(Journal of Coastal Research 296 :H)7 Fort Lauderdale, Florida Spring 1995

Assessment of the t:l7CS Method for EstimatingSediment Accumulation Rates: Louisiana Salt Marshes

Charles S. Milan]. Erick M. Swertson] , R. Eugene Turner'[vt, and James M. Leet

tCoastal Ecology Institute

Louisiana State University

Baton Rouge, LA 7080:1, U.S.A.

.tflllllll:.:e!I3Jle •.. -~ ~ d·

-+i ?

tDepart merit of Oceanography

and Coastal Sciences

Louisiana State I Jniversit y

Baton Rouge. I,A 7080:\, IT.S.A.

ABSTRACT _

MILAN, C.S.; SWENSON, KM.; TlJRNJ.:R, RK, and LEE, ,J.M., 1995. Assessment of the I.17Cs methodfor estimating sediment accumulation rates: Louisiana salt marshes. Journai of Coastal Research, 11(2),296 :Hl7. Fort Lauderdale (Florida). ISSN ()74~)-()2()R

Sediment cores were collected at :l:l salt marsh locations in the Louisiana Deltaic Plain and subjectivelyclassified as impaired or healthy using aerial imagery obtained since the 1950's. Cesium-137 inventoriesfrom these cores were compared with aerial deposition records of nuclear fallout from the southern UnitedStates. A system for assessing quality of the I I.'Cs data was used on these cores. Some cores were foundto have deficits in ILCS inventory, while others had excesses. Therefore, wash-in from outside the basinand erosional loss of II,CS cou)<j be a problem in some of these marshes. Several of these cores hadsignificant amounts of 1Les in the surface layers, whereas aerial deposition of mes has been virtuallyzero since 1982. This pattern may result from recent redistribution of sediments with I.17Cs being depositedonto the top layer (in which case there should be a zero-activity layer in between), or removal of surficialsediments in the last H years. Salt marshes with active depositional or erosional surfaces may not reflectthe aerial deposition of II'CS therefore cores from those markers should be interpreted with caution.

ADDITIONAL INDEX WORDS: Cor« qualit v, sediment accretion, I/;('S iruseni ory, methodology, sed­iment dating,

INTRODUCTION

The radioisotopic activity of l:ncs deposited withsediments over the past several decades is oftenused to calculate rates of sediment accumulation.The l:ncs dating method is attractive because av­erage sediment accumulation rates over severaldecades can be estimated for many samples at areasonable cost. Cesium-IS? found in sedimentsis a result of atmospheric testing of nuclear de­vices. The first significant atmospheric fallout oc­curred in the early 1950's and decreased to nearlyzero in the early 1980's. The highest fallout ofl:ncs occurred between 196~3 and 1964. This max­imum in 1:17CS activity found in sediment recordsis used by many researchers to calculate sedimentaccumulation rates.

The l:ncs dating method. however, yields onlyan estimate of sediment accumulation. Accuratemeasurement of accumulation rates rnay be pre­cluded if, for example, the isotope is significantlymobile or the site is geologically dynarnic (e.g.,

from erosion, subsidence, deposition, etc.), An ex-

94084 received and accepted /H May /994

ample of this mobility is provided by DELAUNEet al. (1978) who observed that l:17CS extendedmuch below the estimated 1954 horizon in thepresence of Spart ina alternitiora roots, but notin a core frorn a nearby lake. Also, bioturbationat streamside locations may compromise a gen­erally good agreement between results from the~1()Pb and l:17CS dating methods obtained in back­marsh locations (e.g., SHAHMA et al., 1987). Smallinvertebrates may move l:r7Cs downward throughmixing or upwards in feeding debris (e.g., ROBBINSet al., 1979). The vertical movement of l:ncs cannot be inferred from other radioisotope data be­cause radioisotopes may act dissimilarly; e.g., thedownward diffusion of 1:17CS may be faster thanfor plutonium (~JAAKKOLA et al., 1983). The nat­ural variability of estuaries within and betweenyears contributes to the variability of Ll7CS in marshsoils. This may be because both salinity and con­tact with particles affect sorption rates, hence spe­cific activity in soil profiles (e.g., SIMPSON et al.,1976). Desorption of radiocesium may occur athigher salinities, and not all the l:17CS delivered tothe estuary may remain where initially deposited,presumably because of desorption (OLSEN, et al.,

Gulf of Mexico

• Sample Site

IllCS Method in Salt Marshes

If

•"\

.- ~~~ <, /o 50 '0·O~ oLJ J / Barat~ria~~~ Bas~n

Kilometers TeBrre~nneas~n

Figure 1. Sampling locations for the salt marsh cores taken in this study.

St. Bernard

~Sil

297

30° 15' N

29° 15' N

1981). Other processes, including delays between137CS delivery and sedimentation, may lead to anunderestimate of sediment accumulation rates(e.g., RITCHIE et al., 197~3). Further, authors donot use the same date for the first deposition of137CS (e.g., 1954 in DELAllNEet al., 1978 and 1950in SCHAFFNER et al., 1987).

