Efficiency of the Sieved-NPMLE in CAR-Missing Data Models

We consider nonparametric missing data models for which the censoring mechanism satisfies coarsening at random and which allow complete observations on the variable X of interest. W show that beyond some empirical process conditions the only essential condition for efficiency of an NPMLE of the distribution of X is that the regions associated with incomplete observations on X contain enough complete observations. This is heuristically explained by describing the EM-algorithm. We provide identifiably of the self-consistency equation and efficiency of the NPMLE in order to make this statement rigorous. The usual kind of differentiability conditions in the proof are avoided by using an identity which holds for the NPMLE of linear parameters in convex models. We provide a bivariate censoring application in which the condition and hence the NPMLE fails, but where other estimators, not based on the NPMLE principle, are highly inefficient. It is shown how to slightly reduce the data so that the conditions hold for the reduced data. The conditions are verified for the univariate censoring, double censored, and Ibragimov-Has'minski models.

Proving Efficiency of NPMLE and Identities

A method is given for proving efficiency of NPMLE directly linked to empirical process theory. The conditions in general are appropriate consistency of the NPMLE, differentiability of the model, differentiability of the parameter of interest, local convexity of the parameter space, and a Donsker class condition for the class of efficient influence functions obtained by varying the parameters. For the case that the model is linear in the parameter and the parameter space is convex, as with most nonparametric missing data models, we show that the method leads to an identity for the NPMLE which almost says that the NPMLE is efficient and provides us straightforwardly with a consistency and efficiency proof. This identify is extended to an almost linear class of models which contain biased sampling models. To illustrate, the method is applied to the univariate censoring model, random truncation models, interval censoring case I model, the class of parametric models and to a class of semiparametric models.

Correlation between the area of increased autofluorescence surrounding geographic atrophy and disease progression in patients with AMD

PURPOSE: To test the hypothesis that the extension of areas with increased fundus autofluorescence (FAF) outside atrophic patches correlates with the rate of spread of geographic atrophy (GA) over time in eyes with age-related macular degeneration (AMD). METHODS: The database of the multicenter longitudinal natural history Fundus Autofluorescence in AMD (FAM) Study was reviewed for patients with GA recruited through the end of August 2003, with follow-up examinations within at least 1 year. Only eyes with sufficient image quality and with diffuse patterns of increased FAF surrounding atrophy were chosen. In standardized digital FAF images (excitation, 488 nm; emission, >500 nm), total size and spread of GA was measured. The convex hull (CH) of increased FAF as the minimum polygon encompassing the entire area of increased FAF surrounding the central atrophic patches was quantified at baseline. Statistical analysis was performed with the Spearman's rank correlation coefficient (rho). RESULTS: Thirty-nine eyes of 32 patients were included (median age, 75.0 years; interquartile range [IQR], 67.8-78.9); median follow-up, 1.87 years; IQR, 1.43-3.37). At baseline, the median total size of atrophy was 7.04 mm2 (IQR, 4.20-9.88). The median size of the CH was 21.47 mm2 (IQR, 15.19-28.26). The median rate of GA progression was 1.72 mm2 per year (IQR, 1.10-2.83). The area of increased FAF around the atrophy (difference between the CH and the total GA size at baseline) showed a positive correlation with GA enlargement over time (rho=0.60; P=0.0002). CONCLUSIONS: FAF characteristics that are not identified by fundus photography or fluorescein angiography may serve as a prognostic determinant in advanced atrophic AMD. As the FAF signal originates from lipofuscin (LF) in postmitotic RPE cells and since increased FAF indicates excessive LF accumulation, these findings would underscore the pathophysiological role of RPE-LF in AMD pathogenesis.

