Holocene History of Catastrophic Hurricane Landfallsalong the Gulf of Mexico Coast Reconstructed from

Coastal Lake and Marsh SedimentsKam-biu Liu

Department of Geography and AnthropologyLouisiana State University, Baton Rouge, LA

Miriam L. FearnDepartment of Geology and Geography,

University of South Alabama, Mobile, AL

Gulf Coast. For example, what is the probabilityfor a major metropolitan area like New Orleans tobe directly hit by a category 5 hurricane? Theanswer to this question would be of great impor-tance to socioeconomic planners, civil engineers,and insurance companies.

Introduction

The Gulf of Mexico coast is highly vulnerable tocatastrophic strikes by landfalling hurricanes.During the past century, over 100 hurricanes ha\'emade landfalls along the northern Gulf of Mexicocoast from the Florida Panhandle to Texas(Neumann et al., 1987). About 44% of these weremajor hurricanes of category 3 or above accordingto the Saffir-Simpson intensity scale. However,only 10 were catastrophic hurricanes of categof)' 4and 5 intensities, including hurricane Camille of1969, the only category 5 hurricane to have struckthe U.S. mainland in the past century of instru-mental record. Thus, landfalls by catastrophichurricanes, here defined as those of category 4and 5 intensities, are very rare occurrences. In thelight of this, several questions are most relevant toscientific issues of Gulf Coast climate changes: (1)What are the landfall probabilities (or alternative-ly, "return periods")* of catastrophic hurricanesalong the Gulf Coast? (2) Did these landfall prob-abilities change temporally, and at what timescale? (3) How are these variations in hurricaneactivity and landfall probabilities related to globaland regional climate changes, such as those asso-ciated with the Little Ice Age and the anticipatedglobal warming? The answers to these questionsare important not only because they are vital to abetter scientific understanding of the patterns andprocesses of global and regional climate change,but also because of their practical value in the pre-diction and assessment of hurricane risks for the

Obviously, these questions cannot be adequatelyanswered by the actuarial approach, which isbased on probability estimates derived from thehurricane landfall statistics of the last 100 years -a period too short for estimating the return peri-ods of these rare but most destructive hurricanes.Therefore, geologic data provide the only meansfor extending the observational database of hurri-cane landfalls from the past century to the lastseveral centuries or millennia. Among the geolog-ical proxies, those that offer some, though as yetunproven, potential for the reconstruction ofpaleo-hurricane activities, include tempestites inmarine and coastal lagoonal sediments (Keen andSlingerland, 1993a, 1993b; Duke, 1985; Davis etal., 1989), oxygen isotopes in rain water and cavedeposits (Lawrence and Gedzelman, 1996;Malmquist, 1997), and pollen in lake and marshsediments (Gathen, 1994; Zhou et al., 1998; Bravoet al., 1997). Recently, Uu and Fearn (1993a) havedemonstrated that a chronologically and strati-graphically distinct record of past catastrophichurricane strikes can be obtained by the detailedstratigraphic study and radiocarbon-dating ofoverwash deposits identified in coastal lake sedi-ments. Since 1990, we have used this approach of

-38 Section 3.2: Holocene History of Catastrophic Hurricane Landfalls

~

hurricane eye. Generally for the northern Gulf ofMexico coast, the coastal segment situated in thenortheastern quadrant (i.e., forward right) of alandfalling hurricane usually experiences thegreatest destruction due to the strongest onshorewinds and the highest storm surge potential,whereas the northwestern quadrant tends toreceive less damage due to the predominantly off-shore wind field. Thus, the sediment-stratigraph-ic proxies recovered from coastal lakes andmarshes along the northern Gulf Coast (a pre-dominantly east-west oriented coastline) are pri-marily sensitive to hurricanes that made landfallwithin < 100 km (the typical radius of hurricane-force wind) to the west of the site.

