07 Mar 2022
07 Mar 2022
Status: this preprint is currently under review for the journal BG.

Carbon isotopic ratios of modern C3 and C4 vegetation on the Indian Peninsula and changes along the plant–soil–river continuum; implications for (paleo-)vegetation reconstructions

Frédérique Kirkels1, Hugo de Boer2, Paulina Concha Hernández1, Chris Martes1, Marcel van der Meer3, Sayak Basu4,a, Muhammed Usman5,b, and Francien Peterse1 Frédérique Kirkels et al.
  • 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Princetonlaan 8a, 3584 CB Utrecht, The Netherlands
  • 2Department of Environmental Sciences, Copernicus Institute of Sustainable Development, Faculty of Geosciences, Utrecht University, Princetonlaan 8a, 3584 CB Utrecht, The Netherlands
  • 3Department of Marine Microbiology and Biogeochemistry, NIOZ Royal Netherlands Institute for Sea Research, PO Box 59, 1790 AB Den Burg, the Netherlands
  • 4Department of Earth Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India
  • 5Geological Institute, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland
  • apresent address: Geological Oceanography Department, National Institute of Oceanography, Dona Paula - 403 004, Goa, India
  • bpresent address: Department of Physical & Environmental Sciences, University of Toronto Scarborough, Toronto, Ontario M1C1A4, Canada

Abstract. The large difference in the fractionation of stable carbon isotopes between C3 and C4 plants is widely used in vegetation reconstructions, where the predominance of C3 plants suggests wetter and that of C4 plants drier conditions. The isotopic composition of organic carbon (OC) preserved in soils or sediments may be a valuable (paleo-)environmental indicator, based on the assumption that plant-derived material retains the carbon isotopic signature of its photosynthetic pathway during transfer from plant to sediment. In this study, we investigated the carbon isotopic signature of C3 and C4 plants (δ13C) and of organic carbon (δ13Corg) in soils, river Suspended Particulate Matter (SPM) and riverbed sediments, to gain insight in the control of precipitation on C3 and C4 plant δ13C values and to assess changes in δ13Corg values along the plant–soil–river continuum. This information allows us to elucidate the implications of different δ13C end-members on C3/C4 vegetation reconstructions. Our analysis was performed in the Godavari River basin, which has mixed C3 and C4 vegetation and is situated in the Core Monsoon Zone in peninsular India, a region that integrates the hydroclimatic and vegetation changes caused by variation in monsoonal strength. The Godavari C3 and C4 plants revealed more negative δ13C values than global average vegetation values, suggesting region-specific plant δ13C signatures. Godavari C3 plants confirmed a strong control by Mean Annual Precipitation (MAP) on their δ13C values, with an isotopic enrichment of ~2.2 ‰ for the interval between ~500 and 1500 mm y-1. Tracing δ13Corg values from plant to soils and rivers revealed that soils and riverbed sediments reflected the transition from mixed C3 and C4 vegetation in the dry upper basin to more C3 vegetation in the humid lower basin. Soil degradation and stabilisation processes and hydrodynamic sorting within the river altered the plant-derived δ13C signal. Phytoplankton dominated the δ13Corg signal carried by SPM in the dry season and year-round in the upper basin. Our analysis revealed that the reconstructed C3/C4 vegetation composition was sensitive to the plant δ13C end-members used as mixing model input. The %C4 plants in the different subbasins was ~10–19 % higher using Godavari-specific end-members than using global averages, and including a correction for drought enrichment in Godavari C3 plants resulted in a 2–10 % lower estimated C4 plant cover. Hence, incorporating region-specific plant δ13C end-members and drought correction of the C3 end-member in mixing models need to be considered to determine C3 and C4 distributions of modern- and paleo-vegetation in monsoonal regions.

Frédérique Kirkels et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-57', Anonymous Referee #1, 20 Apr 2022
  • RC2: 'Comment on bg-2022-57', Sarah Feakins, 12 May 2022

Frédérique Kirkels et al.

Frédérique Kirkels et al.


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Short summary
The distinct carbon isotopic signatures of C3 and C4 plants are widely used to reconstruct past hydroclimate, where more C3 plants represent wetter and C4 plants drier conditions. Here we examine if plant isotopic signatures in the Godavari River basin in peninsular India follow the global patterns. We find that Godavari plants have a distinct isotopic signature and are influenced by drought stress, which introduces a bias in C3/C4 plant estimates and associated hydroclimate reconstructions.