Multiasset Derivatives and Joint Distributions of Asset Prices

Several of multiasset derivatives like basket options or options on the weighted maximum of assets exhibit the property that their prices determine uniquely the underlying asset distribution. Related to that the question how to retrieve this distributions from the corresponding derivatives quotes will be discussed. On the contrary, the prices of exchange options do not uniquely determine the underlying distributions of asset prices and the extent of this non-uniqueness can be characterised. The discussion is related to a geometric interpretation of multiasset derivatives as support functions of convex sets. Following this, various symmetry properties for basket, maximum and exchange options are discussed alongside with their geometric interpretations and some decomposition results for more general payoff functions.

A generalized anisotropic quadric yield criterion and its application to bone tissue at multiple length scales

Nonlinear computational analysis of materials showing elasto-plasticity or damage relies on knowledge of their yield behavior and strengths under complex stress states. In this work, a generalized anisotropic quadric yield criterion is proposed that is homogeneous of degree one and takes a convex quadric shape with a smooth transition from ellipsoidal to cylindrical or conical surfaces. If in the case of material identification, the shape of the yield function is not known a priori, a minimization using the quadric criterion will result in the optimal shape among the convex quadrics. The convexity limits of the criterion and the transition points between the different shapes are identified. Several special cases of the criterion for distinct material symmetries such as isotropy, cubic symmetry, fabric-based orthotropy and general orthotropy are presented and discussed. The generality of the formulation is demonstrated by showing its degeneration to several classical yield surfaces like the von Mises, Drucker–Prager, Tsai–Wu, Liu, generalized Hill and classical Hill criteria under appropriate conditions. Applicability of the formulation for micromechanical analyses was shown by transformation of a criterion for porous cohesive-frictional materials by Maghous et al. In order to demonstrate the advantages of the generalized formulation, bone is chosen as an example material, since it features yield envelopes with different shapes depending on the considered length scale. A fabric- and density-based quadric criterion for the description of homogenized material behavior of trabecular bone is identified from uniaxial, multiaxial and torsional experimental data. Also, a fabric- and density-based Tsai–Wu yield criterion for homogenized trabecular bone from in silico data is converted to an equivalent quadric criterion by introduction of a transformation of the interaction parameters. Finally, a quadric yield criterion for lamellar bone at the microscale is identified from a nanoindentation study reported in the literature, thus demonstrating the applicability of the generalized formulation to the description of the yield envelope of bone at multiple length scales.

Guaranteed greedy routing in overlay networks

Greedy routing can be used in mobile ad-hoc networks as geographic routing protocol. This paper proposes to use greedy routing also in overlay networks by positioning overlay nodes into a multi-dimensional Euclidean space. Greedy routing can only be applied when a routing decision makes progress towards the final destination. Our proposed overlay network is built such that there will be always progress at each forwarding node. This is achieved by constructing at each node a so-called nearest neighbor convex set (NNCS). NNCSs can be used for various applications such as multicast routing, service discovery and Quality-of-Service routing. NNCS has been compared with Pastry, another topology-aware overlay network. NNCS has superior relative path stretches indicating the optimality of a path.

Fully automatic X-ray image segmentation via joint estimation of image displacements.

We propose a new method for fully-automatic landmark detection and shape segmentation in X-ray images. Our algorithm works by estimating the displacements from image patches to the (unknown) landmark positions and then integrating them via voting. The fundamental contribution is that, we jointly estimate the displacements from all patches to multiple landmarks together, by considering not only the training data but also geometric constraints on the test image. The various constraints constitute a convex objective function that can be solved efficiently. Validated on three challenging datasets, our method achieves high accuracy in landmark detection, and, combined with statistical shape model, gives a better performance in shape segmentation compared to the state-of-the-art methods.

