Pre-aged plant waxes in tropical lake sediments and their influence on the chronology of molecular paleoclimate proxy records
Douglas, Peter M. J.
Eglinton, Timothy I.
Hodell, David A.
Curtis, Jason H.
Ma, Keith F.
- Publisher: Elsevier
Sedimentary records of plant-wax hydrogen (δD_(wax)) and carbon (δ^(13)C_(wax)) stable isotopes are increasingly applied to infer past climate change. Compound-specific radiocarbon analyses, however, indicate that long time lags can occur between the synthesis of plant waxes and their subsequent deposition in marginal marine sediments. The influence of these time lags on interpretations of plant-wax stable isotope records is presently unconstrained, and it is unclear whether such time lags also affect lacustrine sediments. We present compound-specific radiocarbon (^(14)C_(wax)) data for n-alkanoic acid plant waxes (n-C_(26) to n-C_(32)) from: (1) a sediment core from Lake Chichancanab, Yucatan Peninsula, Mexico, (2) soils in the Lake Chichancanab catchment, and (3) surface sediments from three other lakes in southeastern Mexico and northern Guatemala. ^(14)C_(wax) ages in the surface sediments are consistently older than modern, and may be negatively correlated with mean annual precipitation and positively correlated with lake catchment area. ^(14)C_(wax) ages in soils surrounding Lake Chichancanab increase with soil depth, consistent with deep, subsoil horizons being the primary source of lacustrine aged plant waxes, which are likely delivered to lake sediments through subsurface transport.
Plant waxes in the Lake Chichancanab core are 350–1200 years older than corresponding ages of bulk sediment deposition, determined by ^(14)C dates on terrestrial plant macrofossils in the core. A δD_(wax) time series is in closer agreement with other regional proxy hydroclimate records when a plant-wax ^(14)C age model is applied, as opposed to the macrofossil-based core chronology. Inverse modeling of plant-wax age distribution parameters suggests that plant waxes in the Lake Chichancanab sediment core derive predominantly from millennial-age soil carbon pools that exhibit relatively little age variance (<200 years).
Our findings demonstrate that high-temporal-resolution climate records inferred from stable isotope measures on plant waxes in lacustrine sediments may suffer from possible chronologic distortions as a consequence of long residence times of plant waxes in soils. They also underscore the importance of direct radiocarbon dating of these organic molecules.