Journal of Paleolimnology 32: 53–66, 2004.# 2004 Kluwer Academic Publishers. Printed in the Netherlands.

53

Inputs of dissolved and particulate 226Ra to lakes and implications for210Pb dating recent sediments

Mark Brenner*, Claire L. Schelske and William F. KenneyDepartment of Geological Sciences, Land Use and Environmental Change Institute, University of Florida,

P.O. Box 112120, Gainesville, FL 32611, USA; *Author for correspondence (e-mail: brenner@ufl.edu)

Received 22 March 2003; accepted in revised form 5 December 2003

Key words: Florida lakes, Groundwater, Lead-210 dating, Paleolimnology, Radioisotopes, Radium-226,

Sediment chronology

Abstract

Gamma spectroscopy was used to measure radioisotope (210Pb, 226Ra, 137Cs) activities in sediment cores

from 20 lakes and a wetland in Florida, USA. Nine profiles display relatively low (<5 dpm g�1) and constant226Ra activities, whereas 12 show high (>5 dpm g�1) and variable 226Ra activities. In the latter group, most

display up-core increases in activity. Upper sediments from two lakes (Round and Rowell) possess very high

(>20 dpm g�1) 226Ra activities that exceed total 210Pb activities, clearly illustrating disequlibrium between226Ra and supported 210Pb. Supported 210Pb activity is generally thought to come from in situ, 226Ra-

containing detrital mineral particles, and is typically assumed to be in secular equilibrium with 226Ra activity.

Since 1966, Round Lake has been augmented hydrologically with 226Ra-rich (�6.2 dpm L�1) groundwaterpumped from the local deep aquifer. Adsorption of dissolved 226Ra to recent Round Lake sediments

probably accounts for the high measured 226Ra activities and the pronounced disequilibrium between226Ra and supported 210Pb in topmost deposits. We suspect that many Florida waterbodies receive some226Ra-rich runoff and seepage from groundwater pumped for irrigation, residential use, industrial applica-

tions, and mining. This may account for up-core increases in 226Ra activity measured in sediment cores from

some Florida lakes. Significant groundwater pumping began within the last century, and there has been

insufficient time for supported 210Pb to come into equilibrium with adsorbed 226Ra in uppermost deposits.

Input of 226Ra-rich groundwater to lakes may occur in any geographic region where local bedrock contains238U and its daughters. When dissolved 226Ra adsorbs to recent sediments, it complicates accurate estimation

of supported 210Pb activity, and confounds calculation of unsupported 210Pb activity that is used in dating

models.

Introduction

Lead-210 dating has become a common procedure

in paleolimnological studies that require age-depth

relations for sediments deposited over the past

100–150 years (Appleby 2001). Forty years ago,

Goldberg (1963) proposed the use of 210Pb as achronological tool in earth science. Since then,

both alpha and gamma counting techniques

have been employed to estimate the stratigraphic

distribution of 210Pb activity in recent lake

sediments (Eakins and Morrison 1978; Appleby

et al. 1986; Schelske et al. 1994). Over the same

time period, several dating models came into wide-

spread use (Appleby and Oldfield 1978, 1983). In

the past four decades, thousands of investigations

have relied on the approach to develop recent sedi-

ment chronologies (Robbins and Herche 1993).Regardless of how 210Pb in sediments is mea-

sured or what dating model is chosen, application

of the method depends on accurate estimation of

unsupported (i.e., excess or atmospherically

derived) 210Pb activity at contiguous depths

throughout the upper portion of a sediment core.

Several analytical mistakes can confound excess210Pb measurement (Joshi 1989), but often these

can be avoided by adhering to careful field andlaboratory procedures. Recent paleolimnological

studies in Florida (USA) show that some natural

processes and human activities can also create

special difficulties for 210Pb dating. For instance,

in some large, shallow Florida lakes, episodic wind

re-suspension and subsequent re-deposition of floc-

culent organic sediments violates assumptions of

dating models (Whitmore et al. 1996). High andvariable 226Ra activity (i.e., supported 210Pb activ-

ity) in some Florida basins requires that measure-

ments be done with gamma counters. In some

cases, disequilibrium between 226Ra and supported210Pb creates problems for dating recent deposits

(Brenner et al. 1994, 1997).

Here we briefly review the geochemistry of 210Pb

in the environment and describe 210Pb dating meth-ods. Next, we present radioisotope stratigraphies

from sediment cores collected in 21 aquatic ecosys-

tems in Florida. These profiles were generated with

gamma detectors. A number of the cores display

fairly low and constant radium activities in all stra-

tigraphic levels whereas others exemplify the

problem that arises when 226Ra-rich particulate

material or radium-bearing groundwater enterslakes. In the latter case, resultant high and variable226Ra activities may violate assumptions of 210Pb

dating models. We discuss the implications with

respect to dating such profiles. We also suggest

alternative ways that radioisotope data might be

used to at least provide a general chronological

framework for recent sediments from such lakes.

210Pb in the environment

Lead-210 is a radioisotope that is a member of the238U decay series. Uranium-238 has a half-life of

4.5 � 109 year and decays through several daugh-

ters to 226Ra, which has a half-life of 1600 year.

