21 research outputs found
Extraction and Analysis of Coral Reef Core Samples from Broward County, Florida.
No full textThe reefs off Broward County exist as three shore-parallel, sequentially deeper terraces named the inner , middle , and outer reefs and also a shallower, nearshore ridge complex. These structures span the continental coast of southeast Florida from Palm Beach County to southern Miami-Dade County and were characterized as relict, early Holocene shelf-edge and mid-shelf reefs along with limestone ridges. Presently, the reefs are colonized by a fauna characteristic of West Atlantic/Caribbean reef systems. Scleractinian coral cover is low except for a few dense patches of Acropora cervicornis, while Acropora palmata is absent except for a few individual living colonies.
Coral reef core-drilling is a useful analytical tool to extract observable and datable geological samples from within reefs. This technique was employed to retrieve 4 cores from the inner reef off Broward County to better understand its age, composition, and Holocene growth history. Sub-samples from corals in cores provided 7 new radiocarbon ages ranging from 7,860-5,560 cal BP, and reef accumulation rates of l.7-2.45 m/1,000 yrs were calculated from these ages. In addition, coral species composition and taphonomic characteristics were analyzed to identify former reef environments/reef zonation, and signals for inner reef termination. Reef zonation was detectable but no clear taphonomic signal for inner reef termination was evident.
Current data and radiocarbon ages from all three Broward County reefs suggest that the outer reef accumulated from ~10.6-8 ka cal BP, the middle reef from at least ~5.8-3.7 ka cal BP, and the inner reef from ~7.8-5.5 ka cal BP. A lack of significant age overlaps between the three reefs has led to the assertion that they represent backstepping reefs in response to Holocene sea-level rise. This study has provided the oldest and youngest ages from the inner reef thus far, and confirms that reef backstepping from the outer reef to the inner reef occurred within just a few hundred years after the termination of the outer reef. The middle reef remains poorly understood and thus a definitive Holocene growth history and ultimately an understanding of their formation are still largely unknown
Accretion History of Mid-Holocene Coral Reefs from the Southeast Florida Continental Reef Tract, USA
No full textSixteen new coral reef cores were collected to better understand the accretion history and composition of submerged relict reefs offshore of continental southeast (SE) Florida. Coral radiometric ages from three sites on the shallow inner reef indicate accretion initiated by 8,050 Cal BP and terminated by 5,640 Cal BP. The reef accreted up to 3.75 m of vertical framework with accretion rates that averaged 2.53 m kyr−1. The reef was composed of a nearly even mixture of Acropora palmata and massive corals. In many cases, cores show an upward transition from massives to A. palmata and may indicate local dominance by this species prior to reef demise. Quantitative macroscopic analyses of reef clasts for various taphonomic and diagenetic features did not correlate well with depth/environmental-related trends established in other studies. The mixed coral framestone reef lacks a classical Caribbean reef zonation and is best described as an immature reef and/or a series of fused patch reefs; a pattern that is evident in both cores and reef morphology. This is in stark contrast to the older and deeper outer reef of the SE Florida continental reef tract. Accretion of the outer reef lasted from 10,695–8,000 Cal BP and resulted in a larger and better developed structure that achieved a distinct reef zonation. The discrepancies in overall reef morphology and size as well as the causes of reef terminations remain elusive without further study, yet they likely point to different climatic/environmental conditions during their respective accretion histories
Past Sea Level & Climate Change during a Critical Time Period from Broward County
