Sea-Level History of the Gulf of Mexico Since the Last Glacial Maximum with Implications for the Melting History of the Laurentide Ice Sheet

ARTICLE IN PRESS

Quaternary Science Reviews 26 (2007) 920–940

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

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

aT. Boone Pickens School of Geology, Oklahoma State University, 105 NRC, Stillwater, OK 74078, USA bResearch 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, USA dInstitute 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 35 m 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 from Mississippi 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 authors have argued for a sea-level lowstand of 120 m within the Gulf of Mexico, bringing the ‘‘global’’ and Gulf Arguing that the Gulf of Mexico was tectonically stable, of Mexico records into agreement at the LGM (Curray, Shepard (1960) produced one of the first ‘‘eustatic’’ sea- 1960; Berryhill et al., 1986). However, none of these level curves for the last 30 ka from radiocarbon dates features have been dated nor corrected for the high obtained from shallow marine mollusks and wood subsidence rates usually present on the outer shelf. fragments. Redfield (1967) and Emery and Garrison Continued work on refining the ‘‘global’’ sea-level record (1967) both noted that the sea-level curves of Shepard by Fairbanks (1989), Chappell and Polach (1991), Bard (1960), and later works by McFarland (1961), obtained et al. (1996), Yokoyama et al. (2000), and Hanebuth et al. from the Gulf of Mexico were up to 35 m shallower at the (2000) and the Gulf of Mexico sea-level record by Nelson Last Glacial Maximum (LGM), and 15 m shallower 10 ka, and Bray (1970) and Bart and Ghoshal (2003) continues to than sea-level curves from other places across the globe. On indicate sea levels within the Gulf of Mexico 35 m the basis of submerged shoreline features and deltas, some shallower during the LGM, and 15 m shallower at 10 ka, than ‘‘global’’ sea-level records (Fig. 1a). A similar disparity has been noted by other studies of sea-level Ã Corresponding author. Tel.: +1 405 744 7725; fax: +1 405 744 7841. indicators from Florida and the Caribbean (e.g. Scholl E-mail addresses: [email protected] (A.R. Simms), et al., 1969; Toscano and Macintyre, 2003). [email protected] (K. Lambeck), [email protected] (A. Purcell), [email protected] (J.B. Anderson), [email protected] Many authors have noted great variability in Late (A.B. Rodriguez). Quaternary/Holocene sea-level records globally. Pirazzoli

compensation from flexural loading of the Gulf of Mexico due to sediments delivered by the Mississippi River. This was the same conclusion that Redfield (1967) and Emery and Garrison (1967) suggested. However, to date, no one has quantitatively tested this hypothesis. Another notable cause of local and regional variations in sea level is the Earth’s differential isostatic response to the waxing and waning of ice sheets since the LGM 18 ka. As the ice sheets grew and melted over the Last Glacial- eustatic cycle, the effective sea-level change at any one point on the globe is not simply a function of the volume of meltwater 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 mass shifted from the polar ice caps and glaciers to the oceans. As a consequence, the isostatic compensation and gravitational potential for the ice and water loads also changed through time. Several authors have found success in quantitatively modeling the Earth’s response to these changes in the distribution of surface loads. Cathles (1975) and Farrell and Clark (1976) formulated some of the earliest numerical models to calculate the Earth’s response to the changing ice sheets. Since then, many authors have advanced the numerical techniques and theory 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 Fig. 1. (a) Sea-level indices from the Gulf of Mexico and from the coral- estuaries along the northwestern Gulf of Mexico from based records of Fairbanks (1989) and Bard et al. (1996) and the Sunda Corpus Christi, Texas to Mobile, Alabama (Fig. 2) as well Shelf shallow marine environments of Hanebuth et al. (2000). Error bars for the coral-based data were calculated by summing the squares of half as other previously published data to test the several the growth range of the corals plus half the tectonic correction as reported possible mechanisms for the disparity between sea level by the original authors. For the Sunda Shelf we assumed an error less than within the Gulf of Mexico and other global sites (Fig. 1a). 2m (Hanebuth et al., 2000). Gulf of Mexico dates come from this study Our new data from each of these three estuaries confirms and Shepard and Moore (1955), Curray (1960), Rehkemper (1969), Nelson earlier works and systematically indicates a sea-level and Bray (1970), and To¨ rnqvist et al. (2004). Blue diamonds represent dates obtained from shell material and green triangles represent dates 10–15 m higher than global-eustatic values for the period 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.