Radium-226 in rocks, soil, and seawater decays to

radon gas (222Rn, t1/2 ¼ 3.8 day), some of which

escapes to the atmosphere. Radon gas undergoes

decay through a series of short-lived daughtersto yield particulate 210Pb, which has a half-life of

22.3 year. This 210Pb in the atmosphere can be

delivered to the earth via rainfall (washout) or dry

fallout (El-Daoushy 1988). Atmospheric 210Pb is

ultimately delivered to waterbodies primarily by

direct fallout, but a fraction of the unsupported210Pb that reaches lakes may also be delivered via

transport from the watershed. After entering awaterbody, particle-reactive 210Pb adsorbs quickly

to materials in the water column and is deposited

on the lake bottom. This 210Pb will decay to 210Bi

(t1/2¼ 5.01 day) which, in turn, decays to 210Po that

has a half-life of 138 day. Decay of 210Po ultimately

yields stable 206Pb. The stratigraphic distribution

of unsupported 210Pb activity must be estimated for

sediment core dating.Presence of supported 210Pb in sediments com-

plicates the dating procedure. Atmospherically

derived (unsupported or excess) 210Pb must be

distinguished from supported 210Pb, which is

produced by decay-series precursors within or

attached to the sediment matrix. Supported 210Pb

activity is assumed to come from in situ 226Ra, with

which it is generally believed to be in secular equili-brium. This 226Ra in the sediments typically comes

from local soil and bedrock particles, which are

delivered to the lake by colluviation, alluviation,

or aeolian transport.

210Pb measurement

Unsupported 210Pb activity can be estimated by

two methods, alpha or gamma counting. Both

approaches require that total 210Pb activity be mea-sured in each stratigraphic sample down to a depth

where no measurable unsupported 210Pb remains.

Supported 210Pb activity, measured or approxi-

mated by an appropriate means, is subtracted

from the total activity to yield an estimate of

unsupported 210Pb activity. The alpha and

gamma counting methods vary with respect to

how total and supported 210Pb activities areassessed. Total 210Pb activity can be measured by

alpha counting of the granddaughter radionuclide,210Po (Eakins and Morrison 1978). This method

has several advantages. The high sensitivity of

alpha detectors makes them suitable for counting

samples with low activity. Small amounts of sample

can therefore be used, which permits high-resolution

(i.e., small depth interval) counting, even in sedi-ments with low bulk density. Furthermore, alpha

detectors are relatively simple and inexpensive

(Appleby 2001). Nonetheless, the alpha counting

54

procedure has several drawbacks. First, samples

must be digested in acid and the acid-extracted210Po must be plated on metal (Ag or Cu) disks.

Second, samples are destroyed during this chemical

process. Third, the wet chemistry method requiresuse of a yield tracer (208Po) to assess the efficiency

of radioisotope plating on the metal planchets.

Fourth, topmost sediments should be held for

almost 2 years before counting to guarantee that

equilibrium between 210Pb and measured 210Po is

achieved. Lastly, supported 210Pb activity is not

measured directly with alpha counters. Instead, it

is generally estimated by the down-core, asymptotictotal 210Pb activity. Typically, it is assumed that

supported 210Pb activity has remained unchanged

over the past 100–150 years. Less commonly, 226Ra

activity is measured by an independent analytical

technique, allowing for subtraction of supported210Pb activity from the total 210Pb activity (i.e.,210Po) on a level-by-level basis.

In the past two decades, low-background, well-type germanium detectors have come into wider

use. These detectors enable assessment of 210Pb

activity in sediments by measurement of gamma

emissions at 46.5 keV. They also provide the

advantage of simultaneously measuring the

gamma ray emissions of other radionuclides. For

instance, 226Ra activity can be evaluated by aver-

aging the 295 and 352 keV gamma ray peaks ofdaughter 214Pb, along with the 609 keV peak of214Bi (Moore 1984). Dried, ground sediment sam-

ples are sealed into counting tubes with epoxy glue

for three weeks to trap any emitted 222Rn gas and

establish equilibrium between parent 226Ra and

the proxies (daughters) used to estimate its activity.

This step mimics conditions within the lake, where

any short-lived radon gas that escapes the mineralmatrix probably does not diffuse up or down very

far in the sediment pore waters. Once a sample is

dried for analysis, the presumably small amount of222Rn that escapes the mineral fraction could be

lost easily to the atmosphere through open spaces

among the sediment grains. Following gamma

counting, whole, dry sediment samples can be

removed from tubes and used for other analyses.The gamma counting method also permits simulta-

neous measurement of anthropogenic radionu-

clides such as 137Cs (Krishnaswami and Lal 1978)

and 241Am (Appleby et al. 1991), that may serve as

horizon markers to check calculated 210Pb dates. In

many cases, the advantages of this technique out-

weigh the disadvantages. The latter include the

high cost of detectors, a need for relatively large

sample masses, difficulties associated with 210Pb

efficiency calibration, and problems with samplegeometry and self-absorption (Schelske et al.

1994; Appleby 2001).

