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The Tibetan Plateau is the largest elevated landmass in the world and profoundly influences atmospheric circulation patterns such as the Asian monsoon system. Therefore this area has been increasingly in focus of palaeoenvironmental studies. This thesis evaluates the applicability of organic biomarkers for palaeolimnological purposes on the Tibetan Plateau with a focus on aquatic macrophyte-derived biomarkers. Submerged aquatic macrophytes have to be considered to significantly influence the sediment organic matter due to their high abundance in many Tibetan lakes. They can show highly 13C-enriched biomass because of their carbon metabolism and it is therefore crucial for the interpretation of δ13C values in sediment cores to understand to which extent aquatic macrophytes contribute to the isotopic signal of the sediments in Tibetan lakes and in which way variations can be explained in a palaeolimnological context. Additionally, the high abundance of macrophytes makes them interesting as potential recorders of lake water δD. Hydrogen isotope analysis of biomarkers is a rapidly evolving field to reconstruct past hydrological conditions and therefore of special relevance on the Tibetan Plateau due to the direct linkage between variations of monsoon intensity and changes in regional precipitation / evaporation balances. A set of surface sediment and aquatic macrophyte samples from the central and eastern Tibetan Plateau was analysed for composition as well as carbon and hydrogen isotopes of n-alkanes. It was shown how variable δ13C values of bulk organic matter and leaf lipids can be in submerged macrophytes even of a single species and how strongly these parameters are affected by them in corresponding sediments. The estimated contribution of the macrophytes by means of a binary isotopic model was calculated to be up to 60% (mean: 40%) to total organic carbon and up to 100% (mean: 66%) to mid-chain n-alkanes. Hydrogen isotopes of n-alkanes turned out to record δD of meteoric water of the summer precipitation. The apparent enrichment factor between water and n-alkanes was in range of previously reported ones (≈-130‰) at the most humid sites, but smaller (average: -86‰) at sites with a negative moisture budget. This indicates an influence of evaporation and evapotranspiration on δD of source water for aquatic and terrestrial plants. The offset between δD of mid- and long-chain n-alkanes was close to zero in most of the samples, suggesting that lake water as well as soil and leaf water are affected to a similar extent by those effects. To apply biomarkers in a palaeolimnological context, the aliphatic biomarker fraction of a sediment core from Lake Koucha (34.0° N; 97.2° E; eastern Tibetan Plateau) was analysed for concentrations, δ13C and δD values of compounds. Before ca. 8 cal ka BP, the lake was dominated by aquatic macrophyte-derived mid-chain n-alkanes, while after 6 cal ka BP high concentrations of a C20 highly branched isoprenoid compound indicate a predominance of phytoplankton. Those two principally different states of the lake were linked by a transition period with high abundances of microbial biomarkers. δ13C values were relatively constant for long-chain n-alkanes, while mid-chain n-alkanes showed variations between -23.5 to -12.6‰. Highest values were observed for the assumed period of maximum macrophyte growth during the late glacial and for the phytoplankton maximum during the middle and late Holocene. Therefore, the enriched values were interpreted to be caused by carbon limitation which in turn was induced by high macrophyte and primary productivity, respectively. Hydrogen isotope signatures of mid-chain n-alkanes have been shown to be able to track a previously deduced episode of reduced moisture availability between ca. 10 and 7 cal ka BP, indicated by a 20‰ shift towards higher δD values. Indications for cooler episodes at 6.0, 3.1 and 1.8 cal ka BP were gained from drops of biomarker concentrations, especially microbial-derived hopanoids, and from coincidental shifts towards lower δ13C values. Those episodes correspond well with cool events reported from other locations on the Tibetan Plateau as well as in the Northern Hemisphere. To conclude, the study of recent sediments and plants improved the understanding of factors affecting the composition and isotopic signatures of aliphatic biomarkers in sediments. Concentrations and isotopic signatures of the biomarkers in Lake Koucha could be interpreted in a palaeolimnological context and contribute to the knowledge about the history of the lake. Aquatic macrophyte-derived mid-chain n-alkanes were especially useful, due to their high abundance in many Tibetan Lakes and their ability to record major changes of lake productivity and palaeo-hydrological conditions. Therefore, they have the potential to contribute to a fuller understanding of past climate variability in this key region for atmospheric circulation systems.
