reducing charcoal abundance in archaeological pollen samples

V.M. Bryant. and R.G. Holloway: Reducing charcoal abundance in archaeological pollen sampling 63REDUCING CHARCOAL ABUNDANCE INARCHAEOLOGICAL POLLEN SAMPLES

VAUGHN M. BRYANTPalynology LaboratoryTexas A&M UniversityCollege Station, Texas 77843-4352U.S.A.e-mail: [email protected]

RICHARD G. HOLLOWAYDepartment of Biological SciencesNorthern Arizona UniversityFlagstaff, Arizona 86011U.S.A.e-mail: [email protected]

Abstract

In the field of archaeological palynology, charcoal and ash debris in sediments have been among the palynologists’ worst enemies. Toooften important pollen information is found in the soils of sites near fire hearths where the members of ancient cultures would gather toeat and conduct social activities, and in winter sleep near the fire. For decades archaeological palynologists have searched for ways toremove the thousands of tiny flecks of charcoal and ash in these samples so the fossil pollen can be observed. Until now, no technique hasproven adequate for successful charcoal removal without the loss of pollen. We have not completely solved this problem, but we havedeveloped an extraction method that significantly reduces the amount of included charcoal and ash in archaeological sediments. We testedthis new extraction procedure on charcoal-laden archaeological sediments from a pueblo site in Arizona. The result demonstrates theadvantage of using this new pollen extraction procedure for certain types of archaeological soils.

Key words: Archaeology; pollen; charcoal removal; palynology preparation techniques; American Southwest.

Palynology, 33(2) 2009: 63–72© 2009 by AASP Foundation ISSN 0191-6122

INTRODUCTION

Pure carbon, in the form of charcoal, is inert and does notreact with the acids typically used to remove debris duringpollen processing. For more than 50 years palynologistshave been studying archaeological sites in the AmericanSouthwest. For some of those studies the pollen analysishad to be abandoned, not because there was a lack of fossilpollen, but because the sediments contained too much inertcharcoal that could not be removed.

As palynologists working in this region of North America,we have been frustrated by an inability to discover somemethod to concentrate fossil pollen in archaeologicalsamples containing large amounts of charcoal. Duringmore than four decades of working in that region we haveexperimented with a wide variety of charcoal removaltechniques, none of which were entirely successful. Initialefforts mainly focused on screening. These included using

brass screens with openings of 150 µm and even finermeshed screens such as ones with openings of 125 and 100µm. However, we discontinued using screen sizes withopenings smaller than 150 µm for fear of losing some of thelarger pollen grains including those of maize, cactus, fir,spruce, and Douglas fir.

To say that we totally ignored the potential benefits ofsieving with screens would not be entirely correct. We didexperiment with various sizes of small-meshed screenswith openings ranging from 5 to 10 µm in diameter.Cwynar et al. (1979) effectively proved the value of usingscreens with small openings to remove debris during cer-tain types of pollen preparations. Using the guidelines setforth in that paper we found that we could effectively usestainless steel screens with openings of 10 µm during theprocessing of archaeological sediments containing largeamounts of charcoal fragments. After using those finemeshed screens we searched the filtrates for lost pollen and

64 PALYNOLOGY, VOLUME 33(2) — 2009

found very few. Most often we found small fragments ofbroken pollen grains and on rare occasions a small pollengrain such as those produced by some species of Mimosa.The rare loss of one or two pollen grains in the filtrate didnot outweigh the apparent benefits of removing some of thecharcoal flecks.

Fine-meshed, stainless steel screens are prohibitivelyexpensive so we also tried using nylon (NITEX) screenswith openings of 5, 7, and 10 µm, but abandoned thatattempt because those screens became clogged too quicklyand we did not believe we were removing enough charcoaleffectively. Our only real success came from using thestainless steel screens with openings of 10 µm, which weremounted in a circular frame with steel sides that were oneinch high. When trying to use this procedure for charcoalremoval we found that the most effective technique was topour small amounts of the residue (i.e. the remainingmaterial after all processing was completed but before theuse of heavy-liquid separation fluids) into the screen andthen spray the screen repeatedly with an EtOH mist from aplastic spray bottle. We also found it was somewhat effec-tive to place the residue on the screen surface and then fillthe screen container half full with EtOH. We then woulduse a sonic probe placed against the outer wall of the metalsides of the screen to encourage the sieving process byproducing a vibration. We found that we could not place thesonic probe directly into the liquid or onto the screensurface because it created splashes from the vibrations.

