Late Holocene relative sea-level changes and the earthquake deformation cycle around upper Cook Inlet, Alaska

ARTICLE IN PRESS

Quaternary Science Reviews 24 (2005) 1479–1498

Late Holocene relative sea-level changes and the earthquake deformation cycle around upper Cook Inlet, Alaska Sarah Hamilton, Ian Shennan Sea Level Research Unit, Department of Geography, University of Durham, Durham, DH1 3LE, UK

Abstract Multiple peat-silt couplets preserved in tidal marsh sediment sequences suggest that numerous great plate boundary earthquakes caused the coast around Cook Inlet, Alaska, to subside over the past 3500 years. Field and laboratory analyses of the two youngest couplets record the well-documented earthquake of AD 1964 and the penultimate one, approximately 850 cal yr BP. Diatom assemblages from a range of modern day estuarine environments from tidal flat through salt marsh to acidic bog produce quantitative diatom transfer function models for elevation reconstructions based on fossil samples. Only nine out of 124 fossil assemblages analysed, including previously published data for the AD 1964 earthquake, have a poor modern analogue. Calibration of fossil samples indicate co-seismic subsidence of 1.5070.32 m for AD 1964, similar to measurements taken after the earthquake, and 1.4570.34 m for the
850 cal yr BP earthquake. Elevation standard errors for individual fossil samples range from
0.08 m in peat layers to
0.35 m in silt units. Lack of a chronology within fossil silt units prevents identification of changes in the rate of recovery and land uplift between the post-seismic and inter-seismic periods. However, preservation of multiple peat-silt couplets indicates no net emergence over multiple earthquake cycles. Glacio-isostatic movements from Little Ice Age glacier advance and retreat explains a
0.15 m relative sea-level oscillation recorded within the peat layer subsequently submerged as a result of the AD 1964 earthquake. Before both this and the
850 cal yr BP earthquake, diatom assemblages suggest pre-seismic relative sea-level rise of
0.1270.13 m, representing possible precursors to great earthquakes. r 2004 Elsevier Ltd. All rights reserved.

1. Context Coasts adjacent to subduction zones experience rapid changes in relative sea level and in associated sedimentary environments during large plate boundary earthquakes (magnitude48). Evidence exists in the form of observational data, especially GPS and tidal information, their modelling and Holocene coastal stratigraphy. From Holocene evidence, it is possible to reconstruct the spatial pattern of co-seismic uplift and subsidence for series of past earthquakes, leading to greater understanding of the earthquake cycle and earthquake hazard.

Corresponding author. Tel.: +44 191 334 1934; fax: +44 191 334 1801. E-mail address: [email protected] (I. Shennan).

0277-3791/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2004.11.003

Southern Alaska, USA, lies above a convergent plate margin, the north dipping Aleutian Megathrust (Bartsch-Winkler and Schmoll, 1992). At this boundary, the Pacific and North American plates converge at approximately 6.3 cm yr1 in a N18 1W direction (e.g., DeMets et al., 1990; Minster and Jordan, 1978). On March 27, 1964, an earthquake of magnitude 9.2 occurred along the south coast of Alaska, dramatically changing many coastal environments. The earthquake was accompanied by vertical and horizontal tectonic deformation and tilting of the land over an area of 170,000–200,000 km2 (Plafker, 1969). Co-seismic subsidence occurred over an elongate region (Fig. 1) including the Kenai Peninsula and most of Cook Inlet. In greater Anchorage, the largest urban area in Alaska, co-seismic subsidence ranged from o0.5 m to 41.5 m (Plafker, 1969). Girdwood, the main site reported in this paper, experienced
1.5 m regional subsidence in AD

ARTICLE IN PRESS 1480

S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

Fig. 1. Location of field sites around Cook Inlet. Map based upon Plafker (1969). Contours (m) show vertical deformation caused by the AD 1964 earthquake. Contours hide many local scale variations, including local subsidence caused by sediment compaction and, in addition, some are based on interpolations over long distances.

1964 and up to
0.9 m localised subsidence due to sediment compaction. In contrast, seaward of the subsidence zone, as much as 11.3 m co-seismic uplift occurred.

Numerous earthquake deformation cycle models exist, for example, those of Thatcher (1984), Nelson et al. (1996) and Shennan et al. (1999). All describe periods of land-level change, as reflected by relative sea-level

ARTICLE IN PRESS S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

change, associated with large plate boundary earthquakes and intervening periods (Fig. 2). Shennan et al. (1999) propose a four-stage earthquake deformation cycle model for a site in southern Alaska experiencing co-seismic subsidence (sudden relative sea-level rise). Rapid post-seismic uplift (relative sea-level fall) occurs in the decades after the earthquake, followed by centuries of slower inter-seismic uplift (relative sea-level fall), then pre-seismic relative sea-level rise immediately before the next earthquake. Litho-stratigraphy of estuarine sediments helps to identify seismic from non-seismic relative sea-level changes (e.g., Long and Shennan, 1994; Nelson et al., 1996). Typically, in areas of co-seismic subsidence, freshwater peat rapidly submerges into the intertidal zone. This results in a peat-silt couplet (Atwater, 1987; Nelson et al., 1996), peat overlain by fine-grained clastic sediment, with a sharp stratigraphic boundary between the two. Assessment of suddenness and amount of subsidence, lateral extent of peat-silt couplets with sharp upper contacts, evidence of tsunami deposits and synchroneity of subsidence with other sites (Nelson et al., 1996) are critical in evaluating a possible co-seismic cause. Around Cook Inlet, Combellick (e.g. 1991, 1994) and Combellick and Reger (1994) report numerous buried peat-silt couplets and radiocarbon evidence of Holocene earthquakes, but offer no microfossil evidence to Prince William Sound Subsidence

Girdwood Anchorage

Uplift

Earlier level Shortening UPPER CONTINENTAL PLATE

SUBDUCTING OCEANIC PLATE Seismogenically locked

(A)

Transition zone Uplift

Subsidence

Level in figure A Extension UPPER CONTINENTAL PLATE SUBDUCTING OCEANIC PLATE Strain release

(B) Fig. 2. Schematic diagrams showing the pattern of (A) inter-seismic and (B) co-seismic deformation associated with a subduction zone earthquake during an earthquake deformation cycle. Adapted from Nelson et al. (1996) to reflect the spatial pattern of co-seismic deformation during the AD 1964 earthquake in Alaska. Site locations are in Fig. 1.

1481

support or quantify co-seismic subsidence. Three different scenarios (Fig. 3A, D and E) result in the preservation of multiple peat-silt couplets beneath a marsh surface. Without any non-seismic (i.e. eustatic or glacio-isostatic) relative sea-level rise, post- and interseismic uplift must be less than co-seismic subsidence (Fig. 3A). If post- and inter-seismic uplift equals coseismic subsidence, different peat layers would be superimposed on top of one another (Fig. 3B). If post- and inter-seismic uplift were greater than co-seismic subsidence (resulting in net uplift during the earthquake deformation cycle), it would lead to net emergence of marshes above present with potential oxidation and decay of peat (Fig. 3C). To allow preservation of buried peat–silt couplets where there is non-seismic relative sealevel rise, inter-seismic uplift can be less than, or equal to co-seismic subsidence (Fig. 3D). If post- and interseismic uplift were greater than co-seismic subsidence, preservation would only occur if uplift were less than the sum of co-seismic subsidence and non-seismic relative sea-level rise (Fig. 3E). Otherwise, it would lead to net emergence of marshes above present with potential oxidation and decay of peat (Fig. 3F). The following equation represents the changes between two earthquakes at a single location: Dxint ðtÞ ¼ Dxrsl ðtÞ þ Dx cos ðtÞ  Dxsed ðtÞ  xpeat ðtÞ: Dxint ðtÞ represents post- and inter-seismic uplift, Dxrsl ðtÞ is non-seismic relative sea-level change over the time period in question (including tidal regime changes), Dxcos ðtÞ equals co-seismic subsidence accompanying a subduction zone earthquake, Dxsed ðtÞ is net sedimentation (including consolidation) between the tops of two peat layers and xpeat ðtÞ is the difference in the formation elevation of the top of the first buried peat (xpeat1 ðtÞ) and the formation elevation of the top of the second buried peat (xpeat2 ðtÞ). Solving this equation would provide a reconstruction for a complete earthquake deformation cycle, in contrast to post-AD 1964 observations that only deal with a small portion of one cycle. This equation will provide quantitative data for testing seismological models of the earthquake deformation cycle. Environmental reconstruction using microfossils contained within estuarine sediments (e.g., diatoms, pollen and foraminifera) distinguish between seismic and nonseismic origins of peat–silt couplets using a sea-level tendency approach (e.g., Shennan, 1986). Microfossils also help to define periods within an earthquake deformation cycle model (e.g., Long and Shennan, 1994). Diatoms are used in this study as they are the most ubiquitous microfossil group, usually present in both marine and freshwater environments. In contrast, pollen grains are often poorly preserved in silt units and foraminifera are mostly absent from peat (e.g., Zong et al., 2003). In broad terms, the order of diatom salinity

