Carbon isotope and organic geochemistry of the Holocene sediments from Rann of Kachchh: implications to the preservation of organic matter and climate during the Indus Valley Civilisation

The large difference in the degree of discrimination of stable carbon isotopes (δ¹³C) between C₃ and C₄ plants has been widely used to retrieve the palaeoenvironmental condition by analysing δ¹³C of bulk sedimentary organic matter (SOM). Underlying in these studies was the assumption that carbon retains the pristine signature of its photosynthetic pathway during later stages of decomposition in soil and sediments. However, there remains considerable uncertainty associated with studies of SOM, especially those from marginal marine environments. The probable presence of organic matter derived from varied sources, e.g., marine sources, terrestrial C_3, and C₄ plants make reconstruction of the paleo-environment difficult using δ¹³C_SOM as a stand-alone tool. The sediments also undergo different stages of microbial decomposition, which can also alter the original organic carbon source signatures. Hence a robust method needs to be developed for identifying the specific phase that can withstand the alteration of the original δ¹³C of SOM. In the present study, we attempted to develop a simple means for identifying a robust oxidation-resistant organic carbon (OROC) phase for bulk isotopic analysis. The data along with the straight-chain n-alkane lipid compound were used to retrieve the Holocene (last 10 Kyr) paleo-environment from a sediment core raised from the Rann of Kachchh, western India. One purpose was to see if the climate had any role in the growth and collapse of an Indus Valley Civilisation (IVC) metropolis Dholavira, a UNESCO heritage site in the vicinity of the core location.

The sediment samples were chemically treated over different oxidation times (24 to 240 hours) following the commonly used dichromate oxidation method (0.1M K₂Cr₂O₇/ 2M H₂SO₄, 60 ⁰C). No more oxidation loss was observed between pre-and post-treatment of SOM after 72 hours suggesting that the remaining organic carbon represents the most resistant phase. The isotopic composition (δ¹³C_OROC)would thus represent the original isotopic signature of the refractory organic carbon. In the specific sediment core, the δ¹³C_OROC values showed no significant difference from the δ¹³C_SOM exhibiting a good down-depth correlation (R² >0.8). The δ¹³C data of the core top sediment along with the modern plants in the Rann suggest that local vegetation dominantly controlled the organic matter composition. The efficacy of the method was also tested by analysing δ¹³C_OROC and δ¹³C_SOM (δ¹³C_SOM ranged from -18.2 ‰ to -20.6 ‰) in ten marine sediment samples from the northern Indian Ocean indicating preservation of marine organic matters after the oxidation experiment. The sediment core data suggest a mixture of terrestrial C₃, C₄, and marine organic matter throughout the Holocene period. A significant increase in the concentration of C₄ photosynthesizing plant groups around 4.2 Kyr is observed and most likely is an expression of enhanced aridity due to the Meghalayan age drought that pervaded the Indian subcontinent and beyond. This is fascinating as the drought has earlier been linked to the collapse of the IVC based on other proxies.