Although we found that this type of sieving did removesome of the very tiny flecks of charcoal, it was ineffectivein removing enough charcoal to warrant the time we spentscreening. We also found that the screens repeatedly be-came clogged and we had to remove the debris often, cleanthe screen surface, and then repeat the process. Even after15–30 minutes of fine screening we usually found that toomuch charcoal still remained in the samples. Thus, undermost circumstances we abandoned this technique becausewe found it too difficult to do, too time-consuming, and noteffective enough to warrant our efforts.

Some of our other experiments revolved around decant-ing samples in various types of liquids, including distilledwater, ethanol (EtOH), and tetra butyl alcohol. We experi-mented using various time limits (Lentfer et al., 2003) aswell as various sizes and shapes of beakers and slendercylinders. We mixed samples with distilled water and thenpoured the mixture into slender glass cylinders that were 7cm wide and 1 meter tall. We then allowed the solidparticles in the mixture to settle to the bottom duringvarious time periods ranging from 30 seconds up to twominutes. At the end of each time period (30, 60, 90, 120seconds) we poured the liquid into a large beaker and thenchecked the material that had settled out for trapped palyno-

morphs. For each test we found that some pollen and tracerspores settled quickly and became trapped in the materialthat settled to the bottom. We tried similar tests with tetrabutyl alcohol and EtOH and found that both, like water,proved ineffective as decanting liquids because pollensettled too quickly and became trapped with the solidmaterials in the bottom.

Swirling techniques (Funkhouser and Evitt, 1959) at onetime were championed as an effective method to removeheavier debris in pollen samples. Their use focuses onputting a small amount of soil in a watch glass, thencarefully swirling the solution. Theoretically, the denserdebris is trapped while the lighter material (including thespores and pollen) remains in suspension around the edgesand can be pipetted off. We found this technique wassomewhat effective and did trap a significant amount ofcharcoal in samples. However, the technique was time-consuming and required skill to prevent spilling the liquid.This technique was also abandoned because some pollenwas always trapped with the heavier charcoal debris.

Our final attempts focused on trying to oxidize thecharcoal without damaging the fossil pollen or spores. Weused various strengths and exposure times of hydrogenperoxide, nitric acid, and sodium hypochlorite. Each provedsomewhat effective in removing charcoal, but each alsooxidized various amounts and different taxa of fossil pollenand spores in the samples, as confirmed by Hopkins andMcCarthy (2002).

CHARCOAL REMOVAL TECHNIQUE

We experimented with various high-density liquids in-cluding sulfuric acid, ethyl iodide, stannic chloride, bromo-form, zinc chloride, and zinc bromide and tried usingdifferent specific gravities and different speeds of centrifu-gation, but each experiment failed to produce satisfactoryresults. When we were able to remove significant amountsof charcoal, some pollen and spores were also lost. Whenwe captured all of the pollen and spores, we couldn’tremove enough charcoal.

During the spring of 2005, we began working on a largenumber of archaeological samples from sites in RidgesBasin, Colorado (excavated by SWCA Inc. in Durango,Colorado). Most samples contained large amounts of char-coal, which made processing and subsequent pollen studiesdifficult. The frustration of not being able to removesufficient amounts of charcoal from those samples renewedour interest in searching for a new charcoal-removingtechnique. After that year’s study was completed, we usedthe remaining soil from the 2005 samples to conductexperiments using variations in the heavy liquid method.Earlier, we had noted that after all chemical extraction

V.M. Bryant. and R.G. Holloway: Reducing charcoal abundance in archaeological pollen sampling 65

procedures were completed, and we conducted heavy liq-uid separation for five minutes of centrifuging at 3000 rpm,we did not get a characteristic black band at the top of thecentrifuge tubes containing the samples. Instead, eachcentrifuge tube consisted of a solid, black, opaque solutionfilled with charcoal flecks at every level in the solution. Wefound it impossible to pipette off only the trapped pollenfloating on top of the zinc bromide without also includinglarge amounts of floating charcoal flecks.