ARTICLE IN PRESS S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

1482

With glacio-isostatic or eustatic relative sea-level rise ξpeat2(τ)

No glacio-isostatic or eustatic relative sea-level rise ξpeat1(τ)

ξpeat2(τ)

∆ξrsl(τ)

∆ξrsl(τ)

∆ξsed(τ)

∆ξcos(τ) ∆ξint(τ) ∆ξcos(τ)>∆ξint(τ)

(A)

ξpeat1(τ)

ξpeat1(τ)

∆ξcos(τ)

ξpeat2(τ)

∆ξrsl(τ)

∆ξint(τ)

∆ξcos(τ)=∆ξint(τ) ∆ξsed(τ)=0

∆ξcos(τ) + ∆ξrsl(τ) > ∆ξint(τ) (E)

ξpeat2(τ)

ξpeat2(τ) ∆ξrsl(τ)

ξpeat1(τ) ∆ξint(τ)

∆ξint(τ)

∆ξcos(τ)

∆ξsed(τ)

∆ξint(τ)

∆ξcos(τ)

∆ξrsl(τ)

(C)

ξpeat2(τ)

ξpeat1(τ)

∆ξcos(τ)

ξpeat1(τ)

∆ξint(τ) ∆ξcos(τ) ≥ ∆ξint(τ)

(D)

∆ξrsl(τ)

(B)

∆ξsed(τ)

∆ξcos(τ)

∆ξcos(τ)<∆ξint(τ)

(F)

∆ξcos(τ) + ∆ξrsl(τ) < ∆ξint(τ)

∆ξint(τ) = post-and inter-seismic uplift ∆ξrsl(τ) = non-seismic sea-level change over the time period in question ∆ξcos(τ) = co-seismic subsidence accompanying an earthquake ∆ξsed(τ) = net sedimentation between the tops of two peat layers (including consolidation) ξpeat1(τ) = formation elevation of the top of the first buried peat ξpeat2(τ) = formation elevation of the top of the second buried peat τ = time

Fig. 3. Schematic peat-silt couplet models of co-seismic subsidence, post- and inter-seismic uplift, sediment accumulation and marsh peat burial with no non-seismic (eustatic or glacio-isostatic) relative sea-level rise (A–C) and with non-seismic relative sea-level rise (D–F).

ARTICLE IN PRESS S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498 Table 1 Halobian classification scheme of diatoms (Hemphill-Haley, 1993) Classification

Salinity range (%)

Environment

Polyhalobous Mesohalobous Oligohalobous halophile

430 0.2–30 o0.2

Oligohalobous indifferent

o0.2

Halophobous

0

Marine Brackish Freshwater—stimulated at low salinity Freshwater—tolerates low salinity Salt-intolerant

classes (Table 1) should reflect change from tidal flat through salt marsh, to freshwater marsh and bog. Marine (polyhalobous) and brackish (mesohalobous) groups usually dominate tidal flat environments and freshwater groups tolerant of different degrees of salinity (oligohalobous halophile and oligohalobous indifferent) become dominant through the transition from salt marsh to freshwater marsh (e.g., Zong et al., 2003). Salt-intolerant species (halophobous) characterise the most landward communities, including those from acidic bogs above the level of highest tides. An increase in marine diatoms in a fossil sequence suggests a relative sea-level rise and a positive sea-level tendency, where as a decrease in marine diatoms indicates a relative sealevel fall and a negative sea-level tendency. In addition, statistical techniques, for example transfer functions, use microfossils for quantitative environmental reconstruction through time (e.g., Gehrels et al., 2001). In this paper we distinguish seismic from non-seismic elements of relative sea-level change and marsh peat burial through the different stages of the earthquake deformation cycle model rather than just the AD 1964 earthquake (e.g., Shennan et al., 1999; Zong et al., 2003). We achieve this by using (i) microfossil data collected from modern intertidal and supratidal environments to produce a quantitative transfer function for fossil reconstructions of relative sea level, (ii) microfossil data from sediments covering the period from the penultimate earthquake, through the AD 1964 earthquake to present, and (iii) a chronology based on new radiocarbon ages and published isotope data.

2. Methods We undertook field sampling at different times between February and October over 5 years, to observe and sample modern sedimentary environments under different seasonal and tidal conditions. No single study site around upper Cook Inlet includes a full range of modern day estuarine environments, from tidal flat through salt marsh to acidic bog. Therefore, we combine new surface diatom sample data from Girdwood, Ocean

1483

View and Kenai with data from Zong et al. (2003) to form our modern data set (Fig. 1). Cleaning away the modern surface film and collecting the top 1 cm of sediment allows for seasonal variations in diatom blooms and effects of winter freezing. Samples were sealed in bags and the sampling site levelled to a temporary benchmark (defined as 100.00 m at each site) using a standard level and staff. Using data from NOAA (http://www.co-ops.nos.noaa.gov), levelled elevations were converted to mean higher high water (MHHW). Elevation relative to mean lower low water (MLLW) was not suitable as a reference tide level because at Kenai base flow of the river dampens out the effect of low tide. Also, the combination of large tidal range (
8.8 m at Anchorage), extent of the intertidal zone and unstable sediments meant that Mean Lower Low Water could not be measured at Girdwood and Ocean View. We measured high tides at each field site and compared them with tide gauge observations from the closest tide station, Anchorage for Girdwood and Ocean View, and Nikiski for Kenai (Fig. 4). From the best-fit solutions, we calculate elevation for each modern sample relative to Mean Higher High Water at Anchorage or Nikiski. Use of tidal observations result in different elevations to those reported by Zong et al. (2003) that were based upon tidal predictions. To accommodate tidal range differences between sites, we standardise elevations relative to Mean Higher High Water and the difference in elevation between mean higher high water and mean sea level (MSL) SWLIn ¼

100ðhn  hMSL Þ þ 100; hMHHW  hMSL

where SWLIn is the standardised water level index for sample n, hn the elevation of sample n, m, hMSL the mean sea level elevation, m, hMHHW the mean higher high water elevation, m. This produces a standardised water level index (SWLI) for each modern sample with 100 representing mean sea level and 200 for mean higher high water. Quantitative diatom transfer functions use the standardised water level index as the environmental variable for each modern sample and produces a reconstructed standardised water level index for each fossil sample. Elevations for the fossil samples are then converted back to mean higher high water at the field site by reversing the calculations. At Girdwood, good exposures of two buried peat layers containing stumps of dead trees (ghost forests) are laterally extensive along a vertical section at the seaward edge of the marsh. Transects of cores and channel exposures on the tidal mudflat were all levelled to a temporary benchmark and used to demonstrate lateral extent of the different layers. Cleaning of a section

ARTICLE IN PRESS S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

1484

10

y = 1.0179x -91.96 R2= 0.9994

9

8

7 98 99 100 Ocean View observed (m TBM)

(A)

Girdwood Anchorage tide gauge observed (m MLLW)

Anchorage tide gauge observed (m MLLW)

Ocean View 10

9

8

7 97 98 99 100 Girdwood observed (m TBM)

(B)

Kenai Flats y = 0.8372x -75.113 R2= 0.9919

6.2

5.8

5.4

Kenai City Pier 6.6 Nikiski tide gauge observed (m MLLW)

Nikiski tide gauge observed (m MLLW)

