TRACE ELEMENTS IN RECENT MOLLUSK SHELLS

April, 1963
LIMNOLOGY
VOLUME
AND
OCEANOGRAPHY
TRACE ELEMENTS
Orrin
Department
IN RECENT
H. Pikey
of Geology,
1
MOLLUSK
NUMBER
VIII
2
SHELLS
and H. G. Goode11
Florida
State University
ABSTRACT
Analyses were performed
for iron, magnesium,
manganese, strontium,
barium,
and
mineralogy in the shells of 7 species of marine mollusks collected over much of their presentday environmental
range. Correlations
between the shell composition
and water temperature and salinity were determined.
While a number of significant
relationships
were observed, these are generally too weak to be used for paleoecological
determinations
and are
not consistent between species. Differences
in salinity cause greater changes in shell composition than differences
in temperature,
but salinities above 25% do not greatly affect
the shell composition.
make possible a clearer delineation of environmental effects. Specific level studies
The chemical composition of invertebrate
skeletons has long been of interest to geolo- have been performed by Pilkey and Hower
( 1960), Krinsley ( 1960)) and Rucker and
gists because of the importance of biologiValentine ( 1961). The latter two studies
cally derived sediments in the geologic
utilized intertidal mollusks wherein environrecord. Recently, attention has been focused
mental
parameters are highly variable and
on the possible determination of ancient endifficult
to estimate. The first mentioned
vironments based on ecological effects on
specific
level
study involved analyses of
skeletal composition.
Early studies of shell composition are re- diagenetically unstable echinoid tests which
limit the geological value of the results.
viewed by Vinogradov ( 1953). Large-scale
The purpose of this paper is to delineate
studies of the composition of calcareous
the
ecological effects on the composition of
skeletal material include Chave ( 1954),
mollusk shells in order to evaluate the
Thompson and Chow ( 1955), and Odum
feasibility of a paleoecology tool. Seven
(1957). The distribution of skeletal strontium
species of mollusks were collected over
and magnesium among the different phyla
much of their present-day environmental
is fairly well understood, but data on other
range
and their shells were analyzed for
elements are scarce. Temperature, salinity,
water composition, phylogenetic level, and magnesium, manganese, barium, strontium,
shell mineralogy among other factors have and iron, as well as mineralogy. The effect
of temperature and salinity on these combeen observed to affect skeletal composition. Studies of shell composition on the positional variables was then statistically
analyzed.
specific level should reduce organic variables which affect shell composition and
The species chosen for this study are
Crepidula
fornicata Conrad, Crepidula
1 Present address: The University
of Georgia
plana
Say,
Oliva
sayana Ravenel, Busycon
Marine Institute, Sapelo Island, Georgia.
137
INTRODUCTION
138
ORRIN
TABLE
Annual
mean
temp.
(“C)
Annual
mean
salinity
(%*)
1
2
21.5
25
30
32.5
3
25
Nun+
ber
_ -----
Summary
AND
of locality
G. GOODELL
data for recent shells
-
it
6
8
9
10A
25
23.5
24
8
11.5
12
11
12
13
15
16
19
20
16
16
16
20.5
19
21
26
16.5
23
27
35
35
33
36
21
27
35
~-----
2.
=z ~-z~z------____-
Total no. of specimens
No. of normal salinity
specimens
Temperature range ( “C )
Salinity range ( % )
____
--________-
Bay mouth bar at mouth of Alligator Harbor, east of Carrabelle, Florida.
2 miles N of Boca Grandc, Florida in vicinity of Three Sisters Key, west of
Fort Myers, Florida.
N end and seaward side of Gasparilla Island, west of Fort Myers.
Small reducing lagoon adjacent to locality no. 3.
Vicinity of Cabbage Key, south of Pass-a-Grille, St. Petersburg, Florida.
Tampa Bay, St. Petersburg, Florida at foot of 15th and 16th avenues south.
Tidal inlet near Prouts Point, Portland, Maine.