Dating models

Two 210Pb dating models have come into general

use. The constant initial concentration (CIC)

model assumes that lakewater has a substantial

reservoir of excess, that is, unsupported 210Pb.According to this model, the water column con-

tains abundant 210Pb, regardless of the bulk sedi-

ment accumulation rate. Sediment scavenging of

the radioisotope is proportional to the rate of sedi-

ment deposition, which proceeds in such a manner

that surface sediments always have the same initial

activity – hence the model name. Alternatively,

the constant rate of supply (CRS) model assumesthat the input of excess 210Pb to the sediments

has remained constant through time. Under this

scenario, high rates of bulk sediment accumulation

dilute incoming excess 210Pb, yielding low activities

in deposits. Conversely, slower bulk sedimentation

rates result in relatively higher 210Pb activities in

the accumulating sediment.

210Pb and 226Ra in Florida lake cores

Binford et al. (1993) selected the CRS model for

dating sediment cores from Florida lakes that were

collected for the Paleoecological Investigations of

Recent Lake Acidification (PIRLA) project. There

were two principal reasons for choosing the CRS

model. First, ongoing studies at the time (e.g.,Deevey et al. 1986) indicated sediment accumula-

tion rates had varied in Florida lakes over the last

century. Second, unsupported 210Pb activities in

surface sediments of Florida lakes were inversely

related to trophic state and, presumably, organic

sediment accumulation rate (Binford and Brenner

1986). This indicated a ‘‘dilution’’ of excess 210Pb by

accumulating organic matter and suggested fairlyconstant delivery of excess 210Pb to the sediments.

In the PIRLA study (Binford et al. 1993), 12

cores from six north Florida lakes were dated

55

using alpha counters. Activities in basal samples

from the profiles were low, with total 210Pb activ-

ities generally <1.0 pCi g�1 (i.e., <2.2 dpm g�1).

The objective of the study was to infer the pH

history of basins thought to have been susceptibleto acid precipitation inputs. Hence, relatively

undisturbed, softwater lakes were examined, of

which five had pH values ranging from 4.48 to

5.07, and one had a pH of 6.33 (Sweets et al. 1990).

In the early 1990s, we began to use paleolimno-

logical techniques to explore the trophic state his-

tories of eutrophic and hypereutrophic Florida

lakes and marshes (e.g., Brenner et al. 1993, 1995,1996, 1999a,b, 2001). Radioisotope activities in

cores from these waterbodies were measured with

gamma detectors (Schelske et al. 1994). Sediment

cores from some lakes displayed high and variable226Ra activities, particularly in sediments deposited

within the last century. We found a strong strati-

graphic correlation between 226Ra activity and

total phosphorus (P) content in 12 cores fromeight lakes and both variables generally displayed

higher values in very recent deposits (Brenner et al.

1997). We suggested that this correlation reflected

recent enhanced delivery to the lakes of wind and

water-borne particles that were rich in both 226Ra

and total P. A number of human activities, includ-

ing phosphate mining, farming, road building,

and home construction disturb soil and bedrock,and promote the mobilization and transport of

inorganic particulate matter from land to water.

Additionally, sewage disposal might be a mechan-

ism for delivery of dissolved radium and

phosphorus to waterbodies, especially urban lakes.

Based on these findings, we suggested that the high226Ra activities in Florida lake cores might be an

indicator of human disturbance in watersheds. Wealso concluded that gamma counting, which

enables simultaneous evaluation of 226Ra activities,

is the required method for 210Pb dating cores from

Florida lakes (Schelske et al. 1994).

Methods

Sediment cores were collected from a marsh and

20 lakes throughout central Florida (Figure 1).Cores were taken with a piston corer designed to

retrieve undisturbed sediment–water interface pro-

files (Fisher et al. 1992). Sediments were generally

extruded vertically, in the field, to avoid sediment

mixing. Stratigraphic samples were collected in a

tray attached to the top of the core barrel.Sectioning intervals varied among cores (2, 4, or

5 cm). Extruded material was transferred to labeled

plastic bags or cups and transported to the labora-

tory where it was refrigerated or frozen prior to

processing. Wet samples were weighed and then

dried in an oven or lyophilizer. Dry mass in each

sample was determined, and dry sediment was

ground in a mortar and pestle for analyses.Weighed sub-samples of dry sediment were

placed in plastic SarstedtTM tubes to a height of

30 mm. A correction for sample geometry (height

correction) was applied in the few cases where there

was insufficient material to fill the tube to the

30 mm height (Schelske et al. 1994). In a few

cases, small sample masses in uppermost deposits

required combining adjacent sample depths. Inthese cases, masses were combined in proportion

to their bulk densities (g dry cm�3 wet). About 1 cm

of epoxy glue was added to the tubes to trap 222Rn

gas that might emanate from in situ 226Ra decay

Figure 1. Florida map showing the locations where sediment

cores were collected. Numbers refer to specific waterbodies as

follows: (1) Rowell, (2) Newnans, (3) Orange, (4) Washington,

(5) Sawgrass, (6) Hell ‘n’ Blazes, (7) Blue Cypress Lake, (8) Blue

Cypress Marsh, (9) Persimmon, (10) Little Jackson, (11)

Howard, (12) Lucerne, (13) Marianna, (14) Hollingsworth,

(15) Parker, (16) Thonotosassa, (17) Round, (18) Halfmoon,

(19) Dosson, (20) Clear, and (21) Panasoffkee.

