Search for: All records

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. 

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. 

Abstract The measured carbon isotopic compositions of carbonate sediments (δ¹³C_carb) on modern platforms are commonly¹³C‐enriched compared to predicted values for minerals forming in isotopic equilibrium with the dissolved inorganic carbon (DIC) of modern seawater. This offset undermines the assumption that δ¹³C_carbvalues of analogous facies in the rock record are an accurate archive of information about Earth's global carbon cycle. We present a new data set of the diurnal variation in carbonate chemistry and seawater δ¹³C_DICvalues on a modern carbonate platform. These data demonstrate that δ¹³C_carbvalues on modern platforms are broadly representative of seawater, but only after accounting for the recent decrease in the δ¹³C value of atmospheric CO₂and shallow seawater DIC due to anthropogenic carbon release, a phenomenon commonly referred to as the¹³C Suess effect. These findings highlight an important, yet overlooked, aspect of some modern carbonate systems, which must inform their use as ancient analogs.