Postglacial Sea level Rise Curve and Meltwater Pulses (MWP)

Meltwater pulse 1B (MWP1b) is the name used by

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million year ...

geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althou ...

paleoclimatologist Paleoclimatology ( British spelling, palaeoclimatology) is the study of climates for which direct measurements were not taken. As instrumental records only span a tiny part of Earth's history, the reconstruction of ancient climate is important t ...

s, and

oceanographer Oceanography (), also known as oceanology and ocean science, is the scientific study of the oceans. It is an Earth science, which covers a wide range of topics, including ecosystem dynamics; ocean currents, waves, and geophysical fluid dynamic ...

s for a period of either rapid or just accelerated

post-glacial The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togethe ...

sea level rise Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cr ...

that some hypothesize to have occurred between 11,500 and 11,200 calendar years ago at the beginning of the

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...

and after the end of the

Younger Dryas The Younger Dryas (c. 12,900 to 11,700 years BP) was a return to glacial conditions which temporarily reversed the gradual climatic warming after the Last Glacial Maximum (LGM, c. 27,000 to 20,000 years BP). The Younger Dryas was the last stag ...

. 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 1A (MWP1a) is the name used by Quaternary geologists, paleoclimatologists, and oceanographers for a period of rapid post-glacial sea level rise, between 13,500 and 14,700 calendar years ago, during which global sea level rose betw ...

meltwater pulse 1C The early Holocene sea level rise (EHSLR) was a significant jump in sea level by about during the early Holocene, between about 12,000 and 7,000 years ago, spanning the Eurasian Mesolithic. The rapid rise in sea level and associated climate ...

, 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 Meltwater is water released by the melting of snow or ice, including glacial ice, tabular icebergs and ice shelves over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing. Meltwater ca ...

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 Fairbanks in his

coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of Colony (biology), colonies of coral polyp (zoology), polyps held together by calcium carbonate. Most coral reefs are built from stony corals, wh ...

studies in

Barbados Barbados is an island country in the Lesser Antilles of the West Indies, in the Caribbean region of the Americas, and the most easterly of the Caribbean Islands. It occupies an area of and has a population of about 287,000 (2019 estima ...

. 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 calendar 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 Tahiti (; Tahitian ; ; previously also known as Otaheite) is the largest island of the Windward group of the Society Islands in French Polynesia. It is located in the central part of the Pacific Ocean and the nearest major landmass is Austra ...

. They concluded that meltwater pulse 1B was, at best, just an acceleration of sea level rise at about 11,300 calendar years ago and it was, at worst, not statistically different from a constant rate sea level rise between 11,500 and 10,200 calendar 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 calendar 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 calendar years ago, a 300-calendar 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 The Antarctic ice sheet is one of the two polar ice caps of Earth. It covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth, with an average thickness of over 2 kilometers. It covers an area of almost and ...

. 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 Deglaciation is the transition from full glacial conditions during ice ages, to warm interglacials, characterized by global warming and sea level rise due to change in continental ice volume. Thus, it refers to the retreat of a glacier, an ice shee ...

in eastern

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...

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 The Western Antarctic Ice Sheet (WAIS) is the segment of the continental ice sheet that covers West Antarctica, the portion of Antarctica on the side of the Transantarctic Mountains that lies in the Western Hemisphere. The WAIS is classified as ...

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 erratic A glacial erratic is glacially deposited rock differing from the type of rock native to the area in which it rests. Erratics, which take their name from the Latin word ' ("to wander"), are carried by glacial ice, often over distances of hundred ...

nunatak A nunatak (from Inuit language, Inuit ''nunataq'') is the summit or ridge of a mountain that protrudes from an ice field or glacier that otherwise covers most of the mountain or ridge. They are also called glacial islands. Examples are natural p ...

s, 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 The East Antarctic Ice Sheet (EAIS) is one of two large ice sheets in Antarctica, and the largest on the entire planet. The EAIS lies between 45° west and 168° east longitudinally. The EAIS holds enough ice to raise global sea levels by and ...