For many methods the contention about whata method does or does not measure lies as much

with the method itself as with the interpretationof the results from its application. Periodic re­consideration of methods, the results, and the in­terpretations of those results is often necessaryin the evolution of ideas as well as methods. Weattempt to do this herein using empirical resultsfrom a variety of salt marsh samples within theLouisiana Deltaic Plain. It is our contention thatthe 1:17CS dating method, as presently used, may

.Iournal of Coastal Research, Vol. 11, No.2, 199fl

298 Milan et al.

be a good method to estimate relative accumu­lation rates in these salt marshes, but may notalways be an accurate enough method to measureactual sediment accumulation rates.

METHODS

Site Selection

Six healthy salt marsh sites and six impairedsalt marsh sites were sampled in each of threeLouisiana coastal watersheds: Terrebonne, Ba­rataria and St. Bernard (Figure 1). Whether a saltmarsh was healthy or impaired was subjectivelydetermined by: (1) the rate of recent land loss,(2) the presence or absence of obvious internalmarsh breakup, and (3) the degree of alterationof natural hydrology or impoundment by canalsand spoil banks. Candidate sites were located us­ing aerial photography, land loss and accretionmaps, and aerial inspection at < 500 m altitude.We had access to an inventory of the NASA over­flights for various time periods within the Loui­siana coastal zone. We used the most recent over­flight (1988-1989) and the U.S. Army Corps ofEngineers land loss maps (MAY and BRITSCH,1987), which show land loss from ~ 1935-1978, todetermine areas that have remained stable andareas that are breaking up. The resulting distri­bution of sites is shown in Figure 1.

Core Collection

In each basin, triplicate cores, 50 m apart, weretaken at one healthy and one impaired salt marshsite. A single core was taken at all other sites. Inaddition, another set of three cores that were 10m apart was taken at a site in the TerrebonneBasin. Cores were collected by inserting a 40 emlong by 12 em diameter plastic core tube into thesediment between plants until the top of the coretube was within 3 em of the marsh surface. Coretubes were inserted using a gentle twisting mo­tion, being careful not to compress the core. Thedepth of the core surface inside the tube and out­side the tube was recorded during core collectionto determine sediment compaction. The cores wereinitially refrigerated upon return to the LSD lab­oratory and later frozen. The frozen core was ex­truded from the core tube by placing the samplein a core defroster (a coil of copper tubing in aninsulated box that fits tightly around the coretube) until the edges had thawed sufficiently toallow the core to be pushed out of the tube (~45

minutes). The core was extruded into a prelabeled

plastic bag. The bag was returned to the freezeruntil it was time to section the core. The distancefrom the top of the tube to the mud surface wasremeasured before the core was extruded to doc­ument compaction or expansion during storage.

Core Sub-Sampling and Analysis

The frozen cores were sectioned into 1 em in­crements using a band saw. The blade thickness(~2 mm) was included in the increment mea­surement to ensure that the depth to the top ofeach consecutive section was always a 1 em in­terval from the top of the previous section. Thethickness of every fifth core sub-section was mea­sured with a digital micrometer to ensure overallaccuracy of the technique. Cut sections were placedinto weighed and prelabeled dishes. The disheswith the core sections were then weighed, driedfor 24 hours at 60°C, and reweighed. This pro­cedure allows for the determination of both wetand dry bulk densities. The dried sections werehomogenized with a Thomas-Wiley Mill E

equipped with a #20 mesh screen. A subsampleof 1 to 2 grams was taken for percent organiccontent determination. The percent organic mat­ter was determined by loss on ignition at 550°Cfor one hour (APHA, 1976). The remaining ho­mogenized sample was placed in a weighed andlabeled 26 em" petri dish with a tight fitting lid,reweighed, and then analyzed for l:ncs.

The mean compaction for 48 salt marsh cores,before extrusion, was 16.8 ",. Ten percent of thecores had a compaction greater then 25r-;,. How­ever, 45°(1 of the cores had compaction less than10 e(). This is not unreasonable owing to the un­consolidated nature of the salt marsh environ­ment. Our data also indicate a compaction rateof only 3.7 (T~) due to core transport (by boat, truck)and storage. Freezing appears to result in a coreexpansion of less than 3 r c .

Two values for the sediment accumulation ratewere calculated based on the compaction mea­surements. One approach assumed that all com­paction occurred above the 1963-1964 layer (min­imum rate). The other approach assumed that allcom paction occurred below the 1963-1964 layer(maximum rate). The maximum rate was ob­tained by adding the amount of compaction backinto the total centimeters above the 1963-1964depth. For example, if the depth to the 1963-1964peak was 15 em and the compaction was 5 em, avalue of 20 cm was used to calculate the accu­mulation rate.

Journal of Coastal Research, Vol. 11, No.2, 1995

me s Method in Salt Marshes 299

I37CS Analysis

Samples were counted using a Princeton Gam­ma-Tech 60 mm di am eter hyperpure germa nium(HPGe) "N" type coa xial detector . The detectorwas inte rfaced to an EG&G Ortec 92X ' spectrummast er integrated gamma -spectrosco py sys te m.Data was acquired using EG&G's Maestro H "soft ware and analyzed with EG&G's Minigam"software.