6.6

5 (C)

y = 1.1273x -102.54 R2= 0.9639

y = 0.9214x -82.934 R2= 0.9413

6.2

5.8

5.4

5 96 97 98 Kenai Flats observed (m TBM)

(D)

95.5 96 96.5 97 Kenai Pier observed (m TBM)

Fig. 4. High tide observations at field sites compared to observations from the closest tide gauge station. Observations at the tide stations are measured relative to MLLW and converted to m MHHW. At Anchorage, MHHW is +8.84 m MLLW and at Nikiski, MHHW is +6.62 m MLLW. These values are used to calculate m MHHW at each site using the best fit equations shown.

allowed description of stratigraphy and sampling of sediment using a variety of monolith tins and tubing. Well preserved in situ macrofossils (mainly herbaceous rootlets) were used for AMS radiocarbon dating. CALIB 4.4 (Stuiver and Reimer, 1993) calibrates radiocarbon results to calendar years before present (cal yr BP) using the atmospheric decadal data set (file INTCAL98.14C, Stuiver et al., 1998) and the 95% probability distribution method. This paper reports calibrated ages as the 95% range. Preparation of diatom samples followed standard laboratory methods (Palmer and Abbott, 1986) with 250 diatom valves counted in most samples. Diatom identification used Van der Werff and Huls (1958–1974) together with supplementary texts of Denys (1991), Hartley et al. (1996), Hemphill-Haley (1993) and Patrick and Reimer (1966, 1975). TILIA (version 2.0 b5; Grimm, 1993) allows plotting of results and implementation of the halobian classification system divides diatom species into five categories of salinity tolerance (Table 1). Transfer functions use regression to quantify the relationship between surface diatom data collected from

modern tidal flats, salt marshes and acidic bog and their associated elevation (m) relative to Mean Higher High water. Calibration then uses this relationship to reconstruct past environments represented by fossil diatom assemblages from Holocene sediment sequences (Birks, 1995). The results provide quantitative estimates of elevation change throughout the studied sediment section. We follow the recommendations of Birks (1995) to identify appropriate numerical methods. Detrended canonical correspondence analysis (DCCA) within CONOCO (version 4.5; ter Braak and Smilauer, 2002) showed what regression methods were suitable. We then used weighted averaging-partial least squares (WA-PLS) within the software package C2 (Juggins, 2003) to produce the transfer function models. In addition, Weighted Averaging within C2 determines each modern diatom species’ optima (the maximum of the response curve) and tolerance (breadth of the response curve) with reference to elevation. Statistical parameters produced during regression and calibration include the coefficient of determination (r2 ) and the root mean square error of prediction (RMSEP).

ARTICLE IN PRESS S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

Both measure the strength of a relationship between observed and inferred values (Birks, 1995). Each model has different components. The first component maximises covariance between the vector of weighted averages and the environmental variable of interest (Birks, 1995). Subsequent components maximise the same criterion but are uncorrelated to earlier components (ter Braak et al., 1993). Selection of the best models and components are based upon a low RMSEP and high r2 : These models are then applied to fossil sequences to give sample-specific reconstructions of elevation 71 standard error (Juggins, 2003). When calculating a relative change between two fossil samples, change in elevation is simply the difference between the two reconstructed values with an error term from the formula (Preuss, 1979) p ðelevation error 12 þ elevation error 22 Þ: We also use the modern analogue technique (MAT) within C2 (Juggins, 2003) to identify fossil samples that have ‘poor’ modern analogues when compared to the modern data set (Birks, 1995; Edwards and Horton, 2000; Zong et al., 2003). We define a good modern analogue as having a dissimilarity coefficient value less than the extreme 5% threshold of the data set. A close modern analogue has a dissimilarity coefficient value within the extreme 5–2.5% range and a poor modern analogue has a dissimilarity coefficient greater than the extreme 2.5% threshold. This is important, as calibration results may be less reliable where ‘poor’ modern analogues exist in fossil sequences.

3. Results: modern samples and transfer function There is a general trend in the 154 modern diatom samples from Girdwood, Ocean View and Kenai relative to elevation (Fig. 5). Polyhalobous species dominate those samples found below a standardised water level index of 100, mean sea level, and unvegetated tidal flat in all cases. At higher elevations, assemblages gradually become less salt tolerant and halophobous species dominate samples found above a standardised water level index of 240 (acidic bog). In addition, diatom species with optima below a standardised water level index of 220 have larger tolerances, where as those with optima above a standardised water level index of 220 generally occur within a smaller elevation range (Fig. 6). We develop the best transfer function to reconstruct elevation relative to Mean Higher High Water for fossil samples using three models. Model 1 uses the full modern data set, model 2 contains only modern samples above a standardised water level index of 180 and model 3 contains only modern samples above a standardised water level index of 225. For each of these models, DCCA suggests that unimodal methods of regression

1485

and calibration are appropriate as gradient lengths are greater than 2 (Table 2). Regression using Weighted Averaging-Partial Least Squares results in r2 and root mean square error of prediction (RMSEP) summarised in Table 3. A high r2 ; low RMSEP and inspection of scatter plots enabled selection of the best component. The relationship between observed and predicted standardised water level index for the best components are in Fig. 7. Regression of model 1, full modern data set, results in a high RMSEP of approximately 22 and a low r2 : This suggests that the predictive abilities and relationship between observed and inferred values are poor, resulting in a high associated error. This model does not successfully predict the elevation of samples found below a standardised water level index of 175 (Fig. 7). To decrease the non-linearity present in the tidal flat samples and to increase the transfer function’s predictive ability, models 2 and 3 include fewer lower elevation samples. Model 3 (component 3) has the lowest RMSEP of 2.90. Calibration of fossil data requires selection of the best regression model or models to estimate elevation changes through time. Different regression models may perform better in certain parts of the environmental range and lithostratigraphy provides an independent assessment over which model or models are applicable. Modern day acidic bog and other peat forming environments occur above a standardised water level index of 230. Therefore, where peat occurs in the stratigraphy, model 3, using samples with a standardised water level index above 225 is the most accurate, as peat only forms above this elevation, allowing for some variation at the lower limits. In this environment, WAPLS component 3 is the best model as it has a high r2 (0.93) and lowest RMSEP (2.90). Model 3, using samples above a standardised water level index above 225, is unsuitable for interpreting changes throughout silt units because it does not include modern day tidal flat diatom assemblages. If the silt contains in situ rootlets, model 2, including only samples above a standardised water level index of 180 is appropriate and more precise, as vegetation does not live below this elevation, again, allowing for some variance at the lower elevation limit. WA-PLS component 3 is best. If the silt does not contain any rootlets then the full data set (model 1) is best using component 2, as it is possible that silt deposition occurred below a standardised water level index of 180. It allows prediction of lower elevations, but has larger associated errors. Using the complete modern data set of 154 diatom samples, a good modern analogue has a dissimilarity coefficient value of o74.01 (extreme 5% threshold). A close modern analogue has a dissimilarity coefficient value of 74.01–92.15 (extreme 5% to 2.5% range) and a poor modern analogue has a dissimilarity coefficient of

Standardised water level index

40 40 80

re C mis oc a D con mb el e ig ph is ua N ine pel av is to i N icul sur des av a i O ic ph rell a d u Pa on la ylle ra tell spe pta li a c Th a s au ies u r Amala lca ita 1 ss t p N h io a av o si ra ra i c N ul c e av a of c ic di fea ce ul gi e nt a to fo ric N s a ra rm a it z lin dia is s R ch ar ta h o ia um Sc p o a b Syolio lod tus i Lu nedneisa o a p t r i a N co f tumerc av la a i u ic m sci da lat ul u cu a tic la a ca a ta ri va N rc av in Ac icul ct a hn a p Ac an ro hn the tra an s cta t h la n Au es c la m eol co i n at se ira utis a si C gr ym an ma D be u la ip lla ta lo n v N eis ent av r ic ov ico N ula alis sa av N ic bro itz ul ck sc a m a hi va a rio nii fru s N t r tic ia it os ta N zsc it z h a i sc a hi pa a lu N pu st it Pi zsc si ris lla nn hi ul a t Pi ari he a rm n la a Pi nu g n l Sy nu aria erslis ne lar m ted E u d r ia m e s tii no a u ic ole tia lna ro pt st a ex au ig ro Pi u n nn a Ta ula be ria l s Ta lari ub be a f c a lla en pit ria es ata flo trat cc a ul os a

Oligohalobous -halophile

40 60 Percent

Oligohalobous-indifferent

40

60

80

100

120

140

160

180

200

220

240 40 40 60 40 40 60 40

Fig. 5. Modern diatom data (415% total diatom valves counted). Samples ordered by elevation (standardised water level index, where 100 ¼ mean sea level and 200 ¼ mean higher high water).

