RAPID BEACHROCK FORMATION: A BIOFILM POWERED GEOLOGIC SNAPSHOT OF HURRICANES

Low-lying islands are at particular risk to rising sea level, increased storm surge, and erosion by large storms. Beachrock, which forms via calcium carbonate cementation between sedimentary grains in the intertidal zone, forms rapidly and thus is capable of stabilizing sediment and potentially armoring coastlines. Little Ambergris Cay (LAC) is a low-lying carbonate island located in the Turks and Caicos Islands that experiences frequent hurricanes (every 5 years). Beachrock is abundant on LAC’s southern coast and includes large, angular locally reworked imbricated clasts, glass bottles, metal fragments, and other human debris, all of which is potentially sourced from hurricane erosion. The relative contributions of physiochemical and biological processes responsible for the formation of beachrock, and the rate at which beachrock can form, are unknown for this study site. We used in situ field experiments to physically (scanning electron microscope, light microscopy, and grain shape and size analysis), biologically (16S rRNA gene sequencing), and chemically (percent total organic carbon) track the process of beachrock formation on LAC and the role of biofilms in this process. Twenty-three replicate ooid pouches (sterilized ooids sewn into a pouch of 75-micron nylon mesh) were deployed in a rubber polyhedral meshwork ball (commercially available as a dog chew toy) onto the beachrock near the cemented bottle location. Samples were collected at three time points: 4 days, 2.5 months, and 5 months of incubation time after deployment in February 2023. After 2.5 months we observed ooids loosely cemented into clumps and dark green coloration on the surface of the clumps and pouch. Incipient meniscus cements were observed in as little as 4 days of incubation, and later mineral precipitation was observed along interwoven microbial filaments between ooids. Networks of thin (1 µm) filaments between ooids were observed after 2.5 months and became denser and smoother after 5 months. 16S rRNA sequencing data show that microbial communities in the experimental ooid clumps match the community seen in in situ beachrock. We interpret that biofilms are critical to the initiation of beachrock by rapidly physically stabilizing sediment while cementation proceeds. This process allows for preservation of hurricane deposits, which include anthropogenic objects, along the coastline of LAC.