Laser-based stable isotopic analyses of carbonates to obtain high-resolution climatic signals and its application to understand Early Harappan climate

Abstract

<p>Laser-based online carbonate analyses open up a simple yet efficient way to explore high-resolution (~125 µm) investigation of different biogenic (<em>viz. </em>bivalves, fish otolith) and abiogenic (viz. speleothem, varves) carbonates for deciphering climatic signals. Stable isotopic sclerochronological analyses on biogenic carbonates that had formed in equilibrium with the ambient water effectively record the temporal changes in the environmental parameters like temperature, salinity and isotopic composition. Similar analysis on archaeological fossil assemblages helps to understand past climate, precipitation variability with respect to cultural changes observed in the archaeological site.</p><p>Dholavira, a UNESCO world heritage site, was a magnificent seven-staged Harappan metropolitan, situated within the Rann of Kutch, Gujarat, India. The city witnessed cultural evolution from Pre- to Post- Harappan (Indus valley civilisation) culture with some minor desertion periods during the late Mature- and early Late-Harappan stages. A previous study has already proposed an estuarine environment, with the presence of a glacial-fed river (ancient Indus distributary), which mixed with the intruding seawater in the Early Harappan Rann. Here, with the high-resolution laser-based technique, we further examined this proposition by analysing and deciphering monthly climatic changes from a 4.6 Kilo year old otolith of a migrating estuarine catfish, <em>Arius dussumieri</em> and a 5.2 Kilo year old freshwater bivalve, <em>Parreysia</em> <em>corruguta</em>. The ẟ<sup>18</sup>O of the otolith carbonate ranges from ~-9 ‰ to ~-1 ‰ (VPDB), corroborating the presence of a highly depleted (ẟ<sup>18</sup>O<sub>water</sub>(VSMOW)~-12 ‰ ) freshwater river debouching into an ancient estuary during summer/monsoonal times. The enriched ẟ<sup>18</sup>O values during winter/ non-monsoonal months result due to evaporative enrichment, reduction in riverine discharge in the semi-enclosed estuarine environment. The winter migration of the fish into deeper water might enrich the otolith ẟ<sup>18</sup>O values further. The monthly climatic signal from the typically freshwater bivalve with a ẟ<sup>18</sup>O variation from ~-10 ‰ to ~-7 ‰ (VPDB) might indicate a presence of a freshwater body (lake/ponds?) receiving monsoonal recharge with ~-7 ‰ (VSMOW; compared to annual weighted modern ẟ<sup>18</sup>O<sub>precipitation</sub>(VSMOW) of -5 ‰) and a slight evaporative enrichment during winter months during Pre- to Early Harappan times. High-resolution climatic studies from different biogenic carbonates thus suggest that the agricultural society of Dholavira flourished in a more conducive environment with riverine discharges and slightly higher precipitation, in contrast to the present day inhabitable arid climate.</p>