An official website of the United States government
The study of modern hurricane deposits is useful both in identifying ancient hurricane deposits in the rock record and predicting patterns of deposition and erosion produced by future storm events. Hurricane deposits on carbonate platforms have been studied less frequently than those along continental coasts. Here we present observations of the characteristics of deposition and scour caused by Hurricane Irma on Little Ambergris Cay, a small uninhabited island located near the southeastern edge of the Caicos platform in the Turks and Caicos Islands. Hurricane Irma passed directly over Little Ambergris Cay on September 7, 2017 as a Category 5 hurricane. We described and sampled multiple types of hurricane deposits and determined that the washover fans were the best sedimentological records for hurricane conditions, as they were subject to very little reworking over time. We compared different model predictions of storm tide and wave height with eyewitness reports and distributions of scour. Examining the washover fans allowed for the construction of a conceptual model for hurricane deposits formed in a high‐energy storm event on a carbonate platform. Characteristics of the washover fans were their small size, the lack of sedimentary structures, and very well‐sorted sediment. The size and distribution of carbonate boulders eroded and transported by the storm are consistent with depth‐averaged flow velocities in the range of 1.5‐5.3 m/s. The strength of the storm and the low‐lying topography, distinct features of a carbonate platform setting, contributed to high levels of sediment bypass and high flow velocities, resulting in small, unstructured deposits.
more »
« less
This content will become publicly available on February 6, 2026
How to Make a Rock in 150 Days: Observations of Biofilms Promoting Rapid Beachrock Formation
ABSTRACT Beachrock is a type of carbonate‐cemented rock that forms via rapid cementation in the intertidal zone. Beachrock is a valuable geological tool as an indicator of paleoshorelines and may protect shorelines from erosion. Previous studies present a range of hypotheses about the processes enabling rapid beachrock formation, which span purely physicochemical mechanisms to a significant role for microbially mediated carbonate precipitation. We designed a set of in situ field experiments to explore the rates and mechanisms of beachrock formation on Little Ambergris Cay (Turks and Caicos Islands). Our field site has evidence for rapid beachrock cementation, including the incorporation of 20th century anthropogenic detritus into beachrock. We deployed pouches of sterilized ooid sand in the upper intertidal zone and assessed the extent of cementation and biofilm development after durations of 4 days, 2.5 months, and 5 months. We observed incipient meniscus cements after only 4 days of incubation in the field, suggesting that physicochemical processes are important in driving initial cementation. After 2.5 months, we observed substantial biofilm colonization on our experimental substrates, with interwoven networks ofHalomicronemafilaments binding clusters of ooids to the nylon pouches. After 5 months, we observed incipient beachrock formation in the form of coherent aggregates of ooids up to 1 cm in diameter, bound together by both networks of microbial filaments and incipient cements. We interpret that the cyanobacteria‐dominated beachrock biofilm community on Little Ambergris Cay plays an important role in beachrock formation through the physical stabilization of sediment as cementation proceeds. Together, this combination of physicochemical and microbial mechanisms enables fresh rock to form in as little as 150 days.
more »
« less
- PAR ID:
- 10570841
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Geobiology
- Volume:
- 23
- Issue:
- 1
- ISSN:
- 1472-4677
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Euendolithic microorganisms, capable of bioerosion in carbonate substrates, play an important role in modern marine ecosystems and have a fossil record extending into deep time. Understanding the factors driving microboring behaviour is essential for interpreting their ecological impact and reconstructing ancient environmental conditions. In this study, we conducted field incubation experiments across multiple sites at Little Ambergris Cay in the Turks and Caicos Islands, examining microboring density in abiotic optical calcite and aragonite under varying conditions of light, subaerial exposure, current energy, substrate mineralogy and trace metal content. We observed sinuous tunnels within 1 week of incubation in transparent calcite, with longer deployment times (2.5–5 months) resulting in meaningful increases in boring density. We also documented boring activity in dark conditions, suggesting potential for enhanced mineral dissolution at night when geochemical conditions are more optimal. Trace metal analysis of our experimental substrates revealed Fe/Ca and Mn/Ca ratios exceeding western Atlantic sea water estimates by 1–3 orders, with calcites more enriched in Mn than aragonites, offering preliminary support for the novel hypothesis that dissolution of CaCO3minerals might be a useful source of trace metals for euendoliths. Sea water chemistry varied across sites, particularly between restricted interior and open platform sites. A comparison of boring densities suggests that trace metal abundance, mineralogy, local sea water CaCO3mineral saturation state (Ω) and subaerial exposure (e.g. intertidal vs. shallow subtidal) may all influence microboring. These findings offer new perspectives on the euendolithic lifestyle, showing how substrate selection and temporal partitioning of dissolution activity balance metabolic costs with environmental constraints. They also enhance our ability to interpret the fossil record and bioerosion dynamics under changing conditions.more » « less
-
