e.g. Thorley and Callaghan, 2019; Agence Nationale de la Recherche, 2019; European Commission, 2021; Deutsche Forschungs Gemeinschaft, 2022) and publishers (e.g. Copernicus Publications, 2018; Wiley Author Service, 2022) have adopted these principles as part of their data management policies, and they have become an integral part of the European Code

<footnote n="1">¹Geography & Earth Sciences, Aberystwyth University, Wales, United Kingdom</footnote>

<footnote n="2">²Archéosciences Bordeaux, UMR 6034, CNRS – Université Bordeaux Montaigne, Pessac, France</footnote>

<footnote n="3">³HUK-Coburg, Coburg, Germany</footnote>

<footnote n="4">⁴Freiberg Instruments GmbH, Freiberg, Germany</footnote>

<footnote n="5">⁵Institute of Physics, Division of Geochronology and Environmental Isotopes, Silesian University of Technology, Gliwice, Poland</footnote>

<footnote n="6">⁶piazza blu² GmbH, Cologne, Germany</footnote>

<footnote>ªpresent address: Institute of Geography, Ruprecht-Karl University of Heidelberg, Heidelberg, Germany</footnote>
of Conduct for Research Integrity (ALLEA, 2017). If inter-
weaved with umbrella terms such as “open data” or “open
science”, the added value of transparency and reproducibil-
ity of modern science comes across as almost self-evident.
Unfortunately, the implementation often seems to fall behind
set goals. For instance, Perkel (2020) vividly covered the
challenge of 35 participants trying to run decades-old com-
puter code and concluded that maintaining reproducibility of
software-based models and analysis pipelines over decades is
a demanding, sometimes impossible, task. Likewise, we can
infer that data formats tied to a small number of (outdated)
programmes run the risk of data becoming inaccessible. An-
other aspect on the data side was considered by Noy and Noy
(2020), who complained that common open data surrogate
statements in articles such as data being “available upon re-
quest” may equate to no data access. Indeed, a pivotal aspect
of the FAIR guidelines is their emphasis on principles foster-
ing automated data processing or enabling such processing in
the first place. The requirement to actively contact the study
authors to request access to the data, e.g. e-mail requests,
therefore inherently undermines the principles of open data
(Noy and Noy, 2020). On the other hand, authors perhaps
refrained from direct sharing because of unclear reporting
guidelines or the effort required to document data with pre-
sumed low demand.

Adhering to the FAIR guidelines with actual benefits for
all parties (e.g. data donors, data users, funding bodies)
involves tackling low-level technical issues, such as defin-
ing an exchange data format enabling study authors to share
their raw data in a manner which is structured, standard-
ised, and ideally effortless, as well as in a format that will
remain accessible long into the future. Here we adopt the
idea that those issues are usually community-specific and
best addressed through discipline-tailored solutions: for in-
stance, for data generated in luminescence-based chronology
studies.

Luminescence dating is a dosimetric dating method of
key importance in Quaternary sciences and archaeology (e.g.
Rhodes, 2011; Roberts et al., 2015; Bateman, 2019; Mur-
ray et al., 2021), covering around the last 300 000 years.
In a nutshell, the datable event is the last sunlight or heat
exposure of natural minerals such as quartz or feldspar.
The dating process determines two parameters: (1) the ab-
sorbed dose (in Gy) accumulated in the minerals since the
last heat or light exposure and (2) the environmental dose
rate (in Gyka⁻¹). The ratio of dose (Gy) divided by dose
rate (Gyka⁻¹) gives the age (ka). Methods frequently ap-
plied in luminescence dating studies are distinguished by
their stimulation mode, e.g. thermally stimulated lumines-
cence (TL; see Aitken, 1985), optically stimulated lumines-
cence (OSL; Huntley et al., 1985), or infrared stimulated lu-
minescence (IRSL; Hütt et al., 1988). Luminescence meth-
ods are also used by adjacent scientific disciplines, e.g. acci-
dent dosimetry and material characterisation (e.g. Yukihara
and McKeever, 2011; Yukihara et al., 2014).

