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Rapid sea-level rise and reef back-stepping at the close of the last interglacial highstand
Widespread evidence of a +4–6-m sea-level highstand during the last interglacial period (Marine Isotope Stage 5e) has led to warnings that modern ice sheets will deteriorate owing to global warming and initiate a rise of similar magnitude by AD 2100. The rate of this projected rise is based on ice-sheet melting simulations and downplays discoveries of more rapid ice loss. Knowing the rate at which sea level reached its highstand during the last interglacial period is fundamental in assessing if such rapid ice-loss processes could lead to future catastrophic sea-level rise. The best direct record of sea level during this highstand comes from well-dated fossil reefs in stable areas. However, this record lacks both reef-crest development up to the full highstand elevation, as inferred from widespread intertidal indicators at +6m, and a detailed chronology, owing to the difficulty of replicating U-series ages on sub-millennial timescales. Here we present a complete reef-crest sequence for the last interglacial highstand and its U-series chronology from the stable northeast Yucatan peninsula, Mexico. We find that reef development during the highstand was punctuated by reef-crest demise at +3m and back-stepping to +6m. The abrupt demise of the lower-reef crest, but continuous accretion between the lower-lagoonal unit and the upper-reef crest, allows us to infer that this back-stepping occurred on an ecological timescale and was triggered by a 2–3-m jump in sea level. Using strictly reliable 230Th ages of corals from the upper-reef crest, and improved stratigraphic screening of coral ages from other stable sites, we constrain this jump to have occurred 121 kyr ago and conclude that it supports an episode of ice-sheet instability during the terminal phase of the last interglacial period.
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Taphonomic differentiation of Acropora palmata facies in cores from Campeche-Bank Reefs, Gulf of México.
A common assumption in the geological analysis of modern reefs is that coral community zonation seen on the surface should also be found in cores from the reef interior. Such assumptions not only underestimate the impact of tropical storms on reef facies development, but have been difficult to test because of restrictions imposed by narrow-diameter cores and poor recovery.
That assumption is tested here using large-diameter cores recovered from a range of common zones across three Campeche Bank reefs. It is found that cores from the reef-front, crest, flat and rubble-cay zones are similar in texture and coral composition, making it impossible to recognize coral assemblages that reflect the surface zonation. Taphonomic signatures imparted by variations in encrustation, bioerosion and cementation, however, produce distinct facies and delineate a clear depth zonation. Cores from the reef-front zone (2–10 m depth) are characterized by sections of Acropora palmata cobble gravel interspersed with sections of in-place (but truncated) A. palmata stumps. Upper surfaces of truncated colonies are intensely bioeroded by traces of Entobia isp. and Gastrochaenolites isp. and encrusted by mm-thick crustose corallines before colony regeneration and, therefore, indicate punctuated growth resulting from a hurricane-induced cycle of destruction and regeneration. Cores from the reef crest/flat (0–2 m depth) are also characterized by sections of hurricane-derived A. palmata cobble-gravels as well as in-place A. palmata colonies. In contrast to the reef front, however, these cobble gravels are encrusted by cm-thick crusts of intergrown coralline algae, low-relief Homotrema and vermetids, bored by traces of Entobia isp. and Trypanites isp. and coated by a dense, peloidal, micrite cement. Cores from the inter- to supratidal rubble-cay zone (+0–5 m) are only composed of A. palmata cobble gravels and, although clasts show evidence of subtidal encrustation and bioerosion, these always represent processes that occurred before deposition on the cay. Instead, these gravels are distinguished on the basis of their limited bioerosion and marine cements, which exhibit fabrics formed in the intertidal zone. These results confirm that hurricanes have a major influence on facies development in Campeche Bank reefs. Instead of reflecting the surface coral zonation, each facies records a distinctive, depth-related set of taphonomic processes, which reflect colonization, alteration and stabilization following the production of new substrates by hurricanes.