Using different experimental variations we discoveredthat we could take the upper portion, which we had pipettedfrom the top of the zinc bromide procedure, and subject thatmaterial to additional steps. We found those additionalsteps would eventually produce a much cleaner samplecontaining all of the original pollen and spores with only afraction of the original amount of charcoal. The onlyproblem was that our new technique was time-consumingand expensive. Time is always a premium, and the heavyliquid, zinc bromide, is too expensive to use except in smallamounts. We do not believe that we can solve the timeproblem, but it might be possible to reduce costs if we canuse a less expensive type of heavy liquid solution.

The new technique we developed is detailed below andwe recommend that it be used to separate much of thecharcoal from the palynomorphs after the removal of car-bonates and silicates in a sample. Further, we suggest thesamples be sonicated for no more than 10 seconds in a Delta5 Sonicator. This procedure seems to dislodge tiny charcoalflecks that become trapped in the reticulate pattern ofLycopodium clavatum spores and on the surface of someornate pollen grains. The sonication also breaks up clumpsof materials in the sample and frees pollen and spores fromother types of attached debris. Other sonicators may workas effectively, but each should be tested to ensure it does notdamage pollen or spores.

Our experiments have revealed two other importantaspects. First, they revealed that the charcoal removalprocedure seems to work best when most of the charcoalflecks are smaller than 50 µm. The procedure does not seemto work as effectively with larger sized particles. Second,we found that if the remaining amount of processed mate-rial in a 15 ml centrifuge tube (before heavy densityseparation) is more than 1.5 ml, this charcoal removalprocedure will not work effectively without some pollenand spore loss. We recommend that excess amounts ofsample be reduced by splitting the sample into two or threeseparate centrifuge tubes before continuing.

Step 1

It is essential that a matrix sample be thoroughly mixedwith zinc bromide (2.0 specific gravity) before centrifuging

begins. We ensure this by putting a small amount (ca. 5 ml)of zinc bromide into a 15 ml conical-shaped centrifuge tubeand then mixing it using a vortex stirrer and a woodenapplicator stick. While holding the centrifuge tube on thestirrer with one hand, we move the wooden applicator upand down along the edges of the centrifuge tube in thesolution. The high speed vortexing of the liquid ensures thatall materials and clumps will come in contact with thewooden applicator stick, and will thus be disaggregated andthoroughly mixed with the zinc bromide solution. Wecontinue this mixing for at least 30 seconds. The speed ofthe vortex mixer should be adjusted to ensure that the liquiddoes not spill out of the top of the tube, but should be fastenough to produce a rapid vortex for mixing purposes.After mixing, additional zinc bromide is added until the 15ml centrifuge tube is approximately two-thirds full. Toensure that no pollen is left on the inside walls of thecentrifuge tube, we spray the inside walls with EtOH untilwe have added about 1–2 ml of EtOH on top of the zincbromide. Those fluids will not mix as long as one does notstir them or turn the tube upside down. When this step iscompleted, a thin, clear band should be visible on top of thezinc bromide solution in the centrifuge tube. As a final stepwe generally add an additional 1–2 ml of distilled water tothe sample. First, we spray a fine mist of distilled water tocoat the inside edges of the centrifuge tube. This washesany trapped pollen on the walls down into the fluid in thecentrifuge tube. We then add another 1 ml of water bycarefully letting it trickle down the inside wall of thecentrifuge tube to prevent it from mixing and thus dilutingthe zinc bromide. We have not tested to confirm that thisfinal spraying and addition of water is essential, but itcleans the inside of the centrifuge tubes and also forms awater seal at the top of the mixture. When we pipette off theupper liquid band that is formed after centrifugation, it iseasier to have the water on top because it helps to define theband of trapped palynomorphs. We always pipette off thewater with the band of palynomorphs.

