Glacial Terminations

Meltwater Pulses: Encyclopedia of Modern Coral Reefs

by Paul Blanchon

Blanchon, P. (2011) Meltwater Pulses. In: Hopley, D. (Ed), Encyclopedia of Modern Coral Reefs: Structure, form and process. Springer-Verlag Earth Science Series, p. 683-690. DOI 10.1007/978-90-481-2639-2

A meltwater pulse is an acceleration in sea-level rise which results from outbursts of pro- or subglacial meltwater... more 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: Encyclopedia of Modern Coral Reefs

by Paul Blanchon

Blanchon, P. (2011) Back-Stepping. In: Hopley, D. (Ed), Encyclopedia of Modern Coral Reefs: Structure, form and process. Springer-Verlag Earth Science Series, p. 77-84. DOI 10.1007/978-90-481-2639-2

Reef back-stepping has been widely recognized from the geological record where it is almost universally interpreted as... more 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)

Variation in deglacial coralgal assemblages and their paleoenvironmental significance: IODP Expedition 310, "Tahiti Sea Level"

by Elizabeth Abbey

Abbey, E., Webster, J.M., Braga, J.C., Sugihara, K., Wallace, C.C., Iryu, Y., Potts, D., Done, T., Camoin, G., Seard, C., In press. Variation in deglacial coralgal assemblages and their paleoenvironmental significance: IODP Expedition 310, "Tahiti Sea Level". Global and Planetary Change.

Keywords: Submerged reefs, terraces, backstepping, sea level rise, deglacial, coralgal assemblages, reef drowning

Fossil reefs are valuable recorders of paleoenvironmental changes during the last deglaciation, and detailed... more 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.

Reef drowning during the last deglaciation: Evidence for catastrophic sea-level rise and ice-sheet collapse

by Paul Blanchon

Elevations and ages of drowned Acropora palmata reefs from the Caribbean-Atlantic region document three catastrophic,... more 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

Multi-stage reef development on Barbados during the Last Interglaciation.

by Paul Blanchon

By mapping the vertical and lateral distribution of reefal facies on the west and south coast of Barbados we have... more 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.

Discovery of a submerged relic reef and shoreline off Grand Cayman: further support for an early Holocene jump in sea level.

by Paul Blanchon

Drilling close to the base of a submerged sea-cliff on the terraced eastern shelf of Grand Cayman has revealed a relic... more 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.