Early Eocene subarctic terrestrial paleoclimatology inferred from stable hydrogen isotope ratios of precipitation using mummified wood
Final Report Abstract
The overall goal of the project was to test the applicability δ²HLM values as paleoclimate proxy for deep-time fossil wood samples. However, deviating from the initial work schedule, an investigation of high-resolution δ²HLM tree-ring series of the earliest Eocene was not feasible due to challenges regarding the sampling of relevant reference tree-ring material in the envisaged study area. Instead, the project investigated δ²HLM values of fossil woods from other Cenozoic epochs to reconstruct climate-sensitive δ²Hprecip values. We were able to analyze δ²HLM values for 43 fossil woods (found as disintegrated wood pieces) originating from various fluvial sediments found across the Canadian Arctic Archipelago (73 – 80° N). Samples came from five sampling sites of the latest Pliocene (~ 3 Ma) and one middle Miocene site (~12 Ma). Combining a calibrated apparent isotope fractionation (εapp value) with the analyzed δ²HLM values of the fossil woods enabled reconstructions of δ²Hprecip values of the Pliocene and Miocene time periods. The reconstructed δ²Hprecip values are strongly elevated (40 to 70 and 80 ‰ for Pliocene and Miocene, respectively) when compared to equilatitudinal modern day δ²Hprecip values. Such increased δ²Hprecip at highlatitudes values are indicative of a decreased Rayleigh distillation occurring during poleward atmospheric water vapor transport induced by a reduced meridional temperature gradient. Thus, this observation gives important insights regarding the global variability of stable water isotopes (both δ²Hprecip and δ18Oprecip values) which are widely used in paleoclimatology. Furthermore, assuming a modern-day relationship between mean annual temperatures (MATs) and δ²Hprecip values, allowed a MAT approximation of corresponding Cenozoic time periods. Averaged MAT reconstructions for the Pliocene and Miocene suggest ~10 and ~15 °C, respectively, warmer temperatures for the Canadian Arctic when compared to today. The estimations also partly match existing reconstructions derived from cellulose δ²H values of fossil woods found at the same sampling sites. However, the here produced MAT estimations require a more detail insight regarding atmospheric circulation and storm track patterns to evaluate whether the modern MAT- δ²Hprecip can validly applied for the corresponding time periods. Overall, δ²HLM values appear promising when investigating deep-time terrestrial paleoclimatology particularly when considering that fossil wood is typically lignin-enriched whereby cellulose is commonly degraded (or fully absent). Thus, lignin-based stable isotope proxies (such as δ²HLM) may be able to unlock the full paleoclimatic potential of deep-time fossil wood specimen.
Publications
- (2020) Tree-ring δ2H values from lignin methoxyl groups indicate sensitivity to European-scale temperature changes. Palaeogeogr. Palaeoclimatol. Palaeoecol. 546, 109665
Anhäuser, T., Sehls, B., Thomas, W., Hartl, C., Greule, M., Scholz, D., Esper, J., Keppler, F.
(See online at https://doi.org/10.1016/j.palaeo.2020.109665)