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 calendar years ago. Although other researchers have concluded that the abrupt decay of the

Laurentide Ice Sheet The Laurentide Ice Sheet was a massive sheet of ice that covered millions of square miles, including most of Canada and a large portion of the Northern United States, multiple times during the Quaternary glacial epochs, from 2.58 million year ...

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 The Mississippi River is the List of longest rivers of the United States (by main stem), second-longest river and chief river of the second-largest Drainage system (geomorphology), drainage system in North America, second only to the Hudson B ...

, can be found in the

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...

s of the Louisiana continental shelf and slope, including the

Orca The orca or killer whale (''Orcinus orca'') is a toothed whale belonging to the oceanic dolphin family, of which it is the largest member. It is the only extant species in the genus '' Orcinus'' and is recognizable by its black-and-white ...

and Pygmy basins, within the

Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of the United ...

.Biller, N.B. (2012
''Evidence for Meltwater Pulse 1a in the Gulf of Mexico based on radiogenic isotopes of leachates.''
Undergraduate Thesis, Departmentof 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

s, and

s to reconstruct both the duration and discharge the mouth of the prehistoric

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

coral record matches quite well with a group of two separate Mississippi River meltwater flood events, MWF-3 (12,600 radiocarbon years ago) and MWF-4 (11,900 radiocarbon years ago). 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 radiocarbon years ago. 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 radiocarbon years ago. 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

sverdrup In oceanography, the sverdrup (symbol: Sv) is a non- SI metric unit of volumetric flow rate, with equal to . It is equivalent to the SI derived unit cubic hectometer per second (symbol: hm3/s or hm3⋅s−1): 1 Sv is equal to 1 hm3/s. It is u ...

s (million cubic meters per second). The superflood at 9450-9290 radiocarbon years ago 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

from the Mississippi River during the preceding thousand years, which is known as the ''cessation event'', that corresponds with the

stadial. The

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

deposits blanketing the

Louisiana Louisiana , group=pronunciation (French: ''La Louisiane'') is a state in the Deep South and South Central regions of the United States. It is the 20th-smallest by area and the 25th most populous of the 50 U.S. states. Louisiana is bord ...

Continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...

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 A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the 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 calendar years ago at the start of the Younger Dryas interval. First, after 12,900 calendar years ago, smectite-rich

sediments Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...

from the Missouri River drainage are progressively and quickly replaced by sediments, which are associated with the

Great Lakes The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes in the mid-east region of North America that connect to the Atlantic Ocean via the Saint Lawrence River. There are five lakes ...

region and further south along the Mississippi River, as indicated by their

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...

mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the proce ...

. Second, after 12,900 calendar years ago, the overall quantity of sediment being transported down the Mississippi River abruptly decreases with a corresponding and significantly increased proportion of locally produced biologically generated carbonate and organic matter. Third, after 12,900 calendar years ago, various analyses, e.g. C/N ratio and Rock–Eval Pyrolysis, indicate that the type of organic matter present changes from organic matter that was reworked from old formations by glacials to well-preserved

organic matter that is mainly of marine origin. Finally, after 12,900 calendar years ago, the presence of reworked

nanofossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

s disappear from sediments accumulating on the Louisiana continental shelf and slope.Montero-Serrano, J.C., V. Bout-Roumazeilles, N. Tribovillard, T. Sionneau, A.Riboulleau, A. Bory, and B. Flower (2009) ''Sedimentary evidence of deglacial megafloods in the northern Gulf of Mexico (Pigmy Basin).'' Quaternary Science Reviews. 28:3333–3347.Sionneau, T.V., B.P. Bout-Roumazeilles, A. Flower, N. Bory, C. Tribovillard, B. Kissel, B. Van Vliet-Lanoë, and J.C. Montero Serrano (2010) ''Provenance of freshwater pulses in the Gulf of Mexico during the last deglaciation.'' Quaternary Research. 74:235–245. The above noted changes in the nature of accumulating sediments indicate that after the start of the

, the southern route for Laurentide Ice Sheet meltwater was largely blocked. On the rare occasions it could flow southward, glacial meltwater flowed through

