Andreas Lücke

ISOTOPE SIGNALS AS CLIMATE PROXIES: THE ROLE OF TRANSFER FUNCTIONS IN THE STUDY OF TERRESTRIAL ARCHIVES

Abstract The use of stable isotopes as climate proxies is widely accepted for climate reconstruction. Re-establishing climate signals, e.g. temperature, from isotope values of biological materials requires some knowledge of the system’s response behaviour. The related problems are discussed in conjunction with stable isotopes of two different terrestrial archives: carbon isotope data of tree rings from southern Germany and oxygen isotope data of diatoms from Lake Holzmaar, Germany. Carbon isotope temperature coefficients (Δδ13C/ΔT) derived from tree rings were chosen as an example for non-linear transformation of environmental signals through biological systems explaining negative and positive temperature coefficients. Thin radial tree ring sections taken from tree rings of different species (poplar, beech and oak) show carbon isotope variations of up to 3‰ with a characteristic, annually recurring isotope pattern. This behaviour is discussed in view of time resolution, isotope signature conservation and the question of storage and remobilisation of photosynthates with time. For Lake Holzmaar it is shown that isotope proxy signals derived from diatoms may not be unambiguously translated into abiotic environmental forcing factors, such as for example temperature. Corresponding measurements reveal that the isotopic input signal varies considerably and nonlinearly with temperature.

Scaling input data by GIS for hydrological modelling

An analysis of scaling effects is performed to evaluate whether data aggregation is a useful regionalization tool or whether it leads to an unacceptable loss of information. One issue concerns the appropriate resolution of digital elevation models (DEMs) used to derive geomorphological input parameters for hydrological models. In particular, the scale problem of watershed division by a channel network and smaller sub-basins is addressed. The investigation involved commercially available data sets with different horizontal and vertical resolutions and systematically aggregated DEMs. A stream network and the contributing subareas were derived from a DEM with a distinct critical support area. By varying this threshold area various watershed configurations were obtained. The sensitivity of surface runoff simulations to all watershed configurations was studied with synthetic storms and by means of an infiltration excess runoff model. The study revealed that elevation data with different resolutions diverge enormously in landscape representation and in the derived parameters such as slopes, flow directions and channel networks. Coarse DEMs show a smoother terrain and shorter flow paths than highly resolved data. The contributing threshold area controls the extent of the watershed configuration and therefore determines the drainage density. These topographic and geomorphological features were used to explain differences in the runoff simulation results. When watershed configurations with a varying extent of the channel network were derived from a distinct DEM and then used to simulate surface runoff, the drainage densities of the configurations correlated with the simulated runoff volume. A distinct drainage density, however, did not necessarily lead to similar simulation results when different DEMs were used. Since the hydrological model permits reinfiltration, the runoff volume depends directly on the lengths of the overland flow. Therefore, the mean length of the overland flow paths might to a certain degree be considered as a scaling factor. Copyright

Paleoenvironmental implications from biomarker and stable isotope investigations on the Pliocene Velenje lignite seam (Slovenia)

A Pliocene lignite seam up to 160 m thick occurs in the Velenje basin (Slovenia). The seam originated in a topogenous mire and evolved within a non-marine, transgressive setting. Differences in soluble organic matter yield and hydrocarbon content of borehole samples from the lignite are related to differences in the composition of free lipids of microbial origin and/or hydrocarbons derived from the biogeochemical degradation of plant tissue. Variations of the redox conditions within the mire are reflected by pristane/phytane ratios. The abundance of terpenoid biomarkers indicates the predominance of gymnosperms over angiosperms, which is consistent with palynomorphic spectra dominated by pollen of the Sequoia-Taxodium-Metasequoia plant community rather than by angiosperms. Evidence is also provided that the content of land plant derived biomarkers and the preservation of plant tissue is controlled by the input of resin-rich, decay-resistant conifers. Sections of the seam characterized by a high degree of gelification of humotelinite (gelification index) show high contents of hop-17(21)-ene but low hopane concentrations. The results suggest that the gelification of plant tissue may be governed by the activity of anaerobic rather than aerobic bacteria. Despite the minor variation in the proportions of gymnosperms versus angiosperms in the peat-forming vegetation, a general influence of the floral assemblage on carbon isotopic composition of the coals (δ13C=−25.3 to −27.0‰) is proposed. Carbon cycling during biogeochemical decomposition of plant tissue by bacteria is assumed to affect the δ13C value of the lignite. Petrographic and geochemical data of gelified and ungelified fossil wood provide evidence that gelification may be governed by microorganisms (e.g. anaerobic bacteria) different from those responsible for decreasing cellulose contents during early diagenetic, aerobic degradation of wood. Based on the molecular composition of terpenoid biomarkers, the wood fragments are identified as derived from gymnosperms. The relative proportions of saturated versus aromatic hydrocarbon fractions of fossil gymnosperms display a general tendency towards lower values in gelified wood remains. This indicates that bacteria involved in gelification of plant tissue may also be involved in aromatisation of diterpenoid hydrocarbons. The chemotaxonomical classification of the macrofossils as gymnosperms is corroborated by the mean carbon isotopic compositions of the macrofossils (δ13C=−24.5‰) and the extracted cellulose (δ13C=−22.0‰). The higher isotopic difference of about 2.5‰ between cellulose and fossil wood, compared to that found in modern trees, can most probably be explained by the smaller effect of 13C discrimination for cellulose when compared with wood during decomposition [Chem. Geol. 158 (1999) 121]. Compared with the coals, the δ13C values of wood and extracted cellulose are affected to a minor extent by microbial activity.