ARTICLE IN PRESS

40

Mesohalobous

S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

Bi

1486

Polyhalobous Halophobous

ARTICLE IN PRESS 1487 Fig. 6. Diatom elevation optima and tolerance from Weighted Averaging within the computer program C2 (Juggins, 2003). Main diagram shows species 410% total diatom valves in at least one sample and inset shows the full data set.

Pinnularia mesolepta Eunotia exigua Frustulia rhomboides Pinnularia subcapitata Eunotia lunaris Nitzschia palustris Aulacoseira granulata Diploneis ovalis Navicula variostriata Pinnularia lagerstedtii Pinnularia microstauron Tabellaria fenestrata Nitzschia palea Navicula begeri Navicula brockmanii Nitzschia fruticosa Nitzschia thermalis Achnanthes lanceolata Nitzschia dubia Navicula protracta Navicula salinarum Nitzschia denticula Nitzschia obtusa Luticola mutica Achnanthes minutissima Nitzschia pusilla Rhopalodia operculata Synedra ulna Tabellaria flocculosa Navicula phyllepta Nitzschia sigma Navicula digitoradiata Navicula cari var. cincta Amphora coffeaeformis Navicula peregrina Scolioneis tumida Synedra fasciculata Gyrosigma wansbeckii Paralia sulcata Cocconeis peltoides Delphineis surirella Achnanthes delicatula Cymbella ventricosa Thalassiosira eccentrica Odontella aurita Thalassionema nitzschioides Biremis ambigua Nitzschia apiculata Navicula species 1

0

50

0

150

100

150

200

250

Standardised water level index

300

300

S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

ARTICLE IN PRESS 1488

S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

bank section, which is approximately 34 cm below the level of the vegetated marsh). From 82 to 91 cm, it contains a small amount of silt. Approximately 100 cm below the top of this peat is peat G. The intervening silt unit ranges in thickness from
70 cm to over 100 cm. At GW-1, Peat G is predominantly herbaceous peat and occurs at a depth of 179 to 188 cm. Coarse sand layers do not overlie any of the buried peat surfaces. AMS dating of in situ herbaceous rootlets found at the base of peat H, and top and base of peat G reveal similar ages for GW-1 and GW-2 (Fig. 8 and Table 4). Peat G formed between
1100 and 850 cal yr BP, and peat H between
450 cal yr BP and AD 1964. In addition, a tephra found beneath the base of peat H correlates with the Augustine tephra, deposited approximately 500 cal yr BP (Combellick, unpublished data), in good agreement with the radiocarbon results. Diatom assemblages at GW-1 (Fig. 9 and Table 5) show abrupt changes around the tops of peats G and H with more gradual changes through the rest of the profile. We quantify elevation changes with the transfer function model appropriate for each fossil sample, with models chosen using litho-stratigraphic constraints (Table 6). The modern analogue technique indicates that one fossil sample, at 110 cm, has a ‘poor’ modern analogue, 11 (23%) have ‘close’ modern analogues and 35 (74%) have ‘good’ modern analogues (Fig. 10). Predicted elevation changes through GW-1 range from 0.3070.38 m MHHW to +1.3870.10 m MHHW in peat H (Fig. 11).

492.15 (extreme 2.5% threshold). These thresholds indicate how similar each individual fossil sample is to a modern sample.

4. Results: Holocene evidence from Girdwood Plafker et al. (1969) suggest that 1.5 m of regional subsidence and up to 0.9 m of local subsidence of unconsolidated sediment accompanied the AD 1964 earthquake at Girdwood. Present day marsh surface consists predominantly of low growing Carex lyngbyei (Lyngby sedge) and Triglochin maritima (Seaside arrowgrass). A vertical bank at the seaward edge of the marsh contains two extensive peat layers (Fig. 8), the upper buried in AD 1964 (peat H) and the lower buried approximately 850 cal yr BP (peat G). The two buried peat layers are laterally extensive and have sharp upper boundaries separating them from overlying silt, suggesting that they may be the result of submergence following co-seismic subsidence (Nelson et al., 1996). This section investigates how the stratigraphy of Girdwood records relative sea- and land-level movements associated with the earthquake deformation cycle model, from Peat G to present. Future analyses will evaluate whether peats A to F record co-seismic subsidence during multiple earthquake deformation cycles over the past 3500 cal yr BP. Peat H is visible along the entire vertical section. Its upper boundary varies in depth from approximately 30 to 100 cm below present day marsh surface. Dead tree stumps visible on the surface today are rooted in this peat. At sampling site GW-1, the upper peat layer is brown herbaceous peat and occurs at a depth of 69–112 cm (all depths relate to the top of the sampled

5. Elevation changes through GW-1 We interpret elevation changes from Peat G through to present in chronological order (Fig. 11). Identification of the four periods of the earthquake deformation cycle model, described earlier, together with the criteria reviewed by Nelson et al. (1996) are important when determining if a buried peat layer results from co-seismic subsidence or non-seismic changes in relative sea level. Peat G started to develop between 1100 and 850 cal yr BP, earlier at GW-2 than GW-1 (Table 4). Its development probably results from the inter-seismic period of the earthquake deformation cycle model (Fig.

Table 2 DCCA results for modern data sets using standardised water level index (SWLI) Model

DCCA axis 1 length (SD units)

1 Full data set 2 SWLI4180 3 SWLI4225

3.28 3.14 3.53

Table 3 Summary statistics for modern data sets using WA-PLS components 1–4 from the computer program C2 (Juggins, 2003) r2 Model 1 Full data set 2 SWLI4180 3 SWLI4225

Component

RMSEP

1

2

3

4

1

2

3

4

0.65 0.71 0.75

0.75 0.83 0.89

0.79 0.89 0.93

0.82 0.91 0.95

24.04 9.61 3.19

21.95 8.35 2.99

21.97 7.79 2.90

22.90 7.85 2.93

Values in bold show the best models, illustrated in Fig. 7.

ARTICLE IN PRESS S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

Predicted Standardised water level index

250

Model 1: full data set Component 2

200

150

100

50

0 0

Predicted standardised water level index

250

50

150

200

250

Model 2: samples with SWLI >180 Component 3

225

200

175 175

Predicted standardised water level index

100

200

250

Model 3: samples

245

with SWLI >225 Component 3

225

250

240 235 230 225 220 220

225

230

235

240

245

250

Observed standardised water level index Fig. 7. Observed against predicted elevation (standardised water level index) for three transfer function models.

2) where strain accumulation at the plate boundary gradually causes land to rise and relative sea level to fall. The transfer function suggests that peat G has a

