(It's not often I get to use my MA (Oxon) in "Agricultural and Forest Sciences" for Stonehenge research so I am please to be able to share this.)
Corroboration of Snoeck et al.’s
References by Recent Research
The landmark study by Snoeck et al.
(2018) in Scientific Reports used strontium and carbon isotope analysis to interpret the
origins of individuals and the wood used in their cremation pyres at
Stonehenge. Their interpretations relied on foundational research into the
“canopy effect”—the phenomenon by which plants in dense forests exhibit
depleted δ¹³C values compared to those in open environments. Seminal studies
such as van der Merwe & Medina (1991) and Drucker et al. (2008) established that this effect is due to a combination of reduced
light intensity and the recycling of ^13C-depleted CO₂ from soil respiration,
with these depleted signatures transferring up the food chain, Vogel (1978).
Recent research has directly measured
the canopy effect in woody tissues, addressing earlier limitations where
extrapolations were made primarily from grasses. For example:
·
van der Sleen et al. (2014) analyzed tree rings of Peltogyne
cf. heterophylla in a Bolivian moist forest, finding that Δ¹³C values
decreased by 1.5–2.5‰ after gap formation due to increased light, confirming
the predominant role of light in the canopy effect.
·
Brienen et al. (2022) studied Cedrela trees
across three tropical forests, observing Δ¹³C reductions of 4–6‰ from
understory (24–25‰) to canopy (17–18‰), with tree height as the main driver
(–0.15 to –0.41‰ per meter).
·
Starkovich et al. (2024) demonstrated that hazelnut shells from denser canopies had δ¹³C
values up to 5‰ lower than those from open settings.
These studies provide direct,
quantitative evidence of the canopy effect in woody tissues, validating the
isotopic principles applied by Snoeck et al. (2018) to infer the origins of
pyre wood at Stonehenge.
In-Depth Explanation of the Canopy
Effect
The canopy effect in isotope ecology
refers to the systematic depletion of δ¹³C values in plants growing under dense
woodland canopies. This is primarily due to:
Reduced light intensity: Limits photosynthesis, increasing the ratio of intercellular to ambient CO2 (Ci/Ca), and thus enhancing discrimination against 13C.
Recycling of 13C-depleted CO2: Soil respiration under forest cover releases CO2 with δ13C around –27‰, further depleting plant isotopic signatures.
The δ13C value, expressed in per mil (‰) relative to the Vienna Pee Dee Belemnite (VPDB) standard, is governed by carbon isotope discrimination (Δ13C) during photosynthesis in C3 plants (which dominate UK woodlands). The standard model, after Farquhar et al. (1982), is:
Δ13C = a + (b - a) ⋅ Ci/Ca
where:
- a ≈ 4.4‰ (fractionation during diffusion),
- b ≈ 27‰ (fractionation during carboxylation),
- Ci/Ca is the ratio of intercellular to ambient CO2 concentration.
In dense canopies, low light raises Ci/Ca (e.g., 0.7–0.9), leading to greater discrimination and lower δ13C values (–30‰ to –32‰). In open landscapes, higher light reduces Ci/Ca (e.g., 0.5–0.7), resulting in higher δ13C values (–25‰ to –27‰).
Bonafini et al. (2013) quantified this in Wytham Wood, UK, finding up to 5‰ δ¹³C
depletion in grasses under closed canopies, primarily due to shading. Recent
timber studies, such as Brienen et al. (2022), confirm similar depletions in tree
rings, with Δ¹³C shifts reflecting canopy density. However, water stress can
also influence δ¹³C in drier sites, potentially confounding purely light-driven
effects.
Archaeological Application
In cremation contexts, the canopy
effect is preserved in bone apatite, as carbon from pyre wood is incorporated
during high-temperature burning. Lower δ¹³C values in cremated remains suggest
wood from dense woodlands; higher values indicate more open landscapes. This
isotopic signature, combined with strontium isotope analysis, enables
reconstruction of both human mobility and the environmental context of
cremation practices.
Link to Snoeck et al.’s Stonehenge Work
Snoeck et al. (2018) analyzed strontium (87Sr/86Sr) and carbon (δ¹³C)
isotopes in 25 cremated human remains from Stonehenge. Strontium isotopes in
tooth enamel indicated that 10 individuals had ratios consistent with west
Wales (e.g., Preseli Hills, the source of Stonehenge’s bluestones), while others
matched the local chalk geology of Salisbury Plain. δ¹³C analysis of cremated
bone apatite revealed that individuals from Wales had lower δ¹³C values,
suggesting cremation with wood from denser woodlands, while those from Wessex had
higher values, indicating wood from open downlands.