Woods Hole, Massachusetts within town limits just south of Quissett Harbor.
Bay mouth bar at Barncgat Lighthouse, Long Beach Island, New Jersey, north
of Atlantic City, New Jersey.
Hammock Point, Jersey Island, Crisfield, Maryland.
Adjacent to breakwater,
Cape Charles, Virginia.
Tip of Cape Charles, south of Kiptopcke (20 miles south of location 12).
Tip of island just south of Hatteras, North Carolina.
Mouth of Murrels Inlet near Myrtle Beach, South Carolina.
Vicinity of inlet at N end of Sea Island, Georgia near Brunswick, Georgia.
Tavernier Key, seaward from the town of Tavernier on Key Largo, Florida
Keys.
Vicinity of Mayaquez, Puerto Rico.
---.--_ -~_---__---
contrarium
Lamark, Polinices duplicatus
Say, Tagelus clivisus Spengler, and Anemia
simplex Orbigny. The last two named are
pelecypods and the remainder are gastropods. All are shallow water forms, but
Polinices duplicatus is the only species collected which lives primarily in the intertidal
environment.
The writers gratefully acknowledge the
co-operation and assistance rendered in
various forms by Drs G. W. DeVore, D. S.
Gorsline, and J. K. Osmond. Mr Norman
Weisbord was most helpful in confirming
species identification, and Mr Robert Harriss
assisted in the field collection of specimens.
This study was supported by a grant from
the petroleum research fund of the American Chemical Society. A grant-in-aid from
the research council of the American AssoTABLE
II.
Lcxatlon
35
35
35
26.5
30
32
30
.-
1.
H. PILKEY
Sample size and observed
- - --.-A. simplex
33
28
11.5-27
23-36
__.__--
C. fornicata
38
32
8-25
16.5-35
__.---
ciation of Petroleum Geologists provided
travelling
expenses. Partial support for
computer time was from a National Science
Foundation grant to the computing center
of Florida State University.
PROCEDURES
The collection of shells was accomplished
mainly by shallow water dredging at a
number of stations between Mainc and
Puerto Rico. The majority of the shells
were collected alive. Table 1 gives a list of
collecting localities together with the available data on annual mean temperature and
salinity.
Annual mean temperatures and
salinities were established from data published by U. S. Coast and Geodetic Survey
and U. S. Fish and Wildlife Service and
from unpublished manuscripts. Because of
environmental
ranges for each species
- - --------
C. plana
0. sayuna
26
22
22
8-25
26.5-35
-
22
19-25
30-35
- ---
P. duplicatus
28
B. contrarium
18
24
18
11.5-25
19-26
27-35
30-36
.___-________
-
_
T. dioisus
9
5
21.5-27
26.5-35
TRACE
ELEMENTS
IN RECENT
MOLLUSK
139
SHELLS
TABLE 3. Summary of observed compositional-temperature
relationships.
Significant
relationships
denoted by Direct and Inverse, depending on the nature of the trend. NS = non-significant
(9570 level of confidence)
.- -_ _.-A. simpler
C. fornicata
C. plana
0. sayana
P. duplicatus
Mn/Mg
NS
NS
NS
NS
NS
Mn/Ba
Mn/GFe/Mg
Fe/Ba
Fe/Sr
Mg/f+
WMg
Ba/Sr
Mn+Fe
Mg+Ba+Sr
Mn+Mg+Ba+Fe
-Sr
Mn + Mg + Ba
Sr
Mn
Fe
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
Inverse
Direct
NS
Direct
NS
Inverse
Inverse
Invcrsc
Invcrsc
NS
Inverse
NS
NS
NS
NS
Inverse
NS
NS
Mg
Ba
Sr
“/o Calcite
NS
NS
NS
NS
Inverse
Direct
B. contrariunt
are
T. divisus
NS
Inverse
Inverse
Invcrsc
Inverse
Inverse
Inverse
NS
NS
NS
Direct
Direct
Direct
NS
NS
NS
Inverse
Inverse
NS
Invcrsc
NS
Inverse
NS
NS
Inverse
NS
Inverse
NS
Inverse
NS
Inverse
NS
Inverse
NS
NS
NS
NS
NS
Direct
Direct
NS
NS
NS
Direct
Direct
NS
NS
Inverse
Inverse
NS
Direct
NS
NS
Inverse
Inverse
Inverse
NS
NS
NS
Direct
NS
NS
NS
NS
NS
the necessity of extrapolating regional environmental data to specific localities, the
figures used must be considered approximations. Table 2 lists the number of specimens analyzed and the temperature and
salinity ranges within which each species
was collected.