\\

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Fig. 1. Location of lakes and marshes along the northernGulf of Mexico coast cored for the hurricane paleoclimatestudy, in relation to the storm tracks of major landfallinghurricanes (categories 3, 4, 5) recorded in this century.

1. Coastal Lakes

using storm deposits in coastal lakes and marshesas a proxy for paleo-hurricane landfalls in a com-prehensive study to reconstruct the Holocene his-tory of catastrophic hurricane activities along thenorthern Gulf of Mexico coast. So far, more than adozen coastal lakes and marshes from Texas to theFlorida Panhandle have been cored for this study(Liu and Fearn, 1993a, 1993b, 1997, 1998; Gathen,1994; Li, 1994; Zhou et al., 1998) (Fig. 1). In thispaper, we will present an overview of our meth~ods and a summary of our key findings in the lightof the above questions relating to Holocene cli-mate change and long-term variations in hurri-cane activity and landfall probabilities along theGulf of Mexico coast.

Coastal lakes situated behind sandy barriers (e.g.,barrier beaches, sand dunes, beach ridges) aresubject to overwash if the storm surge heightcaused by a landfalling hurricane exceeds the

0..,--~

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Rationale and Methods

For a given coastal site during a hurricane strike,the geomorphic impact (e.g., storm surge height)depends on the interplay of a number of factors(Simpson and Riehl, 1981). Among the moreimportant factors are: (1) the intensity, size, andstructure of the hurricane; (2) the duration andtiming of the hurricane landfall, especially in rela-tion to tidal parameters; (3) the angle of hurricaneapproach in relation to the configuration of thecoastline; and (4) the proximity and direction ofthe site in relation to the location of landfall or the

Fig. 2. A model of sediment stratigraphy in a coastal lakecontaining multiple sand layers representing overwashevents caused by past catastrophic hurricane strikes.These sand layers can be identified in sediment corestaken from different parts of the lake. Cores taken fromnearshore sites should contain more and thicker sandlayers than those taken farther away from shore. Onlysand layers that are the most widespread, probablyreflecting the severest overwash caused by the strongesthurricanes, are likely to be recorded in cores taken fromthe lake center. However, sand layers may coalesce toform a massive sand unit at sites too close to shore, sothat individual overwash events may not be distinguish-able in the core.

Section 3.2: Holocene History of Catastrophic Hurricane Landfalls 39

2. Coastal Marshesheight of the coastal barrier. During an overwashevent, much sand is transported into the lake bywaves, currents, and wind. The model we use asa working hypothesis (Fig. 2) is that a sand layerwill be deposited in the lake bottom when anoverwash occurs due to a hurricane strike. Thissand layer, an overwash deposit, can be distin-guished from the more organic and finer sedi-ments that accumulate continuously throughoutthe year. Generally, this sand layer is thickest nearthe edge of the lake and thins out towards the cen-ter. If other factors are more or less equal, impactsby stronger or major hurricanes are more likely toproduce a thicker and more extensive sand layerthan minor hurricanes and winter storms. Thus,cores taken from the center of the lake should con-tain fewer, and generally thinner, sand layers,which record impacts only by the most intensehurricanes. On the other hand, cores taken fromcloser to shore should contain more, and thicker,sand layers, which may provide a more sensitiverecord of smaller hurricane impacts. However,sediments from the littoral zone too close to shorewould be more susceptible to reworking and dis-turbance by subsequent hurricanes so that thestratigraphy may not be preserved, or the thicksand layers may coalesce to form one massivesand unit so that individual events may not be dis-tinguishable. Moreover, cores taken fromthe nearshore zone would tend to be lessorganic and the sedimentary record there islikely to be shorter or incomplete due towave disturbance and sediment focusing(Davis and Ford, 1982).

