Forms, response times and variability of relative sea-level curves, glaciated North America

Ž .Geomorphology 32 2000 315–333

Forms, response times and variability of relative sea-level curves,glaciated North America

Arthur S. Dyke a,), W.R. Peltier b

a Terrain Sciences DiÕision, Geological SurÕey of Canada, 601 Booth St, Ottawa, Ontario, Canada K1A 0E8b Department of Physics, UniÕersity of Toronto, Toronto, Ontario, Canada M5S 1A7

Received 1 July 1998; received in revised form 1 March 1999; accepted 16 August 1999

Abstract

Relative sea level curves from glaciated North America reveal coherent spatial patterns of response times. In theLaurentide Ice Sheet area, curve half-lives range from 1.2–1.4 ka at the uplift centre to 1.7–2 ka in a ridge of high valuesinboard of the glacial limit. Half-lives decline from this ridge to less than 1 ka along the margin. In the Innuitian Ice Sheetarea, half-lives are about 2 ka at the uplift centre and decline to less than 1 ka at the margin. The central Laurentide responsetimes are about half those of central Fennoscandia. This accords with the theoretical expectation that central response timesare inversely proportional to ice sheet radius for ice loads large enough that rebound at the centre is insensitive tolithospheric thickness. The Innuitian central response time indicates that rebound at the centre of this ice sheet, which ismuch smaller than the Fennoscandian Ice Sheet, remains sensitive to lithospheric thickness. Radial gradients in responsetimes reflect the increasing influence of the lithosphere at sites increasingly closer to the margin. Along this gradient,rebound progresses as though at the centres of smaller and smaller ice sheets. That is, the effective spatial scale of the iceload decreases toward the margin. Near the glacial limit, postglacial isostatic adjustment is complicated by forebulgemigration and collapse. This is seen most strongly in the relative sea level record of Atlantic Canada, which has subsidedduring the Holocene more than 20 m more than the adjacent American seaboard. The relative sea level history of someareas, notably the St. Lawrence Estuary, is complicated by tectonic processes. q 2000 Elsevier Science B.V. All rightsreserved.

Keywords: uplift; isostasy; Laurentide; Innuitian; Cordilleran tectonics

1. Introduction

Ž .Postglacial relative sea level RSL curves in andnear glaciated regions are of three general formsŽ .e.g., Clark, 1980; Andrews and Peltier, 1989 . Most

) Corresponding author. Fax: q1-613-992-0190.Ž .E-mail address: [email protected] A.S. Dyke .

sites that were heavily ice loaded during the lastŽ .glacial maximum LGM exhibit continuous post-

glacial emergence; sites near the LGM ice limit, bothinside and outside of it, exhibit initial emergencefollowed by submergence, a ‘‘J-shaped’’ curve; andmore distal sites exhibit continuous submergence.This pattern is well established by observations and

Žunderstood theoretically e.g., Farrell and Clark,1976; Peltier, 1976; Peltier and Andrews, 1976;

0169-555Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-555X 99 00102-6

( )A.S. Dyke, W.R. PeltierrGeomorphology 32 2000 315–333316

.Quinlan and Beaumont, 1981 as the net result ofpostglacial rebound, forebulge migration and col-lapse, eustatic sea level rise and hydroisostatic defor-mation.

The purpose of this paper is to examine thevariability of North American RSL curves, whichinclude examples of each of the three types, and tocomment on the probable causes and significance ofthese variations. We examine only the forms of RSLcurves, rather than derivatives such as ‘‘uplift’’

Ž .curves. There are two reason for this: 1 there is nouniversally applicable eustatic correction needed toconvert RSL curves to uplift curves, due to hydroiso-

Ž .static deformation, and 2 current geophysical mod-els of glacial isostatic adjustment incorporate theeffects of changing ice and water loads everywhereon earth and aim to replicate local RSL histories.This aim is legitimately restricted to RSL historiesthat do not contain signals other than that of isostaticadjustment. In this respect, however, it is helpful toidentify those records that may contain other signals,and we attempt to do this below. First we examinethe response times of emergence curves from theareas of the Laurentide and Innuitian ice sheets. Aspecific objective is to see whether there are broadpatterns in these response times. We then discusscurves of more complex form that do not fit into theexpected tripartite sequence. The major known com-plexities are the transgressions and regressions dur-ing the Holocene in the St. Lawrence Estuary ofQuebec, the proposed deep early Holocene lowstandsof the southeastern glaciated region, and the ‘‘in-verted J’’ form of RSL curves for outer coastalBritish Columbia and adjacent Alaska.

The data presented in this paper come from theRSL database for northern North America main-tained by the first author for the Geological Surveyof Canada. We present mainly a synthesis of the rawdata rather than a review of the literature. Thedatabase currently contains over 7000 radiocarbonage determinations and is available on written re-quest. We use the radiocarbon time scale throughout.

2. General postglacial emergence

Most published RSL curves for glaciated NorthAmerica illustrate either continuous postglacial

emergence or, near the glacial limit, emergence fol-lowed by submergence. A few more complex curveshave been proposed. For example, Mathews et al.Ž .1970 illustrated a re-submergence by about 100 minterrupting the late Pleistocene emergence of south-western British Columbia. Although no such eventwas illustrated in a more recent interpretation of the

Ž .RSL history of this region Clague et al., 1982 , thestratigraphic evidence of a large, double, late Pleis-tocene transgression in adjacent Washington StateŽ .Easterbrook, 1963, 1992 seems firm. Similarly, are-submergence interrupting general emergence,though by only 10 m, has been suggested for the

Žsoutheast tip of Baffin Island Andrews and Miller,.1985; Andrews, 1989 . Stable early Holocene sea

levels, so-called flat-topped emergence curves, haveŽalso been proposed for two regions. England 1983,

.1992, 1997 proposed that RSL remained close to themarine limit on various parts of Ellesmere Island forabout a millennium before postglacial emergenceoccurred at more normal rates. This conclusion wasaccommodated by the model of limited LGM icecover in the region then favoured, but is probablyunsustainable if the ice cover was much larger, as

Ž .currently thought Dyke 1998, 1999; England, 1998 .Another flat-topped curve has been proposed for a

Žsite on the Hudson Strait shore of Quebec Gray et.al., 1993 , with sea level remaining at or near marine

limit for 5 ka after deglaciation. This interpretation isŽ .problematic, as discussed by Dyke et al. 1996 .