We present observations from a continuous exposure of an ancient plate interface in the depth range of its former seismogenic zone in the central Alps of Europe related to Late Cretaceous-early Tertiary subduction and accretion of the South Penninic lower plate underneath the Adriatic upper plate. The material forming the exposed plate interface zone has experienced flow and fracturing over an extended period of time followed by syncollisional exhumation, thus reflecting a multistage evolution. Fabric formation and metamorphism, however, chiefly record the deformation conditions of the precollisional setting along the plate interface. We identify an unstable slip domain from pseudotachylytes occurring in the temperature range between 200 and 300 degrees C. This zone coincides with a domain of intense veining in the subduction melange with mineral growth into open cavities, indicating fast, possibly seismic, rupture. Evidence for transient near-lithostatic fluid pressure as well as brittle fractures competing with mylonitic shear zones continues into the region below the occurrence of pseudotachylytes, possibly reflecting a zone of conditionally stable slip. The zone above the unstable slip area is devoid of veins but displays ample evidence of fluid-assisted processes similar to the deeper zone: solution-precipitation creep and dehydration reactions in the melange matrix, hydration, and sealing of the base of the upper plate. Seismic rupture here is possibly expressed by ubiquitous localized deformation zones. We hypothesize that trenchward sealing of parts of the plate interface as well as reaction-enhanced destruction of upper plate permeability is an important component, localizing the unstable slip zone. This relation may result from the competition of the pervasive, presumably interseismic, pressure solution creep destroying permeability and building elevated fluid pressure until the strength threshold is reached with seismic failure.
Accretionary orogens are considered major sites of formation of juvenile continental crust. In the central and southern Andes this is contradicted by two observations: siliciclastic fills of Paleozoic basins in the central Andean segment of the accretionary Terra Australis Orogen consist almost exclusively of shales and mature sandstones; and magmatic rocks connected to the Famatinian (Ordovician) and Late Paleozoic magmatic arcs are predominantly felsic and characterized by significant crustal contamination and strongly unradiogenic Nd isotope compositions. Evidence of juvenile crustal additions is scarce. We present laser ablation (LA)-ICPMS U-Pb ages and LA-MC-ICPMS Hf isotope data of detrital zircons from seven Devonian to Permian turbidite sandstones incorporated into a Late Paleozoic accretionary wedge at the western margin of Gondwana in northern Chile. The combination with Nd whole-rock isotope data permits us to trace the evolution of the South American continental crust through several Proterozoic and Paleozoic orogenic cycles. The analyzed detrital zircon spectra reflect all Proterozoic orogenic cycles representing the step-wise evolution of the accretionary SW Amazonia Orogenic System between 2.0 and 0.9 Ga, followed by the Terra Australis Orogen between 0.9 and 0.25 Ga. The zircon populations are characterized by two prominent maxima reflecting input from Sunsas (Grenville) age magmatic rocks (1.2-0.9 Ga) and from the Ordovician to Silurian Famatinian magmatic arc (0.52-0.42 Ga). Grains of Devonian age are scarce or absent from the analyzed zircon populations. The Hf isotopic compositions of selected dated zircons at the time of their crystallization (epsilon Hf-(T); T = 3.3-0.25 Ga) vary between -18 and +11. All sandstones have a significant juvenile component; between 20 and 50% of the zircons from each sedimentary rock have positive epsilon Hf-(T) and can be considered juvenile. The majority of the juvenile grains have Hf-depleted mantle model ages (Hf T-DM) between 1.55 and 0.8 Ga. the time of the Rondonia-San Ignacio and Sunsas orogenic events on the Amazonia craton. The corresponding whole-rock epsilon Nd-(T) values fot these same rocks are between -8 and -3 indicating a mixture of older evolved and juvenile sources. Nd-depleted mantle model ages (Nd T-DM*) are between 1.5 and 1.2 Ga and coincide broadly with the zircon Hf model ages. Our data indicate that the Paleo- and Mesoproterozoic SW Amazonia Orogenic System, and the subsequent Neoproterozoic and Paleozoic Terra Australis Orogen in the region of the central and southern Andes, developed following two markedly different patterns of accretionary orogenic crustal evolution. The SW Amazonia Orogenic System developed by southwestward growth over approximately 1.1 Ga through a combination of accretion of juvenile material and crustal recycling typical of the extensional or retreating mode of accretionary orogens. In contrast, the central Andean segment of the Terra Australis Orogen evolved from 0.9 to 0.25 Ga in the compressional or advancing mode in a relatively fixed position without the accretion of oceanic crustal units or large scale input of juvenile material to the orogenic crust. Here, recycling mainly of Mesoproterozoic continental crust has been the dominant process of crustal evolution. (C) 2009 Elsevier B.V. All rights reserved.