The detector was ca librated for t he resp ect ivegeomet ry and sa mple mat rix using a certifiedm ixed standa rd (A me rs ha m Co r pora tio n;QCY.44). T he calibra t ion was chec ked mon thly.Sa mples from eac h core were counted in iti all y for30 min to screen for t he presen ce of 1:l7CS. Afterdetermin ing the limits of 1:l7CS in the core, thesamples were counted for 4 hr . T h is count t imeyield ed a counting erro r (based on counti ng sta­t istics) of 10- 20 ",· for sa mples arou nd t he 1963­1964 peak. Resul ts for 1:l7CSare ex pressed as pCi/g(dry weigh t ). Detecti on limi t for 1:l7CS, dep endingon sa mple weight, was "='0.5 pCi/g with a countingerror of "='45 ";- . Longer counti ng ti mes would haveyielde d better counti ng error stat istics, bu t noalte ra t ion in acc umulation rates was obtaine d byreplicate ana lyses of sa mples around t he 196:3­1964 peak fro m 12 cores.

The m c s inven tor y expected in these cores wasbased on precip itat ion data an d th e atmos phericfallout record s of m c s and ""Sr conce nt ra ti ons inair samples and ""Sr dep osit ion . T he concent ra­tion in air of 1:l7CS and ''''S r at a station in Miami ,Florida, was mon itored from 196:3-1 985. Moni ­toring of "'Sr was sus pende d in mid -1985 be ­cause the conce nt ra t ions were too low to measure(FEELY et al., 1981 ,1985,1988). The ra tio of 1:l7CSto ""Sr in ai r was used to estimate the deposi tionof m c s from records of !l"S r de position (M ILLERand HEIT, 1986) . The dep osition of ''''Sr was mon ­itored at stations in New Orl ean s, LA fro m 1961­1975, in Houston , TX from 1959- pr esen t, and inNew York, NY from 1954-presen t (HARDY, 1977;LARSEN, 1983; J UZDAN, 1988; MONETTI and LAH­SEN, 1991). The ratio of J:17 Cs:''''Sr in air from 1952­1962 was esti ma te d to be 1.3. The dep osition of90Sr near New Orleans was estimated fr om thedeposit ion rates at Houston from 1959-1960 and1976-1986 and from New York during 1954-1958.The deposition of '"'S r for the peri od 1952-1953was estimated from the annual fission yields ofatmospher ic weap ons tests during 19fil and 19fi2.The deposit ion of "" S r resul ted from at mos phe ric I

test ing during the prior year (J UZDAN, 1988;MONETTI and LARSEN, 1991). T he total mc s es­timated dep osited fro m 1952-1986 in south Lou­isiana from t hese data was 83 mCi/km", and whencorrected for subseq uen t decay, it was 45 mCi/km-. This va lue will, of course, va ry some whatdepending on rainfall.

RESULTS

A mc s peak was detected in 46 out of 48 corest ha t we assume represen ts t he J:17CS deposite d in1963-1964. T he atmos phe ric dep osit ion of 1:l7CSin these wetlands, correc ted for decay, is esti­mated to be 45 mCi/km" since t he mid 1940 's and29 mCi/km" from 1963 to t he presen t . T he totalinve ntory of ' :l7CS measured in these cores ra nge dfrom 19 to 105 mfl i/krn" and ran ged fro m 9 to 98mCi/km" above the 1963- 1964 peak. T he fre­qu en cy di stributi on of both post 1963-1 964 andto ta l 1:l7CS inven tori es was determined to be nor­mal using the Shapiro- Wilk statistical test (SAS ,1990) . The mean inventory for all cores was 50mfli/km" for t he total and 36 mfl i/km" for t hepost 1963-1964 1:l7CS inventory Crable l a) , wh ichis somew hat higher t ha n the estimated fallout .The coeffic ient of va ria t ion (CV) in 1:l7CS inven­to ries for cores 50 m apart ran ged from 7 'I , t o39 "(" whil e t he va riation amo ng cores 10 m apa rtwas slightly bet ter (Table l b). Approximately 40 ~;'

of the cores from all basins contained inventorieswithin ±25"i· (counti ng er ro r) of t he expectedin ven tory for t he post 1963-1 964 t ime pe riod (Ta­ble 2). For t he to tal inventory, approximately 60 ~'i,

of t he cores from St . Bernard and TerrebonneBasins were within 25':i·, but on ly 44"i. of t hecores from the Ba rataria Basin were within 25 'Iiiof t he expecte d inventory acc umulat ed since 1950.T he mcs inv entor ies found in cores from Bara­taria Basin wer e, on the ave rage, 7";' higher t ha nt he expected inve ntory for t he post 1963-1964peri od , but were 7"i. lower in the total 1:l7CS an­t icipated dep osit ed from atmos phe ric depositi on .Cores from St. Bernard and T errebonne Basinscontai ned 1:l7CS inventories that wer e generallyhigh er t ha n the expected inventory.