Asphalt volcanism and chemosynthetic life, Campeche Knolls, Gulf of Mexico
In the Campeche Knolls, in the southern Gulf of Mexico, lava-like flows of solidified asphalt cover more than 1 square kilometer of the rim of a dissected salt dome at
a depth of 3000 meters below sea level. Chemosynthetic tubeworms and bivalves colonize the sea floor near the asphalt, which chilled and contracted after discharge.
The site also includes oil seeps, gas hydrate deposits, locally anoxic sediments, and slabs of authigenic carbonate. Asphalt volcanism creates a habitat for chemosynthetic life that may be widespread at great depth in the Gulf of Mexico.
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Are catch-up reefs and artifact of coring?
Drill cores through modern coral reefs commonly show a time lag in reef initiation followed by a phase of rapid accretion to sea level from submerged foundations – the so-called ‘catch-up response’. But because of the difficulty of drilling in these environments, core distribution is usually restricted to accessible areas that may not fully represent reef history, especially if the reef initiated in patches or developed with a prograde or retrograde geometry. As a consequence, core data have the potential to give a misleading impression of reef development, particularly with respect to the timing of initiation and response to sea-level rise. Here, we use computer models to simulate keep-up reef development and, from them, quantify variations in the timing of reef initiation and accretion rate using mock cores taken through the completed simulations. The results demonstrate that cores consistently underestimate the timing of reef initiation and overestimate the reef accretion rate so that, statistically, a core through a keep-up reef will most likely produce a catch-up pattern – an initiation lag followed by a phase of rapid accretion to sea level. This implies that catch-up signatures may be an artefact of coring and that keep-up reefs are significantly more common than previous core studies claim.
Discovery of a submerged relic reef and shoreline off Grand Cayman: further support for an early Holocene jump in sea level.
Drilling close to the base of a submerged sea-cliff on the terraced eastern shelf of Grand Cayman has revealed a relic Acropora palmata reef at a depth of 21 m below msl. Ten cores from its crest are principally composed of cobble-sized clasts of A. palmata set in a matrix of cemented skeletal grainstone. The clasts have a distinctive succession of encrusters that indicate rapid burial: a photophilic association of crustose coralline algae, foraminifera and vermetid gastropods superimposed by a cryptic association of sclerosponges, foraminifera and serpulids. In addition to rapid burial, U–Th TIMS dating of coral clasts within 1m of the relic-reef surface indicates minor temporal mixing with ages between 8.9 and 8.1 ka. Such mixing and rapid burial is consistent with a hurricane deposit and is identical to deposits found on the crests of modern reefs. In relation to its age, the preservation of an 18.5-m intertidal notch cut into the submerged sea-cliff on the western shelf of Grand Cayman implies that the crest of the relic reef has been lowered 1.5–2 m by marine abrasion/bioerosion at a rate of 0.25 mm/yr. Reconstructing this eroded section using average Holocene accretion rates indicates that the reef likely ceased accreting at 7.6 ka at a depth of 19 m. Comparing these data with other relic reefs in the Caribbean indicates that the relic reef on Grand Cayman died within 160 years of relic reefs on Barbados, St. Croix, St. Thomas and north Florida. This narrow interval of reef demise also coincides with the time when modern reefs were establishing themselves some 4–9 m higher upslope—a fact that can only be resolved by invoking a rapid 6-m jump in sea level 7.5 ka ago. Such a jump would also account for two unexplained events around this time: the restricted interval of global delta initiation and the catastrophic flooding of the glacially lowered Black Sea.
Multi-stage reef development on Barbados during the Last Interglaciation.