56

within the sediment. Tubes were allowed to set for

at least 20 days before counting to ensure secular

equilibrium between 226Ra and the daughter

nuclides that are counted to estimate its activity

(214Pb and 214Bi). Total 210Pb, 226Ra and 137Csactivities in 20 cores were measured with

ORTECTM intrinsic well-type germanium detec-

tors (i.e., gamma counters) connected to a 4096-

channel, multi-channel analyzer (Appleby et al.

1986; Schelske et al. 1994). The Orange Lake core

was measured with a CanberraTM well-type germa-

nium detector. Total 210Pb activity was obtained

from the photopeak at 46.5 keV. Radium-226activity was estimated from the 214Bi photopeak

at 609.3 keV (Schelske et al. 1994), or by averaging

the activities of 214Pb (295.1 keV), 214Pb (351.9keV),

and 214Bi (609.3 keV) (Moore 1984). Cesium-137

activity was determined from the 661.7 keV photo-

peak in an attempt to identify the ca. 1963 fallout

maximum that was a consequence of atmospheric

bomb testing (Krishnaswami and Lal 1978). Allactivities are expressed as decays per minute per

gram dry sediment (dpm g�1).

Results

Sediment cores from nine Florida lakes display

relatively low and constant 226Ra activities over

the lengths of the sections (Figure 2). Within each

of these profiles, 226Ra activities are generally<5 dpm g�1. These activities are low relative to

the highest measured total 210Pb activities and typi-

cally represent <15% of the total 210Pb activities in

surface sediments. Lake Panasoffkee is an excep-

tion. Although it has relatively low 226Ra activity,

about 2–4 dpm g�1, it also displays low total210Pb activity throughout the core. Panasoffkee is

unique among the study lakes in being partiallyspring-fed, and possessing dense, carbonate-rich

deposits. The other study lakes have sediments

dominated by organic matter and quartz sand.

Stratigraphic variability in 226Ra activity is also

low in these nine cores, with values generally fluc-

tuating by <1 dpm g�1 about the mean (Figure 2).

In these profiles, there is little difference in calcu-

lated dates, regardless of how supported 210Pb isestimated (Table 1 and Figure 4). For instance,

excess 210Pb activity can be calculated by subtracting

supported 210Pb (i.e., 226Ra) activity from total

210Pb activity on a level-by-level basis. Alter-

natively, the mean supported 210Pb activity for the

whole core could be subtracted from each total210Pb measurement. Finally, the mean, down-

core, asymptotic 210Pb value could be subtractedfrom each total 210Pb measurement, as might have

been done had the samples been measured by alpha

counting. All three approaches yield similar dating

outcomes. In these lakes, alpha counting would

provide reasonable core chronologies. It is not

known a priori, however, whether 226Ra activities

remain relatively low and constant over the length

of the cores. Therefore, gamma counting is recom-mended for Florida cores and for profiles from any

system in which the 226Ra stratigraphy is unknown.

Sediment cores from 12 lakes are characterized

by high and variable 226Ra activities (Figure 3) and

illustrate the need for gamma counting. With the

exception of the Lake Washington section, all

the profiles contain stratigraphic samples with

>5 dpm g�1. More importantly, all the coresdisplay stratigraphic changes in 226Ra activity,

with ranges sometimes exceeding an order of mag-

nitude. These profiles illustrate the problematic

nature of using the measured down-core 226Ra

activity or total 210Pb activity as an estimate of

supported 210Pb throughout the section (see Table 1

and Figure 4). Radium-226 activity exceeds total210Pb activity in near-surface samples from theRound Lake and Lake Rowell cores (Figure 3),

and provides definitive evidence for disequilibrium

between 226Ra and supported 210Pb.

Discussion

Several processes were proposed to account for the

disequilibrium in recent Lake Rowell deposits(Brenner et al. 1994). First, high 222Rn loss during

transport of 226Ra-rich soil particles in the

watershed could account for the problem. When

these detrital particles reach the lake, they require

substantial time for in-growth of supported 210Pb.

Equilibrium is established quickly between in situ226Ra and the daughters employed to estimate it

(214Pb and 214Bi) once the Sarstedt tubes are sealed,but in-growth of 210Pb requires a much longer time,

dictated by its 22.3-year half-life. Assuming that

there is no supported 210Pb in the sediment at the

57

time of deposition, it will require approximately

110 years, or about five 210Pb half-lives for 226Ra

and supported 210Pb to equilibrate. Second, a

similar problem may occur if radon gas is lost

from near-surface sediments once they are depos-

ited in the lake. Only when the sediments are buriedsufficiently will 222Rn gas be trapped effectively in

place. Again, substantial time is required to bring

the supported 210Pb into equilibrium with 226Ra.

Lastly, dissolved radium that was delivered to the

lake in recent years may adsorb to particles in the

water column and to surface sediments. This

dissolved 226Ra, separated from its parent radio-

nuclide and later adsorbed to lake sediment, will

require about 110 years (i.e., five 210Pb half-lives)

to equilibrate with supported 210Pb.

