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Antarctica's Contribution to Eustasy During the Past Glacial/Intergiaciai Cycie

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Antarctica's Contribution to Eustasy During the Past Glacial/Intergiaciai Cycie CoastalChange on the TimeSea/e pl' Oecades to Millennia Antarctica's contribution to eustasy during the past glacial/intergiaciai cycie John B. Anderson DepartmentOf Geologyand GeophysicsRice Llniirersity, HOOSten, TexaS, 77251, U,S,A, e-mail:tohnaOrice.edu Knowledgeof Antarctica'scontribution to eustasyduring the past glacial/interglacialcycle has recently improved as a resultof onshoreand marinegeological surveys. Marine geological stud es haveyielded compelling evidence that all three componentsof the Antarctic cryosphere,the EastAntarctic Ice Sheet EAIS!,West Antarctic Ice Sheet WAIS!, and Antarc- tic Peninsulaice sheet,advanced across the continentalshelf during the last glacialcycle. There is also new evidenceto suggestthat the EAISretreated from the continentashelf prior to 24,000yrs BP,possibly during OxygenIsotope Stage 3. The WAIS retreated from the shelf after the I GM Oxygen Isotope Stage 2!. Retreat of the Antarctic Peninsulaice sheet from the shelfwas completeby the mid-Holocene. Marinegeological surveys conducted during the past five yearshave focused on the individua drainagesystems of the West Antarctic ce sheet. These studies have shown that the retreat of the WAIS was diachronous around the continent, with differentglacial drainage systems behaving more or lessindependently during recession,Some drainage systems, suchas those of the westernRoss Sea, retreated slowly and continuouslyfrom the shelf,whereas other segments,such asthe PineIsland Bay drainage system, t'etreated rapidly. Collapse of someWAIS drainage systems, such as the PineIsland Baysystem, could havecaused eustatic rises of severaldecimeters over a fewcenturies. There were still significantvolumes of ice left on the Antarctic continental shelf after 9000 years to have contributed to the observed sea-levelnse of the late Holocene.There is someevidence for rapidphases of retreatduring this time interval, Thecomplex retreat history of the Antarctic Ice Sheetsindicates an episodic nature to sea-levelrise with potentially rapid phasesof retreatthat could havesignificantly altered coastal systems, Thereis no evidencefor widespreaddeglacial episodesthat would have caused the kinds of meter-scalerapid risesinvoked by some researchers. 29 CoastaiChange on the TimeScare of Decadesto Mi lennia Relative variation curves in sea-level during the last 7000 years in Brazil: A review RodolfoJosh Angulo and MariaCristina de Souza Departamentode Geoogia, univerSidade Federal dO Parana, Caixa Postal 19001, 81 93 1-990 Curitiba-PR, BRASIL e-mail Angulo!.angulotgao logla.uf pr.br e-mail de Souza!:cristinaegeologla.ufpribr In the last three decades hundreds of paleosea-levelindicators from the Brazilian coast were dated and several pale- osea-levelcurves, for the last 7,000 years,were published for different sectorsaf the coastline Figs. 1 and 2!. Thesecurves presentsignificant differences and contradictions, which are; i! the existenceor not of 3-4 rn oscillations between 4100- 3800 years B.P.and between 3000-2700 years B.P. Suguio et al. 1985, Angulo & Lessa1997, Martin et al. 1998, Lessa& Angulo1998!; ii! the existenceor not of a sea-levelrise in the lasttwo thousandyears along the coastof RioGrande do Sul State, and Tomazelli& Villwock 1989 apud Tomazelli 1990, Tomazelli 1990, Angulo & Giannini 1996!, iii! the maxi- rnum altitude attained by the post-glacial marine transgressionon the State of Parana Martin et al. 1988, Angulo 1992, Angulo & Suguio 1995!, Thereare still other aspectsthat have not yet been discussed,such as a! when did the sea reach a level similar to the cur- rent one for the first time during the Holocene,and b! what was the maximum altitude of the post-glacial marine trans- gression PMT! in the country? Previousstudies suggested different times when sea-leveleventually reached the present mean sea-levelafter the PMT,and that would span between 7100 and 6600 years B.P.Holoceno Suguio et al 1985!. It is also proposedthat the maximum sea-levelat the end of the PMTwould have occurred around 5100 years B.P.,and that it would have reachedaltitudes that vary from 4.8 x 0 5 m to lessthan 2.5 m. These differences in age and elevation are explainedby the shifting of a geoidal depressionnow located close to Cananeia, Sao Paulo State Martin et al. 1985!. A critical analysiso the literature suggests that elevations of sea-level maximum higher than 4 m are probably overesti- mated. This s mainly due to the fact that there are more than one hundred vermets samplesdated in the Brazilian coast, and only three indicate paleo-sea-levelshigher than 4 m Fig. 3!. The majority of the samples were obtained on cliffs exposedto strong wave action, in