Low rank subspace clustering (LRSC)

We consider the problem of fitting a union of subspaces to a collection of data points drawn from one or more subspaces and corrupted by noise and/or gross errors. We pose this problem as a non-convex optimization problem, where the goal is to decompose the corrupted data matrix as the sum of a clean and self-expressive dictionary plus a matrix of noise and/or gross errors. By self-expressive we mean a dictionary whose atoms can be expressed as linear combinations of themselves with low-rank coefficients. In the case of noisy data, our key contribution is to show that this non-convex matrix decomposition problem can be solved in closed form from the SVD of the noisy data matrix. The solution involves a novel polynomial thresholding operator on the singular values of the data matrix, which requires minimal shrinkage. For one subspace, a particular case of our framework leads to classical PCA, which requires no shrinkage. For multiple subspaces, the low-rank coefficients obtained by our framework can be used to construct a data affinity matrix from which the clustering of the data according to the subspaces can be obtained by spectral clustering. In the case of data corrupted by gross errors, we solve the problem using an alternating minimization approach, which combines our polynomial thresholding operator with the more traditional shrinkage-thresholding operator. Experiments on motion segmentation and face clustering show that our framework performs on par with state-of-the-art techniques at a reduced computational cost.

Spider Venoms Potentially Lethal to Humans

Spiders have one pair of venom glands, and only a few families have reduced them completely (Uloboridae, Holarchaeidae) or modified them to another function (Symphytognathidae or Scytodidae, see Suter and Stratton 2013). All other 42,000 known spider species (99%) utilize their venom to inject it into prey items, which subsequently become paralysed or are killed. Spider venom is a complex mixture of hundreds of components, many of them interacting with cell membranes or receptors located mainly in the nervous or muscular system (Herzig and King 2013). Spider venom, as it is today, has a 300-million-yearlong history of evolution and adaptation and can be considered as an optimized tool to subdue prey. In Mesothelae, the oldest spider group with less than 100 species, the venom glands lie in the anterior part of the cheliceral basal segment. They are very small and do not support the predation process very effectively. In Mygalomorphae, the venom glands are well developed and fill the basal cheliceral segment more or less completely. Many of these 3,000 species are medium- to large-/very large-sized spiders, and they have created the image of being dangerous beasts, attacking and killing a variety of animals, including humans. Although this picture is completely wrong, it is persistent and contributes considerably to human arachnophobia. The third group of spiders, Araneomorphae or “modern spiders”, comprises 93% of all spider species. The venom glands are enlarged and extend to the prosoma; the openings of the venom ducts are moved from the convex to the concave side of the cheliceral fangs and enlarged as well. These changes save the chelicerae from the necessity of being large, and hence, on the average, araneomorph spiders are much smaller than mygalomorphs. Nevertheless, they possess relatively large venom glands, situated mainly in the prosoma, and may also have rather potent venom.

Eye examining instrument

An eye examining instrument comprises a projection device and a concave screen. The eye examining instrument furthermore comprises a convex reflector, wherein an image can be projected by the projection device onto the convex reflector and reflected by the convex reflector onto the concave screen.

Cell and matrix morphology in articular cartilage from adult human knee and ankle joints suggests depth-associated adaptations to biomechanical and anatomical roles

OBJECTIVE Marked differences exist between human knee and ankle joints regarding risks and progression of osteoarthritis (OA). Pathomechanisms of degenerative joint disease may therefore differ in these joints, due to differences in tissue structure and function. Focussing on structural issues which are design goals for tissue engineering, we compared cell and matrix morphologies in different anatomical sites of adult human knee and ankle joints. METHODS Osteochondral explants were acquired from knee and ankle joints of deceased persons aged 20 to 40 years and analyzed for cell, matrix and tissue morphology using confocal and electron microscopy and unbiased stereological methods. Variations associated with joint (knee versus ankle) and biomechanical role (convex versus concave articular surfaces) were identified by 2-way analysis of variance and post-hoc analysis. RESULTS Knee cartilage exhibited higher cell densities in the superficial zone than ankle cartilage. In the transitional zone, higher cell densities were observed in association with convex versus concave articular surfaces, without significant differences between knee and ankle cartilage. Highly uniform cell and matrix morphologies were evident throughout the radial zone in the knee and ankle, regardless of tissue biomechanical role. Throughout the knee and ankle cartilage sampled, chondron density was remarkably constant at approximately 4.2×10(6) chondrons/cm(3). CONCLUSION Variation of cartilage cell and matrix morphologies with changing joint and biomechanical environments suggests that tissue structural adaptations are performed primarily by the superficial and transitional zones. Data may aid the development of site-specific cartilage tissue engineering, and help identify conditions where OA is likely to occur.