Our experiments have demonstrated that separation oc-curs best if the sample is allowed to sit for about 5–10minutes in a test tube rack. This allows the heaviest par-ticles in the solution to sink to the bottom of the centrifugetube without dragging down pollen and spores in theprocess. Next, we centrifuge the test tubes for 5 minutes ata speed no greater than 200–300 rpm. This very slowspeed seems to allow other medium-heavy materials in thematrix to settle to the bottom of the centrifuge tube withoutalso ‘carrying down’ or trapping pollen and spores in theprocess. After 5 minutes at this slow speed, the solutionshould be centrifuged at a higher speed of 2000–3000 rpmfor another 5 minutes. This second step sometimes, but notalways, will make the separation cleaner of charcoal debris.


When centrifugation is completed, the liquid fraction atthe top should show a dark band of particles below thethin, clear band of EtOH mixed with water. That darkband, usually 3–5 mm wide, will consist of pollen, spores,and other debris with a specific gravity less than 2.0. Thatband should be carefully pipetted off along with all of theEtOH and water on top of the band. All pipetted materialshould be placed into a clean 15 ml centrifuge tube that iscorrectly labeled. Unfortunately, when working withsamples full of charcoal, it is rare to see a defined band atthe top of the zinc bromide after centrifugation. In thattype of situation, we recommend that the uppermostportion of the ‘black’ solution, below the water and EtOH,be pipetted off and saved. We generally pipette off theupper 2 ml of the black layer to ensure that we capture allthe pollen and spores.

Fill the centrifuge tube containing the sub-sampled ma-terial with EtOH, stir thoroughly, and then centrifuge thesolution at 2000–3000 rpm for one minute. We use EtOHinstead of distilled water because EtOH has a lower specificgravity (0.7) than water, and it will ensure that the removedzinc bromide liquid, when mixed, will have a low specificgravity (ca. 1.0). This ensures that all pollen and spores willsink to the bottom during centrifugation.

Step 2

When working with samples rich in charcoal, we find itessential to repeat step 1 in its entirety a second time. Thisshould be done regardless of any future attempts to re-move charcoal. During the examination of the bottomportions of charcoal-laden samples we have discoveredthat after a typical zinc bromide separation process thereare often a few tracer spores of Lycopodium clavatumtrapped in that material, and less often a few pollen grains.The spores are normally added at the beginning of ourextraction procedure to enable the analyst to calculatepollen concentration values. We suspect the spores be-come trapped for several reasons. First, charcoal frag-ments sometimes trap one or more spores as they movedownward through the liquid during centrifugation. Sec-ond, we have noticed on some of the trapped Lycopodiumspores that their reticulate ornamentation has trapped anumber of charcoal particles, which may change thespecific gravity of the spores enough to allow them to sinkwith other heavy fragments.

We found that we could solve both problems by sonica-tion. If the sample is sonicated prior to using zinc bromideseparation, then fewer spores seem to sink to the bottomwith the heavier materials. However, some spores seem tosink even if sonication is used prior to the zinc bromideseparation. Those few spores can, however, be recovered

by repeating step one in its entirety and then combining theupper portion pipetted off during both procedures beforeproceeding to the next step.

After completing the second procedure in which weagain conducted the zinc bromide separation on the mate-rial that remained at the bottom of the centrifuge tube afterthe first separation, the material still remaining in thebottom of the centrifuge tubes is removed and examined at100× magnification. We do this because it is essential thatall material at the bottom of each centrifuge tube be checkedto ensure that no pollen or tracer spores remain trapped inthe debris. If we find any, the procedure outlined in step 1and 2 is repeated a third time to recover those additionalpollen grains. When using this charcoal removal proce-dure, we rarely find any pollen or tracer spores remainingin the bottom debris after the second zinc bromide separa-tion at 2.0 specific gravity.

The difference in the amount of unwanted charcoal thatcan be removed by our procedure is illustrated in Text-Figures 1 and 2. Text-Figure 1 is taken at 400× magnifica-tion and shows a typical sample we prepared from the 2006field season after we had completed the pollen extractionprocedure to this point. Text-Figure 2 is a view at 400×magnification of the same sample after we completed theextended charcoal removal process detailed below.

