D/H ratios of methoxyl groups of the sedimentary organic matter of Lake Holzmaar (Eifel, Germany): A potential palaeoclimate/-hydrology proxy

Abstract

Stable hydrogen isotope ratios (δD values) of methoxyl groups in lignin and pectin from a variety of plant types from different locations have been shown to mirror δD values of precipitation, with a mean uniform isotopic fractionation. Since δD in precipitation is mainly influenced by climatic conditions, including temperature, evaporation and precipitation amount, methoxyl groups of organic matter have been proposed as a potential palaeoclimate proxy. Here, we measured the δD values of the methoxyl groups of sedimentary organic matter derived from sediment core segments of Lake Holzmaar (Eifel, Germany). The sediment core covers the entire Holocene and part of the Late Glacial (until 13,650years BP). Since the sedimentary organic matter was indicated by organic source proxies of predominantly phytoplanktonic origin, we considered the δD values of the methoxyl groups as a proxy for the δD value of the lake water. Presently and probably during most of the Holocene, the δD value of precipitation was primarily controlled by temperature (2–4.5‰/°C) as well as in isotopic equilibrium with the δD value of the lake water. We used these modern temperature sensitivities of the Eifel region together with the measured δD values of the methoxyl groups of the sedimentary organic matter to evaluate temperature changes recorded for the Holocene. The resulting mean temperature variance was found to be consistently less than 1.8–4°C and is broadly in line with that reported from the NGRIP ice core for the same period. The transition from the Holocene to the Late Glacial period was accompanied with more negative δD values of the methoxyl groups (∼13‰ for the Younger Dryas). Besides the association with temperature we assumed a higher susceptibility of the δD value of Lake Holzmaar to evaporative enrichment during the Younger Dryas which could serve as an explanation for discrepancies reported for existing palaeoclimate/-hydrology records of the Eifel region. Our findings indicate that the δD values of methoxyl groups have great potential for assisting with palaeoclimate and palaeohydrology, particularly when the organic matter is well characterized and of uniform origin.