1489

reconstructed elevation of approximately +1.277 0.11 m MHHW (zone A). Within the upper 3 cm of peat G, diatom assemblages suggest a positive sealevel tendency (Figs. 9 and 11). Quantitative reconstructions suggest an elevation change of 0.1170.13 m, from a ground level of +1.3070.10 m MHHW to +1.1970.09 m MHHW. Evidence to suggest that these assemblages are not a result of diatoms filtering down from the overlying silt includes an increase in Navicula variostriata and Nitzschia pusilla within this zone (Fig. 9). The stratigraphic boundary between peat G (zone A) and the overlying silt (zone B) represents submergence following co-seismic subsidence associated with the penultimate earthquake approximately 850 cal yr BP. Evidence suggesting co-seismic subsidence includes a laterally extensive buried peat layer with a sharp upper boundary together with polyhalobous diatoms dominating the overlying silt (Fig. 9). This indicates a sudden change from a freshwater peat environment to one regularly inundated by tides. There is no evidence of any tsunami deposit and synchroneity of subsidence awaits analyses from other sites. Quantitative reconstructions indicate rapid subsidence, 1.4570.34 m, to a new ground height of 0.2670.33 m MHHW, assuming the first couple of elevation reconstructions within the silt unit are due to mixing following burial (Fig. 11). Previous investigations (e.g. Shennan et al., 1996; Zong et al., 2003) suggest calculation of the amount of subsidence equals the maximum difference estimated from the marsh top and the minimum value indicated in the basal part of the overlying silt since accumulation after the earthquake can be very rapid (Atwater et al., 2001). Relative sea-level fall follows co-seismic subsidence, allowing development of peat H approximately 450 cal yr BP (Fig. 11). Quantitative reconstructions indicate an elevation change of 1.6470.33 m, to a new ground height of +1.3870.09 m MHHW (zone C), representing post-seismic and inter-seismic recovery of the land combined with sediment accumulation. Within peat H, there is an oscillation in relative sea level (zone D). This is significant because it also corresponds to a slight increase in silt content between 82 and 91 cm. Quantitative reconstructions suggest a relative sea-level rise, an elevation change of 0.1670.13 m, followed by a relative sea-level fall, an elevation change of 0.1270.13 m. Hamilton (2003) reports a similar oscillation in the diatom stratigraphy at Kenai, below the sequence reported by Zong et al. (2003) suggesting a regional rather than local scale process. Within the upper few centimetres of peat H, quantitative reconstructions indicate an increase in tidal inundation with an elevation change of 0.1470.13 m to a new ground height of +1.2070.10 m MHHW (zone

ARTICLE IN PRESS S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

1490

Fig. 8. Location of sampling sites (A) and composite section (B) showing buried peat layers beneath the marsh surface at Girdwood site GW-2. Peats G and H were also exposed and sampled at GW-1. GW-33 and 34 are sites reported by Zong et al. (2003) and GW-99 by Shennan et al. (1999). Table 4 Radiocarbon results from Girdwood, including two dates on tree stumps rooted in peat G near sampling site GW-33 (from Combellick and Reger, 1994) Site

Lab code

Stratigraphic context

Laboratory reported 14C age71s

Calibrated age BP Median age followed by minimum and maximum ages of 95% range

GW-1 GW-33 GW-33 GW-1 GW-2 GW-1 GW-2 GW-2 GW-2 GW-2 GW-2 GW-2 GW-2 GW-2 GW-2 GW-2

CAMS-93957 Beta-45197 Beta-45199 CAMS-93958 Beta-184321 CAMS-93959 Beta-184322 Beta-184326 Beta-184323 Beta-184324 Beta-184325 Beta-184327 Beta-184331 Beta-184330 Beta-184328 Beta-184329

Base of peat H Rooted wood top of peat G Rooted wood top of peat G Top of peat G Top of peat G Base of peat G Base of peat G Top of peat F Base of peat F Top of peat E Base of peat E Top of silty peat D Within silty peat D Top of silty peat C Top of peat B Top of peat A

395740 860760 940760 955740 890740 945740 1170740 1540740 2080740 2120750 2480750 2560740 2600740 2530740 2710740 3040740

455 780 848 856 817 853 1087 1435 2047 2095 2560 2628 2742 2588 2812 3252

E). It is unlikely that these diatom assemblages are a result of diatoms filtering down from the overlying silt because species such as Nitzschia obtusa, Navicula begeri, Navicula brockmanii and Pinnularia lagerstedtii

318 677 731 764 714 744 971 1334 1934 1949 2360 2472 2495 2383 2751 3080

515 913 954 948 920 932 1175 1523 2148 2304 2727 2763 2782 2749 2916 3356

increase and do not occur in the overlying silt (Fig. 9). Three other exposures of the AD 1964 peat at Girdwood record the same trend in diatom and pollen assemblages (Shennan et al., 1999; Zong et al., 2003). Zong et al.

ARTICLE IN PRESS S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

Fig. 9. Bio-stratigraphy of GW-1, showing diatoms that account for 42% total counted.

1491

ARTICLE IN PRESS S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

1492

Table 5 Diatom descriptions through GW-1 Depth (cm)

Description

Lithology

F

0–68.5

Silt with herbaceous rootlets

E

68.5–74

D

74–94

C

94–171

B

171–178.5

A

178.5–188

Polyhalobous diatoms dominate, e.g. Cocconeis peltoides, Delphineis surirella and Paralia sulcata. Towards the surface, the assemblage becomes more freshwater, with dominance by Achnanthes minutissima and Nitzschia pusilla. Polyhalobous species increase and halophobous taxa decrease towards the top. Nitzschia obtusa and Pinnularia lagerstedtii peak. In the middle of zone D, polyhalobous species increase slightly and Navicula begeri, Navicula pusilla and Pinnularia lagerstedtii peak. On either side of this change, halophobous species increase, especially Eunotia exigua. Polyhalobous species decrease towards the top (e.g., Delphineis surirella) and oligohalobous-indifferent and halophobous taxa increase (e.g., Eunotia lunaris, Nitzschia fruticosa, Eunotia exigua and Pinnularia subsolaris). Polyhalobous species dominate, especially, Actinoptychus senarius, Delphineis surirella and Paralia sulcata. Oligohalobous-indifferent diatoms dominate (e.g., Navicula begeri and Pinnularia lagerstedtii). Halophobous species increase towards the top (e.g., Eunotia exigua and Navicula contenta). Navicula variostriata and Nitzschia pusilla peak immediately below the peat-silt boundary.

Model used

Reason

0–68 116–178 69–112 179–188

2 SWLI4180 3 SWLI4225

Lithology is silt with rootlets Lithology is peat

Poor

Silt with herbaceous rootlets through to Peat H Silt with herbaceous rootlets Peat G

20 40 60 80 100 120

Peat G

(2003) suggest that this change started around the beginning of the 1950s, dated using initial 137Cs concentrations within the peat. They calculated a relative sea-level rise of 0.1670.13 m using diatom transfer functions and 0.1570.11 m using pollen transfer functions. However, Zong et al. (2003) used an incorrect calculation for the error term leading to an underestimate. The stratigraphic boundary between peat H and overlying silt represents co-seismic subsidence associated with the AD 1964 earthquake (zone F). Evidence includes a laterally extensive sharp boundary between peat H and overlying silt together with diatom assemblages showing a rapid change to polyhalobous species. Peaks in 137Cs concentration date this contact to AD 1964 (Zong et al., 2003). Quantitative reconstructions indicate rapid subsidence of 1.5070.32 m to a new ground level of 0.3070.30 m MHHW. This is the same value as Plafker’s estimate of regional subsidence,
1.5 m, but does not record up to 0.9 m local subsidence of unconsolidated sediment (Plafker et al., 1969). These figures also compare favourably with the results of Zong et al. (2003) who estimate co-seismic subsidence between 1.5970.22 and 1.9870.23 m using diatom transfer functions and greater than 1.0770.42 m using pollen

Peat H with slight increase in silt

0

Peat H

Depth ranges (cm)

Peat H

Close

Table 6 Transfer function models used to reconstruct elevation changes through GW-1

Good

Depth (cm)

Zone

140 160 180 200 0

20 40 60 80 100 Minimum Dissimilarity Coefficient Silt

120

Herbaceous peat

Fig. 10. Minimum dissimilarity coefficient values from the modern analogue technique for each fossil sample from GW-1.

transfer functions. However, Zong et al. (2003) use a less accurate estimate for tidal regime differences between Kenai and Girdwood that did not account for site-scale

ARTICLE IN PRESS S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498 Depth (cm) Lithology 0

Zone

1493

reconstructions suggest an elevation change of 0.7470.42 m during the accumulation of 0.69 m of sediment (Fig. 11). This indicates the combined effect of rapid sediment accumulation and post-seismic uplift of the land.