This latter scenario almost certainly accounts for

the pronounced disequilibrium between 226Ra and

supported 210Pb in recent Round Lake sediments(Figure 3). Round Lake lies in Hillsborough

County, where long-term pumping of the

Floridan Aquifer caused stage declines in some

local waterbodies. In 1966, Round Lake began to

receive groundwater supplements (hydrologic

Figure 2. Total 210Pb, 226Ra, and 137Cs activities in sediment cores from eight Florida lakes and the Blue Cypress Marsh. Activities are

plotted at the mid-depth for each sample interval. Depth and activity scales vary among the plots. These cores are characterized by

generally low 226Ra activities and display minor stratigraphic variability in 226Ra activity.

58

augmentation) to maintain its water level (Stewart

and Hughes 1974). Water was provided from a

deep well drilled into the Floridan Aquifer.

Geologic deposits in the area contain carbonate-

fluorapatite deposits that are rich in 238U

(Kaufmann and Bliss 1977; Upchurch and

Randazzo 1997). These 238U-rich deposits giverise to 226Ra, which dissolves readily in ground-

water. Radium-226 activity in groundwater that

enters the lake is on the order of 6.2 dpm L�1

(Brenner et al. 2000). Dissolved radium binds to

water-column particles and surface sediments,

accounting for the high 226Ra activities in recent

deposits of hydrologically augmented lakes.

Although few lakes in Florida are deliberatelyaugmented with groundwater, numerous water-

bodies throughout the state probably receive inad-

vertent groundwater inputs. This groundwater is

pumped to the surface for domestic, agricultural

(irrigation), industrial, and mining use, and enters

lakes in runoff and seepage. Pumped groundwater

may represent an important pathway for dissolved226Ra to enter some Florida lakes. Several factors

control whether a lake will receive significant

amounts of 226Ra-rich groundwater. These

include: (1) the concentration (activity) of 226Ra

in the local aquifer, (2) the quantity of water

withdrawn from the aquifer, and (3) the efficiency

of 226Ra transfer from pumped groundwater to

the lake. If lakes lack overland outflows, as domany Florida waterbodies, most of the 226Ra that

reaches a lake may ultimately find its way into the

sediments.

If large quantities of dissolved 226Ra were incor-

porated into the sediments of some Florida lakes in

recent decades, what are the implications for 210Pb

dating cores from these lakes? Introduction to

lakes of dissolved 226Ra that is not in equilibriumwith 210Pb creates dating problems because it

compromises accurate estimation of supported210Pb activity. The magnitude of disequilibrium

will vary with depth (age) in the section.

This problem is illustrated clearly by the Round

Lake and Lake Rowell radioisotope stratigraphies

(Figure 3). In the uppermost deposits from these

two cores, 226Ra activity exceeds total 210Pb activity,

Table 1. Results of the CRS dating model when applied to four sediment cores from Florida lakes.

Newnans (cm) Little Jackson (cm) Persimmon (cm) Rowell (cm)

Depth Date 1 Date 2 Depth Date 1 Date 2 Depth Date 1 Date 2 Depth Date 2

0 1997 1997 0 1996 1996 0 1998 1998 0 1992

4 1996 1996 5 1993 1994 5 1991 1989 2 1991

8 1994 1994 10 1989 1990 10 1980 1976 4 1989

12 1992 1992 15 1984 1985 15 1967 1956 6 1987

16 1989 1989 20 1978 1979 20 1950 1926 8 1984

20 1985 1985 25 1972 1972 25 1934 1899 10 1981

24 1981 1982 30 1964 1964 30 1921 1873 12 1978

28 1976 1977 35 1951 1950 35 1911 1865 14 1974

32 1971 1972 40 1935 1931 40 1901 1865 16 1969

36 1964 1965 45 1907 1895 18 1964

40 1957 1957 20 1958

44 1945 1946 22 1952

48 1930 1930 24 1946

52 1914 1913 26 1938

56 1904 1903 28 1928

30 1917

32 1908

34 1898

All dates were rounded to the nearest year. For the first model run (Date 1), unsupported 210Pb activity was calculated as total 210Pb

activity minus the supported 210Pb activity (226Ra activity) measured at each depth. For the second calculation (Date 2), unsupported210Pb activity was computed by subtracting the mean, down-core, asymptotic total 210Pb activity from total 210Pb activity, as would be

done if total 210Pb activity had been measured by alpha counting and there were no independent estimate of 226Ra activity. A Date 1

calculation could not be run for the Rowell core because 226Ra activity exceeded total 210Pb activity in the upper samples from the core.

See Figure 4.

59

indicating obvious, pronounced disequilibrium.

Total 210Pb activities are not particularly low in

either core, and the total 210Pb activity in near-

surface sediments presumably contains a significant

amount of unsupported, that is, atmospherically

derived 210Pb. Nevertheless, the 226Ra activity

near the tops of these cores exceeds the activity of

the combined sources of both unsupported and

Figure 3. Total 210Pb, 226Ra, and 137Cs activities in sediment cores from twelve Florida lakes. Activities are plotted at the mid-depth for

each sample interval. Depth and activity scales vary among the plots. Some cores show high and variable 226Ra activities, and most

display an up-core increase in 226Ra content. Radioisotope disequilibrium is evident in the Round Lake and Lake Rowell cores, where226Ra activity exceeds total 210Pb activity.