Like their lacustrine counterparts, coastal or estu-arine marshes are also highly susceptible to hurri-cane impacts. Storm surges caused by landfallinghurricanes typically result in increased sedimenta-tion in coastal marshes, although here the patternof sedimentation could be more complex andmore variable both spatially and temporally thanin lacustrine environments (Cahoon et al., 1995;Gunterspergen et al., 1995). In the MississippiRiver deltaic plain of Louisiana, where normalriverine flooding contributes very little to marshaccretion, hurricanes may result in significantdeposition of clastic sediments on coastal marsh-es due to the resuspension of sediments from shal-low bays and estuaries (Rejmanek et al., 1988;Baumann et al., 1984). Remarkably, sand layershave been reported to occur in coastal marsh sed-iments along the tectonically active Pacific coastof North America where hurricanes are absent;these sand layers were attributed to tsunamis trig-gered by submarine or nearshore earthquakes(Benson et al., 1997). It is reasonable to infer thattsunamis and hurricane-driven storm surgeswould probably have similar sedimentologicalimpacts in coastal marshes, even though thesetwo types of extreme events are somewhat differ-

Therefore, the frequency, extent, and thick-ness of these sand layers, which can betraced and cross-correlated in a suite ofcores taken in different parts of the lake,can be used to reconstruct a history ofpaleo-hurricane activities. The organic sed-iments adjacent to these sand layers can beradiocarbon-dated either by the conven-tional radiometric method or by the accel-erator mass-spectrometric CAMS) methodto provide a chronology of these past hur-ricane landfalls.

Fig. 3. A model of sediment stratigraphy in a coastal marsh con-taining multiple storm deposits. The storm deposits, in the form ofclastic layers embedded in marsh peat or organic-rich sediments,can be identified in sediment cores. Each storm deposit may varyspatially in thickness and in texture and mineral composition dueto variations in the direction and source of sediment supply, and inthe hydrology and topography of the marsh surface. The environ-mental impacts of past hurricanes can also be detected by meansof biostratigraphic (e.g., pollen, diatom, foraminifera) studies ofcores taken along salinity and ecological gradients from fresh

marsh to saltmarsh.

40 Section 3.2: Holocene History of Catastrophic Hurricane Landfalls

-~~~

ent in duration and intensity. The occurrence oftsunamis is highly unlikely, however, along thetectonically and seismically stable Gulf of Mexicocoast, so that a non-storm origin for these sand orsilt layers can be discounted. Along the GulfCoast, winter storms may also contribute signifi-cantly to marsh accretion and clastic deposition(Reed, 1989). However, except in some extraordi-nary cases such as the 1993 "Storm of theCentury" (Goodbred and Hine, 1995), winterstorms' surge heights are typically lower and theirgeomorphic and sedimentological impacts lesspronounced than those of catastrophic hurricanes.Storm deposits attributable to past winter stormshave not been identified from sub-recent or oldermarsh sediments (Goodbred and Hine, 1995).

Results

During the past eight years we have retrieved mul-tiple cores from over a dozen coastal lakes andmarshes along the northern Gulf of Mexico coastfrom Texas to northwestern Florida (Fig. 1). Thesediment- and bio-stratigraphies of these coreswere studied by means of loss-on-ignition analy-sis, pollen analysis, and diatom analysis to helpidentify storm deposits and overwash sand layers,and to detect any vegetational and environmentalchanges associated with such hurricane impacts.Most of these cores were supported by radiocar-bon dates, including many AMS dates. The fol-lowing is a summary and highlight of the key find-ings from selected study sites.

Based on these studies, we may hypothesize thatthe sedimentological and stratigraphic signaturesof paleo-hurricanes can be identified from coastalmarshes in the form of distinct clastic (clay, silt, orsand, depending on sediment source) or inorgan-ic layers embedded in peat deposits. Because ofthe topographic and hydrographic complexity ofthe marsh $urface in relation to the source anddirection of sediment supply during and after as(orm surge event, this storm deposit may beuneven in thickness or spatially discontinuous(Fig. 3). Hurricane impacts may also result in hia-tus in the marsh stratigraphy because certainareas of the marsh may be subjected to significantsediment compaction and lateral compression,wrack deposition, reworking, and erosion(Cahoon et al., 1995; Gunterspergen et al., 1995).Therefore, multiple cores from different parts ofthe marsh and adequate radiocarbon dating con-trol are necessary for reconstructing a more com-plete history of prehistoric hurricane activities.Finally, since storm surge events often cause dras-tic physical, geochemical (e.g., salinity), and eco-logical changes in the coastal marsh ecosystems(Gunterspergen et al., 1995; Jackson et al., 1995;Conner et al., 1989; Valentine, 1977; Chabreck andPalmisano, 1973), such changes should be readilydetectable by means of pollen and diatom analy-ses of marsh sediment stratigraphies (Zhou et al.,1998; Li, 1994; Hemphill-Haley, 1996).

