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Bioturbation beyond Earth: potential, methods and models of Astroichnology
No full textTraces – burrows, borings, footprints – are major evidences of biological behaviour on Earth, yet they received little attention in the field of astrobiology. This study aims to discuss the application of ichnology (i.e. the study of life activity traces) to the search for past and modern life beyond Earth (i.e. herein called Astroichnology).
Why to look for traces is a central question, given that organisms (and their body fossils) apparently represent a more direct evidence of life in present and past times. The reason is fourfold. First, the Earth’s ichnological record shows that traces record accurately the activity of soft-bodied organisms – from annelids to bacteria – that are comparatively underrepresented in the fossil record but that constitute the most part of the benthic biomass. Second, trace fossils are commonly preserved in sediments that are otherwise unfossiliferous. Third, bioturbation and bioerosion change permanently the physico-chemical properties of the substrate and leave geochemical, petrographic and geotechnical signals that enable to relate with the presence of life. In addition, the bioturbating activity of organisms commonly results in structures that are far more abundant and more visible than their tracemakers (e.g. several arthropod taxa produce km-scale mounded topographies in aquatic and continental environments; ectomycorrhizal fungi are responsible for soil microbioturbation; cm-sized organisms shaped the geochemistry of the Earth’s benthic ecosystem during the Cambrian Agronomic revolution).
With increasing availability of high-resolution imagery, the search for past and modern traces is possible for several terrestrial bodies, among which the Moon, Mars, Venus, Titan and Mercury. Nevertheless, finding ichnological evidences beyond Earth is still a significant challenge because of (a) resolution issues, (b) relative paucity of bedding-plane imagery, (c) lack of core data, (d) lack of method-specific instrumentation.
For these reasons, there is a great potential for developing tools that incorporate ichnology into an astrobiological framework. Specifically, tools for the analysis of sediment and/or rock cores are needed for observing biogenically-produced sedimentary fabrics (ichnofabrics) beyond Earth. Applied ichnology provides a vast set of practical tools (i.e. CT-scanning, borehole imagery) for studying ichnofabrics.
Finally, a question might arise: What to expect? Models of bioturbation beyond Earth are extremely complex and variable due to the variety of geodynamical conditions existing on exoplanets. Nevertheless, burrowing and boring behaviours are expected to be a general pattern for life because they allow to face harsh surficial physico-chemical conditions (e.g. cosmic rays) and/or evolutionary pressures, both for mineralized or soft-bodied organisms. Identifying more precisely the forms and variation of Earth’s traces in extreme environments and their evolutionary paths is likely to provide a more robust predictive model for bioturbation beyond Earth
Geographic information systems for ichnofabric analysis: modelling a modern lagoon (Grado, Italy) with the IchnoGIS method
No full text1. Introduction
The Grado-Marano lagoon is one of the major transitional systems of the Adriatic Sea, consisting of a barrier island system extended for over 30 km (Baucon, 2008a; Turri, 1999). Characterized by significant biodiversity and heterogeneous environments, this area provide optimal conditions to assess the ichnologic and sedimentary features of siliciclastic, marginal-marine settings.
The complex relationships that exist among ichnological, physical, and environmental proprieties require advanced, integrated analysis techniques to visualize spatial patterns and determine the factors controlling trace distribution. For these reasons, a new method for quantitative ichnosedimentological analysis (IchnoGIS) has been developed.
The goal of this work is to discuss a quantitative ichnological model of the external margin of the Grado lagoon and test the application of the IchnoGIS method for ichnofabric analysis.
2. Geographical and geological setting
The study area (Fig.1) is located on the external margin of the Grado basin, between Grado town and the locality Pineta. Tides, which are the main driving forces of the lagoon hydrodynamics, created a composite mosaic of marginal marine environments, among which vast siliciclastic intertidal flats. A very peculiar environment is represented by microbial-related settings: large sections of the tidal flat are colonized by microbial mats, which are presenting a diverse ichnofauna, preliminary described by Baucon (2008a) and discussed in this study.
Fig. 1 – Study area. Modified from Baucon, 2008a.