Depositional environment of the Late Miocene Hausruck lignite (Alpine Foreland Basin): Insights from petrography, organic geochemistry, and stable carbon isotopes

Abstract Lignites and fossil wood from two boreholes in the late Miocene Hausruck district (Alpine Foreland Basin, Austria) were investigated with respect to organic carbon and total sulfur contents, ash yields, maceral composition, organic geochemistry, and stable carbon isotope ratios. The lignites from the two sampled profiles differ in sulfur contents and petrography-based facies indicators (gelification index [GI], tissue preservation index [TPI]). The results point to drier and more acidic conditions in the part of the mire sampled at drill site Lukasberg, whereas the lignites from Kalletsberg formed under near neutral conditions due to a raised (ground) water table. The different depositional environments are supported by the paleogeographic positions of the boreholes: borehole Lukasberg is located within a paleo-valley, whereas borehole Kalletsberg is located at the deeper part of a basin situated south and southwest of the Hausruck area. Environmental changes with time within the mire are confirmed by the molecular composition of the soluble organic matter (SOM). Differences in pristane/phytane ratio, isomerisation of hopanes at position 17, concentrations of hopanes, and in the ratio of hopanes to hop-17(21)-enes are related to the extent of gelification of plant tissue, sulfur content and pH values, caused by differences in (ground) water table. The variable contents of individual terpenoid hydrocarbons derived from land plants indicate that angiosperms and conifers contribute to peat formation in the Hausruck district in variable proportions. Overall, angiosperm biomarkers slightly predominate. The tissue preservation index (TPI) within the Kalletsberg profile is probably governed by the proportion of decay-resistant conifers, as indicated by the correlation between TPI and biomarker composition of the coals. Correlations between δ 13 C values of the lignites (between −24.9‰ and −27.4‰) and their non-hopanoid terpenoid composition indicate the major influence of changes in peat-forming vegetation on the carbon isotopic composition of the coals in the Hausruck district. Carbon isotope data of macrofossils are consistent with their taxonomical classification as conifers ( δ 13 C between −22.7‰ and −25.1‰) and angiosperms ( δ 13 C between −25.5‰ and −26.6‰), respectively. The δ 13 C data of the extracted cellulose reveal clear differences between fossil wood from gymnosperms (average δ 13 C=−21.0‰) and angiosperms (mean δ 13 C=−23.4‰).

Environmental history of the German Lower Rhine Embayment during the Middle Miocene as reflected by carbon isotopes in brown coal

Stable carbon isotope investigations have been carried out on Miocene brown coal from the Garzweiler Seam of the German Lower Rhine Embayment. Material studied included fossil wood from seven different taxa, and brown coal matrix. Isotope results from macrofossil analysis show variations of more than 6‰ within individual samples and reveal a general isotopic difference between angiosperm and gymnosperm wood specimens. According to mean carbon isotope values found for gymnosperms, angiosperms and brown coal matrix (−23.3‰, −26.0‰ and −25.8‰), the peat-forming vegetation of the Garzweiler Seam was dominated by angiosperm taxa. Results from brown coal matrix establish a continuous high-resolution depth profile of carbon isotope variations during the late Middle Miocene. They show a significant and characteristic isotope pattern with distinct medium- and short-term cycles (high-frequency variations) in the two main units of Garzweiler Seam (locally split into three units). The medium-term δ13C variations are most likely caused by varying proportions of gymnosperms within the peat-forming vegetation while high-frequency oscillations seem to be a direct signal of environmental changes. A long-term decline of carbon isotope values observed within the complete Garzweiler Seam from base to top is presumably due to a cooling trend in the Miocene.