1. Lake Shelby, Alabama

Lake Shelby, Alabama, was the site of our pioneerstudy to use overwash deposits in lake sedimentsas a proxy for paleo-hurricane reconstruction (Liuand Fearn, 1993a), This lake is ideally situated fortesting our model because it is a closed-basinfreshwater lake isolated from the Gulf; it is sur-rounded by systems of beach ridges and sanddunes and separated from the Gulf by a relativelynarrow barrier beach; and it was directly struck bya category 3 hurricane (Hurricane Frederic) in1979 which can provide a "modern analog" forreconstructing the intensity of prehistoric hurri-canes.

Sediment cores retrieved from the center of LakeShelby contained multiple sand layers that wereattribut:able to catastrophic impacts by prehistorichurricanes of category 4 or 5 intensity (Liu andFearn, 1993a). Validation that these sand layers doindeed represent category 4 or 5 hurricanes camefrom a study of the geomorphic impacts and strati-graphic signatures of Hurricane Frederic (a strongcategory 3 hurricane), whose overwash sand layerwas confined to the nearshore sediments of LakeShelby. Based on the radiocarbon chronology ofthese older sand layers, it was established that cat-astrophic hurricanes directly struck Lake Shelby atca. 3200 yr BP (radiocarbon years before present),

Section 3.2: Holocene History of Catastrophic Hurricane Landfalls 41

(Liu and Fearn, 1993a). Several less prominentsand layers were not radiocarbon-dated, and sev-eral distinct sand layers dated to about 3200-3000yr BP were conservatively regarded as one event(Fig. 4). If all the sand layers had been radiocar-bon-dated and counted, the frequency of cata-strophic hurricane strikes would have been 9 inthe last 3200 years; hence a "return period" ofapproximately 350 years or a landfall probabilityof 0.28% per year. This estimate is not unreason-able, considering the fact that no hurricane of cat-egory 4 or 5 intensity has made landfall on theAlabama coast during the past 120 years of docu-mentary record, even though this coast was direct-ly struck by category 3 hurricanes four times-in1906, 1916, 1926, and 1979 (Neumann et al.,1987).

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\ The absence of sand layers in the Lake Shelbycores prior to 3200 yr BP implies a significantenvironmental change occurring around that time.The discovery of corn (Zea mays) pollen in theLake Shelby sediments dating to 3500 yr BP sug-gests that the Lake Shelby area had been settledbefore this environmental change took place(Fearn and Uu, 1995, 1997). This environmentalchange was at least regional in scale and probablyaffected the whole Gulf Coast, because similarstratigraphic changes also occur in the sedimentsof Western Lake, Florida, dating to about the sametime (Uu and Fearn, 1993a, 1997). It is likely thata significant shift in global climate and oceaniccirculation occurred during 3500-3200 yr BP, asevidenced from broadly synchronous changesdocumented from various paleoclimatic proxies atdifferent sites around the world (Uu and Fearn,1993a; Uu et al., 1994).

860. D880-

900-' 4760t601

Fig. 4. Stratigraphic columns of Lake Shelby cores E, 8, A,S, and L showing the occurrence of overwash sand layers.Core L is a short core taken from a nearshore site wherethe Hurricane Frederic sand layer was deposited. Core E isa 9 m long core taken from the center of the lake. The mostprominent sand layers in these cores were radiocarbon-dated. The date in brackets was rejected as being too olddue to contamination by shell fragments. All dates shownare uncalibrated "C ages (After Liu and Fearn, 1993a).Inset photograph shows an overwash sand layer in core E.

