Neoichnological Framework of Coastal Habitat Shifts Represented by Bahamian Decapod Burrows

In coastal settings, the cumulative spatial and temporal impacts of burrowing, ichnofabric formation, and biodeposition by large crustaceans have been largely neglected at zoogeomorphic scales. These traces also serve as important (paleo-)environmental and (paleo)hydrologic indicators, both vertical (tidal or groundwater level) and lateral (areal wetland or basin extent). To date, 354 only few studies have addressed the comparative value of decapod ichnites generated by land crabs in carbonate settings. The aim of this paper is to introduce a general conceptual framework for burrows created by three crab species in the Bahama Archipelago, using examples from San Salvador Island

Storm-Generated Molluscan Thanatocoenosis along a Carbonate Paleoshoreline: Southern Eleuthera Island, The Bahamas

Concentrations of large mollusc shells in coastal deposits provide important information about the local malacofauna and potential transport agents, including extreme events [1-4]. Such accumulations are common in the rock record [5,6], with Quaternary examples serving as good time-averaged examples by combining aspects of both the modern biocoenoses and the fossil record. Death assemblages of local organisms (thanatocoenosis) and their preserved record (taphocoenosis) in carbonate settings, where granulometric spectrum may be very limited (e.g., ooilitic sand), can serve as important paleo-environmental indicators, especially when considered in combination with primary sedimentary structures (in outcrops or geophysical images) and in situ biogenic structures (trace fossils)[7]. Along prograded beach/dune ridge complexes (strandplains) [8], extensive accumulations of large nearshore mollusc shells are likely related to extreme events, such as intense storms [1]. This study reports on an anomalous accumulation of mostly juvenile conch shells (Aliger sp.) along one of the oldest (landwardmost) paleoshorelines of the Plum Creek Beach in Freetown, southern Eleuthera Island, The Bahamas (Fig. 1). Shell preservation is assessed using semi-quantitative taphonomic grades

Ecohydrological interactions within “fairy circles” in the Namib Desert: Revisiting the self-organization hypothesis

Vegetation patterns such as rings, bands, and spots are recurrent characteristics of resource-limited arid and semiarid ecosystems. One of the most recognizable vegetation patterns is the millions of circular patches, often referred to as “fairy circles,” within the arid grassland matrix extending over hundreds of kilometers in the Namib Desert. Several modeling studies have highlighted the role of plant-soil interactions in the formation of these fairy circles. However, little is known about the spatial and temporal variabilities of hydrological processes inside a fairy circle. In particular, a detailed field assessment of hydrological and soil properties inside and outside the fairy circles is limited. We conducted extensive measurements of infiltration rate, soil moisture, grass biometric, and sediment grain-size distribution from multiple circles and interspaces in the Namib Desert. Our results indicate that considerable heterogeneity in hydrological processes exists within the fairy circles, resulting from the presence of coarser particles in the inner bare soil areas, whereas concentration of fine soil occurs on the vegetated edges. The trapping of aeolian and water-borne sediments by plants may result in the observed soil textural changes beneath the vegetation, which in turn, explains the heterogeneity in hydrological processes such as infiltration and runoff. Our investigation provides new insights and experimental data on the ecohydrological processes associated with fairy circles, from a less studied location devoid of sand termite activity within the circles. The results seem to provide support to the “self-organization hypothesis” of fairy circle formation attributed to the antiphase spatial biomass-water distributions

Coastal impacts due to sea-level rise

Author Posting. © Annual Reviews, 2007. This is the author's version of the work. It is posted here by permission of Annual Reviews for personal use, not for redistribution. The definitive version was published in Annual Review of Earth and Planetary Sciences 36 (2008): 601-647, doi:10.1146/annurev.earth.35.031306.140139.Recent estimates by Intergovermental Panel on Climate Change (2007) are that global sea level will rise from 0.18 to 0.59 m by the end of this century. Rising sea level not only inundates low-lying coastal regions, but it also contributes to the redistribution of sediment along sandy coasts. Over the long-term, sea-level rise (SLR) causes barrier islands to migrate landward while conserving mass through offshore and onshore sediment transport. Under these conditions, coastal systems adjust to SLR dynamically while maintaining a characteristic geometry that is unique to a particular coast. Coastal marshes are susceptible to accelerated SLR because their vertical accretion rates are limited and they may drown. As marshes convert to open water, tidal exchange through inlets increases, which leads to sand sequestration on tidal deltas and erosion of adjacent barrier shorelines

Cylindrical Mega-Voids in Quaternary Aeolianites, Little Exuma Island, The Bahamas: Georadar Response

