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On the formation of perylene in recent sediments: kinetic models
On the formation of perylene in recent sediments: kinetic models PHILIP M. GSCHWEND~, PAUL H. CHEN and RONALD A. MITES* School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405 (Received February 17, 1983; accepted in rwised.form August 26, 1983)
Abstract-The concentration of perylene increases regularly with depth in a radiometrically dated sediment core from Mountain Pond, Cobum Mountain, Maine, U.S.A. This concentration profile was fit to simple reaction kinetic models which indicate that perylene forms in this core either by a first order reaction characterized by a rate constant of 0.012 yr-* or by a second order reaction characterized by a rate constant of 0.00 13 nmole-‘g yr-‘.
INTRODUCTION PERYLENE IS A pentacyclic aromatic hydrocarbon which has been found in many recent sediments (ORR and GRADY, 1967; BROWN el al., 1972; AIZENSHTAT, 1973; ISHIWATARI and HANYA, 1975; WAKEHAM, 1977; LAFLAMME and HITES, 1978; PRAHL and CARPENTER, 1979; WAKEHAM el al., 1979; HITES et al., 1980; WA~HAM et al., 1980a,b; TAN and HEIT, 1981; PRAHL and CARPENTER, 1983). It is, in fact, unusual not to find perylene as /\ \I m
/\ / , _
perylene
the most abundant polycyclic aromatic hydrocarbon (PAH) in sections of anoxic cores over 100 years old. Perylene may be introduced into these sediments from several sources: (a) the inclusion ofcombustionderived PAH, (b) the input of weathered sedimentary rocks confining perylene, (c) the inco~ration of PAH from fossil fuels such as petroleum, or (d) in situ production by diagenesis. The last source clearly predominates in older core sections of anoxic sediments (see references cited above). This diagenetic source has been an intriguing mystery both in terms of the identity of the precursor and the mechanism by which the transformation occurs. Early on, there had been a consensus that perylene was formed only under reducing conditions from a terrigenous precursor (AIZENSHTAT, 1973). However, its occurrence in Walvis Bay sediment (a site thought to be largely free of inputs of terrestrial organic matter) led WAKEHAM et a/. (1979) to question whether the precursor must be terrestrial. In addition, HITES et al. (1980) found high perylene concentrations in an anoxic, coastal, diatomaceous ooze, and therefore. suggested that these phytoplankton may
t Present Address: Department of Civil Engineering, Room 48-42 1, Massachusetts Institute ofTechnology, Cambridge, MA 02 I39 * To whom correspondence should be addressed.
produce the precursor. In spite of these observations, all that we may presently say about perylene’s precursor is that it has a widespread dist~bution, and that the diagenetic formation of perylene requires deposition of this precursor into suitably reducing sites. In the course of an investigation of PAH fluxes to sediments in the northeastern United States (GSCHWEN~ and HITES, 198 l), we were fortunate to obtain samples from a lacustrine core which, based on extensive geochemical examination by other workers (NORTON et al., 1981: HANSON et al,. 1982), proved to be very well suited for a study of the diagenetic formation of perylene. The sediment was anoxic below about two centimeters and showed no evidence of deep (~2 cm) bioturbation. Additionally, the deposit accumulated regularly and had no major compositional variations. Finally, the location of the pond precluded large interfering inputs of anthropogenic perylene or perylene from weathered rocks. This core gave perylene data which was examined in terms of a rate constant of formation, the initial precursor concentration deposited in the sediment, and the molecularity of the diagenetic reaction. We believe this core provided a unique opportunity as an in situ “reaction vessel” which allowed the kinetics of this organic geochemi~al transformation to be evaluated. EXPERIMENTAL The sediment core (10 cm dia.) was obtained in July, 1978 from the deepest part of Mountain Pond, Cobum Mountains Maine, U.S.A. (48”28‘N X 70”8W) by Drs. Stephen A. Norton and Ronald B. Davis of the University of Maine at Orono (NORTONet al.. 1981; HANSON et ul., 1982). This high altitude pond (837 m above sea level) is small (2.6 hectares), shallow (about 3 meters maximum depth), and oligotrophic. Mountain Pond was initially chosen for study because the neighboring region is fairly pristine, forested primarily with spruce and fir, although some cutting may have occurred approximateIy 40 to 50 years ago. Extensive geochemicai data have been obtained for this core including: water content, organic matter content, Pb210, Cs-137, Fe203, MnO, Pb, Zn. TiOZ. and AI203 (NORTON et al., 1981). Some of these data are reproduced in Table 1. columns 4 to 6.