(1991) compiled an atlas of Late Quaternary/Holocene sea- level curves from across the world and noted their wide range of magnitudes and histories. Pirazzoli (1991) also explored many of the possible causes for regional variations in sea level. One of the most common causes for regional variations is tectonic activity. In many areas, Fig. 2. Map of the study area showing the location of the three estuaries tectonic activity can keep pace and even exceed rates of sea- examined in this study as well as the location of two R. cuneata shells level change. Bart and Ghoshal (2003) argued that sea level obtained in a core by Curray (1960). Also shown is the size and thickness in the Gulf of Mexico never reached the full 120 m during 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 the LGM based on their observation that no shelf margin a contour interval of 50 m. Stars indicate locations of sea-level predictions deltas have been incised by fluvial systems to depths greater and cores with radiocarbon dates listed in Table 1. Diamonds represent than 90 mbsl. They attributed this disparity to isostatic locations of dates shown in Fig. 1a. ARTICLE IN PRESS 922 A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940 between 8 and 10 ka (Fig. 1b). This trend is also found in the same depth that were independently dated within 200 data from other studies from the Gulf of Mexico dating years of one another by Neumann (1958) and reported by back to the LGM in which no published dated sea-level Curray (1960). R. cuneata is a mollusk that lives only indices from LGM times are found deeper than 92 m within shallow freshwater influenced estuarine environ- (‘‘freshwater’’ shells from McFarland, 1961, Table S1 in ments (Parker, 1959). The other dates from similar studies supplementary materials). Using quantitative models de- conducted across the Gulf of Mexico were also used to veloped at the Australian National University (Nakada supplement the new data and better establish the misfit and Lambeck, 1987; Johnston, 1993; Lambeck and between the ‘‘global’’ eustatic records of Fairbanks (1989), Johnston, 1998; Lambeck et al., 2003) and observational Bard et al. (1996), and others and the Gulf of Mexico sea- data, we investigate the possible explanations for the level indicators (Fig. 1a). observed disparity. The possibilities considered in this The new radiocarbon dates were obtained from two paper include: a large radiocarbon reservoir for the sources: mollusks and wood fragments (Table 1). The northwestern Gulf of Mexico, tectonic uplift over the sedimentology of these deposits is described in detail for entire region, isostatic compensation for the large sedi- Corpus Christi Bay in Simms (2005), for Galveston Bay in mentary load of the Mississippi River, and glacio-hydro- Rodriguez et al. (2004), and for Mobile Bay in Metcalf and isostatic adjustments to the ice sheets that melted since Rodriguez (2003). Most of the mollusks used for dating the LGM. were articulated. In order to test the reliability of dates obtained from wood fragments and unarticulated mol- 2. Methods lusks, dates were only considered from cores with several radiocarbon dates from a variety of depths. The chronos- 2.1. Sea-level observations tratigraphic consistency with depth increased the confi- dence that the material dated was not reworked by coastal Sea-level observations were obtained from shallow or estuarine processes. marine deposits from cores taken in Corpus Christi Bay, The deposits containing the age-dated material represent Texas, Galveston Bay, Texas, and Mobile Bay, Alabama either an upper- or middle-bay environment based on aboard the R.V. Trinity (Fig. 2). These three areas were coarse-fraction analysis (Shepard and Moore, 1954; chosen for two reasons: (1) their location within an incised Metcalf and Rodriguez, 2003; Rodriguez et al., 2004; valley providing the best preservation potential for shallow Simms, 2005). In addition, with the exception of Mulinia marine deposits in light of transgressive ravinement lateralis, the mollusks in which dates were obtained (e.g. (Belknap and Kraft, 1981, 1985) and (2) their relatively Rangia sp.; Macoma mitchelli, Nuculana sp.) are restricted large distances from major depo-centers, most notably the to upper- or middle-bay environments (Parker, 1959; Mississippi River, Brazos-Colorado delta complex, and the Andrews, 1971; White et al., 1983). M. lateralis is known Rio Grande, minimizing the effects of compaction and to inhabit the inner shelf; however, when found along the subsidence. The one limitation to observations obtained inner shelf it is found in association with other species from these cores is their relatively young nature, no endemic to inner shelf conditions such as Anadara sp. material older than 9.8 ka, a consequence of sea level being When found alone, M. lateralis is almost always an lower than these positions before 10 ka. Most of the misfit indicator of an enclosed lagoon characterized by wide described in previous work (i.e. Redfield, 1967; Bart fluctuations in salinities (Parker, 1959; Andrews, 1971; and Ghoshal, 2003) is from time periods older than this White et al., 1983). Today, these environments are in water (Fig. 1a). depths less than 5.5 m and the mollusks indicative of upper- In order to supplement our dataset and extend the time bay/river-influenced regions (e.g. Rangia sp.) are not found span in which the various hypotheses for the disparity in water depths greater than 2.0 m (White et al., 1983; could be tested, we included other reliable indicators of sea Parker, 1959). The deposits in which plant/wood fragments level within the Gulf of Mexico made available by previous were used to date were interpreted as bayhead deltaic in studies in our analysis. Of over 250 radiocarbon dates origin based on their sedimentary characteristics (e.g. assembled from the literature and other studies by the peats, R. cuneata, course and lithic sand component, authors (Table S1 in supplementary materials), we and prevalent regular bedding). These environments narrowed this list to 98 dates by only including points today are found in water depths not exceeding 2 m bsl. that were obtained from a species whose relationship to The present tidal range within these estuaries is less than sea-level was well constrained (e.g. an estuarine species) 1.0 m. For this study, we compared the predicted sea levels and whose age was determined from a single specimen with the observed depth of the sea-level indicator. As each (Fig. 1a and Table S2 in supplementary materials). For of the observations represent deposition within an inter- comparisons with numerical models, we limited our tidal to subtidal (0 to 2 m) depositional setting, they analyses to our three new sites and the most reliable should always lie close to or below predictions, but not indicator of sea levels between 18 and 10 ka available in the above. literature. We chose a sea-level indicator obtained from Although some variability in the age–depth relationship two articulated Rangia cuneata shells from the same core at among the sea-level indices can be seen among the ARTICLE IN PRESS A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940 923

Table 1 Radiocarbon dates utilized in this study

Core # Material Lat. Long. Depth Depth C14 Error C14 Res Calender 2s 2s bsl error (7) years Res error (7) years older younger