In our database, about 130 RSL curves for heavilyice-loaded sites illustrate apparently continuous post-

Ž .glacial emergence e.g., Fig. 1a–c; Fig. 2 . Thecurves can be divided qualitatively into two groups:those based on dated floatage and other fossils thatcan be related to a specific paleoshore; and thosebased mainly on dated materials that must fall eitherabove or below the paleoshore. The qualitative dif-ference requires different approaches in manipulatingthe data statistically.

RSL curves of the first group are based primarilyon radiocarbon age determinations for driftwood andsea mammal remains. For North America, curvesbased mainly on floatage are available only from the

Žcentral and eastern Canadian Arctic Archipelago Fig..2 , where driftwood and whalebone are abundant on

Žraised beaches e.g., Blake, 1975; Dyke et al., 1991;.Hooper, 1996; Dyke, 1998 . Fifty-one curves, about

( )A.S. Dyke, W.R. PeltierrGeomorphology 32 2000 315–333 317

Fig. 1. Relative sea level data for sites exhibiting general post-Ž .glacial emergence: a Poste-de-la-BaleinerManitounuk Sound,

Quebec; 55803X to 55845X N, 75824X to 77847X W; marine limitŽ .about 270 m. b Tikiraq River, Admiralty Inlet, Baffin Island;

X X X X Ž .71844.5 to 71851 N, 86801 to 86818 W; marine limit 95 m. cLavoie Point, Navy Board Inlet, Baffin Island; 72843X to 72854X N,80818X to 80832.5X W; marine limit 61 m. See Fig. 3 for detail ofarctic sites.

30 of which are previously unpublished, are nowavailable from this region for sites arrayed betweenthe centres and the margins of former ice loads.These primary data can be evaluated statistically byfitting least-squares regressions to them after remov-ing evident outliers that result from post-strandingdisplacement or from occasional sinking of datedmaterials. Unculled data sets from Tikiraq River

Ž . Ž .Fig. 1b and Lavoie Point Fig. 1c , for example,illustrate strong accordance of ages of the uppermostsamples, which are remains of bowhead whales re-covered from raised beaches. The old-but-low sam-ples are bones of whales that sank in 5–20 m of

Ž . Ž .water Fig. 1b or are marine shells Fig. 1c . Afterculling, these 51 curves retain 5 to 31 samples each,with an average of 13.

Data of the sort shown in Fig. 1b and 1c can beŽdescribed well by a simple exponential function y

b x Ž . Ž .sae , where y is elevation m , x is age ka and.b is the proportionality constant , with coefficients

Ž 2 .of determination r rarely less than 0.95 and com-monly greater than 0.99. For a few sites, a slightlyimproved fit is provided by a double exponential

Ž b x d x .function ysae qce . However, reinterpreta-tion of the RSL assignment of one or a few datedsamples, which falls well within the latitude allowedby observations, improves the fit of the simplerfunction at these sites.

At none of these well studied sites has any strati-graphic evidence of transgressions superimposed onthe general emergence, been described. Furthermore,if the scatter within the data is assumed to be due tominor oscillations of RSL, no coherent regional pat-tern emerges. That is, the putative transgressions ofone curve are not matched in neighbouring curvesŽ .Dyke, 1998 . Therefore, the scatter in the data is

Žprobably due to errors of measurement elevation,.age and to natural processes that displace dated

materials slightly from their stranding sites, primarilyŽ .sea-ice push and solifluction Dyke et al., 1991 .

The second group of curves is derived primarilyfrom age determinations of marine mollusc shells,which must fall on or below the RSL curve, and ofterrestrial deposits and archaeological sites, whichmust fall on or above it. Among dated shells in ourdatabase, Mytilus edulis is the only common specieswith a usefully restricted depth range, preferringmainly shallow water and the intertidal zone. Thelast 3.5 ka of the emergence illustrated in the Poste-

Žde-la-Baleine data Fig. 1a; Allard and Tremblay,.1983 is defined by dates on this species. Otherwise,

the reliability of a curve based on these kinds ofmaterials depends on assembling a cloud of datapoints on at least one side of the curve, preferably onboth sides. For data sets of this type, we drewsubjective curves assuming that they are generally


Fig. 2. Map of half-lives of relative sea level curves, rounded off to 100 years. The values in boxes are for the emergence limbs of morecomplex curves. PB: Poste-de-la-Baleine, FD: Fraser Delta, V: Victoria, VI: Vancouver Island, QC: Queen Charlotte Islands, SF: SonderStromfjord, DI: Disko Island.

exponential; that is, that they are like the curves thatcan be defined objectively. We used all availableinformation on the sedimentological context of thedated shells to assign minimum RSLs and took careto intersect the marine limit at the most reasonable

Ž .age deglaciation date . These subjective curves werethen sampled, some irregularly to emphasize thebest-controlled segments, and fitted by exponentialregression in order to calculate their response times.

RSL curves can also be divided qualitatively onthe basis of the geographic spread of the datedsamples. The ideal curve represents a point on theearth’s surface; the quality of the curve degrades asthe sampled area increases. This is most critical near

Ž .the glacial limit where delevelling tilting isstrongest, and it follows that extending the samplearea in the direction of tilt more seriously compro-mises curve quality than extending it normal to tilt.In assembling the curves used herein, the RSLdatabase was sorted and partitioned to minimize this

effect. No attempt is made to use all of the availabledata; nevertheless, ‘‘curve areas’’ range from lessthan 100 km2 to about 60 000 km2. Canadian ArcticArchipelago curves generally have the smallest sam-ple areas and those around Hudson Bay the largest.Because the response times of curves near the centreof postglacial uplift have special geophysical signifi-

Ž .cance see below , it is unfortunate that none of theavailable curves from Hudson Bay are of the bestquality in this respect. Increasing the area over whichsamples are taken for a curve has two potentialnegative effects on the form of the curve. If theyounger dated samples come from the more upliftedpart of the sampled area, the apparent slope of thecurve will be decreased; the slope will be increasedif the opposite pertains.