Lake Agassiz Lake Agassiz was a large glacial lake in central North America. Fed by glacial meltwater at the end of the last glacial period, its area was larger than all of the modern Great Lakes combined. First postulated in 1823 by William H. Keating, i ...

and sometimes the Great Lakes to the Mississippi River. As the water moved through either Lake Agassiz or other

proglacial lake In geology, a proglacial lake is a lake formed either by the damming action of a moraine during the retreat of a melting glacier, a glacial ice dam, or by meltwater trapped against an ice sheet due to isostatic depression of the crust around th ...

s, they completely trapped and removed any glacial outwash and the older, reworked organic material and reworked nanofossils that the outwash contained. As a result, the sediment carried by the Mississippi River after the start of the Younger Dryas consisted of

illite Illite is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – alumina ...

and

chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorou ...

enriched sediments from the Great Lakes region that lacked any reworked nanofossils. These changes argue that the superfloods of MWF-5 which fed Meltwater Pulse B are related to either rare periods of southerly discharge of meltwater through Lake Agassiz, nonglacial periods of climate-enhanced discharge within the

Mississippi River Basin The Mississippi River is the second-longest river and chief river of the second-largest drainage system in North America, second only to the Hudson Bay drainage system. From its traditional source of Lake Itasca in northern Minnesota, it f ...

, or a combination of both.

Antarctic iceberg discharge events

In case of the

, an equivalent well-dated, high-resolution record of the discharge of

iceberg An iceberg is a piece of freshwater ice more than 15 m long that has broken off a glacier or an ice shelf and is floating freely in open (salt) water. Smaller chunks of floating glacially-derived ice are called "growlers" or "bergy bits". The ...

s from various parts of the Antarctic Ice Sheet for the past 20,000 calendar years is also available. Research by Weber and others constructed a record from variations in the amount of iceberg-rafted debris versus time and other environmental proxies in two cores taken from the ocean bottom within Iceberg Alley of the Weddell Sea. The cores of ocean bottom

within Iceberg Alley provide a spatially integrated signal of the variability of the discharge of icebergs into the marine waters by the Antarctic Ice Sheet because it is a confluence zone in which icebergs calved from the entire Antarctic Ice Sheet drift along currents, converge, and exit the Weddell Sea to the north into the

Scotia Sea The Scotia Sea is a sea located at the northern edge of the Southern Ocean at its boundary with the South Atlantic Ocean. It is bounded on the west by the Drake Passage and on the north, east, and south by the Scotia Arc, an undersea ridge and ...

.Weber, M.E., P.U. Clark, G. Kuhn, A. Timmermann, D. Sprenk, R. Gladstone, X. Zhang, G. Lohmann, L. Menviel, M.O. Chikamoto, T. Friedrich, and C. Ohlwein (2014) ''Millennial-scale variability in Antarctic ice-sheet discharge during the last deglaciation.'' Nature. 510(7503):134–138. Between 20,000 and 9,000 calendar years ago, Weber and others documented eight well-defined periods of increased

calving and discharge from various parts of the Antarctic Ice Sheet. Five of these periods, AID5 through AID2 (Antarctic Iceberg Discharge events), are comparable in duration and have a repeat time of about 800–900 calendar years. The largest of the Antarctic Iceberg Discharge events is AID2. Its peak intensity at about 11,300 calendar years ago, which is synchronous with meltwater pulse 1B in the

sea-level record, is consistent with a significant Antarctic contribution to meltwater pulse 1B. The lack of a sea level response in the Tahiti coral record might indicate a regionally specific sea-level response to a

event only from the Pacific sector of the Antarctica Ice Sheet.

References

{{reflist, 2

External links

* Gornitz, V. (2007
Sea Level Rise, After the Ice Melted and Today
Science Briefs, NASA's Goddard Space Flight Center. (January 2007) * Gornitz, V. (2012
The Great Ice Meltdown and Rising Seas: Lessons for Tomorrow
Science Briefs, NASA's Goddard Space Flight Center. (June 2012) * Liu, J.P. (2004
Western Pacific Postglacial Sea-level History.River, Delta, Sea Level Change, and Ocean Margin Research Center
Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC. Glaciology Oceanography Paleoclimatology Sea level 10th millennium BC