Step 3

Once all the material recovered by pipetting off duringboth heavy density procedures in the 2.0 zinc bromide hasbeen diluted and then concentrated into one 15 centrifugetube, we repeat all the procedures outlined in step 1 again.However, this time we use only the recovered material asour new sample for heavy density separation. During thisthird step all procedures are the same except we use zincbromide with a specific gravity of 1.9 instead of 2.0. Byusing a lower specific gravity during this procedure we areable to remove additional amounts of charcoal without theloss of pollen or spores. Nevertheless, we recommend thatthis procedure should be repeated twice using 1.9 as thespecific gravity of the zinc bromide. Those two separateprocedures will ensure that no pollen and spores are lostwhen using the 1.9 specific gravity.

Step 4

Step 4 is a repeat of step 3 except we substitute the 1.9specific gravity zinc bromide with 1.8 specific gravity zincbromide. As recommended for the previous two solutionsof zinc bromide (2.0 and 1.9 specific gravity), we againrecommend that the procedure be repeated twice using the1.8 specific gravity separation.


Step 5

Finally, using only the recovered material pipetted offafter the 1.8 separation procedure, we repeat the procedureoutlined in Step 1 using a specific gravity of 1.7. As werecommend for each of the previous cases, one should dothis twice to ensure no pollen or spore loss.

We do not recommend conducting any specific gravityseparations using fluids less than 1.7 specific gravity eventhough the Exxon petroleum processing lab at Houston,Texas during the 1970s and 1980s routinely used zincbromide heavy liquid separation at 1.65 specific gravity aspart of their pollen and spore separation techniques. Duringthe mid-1970s, Jack Bruce, the director of the Exxonprocessing facility, told one of us (VMB) that they hadconducted extensive tests using zinc bromide and otherheavy density fluids at different specific gravities and haddetermined that the lowest specific gravity that could beused safely on samples from Mesozoic and Tertiary depos-its, without any loss of palynomorphs was 1.65. Thisresearch was later revisited by Forster and Flenley (1993)who demonstrated that different pollen taxa will fractionateout at different gradient levels in heavy density fluids. Intheir experiments they found that in all cases both modernand fossil pollen and spores will separate out and float onthe top of fluids that have a specific gravity of 1.7 or greater.This is one reason why we do not use any specific gravityseparations lower than 1.7. In both the Forster and Flenleypollen separation experiments, and in our own work, we havefound it safe to use heavy liquid separation with a specificgravity of 1.7 or greater without losing palynomorphs.

Charcoal Removal:An Experiment Using New Samples

During 2005 and 2006, a multiyear archaeological inves-tigation of the Animas-La Plata Project was conducted bySWCA Inc. For that project we were asked to examinepalynological samples from one of the sites located inNorthern Arizona dating from the Pueblo I Period (A.D.750–900). For all samples from both years we used 15 mlof sediments from each collected sample and we addedthree tablets of Lycopodium clavatum tracer spores, witheach tablet containing 13,500 ± 400 spores.

For the 2005 season we were asked to process and thenexamine 26 pollen samples that had been collected fromsite 5 LP 0245. However, because of the excessive amountsof charcoal in the pollen samples from the 2005 season, wewanted to develop a better extraction technique beforeprocessing the samples from the 2006 season. Since webelieved that the charcoal problem in the samples from the2006 season would be similar to those found during the

2005 season, we asked for, and received, permission to testour newly-developed experimental extraction technique ona new series of samples from the 2006 season.