Thus, RSL curves presently available to us forglaciated North America range widely in quality. Onthe other hand, for this paper we use only sites withdata that define reasonably complete curves.


3. Response times of emergence curves

The response times of RSL curves can be de-scribed with respect to their half-lives or their expo-nential relaxation times. The half-life is 0.693rb, thenatural logarithm of 2 divided by the proportionalityconstant in the exponential equation; it is the timetaken to accomplish half of the remaining emergenceand is the parameter commonly used in geologicaldiscussions. The relaxation time is 1rb, which is thetime taken to accomplish 1y1re of remainingemergence, and is the parameter commonly used ingeophysical discussions. The response times of RSLcurves are of fundamental significance as an index of

Ž .mantle viscosity e.g., Peltier, 1980; Walcott, 1980 .

3.1. Spatial patterns

Our interest here stems from the recent finding ofŽ .Dyke 1998 that the half-lives of RSL curves for

Ž .Devon Island Fig. 2 range from about 2 ka near thecentre of uplift of the Innuitian Ice Sheet to about 1ka close to the margin, with the decrease concen-trated near the margin. The most recent comprehen-sive review of geological data for glaciated NorthAmerica that attempted to address the variability of

Ž .RSL response times is that of Andrews 1970 . Heconcluded that response times are invariant withinthe area of an individual ice sheet, which implies thata complete emergence history for a site can bederived from a single, accurately dated paleoshore.Hence, it was not clear whether the Devon Islandresults are illustrative of a general feature of glaciatedareas, are a regional feature, or are simply spurious.Andrews’ analysis relied on 89 radiocarbon agesfrom 21 sites, with two to nine samples per site.Since then, the database has increased by two ordersof magnitude. A reassessment of his conclusions arethus in order.

The half-lives of all available North AmericanRSL curves that illustrate continuous postglacialemergence are plotted in Fig. 2. We also include inFig. 2 a few curves, generally the most distal sitesplotted, that exhibit slight Holocene submergence.Their forms do not depart seriously from the expo-nential but by extending the period of emergenceŽ .i.e., by ignoring the submergence we are slightlyoverestimating these half-lives. For two sites, Meri-

Ž .mack River, MA Oldale et al., 1993 andŽStephenville, Newfoundland Shaw and Forbes,

.1995; data points inside boxes in Fig. 2 , we havecalculated half-lives for the emergence phases oflocal J-shaped curves. These are the only two sites inthe study area for which both the depths and ages ofHolocene lowstands are well documented. We havealso included a few curves that show more complex

Ž .forms see below but that can, nevertheless, bedescribed as primarily exponential with high coeffi-

Ž .cients of determination )0.9 .Ž .Contouring of the half-lives Fig. 2 is subjective,

because it takes into consideration the likelihood thatsome differences between values at neighbouringsites are probably due to inadequacies in the data.We, therefore, give preference to values derivedfrom the best curves. For example, the curve with a1.4 ka half-life within the highest contour in the

Žcentral arctic islands Resolute Bay, Cornwallis Is-.land; Washburn and Stuiver, 1985 is based mainly

on shell ages, whereas most of the neighbouringcurves with about 2 ka half-lives are based on drift-

Ž .wood ages Dyke, 1993, 1998; Dyke et al., 1991 .This example illustrates clearly the risk that mayarise from relying mainly on shell ages. The Reso-lute Bay curve shows an apparently too rapid emer-gence despite a large number of age determinationsfrom a small area. Most shell debris in late Holoceneraised beaches here is considerably older than thebeaches.

Two broad spatial patterns are evident from theplotted half-life data. In the High Arctic islands, thehalf-lives increase from less than 1 ka at the former

Ž .ice sheet margins generally, the continental marginto about 2 ka at the centre. The long, central ridge ofhigh values coincides in part with the axis of uplift

Žof the Innuitian Ice Sheet Blake, 1970; Dyke, 1996,.1998 . Farther south, in the area covered by the

Laurentide Ice Sheet, the contoured half-lives form acoherent, but different pattern. Values increase fromless than 1 ka along the former ice sheet margin,except along the Labrador coast where they arehigher, to a ridge of between 1.5 and 2 ka inboard ofthe margin. Behind this ridge, which, with currentlyavailable data, appears to be breached at HudsonStrait, is a broad basin of lower values, declining toabout 1.2–1.4 ka at the main centre of Laurentideuplift in southeastern Hudson Bay. Despite the rather


large sample areas for these central curves, the simi-larity of apparent response times throughout HudsonBay and Foxe Basin suggests that the results arereliable.

There are too few Pacific coast sites of docu-mented general postglacial emergence to define acoherent pattern. The Fraser Delta and Victoria sitesŽ .Fig. 2 both show some Holocene submergencefollowing Late Pleistocene to early Holocene emer-

Ž .gence 200 m at the Fraser Delta . The RSL curves,therefore, are not truly exponential. However, be-cause the Holocene lowstands at both sites may besimilar in level and age, the difference in the half-lives of these two sites suggested on Fig. 2 isprobably valid.

With the data available to us for western Green-land, four RSL curves could be constructed. Two ofthese curves are from Sonder Stromfjord and our

Ž . Ž .Fig. 3. Plots of half-lives of relative sea level curves along probable ice sheet flowlines: a All central-eastern Arctic sites. b PrinceŽ . Ž . Ž . Ž .Regent Inlet east flowline. c Admiralty Inlet flowline. d Wellington Channel flowline. e Jones Sound south flowline. f Location of

flowlines.


interpretation of these data indicates short half-livesof 0.9–1 ka, similar to those inferred by TenBrinkŽ .1974 . However, the curve for western Disko Is-land, which is more distal to the ice load than theSonder Stromfjord sites, has a much longer half-lifeŽ .1.6 ka . The curve for eastern Hall Land, northwestGreenland, resembles those on adjacent EllesmereIsland.