Large shells were boiled to remove soft
parts and small shells (most of the spccimens ) were air dried. The shells were then
powdered and heated to 350°C to remove
organic matter. Analyses of shell materials
before and after heating showed that no
alteration in shell mineralogy had occurred.
Analytical precision figures in the following
discussion are presented in terms of the
amount present and arc calculated at the
95% confidence level.
Analyses for strontium were determined
by X-ray fluorescence utilizing fixed count
with a precision of -t- 3%. Analyses for the
magnesium, manganese, iron, and barium
contents were accomplished by an emission
spectrograph technique.
Calcium was as-
NS
NS
NS
NS
NS
NS
NS
NS
Inverse
sumed to be constant and was used as the
in ternal standard. External standards were
mixed using spectrographically pure calcium
carbonate. The analytical precision of the
iron and barium concentration is + 24% of
the amount present, and the manganese and
magnesium contents were determined to
within -c-34% and 2 19%, respectively.
The mineralogy was determined by peak
intensity of X-ray diffraction patterns. Six
out of the seven species are primarily composed of aragonite and contain barely detectable amounts of calcite. The limit of
detection of calcite by this technique is not
known, hence the calcite figures reported
herein are meaningful only in a relative
sense. However, it is believed that the true
concentrations are at the most 1 or 2%
higher than the reported concentrations.
Below what is reported as 1% calcite, the
precision of the determinations
is + 30%.
This rapidly improves to + 15% above 1%
calcite. The percentage of calcite in Anomin
simplex is precise to * 10% of the amount
140
ORRIN
II.
PLLKEY
ANOMIA
AND
II.
G. GOODELL
SIMPLEX
.
FIG. 1. The relationships
between shell composition
and annual mean water temperatures.
Least
squares regression lines were calculated only on the basis of shells collected from normal salinity locarelationships.
tions. Dashed lines designate non-significant
Open circles = annual mean salinity <
25%,, x I annual mean salinity between 25 and 30%,, dark circles = salinity > 30 go.
present. A total of 174 Recent shells was
analyzed.
RESULTS
Table 5 presents the analytical results.
The figures reported arc average conccntrations of all the shells collected from each
locality. Wherever possible, at least three
shells from each locality were analyzed in
order to obtain a reasonable mean compositional figure.
The results of the statistical analysis of
the relationship between shell composition
and temperature are presented in Table 3.
The element ratios in the table are atom
ratios. Figures 1 through 7 are plots of
composition
US. temperature
for the 7
species studied.
Since only 5 Tagelus divisus shells from
normal salinity waters were analyzed, the
4. Summary of non-linear (cubic and
quadratic) relationships between composition and
annual mean water temperature. NS = nonsignificant, C = cubic, Q = quadratic (95%
level of confidence).
TABLE
Temperature
effects
In order to facilitate the study of the effect of temperature on shell composition,
correlation coefficients between temperature and composition
were determined.
These were computed on the basis of shells
from waters of annual mean salinity greater
than 30% (considered normal salinity for
the purposes of this study) in order not to
confuse temperature and salinity effects.
Correlation coefficients are an indication of
the degree of inter-dependency
between
two variables assuming a linear relationship.
