**1. Introduction**

Stratigraphic documentation and interpretation are crucial parts of archaeological research. Since the emergence of the archaeological discipline, researchers have been concerned with the identification of methods that allow more objectivity in stratigraphic delineation and interpretation. The fundamental work of, e.g., Harris [1] is meanwhile complemented by a progressing digitisation of fieldwork (e.g., [2]) and a growing number of on-site measurements and analyses carried out in various

laboratories in the aftermath of an excavation. These techniques comprise, e.g., the establishment of soil colour standards (cf. [3,4]), geochemical analyses of sediments (e.g., [5–7]), the analysis of thin sections (e.g., [8]), or the analysis of pollen [9] and phytoliths [10].

The overall objectivity of stratigraphic interpretation clearly benefits from these techniques. Nevertheless, the initial on-site delineation of stratigraphic layers (i.e., stratigraphic documentation) remains partly influenced by the perception of the respective researcher at work. As pointed out by Zhang and Hartemink [11] and Haburaj et al. [12], statistical analysis of RGB and multispectral images of soil profiles can be utilised to semi-automatically delineate stratigraphic units. These derived units are based on the physical measurement of the sediments' spectral reflectance and allow the respective researcher to critically face up to his or her delineation.

Building upon these developments, our study further explores the potential of visible and near-infrared (VIS-NIR, 400–700–2500 nm) spectral data for archaeological fieldwork. Due to its non-destructive character and the moderate extent of necessary sample preparation, spectroscopy in general has the potential to save time and costs during fieldwork (e.g., [12–15]). Working with an archaeological cross-section from a Bronze Age burial mound from Brandenburg, northern Germany, we investigate how traditional geochemical sediment analyses, selective spectroscopic measurements, and extensive RGB and multispectral image data can be used to objectivise archaeological field documentation by quantitatively capturing stratigraphic layers. Using a detailed on-site description supported by quantitatively measured sediment properties (grain size composition, water content, and soil organic carbon content) as a measure of quality, we compare clustering results of (i) extensive colour measurements (i.e., image data) acquired with an RGB and a multispectral camera during fieldwork, as well as clustering results of selectively sampled (ii) portable energy-dispersive X-ray fluorescence spectrometer (p-ED-XRF) data and (iii) visible and near infrared (VIS-NIR) hyperspectral data, both acquired in the laboratory. This experimental approach allows us to illustrate the benefits and shortcomings of systematic spectral measurements as a complementary or alternative method for stratigraphic documentation, delineation, and interpretation.

The characteristics of the recorded diffuse reflectance spectra of the sediments are directly linked to their chemical and physical composition (e.g., [13]). Within the last two decades, an increasing number of publications were devoted to the prediction of soil properties via VIS-NIR spectroscopy (e.g., [16–20]). Soriano-Disla et al. [21] deliver an extensive overview on the predictability of multiple soil properties via visible, near-infrared, and mid-infrared spectral data. We therefore examined how the conducted image classification performs when using additional input data: We were able to calibrate a prediction model for soil organic carbon (SOC) based on the recorded spectral signal and created a raster containing predicted SOC values for the entire profile. Several authors have argued that the CIELAB colour space (Commission Internationale de l'Eclairage) shows a clear benefit regarding quantitative analyses (e.g., [22–27]). The CIELAB colour space consists of three values: L\* is the vertical axis and is defined by lightness, while a\* and b\* represent the chromaticity (a\*: Red/green; b\*: Yellow/blue). We examined colour transformation to the CIELAB colour space, as it was found to be highly effective when analysing soil profiles (e.g., [28]). Hereby, our overarching aim remains the delineation of stratigraphic layers rather than the quantitative examination of sediment properties.

Our results indicate the importance of quantitatively measuring sediment colours—be it selective or extensive—as they constitute an additional layer of documentation during an archaeological excavation, which can be acquired easily and render the on-site archaeological documentation and interpretation more transparent and reproducible. The study at hand is presented as an experimental investigation, which allows us to assess the potential of VIS-NIR spectral sensors for archaeological fieldwork.