60

supported 210Pb. We argue that dating such

profiles requires several indefensible assumptions

(Brenner et al. 1994) and these cores are better

deemed ‘‘undatable’’, at least using traditional

approaches.

Perhaps even more problematic are cores thatdisplay high and variable 226Ra activity, but in

which 226Ra activity remains lower than total210Pb activity throughout the entire section

(Figure 3). If all the 226Ra were associated with

detrital material, in which 222Rn were trapped

quantitatively in the mineral matrix, one could

safely assume that supported 210Pb had equili-

brated with the measured 226Ra. In such cases,

subtraction of the 226Ra activity (supported 210Pb

activity) from measured total 210Pb activity on a

level-by-level basis is justified and should yield anaccurate estimate of unsupported 210Pb activity

throughout the core. If, however, the 226Ra in det-

rital material is not in equilibrium with 210Pb, or if

inputs of dissolved radium contributed to the high

and fluctuating 226Ra activity in a sediment core,

Figure 4. Date–depth plots for four Florida lakes: (a) Newnans, (b) Little Jackson, (c) Persimmon, and (d) Rowell. Dates were

calculated using the CRS model. Closed circles (*) represent results of model runs in which unsupported 210Pb activity was

calculated as total 210Pb activity minus supported 210Pb activity (226Ra activity) measured at each depth. Open squares (&) represent

results of model runs in which the mean down-core, asymptotic total 210Pb activity was subtracted from each total 210Pb value to obtain

the unsupported activity for each depth, as is required if total 210Pb activity is measured by alpha counting. For the Newnans core, with

relatively low and constant 226Ra activity, the two approaches yield nearly identical results. Little Jackson shows a slight up-core

increase in 226Ra activity. Results of the two approaches only begin to diverge about 1900. Persimmon displays a dramatic up-core 226Ra

increase and the two dating approaches yield very different ages in sediments older than several decades. For such cores, neither

approach is arguably correct (see Figure 5). Lastly, had total 210Pb activity in the Rowell core been measured by alpha counting, a

perfectly reasonable date-depth plot would have been generated, but would have certainly been in error given the high and fluctuating226Ra in this core.

61

then accurate estimates of supported 210Pb activity

are difficult or impossible to obtain.

We employed the CRS dating model to generate

date–depth relations for four selected cores in our

data set. The outcomes illustrate the necessity forgamma counting in Florida (Table 1 and Figure 4).

For three cores (Newnans, Little Jackson,

and Persimmon), unsupported 210Pb activity was

estimated in two ways. For the first model run,

measured 226Ra activities were subtracted from

total 210Pb activities on a level-by-level basis. For

the second model run, supported 210Pb activity was

estimated by the mean of down-core, asymptotictotal 210Pb activities, as if the cores had been alpha-

counted and there were no independent measures

of 226Ra activity. The fourth core (Rowell) was

‘‘dated’’ using only the latter approach, because226Ra activity exceeded total 210Pb activity in

uppermost deposits. The Newnans lake core dis-

played relatively low and constant 226Ra activity.

Consequently, the two dating approaches yield vir-tually identical results, with less than a 1-year dis-

crepancy at each dated interval over the past �100

years. Little Jackson shows a slight up-core

increase in 226Ra activity and the two dating

approaches yield similar results for the upper part

of the section, but differ by about a decade around

1900. In the above two cases, similar dates would

have been obtained had either alpha or gammacounting been done. Nevertheless, there would

have been no a priori way to know that 226Ra

activities did not display high stratigraphic varia-

bility without the results of gamma counting.

Lake Persimmon displays a pronounced up-core

increase in 226Ra activity, and the two dating

approaches yield rather divergent outcomes. At

some depths, ages calculated by the two methodsdiffer by as much as 48 years. We note that neither

approach is arguably correct. In fact, both

approaches likely yield erroneous dates.

Nevertheless, gamma counting alerted us to the

potential problem with dating this section. The

Rowell core, ‘‘dated’’ without regard to its high

and stratigraphically variable 226Ra activities,

yields what appears to be a perfectly acceptableage–depth profile, but that is almost certainly

wrong.