20

6. Discussion 40

6.1. Diatom transfer function for Cook Inlet F

60

E 80 Peat H

D

100

120

140

C

160

Peat G

B 180 A

200 -1.0 -0.5 0.0 0.5 1.0 Predicted elevation (m) relative to MHHW Poor Modern Analogue

Silt

1.5

Herbaceous peat

Fig. 11. Changes in predicted elevation (m) relative to mean higher high water for GW-1, using the transfer function models specified in Table 6. Sample 110 cm has a ‘poor’ modern analogue (Fig. 10).

modifications of tide levels along Kenai River and Turnagain Arm. Immediately after the AD 1964 earthquake, rapid uplift of the land and rapid sedimentation occurred, continuing to the present day (e.g., Brown et al., 1977; Bartsch-Winkler, 1988; Bartsch-Winkler and Schmoll, 1992; Plafker et al., 1992; Combellick, 1997; Cohen and Freymueller, 1997, 2001; Atwater et al., 2001; Cohen, 1996, 1998). Repeated levelling at Girdwood by Brown et al. (1977) suggests a cumulative uplift of approximately 2 cm from AD 1964 to 1965, 12 cm from 1964 to 1968 and 40 cm from 1964 to 1975. Transfer function

Surface diatom assemblages from intertidal and supratidal environments at three sites around Cook Inlet form our modern data set used to quantify elevation changes during the late Holocene. Initially, the full range of diatom assemblages was summarised using the halobian classification. This illustrated the need for quantitative reconstructions as interpretations of summary salinity classifications (e.g. Long and Shennan, 1994; Shennan et al., 1999) derived from samples in north-west Europe may not be a sound basis to reconstruct elevation since different processes may control diatom distributions in Alaska. A primary example is Navicula cari var. cincta that occurs on the tidal flat and low marsh at all three sites yet in the halobian classification for north-west Europe it is classified as freshwater within the oligohalobous–halophile group. The modern data set produced a quantitative diatom transfer function. Use of a standardised water level index, based upon the difference between mean higher high water and mean sea level, allows for differences in tidal range in this macrotidal environment. This is a substantial improvement to initial analyses that used a transfer function from Kenai only and depended upon upscaling of reconstructed elevations due to differences in predicted rather than observed tide levels (Hamilton, 2003; Zong et al., 2003). The full modern data set (model 1) produces a non-linear relationship between observed and predicted elevation for the tidal flat samples. Possible explanations of this trend may include the effect of river discharge at Kenai or intense mixing of tidal flat sediment during the winter months by ice or throughout the year by tides. Applying a lithological constraint to fossil samples determines the best model for calibrating palaeoelevation. In this study, sample specific errors for elevation reconstructions from fossil sequences range from a minimum of
0.08 m in peat layers to a maximum of
0.35 m in tidal silt sequences. 6.2. Application to other datasets Application of the transfer function developed in this paper to other fossil sequences at Girdwood (Shennan et al., 1999; Zong et al., 2003) records clear co-seismic

ARTICLE IN PRESS S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

1494

subsidence (
1.5 m) associated with the AD 1964 earthquake (Fig. 12). They agree with our results from GW-1 and the 1.5 m land subsidence observed by Plafker et al. (1969). Mixing of sediment within the lowest 1 cm or 2 cm of the silt above the stratigraphic boundary gives intermediate reconstructions. Diatom assemblages and pollen analyses offer strong evidence

for a pre-seismic relative sea-level rise at all three sites (Shennan et al., 1999; Zong et al., 2003). Girdwood-99, the only sequence where diatoms were sampled up to the present marsh surface, clearly records post-seismic recovery. Application of the transfer functions to sequences beneath the salt marsh at Kenai (Kenai-3 and Kenai-8),

Kenai-3

0

0 Relative depth (cm)

Relative depth (cm)

Girdwood-99

-5 -10 -15

Silt Peat

-20 -25 -30 -0.5

-2 -4

Silt Peat

-6 -8

0.0

0.5 1.0 m MHHW

1.5

0.0

2.0

0.5

-5 -10

Silt Peat

-15 -20 -1.0

-0.5

0.0

0.5 1.0 m MHHW

1.5

-2 -4

Silt Peat

-6 -8 -10 0.0

2.0

0.5 1.0 m MHHW

1.5

Kenai-13

0

0 Relative depth (cm)

Relative depth (cm)

2.0

0

Girdwood-34

-5

1.5

Kenai-8

0

Relative depth (cm)

Relative depth (cm)

Girdwood-33

1.0 m MHHW

Silt Peat

-10

-15 -1.0

-2 -4

Herbaceous Peat

-6

Bryophyte Peat

-8 -10 -12

-0.5

0.0

0.5 m MHHW

1.0

1.5

2.0

0.5

0.7

0.9

1.1

1.3

1.5

m MHHW

Fig. 12. Elevation reconstructions associated with the AD 1964 earthquake based on the diatom transfer function described in Section 3 and fossil data reported by Shennan et al. (1999), for GW-99, and Zong et al. (2003) for the other sites. In each graph, the horizontal line depicts the stratigraphic boundary that represents the AD 1964 earthquake. Solid circles indicate samples with poor modern analogues and depths relate to the top of the sampled section. Pre-seismic relative-sea level rise usually starts 4–5 cm below the peat–silt boundary apart from in GW-33.

ARTICLE IN PRESS S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

record
0.3 m co-seismic subsidence (allowing for sediment mixing 1 cm above the boundary in Kenai-3) followed by post-seismic recovery. Plafker (1969) recorded 0.27 m co-seismic subsidence at Nikiski, 15 km away (Fig. 1). Both of these sites record preseismic relative sea-level rise of
0.170.1 m. Kenai-13, a site within the acidic bog, records a pre-seismic relative sea-level rise of similar magnitude, but not a quantifiable co-seismic change. This reinforces the conclusion of Hamilton (2003) that in order to identify small changes in elevation it is necessary to select sites close to thresholds along an environmental gradient. Since such thresholds are difficult to identify in the field, it is essential to apply quantitative reconstructions for each earthquake on samples from at least two different locations from each site (Hamilton, 2003). 6.3. Subsidence events and the earthquake deformation cycle model We assess three possible subsidence events with reference to the four periods (co-seismic, post-seismic, inter-seismic, pre-seismic) of the earthquake deformation cycle model. The AD 1964 earthquake at Girdwood (peat H, four locations) and Kenai (three locations) and the penultimate earthquake at Girdwood (peat G) show a series of changes compatible with the earthquake deformation cycle model. All reveal pre-seismic relative sea-level rise and only the acidic bog sequence at Kenai-13 fails to quantify co-seismic subsidence. In all cases, it is difficult to differentiate between post-seismic and inter-seismic relative sea-level fall because of the lack of knowledge on sedimentation rates throughout clastic units. However, observations from the AD 1964 earthquake (e.g., Brown et al., 1977) indicate that post- and inter-seismic periods are separate. An additional co-seismic event may occur at GW-1, within zone D (Fig. 11), where there is a relative sealevel oscillation and an increase in silt within the stratigraphy. As the elevation changes are small, coseismic subsidence may be indistinguishable from nonseismic relative sea-level change. Further investigation of other cores is required to see if it becomes a sharp peat-silt boundary elsewhere beneath the marsh at Girdwood, to investigate lateral extent of the unit and to quantify the amount and suddenness of subsidence. The simplest explanation at present is that isostatic movements occurred during the Little Ice Age, 650–100 cal yr BP. Advance of the surrounding glaciers in Portage Valley and adjacent mountains reached their maximum extent before 140 cal yr BP (Crossen, 1992) causing depression of the Earth’s crust. Following melting, isostatic uplift returned the land to its previous position. These responses would be rapid (i.e. decades to centuries rather than the seconds to minutes for co-