2115
P. M. Gschwend, P. H. Chen and R. A. Hites
2116
Table
1
Perylene and corrected perylene concentrations in sectlons of a core from Mountain Pond, Coburn Mountain, Nalne. Rlsa given are the estimated dnd corrected section ages calculated frMn the sedimentation rate (0.4 cmlyr for the top 9 cm and 0.2 cmlyr below) and the water. organic matter, Cs-137, and Pb-210 concentrations. .Correcteda Section
Age
$9terb
Age
_LaLrrL.kY.Q
O-2 3-4 5-6 7-8 9-10 10-12 14-16 17-18 20-21 23-24 26-27 32-33 37-39 40-42 43-45
see
2.5 8.8 14 :", 33 53 65 80 95 110 140 168 183 198
text
reflect
3.8 9 14 20 28
75 90 105 135 163 '79 193
;:
data
c. d.
et al., 1981). %y%ight basis duplicate measurements.
averages
96.5 94.5 94.6 94.6 94.5 94.9 95.9 95.7 95.3 _-
49.1 48.7 46.8 47.1 50.4 53.4 52.0 50.9 51.1
94.5 95.1 94.4 94.3
51.2 53.5 51.0 52.5
over
error
~eryiene‘
Corrected".' Perylene
(pci
(nmoleig)
(nmoleja
#as
less
AND DISCUSSION
The perylene concentration data for the sediment core from the Mountain Pond site are given in col-
(pcl/g)
8.5 6.1 13.7 5.4 3.2 0.8 1.0
:: 23 19
0.42 0.43 0.29 :.11
3.56 " 1.w
14 II
2.06
1.35
2.62 3.29 3.57 3.89 3.61 4.17 4.37 4.38
1.9P :.5i 3.25 1.5? 3.85 ?.6' 4.17 4.3; 3.78
6 4 3
sublnterva?s
Based on the Pb-2 10 activity profile, the Cs- 137 activity maximum, and our own combustion-derived PAH data from this site (GSCHWEND and HITES, 198 I), we estimate the upper 9 centimeters of the core accumulated since about 1955, and the deeper material was deposited somewhat slower such that the 20-2 1 cm section is about 80 years old. Solid phase FezOX and MnO concentration maxima (not seen for AlzOl or TiOz) were observed in the uppermost 23 cm indicating that the sediment was reducing below this depth. In addition, deeper core sections were black. Several lines of evidence, besides the apparent exclusion of oxygen below 2-3 cm, suggest that bioturbation was limited to less than a few centimeters: (a) The sedimentary water content was constant below 2 cm. (b) The Cs-I 37 activity maximum was confined to a 2 cm thick layer, was not displaced below the 1963 horizon identified by Pb-2 10dating (ROBBINSand EDGINGTON, 1975), and the Cs-137 activity decreased sharply to values indistinguishable from zero in core sections older than 1950. (c) Similarly sharp changes in the depth profiles of the anthropogenic pollutants, Pb, Zn, and PAH, were observed in core sections corresponding to times when these contaminants began to be widely dispersed in the environment (NORTON et al., I98 1; HANSON et al.. 1982; GSCHWENDand HITES, 1981). The organic matter content and the Ti02 and A&O9 concentrations (indicative of detrital inputs) were observed to remain quite constant throughout the core, suggesting inputs to this depositional site did not vary widely. The sediment was extracted by Dr. Philip A. Meyers, University of Michigan. Aliquofs were provided to us, as were Dr. Norton’s and Davis’ Pb-210 data from which we estimated the sedimentation rate. The PAH analytical methods have been described elsewhere (GSCHWENDand HITES, 198 1). Briefly, two internal standards (I, I’-binaphthyl and dlz-perylene) were added to the thawed sediment, and the sediment was soxhlet extracted with toluene-methanol or dichloromethane-methanol. After rotary evaporation to a small volume, the extract was partitioned between water and dichloromethane, and then subjected to both silica and Sephadex LH-20 column chromatography (GIGER and SCHAFFNER,1978). Finally, quantitation was performed by glass capillary gas chromatography or by gas chromatographic mass spectrometry operating in the selected ion monitoring mode. RESULTS
cs-l@>cPb-210‘
analyzed
1.d 2.14d
by others
(see
‘.n*
Nortsr.
than :10X.
umn 8 of Table 1 along with the average age of each core section (column 2). In situ formation is strongly indicated since the perylene concentration increases with depth. However, surface sediment sections contain significant amounts of perylene derived from anthropogenic combustion inputs, as judged by the co-occurrence of several other PAH. In order to estimate the perylene derived from in situ production only, we have substracted an amount of perylene based on a fixed ratio (0.4) of pyrene in the same core sections (see “Corrected Perylene”, column 9, Table 1). We feel this is justified since, to a first approximation, we have found that the relative abundances of the major anthropogenic PAH correlate with each other in most of the environmental samples we have investigated (HITES et al., 1980, and references therein; GSCHWEND and HITES, 198 1). We also corrected the ages by subtracting a time (5 yr) corresponding to the 2-3 cm mixed, oxygenated layer at the top of the core. We felt that this was a necessary correction since perylene formation would not begin until the sediment had become anoxic. These corrected ages are given in column 3 of Table 1. When the corrected perylene concentrations are plotted as a function of corrected interval age (see Fig. l), a record of the in situ production of perylene can be seen. Our approach to interpreting these data is to treat the core sections as a series of identical “reaction vessels” incubated in situ for varying times and, therefore, reflecting the kinetics of the perylene formation reaction. Certain assumptions are necessary for this approach to apply. First, over the time periods in question, there can be no significant perylene transport within the core or decomposition after its formation. Second, we must have essentially constant inputs, especially of the precursor compound, to the site. Given the nature of perylene and the other geochemical information from this core, these assumptions appear quite reasonable. Thus, we feel justified in modelling this perylene core profile
2117
Perylene in recent sediments the results of this kinetic calculation solid curve in Fig. 1.