3. Method and approach
Similarly to a geographic information system, the proposed approach integrates hardware, software, and data for capturing, managing, analyzing, and displaying geographically referenced ichnological data. For this reason, the method has been named ‘IchnoGIS’. Its development derived from previous work on the application of GPS and GIS techniques to neoichnology (Baucon, 2008b; 2008a). IchnoGIS is an orderly procedure consisting of 6 steps:
a. Survey design: The starting point is defining the objects of interest and the sampling size.
b. Sampling: If we want to know how traces are distributed in a particular habitat, it is usually impossible to count each and every one present. For this reason, the second step of IchnoGIS is based on quadrat sampling, a method widely used in the interpretation of large ecological data sets with environmental gradients (McIntyre & Eleftheriou, 2005). It consists of characterizing ichnological, sedimentological, environmental attributes (i.e. number of Arenicolites, grain size, salinity) contained in a square frame (in this study: 0.25 m2; Fig.2).
c. Significance test: In the simplest case, the result of the sampling process is a spreadsheet including X Y coordinates, facies type and abundance of each structure (Fig. 3). For this reason, nearest neighbour analysis (Borradaile, 2003) is an efficient method to assess the sampling quality.
d. Descriptive statistics. One of the primary goals is to describe the influence of the sedimentological features on the numbers and types of traces. This aim can be achieved by cross-tabulating frequency counts of ichnotaxa respect to facies. Another possibility is to provide a measure of central tendency (i.e. Fig.5A) and/or distribution.
e. Ichnoassemblage analysis. Ichnoassemblages are verified by cross tabulating the abundance of a trace in relation to that of another trace.
f. Spatial analysis. Spatial analysis is performed through (a) classed post maps (b) geostatistical interpolation techniques (i.e. Fig.4B, 5B). Classed post maps are simpler to implement, but interpolation can estimate the value of a variable (facies type, number of traces) in unsampled positions, delivering accurate ichnosedimentary maps.
It appears manifest that IchnoGIS emphasizes the recognizement of distinct structures on the sediment surface. This is apparently in contrast with the ichnofabric approach, whose application is usually related to vertical rock slabs/core samples. However, the approach of this study aims to integrate these philosophies by complementing ichnofabric analysis with the quantitative study of discrete ichnofabric-forming ichnotaxa.
Fig. 2 – Quadrat sampling in IchnoGIS. Frame area: 0.25m2. A – Overview of the tidal flat with the sampling frame (quadrat). B – Spatial, sedimentological and ichnological attributes are collected for each sampling site and stored in a spreadsheet.
4. Ichnofabrics
The studied area comprises six ichnofabrics, which are named for their most representative ichnotaxa. Some structures presents doubtful affinities with existing ichnogenera, therefore the corresponding ichnofabrics are named for their producer.
• Arenicolites (large type) ichnofabric: This ichnofabric mainly occurs within medium- to fine-grained sands with abundant ripple marks (facies B in Figs.4,5). The ichnofabric is characterized by large U-shaped burrows (Arenicolites) penetrating for 20-40 cm into the substrate. Thalassinoides and Siphonichnus can be present, although in rare clusters. Intensity of bioturbation is variable, usually low (BI 1-2).
• Thalassinoides-Arenicolites (small type) ichnofabric: This ichnofabric is commonly associated to sandy muds (facies C in Figs.4,5). The predominant component of this ichnofabric is Thalassinoides, at times associated to small Arenicolites (penetration depth: 5-8 cm; Fig. 3A). The degree of bioturbation is moderate to high (BI 3-6).
• Thalassinoides ichnofabric: This ichnofabric consists of monotypic Thalassinoides-dominated firmgrounds (facies F, Figs.4, 5). The degree of bioturbation is low to moderate (BI 1-3).
• ‘Insect burrows’ ichnofabric: This ichnofabric is present within microbial-bound deposits, consisting of laminated sands with an upper, organic-rich layer and a lower mineral-rich one (facies E, Figs.4, 5). The ichnoassemblage is dominated by vertical clavate burrows (Fig.3B) and horizontal unbranched burrows, respectively produced by coleopterans and larvae of Diptera. Small Arenicolites can be present. Intensity of bioturbation is generally low (BI 1-2).
