Microbial and abiotic sedimentary surface textures in the Murchison River. Images A–F and H–I from ponds at Stonewall, scale bar = 1 cm. Image G from alluvial sand at Four Ways, scale bar = 5 cm. See Online Supplemental File 1 for high-resolution, zoomable images. A) Submerged biofilm with oxygen bubbles. Arrow indicates how a thicker, filamentous microbial community has developed along the edge of a submerged desiccation crack and acts as a locus for a greater density of bubbles. B) Dried equivalent of A, where cavities left by bubble have been left in clay film. Arrow points to greater density of bubble marks along desiccation crack. C) Elongate bubble impressions in dried clay on slope slightly inclined in direction of arrow. Arrow indicates how ellipsoidal bubble marks are aligned downslope, presumably due to retreat as water dried, and show convergence and divergence of bubble tracks around an obstacle—in this instance a small raised mound of sediment. D) Dried biofilm on clay substrate demonstrating original organic material with cavities from burst bubbles aligned in direction of water retreat. E) Raindrop impressions in dried clay. Impressions formed when clay was wet; they have greater dimensions, overlap one another, and display crater rims (arrow), which distinguish them from bubble marks. F) Neoichnological, microbial, and abiotic sedimentary surface textures in close association. Raindrop impressions formed on a substrate with a veneer of water at the time of precipitation have wider dimension than those in damp sediment, and less pronounced rims, although craters still exhibit interference with one another (black arrow). Branching insect burrow superimposed by small biofilm bubble marks (white arrow), which also occur on sediment away from burrow. Small “Cochlichnus”-type trail, formed when sediment had a thin film of water, has been offset by later desiccation cracking (gray arrow). G) Raindrop impressions in sand, stabilized by salt crust. Water was poured onto the impressions from a bottle, but the impressions were only destroyed when they were directly under the poured stream (arrow). Where sediment was dampened, salt crust preserved original morphology (inset). H) Branching insect burrow (black arrow) and rolled up preserved biofilm (white arrow). Superimposition of the former onto the latter indicates that it post-dated the degradation and roll-up of the biofilm (gray arrow). I) Sinusoidal trail (“Cochlichnus”) likely made by insect larva or worm in thin veneer of water on sediment (Metz 1987) (black arrow). Close association with both microbial bubble marks (white arrow) and partial desiccation crack (gray arrow).
Margin of deep pool of standing water on dry river bed of the Murchison River, west of The Loop. Image shows close spatial proximity of sedimentary signatures developed by abiotic (A), microbial (M) and ichnological (I) means. Abbreviations: A.i. = adhesion marks developed on the bottomset of a 1.5 m-tall fluvial sand bar; A.ii. = tepee structures developed in salt crust; M.i. = wrinkled surface developed on top of 0.5 cm-thick microbial mats; M.ii. = rolled-up and curled fragments of microbial mat in shallower water; M.iii. = reduced layer of black sediment exposed where mats were previously active; I.i. = bird footprints; I.ii. = branching insect burrow developed in thin layer of oxidized sediment underneath salt crust, does not penetrate into reduced sediment.
Temporal variation in sedimentary surface textures at Stonewall between 28 April (following a 48-hour interval of 9.7 mm precipitation) and 10 May 2016 (following a 12-day interval of 0.2 mm precipitation). Scale bar = 5 cm in A–D and 1 cm in images E–H. See Online Supplemental File 2 for high-resolution, zoomable images. A, B) Partly submerged substrate which dried completely during the interval, showing: 1) complete desiccation and detachment of mud clasts formerly colonized by biofilm; 2) relict desiccation cracks (existing prior to filling of puddle in A) maintaining their form; 3) actively photosynthesizing mats creating bubbles in (A) that are preserved as bubble impressions in (B); 4) elongate bubble marks, the most pronounced of which are preserved over the period of study; 5) increased density of desiccation cracks; and 6) raindrop impressions in clayey substrate at margin of submerged area maintaining their form. C, D) Damp substrate which dried completely during the interval, showing: 1) biofilm roll-up maintaining form; 2) active biofilm (green) which left no trace; 3) vertebrate tracks degrading but largely maintaining form; 4) increasing depth and degree of desiccation; 5) preservation of bubble impressions; 6) preservation of subdued raindrop marks made in sediment that was submerged under a film of water during precipitation event; and 7) preservation of sharply defined raindrop marks made in subaerially exposed sediment during precipitation event. E, F) Microbial roll-up (arrow), containing bubble marks, maintaining form despite desiccation. G, H) Microbial mat composed of both sediment and filamentous microbiota drying from damp state, exhibiting: 1) preservation of burst bubble marks made while mat was photosynthesizing; 2) incomplete desiccation due to binding effect of filaments; and 3) raised, unburst bubbles which have developed longitudinal cracks during drying.