Luminescence (dating) does not measure the absorbed dose directly but infers an equivalent dose (D~e~) from the minerals' natural light output (luminescence) compared to a laboratory dose of known size. Luminescence dating studies and research building on such work routinely tabulate only a fraction of the recorded data in the form of aggregated parameters. One could think of a pyramidal information hierarchy with the age on the top (Fig. 1). The base is made out of minimally processed luminescence data, i.e. measured luminescence (for the purpose of this paper we "neglect" the dose rate information). Original dating studies ideally report the full information pyramid. However, the further an age is carried forward through subsequent studies or collected in data repositories, the higher the level of data aggregation. Good examples for aggregated luminescence data are repositories such as Lancaster et al. (2015) and Codilean et al. (2018). Such archives are excellent places to find locations of dating studies, but it is not easy to spatially link different ages without accessing the original studies with primary data.

Original, minimally processed luminescence data (see
Fig. 1), i.e. measured luminescence, are hardly ever pub-
lished along with a study. However, sharing of unprocessed
luminescence data, accessible to others after the completion
of a dating study, is desirable for several reasons.

1. Luminescence ages are end members of long measurement series involving various protocols, tests, and analysis steps with potentially different hardware and software tools. Once aggregated, it is challenging for others to revalidate published luminescence dates beyond plausibility checks. Shared raw data will potentially lead to better reproducibility and data quality.

2. Access to luminescence data on a single curve level supports the application of advanced analysis tools employing hierarchical Bayesian models such as the R package *BayLum* (Philippe et al., 2019) or the model “baSAR” (Combès et al., 2015; Mercier et al., 2016). Both approaches start with individual luminescence curves to integrate different parameters into a holistic model using Bayesian statistics to derive equivalent doses based on prior knowledge.

Other work has shown examples of how to study sedi-
ment pathways by tracing the bleaching histories of sed-
iment grains (Chamberlain and Wallinga, 2019). If such
data are never shared, their full potential remains un-
tapped.

Recently, Balco (2020) advocated for a transparent and
open middle-layer concept, disconnecting measured
quantities from processed ages to account for changed,
perhaps improved calculation procedures. His proposal
was specific to cosmogenic nuclide exposure dating, but
the general idea appears to be valid for other dating
techniques, such as luminescence dating. For instance, it
would enable others to test the impact of alternative ap-
<figure><img src="image_4.png" /><figcaption>**Figure 1.** A luminescence age is the result of data aggregation. In order to reproduce all steps, access to primary data (the base level) is indispensable. However, such primary data are seldom published or otherwise accessible. Republishing usually leads to information loss. The number of information and/or process levels in the graph is arbitrary.</figcaption></figure>

plied statistical parameters on the calculated D~e~ in the future.

3. The approach of Balco (2020) renders ages moving targets; i.e. they may change with time due to different calculation procedures. Balco's approach emphasizes the data treasure character of measured physical quantities (with "data are described with rich metadata"; FORCE11, 2014), which need to be preserved and shared instead of processed numbers. This approach holds for luminescence dating studies, which create, somewhat as a by-product, a vast amount of luminescence data on minerals from different origins. Such data are of potential interest, for instance, to geoscientists working on provenance analysis (e.g. Sawakuchi et al., 2018; Tsukamoto et al., 2011), to physicists focusing on luminescence models, or to data scientists trying to develop new approaches to enable exploratory luminescence data analysis to constrain physical parameters of OSL curves (e.g. Burow et al., 2016) or seeking training datasets to test machine learning approaches (e.g. Kröninger et al., 2019).

4. Broadly shared and accessible through a standard format, luminescence curve data will help establish a comprehensive repository for luminescence data, enabling studies and meta-studies not covered by the abovementioned examples.

Data sharing requests can only be reasonably accommodated if luminescence data can be easily exchanged, sufficiently archived, and analysed independently of proprietary software or file formats. We argue that one particular factor hampering the exchange and reuse of luminescence data is the absence of a suitable data format supporting long-term data preservation and fostering data exchange. To the best of our knowledge, long-term data preservation is an unresolved issue in the luminescence (dating) community. After being analysed and published, one can expect original primary data

to be archived in compliance with scientific standards, but they may become inaccessible or incompatible with new data over time when the reanalysis is wanted. Such data are often lost to the public and need to be measured again. Hence, the first step of chronological data sharing and archiving is a data format that qualifies to serve that purpose.

