Sea-level history of the Gulf of Mexico since the Last Glacial...

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0277-3791/$ - se

doi:10.1016/j.qu

�CorrespondE-mail addr

kurt.lambeck@

(A. Purcell), jo

(A.B. Rodrigue

Quaternary Science Reviews 26 (2007) 920–940

Sea-level history of the Gulf of Mexico since the Last Glacial Maximumwith implications for the melting history of the Laurentide Ice Sheet

Alexander R. Simmsa,�, Kurt Lambeckb, Anthony Purcellb,John B. Andersonc, Antonio B. Rodriguezd

aT. Boone Pickens School of Geology, Oklahoma State University, 105 NRC, Stillwater, OK 74078, USAbResearch School of Earth Sciences, Building 61 Mills Road, The Australian National University, Canberra ACT 0200, Australia

cDepartment of Earth Science, Rice University, 6100 S. Main, MS-126, Houston, TX 77005, USAdInstitute of Marine Sciences, University of North Carolina, at Chapel Hill, 3431 Arendell Street, Morehead City, NC 28557, USA

Received 26 January 2006; received in revised form 8 January 2007; accepted 9 January 2007

Abstract

Sea-level records from the Gulf of Mexico at the Last Glacial Maximum, 20 ka, are up to 35m higher than time-equivalent sea-level

records from equatorial regions. The most popular hypothesis for explaining this disparity has been uplift due to the forebulge created by

loading fromMississippi River sediments. Using over 50 new radiocarbon dates as well as existing published data obtained from shallow-

marine deposits within the northern Gulf of Mexico and numerical models simulating the impact of loading due to the Mississippi Fan

and glacio-hydro-isostasy, we test several possible explanations for this sea-level disparity. We find that neither a large radiocarbon

reservoir, sedimentary loading due to the Mississippi Fan, nor large-scale regional uplift can explain this disparity. We do find that with

an appropriate model for the Laurentide Ice Sheet, the observations from the Gulf of Mexico can be explained by the process of glacio-

hydro-isostasy. Our analysis suggests that in order to explain this disparity one must consider a Laurentide Ice Sheet reconstruction with

less ice from 15 ka to its disappearance 6 ka and more ice from the Last Glacial Maximum to 15 ka than some earlier models have

suggested. This supports a Laurentide contribution to meltwater pulse 1-A, which could not have come entirely from its southern sector.

r 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Arguing that the Gulf of Mexico was tectonically stable,Shepard (1960) produced one of the first ‘‘eustatic’’ sea-level curves for the last 30 ka from radiocarbon datesobtained from shallow marine mollusks and woodfragments. Redfield (1967) and Emery and Garrison(1967) both noted that the sea-level curves of Shepard(1960), and later works by McFarland (1961), obtainedfrom the Gulf of Mexico were up to 35m shallower at theLast Glacial Maximum (LGM), and 15m shallower 10 ka,than sea-level curves from other places across the globe. Onthe basis of submerged shoreline features and deltas, some

e front matter r 2007 Elsevier Ltd. All rights reserved.

ascirev.2007.01.001

ing author. Tel.: +1405 744 7725; fax: +1 405 744 7841.

esses: [email protected] (A.R. Simms),

anu.edu.au (K. Lambeck), [email protected]

[email protected] (J.B. Anderson), [email protected]

z).

authors have argued for a sea-level lowstand of �120mwithin the Gulf of Mexico, bringing the ‘‘global’’ and Gulfof Mexico records into agreement at the LGM (Curray,1960; Berryhill et al., 1986). However, none of thesefeatures have been dated nor corrected for the highsubsidence rates usually present on the outer shelf.Continued work on refining the ‘‘global’’ sea-level recordby Fairbanks (1989), Chappell and Polach (1991), Bardet al. (1996), Yokoyama et al. (2000), and Hanebuth et al.(2000) and the Gulf of Mexico sea-level record by Nelsonand Bray (1970) and Bart and Ghoshal (2003) continues toindicate sea levels within the Gulf of Mexico 35mshallower during the LGM, and 15m shallower at 10 ka,than ‘‘global’’ sea-level records (Fig. 1a). A similardisparity has been noted by other studies of sea-levelindicators from Florida and the Caribbean (e.g. Schollet al., 1969; Toscano and Macintyre, 2003).Many authors have noted great variability in Late

Quaternary/Holocene sea-level records globally. Pirazzoli

dx.doi.org/10.1016/j.quascirev.2007.01.001mailto:[email protected]:[email protected]:[email protected]:[email protected]

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Fig. 1. (a) Sea-level indices from the Gulf of Mexico and from the coral-

based records of Fairbanks (1989) and Bard et al. (1996) and the Sunda

Shelf shallow marine environments of Hanebuth et al. (2000). Error bars

for the coral-based data were calculated by summing the squares of half

the growth range of the corals plus half the tectonic correction as reported

by the original authors. For the Sunda Shelf we assumed an error less than

2m (Hanebuth et al., 2000). Gulf of Mexico dates come from this study

and Shepard andMoore (1955), Curray (1960), Rehkemper (1969), Nelson

and Bray (1970), and Törnqvist et al. (2004). Blue diamonds represent

dates obtained from shell material and green triangles represent dates

obtained from wood fragments or peat. (b) New sea-level observations

collected during this study. See text for corrections applied to radiocarbon

dates. Vertical bars encompass the uncertainty in the relationship between

the sea-level observations and paleo-sea levels. All GOM SL Data refers to

all the sea-level indicators from the Gulf of Mexico as given in Table S2 in

supplementary materials.

Fig. 2. Map of the study area showing the location of the three estuaries

examined in this study as well as the location of two R. cuneata shells

obtained in a core by Curray (1960). Also shown is the size and thickness

of the youngest lobe of the Mississippi Fan as mapped by Stelting et al.

(1986). Contours for the Mississippi Fan thickness are given in meters with

a contour interval of 50m. Stars indicate locations of sea-level predictions

and cores with radiocarbon dates listed in Table 1. Diamonds represent

locations of dates shown in Fig. 1a.