CCB03-01 M. lateralis or R. fiexusob 27.776 97.278 7.85 1.0 2390 30 760 350 1590 850 845 CCB03-01 M. lateralisb 27.776 97.278 10.99 1.0 4470 35 760 350 4100 1015 1165 CCB03-01 R. flexusa 27.776 97.278 11.97 1.0 5010 30 760 350 4800 1065 930 CCB03-01 Mollusk fragment 27.776 97.278 17.04 1.1 6520 55 760 350 6590 840 830 CCB03-01 M. campechiensis texana 27.776 97.278 17.81 1.1 6780 45 760 350 6870 865 725 CCB03-01 M. campechiensis texana 27.776 97.278 19.06 1.1 7500 45 760 350 7600 805 745 CCB03-01 N. concentrica 27.776 97.278 19.77 1.1 7890 60 760 350 7980 725 955 CCB03-01 A. aequalisb 27.776 97.278 20.62 1.1 8050 40 760 350 8140 700 860 CCB03-01 N. concentricab 27.776 97.278 21.58 1.1 8470 40 760 350 8590 755 880 CCB03-01 N. concentrica 27.776 97.278 24.42 1.1 9240 65 760 350 9480 1000 1000 NB03-01 M. lateralisb 27.849 97.486 2.02 1.0 2630 45 760 350 1850 785 895 NB03-01 M. lateralis 27.849 97.486 3.27 1.0 3060 45 760 350 2340 920 910 NB03-01 M. lateralis 27.849 97.486 5.32 1.0 5640 60 760 350 5580 1000 855 NB03-01 M. Mitchellib 27.849 97.486 6.72 1.0 5980 60 760 350 5980 995 870 NB03-01 N. acuta 27.849 97.486 8.82 1.0 6640 50 760 350 6720 810 780 NB03-01 N. concentricab 27.849 97.486 11.53 1.0 7900 100 760 350 7800 820 755 NB03-01 Rangia flexusob 27.849 97.486 10.68 1.0 7700 70 760 350 7990 735 985 GB-00-1.1 C. virginica 29.571 94.877 9.42 1.0 6620 64 225 103 7280 300 215 GB-00-1.1 Wood 29.571 94.877 9.58 1.0 6454 51 0 0 7370 95 70 GB-00-1.1 Wood 29.571 94.877 10.27 1.0 6900 35 0 0 7730 65 100 GB-00-1.2 Wood 29.571 94.877 11.29 1.1 7413 53 0 0 8250 195 115 GB-00-1.2 Plant/Wood 29.571 94.877 12.93 1.1 7660 40 0 0 8450 55 90 GB-00-1.3 Plant/Wood 29.571 94.877 16.8 1.1 8330 45 0 0 9350 210 120 MD-02-1 Plant/Wood 30.677 87.969 2.17 1.0 720 30 0 0 675 105 50 MD-02-1 Plant/Wood 30.677 87.969 9.42 1.0 5070 30 0 0 5820 70 85 MD-02-1 Plant/Wood 30.677 87.969 9.42 1.0 5180 30 0 0 5940 30 55 MD-02-1 Plant/Wood 30.677 87.969 11.16 1.0 6220 50 0 0 7120 120 140 MD-02-1 Plant/Wood 30.677 87.969 12.29 1.0 7190 35 0 0 8000 60 155 MD-02-1 Plant/Wood 30.677 87.969 13.1 1.0 8000 55 0 0 8860 215 150 MD-02-1 Plant/Wood 30.677 87.969 14.92 1.1 8210 40 0 0 9180 145 115 MD-02-1 Plant/Wood 30.677 87.969 15.54 1.1 8480 35 0 0 9500 50 35 MD-02-1 Plant/Wood 30.677 87.969 17.26 1.1 8710 35 0 0 9660 110 230 MD-02-1 Plant/Wood 30.677 87.969 18.09 1.1 8770 50 0 0 9780 220 340 57-9a R. cuneatab 28.157 94.293 57.6 1.2 13360c 470 450 225 15200 1415 1420 57-9a 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 Galveston Bay, MB ¼ Mobile Bay. Depths are given in meters. Res ¼ Reservoir. aCurry (1960). bArticulated. cCorrected for 13C. three bays, a remarkable consistency is found within paleo sea levels was assigned to sea-level indices from the the sea-level indices of the three estuaries (Fig. 1b). longer core. The one exception to this is Corpus Christi Bay where Fluvial waters can carry carbon with an anomalously the longest core was taken in deeper water than cores low abundance of 14C(Broecker and Walton, 1959; from the other three bays and the dated deposits Raymond and Bauer, 2001). As a result, deposits from represent deeper-water environments (4–5 m versus areas influenced by rivers, such as estuaries, may have a 1–2 m). In order to compare sea-level indicators from like larger radiocarbon-reservoir effect than the ‘‘normal’’ environments, we also considered another but shorter core seawater correction of 440 years (Stuiver et al., 1998), from the Corpus Christi Bay system—NB03-01. Sea-level resulting in anomalously old ages. This effect could affect indicators from this shorter core are more consistent with carbon in organisms precipitating material from the water indicators from the other two bays (Fig. 1b). However, column (i.e. mollusks), although it should not affect because the data from the longest core in Corpus organic carbon in plants that sequester their carbon from Christi Bay extends further back in time than NB03-01, the atmosphere. Aten (1983), in a study of archeological the longest core was used in comparison to sea-level sites, established the carbon reservoir effect for the mollusk predictions. However, a larger uncertainty with respect to R. cuneata from Galveston Bay, Texas and from several ARTICLE IN PRESS 924 A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940 other estuaries and rivers along the Texas/Louisiana Coast. In order to test if the loading due to the Mississippi Fan Aten’s (1983) correction of 2257103 years within the is of sufficient magnitude to explain the disparity between Trinity Delta of Galveston Bay fits a linear trend the sea-level indicators from the Gulf of Mexico and global (Pearson’s R ¼ 0.985) without significant variation for records, we simulated the Earth’s response to loading of the last 2 ka. One of our study sites, Corpus Christi Bay, the Mississippi Fan using a simple model for the was not included in Aten’s (1983) study. We obtained a sedimentary-load history. The rebound model used is radiocarbon date from an articulated barnacle attached to based on the same numerical models as that used for a wood fragment. Both the barnacle and wood fragment glacial loading and unloading. However, the effect of were dated with an age of 8430745 and 7670745, global loading of the sea floor by the sediment-displaced respectively. The difference between the two is 760 years. water is ignored. The time scale of recent sediment loading This value is consistent with other values for radiocarbon is similar to that for glacial loading so that the same reservoirs established from other estuaries along the Texas rheological parameters can be used for both sediment and Coast (Aten, 1983). Assigning a radiocarbon reservoir of glacial loading problems. As noted above, core sites lie 760 years to dates obtained from mollusks in cores from beyond the sediment depocenter such that the rebound will Corpus Christi Bay near the surface (o0.5 m) also brings be that of a peripheral bulge around the load and one of their radiocarbon age to modern values. Although, the uplift. radiocarbon reservoir is species and time dependent, we We assumed an axi-symmetric load of a parabolic found that a value of 760 years, when applied to a variety function with a maximum sediment thickness of 400 m, a of mollusks within Corpus Christi Bay, results in a radius of 3.01 (330 km), and a density of 2.3 g/cm3. The remarkable agreement among all the radiocarbon dates density values obtained in cores are significantly lower, obtained. These values of a radiocarbon reservoir were ranging from 1.7 to 2.0 g/cm3 due to their unconsolidated used instead of Stuiver et al. (1998) ‘‘normal’’ seawater nature and inherent porosity, which in some cores reservoir of 440 years. With the exception of one date from approach 50% (Shipboard Scientific Party, 1986). In Galveston Bay, all other dates from Galveston and Mobile addition, the effective load would also be reduced by the bays were taken from wood or plant fragments which density of the water displaced by the sediments, thus 3 3 should not be subject to radiocarbon reservoir affects r–rw g/cm . We adopt a value of r–rw ¼ 2.3 g/cm as a (Table 1). maximum value and any deflection estimate will be All dates obtained in this study were corrected for their overestimated. In order to obtain the maximum amount appropriate radiocarbon reservoir, calibrated using Calib of uplift possible within the forebulge, we allowed the 5.0 (Stuiver and Braziunas, 1993; Stuiver et al., 1998), and sedimentary loading history to vary between 18 and 10 ka. reported in calendar years BP. The same procedure was Assuming a loading history commencing prior to 18 ka followed for the radiocarbon dates obtained from the two would only diminish estimates for the magnitude of uplift. R. cuneata shells of Curray (1960) and all other data We also allowed the onset and duration of loading to vary presented in Fig. 1. In addition, all dates from other over that 8 ka (18–10 ka) time span in order to determine its sources that were not corrected for the ratio of 13C/12C effect. were corrected for 13C using the formula given on the In addition to the size of the load, the Earth’s response NOSAMS website (http://www.nosams.whoi.edu/clients/ to loading is dependent on the structure of the Earth. One data_d13correction.php3) and assuming the average values critical parameter is the structure and viscosity of the of d13C obtained from our data set of 27.5 for plant mantle. We used a three layer mantle with 6 different material and 0.66 for shell material. These corrections combinations of rheological parameters that are represen- were applied in order to assure that all radiocarbon dates tative of values found in other regions across the globe (e.g. were calibrated to the same time scale. Lambeck and Chappell, 2001). The boundary between the upper and lower mantle was assumed to be at 670 km. The 2.2. Numerical Modeling of the Mississippi Fan Earth model parameters for each of the runs are summarized in Table 2. The largest depositional feature formed over the last glacial-eustatic cycle in the Gulf of Mexico is the Mississippi sea-floor fan. The fan is roughly 600 km long Table 2 and 400 km wide, attaining a maximum thickness of over Earth parameters for model results shown in this paper 400 m (Bouma et al., 1983; Fig. 2). The timing, extent, and E1 120 10 30 depositional history of this feature have been studied in E2 80 3 30 great detail by a number of authors (Curray, 1960; Bouma E3 50 1 20 et al., 1983; Stelting et al., 1986; O’Connell and Normark, E4 50 1 1 1986). According to Stelting et al. (1986), the youngest lobe E5 120 10 100 of the Mississippi Fan was deposited sometime after 55 ka. E6 65 4 10 By 10 ka, most of the Mississippi Fan was deposited m ¼ viscosity. The boundary between the upper and lower mantle was (Wetzel and Kohl, 1986). assumed to be 670 km. ARTICLE IN PRESS A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940 925