Variations in organic matter composition in sediments from Lake Huguang Maar (Huguangyan), south China during the last 68 ka: implications for environmental and climatic change

Abstract This study presents a continuous and long-term palaeoenvironmental and palaeoclimatic record from a lacustrine sediment core (Lake Huguang Maar) at the northern coast of the South China Sea, extending back to about 68 cal ka BP. We provide a comprehensive ecosystem analysis from the opportunity to combine organic geochemical and palynological data. Several climatically induced changes to the flora and fauna have been recognised. From 68 to 58 and 48 to 40.5 cal ka BP, relatively depleted δ 13 C TOC (−31.3 to −25.0‰) and δ 13 C values of mid- and long-chain n -alkanes (−34.3 to −30.2‰), as well as high percentages of tropical pollen, indicate a vegetation dominated by subtropical/tropical forest (C 3 plants). An expansion of a mixed C 3 /C 4 open grassland between 58 and 48 and particularly after 40.5 cal ka BP mirrors drier climatic conditions, in conjunction with a lowering of atmospheric CO 2 concentrations; δ 13 C TOC (−24.1 to −16.8‰) and δ 13 C values of n -alkyl lipids (−28.9 to −24.1‰) are heavier, higher percentages of non-arboreal pollen were observed and the wood/grass lignin index (WGLI) from open pyrolysis (Py–GC) indicates a higher input of grass lignin. Abundant oxidised terrigenous particles that resulted from increased fire activity and/or greater erosion rates are a further hint of enhanced dryness. The clear drop in moisture availability suggests that this site is sensitive in fluctuations of the summer and winter monsoon activity in accordance with other proxy records in south-east Asia. The response of the aquatic ecosystem is documented by an extremely large δ 13 C shift of algal-derived botryococcenes from −35.4 to −6.2‰, which records a change towards a bicarbonate carbon source at low dissolved CO 2 concentrations.

Parameters determining the carbon isotopic composition of coal and fossil wood in the Early Miocene Oberdorf lignite seam (Styrian Basin, Austria)

Abstract Petrographical and geochemical data of gelified and ungelified fossil wood from the Early Miocene Oberdorf lignite seam (Styrian Basin, Austria) provide evidence that the early diagenetic, aerobic degradation of wood by fungi may be followed by further decomposition under reducing conditions by the activity of anaerobic bacteria. Based on the molecular compositions of terpenoid hydrocarbons, the wood fragments in the lignite are identified as gymnosperms. This result is corroborated by the mean isotopic composition (δ13C=-24.2‰) of the macrofossils. The isotopic difference between fossil wood and extracted cellulose of most samples (3.9–2.5‰) is higher than in modern trees, which can be explained by the smaller effect of 13C discrimination during decomposition of cellulose when compared with wood. The mean carbon isotope values found for gymnosperms and coals (−24.2‰ and –24.7‰, respectively) confirm the results from biomarker analyses indicating that the peat-forming vegetation of the Oberdorf seam was dominated by gymnosperm taxa. Minor variation in the relative contributions of gymnosperms and angiosperms to coal deposition is of negligible influence on the isotopic composition of the coals. Cross-correlations between δ13C of the coals, soluble organic matter yield, and the proportions of liptinite macerals of higher-plant origin imply that the carbon isotopic composition may be affected by the contents of plant lipids because of their δ13C values ranging from 5 to 10‰ less than whole-plant tissue values. An isotopic shift of the coals towards more negative values due to the activity of prokaryotes (i.e. anaerobic bacteria) in organic matter degradation is proposed. The results obtained from the Early Miocene Oberdorf lignite indicate that the carbon isotope ratios of the coals are primarily affected by varying contributions of different parts of whole-plant tissue, due to their different isotopic and molecular compositions (e.g. epicuticular leaf waxes, resins, wood) and their different decay-resistance against the early diagenetic changes involved in organic matter decomposition. Carbon cycling during anoxic decomposition of plant-derived organic matter is assumed to affect the δ13C values of coal. δ13C values of wood and extracted cellulose are affected only to a minor extent.