Recent studies directly corroborate
this interpretation. Starkovich et al. (2024) showed that woody tissues from dense
canopies have δ¹³C values up to 5‰ lower, matching the lower δ¹³C in some
Stonehenge remains. Brienen et al. (2022) confirmed that tree rings in shaded
understories exhibit significant δ¹³C depletion, supporting the idea that Welsh
woodlands produced the wood for some pyres. In contrast, the open Wessex
downlands, with higher δ¹³C values, align with the isotopic signatures of local
cremations.
Archaeological
Context: Transport of Cremated Remains
The physical context of the burials
reinforces this interpretation. Excavations by Hawley and later researchers
found that many cremation deposits at Stonehenge were clustered in the Aubrey
Holes and were often contained within circular margins, suggesting they had
been placed in organic containers—most likely leather bags—before burial. These
organic containers have long since decayed, but their impressions remain,
supporting the hypothesis that the cremated remains were transported as
discrete packages.
As summarized by recent overviews
and the Stonehenge Riverside Project, Willis, C. et al. (2016), this practice fits with the idea that
Stonehenge was a ceremonial centre where people from distant regions—including
west Wales—brought their dead for burial. The movement of both
the bluestones and people from the Preseli region underscores the monument’s
role as a focal point for inter-regional connections during the Neolithic.
Synthesis: Cremation in Wales,
Burial at Stonehenge
Given the combined strontium and
carbon isotope evidence, and the archaeological context of the cremation
deposits, the most parsimonious explanation is that the individuals with Welsh
isotopic signatures were cremated in west Wales using local woodland fuel.
Their remains were then carefully collected—likely in leather bags or similar
containers—and transported to Stonehenge for burial. This scenario is
supported by the absence of local pyre debris at Stonehenge, the preservation
of distinct isotopic signatures, and the physical evidence for organic
containers.
This interpretation is now widely favoured
over the alternative hypothesis that non-local wood was transported to
Stonehenge for use in cremations. It also fits with the broader pattern of
Neolithic mobility and ritual, where both people and materials—including the
bluestones—were moved over considerable distances.
References
·
Bonafini,
M., Pellegrini, M., Ditchfield, P., & Pollard, A. M. (2013). Investigation of the ‘canopy effect’ in the isotope
ecology of temperate woodlands. Journal of
Archaeological Science, 40, 3926–3935.
·
Brienen,
R. J. W., Schöngart, J., Zuidema, P. A., et al. (2022). Paired analysis of tree ring width and carbon isotopes
indicates when controls on tropical tree growth change from light to water
limitations. Tree Physiology, 42(6), 1137–1150.
·
Drucker,
D. G., Bridault, A., Hobson, K. A., et al. (2008). Can carbon-13 in large
herbivores provide an insight into palaeoenvironmental conditions? Palaeogeography, Palaeoclimatology,
Palaeoecology, 266(3–4), 183–191.
·
Snoeck,
C., Pouncett, J., Claeys, P., et al. (2018). Strontium isotope analysis on cremated human remains
from Stonehenge support links with west Wales. Scientific
Reports, 8, 10790.
·
Starkovich,
B. M., Krauß, R., & Britton, K. (2024). Carbon isotope values of hazelnut shells: a new proxy
for canopy density. Frontiers in Environmental Archaeology,
3, 1351411.
·
van der
Merwe, N. J., & Medina, E. (1991). The canopy effect, carbon isotope ratios and foodwebs
in Amazonia. Journal of Archaeological Science,
18(3), 249–259.
·
van der
Sleen, P., Groenendijk, P., Vlam, M., et al. (2014). Understanding causes of tree growth response to gap
formation: Δ¹³C-values in tree rings reveal a predominant effect of light. Trees,
29, 439–448.
·
Vogel, J.
C. (1978). Recycling of carbon dioxide in a forest environment. Oecologia Plantarum, 13, 89–94.
·
Willis,
C. et al. (2016) ‘The dead of Stonehenge’, Antiquity,
90(350), pp. 337–356. doi:10.15184/aqy.2016.26.
⁂