2.3. Hydro-glacio-isostasy Licciardi et al., 1998; Marshall et al., 2002). These ice models can then be used as loads in the inverting of sea 2.3.1. Theory level data for obtaining rheological parameters. In reality, The groundwork for quantitative formulations of the ice-sheet models remain sufficiently uncertain that the Earth’s response to glacio-hydro-isostasy was most exten- rebound data is used to constrain both the ice sheets and sively laid out by Cathles (1975), Farrell and Clark (1976), the mantle’s rheology (e.g. Lambeck, 1993a; Wu, 1993; and Clark et al. (1978). One of the important advance- Peltier, 1994). Thus the inversion procedure is an iterative ments of Farrell and Clark’s (1976) work is the concept one with initial ice models based on glaciological models that the ocean surface remains an equipotential at all times scaled to fit the ice volumes required by sea-level records due to the Earth’s changing gravitational field in light of from locations far removed from the ice sheets (e.g. changing mass distributions. In their formulations, Farrell Lambeck and Purcell, 2005). Earth parameters are then and Clark (1976) also assured that mass is conserved and estimated using sea-level data from other locations both took into account the deformation of the Earth’s surface far from and under former ice sheets. Using appropriate not only in response to ice loads but also water loads Earth parameters, corrections for glacio- and hydro- (a concept introduced earlier by Bloom, 1967 and others). isostatic components are applied to locations far from the The theory used in this study follows the procedure ice sheets and improved estimates of global ice volume outlined by Nakada and Lambeck (1987) with later changes are made. After several iterations consistent modifications by Johnston (1993), Lambeck (1993a), solutions are emerging (Lambeck and Purcell, 2005). All Lambeck and Johnston (1998), Lambeck et al. (2002, relative sea-level calculations were run with 3 or 5 2003), and Lambeck and Purcell (2005). These methods iterations of Eq. (1) to take into account the affects of have been called into question by Peltier (2002b), but the differences in water load felt by areas receiving more or according to Mitrovica (2003), the theory developed by less meltwater than the global averaged values (‘‘eustatic’’ these authors represents, along with Milne et al. (1999) and sea level) due to the effects of ice loading (Johnston, 1993; Mitrovica and Milne (2003), the most complete and Milne et al., 1999; Lambeck et al., 2003) and to take rigorous theory available. From this work, the sea-level into account the migration of shorelines during the equation for a tectonically stable area can be schematically deglaciation phase (Lambeck and Nakada, 1990; Milne written as et al., 1999). The Gulf of Mexico lies within an intermediate site with Dz ðf; tÞ¼z ðtÞþz ðf; tÞþz ðf; tÞ, (1) rsl e i w respect to the ice sheets of the LGM (Clark et al., 1978). where Dzrsl(j, t) represents the relative sea-level at a Within this location, vertical movements are dominated by location j and time t, ze(t) represents the global meltwater the collapse of the glacial forebulge and the effects of water added to the oceans as a result of the melting of ice sheets loading due to the addition of meltwater to the ocean and is a function of time t, zi(j, t) represents the glacio- basins (Clark et al., 1978; Clark and Lingle, 1979; Nakada isostatic component at location j and time t, and zw(j, t) and Lambeck, 1987). Fig. 3 shows the respective compo- represents the hydro-isostatic component at location j and nents of both the glacial or ice load, zi(j, t), and water time t. We do not discuss here the full procedure in detail load, zw(j, t), for Earth model E1 and ice model I0 for but refer to earlier works (e.g. Nakada and Lambeck, 1987; Mobile Bay and the location of the Curray (1960) core. It Johnston, 1993; Lambeck et al., 2002; Lambeck and should be noted that, for the time period covered by most Purcell, 2005). Solving the sea-level equation (Eq. (1)) requires a convolution of the Earth’s response to deformation with the ice and water loads. Included in both zi(j, t)andzw(j, t) are considerations for radial deformation, tangential deformation, changes in the gravitational field (equipotential) of the Earth due to a redistribution of mass (both the water mass as well as mass of displaced mantle), rotational changes, and a stress component. Of these the most important are the radial deformation and changes in the gravitational field. The ice and water loads are the ice-sheet reconstruction and melting history, respectively. Ideally, one has an independent method of obtaining either the ice-sheet reconstruction and melting history or the Earth parameters. Independent methods of obtaining the ice-sheet reconstruction and melting history include the mapping of Fig. 3. Response of ice loading and water loads through time for Earth moraines and nanatuks (i.e. Denton and Hughes, 1981)or model E1 for Mobile Bay (MB) and the location of Curray (1960) data the use of glaciological models (i.e. Ritz et al., 1997; (CD). ARTICLE IN PRESS 926 A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940 of the observations (i.e. 14 ka—present), the ice load 1021 Pa s, and lower mantle viscosities, from 670 km to the component is similar between the two sites. However, the outer core, ranging from 1021 to 1023 Pa s. water loads do vary significantly over the last 20 ka (Fig. 3). The water load component is largely responsible for the 2.3.3. Ice models different RSL values experienced by the two sites (Fig. 1;at Other complicating factors in solving Eq. (1) are the the LGM the ice load also contributes) due to the variable unknowns in the reconstruction of the ice sheets and their distances of the two locations from the edge of the water melting histories. Sea-level records from far-field sites such load—the effects of time-dependent shorelines (Johnston, as Barbados and Tahiti are thought to be removed from 1993, 1995; Milne et al., 1999; Lambeck et al., 2003). Fig. 4 most of the effects of glacio-isostasy and when corrected illustrates the modeled sea-level history for a transect of for hydro-isostasy, provide a first-order constraint on the sites across the central Texas shelf using Earth model E1 sea-level history and thus melting history of the ice sheets and ice model I0, defined below. (Peltier, 1998; Lambeck et al., 2002). However, these sea- level histories provide a constraint on the entire meltwater 2.3.2. Earth models budget, not just the North American ice sheets, which have As in most glacial rebound studies, the Earth was the largest influence on the Gulf of Mexico. modeled as a Maxwell viscoelastic sphere, which has been Seven different ice-sheet reconstructions for the Lauren- found to best simulate the Earth’s response to loads at the tide Ice Sheet were considered that span the range of time scale of 1–100 ka (Peltier, 1974; Cathles, 1975; Clark et plausible models or that have particular features that help al., 1978; Nakada and Lambeck, 1987). This means that at understand the dependence of the sea-level predictions for short time scales (i.e. o1 y) the Earth behaves effectively as the Gulf of Mexico on the ice distribution. The first is the an elastic solid while on longer time scales (i.e. 41 ka) the nominal ice model, I0, based on ARC3 of Nakada and Earth behaves as a viscous fluid. Lambeck (1987) which was derived from the ICE-1 model No single set of radially dependent Earth parameters of Peltier and Andrews (1976). The second ice model, I1, is will effectively simulate the Earth’s response to loading the same as I0 only it has been reduced in thickness by a across the globe due to known lateral heterogeneities scalar multiple, b, of 0.42. This ice model is only within the Earth’s interior. However, by using a variety of appropriate when used with Earth model E3. A maximum plausible Earth models it is possible to determine a range of ice sheet reconstruction, I2, was based on the ICE-3G sea-level histories that should encompass the sea-level model of Tushingham and Peltier (1991). A minimum ice- record for the Gulf of Mexico. We simulated the response sheet reconstruction, I3, was that published as the to over 600 different Earth parameter combinations. We minimum ice model of Licciardi et al. (1998) with the considered elastic lithospheric thicknesses ranging from 50 addition of a Cordilleran and Innuitian Ice Sheet model. to 120 km, upper mantle viscosities, from the lithosphere/ These two models should bracket the sea-level predictions mantle boundary to 670 km depth, ranging from 1020 to for the Gulf of Mexico. The fifth ice model, I4, is the same