A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940 921

(1991) compiled an atlas of Late Quaternary/Holocene sea-level curves from across the world and noted their widerange of magnitudes and histories. Pirazzoli (1991) alsoexplored many of the possible causes for regional varia-tions in sea level. One of the most common causes forregional variations is tectonic activity. In many areas,tectonic activity can keep pace and even exceed rates of sea-level change. Bart and Ghoshal (2003) argued that sea levelin the Gulf of Mexico never reached the full �120m duringthe LGM based on their observation that no shelf margindeltas have been incised by fluvial systems to depths greaterthan �90mbsl. They attributed this disparity to isostatic

compensation from flexural loading of the Gulf of Mexicodue to sediments delivered by the Mississippi River. Thiswas the same conclusion that Redfield (1967) and Emeryand Garrison (1967) suggested. However, to date, no onehas quantitatively tested this hypothesis.Another notable cause of local and regional variations in

sea level is the Earth’s differential isostatic response to thewaxing and waning of ice sheets since the LGM 18ka. Asthe ice sheets grew and melted over the Last Glacial-eustatic cycle, the effective sea-level change at any onepoint on the globe is not simply a function of the volume ofmeltwater added to or taken away from the ocean (Bloom,1967; Cathles, 1975; Walcott, 1972; Lambeck, 1988; Peltier,1998, 2002a; Milne et al., 2002; Mitrovica and Milne,2003). As the ice sheets melted, the distribution of massshifted from the polar ice caps and glaciers to the oceans.As a consequence, the isostatic compensation and gravita-tional potential for the ice and water loads also changedthrough time. Several authors have found success inquantitatively modeling the Earth’s response to thesechanges in the distribution of surface loads. Cathles(1975) and Farrell and Clark (1976) formulated some ofthe earliest numerical models to calculate the Earth’sresponse to the changing ice sheets. Since then, manyauthors have advanced the numerical techniques andtheory for modeling glacio-hydro-isostasy (Peltier et al.,1978; Nakada and Lambeck, 1987; Mitrovica and Peltier,1991; Lambeck, 1993a, 1995a; Milne et al., 1999; Peltier,2002a; Lambeck et al., 2003; Mitrovica and Milne, 2003).In this paper, we use newly acquired data from three

estuaries along the northwestern Gulf of Mexico fromCorpus Christi, Texas to Mobile, Alabama (Fig. 2) as wellas other previously published data to test the severalpossible mechanisms for the disparity between sea levelwithin the Gulf of Mexico and other global sites (Fig. 1a).Our new data from each of these three estuaries confirmsearlier works and systematically indicates a sea-level10–15m higher than global-eustatic values for the period

ARTICLE IN PRESSA.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940922

between 8 and 10 ka (Fig. 1b). This trend is also found indata from other studies from the Gulf of Mexico datingback to the LGM in which no published dated sea-levelindices from LGM times are found deeper than �92m(‘‘freshwater’’ shells from McFarland, 1961, Table S1 insupplementary materials). Using quantitative models de-veloped at the Australian National University (Nakadaand Lambeck, 1987; Johnston, 1993; Lambeck andJohnston, 1998; Lambeck et al., 2003) and observationaldata, we investigate the possible explanations for theobserved disparity. The possibilities considered in thispaper include: a large radiocarbon reservoir for thenorthwestern Gulf of Mexico, tectonic uplift over theentire region, isostatic compensation for the large sedi-mentary load of the Mississippi River, and glacio-hydro-isostatic adjustments to the ice sheets that melted sincethe LGM.

2. Methods

2.1. Sea-level observations

Sea-level observations were obtained from shallowmarine deposits from cores taken in Corpus Christi Bay,Texas, Galveston Bay, Texas, and Mobile Bay, Alabamaaboard the R.V. Trinity (Fig. 2). These three areas werechosen for two reasons: (1) their location within an incisedvalley providing the best preservation potential for shallowmarine deposits in light of transgressive ravinement(Belknap and Kraft, 1981, 1985) and (2) their relativelylarge distances from major depo-centers, most notably theMississippi River, Brazos-Colorado delta complex, and theRio Grande, minimizing the effects of compaction andsubsidence. The one limitation to observations obtainedfrom these cores is their relatively young nature, nomaterial older than 9.8 ka, a consequence of sea level beinglower than these positions before 10 ka. Most of the misfitdescribed in previous work (i.e. Redfield, 1967; Bartand Ghoshal, 2003) is from time periods older than this(Fig. 1a).

In order to supplement our dataset and extend the timespan in which the various hypotheses for the disparitycould be tested, we included other reliable indicators of sealevel within the Gulf of Mexico made available by previousstudies in our analysis. Of over 250 radiocarbon datesassembled from the literature and other studies by theauthors (Table S1 in supplementary materials), wenarrowed this list to 98 dates by only including pointsthat were obtained from a species whose relationship tosea-level was well constrained (e.g. an estuarine species)and whose age was determined from a single specimen(Fig. 1a and Table S2 in supplementary materials). Forcomparisons with numerical models, we limited ouranalyses to our three new sites and the most reliableindicator of sea levels between 18 and 10 ka available in theliterature. We chose a sea-level indicator obtained fromtwo articulated Rangia cuneata shells from the same core at

the same depth that were independently dated within 200years of one another by Neumann (1958) and reported byCurray (1960). R. cuneata is a mollusk that lives onlywithin shallow freshwater influenced estuarine environ-ments (Parker, 1959). The other dates from similar studiesconducted across the Gulf of Mexico were also used tosupplement the new data and better establish the misfitbetween the ‘‘global’’ eustatic records of Fairbanks (1989),Bard et al. (1996), and others and the Gulf of Mexico sea-level indicators (Fig. 1a).The new radiocarbon dates were obtained from two

sources: mollusks and wood fragments (Table 1). Thesedimentology of these deposits is described in detail forCorpus Christi Bay in Simms (2005), for Galveston Bay inRodriguez et al. (2004), and for Mobile Bay in Metcalf andRodriguez (2003). Most of the mollusks used for datingwere articulated. In order to test the reliability of datesobtained from wood fragments and unarticulated mol-lusks, dates were only considered from cores with severalradiocarbon dates from a variety of depths. The chronos-tratigraphic consistency with depth increased the confi-dence that the material dated was not reworked by coastalor estuarine processes.The deposits containing the age-dated material represent

either an upper- or middle-bay environment based oncoarse-fraction analysis (Shepard and Moore, 1954;Metcalf and Rodriguez, 2003; Rodriguez et al., 2004;Simms, 2005). In addition, with the exception of Mulinialateralis, the mollusks in which dates were obtained (e.g.Rangia sp.; Macoma mitchelli, Nuculana sp.) are restrictedto upper- or middle-bay environments (Parker, 1959;Andrews, 1971; White et al., 1983). M. lateralis is knownto inhabit the inner shelf; however, when found along theinner shelf it is found in association with other speciesendemic to inner shelf conditions such as Anadara sp.When found alone, M. lateralis is almost always anindicator of an enclosed lagoon characterized by widefluctuations in salinities (Parker, 1959; Andrews, 1971;White et al., 1983). Today, these environments are in waterdepths less than 5.5m and the mollusks indicative of upper-bay/river-influenced regions (e.g. Rangia sp.) are not foundin water depths greater than 2.0m (White et al., 1983;Parker, 1959). The deposits in which plant/wood fragmentswere used to date were interpreted as bayhead deltaic inorigin based on their sedimentary characteristics (e.g.peats, R. cuneata, course and lithic sand component,and prevalent regular bedding). These environmentstoday are found in water depths not exceeding 2mbsl.The present tidal range within these estuaries is less than1.0m. For this study, we compared the predicted sea levelswith the observed depth of the sea-level indicator. As eachof the observations represent deposition within an inter-tidal to subtidal (0 to �2m) depositional setting, theyshould always lie close to or below predictions, but notabove.Although some variability in the age–depth relationship

