Early Eocene subarctic terrestrial paleoclimatology inferred from stable hydrogen isotope ratios of precipitation using mummified wood

始新世早期（距今5500万年~ 5000万年[Ma]）由于大气温室气体浓度高而具有“温室”气候的特征。这个区间被认为是近未来气候条件的类似物，因此一直是激烈的（古）气候研究的主题。然而，大多数始新世气候动力学是利用海洋气候档案重建的，而由于缺乏适当的档案，陆地领域的了解较少。在始新世火山碎屑金伯利岩中发现的干化（非石化）木材可能有助于陆地气候调查。根据保存状态的气候代用物，如树木年轮宽度和化合物特定的稳定氢同位素比率（以δ 2H值表示），可以确定年分辨率。加拿大多伦多大学（University of Toronto）收藏了大量的干尸木材（由Jochen Halfar教授监督），其中包括12个年代际到百年的年轮系列，每年到五旬的分辨率。在加拿大西北亚北极地区的Diavik A-154 （55.5 Ma）、Diavik A-418 （55.2 Ma）和Ekati Panda （53.3 Ma）三个早始新世金伯利岩中发现了标本。在最近的一项先驱研究中（使用了12个干枯树木年轮系列中的3个），我提出木质素甲氧基作为干枯木材的有希望的气候代用物，因为(I)木质素是主要的剩余木材化合物，在很大程度上排除了常用纤维素的提取；（ii）在现代树木中，它们的δ 2H值反映了由均匀同位素分馏（以epsilon值表示）调节的降水中的气候敏感δ 2H值。然而，利用发现的古代分类群（Taxodioxylon）重建降水中的δ 2H在研究的早期阶段受到阻碍，因为最近的现存亲戚水杉（Metasequoia glyptostroboides）的epsilon值尚未确定。在这里，我建议在加拿大的一个研究地点校准水杉的物种特异性epsilon值，以加强木质素甲氧基的δ 2H值作为气候代理的潜力。随后，校准后的epsilon值将被用于前所未有的高分辨率，通过利用多伦多大学可获得的完整干化木材收集来重建始新世早期陆地气候的多个世纪长度。因此，这项研究不仅有望建立木质素甲氧基的δ 2H值，作为干尸木样品的“替代选择”，而且还将显著推进对始新世早期陆地古气候学的认识，这是一个对了解高二氧化碳世界近未来气候条件具有重要气候意义的时期。

The early Eocene (~55-50 million years before present [Ma]) was characterized by a 'hothouse' climate as a result of high atmospheric greenhouse gas concentrations. This interval is regarded as analogue for near-future climatic conditions and has therefore been subject of intense (paleo-) climate studies. However, most of Eocene climate dynamics is reconstructed using marine climate archives whereby the terrestrial realm is less understood due to a lack of appropriate archives. Helpful for terrestrial climate investigations may be the use of mummified (non-petrified) wood found in Eocene volcaniclastic kimberlites. Depending on the state of preservation climate proxies such as tree-ring width and compound-specific stable hydrogen isotope ratios (expressed as Delta 2H values) at annual resolution can be determined. The University of Toronto (Canada) holds a large mummified wood collection (supervised by Prof. Jochen Halfar) comprising 12 multi-decadal to -centennial length tree-ring series at annual to pentadal resolution. Specimens were found in three early Eocene kimberlites such as Diavik A-154 (55.5 Ma), Diavik A-418 (55.2 Ma) and Ekati Panda (53.3 Ma) in the subarctic Northwest Territories (Canada). In a recent pioneer study (using three of the 12 mummified tree-ring series) I proposed the lignin methoxyl groups as promising climate proxy for mummified wood as (i) lignin is the dominant remaining wood compound largely excluding the extraction of commonly used cellulose and (ii) in modern trees their delta 2H values reflect the climate-sensitive delta 2H value in precipitation modulated by a uniform isotope fractionation (expressed as epsilon value). However, reconstructions of delta 2H in precipitation using the found ancient taxa (Taxodioxylon) are - at this early stage of research - hampered since the epsilon value of the nearest living relatives, Metasequoia glyptostroboides, has yet to be determined. Here, I propose to calibrate the species-specific epsilon value of Metasequoia glyptostroboides at a Canadian study site to strengthen the potential of delta 2H values of lignin methoxyl groups as a climate proxy. Subsequently, the calibrated epsilon value will be used to produce unprecedented high-resolution, multi-centennial length reconstructions of early Eocene terrestrial climate by exploiting the full mummified wood collection available at the University of Toronto. Hence, this research is not only expected to establish the delta 2H values of lignin methoxyl groups as 'proxy of choice' for mummified wood samples but also significantly advance knowledge of terrestrial paleoclimatology of the early Eocene, a climatically important period for the understanding of near-future climate conditions in a high-CO2 world.