which case the ecological zone of the vermets can be displaced 1 rn upwards Laborel 7986!. In two places, "in situ" oyster shells would suggest a sea-level maximum of 4,8 m. However, Delibrias & Laborel 969! consider that this elevation would also be overestimated since the reference level used was the mean sea-level insteadof the ecological zone of the oysterswhich in some situations can be 3 m higher than that level. It would then be necessaryto know the characteristicsof the place where the oysters were collected to define the precise paleo-sea-level. Finally,higher than 4 m estimatesof the sea-levelmaximum come from shellfragments found on paleo-beachdeposits or the elevation of the paleo-beachesterraces. In these cases it is well known that wave run-up can form sedimentary depositsseveral meters above mean sea-level, Theanalysis of the data usedto producethe curvesreveal that, besidesthe scarcityof the data, the consideredmargins of errorare frequently extremely small for the type of indicatorused. Thus, the curvesare not too precisedue to the qual- ity of the data utilizedin their construction.As for the time when sea-levelfirst reachedthe presentone, the published dataindicate only that this happenedabout 7000 yearsA.P. The data are not sufficientlyprecise for an adequateidentifi- cationof the proposedregional differences. For example, wood fragmentsdeposited in clay-sandysediments attributed to mangroves,which would more likely indicate a lowestpossible paleo-sea-level, isused to determinemean sea-levei with a margin of error of only 0.3 m to 0,4 m. Consideringthe maximumsea-level altitude after the PMT,the publisheddata, in mostcases, does not allowfor a deter- minationwith an accuracy of less than 1 meter.Moreover, they do not have enough precision to identify possible signifi- cantdifferences in altitude at severalsections of the Brazliancoast as previously proposed. The data only indicate altitudes around 2.5 to 4.0 m in the penod between 5000 and 5400 years B.P. References Angulo,R.J., 1992, Geoiogia daPianicie Costeira doEstado do Parana. Unpublished Ph.D. Thesis, Geoscience Institute, Universityof SaoPaulo, Brazil, 334 p, Angulo,R.J. and Giannini, P.C. F., 1996. VariaCao donivel relativo domar nos ultimos dois rnil anos na regiao suldo Biasil: umadiscussao, aofetim Paranaense de GeocienciasCuri tiba. 44:67-75. Angulo,R.J.and Lessa, G.C., 1997. The Brazilian sea-level curves: a critical review with emphasis onthe curves from Paranaguland Cananeia regions. Mar. Geol., 140 l41-166. AnguloR.J, and Suguio K.,1995. Re-evaluation ofthe Holocene sea-level maxima forthe state ofParanh, Brazil. Paleo- geog.,Paieoclim., Paleontology, 113;385-393. 31 Delibirias,C. and Laborel,J. 1969. Recentvariations of the sea-levelalong the Braziliancaast. Quaternana. 14:45-49. LaborelJ, 1986.Vermetid gastropods assea-evel indicators. In:Van de Plassche, O.ed, Sea-level research: a manual for the collection and evaluation of data. Norwich. Geo Books, p,281-310. Lessa,G.C, and Angulo, R,J., 1998. Oscillations or not oscillations:that is thequestion - Reply,Mar Geol.,150:189-196. Martin,L.; Bittencourt, A. C. da S P.;Dominguez, J. M. L.;Flexor, J. M.; Suguia,K. 1998.Oscillations or not oscillations, thatis the question: Comment on Angulo, R. J. and Lessa, G. C, The Brazilian sea-level curves: a criticalreview with empha- sison the curvesfrom the Paranaguhand Cananeiaregions. Mar. Geol., 140:141-166. Marine Geology. Amsterdam, 150:179-187, Martin,I; Flexar,J.M.; Blitzkow, D. andSuguio, K. 1985.Geoid change indications along the braziliancoast during the last 7,000 years.In lnt. CoralReef Congress, 5. Tahiti,1985, Proc... Tahiti, v.3, p,85-90. Martin,L.; Suguio, K.; Flexor, J.M. and Azevedo, A.E,G.. 1988. Mapa Geol6gico do QuaternarioCosteiro das Estados do Paranhe Santa Catarina. Serie Geal6gica DNPM, no. 28, 40 p. Suguio,K; Martin, L.; Bittencourt, A.C.S.P., Dorningues, J.M.L.; Flexor, J,M. and Azevedo, A.E.G., 1985 Flutuaqoes donlvel relativodo mardurante o QuaternkrioSuperior ao longo do litoral brasileiroe suasimplicaCoes na sedimentaqaocosteira. Rev. 8ras, Geoc., 15:273-286. Tomazelli,L.J. 1990.Contribuiqao ao estudodos sistemas deposiaonais holocenicos do nordesteda provlncia costeira do RioGrande da Sul,corn enfase na sistemae6lico. Unpublished Ph.D. Thesis, Geoscience Institute, Federal University of Rio Grande do Sul, Porto Alegre, Brazil, 270p. Fig.t. Location map 32 F>g.2. The Braziliansea level curvesfor the last 7000 years after Suguio et al., 1985 and Villwock and Tomazel- li, 1989, in Tomazelli, 1990!. See Figure 1 for location. 33 Fig. 3. Elevationof the publishedpaleo-sea level indicatorsin Brazilderived from vermetidradiocarbon dates. Shaded areas indicate the time for the proposed secondary oscillations proposed by other authors seeSuguio et al. 1985!.The line is a 4th order polynomialbest fit. Angulo5 Lessa1997!
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