The percent of the inv entory deposit ed aboveth e pr esumed 1963-64 peak ranged from 30 % to97 % , whi ch compa res to the estimated per cen tagebased on actua l fallout records of 62 "i,. T he rewere no significa nt d iffer en ces in this percen t agebetween basins. The J:17Cs inve ntories since 1963­1964 of a major ity of cores wer e higher than that

.Journal of Coas tal Hesear ch , Vo l. II , No.2, 199.';

300 Milan et al.

Table 1a. Basic statistical properties of the cores taken in the three different basins studied: compaction rate, percent organiccontent, total and post 1963 137CS inventory, total accumulation of organic and inorganic material above the estimated 1963-1964horizon, and time resolution (7) an estimate of 137CS mobility used by MILLER and HEIT (1986).

Post 1963 137CS Inventory Minimum MaximumOrganic Content (mfli/km-) Accumulation Accumulation

Compaction (g, dry wt.) Rate Rate Time ResolutionSample Site (%) (% of total) Post 1963 Total (em/year) (em/year) (7)

Barataria

Avg ± SDa 19.5 ± 13.5 35.7 ± 11.2 31 ± 7 42 ± 10 0.52 ± 0.22 0.80 ± 0.25 7.2 ± 3.7CVb 68.9% 31.5% 22.6% 23.8% 43.5% 31.7% 51.7%

St. Bernard

Avg ± SDa 13.7 ± 7.0 22.9 ± 7.9 37 ± 8 54 ± 13 0.50 ± 0.13 0.69 ± 0.16 10.0 ± 4.9CVb 51.0% 34.4% 21.6% 24.1% 25.9% 22.6% 48.8%

Terrebonne

Avg ± SDa 15.1 ± 9.5 27.9 ± 9.0 41 ± 8 55 ± 12 0.68 ± 0.20 0.90 ± 0.18 7.0 ± 4.4CVb 62.7% 32.1 % 19.5% 21.8% 29.2% 20.3% 63.4%

All cores

Avg ± SDa 16.3 ± 10.6 29.2 ± 10.7 36 ± 8 50 ± 11 0.57 ± 0.21 0.80 ± 0.22 7.9 ± 4.5CVb 65.0% 36.6% 22.2% 22.0% 36.0% 26.8% 56.1%

"Ideal" Coree 29 45

aSD = counting error used for 137CS inventories instead of statistical standard deviationbCV = coefficient of variation, expressed as a percentagee"Ideal" Core: Cs-137 distribution based on fallout records

Table lb. Basic statistical properties of replicate cores taken: compaction rate, percent organic content, total and post 1963 137CS

inventory, total accumulation of organic and inorganic material above the estimated 1963-1964 horizon, and time resolution (7)an estimate of 137CS mobility used by MILLER and HElT (1986).

Post 1963Organic 137CS Inventory Minimum MaximumContent (mfli/km-) Accumulation Accumulation Time

Compaction (g, dry wt.) Rate Rate ResolutionSample Site (%) (% of total) Post 1963 Total (em/year) (em/year) (7)

Replicates 50 m apart

BH2

Avg ± SDa 14.2 ± 3.8 40.1 ± 2.4 39 ± 10 46 ± 12 0.57 ± 0.12 0.77 ± 0.14 4.4 ± 2.9CVb 27.0% 5.9% 26.5% 26.1% 21.5% 18.6% 66.1%

BI2

Avg ± SDa 38.3 ± 6.3 48.9 ± 2.0 16 ± 6 27 ± 8 0.37 ± 0.04 0.92 ± 0.12 7.5 ± 4.9CVb 16.4% 4.1% 39.3% 30.3% 11.1% 12.5% 65.8%

SH6

Avg ± SDa 17.5 ± 5.0 19.8 ± 4.5 46 ± 9 64 ± 11 0.63 ± 0.08 0.88 ± 0.06 7.3 ± 4.2CVb 28.6% 22.9% 20.0% 17.6% 13.3% 7.3% 58.0%

SI2

Avg ± SDa 18.3 ± 9.5 26.1 ± 4.6 37 ± 3 52 ± 3 0.41 ± 0.05 0.67 ± 0.10 13.5 ± 3.7CVb 51.6% 17.5% 8.5% 6.5% 11.2% 14.4% 27.7%

TH2

Avg ± SDa 13.3 ± 3.8 18.5 ± 1.7 41 ± 15 69 ± 19 0.60 ± 0.18 0.79 ± 0.16 10.7 ± 5.2CVb 28.6% 9.2% 35.6% 27.1% 30.6% 20.6% 48.7%

Replicates 10 m apart

Avg ± SDa 13.3 ± 8.8 38.4 ± 2.9 35 ± 5 55 ± 2 0.67 ± 0.04 0.86 ± 0.09 5.7 ± 0.96CVb 65.8% 7.6% 15.3% 4.3% 6.7% 10.4% 17.0%

aSD = counting error used for 137CS inventories instead of statistical standard deviationbCV = coefficient of variation, expressed as a percentage

Journal of Coastal Research, Vol. 11, No.2, 1995

137CS Method in Salt Marshes 301

Table 2. An outline of several parameters of core quality, grouped by watershed basin.

