Hurricanes cause substantial loss of life and resources in coastal areas. Unfortunately, historical hurricane records are too short and incomplete to capture hurricane‐climate interactions on ...multi‐decadal and longer timescales. Coarse‐grained, hurricane‐induced deposits preserved in blue holes in the Caribbean can provide records of past hurricane activity extending back thousands of years. Here we present a high resolution record of intense hurricane events over the past 1500 years from a blue hole on South Andros Island on the Great Bahama Bank. This record is corroborated by shorter reconstructions from cores collected at two nearby blue holes. The record contains coarse‐grained event deposits attributable to known historical hurricane strikes within age uncertainties. Over the past 1500 years, South Andros shows evidence of four active periods of hurricane activity. None of these active intervals occurred in the past 163 years. We suggest that Intertropical Convergence Zone position modulates hurricane activity on the island based on a correlation with Cariaco Basin titanium concentrations. An anomalous gap in activity on South Andros Island in the early 13th century corresponds to a period of increased volcanism. The patterns of hurricane activity reconstructed from South Andros Island closely match those from the northeastern Gulf of Mexico but are anti‐phased with records from New England. We suggest that either changes in local environmental conditions (e.g., SSTs) or a northeastward shift in storm tracks can account for the increased activity in the western North Atlantic when the Gulf of Mexico and southeastern Caribbean are less active.
Key Points
Sediment cores from blue holes on Andros Island record intense hurricane activity over the past millennium and a half
Multi‐decadal shifts in Intertropical Convergence Zone position and volcanic activity modulate the hurricane patterns observed on Andros
Hurricane patterns on Andros match patterns from the northeastern Gulf of Mexico but are anti‐phased with patterns from New England
The white shark, Carcharodon carcharias , is an iconic apex predator, playing an important ecological role across its range. Persistent bycatch and overfishing led to white shark declines, but recent ...studies in the North Western Atlantic (NWA) revealed evidence for regional recovery, and highlighted the importance of Southeastern Florida and the Gulf of Mexico as overwintering grounds for maturing white sharks. However, despite its proximity to Florida and comparably productive habitats, records of white sharks in The Bahamas are extremely rare, with a comprehensive survey of sightings and captures describing only one white shark between 1800 - 2010. Here, we reveal acoustic tracking detections of ten white sharks from 2020 - 2024 along the western edge of the Tongue of the Ocean off Central Andros Island, The Bahamas. White sharks were originally tagged off the coast of the United States and Canada, and detected off Andros Island, The Bahamas from November-May. White sharks were detected along the drop-off zone of the reef at ca. 25 m, exclusively between dusk and dawn, with the number of detections suggesting transient behavior. These findings expand our knowledge of white shark distribution in the NWA, highlighting data gaps in The Bahamas and underlining the importance of collaborative protective measures for species recovery.
This study aims at clarifying the relationship between the Cape Steno mélange, southern Andros, and the main tectonic units of the Attic-Cycladic Crystalline Belt. Jurassic protolith ages and ...geochemical characteristics indicate a Pelagonian affinity and point to a correlative relationship with the Tsiknias Ophiolite on Tinos Island. However, jadeitites and high-Si phengite in the gneisses clearly indicate a high-pressure metamorphic overprint that is unknown from the Tsiknias outcrop and other occurrences of the Upper Cycladic Unit. A correlation with the Cycladic Blueschist Unit (CBU) is an obvious assumption, but initially seemed difficult to reconcile with the Cretaceous protolith ages of meta-ophiolitic rocks from the CBU and distinct geochemical characteristics of associated jadeitites. The Jurassic ages of the Cape Steno rock suite either document a broader spectrum of source rocks than previously known from the CBU, or the existence of a distinct tectonic unit. We assume that the geological and tectono-metamorphic evolution of the Cape Steno occurrence is similar to that of the Makrotantalon Unit of NW Andros, which represents a Pelagonian subunit in the nappe stack of the CBU, with abundant slices of serpentinites, rare meta-gabbro and a metamorphic history comprising both Cretaceous and Eocene HP/LT episodes.