among the sea-level indices can be seen among the

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Table 1

Radiocarbon dates utilized in this study

Core # Material Lat. Long. Depth

bsl

Depth

error (7)C14

years

Error C14

Res

Res

error (7)Calender

years

2solder

2syounger

CCB03-01 M. lateralis or R. fiexusob 27.776 �97.278 7.85 1.0 2390 30 760 350 1590 850 845CCB03-01 M. lateralisb 27.776 �97.278 10.99 1.0 4470 35 760 350 4100 1015 1165CCB03-01 R. flexusa 27.776 �97.278 11.97 1.0 5010 30 760 350 4800 1065 930CCB03-01 Mollusk fragment 27.776 �97.278 17.04 1.1 6520 55 760 350 6590 840 830CCB03-01 M. campechiensis texana 27.776 �97.278 17.81 1.1 6780 45 760 350 6870 865 725CCB03-01 M. campechiensis texana 27.776 �97.278 19.06 1.1 7500 45 760 350 7600 805 745CCB03-01 N. concentrica 27.776 �97.278 19.77 1.1 7890 60 760 350 7980 725 955CCB03-01 A. aequalisb 27.776 �97.278 20.62 1.1 8050 40 760 350 8140 700 860CCB03-01 N. concentricab 27.776 �97.278 21.58 1.1 8470 40 760 350 8590 755 880CCB03-01 N. concentrica 27.776 �97.278 24.42 1.1 9240 65 760 350 9480 1000 1000NB03-01 M. lateralisb 27.849 �97.486 2.02 1.0 2630 45 760 350 1850 785 895NB03-01 M. lateralis 27.849 �97.486 3.27 1.0 3060 45 760 350 2340 920 910NB03-01 M. lateralis 27.849 �97.486 5.32 1.0 5640 60 760 350 5580 1000 855NB03-01 M. Mitchellib 27.849 �97.486 6.72 1.0 5980 60 760 350 5980 995 870NB03-01 N. acuta 27.849 �97.486 8.82 1.0 6640 50 760 350 6720 810 780NB03-01 N. concentricab 27.849 �97.486 11.53 1.0 7900 100 760 350 7800 820 755NB03-01 Rangia flexusob 27.849 �97.486 10.68 1.0 7700 70 760 350 7990 735 985GB-00-1.1 C. virginica 29.571 �94.877 9.42 1.0 6620 64 225 103 7280 300 215GB-00-1.1 Wood 29.571 �94.877 9.58 1.0 6454 51 0 0 7370 95 70GB-00-1.1 Wood 29.571 �94.877 10.27 1.0 6900 35 0 0 7730 65 100GB-00-1.2 Wood 29.571 �94.877 11.29 1.1 7413 53 0 0 8250 195 115GB-00-1.2 Plant/Wood 29.571 �94.877 12.93 1.1 7660 40 0 0 8450 55 90GB-00-1.3 Plant/Wood 29.571 �94.877 16.8 1.1 8330 45 0 0 9350 210 120MD-02-1 Plant/Wood 30.677 �87.969 2.17 1.0 720 30 0 0 675 105 50MD-02-1 Plant/Wood 30.677 �87.969 9.42 1.0 5070 30 0 0 5820 70 85MD-02-1 Plant/Wood 30.677 �87.969 9.42 1.0 5180 30 0 0 5940 30 55MD-02-1 Plant/Wood 30.677 �87.969 11.16 1.0 6220 50 0 0 7120 120 140MD-02-1 Plant/Wood 30.677 �87.969 12.29 1.0 7190 35 0 0 8000 60 155MD-02-1 Plant/Wood 30.677 �87.969 13.1 1.0 8000 55 0 0 8860 215 150MD-02-1 Plant/Wood 30.677 �87.969 14.92 1.1 8210 40 0 0 9180 145 115MD-02-1 Plant/Wood 30.677 �87.969 15.54 1.1 8480 35 0 0 9500 50 35MD-02-1 Plant/Wood 30.677 �87.969 17.26 1.1 8710 35 0 0 9660 110 230MD-02-1 Plant/Wood 30.677 �87.969 18.09 1.1 8770 50 0 0 9780 220 34057-9a R. cuneatab 28.157 �94.293 57.6 1.2 13360c 470 450 225 15200 1415 142057-9a R. cuneatab 28.157 �94.293 57.6 1.2 13220c 390 450 225 15000 1215 1270

14C dates includes a correction for 13C/12C. CCB ¼ Corpus Christi Bay, NB ¼ Nueces Bay (landward portion of Corpus Christi Bay), GB ¼ GalvestonBay, MB ¼Mobile Bay. Depths are given in meters.Res ¼ Reservoir.

aCurry (1960).bArticulated.cCorrected for 13C.


three bays, a remarkable consistency is found withinthe sea-level indices of the three estuaries (Fig. 1b).The one exception to this is Corpus Christi Bay wherethe longest core was taken in deeper water than coresfrom the other three bays and the dated depositsrepresent deeper-water environments (4–5m versus1–2m). In order to compare sea-level indicators from likeenvironments, we also considered another but shorter corefrom the Corpus Christi Bay system—NB03-01. Sea-levelindicators from this shorter core are more consistent withindicators from the other two bays (Fig. 1b). However,because the data from the longest core in CorpusChristi Bay extends further back in time than NB03-01,the longest core was used in comparison to sea-levelpredictions. However, a larger uncertainty with respect to

paleo sea levels was assigned to sea-level indices from thelonger core.Fluvial waters can carry carbon with an anomalously

low abundance of 14C (Broecker and Walton, 1959;Raymond and Bauer, 2001). As a result, deposits fromareas influenced by rivers, such as estuaries, may have alarger radiocarbon-reservoir effect than the ‘‘normal’’seawater correction of 440 years (Stuiver et al., 1998),resulting in anomalously old ages. This effect could affectcarbon in organisms precipitating material from the watercolumn (i.e. mollusks), although it should not affectorganic carbon in plants that sequester their carbon fromthe atmosphere. Aten (1983), in a study of archeologicalsites, established the carbon reservoir effect for the molluskR. cuneata from Galveston Bay, Texas and from several

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Table 2

Earth parameters for model results shown in this paper

E1 120 10 30

E2 80 3 30

E3 50 1 20

E4 50 1 1

E5 120 10 100

E6 65 4 10

m ¼ viscosity. The boundary between the upper and lower mantle wasassumed to be 670km.

A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940924

other estuaries and rivers along the Texas/Louisiana Coast.Aten’s (1983) correction of 2257103 years within theTrinity Delta of Galveston Bay fits a linear trend(Pearson’s R ¼ 0.985) without significant variation forthe last 2 ka. One of our study sites, Corpus Christi Bay,was not included in Aten’s (1983) study. We obtained aradiocarbon date from an articulated barnacle attached toa wood fragment. Both the barnacle and wood fragmentwere dated with an age of 8430745 and 7670745,respectively. The difference between the two is 760 years.This value is consistent with other values for radiocarbonreservoirs established from other estuaries along the TexasCoast (Aten, 1983). Assigning a radiocarbon reservoir of760 years to dates obtained from mollusks in cores fromCorpus Christi Bay near the surface (o0.5m) also bringstheir radiocarbon age to modern values. Although, theradiocarbon reservoir is species and time dependent, wefound that a value of 760 years, when applied to a varietyof mollusks within Corpus Christi Bay, results in aremarkable agreement among all the radiocarbon datesobtained. These values of a radiocarbon reservoir wereused instead of Stuiver et al. (1998) ‘‘normal’’ seawaterreservoir of 440 years. With the exception of one date fromGalveston Bay, all other dates from Galveston and Mobilebays were taken from wood or plant fragments whichshould not be subject to radiocarbon reservoir affects(Table 1).

