CC BY
8
ISSN 2617-2909 (print) ISSN 2617-2119 (online)
Journ.Geol. Geograph.
Geology, 29(2), 252-257. doi: 10.15421/112023
Ilya V. Buynevich Journ. Geol. Geograph. Geoecology, 29 (2), 252-257.
Detection of mineralogically accentuated biogenic structures with high-resolution geophysics: implications for ichnology and geoecology
Ilya V. Buynevich
Temple University, Philadelphia, USA, coast@temple.edu
Received: 19.04.2020 Abstract. Identification and mapping of small-scale physical and biogenic structures in
Received in revised form: 02.05.2020 sand has been a challenge to sedimentologists and ichnologists. Under natural conditions, Accepted: 10.05.2020 biogenic activity (trampling tracks, burrows) alter primary sedimentary structures, but also
serve as important paleoenvironmental indicators of geotechnical properties of sediments, omission surfaces, and ecosystem dynamics. Therefore, the ability to recognize such structures as anomalies in shallow subsurface, especially when using indirect non-invasive methods, such as geophysical imaging, is an important aspect of assessing their relative contribution to the overall erosional-depositional record. This study presents experimental evidence of the viability of two highresolution geophysical methods in detecting sediment deformation that mimics shallow animal traces. High-frequency (800 MHz) ground-penetrating radar (GPR) imaging aided in visualizing a buried depression produced by a deer hoofprint cast indenter, with high-amplitude reflection return enhanced by a heavy-mineral concentration (HMC). Bulk in situ low-frequency (930 Hz), low-field magnetic susceptibility (MS) experiment supported the theoretical pattern of a decrease in MS over the thickest cover sand (maximum indentation depth) to ~0 mSI and the highest values over raised HMC horizon (marginal ridge; >8 mSI). Because both methods are affected by the presence and relative abundance of heavy minerals, the present approach can be applied in most siliciclastic settings. This study demonstrates the promise of extending the 2D visualization of subsurface targets to 3D datasets, with potential implications for sedimentological, ichnological, archaeological, and geoecological research that involves animal-sediment interaction at different scales.
Keywords: Georadar, magnetic susceptibility, heavy minerals, ichnology
Виявлення мшерально-акцентованих бюгенних структур з використанням геоф1зичних метод1в високот роздшьнот здатностк наслщки для 1хнологн та геоекологн
1лля В. Буйневич
Темпльский ун1верситет, Ф1ладельф1я, США, coast@temple.edu
Анотащя. 1дентифжацш та воображения дрiбномасштабних фiзичних та бюгенних структур у теку е викликом для седиментолопв та ixrnoioriB. У природних умовах бюгенна актившсть (трамбування, сшди, нори) змшюе первинш осадовi структури, але також слугуе важливими палеоеколопчними показниками геотехшчних властивостей опадiв, ерозшних поверхонь та динамжи екосистем. Тому здаттсть розтзнавати таю структури як аномали в неглибоких шарах, особливо при використанш непрямих нешвазивних методгв, таких як геофiзична вiзуалiзацiя, е важливим аспектом оцшки 1х вдаосного внеску в загальний ерозшно-осадовий запис. У цьому дослiдженнi представлено експериментальш докази життездатностi двох геофiзичниx методгв високо! роздiльноi здатностi при виявленш деформацii осаду, що iмiтуе неглибокi сшди тварин. Високочастотний (800 МГц) наземний георадар (GPR) дозволяе допомогти вiзуалiзувати поховану западину, вироблену ш-дектором вiдпиву копита ввдбиткгв оленя, з вiддачею вiдбиття з високою амплiтудою, посиленим концентрацiею важких мь нералiв (HMC). Експеримент з низькою частотною (930 Гц) низькопольовою магнiтиою сприйнятливiстю (МС) пiдтримував теоретичну схему зменшення МС вiд ~ 0 mSI над покривним пiском (максимальна глибина вiдступу) до найвищих значень над пiдштовxиутим HMC горизонтом (граничний хребет; > 8 mSI). Оскiльки обидва способи впливають на наявшсть та вдаосне надходження важких мшератв, даний пiдxiд може використовуватися в рiзниx субстратах. Це дослвдження демонструе перп-сектививу розширити двовимiрну вiзуалiзацiю пiдповерxневиx цiлей до 3D, з потенцшними наслiдками для седиментолопчних, ixнологiчниx, арxеологiчниx та геоекологiчниx дослвджень, що передбачае взаемодiю тварин та опадiв в рiзниx масштабах.