3.2. Flowline patterns

Fig. 3 plots half-life against approximate flowlinedistance behind the LGM ice limit. Here we use onlycurves from the central and eastern Arctic because ofthe high density of sites and large numbers of datedfloatage. The LGM ice limit is not known precisely.

Ž .Taking all these sites together ns47 , the patternshows an exponential increase of half-life up flow-

Ž .line Fig. 3a . The expression for an exponential risew Ž y b x .xto a maximum ysa 1ye provides a least-

squares regression fit to these data with r 2 s0.247Ž .not shown . An improved fit is provided by a log

Ž 2 .normal expression Fig. 3a; r s0.373 , because ofthe decrease in half-life seen in curves farthest be-hind the margin. The scatter in the data is such,however, that the null hypothesis that distance and

Ž .half-life are unrelated cannot be rejected Table 1 .When these data are arranged along probable,

individual ice sheet flowlines, the scatter stronglydecreases, the coefficients of determination providedby the log normal expression increases to 0.81–0.91,and the probabilities of falsely rejecting the null

Žhypothesis are acceptably low Table 1; Fig. 3b–d;.see Fig. 3f for flowline locations . The pattern is less

convincing for the putative flowline along JonesŽ .Sound Fig. 3e , though even here there is a definite

increase of half-life up the flowline. It is probablethat Jones Sound was not occupied by a simple axialice stream and that a more complex ice flow pattern

Ž .occurred in this area Dyke, 1998 .The fact that the same or similar expressions

provide the best fit from flowline to flowline sug-gests that this is a valid general relationship for theLaurentide and Innuitian ice sheets. Of more signifi-cance glaciologically, the differences in slopes of thelines, in this case drawn along adjacent marine chan-nels, suggests that the rebound data hold the memoryof significant differences in the surface slope of the

Table 1wStatistical parameters for log normal regressions y s

Žy0.5)ŽlnŽ x r x 0.r b.2 xae of Fig. 3

Value Standard error t statistic P

Fig. 3a2r 0.373

n 47a 98745.5 8 618618.8 0.0115 0.99b 9.645 492.4 0.0196 0.98X 499.04 60.96 8.1864 -0.00010

Fig. 3b2r 0.814

n 15a 1875.9 97.5 19.25 -0.0001b 0.7321 0.08 8.99 -0.0001X 545.4 34.8 15.65 -0.00010

Fig. 3c2r 0.911

n 12a 1684.5 548.11 3.12 0.0141b 0.4689 0.21 2.27 0.0525X 358.8 20.02 17.88 -0.00010

Fig. 3d2r 0.919

n 7a 1883.5 91.68 20.54 -0.0001b 0.8460 0.1621 5.22 0.0064X 489.0 88.45 5.53 0.00520

Fig. 3e2r 0.396

n 7a 1673.9 131.63 12.7 0.0002b 1.1119 0.74 1.5 0.2078X 376.1 169.26 2.2 0.09040

ice sheet along these former ice streams. Hence,analyses of this type may be profitable in recon-structing detailed ice thickness histories.

4. Interpretation of response time patterns

The most probable explanation of the broad basinof somewhat reduced half-lives in the central Lau-rentide Ice Sheet region lies in the depth dependenceof mantle viscosity. Clearly, the larger the horizontalspatial scale of the ice load, the greater the depthover which the rebound process will be sensitive to

Žmantle viscosity. For an isoviscous mantle Peltier,


.1974, 1982 , the natural tendency is for the relax-ation time to increase at the centre of rebound as thedominant spatial scale of the load decreases. How-ever, this tendency holds only for loads whose spa-tial scales are sufficiently large that the influence oflithospheric thickness plays no significant role indetermining the relaxation time at the centre ofuplift. Very broadly speaking, this depth dependenceof mantle viscosity accounts for the observed differ-ence in relaxation time between central Fennoscan-dia and central Laurentia. In Fennoscandia, the expo-nential relaxation time at the Angerman River site,near the centre of the former ice sheet, is 4.6 kaŽ .e.g., Peltier, 1996 . This load had a dominant spatialscale corresponding to a spherical harmonic degree

Ž .near 14 e.g., McConnell, 1968 . At the centre ofLaurentide uplift, whose spatial scale corresponds toa dominant spherical harmonic degree near 8, ac-cording to the analysis presented herein, half-livesŽ .0.693rb are 1.2–1.4 ka, which corresponds to an

Žexponential relaxation time of about 2 ka half-.lifer0.693 . Given that the spatial scale of the Lau-

rentide load is approximately double that of theFennoscandian load, the ratio of their relaxation

Ž .times about 1r2 suggests that there is no largedifference in viscosity between the upper mantle andthe transition zone on the one hand, and between thetransition zone and the upper part of the lowermantle on the other.

The Innuitian Ice Sheet was considerably smallerthan the Fennoscandian Ice Sheet. Yet, according toour analysis, exponential relaxation times near the

Ž .centre of Innuitian uplift ca 2.9 ka are lower thanthose in central Fennoscandia. The lower Innuitianrelaxation time is entirely explicable as an effect oflithospheric thickness. For a given lithospheric thick-ness, there exists a particular ice sheet size belowwhich the relaxation time decreases along with the

Ž .size or with increasing spherical harmonic degree .The Innuitian RSL response times provide us with ameans of quantitatively estimating lithospheric thick-ness.

ŽThe zone of slower rebound ridge of longer.half-lives inboard of the margin of the Laurentide

Ice Sheet, where response times are similar to thoseobserved in central Innuitia, is explained by upliftprocesses, which are controlled by position relativeto the ice sheet centre. As one approaches the edge

of the ice sheet from the interior, uplift occurs as ifat the centre of a smaller ice sheet; i.e., the dominantspatial scale of the rebound process diminishes. Onlyat the centre of the load is the response governed bythe relaxation time appropriate to the scale of theload itself. This radial variation is further compli-cated, of course, by the fact that the location of theedge of the ice sheet, and hence the duration of

Žloading, changes through time e.g., Dyke and Prest,.1987 .