We realized that trying our new extraction proceduremight prove beneficial, but that it would also presentseveral new problems for us as well as the archaeologistexcavating the pueblo site. First, we realized that to vali-date the scientific merits of our new procedure we wouldneed to conduct parallel experiments using the same set ofsamples and use the same extraction procedures, changingonly the final steps for one set of samples. After that, wewould need to conduct pollen counts from both sets ofsamples and determine the time spent as well as the differ-ent results in terms of total pollen found, pollen percent-ages, pollen concentration values, and the total numbers oftaxa found in the pairs of samples. This would be a worthyexperiment and it should be done. However, because wewere working on this archaeological project under criticaldeadlines, and also lacked the extra time and funding toconduct a new study using a large number of paired samplesfrom the same sediments, we decided instead to try our newprocedure on the next year’s (2006) samples. We knew thatusing this new technique on a different set of samples fromthe same pueblo site might provide much better results butwe also knew that those results would not ‘validate’ the realmerits of our new technique. That validation will have towait for someone to conduct the needed experiment. Nev-ertheless, we offer this technique paper because we areconvinced that this new procedure is effective at safelyremoving excessive amounts of charcoal from pollensamples. The second problem created by using differentextraction techniques is that it made interpretations moredifficult for the archaeologist when trying to compare theimplications presented by the pollen data from each of thetwo field seasons (2005 and 2006).

We present the following pollen data as an example ofwhat we believe can be gained by using the new extractiontechnique. We acknowledge that the two sets of data (2005and 2006) do not come from the same set of samples, butalso know that both sets of samples were collected fromsimilar types of pits, hearths, and rooms in the same puebloand that the color and texture of samples collected duringboth seasons were visually identical. Later, while process-ing both sets of samples, the amount of charcoal remainingin each of the samples from both seasons appeared to benearly identical. Finally, we emphasize that we believe thistechnique enabled us to gain considerably more polleninformation from the 2006 season samples (processedusing the new technique) than from the previous set ofsamples from the 2005 season (processed using the stan-dard technique), but we acknowledge that we cannot quan-titatively verify this to be true.


Text-Figure 1. View at 400× magnification of a typical pollen sample from the 2006 season after the end of normal processing andheavy liquid separation using only zinc bromide with 2.0 specific gravity.

Text-Figure 2. View of the same sample shown in Text-Figure 1 at 400× magnification after completion of the extended processingprocedures using multiple heavy-liquid separation steps with zinc bromide at various specific gravities, as described in this paper.


Pollen Counting Procedure

The initial step one must consider in any pollen analysisis the type of counting procedure desired in order to obtaina pollen sum. In this analysis our counting procedureconsisted of a two-step approach. The initial step consistedof a preliminary scan of each sample. This initial countingprocedure was conducted to determine two things. First, wewanted to obtain an estimate of the ratio of fossil pollen tomarker grains in each sample. Second, we wanted todetermine how long it might take to complete a 200+ grainpollen count for each sample. From past experience wehave found that a convenient way to obtain both estimatesis to count all fossil pollen in sequential microscope transectsat 400× until we have encountered and tabulated a total of50 Lycopodium marker grains. At that point we can esti-mate the approximate pollen concentration for that sample,and we can also estimate the length of time it might take toobtain a 200–300 grain pollen count. If during this initialstep we found that the sample had a high pollen concentra-tion, then we continued counting for at least four micro-scope slide transects, regardless of the total number ofpollen or tracer spores viewed. To avoid the edge effect thatcan sometimes lead to an uneven distribution of pollengrains under a microscope cover slip, we spaced our transectsevenly over the surface of the cover slip. If, after fourtransects, we had counted either fewer than 200 pollen

grains or fewer than 50 marker grains, then we continuedcounting additional transects until obtaining either 200+pollen grains or a minimum of 50 tracer spores. During thisinitial counting step, we were careful to identify all fossilpollen taxa including highly degraded and unidentifiablepollen grains. All degraded pollen that could not be identi-fied was included in our total counts as recommended fordegraded pollen by Cushing (1967).

Once we reached 50 tracer spores, the estimated ratio ofpollen to tracer spores provided us with a general indicationof the overall pollen preservation in the assemblage, andalso gave us an indication of the general concentration offossil pollen against the amount of charcoal debris presentin the sample. For samples containing an extremely largeamount of charcoal, if we had found fewer than 25 pollengrains upon reaching the 50 tracer spore level of counting,we would estimate the additional time it would take toobtain a 200+ fossil pollen grain count from that sample. Ifwe calculated that it might take longer than two hours, wewould not attempt to count more pollen from that sampleunless the archaeologist believed the sample was of criticalimportance and needed to be counted regardless of theadditional time needed. After completing this first step inour analysis, we sent the results to the archaeologist incharge of the project. With those results were our estimatesas to the amount of time it might reasonably take tocomplete a standard pollen count for each sample. The