Mn
PC
Mg
Ba
Sr
% Calcite
C
NS
NS
NS
NS
NS
C
NS
Q
Q
C
NS
C
NS
NS
NS
C
NS
NS
NS
C
Q
NS
NS
NS
NS
NS
NS
NS
NS
C
C
Q
NS
NS
NS
NS
NS
NS
NS
NS
NS
TRACE
ELEMENTS
IN RECENT
CREPIDULA
T “C
MOLLUSK
SIIELLS
FORNICATA
T “C
T “c
o
6
4
2
x
2ood
11
15‘
T “C
19
23
1
27
See Figure
observed trends between temperature and
composition will not be considered for this
species in the following discussion,
The composition of the shells of Polinices
cluplicatus exhibits fewer significant linear
relationships with temperature than any of
the other species studied. Only the percentage of calcite is related to temperature.
This is probably due to the intertidal nature
of this species in which situation environmental factors vary widely and are difficult
to estimate. The only species studied that
deposits a primarily calcitic shell, Anomia
simplex, exhibits only two significant trends
with temperature. The most numerous significant compositional-temperature
rclationships are observed in Busycon contrarium
and Oliva sayana shells.
No compositional variable is related to
temperature in more than 4 species. The
strontium content of shells is most consistent
in this respect. The atom ratios Mg/%, Mn +
Mg + Ba + Fe/Sr, Mn + Mg + Ba/Sr, and
the percentage of calcite are related to temperature in 3 species, while other ratios or
percentages are related in two or less instances. Every compositional variable exhibits a significant trend with tcmpcraturc
in the shells of at least one species.
1
1
1
%23
7
T “C
T ‘C
FIG. 2.
t-
1.
With two exceptions, the nature of the
relationships between temperature and any
single compositional variable are consistent.
In other words, the correlations are always
inverse or always direct. The exceptions arc
in the variables strontium and the per cent
calcite. Strontium is inversely related to
temperature in calcitic Anomia simplex, but
directly related in 3 of the aragonitic
species. This difference is probably a mineralogical effect. Pilkey and Hower ( 1960)
noted a similar inverse relationship between
temperature and strontium in the calcite
tests of an echinoid species, The percentage
of calcite is directly related to temperature
in Anomin simplex and Crepidula fornicata
shells, but inversely related in Polinices
duplicatus shells.
Basically, changes in the composition of
recent shells must bc related to crystal
growth rate and composition of external ( sea
water ) and internal ( depositional tissues )
depositional mediums. Explanation of the
temperature effects on skeletal composition
observed in this study is complicated by the
fact that the shells of each species react
uniquely to temperature. Thus, it would be
necessary to hypothesize on each case sepa-
142
ORRIN
TABLE
-.
~-.
Lot.
NL;s’‘ . Mn
1
4
:
5
6
8
9
10A
11
12
13
15
16
19
21
1
3
4
4
4
1
1
ii
1
9
10A
12
13
15
16
19
20
21
1
3
9
10A
13
15
19
5.
Summary
-~.
4
4
4
4
3
4
1
4
4
4
3
2
4
5
4
3
4
4
4
Pe
Crepidula
9
25
2
1
11
4
9
19
78
21
26
5
7
16
13
56
187
157
63
100
135
232
21
438
4
1
2
24
17
4
86
Mg
Ba
fornicata
267
7
I-1. PILKEY
Sr
Anomia simplex
84 2,350
10
183 2,758
17
265 2,097
7
38 3,093 24
72 2,620
16
141 2,353
10
139 2,483 22
1,059 3,114
10
91 2,653 21
298 2,240
5
1,965
1,960
1,917
2,467
2,210
1,901
1,992
1,857
1,875
1,540
Polinices
22
14
17
47
37
63
69
duplicatus
237
6
218
3
192
3
285
5
236
4
254
5
278
6
G. GOODELL
%
calcite
Lot.