2600 yr BP, 2200 yr BP, 1400 yr BP, and 800 yr BP,or 5 times during the past 3200 years (Uu and

Fearn, 1993a) (Fig. 4). This implies a "return peri-od" of approximately 600 years for hurricanes ofcategory 4 or 5 intensity, or a landfall probabilityof about 0.16% per year for the Alabama coastnear Lake Shelby.

2. Western Lake, FloridaThis "return period" derived from the Lake Shelbysedimentary record is the first empirically basedestimate of landfall probability for catastrophichurricanes of category 4 or 5 intensity. This isprobably a minimum estimate, because only themost prominent sand layers in the Lake Shelbysediment cores were used in the reconstruction

Western Lake in the Florida Panhandle has yield-ed another detailed record of catastrophic hurri-cane strikes during the Holocene. Since 1992, wehave raised 16 sediment cores from this lake forour hurricane paleoclimate study. The WesternLake area was severely impacted by Hurricane

42 Section 3.2: Holocene History of Catastrophic Hurricane Landfalls

Opal in October 1995 when this category 3 hurri-cane made landfall about 75 km to the west. TheOpal sand layer was found only in a core takenclose to the south shore of the lake (Liu and Fearn,1997). Using that as a modern analog, we inter-pret that the prominent sand layers occurring incores taken farther away from the south shoremust have been caused by prehistoric hurricanesof category 4 or 5 intensity. If only the mostprominent sand layers are counted, at least eightsuch events occurred during the last 3400 years,suggesting a "return period" of approximately 400years for these catastrophic hurricane strikes. Thistranslates to a landfall probability of about 0.25 %per year (Liu and Fearn, 1997). Notably, even thisis a minimum estimate. If four other distinct butless prominent sand layers in the core are count-ed, the "return period" would be about 280 years(12 strikes in 3400 years) (Fig. 5).

matic changes? Answers to these questions are ofutmost importance to the scientific community,risk managers, insurance companies, and socio-economic planners.

3. Horn Island, Mississippi

Horn Island ,is one of the barrier islands situatedoffshore from the coast of Mississippi. The lakeson Horn Island are very sensitive to catastrophichurricane strikes because the barrier island formsthe "frontline" to the Mississippi coast againstlandfalling hurricanes. The beaches and sanddunes on the island were completely overwashedby the storm surge associated with an intense hur-ricane strike, such as the one by HurricaneCamille (the only category 5 hurricane instrumen-tally recorded to have directly struck the u.s.coast) in 1969. We cored nine lakes on HornIsland and found a sand layer near the top of each

0, ,

The radiocarbon chronology of the sand layersfrom Western Lake reveals significant variations incatastrophic hurricane frequencies at millennialtimescales. Of the 12 paleo-hurricane strikes dis-cussed above, 11 occurred during the two millen-nia between 3400 and 1400 yr BP, while only onestruck during the past 1400 years (Fig. 5).Sedimentary records from two adjacent lakes(Campbell Lake, Camp Creek Lake), plus newdata from other cores taken from Western Lake,seem to suggest that the first millennium AD(2000-1000 yr BP) may be one of the "stormiest"periods during the late-Holocene, with multiplestrikes on the Florida Panhandle coast by cata-strophic hurricanes at an average frequency ofonce every 200 years, or an annual landfall prob-ability of 0.5% (Liu and Fearn, 1998).