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Text-Figure 3. Pollen sum by categories, showing maximum number of pollen grains that could be counted within a reasonablelength of time for samples from the 2005 and 2006 seasons. The total numbers of samples in each of the categories for both seasonsare plotted against each other for comparison. Note that none of the 2005 season samples could be counted beyond a total of 400pollen grains per sample.


archaeologist in charge then determined which samplesneeded to be counted. These decisions were based on thearchaeological importance of the assemblage, the condi-tion of the pollen assemblage as indicated by the number ofindeterminate pollen grains found during each preliminarycount, the estimates we provided about the potential timeneeded for obtaining a statistically-reliable pollen sum, andthe level of available funding.

Pollen Sums

Text-Figure 3 shows the total number of fossil pollensamples in each category for each of the two seasons. Thesamples for each season are divided into 11 different 50-grain units, and within each unit we indicate the totalnumber of samples from each season for which that cat-egory was the highest possible level of fossil pollen countswe could obtain within a reasonable length of time (e.g., nomore than two hours). For example, during the 2005 seasonfour of the total of 26 samples contained so much charcoalthat we could not count more than 50 pollen grains withina reasonable length of time (i.e. no more than two hours).During the counting of the 24 samples from the 2006season, only two samples fell into the same category (Text-Figure 3).

The distribution and changes in the pollen sums are quiteremarkable. In the 2005 group of 26 samples, processedusing only the standard procedures, the quantity of charcoaldebris remaining in the samples prevented us from reachingpollen sums of more than 100 pollen grains in 10 of the 26samples (38%). As first noted by Barkley (1934) andconfirmed by others, including Martin (1963) and Traverse(2007), pollen sums below 100 grains are statisticallyunreliable. Of the remaining 2005 season samples, twoadditional samples (7%) could be counted to levels ofbetween 101–150 pollen grains in a reasonable amount oftime. In contrast to the samples from the 2005 season, afterusing our newly-developed extraction procedure on thenext group of 24 pollen samples from the 2006 season, wewere able to reach acceptable pollen counts of 200 or morepollen grains per sample in 92% of the 24 samples, whereasin the previous set we reached that level in only 54% of thesamples. Another benefit of the newly-developed extrac-tion procedure was our ability to obtain a pollen sum inexcess of 250 grains fairly easily in 79% of the samples,whereas in the previous set that level was only 35%.

Even though alternative explanations are possible, weconsider that the differences between the ability to reachhigher pollen counts in the samples from the 2006 seasonas compared to those from the previous season is primarilyrelated to charcoal removal. The initial amounts of charcoalin the collected samples from both the 26 original samples

and the later 24 samples were nearly identical. We observedthis by noting the amount and size of the remaining extrac-tion residue in the 26 completed samples where only thestandard extraction procedure was used, compared to thetotal residue that existed in the 24 new samples where theextended extraction procedure was used. After the initialheavy liquid separation, explained in step 1 above, the totalamount of charcoal remaining in the 24 pollen samplesfrom the 2006 season was essentially the same as hadremained in the 26 samples from the previous season.However, after completing the extended extraction proce-dure on the 24 samples from the 2006 season, the amountof charcoal remaining in each sample was significantlyreduced.

Number of Taxa

The total number of taxa found in each of the samplesfrom both seasons is listed in Table 1 according to each ofthe main pollen sum categories recorded in Text-Figure 3.The number of taxa present in each sample was calculatedusing Simpson’s Index of Diversity available within MVSP(Kovach, 2002), a computer program specifically designedfor use with palynological data. For each group of samplesfrom each of the two seasons, we list the minimum, maxi-mum, and average number of pollen taxa recovered frompollen samples in each of six categories in Table 1. Thesedata show a small, yet significant increase in the number ofpollen taxa identified within each category during thecounting of samples from the second season over those ofthe first season.

