Lindsay McHenry

The lithic assemblages at the Oldowan-Acheulean transition in Bed II of Olduvai Gorge, Tanzania, represent a wide variety of raw materials reflecting both the diversity of volcanic, metamorphic, and sedimentary source materials available in the Olduvai basin and surroundings and the preferences of the tool-makers. A geochemical and petrographic systematic analysis of lava-derived archaeological stone tools, combined with textural and mineralogical characterization of quartzite, chert, and other metamorphic and sedimentary raw materials from two Middle and Upper Bed II sites, has enabled us to produce a comprehensive dataset and characterization of the rocks employed by Olduvai hominins, which is used here to establish a referential framework for future studies on Early Stone Age raw material provenancing. The use of rounded blanks for most lava-derived artifacts demonstrates that hominins were accessing lava in local stream channels. Most quartzite artifacts appear to derive from an...

Introduction: Evaporitic minerals on Mars have been identified by orbital instruments such as CRISM [1] and OMEGA [2], from Mars meteorites such as ALH84001 [3] and nakhlites [4], and by the in-situ rover missions such as Opportunity [5] and Curiosity [6]. The identification and characterization of evaporative environments are important to reconstruct the climatic history and to characterize the ancient aqueous fluids once present at the Martian surface and subsurface [5]. The CRISM summary products (simple band ratios and index maps) are commonly used to identify a diverse range of aqueous minerals, including sulfates, carbonates, phyllosilicates, and hydrated silica [7]. However, these methods can be time consuming and require attention to detail. Therefore, in this study, we demonstrate the capability of statistical learning algorithms (also called Machine Learning (ML) algorithms) to better identify the different evaporitic spectra from common surface mineralogies on Mars using ...

Introduction: Zeolites are among the most common and widespread authigenic silicate minerals found in saline-alkaline paleolacustrine sedimentary deposits on Earth [1]. The presence of certain zeolite mineral species or assemblages in a saline-alkaline paleolake can be attributed to thermodynamic equilibrium reactions due to changing water chemistry and to kinetically controlled nonequilibrium dissolution and precipitation reactions. Therefore, the spatial and stratigraphic patterns of zeolites and other authigenic minerals in paleolake deposits can be used to track hydrological changes over their history (e.g., [2]). Zeolites have been detected on the surface of Mars using orbital spectral data (e.g., [3,4]). [5] used a geochemical model to explore the formation and fate of phyllosilicates on the surface of Mars using Martian olivine basalts under acid weathering with different water-rock (W/R) ratios and PCO2, and they observed the formation of zeolites (analcime, stilbite, stelle...

Introduction: Prediction of mineral solubilities in natural waters has been a major area of research by geochemists for over 50 years (e.g. [1]). Solubility of evaporite minerals not only depends on the presence of species in solution and the activity of water, but also the environmental conditions such as temperature and pressure [2]. Calculations of mineral precipitation and solubilities based on thermodynamic data provide important constraints on geochemical systems of interest. Geochemical and numerical models have been developed to understand the formation and dissolution kinetics evaporitic minerals on Earth [3,4] and Mars [5,6]. Cyclic wetting and drying processes, especially in evaporitic playa environments, increase the complexity of predicting the evaporite mineral assemblages using geochemical data [7]. Here we present initial results of dissolution kinetic experiments on evaporite minerals from Panamint Basin, California.

Introduction: Venus has a volcanically active surface, with evidence for recent or even current volcanic activity. Analyses of surface rocks by Venera landers suggests a basaltic composition, though silicic materials have been inferred for some regions, based in part on differences in emissivity. ESA’s Venus Express (2006-2014) had a spectrometer [1] that analyzed emissivity in the near infrared (around 1 micron), distinguishing between low emissivity and high emissivity areas, which could indicate basaltic compositions in the lowlands and more silica-rich compositions in highland areas, respectively, and also suggest that some tesserae are more silica-rich than the surrounding likely basaltic plains [2]. However, these interpretations are complicated by surface alteration, since the oxidation of basaltic surfaces at high temperatures (a likely process, given the oxidizing conditions on the surface of Venus) could form a veneer of hematite, which has a lower emissivity than the orig...