Event‐based paleohurricane reconstructions of the last millennium indicate dramatic changes in the frequency of landfalling hurricanes on centennial timescales. It is difficult to assess whether the ...variability captured in these paleorecords is related to changing climate or randomness. We assess whether centennial‐scale active and quiet intervals of intense hurricane activity occur in a set of synthetic storms run with boundary conditions from an earth system model simulation of the last millennium. We generate 1,000 pseudo sedimentary records for a site on South Andros Island using a Poisson random draw from this synthetic storm data set. We find that any single pseudo sedimentary record contains active and quiet intervals of hurricane activity. The 1,000‐record ensemble average, which reflects the common signal of climate variability, does not. This suggests that the record of paleohurricane activity from The Bahamas reflects variability in hurricane frequency dominated by randomness and not variability in the climatic conditions.
Plain Language Summary
Sedimentary records of past hurricane strike from coastal basins in The Bahamas indicate dramatic changes in hurricane frequency over the past millennium with century‐scale periods of time with significantly different, and higher, levels of hurricane activity than has been observed during the last century. It is still unknown whether these anomalous hurricane patterns are a result of changes in climate or simply due to randomness. Here, we recreate these century scale patterns of hurricane activity for the past millennium using synthetic tropical cyclones and a model of how the passage of each storm is documented in a sediment record. We prove these patterns of activity are predominantly a result of randomness not climate changes. This work has important implications for future research into past millennium hurricanes. We cannot use single sediment records from The Bahamas to infer climate impacts on hurricane activity. Instead, we need to use regional or larger scale compilations of records to characterize hurricane activity and its drivers.
Key Points
Pseudo records of past millennium hurricane activity closely replicate centennial‐scale variability found in published sediment records
Most of the centennial‐scale signal captured in event‐based paleohurricane records from The Bahamas is due to randomness not climate
There is promise in using compilations of paleohurricane records to characterize hurricane activity and its drivers
Satellite imagery and an extensive set of water‐depth measurements have been used to map and critically evaluate the magnitude and patterns of bathymetry across Great Bahama Bank. Descriptions of ...previously collected sediment samples were combined with satellite imagery to map and refine the interpreted distribution of surficial carbonate sediments (depositional facies). Data reveal that 60% of Great Bahama Bank lies in 5 m or less of water. The deep portion occurs mainly in a generally east–west trending area in the southern portion of the platform. The re‐evaluation of the facies reveals that Great Bahama Bank is essentially a very grainy platform with muddier accumulations primarily in the lee of Andros Island. This area of Great Bahama Bank also experiences currents related to an excursion of the Florida Current onto the platform top; possibly enhancing sediment mud production through the generation of whitings. Sediment equivalents to mudstones, wackestones and mud‐rich packstones cover 8%, 5% and 14%, respectively, of the platform top, whereas sediment equivalents to mud‐poor packstones, grainstones and rudstones account for 20%, 45% and 3% of the surface area. Boundstones (reefs) were not specifically mapped in this study due to the resolution of the mapping. There is a poor relationship between the occurrence of the depositional texture and water depth, in that the grainier sediment types are abundant across the full range of water depths. The most abrupt lateral facies changes portrayed on the facies maps are observed leeward of islands, areas which also hold the highest diversity in facies type. The majority of the islands on the platform align with the north‐west/south‐east strike of the platform margin and these islands, in turn, exert control on the shape and orientation of facies belts that develop in proximity to them. For this reason, regions of the platform that contain principal islands host facies belts that align with the principal axis of the platform, whereas for regions lacking islands, the facies belts adopt an east–west trend consistent with prevailing winds and currents. There is a clear trend that the wide southern portion of the platform hosts the most continuous expanses of grain‐rich sediments.
Integrated time-separated remote sensing, geographic information systems (GIS), microbiology, and carbonate sedimentology of the tidal-flat portion of Andros Island (Great Bahama Bank) quantify the ...geomorphologic change of tidal-flat facies through time and illustrate how sea-level rise, among other controls, is recorded within these deposits. In the 75 years spanned by the remote-sensing datasets, sea level has risen by approximately 10 cm. In the face of this change: a) the seaward margin of the tidal flats has remained static, while b) the internal boundary, which separates the channelized and supratidal marsh deposits, retrograded up to 220 m, broadening the channelized zone, and c) in unison to the broadening of the channelized zone, the abundance of laminated (Scytonema) cyanobacterial fabrics decreased, d) accompanied by the lengthening and avulsion of the network of tidal channels that traverse this zone. Given the amount of sea level rise during the period of observation, such broad-scale restructuring of the Andros tidal-flat architecture is surprising. The muddy tidal flats did not prograde, but instead aggraded and locally retrograded. Whereas retrogradation was minimal at the coastline, the channelized zone retrograded substantially over the more landward supratidal inland marsh. These results question aspects of the autocyclic model for the development of peritidal carbonate cycles, which is underpinned by a dominant behavior of tidal-flat progradation. Whereas other controls cannot be explicitly excluded, sea-level oscillations seemingly exerted considerably more impact on the Andros tidal flat in the last decades than did major storm events.