Correction to Variations of Ice Bed Coupling Beneath and Beyond Ice Streams: The Force Balance

A geometrical force balance that links stresses to ice bed coupling along a flow band of an ice sheet was developed in 1988 for longitudinal tension in ice streams and published 4 years later. It remains a work in progress. Now gravitational forces balanced by forces producing tensile, compressive, basal shear, and side shear stresses are all linked to ice bed coupling by the floating fraction phi of ice that produces the concave surface of ice streams. These lead inexorably to a simple formula showing how phi varies along these flow bands where surface and bed topography are known: phi = h(O)/h(I) with h(O) being ice thickness h(I) at x = 0 for x horizontal and positive upslope from grounded ice margins. This captures the basic fact in glaciology: the height of ice depends on how strongly ice couples to the bed. It shows how far a high convex ice sheet (phi = 0) has gone in collapsing into a low flat ice shelf (phi = 1). Here phi captures ice bed coupling under an ice stream and h(O) captures ice bed coupling beyond ice streams.

Variations of Ice Bed Coupling Beneath and Beyond Ice Streams: The Force Balance

A geometrical force balance that links stresses to ice bed coupling along a flow band of an ice sheet was developed in 1988 for longitudinal tension in ice streams and published 4 years later. It remains a work in progress. Now gravitational forces balanced by forces producing tensile, compressive, basal shear, and side shear stresses are all linked to ice bed coupling by the floating fraction phi of ice that produces the concave surface of ice streams. These lead inexorably to a simple formula showing how phi varies along these flow bands where surface and bed topography are known: phi = h(O)/h(I) with h(O) being ice thickness h(I) at x = 0 for x horizontal and positive upslope from grounded ice margins. This captures the basic fact in glaciology: the height of ice depends on how strongly ice couples to the bed. It shows how far a high convex ice sheet (phi = 0) has gone in collapsing into a low flat ice shelf (phi = 1). Here phi captures ice bed coupling under an ice stream and h(O) captures ice bed coupling beyond ice streams.

Schmiede ─ Heiligtum ─ Wassermühle. Cham-Hagendorn (Kanton Zug) in römischer Zeit. Ausgrabungen 1944/45 und 2003/04