All dates obtained in this study were corrected for theirappropriate radiocarbon reservoir, calibrated using Calib5.0 (Stuiver and Braziunas, 1993; Stuiver et al., 1998), andreported in calendar years BP. The same procedure wasfollowed for the radiocarbon dates obtained from the twoR. cuneata shells of Curray (1960) and all other datapresented in Fig. 1. In addition, all dates from othersources that were not corrected for the ratio of 13C/12Cwere corrected for 13C using the formula given on theNOSAMS website (http://www.nosams.whoi.edu/clients/data_d13correction.php3) and assuming the average valuesof d13C obtained from our data set of �27.5 for plantmaterial and �0.66 for shell material. These correctionswere applied in order to assure that all radiocarbon dateswere calibrated to the same time scale.

2.2. Numerical Modeling of the Mississippi Fan

The largest depositional feature formed over the lastglacial-eustatic cycle in the Gulf of Mexico is theMississippi sea-floor fan. The fan is roughly 600 km longand 400 km wide, attaining a maximum thickness of over400m (Bouma et al., 1983; Fig. 2). The timing, extent, anddepositional history of this feature have been studied ingreat detail by a number of authors (Curray, 1960; Boumaet al., 1983; Stelting et al., 1986; O’Connell and Normark,1986). According to Stelting et al. (1986), the youngest lobeof the Mississippi Fan was deposited sometime after 55 ka.By 10 ka, most of the Mississippi Fan was deposited(Wetzel and Kohl, 1986).

In order to test if the loading due to the Mississippi Fanis of sufficient magnitude to explain the disparity betweenthe sea-level indicators from the Gulf of Mexico and globalrecords, we simulated the Earth’s response to loading ofthe Mississippi Fan using a simple model for thesedimentary-load history. The rebound model used isbased on the same numerical models as that used forglacial loading and unloading. However, the effect ofglobal loading of the sea floor by the sediment-displacedwater is ignored. The time scale of recent sediment loadingis similar to that for glacial loading so that the samerheological parameters can be used for both sediment andglacial loading problems. As noted above, core sites liebeyond the sediment depocenter such that the rebound willbe that of a peripheral bulge around the load and one ofuplift.We assumed an axi-symmetric load of a parabolic

function with a maximum sediment thickness of 400m, aradius of 3.01 (�330 km), and a density of 2.3 g/cm3. Thedensity values obtained in cores are significantly lower,ranging from 1.7 to 2.0 g/cm3 due to their unconsolidatednature and inherent porosity, which in some coresapproach 50% (Shipboard Scientific Party, 1986). Inaddition, the effective load would also be reduced by thedensity of the water displaced by the sediments, thusr–rw g/cm

3. We adopt a value of r–rw ¼ 2.3 g/cm3 as amaximum value and any deflection estimate will beoverestimated. In order to obtain the maximum amountof uplift possible within the forebulge, we allowed thesedimentary loading history to vary between 18 and 10 ka.Assuming a loading history commencing prior to 18 kawould only diminish estimates for the magnitude of uplift.We also allowed the onset and duration of loading to varyover that 8 ka (18–10 ka) time span in order to determine itseffect.In addition to the size of the load, the Earth’s response

to loading is dependent on the structure of the Earth. Onecritical parameter is the structure and viscosity of themantle. We used a three layer mantle with 6 differentcombinations of rheological parameters that are represen-tative of values found in other regions across the globe (e.g.Lambeck and Chappell, 2001). The boundary between theupper and lower mantle was assumed to be at 670 km. TheEarth model parameters for each of the runs aresummarized in Table 2.

http://www.nosamshttp://www.nosams

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Fig. 3. Response of ice loading and water loads through time for Earth

model E1 for Mobile Bay (MB) and the location of Curray (1960) data

(CD).


2.3. Hydro-glacio-isostasy

2.3.1. Theory

The groundwork for quantitative formulations of theEarth’s response to glacio-hydro-isostasy was most exten-sively laid out by Cathles (1975), Farrell and Clark (1976),and Clark et al. (1978). One of the important advance-ments of Farrell and Clark’s (1976) work is the conceptthat the ocean surface remains an equipotential at all timesdue to the Earth’s changing gravitational field in light ofchanging mass distributions. In their formulations, Farrelland Clark (1976) also assured that mass is conserved andtook into account the deformation of the Earth’s surfacenot only in response to ice loads but also water loads(a concept introduced earlier by Bloom, 1967 and others).The theory used in this study follows the procedureoutlined by Nakada and Lambeck (1987) with latermodifications by Johnston (1993), Lambeck (1993a),Lambeck and Johnston (1998), Lambeck et al. (2002,2003), and Lambeck and Purcell (2005). These methodshave been called into question by Peltier (2002b), butaccording to Mitrovica (2003), the theory developed bythese authors represents, along with Milne et al. (1999) andMitrovica and Milne (2003), the most complete andrigorous theory available. From this work, the sea-levelequation for a tectonically stable area can be schematicallywritten as

Dzrslðf; tÞ ¼ zeðtÞ þ ziðf; tÞ þ zwðf; tÞ, (1)

where Dzrsl(j, t) represents the relative sea-level at alocation j and time t, ze(t) represents the global meltwateradded to the oceans as a result of the melting of ice sheetsand is a function of time t, zi(j, t) represents the glacio-isostatic component at location j and time t, and zw(j, t)represents the hydro-isostatic component at location j andtime t. We do not discuss here the full procedure in detailbut refer to earlier works (e.g. Nakada and Lambeck, 1987;Johnston, 1993; Lambeck et al., 2002; Lambeck andPurcell, 2005).

Solving the sea-level equation (Eq. (1)) requires aconvolution of the Earth’s response to deformation withthe ice and water loads. Included in both zi(j, t) and zw(j,t) are considerations for radial deformation, tangentialdeformation, changes in the gravitational field (equipoten-tial) of the Earth due to a redistribution of mass (both thewater mass as well as mass of displaced mantle), rotationalchanges, and a stress component. Of these the mostimportant are the radial deformation and changes in thegravitational field.