!&a?;
Mn
Fc
Mg
-. -
Ba
Sr
.___.~
%*
calcltc
-
Oliva sa yana
73
6
6
3
8
4
26
11
11
7
10
10
287
323
323
451
392
315
247
2,050
285
300
166
199
349
II.
of the mean composition of shells from each locality
_
-----.- __
~
- ~_.~-~
.~-
2,615
2,870
2,525
2,750
2,850
2,398
2,216
2,521
3,260
3,260
2,890
3,064
2,717
3,047
5
2;
36
34
56
97
620
77
179
140
22
325
AND
2,060
1,957
2,125
2,266
2,263
2,335
2,300
0.8
0.7
0.7
0.5
0.3
1
3
4
5
15
16
14
13
4
9
74
47
3
3
2
3
4
3
2,511
2,645
2,693
2,700
2,855
2,393
0.4
0.3
0.3
0.6
0.5
0.4
7
11
6
10
4
8
4
8
11
10
2,610
3,400
2,740
3,240
2,092
2,207
2,545
2,497
2,605
2,877
0.3
0.7
0.2
0.9
0.2
0.2
0.2
0.3
1.6
0.2
0.6
0.3
0.3
0.4
0.5
0.4
0
0.1
0.2
147
133
142
123
157
183
::i
;-ii
0:5
1.1
Ei
1:o
93
88
94
83
85
96
94
95
94
95
0.6
0.2
1.2
1.5
0.8
0.8
1.1
rately. However, some general statements
can be made.
The composition of sea water is probably
not an important factor to be considered as
far as the normal salinity temperature effect
is concerned, because the composition would
not bc expected to change progressively
Crystal
with latitude
and temperature.
growth rate cannot be the only factor involved because all the elements would increase or decrease simultaneously
if this
was the case. However, there appears to be
some relationship bctwcen the nature of the
temperature effect and the lattice stability
contribution of a particular clement. Wray
and Daniels ( 1957) and others have prescnted evidence that the large strontium ion
Crepidula
1
2
5
6
8
9
10A
13
16
19
3
1
1
1
4
4
4
3
2
3
17
3
9
7”
28
36
24
21
17
35
49
5
21
70
253
56
34
65
659
plann
354
300
307
395
310
250
195
212
409
236
Busycon contrarium
1
2
4
4
2
1
11
45
132
174
3
4
z
16
20
32
2
3
1
:
1
16
3
304
12
210
137
365
194
3
4
3
2,719
2,789
2,770
2,793
2,700
2,590
1
6
21
3
4
2
8
3
29
48
12
15
4,097
2,995
3,318
Tagelus
111
45
55
divisus
262
311
323
adds to the stability of aragonite. In the
aragonitic shells, the cations inversely related to temperature are those smaller than
calcium, while those directly related to
temperature are larger. Barium and strontium, the larger ions, also exhibit higher
heats of formation and free energies of carbonate formation and percentages of ionic
nature of oxide bonds than do the other 3
impurity ions. Hence, from a strictly inorganic standpoint, it is possible that a more
stable crystal is formed with increasing tcmpera ture.
The calcium in aragonite exhibits ninefold co-ordination
with oxygen, while in
calcite the calcium is six co-ordinated. Thus,
in aragonite, the large nine co-ordinated
TRACE
ELEMENTS
IN RECENT
CREPIDULA
MOLLUSK
143
SHELLS
PLANA
T ‘C
1: ‘C
T ‘C
T ‘C
T “C
T “C
FIG. 3.
See Figure
site is more favorable for the large strontium ion while the smaller co-ordinated site
in calcite would be rendered unstable by
the large ion. In fact, recrystallization
of
aragonite to calcite always results in a lowering of the Sr/Ca ratio (Odum 1957). The
OLIVA
1.
inverse relationship
of strontium to tempcrature in the calcite shells of Anon& simpZex perhaps indicates that a more stable
crystal is attained in warmer waters in this
instance also.
In the foregoing discussion, it has been
SAYAN
16
i
8’
P
H
9
I
k
.
16
0
.
.
14
.
‘2 LislL 1
4
20
T-C
T ‘C
T “C
T “C
FIG. 4.
See Figure
1.
144
ORRIN
II.
PILKEY
AND
BU5YCON
I-1. G. GOODELL
CONTRARllJM
T-C
T %
T “C
T
FIG. 5.