At the ice margin itself, the influence of thelithosphere is extreme, irrespective of the scale of theice sheet. This influence contributes very signifi-cantly to the diminution of response time that occursthere, as possibly does the decrease of crustal thick-ness near the continental margin. Well beyond theice margin, of course, the RSL history is one ofmonotonic submergence associated with the collapseof the proglacial forebulge and with eustatic sealevel rise. RSL curves for sites at the ice margin arefurther complicated by the inward migration of theforebulge that occurs subsequent to deglaciationŽ .Peltier, 1974 . This, together with recession of theice edge itself, adds further complexity to the recov-ery process at such locations.

5. Variations on continuous emergence

To this point, we have discussed RSL curves thatshow continuous postglacial emergence. Some curvesfrom southeastern Quebec, which fall in the zonethat is normally characterized by continuous emer-gence, instead show one or more substantial Holocenefluctuations of RSL. The well dated stratigraphicsuccession at Montmagny and neighbouring sites, forexample, shows that RSL dropped from the marine

Ž .limit about 160 m to below present sea level by 9Ž .ka BP Fig. 4a , in the process forming an uncon-

formity with a widespread boulder lag on glacioma-rine sediment. RSL rose slightly above present level8.1–7 ka BP and deposited marine sediment on theunconformity. It then fell again far enough to allowtrees to grow at present low tide level and lowerŽ .probably to y10 m 7–5.8 ka BP. It rose to about 9m, depositing marine sediment over peat, between5.8–5.2 ka BP and probably fluctuated somewhat

Žirregularly thereafter Dionne, 1985, 1988a,b, 1997,.1998 . Similarly, at Matane, RSL dropped from the


Ž .marine limit about 110 m to present sea level orlower by 7 ka BP, then rose to about 9 m asl at about

Ž . Ž5 ka BP Fig. 4b; Dionne, 1995 . At Tadoussac Fig..4c , a low RSL was attained by 6 ka BP. After that,

it remained near, possibly below, present until atransgression of several metres occurred about 2 ka

Ž .BP Dionne, 1996a; Dionne and Occhietti, 1996 . AtŽ .Riviere du Loup, Dionne 1990 documented a drop`

of RSL below sea level present before 7 ka BP, andŽ .a rise back above present by 5.7 ka BP Fig. 5a . In

Ž .northeastern Gaspesie, Gray 1987 recognized a´transgression above present sea level starting about 5

Ž .ka BP Fig. 5b .

Fig. 4. Relative sea level data for the upper St. Lawrence Estuary,Ž .Quebec indicating middle and late Holocene transgressions: a

Montmagny; 46845X to 47830X N, 70800X to 70852X W; marine limitŽ . X X X Xabout 160 m. b Matane; 48830 to 48854 N, 67822 to 68817 W,

Ž . X X Xmarine limit 110–120 m. c Tadoussac; 48804 to 48814 N, 69831to 69845X W; marine limit 130–140 m.

Fig. 5. Relative sea level data for the lower St. Lawrence Estuary,Ž .Quebec indicating middle and late Holocene transgressions: a

Riviere du Loup; 47827X to 48803X N, 69812X to 69854X W, marine`Ž . X X Xlimit 155 m. b Gaspesie east; 49812 to 49815 N, 65802 to´

65827X W. See Fig. 11 for place names.

Thus, there is widespread evidence of low- toŽ .moderate-amplitude as much as 20 m , millennial-

scale transgressions and regressions along most ofthe St. Lawrence estuary below Quebec City. Indeed,this is probably the most significant recent discoveryarising from RSL studies in eastern Canada. How-ever, the causes of these RSL variations are not yetunderstood. It is not clear that these events are

Ž .correlative from site to site Fig. 6 , as might beexpected for eustatic transgressions. Furthermore, atransgression of about 20 m between 6 and 5.5 ka BPraises the problem of a meltwater source. And if thefirst Holocene transgression is attributed to glacial

Ž .forebulge migration or collapse cf. Dionne, 1998 ,we are left without a glacioisostatic mechanism for

Ž .the subsequent uplift s and transgressions. The re-gion is one of the most seismically active in eastern

ŽCanada Adams and Clague, 1993; Lamontagne,.1998 , and the variability in timing of transgressions

along the estuary may be due to earthquakes oraseismic tectonic processes inducing local changes


Fig. 6. Holocene transgressions above present sea level and regressions below present sea level recorded along the St. Lawrence Estuary,Ž .Quebec cf. Figs. 4 and 5 .

Ž .of RSL. Indeed, Dionne 1996b invoked tectonics toexplain the different ages of low marine terraceremnants just east of Quebec City. In this view, the

Ž .anomalous non-isostatic departures from the ex-pected exponential curves are the lowstands, whichmay record tectonic uplift, rather than the high-stands.

These Holocene oscillations notwithstanding, theRSL curves for the region are still nearly exponentialin form. However, there is an exceptionally largerange of half-lives among curves in this region, from

Ž . Ž .about 0.6 to 2.3 ka Fig. 2 . Montmagny Fig. 4a ,where the RSL oscillations are best documented and

Ž .dated, has the shortest half-life 0.6 ka . Even if thefirst lowstand is removed, the half-life only increasesto 0.9 ka. In light of the unusual complexity of RSLcurves in this region, the spatial pattern of responsetimes may be real and may be a further indication oftectonism and potentially a guide to specific mecha-nisms and rates.

6. Variability of Holocene submergence

Sites near and beyond the LGM ice limit innorthern North America have submerged during theHolocene, but at different rates. Below we examinethe best documented sites from the Pacific-Beaufortcoast and then those from the Atlantic coast. There isalso clear geomorphological evidence of middle tolate Holocene submergence of much of the outer-most northeast coast of Canada from Hudson Strait

Žto northern Baffin Bay Dyke, 1979; Andrews and.Miller, 1985; Andrews, 1989; Dyke, 1998 . Rates

and amount of submergence have yet to be estab-lished for that region, though we can in places trackthe landward expansion of the zone of submergence.

