Climate plays a central role in coral-reef development, especially in marginal environments. The high-latitude reefs of southeast Florida are currently non-accreting, relict systems with low coral cover. This region also did not support the extensive Late Pleistocene reef development observed in many other locations around the world; however, there is evidence of significant reef building in southeast Florida during the Holocene. Using 146 radiometric ages from reefs extending ~ 120 km along Florida’s southeast coast, we test the hypothesis that the latitudinal extent of Holocene reef development in this region was modulated by climatic variability. We demonstrate that although sea-level changes impacted rates of reef accretion and allowed reefs to backstep inshore as new habitats were flooded, sea level was not the ultimate cause of reef demise. Instead, we conclude that climate was the primary driver of the expansion and contraction of Florida’s reefs during the Holocene. Reefs grew to 26.7° N in southeast Florida during the relatively warm, stable climate at the beginning of the Holocene Thermal Maximum (HTM) ~ 10,000 years ago, but subsequent cooling and increased frequency of winter cold fronts were associated with the equatorward contraction of reef building. By ~ 7800 years ago, actively accreting reefs only extended to 26.1° N. Reefs further contracted to 25.8° N after 5800 years ago, and by 3000 years ago reef development had terminated throughout southern Florida (24.5–26.7° N). Modern warming is unlikely to simply reverse this trend, however, because the climate of the Anthropocene will be fundamentally different from the HTM. By increasing the frequency and intensity of both warm and cold extreme-weather events, contemporary climate change will instead amplify conditions inimical to reef development in marginal reef environments such as southern Florida, making them more likely to continue to deteriorate than to resume accretion in the future.

The global‐scale degradation of coral reefs has reached a critical threshold wherein further declines threaten both ecological functionality and the persistence of reef structure. Geological records can provide valuable insights into the long‐term controls on reef development that may be key to solving the modern coral‐reef crisis. Our analyses of new and existing coral‐reef cores from throughout the Florida Keys reef tract (FKRT) revealed significant spatial and temporal variability in reef development during the Holocene. Whereas maximum Holocene reef thickness in the Dry Tortugas was comparable to elsewhere in the western Atlantic, most of Florida's reefs had relatively thin accumulations of Holocene reef framework. During periods of active reef development, average reef accretion rates were similar throughout the FKRT at ~3 m/ky. The spatial variability in reef thickness was instead driven by differences in the duration of reef development. Reef accretion declined significantly from ~6,000 years ago to present, and by ~3,000 years ago, the majority of the FKRT was geologically senescent. Although sea level influenced the development of Florida's reefs, it was not the ultimate driver of reef demise. Instead, we demonstrate that the timing of reef senescence was modulated by subregional hydrographic variability, and hypothesize that climatic cooling was the ultimate cause of reef shutdown. The senescence of the FKRT left the ecosystem balanced at a delicate tipping point at which a veneer of living coral was the only barrier to reef erosion. Modern climate change and other anthropogenic disturbances have now pushed many reefs past that critical threshold and into a novel ecosystem state, in which reef structures built over millennia could soon be lost. The dominant role of climate in the development of the FKRT over timescales of decades to millennia highlights the potential vulnerability of both geological and ecological reef processes to anthropogenic climate change.

Five seismic units may be identified in the ~8m thick Holocene sediment package at the bottom of the Blue Hole, a 120m deep sinkhole located in the atoll lagoon of Lighthouse Reef, Belize. These units may be correlated with the succession of an existing 5.85-m-long sediment core that reaches back to 1.385kyrsBP. The identification of seismic units is based on the fact that uniform, fine-grained background sediments show weak reflections while alternating background and coarser-grained event (storm) beds exhibit strong reflections in the seismic profiles. The main source of sediments is the marginal atoll reef and adjacent lagoon area to the east and north. Northeasterly winds and storms transport sediment into the Blue Hole, as seen in the eastward increase in sediment thickness, i.e., the eastward shallowing of the Blue Hole. Previous assumptions of much thicker Holocene sediment packages in the Blue Hole could not be confirmed. So far, close to 6-m-long cores were retrieved from the Blue Hole but the base of the sedimentary succession remains to be recovered. The nature of the basal sediments is unknown but mid-Holocene and possibly older, Pleistocene sinkhole deposits can be expected. The number of event beds identified in the Blue Hole (n=37) during a 1.385kyr-long period and the number of cyclones listed in historical databases suggest that only strong hurricanes (categories 4 and 5) left event beds in the Blue Hole sedimentary succession. Storm beds are numerous during 1.3–0.9kyrsBP and 0.8–0.5kyrsBP.