Set Figure
assumed that the dependency between
water temperature and shell composition is
linear. This assumption may not be valid.
The composition of shells is at least in part
due to a vital effect of the organism secreting the shells and since most biological rate
processes are not linear over the entire
range of tolerated environmental tempcratures, the compositional-environmental
relationships may be complex. Because many
details of shell secreting mechanisms arc
not fully understood, the meaning of nonlinear relationships would be difficult
to
evaluate and for this reason, only the linear
trends are discussed in detail.
Quadratic
(Y = A + BX + CX2) and
cubic (Y = A + BX + CX2 + DX3 ) regression analyses were performed on the data
and statistically tested for significance both
as to the curve itself and as to whether an
additional significant reduction of the variability of the data was accomplished by the
fit of the higher over the next lower polynomial. Both the polynomial and the additional reduction of variability
must be
significant for a given polynomial curve to
be accepted. Table 4 presents the results of
this statistical analysis and significant quad-
T “C
‘C
1.
ratic and cubic curves are drawn in Figures
1 through 7.
Within the limits of the accuracy of the
available environmental data, it is apparent
that annual mean temperature is only one
of several other unknown factors involved
in determining changes in shell composition That is, observed correlations with
temperature are generally weak. Since shell
growth is not a year-round constant, some
of the correlations might be improved if
seasonal temperatures were used, but such
data are not available.
Comparison of temperature trends
Figure 8 is a plot of the linear rcgrcssion
lines of temperature vs. composition for
each of the species studied. The Anomin
simplex and Tngelus clivisus trends are not
shown in some cases for reasons of scale.
The regression lines are extended only over
the range of temperatures from which the
shells were collected. Since many of the
trends are statistically insignificant,
these
are only intended to give some idea of general concentration levels of the clemcnts in
the shells of each species.
From these plots it is apparent that a
strong generic effect is present. That is, dif-
TRACE
ELEMENTS
IN RECENT
TAEGULUS
MOLLUSK
SHELLS
DIVEUS
25
T ‘c
T *C
T OC
T ‘C
T ‘C
T ‘C
FIG. 6.
Set Figure
1.
creases in salinities of 27%0and lower (Fig.
1) . A similar relationship was suggcstcd by
Lowenstam (1954). The percentage of calcite radically increases in the predominantly
aragonitic Crepiduln fornicatn shells collected from 16.5%0 salinity, but no significant increase was observed in shells collected from waters of 23%0 (Fig. 2).
A strong salinity effect was observed in
only
two species and specimens collected
Salinity effects
from a wide range of salinities were obThe effect of low salinity on shell eletained only from Chesapeake Ray. Hence,
mental composition, wherever observed, is
the gcncral applicability
of the observed
to increase the minor element content of the
salinity effect is not known, although Rucker
shell. However, salinities above 25%0 have
and Valentine ( 1961) noted a similar inlittle effect on shell composition. Ano,min
crease in trace elements in low salinity
simplex and Crepidula fornicata specimens
shells of Crnssostrea uirginica. Also, since
which were collected in waters of 23%0
the water composition at these low salinity
salinity exhibit increased barium and stronstations is unknown, the effect of the comtium contents (Figs. 1 and 2). Crepiduln
position
of the external medium cannot be
fornicata shells from location 11 of an anevaluated.
nual mean salinity of 16.5 exhibit a radical
Two possible explanations of the salinity
increase in all trace elements (about 50
effect are presented. From a growth-rate
dwarfed Crepidula fornicata shells from
this low salinity sampling location were standpoint, the increase in minor elements
needed in order to obtain sufficient sample is reasonable if it can be assumed the rate
of growth is fast. The reasoning is that
for analysis ) .
In the predominantly calcitic Anomia sim- rapid crystal growth rate traps impurities.
plex shells, the percentage of calcite dc- Growth rate is difficult to evaluate, howferent genera exhibit different fractionation
rates of trace elements from sea water. The
concentration levels of magnesium, barium,
and strontium
in the two species of
CrepiduZa are statistically indistinguishable.