No full textOf the many profound and contentious consequences Greenhouse warming poses, sea-level rise and large-scale coastal flooding will have the greatest impact on societies globally. There has been considerable debate as to the nature and magnitude of sea-level rise in the near future, but also in the recent past, since the last ice age (last 11,000 years). Specifically, the debate centers on whether changes in past sea-level position occurred in rapid meter-scale jumps or a general smooth rise. Evidence for both exists, however no clear defining trend has been totally resolved. It is therefore of importance to understand how changes in sea level actually occurred in the recent past during a time period of changing climatic conditions in order to gain an understanding of how it may act in the near future. The coral reefs of Broward County Florida are ideally poised to test these ideas since they were actively growing during this critical time period, and were also the basis of the argument for the rapid jumps in sea level. Our recent studies regarding the internal composition and geomorphology of the Broward County reefs revealed that potentially significant altered environmental conditions did occur, but for reasons still largely unknown. We propose to conduct intensive laboratory analyses of previously retrieved reef core coral material in order to constrain the rates of sea-level rise as well as the proxy environmental conditions during this time period of reef growth. This will be accomplished using newly obtained radiocarbon ages and geochemical isotope chronologies produced from coral sample material already in possession. These new analyses yield the potential to provide a better understanding of the environmental conditions that operated during that time, including changes in sea-level as well as significant climatic excursions
Evidence for Catastrophic Sealevel Rise in Broward County
No full textCatastrophic sealevel rise due to ice-sheet collapse is one of the major worries for coastal communities like S-Florida. Using the past as a key to the present and future, we will use the growth history of S-Florida coral reefs to test whether such catastrophic sealevel (~5-6m in ~100 years) rise did indeed occur in Broward County 8000 years (8ka) ago. Such a hypothesis has been put forward based on Broward shelf geomorphology, but preliminary data could neither verify nor falsify it. We propose to drill submerged coral reefs in Broward County to obtain detailed age-with-depth data to reconstruct sealevel rise and find ultimate clarification whether final instability of the Laurentide icesheet caused catastrophic sealevel rise 8ka ago. Such information is valuable for forecast models, since instability of the Greenland ice-sheet has been implicated with theoretical future catastrophic sealevel rise scenarios. For this project, we leverage infrastructure and equipment built from other grant sources. Collaboration with Manchester Metropolitan University and the University of Kiel will allow access to the latest radiometric dating techniques and will provide training for a PhD candidate at NSU. The project will also pay for the data required for this NSU dissertation, and will provide a visiting student from ETH Zurich with an internship opportunity
A Revised Holocene Coral Sea-Level Database from the Florida Reef Tract, USA
No full textThe coral reefs and mangrove habitats of the south Florida region have long been used in sea-level studies for the western Atlantic because of their broad geographic extent and composition of sea-level tracking biota. The data from this region have been used to support several very different Holocene sea-level reconstructions (SLRs) over the years. However, many of these SLRs did not incorporate all available coral-based data, in part because detailed characterizations necessary for inclusion into sea-level databases were lacking. Here, we present an updated database comprised of 303 coral samples from published sources that we extensively characterized for the first time. The data were carefully screened by evaluating and ranking the visual taphonomic characteristics of every dated sample within the database, which resulted in the identification of 134 high-quality coral samples for consideration as suitable sea-level indicators. We show that our database largely agrees with the most recent SLR for south Florida over the last ∼7,000 years; however, the early Holocene remains poorly characterized because there are few high-quality data spanning this period. Suggestions to refine future Holocene SLRs in the region are provided including filling spatial and temporal data gaps of coral samples, particularly from the early Holocene, as well as constructing a more robust peat database to better constrain sea-level variability during the middle to late Holocene. Our database and taphonomic-ranking protocol provide a framework for researchers to evaluate data-selection criteria depending on the robustness of their sea-level models
Accretion History and Stratigraphy of mid-Holocene Coral Reefs from Southeast Florida, USA