Introduction: The Hengill Volcanic Complex (HVC) of SW Iceland boasts a wide range of surface hydrothermal conditions, from acid-sulfate fumaroles (pH as low as 0.5), mud pots, and hot springs to nearneutral travertine or silica sinter depositing hot springs (pH as high as 8.3), all likely associated with a deep hydrothermal aquifer with pH ~9 [1] and a highFe tholeiitic basaltic substrate (Fe2O3T= 13.2%). The Nesjavellir Geothermal Field on the NW side of the

Olduvai Gorge, Tanzania, often cited as a textbook example of continental stratigraphic study (as depicted by Hay 1976 and others) is well known for its abundant Plio-Pleistocene fauna, hominin fossils, stone artifacts, and well-dated tephra layers used widely in the calibration of human evolution. However, precise stratigraphic correlation between various sites within Olduvai has been hampered by the rarity of unaltered volcanic glass suitable for geochemical characterization. Geochemical characterization of tephra for stratigraphic correlation purposes is difficult when fresh glass is absent or secondarily altered. A multi-component approach using the major and minor element compositions of phenocrysts and glass (where present) provides successful results at Olduvai and is proposed here as a viable methodology for use elsewhere. Six widespread Bed I (~2.1-1.79 myr) tephra layers of the Olduvai area are geochemically fingerprinted and used here to show the viability of the multi-co...

Introduction: Hydrothermal environments have been postulated for early Mars, based on abundant evidence for water and volcanism [1,2], as well as direct detection of likely hydrothermal deposits at Gusev Crater and elsewhere by orbital and rover platforms [3,4]. Such environments could have provided both a source of water and a source of heat for life, though their habitability depends on many other factors [5,6]. One important factor is whether a hydrothermal environment is more akin to acid-sulfate fumaroles (less habitable) or more neutral sinterdepositing hot springs (more habitable). Both have been proposed to explain features of the Columbia Hills [7,8]. Studies of modern terrestrial fumarole and hot spring environments and their deposits can help identify diagnostic mineralogical and geochemical signatures. In this abstract we report preliminary results for a geochemical and mineralogical study of modern fumaroles and hot springs in the vicinity of Lassen volcano in the south...

Introduction: Iceland has long been suggested as an analog for Mars volcanological, petrological, glacial, and surface alteration processes. Its hydrothermal system, and the geochemical and mineralogical signatures it produces as it interacts with the high-Fe basaltic substrate, provides an exceptional analog for potential hydrothermal alteration on Mars. We introduce multiple hydrothermal sites in southwest and northeast Iceland (Fig. 1), and discuss their suitability for Mars analog research based on the results of our pilot study.

DEPOSIT: IMPLICATIONS FOR THE DISTRIBUTION AND LONGEVITY OF JAROSITE ON MARS. L. J. McHenry, V. F. Chevrier, and C. Schröder, Department of Geosciences, U. WisconsinMilwaukee, PO Box 413, Milwaukee, WI 53201, lmchenry@uwm.edu, W.M. Keck Laboratory for Space and Planetary Simulation, Arkansas Center for Space and Planetary Science, MUSE 202, University of Arkansas, Fayetteville, AR 72701, Center for Applied Geoscience, Eberhard Karls Universität, Sigwartstr. 10, D-72076 Tübingen, Germany, christian.schroeder@ifg.uni-tuebingen.de.

Introduction: Lassen Volcanic National Park has the largest hydrothermal system in the Cascades, including a variety of hydrothermal environments [1]. Little Hot Springs Valley (LHSV) in particular hosts a wide range of hydrothermal conditions and associated deposits, including acid-sulfate fumaroles, mud pots, and hot springs, near-neutral, travertine-depositing hot springs, and near-neutral gypsum-depositing hot springs (Fig. 1) [2]. These varied sources originate as part of the same hydrothermal system in some cases within meters of each other, revealing a complex system.

Introduction: Martian basalts differ in composition from most terrestrial basalts in their high Fe abundances. The high-Fe products of hot spot basaltic volcanism, including lavas from Craters of the Moon (COM), Idaho [1], are the closest analogs (Table 1) and provide an opportunity to observe how Mars-like basalts behave in different alteration environments. This is critical, as the products of alteration depend heavily on the composition of the substrate (e.g. [2]).