Fig. 4. Relative sea levels along a transect of the central Texas coast and shelf for Earth model E1. mbpsl ¼ meters below present sea level. Contours represent bathymetry on the Texas shelf with an interval of 10 m. Deepest contour is 100 m. ARTICLE IN PRESS A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940 927 as I3 except for the time period between 25 and 15.1 ka depending on the Earth model used. Mobile Bay falls where the thickness of the North American ice sheets has within this range of distances. The effects felt in the region been increased by 58%. The sixth ice model, I5, considered of Curray’s (1960) data would be less than 5 m (6.81 on in this study is the same as the minimum reconstruction, I3, Fig. 5a). The effects felt in Galveston Bay would be less but with more ice placed in the Antarctic Ice Sheet. The than 4.5 m (7.51 on Fig. 5a) and Corpus Christi Bay would final ice sheet reconstruction, I6, is the same as I0 except experience less than 2.0 m of uplift (9.251 on Fig. 5). These that all ice on the Laurentide Ice Sheet west of 751 W and values alone fall well short of the 10–15 m (35 m in the case south of 501 N has been removed. In all ice model cases, a of Curray’s (1960) data) needed to explain the disparity pre-LGM ice history was assumed based on the global ice- between their sea-level histories and time equivalent global volume curve of Lambeck and Chappell (2001) and on the records. However, as the sediment loading may be a assumption that the distribution of ice during the pre- portion of the solution, a time-dependent correction for LGM period was similar to that during and after the this signal (Fig. 5b) has been included in all subsequent LGM. model predictions. All ice models and modeling results are defined in calendar years BP using the time scale of Calib 4.4 (Stuiver 3.2. Glacio-hydro-isostasy and Braziunas, 1993; Stuiver et al., 1998). The other ice sheets were included in each model run. The Antarctic Earth model E1 fits the data best, falling within the error Ice Sheet reconstruction was based on the model as bars of the observations in Galveston and Corpus Christi originally outlined by Nakada and Lambeck (1987) with Bays. However, the predictions for Mobile Bay and the subsequent revisions assuming that meltwater volume not location of the Curray (1960) data both fall outside the accounted for in the other ice sheets came from Antarctica error bars of their respective observations. Model E2, a with a spatial distribution of ice based on the LGM preferred Earth model from other studies (i.e. Nakada and reconstruction of Denton and Hughes (1981). The Fen- Lambeck, 1987), does not fit the data as well as E1, but noscandian Ice Sheet is based on the model discussed in does explain most of the misfit between the ‘‘global’’ sea- Lambeck et al. (1998), the British Ice Sheet is based on the level data and the data within the Gulf of Mexico. The reconstruction of Lambeck (1993b), the Barents-Kara Sea remaining misfit in all Earth models suggests that, at least of Lambeck (1995b), the Greenland ice sheet of Fleming with the Earth parameters we considered, a solution cannot and Lambeck (2004), and a northern and southern be found by varying the Earth parameters alone. It is of hemisphere mountain glacier model described in Lambeck interest to note that of the Earth model parameters and Purcell (2005). considered, the best fit found, using Earth model E1, uses an extreme value of lithospheric thickness (120 km). Peltier 3. Results (1988) was able to fit the Gulf of Mexico observational data to his models using a lithospheric thickness of over 3.1. Mississippi loading 200 km. However, the thick lithosphere models do not predict the shorter wavelength changes in sea level seen The modeling results from the flexure due to the across continental margins or across smaller ice sheets and Mississippi Fan are illustrated in Fig. 5a. Again, it must are, therefore, ruled out (e.g. Nakada and Lambeck, 1988; be stressed that values used to obtain these results were Lambeck and Nakada, 1990; Lambeck et al., 1998). chosen so as to maximize the possible effects of loading due E3 produced the minimum variance between observa- to the Mississippi Fan. Our results indicate that, since the tions and predictions when allowing the ice thickness of the LGM, the Mississippi Fan depressed the lithosphere by Laurentide ice sheet to be scaled by a single scaling 200 m in the center of the load and nearly 50 m at the parameter (Lambeck et al., 2000). However, this solution load’s edge (Fig. 5a). Our results also indicate that the leads to a reduction of the ice thickness to 42% of the ice maximum amount of uplift experienced within the fore- thickness in the nominal model and illustrates the trade- bulge of the Mississippi Fan is 5 m. The maximum offs that can occur between Earth- and ice-model magnitude of the bulge is not strongly dependent on the parameters in the absence of independent constraints on loading history as decreasing the time frame in which ice thickness. loading occurred from 8 to 4 ka only increased the Predictions of relative sea levels from the Late glacial magnitude of the forbulge from 5.0 to 5.2 m. However, and early Holocene times are sensitive to the total ice load, changing the loading history does affect the shape and the melting history of the ice, and the rheologic parameters timing of the bulge. Assuming the loading occurred of the Earth, whereas the mid to late Holocene sea levels between 14 and 10 ka creates the largest magnitude of are sensitive primarily to the bulk characteristics of the ice uplift that can be applied over the longest time period. load and to the rheology of the Earth. Therefore, we will The magnitude of uplift due to the Mississippi Fan is first compare predictions and observations from the latter also a function of the distance from the load with the interval to obtain constraints on the Earth rheology and maximum amount of uplift occurring between 150 and then use the data from the earlier period to improve upon 475 km from the edge of the load (4.5–71 on Fig. 5a), the ice model. ARTICLE IN PRESS 928 A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940