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The notion that the current millennium (1000 yrBP to present) is a relatively quiescent episode incatastrophic hurricane activities has significantscientific as well as practical implications. Whatcaused such millennial-scale variations in cata-strophic hurricane activities? What mechanismstriggered the switch from a hyperactive to a qui-escent hurricane regime, and vice-versa? How arethese apparent "mega-cycles" related to global cli-

3000-

3500-1

Fig. 5. Abbreviated chronostratigraphic column of WesternLake core I for the past 3500 years showing the radiocar-bon-dated ages of the major sand layers (solid lines) andminor sand layers (stippled lines). The vertical axis is timemeasured in uncalibrated 14C years before present. At least12 sand layers occur within the past 3400 years.

Section 3.2: Holocene History of Catastrophic Hurricane Landfalls 43

~

core that was dated to the Camille strike by meansof the Cesium-137 OOCs) dating method (Gathen,1994). Radiocarbon dating of the sediment coresshows that the lakes were formed about 600 yearsago. The sediment record from Fearn Lake seemsto suggest that few intense hurricanes struck HornIsland between 240 and 600 yr BP-a time coinci-dent with part of the Little Ice Age, a climaticepisode of global cooling (Grove, 1988). Pollenanalysis of a core from Momma's Diner Lakereveals that the lake environment changes episod-ically from fresh to brackish in a way consistentwith repeated saltwater intrusions caused by over-wash events associated with catastrophic hurri-cane strikes (Gathen, 1994).

approximately once evef)' 450 years. The landfallprobability, about 0.22 % per year, is consistentwith the estimates derived from the Lake Shelbyand Western Lake records.

S. Atchafalaya Marsh, Louisiana

The Atchafalaya Marsh in southern Louisiana wasseverely impacted by Hurricane Andrew in August

Pearl River Marsh

Depth(cm)

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4. Pearl River Marsh,Louisiana/Mississippi

The Pearl River Marsh is an estuarine marsh situ-ated along the border between Louisiana andMississippi. In August 1969 when HurricaneCamille (a category 5 hurricane) made landfallnear Pass Christian on the Mississippi coast about16 km to the east, the Pearl River Marsh wasaffected by the 6 m storm surge recorded near thehurricane eye, although the hurricane-force windwas predominantly offshore. We have identifiedthe Camille-induced overwash sand layer in atransect of cores taken from the Buccaneer Marsh,Mississippi, situated directly under the path ofHurricane Camille at landfall (Uu and Fearn,1993b). In the Pearl River Marsh, Camille left aclay-rich layer in a core taken at a site about 4miles inland from the coast (U, 1994). The 8.5mcore, however, contains other distinct clay or siltlayers that were probably caused by storm surgesassociated with prehistoric hurricane strikes. Atleast 9 such layers are present in the upper half ofthe sediment core spanning the last 4000 years(Fig. 6). If these clay or silt layers were all formedby catastrophic hurricanes comparable to Camillein terms of storm surge height and sedimentolog-ical impacts, as it seems likely, then the recordimplies that catastrophic hurricanes of category 4or 5 intensity directly struck the Pearl River Marsh

Fig. 6. Loss-on-ignition curves for a core from the Pear!River Marsh, Louisiana, showing the organic matter con-tents (dark) and water contents (stippled) of the sediments.The occurrence of clastic layers (storm deposits), denotedby arrows, is reflected by an abrupt drop in both watercontents and organic matter contents. At least 9 such clas-tic layers occur within the last 4000 years.

44 Section 3.2: Holocene History of Catastrophic Hurricane Landfalls

1992 when this category 3 hurricane passedimmediately to the west of this coastal marshbefore making landfall near Morgan City,Louisiana. In a series of short cores taken along atransect from the fresh marsh to the saltmarsh, wefound a 5-16 cm thick sand or silt layer at or nearthe top that is attributable to the impact ofHurricane Andrew and the 1-2 in storm surge thatit caused, thus confirming earlier findings report-ed by Cahoon et al. (1995) and Gunterspergen etal. (1995) on Andrew's sedimentological and eco-logical impacts. Data from pollen and diatomanalyses suggest that this storm deposit was prob-ably derived from the estuarine and bay-bottomsediments in the Atchafalaya Bay that were resus-pended and redeposited by waves and currentsduring the storm surge.

undoubtedly being a minimum estimate. If theseestimates are representative of places along thenorthern Gulf of Mexico coast-an assumption tobe tested with proxy records from more sites-then the annual.landfall probabilities for the cata-strophic hurricanes are in the order of 0.16 -0.33%.