The ice and water loads are the ice-sheet reconstructionand melting history, respectively. Ideally, one has anindependent method of obtaining either the ice-sheetreconstruction and melting history or the Earth para-meters. Independent methods of obtaining the ice-sheetreconstruction and melting history include the mapping ofmoraines and nanatuks (i.e. Denton and Hughes, 1981) orthe use of glaciological models (i.e. Ritz et al., 1997;

Licciardi et al., 1998; Marshall et al., 2002). These icemodels can then be used as loads in the inverting of sealevel data for obtaining rheological parameters. In reality,ice-sheet models remain sufficiently uncertain that therebound data is used to constrain both the ice sheets andthe mantle’s rheology (e.g. Lambeck, 1993a; Wu, 1993;Peltier, 1994). Thus the inversion procedure is an iterativeone with initial ice models based on glaciological modelsscaled to fit the ice volumes required by sea-level recordsfrom locations far removed from the ice sheets (e.g.Lambeck and Purcell, 2005). Earth parameters are thenestimated using sea-level data from other locations bothfar from and under former ice sheets. Using appropriateEarth parameters, corrections for glacio- and hydro-isostatic components are applied to locations far from theice sheets and improved estimates of global ice volumechanges are made. After several iterations consistentsolutions are emerging (Lambeck and Purcell, 2005). Allrelative sea-level calculations were run with 3 or 5iterations of Eq. (1) to take into account the affects ofthe differences in water load felt by areas receiving more orless meltwater than the global averaged values (‘‘eustatic’’sea level) due to the effects of ice loading (Johnston, 1993;Milne et al., 1999; Lambeck et al., 2003) and to takeinto account the migration of shorelines during thedeglaciation phase (Lambeck and Nakada, 1990; Milneet al., 1999).The Gulf of Mexico lies within an intermediate site with

respect to the ice sheets of the LGM (Clark et al., 1978).Within this location, vertical movements are dominated bythe collapse of the glacial forebulge and the effects of waterloading due to the addition of meltwater to the oceanbasins (Clark et al., 1978; Clark and Lingle, 1979; Nakadaand Lambeck, 1987). Fig. 3 shows the respective compo-nents of both the glacial or ice load, zi(j, t), and waterload, zw(j, t), for Earth model E1 and ice model I0 forMobile Bay and the location of the Curray (1960) core. Itshould be noted that, for the time period covered by most


of the observations (i.e. 14 ka—present), the ice loadcomponent is similar between the two sites. However, thewater loads do vary significantly over the last 20 ka (Fig. 3).The water load component is largely responsible for thedifferent RSL values experienced by the two sites (Fig. 1; atthe LGM the ice load also contributes) due to the variabledistances of the two locations from the edge of the waterload—the effects of time-dependent shorelines (Johnston,1993, 1995; Milne et al., 1999; Lambeck et al., 2003). Fig. 4illustrates the modeled sea-level history for a transect ofsites across the central Texas shelf using Earth model E1and ice model I0, defined below.

2.3.2. Earth models

As in most glacial rebound studies, the Earth wasmodeled as a Maxwell viscoelastic sphere, which has beenfound to best simulate the Earth’s response to loads at thetime scale of 1–100 ka (Peltier, 1974; Cathles, 1975; Clark etal., 1978; Nakada and Lambeck, 1987). This means that atshort time scales (i.e. o1 y) the Earth behaves effectively asan elastic solid while on longer time scales (i.e. 41 ka) theEarth behaves as a viscous fluid.

No single set of radially dependent Earth parameterswill effectively simulate the Earth’s response to loadingacross the globe due to known lateral heterogeneitieswithin the Earth’s interior. However, by using a variety ofplausible Earth models it is possible to determine a range ofsea-level histories that should encompass the sea-levelrecord for the Gulf of Mexico. We simulated the responseto over 600 different Earth parameter combinations. Weconsidered elastic lithospheric thicknesses ranging from 50to 120 km, upper mantle viscosities, from the lithosphere/mantle boundary to 670 km depth, ranging from 1020 to

Fig. 4. Relative sea levels along a transect of the central Texas coast and she

represent bathymetry on the Texas shelf with an interval of 10m. Deepest con

1021 Pa s, and lower mantle viscosities, from 670 km to theouter core, ranging from 1021 to 1023 Pa s.

2.3.3. Ice models

Other complicating factors in solving Eq. (1) are theunknowns in the reconstruction of the ice sheets and theirmelting histories. Sea-level records from far-field sites suchas Barbados and Tahiti are thought to be removed frommost of the effects of glacio-isostasy and when correctedfor hydro-isostasy, provide a first-order constraint on thesea-level history and thus melting history of the ice sheets(Peltier, 1998; Lambeck et al., 2002). However, these sea-level histories provide a constraint on the entire meltwaterbudget, not just the North American ice sheets, which havethe largest influence on the Gulf of Mexico.Seven different ice-sheet reconstructions for the Lauren-

tide Ice Sheet were considered that span the range ofplausible models or that have particular features that helpunderstand the dependence of the sea-level predictions forthe Gulf of Mexico on the ice distribution. The first is thenominal ice model, I0, based on ARC3 of Nakada andLambeck (1987) which was derived from the ICE-1 modelof Peltier and Andrews (1976). The second ice model, I1, isthe same as I0 only it has been reduced in thickness by ascalar multiple, b, of 0.42. This ice model is onlyappropriate when used with Earth model E3. A maximumice sheet reconstruction, I2, was based on the ICE-3Gmodel of Tushingham and Peltier (1991). A minimum ice-sheet reconstruction, I3, was that published as theminimum ice model of Licciardi et al. (1998) with theaddition of a Cordilleran and Innuitian Ice Sheet model.These two models should bracket the sea-level predictionsfor the Gulf of Mexico. The fifth ice model, I4, is the same

lf for Earth model E1. mbpsl ¼ meters below present sea level. Contourstour is 100m.

ARTICLE IN PRESSA.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940 927

as I3 except for the time period between 25 and 15.1 kawhere the thickness of the North American ice sheets hasbeen increased by 58%. The sixth ice model, I5, consideredin this study is the same as the minimum reconstruction, I3,but with more ice placed in the Antarctic Ice Sheet. Thefinal ice sheet reconstruction, I6, is the same as I0 exceptthat all ice on the Laurentide Ice Sheet west of 751 W andsouth of 501 N has been removed. In all ice model cases, apre-LGM ice history was assumed based on the global ice-volume curve of Lambeck and Chappell (2001) and on theassumption that the distribution of ice during the pre-LGM period was similar to that during and after theLGM.

All ice models and modeling results are defined incalendar years BP using the time scale of Calib 4.4 (Stuiverand Braziunas, 1993; Stuiver et al., 1998). The other icesheets were included in each model run. The AntarcticIce Sheet reconstruction was based on the model asoriginally outlined by Nakada and Lambeck (1987) withsubsequent revisions assuming that meltwater volume notaccounted for in the other ice sheets came from Antarcticawith a spatial distribution of ice based on the LGMreconstruction of Denton and Hughes (1981). The Fen-noscandian Ice Sheet is based on the model discussed inLambeck et al. (1998), the British Ice Sheet is based on thereconstruction of Lambeck (1993b), the Barents-Kara Seaof Lambeck (1995b), the Greenland ice sheet of Flemingand Lambeck (2004), and a northern and southernhemisphere mountain glacier model described in Lambeckand Purcell (2005).