are plotted as the
Second order kinetics This model considers an alternative hypothesis that perylene is formed from its precursor by a bimolecular rate determining step: two molecules of an unknown precursor yield perylene. Like the first order model. this second order model does not require that all steps in the reaction sequence be biomolecular but only that the slowest step is bimolecular. Mathematically, we have dP dt
0
50
100 Age
150
200
(years)
FIG. 1. Concentration of perylene vs. age of deposition in a core from Mountain Pond, Cobum Mountain, Maine (data points from columns 3 and 9 of Table 1) and curves showing fit of these perylene data to a first order kinetic model (solid line) and a second order kinetic model (dotted line). These models are described in the text. to evaluate the kinetic factors associated with this diagenetic reaction; these include the rate constant, the initial precursor concentration, and the molecularity of the reaction with respect to the precursor.
First order kinetics This model is based on the hypothesis that perylene is formed from its precursor by a unimolecular rate determining step: one molecule of an unknown precursor yields perylene. This does not require that all steps in the reaction sequence forming perylene be unimolecular but only that the slowest step is unimolecular. Mathematically. we have dP
z
= k,(C, - P)
(I)
where P is the concentration of perylene at time f, k, is the first order rate constant, and Co is the precursor concentration. Integrating Eqn. (1) with the initial condition of P = 0 at t = 0, we have -In (I ~ P/C,,) = k,r
(2)
which rearranges to P = CO[I - exp (-k,f)].
(3)
Thus, from the set of data (P,, t,). we can calculate the precursor concentration (C,) and the rate constant (k,). This calculation was implemented on an IBM/PC by an iterative computer program (ROSENBROCK and STORY, 1966) which searched for the combination of C,, and k, which minimized the sum of squares: SOS = 2,{ P, ~ C,[ 1 ~ exp (-k,t,)]}*
=
k,(C,
- P)’
250
(4)
The calculated precursor concentration was 4.9 nmole/g dry sediment, and the first order rate constant was 0.0 12 yr-’ (precursor half-life = 58 yr). The standard deviation about the regression was 0.29 nmole/ g. The fit of our first order model to the data is excellent;
Integrating Eqn. (5) with the initial condition of P = 0 at t = 0, we have P/[CO(C, - P)] = kg (6) which arranges to P = C,,[l - l/(k,Cot + I)]
(7)
Using the same interative program, k2 and Co were determined such that SOS = C,{P, - Co[l - l/(k,C,r,
+ I)]}’
(8)
was minimized. The calculated precursor concentration was 7.1 nmole/g dry sediment, and the second order rate constant was 0.0013 nmole-‘g yr-’ (precursor half life = 110 yr). The standard deviation about the regression was 0.3 1 nmole/g which is not significantly different from the first-order fit (F = I. 16, p < 0.40). The fit of this second order model is compared to the first order model and the data in Fig. 1 (see dotted curve). It is not possible to distinguish whether the first order or second order model is more applicable to the data. Divergence between models is noted only at core depths greater than 40 cm. Unfortunately, this core was not deep enough to allow us to investigate this region of divergence.
Other kinetic models We also attempted to fit other kinetic models to the data in Table 1. A zero-order model gave a significantly (F = 3.87, p < 0.013) higher standard deviation about the regression than the first order model. A model of two consecutive first order reactions was not successful either because it gave converging rate constants, or in other words, reduced to our first order case.
Implications qf the model results The calculated perylene precursor concentrations for either the first or second order kinetic models are quite reasonable. These precursor concentrations (C,) can be judged by comparing them to maximum perylene concentrations observed in other sediment cores since the precursor concentration must be greater than or equal to these maximum perylene
P. M. Gschwend, P. H. Chen and R. A. Hites
2118
levels. Perylene has been observed at numerous locations (see references in first paragraph) at maximum levels ranging from a few to a few tens of nanomoles per gram dry sediment. Our calculated values (4.9 and 7.1 nmole/g) fall well within this range. Thus, perylene’s precursor must be relatively abundant, typically on the order of a few micrograms (assuming a molecular weight near that of perylene) per gram dry sediment. It is much more difficult to evaluate the reaction rate constants and precursor molecularities given by these kinetic models. Unfortunately, we do not know exactly what range of temperature our core reflects, although we would estimate it was near 10°C. Also, perylene formation mechanisms have been proposed which would account for either a first or second order molecularity with respect to the precursor molecule. For example, the rate limiting step may involve the unimolecular, reductive conversion of an extended quinone to perylene (ORR and GRADY, 1967; AIZENSHTAT, 1973) or may involve the biomolecular condensation of two naphthalene-like precursor molecules (S. G. WAKEHAM, pers. commun.). Our present data do not allow us to distinguish between possibilities, but further work with this kinetic approach should be fruitful in resolving these questions. Perylene profiles from the literature do not help resolve these issues and are generally inappropriate for the kinetic approach used here. Often, the perylene concentrations down a core are quite variable (see for example Fig. 1 in WAKEHAM et al., 1980b, and Fig. 2 in HITES et al., 1980) and sometimes decrease substantially in the deepest core sections. In other cases, there are short-term anomalies in the perylene profile suggesting that the precursor input, anoxia, or sedimentation has varied. For example, the Saanich Inlet study of AIZENSHTAT ( 1973) reflects sediment accumulation riod (approximately
over an extremely long pe-
10,000 yr) but with widely vary-
ing Eh conditions (E,, at O-2 m approximately - 100 mV, Eh
at 34 m approximately +370 mV; NISSEN1972). The perylene core data from an anoxic basin in the Pettaquamscutt River (HITES d BAUM et al.,
al., 1980) are also difficult to interpret because the upper 10 cm do not show regularly declining Pb-2 IO activity (GOLDBERG et al., 1977) and deeper layers show important compositional changes (e.g., 6C’” and organic content). Finally, much perylene data has been accumulated either whh a very limited number of core intervals or at sites without the requisite ancillary information to allow a kinetic interpretation. Indeed, we are aware of no other natural organic compound profile in the literature where a kinetic model could have been applied. We suggest that these problems be carefully considered in the future. Since simple kinetic models describe the formation of perylene in recent sediments, the determination of perylene in cores may provide useful additional insight to the depositional history at those sites. Such perylene data may be used to judge the constancy, not only of
amount but also of kind, of organic matter accumulated (as A1203 or TiOz reflect detrital deposition). The depth of initial perylene formation may indicate an important reductive horizon in the core while a regularly increasing perylene profile may demonstrate that the anoxic conditions have not varied. Analysis of perylene is not difficult and does not appear to be influenced by sampling or storage artifacts (e.g. allowing O2 to contact the sample). Thus, perylene could be a very useful geochemical indicator. Acknowledgments-We thank Drs. S. A. Norton and R. B. Davis (University of Maine at Orono) for the Mountain Pond sediment samples. Their work was supported by the National Science Foundation, Grant DEB-78- 1064 1. We also thank Drs. Stuart Wakeham, Walter Giger, and R~oy Carpenter for helpful comments. Our work was supported by the National Science Foundation, Grant GCE-80-05997. and by the U.S. Department of Energy, Grant 80-EV- 10449. REFERENCES AIZENSHTATZ. (1973) Perylene and its geochemical significance.Geochim. Cosmochim. Acta 37, 559-567. BROWN F. S., BAEDECKERM. J., NISSENBAUMA. and KAPLANI. R. (1972) Early diagenesisin a reducingIjord,
Saanich Inlet, British Columbia, III. Changes in organic constituents of sediment. Geochim. Cosmochim. Acfa 36, 1185-1203. GIGER W. and SCHAFFNERC. (1978) Determination of polycyclic aromatic hydrocarbons in the environment by glass capiliary gas chromatography. Anal. (‘hem. 50.24% 249. GOLDBERGE. D., GAMBLEE., GRIFFINJ. J. and KOIDE M. (1977) Pollution history of Narragansett Bay as recorded in its sediments. Estuarine Coastal Mar. Sci. 5, 549-56 I. GSCHWENDP. M. and Hrms R. A. (1981) FIuxesof polycyclic aromatic hydrocarbonsto marine and lacustrine sediments in the northeasternUnited States. Geochim.
Cosmochim. Acta 45, 2359-2367. HANSOND. W., NORTONS. A. and WILLIAMSJ. S. (1982) Modem and paIeolimnologica1evidence for accelerated
leaching and metal accumulation in soils in New England, caused by atmospheric deposition. Water, .lir. and Soil Pollution 18, 227-239. HITESR. A., LAKAMME R. E., WINDSORJ. G., JR., FAKRINGTONJ. W. and DEUSER W. G. (1980) Polycyclic aromatic hydrocarbons in an anoxic sediment core from the Pettaquamscutt River (Rhode Island, U.S.A.). Geochim. Cosmochim. Acta 44, 873-878. ISHIWATARIR. and HANYAT. (1975) Organic geochemistry of a 200 m core sample from Lake Biwa, II. Vertical distribution of mono- and dicarboxylic acids and polynuclear aromatic hydrocarbons. Proc. Japan. .4cad, 51, 436-44 1. LAFLAMMER. E. and HITES R. A. (1978) The global distribution of polycyclic aromatic hydrocarbons in recent sediments. Geochim. Cosmochim. Acta 42, 289-303. N~SSENBAUM A., PRESLEYB. J. and KAPLAN I. R. (1972) Early d&genesis in a reducing ljord, Saanich Inlet, British Columbia I. Chemical and isotopic changes in major components of interstitial water. Geochim. Cosmochim, Acta 36, 1007-1027. NORTON S. A., HESS C. T. and DAVIS R. B. (lY8 1) Rates of accumulation of heavy metals in pre- and post-European sediments in New England lakes. In Atmuspheric Pollutants in Natural Waters, (Ed. S. J. Eisenreich) Ann Arbor Science, pp. 409-42 1. ORR W. L. and GRADY J. R. (1967)Perylene in basin sediments off southern California. Geochim. Casmochim Acta 31, 1201-1209.