The maximum middle to late Holocene transgres-sion proposed, to date, for northern North Americaoccurred in the Mackenzie Delta region of the Beau-fort Sea. Here RSL appears to have risen by about 60m since 8 ka BP and by about 25 m since 4 ka BPŽ .Fig. 7a; cf. Hill et al., 1985 . In contrast, at Barrow,Alaska, also on the Beaufort coast but well beyondthe glacial limit, RSL has risen by less than 5 m

Ž .since 4 ka BP Fig. 7b . Along Kotzebue Sound,near Bering Strait, RSL appears to have changed

Ž .little during the last 4 ka Fig. 7c . Here, accre-tionary beach forelands have formed under stableRSL; radiocarbon age determinations on ancient en-campments show that these beaches extend back in

Ž .time to about 4 ka BP Mason and Jordan, 1993 .Two sites at the marginal zone of the southwestern

Ž .Cordilleran Ice Sheet, Victoria Fig. 8a and FraserŽ .Delta Vancouver, Fig. 8b , exhibit Holocene sub-

mergence from early Holocene lowstands at 8 ka BP.These sites have seen little or no change in RSL over

Žthe last 4–5 ka e.g., Clague, 1989; Clague et al.,.1982 .

Neither the similarities nor the differences amongthese western RSL curves have self-evident explana-tions. First, sites exhibiting little or no change ofRSL over the last 4 ka range from the heavily

Ž .ice-loaded Fraser Delta marine limit 200 m to moreŽ .lightly loaded Victoria marine limit 75 m , to sites

Žfar beyond the limit of glaciation Kotzebue Sound.and Barrow . The exceptional submergence of the

Mackenzie Delta area may be partly due to the load


Fig. 7. Relative sea level data for Beaufort Sea to Bering StraitŽ .sites of Holocene submergence: a Mackenzie Delta and shelf,

X X X X Ž .NWT; 68818 to 70830 N, 128820 to 141800 W. b Point Bar-X X X X Ž .row, Alaska; 70800 to 71823 N, 148847 to 166800 W. c Kotze-

bue Sound, Alaska; 65845X to 67810X N, 161858X to 168800X W.

of Late Pleistocene and Holocene sediment in thisŽ 2 .large delta 12 000 km , thought to be between 100

Žm and 300 m thick A. Duk-Rodkin, personal com-. Žmunication, 1998 . The Fraser Delta is smaller 1000

2 .km but also contains )100 m of Late Pleistocene.and Holocene sediment Clague et al., 1998 . It ap-

parently has caused negligible subsidence, unless thiseffect has been fortuitously balanced by glacial re-bound. The similarity of the Fraser Delta and Victo-ria records, where there is no delta, argues againstany important delta-loading effect for deltas of thissize. Finally, the similarity of the Fraser Delta andVictoria records for the last 8 ka, and the apparentsimilarity of these to other regional records, suggest

that glacial rebound was finished at both sites in theearly Holocene. It is unlikely that glacial reboundwould precisely cancel eustatic sea level rise at sites

Žwith such divergent ice load histories cf. Easter-.brook, 1992 .

Atlantic sites can be divided into two contrastingŽ .groups: 1 sites from Portland, Maine to New Jer-Ž .sey, and 2 sites from the Bay of Fundy to Gulf of

St. Lawrence. Within the former group, individualŽ .RSL curves Fig. 9a–f are very similar or indistin-

Ž .guishable Table 2 . There is scarcely any increase inscatter when all of these data are grouped into oneset. In this region, RSL rose from about y20 m at 8ka, to y11 m at 6 ka, y7 m at 4 ka, and y2.5 m at2 ka BP. The Atlantic Canada curves, however,

Ž .show greater submergence Fig. 10a–d; Table 2 . Atthese sites, the RSL rise since 8 ka and since 6 ka BP

Žis twice that at the American sites note, however,

Fig. 8. Relative sea level data for Pacific sites of HoloceneŽ . X Xsubmergence: a Victoria, British Columbia; 48818 to 48840 N,

X X Ž . X123817 to 123852 W. b Fraser Delta, British Columbia; 48858to 49835X N, 121848X to 123819X W.


Ž . X X X XFig. 9. Relative sea level data for American Atlantic sites of Holocene submergence: a New Jersey; 38849 to 40827 N, 72807 to 74843 W.Ž . X X X X Ž . X X X X Ž .b New York; 40827 to 41837 N, 70852 to 74811 W. c Connecticut; 41806 to 41822 N, 71858 to 73823 W. d Barnstable marsh, MA;

X X X X Ž . X X X X Ž . X X40859 to 41857 N, 68843 to 71800 W. e Boston, MA; 42805 to 42853 N, 69859 to 71804 W. f Portland, Maine; 43816 to 43859 N,69838X to 70836X W.

.that it was much less than at Mackenzie Delta . Thisdifference has long been noted, for example by Grant

Table 2Ž .Depth below present sea level m of Holocene shorelines, north-

eastern USA, Atlantic Canada and Mackenzie Delta

Site 2 ka 4 ka 6 ka 8 ka

New Jersey 2.5 8 12 20New York 3 8 12 20Connecticut 2 7 10Barnstable marsh, MA 2.5 8 11 20Boston, MA 2 5 11Portland, Maine 1.5 4 10–12 15–18Halifax, Nova Scotia 4 12 25 38FundyrMinas Basin, Nova Scotia 5 11Moncton, New Brunswick 5 11 23 40Prince Edward Island 4 12 35?Mackenzie Delta, NWT 8 25 42 60

Ž .1970 in his work on the drowned forests of the Bayof Fundy. Grant suggested that the greater apparentsubmergence there might be due to a Late Holoceneincrease in tidal range, culminating in Fundy’s fa-mous 15 m tides, which would produce a transgres-sion without a change in mean sea level. However,the similar RSL rise at Halifax, which is on the openAtlantic coast with normal tides, and around PrinceEdward Island, in the Gulf of St Lawrence, arguesagainst this explanation. Hydro-isostatic depressionshould have been similar in magnitude along boththe Canadian and American coasts. Finally, glacialrebound should have been larger at the Canadiansites because they were farther behind the ice limitand were deglaciated later. Both of these factorsfavour higher Holocene shorelines in the Canadianregion.