1944/1945 wurde in Cham-Hagendorn eine Wassermühle ausgegraben, die dank ihrer aussergewöhnlich guten Holzerhaltung seit langem einen prominenten Platz in der Forschung einnimmt. 2003 und 2004 konnte die Kantonsarchäologie Zug den Platz erneut archäologisch untersuchen. Dabei wurden nicht nur weitere Reste der Wassermühle, sondern auch Spuren älterer und jüngerer Anlagen geborgen: eine ältere und eine jüngere Schmiedewerkstatt (Horizont 1a/Horizont 3) sowie ein zweiphasiges Heiligtum (Horizonte 1a/1b). All diese Anlagen lassen sich nun in das in den neuen Grabungen erkannte stratigraphische Gerüst einhängen (s. Beil. 2). Dank der Holzerhaltung können die meisten Phasen dendrochronologisch datiert werden (s. Abb. 4.1/1a): Horizont 1a mit Schlagdaten zwischen 162(?)/173 und 200 n. Chr., Horizont 1b um 215/218 n. Chr. und Horizont 2 um 231 n. Chr. Ferner konnten in den neuen Grabungen Proben für mikromorphologische und archäobotanische Untersuchungen entnommen werden (Kap. 2.2; 3.11). In der vorliegenden Publikation werden der Befund und die Baustrukturen vorgelegt, (Kap. 2), desgleichen sämtliche stratifizierten Funde und eine umfassende Auswahl der 1944/1945 geborgenen Funde (Kap. 3). Dank anpassender Fragmente, sog. Passscherben, lassen sich diese zum Teil nachträglich in die Schichtenabfolge einbinden. Die mikromorphologischen und die archäobotanischen Untersuchungen (Kap. 2.2; 3.11) zeigen, dass der Fundplatz in römischer Zeit inmitten einer stark vom Wald und dem Fluss Lorze geprägten Landschaft lag. In unmittelbarer Nähe können weder eine Siedlung noch einzelne Wohnbauten gelegen haben. Die demnach nur gewerblich und sakral genutzten Anlagen standen an einem Bach, der vermutlich mit jenem Bach identisch ist, der noch heute das Groppenmoos entwässert und bei Cham-Hagendorn in die Lorze mündet (s. Abb. 2.4/1). Der antike Bach führte wiederholt Hochwasser ─ insgesamt sind fünf grössere Überschwemmungsphasen auszumachen (Kap. 2.2; 2.4). Wohl anlässlich eines Seehochstandes durch ein Überschwappen der Lorze in den Bach ausgelöst, müssen diese Überschwemmungen eine enorme Gewalt entwickelt haben, der die einzelnen Anlagen zum Opfer fielen. Wie die Untersuchung der Siedlungslandschaft römischer Zeit rund um den Zugersee wahrscheinlich macht (Kap. 6 mit Abb. 6.2/2), dürften die Anlagen von Cham-Hagendorn zu einer in Cham-Heiligkreuz vermuteten Villa gehören, einem von fünf grösseren Landgütern in diesem Gebiet. Hinweise auf Vorgängeranlagen fehlen, mit denen die vereinzelten Funde des 1. Jh. n. Chr. (Kap. 4.5) in Verbindung gebracht werden könnten. Diese dürften eher von einer der Überschwemmungen bachaufwärts weggerissen und nach Cham-Hagendorn eingeschwemmt worden sein. Die Nutzung des Fundplatzes (Horizont 1a; s. Beil. 6) setzte um 170 n. Chr. mit einer Schmiedewerkstatt ein (Kap. 2.5.1). Der Fundanfall, insbesondere die Schmiedeschlacken (Kap. 3.9) belegen, dass hier nur hin und wieder Geräte hergestellt und repariert wurden (Kap. 5.2). Diese Werkstatt war vermutlich schon aufgelassen und dem Verfall preisgegeben, als man 200 n. Chr. (Kap. 4.2.4) auf einer Insel zwischen dem Bach und einem Lorzearm ein Heiligtum errichtete (Kap. 5.3). Beleg für den sakralen Status dieser Insel ist in erster Linie mindestens ein eigens gepflanzter Pfirsichbaum, nachgewiesen mit Pollen, einem Holz und über 400 Pfirsichsteinen (Kap. 3.11). Die im Bach verlaufende Grenze zwischen dem sakralen Platz und der profanen Umgebung markierte man zusätzlich mit einer Pfahlreihe (Kap. 2.5.3). In diese war ein schmaler Langbau integriert (Kap. 2.5.2), der an die oft an Temenosmauern antiker Heiligtümer angebauten Portiken erinnert und wohl auch die gleiche Funktion wie diese gehabt hatte, nämlich das Aufbewahren von Weihegaben und Kultgerät (Kap. 5.3). Das reiche Fundmaterial, das sich in den Schichten der ersten Überschwemmung fand (s. Abb. 5./5), die um 205/210 n. Chr. dieses Heiligtum zerstört hatte, insbesondere die zahlreiche Keramik (Kap. 3.2.4), und die zum Teil auffallend wertvollen Kleinfunde (Kap. 3.3.3), dürften zum grössten Teil einst in diesem Langbau untergebracht gewesen sein. Ein als Glockenklöppel interpretiertes, stratifiziertes Objekt spricht dafür, dass die fünf grossen, 1944/1945 als Stapel aufgefundenen Eisenglocken vielleicht auch dem Heiligtum zuzuweisen sind (Kap. 3.4). In diesen Kontext passen zudem die überdurchschnittlich häufig kalzinierten Tierknochen (Kap. 3.10). Nach der Überschwemmung befestigte