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•The two mineralized sites differ in their Mn ore mineralogy (braunite versus todorokite).•Manganiferous samples from both localities show similar bulk-rock geochemistry of the ...tuffaceous host.•Mn-oxide accumulation in the CBU is genetically linked to hydrothermal venting in a Triassic back-arc rift setting.
The Triassic volcaniclastic sequence in the Cycladic Blueschist Unit (CBU) of Greece is known to contain several occurrences of metamorphosed Mn mineralization. Despite previous literature reports, a comprehensive and holistic review of the origin of these occurrences is hitherto lacking. Here we revisit the Mn metallogenic system of the CBU through a comprehensive study of Mn mineralisation at Varnavas area, Northern Attica, and its correlation with a similar occurrence at Mparades Hill, Andros island. Manganese mineralogy at both localities is manifested in a typical high-P metamorphic silicate assemblage dominated by piemontite, spessartine garnet, and minor pyroxmangite (rhodonite). Whereas at Andros Mn-rich subdomains contain brecciated braunite micronodules, preservation of tetravalent Mn oxides in a similar nodular form is documented from Varnavas, comprising dominant todorokite along with lesser hollandite, pyrolusite, and minor Mn-bearing hematite. The contrasting Mn oxide mineralogy and comparable textural characteristics at the two sites are tentatively interpreted as the result of locally incomplete reduction of precursor Mn(IV) phases during metamorphism. Bulk Mn concentrations of the studied materials generally do not exceed the value of 10 wt% Mn, unless small-scale subsampling of Mn-rich domains is undertaken. Key geochemical characteristics of the Mn-rich rocks include low transition metal concentrations (sum of Co + Ni + Cu + Zn between 0.02 and 0.06 wt%); positive-sloping, PAAS-normalised REE spidergrams at a maximum Ndsn/Ybsn ratio of 0.3; positive Ce anomalies, albeit of variable magnitude across individual samples; and high As (up to 1930 ppm) and Ba (up to 2767 ppm) contents. A combination of structural, geochemical and mineralogical evidence from the two localities along with published results from similar occurrences elsewhere (e.g., Kythnos) suggest that Mn-oxide accumulation in the CBU is genetically linked to hydrothermal venting in a back-arc rift setting during the Triassic. The geochemical variability recorded is attributed primarily to the variable mixing of a hydrothermal-sourced, hydrogenous metalliferous component that precipitated broadly contemporaneously with the deposition of the host tuffs. Low transition metal contents coupled with substantial enrichments in elements such as Ba, As, Pb and REY, attest to the felsic/intermediate character of back-arc magmatism/volcanism and associated hydrothermal activity. Primary Mn precipitates are thought to have been in the form of tetravalent Mn assemblages, which may locally be partially preserved through metamorphism, as appears to be the case in the Varnavas occurrence. The fully oxic and hydrogenous character of the precursor Mn(IV) oxy-hydroxides is supported by the consistently positive Ce anomalies observed in practically all samples from both sites. Although the Mn concentrations and distribution of the studied assemblages are sub-economic at best, we consider possible that their geographically widespread occurrence may still hold untapped potential for future discovery of commercially viable Mn ores sensu stricto, both in the Hellenic arc and in other regions of similar geotectonic and metallogenic activity.