In this contribution, we first briefly list existing data formats commonly used to store luminescence data. We then outline identified general technical requirements for a data format for the long-term preservation of luminescence data. Hereafter, we highlight features of a new XML-based file format, XLUM, developed for long-term preservation and exchange of luminescence data. The remainder provides examples and illustrates a reference implementation in R and Python, showing how existing data can be converted effortlessly into the new XLUM format. The discussion addresses potential shortcomings and challenges and canvasses future directions. We consider our contribution to be an initial definition, and the format blueprint is open to discussion within the luminescence community.

The XLUM file format is published and fully detailed at https://r-lum.github.io/xlum_specification (last access: 7 March 2023), a GitHub™ repository, and on Zenodo (European Organization For Nuclear Research and OperaNARE, 2013; Kreutzer et al., 2022b). The chosen open-source MIT licence (https://opensource.org/licenses/MIT, last access: 2 May 2023) allows reuse in open-access and closed-source software projects.

In the remainder of this paper, we will use monospace letters for format and code snippets, as well as file format arguments. XML elements (nodes), if not accompanied by a closing tag, are contracted into one short tag, for instance `<node/>` instead of `<node> ... </node>`.
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Simplified illustration of two approaches to store a typical TL curve. (a) In the “conventional way” count data of the PMT are recorded channel-wise (1), and the temperature values are recalculated according to the equations in step (2) based on the minimum and maximum temperature values to obtain the (3) final TL curves, while data are imported in a programme. (b) In the approach suggested here, the luminescence signal and the temperature are recorded by two independent technical components, e.g. a PMT (1) and a temperature sensor (2) monitoring the heating process. While importing, the resulting TL curve (3) matches both recorded signals on the time domain. Such an import routine is e.g. available in the R package *Luminescence* (Kreutzer et al., 2012).</figcaption></figure>

- The FAIR guidelines are supported by design and facilitate the creation of large repositories for long-term preservation and exchange of luminescence measurement and metadata.

We identified an XML (Extensible Markup Language)-based (W3C XML Core Working Group, 2008) format as the most suitable structure serving the outlined requirements.

The idea of introducing an XML-based format for storing luminescence data is not new. Bortolot and Bluszcz (2003) sketched a few general requirements for such a format 20 years ago, although this approach has not been widely adopted. An XML-based format is rather memory-inefficient, particularly if compared to binary formats, leading to relatively large files (tens of megabytes or more instead of megabytes). However, we believe that this aspect is of limited relevance because of the following. (1) Mass data storage is inexpensive, particularly if costs are compared to the year of Bortolot and Bluszcz (2003). (2) The overall amount of data produced in luminescence dating is negligible compared

to other disciplines working with XML-based formats (e.g. Martens et al., 2011; Röst et al., 2015). (3) Modern storage systems of data repositories usually employ highly efficient low-level data compression methods (e.g. lossless data compression) independent of any file format, reducing the data footprint regardless of the exchange format.

## 4 Format description

In the following, we outline the conceptual structure of the XLUM format. To minimise verbosity, we only focus on key design concepts. For full details, we refer to our reference document on GitHub (https://xlum.r-luminescence.org, last access: 30 June 2022). The GitHub™ repository also contains a formal format description following the XML Schema Definition (XSD) for automated validation. XLUM defines a substructure, which can be part of a file or any other XML structure (W3C XML Core Working Group, 2008) that acts as a container or constitutes a file of its own, for instance
<figure><img src="image_6.png" /><figcaption>**Figure 3.** The figure is a graphical representation of the data storage concept with the different node levels of the XLUM format. Data are stored sequentially over time. Dashed lines are used to indicate the possibility of multiple instances. For example, one XLUM file can contain many `<xlum/>` nodes, and one `<xlum/>` node may contain many `<sample/>` nodes.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Listing 1.** Basic hierarchical structure of the XLUM format following the XML scheme in version 1.0 with UTF-8 encoding. The three dots (`...`) indicate node attributes.</figcaption></figure>

with the file extension `*.xlum`, although the XLUM format does not enforce a specific file extension.