By mapping the vertical and lateral distribution of reefal facies on the west and south coast of Barbados we have produced a revised model of reef development for the Last Interglaciation. We find that reef architecture around Barbados has significant complexity including evidence for wave exposure-related variations in reef geometry and at least 3 stages of reef development that were controlled by variations in sea level. During the main stage of development, an Acropora palmata-dominated reef-crest aggraded 22m in response to a minimum sea-level rise of 20m. During stage 2, a sea cave was cut 3-4m above the fossil reef-crest, possibly indicating that reef growth was terminated before sea level reached the highstand. Similar sequences elsewhere in the Caribbean indicate that this early reef demise may not be a local phenomenon: several reefs apparently stopped growing between +2 and +4m and only in sheltered areas did they reach the highstand at +6m, as recorded by intertidal notches. This pattern of reef demise has previously been related to rapid sea-level rise at the end of the interglacial, but stratigraphic data are equivocal. The final stage of reef development on Barbados occurred when sea level began to fall. This fall was rapid, leaving a thin but widespread veneer of reef-crest deposits over the proximal reef-front, and discontinuous intertidal and shallow subtidal deposits capping the distal reef-front. Although further dating is required to diferentiate these three stages, our only reliable U/Th TIMS date indicates that almost 50% of the exposed reef had accreted by 129 ka, giving an estimate of 15 ka for the main aggradational stage. Furthermore, reports of relict reef-crests buried beneath these exposed deposits indicate that our revised model is incomplete and that earlier stages of reef growth occurred during the Last Interglaciation. These earlier stages imply that sea-level was at an interglacial level for as long as 20ka supporting the Devils Hole record of interglacial duration. Unfortunately, these estimates could not be verified directly because most of our U/Th data show major stratigraphic age reversals attributed to diagenesis. This pattern is also evident in all other well-dated reefal units in the Caribbean and leads us to conclude that only diagenetically screened, precise, stratigraphically consistent coral dates can be used to directly estimate the duration of the Last Interglaciation.
Hurricane control on shelf-edge-reef architecture around Grand Cayman
Rimming the outer shelf of Grand Cayman is a submerged, 87-km long shelf-edge reef that rises to within 12 m of mean sea level. It consists of an array of coral-armoured buttresses aligned perpendicular to shore and separated by steep-sided sediment-floored canyons. Individual buttresses have a diverse coral-dominated biota and consist of 3 architectural elements: a shield-like front wall colonized by platy corals, a dome-shaped crown colonized by head corals, and a shoreward-projecting spur covered by varying amounts of branching coral. Buttresses are commonly fronted by coral pinnacles that, in some areas, have amalgamated with buttress walls to produce pinnacle-and-arch structures.
As margin orientation changes, shelf-edge-reef architecture shows systematic variations that are consistent with changes in fetch and height of hurricane waves. Along margins exposed to fully-developed storm waves, shelf-edge-reef buttresses are deep, have large amplitudes, and are dominated by robust head corals. These characteristics are consistent with hurricane-induced pruning of branching corals and the flushing of significant quantities of sand from buttress canyons by return flows. Along margins impacted by fetch-limited storm waves, reef buttresses are shallower, have intermediate-amplitudes, and have a significantly higher proportion of branching corals. These characteristics are consistent with less coral pruning and sand flushing by weaker hurricane waves. Along margins fully protected from storm waves, the buttresses-canyon architecture of the shelf-edge reef breaks down producing a series of shallow, undulating, branching-coral-dominated ridges that merge laterally into an unbroken belt of coral. These characteristics correspond with negligible amounts of pruning and flushing during hurricanes.
In addition to differences between margins, local intra-marginal changes in shelf-edge reef architecture are consistent with changes in the angle of hurricane-wave approach. Open sections of the shelf-edge reef, which face directly into storm waves, are pruned of branching corals and the fragments swept back onto the shelf producing extensive spurs. By contrast, on more sheltered, obliquely-oriented sections, storm-waves sweep debris along and off shelf producing little or no spur development. Instead, the debris shed seawards accumulates in front of the buttress walls and initiates the development of coral pinnacles.
Over time, repeated buttress pruning and canyon flushing during hurricanes not only controls reef architecture but may also influence accretion patterns. Vertical accretion is limited by the effective depth of storm-wave fragmentation. Once this hurricane-accretion threshold is reached the reef moves into a shedding phase and accretes laterally via pinnacle growth, amalgamation, and infilling. Consequently, the reef steps out over its own debris in a kind of balancing act between lateral growth and slope failure—a pattern widely recognized in ancient reefs.