Perylene in recent sediments PRAHL F. G. and CARPENTERR. (1979) The role of zooplankton fecal pellets in the sedimentation of polycyclic aromatic hydrocarbons in Dabob Bay, Washington. Geochmz
Cosmochim.
Acta 43,
1959-1972.
PRAHLF. G. and CARPENTERR. (1983) Polycyclic aromatic hydrocarbon-(PAH)-phase associations in Washington coastal sediment. Geochim. Cosmochim. Acta 47, 10131023. ROBBINSJ. A. and EDGINGTOND. N. (1975) Determination of recent sedimentation rates in Lake Michigan using Pb210 and Cs-137. Geochim. Cosmochim. Acta 39, 2855 304.
ROSENBROCKH. H. and STORY C. (1966) Computational Techniques for Chemical Engineers. Pergamon Press, pp. 64-68. TAN Y. L. and HEIT M. (1981) Biogenic and abiogenic polynuclear aromatic hydrocarbons in sediments from two remote Adirondack lakes. Geochim. Cosmochim Actu 45, 2267-2279.
2115
WAKEHAM S. G. (1977) Synchronous fluorescence spectroscopy and its application to indigenous and petroleumderived hydrocarbons in lacustrine sediments. Environ. Sci. Technol.
I I, 272-216.
WAKEHAM S. G., SCHAFFNERC., GIGER W.. BOON J. J. and DE LEEUWJ. W. (1979) Perylene in sediments from the Namibian Shelf. Geochrm. Cosmochim. .4cta 43, 1141-I
144.
WAKEHAM S. G., SCHAFFNERC. and GIGER W. (1980a) Polycyclic aromatic hydrocarbons in Recent lake sediments, 1. Compounds having anthropogenic origins. Gcochim.
Cosmochim.
Acta 44, 403-4
13.
WAKEHAM S. G.. SCHAFFNERC. and GIGER W. (1980b) Polycyclic aromatic hydrocarbons in Recent lake sediments, II. Compounds derived from biogenic precursors during early diagenesis. Geochim. Cosmochim. Acta 44, 4 15-429.
Abstract-The concentration of perylene increases regularly with depth in a radiometrically dated sediment core from Mountain Pond, Cobum Mountain, Maine, U.S.A. This concentration profile was fit to simple reaction kinetic models which indicate that perylene forms in this core either by a first order reaction characterized by a rate constant of 0.012 yr-* or by a second order reaction characterized by a rate constant of 0.00 13 nmole-‘g yr-‘.
INTRODUCTION PERYLENE IS A pentacyclic aromatic hydrocarbon which has been found in many recent sediments (ORR and GRADY, 1967; BROWN el al., 1972; AIZENSHTAT, 1973; ISHIWATARI and HANYA, 1975; WAKEHAM, 1977; LAFLAMME and HITES, 1978; PRAHL and CARPENTER, 1979; WAKEHAM el al., 1979; HITES et al., 1980; WA~HAM et al., 1980a,b; TAN and HEIT, 1981; PRAHL and CARPENTER, 1983). It is, in fact, unusual not to find perylene as /\ \I m
/\ / , _
perylene
the most abundant polycyclic aromatic hydrocarbon (PAH) in sections of anoxic cores over 100 years old. Perylene may be introduced into these sediments from several sources: (a) the inclusion ofcombustionderived PAH, (b) the input of weathered sedimentary rocks confining perylene, (c) the inco~ration of PAH from fossil fuels such as petroleum, or (d) in situ production by diagenesis. The last source clearly predominates in older core sections of anoxic sediments (see references cited above). This diagenetic source has been an intriguing mystery both in terms of the identity of the precursor and the mechanism by which the transformation occurs. Early on, there had been a consensus that perylene was formed only under reducing conditions from a terrigenous precursor (AIZENSHTAT, 1973). However, its occurrence in Walvis Bay sediment (a site thought to be largely free of inputs of terrestrial organic matter) led WAKEHAM et a/. (1979) to question whether the precursor must be terrestrial. In addition, HITES et al. (1980) found high perylene concentrations in an anoxic, coastal, diatomaceous ooze, and therefore. suggested that these phytoplankton may
t Present Address: Department of Civil Engineering, Room 48-42 1, Massachusetts Institute ofTechnology, Cambridge, MA 02 I39 * To whom correspondence should be addressed.