The two key features of the XLUM format are (1) information nesting, with measurement data only stored in the lowest node, and (2) support of data sharing by design.

## 4.1 Nesting of information on five node levels

The format consists of five levels (nodes) (Listing 1, Fig. 3), indicated by so-called tags. The correct formal description requires an opening tag (`<...>`) and a closing tag (`</...>`) (see Listing 1). Each tag allows various attributes (`<tag attribute="" ...>`) for metadata, a few of which we will detail below. The number of attributes is not limited, and additional user-defined attributes, not covered by the format definition, are explicitly allowed. However, the format definition insists on mandatory attributes, with a few accepting the non-empty string `NA` for not available or not applicable. The first four upper nodes structure the data and provide metadata to describe the dataset. The lowest node (`<curve/>`) contains the raw (or minimal processed) measurement data.

1. `<xlum/>` is the root node. It wraps all other data and is the parent to all other child nodes. The number of child nodes of `<xlum/>` is unlimited. Everything within one `<xlum/>` is considered a collection of data for different samples for which e.g. author names, a digital object identifier (DOI), and a licence can be assigned through attributes.

2. `<sample/>` is the first child node to `<xlum/>`. It is the parent structure for luminescence data collected for a single sample. Hence, everything wrapped between `<sample/>` refers to a specific sample. Amongst others, expected attributes are the name of the sample and the geographic coordinates (`latitude`, `longitude`).

3. `<sequence/>` is the first child node to `<sample/>`. It sets the structure for measurement data defined through (measurement) sequences, e.g. a single aliquot regenerative (SAR) dose (Murray and Wintle, 2000) measurement sequence or any measurement data arranged in a particular order. Typical attributes are `position`, `filename`, or `readerName`.

4. `<record/>` holds all records of a sequence of a particular sample. A record is not necessarily limited to a single measured `xy` curve but a collection of `xyz` data created by technical components, which together define a record, e.g. a TL measurement. A typical argument is `recordType`.

5. `<curve/>` is the lowest node and the only node containing measurement data. All data stored in this node refer to a single technical component, e.g. a photomultiplier. One or many curves define one record. Numerical values in this node are separated by whitespace and span an array with three dimensions (see Eq. 1). Alternatively, this node allows data encoded as base-64 strings.
The crucial concept of the format is that data are stored only in <curve/> nodes, defined by technical components (actual or virtual) **measuring or simulating physical quantities over time**. Data in <curve/> are numerical (measurement/simulation) values of a physical quantity $v_1, ..., v_n; n \in \mathbb{R}$ (discrete/continuous) spanning an array A of the form

$$A_{[x \times y \times 1]}, \dots, A_{[x \times y \times t]} | x, y, t \in \mathbb{Z}; \qquad (1)$$

with $n = \max(x) \times \max(y) \times \max(t)$, where $t$ is the extension of the array with respect to time (i.e. channels per time instant), and $x$ and $y$ define the lateral geometry of the detector. Data are stored column-wise starting from $A_{(1,1,1)}$, $A_{(2,1,1)}$, ..., $A_{(x,y,1)}$ before continuing in the time dimension. For instance, for a measurement over 100 channels with a photomultiplier tube $x = y = 1$ and $t_1, ..., t_{100}$. In contrast, for a measurement with a camera with a lateral resolution of 512 pixels $\times$ 512 pixels, $x, y \in 1, ..., 512$, where $t_1, ..., t_{100}$ remains the same. The dimensional information is stored in the node attributes `xValues`, `yValues`, and `tValues`. All quantities, except for $t$, are dimensionless (i.e. they have no default unit). However, the attributes `xUnit`, `yUnit`, and `tUnit` allow setting SI units. For more attributes and their meaning, we refer to the detailed format description (https://github.com/R-Lum/xlum_specification, last access: 4 July 2022).

