Rainfall Reconstruction through Isotope Signatures of Mollusc Micro-fossils from a Late Holocene Archaeological Site in North West India

Mollusc shells, though often microscopic in size, are preserved ubiquitously in archaeological sites due to their robust calcareous composition that withstands post-depositional stresses. Invariably, ‘flotation’ – a method used conventionally to retrieve botanical remains in archaeological sites – also usually recovers tiny but intact mollusc shells. In archaeological research practice, morphometry-based species identification of mollusc shells has, over the past few decades, proven to be an accessible tool to reconstruct locale-specific past environments at ancient human settlements. Broadening the ambit of the utility of mollusc microfossils, isotopic studies have further allowed us to tap into the chemical composition of these shells to yield insights into the environments in which they lived and formed.

In particular, the stable oxygen isotope composition of the shells (δ¹⁸O_shell) of freshwater and terrestrial mollusc is directly contingent on the oxygen isotope composition of the water body (δ¹⁸O_water) in which the organism lived. The latter, in turn, is driven primarily by the rainfall received, the evaporation dynamics vis-à-vis precipitation as well as the ambient temperature. Therefore, δ¹⁸O_shell in archaeological contexts – and otherwise – has been used extensively in recent decades to retrieve information about past hydrological conditions. That said, however, their immense potential as palaeo-environmental proxies has remained under-utilised in Indian and South Asian archaeological contexts, where most mollusc recoveries rarely find mention in archaeological literature or, if they do, are limited solely to morphology-based species identification.

Here, we present a high-resolution record of δ¹⁸O_shell from the Neolithic/Chalcolithic site of Tigrana, Haryana, a site that falls in the wider network of other Mature Indus Valley Civilisation sites (~5200-3900 BP). Shells under examination here were recovered from well-marked stratums during the excavation seasons through 2019-2024; alongside botanical remains (grains, wood). Of the three morphotypes (or, species/genera) identified from those recovered, only one (here, Bithynia sp.) has been used for stable oxygen and carbon isotope analyses to pre-empt any interference potentially arising from species-based fractionation. The same single-specie aliquots were used for radiocarbon dating. Additionally, the inorganic δ¹⁸O data has been supplemented with that of organic plant matter (δ¹³C) wherever possible.

We observe values ranging from -5.07‰ VPDB to -0.66% VPDB between a period of 4500 to 3800 years BP. The period 4300 BP to 4150 years BP, in particular, witnesses rapid fluctuations of the order of 3-4‰ VPDB, indicating abrupt changes in the rainfall and local evaporative regimes in the location in the above timeframe.

This work carries importance not only in terms of utilising micropalaeontological recoveries for palaeo-environmental reconstruction; but also in-terms of ascribing a climatic agency for the gradual decline of the Indus Valley Civilisation. It is noteworthy that most climate records from North-West India, based on isotopic assessments of molluscs, reconstructed for this purpose have been constructed from lake cores. These records, inevitably, carry ‘averaged-out’ signatures, for lakes collect waters through relatively large time-scales. This study, by contrast, is one of the first few attempts at reconstructing a climate-record directly from mollusc shells recovered in-situ from the archaeological site itself.