Ключовi слова: Георадар, магнтна сприйнятливктъ, важт мтерали, iхнологiя
Journal of Geology, Geography and Geoecology
Journal home page: geology-dnu-dp.ua
Introduction. A wide suite of vertebrate and invertebrate organisms is responsible for generating billions of biogenic structures (surface traces and burrows) in both modern and ancient depositional settings (Vi-alov, 1966; Frey and Pemberton, 1986; Loope 1986; Fornos et al. 2002; Roberts 2003; Hasiotis et al., 2007; Lockley et al. 2007; Milan et al. 2007a,b; Zonneveld, 2016). However, both preservation and detection of these structures, particularly in unconsolidated sandy substrates (= media) have been long considered to be challenging aspects in neo- and paleo-ichnological research (Loope, 1986; Allen, 1997; Fanelli et al., 2007; Buynevich, 2015). However many such traces rework the substrate, sometimes resulting in zoogeomorphic-scale impact (Laporte and Behrensmeyer, 1980; Butler, 1995; Scott et al., 2008). In recent decades, new applications of high-resolution geophysical techniques, such as ground-penetrating radar (GPR or georadar),
high-amplitude signal return in georadar images and can be detected using low-field magnetic susceptibility (MS) surveys (Buynevich, 2012). The latter is especially sensitive to ferrimagnetic (magnetite) content. Therefore, these techniques are potentially applicable to detecting near-surface biogenic structures accentuated by HMCs. This paper presents examples of laboratory experiments using an ungulate cast track indenter, with the aim of demonstrating the ability to detect the near-surface structure using high-resolution georadar imaging and magnetic susceptibility trends. Materials and methods. The laboratory experiment consisted of two separate stages (GPR and MS survey techniques) that simulate natural conditions (Table 1; Fig. 2). For georadar imaging (Fig. 2B), a quartz-rich sand surface (10-cm-thick layer in a plastic box) was imprinted with a cast of a white-tailed deer (Odocoi-leus virginianus) hoofprint (length = 7 cm; maximum
Fig. 1. Examples of ungulate tracks in sand: A) Deer hoofprints in coarse fluvial sand (Delaware River, Pennsylvania, USA; MR - marginal ridge); B) Large and small ungulate tracks (deer, boar) on a lagoon shoreline, with slight heavy-mineral (almandine) enrichment (Curonian Lagoon, Lithuania); C) Feral horse footprint in coastal dune sand, with heavy-mineral concentration (HMC+) composed of magnetite along the marginal ridge (Assateague Island, Maryland, USA)
have demonstrated success in rapid continuous imaging of shallow tracks and large burrows (Stott 1996; Buynevich, 2010; Buynevich, 2011b,c; Buynevich, 2012; Urban et al., 2019). The presence of mineral-ogical anomalies, such as heavy-mineral concentrations (HMCs), which consist of magnetite, ilmenite, garnet and other minerals denser than the background quartz and feldspar matrix often visually accentuates the traces in plan view and cross-section (Fig. 1; Van der Lingen and Andrews, 1969; Lewis and Titheridge, 1978). These anomalies also produce characteristic
width: 4 cm) to a depth of ~1-2 cm (heel-to-toe) to simulate traces observed in natural settings (Fig. 1B). The imprint produced slight marginal ridges (MR) or expulsion rims (Fig. 2A, Part 1). The entire track surface was then covered by a thin (1-2 mm) layer of mixed almandine garnet and magnetite heavy-mineral concentration (Fig. 2A, Part 2; Fig. 3A). Subsequently, this surface was capped by a layer of quartzose sand to a depth of 1-2 cm in order to bury the track (Fig. 2A, Part 3; Fig. 3B). A survey transect was collected along the central longitudinal path of the track
Table 1. Flowchart of experimental design of GPR and MS experiments
Stage Georadar (GPR) Magnetic Susceptibility (MS) Example (Figure)
Imprinting Track cast indent into 10 cm sand bed Track cast and sensor indents 2A(1), 4A
Accentuation Garnet-magnetite mix (1-2 mm HMC) Garnet-magnetite mix (1-2 mm HMC) Random MS sensing of HMC 2A(2), 3A, 4B
Burial 1 cm of quartz sand (more over indent) 2 mm of quartz sand (more over indent) 2A(3), 3B
Imaging 800 MHz monostatic antenna profile MS2K field sensor profile 2B, 2D, 3C, 4C
MR
MR