137CS Activity" Percent 1963 Presence ofCompaction No mcs First 137CS 137CS Activity 1959 Peak

Sample Site < 10% at Surface > 19508 Post 1963 Total of Totalc T < 4.0

Barataria

Frequency 7 7 11 5 7 8 4% of total 44% 44% 69% 31 % 44% 50% 25%

St. Bernard

Frequency 6 8 4 5 8 8 2% of total 46% 62% 31 % 38% 62% 62% 15%

Terrebonne"

Frequency 5 9 11 7 9 10 3% of total 29% 53% 65% 41% 64% 59% 18%

All samples''Frequency 18 24 26 17 24 26 9% of total 40% 53% 58% 37% 56% 58% 20%

aBased on accumulation rate from 1963 peakbWithin ± 25 % (average counting error) of ideal accumulationcWithin ± 20% of ideal ratiodFor three cores from Terrebonne Basin, the end of mcs activity not reached, n = 13 and n = 43 used for number of cores tocalculate % of total for total mcs inventoryT = time resolution function (MILLER and HEIT, 1986)

based on the estimated atmospheric deposition of137CS. The average for all cores for the total in­ventory of 137CS was about equal to the total de­livery to the marsh surface. This .is in contrast tothe results of OLSEN et al. (1981) who estimatedthat only 10 to 30% of the 137CS delivered to theHudson River estuary was deposited in the sed­iments and CASEY et al. (1986) who found thatcores from a Virginia marsh contained inventoriesthat were less than 70% of the expected inven­tory.

The vertical accumulation of sediments in allcores ranged from 0.24 to 1.12 cm/yr assumingcompaction occurred entirely above the 1963-1964horizon (the minimum), to 0.31 to 1.33 cm/yr as­suming compaction below the presumed 1963­1964 horizon (the maximum). The average of themaximum and minimum accumulation rates forcores from the three basins studied are presentedin Table la and for replicate cores in Table 1b.

Table 3 contains correlations between 137Cs in­ventories and organic and inorganic content. Grainsize analysis of the inorganic portion was not done.There was a strong correlation between the post1963-1964 inventory of 137CS (pCi) and organiccontent (g, dry wt.), but a weak relationship withinorganic content (g, dry wt.). The relationshipbetween the post 1963-1964 137CS inventory andof organic matter was highest in cores from theSt. Bernard Basin and lowest in cores from Ba-

rataria Basin. The correlation between the totalorganic matter and the total 137CS inventory (allcores) was weaker than it was for the post 1963­1964 period. The correlation between the totalinorganic matter and the total 137CS inventory (allcores) was slightly stronger than for the post 1963­1964 period.

Figure 2 shows the pattern of 137CS activity inan "ideal" core that would result from an envi­ronment with a high sediment accumulation rate,> 1 cm/yr, (SCHAFFNER et al., 1987) and no mix­ing, wash-in, or erosion (MILLER and HEIT, 1986).Note that the peak in 137CS activity occurred dur­ing 1964 in south Louisiana, and that deposition

Table 3. Correlations (r) between 137CS inventories and or­ganic and inorganic content.

St.All Bara- Ber- Terre­

Data taria nard bonne

Post 1963-1964 137CS inventory(pCi) us. organic content (g, drywt.) 0.86 0.82 0.94 0.87

Post 1963-1964 137CS inventory(pCi) us. inorganic content (g,dry wt.) 0.53 0.40 0.68 0.52

Total 137CS inventory (pCi) us. or-ganic content (g, dry wt.) 0.76 0.69 0.85 0.75

Total 137CS inventory (pCi) us. inor-ganic content (g, dry wt.) 0.55 0.45 0.51 0.47

Journal of Coastal Research, Vol. 11, No.2, 1995

302

0.20

Milan et al.

~o:j 0.15ocd~fq

~~

.s O. 10~Q)

:>~H

Ul

['oU 0.05C"l...-l

0.00'"1 ,............--- , , , , , , , , , , , , , , , ,

90 88 86 84 82 80 78 76 74 72 70 68 66 64 62 60 58 56 54 52

YearFigure 2. The pattern of mcs activity in an "ideal" core, based on a reconstruction of atmospheric fallout patterns in the southernUnited States.