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 ¼ Mobile 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 last viscosities, respectively: 8 ka, the time by which most of the Laurentide Ice Sheet Z X3 1020, was melted (Dyke and Prest, 1987; Clark et al., 2000; um England et al., 2004; Dyke et al., 2003) and recalling the Z X2 1022. subtidal nature of the data such that predictions should lm always lie above observations, several possible Earth We were unable to put any constraints on the litho- models can be eliminated. During this period, predicted spheric thickness. Over the last 8 ka, sea-level predictions sea levels should at no point be lower than the sea-level from models with upper and lower mantle viscosities less observations as they were all deposited in subtidal settings. than these values, such as E3, lie below sea-level observa- Using this procedure and assuming the current ANU ice tions while sea-level predictions from models with upper model (I0) is correct, we were able to determine the and lower mantle viscosities greater than these values, following constraints on the upper and lower mantle such as models E1 and E2, lie above sea-level observations ARTICLE IN PRESS A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940 929

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 ice 3.3. Ice models model as the same relationship is found between observations and predictions over the last 8 ka using two other ice The other variable in the modeling procedure is the ice- models, I2 and I3 (Fig. 7c). Therefore, we limit our search sheet reconstructions. Fig. 7a illustrates the results when to Earth models E1 and E2. using a maximum (I2) and minimum (I3) Laurentide Ice ARTICLE IN PRESS 930 A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940