One of the most important findings of our studiesis that there are long-term variations in cata-strophic hurricane activities at millennialtimescales. The inter-millennial variability issuperimposed on the inter-decadal and inter-annual variability that has been documentedbased on the instrumental hurricane record of thiscentury (Gray, 1990; Landsea et al., 1996). Datafrom Western Lake and adjacent lakes suggest thatcatastrophic hurricanes struck the FloridaPanhandle much more frequently during the firstmillennium AD than the last 1000 years. Thenotion that the current millennium is relativelyquiescent in catastrophic hurricane activities hasimportant scientific and practical implications. Iffuture global climate change results in a return tothe hyperactive hurricane regime typical of thefirst millennium AD, the Gulf of Mexico coastwould experience a dramatic increase in the riskof catastrophic hurricane landfalls. Understandingthe climatological mechanisms controlling suchlong-term variations in hurricane activities is oneof the most challenging tasks for climate scientistsand global change researchers.

Long cores taken from two saltmarsh sites in thesouthernmost part of the Atchafalaya Marsh per-mit reconstruction of a millennial history of cata-strophic hurricane strikes in southern Louisiana.While this work is still in progress (Zhou et al.,1998), preliminary results from a 2.85 m core nearGoreau suggest that intense hurricanes equal to orexceeding Andrew in intensity (i.e., categories 3,4, 5) directly struck the Atchafalaya Marsh at least9 times during the past 1200 years. This implies a"return period" of 133 years, or a landfall proba-bility of about 0.75 % per year. Stratigraphic workis being conducted on another 7.9 m long core toconfirm this preliminary reconstruction.

*Landfall probability is defined as the reciprocalof the "return period" of a specified intensity cat-egory of hurricanes. Note that the term "returnperiod" is used here loosely and in quotes todenote the long-term average in the frequency ofhurricane landfalls; it does not imply cyclicity orperiodicity in the occurrence of these events.

Conclusions

By using overwash sand layers and storm surgedeposits in coastal lake and marsh sediments asproxies of paleo-hurricane strikes, we were able toextend the record of catastrophic hurricane land-falls from the past century to the last several mil-lennia. Thus the geological record offers the onlymeans by which empirically based, realistic land-fall probabilities can be estimated for these rarebut most destructive hurricanes. Our data fromLake Shelby and Western Lake suggest that thesecatastrophic hurricanes typically have "returnperiods" of 600 to 300 years, with the former

Acknowledgments

Our hurricane research has been supported by theRisk Prediction Initiative (RPI) of the BermudaBiological Station for Research (RPI-96-01S), andby the National Science Foundation (SES-8922033,

Section 3.2: Holocene History of Catastrophic Hurricane Landfalls 45

SES-91220S8). We thank Caiming Shen, XinyuZhou, Zuju Yao, Don Cahoon, and StephenFournet for assistance in the field and in the labo-ratory. Over the years many graduate and under-graduate students helped in the field work at var-ious locations. We also thank the staff of Gray tonBeach State Park, Florida, and Gulf State Park,Alabama, for logistical support and field assis-tance.

Florida. Journal of Sedimentary Petrology 59: 1052-1061.

Duke, W. L., 1985. Hummocky cross-stratification,tropical hurricanes, and intense winter storms.Sedimentology 32: 167-194.

Fearn, M.L. and Uu, K-b., 1995. Maize pollen of 3500B.P. from southern Alabama. American Antiquity60: 109-117.

Fearn, M. L. and Uu, K-b., 1997. Identification ofmaize pollen: Reply to Eubanks, AmericanAntiquity 62: 146-148.

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Section 3.2: Holocene History of Catastrophic Hurricane Landfalls 47

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