ABSTRACTIncreasing stable carbon isotopic ratio (δ13C) of sedimentary organic matter (SOM) has traditionally been interpreted to reflect an increase in C4 vegetation abundance, though microbial degradation or increasing δ13C values of C3 plants in response to precipitation change can also cause a similar effect. Therefore, δ13CSOM values alone cannot reveal the true origin of the observed 13C 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 (~10‰ positive shifts in δ13CSOM values). However, the present study showed that ~60% (~6‰) of the observed positive shift in δ13CSOM values actually register a temporal change in the C3 plant end‐member δ13C value in response to reduction in Indian summer monsoon precipitation.