1495

seismic subsidence) due to Girdwood’s proximity to the subduction zone, similar to the glacio-isostatic response recorded in SW Canada and SE Alaska. James et al. (2000) and Clague and James (2002) conclude that postglacial rebound at the northern Cascadia subduction zone was rapid following disappearance of the Cordilleran ice sheet at the end of the Pleistocene, and that it was complete by the middle Holocene. Motyka (2003) reports significant crustal deformation attributed to Little Ice Age glacier expansion and contraction in SE Alaska, around Juneau to Glacier Bay. Because we cannot identify the four periods of the earthquake deformation cycle model within the relative sea-level oscillation observed within zone D, we infer the oscillation to be the result of glacioisostatic changes due to ice cap expansion during the Little Ice Age. 6.4. Long-term tectonic movement Preservation of multiple peat–silt couplets beneath ground level (Fig. 3) limits the amount of post-seismic and inter-seismic uplift for each earthquake deformation cycle. Post-seismic and inter-seismic uplift must be less than the sum of co-seismic subsidence during the preceding earthquake and non-seismic relative sea-level change between the two earthquakes. We can obtain broad estimates for the sum of post-seismic and interseismic uplift from Dxint ðtÞ ¼ Dxrsl ðtÞ þ Dx cos ðtÞ  Dxsed ðtÞ  xpeat ðtÞ: Peltier (2002) estimates Dxrsl ðtÞ for the area at 0.570.5 mm yr1. We measured Dxsed ðtÞ in the field to be 1.1 m between the top of peat G and H. The transfer function reconstructions of elevation provide estimates for Dxcos ðtÞ and xpeat ðtÞ; 1.45 and 0.07 m respectively. We solve this equation using these values and a range of non-seismic relative sea-level changes (0 to 1 mm yr1) over approximately 860 years. This results in 19% postand inter-seismic uplift as a proportion of co-seismic subsidence for zero non-seismic relative sea-level rise, 49% recovery for 0.5 mm yr1 non-seismic relative sealevel rise and 79% recovery for 1 mm yr1 non-seismic relative sea-level rise. However, these calculations make no allowance for sediment consolidation. If there is a similarity between successive cycles, the rate of post-seismic uplift observed in the decade after the AD 1964 earthquake,
0.4 m by 1975 at Girdwood (Brown et al., 1977), cannot continue through a whole inter-seismic period. If it did, 100% recovery would occur within approximately 40 years, raising the marshes above high tide level and leading to net emergence. Comparison with the longer-term evidence indicates a decrease in uplift rate through the post- and inter-seismic periods.

ARTICLE IN PRESS 1496

S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

6.5. Pre-seismic relative sea-level rise as a precursor to a great earthquake Hamilton (2003), Shennan et al. (1996, 1998, 1999) and Zong et al. (2003) record a pre-seismic relative sealevel rise before some episodes of co-seismic subsidence in Oregon and Washington and before the AD 1964 earthquake in Alaska. Tide gauge records before the AD 1964 earthquake are available from Women’s Bay (Kodiak Island) and Seward (Kenai Peninsula). Both locations experienced co-seismic subsidence associated with the AD 1964 earthquake but the tide gauge measurements did not register any pre-seismic relative sea-level rise (Savage and Plafker, 1991). One explanation is that there is differential movement between sites in the pre-seismic period as well as during the co-seismic, post-seismic and inter-seismic periods. For example, Cohen and Freymueller (2001) use GPS measurements to establish a different direction and rate of present day movement between Seward and Anchorage against sites along the western Kenai Peninsula. Further support for relative sea-level rise before the AD 1964 earthquake comes from observational data. Karlstrom (1964) reports that at Girdwood, storm tides, which had never flooded the marsh surface before, began depositing a thin surface silt layer in 1953 that became progressively thicker each year. This date corresponds to the start of the pre-seismic signal identified from the microfossil data at Girdwood, dated using 137Cs. Pre-seismic relative sea-level rise appears to be a common feature in many sequences that record late Holocene great earthquakes. Zong et al. (2003) consider alternative explanations, such as sediment mixing or a temporary change in sea level due to the El Nino Southern Oscillation (ENSO), but suggest that neither can explain the litho- and bio-stratigraphic evidence from Kenai and Girdwood. Overall, we conclude that pre-seismic relative sea-level rise is part of the earthquake deformation cycle model and possible mechanisms include aseismic slip (e.g., Dragert et al., 2001; Miller et al., 2002; Katsumata et al., 2002; Uchida et al., 2003). It remains a challenge for seismological models of the earthquake deformation cycle developed on observational data since the AD 1964 earthquake to take account of late Holocene relative sea-level movements through complete cycles and to evaluate whether preseismic relative sea-level rise is a precursor to great subduction zone earthquakes.

Acknowledgements This research was supported by: the US Geological Survey, Department of the Interior, under USGS award number 02HQGR0075 (The views and conclusions contained in this document are those of the authors

and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the US Government); NERC Research Studentship GT 04/ 99/ES/57; NERC radiocarbon allocation number 935 0901; Technician support from the Geography Department, University of Durham; Design and Imaging Unit, University of Durham; and Sir James Knott Foundation Fellowship. We thank Smiley Shields and Rod Combellick for support and advice. John Clague and Alan Nelson provided perceptive and helpful comments. References Atwater, B.F., 1987. Evidence for great Holocene earthquakes along the outer coast of Washington State. Science 236, 942–944. Atwater, B.F., Yamaguchi, D.K., Bondevik, S., Barnhardt, W.A., Amidon, L.J., Benson, B.E., Skjerdal, G., Shulene, J.A., Nanayama, F., 2001. Rapid resetting of an estuarine recorder of the 1964 Alaska Earthquake. Bulletin of the Geological Society of America 113, 1193–1204. Bartsch-Winkler, S., 1988. Cycle of earthquake induced aggradation and related tidal channel shifting, upper Turnagain Arm, Alaska, USA. Sedimentology 35, 621–628. Bartsch-Winkler, S., Schmoll, H.R., 1992. Utility of radiocarbon dated stratigraphy in determining late Holocene earthquake recurrence intervals, upper Cook Inlet region, Alaska. Geological Society of America Bulletin 104, 684–694. Birks, H.J.B., 1995. Quantitative palaeoenvironmental reconstructions. In: Maddy, D., Brew, J.S. (Eds.), Statistical Modelling of Quaternary Science Data. Technical Guide No. 5. Quaternary Research Association, London, pp. 161–254. Brown, L.D., Reilinger, R.E., Holdahl, S.R., Balazs, E.I., 1977. Postseismic crustal uplift near Anchorage, Alaska. Journal of Geophysical Research 82, 3369–3378. Clague, J.J., James, T.S., 2002. History and isostatic effects of the last ice sheet in southern British Columbia. Quaternary Science Reviews 21, 71–87. Cohen, S., 1996. Time-dependent uplift of the Kenai Peninsula and adjacent regions of south central Alaska since the 1964 Prince William Sound earthquake. Journal of Geophysical Research 101, 8595–8604. Cohen, S., 1998. On the rapid postseismic uplift along Turnagain Arm, Alaska, following the 1964 Prince William Sound earthquake. Geophysical Research Letters 25, 1213–1215. Cohen, S.C., Freymueller, J.T., 1997. Deformation of the Kenai Peninsula, Alaska. Journal of Geophysical Research 102, 20,479–20,487. Cohen, S.C., Freymueller, J.T., 2001. Crustal uplift in the south central Alaska subduction zone: new analysis and interpretation of tide gauge observations. Journal of Geophysical Research 106, 11,259–11,270. Combellick, R.A., 1991. Palaeoseismicity of the Cook Inlet region, Alaska: evidence from peat stratigraphy in Turnagain and Knik Arms. Alaska Division of Geological and Geophysical Surveys Professional Report 112, 1–52. Combellick, R.A., 1994. Investigation of peat stratigraphy in tidal marshes along Cook Inlet, Alaska, to determine the frequency of 1964style great earthquakes in the Anchorage region. Alaska Division of Geological and Geophysical Surveys Report of Investigations 94–7, 1–24. Combellick, R.A., 1997. Evidence of prehistoric great earthquakes in the Cook Inlet region, Alaska. In: Hamilton, T.D. (Ed.), IGCP Project 367. Late Quaternary Coastal Records of Rapid Change: Application to Present and Future Conditions. Field Trip Guide to