# 5 Data representation: example

To illustrate data storage in the XLUM format, we will pick one TL and one green stimulated luminescence (GSL) record belonging to a test sequence measured for one sample. For simplicity, we limit the number of values for each curve to 10 and substitute NA with “...” (three dots). We provide the complete file that can be imported correctly as a Supplement.

**Line 1** Mandatory entry, announcing the XML format. The format follows the Unicode® (The Unicode Consortium, 2022) UTF-8 and must not be changed. In a nutshell, it tells programmes for file parsing the character encoding and ensures that characters are interpreted correctly.

**Lines 2–5** Start of the XLUM record, with mandatory entries, e.g. for the namespace (`xmlns::`...) and the format version used (here: `1.0`), as well as metadata related to the data, e.g. `author` and `license`. Those attributes apply to all child nodes and clarify the data sharing rights in simple and unequivocal terms. In the example, we have applied the Creative Commons (CC) Licence CC-BY (https://creativecommons.org/about/cclicenses/, last access: 26 November 2022). This licence allows unrestricted data reuse, mixing, and sharing with the requirement to credit the data creators.

**Lines 6–7** The `<sample/>` node allows providing information about the sample, e.g. `mineral`, `latitude`. Those data are helpful for explorative data analysis with data from different geographical origins.

**Lines 8–9** The `<sequence/>` node, with general information that remains unchanged for a sequence, e.g. `position` for referring to a position in the equipment.

**Line 10** The first record in the dataset, here of type TL.

**Lines 11–16** The complete entry for data as recorded by the thermocouple, i.e. the sensor monitoring the temperature of the period of 10 s (`tValues`, `tUnit`) recorded in Kelvin (K, `vUnit`). The data (temperature values) are the content of the `<curve/>` node in line 15. For the other two curves, the measured values are listed in line 22 (PMT TL curve) and line 32 (count values GSL).

**Lines 17–23** The second `<curve/>` node with values measured with a PMT during heating (component). Here attributes for `detectionWindow` and `filter` are set. However, those attributes can be set to NA if information is not available or not applicable.

**Lines 25–35** The second record in the sequence with one file with GSL measured with a PMT. While the record may contain information about the stimulation power or settings of the heating during stimulation, that information was not recorded for our example.

**Lines 35–37** The closing tags for the nodes `<sequence/>`, `<sample/>`, and `<xlum/>`.

The example shows that a considerable part of the information refers to data giving information about the data measured with a technical component (PMT or thermocouple), rendering the dataset self-contained and portable without requiring additional documentation.

# 6 Format support in R and Python

Beyond a community-tailored open data solution for storage and sharing, an open exchange format expands the availability of luminescence data beyond the target community. This enables experts from other disciplines to work with luminescence data. Data analytics is a field in the realm of artificial intelligence, machine learning, and data science. Here, two programming languages have gained significant popularity: R (R Core Team, 2022) and Python™ (Python Software Foundation, 2022). To foster the distribution and usage of XLUM, we developed two prototype interfaces: one for R and one for Python™.
<figure><img src="image_8.png" /><figcaption>Listing 2. Example luminescence data representation in the XLUM format.</figcaption></figure>

## 6.1 Programming environment R

We realised support for the XLUM format for the statistical programming language R (R Core Team, 2022) through a new package called *xlum* (Kreutzer and Burow, 2022). The package has reached a stable development status with a test coverage of nearly 100%; i.e. package functionalities are continuously tested in unit tests (see Kreutzer et al., 2017, for more details) on different operating systems. Within the R environment, data elements can be accessed through base R functionality and data types (e.g. *list*; see documentation of the packages *xlum* and *xml2*).

In *xlum*, all file interactions (functions *xlum::read_xlum()*, *xlum::write_xlum()*, *xlum::validate_xlum()*) are realised through the R package *xml2* (Wickham et al., 2021) interfacing the XML parser library *libxml2* (https://gitlab.gnome.org/GNOME/libxml2, last access: 5 July 2022). The generation of XLUM files (function: *xlum::write_xlum()*) follows the workflow outlined in Fig. 4.

