Melt-water pulses

A meltwater pulse is an acceleration in sea-level rise which results from outbursts of pro- or subglacial meltwater and/or surging of ice-streams into the ocean during ice- sheet disintegration. Radiometric ages of coral-reef drowning and back-stepping indicate that rates of sea- level rise during these meltwater pulses were at least 35 mm/yr and may have been as much as 60 mm/yr, and that these rises persisted for 300–500 years. (...Continues)

Reef back-stepping has been widely recognized from the geological record where it is almost universally interpreted as a result of rapid rise in relative sea level. Yet ironically, there has been considerable argument over its existence and significance in late Pleistocene and Holocene reefs. (...Continues)

During the LIG, elevated temperatures, together with the smaller latitudinal temperature gradient, led to more widespread reef development than present, and reefs not only extended further into coastal lowlands but significantly expanded their latitudinal range. The elevation and age of fossil reef-crests from tectonically stable terrains in several areas of the world indicate that the sea level was 3 m higher than the present by 125 ka and peaked at 6 m sometime thereafter (...Continues).

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.

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

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.

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 sedimentological 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.

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.

Reviewer Comment:
“This is the most comprehensive documentation of the last Interglacial 'double peak' available—one that is based upon the two necessary and sufficient pillars of age dating and paleoecology. This paper should finally lay to rest any notion that the last Interglacial was NOT characterized by a rapid sea-level event superimposed upon a normal Milankovitch cycle.”

Fossil reefs are valuable recorders of paleoenvironmental changes during the last deglaciation, and detailed characterizations of coralgal assemblages can improve understanding of the behavior and impacts of sea-level rise. Drilling in 2005 by the Integrated Ocean Drilling Program (IODP) Expedition 310 explored submerged offshore reefs from three locations around Tahiti, French Polynesia and provides the first look at island-wide variability of coralgal assemblages during deglacial sea-level rise. We present the first detailed examination of coral and coralline algal taxonomy and morphology from two sites on Tahiti (offshore Tiarei and offshore Maraa). Sixteen cores ranging in depth from 122 m to 45 m below sea-level represent reef growth from 16 ka to ca. 8 ka (Camoin, G.F., Iryu, Y., McInroy, D.B. and the IODP Expedition 310 Scientists, 2007. IODP Expedition 310 reconstructs sea level, climatic, and environmental changes in the South Pacific during the last deglaciation. Scientific Drilling, 5: 4-12). Twenty-six coral species, twelve coral genera and twenty-eight coralline algal species were identified from 565 m of core and over 400 thin sections. Based on these data, and in comparison with modern and fossil analogs, seven coral and four algal assemblages have been identified in the deglacial sequences in Tahiti, representing a range of environments from less than 10 m to greater than 20-30 m water depth. Deglacial reef initiation varied at sites based on the available substrate, and early colonizers suggest water conditions at all sites were unfavorable to sensitive corals, such as Acropora, prior to ca. 12.5 ka. Mainly shallowwater (b 10-15 m) corals and coralline algal assemblages developed continuously throughout both sites from 16 ka to ca. 8 ka, suggesting that coralgal assemblage variation is more influenced by factors such as turbidity and water chemistry than sea-level rise alone.

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 subglacial 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 in West Antarctica-is in question.

Fossil reefs are valuable recorders of paleoenvironmental changes during the last deglaciation, and detailed characterizations of coralgal assemblages can improve understanding of the behavior and impacts of sea-level rise. Drilling in 2005 by the Integrated Ocean Drilling Program (IODP) Expedition 310 explored submerged offshore reefs from three locations around Tahiti, French Polynesia and provides the first look at island-wide variability of coralgal assemblages during deglacial sea-level rise. We present the first detailed examination of coral and coralline algal taxonomy and morphology from two sites on Tahiti (offshore Tiarei and offshore Maraa). Sixteen cores ranging in depth from 122 m to 45 m below sea-level represent reef growth from 16 ka to ca. 8 ka (Camoin, G.F., Iryu, Y., McInroy, D.B. and the IODP Expedition 310 Scientists, 2007. IODP Expedition 310 reconstructs sea level, climatic, and environmental changes in the South Pacific during the last deglaciation. Scientific Drilling, 5: 4-12). Twenty-six coral species, twelve coral genera and twenty-eight coralline algal species were identified from 565 m of core and over 400 thin sections. Based on these data, and in comparison with modern and fossil analogs, seven coral and four algal assemblages have been identified in the deglacial sequences in Tahiti, representing a range of environments from less than 10 m to greater than 20-30 m water depth. Deglacial reef initiation varied at sites based on the available substrate, and early colonizers suggest water conditions at all sites were unfavorable to sensitive corals, such as Acropora, prior to ca. 12.5 ka. Mainly shallow-water (< 10-15 m) corals and coralline algal assemblages developed continuously throughout both sites from 16 ka to ca. 8 ka, suggesting coralgal assemblage variation is more influenced by factors such as turbidity and water chemistry than sea-level rise alone.