The Younger Dryas event, which began approximately 12,900 years ago, was a period of rapid cooling in the Northern Hemisphere, driven by large-scale reorganizations of patterns of atmospheric and oceanic circulation(1-3). Environmental changes during this period have been documented by both proxy-based reconstructions(3) and model simulations(4), but there is currently no consensus on the exact mechanisms of onset, stabilization or termination of the Younger Dryas(5-8). Here we present high-resolution records from two sediment cores obtained from Lake Krakenes in western Norway and the Nordic seas. Multiple proxies from Lake Krakenes are indicative of rapid alternations between glacial growth and melting during the later Younger Dryas. Meanwhile, reconstructed sea surface temperature and salinity from the Nordic seas show an alternation between sea-ice cover and the influx of warm, salty North Atlantic waters. We suggest that the influx of warm water enabled the westerly wind systems to drift northward, closer to their present-day positions. The winds thus brought relatively warm maritime air to Northern Europe, resulting in rising temperatures and the melting of glaciers. Subsequent input of this fresh meltwater into the ocean spurred the formation of sea ice, which forced the westerly winds back to the south, cooling Northern Europe. We conclude that rapid alternations between these two states immediately preceded the termination of the Younger Dryas and the permanent transition to an interglacial state.
Foreland-basin systems are excellent archives to decipher the feedbacks between surface and tectonic processes in orogens. The sedimentary architecture of a foreland-basin system reflects the balance between tectonic subsidence causing long-term accommodation space and sediment influx corresponding to efficiency of erosion and mass-redistribution processes. In order to explore the effects of climatic and tectonic forcing in such a system, I investigated the Oligo-Miocene foreland-basin sediments of the southern Alborz mountains, an intracontinental orogen in northern Iran, related to the Arabia-Eurasia continental collision. This work includes absolute dating methods such as 40Ar/39Ar and zircon (U-Th)/He thermochronology, magnetostratigraphy, sedimentological analysis, sandstone and conglomerate provenance study, carbon and oxygen isotope analysis, and clay mineralogy study. Results show a systematic correlation between coarsening-upward cycles and sediment accumulation rates in the basin on 105 to 106yr time scales. During thrust loading phases, the coarse-grained fraction supplied by the uplifting range is stored in the proximal part of the basin (sedimentary facies retrogradation), while fine-grained sediments are deposited in distal sectors. Variations in sediment provenance during these phases of enhanced tectonic activity give evidence for erosional unroofing phases and/or drainage-reorganization events. In addition, enhanced tectonic activity promoted the growth of topography and associated orographic barrier effects, as demonstrated by sedimentologic indicators and the analysis of stable C and O isotopes from calcareous paleosols and lacustrine/palustrine samples. Extensive progradation of coarse-grained deposits occurs during phases of decreased subsidence, when the coarse-grained fraction supplied by the uplifting range cannot be completely stored in the proximal part of the basin. In this environment, a reduction in basin subsidence is associated with laterally stacked fluvial channel deposits, and is related to intra-foreland uplift, as documented by growth strata, tectonic tilting, and sediment reworking. Increase in sediment accumulation rate associated with progradation of vertically-stacked coarse-grained fluvial channels also occurs. Paleosol O-isotope data shows that this increase is related to wetter climatic phases, suggesting that surface processes are more efficient and exhumation rates increase, giving rise to a positive feedback. Furthermore, isotopic and sedimentologic data show that starting from 10-9 Ma, climate became less arid with an increase in seasonality of precipitation. Because important changes were also recorded in the Mediterranean Sea and Asia at that time, the evidence for climatic variability observed in the Alborz mountains most likely reflects changes in Northern Hemisphere atmospheric circulation patterns. This study has additional implications for the evolution of the Alborz mountains and the Arabia-Eurasia continental collision zone. At the orogenic scale, the locus of deformation did not move steadily southward, but stepped forward and backward since Oligocene time. In particular, from ~ 17.5 to 6.2 Ma the orogen grew by a combination of frontal accretion and wedge-internal deformation on time scales of ca. 0.7 to 2 m.y. Moreover, the provenance data suggest that prior to 10-9 Ma the shortening direction changed from NW-SE to NNE-SSW, in agreement with structural data. On the scale of the entire collision zone, the evolution of the studied basins and adjacent mountain ranges suggests a new geodynamic model for the evolution of the Arabia-Eurasia continental collision zone. Numerous sedimentary basins in the Alborz mountains and in other locations of the Arabia-Eurasia collision zone record a change from a tensional (transtensional) to a compressional (transpressional) tectonic setting by ~ 36 Ma. I interpret this to reflect the onset of subduction of the stretched Arabian continental lithosphere beneath central Iran, leading to moderate plate coupling and lower- and upper-plate deformation (soft continental collision). The increase in deformation rates in the southern Alborz mountains from ~ 17.5 Ma suggests that significant upper-plate deformation must have started by the early Miocene most likely in response to an increase in degree of plate coupling. I suggest that this was related to the subduction of thicker Arabian continental lithosphere and the consequent onset of hard continental collision. This model reconciles the apparent lag time of 15-20 m.y between the late Eocene to early Oligocene age for the initial Arabia-Eurasia continental collision and the onset of widespread deformation across the collision zone to the north in early to late Miocene time.