HMC+ / HMC
E
_ a
shallow HMC+
B
t^s. MA.L.M
cover sand
D
GPR
k2>k: trace k3 ~ k -i-♦
cover quartz sand Kmin (~0 mSI)
HMC(Kmax)
TRACK
UNDERTRACK
■ - "deep HMC
laminated quartz sand Kmln (~0 mSI)
Fig. 2. A) A conceptual sequence of a buried tracking surface with a mineralogical anomaly: (1) track (T) emplacement and marginal-ridge (MR) formation on a sand surface with minor heavy-mineral content (UT - undertracks), (2) heavy-mineral concentration (HMC) formation due to aeolian action (HMC+ - enhanced enrichment over MR), and (3) burial of the accentuated track by quartz-rich cover sand. B) a monostatic 800 MHz GPR antenna over a cover sand; C) Bartington low-field magnetic susceptibility (MS) control unit with an MS2K sensor; D) idealized cross-section of a buried track (see location in A3). Oscilloscope shows higher magnitude electromagnetic georadar response to HMC. Bulk MS values (k) are shown for different media and expected surface result (see Table 1 for experimental flowchart)
using a digital MALÄ Geoscience radar system with a monostatic 800 MHz antenna (Fig. 2B). The resulting radargram was not post-processed due to lack of surface topography and the need for detection of small-scale subsurface sediment deformation (for application GPR technique in neo-ichnological research see Buynevich, 2011a). The heavy mineral layer was used to accentuate the electromagnetic radar signal response (Fig. 2D).
In a second experiment, the in situ low-field magnetic susceptibility surveys, a Bartington MS meter with an MS2K field sensor (930 Hz; Fig. 2C) was used. Similar to georadar setup, a track cast (Fig. 4A) was imprinted into a quartz sand surface with low bulk susceptibility (k ~0 mSI; Fig. 2D). In addition, a shallow (2-3 mm) imprint was produced by pushing the 2.5-cm-wide sensor into the sand behind the hoofprint, thereby simulating a small trace or an upper part of a burrow shaft (Table 1; Fig. 4B). The detection sensitivity of the sensor is a 50% signal decay at a depth of 0.3 cm. Several random measurements of heavy mineral layer were taken prior to burial. Conceptually, due to variations in the thickness of
cover sediment over the HMC-accentuated tracking surface, it is expected that the bulk susceptibility will range from near-minimum background over deepest parts of the indented features to near-maximum over shallow elements, such as marginal ridge (Fig. 2D). Results and Discussion. The results of this study show success of both near-surface geophysical methods in detecting sediment deformation. Indenter mechanics. The impression of a track cast is manifested as a downward-dipping reflection in the GPR image (Fig. 3D), consistent with high-amplitude response of a heavy-mineral lamina (as indicated in Fig. 2D). The mechanical deformation is typical for that of an indenter, with the likely formation of undertracks (Allen, 1989; Allen, 1997; Milan and Bromley, 2006.). These are likely expressed by truncated sediment layers below the main hoofprint image, at a depth of at least 4 cm (Fig. 3D). Therefore, even an 800 MHz frequency is sufficient to detect the buried feature, with even better results expect for higher-frequency setups. It is important to note that detection, rather than vertical resolution, was the focus of this study. The latter is limited by incoming electromagnetic pulse frequency
Fig. 3. Georadar imaging of a buried track: A) hoofprint cast emplaced into a laboratory sand and covered by a thin heavy-mineral concentration (HMC); B) surface of cover sand with a GPR antenna survey line; C) Radargram (800 MHz) showing the downward-dipping reflections at the buried track location, with undertrack(s) below. The unaffected sediment layers are at right, with a relatively low-amplitude signature of the overlying cover sand (t - two-way travel time in nanoseconds (10-9 s), using v ~ 15 cm/ns)
and is the ability to discriminate between multiple overlying targets (e.g., two or more closely spaced horizons, top and bottom of a point-source anomaly; Buynevich et al., 2014). The difference in signal amplitude between HMC (strong) and overlying cover sand (weak) is also expressed in the radargram, to the right of the imprint (Fig. 3D).