3. Results

3.1. Mississippi loading

The modeling results from the flexure due to theMississippi Fan are illustrated in Fig. 5a. Again, it mustbe stressed that values used to obtain these results werechosen so as to maximize the possible effects of loading dueto the Mississippi Fan. Our results indicate that, since theLGM, the Mississippi Fan depressed the lithosphere by�200m in the center of the load and nearly 50m at theload’s edge (Fig. 5a). Our results also indicate that themaximum amount of uplift experienced within the fore-bulge of the Mississippi Fan is �5m. The maximummagnitude of the bulge is not strongly dependent on theloading history as decreasing the time frame in whichloading occurred from 8 to 4 ka only increased themagnitude of the forbulge from 5.0 to 5.2m. However,changing the loading history does affect the shape andtiming of the bulge. Assuming the loading occurredbetween 14 and 10 ka creates the largest magnitude ofuplift that can be applied over the longest time period.

The magnitude of uplift due to the Mississippi Fan isalso a function of the distance from the load with themaximum amount of uplift occurring between 150 and475 km from the edge of the load (4.5–71 on Fig. 5a),

depending on the Earth model used. Mobile Bay fallswithin this range of distances. The effects felt in the regionof Curray’s (1960) data would be less than 5m (6.81 onFig. 5a). The effects felt in Galveston Bay would be lessthan 4.5m (7.51 on Fig. 5a) and Corpus Christi Bay wouldexperience less than 2.0m of uplift (9.251 on Fig. 5). Thesevalues alone fall well short of the 10–15m (35m in the caseof Curray’s (1960) data) needed to explain the disparitybetween their sea-level histories and time equivalent globalrecords. However, as the sediment loading may be aportion of the solution, a time-dependent correction forthis signal (Fig. 5b) has been included in all subsequentmodel predictions.

3.2. Glacio-hydro-isostasy

Earth model E1 fits the data best, falling within the errorbars of the observations in Galveston and Corpus ChristiBays. However, the predictions for Mobile Bay and thelocation of the Curray (1960) data both fall outside theerror bars of their respective observations. Model E2, apreferred Earth model from other studies (i.e. Nakada andLambeck, 1987), does not fit the data as well as E1, butdoes explain most of the misfit between the ‘‘global’’ sea-level data and the data within the Gulf of Mexico. Theremaining misfit in all Earth models suggests that, at leastwith the Earth parameters we considered, a solution cannotbe found by varying the Earth parameters alone. It is ofinterest to note that of the Earth model parametersconsidered, the best fit found, using Earth model E1, usesan extreme value of lithospheric thickness (120 km). Peltier(1988) was able to fit the Gulf of Mexico observationaldata to his models using a lithospheric thickness of over200 km. However, the thick lithosphere models do notpredict the shorter wavelength changes in sea level seenacross continental margins or across smaller ice sheets andare, therefore, ruled out (e.g. Nakada and Lambeck, 1988;Lambeck and Nakada, 1990; Lambeck et al., 1998).E3 produced the minimum variance between observa-

tions and predictions when allowing the ice thickness of theLaurentide ice sheet to be scaled by a single scalingparameter (Lambeck et al., 2000). However, this solutionleads to a reduction of the ice thickness to 42% of the icethickness in the nominal model and illustrates the trade-offs that can occur between Earth- and ice-modelparameters in the absence of independent constraints onice thickness.Predictions of relative sea levels from the Late glacial

and early Holocene times are sensitive to the total ice load,the melting history of the ice, and the rheologic parametersof the Earth, whereas the mid to late Holocene sea levelsare sensitive primarily to the bulk characteristics of the iceload and to the rheology of the Earth. Therefore, we willfirst compare predictions and observations from the latterinterval to obtain constraints on the Earth rheology andthen use the data from the earlier period to improve uponthe ice model.

ARTICLE IN PRESS

Fig. 5. (a) Modeling results of the deflection of the lithosphere due to a load approximating the dimensions of the Mississippi Fan as mapped by Stelting et

al. (1986). Distances are in degrees of latitude from the center of the load. Inset is a closer view of the deflection adjusted to highlight the forebulge created

by the load. The parameters for each of the Earth models are given in Table 2. Triangles represent the locations of the estuaries and sites discussed in the

text. MB ¼Mobile Bay, CD ¼ location of Curray (1960) data, GB ¼ Galveston Bay, CCB ¼ Corpus Christi Bay (See Fig. 2 for locations). (b) Time-dependent correction applied to models to account for loading due to the Mississippi Fan for each of the four locations discussed in the text.


By comparing predictions to observations over the last8 ka, the time by which most of the Laurentide Ice Sheetwas melted (Dyke and Prest, 1987; Clark et al., 2000;England et al., 2004; Dyke et al., 2003) and recalling thesubtidal nature of the data such that predictions shouldalways lie above observations, several possible Earthmodels can be eliminated. During this period, predictedsea levels should at no point be lower than the sea-levelobservations as they were all deposited in subtidal settings.Using this procedure and assuming the current ANU icemodel (I0) is correct, we were able to determine thefollowing constraints on the upper and lower mantle

viscosities, respectively:

ZumX3� 1020,

ZlmX2� 1022.

We were unable to put any constraints on the litho-spheric thickness. Over the last 8 ka, sea-level predictionsfrom models with upper and lower mantle viscosities lessthan these values, such as E3, lie below sea-level observa-tions while sea-level predictions from models with upperand lower mantle viscosities greater than these values,such as models E1 and E2, lie above sea-level observations

ARTICLE IN PRESS

Fig. 6. Modeling results compared to sea-level observations for each of the respective Earth parameters at locations (i) Corpus Christi Bay, Texas,

(ii) Galveston Bay, Texas, (iii) Mobile Bay, Alabama, (iv) location of Curray (1960) data, and (v) Barbados. With the exception of Barbados, each model

has a correction applied to take into account the loading due to the Mississippi Fan (see Fig. 5b).


(Fig. 6). This trend appears to be independent of the icemodel as the same relationship is found between observa-tions and predictions over the last 8 ka using two other icemodels, I2 and I3 (Fig. 7c). Therefore, we limit our searchto Earth models E1 and E2.

3.3. Ice models

The other variable in the modeling procedure is the ice-sheet reconstructions. Fig. 7a illustrates the results whenusing a maximum (I2) and minimum (I3) Laurentide Ice

ARTICLE IN PRESS

Fig. 7. (a) Relative sea level for Earth model E1 for a maximum (I2) and minimum (I3) ice model reconstruction at locations (i) Corpus Christi Bay,

Texas, (ii) Galveston Bay, Texas, (iii) Mobile Bay, Alabama, (iv) location of Curray (1960) data, and (v) Barbados With the exception of Barbados, each

model has a correction applied to take into account the loading due to the Mississippi Fan (see Fig. 5b). (b) Relative sea level for Earth model E2 for a

maximum (I2) and minimum (I3) ice model reconstruction at locations (i) Corpus Christi Bay, Texas, (ii) Galveston Bay, Texas, (iii) Mobile Bay,

Alabama, (iv) location of Curray (1960) data, and (v) Barbados. With the exception of Barbados, each model has a correction applied to take into account

the loading due to the Mississippi Fan (see Fig. 5b). (c) Relative sea level for Earth model E3 for a maximum (I2) and minimum (I3) ice model

reconstruction at locations (i) Corpus Christi Bay, Texas, (ii) Galveston Bay, Texas, (iii) Mobile Bay, Alabama, (iv) location of Curray (1960) data, and (v)

Barbados. With the exception of Barbados, each model has a correction applied to take into account the loading due to the Mississippi Fan (see Fig. 5b).


