Meltwater pulse 1B (MWP1b) is the name used by Quaternary geologists, paleoclimatologists, and oceanographers for a period of either rapid or just accelerated post-glacial sea level rise that some hypothesize to have occurred between 11,500 and 11,200 years ago at the beginning of the Holocene and after the end of the Younger Dryas. Meltwater pulse 1B is also known as catastrophic rise event 2 (CRE2) in the Caribbean Sea.
Other named, postglacial meltwater pulses are known most commonly as meltwater pulse 1A0 (meltwater pulse19ka), meltwater pulse 1A, meltwater pulse 1C, meltwater pulse 1D, and meltwater pulse 2. It and these other periods of proposed rapid sea level rise are known as meltwater pulses because the inferred cause of them was the rapid release of meltwater into the oceans from the collapse of continental ice sheets.Cronin, T.M. (2012) Rapid sea-level rise. Quaternary Science Reviews. 56:11-30.
Sea level There is considerable unresolved disagreement over the significance, timing, magnitude, and even existence of meltwater pulse 1B. It was first recognized by Richard G. Fairbanks in his coral reef studies in Barbados. From the analysis of data from cores of coral reefs surrounding Barbados, he concluded that during meltwater pulse 1B, sea level rose in about 500 years about 11,300 years ago.Fairbanks, R.G . (1989) "A 17 000 year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation". Nature. 342:637–642.
However, in 1996 and 2010, Bard and others published detailed analysis of data from cores from coral reefs surrounding Tahiti. They concluded that meltwater pulse 1B was, at best, just an acceleration of sea level rise at about 11,300 years ago and it was, at worst, not statistically different from a constant rate sea level rise between 11,500 and 10,200 years ago. They argued that meltwater pulse 1B was certainly not an abrupt jump in sea level, which they would consider to be a meltwater pulse. They argue that the rise in sea level estimated by Fairbanks from cores is an artifact created by differential tectonic uplift between different sides of a tectonic structure lying between the two Barbados cores used to identify meltwater pulse 1B and calculate its magnitude.Bard, E., B. Hamelin, M. Arnold, L. Montaggioni, G. Cabioch, and others (1996) Deglacial sea-level record from Tahiti corals and the timing of global meltwater discharge. Nature. 382: 241-244.Bard, E., B. Hamelin, and D. Delanghe-Sabatier (2010) Deglacial Meltwater Pulse 1B and Younger Dryas Sea Levels Revisited with Boreholes at Tahiti. Science. 327:1235-1237.
Other differing estimates about the magnitude of meltwater pulse 1B have been published. In 2010, Standford and others found it to be "robustly expressed" as a multi-millennial interval of enhanced rates of sea-level rise between 11,500 and 8,800 years ago with peak rates of rise of up to 25 mm/yr.Stanford, J.D., R. Hemingway, E.J. Rohling, P.G. Challenor, M. Medina-Elizalde, and A.J. Lester (2011) Sea-level probability for the last deglaciation: a statistical analysis of far-field records. Global and Planetary Change. 79:193–203. In 2004, Liu and Milliman reexamined the original data from Barbados and Tahiti and reconsidered the mechanics and sedimentology of reef drowning by sea level rise. They concluded that meltwater pulse 1B occurred between 11,500 and 11,200 years ago, a 300-year interval, during which sea level rose from to , giving a mean annual rate of around 40mm/yrLiu, J.P., and J.D. Milliman (2004) Reconsidering Melt-water Pulses lA and lB: Global Impacts of Rapid Sea- level Rise. Journal of Ocean University of China. 3(2):183–190. Other studies have revised the estimated magnitude of meltwater pulse 1B downward to between and less than .Blanchon, P., and J. Shaw (1995) Reef drowning during the last deglaciation: Evidence for catastrophic sea-level rise and ice-sheet collapse. Geology. 23(1):4-8.Whitehouse, P.L., and S.L. Bradley (2013) Eustatic sea-level changes since the Last Glacial Maximum. In: Elias, S.A., ed., pp. 439–451. Encyclopedia of Quaternary Sciences, 2nd Edition. Elsevier (Amsterdam).
Source(s) of meltwater pulse 1B Given the disagreement over its timing, magnitude, and even existence, it has been very difficult to constrain the source of meltwater pulse 1B. In his modeling of global glacial isostatic adjustment, Peltier assumed that the predominant source for MWP-1B was the Antarctic Ice Sheet. However, no justification for this assumption is provided in his papers.Peltier, W.R. (1994), Ice age paleotopography. Science. 265:195–201.Peltier, W.R. (2004), Global glacial isostasy and the surface of the ice-age Earth: The ICE-5 G (VM2) model and GRACE. Annual Review Earth and Planetary Science. 32:111–149. In addition, Leventer and others argue that the timing of deglaciation in eastern Antarctica roughly coincides with the onset of meltwater pulse 1B and the Antarctic Ice Sheet is a likely source.Leventer, A., E. Domack, R. Dunbar, J. Pike, C. Stickley, E. Maddison, S. Brachfeld, P. Manley, and C. McClennen (2006) Marine sediment record from the East Antarctic margin reveals dynamics of ice sheet recession. GSA Today. 16(12):4-10. Finally, McKay and others suggested that recession of the West Antarctic Ice Sheet may have supplied the meltwater needed to the start meltwater pulse 1B.McKay, R.M., G.B. Dunbar, T.R. Naish, P.J. Barrett, L. Carter, and M. Harper (2008) Retreat history of the Ross ice sheet (Shelf) since the last glacial maximum from deep-basin sediment cores around Ross island. Palaeogeography, Palaeoclimatology, Palaeoecology. 260:245-261.