Ž . X X XFig. 10. Relative sea level data for sites of Holocene submergence in Atlantic Canada: a Halifax, Nova Scotia; 43856 to 45806 N, 62857X Ž . X X X X Ž .to 64835 W. b Fundy HeadrMinas Basin, Nova Scotia; 45800 to 45827 N, 63819 to 64843 W. c MonctonrBaie Verte, New Brunswick;

X X X X Ž . X X X X45837 to 46810 N, 63838 to 64844 W. d southeastern Prince Edward Island; 45823 to 46845 N, 61844 to 62854 W.

The greater subsidence of the Atlantic Canadasites from 8 ka BP to the present probably indicatesthat the glacial forebulge has been actively collaps-ing in that region and that it had retreated inland ofthe American seaboard prior to 8 ka BP. Forebulgecollapse in the Nova Scotia to southern NewBrunswick region has apparently contributed about18–20 m of subsidence since 8 ka BP. Variable ratesof Holocene submergence of the American seaboardsouth to Florida were discussed by Newman et al.Ž .1980 . They concluded that geosynclinal subsidenceand loading by Late Wisconsinan glacial sedimenton the continental slope contributed some of thecomplexity. Possibly, areas with relatively high

Ž .Holocene shorelines i.e., less submerged , such asFlorida and South Carolina, were affected by anoceanic forebulge.

7. Late Wisconsinan to early Holocene lowstands

In the marginal zone of the ice sheets, but wheredeglaciation was generally delayed until 14–12 ka

BP, late Pleistocene rebound caused emergence toproceed to a lowstand below present sea level. Inmany areas, these lowstands appear to have reached

Ž0 to y45 m e.g., Clague et al., 1982; Oldale et al.,.1993; Shaw and Forbes, 1995; Dyke, 1998 . How-

ever, deeper lowstands have been proposed for fourareas along the southeastern margin of the Lauren-

Žtide and Appalachian ice sheets: off Halifax Stea et. Žal., 1994 , off Maine Belknap et al., 1986; Kelley et. Ž .al., 1992 , in Baie des Chaleurs Syvitski, 1992 , and

on the southern Grand Banks of NewfoundlandŽ .Dalrymple et al., 1992 . Some of these lowstandsare problematic.

Ž .Stea et al. 1994 proposed that sea level reachedy65 m off the outer coast of Nova Scotia about 11.2ka BP, following a brief and poorly defined intervalof emergence by about 20 m that attended localdeglaciation. Subsequent submergence resembles that

Ž .at Barbados Fairbanks, 1989 and is, therefore, notproblematic. This record does, however, indicateearly completion of only a small amount of isostaticrebound, which, therefore, limits the probable amountand duration of Late Wisconsinan ice loading.


Ž . Ž .Schnitker 1974 and Belknap et al. 1986 , amongothers, proposed a y65 m lowstand off central andwestern Maine at about 9 ka BP. This proposal wasproblematic because the proposed subsequent sub-

Žmergence exceeded that at more ice-distal sites e.g.,.Boston and the 9 ka paleoshore, therefore, locally

declined in elevation toward the loading centre.However, a subsequent revision of the age estimate

Žof this lowstand to about 11 ka BP Kelley et al.,.1992 brings this record more in line with that from

Nova Scotia. The preceding emergence of about 120m was completed in only 1.5 ka, indicating a half-lifeof less than 1 ka.

Ž .Syvitski 1992 proposed that the shoreline inBaie des Chaleurs regressed to y90 m around 9 kaBP. This proposal was based on the occurrence ofnumerous sea-floor gullies eroded into glaciomarinesediment and on possible submerged shoreline fea-tures, including a delta-like feature and terraces.However, such a low level for the 9 ka BP shorelineis anomalous in the context of other available data

Žfrom the Gulf of St. Lawrence region Table 3; Fig..11 . These other data describe a systematic, though

nonlinear, rise of the 9 ka shoreline from an eleva-tion of y40 to y45 m in the southeast to elevationsin excess of 100 m in the northwest. Baie desChaleurs lies midway on this transect and the 9 kashoreline there is probably below present sea level,but by less than 20 m, and surely by less than 40 mŽ .Fig. 11 . Furthermore, the general global RSL at 9

Žka BP was around y35 to y40 m Fairbanks,.1989 . If rebound carried the crust around Baie des

Chaleurs 50 m higher than that, the subsequentexcessive submergence could only be seen as theresult of isostatic over-compensation, and it is diffi-cult to imagine why the crust would have reboundedmore than it ought to have to achieve isostaticequilibrium. Therefore, until there is firm strati-graphic evidence of such a deep lowstand, this pro-posal should be treated cautiously.

A similarly problematic age has been suggestedfor a y110 to y120 m lowstand along the southernGrand Banks of Newfoundland. Dalrymple et al.Ž .1992 proposed that the banks were submergedduring deposition of the Downing Silt betweendeglaciation of the shelf edge about 12 to 11.5 ka BPŽ .Bonifay and Piper, 1988 and sometime after 11 kaBP. This requires that local RSL was higher than

Table 3Ž .Minimum elevations m of the 9 ka BP shoreline in southeastern

Canada

Site Elevation

St. Anthony, Newfoundland 48Port au Choix, Newfoundland 35St. Paul’s Inlet, Newfoundland 16Bonne Bay, Newfoundland 10Stephenville, Newfoundland y25Blanc Sablon, Quebec 45Romaine River, Quebec 65Western Anticosti Island, Quebec 47Eastern Anticosti Island, Quebec 40Sept Isle, Quebec 120Baie Comeau, Quebec 70Tadoussac, Quebec 63Lac St. Jean, Quebec 175Quebec City, Quebec 47Montmagny, Quebec 40Riviere du Loup, Quebec 48`Rimouski, Quebec 65Matane, Quebec 30St. Anne-des-Monts, Quebec 20Mont-St.-Pierre, Quebec 17Eastern Gaspesie, Quebec 10´Acadian Peninsula, New Brunswick -0Southeastern Prince Edward Island y45Halifax, Nova Scotia y40

y60 m during that interval. Crustal rebound is thenthought to have brought RSL to the lowstand about10 ka BP. The proposed subsequent submergence

Žthus is about double the RSL rise at Barbados Fair-.banks, 1989 . In fact, however, both the glacial and

RSL history of the Grand Banks are poorly known.The only firm evidence presently available is thatRSL was above y80 m at 19 ka and 17.1 ka BPŽMuller and Milliman, 1973; radiocarbon ages of¨

.barnacles and that walrus occupied the southeasternGrand Banks where water depths are presently 60 m

Žat 12 550"50 ka BP Dyke et al., in press; Beta-.83414 .