ARTICLE IN PRESS

Fig. 7. (Continued)


Sheet model with the Earth model whose initial predictionsbest fit the observations, E1. Of the two scenarios, icemodel I3 fits the data best. When the appropriatecorrection for the time-dependent deformation due to theMississippi Fan is included the predictions fall within theerror bars of the observational data for sites within the

Gulf of Mexico (Fig. 7a). This trend appears to beindependent of the Earth model considered, as a similarpattern is found when using the maximum (I2) andminimum (I3) ice models with Earth model E2 (Fig. 7b).In addition, when multiplying the thickness of the entireLaurentide ice sheet by a scalar to obtain the best fit

ARTICLE IN PRESS

Fig. 7. (Continued)


between the observations and predictions, a value for b ofo1 (0.42) brings the closest fit (I1; Fig. 8). However, if theother ice-sheet reconstructions (i.e. Fennoscandian, Ant-arctic, etc.) are approximately correct, ice models I1 and I3largely underestimate the meltwater volume delivered tothe oceans since the LGM, particularly for the time period

prior to 15 ka. This underestimation of meltwater volumesis best illustrated by examining the model results forBarbados, which lies outside the area of major isostaticadjustment to the Laurentide Ice Sheet (Figs. 7a, b, and 8).In order to compensate for this discrepancy in meltwatervolumes, two scenarios are considered.

ARTICLE IN PRESS

Fig. 8. Relative sea levels for Earth model E3 and a scaled ice sheet model I1 that is based on the nominal ice model (I0) with a scalar multiple of 0.42

applied to the ice sheet thickness across its entire area at locations (i) Corpus Christi Bay, Texas, (ii) Galveston Bay, Texas, (iii) Mobile Bay, Alabama, (iv)

location of Curray (1960) data, and (v) Barbados. With the exception of Barbados, each model has a correction applied to take into account the loading

due to the Mississippi Fan (see Fig. 5b).


The first scenario is increasing the volume of ice on theLaurentide Ice Sheet within ice model I3 to roughlybalance the global meltwater volume. As the deficit inmeltwater volume obtained from ice model I3 is only found

prior to the first meltwater pulse 1-A (MWP1-A) between14 and 15 ka (Fig. 7), we increased the volume of ice withinthe Laurentide Ice Sheet in ice model I4 by 58% over I3from 25 to 15 ka. Thus, ice volumes of the Laurentide Ice

ARTICLE IN PRESS

Fig. 9. Relative sea levels for Earth model E1 and ice sheet model I4 at locations (i) Corpus Christi Bay, Texas, (ii) Galveston Bay, Texas, (iii) Mobile Bay,

Alabama, (iv) location of Curray (1960) data, and (v) Barbados. With the exception of Barbados, each model has a correction applied to take into account

the loading due to the Mississippi Fan (see Fig. 5b).


Sheet in ice model I4 are in close agreement with otherNorth American ice sheet models from the time periodbetween MWP1-A and the LGM (i.e. I0 of this study;Nakada and Lambeck, 1991; Tushingham and Peltier,1991; Peltier, 1994; Peltier, 2005), but are significantly

reduced from the time period between MWP1-A and 8 ka.Other studies have come to similar conclusions requiring areduction in the thickness of ice over the Laurentide IceSheet during the early Holocene in order to fit rebounddata from across North America (Lambeck et al., 2000).

ARTICLE IN PRESS

Fig. 10. Relative sea levels for Earth model E1 and ice sheet model I3 with a source for meltwater pulse 1-A in an ice sheet other than the Laurentide Ice

Sheet at locations (i) Corpus Christi Bay, Texas, (ii) Galveston Bay, Texas, (iii) Mobile Bay, Alabama, (iv) location of Curray (1960) data, and (v)

Barbados. With the exception of Barbados, each model has a correction applied to take into account the loading due to the Mississippi Fan (see Fig. 5b).


Ice model I4 with Earth model E1 produces a reasonable fitbetween model predictions and observations for our foursites within the Gulf of Mexico, including Curray’sobservation, as well as for Barbados (Fig. 9). The second

scenario that would balance the global meltwater volumesbased on the North American portion of ice model I3 is toincrease the ice volumes in other ice sheets, the bestcandidate being Antarctica. However, this scenario does


not produce as good a fit between observations andpredictions within the Gulf of Mexico, most notablyCurray’s (1960) data (Fig. 10), as does the first scenario.

4. Discussion

4.1. Tectonic movements and compaction

Paine (1993) established the vertical movement oflocations across the northwestern Gulf of Mexico, includ-ing our study sites, based on the position of the LastInterglacial shoreline (OIS 5e) where it is known as theIngleside barrier along the Texas coast. He concludes that,based on this datum, and the assumption that in theabsence of tectonics its expected location would be at5–8m, the Gulf of Mexico has subsided �6m over the last120 ka, not risen. However, given the importance of glacio-hydro-isostasy within the Gulf of Mexico during the LGM,applying a sea-level datum from other parts of the globeto the Gulf of Mexico during the Last Interglacial maynot be appropriate. Many other studies of the Gulf ofMexico shelf’s stratigraphic architecture suggest subsidence

Fig. 11. Relative sea levels for Earth model E1 if the Laurentide Ice Sheet w

(i) Corpus Christi Bay, Texas, (ii) Galveston Bay, Texas, (iii) Mobile Bay, Alab

applied to take into account the loading due to the Mississippi Fan (see Fig.

over the last 120 ka (Berryhill et al., 1986; Paine, 1993;Abdulah et al., 2004). Also tide-gauge data indicate apresent-day subsidence of 4–8mm/yr at Galveston Bay(Paine, 1993).One possible cause of vertical motion is sediment

compaction. None of the above sea-level indicators havebeen corrected for this. Pizzuto and Schwendt (1997)calculated the total amount of compaction within a verysimilar estuarine setting along the Delaware coast to thatfound within the Gulf of Mexico and found that thecompaction was on the order of 2m for a 10m-thickHolocene succession. However, correcting for compactionreduces the depth of sea-level indicators and furtherincreases the disparity between sea-level indicators in theGulf of Mexico and equatorial regions.

4.2. Radiocarbon reservoir

Another possible explanation for the disparity betweenthe observed sea-level curves generated within the Gulf ofMexico and those generated elsewhere (Fig. 1a) could beincorrect ages for markers of sea level. In particular,

est of 751 W and south of 501 N were completely removed at locationsama, and (iv) location of Curray (1960) data. Each model has a correction

5b).