Introduction: Here we demonstrate the application of advanced Machine Learning (ML) algorithms for the identification of mineral assemblages using the Panoramic Camera (PanCam) instrument onboard on the upcoming ExoMars Rover mission. The PanCam camera uses a dedicated “geology” filter set consisting of twelve narrowband filters of pre-determined wavelength between 440 nm and 1000 nm to map the mineralogy of the surface [1]. 200 reflectance spectra covering five mineral groups with twenty mineral species, resampled by the Geological filters on the Exomars PanCam instruments, were used to demonstrate the capability of ML methods to identify mineralogy using twelve spectral bands in the visible to near infrared wavelength region.

Abstract The Upper Laetolil marker Tuffs 1 to 8 are mineralogically similar rocks, but heterogeneous in terms of their texture, structure, proportion of primary minerals, volume of cement and degree of low-temperature alteration. Originally they were deposited as crystal and/or vitric ash of evolved melilite-nephelinitic composition and not as melilitite-(natro)carbonatite. Occurrence of carbonate-silicate melt inclusions in primary minerals supports R. Hay's conclusion that the ash could have erupted from a carbonatitic volcanic source. Primary minerals (melilite, clinopyroxene, garnet, perovskite, magnetite) in the tuffs are characterised by wide variations in their compositions and two and even more mineral populations are present within each marker tuff. Thus, any correlation between the tuffs from different localities on the basis of mineral composition is very difficult to impossible. Tuff 7, with footprints of Australopithecus afarensis, is a very heterogeneous unit both vertically and laterally that formed during four major eruption events. Trace-element geochemistry and Sr–Nd isotopic data for Tuffs 6, 7 and 8 suggest that compositionally different volcanic sources were involved in their formation. Initial 87Sr/86Sr and 144Nd/143Nd ratios also show that the Sadiman volcano should not be considered as a source for these three marker tuffs at Laetoli. Only Essimingor and Mosonik volcanoes produced rocks that are mineralogically and geochemically similar to the Upper Laetolil marker tuffs, though these volcanoes lie about 100 km from Laetoli.

Tuffaceous marker beds, derived from volcanic products from the Ngorongoro Volcanic Highlands, help define a stratigraphic framework for the world-renowned fossil and stone tool record exposed at Olduvai Gorge, Tanzania. However, previous efforts to constrain this tuff record, especially for Olduvai Bed II, have been limited because of erosion, contamination, reworking, and the alteration of volcanic glass under saline-alkaline conditions. This paper applies previously defined geochemical and mineralogical "fingerprints" for several major Bed II marker tuffs, based on glass (where available) and phenocrysts more resistant to alteration (feldspar, hornblende, augite, and titanomagnetite), to tuffs from stratigraphic sections in the Olduvai Junction Area, including previously and recently excavated Acheulean and Oldowan sites (HWK EE (Locality (Loc) 42), EF-HR (Loc 12a), FLK (Loc 45), and MNK (Loc 88)). The Middle Bed II Bird Print Tuff (BPT) is found to be more compositionally variable than previously reported but is still valuable as a stratigraphic marker over short distances. The confirmation of blocks of Tuff IID in conglomerate helps constrain Upper Bed II stratigraphy at sites where in-situ tuffs are absent. This paper also compiles the results of published geochronological research, providing stratigraphic context and updating previously reported dates using a consistent 40Ar/39Ar reference standard age. The results of this work support the following paleoanthropologically relevant conclusions: 1) the early Acheulean site EF-HR (Loc 12a) is situated above the level of Hay's Tuff IIC, and thus sits in Upper rather than Middle Bed II, (2) the HWK EE (Loc 42) Oldowan site is constrained between Tuff IIA and Tuff IIB, just above the boundary between Lower and Middle Bed II, and 3) the Acheulean site at FLK W most likely lies within the Middle Augitic Sandstone, above Tuff IIB, similar to the placements by Leakey and Hay for the earliest Acheulean at Olduvai.