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Anatomy of a fringing reef around Grand Cayman: Storm rubble, not coral framework
Our fair-weather perception of modern reefs has led to the implicit assumption that their development is controlled by processes that govern the siting of in-place coral growth. Yet more ephemeral processes, such as storms and hurricanes, assume much greater importance over longer time scales because few reefs escape their influence. To discover the importance of storms on reef development, we analyze the zonation, anatomy, and architecture of a fringing-reef complex around Grand Cayman. We find that the surface zonation of in-place corals is merely a facade and the reef core is in fact composed of meter-thick layers of coral-cobble rudstone capped by crusts of coralline algae. The large size and abraded condition of the rudstone clasts shows these layers are not the product of fair-weather processes but the result of destruction and deposition during hurricanes. As hurricane waves cross coral-mantled zones of the inner shelf, they destroy live coral stands and deposit the clasts as a rubble layer covering the entire reef complex. Between storms, this rubble foundation is stabilized by coralline-algal crusts and recolonized by rapidly growing corals, leading eventually to full reef regeneration before the next hurricane. This cyclic pattern of destruction and regeneration consequently produces a fringing-reef complex with a core composed of hurricane-generated rubble—not coral framework as previously assumed.
In addition to explaining reef anatomy, hurricane control also explains the variation in reef architecture along shelf, uniform reef location across shelf, and reef absence along certain shelf sections. As hurricane waves cross a mid-shelf scarp, they start to break and destroy coral growth over most of the inner shelf. Coral rubble generated by these waves is deposited 350 (± 50) m from the mid-shelf scarp on margins exposed to the largest waves, but only 250 (± 50) m on semiprotected margins that experience smaller, fetch-limited waves. In areas where the width of the inner shelf is < 250 m, hurricane waves throw rubble ashore and a fringing reef does not develop. During sea-level rise, this influence of shelf width on rubble deposition controls the timing of reef initiation, and that in turn controls reef architecture. Reefs initiate first on low-gradient coasts with wide shelves, and gradually extend around higher-gradient coasts as sea level rises and shelf width increases. Thus, older reefs are located farther offshore, front deeper lagoons, and have thicker and narrower profiles than younger reefs.
Reef drowning during the last deglaciation: Evidence for catastrophic sea-level rise and ice-sheet collapse
Elevations and ages of drowned Acropora palmata reefs from the Caribbean-Atlantic region document three catastrophic, metre-scale sea-level-rise events during the last deglaciation. These catastrophic rises were synchronous with (1) collapse of the Laurentide and Antarctic ice sheets, (2) dramatic reorganization of ocean-atmosphere circulation and, (3) releases of huge volumes of sub- and proglacial meltwater. This correlation suggests that release of stored meltwater periodically destabilized ice sheets, causing them to collapse and send huge fleets of icebergs into the Atlantic. Massive inputs of ice not only produced catastrophic sea-level rise, drowning reefs and destabilizing other ice sheets, but also rapidly reduced the elevation of the Laurentide ice sheet, flipping atmospheric circulation patterns and forcing warm equatorial waters into the frigid North Atlantic. Such dramatic evidence of catastrophic climate and sea-level change during deglaciation has potentially disastrous implications for the future, especially as the stability of remaining ice sheets—such as west Antarctica—is in question
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Marine-planation terraces on the shelf around Grand Cayman: A result of stepped Holocene sea-level rise
The shelf around Grand Cayman consists of two seaward-sloping terraces separated by a mid-shelf scarp. Except along the exposed-windward margin where coral growth is dominant, the upper terrace (0-10 m bsl) largely consists of a barren rocky pavement traversed by erosional furrows. Exposure-related trends in the morphology and distribution of these erosional features, and the lack of coral growth, demonstrates that the terrace is the result of contemporary erosion during seasonal storms. The upper terrace is terminated by a mid-shelf scarp (10-20 m bsl) that, in most areas, is partially to completely buried by modern carbonate deposits. Along narrow sections of the leeward shelf however, the scarp is commonly exposed and displays an erosional intertidal notch at -18.5 m. The lower terrace (12-40 m bsl) extends from the mid-shelf scarp to the shelf edge. Its surface is a modern reef-and-sediment wedge that thickens toward the shelf edge, reaching up to 40 m in thickness. These deposits are underlain by a seaward-sloping bedrock terrace (20-40 m bsl). This buried terrace and the mid-shelf scarp, which are geomorphic equivalents of the upper terrace and coastal cliff, represent an earlier episode of marine planation when sea-level was stabilized at a lower position.