First, a format blueprint is derived from a prototype shipped with the package *xlum*. The prototype is then expanded and filled with data. Before export, the file is validated (*xlum::validate_xlum()*) against an XSD schema to ensure that the produced XLUM file follows the correct format specification. Both the prototype and the XSD are copies of files available as part of the XLUM file format definition.

Other than basic import and export functionality to and from R of data shared as *.xlum files, the package *xlum* supports the conversion of various file formats (e.g. *.binx, *.xsyg) to the XLUM format proposed here using R functions commencing with *convert_*(e.g. *xlum::convert_binx2xlum()*) - see Fig. 5. Internally, the conversion uses the R package *Luminescence* (Kreutzer et al., 2012, 2022a) to create R-specific object structures (S4-class objects) tailored to luminescence data, called RLum objects (see Kreutzer et al., 2017, their Fig. 2), and converts these structures using
<figure><img src="image_9.png" /><figcaption>Listing 4. Minimal script for the usage of the Python package *xlum*.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 6. Graphical representation of an XLUM example file, imported with the Python package *xlum* and visualised using the Python library *VESIOT*. Panel (a) shows a TL curve with data from two technical components (photomultiplier and temperature sensor), and panel (b) displays a so-called shine-down luminescence curve, i.e. photomultiplier data.</figcaption></figure>

tages of XML vs. JSON are considered on numerous IT websites, blogs, and platforms such as Stack Overflow (https:// stackoverflow.com, last access: 15 May 2023) or in technology magazines. Without delving into their technical details,

JSON has gained popularity over XML in recent years (e.g. Patrizio, 2016). Nevertheless, given the widespread use (e.g. Copernicus Publications, 2014) and support of XML schemas for data representation in various fields (see examples in Nolan and Lang, 2013), we opted for XML as a robust basis. XML provides standard grammar but remains flexible enough to be tailored to our purpose. If wanted and needed, luminescence data will remain easily transferable to other formats once standardised and archived as XLUM files. Another possibility is an amendment of XLUM, for instance, to facilitate better image data, for which storage is already possible today, and the optional support of base-64 string encoding enables a more efficient representation of those data.

Throughout the paper, we implicitly carried forward the limitation that the XLUM concerns luminescence data only, although a luminescence age is obtained from a luminescence-derived equivalent dose divided by a dose rate. Radionuclide concentration values used to calculate dose rates are derived using different methods, e.g. high-resolution γ-ray spectrometry, in situ γ-ray spectrometry, alpha or beta counting, and inductively coupled plasma mass spectrometry. Different methods make it challenging to develop a data format applicable to all methods. In contrast, nearly every luminescence dating laboratory has access to luminescence readers (recording luminescence data) with comparable technical capabilities. Those readers enable accurate and precise records of dim light emissions down to a single-grain level. Protocols and methods differ. The primary data are luminescence (light) in all cases. Still, a concept development, perhaps again focusing on luminescence dating, might be a reasonable attempt in the future.

The concept of open data has the notion of accessibility and data insight. However, shared data are not becoming automatically accessible, and not every dataset may provide similar valuable insight, but it depends on the experimental design. Making data accessible instantaneously with each study published appears to be advantageous for data users and disadvantageous for donors. For instance, Mills et al. (2015) discuss potential adverse effects on the availability of primary data in the field of biology if investigators of long-term studies are obliged to share their data. Although XLUM is merely a data format that sets no sharing rules, comparisons with different disciplines quickly wear thin. The fear of study authors that others may use their hard work to publish more quickly might be one of the reasons for the “upon reasonable request” data availability statements (Sect. 1). However, in the case of luminescence studies,
ies, long-term studies running over many years are scarce (e.g. Guérin and Visocekas, 2015, for an excellent example of such a study), and single datasets, even such as from a whole stratigraphic section as typical in palaeoenvironmental results studies, are of limited use to others. The true benefit of data sharing lies within many accessible and findable single datasets meaningfully linked through metadata, forming large datasets. With its component-focused design with minimum required metadata, XLUM does the groundwork for datasets aligned in luminescence-based chronologies across different sites in data mining projects concerned with luminescence model development and validation or in any explorative data analysis study.