in 1963 was slightly lower. Sixty-five percent ofthe 137CS fallout was during the first 4 months of1964 while 56 ~o of the 137CS fallout for 1963 oc­curred in the middle 4 months (May-August).There was also a peak in 137CS activity during 1959.Samples were collected in late summer of 1991and since the peak fallout of l:n Cs was between1963 and 1964, approximately 28 years were as­sumed to have elapsed between peak l:ncs falloutand sampling. Most l:ncs activity profiles showthe maximum peak at 1963 (28 years from 1991),since the peak fallout was somewhere between1963 and 1964, 1963 was chosen as the "peak"since it conforms to other investigations. Very fewof the 46 cores examined in this study exhibitedprofiles close to the ideal core. Only 9 profilesdisplayed a peak that we thought could be inter­preted to represent the 1959 deposition peak. Fig­ure 3 shows example cores which represent threedata groups determined from the time resolution(7) scheme of MILLER and HEIT (1986): (1) a pro­file representing 20 C)O of all cores which had thebest fit to an ideal core (7'S < 4); (2) a core rep-

resenting 47% of the cores which displayed a dis­tinct maxima, but no 1959 peak (7'S between 4and 10); and (3) a core representing the one thirdof the cores that had a maxima, but where mostof the l:ncs activity was distributed throughoutthe core (7'S > 10). The time resolution function"7" is calculated from the number of years (i.e.,

centimeters of depth) in which 68 r,o of the l:ncsactivity occurs in a core and from constants de­rived from the ideal l:ncs distribution (MILLERand HEIT, 1986). This function is a measure ofpeak spread either increasing or decreasing withcore depth. An increasing degree of peak spread(larger "7") indicates mixing and/or migration ofl:ncs within a core. This peak spreading couldmean that the 1963-1964 peak has moved fromthe sediment layer it originally occupied or thatthere is a diffusion of material that has caused adecrease in peak amplitude. Shifting of the l:ncspeak would corrupt calculations of sediment ac­cumulation rates. Therefore, accumulation ratedata obtained from cores with "7" values greaterthan ideal should be interpreted with caution.

Journal of Coastal Research, Vol. 11, No.2, 1995

mcs Method in Salt Marshes 303

tJ'l 4.0<, a) TH1A·rtUc,

3.0

>,+J·rt 2.0>·rt+J0~ 1.0

ft)

Ur-

0.0Mri

90 88 86 84 82 80 78 76 74 72 70 68 66 64 62 60 58 56 54 52

Year

0'1 4.0<, TH2A·rtUc,

3.0

>.+J·rt 2.0>

·rt+J0

ICC 1.0

enU

r- 0.0C'1ri

90 88 86 84 82 80 78 76 74 72 70 68 66 64 62 60 58 56 54 52

YeartJ'l 4.0<, c) TH2B·rtUc,

3.0

>,+J·ri 2.0>·rt+>0~ 1.0

ft)

Ur- 0.0Mri 90 88 86 84 82 80 78 76 14 72 70 68 66 64 62 60 58 56 54 52

YearFigure 3. The distribution of mcs in cores from the Terrebonne Basin that represent three ranges in the time resolution function(T) found in this study (MILLER and HEIT, 1986); a) T'S < 4, b) T'S between 4 and 10, and c) r 's > 10.

There was an inverse and statistically significantlinear correlation (r = 0.52) between accumula­tion rate and the time resolution within the core.This indicates that dating the 1963-64 time ho-

rizon is more precise at higher sediment accu­mulation rates and especially above 1 cm/yr ac­cumulation rates as others have found (e.g.,SCHAFFNER et al., 1987).

Journal of Coastal Research, Vol. 11, No.2, 1995

304 Milan et al.

DISCUSSION

The utility of using l:n(~s to assess salt marshaccumulation rates is dependent upon the follow­ing assumptions: (1) the marsh is rapidly accret­ing; (2) there is no import or export of I:ncs intothe system other than from atmospheric fallout,and; (3) the l:ncs is fixed in the sediment columnand does not migrate vertically in either direction.The quality of a core should be assessed beforeusing the l:ncs information on the vertical distri­bution to calculate accumulation rates. The term"quality" reflects a grading of a core based on theevaluation of the characteristics or parametersmeasured. MILLER and HErr (1986) assessed corequality in determining the utility of using a par­ticular core to reconstruct pollution histories. Someparameters to consider in assessing the quality ofa core are: (1) core compaction, (2) the presenceof l:ncs at the surface of the core, (3) where in thecore the first I:ncs activity is detected in relationto the 1963-1964 peak, (4) the l:ncs inventory forboth post 1963-1964 and total accumulations, (5)the percentage of the post 196:3--1964 inventoryof the total, and (6) the presence of a 1959 peak,which is illustrated by the time resolution of thecore profile (MILLER and HEIT, 1986).

It is important to determine the core compac­tion, even if it is relatively small. For example, ifthe core compaction is 5 c( and the accurnulationrate is <0.3 cm/yr, then the difference in the min­imum and maximum rates (see methods section)is 30('0. If the compaction was 10 l

(' in the samecore, then the difference would be 60 ", . If thereis no way to avoid compaction during sampling,at least by measuring it one can report a range ofaccumulation rates. Besides compaction of the coreduring sampling, a correction for compaction ofthe sediment column due to the consolidation mustbe made (CHRISTENSEN, 1982; LYNCH et al., 1989).