Sedimentation patterns of diatoms in Lake Holzmaar, Germany - (on the transfer of climate signals to biogenic silica oxygen isotope proxies)

The seasonal sedimentation pattern of diatom valves in Lake Holzmaar was investigated during 1995 by deploying sediment traps at three different lake depths. According to the sedimentation pattern, the major reproduction zone of diatoms was restricted to the upper 6 m of the water body. The population growth started late in April and blooms of Cyclotella cf. comensis Grun., which dominates the plankton diatoms, and Fragilaria crotonensis Kitton were collected in traps during June and September, and July, respectively. During summer, the seasonal sedimentation pattern of each taxon, as collected in the upper traps, was reflected in the concentrations in the lowest trap. However, in May and from September onwards, the community composition in the lowest trap and augmented trapping rates suggest both sediment focusing and resuspension of bottom sediments.The temperature signals as recorded by δ18O values of diatom valves should, therefore, reflect integrated temperatures between 0 and 6 m depth. However, temperatures during summer and autumn are expected to be accentuated in the sedimentary record since the isotopic signal is weighted by both the number and the weight-mass of the valves. During summer, the transfer of these signals by the sedimenting diatoms retains the information pattern recorded, while in spring and autumn/winter additional influxes caused by resuspension may somewhat alter those temperature informations. The proxy signals finally stored in the sediments, may, therefore, not precisely represent the successive temperatures currently recorded during 1995 within mid-lake.

A terrestrial observatory approach to the integrated investigation of the effects of deforestation on water, energy, and matter fluxes

Integrated observation platforms have been set up to investigate consequences of global change within a terrestrial network of observatories (TERENO) in Germany. The aim of TERENO is to foster the understanding of water, energy, and matter fluxes in terrestrial systems, as well as their biological and physical drivers. Part of the Lower Rhine Valley-Eifel observatory of TERENO is located within the Eifel National Park. Recently, the National Park forest management started to promote the natural regeneration of near-natural beech forest by removing a significant proportion of the spruce forest that was established for timber production after World War II. Within this context, the effects of such a disturbance on forest ecosystem functioning are currently investigated in a deforestation experiment in the Wüstebach catchment, which is one of the key experimental research sites within the Lower Rhine Valley-Eifel observatory. Here, we present the integrated observation system of the Wüstebach test site to exemplarily demonstrate the terrestrial observatory concept of TERENO that allows for a detailed monitoring of changes in hydrological and biogeochemical states and fluxes triggered by environmental disturbances. We present the observation platforms and the soil sampling campaign, as well as preliminary results including an analysis of data consistency. We specifically highlight the capability of integrated datasets to enable improved process understanding of the post-deforestation changes in ecosystem functioning.

New insights into paleoenvironmental changes in Laguna Potrok Aike, southern Patagonia, since the Late Pleistocene: The PASADO multiproxy record

A series of long sediment cores was retrieved from Laguna Potrok Aike, Southern Patagonia, within the framework of PASADO (Potrok Aike Maar Lake Sediment Archive Drilling Project), an ICDP (International Continental Scientific Drilling Program) lake drilling project. This maar lake, located at 52°S, 70°W in the Province of Santa Cruz (Argentina), in the southernmost continental area of the world, is one of the few permanent lakes in the region, providing a unique continuous paleoclimatic and paleoecological lacustrine record for the last glacial cycle. Previous multiproxy studies of this site have characterized the environmental history of these dry lands in the Patagonian Steppe for the last 16 cal. ka BP. This new series of sediment cores provides a much longer record of climate variability in Southern Patagonia since 51.3 cal. ka BP. Using a multiproxy strategy, a set of samples (mostly from core catcher material) was analyzed for physical properties, rock magnetism, geochemistry, CNS elemental analysis, stable isotopes, pollen and diatoms. This preliminary multiproxy limnogeological interpretation sheds new light on the regional Pleistocene and Holocene environmental history, revealing lake-level variations through time and identifying time windows of interest where higher resolution analyses will be carried out.