We examined the use of δ13C, TOC and C/N geochemistry of sedimentary organic matter to reconstruct former sea levels and paleoenvironments in the absence of suitable microfossil data. The modern distribution of δ13C, TOC and C/N of 33 vegetation and 74 surface sediment samples collected from four coastal wetlands in the Thames Estuary and Norfolk, UK are described. The δ13C, TOC and C/N geochemistry of sediments varied in relation to the input of in situ vascular vegetation versus allochthonous particulate organic matter and algae, which was controlled primarily by tidal inundation. We reviewed published and unpublished studies to produce an English database of vegetation (n = 257) and sediment (n = 132) δ13C, TOC and C/N geochemistry. Four elevation‐dependent environments in the database had statistically distinct δ13C, TOC and C/N values: (1) tidal flat/low marsh (δ13C: −24.9 ± 1.2‰; TOC: 3.6 ± 1.7%; C/N: 9.9 ± 0.8); (2) middle marsh/high (δ13C: −26.2 ± 1.0‰; TOC: 9.8 ± 6.7%; C/N: 12.1 ± 1.8); (3) reed swamp (δ13C: −27.9 ± 0.7‰: TOC: 36.5 ± 11.5%; C/N: 13.9 ± 1.2); and (4) fen carr (δ13C: −29.0 ± 0.6‰; TOC: 41.6 ± 5.7%; C/N: 17.4 ± 3.1). The δ13C, TOC and C/N geochemistry database was applied to a Holocene sediment core collected from the Thames Estuary to produce three new sea‐level index points and one limiting date, illustrating the utility of δ13C, TOC and C/N values to reconstruct Holocene relative sea levels. Copyright © 2015 John Wiley & Sons, Ltd.