This review of published literature examines the nature of modern carbonate coastal barrier systems, concentrating on barrier islands and their related shoals and lagoons in light of concepts derived ...from their siliciclastic counterparts. The observations show that subaerial coastal (C-type) barriers are common along transgressing carbonate shorelines and are more abundant than largely subtidal shoals. The carbonate sediment composition and associated contemporaneous marine and meteoric cementation do not prevent landward migration during transgression, in contrast to R-type barriers that occur at platform margins to which they are genetically linked. As a result, carbonate C-type barrier systems act like siliciclastic barriers during transgression, showing both continuous (rollover) and discontinuous (overstepping) landward movement. The widely cited examples of immobile carbonate barriers in the United Arab Emirates, Persian Gulf, and the Coorong, Australia, are believed to be stable because of a recent sea-level fall. The sandy shoals on the Great Pearl Bank, Persian Gulf, and Faure Sill, Shark Bay, Australia, are likely the remnants of overstepped barriers that have been reworked after Holocene drowning; the transgressive history of Faure Sill is suggested by the presence of (lagoonal?) muds beneath the sandy shoal. Several examples of preserved, post-glacial overstepped coastal barriers have been reported from modern carbonate shelves, and evidence of preserved, back-barrier lagoonal deposits is reported from the Persian Gulf ramp and the southern Australian margin. This implies that transgressive successions that begin with tidal-flat deposits overlying the sequence boundary, passing upward into more offshore sediments, should be more common than currently appreciated; the shoreline of NW Andros Island, Bahamas, is transgressing and provides an example of this succession. Such successions might contain two wave-generated erosional discontinuities: a lower one generated by a low-energy ‘nested’ beach-ridge barrier along the landward side of the lagoon (= the ‘bay ravinement surface'), and a more significant surface formed by the passage of the main barrier shoreface (the ‘master ravinement surface'). Preservation of the barrier's subaerial cap is not expected, except in rare cases, because it is removed by the master ravinement surface. Meteoric diagenesis of coastal-barrier grainstones is likely to be limited because of the presence of an underlying layer of impermeable lagoonal mud and the short residence time of any location within the freshwater lens due to barrier migration. The ‘lag depth’ or ‘lag time' that is invoked to account for a delay in sediment accumulation is thought to be due to passage of the erosional shoreface, the depth of which is approximately equal to fair-weather wave base. Ancient ooid shoal complexes might be analogous to the amalgamated, highstand coastal barrier systems that have been stranded along many siliciclastic shorelines. The hypersaline conditions expected in arid coastal settings are most easily obtained if a continuous subaerial barrier exists; subaqueous shoals typically have too much tidal exchange to permit large departures from normal-marine salinity. These findings offer an alternative to the classic upward-shallowing model for coastal sedimentation in carbonate systems that might have been employed uncritically in some cases.
Ooid formation remains elusive despite their importance as palaeoclimatic indicators and important contributors to global carbonate budget. Based on stable isotopes, nutrient and elemental analyses ...on solid components and ooidal leachates, this study supports the notion of microbial involvement in the development of ooids from Great Bahama Bank. Carbon and nitrogen isotopic analyses on organic fractions identified geochemical signatures of microbial activity. The δ13C values for organic carbon in the bulk (−11·9 to −16·9‰); intercrystalline/intracrystalline (−11·9 to 16·7‰); and intracrystalline phases (−12·4 to −17·7‰) were similar and, except for the more enriched values of ooids from Butterfly Beach, were within the range of photosynthesisers. The δ15N values for the bulk (+0·5 to −0·2‰); intercrystalline/intracrystalline (−0·3‰ to −0·7‰) and intracrystalline organic matter (−0·3 to −1·7‰) showed a narrow range consistent with nitrogen fixation. While positive δ15N and δ18O values of the NO3− leached from the ooids provided evidence of denitrification, the carbonate associated sulphate δ34SCAS of the bulk sediments (+19·2 to +19·6‰) and δ34S of the leachates (+16·6 to +18·3‰) provided weak indication of sulphate reduction, suggesting either that high concentrations of isotopically enriched S are overriding bio‐signatures of sulphate reduction or that microbes are preferentially using NO3− as an electron acceptor. In contrast, the elevated sulphate concentrations of the leachates suggest the occurrence of microbial sulphide oxidation within ooids. The high Mg/Ca of the leachates and scanning electron microscope analyses provide putative evidence of amorphous calcium carbonate and a formative role in CaCO3 precipitation. Together, these findings indicate that a redox dependent microbial consortium may influence CaCO3 precipitation in the form of ooid accretion, cementation and micritization. It is also inferred that ooid deposits are not suitable indicators of palaeoclimate because ooids are affected throughout their life by a complex chain of abiotic and biological processes which can lead to large geochemical offsets.
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