However, later studies involving the surface exposure dating of glacial erratics, nunataks, and other formerly glaciated exposures using cosmogenic dating contradicted the above arguments and assumptions.Carlson, A.E., and P.U. Clark (2012) Ice sheet sources of sea level rise and freshwater discharge during the last deglaciation. Reviews of Geophysics. 50(4):1944-9208. These studies tentatively concluded that the actual amount of thinning of the East Antarctic Ice Sheet is too small, , and likely too gradual and too late to have contributed any significant amount of meltwater to meltwater pulse 1B. They also concluded that the ice sheet retreat and thinning accelerated for the West Antarctic Ice Sheet only after 7,000 years ago. Although other researchers have concluded that the abrupt decay of the Laurentide Ice Sheet might have been sufficient to have been responsible for meltwater pulse 1B, its sources remain an unresolved mystery. For example, recent research in West Antarctica found that sufficient deglaciation contemporaneous with meltwater pulse 1B occurred to readily explain this rapid period of global sea level rise.Fogwill, C., C. Turney, N. Golledge, D. Etheridge, M. Rubino, J. Woodward, K. Reid, T. van Ommen, A. Moy, M. Curran, D. Thornton, C. Rootes, and A. Rivera, Andrés (2015) Direct evidence for significant deglaciation across the Weddell Sea embayment during Melt Water Pulse-1B. EGU General Assembly 2015, held 12–17 April 2015 in Vienna, Austria. id.2572.
Mississippi River superflood events MWF-5 A variety of paleoclimate and paleohydrologic proxies, which can be used to reconstruct the prehistoric discharge of the Mississippi River, can be found in the sediments of the Louisiana continental shelf and slope, including the Orca and Pygmy basins, within the Gulf of Mexico.Biller, N.B. (2012) Evidence for Meltwater Pulse 1a in the Gulf of Mexico based on radiogenic isotopes of leachates. Undergraduate Thesis, Department of Geological Sciences University of Florida, Tallahassee, Florida. 39 pp.Meckler, A.N., C.J. Schubert, P.A. Hochuli, B. Plessen, D. Birgel, B.P. Flower, K.-U. Hinrichs, and G.H. Haug (2008) Glacial to Holocene terrigenous organic matter input to sediments from Orca Basin, Gulf of Mexico. Earth and Planetary Science Letters 272 (2008) 251–263. These proxies have been used by Quaternary geologists, paleoclimatologists, and oceanographers to reconstruct both the duration and discharge the mouth of the prehistoric Mississippi River for the Late glacial and postglacial periods, including the time of meltwater pulse 1B.Kennett, P., K. Elmstrom, and N. Penrose (1985) The last deglaciation in orca basin, gulf of Mexico: High-resolution planktonic foraminiferal changes. Palaeogeography, Palaeoclimatology, Palaeoecology. 50(1):189-216.Aharon, P. (2003) Meltwater Flooding Events in the Gulf of Mexico Revisited: Implications for Rapid Climate Changes During the Last Deglaciation. Paleoceanography. 18(4):3-1 to 3-13.Aharon, P. (2006) Entrainment of meltwaters in hyperpycnal flows during deglaciation superfloods in the Gulf of Mexico. Earth and Planetary Science Letters. 241:260–270.Marchitto, T.M. K.-Y. Wei (1995) History of Laurentide meltwater flow to the Gulf of Mexico during the last deglaciation, as revealed by reworked calcareous nannofossils Geology. 23(9):779-782. The chronology of flooding events found by the study of cores on the Louisiana continental shelf and slope are in agreement with the timing of meltwater pulses. For example, meltwater pulse 1A in the Barbados coral record matches quite well with a group of two separate Mississippi River meltwater flood events, MWF-3 (12,600 ) and MWF-4 (11,900 ). In addition, meltwater pulse 1B in the Barbados coral record matches a cluster of four Mississippi River superflood events, MWF-5, that occurred between 9,900 and 9,100 . In 2003, Aharon reported that flood event MWF-5 consists of four separate and distinct superfloods at 9,970-9,870; 9,740-9,660; 9,450-9,290; and 9,160-8,900 . The discharge at the mouth of the Mississippi River during three of the four superfloods of MWF-5 is estimated to have varied between 0.07 and 0.08 sverdrups (million cubic meters per second). The superflood at 9450-9290 is estimated to have had a discharge of 0.10 sverdrups (million cubic meters per second). This research also shows that the Mississippi superfloods of MWF-5 occurred during the Preboreal. The same research found an absence of either meltwater floods or superfloods discharging into the Gulf of Mexico from the Mississippi River during the preceding thousand years, which is known as the cessation event, that corresponds with the Younger Dryas stadial.
The Pleistocene deposits blanketing the Louisiana Continental shelf and slope between the mouth of the Mississippi River and Orca and Pygmy basins largely consist of sediments transported down the Mississippi River mixed with variable additions of local biologically generated carbonate. Because of this, the provenance of the meltwater and superfloods can be readily inferred from the sediment's composition. The composition of the sediments brought into the Gulf of Mexico and deposited on the Louisiana continental shelf and slope during the superfloods of MWF-5 reflect an abrupt change in mineralogy, fossil content, organic matter, and amount after 12,900 years ago at the start of the Younger Dryas interval.