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 A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940 931

Fig. 7. (Continued)

Sheet model with the Earth model whose initial predictions Gulf of Mexico (Fig. 7a). This trend appears to be best fit the observations, E1. Of the two scenarios, ice independent of the Earth model considered, as a similar model I3 fits the data best. When the appropriate pattern is found when using the maximum (I2) and correction for the time-dependent deformation due to the minimum (I3) ice models with Earth model E2 (Fig. 7b). Mississippi Fan is included the predictions fall within the In addition, when multiplying the thickness of the entire error bars of the observational data for sites within the Laurentide ice sheet by a scalar to obtain the best fit ARTICLE IN PRESS 932 A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940

Fig. 7. (Continued) between the observations and predictions, a value for b of prior to 15 ka. This underestimation of meltwater volumes o1 (0.42) brings the closest ﬁt (I1; Fig. 8). However, if the is best illustrated by examining the model results for other ice-sheet reconstructions (i.e. Fennoscandian, Ant- Barbados, which lies outside the area of major isostatic arctic, etc.) are approximately correct, ice models I1 and I3 adjustment to the Laurentide Ice Sheet (Figs. 7a, b, and 8). largely underestimate the meltwater volume delivered to In order to compensate for this discrepancy in meltwater the oceans since the LGM, particularly for the time period volumes, two scenarios are considered. ARTICLE IN PRESS A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940 933

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 the prior to the first meltwater pulse 1-A (MWP1-A) between Laurentide Ice Sheet within ice model I3 to roughly 14 and 15 ka (Fig. 7), we increased the volume of ice within balance the global meltwater volume. As the deficit in the Laurentide Ice Sheet in ice model I4 by 58% over I3 meltwater volume obtained from ice model I3 is only found from 25 to 15 ka. Thus, ice volumes of the Laurentide Ice ARTICLE IN PRESS 934 A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940

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 other reduced from the time period between MWP1-A and 8 ka. North American ice sheet models from the time period Other studies have come to similar conclusions requiring a between MWP1-A and the LGM (i.e. I0 of this study; reduction in the thickness of ice over the Laurentide Ice Nakada and Lambeck, 1991; Tushingham and Peltier, Sheet during the early Holocene in order to ﬁt rebound 1991; Peltier, 1994; Peltier, 2005), but are signiﬁcantly data from across North America (Lambeck et al., 2000). ARTICLE IN PRESS A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940 935

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 fit scenario that would balance the global meltwater volumes between model predictions and observations for our four based on the North American portion of ice model I3 is to sites within the Gulf of Mexico, including Curray’s increase the ice volumes in other ice sheets, the best observation, as well as for Barbados (Fig. 9). The second candidate being Antarctica. However, this scenario does ARTICLE IN PRESS 936 A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940 not produce as good a fit between observations and over the last 120 ka (Berryhill et al., 1986; Paine, 1993; predictions within the Gulf of Mexico, most notably Abdulah et al., 2004). Also tide-gauge data indicate a Curray’s (1960) data (Fig. 10), as does the first scenario. present-day subsidence of 4–8 mm/yr at Galveston Bay (Paine, 1993). 4. Discussion One possible cause of vertical motion is sediment compaction. None of the above sea-level indicators have 4.1. Tectonic movements and compaction been corrected for this. Pizzuto and Schwendt (1997) calculated the total amount of compaction within a very Paine (1993) established the vertical movement of similar estuarine setting along the Delaware coast to that locations across the northwestern Gulf of Mexico, includ- found within the Gulf of Mexico and found that the ing our study sites, based on the position of the Last compaction was on the order of 2 m for a 10 m-thick Interglacial shoreline (OIS 5e) where it is known as the Holocene succession. However, correcting for compaction Ingleside barrier along the Texas coast. He concludes that, reduces the depth of sea-level indicators and further based on this datum, and the assumption that in the increases the disparity between sea-level indicators in the absence of tectonics its expected location would be at Gulf of Mexico and equatorial regions. 5–8 m, the Gulf of Mexico has subsided 6 m over the last 120 ka, not risen. However, given the importance of glacio- 4.2. Radiocarbon reservoir hydro-isostasy within the Gulf of Mexico during the LGM, applying a sea-level datum from other parts of the globe Another possible explanation for the disparity between to the Gulf of Mexico during the Last Interglacial may the observed sea-level curves generated within the Gulf of not be appropriate. Many other studies of the Gulf of Mexico and those generated elsewhere (Fig. 1a) could be Mexico shelf’s stratigraphic architecture suggest subsidence incorrect ages for markers of sea level. In particular,

Fig. 11. Relative sea levels for Earth model E1 if the Laurentide Ice Sheet west of 751 W and south of 501 N were completely removed at locations (i) Corpus Christi Bay, Texas, (ii) Galveston Bay, Texas, (iii) Mobile Bay, Alabama, and (iv) location of Curray (1960) data. Each model has a correction applied to take into account the loading due to the Mississippi Fan (see Fig. 5b). ARTICLE IN PRESS A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940 937