The distribution of l:ncs throughout the coremay be used to test various interpretations aboutthe environment or core quality. Atmosphericfallout of l:ncs was negligible after 1982 in thesemarshes. Surface erosion may be indicated if l:ncsis detected at the top of cores taken in the 1990'sand there is a deficit in the post 196:3--1964 in­ventory. On the other hand, if there is surfacel:ncs activity and an excess of l:l7CS inventory,then wash-in may be indicated. Finally, if I:l7Cs ispresent in the surface layer and the inventoryequals the estimated atmospheric deposition, thenthere is the possibility of upward migration

(DOMINIK et al., 1981; EDGINGTON et al., 1991;JAAKKOLA et al., 198:~; LONGMORE et al., 1983).

Atmospheric testing of nuclear weapons begansometime in the mid 1940's (CARTER and MOGHIS­SI, 1977; MONETTI and LARSEN, 1991). The firstsignificant megaton yield was in 1952, which was~30 times higher than the total of the previousseven years (CARTEH and MOGHISSI, 1977). It canbe assumed from this data that fallout of l:ncsoccurred in the early 1950's even though 90Sr de­position monitoring was not started until 1954 bythe Environmental Measurements Laboratory,lJ.S. Department of Energy, New York, NY.Therefore, there should be no l:ncs detected atdepths dated by the 196:3-1964 peak earlier than1950. If there is, then there has been either mixingof the surface sediments (either physical or bio­logical) when the 1:l7 CS was first deposited or adownward migration of I:l7Cs in the sediment col­umn. Dating cores using the first appearance ofI:l7Cs as the 1954 horizon is of questionable meth­odology.

The inventory of 1:l7CS found in cores should beevaluated to determine if there are severe deficitsor excesses. Deficits suggest the possibility of ero­sional events, while excesses suggest wash-in. Ei­ther of these conditions violates the assumptionthat there has been no transport of 1:17CS in or outof the system. Deficits in l:ncs inventories havebeen noted in saline environments (CASEY et al.,1986), which are attributed to the low sorptioncoefficient of dissolved I:l7Cs in these environ­rnents. The cores that we obtained from varioussalt marsh environments in southern Louisianashowed both deficits and excesses, with the meanbeing slightly higher than what would be expectedfrom fallout records. This suggests that in thesemarshes a salinity effect is not as dramatic asother investigators have reported (CASEY et al.,1986). Erosion of sediment above the 1963-1964peak will cause the peak to be detected closer tothe surface than where is was originally deposited.If this is localized to the particular area where thecore was obtained, then using accumulation ratedata from such a core for evaluation of a basinwill result in underestimation of accumulationrates for that basin. Wash-in of I:17Cs, from eitherthe surrounding watershed or from deposition onvegetation that was later released to the sedi­ments, could cause a peak to be closer to the coresurface than the actual 196:3-1964 peak and resultin an erroneous calculation of the accumulationrate. The percentage of the post 1963-1964 to the

.lournal of Coastal Research, Vol. 11, No.2, 199:)

I.l7Cs Method in Salt Marshes

total inventory of I:17Cs is another indicator ofwhether l:ncs deposition was constant or if therehas been wash-in or erosion over time. A higherpercentage of the total 1:17CS inventory detectedabove the 1963-1964 peak would suggest wash­in, while a lower percentage could be the resultof erosion.

The presence or absence of a 1959 peak, whichis prominent in the ideal core profile (Figure 2),may indicate whether 1:17CS has undergone mixingand/or diffusion. This is best illustrated by thebroad shaped profiles (7 > 10) that have down­ward tails that suggest extensive 1:17CS movementdown the sediment column. Another cause of peakbroadening is delayed wash-in that would causethe profile to broaden in the opposite direction.The core profiles with "7" between 4 and 10 in­dicate either some diffusion of l:17CS down core ordelayed wash-in. A core profile of l:l7CS activityshould resemble that of the ideal core if there hasbeen no wash-in or erosion. The downward dif­fusion of l:ncs would not be possible if 1:17CS wasimmobile and has not undergone some sort ofmixing. Cesium-137 deposited in the salt marshattaches to both organic matter and clays. It isbelieved the attachment of 1:17CS to organic matteris not permanent, because the 1:17CS can be re­mobilized when the organic matter decomposes(e.g., DAVIS et al., 1984). The strong correlationsbetween the inventories of 1:17CS and organic mat­ter found in these cores (Table 3) either confirmsthat I:~7CS is associated with organic matter or itis coincidental to the accumulation of organicmatter. The attachment of J:17Cs to some clay typesis also not permanent. Cesium-Ll? is known tostrongly fix to illite and vermiculite, but can beexchanged from minerals such as montmorilloniteand kaolinite by ions such as K ! and NH4 t , whichare common in salt marshes (SAWHNEY, 1972;SCHULZ, 1965; SCHULZ et al., 1960; rrAMtlHA andJACOBS, 1960). Although the fixation of 1:17CS intothe clay lattice of illite may be permanent, it oc­curs slowly, over months and years (EVANS ei al.,1983; ZUCKER et al., 1984). The clay fraction ofLouisiana salt marsh sediments probably closelyresembles the composition of Mississippi Riverclays, which contain smectite (montmorillonitefamily), kaolinite, and illite in a ratio of approx­imately 3:1:1 (JOHNS and GRIM, 1958). These factssuggest that a portion of the 1:17CS deposited insalt marshes could be desorbed and migrate cre­ating 137CS profiles that are wide and not welldefined.