Carbonate sand beaches and spits nurtured by longshore currents in the Arabian Gulf commonly form sheltered areas favourable for evaporite and carbonate sedimentation. Promontories and embayments created by pre-Holocene topography favour such spit formation, especially in areas where contours are perpendicular to prevailing winds and longshore currents. Ras Umm Said on the NE coast of the Qatar Peninsula provides a useful model for this style of sedimentation. In the mid-1960s, three 12 km long chenier-like Holocene beaches had developed en echelon at Ras Umm Said. Each chenier had several landward-projected carbonate sand bodies that originated as terminal fish-hook-shaped spits during southward migration. The flat-lying channelized sabkha and intertidal areas that formed landward of, and between, the beaches provided low-energy areas suitable for fine-grained carbonate sedimentation and local evaporite deposition. These combined cheniers and channelized sabkhas have continued to grow at a remarkable rate over the last 40 years. Each chenier, with its recurved spits, reaches a stable phase and is then protected from wave action when a new chenier forms on its seaward side.The chenier beach and recurved spit create conditions favourable for rapid beachrock formation on their low-energy landward side; cementation is apparent within 1 year. The narrow beachrock then becomes exposed as linear ridges on the seaward side of the cheniers as they are eroded and migrate landward, creating a characteristic set of beach laminations. Wave erosion of these exposed rock ridges creates locally abundant lithoclasts that are incorporated into the beach as it transgresses landward. Storm tides and currents break through the curved landward-directed spits allowing flooding and extension of tidal channels into the sabkha and intertidal zones between the cheniers. Periodically, the entire beach/sabkha complex "jumps" and accretes seaward over offshore areas of submarine-cemented sediment. A new "jump" is beginning and growing today.The bored submarine-cemented layers (hardgrounds) that become buried under the intertidal and sabkha sediments often constrain the depth to which later tidal channels may erode. Such bored surfaces could easily be misidentified as flooding surfaces during sequence stratigraphic analysis. The cemented layers also serve as impermeable seals, preventing or retarding vertical movement of fluids; such early formation of aquitards may explain the distribution of fluids (water or oil) in ancient analogue settings. Creation of mainly low-angle landward-clipping beach bedding, overlying a basal, more steeply dipping, set of landward-dipping beds, is indicative of the migrating chenier sedimentary model. The spits will eventually seal off the lagoon, which will then evolve into a large sabkha possibly composed of wind-blown sand. Chenier migration and marine cementation have considerable significance for coastal development and civil engineers projects in the region.

Using mainly satellite images of both the Arabian Gulf and portions of the Red Sea, [Purkis et al. (2010)][1] applied sophisticated mathematical modeling to explain large polygonal seafloor features, “templates,” that are amplified in some areas by modern coral growth. They conclude from their

Pleistocene carbonates of south Florida and islands of the Florida Keys are currently divided into five marine sequences designated, from oldest to youngest, the Q1-Q5 units. The units include a mosaic of freshwater and shallow marine deposits that accumulated on the Florida platform during high sea-level stands. The units are separated by regional-scale subaerial-exposure surfaces that formed during glacioeustatic lowstands. Analyses of cores recovered at Grossman Ridge Rock Reef and Joe Rae Rock Reef in the Florida Everglades reveal additional subaerial-exposure surfaces that are used to delineate subdivisions within units Q1 (Q1a-Q1b), Q2 (Q2a-Q2d), and Q4 (Q4a-Q4b). Units Q1-Q5 preserve evidence of at least 10 separate sea-level highstands, rather than 5 as indicated by previous studies.Compilation of available uranium-series dates on corals recovered from the Florida Keys indicates that the Q4 unit accreted during sea-level maxima associated with marine oxygen-isotope Stage 9 (Q4a) and isotope Stage 7 (Q4b). The Q5 unit formed during isotope Stage 5. No reliable dates are available for units Q1-Q3. We infer that unit Q3 was formed during the extended sea-level highstand of isotope Stage 11 and that units Q2 and Q1 predate isotope Stage 11.

Parallel rows of intertidal beachrock composed of large (up to 4m), oblong, pillow-shape blocks submerged by rising Holocene sea level occur throughout the Caribbean. Such blocks have recently been discovered in 90m of water off southwest Florida. The best known example is in 5-7m of water off the northwest side of Bimini, Bahamas, where the feature is known as the Bimini Road. 'New-Age' alternative thinkers assert that the arrangement of the Bimini stones, and similar stones off Andros Island in the Bahamas, is actually man-made. The theory holds that ancient humans, principally citizens of the mythical city of Atlantis rearranged blocks of beachrock to form a harbour. This paper demonstrates that the stones and their arrangement are natural and suggests that some villages protected by ancient harbours in the Mediterranean may in fact have been developed on naturally occurring beachrock.

Sediment cores (up to 6 m in length) from the bottom of the Blue Hole, a 125 m deep Pleistocene sinkhole located in the lagoon of Lighthouse Reef Atoll, Belize, consist of undisturbed, annually layered biogenic carbonate muds and silts with intercalated coarser grained storm beds. The sedimentation rate of the layered sections is 2.5 mm/y on average, and the long cores span the past 1500 years. Oxygen isotopes of laminated sediment provide a late Holocene climate proxy: A high-resolution delta O-18 time series traces the final Migration Period Pessimum, the Medieval Warm Period, the Little Ice Age, and the subsequent temperature rise. Carbon isotopes (delta C-13) decrease up core and show the impacts of the decline of the Mayan culture and the Suess effect. Time series analyses of delta O-18 and delta C-13 content reveal 88-, 60-, 52-, and 32-year cyclicities, and suggest solar forcing. Storm event beds are most common during AD 650-850, around AD 1000, during AD 1200-1300, and AD 1450-1550. Major storm beds are rare during the past 500 years BP.

Potential effects of climate change and ocean acidification have energized much discussion among coral scientists, especially biologists. Will corals go extinct, lose their skeletons, or migrate po...