The problem created by dissolved radium input

is best understood if one considers a hypothetical

case in which all the elevated 226Ra activity in

recent deposits (i.e., 226Ra above ‘‘background’’

activities in deep deposits) comes from input of

dissolved 226Ra (Figure 5). In such a situation, the

only 226Ra in surface sediments that would be in

Figure 5. Radioisotope plot illustrating how 226Ra/supported210Pb disequilibrium causes dating errors. Hypothetical

activities are plotted at the mid-point of 2-cm intervals and are

based on gamma counting a sediment core from a lake that began

to receive inputs of 226Ra-rich groundwater ca. 1960 (arrow at 9

cm depth). Assumptions include: (1) input of 226Ra-containing

mineral particles has been low and relatively constant, (2) detrital226Ra is in equilibriumwith 210Pb, (3) input of dissolved 226Ra has

been relatively constant since it began, and (4) sediment

accumulation has been fairly steady. Had this core been dated

by alpha counting, the down-core asymptotic total 210Pb activity

(�2 dpm g�1) would have been used to estimate supported 210Pb

activity and subtracted from each total 210Pb value over the

length of the core to calculate excess 210Pb activity. This

approach fails to account for supported 210Pb coming from

dissolved 226Ra in groundwater. Alternatively, with gamma

counting, 226Ra activities (open circles and squares) are typically

used to estimate supported 210Pb and would be subtracted from

total 210Pb values on a level-by-level basis over the length of the

core to estimate excess 210Pb activity. This approach fails to

recognize that radium that entered the lake in dissolved form is

not in secular equilibrium with supported 210Pb. Triangles denote

actual supported 210Pb activities, derived from both detrital 226Ra

and ingrowth of 210Pb from adsorbed radium. 226Ra-rich surface

sediments lack supported 210Pb from groundwater-borne 226Ra,

but there is substantial ingrowth of 210Pb from this source in

deeper, older sediments. Hence, mis-estimation of supported210Pb activity changes with sediment age. The true supported210Pb values that should be subtracted from total 210Pb activity

are described by the horizontal bars (open squares and triangles)

at each sample depth. It is, however, no simple matter to

determine the supported 210Pb values denoted by triangles.

62

equilibrium with supported 210Pb is the small

amount associated with the ‘‘background’’, detrital

component. Nevertheless, one would measure high226Ra activity in the surface sample. Once the sam-

ple is sealed inside a Sarstedt tube, all the 226Ra inthe surface slice, much of which entered the lake in

dissolved form, would quickly (�3 weeks) equili-

brate with the proxies used to estimate radium

activity (214Pb and 214Bi). Subtraction of this

apparently high 226Ra (supported 210Pb) activity

from the total 210Pb activity would yield an under-

estimate of unsupported 210Pb activity in the sur-

face sample.At greater depths in a sediment column, dis-

solved 226Ra that adsorbed to particles will have

begun the process of establishing equilibrium with

supported 210Pb, assuming that 222Rn gas pro-

duced by decay of adsorbed 226Ra remains in stra-

tigraphic position (see Imboden and Stiller 1982).

If, for purposes of illustration, one assumes that:

(1) sediment accumulation rate remained constant,(2) anthropogenic dissolved 226Ra input has been

constant since it began�50 years ago, and (3) there

has been no post-depositional migration of radium

or its daughters, then there will be systematic error

in the calculation of unsupported 210Pb activity.

Deeper in the section, sediments will be approach-

ing 226Ra/supported 210Pb equilibrium, whereas

virtually none of the dissolved radium will havegiven rise to supported 210Pb at the sediment sur-

face. Errors in the 226Ra-based estimate of sup-

ported 210Pb will hence increase up-core (Figure 5).

That is to say, the magnitude of underestimation of

unsupported 210Pb increases up-core. When the

CRS model is applied, this systematic error will

result in an apparent increase in bulk sediment

accumulation rate through time. This, of course,will also influence the calculated accumulation

rates of sediment constituents such as nutrients or

microfossils.

If the assumed conditions in the above example

could be met, it might be possible to model the

stratigraphic distribution of supported 210Pb.

Unfortunately, these assumptions probably do

not hold in most situations and it is thus not asimple matter to discern the degree of disequili-

brium between 226Ra and supported 210Pb at var-

ious depths. Several processes complicate the

matter. First, input rates of dissolved 226Ra to

Florida lakes change over time as a consequence

of shifts in the amount of groundwater pumping.

Second, bulk sedimentation rates in Florida lakes

have fluctuated due to changing human activities in

watersheds. Third, 222Rn gas may be fairly mobile

(ImbodenandStiller 1982) inFlorida’s high-porositylake sediments. Lastly, human-mediated delivery

of detrital, 226Ra-containing particles probably

varied in recent decades as a consequence of chan-

ging disturbances in the drainage basin.

Although anthropogenic input of dissolved226Ra causes dating complications in some

Florida lakes, it is not problematic in all cases.

Radioisotope stratigraphies in Figure 2 illustratethat many Florida lakes do not receive substantial

inputs of 226Ra. Lake Panasoffkee is a ground-

water-fed system, but geologic deposits through

which its inflow waters pass are evidently low in

radium content. Many of the other waterbodies,

Blue Cypress Marsh being one of the best exam-

ples, have hydrologies that are almost certainly

dominated by surface waters. It is also unlikelythat the dated cores in the PIRLA project

(Binford et al. 1993) were affected by dissolved226Ra inputs because the lakes all display low pH

values, even at present. Lakes that receive 226Ra-

rich Floridan aquifer supplements have relative ion

concentrations similar to groundwater and are high

in pH, bicarbonate concentration, and hardness

(Martin et al. 1976; Dooris and Martin 1979).There are several options for dealing with cores

that display high and variable 226Ra activity. In

some regions, a 137Cs peak can be used to identify

the period of maximum atmospheric cesium fall-

out, ca. 1963 (Krishnaswami and Lal 1978).