Changes to living microbial mat in response to environmental changes, within an isolated rain-fed pothole at Four Ways. Scale bar = 5 cm. See Online Supplemental File 3 for high-resolution, zoomable images. A) Detached mat containing oxygenic microbial community floating in pond and exhibiting doming (black arrow) and small surficial bubbles of oxygen (white arrow). Photograph taken on 27 April 2016, following 9.7 mm rain in the previous 24 hours. B) Same mat 12 days later (9 May 2016) with only 0.2 mm rain in intervening days. Pond has dried slightly. Mat dome has collapsed (black arrow) and mat has fragmented further. Fewer oxygen bubbles visible, but ‘lizard-skin' domes (sensuEriksson et al. 2007) have developed in separate mat fragment floating deeper under water (white arrow). New microbial community seen as reddened biofilm (gray arrow) has developed (possibly indicating a community of iron-oxidizing bacteria). C) Mat on 16 May 2016, following seven days without precipitation. No surficial oxygen bubbles. Lizard-skin lower mat now subaerially exposed at water-air interface, but maintaining form (black arrow). Red biofilm has expanded (white arrow). D) Mat on 23 May 2016, immediately after 5.2 mm precipitation. Pond has refilled, but mats have not yet ‘caught-up' with new water level. Mats are apparently temporarily dormant with no evidence for active production of oxygen bubbles.
Evidence for preservation of primary sedimentary surface textures in the geological record, shown through comparison of ancient and modern surfaces. A) Bedding plane exposure of the Silurian Tumblagooda Sandstone, preserved in the wall of the Murchison River Gorge at The Loop. Ruler for scale is 1 meter. B) Modern pond of standing water in the Murchison River, west of The Loop. Scale bar = 20 cm. Both the modern and ancient examples reveal analogous substrate conditions, exhibiting ripple formation in deeper parts of a submerged pond (arrow R), a smooth substrate on the recently emergent drainage slope at the pond margin (arrow S), adhesion marks on the emergent margin of the pond (arrow AM), and a dislodged raft of adhesion marked sediment (arrow D). In the modern example, the raft was dislodged artificially, while in the ancient example it appears to have been dislodged by an arthropod; arthropod tracks are common in the formation and a series of grooves (small white arrows) at this location may be repichnial.
Conceptual model illustrating how sedimentary surface textures in the rock record reveal features developed during stasis, how they differ from hydrodynamic bedforms deposited during stasis, erosion, deposition and transport, and how the potential morphology of their signatures may vary significantly depending on the instant of interment. Time scale on left is representative only, but all four examples could be considered discrete components of the same deposystem. A) Hydrodynamic bedforms, developed during erosion, deposition, stasis, and transport. B) Abiotic and ichnologic sedimentary surface textures developed as events during stasis. C) Abiotic and ichnologic sedimentary surface textures developed as events during stasis, but later destroyed by erosion and deposition. D) Microbial sedimentary surface textures developed as ongoing morphological evolution during stasis, with resultant greater potential variability. Note that if the observable end-products B or D are seen in the rock record, then they must be a palimpsested record of multiple events during an interval of sedimentary stasis.