produce the precursor. In spite of these observations, all that we may presently say about perylene’s precursor is that it has a widespread dist~bution, and that the diagenetic formation of perylene requires deposition of this precursor into suitably reducing sites. In the course of an investigation of PAH fluxes to sediments in the northeastern United States (GSCHWEN~ and HITES, 198 l), we were fortunate to obtain samples from a lacustrine core which, based on extensive geochemical examination by other workers (NORTON et al., 1981: HANSON et al,. 1982), proved to be very well suited for a study of the diagenetic formation of perylene. The sediment was anoxic below about two centimeters and showed no evidence of deep (~2 cm) bioturbation. Additionally, the deposit accumulated regularly and had no major compositional variations. Finally, the location of the pond precluded large interfering inputs of anthropogenic perylene or perylene from weathered rocks. This core gave perylene data which was examined in terms of a rate constant of formation, the initial precursor concentration deposited in the sediment, and the molecularity of the diagenetic reaction. We believe this core provided a unique opportunity as an in situ “reaction vessel” which allowed the kinetics of this organic geochemi~al transformation to be evaluated. EXPERIMENTAL The sediment core (10 cm dia.) was obtained in July, 1978 from the deepest part of Mountain Pond, Cobum Mountains Maine, U.S.A. (48”28‘N X 70”8W) by Drs. Stephen A. Norton and Ronald B. Davis of the University of Maine at Orono (NORTONet al.. 1981; HANSON et ul., 1982). This high altitude pond (837 m above sea level) is small (2.6 hectares), shallow (about 3 meters maximum depth), and oligotrophic. Mountain Pond was initially chosen for study because the neighboring region is fairly pristine, forested primarily with spruce and fir, although some cutting may have occurred approximateIy 40 to 50 years ago. Extensive geochemicai data have been obtained for this core including: water content, organic matter content, Pb210, Cs-137, Fe203, MnO, Pb, Zn. TiOZ. and AI203 (NORTON et al., 1981). Some of these data are reproduced in Table 1. columns 4 to 6.
2115
P. M. Gschwend, P. H. Chen and R. A. Hites
2116
Table
1
Perylene and corrected perylene concentrations in sectlons of a core from Mountain Pond, Coburn Mountain, Nalne. Rlsa given are the estimated dnd corrected section ages calculated frMn the sedimentation rate (0.4 cmlyr for the top 9 cm and 0.2 cmlyr below) and the water. organic matter, Cs-137, and Pb-210 concentrations. .Correcteda Section
Age
$9terb
Age
_LaLrrL.kY.Q
O-2 3-4 5-6 7-8 9-10 10-12 14-16 17-18 20-21 23-24 26-27 32-33 37-39 40-42 43-45
see
2.5 8.8 14 :", 33 53 65 80 95 110 140 168 183 198
text
reflect
3.8 9 14 20 28
75 90 105 135 163 '79 193
;:
data
c. d.
et al., 1981). %y%ight basis duplicate measurements.
averages
96.5 94.5 94.6 94.6 94.5 94.9 95.9 95.7 95.3 _-
49.1 48.7 46.8 47.1 50.4 53.4 52.0 50.9 51.1
94.5 95.1 94.4 94.3
51.2 53.5 51.0 52.5
over
error
~eryiene‘
Corrected".' Perylene
(pci
(nmoleig)
(nmoleja
#as
less
AND DISCUSSION
The perylene concentration data for the sediment core from the Mountain Pond site are given in col-
(pcl/g)
8.5 6.1 13.7 5.4 3.2 0.8 1.0
:: 23 19
0.42 0.43 0.29 :.11
3.56 " 1.w
14 II
2.06
1.35
2.62 3.29 3.57 3.89 3.61 4.17 4.37 4.38
1.9P :.5i 3.25 1.5? 3.85 ?.6' 4.17 4.3; 3.78
6 4 3
sublnterva?s
Based on the Pb-2 10 activity profile, the Cs- 137 activity maximum, and our own combustion-derived PAH data from this site (GSCHWEND and HITES, 198 I), we estimate the upper 9 centimeters of the core accumulated since about 1955, and the deeper material was deposited somewhat slower such that the 20-2 1 cm section is about 80 years old. Solid phase FezOX and MnO concentration maxima (not seen for AlzOl or TiOz) were observed in the uppermost 23 cm indicating that the sediment was reducing below this depth. In addition, deeper core sections were black. Several lines of evidence, besides the apparent exclusion of oxygen below 2-3 cm, suggest that bioturbation was limited to less than a few centimeters: (a) The sedimentary water content was constant below 2 cm. (b) The Cs-I 37 activity maximum was confined to a 2 cm thick layer, was not displaced below the 1963 horizon identified by Pb-2 10dating (ROBBINSand EDGINGTON, 1975), and the Cs-137 activity decreased sharply to values indistinguishable from zero in core sections older than 1950. (c) Similarly sharp changes in the depth profiles of the anthropogenic pollutants, Pb, Zn, and PAH, were observed in core sections corresponding to times when these contaminants began to be widely dispersed in the environment (NORTON et al., I98 1; HANSON et al.. 1982; GSCHWENDand HITES, 1981). The organic matter content and the Ti02 and A&O9 concentrations (indicative of detrital inputs) were observed to remain quite constant throughout the core, suggesting inputs to this depositional site did not vary widely. The sediment was extracted by Dr. Philip A. Meyers, University of Michigan. Aliquofs were provided to us, as were Dr. Norton’s and Davis’ Pb-210 data from which we estimated the sedimentation rate. The PAH analytical methods have been described elsewhere (GSCHWENDand HITES, 198 1). Briefly, two internal standards (I, I’-binaphthyl and dlz-perylene) were added to the thawed sediment, and the sediment was soxhlet extracted with toluene-methanol or dichloromethane-methanol. After rotary evaporation to a small volume, the extract was partitioned between water and dichloromethane, and then subjected to both silica and Sephadex LH-20 column chromatography (GIGER and SCHAFFNER,1978). Finally, quantitation was performed by glass capillary gas chromatography or by gas chromatographic mass spectrometry operating in the selected ion monitoring mode. RESULTS
cs-l@>cPb-210‘
analyzed
1.d 2.14d
by others
(see
‘.n*
Nortsr.