In addition to karst features, tropical carbonates contain a wide range of smaller cylindrical voids (“pipes”) attributed to bioturbation, tree molds, or dissolution, among others. During geophysical investigation of the Little Exuma Island, The Bahamas, several sites with enigmatic voids were investigated using a high-frequency ground-penetrating radar (GPR) imaging. The aim of the paper is to assess the feasibility of GPR to detect voids within lithified Holocene calcarenites of the Hannah Bay Membe

Lithological anomalies in a relict coastal dune : geophysical and paleoenvironmental markers

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 34 (2007): L09707, doi:10.1029/2007GL029767.Ground exposures of migration surfaces (slipfaces) of a relict Holocene coastal dune along the southeastern Baltic Sea coast provide an ideal opportunity for establishing the causes of prominent reflections on geophysical profiles. High-amplitude reflections on high-resolution ground-penetrating radar (GPR) images correlate well with two major lithological anomalies: 1) paleosols developed on dune slipfaces, and 2) slipfaces consisting of heavy-mineral concentrations (HMCs). Paleosols serve as indicators of dune stability, represent datable chronostratigraphic surfaces, and help reconstruct dune paleo-morphology. HMCs have substantially higher magnetic susceptibility values than background quartz-rich sands and, where they are well-developed, can be also used for spatial correlation. Based on their occurrence at the study site, these enriched horizons likely represent periods of increased wind activity (storminess). Multiple HMCs upwind of paleosol P1 (800–670 cal years BP) likely reflect periods of intensified wind activity along the southeast Baltic region during the Medieval Warm Period.This research was funded by the Ocean and Climate Change Institute and The J. Lamar Worzel Assistant Scientist Fund of the Woods Hole Oceanographic Institution

Subsurface signatures and timing of extreme wave events along the Southeast Indian coast

Written history's limitation becomes apparent when attempting to document the predecessors of extreme coastal events in the Indian Ocean, from 550-700 years in Thailand and 1000 years in Indonesia. Detailed ground-penetrating radar (GPR) surveys in Mahabalipuram, southeast India, complemented with sedimentological analyses, magnetic susceptibility measurements, and optical dating provide strong evidence of extreme wave events during the past 3700 years. The diagnostic event signatures include the extent and elevation of the deposits, as well as morphologic similarity of buried erosional scarps to those reported in northern Sumatra region. Optical ages immediately overlying the imaged discontinuities that coincides with high concentration of heavy minerals date the erosional events to 340 ± 35, 350 ± 20, 490 ± 30, 880 ± 40, 1080 ± 60, 1175 ± 188, 2193 ± 266, 2235 ± 881, 2489 ± 293, 2450 ± 130, 2585 ± 609, 3710 ± 200 years ago. These evidences are crucial in reconstructing paleo extreme wave events and will pave the way for regional correlation of erosional horizons along the northern margin of Indian Ocean

Non-invasive (georadar) investigation of groundhog (Marmota monax) burrows, Pennsylvania, USA

Zoogenic impact plays a critical role in stream processes, especially bank stability and resulting channel dynamics. This study focuses on bioturbation by groundhogs (Marmota monax) along the riparian zone of Mill Creek (Bucks County, Pennsylvania, USA). Several complexes comprising at least 32 active burrows (average diameter: 25.9 cm) were geolocated, with morphometric measurements obtained at selected sites. Two networks were imaged using high-frequency 800 MHz ground-penetrating radar (GPR) and included: 1) a grid of parallel 3-m-long transects on the south bank, and 2) an 11-m-long profile on the north bank. Post-processed electromagnetic signal traces (A-scans) comprising 2D radargrams (B-scans) revealed voids as reverse-polarity anomalies (hollow inclined shafts and tunnels), allowing for a general assessment of burrow depth and orientation. At the southern cutbank site, a large burrow had an entrance diameter of 0.3 m and a westerly dip. A sloping tunnel section was detected at ~0.5 m depth, based on the geometry of point-source (transverse) hyperbolic diffractions corresponding to the roof and a floor ‘pull-up’. The second locality traversed three open burrow entrances adjacent to large tree roots. This survey along a tributary channel shows multiple hyperbolics below adjacent openings, with the latter showing the characteristic signal ‘breakout’. GPR data show hyperbolic signatures ~0.3–0.4 m below the ground surface. Along this transect, burrowing activity appears to increase with proximity to the northern bank of Mill Creek. An example of a depth slice (bedding-plane view) from a nearby riverbank demonstrates the potential for 3D visualization (C-scans) of burrow networks using a grid of closely spaced GPR profiles. Groundhog burrows constrain maximum long-term level of the groundwater table and serve as important zoogeomorphic structures in diverse ecotones, including developed landscapes. Abundant evidence of bank slumping, incision, and treefall suggests that burrowing activity likely weakens root systems and enhances groundwater flow, thereby initiating or accelerating geomorphic cascades leading to slope failure