Is it C4 or 'C3 in disguise'? Disentangling the ambiguity in palaeovegetation estimation

Increasing stable carbon isotopic ratio (delta C-13) of sedimentary organic matter (SOM) has traditionally been interpreted to reflect an increase in C4 vegetation abundance, though microbial degradation or increasing delta C-13 values of C3 plants in response to precipitation change can also cause a similar effect. Therefore, delta C-13(SOM) values alone cannot reveal the true origin of the observed C-13 enrichment in SOM. Here, employing a wet-oxidation method on modern sediment, we have demonstrated that this treatment removes partly degraded and degradation-prone components of the C3 plant-derived organic matter (OM). Therefore, it helps to understand the real contribution of C4-derived organic carbon (OC) in the modern sediment and identify the C3 plant-derived OC in disguise. As a test case, we extend our inference to Middle to Late Holocene lower Gangetic floodplain records, which were supposed to register a complete switchover from a C3-dominated to a C4-dominated system (similar to 10 parts per thousand positive shifts in delta C-13(SOM) values). However, the present study showed that similar to 60% (similar to 6 parts per thousand) of the observed positive shift in delta C-13(SOM) values actually register a temporal change in the C3 plant end-member delta C-13 value in response to reduction in Indian summer monsoon precipitation. (C) 2022 John Wiley & Sons, Ltd.