man für 215 n. Chr. (Kap. 4.2.4) das unterspülte Bachufer mit einer Uferverbauung (Kap. 2.6.1). Mit dem Bau eines weiteren, im Bach stehenden Langbaus (Kap. 2.6.2) stellte man 218 n. Chr. das Heiligtum auf der Insel in ähnlicher Form wieder her (Horizont 1b; s. Beil. 7). Von der Pfahlreihe, die wiederum die sakrale Insel von der profanen Umgebung abgrenzte, blieben indes nur wenige Pfähle erhalten. Dennoch ist der sakrale Charakter der Anlage gesichert. Ausser dem immer noch blühenden Pfirsichbaum ist es ein vor dem Langbau aufgestelltes Ensemble von mindestens 23 Terrakottafigurinen (s. Abb. 3.6/1), elf Veneres, zehn Matres, einem Jugendlichen in Kapuzenmantel und einem kindlichen Risus (Kap. 3.6; s. auch Kap. 2.6.3). In den Sedimenten der zweiten Überschwemmung, der diese Anlage um 225/230 n. Chr. zum Opfer gefallen war, fanden sich wiederum zahlreiche Keramikgefässe (Kap. 3.2.4) und zum Teil wertvolle Kleinfunde wie eine Glasperle mit Goldfolie (Kap. 3.8.2) und eine Fibel aus Silber (Kap. 3.3.3), die wohl ursprünglich im Langbau untergebracht waren (Kap. 5.3.2 mit Abb. 5/7). Weitere Funde mit sicherem oder möglichem sakralem Charakter finden sich unter den 1944/1945 geborgenen Funden (s. Abb. 5/8), etwa ein silberner Fingerring mit Merkurinschrift, ein silberner Lunula-Anhänger, eine silberne Kasserolle (Kap. 3.3.3), eine Glasflasche mit Schlangenfadenauflage (Kap. 3.8.2) und einige Bergkristalle (Kap. 3.8.4). Im Bereich der Terrakotten kamen ferner mehrere Münzen (Kap. 3.7) zum Vorschein, die vielleicht dort niedergelegt worden waren. Nach der zweiten Überschwemmung errichtete man um 231 n. Chr. am Bach eine Wassermühle (Horizont 2; Kap. 2.7; Beil. 8; Abb. 2.7/49). Ob das Heiligtum auf der Insel wieder aufgebaut oder aufgelassen wurde, muss mangels Hinweisen offen bleiben. Für den abgehobenen Zuflusskanal der Wassermühle verwendete man mehrere stehen gebliebene Pfähle der vorangegangenen Anlagen der Horizonte 1a und 1b. Obwohl die Wassermühle den 28 jährlichen Überschwemmungshorizonten (Kap. 2.2) und den Funden (Kap. 4.3.2; 4.4.4; 45) zufolge nur bis um 260 n. Chr., während gut einer Generation, bestand, musste sie mindestens zweimal erneuert werden – nachgewiesen sind drei Wasserräder, drei Mühlsteinpaare und vermutlich drei Podeste, auf denen jeweils das Mahlwerk ruhte. Grund für diese Umbauten war wohl der weiche, instabile Untergrund, der zu Verschiebungen geführt hatte, so dass das Zusammenspiel von Wellbaum bzw. Sternnabe und Übersetzungsrad nicht mehr funktionierte und das ganze System zerbrach. Die Analyse von Pollen aus dem Gehhorizont hat als Mahlgut Getreide vom Weizentyp nachgewiesen (Kap. 3.11.4). Das Abzeichen eines Benefiziariers (Kap. 3.3.2 mit Abb. 3.3/23,B71) könnte dafür sprechen, dass das verarbeitete Getreide zumindest zum Teil für das römische Militär bestimmt war (s. auch Kap. 6.2.3). Ein im Horizont 2 gefundener Schreibgriffel und weitere stili sowie eine Waage für das Wägen bis zu 35-40 kg schweren Waren aus dem Fundbestand von 1944/1945 könnten davon zeugen, dass das Getreide zu wägen und zu registrieren war (Kap. 3.4.2). Kurz nach 260 n. Chr. fiel die Wassermühle einem weiteren Hochwasser zum Opfer. Für den folgenden Horizont 3 (Beil. 9) brachte man einen Kiesboden ein und errichtete ein kleines Gebäude (Kap. 2.8). Hier war wohl wiederum eine Schmiede untergebracht, wie die zahlreichen Kalottenschlacken belegen (Kap. 3.9), die im Umfeld der kleinen Baus zum Vorschein kamen. Aufgrund der Funde (Kap. 4.4.4; 4.5) kann diese Werkstatt nur kurze Zeit bestanden haben, höchstens bis um 270 n. Chr., bevor sie einem weiteren Hochwasser zum Opfer fiel. Von der jüngsten Anlage, die wohl noch in römische Zeit datiert (Horizont 4; Beil. 10), war lediglich eine Konstruktion aus grossen Steinplatten zu fassen (Kap. 2.9.1). Wozu sie diente, muss offen bleiben. Auch der geringe Fundanfall spricht dafür, dass die Nutzung des Platzes, zumindest für die römische Zeit, allmählich ein Ende fand (Kap. 4.5). Zu den jüngsten Strukturen gehören mehrere Gruben (Kap. 2.9.2), die vielleicht der Lehmentnahme dienten. Mangels Funden bleibt ihre Datierung indes ungewiss. Insbesondere wissen wir nicht, ob sie noch in römische Zeit datieren oder jünger sind. Spätestens mit der fünften Überschwemmung, die zur endgültigen Verlandung führte und wohl schon in die frühe Neuzeit zu setzen ist, wurde der Platz aufgelassen und erst mit dem Bau der bestehenden Fensterfabrik Baumgartner wieder besetzt.