than :10X.
umn 8 of Table 1 along with the average age of each core section (column 2). In situ formation is strongly indicated since the perylene concentration increases with depth. However, surface sediment sections contain significant amounts of perylene derived from anthropogenic combustion inputs, as judged by the co-occurrence of several other PAH. In order to estimate the perylene derived from in situ production only, we have substracted an amount of perylene based on a fixed ratio (0.4) of pyrene in the same core sections (see “Corrected Perylene”, column 9, Table 1). We feel this is justified since, to a first approximation, we have found that the relative abundances of the major anthropogenic PAH correlate with each other in most of the environmental samples we have investigated (HITES et al., 1980, and references therein; GSCHWEND and HITES, 198 1). We also corrected the ages by subtracting a time (5 yr) corresponding to the 2-3 cm mixed, oxygenated layer at the top of the core. We felt that this was a necessary correction since perylene formation would not begin until the sediment had become anoxic. These corrected ages are given in column 3 of Table 1. When the corrected perylene concentrations are plotted as a function of corrected interval age (see Fig. l), a record of the in situ production of perylene can be seen. Our approach to interpreting these data is to treat the core sections as a series of identical “reaction vessels” incubated in situ for varying times and, therefore, reflecting the kinetics of the perylene formation reaction. Certain assumptions are necessary for this approach to apply. First, over the time periods in question, there can be no significant perylene transport within the core or decomposition after its formation. Second, we must have essentially constant inputs, especially of the precursor compound, to the site. Given the nature of perylene and the other geochemical information from this core, these assumptions appear quite reasonable. Thus, we feel justified in modelling this perylene core profile
2117
Perylene in recent sediments the results of this kinetic calculation solid curve in Fig. 1.
are plotted as the
Second order kinetics This model considers an alternative hypothesis that perylene is formed from its precursor by a bimolecular rate determining step: two molecules of an unknown precursor yield perylene. Like the first order model. this second order model does not require that all steps in the reaction sequence be biomolecular but only that the slowest step is bimolecular. Mathematically, we have dP dt
0
50
100 Age
150
200
(years)
FIG. 1. Concentration of perylene vs. age of deposition in a core from Mountain Pond, Cobum Mountain, Maine (data points from columns 3 and 9 of Table 1) and curves showing fit of these perylene data to a first order kinetic model (solid line) and a second order kinetic model (dotted line). These models are described in the text. to evaluate the kinetic factors associated with this diagenetic reaction; these include the rate constant, the initial precursor concentration, and the molecularity of the reaction with respect to the precursor.
First order kinetics This model is based on the hypothesis that perylene is formed from its precursor by a unimolecular rate determining step: one molecule of an unknown precursor yields perylene. This does not require that all steps in the reaction sequence forming perylene be unimolecular but only that the slowest step is unimolecular. Mathematically. we have dP
z
= k,(C, - P)
(I)
where P is the concentration of perylene at time f, k, is the first order rate constant, and Co is the precursor concentration. Integrating Eqn. (1) with the initial condition of P = 0 at t = 0, we have -In (I ~ P/C,,) = k,r
(2)
which rearranges to P = CO[I - exp (-k,f)].
(3)
Thus, from the set of data (P,, t,). we can calculate the precursor concentration (C,) and the rate constant (k,). This calculation was implemented on an IBM/PC by an iterative computer program (ROSENBROCK and STORY, 1966) which searched for the combination of C,, and k, which minimized the sum of squares: SOS = 2,{ P, ~ C,[ 1 ~ exp (-k,t,)]}*
=
k,(C,
- P)’
250
(4)
The calculated precursor concentration was 4.9 nmole/g dry sediment, and the first order rate constant was 0.0 12 yr-’ (precursor half-life = 58 yr). The standard deviation about the regression was 0.29 nmole/ g. The fit of our first order model to the data is excellent;
Integrating Eqn. (5) with the initial condition of P = 0 at t = 0, we have P/[CO(C, - P)] = kg (6) which arranges to P = C,,[l - l/(k,Cot + I)]
(7)
Using the same interative program, k2 and Co were determined such that SOS = C,{P, - Co[l - l/(k,C,r,
+ I)]}’
(8)
was minimized. The calculated precursor concentration was 7.1 nmole/g dry sediment, and the second order rate constant was 0.0013 nmole-‘g yr-’ (precursor half life = 110 yr). The standard deviation about the regression was 0.3 1 nmole/g which is not significantly different from the first-order fit (F = I. 16, p < 0.40). The fit of this second order model is compared to the first order model and the data in Fig. 1 (see dotted curve). It is not possible to distinguish whether the first order or second order model is more applicable to the data. Divergence between models is noted only at core depths greater than 40 cm. Unfortunately, this core was not deep enough to allow us to investigate this region of divergence.
Other kinetic models We also attempted to fit other kinetic models to the data in Table 1. A zero-order model gave a significantly (F = 3.87, p < 0.013) higher standard deviation about the regression than the first order model. A model of two consecutive first order reactions was not successful either because it gave converging rate constants, or in other words, reduced to our first order case.