Abstract During the mid-Carboniferous, ice centers located in present-day western Argentina disappeared until the late Cenozoic with glaciation of the Andes. The disappearance of mid-Carboniferous glaciers and the subsequent climate shift, recorded in the Paganzo Basin, has been attributed to global events and drivers, such as increased atmospheric CO2 concentrations and the shifting position of Gondwana across the South Pole. However, glaciers continued at the same paleolatitude in eastern South America and did not disappear from Gondwana until the Late Permian. This study investigates links to local drivers that acted in combination with other global drivers to explain the early deglaciation along the western margin of Gondwana. To do this, several outcrops within the eastern portion of the Paganzo Basin in western Argentina were sampled for the Chemical Index of Alteration (CIA) geochemical analyses. Here, we test the applicability of the CIA as a paleoclimate proxy on strata in the Olta-Malanzan paleovalley that historically was thought to have been glaciated. A recent study by the authors has shown that the paleovalley was not glaciated, but owes its origin to extension and excavation by fluvial processes. However, the late Paleozoic stratigraphy of this paleovalley system is similar to the rest of the Paganzo Basin. The results from the paleovalley samples show that this area was intermittently humid and arid through time, but with an overall arid profile. This signature is predominantly due to the nature of the paleovalley, which was subjected to rapid burial from frequent rock falls, progradation alluvial fans/fan deltas, and lacustrine sediment gravity flows (Malanzan Fm.), which prevented any significant chemical weathering. While the overall Pennsylvanian climatic signature appears to be relatively arid (Malanzan, Loma Larga, and Solca Fms.), it seems that the climate during the deposition of the late Pennsylvanian and Permian La Colina Formation was more humid than previously thought.

Archaeological excavations at EF-HR and HWK EE allow reassessment of Bed II stratigraphy within the Junction Area and eastern Olduvai Gorge. Application of Sequence Stratigraphic methods provides a time-stratigraphic framework enabling correlation of sedimentary units across facies boundaries, applicable even in those areas where conventional timelines, such as tephrostratigraphic markers, are absent, eroded, or reworked. Sequence Stratigraphically, Bed II subdivides into five major Sequences 1 to 5, all floored by major disconformities that incise deeply into the underlying succession, proving that simple "layer cake" stratigraphy is inappropriate. Previous establishment of the Lemuta Member has invalidated the use of Tuff IIA as the boundary between Lower and Middle Bed II, now redefined at the disconformity between Sequences 2 and 3, a lithostratigraphic contact underlying the succession containing the Lower, Middle, and Upper Augitic Sandstones. HWK EE site records Old...

This paper reports the results of renewed fieldwork at the HWK EE site (Olduvai Gorge, Tanzania). HWK EE is positioned across the boundary between Lower and Middle Bed II, a crucial interval for studying the emergence of the Acheulean at Olduvai Gorge. Our excavations at HWK EE have produced one of the largest collections of fossils and artefacts from any Oldowan site, distributed across several archaeological units and a large excavation surface in four separate trenches that can be stratigraphically correlated. Here we present the main stratigraphic and archaeological units and discuss site formation processes. Results show a great density of fossils and stone tools vertically through two stratigraphic intervals (Lemuta and Lower Augitic Sandstone) and laterally across an area of around 300 m(2), and highlight the confluence of biotic and abiotic agents in the formation of the assemblage. The large size and diversity of the assemblage, as well as its good preservation, qualify HWK...

This paper reports the results of renewed fieldwork at the HWK EE site (Olduvai Gorge, Tanzania). HWK EE is positioned across the boundary between Lower and Middle Bed II, a crucial interval for studying the emergence of the Acheulean at Olduvai Gorge. Our excavations at HWK EE have produced one of the largest collections of fossils and artefacts from any Oldowan site, distributed across several archaeological units and a large excavation surface in four separate trenches that can be stratigraphically correlated. Here we present the main stratigraphic and archaeological units and discuss site formation processes. Results show a great density of fossils and stone tools vertically through two stratigraphic intervals (Lemuta and Lower Augitic Sandstone) and laterally across an area of around 300 m2, and highlight the confluence of biotic and abiotic agents in the formation of the assemblage. The large size and diversity of the assemblage, as well as its good preservation, qualify HWK E...

Excellent agreement was noted in the concentration of major and trace elements in five NIST soil reference materials (NIST SRM 2586, 2587, 2709a, 2710a and 2711a) between measurement results from wavelength dispersive-XRF and ICP-MS from two independent laboratories, and NIST certificate of analysis and literature data. We describe the variability in concentrations of up to forty-nine elements (plus loss on ignition) and provide values for up to twenty-one elements previously uncharacterised by NIST in these soil RMs. The additional characterisation provided in this investigation can be utilised to reduce the measurement bias of custom calibration routines and improve the quality of control checks developed using these NIST RMs.

