Impact of contamination and pre-treatment on stable carbon and nitrogen isotopic composition of charred plant remains

Abstract

Rationale: Stable isotope analysis of archaeological charred plants has become a useful tool for interpreting past agricultural practices and refining ancient dietary reconstruction. Charred material that lay buried in soil for millennia, however, is susceptible to various kinds of contamination, whose impact on the grain/seed isotopic composition is poorly understood. Pre-treatment protocols have been adapted in distinct forms from radiocarbon dating, but insufficient research has been carried out on evaluating their effectiveness and necessity for stable carbon and nitrogen isotope analysis.

Methods: The effects of previously used pre-treatment protocols on the isotopic composition of archaeological and modern sets of samples were investigated. An archaeological sample was also artificially contaminated with carbonates, nitrates and humic acid and subjected to treatment aimed at removing the introduced contamination. The presence and removal of the contamination were investigated using Fourier transform infrared spectroscopy (FTIR) and δ(13)C and δ(15)N values.

Results: The results show a ca 1‰ decrease in the δ(15)N values of archaeological charred plant material caused by harsh acid treatments and ultra-sonication. This change is interpreted as being caused by mechanical distortion of the grains/seeds rather than by the removal of contamination. Furthermore, specific infrared peaks have been identified that can be used to detect the three types of contaminants studied. We argue that it is not necessary to try to remove humic acid contamination for stable isotope analysis. The advantages and disadvantages of crushing the grains/seeds before pre-treatment are discussed.

Conclusions: We recommend the use of an acid-only procedure (0.5 M HCl for 30 min at 80 °C followed by three rinses in distilled water) for cleaning charred plant remains. This study fills an important gap in plant stable isotope research that will enable future researchers to evaluate potential sources of isotopic change and pre-treat their samples with methods that have been demonstrated to be effective.

Figure 1

Offsets in δ¹³C values between untreated and chemically pre-treated archaeological (a: pea, b: barley) and modern (c: lentil, d: bread wheat) samples compared in stage I of this experiment. Offset was calculated as treated – untreated.

Figure 2

Offsets in δ¹⁵N values between untreated and chemically pre-treated archaeological (a: pea, b: barley) and modern (c: lentil, d: bread wheat) samples compared in stage I of this experiment. Offset was calculated as treated – untreated.

Figure 3

δ¹³C and δ¹⁵N values of untreated and ultra-sonicated archaeological samples (a, b: pea; c, d: barley) compared in stage I of this experiment.

Figure 4

FTIR spectra of (a) untreated and carbonate contaminated (at 5%, 10% and 50% by dry mass) archaeological pea sample, and (b) carbonate contaminated samples from above treated with 0.5 M HCl.

Figure 5

FTIR spectra of (a) untreated and nitrate contaminated (at 5%, 10% and 50% by dry mass) archaeological pea sample, and (b) nitrate contaminated samples from above washed with Milli-U water.

Figure 6

FTIR spectra of (a) untreated and humic salt contaminated (at 5%, 10% and 50% by dry mass) archaeological pea sample, (b) humic salt contaminated samples from above treated with an acid-base-acid protocol, and (c) humic salt contaminated samples from above treated with a base-acid protocol (for details on the treatment methods, refer to the text). All contaminated samples are PEAar unless specified otherwise.