Together with the supported/unsupported 210Pb

boundary (�1900), one could at least assign two

dated horizons to a profile. Unfortunately, the1963 137Cs peak is preserved poorly or not at all

in many Florida lake and wetland sediments

(Figures 2 and 3). Most Florida deposits are domi-

nated by organic matter and quartz sands, and lack

2 : 1 lattice clays that are required to bind highly

mobile cesium. Alternatively, if one knew with cer-

tainty when groundwater pumping began, it might

be possible to assign a date to the onset of the risein 226Ra activity.

Experiments might also be done to ascertain

whether a large proportion of 226Ra counted in

sediments has equilibrated with supported 210Pb.

If stratigraphic variation in 226Ra activity were

63

due solely to fluctuations in the content of mineral

particles, which probably trap 222Rn gas fairly

effectively, then 226Ra and supported 210Pb should

be in equilibrium throughout a core. To test this,

one could fill a Sarstedt tube with a dry sedimentsample, seal it with epoxy glue, and count it imme-

diately by gamma spectroscopy for 226Ra proxies

(i.e., 214Pb and 214Bi). The same sample could be

counted again after 3 weeks. If there is little differ-

ence between the two counts, it suggests that there

is secular equilibrium between 226Ra and supported210Pb. If, however, there is a large difference in the

activities between the two counts, this disparityrepresents in-growth of daughters 214Pb and 214Bi

over the time since the sediment sample was sealed.

A series of counts on samples from different strati-

graphic depths could then be used to determine the

degree of disequilibrium with 210Pb at each level in

the core. In cases where much of the radium enters

the lake in dissolved form, one might expect to find

the highest disparities, that is, greatest disequili-brium, in very recent sediments with high activities

coming largely from adsorbed 226Ra.

Another analytical procedure may provide

insight into whether a large component of 226Ra

activity in the lake sediment comes from the

adsorbed fraction. Sub-samples of dried sediment

from each stratigraphic level could be sealed in

Sarstedt tubes, equilibrated for 3 weeks, andcounted by gamma spectroscopy. A second set of

sub-samples could be soaked in a saline solution

(e.g., NH4Cl or NaCl) to de-sorb adhering radium.

After removing the supernatant, the samples could

be dried, sealed in Sarstedt tubes, and counted after

3 weeks. If sediments possess high concentrations

of adsorbed 226Ra, untreated samples should dis-

play high measured 226Ra activity whereas samplesthat were soaked in saline solution will have lost

this labile radium and will display lower activities.

Although this method cannot establish the degree

of disequilibrium in each stratigraphic sample, it

may help discern whether adsorbed 226Ra activity

is significant in a sample. Use of this approach may

not be appropriate in lakes where authigenic car-

bonate formation may sequester dissolved radiumwithin particles that are deposited on the lake bot-

tom. Furthermore, interpretation of this procedure

may be complicated if some 226Ra associated with

the mineral matrix is de-sorbed, even by a weak salt

solution.

Input of 226Ra-rich groundwater to lakes

presents special difficulties for 210Pb dating recent

sediments. Groundwater-augmented Round Lake

clarified the issue, in that it is deliberately augmen-

ted with water that contains high quantities of226Ra and a large fraction of its hydrologic budget

comes from the deep aquifer (Stewart and Hughes

1974). Radium-rich groundwater input may also

account for high and variable activities in recent

deposits of many Florida lakes. It is unlikely that

this problem is restricted to Florida. In any region

where local bedrock has substantial activities of

radionuclides in the 238U series, groundwaterdischarge to lakes, whether natural or human-

mediated, may confound assessments of supported

and unsupported 210Pb. Groundwater inputs of

radionuclides other than 226Ra can also create

dating problems. For instance, in some Rocky

Mountain lakes, groundwater-borne 222Rn yielded

unusually high influxes of unsupported 210Pb to

sediments (Norton et al. 1985). We strongly recom-mend that gamma detectors be utilized to count

sediment samples in districts with radium-rich bed-

rock. This counting method is not a panacea, in

that it cannot necessarily solve the difficulties

associated with high and variable 226Ra activity,

or yield reliable 210Pb dates. Nevertheless, it can

alert investigators to the fact that a problem exists

and will help avoid erroneous dating of cores thatmight otherwise appear to have a valid excess 210Pb

inventory.

Acknowledgements

We thank our many colleagues who assisted with

core collection, sediment processing, and radioiso-

tope activity measurement. They include Jaye

Cable, Pete Cable, Jason Curtis, Jason Kahne,

Lawrence Keenan, Margaret Lasi, Doug Leeper,

Arthur Peplow, Donny Smoak, and Tom

Whitmore. Some cores were analyzed under pro-jects funded by the St. Johns River Water

Management District, the Southwest Florida

Water Management District, the US Geological

Survey, and the Highlands County Commission.

We thank Yongsong Huang for helpful comments

on the manuscript and Dan Engstrom for discus-

sions relating to radium in groundwater. A version

of this paper was presented at the 9th International

64

Symposium on Paleolimnology, Espoo, Finland,

24–28 August 2003.

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