• Macanopsis-Arenicolites (small type) ichnofabric: This ichnofabric occurs in sandy muds colonized by filamentous algal turf (facies D). The ichnoassemblage is dominated by crab traces, consisting of gently bending unbranched burrows with a circular-to-oval cross (Macanopsis). Crab burrows are often accompanied by small Arenicolites. Intensity of bioturbation is generally high (BI 3-6).
• Unbioturbated deposits: Sands with indisturbed lamination are common in the study area (i.e. facies A, Figs.4, 5). Intensity of bioturbation is low (BI 0).
Fig. 3 – Ichnofabrics from the Grado lagoon. A – Thalassinoides-Arenicolites ichnofabric. B – ‘Insect burrows’ ichnofabric.
5. Data analysis
This section presents the results of the IchnoGIS method.
• Callianassid mounds and openings. Callianassid shrimps are responsible for producing Thalassinoides burrow systems, whose surface expression is represented by sediment mounds and characteristic funnel-linke openings. When abundant, they are responsible for the Thalassinoides–Arenicolites and Thalassinoides ichnofabrics. IchnoGIS revealed two peculiar trends in their distribution:
a. Distance from the coastline. These structures are absent in the upper (landward) foreshore. This phenomenon is probably linked to prolonged subaerial exposure during low tide, which is a stressful condition for the Thalassinoides producers (Fig. 4B).
b. Facies-dependent distribution. Such structures are restricted in sedimentological range, being more abundant in protected conditions with disposability of organic material. However, they can be also associated to firmgrounds (Fig. 5A).
Fig. 4 – Spatial analysis. A – Facies map, derived from the information gathered during quadrat sampling. Coordinates in metres (Datum: WGS84). B – Exposure time controls the abundance of Thalassinoides. Area corresponds to the rectangle in A.
• Large Arenicolites. High numbers of Arenicolites are mainly related to sandy sediments (Fig.5B) with moderate to low exposure times. Such conditions correspond to the Arenicolites ichnofabric.
• Macanopsis. From the ichnologic characterization of sedimentary facies (Fig. 5A), it emerges a clear facies-related distribution of crab traces (Macanopsis). Crab burrows are present specifically within the algal turf zone.
Fig. 5 – Traces and facies. A – Ichnologic characterization of sedimentary facies. Facies codes refer to Fig.4. B – Interpolated distribution of Arenicolites (large) and Thalassinoides (openings) stacked on facies map. The dashed areas correspond to Arenicolites≥1 per sampling unit; the same is valid for Thalassinoides.
6. Discussion and Conclusions
Ichnofabric analysis, integrated with the IchnoGIS approach, revealed four main environmental controls: exposure time, hydrodynamism, sediment binding (algal or microbial) and firmness. Environmental significance of each ichnofabric is shown in Fig.6. It should be noted that the application of quadrat sampling alone would not cover all the ichnofabric-forming ichnogenera (i.e. insect traces). Hence the necessity of complementing the quadrat sampling approach with observations of vertical sections or, more accurately, with quantitative measurements in section. One possible technique could be quantifying burrow type and abundance within sections of a given area. As Gingras et al. (2011) argued, the models we have for animal-sediment relations are largely based on neoichnological studies of the 1950s, 1960s and 1970s. For higher-resolution models, new studies on modern environments are required. The IchnoGIS method could contribute to solve these issues by producing realistic models of trace distribution in modern environments, that are immediately comparable with examples from the fossil record.
Fig. 6 – Environmental significance of the studied ichnofabrics. Firmness measured with the modified Brinnell test (Gingras & Pemberton, 2000); main processes derived from field observations and examination of the main geomorphic features (island, shoreline) in Fig.4A.
References
Baucon, A. (2008a). Neoichnology of a microbial mat in a temperate, siliciclastic environment: Studi Trent. Sci. Nat. Acta Geol., 83, 183-203.