Renewed fieldwork at the early Acheulean site of EF-HR (Olduvai Gorge, Tanzania) has included detailed stratigraphic studies of the sequence, extended excavations in the main site, and has placed eleven additional trenches within an area of nearly 1 km(2), to sample the same stratigraphic interval as in the main trench across the broader paleo-landscape. Our new stratigraphic work suggests that EF-HR is positioned higher in the Bed II sequence than previously proposed, which has implications for the age of the site and its stratigraphic correlation to other Olduvai Middle Bed II sites. Geological research shows that the main EF-HR site was situated at the deepest part of an incised valley formed through river erosion. Archaeological excavations at the main site and nearby trenches have unearthed a large new assemblage, with more than 3000 fossils and artefacts, including a hundred handaxes in stratigraphic position. In addition, our test-trenching approach has detected conspicuous d...

The early Cambrian Indian Springs biota of western Nevada, USA, exhibits Burgess Shale-type (BST) preservation of a diverse array of animal phyla, including the earliest definitive echinoderms. It therefore provides an important window on animal life during the Cambrian radiation. The objective of this study was to analyse the trace metal palaeoredox geochemistry and bioturbation levels of this BST deposit in order to characterize the palaeoenvironmental conditions in which these animals lived and their remains preserved. A total of 28 rock samples were collected from outcrops at three previously reported intervals of exceptional preservation at the Indian Springs locality, as well as from one interval not exhibiting such preservation. An additional 20 random samples were collected from talus for comparison. In the laboratory, the samples were analysed for trace metal palaeoredox indices (V/Cr and V/(V + Ni) ratios). Bioturbation levels were assessed through X-radiography and petrographic thin sections using the ichnofabric index (ii) method. Additional samples from coeval strata of the Poleta Formation in the White-Inyo Mountains, CA, that lack BST preservation were also analysed with the same methodology. Results indicate that oxic bottom water conditions dominated during deposition of these strata, despite consistently low bioturbation levels. This pattern holds for intervals with BST preservation and those without. Although ephemeral incursions of low-oxygen waters may have taken place, there is no evidence for persistent oxygen restriction in these palaeoenvironments. The low levels of bioturbation indicate limited mixed layer development and a redox boundary near the sediment–water interface, likely allowing post-burial BST preservation to occur even in this setting dominated by oxic bottom waters. Palaeoecological reconstructions and taphonomic hypotheses relating to the Indian Springs Lagerstatte must consider the palaeoredox conditions revealed in this study. With the dispensing of anoxic bottom waters as a requirement for BST preservation, other models proposing a role for clay minerals, the presence of hypersaline brines and the actions of Fe-reducing bacteria as mechanisms for exceptional preservation warrant renewed consideration.

Basaltic caves and lava tubes offer stable physicochemical conditions for formation of secondary minerals. Such features, putatively observed on Mars, intercept groundwater to weather country rock, leading to formation of secondary minerals. Further, caves are stable environments to search for evidence of past life, as they could offer protection from the oxidizing martian atmosphere. Searching for signs of life in a cave that could protect bio/organic compounds would preclude the need for risky drilling on Mars. Craters of the Moon National Monument (COM) offers an opportunity to study caves in Holocene iron-rich basalt flows to characterize secondary mineral deposits and search for organic compounds associated with secondary minerals; COM basalts are a good analog for martian basalts because of their high iron but other elements are higher at COM than on Mars. The Blue Dragon flow (~2.1 ka) contains the majority of the accessible caves and lava tubes. Two types of secondary mineral deposits were observed in these caves: ceiling coatings and crack or floor precipitates. Hematite, silica, and calcite comprise ceiling coatings. The crack and floor precipitates are white, efflorescent deposits in cavities along cave walls and ceilings or in localized mounds on cave floors. The secondary minerals in crack and floor precipitates are mainly thenardite and mirabilite with some minor concentrations of trona and/or burkeite. Organic compounds were found associated with the efflorescent deposits. Formation of the deposits is likely due to chemical leaching of basalt by meteoritic water. To test this, fluids collected from the ceiling and walls of the caves were analyzed. Solutions were modeled with the geochemical code, PHREEQC. The model tracked composition as water evaporated. Selected minerals were allowed to precipitate as they became oversaturated. Among the first minerals to become oversaturated were quartz and calcite, which are observed in ceiling deposits. Iron minerals were not included as no iron was detected in solution. Results compared well with evaporation of solutions generated by simulating chemical weathering of minerals found in the basalt; this approach allowed iron minerals to precipitate during evaporation because minerals in the basalt contained iron. The minerals modeled upon evaporation included the minerals observed in the actual deposits - hematite, calcite, and quartz. Na-minerals neared saturation in simulations but were normally not saturated, leaving open the question of their origin. One possible explanation for the presence of Na-minerals could be seasonal ice formation in the caves followed by sublimation, leaving more concentrated solutions behind than were sampled here. A seasonal model for mineral deposition in caves could be relevant to deposits in martian caves. While the formation mechanism for the secondary minerals at COM is not completely understood, the presence of secondary minerals that harbor organic compounds in a cave environment that may be analogous to Mar has implications for where to search for signs of martian life.