The contemporary erosional features of the upper-shelf terrace, and the presence of identical terraces around recently uplifted islands, demonstrates that the terraces on Grand Cayman were sculptured by marine erosion during the last deglacial sea-level rise. The lower terrace and the mid-shelf scarp were eroded during a slow-rise episode from 11-7 ka and were subsequently drowned by an extremely rapid, 5 m rise-event at ~7 ka. Following this catastrophic event, which drowned fast-growing Acropora reefs in other areas of the Caribbean, sea-level stabilized and rose slowly to its present position, producing the upper terrace. This pronounced stepped pattern in Holocene sea-level rise remains to be confirmed from outside the Caribbean-Atlantic reef province but is consistent with the stepped nature of pre-Holocene sea-level curves.
The presence of seaward sloping terraces on many shelves around the world suggests that erosional terrace cutting is a common phenomenon during sea-level rise. In contrast, terraces in areas that have undergone relative sea-level fall are constructional in origin, being produced entirely by reef accretion. This suggests that there is a genetic relationship between the sea-level cycle and terrace type, with erosional terraces forming during rise and constructional terraces during fall.
Reef demise and back-stepping during the last interglacial, northeast Yucatan.
The elevation of reefs and coastal deposits during the last Interglaciation (MIS-5e) indicates that sea level reached a highstand of as much as 6 m above the
present, but it is uncertain how rapidly this level was
attained and how it impacted reef development. To investigate this problem, I made a detailed sediment-ological analysis of a well-dated reef from the northeast coast of the stable Yucatan Peninsula. Two linear reef tracts were delineated which are offset and at different elevations. The lower reef tract crops out along northern shore for 575 m and extends from below present mean sea level to +3 m. The reef crest facies consists of large Acropora palmata colonies dispersed within a coral boulder-gravel and is flanked by an A. cervicornis-dominated reef-front and a large area of lagoonal framework formed by coalesced patches of A. cervicornis and Montastraea spp. Constituents in the upper metre of the lower tract are heavily encrusted by a cap of crustose corallines and, in places, are levelled by a discontinuous marine-erosion surface. The upper reef tract crops out ~150 m inland up to an elevation of +5.8 m and parallels the southern section of shore for ~400 m. It also consist of an A. palmata-dominated crest facies flanked by reef-front, back-reef and lagoonal frameworks. In this case, however, lagoonal frameworks are dominated by a sediment-tolerant assemblage of branching coralline algae. Also different is the lack of encrustation by corallines, and the infiltration of upper tract facies by beach-derived shell-gravels from regressive shoreface deposits above. These results indicate that the lower reef tract and lagoonal patch-reefs formed at a sea level of +3 m. Final capping by crustose corallines and discontinuous marine erosion indicates that the lower tract was terminated by the complete demise of corals on the crest but only patchy demise in the lagoon. Areas of continuous framework accretion between the lagoonal patch reefs and the upper reef-tract, however, require that the demise of this reef was ecologically synchronous with initiation of the upper reef-tract, which had back-stepped 100 m into the lagoon. In this new position, the upper tract developed a reef crest that corresponded to a final sea-level position of +6 m. Reef flat development at +5 m and large in-place colonies of A. palmata at the base of the crest unit indicate, however, that sea level must have risen rapidly from +3 to more than +5 m to accommodate back-stepping. This sea-level jump created a higher energy wave field that mobilized back-reef and lagoonal sediments, and the resulting high sediment flux eroded lagoonal framework and prevented the recovery of the submerged lower reef crest. So this single jump in sea level was responsible not only for reef demise and back-stepping but also for marine erosion and suppression of subsequent reef development—features that elsewhere have been used to support multiple sea-level excursions during the last interglacial.
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