The bulk magnetic susceptibility experiment was similarly successful in revealing the two indented structures (hoofprint cast and sensor imprint; Fig. 4). Several random pre-burial HMC measurements yielded values between k = 4.66-9.92 mSI (triangles in Fig. 4C). Following burial by diamagnetic (low to negative MS) quartz sand, the pattern of MS values clearly show broad depression associated with the footprint (0.008 mSI) and a narrow dip (0.025 mSI) at the location of the sensor indent. Bulk MS readings greater than the mean of k = 3.3-3.5 mSI, are associated with the relatively undisturbed parts of the profile, with the highest value of k = 8.23 just behind the footprint (Fig. 4C). This anomaly is located at the point of a relatively low pre-burial HMC concentration
and is interpreted as a marginal ridge (MR) produced during cast emplacement (Figs. 1C; 2A; 2D).
In summary, the findings of this study demonstrate the viability of both GPR imaging and MS surveys can be used to detect and visualize shallow subsurface sedimentary structures similar to those generated by animals. Recognition and mapping will be improved by collection of closely spaced GPR transects and MS profiles to generate pseudo-3D and true 3D images of biogenic structures. Whereas the latter depends heavily on the presence of ferrimagnetic (e.g., magnetite) and paramagnetic minerals, georadar pulse responds to fabric-related water retention and other fine-scale properties, as indicated by successful footprint imaging in carbonate and evaporate settings (Buynevich et al., 2014; Urban et al., 2019). In addition to neo- and paleo-ichnological applications, this research can also aid in geoecological assessment of animal-landscape interaction and conservation of important footprint sites (Loope, 1986; Zonneveld, 2016). Future experiments must therefore include a spectrum of substrate lithologies in both field
anterior (front)
posterior (hind)
MS2K sensor imprint
з , . A -i 1 ■,—,■■ T- « 1
CO "1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
nietísn^ü fr>m\
Distance (cm)
Fig. 4. Low-field magnetic susceptibility (MS) transect: A) hoofprint cast; B) track (at left) and MS2K sensor (at right) imprints into laboratory sand covered by a thin heavy-mineral concentration (dashed line shows the MS sampling transect); B) surface of cover sand with a GPR antenna survey line; C) MS values (2 cm interval) of sediment surface following burial by quartz-rich cover sand (triangles show random point measurements of HMC prior to burial). Note the low values below the two imprints and a peak likely related to the marginal ridge
and laboratory settings, with the ultimate goal of assembling diagnostic criteria for distinguishing physical and biogenic structures. Acknowledgments
This research was funded by the National Geographic Society CRE Grant #8060-06 and the College of Science and Technology, Temple University. The author thanks Christopher Seminack, Zachary Grimes, Donatas Pupienis, Albertas Bitinas, and Aldona Damusyté for assistance in the field and Justin Darrow for help with laboratory experiments.
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