However, the levels of manganese and iron
concentration arc distinctly different in the
two species, indicating a fractionation effect
at the specific level.
146
ORRIN
I-1. PILKEY
AND
FWNICES
II.
G. GOODELL
DUPLICATUS
T ‘C
T ‘C
T ‘C
T ‘C
T ‘C
T “C
FIG. 7.
See Figure 1.
ever, because growth rate of the individual
crystallites forming within the shell is the
important consideration,
and not growth
rate of the whole shell.
Perhaps the argument used by Odum
( 1957) with regard to strontium uptake can
be applied to other elements. Odum states
that as strontium is excluded from the crystallite during its initial growth, the surrounding tissues become strontium-rich.
If
restricted free exchange exists with the surrounding environment, then the strontium/
calcium ratio of the crystallite ultimately is
forced to higher values by the concentration
of strontium in depositional tissues. In the
cast of the salinity effect under present consideration, let us assume that exchange bctween low salinity waters and depositional
tissues is restricted. This may not be unreasonable, since the conchiolin content of
the Crepiduln fornicnta shells is considerably higher in low salinity waters relative to
the normal salinity environment,
which
means that the thickness of tissue between
shell material and sea water is increased.
Thus, as deposition proceeds, the element/
calcium ratios increase in the surrounding
tissues, ultimately increasing the same elc-
ment/calcium ratios in the calcium carbonate crystalline phase.
Element-element relationships
When the amount of one element is directly related to the amount of another elemcnt, the two are probably following
a
similar sea-water-to-shell cycle. In order to
determine element-element
relationships,
correlation coefficients were computed for
element ws. element in individual shells of
each of the 7 species. The meaning of these
correlations, however, depends upon where
the elements are held in the shell. Elements
such as barium, manganese, and iron which
are found in relatively small amounts could,
conceivably, be held within the organic
matrix. This is particularly true of iron and
manganese, which, because of empty 3 d
electron orbitals, show a strong tendency to
form covalent bonds and to complex with
organic molecules ( Goldberg 1957). On
the other hand, the 5 elements determined
in this study are sufficiently closely related
geochemically to calcium to account for
their presence in trace amounts within the
crystal structure. Furthermore, analyses of
conchiolin
do not report any of these
elements as being present in significant
TRACE
ELEMENTS
IN RECENT
MOLLUSK
147
SIIELLS
T "C
T 'C
T lC
T ‘C
FIG. 8. Comparison
of composition-temperature
trend lines of the various species. Non-significant
relationships
are designated by dashed lines. Some Anomia simplex and Taegulus divkus trends are not
illustrated for reasons of scale.
amounts. It is assumed, therefore, that all
5 elements are incorporated in the crystal
lattice.
The elements in Polinices duplicatus,
Tagelus divisus, and Anomiu simplex shells
show no significant inter-relationships.
In
the remaining species, most of the significant correlations are weak and lack consistency between different species. All relationships are direct.
In Crepidula fornicata shells, significant
inter-dependencies
exist bctwecn manganese and iron, strontium and iron, and
strontium and barium. In Crepidula plana
shells, barium and iron, and strontium and
barium are related. The barium and iron
contents of Oliva sayana shells are related,
an d in Busycon contrarium shells, magnesium and manganese, magnesium and iron,
and iron and manganese are intcr-dependent.
The barium and iron, strontium
and
barium, magnesium and manganese, an d
iron and manganese contents exhibit intcrdependencies in 2 out of 7 species each. As
a general conclusion, it is apparent that the
elements follow independent
water to shell.
paths from sea
CONCLUSIONS
Changes in the composition of the shells of
the 7 species of mollusks studied are related, at least, in part, to the environmental
conditions. Differences in salinity are more
important in this respect than differences in
temperature. Within the limits of analytical
precision, and the accuracy of the environmental data used in this study, these compositional-environmental
relationships
are
too weakly defined to be of use as paleoecological tools.
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