Building biogenic beachrock: Visualizing microbially-mediated carbonate cement precipitation using XFM and a strontium tracer

Abstract

The fate of reef islands is a topic of ongoing debate in the face of climate change-induced sea-level rise and increased cyclone intensity. Increased erosion and changes to the supply of reef-derived sediment may put sand reef cays at risk of dramatic morphological changes.These changes may have negative implications for the existence of reef cay environments, which host vital sea turtle and bird rookery habitats. Beachrock, consolidated carbonate beach sediment in the intertidal zone, forms naturally on many tropical beaches and reduces the erosion rates of these beaches when compared to unconsolidated sand. In spite of the critical role beachrock plays in stabilizing some reef cay shores, the method of beachrock formation is still incompletely understood. In this investigation, beachrock was synthesized using beach sand and beachrock samples from Heron Island (Great Barrier Reef, Australia) in aquarium experiments in which natural beachrock formation conditions were simulated. Beachrock was produced in two aquaria wherein the water chemistry was influenced by microorganisms derived from the natural beachrock 'inoculum', whereas no cementation occurred in an aquarium that lacked a microbial inoculum and was controlled only by physicochemical evapoconcentration. The new cements in the synthesized beachrock were analyzed using synchrotron-based X-ray fluorescence microscopy and were identified using a Sr-tracer added to the experimental seawater. The resulting precipitates cement sand grains together and contain abundant microfossils of the microorganisms on whose exopolymer they nucleated, demonstrating the fundamental role microbes play in beachrock formation. These results are of interest because beachrock could be utilized as a natural coastline stabilization strategy on sand reef cays, and in turn, protect the unique habitats reef islands support.

Advanced biofilm staining techniques for TEM and SEM in geomicrobiology: Implications for visualizing EPS architecture, mineral nucleation, and microfossil generation

“…Microbial mats and biofilms found on, and within, the beachrock on Heron Island (Capricorn Group, Great Barrier Reef, Australia) have been previously studied for their ability to precipitate calcium carbonate cements and form microbialites (McCutcheon et al, 2016a;McCutcheon et al, 2017a;Webb et al, 1999). A sample of the cyanobacteria-dominated microbial mat hosting abundant carbonate mineral grains was collected from the surface of the beachrock and prepared using the protocol outlined in Section 2.1.…”

Synergistic Use of UAV and USV Data and Petrographic Analyses for the Investigation of Beachrock Formations: A Case Study from Syros Island, Aegean Sea, Greece

“…The precipitation and composition of these phases depend on a variety of physicochemical parameters, e.g., temperature, pH, salinity, and Mg 2+ concentration. Besides these factors, recent studies revealed that the presence of microorganisms play a severe role in the formation of beachrocks as these enhance the dissolution of carbonate grains, increasing the cation in solution leading to precipitation [4,5]. In the case of area 1, textural characteristics of cement in combination with its HMC composition are probably indicating that the beachrock was formed under marine-vadose conditions [1,13].…”

“…In the case of area 1, textural characteristics of cement in combination with its HMC composition are probably indicating that the beachrock was formed under marine-vadose conditions [1,13]. The contribution of organism activity to cement formation should not be rejected but further investigation with high-resolution techniques is demanded, e.g., References [4,5]. Beachrock samples of area 2, as shown in Figure 13A-G, can be described as conglomerates comprising mainly of sub angular to sub rounded lithoclasts (epidote-chlorite schists, mica schists, carbonate rocks).…”

“…Beachrock is an intertidal beach deposit with a carbonate cement. There are several processes involved in beachrock formation, including direct precipitation from meteoric waters, mixing of marine and meteoric waters, degassing of CO 2 , evaporation, and microbial carbonate mineral precipitation [1][2][3][4][5]. It is composed in shallow water either on the surface or, more commonly, beneath the littoral/intertidal sediments, by carbonate cementation of beach deposits during stages of shoreline stabilization and it is used as a marker of sea level change [6][7][8][9].…”

Geochronology and palaeoclimatic context of submerged siliciclastic beachrock formation in the western Mediterranean Sea

“…The main primary cementing mechanisms proposed are physicochemical, linked to (1) the common ion effect in the mixture of meteoric and sea water, (2) evaporation and (3) degassing of CO 2 . Recent studies show that carbonate precipitation induced by the activity of microorganisms plays an important role in the cementation of beachrocks (McCutcheon et al 2016, 2017, Ramachandran et al 2020, Daryono et al 2020.…”