ARTICLE IN PRESS

Fig. 12. Modeling results compared to sea-level observations for each of the respective Earth parameters at locations (i) Ungava Bay and (ii) Cape

Henrietta Maria. Data for Ungava Bay is from Gray et al. (1980) and dates were obtained from the mollusks Mya truncata and Balanus crenatus. Error

bars are set as the depth range of the respective species. Data from Cape Henrietta Maria were obtained from peats by Skinner (1973). Depth error bars

were assigned assuming the peat was deposited within 10m of sea level. In both cases ages were calibrated using Calib 5.0 (Stuiver and Braziunas, 1993;

Stuiver et al., 1998) and errors reported in 2 sigma.


reservoir effects may be larger than assumed. However,two arguments can be made to refute this. First, aconsistent radiocarbon reservoir effect for these estuarieshas been independently established (as discussed pre-viously). Second, based on their age/depth relationships,no distinction between a wood and shell population is seen(Fig. 1a).

4.3. Laurentide Ice Sheet

The best agreement between observed and predicted sealevels occurs with ice model I4. Ice model I4 has a thinnerLaurentide Ice Sheet (the ice sheet of Licciardi et al., 1998)after 15 ka than the other ice models, but a similarthickness before 15 ka. This implies a significant thinningof the Laurentide Ice Sheet around the time period ofMWP1-A (Fig. 9). Debate continues on whether theLaurentide Ice Sheet was the source of MWP1-A. Keigwinet al. (1991), Charles and Fairbanks (1992), Fairbankset al. (1992), Lehman and Keigwin (1992) and Flower et al.(2004) support increased amounts of meltwater from theLaurentide Ice Sheet during MWP1-A based on oxygenisotopes from cores in the Gulf of Mexico and AtlanticOcean. From the relative abundances of reworked calcar-eous nannofossils within the Orca Basin of the Gulf ofMexico, Marchitto and Wei (1995) showed an increase inthe amount of meltwater traveling down the MississippiRiver during MWP1-A, suggesting a contribution fromsome portion of the Laurentide Ice Sheet. Peltier (1994,2004, 2005) supports a Laurentide source for MWP1-Abased on relative sea-level changes across the globe, butincludes little sea-level data from North America prior to8 ka (Peltier, 1994, 2004; Peltier, 2005). Tarasov and Peltier(2004, 2005) support at least some contribution toMWP1-A from the Laurentide Ice Sheet based on aglacial-systems model.

Clark et al. (1996, 2002), Weaver et al. (2003), andBassett et al. (2005) have made convincing arguments for atleast some Antarctic contribution to MWP1-A. Ouranalysis for sea-level predictions within the Gulf of Mexicoagree with Clark et al. (2002), specifically, that MWP1-Acould not have solely come from the southern sector of theLaurentide ice sheet, as removing all the ice within theLaurentide Ice Sheet below 501 N (ice model I6) fails toreconcile the predictions and observations, particularly forthe location of the Curray (1960) core (Fig. 11). AlthoughAntarctic Ice Sheet models vary greatly (Nakada et al.,2000) and the model used in Fig. 10 may not be the mostaccurate, it is the isostatic uplift of the Gulf of Mexicoprovided by Laurentide ice melt during the time of MWP1-A that leads us to favor a large contribution from theLaurentide Ice Sheet at this time. Ice model I4 has asignificant reduction in ice thickness for most of theLaurentide Ice Sheet and not just its southern margins.Physically this could be a result of a change from a frozen-bed ice sheet to a more soft-bedded ice sheet similar to aform described by Licciardi et al. (1998), possibly via ice-streaming (e.g. Andrews et al., 1998; Miller et al., 2002;Stokes et al., 2005) as has been inferred to have occurred inthe Scandinavian Ice Sheet (Lambeck et al., 1998). A rapiddeflation of the entire ice sheet with little change to its aerialextent would also help explain the absence of an earlierintroduction of meltwater to the eastern outputs such as theGulf of St. Lawrence. A deflation of the entire LaurentideIce Sheet as a source for MWP1-A falls within the errorbars of Clark et al. (2002) ‘‘fingerprint’’ test of the event.It should be noted that the ice-sheet reconstruction

advocated by this study (I4), with a Laurentide source forMWP1-A, best describes the bulk characteristics of the icesheet and needs to be further tuned when applied to specificareas that were once overridden by the former ice sheets.For some areas such as Ungava Bay the model works well


(Fig. 12a) and for other regions such as Cape HenriettaMaria, the model needs further improvement (Fig. 12b).The misfit in some areas may also be due to mantleheterogeneities and the uncertainties of the Canadian sea-level indices relationship to former sea levels (Mitrovicaet al., 2000).

5. Conclusions

The sea-level records from the northern Gulf of Mexicoare up to 35m shallower than ‘‘global’’ records at 18 kaand decrease to 15m at 10 ka. Sedimentary loading, alarger than assumed radiocarbon reservoir correction, ortectonic activity alone cannot explain this difference.Glacio-hydro-isostasy, however, can explain this differ-ence. Its magnitude will be a function of earth rheology andof changes in the ice load. Thus, we have used this methodto infer the distribution of ice within the Laurentide icesheet during the LGM and Late glacial periods. The best fitbetween the glacio-hydro-isostatic models and the observa-tional data from the Gulf of Mexico is found when using aminimum ice-sheet reconstruction for the Laurentide IceSheet for the time period between 15 ka to the present witha significant increase in the thickness of the ice sheetbetween 25 and 15 ka and a small time-dependentcorrection for the uplift associated with loading due tothe Mississippi Fan. The data is consistent with aLaurentide Ice Sheet whose ice volume is greatly reducedat the time of MWP1-A. However, a meltwater sourcefrom the southern Laurentide Ice Sheet alone cannotreconcile the sea-level observations with the model predic-tions, and a reduction in ice thickness across much of theLaurentide Ice Sheet is required.

Acknowledgements

The authors would like to thank Chip Anderson, PatrickTaha, Kristy Milliken, and Jessica Maddox for their helpin collecting cores in the field. We also wish to thank HerbMcQueen and Jason Zao with their help with thecomputing facilities. Adrian Lenardic and Jerry Dickensprovided helpful insights on earlier versions of thismanuscript as did Thomas Cronin, Jerry Mitrovica, andan anonymous reviewer. This research was funded by theNational Science Foundation Grant EAR-0107650, aNational Science Foundation Graduate Research Fellow-ship, GCSSEPM Ed Picou Graduate Research Grant, aStudent Grant from the International Association forMathematical Geology, the Australian National Universityand the Australian Research Council.

Appendix A. Supplementary Materials

Supplementary data associated with this article can befound in the online version at doi:10.1016/j.quascirev.2007.01.001.

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Sea-level history of the Gulf of Mexico since the Last Glacial Maximum with implications for the melting history of the Laurentide Ice SheetIntroductionMethodsSea-level observationsNumerical Modeling of the Mississippi FanHydro-glacio-isostasyTheoryEarth modelsIce models

ResultsMississippi loadingGlacio-hydro-isostasyIce models

DiscussionTectonic movements and compactionRadiocarbon reservoirLaurentide Ice Sheet

ConclusionsAcknowledgementsSupplementary MaterialsReferences

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