SUMMARY

Palynologists have been plagued for decades with theproblem of trying to obtain pollen counts from archaeologi-cal samples that contained large amounts of charcoal andash. Both charcoal flecks and ash from fire hearths producecharred fragments of wood, which are inert and thus do notreact to various types of chemical extraction procedures.Likewise, because ash and charcoal fragments are often thesame size as pollen grains, screening techniques haveproven ineffective for removing only the unwanted char-coal and ash without also removing fossil pollen. Swirlingand decanting techniques are used to remove various typesof heavy debris from archaeological pollen samples. How-ever, that technique is ineffective for charcoal and ashfragments because most of them have a slow sinking speedin water equal to that of pollen. In most cases, the use ofheavy-liquid separation techniques have also failed toremove adequate amounts of charcoal and ash fragmentsfrom archaeological samples. The results of these failures


have often led palynologists to abandon attempts to countfossil pollen from a variety of archaeological sites wherecharcoal and ash flecks are common in the site’s sediments.

Experimentation has finally produced an extractionmethod whereby a great deal of charcoal and ash materialcan be removed successfully from archaeological samples,without the concurrent loss of fossil pollen or tracer spores.This new charcoal extraction method is time-consumingand requires the use of expensive chemicals, but has provento be effective. Our efforts demonstrate that using the newextraction procedure enables us to reach valid pollen sumsin a much higher percentage of archaeological samples,which in the past would have been abandoned due to thehigh percentages of included charcoal and ash fragments.Other added benefits of our new extraction techniqueinclude finding higher numbers of different pollen taxa persample because of less occurring debris. Concentrationvalues per sample are also higher, which we believe resultfrom our ability to see both pollen and tracer spores moreclearly in samples freed from much of the charcoal and ashdebris. Nevertheless, we recognize there may be otherfactors creating the increases in concentration values forthe samples from season 2006.

In addition to the previously mentioned benefits, wediscovered that the new extraction procedure resulted inour finding higher percentages of maize pollen (Zea mays)in samples. This aspect surprised us because maize pollenis very large and usually easy to identify, even when those

pollen grains are partially obscured by charcoal debris. It ispossible, however, that the increases in maize pollen mightbe caused by natural differences of maize pollen in the twosets of samples. We also found that, in general, the concen-trations of most other pollen types of economic plantsincreased in the 2006 samples, regardless of their size.Again, we cannot be certain, but we suspect this increase isalso directly related to the extended processing proceduresduring the second year.

Our study is based on a two-year study of pollen samplesfrom one large pueblo site located in the American South-west. We recognize and acknowledge that by using differ-ent sets of samples for the 2005 and 2006 field seasons, andby applying different extraction procedures to each set, wecannot be certain that our new technique will alwaysproduce the results we have discussed. We would encour-age others to compare the old technique with this newprocedure on a single set of samples in order to confirm ourpredicted results. Nevertheless, we believe our newly de-veloped extraction procedure can be applied to archaeo-logical samples that contain significant amounts of char-coal and ash, and that it will produce cleaner samples withless charcoal, which can then be analyzed more quickly andwith a greater degree of accuracy.

ACKNOWLEDGMENTS

We are grateful to SWCA for providing the samples andfor permitting us to conduct the extractions on duplicatesamples. In particular we wish to acknowledge the contri-butions of James Potter and Karen Adams whose input andsuggestions were invaluable in this research. FrancineMcCarthy and an anonymous reviewer are thanked for theirconstructive comments on the manuscript.

References Cited

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283–289.CUSHING, E.J.

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Number of TaxaMinimum Average Maximum

2005 Season< 50 7 8.25 1051–100 7 10.33 16101–150 8 8.5 9201–250 11 14 17> 250 12 15.22 22

2006 Season< 50 8 9 1051–100101–150201–250 14 16 19> 250 13 16.36 21

TABLE 1. Data showing the number of fossil pollen taxafound in each category of sample (e.g. 50 grain counts,100 grain counts, etc.) for the 2005 and 2006 seasons.Note that for samples in the 2006 season, each categoryshows an increase in the number of recovered fossilpollen taxa, which we believe results from the improvedvisibility on microscope slides after reducing the amountof charcoal debris.


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reducing charcoal abundance in archaeological pollen samples

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