No full texthe southeast Florida shelf is a well-studied coral reef region previously used in studies of late Quaternary sea-level, reef geomorphology, and paleoecology in the sub-tropical Atlantic. Situated on the shelf is the southeast Florida continental reef tract; a ~125 km long Holocene fringing/barrier coral reef complex, composed of three shore-parallel linear reefs (‘outer’, ‘middle’, and ‘inner’ reefs) of varying age. Since few detailed stratigraphic descriptions exist, drill cores were extracted to further understand the composition, character, and radiometric ages of reef material in order to reconstruct the accretion history. Sixteen reef cores from the shallow inner reef were collected along and across the reef axes and were combined with lidar bathymetric data for stratigraphic and geomorphologic analyses. Macroscopic and microscopic (petrographic thin sections) examinations of reef clasts were performed to identify coral and reef infauna species compositions, diagenetic facies, and taphonomic features for interpretation of former reef environments/zonation. The southeast Florida continental reef tract was characterized by dynamic reef terminations, backstepping, and re-initiation in response to post-glacial sea-level rise and flooding of topography suitable for reef initiation and growth. Results suggest that the outer reef accreted from ~10.6-8.0 ka cal BP, the middle reef from at least ~5.8-3.7 ka cal BP, and the inner reef from ~7.8-5.5 ka cal BP. The outer reef is the best-developed reef, followed by the inner reef, while the middle reef apparently has relatively little framework buildup. New data from this study and a lack of significant age overlaps confirm that reef backstepping from the outer to the inner reef occurred within a few hundred years after outer reef termination. This is consistent with temporal and spatial scales reported from backstepped reefs in St. Croix and Puerto Rico. The cause of the backstep is still unknown however some studies suggest the mid-Holocene (~8-5 ka) was punctuated by a transition to a more moist and warm climate and/or a potentially rapid sea-level rise. The color and texture of cements support increased freshwater input as a likely agent of reef demise. We also observed that the once-dominant Caribbean reef builder Acropora palmata was mostly present throughout the early and mid-Holocene but absent thereafter. Reef geomorphology was strongly determined by the length of presence of this species, as the thickness, size, and shape of the three linear reefs clearly reflect its declining importance during the Holocene in Florida
Shifting baselines of coral‐reef species composition from the Late Pleistocene to the present in the Florida Keys
No full textAbstract The ongoing global‐scale reassembly of modern coral reefs is unprecedented compared with the observed stability of most late Quaternary reef assemblages. One notable exception is the marine isotope stage (MIS) 5e (ca 130–116 thousand years ago [ka]) reefs in the Florida Keys, where the ubiquitous shallow‐water coral, Acropora palmata, was near absent. Little is known, however, about reefs that grew during MIS5d–a (ca 116–74 ka), between MIS5e and the Holocene. It is therefore unclear whether Florida's unique MIS5e coral assemblages represent a geologically brief anomaly or a more persistent departure from the western Atlantic coral‐reef archetype. We addressed that question by reconstructing the composition of MIS5d–a reefs within 29 coral‐reef cores collected throughout the Florida Keys. We then compared the relative composition of corals during MIS5d–a to existing datasets from MIS5e, Holocene and modern (1996 and 2022) reefs to evaluate how far today's reef assemblages have diverged from geological baselines. We show that although the proportion of reef frameworks built by corals was remarkably consistent (ca 38%), species composition changed significantly through time. Acropora palmata was rare throughout MIS5, which we hypothesise was due to greater cold‐temperature stress in Florida's subtropical reefs compared with the more climatically stable tropics. In contrast, the massive reef‐building coral, Orbicella spp., was regionally dominant throughout the late Quaternary, but has become increasingly rare on modern reefs. By 2022, reefs in the Florida Keys were characterised by a truly novel coral assemblage dominated by Porites astreoides and Siderastrea siderea. In many ways, Florida's reefs defy the concept of a natural baseline; instead, their most persistent characteristic since the Late Pleistocene is their uniqueness. Yet, as reefs are increasingly subjected to unprecedented levels of environmental change, the exceptions to what was normal in the past could, paradoxically, provide the best geological analogues for the future
A 3,000‐Year Lag between the Geological and Ecological Shutdown of Florida\u27s Coral Reefs
No full textThe 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\u27s 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\u27s 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