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 10 m 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, Clark et al. (1996, 2002), Weaver et al. (2003), and two arguments can be made to refute this. First, a Bassett et al. (2005) have made convincing arguments for at consistent radiocarbon reservoir effect for these estuaries least some Antarctic contribution to MWP1-A. Our has been independently established (as discussed pre- analysis for sea-level predictions within the Gulf of Mexico viously). Second, based on their age/depth relationships, agree with Clark et al. (2002), specifically, that MWP1-A no distinction between a wood and shell population is seen could not have solely come from the southern sector of the (Fig. 1a). Laurentide ice sheet, as removing all the ice within the Laurentide Ice Sheet below 501 N (ice model I6) fails to 4.3. Laurentide Ice Sheet reconcile the predictions and observations, particularly for the location of the Curray (1960) core (Fig. 11). Although The best agreement between observed and predicted sea Antarctic Ice Sheet models vary greatly (Nakada et al., levels occurs with ice model I4. Ice model I4 has a thinner 2000) and the model used in Fig. 10 may not be the most Laurentide Ice Sheet (the ice sheet of Licciardi et al., 1998) accurate, it is the isostatic uplift of the Gulf of Mexico after 15 ka than the other ice models, but a similar provided by Laurentide ice melt during the time of MWP1- thickness before 15 ka. This implies a significant thinning A that leads us to favor a large contribution from the of the Laurentide Ice Sheet around the time period of Laurentide Ice Sheet at this time. Ice model I4 has a MWP1-A (Fig. 9). Debate continues on whether the significant reduction in ice thickness for most of the Laurentide Ice Sheet was the source of MWP1-A. Keigwin Laurentide Ice Sheet and not just its southern margins. et al. (1991), Charles and Fairbanks (1992), Fairbanks Physically this could be a result of a change from a frozen- et al. (1992), Lehman and Keigwin (1992) and Flower et al. bed ice sheet to a more soft-bedded ice sheet similar to a (2004) support increased amounts of meltwater from the form described by Licciardi et al. (1998), possibly via ice- Laurentide Ice Sheet during MWP1-A based on oxygen streaming (e.g. Andrews et al., 1998; Miller et al., 2002; isotopes from cores in the Gulf of Mexico and Atlantic Stokes et al., 2005) as has been inferred to have occurred in Ocean. From the relative abundances of reworked calcar- the Scandinavian Ice Sheet (Lambeck et al., 1998). A rapid eous nannofossils within the Orca Basin of the Gulf of deflation of the entire ice sheet with little change to its aerial Mexico, Marchitto and Wei (1995) showed an increase in extent would also help explain the absence of an earlier the amount of meltwater traveling down the Mississippi introduction of meltwater to the eastern outputs such as the River during MWP1-A, suggesting a contribution from Gulf of St. Lawrence. A deflation of the entire Laurentide some portion of the Laurentide Ice Sheet. Peltier (1994, Ice Sheet as a source for MWP1-A falls within the error 2004, 2005) supports a Laurentide source for MWP1-A bars of Clark et al. (2002) ‘‘fingerprint’’ test of the event. based on relative sea-level changes across the globe, but It should be noted that the ice-sheet reconstruction includes little sea-level data from North America prior to advocated by this study (I4), with a Laurentide source for 8ka(Peltier, 1994, 2004; Peltier, 2005). Tarasov and Peltier MWP1-A, best describes the bulk characteristics of the ice (2004, 2005) support at least some contribution to sheet and needs to be further tuned when applied to specific MWP1-A from the Laurentide Ice Sheet based on a areas that were once overridden by the former ice sheets. glacial-systems model. For some areas such as Ungava Bay the model works well ARTICLE IN PRESS 938 A.R. Simms et al. / Quaternary Science Reviews 26 (2007) 920–940

(Fig. 12a) and for other regions such as Cape Henrietta References Maria, the model needs further improvement (Fig. 12b). The misfit in some areas may also be due to mantle Abdulah, K.C., Anderson, J.B., Snow, J.N., Holdford-Jack, L., 2004. The heterogeneities and the uncertainties of the Canadian sea- Late Quaternary Brazos and Colorado Deltas, offshore Texas—their evolution and the factors that controlled their deposition. In: level indices relationship to former sea levels (Mitrovica Anderson, J.B., Fillon, R.H. (Eds.), Late Quaternary Stratigraphic et al., 2000). Evolution of the Northern Gulf of Mexico Margin. SEPM Special Publication 79, pp. 237–270. Andrews, J., 1971. Sea Shells of the Texas Coast. The Elma Dill Russell 5. Conclusions Spencer Foundation Series 5. University of Texas at Austin, 298pp. Andrews, J.T., Kirby, M.E., Aksu, A., Barber, D.C., Meese, D., 1998. The sea-level records from the northern Gulf of Mexico Late Quaternary detrital carbonate (DC-) events in Baffin Bay are up to 35 m shallower than ‘‘global’’ records at 18 ka (67–74 N): do they correlate with and contribute to Henirich events and decrease to 15 m at 10 ka. Sedimentary loading, a in the North Atlantic? Quaternary Science Reviews 17, 1125–1137. Aten, L.E., 1983. Indians of the Upper Texas Coast. Academic Press, New larger than assumed radiocarbon reservoir correction, or York, 370pp. tectonic activity alone cannot explain this difference. Bard, Ed., Hamelin, B., Arnold, M., Montaggioni, L., Cabioch, G., Faure, Glacio-hydro-isostasy, however, can explain this differ- G., Rougerie, F., 1996. Deglacial sea-level record from Tahiti corals and ence. Its magnitude will be a function of earth rheology and the timing of global meltwater discharge. Nature 382, 241–244. of changes in the ice load. Thus, we have used this method Bart, P.J., Ghoshal, S., 2003. Late Quaternary shelf-margin deltas in the northern Gulf of Mexico: implications for the Late Quaternary sea- to infer the distribution of ice within the Laurentide ice level elevation at the culmination of the Last Glacial Maximum. In: sheet during the LGM and Late glacial periods. The best fit Roberts, H.H., Rosen, N.C., Fillon, R.H., Anderson, J.B. (Eds.), Shelf between the glacio-hydro-isostatic models and the observa- Margin Deltas and Linked Down Slope Petroleum Systems: Global tional data from the Gulf of Mexico is found when using a Significance and Future Exploration Potential. 23rd Annual minimum ice-sheet reconstruction for the Laurentide Ice GCSSEPM Foundation Bob F. Perkins Research Conference, Programs and Abstracts, p. 19. Sheet for the time period between 15 ka to the present with Bassett, S.E., Milne, G.A., Mitrovica, J.X., Clark, P.U., 2005. Ice sheet a significant increase in the thickness of the ice sheet and solid earth influences on far-field sea-level histories. 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