The deepest postglacial lowstands are recordedoff the coast of British Columbia. The sea at thenorth end of Vancouver Island regressed to y95 mor lower at 10.5 ka BP following a highstand of 27

Žm or more at about 13 ka BP Luternauer et al.,.1989 . The lowstand is demonstrated by a paleosol

with in situ plant roots. In the Queen CharlotteIslands, a region extensively glaciated but perhaps


Fig. 11. Minimum elevations of the 9 ka BP shoreline in the Gulf of St. Lawrence region. M: Montmagny, Mt: Matane, T: Tadoussac, R:Riviere du Loup, G: northeastern Gaspesie.` ´

lightly loaded at LGM, the shoreline stood at y153m at 12.4 ka BP, rising toq14 m at 9 ka BP

Ž .Fig. 12 . Deglaciation here occurred mainly be-Ž .tween 14 and 13 ka BP Josenhans et al., 1995 . The

Fig. 12. Relative sea level data for Moresby Island, Queen Charlotte Islands, British Columbia; 52829X to 53815X N, 131820X to 132826X W;marine limit 14 m at 9 ka BP; lowstand y153 m.


submergence record has been established from abun-dantly dated cores from drowned lake basinsŽ .Josenhans et al., 1997 . The subsequent Holocenelowering of RSL must be due to tectonic uplift along

Ž .this active plate margin Clague, 1989 , which hasbeen proceeding at a rate of about 1–2 m per kasince 6 ka BP.

The hyper-submergence of the outer BritishColumbia coast has been explained as the result offorebulge migration and collapse augmenting the

Žgeneral eustatic rise Clague, 1983; Luternauer et al.,.1989; Josenhans et al., 1997 . If we use the Barbados

curve to approximate eustatic rise, about 61 m be-tween 12.5 and 9 ka BP, substantial forebulge col-lapse is needed to account for the remaining 106 mof submergence of Queen Charlotte Islands in thatinterval. If deglaciation occurred between 15 and12.5 ka BP, rebound must have been confined to thatsame interval in order to allow forebulge migrationand collapse starting at 12.5 ka BP. In this vein, itseems improbable that the rebound was small if theforebulge was so high. This interpretation, therefore,calls for very short response times, not presentlyquantifiable, and sharp crustal flexure, both indica-tive of a thin lithosphere or low mantle viscosity.Alternatively, the plate-tectonic stress field may havebeen distorted by glacioisostatic stress, such thattectonic subsidence occurred in the Late Pleistocene.

8. Conclusion

The relative sea level curves currently availablefor the area of postglacial emergence in North Amer-ica reveal spatial patterns in response times that canbe related to ice sheet size and to the variableinfluence of lithospheric thickness on the isostaticadjustment process. The central Laurentide responsetime is about half the central Fennoscandian re-sponse time, because the Laurentide load was doublethe length of the Fennoscandian load and, hence, agreater thickness of the mantle responded to loadingand unloading under Laurentia. Response times in-crease away from the Laurentide load centre, as theeffective load decreases and the influence of litho-spheric thickness on the rebound process increases.

Near the margin, where the lithospheric influence isdominant, response times decrease sharply.

Response times for the Innuitian Ice Sheet, whichwas much smaller than the Fennoscandian Ice Sheet,indicate that lithospheric thickness was a significantcontrol on rebound even at the centre of the icesheet. The lower limit of ice sheet size for whichcentral response time is insensitive to lithosphericthickness apparently falls between these two icesheets.

The postglacial isostatic adjustment process inCanada was complicated by additional factors. Mostsignificantly, the record of multiple, and evidentlynon-synchronous, Holocene transgressions and re-gressions in the St. Lawrence Estuary suggests com-plexity due to local tectonic vertical movements of10–20 m. Similarly, the large and rapid submergenceof the outer British Columbia coast in the late Pleis-tocene and early Holocene, which has been attributedto forebulge collapse, may indicate abnormal crustaland mantle conditions under that region.

Isostatic adjustment in the vicinity of the LGMlimit is complicated by inward migration and col-lapse of the forebulge. The geological record of thisis best seen in Atlantic Canada and along parts of theeastern Canadian Arctic seaboard. In AtlanticCanada, forebulge collapse since 8 ka BP can ex-plain the 20 m of excess submergence there ascompared to that along the adjacent Americanseaboard. The far greater Holocene submergence ofthe Mackenzie Delta region and the comparativestability of north Pacific shorelines at the same timeremain unexplained.

Acknowledgements

Dyke’s field work in arctic Canada has beensupported logistically since 1975 by Polar Continen-tal Shelf Project, Natural Resources Canada. RogerMcNeely of the Geological Survey of Canada Radio-carbon Laboratory, which has produced the lion’sshare of the age determinations used here, has pro-vided constant support for Dyke’s efforts in main-taining and improving the RSL database. Peltier’swork on isostatic adjustment over the same periodhas been supported by Natural Sciences and Engi-neering Research Council of Canada. The manuscript


benefitted from early reviews by Douglas Hodgsonand Larry Dyke, Geological Survey of Canada, Ot-tawa, from formal reviews by Jim Teller, Universityof Manitoba, and John Andrews, University of Col-orado, and from the editorial pen of John Clague,Simon Fraser University.

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Forms, response times and variability of relative sea-level curves, glaciated North America

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