Baucon, A. (2008b). GPS and GIS techniques applied to neoichnology: a case study from a temperate, lagoonal microbial-mat (Adriatic Sea, Italy). Proceedings of Ichnia 2008, 2nd International Congress on Ichnology, Kracow.
Borradaile, G. J. (2003). Statistics of earth science data. Springer, Amsterdam, 351 pp.
Gingras, M. K., & Pemberton, S. G. (2000). A field method for determining the firmness of colonized sediment substrates. Journal of Sedimentary Research, 70(6), 1341-1344.
Gingras, M. K., MacEachern, J. a, & Dashtgard, S. E. (2011). Process ichnology and the elucidation of physico-chemical stress. Sedimentary Geology. In press, doi: 10.1016/j.sedgeo.2011.02.006.
McIntyre, A. D., & Eleftheriou, A. (2005). Methods for the study of marine benthos. Wiley-Blackwell, London, 418 pp.
Turri, E. (1999). Lagune d’Italia: visita alle zone umide lungo le coste dei nostri mari. Touring Editore, Torino, 144 pp.
IchnoGIS : a novel method for analyzing neoichnological and sedimentological data
No full textThe need to understand the distribution of traces and sediments requires a method to collect, organize and synthesize the information, and to communicate ichnological and sedimentological data effectively. With these aims in mind, a new method is proposed for surveying and analyzing modern environments under an ichnological and sedimentological perspective.
IchnoGIS is a system that captures, stores, analyzes, manages, and presents neoichnological and sedimentological data that are spatially referenced. It allows to analyze spatial information, determine ichnoassociations, inspect the environmental and sedimentological significance of traces.
The IchnoGIS method consists of 6 steps:
Survey design. The first step is defining the objects of interest (ichnogenera, facies) and the sampling size.
Quadrat sampling. For each sampling site, a frame (quadrat) of a set size is placed on the substrate. Spatial coordinates, sedimentological (i.e. facies, granulometry, sedimentary structures) and ichnological attributes (i.e. abundance of each ichnogenus of interest) are recorded.
Significance tests. The method includes a number of significance tests (nearest neighbour analysis, kernel density) to assess the sampling quality. If sampling is optimal, data are investigated through statistical techniques in order to characterize ichnoassemblages and to formulate hypotheses on the factors that control their distribution.
Descriptive statistics. IchnoGIS uses spatial location as the key index variable for all other information, which are – in the simplest case – ichnogenus abundance and facies type. Several techniques are available to determine the relationship between the abundance of each trace and a specific facies. Standard practice is to create a cross tabulation containing frequency counts of ichnogenera respect to facies. Another possibility is to provide a measure of central tendency and distribution.
Ichnoassemblage analysis. The next step concerns ichnoassemblages, i.e. determine whether there are groups of traces that are recurrently associated with each other. This can be verified by cross tabulating the abundance of a trace in relation to that of another.
Spatial analysis. The most immediate way to analyze spatial relationships is a classed post map, although geostatistical interpolation techniques, based on varigram analysis, can estimate the value assumed by a variable (facies type, number of traces) in unsampled positions, producing accurate ichnosedimentary maps.