Implications qf the model results The calculated perylene precursor concentrations for either the first or second order kinetic models are quite reasonable. These precursor concentrations (C,) can be judged by comparing them to maximum perylene concentrations observed in other sediment cores since the precursor concentration must be greater than or equal to these maximum perylene
P. M. Gschwend, P. H. Chen and R. A. Hites
2118
levels. Perylene has been observed at numerous locations (see references in first paragraph) at maximum levels ranging from a few to a few tens of nanomoles per gram dry sediment. Our calculated values (4.9 and 7.1 nmole/g) fall well within this range. Thus, perylene’s precursor must be relatively abundant, typically on the order of a few micrograms (assuming a molecular weight near that of perylene) per gram dry sediment. It is much more difficult to evaluate the reaction rate constants and precursor molecularities given by these kinetic models. Unfortunately, we do not know exactly what range of temperature our core reflects, although we would estimate it was near 10°C. Also, perylene formation mechanisms have been proposed which would account for either a first or second order molecularity with respect to the precursor molecule. For example, the rate limiting step may involve the unimolecular, reductive conversion of an extended quinone to perylene (ORR and GRADY, 1967; AIZENSHTAT, 1973) or may involve the biomolecular condensation of two naphthalene-like precursor molecules (S. G. WAKEHAM, pers. commun.). Our present data do not allow us to distinguish between possibilities, but further work with this kinetic approach should be fruitful in resolving these questions. Perylene profiles from the literature do not help resolve these issues and are generally inappropriate for the kinetic approach used here. Often, the perylene concentrations down a core are quite variable (see for example Fig. 1 in WAKEHAM et al., 1980b, and Fig. 2 in HITES et al., 1980) and sometimes decrease substantially in the deepest core sections. In other cases, there are short-term anomalies in the perylene profile suggesting that the precursor input, anoxia, or sedimentation has varied. For example, the Saanich Inlet study of AIZENSHTAT ( 1973) reflects sediment accumulation riod (approximately
over an extremely long pe-
10,000 yr) but with widely vary-
ing Eh conditions (E,, at O-2 m approximately - 100 mV, Eh
at 34 m approximately +370 mV; NISSEN1972). The perylene core data from an anoxic basin in the Pettaquamscutt River (HITES d BAUM et al.,
al., 1980) are also difficult to interpret because the upper 10 cm do not show regularly declining Pb-2 IO activity (GOLDBERG et al., 1977) and deeper layers show important compositional changes (e.g., 6C’” and organic content). Finally, much perylene data has been accumulated either whh a very limited number of core intervals or at sites without the requisite ancillary information to allow a kinetic interpretation. Indeed, we are aware of no other natural organic compound profile in the literature where a kinetic model could have been applied. We suggest that these problems be carefully considered in the future. Since simple kinetic models describe the formation of perylene in recent sediments, the determination of perylene in cores may provide useful additional insight to the depositional history at those sites. Such perylene data may be used to judge the constancy, not only of
amount but also of kind, of organic matter accumulated (as A1203 or TiOz reflect detrital deposition). The depth of initial perylene formation may indicate an important reductive horizon in the core while a regularly increasing perylene profile may demonstrate that the anoxic conditions have not varied. Analysis of perylene is not difficult and does not appear to be influenced by sampling or storage artifacts (e.g. allowing O2 to contact the sample). Thus, perylene could be a very useful geochemical indicator. Acknowledgments-We thank Drs. S. A. Norton and R. B. Davis (University of Maine at Orono) for the Mountain Pond sediment samples. Their work was supported by the National Science Foundation, Grant DEB-78- 1064 1. We also thank Drs. Stuart Wakeham, Walter Giger, and R~oy Carpenter for helpful comments. Our work was supported by the National Science Foundation, Grant GCE-80-05997. and by the U.S. Department of Energy, Grant 80-EV- 10449. REFERENCES AIZENSHTATZ. (1973) Perylene and its geochemical significance.Geochim. Cosmochim. Acta 37, 559-567. BROWN F. S., BAEDECKERM. J., NISSENBAUMA. and KAPLANI. R. (1972) Early diagenesisin a reducingIjord,
Saanich Inlet, British Columbia, III. Changes in organic constituents of sediment. Geochim. Cosmochim. Acfa 36, 1185-1203. GIGER W. and SCHAFFNERC. (1978) Determination of polycyclic aromatic hydrocarbons in the environment by glass capiliary gas chromatography. Anal. (‘hem. 50.24% 249. GOLDBERGE. D., GAMBLEE., GRIFFINJ. J. and KOIDE M. (1977) Pollution history of Narragansett Bay as recorded in its sediments. Estuarine Coastal Mar. Sci. 5, 549-56 I. GSCHWENDP. M. and Hrms R. A. (1981) FIuxesof polycyclic aromatic hydrocarbonsto marine and lacustrine sediments in the northeasternUnited States. Geochim.
Cosmochim. Acta 45, 2359-2367. HANSOND. W., NORTONS. A. and WILLIAMSJ. S. (1982) Modem and paIeolimnologica1evidence for accelerated
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