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...

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 subglacial 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 in West Antarctica-is in question.

Fossil reefs are valuable recorders of paleoenvironmental changes during the last deglaciation, and detailed characterizations of coralgal assemblages can improve understanding of the behavior and impacts of sea-level rise. Drilling in 2005 by the Integrated Ocean Drilling Program (IODP) Expedition 310 explored submerged offshore reefs from three locations around Tahiti, French Polynesia and provides the first look at island-wide variability of coralgal assemblages during deglacial sea-level rise. We present the first detailed examination of coral and coralline algal taxonomy and morphology from two sites on Tahiti (offshore Tiarei and offshore Maraa). Sixteen cores ranging in depth from 122 m to 45 m below sea-level represent reef growth from 16 ka to ca. 8 ka (Camoin, G.F., Iryu, Y., McInroy, D.B. and the IODP Expedition 310 Scientists, 2007. IODP Expedition 310 reconstructs sea level, climatic, and environmental changes in the South Pacific during the last deglaciation. Scientific Drilling, 5: 4-12). Twenty-six coral species, twelve coral genera and twenty-eight coralline algal species were identified from 565 m of core and over 400 thin sections. Based on these data, and in comparison with modern and fossil analogs, seven coral and four algal assemblages have been identified in the deglacial sequences in Tahiti, representing a range of environments from less than 10 m to greater than 20-30 m water depth. Deglacial reef initiation varied at sites based on the available substrate, and early colonizers suggest water conditions at all sites were unfavorable to sensitive corals, such as Acropora, prior to ca. 12.5 ka. Mainly shallowwater (b 10-15 m) corals and coralline algal assemblages developed continuously throughout both sites from 16 ka to ca. 8 ka, suggesting that coralgal assemblage variation is more influenced by factors such as turbidity and water chemistry than sea-level rise alone.

Fossil reefs are valuable recorders of paleoenvironmental changes during the last deglaciation, and detailed characterizations of coralgal assemblages can improve understanding of the behavior and impacts of sea-level rise. Drilling in 2005 by the Integrated Ocean Drilling Program (IODP) Expedition 310 explored submerged offshore reefs from three locations around Tahiti, French Polynesia and provides the first look at island-wide variability of coralgal assemblages during deglacial sea-level rise. We present the first detailed examination of coral and coralline algal taxonomy and morphology from two sites on Tahiti (offshore Tiarei and offshore Maraa). Sixteen cores ranging in depth from 122 m to 45 m below sea-level represent reef growth from 16 ka to ca. 8 ka (Camoin, G.F., Iryu, Y., McInroy, D.B. and the IODP Expedition 310 Scientists, 2007. IODP Expedition 310 reconstructs sea level, climatic, and environmental changes in the South Pacific during the last deglaciation. Scientific Drilling, 5: 4-12). Twenty-six coral species, twelve coral genera and twenty-eight coralline algal species were identified from 565 m of core and over 400 thin sections. Based on these data, and in comparison with modern and fossil analogs, seven coral and four algal assemblages have been identified in the deglacial sequences in Tahiti, representing a range of environments from less than 10 m to greater than 20-30 m water depth. Deglacial reef initiation varied at sites based on the available substrate, and early colonizers suggest water conditions at all sites were unfavorable to sensitive corals, such as Acropora, prior to ca. 12.5 ka. Mainly shallowwater (b 10-15 m) corals and coralline algal assemblages developed continuously throughout both sites from 16 ka to ca. 8 ka, suggesting that coralgal assemblage variation is more influenced by factors such as turbidity and water chemistry than sea-level rise alone.

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.