Abstract Low‐lying islands in tropical regions are vulnerable to near‐term sea‐level rise and hurricane‐induced flooding, with substantial human impact. These risks motivate researchers to elucidate the processes and timescales involved in the formation, growth and stabilization of coastlines through the study of Holocene shoreline dynamics. Little Ambergris Cay (Turks and Caicos Islands) is a low‐lying carbonate island that provides a case study in the nucleation and growth of such islands. This study investigates the sedimentology and radiocarbon chronology of the island's lithified sediments to develop a model for its history. The island's lithified rim encloses a tidal swamp populated by microbial mats and mangroves. Preliminary radiocarbon data supported a long‐standing inference that the island is Holocene in age. This study integrates petrographic, sedimentological and new radiocarbon data to quantify the age of the island and develop a model for its evolution. Results indicate that the ages of most lithified sediments on the island are <1000 cal yrbp, and the generation and lithification of carbonate sediment in this system supports coastline growth of at least 5 cm/year. The lithification of anthropogenic detritus was documented, consistent with other evidence that in recent centuries the lithified rim has grown by rates up to tens of centimetres per year. A unit of mid‐Holocene age was identified and correlated with a similar unit of early transgressive aeolianite described from San Salvador, The Bahamas. It is proposed that this antecedent feature played an important role in the nucleation and formation of the modern island. Results extend an established Bahamian stratigraphic framework to the south‐western extreme of the Lucayan archipelago, and highlight the dynamism of carbonate shorelines, which should inform forward‐looking mitigation strategies to increase coastal resiliency to sea‐level rise. These results inform interpretation of the palaeoenvironmental record of carbonate environments, underscoring their geologically rapid pace of lithification.more » « less
-
Abstract Because quartz veins are common in fault zones exhumed from earthquake nucleation temperatures (150°C–350°C), quartz cementation may be an important mechanism of strength recovery between earthquakes. This interpretation requires that cementation occurs within a single interseismic period. We review slip‐related processes that have been argued to allow rapid quartz precipitation in faults, including: advection of silica‐saturated fluids, coseismic pore‐fluid pressure drops, frictional heating, dissolution‐precipitation creep, precipitation of amorphous phases, and variations in fluid and mineral‐surface chemistry. We assess the rate and magnitude of quartz growth that may result from each of the examined mechanisms. We find limitations to the kinetics and mass balance of silica precipitation that emphasize two end‐member regimes. First, the mechanisms we explore, given current kinetic constraints, cannot explain mesoscale fault‐fracture vein networks developing, even incrementally, on interseismic timescales. On the other hand, some mechanisms appear capable, isolated or in combination, of cementing micrometer‐to‐millimeter thick principal slip surfaces in days to years. This does not explain extensive vein networks in fault damage zones, but allows the involvement of quartz cements in fault healing. These end‐members lead us to hypothesize that high flux scenarios, although more important for voluminous hydrothermal mineralization, may be of subsidiary importance to local, diffusive mass transport in low fluid‐flux faults when discussing the mechanical implications of quartz cements. A renewed emphasis on the controls on quartz cementation rates in fault zones will, however, be integral to developing a more complete understanding of strength recovery following earthquake rupture.more » « less
-
The Great Bank of Guizhou (GBG) is an isolated carbonate platform in the Nanpanjiang Basin. Contacts between limestone and dolomitized material cross-cut bedding and form an irregular lobe shaped body that extends from the interior and terminates toward the basin. Precursor carbonate facies include oolite grainstone and packstone (with some containing a microbial matrix), clotted microbialite boundstone, and massively dolomitized carbonate mudstone containing stromatolitic and fenestral fabrics indicating a range of high-energy subtidal shoal to intertidal tidal flat environments. The mudstones are present lower in the outcrop, followed by grainstones and packstones moving upwards. Samples range from partially to fully dolomitized, with partially dolomitized samples occurring near the dolomitization front. In partially dolomitized oolite, the ooids are selectively replaced by dolomite. The paragenetic sequence interpreted from petrography includes early marine intergranular cement, early fracturing associated with possible tepee formation, neomorphism of aragonitic mollusk shells and cortical laminae in ooids, anhedral replacement dolomite, dissolution forming dissolved ooids and vugs, late stage coarse euhedral dolomite cement within intergranular pores, vugs, and early fractures, twinned calcite fill of early fractures and vugs, late stage fractures filled with twinned calcite, and finally stylolites. Previous data from the western GBG using fluid inclusion and clumped isotope geothermometry indicates that dolomitization occurred with high temperatures at burial, within a spatially variable range of 90°C to 185°C. 87Sr/86Sr values of 0.707677 to 0.708601 are similar to the ratios found in Triassic seawater. δ18O (VPDB) values for dolomite range from -7.68‰ to -1.53‰, consistent with evaporative enrichment of seawater and high burial temperatures. The spatial distribution of the dolomite, strontium isotopes and oxygen isotopes are consistent with reflux dolomitization. In other areas, the platform interior contains evidence for hypersaline conditions and evaporite mineral precipitation also consistent with the reflux model. The geothermometry data indicates that early reflux dolomite was replaced at high temperatures during burial.more » « less
