RECONSTRUCTING ARCTIC PALEO-ENVIRONMENT: WHAT LIGHT HAS TO DO WITH IT?

Reconstructing paleo-environments in the Arctic plays a critical role in understanding Earth system dynamics in the past and holds a key in predicting future climate and ecosystem changes. Fossil floras, plant-derived biomarkers, and their stable isotopes are among routinely applied proxies to investigate hydrological, atmospheric, and biotic changes in the Arctic. The unique light regime in the Arctic has long been recognized to influence plant photosynthesis, thus critically regulating almost all morphological, physiological, and biochemical development processes. However, only recently, quantitative assessment of these impacts on their paleo-environmental signals has become available.

Using modern and fossil deciduous conifers that once widely distributed in high latitudes, we investigate how Artic light impacts on plant cuticle micro-morphology as well as stable carbon and hydrogen isotope fractionations. We found that comparing with their low latitudinal counterparts, the whole leaf stomatal density (WL-SD) is noticeably lower under the Arctic light while plant guard cell length (GCL) and width (CGW) show little difference. Leaves developed under the Arctic light tend to exhibit 1.5–4‰ relatively depleted δ¹³C values but yield 25–35‰ more positive δD values, leading to a smaller apparent hydrogen isotope fractionation factor between their lipid and environmental water (ε_lipid-water). Our data further shows that light intensity and duration may affect plants differently, and the degree of light impact may vary for different plant taxa.

As both WL-SD and δ¹³C values are key parameters used in the Franks Model to reconstruct ancient atmospheric CO₂, better understanding the impacts of Arctic light on these values and the dynamic balance of these key parameters under different light regimes will improve the accuracy of paleo-CO₂ estimation. While the precise mechanism for the smaller ε_lipid-water under Arctic light remains speculative, such an isotope behavior, well-documented in both greenhouse studies, field observations, and geological samples, will impact on paleo-hydrology inference. Taxon-specific knowledge on the impact of Arctic light should be taken into account in paleo-environmental reconstruction using Arctic plant material.