Abstract The Cambrian radiation of complex animals is a fundamental event in the history of life on Earth. Much of our understanding of this event is made possible through the study of exceptionally preserved fossils in Burgess Shale-type (BST) deposits. Based on bioturbation levels in BST deposits, they are often interpreted as representing restricted oxygen settings. This study tests the low-oxygen interpretation through analysis of geochemical paleoredox proxies and bioturbation levels in three BST deposits: the early Cambrian Maotianshan Shale (China), the middle Cambrian Wheeler Shale (USA), and the middle Cambrian Spence Shale (USA). Results from 96 samples show fine-scale geochemical evidence for oxic bottom water conditions during the deposition of these three BST deposits. Trace element paleoredox indices (PI) give consistent oxic signals in all three shales. In addition, total organic carbon (TOC) and total sulfur (TS) levels in the Spence Shale are extremely low. The average C/S ratio is 3.1, comparable to values for modern oxic marine water. Bioturbation levels in all three shales are low, with rare development of minimal to moderate horizontal bioturbation. While short-term oxygen fluctuations, or poikiloaerobic conditions, could still have sporadically existed in these settings, this study shows that these BST deposits were deposited in a dominantly oxic paleoredox setting. Evidence for low bioturbation levels coupled with paleoredox geochemistry indicative of oxic bottom waters suggests that pervasive exaerobic conditions, with an oxic water column positioned above oxygen-depleted sediment pore waters, may have existed in many Cambrian BST deposit paleoenvironments.

Abstract The Cambrian radiation of complex animals is a fundamental event in the history of life on Earth. Much of our understanding of this event is made possible through the study of exceptionally preserved fossils in Burgess Shale-type (BST) deposits. Based on bioturbation levels in BST deposits, they are often interpreted as representing restricted oxygen settings. This study tests the low-oxygen interpretation through analysis of geochemical paleoredox proxies and bioturbation levels in three BST deposits: the early Cambrian Maotianshan Shale (China), the middle Cambrian Wheeler Shale (USA), and the middle Cambrian Spence Shale (USA). Results from 96 samples show fine-scale geochemical evidence for oxic bottom water conditions during the deposition of these three BST deposits. Trace element paleoredox indices (PI) give consistent oxic signals in all three shales. In addition, total organic carbon (TOC) and total sulfur (TS) levels in the Spence Shale are extremely low. The average C/S ratio is 3.1, comparable to values for modern oxic marine water. Bioturbation levels in all three shales are low, with rare development of minimal to moderate horizontal bioturbation. While short-term oxygen fluctuations, or poikiloaerobic conditions, could still have sporadically existed in these settings, this study shows that these BST deposits were deposited in a dominantly oxic paleoredox setting. Evidence for low bioturbation levels coupled with paleoredox geochemistry indicative of oxic bottom waters suggests that pervasive exaerobic conditions, with an oxic water column positioned above oxygen-depleted sediment pore waters, may have existed in many Cambrian BST deposit paleoenvironments.

DEPOSIT: IMPLICATIONS FOR MARS L. J. McHenry 1 , V. F. Chevrier 2 , and C. Schroder 3 , 1 Department of Geosciences, University of WisconsinMilwaukee, PO Box 413, Milwaukee, WI 53201, lmchenry@uwm.edu, 2 W.M. Keck Laboratory for Space and Planetary Simulation, Arkansas Center for Space and Planetary Science, MUSE 202, University of Arkansas, Fayetteville, AR 72701, 3 Center for Applied Geoscience, Eberhard Karls Universitat, Sigwartstr. 10, D-72076 Tubingen, Germany, christian.schroeder@ifg.uni-tuebingen.de.