Last, a significant obstacle to our initiative towards success is the question of broad community acceptance of the new format. Reasonable predictions are difficult to make. We tried to improve the chance of success of our initiative by implementing the first support in the programming language R and Python and by keeping all documents open-access. Furthermore, with the publication of this paper after peer review, the XLUM format will be supported by LexStudio2, the software running lexysg luminescence readers, and could be supported by and/or adopted by other luminescence and dosimetry manufacturers. Further versions of this format will be developed transparently using the GitHub™ repository and are open to comments and contributions. Additionally, we propose allocating future format developments to a dedicated working group under the umbrella of a (to be formed) trapped-charge dating association.

## 8 Conclusions

Our contribution suggests an exchange and long-term data preservation format (XLUM) tailored to the specific requirements of the luminescence (dating) community. The format is XML-based and intended to store primary luminescence data and metadata self-consistently. The format implements (but does not enforce) the FAIR guidelines, facilitating a focus on accessibility and findability.

1. On the data storage level, XLUM does not constrain the amount of data stored for each measurement by an arbitrary format limitation; i.e. the number of components monitored is not limited by the file format. Furthermore, with this approach, the raw data are self-consistent and inherently contain all relevant information returned by a technical component.

2. On the data analysis level, the format design allows better data quality, as the data wanted for the analysis can be combined with additional information from other technical sensor data. These later data, e.g. stemming from a feedback system monitoring a particular instrument setting, might not be needed to answer the research question; however, it allows data validation and

increases confidence in the result. For instance, failure of technical components may have invalidated the measurements and created artefacts. Such records can be excluded in the post-processing.

3. On the data exchange level, data can now be easily exchanged and combined, even though the file format version might be modified in the future, which might increase the overall transparency and value of measurement data.

4. On the preservation level, XLUM is suitable for long-term preservation of measurement data because it uses widely used and supported standard techniques (XML), and it is human-readable.

With XLUM, researchers can share their data using a unified file format or make them accessible via publisher websites to comply with various stakeholder guidelines, such as funding bodies. For the future, we hope that this approach can pave the way toward a new type of luminescence data repository, providing access to primary data on a single component level.

**Code availability.** XLUM format specification: https://doi.org/10.5281/zenodo.7362438 (Kreutzer et al., 2022b). R package *xlum*: https://doi.org/10.5281/zenodo.7362364 (Kreutzer and Burow, 2022). Python package *xlum*: https://github.com/SteveGrehl/xlum-python (last access: 1 July 2022).

**Author contributions.** SK wrote the first draft of the paper, contributed to the original format specification, and developed and tested its implementation for R via *xlum*. SG led the original format design process and wrote down the first format specifications for the XSYG format. He later developed the Python application "xlum-python" to provide support for XLUM. MH contributed to the format specification and implemented its support as output format for the *lexsyg* TL and OSL readers. OS enhanced and refined the original format description. KD raised funds for the development of the predecessor XSYG 2013. GA added critical input to initial versions of this paper and its aims. CB validated and contributed to the *xlum* package. HR and GD contributed to the new format specification in the framework of the Marie Curie Action CREDit. All authors discussed and finalised the paper.

**Competing interests.** Freiberg Instruments GmbH is a manufacturer of commercial luminescence readers. Sebastian Kreutzer and Steve Grehl were employed at Freiberg Instruments GmbH when the XSYG file format, the predecessor of XLUM, was specified in early 2013. The initiative for this paper was undertaken independently by Sebastian Kreutzer in the framework of the Marie Curie Action CREDit to support the distribution of chronological reference data.
**Disclaimer.** Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Acknowledgements.** The XLUM file format has a predecessor. XSYG, originally designed in the course of a 2-month software project in early 2013. The two referees are thanked for their constructive comments on this paper.

**Financial support.** This research has been supported by the Horizon Europe Marie Skłodowska-Curie Actions (grant no. 844457) (CREDit).

**Review statement.** This paper was edited by Pieter Vermeesch and reviewed by Shannon Mahan and one anonymous referee.

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