There are far-reaching conceptual similarities between bi-static surface georadar and post-stack, "zero-offset" seismic reflection data, which is expressed in largely identical processing flows. One important difference is, however, that standard deconvolution algorithms routinely used to enhance the vertical resolution of seismic data are notoriously problematic or even detrimental to the overall signal quality when applied to surface georadar data. We have explored various options for alleviating this problem and have tested them on a geologically well-constrained surface georadar dataset. Standard stochastic and direct deterministic deconvolution approaches proved to be largely unsatisfactory. While least-squares-type deterministic deconvolution showed some promise, the inherent uncertainties involved in estimating the source wavelet introduced some artificial "ringiness". In contrast, we found spectral balancing approaches to be effective, practical and robust means for enhancing the vertical resolution of surface georadar data, particularly, but not exclusively, in the uppermost part of the georadar section, which is notoriously plagued by the interference of the direct air- and groundwaves. For the data considered in this study, it can be argued that band- limited spectral blueing may provide somewhat better results than standard band-limited spectral whitening, particularly in the uppermost part of the section affected by the interference of the air- and groundwaves. Interestingly, this finding is consistent with the fact that the amplitude spectrum resulting from least-squares-type deterministic deconvolution is characterized by a systematic enhancement of higher frequencies at the expense of lower frequencies and hence is blue rather than white. It is also consistent with increasing evidence that spectral "blueness" is a seemingly universal, albeit enigmatic, property of the distribution of reflection coefficients in the Earth. Our results therefore indicate that spectral balancing techniques in general and spectral blueing in particular represent simple, yet effective means of enhancing the vertical resolution of surface georadar data and, in many cases, could turn out to be a preferable alternative to standard deconvolution approaches.
The rhodolithic slope deposits of a Burdigalian carbonate platform in Sardinia near Sedini were analyzed to reconstruct facies and palaeobathymetry. There is a distinct red-algal growth zonation along the platform slope. The clinoform rollover area consists of coralline-algal bindstones, which downslope change into a zone where rhodoliths are locally fused by progressive encrustation. Mid-slope rhodoliths are moderately branched, and downslope rhodoliths have fruticose protuberances, resulting in branching rhodolith growth patterns. There is a sharp change from the rhodolitic rudstones to the basinal, bivalve-dominated rudstones at the clinoform bottomsets. Red-algal genera identified include Sporolithon, Lithophyllum, Spongites, Hydrolithon, Mesophyllum, Lithoporella, Neogoniolithon, and other mastophoroids and melobesioids. Genera and subfamilies show a zonation along the clinoforms, allowing palaeobathymetric estimates. The clinoform rollovers formed at a water depth of around 40 m and the bottomsets around 60 m. Results from geometrical reconstruction show that coral reefs in the inner platform formed at water depths of around 20 m. Therefore, the Sedini carbonate platform is an example of a reef-bearing platform in which the edge or the platform-interior reefs do not build up to sea level.
Tectonic and climatic control on evolution of rift lakes in the Central Kenya Rift, East Africa
(2009)
The long-term histories of the neighboring Nakuru-Elmenteita and Naivasha lake basins in the Central Kenya Rift illustrate the relative importance of tectonic versus climatic effects on rift-lake evolution and the formation of disparate sedimentary environments. Although modem climate conditions in the Central Kenya Rift are very similar for these basins, hydrology and hydrochemistry of present-day lakes Nakuru, Elmenteita and Naivasha contrast dramatically due to tectonically controlled differences in basin geometries, catchment size, and fluvial processes. In this study, we use eighteen C-14 and Ar-40/Ar-39 dated fluvio-lacustrine sedimentary sections to unravel the spatiotemporal evolution of the lake basins in response to tectonic and climatic influences. We reconstruct paleoclimatic and ecological trends recorded in these basins based on fossil diatom assemblages and geologic field mapping. Our study shows a tendency towards increasing alkalinity and shrinkage of water bodies in both lake basins during the last million years. Ongoing volcano-tectonic segmentation of the lake basins, as well as reorganization of upstream drainage networks have led to contrasting hydrologic regimes with adjacent alkaline and freshwater conditions. During extreme wet periods in the past, such as during the early Holocene climate optimum, lake levels were high and all basins evolved toward freshwater systems. During drier periods some of these lakes revert back to alkaline conditions, while others maintain freshwater characteristics. Our results have important implications for the use and interpretation of lake sediment as climate archives in tectonically active regions and emphasize the need to deconvolve lacustrine records with respect to tectonics versus climatic forcing mechanisms.