Geophysical and Hydrological Evaluation of Two Bog Complexes in a Northern Peatland: Implications for the Distribution of Biogenic Gases at the Basin Scale

Ground penetrating radar (GPR) was used to determine peat basin geometry and the spatial distribution of free-phase biogenic gasses in two separate units of a northern peatland (Central and Southern Unit of Caribou Bog, Maine). The Central Unit is characterized by a deep basin structure (15 m maximum depth) and a raised (eccentric) bog topographic profile (up to 2 m topographic variation). Here numerous regions of electromagnetic (EM) wave scattering are considered diagnostic of the presence of extensive free-phase biogenic gas. In contrast, the Southern Unit is shallower (8 m maximum depth) and has a slightly convex upwards bog profile (less than 1 m topographic variation), and areas of EM wave scattering are notably absent. The biogenic gas zones interpreted from GPR in the Central Unit are associated with: (1) wooded heath vegetation at the surface, (2) open pools at the surface, (3) high water table elevations near the center of the basin, and (4) a region of overpressure (at approximately 5 m depth) immediately below the zone of free-phase gas accumulation. The latter suggests (1) a transient pressure head associated with low hydraulic conductivity resulting from the biogenic gasses themselves or confining layers in the peat that restrict both gas release and groundwater flow and/or (2) overpressure in the peat column as a result of the gas buildup itself. In contrast, the Southern Unit, where zones of EM scattering are absent, is characterized by: (1) predominantly shrub vegetation, (2) a lack of open pools, (3) only minor variations (less than 1 m) in water table elevation throughout the entire unit; and (4) generally upward groundwater flow throughout the basin. The results illustrate the nonuniformity of free-phase biogenic gas distribution at the peat basin scale and provide insights into the processes and controls associated with CH4 and CO2 accumulation in peatlands.