04/01202 13C-depleted charcoal from C4 grasses and the role of occluded carbon in phytoliths : Krull, E. S. et al. Organic Geochemistry, 2003, 34, (9), 1337–1352

Abstract

We synthesise records of stable carbon isotopes (δ13C values), TOC/TN, magnetic susceptibility (χlf), palynology, and phytoliths for a late Quaternary lacustrine archive from the northern Gangetic Plain, India to determine the primary driver(s) of past C3/C4 plant variability. The study reveals nine climate-driven shifts in vegetation over the last 15.2 ka and provides a comprehensive picture of the evolution of the lake sequence. During 15.2–14.5 ka, lower δ13C values, relatively higher TOC/TN ratios and χlf values suggest increasing contribution of C3 plant-derived organic matter into the lake and enhanced hydroclimatic conditions in the northern Gangetic Plain. Subsequently, a warm and humid phase corresponding to the Bølling–Allerød interstadial is inferred during 14.5–12.8 ka, followed by evidence of a waning of the Indian Summer Monsoon (ISM) during the Younger Dryas from 12.8–11.7 ka. Despite a strong ISM regime during the early-Greenlandian (11.7–11.0 ka), vegetation shifted from a mixed C3-C4 to a C4 dominated composition as inferred from higher δ13C values, TOC/TN ratios and χlf values. However, a weakened ISM phase is noted between 11.0 and 10.2 ka. On return of warm and humid climatic conditions during 10.2–8.3 ka, tall C4 Panicoideae grasses flourished around the lake. Two weaker hydroclimatic phases between 8.3 and 6.5 ka and 4.2–2.5 ka favoured C4-short Chloridoideae grasses over C4-tall Panicoideae members. Moderate hydroclimatic conditions after 0.6 ka favoured C4-tall moist-loving grasses. This study shows that before shifting to a C4-dominated vegetation during the early-Greenlandian to early-Northgrippian, C3 plants used to dominate the northern Gangetic Plain throughout the late-Pleistocene period. We infer that temperature and rainfall jointly influenced the diversity and distribution of C4 plants in the northern Gangetic Plain.