ARTICLE IN PRESS S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498 Portage-Girdwood Area, Anchorage lowland and Kenai Fjords. Anchorage, Alaska, pp. 23–40. Combellick, R.A., Reger, R.D., 1994. Sedimentological and radiocarbon age data for tidal marshes along eastern and upper Cook Inlet, Alaska. Alaska Division of Geological and Geophysical Surveys Report of Investigations 94–6, 1–60. Crossen, K.J., 1992. Guide to the little ice age landforms and glacial dynamics of portage glacier. In: Crossen, K.J. (Ed.), Guide to the Little Ice Age Landforms and Glacial Dynamics in Portage Valley and Portage Pass. Alaska Geological Society, Anchorage, Alaska, pp. 3–15. DeMets, C., Gordon, R.G., Argus, D.F., Stein, S., 1990. Current plate motions. Geophysical Journal International 101, 425–478. Denys, L., 1991. A checklist of the diatoms in the Holocene deposits of the western Belgian coastal plain with a survey of their apparent ecological requirements. Belgian Geological Survey Professional Paper 1991/2 246, 1–41. Dragert, H., Wang, K., James, T.S., 2001. A silent slip event on the deeper Cascadia subduction interface. Science 292, 1525–1528. Edwards, R.J., Horton, B.P., 2000. Reconstructing relative sea-level change using UK salt-marsh foraminifera. Marine Geology 169, 41–56. Gehrels, W.R., Roe, H.M., Charman, D.J., 2001. Foraminifera, testate amoebae and diatoms as sea-level indicators in UK salt marshes: a quantitative multiproxy approach. Journal of Quaternary Science 16, 201–220. Grimm, E., 1993. TILIA: A Pollen Program for Analysis and Display. Illinois State Museum, Springfield. Hamilton, S.L., 2003. Late Holocene relative sea-level changes and earthquakes around the upper Cook Inlet, Alaska, USA. Unpublished Ph.D. Thesis, University of Durham, Durham, UK. Hartley, B., Barber, H.G., Carter, J.R., 1996. An Atlas of British Diatoms. Biopress, Bristol 601pp. Hemphill-Haley, E., 1993. Taxonomy of recent and fossil (Holocene) diatoms (Bacillariophyta) from northern Willapa Bay, Washington. US Geological Survey Open File Report 93–289, 1–151. James, T.S., Clague, J.J., Wang, K., Hutchinson, I., 2000. Postglacial rebound at the northern Cascadia subduction zone. Quaternary Science Reviews 19, 1527–1541. Juggins, S., 2003. C2 User Guide. Software for Ecological and Palaeoecological Data Analysis and Visualisation. University of Newcastle, Newcastle upon Tyne, UK 69pp. Karlstrom, T.N.V., 1964. Quaternary geology of the Kenai lowland and the glacial history of the Cook Inlet region, Alaska. US Geological Survey Professional Paper 443, 1–69. Katsumata, K., Kasahara, M., Ozawa, S., Ivashchenko, A., 2002. A five years super-slow aseismic precursor model for the 1994 M8.3 Hokkaido-Toho-Oki lithospheric earthquake based on tide gauge data. Geophysical Research Letters, 29, 3-21–3-24. Long, A.J., Shennan, I., 1994. Sea-level changes in Washington and Oregon and the ‘‘Earthquake deformation cycle’’. Journal of Coastal Research 10, 825–838. Miller, M.M., Melbourne, T., Johnson, D.J., Sumner, W.Q., 2002. Periodic slow earthquakes from the Cascadia subduction zone. Science 295, 2423. Minster, J.B., Jordan, T.H., 1978. Present day plate motions. Journal of Geophysical Research 83, 5331–5354. Motyka, R.J., 2003. Little ice age subsidence and post little ice age uplift at Juneau, Alaska, inferred from dendrochronology and geomorphology. Quaternary Research 59, 300–309. Nelson, A.R., Shennan, I., Long, A.J., 1996. Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia subduction zone of western North America. Journal of Geophysical Research 101, 6115–6135.

1497

Palmer, A.J., Abbott, W.H., 1986. Diatoms as indicators of sea-level change. In: Van de Plassche, O. (Ed.), Sea Level Research: A Manual for the Collection and Evaluation of Data. Geobooks, Norwich, pp. 457–488. Patrick, R., Reimer, C.W., 1966. The Diatoms of the United States, Vol. 1. Monographs of The Academy of Natural Sciences of Philadelphia No 13. Academy of Natural Sciences of Philadelphia, Philadelphia 688pp. Patrick, R., Reimer, C.W., 1975. The Diatoms of the United States, Vol. 2, Part 1. Monographs of The Academy of Natural Sciences of Philadelphia No 13. The Academy of Natural Sciences of Philadelphia, Philadelphia 213pp. Peltier, W.R., 2002. Global glacial isostatic adjustment: palaeogeodetic and space-geodetic tests of the ICE-4G (VM2) model. Journal of Quaternary Science 17, 491–510. Plafker, G., 1969. Tectonics of the March 27, 1964, Alaska earthquake. US Geological Survey Professional Paper 543–1, 1–74. Plafker, G., Kachadoorian, R., Eckel, E.B., Mayo, L.R., 1969. Effects of the earthquake of March 27, 1964 on various communities. US Geological Survey Professional Paper 542–G, 1–50. Plafker, G., Lajoie, K.R., Rubin, M., 1992. Determining intervals of great subduction zone earthquakes in southern Alaska by radiocarbon dating. In: Taylor, R.E., Long, A., Kra, R.S. (Eds.), Radiocarbon after Four Decades. An Interdisciplinary Perspective. Springer, New York, pp. 436–452. Preuss, H., 1979. Progress in computer evaluation of sea-level data within the IGCP project no. 61. In: Suguio, K., Fairchild, T.R., Martin, L., Flexor, J. (Eds.), International Symposium on Coastal Evolution in the Quaternary, September 11–18, 1978. Sao-Paulo, Brazil, pp. 104–134. Savage, J.C., Plafker, G., 1991. Tide gage measurements of uplift along the south coast of Alaska. Journal of Geophysical Research 96, 4325–4336. Shennan, I., 1986. Flandrian sea-level changes in the Fenland II. Tendencies of sea-level movement, altitudinal changes and local and regional factors. Journal of Quaternary Science 1, 155–179. Shennan, I., Long, A.J., Rutherford, M.M., Green, F.M., Innes, J.B., Lloyd, J.M., Zong, Y., Walker, K., 1996. Tidal marsh stratigraphy, sea-level change and large earthquakes, I: a 5000 year record in Washington USA. Quaternary Science Reviews 15, 1023–1059. Shennan, I., Long, A.J., Rutherford, M.M., Kirkby, J., Green, F.M., Innes, J.B., Walker, K., 1998. Tidal marsh stratigraphy, sea-level change and large earthquakes, II: events during the last 3500 years at Netarts Bay, Oregon, USA. Quaternary Science Reviews 17, 365–393. Shennan, I., Scott, D., Rutherford, M., Zong, Y., 1999. Microfossil analysis of sediments representing the 1964 earthquake, exposed at Girdwood Flats, Alaska, USA. Quaternary International 60, 55–73. Stuiver, M., Reimer, P.J., 1993. Extended 14C database and revised CALIB 3.0 radiocarbon calibration program. Radiocarbon 35, 215–230. Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., Hughen, K.A., Kromer, B., McCormac, F.G., v.d.Plicht, J., Spurk, M., 1998. INTCAL98 Radiocarbon age calibration 24,000–0 cal BP. Radiocarbon 40, 1041–1083. ter Braak, C.J.F., Smilauer, P., 2002. CANOCO Reference Manual and CanoDraw for Windows User’s Guide: Software for Canonical Community Ordination (version 4.5). Microcomputer Power, Ithaca, NY, USA 500pp. ter Braak, C.J.F., Juggins, S., Birks, H.J.B., van der Voet, H., 1993. Weighted averaging partial least squares regression (WA-PLS): definition and comparison with other methods for speciesenvironment calibration. In: Patil, G.P., Rao, C.R. (Eds.), Multivariate Environmental Statistics. Elsevier Science Publishers, Amsterdam, pp. 525–560.

ARTICLE IN PRESS 1498

S. Hamilton, I. Shennan / Quaternary Science Reviews 24 (2005) 1479–1498

Thatcher, W., 1984. The earthquake deformation cycle, recurrence and the time predictable model. Journal of Geophysical Research 89, 5674–5680. Uchida, N., Hasegawa, A., Matsuzawa, T., Igarashi, T., 2003. Pre- and post-seismic slow slip on the plate boundary off Sanriku, NE Japan associated with three interpolate earthquakes as estimated from small repeating earthquake data. Tectonophysics 385, 1–15.

Van der Werff, A., Huls, H., 1958–1974. Diatomeeenflora van Nederland. 8 parts. Published privately, De Hoef, The Netherlands. Zong, Y., Shennan, I., Combellick, R.A., Hamilton, S.L., Rutherford, M.M., 2003. Microfossil evidence for land movements associated with the AD 1964 Alaska earthquake. The Holocene 13, 7–20.