The parameters used to assess core quality inthis study Crable 2) show no meaningful differ­ences between basins, except that cores from Ba­rataria Basin scored somewhat lower in the pres­ence of 1:17C~S at the surface and 1:17CS inventory;while a large majority of cores from the St. Ber­nard Basin contained l:ncs at depths dated by the196~3-1964 peak earlier than 1950. Cesium-137 atdepths earlier than 1950 suggest diffusion of l:17CSdown core or a change in accumulation rates after196:t I t would appear from these data that thequality of a core is not a function of the basinwhere it is located but is affected by local con­ditions. These local conditions include wash-in,erosion, bioturbation, physical mixing, and l:17CSmigration. Even samples within 50 m of each oth­er displayed differences in 1:17CS inventory andposition of 1:17CS in the sediment column.

In this study the evaluation of the quality ofthe core was used to indicate whether there wasmigration of the 1:17CS in the sediment column ormixing has occurred. However, there are othertechniques that can be used to determine if the1:17CS profile in a core accurately represents sedi­ment accumulation. Two techniques that can beused to assess if mixing has occurred are X -ra­diographs and vertical profiles of other radioiso­topes ii.e., ~J()Pb). X-radiographs will help deter­rnine if bioturbation occurred in the sedimentcolumn and assess the stratigraphy in non-ho­mogenous sediments (SCHAFFNER et al., 1987).Lead-210 profiles are useful to locate areas of ho­mogenous activity in the sediment column (DAVISct al., 1984; DOMINIK et al., 1981; EDGINGTON etal., 1991; LYNCH et al., 1989; ROBBINS and ED­<;{N<;TON, 1975). If it can be determined that mix­ing has occurred, there are numerous mixing mod­els that have been applied to deal with this problem(BENNINGEH et al., 1979; CHRISTENSEN, 1982; ED­(~INCT()N et al., 1991; C;UINASSO and SCHINK, 1975;LYNCH et al., 1989; OFFICER, 1982; ROBBINS andEn(~INGTON, 1975).

CONCLUSI()NS

The total inventory of I:ncs in these cores variedfrom 42 ". to 2~3:3 ", of the estimated fallout. Themean value for all 46 samples was not statisticallydifferent from the estimated aerial fallout rate as56 ". of the cores had inventories within the av­erage counting error (25 ", ). Sorne cores had ob­vious loss or accumulation of I:ncs that was dif­ferent from that anticipated from aerial depositionalone. Wash-in from within or outside the basin

.lournal of Coastal Research, Vol. 11, No.2, 199f}

306 Milan et at.

as well as erosional loss of 137CS could be a problemin these marshes.

Several cores had detectable amounts of 1:17CSin the surface layers, whereas since 1982 the aerialdeposition rates are virtually zero. This patternmay result from at least three different situations:(1) irregular and recent redistribution of l:ncs de­posited onto the top layer, that will result in l:ncsactivity in the surface layer and minor t:ncs ac­tivity between the surface and the 1963-1964 ho­rizon; (2) removal of surficial sediments in the last8 years (when 137CS fallout was minimal); and (3)no sediment deposition during the last 8 years. Ifthere was a recent redistribution of sediments,there should be a zero-activity layer below thesurface layer with 137CS activity. Salt marshes withactive depositional or erosional surfaces may notreflect the aerial deposition of 1:17CS, therefore coresfrom those marshes should be interpreted withcaution.

The examples of accumulation rates from thisstudy demonstrate a CV of 10lJ(] for a sample sizeof 3 within 10 m and a CV with a range between7 C:o and 20% for samples within 50 m (5 sitesfrom 3 basins with replicates). The replication,although gratifying in many ways, is troublesomeif one is trying to determine small differences (10lJoor less) between the apparent rise in sea level andsediment accumulation. Backup cores should becollected and analyzed routinely to give credenceto the interpretation of small changes in sedimentconstituent accumulations over time. Backup corescan be archived and analyzed if the quality of thefirst core analyzed is considered poor.

There are a variety of measurements that areuseful in interpreting the integrity of sedimentcores with the 137CS dating method. Not all ap­proaches give consistent results and subjective es­timations of core integrity remain an integral partof dating sediments using this methodology. Theuse of other methods, such as X-radiography and21°Pb dating, should be used in conjunction with137CS inventory determination to obtain accurateaccumulation rates in marsh sediments.

ACKNOWLEDGEMENTS

This analysis was supported by the Environ­mental Monitoring and Assessment Program(EMAP), administered by the U.S. Environmen­tal Protection Agency, Corvallis, Washington. Dr.R. Novitski and Ms. L. Squires provided signifi­cant administrative support. Mr. James Lee's par­ticipation was funded by the National Marine

Fisheries Service through an intergovernmentalpersonal agreement. We thank several studentworkers, especially Ms. L. Brunet and Mr. Z. Ying,for laboratory assistance.

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