The use of IchnoGIS method has many advantages, as evidenced by its application to a case study (intertidal flats of the Grado lagoon, Italy). It reveals a precise view of biogenic structures in space, allowing the collection of ichnosedimentary data for use in sedimentological studies, palaeoenvironmental reconstitutions and quantitative modelling. Finally, the IchnoGIS is fast and cost effective: it requires only a GPS unit, a plastic or wooden frame, and standard geostatistical software (open-source or commercial)
Neoichnology of a barrier-island system: The Mula di Muggia (Grado lagoon, Italy)
No full textBarrier-islands are common landforms and biodiverse habitats, yet they received scarce neoichnological attention. This gap is tackled by studying the Mula di Muggia barrier-island system (Grado lagoon, Italy), focusing on morphology, ecology and ethology of individual traces. The following incipient ichnotaxa are identified: Archaeonassa, Arenicolites, Bergaueria, 'diverging shafts', Helminthoidichnites, Lockeia, Macanopsis, Monocraterion, Nereites, Parmaichnus, Polykladichnus, Skolithos, Thalassinoides and 'squat burrows'. Vertebrate (Avipeda-/Ardeipeda-like, Canipeda) and invertebrate tracks ('parallel furrows') are also described.For each ichnotaxon, tracemaker and behavior are discussed, together with their position with respect to sediment barriers. Results suggest that sediment barriers impose a sharp contrast in terms of ichnological composition. Back-barrier is dominated by branched burrows (i.e. Thalassinoides, Parmaichnus), while the fore-barrier presents vertical and U-shaped burrows (Arenicolites, Skolithos). The environmental conditions of the back-barrier show that low-oxygen substrates favor intense bioturbation, provided that the water column is sufficiently oxygenated
Sharing Ichnofabrics: theory, design and implementation of an open-acess ichnological database (Ichnobase)
No full textThe ICHNOBASE project aims to create the first comprehensive database on trace fossils, allowing to organize, store, and retrieve large amounts of ichnological information (Fig.1). The structure of the project is based on a linked database design. According to this approach, the architecture of ICHNOBASE consists of three interconnected levels, corresponding to bibliographic, taxonomic and morphological data (Fig. 2).
The first two levels refer to bibliographic data and taxonomic identification (diagnoses, synonymies, ...), while the morphological level encompasses images of ichnotaxa, pictures of ichnofabrics and three-dimensional models achieved by laser-scanning. When fully developed, ICHNOBASE will allow users to interactively interrogate the database through an open-access web-interface, analyze its data and update it according to the users privileges.
The goal of this work is not only to present the ICHNOBASE project, but also to encourage ichnofabric workers to take part into it by submitting relevant information (e.g. ichnofabric pictures, bibliographic information)
Towards an open-source software for ichnological analysis
No full textTrace fossils are widely accepted as an invaluable investigative tool in facies recognition, event correlation and palaeoecological reconstruction. However, comparatively little work has been done on software development. This work present a software project which aims to apply computational methods to study the relationship between environment and traces, resulting in a software especially dedicated to ichnological statistics.
The software design builds on the experience with the IchnoGIS method (Baucon et al., this volume) and the corresponding outcome will include ichnoassemblage analysis, ichnologic characterization of facies, spatial statistics as well as ANOVA and other classic statistical methods. It will provide a wide variety of statistical and graphical techniques, including clustered bar charts, pie charts, single and multiple line charts, histograms and scatterplots. Tabular output will be in HTML, which means an immediate compatibility with the World Wide Web.
The source code of this software environment will be freely available at www.ichnogis.com under the GNU Affero General Public License, and pre-compiled binary versions will be provided for various operating systems
The IchnoGIS method : Network science and geostatistics in ichnology. Theory and application (Grado lagoon, Italy)
No full textA new method is proposed for capturing, managing, analyzing, and displaying geographically referenced ichnological data: IchnoGIS. This approach is based on the integration of spatial, geostatistical techniques with network theory, aiming to characterize the environmental significance of recent traces. The efficiency of the IchnoGIS method is tested against a case-study: the Grado lagoon (Italy). The studied site, located within the epeiric Northern Adriatic Sea, consists of a complex mosaic of peritidal environments in a barrier-island context. Here, a diverse ichnofauna includes the following incipient ichnotaxa: Arenicolites, Helminthoidichnites, Lockeia, Macanopsis, Monocraterion, Parmaichnus, Polykladichnus, Skolithos, Thalassinoides and 'squat burrows'. Ichnofaunal distribution is described by the spatial and geostatistical tools proper of the IchnoGIS approach. Additionally, the application of network theory documents the emergence of organized structures (ichnoassociations) from interactions driven by environmental factors. Our results elucidate the role that environmental processes play in producing the complex ichnological patterns of the Grado site. In particular, emersion time, hydrodynamics, substrate firmness and microbial binding are the major control factors determining the structure and distribution of trace associations. These structuring factors are used to define a predictive model of ichnoassociation composition, providing an immediate tool for future palaeoenvironmental reconstitutions
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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