mufm
mltRoPnLMmOLOGV
ELSEVIER
Marine Micropaleontology 26 ( 1995) 187-206
Taphonomy and time-averaging of foraminiferal
assemblages in Holocene tidal flat sediments, Bahia
la Choya, Sonora, Mexico (northern Gulf of
California)
Ronald E. Martin”, M. Scott Harris”, W. David Liddellb
” Depwttnmt
h Deprtment
ofGeology, University
c.fGeology,
of’Delawure,
Utah Stclte University,
Newark,
Logpn,
DE 19716, USA
UT 84322. USA
Received 5 September 1994; accepted after revision 5 January I995
Abstract
Foraminiferal reproduction and preservation have been studied in Holocene tidal flat sediments of Bahia la Choya, Sonora,
Mexico ( northern Gulf of California). Foraminiferal reproduction at Choya Bay tends to occur in discrete ( -a few weeks)
seasonal pulses. which are then followed by periods of homogenization and dissolution of several months duration. Foraminiferal
number (number of tests/gram sediment) increases northward across the flat primarily because of decreasing intensity of
hioturbation and increasing total carbonate weight percent (shell content) of sediments.
Despite intensive dissolution of foraminiferal reproductive pulses, tests which appear to be relatively fresh are actually quite
old (up to - 2000 years based on 14C dates). We hypothesize that after reproduction some tests survive dissolution because of
rapid advection (burial) downward by conveyor belt deposit feeders (e.g., callianassid shrimp, polychaete worms) into a
subsurface shell layer, where tests are preserved until exhumation much later by biological activity or storms. Thus, taphonomic
grade (surface condition) of foraminiferal tests in these sediments is not an infallible indicator of shell age (time since death).
The condition of the test surface is indicative of the residence time of the test at the sediment-water interface
( “taphonomically
active Lone”) and not test age.
1. Introduction
For more than half a century, microfossils-especially foraminifera-have
been widely used as stratigraphic and paleoenvironmental
indicators. Despite
countless studies of foraminiferal
distribution
and
diversity in modern sediments (see Murray, 1991, for
review), and their wide usage in stratigraphic, paleoenvironmental,
paleoceanographic,
and paleoclimatic
studies; surprisingly little attention has been paid to the
formation and preservation of foraminiferal
assemblages, especially in continental shelf and slope set0377-839X/95/$09.50
0 1995
SSD/O377-X398(95)00009-7
Elsevier Science B.V. All rights reserved
tings, where much of the fossil record occurs (see
Martin, 1993, for review). Differential preservation of
foraminiferal assemblages likely varies according to
depositional setting (Martin, 1993; see also Kidwell
and Bosence, 199 1; Powell, 1992). The frequency and
amount of shell (micro- and macrofossil) input to the
surface mixed layer and rates of SO:- reduction (alkalinity buildup), sedimentation,
and bioturbation
all
play a role in the modification of the surficial mixed
layer into a time-averaged fossil assemblage and its
incorporation into the historical layer below (Martin,
1993).
R.E. Murtin et 01. /Munne Micropalecjnt~/i~~~ 26 (1995) 1X7-206
188
r
CALIFORNIA
Fig. I. Location
1987).
I
of Choya Bay (adapted
from Ftirsich and Flessa,
Based on experimental
analyses of
dwelling foraminifera from Discovery
Martin and Liddell (1991) and Kotler
1992) concluded that, once produced,
A)
tests in carbonate environments persist for relatively
long periods of time (up to hundreds or thousands of
years, and perhaps longer; Martin, 1993; see also Kidwell and Behrensmeyer, 1993). Despite intensive, deep
( 2 1 m) bioturbation in such environments
(Walter
and Burton, 1990)) the high shell content of the sediment apparently slows dissolution (Aller, 1982; Kidwell, 1989), and allows many foraminiferal tests to
persist (Kotler et al., 1991, 1992).
We test our findings for carbonate environments in
siliciclastic regimes that vary in shell content and, presumably, foraminiferal preservation. Extensive Holocene tidal flat sediments ( - 10 km2 exposed during
spring tides; Fiirsich and Flessa, 1987, 1991) at Bahia
la Choya ( “Choya Bay”), Sonora, Mexico (northern
Gulf of California; Figs. 1 and 2A), offer a variety of
easily accessible environments in which to study the
subtle interplay of reproduction (shell input), shell
content, bioturbation and pore water chemistry during
the formation of foraminiferal assemblages. Choya Bay
was also chosen because its environments had already
LOCATION OF CORE SITES 1-9
modern reefBay, Jamaica,
et al. (1991,
foraminiferal
6)
DEPTH TO SHELL LAYER (km)
Fig. 2. (A) Location of core sites at Choya Bay; distances (in meters) measured from permanent stations located above high tide; distances to
outer flat sites varied according to tide (season). (B) Depth (in cm) to shell layer for each sampling season; contact between shell layer and
overlying shell-poor mixed layer was typically sharp, but sometimes gradational ( = G).
R. E. Martin
et al. /Marine
Microll’aleontol~~~~
been documented by other workers (Flessa, 1987; Ftirsich and Flessa, 1987) and were the subject of ongoing
taphonomic research (Ftirsich and Flessa, 1987, 1991;
Meldahl, 1987, 1990; Flessa, 1993; Flessa et al., 1993;
Flessa and Kowalewski, 1994).
2. Oceanographic
and geologic setting
Choya Bay lies at the northern extreme of the Gulf
of California adjacent to the Sonoran Desert. Nearby
Puerto Peiiasco receives an annual average rainfall of
74 mm (evaporation exceeds rainfall; Maluf, 1983),
and air (water)
temperatures
range from 11.6”C
( 13.8”C) in January to 30°C (29.4”C) in August (Fursich and Flessa, 1987); offshore surface salinities in
the northern Gulf range from -35.5
to 37.5%0,
although they may range higher in restricted areas
(Maluf, 1983). Tides are semidiurnal and spring tides
range up to - 8 m (Fursich and Flessa, 1987). Hurricanes normally occur between late May and early
November, although they are most common in September and October (Roden, 1964). There is seasonal
overturn of the nutrient-rich thermocline in the northern
Gulf (as indicated by depth to the thermocline; Roden,
1964; Robinson, 1973)) which causes seasonal pulses
of phytoplankton reproduction (Maluf, 1983; Pride et
al., 1994).
The tidal flat at Choya Bay is a potentially useful
modern analog for studying the formation of shell concentrations
on ancient
shallow,
sediment-starved
shelves: sedimentation is held constant while hardpart
input varies seasonally (cf. Kidwell, 1986a). There has
been little sediment input to the northern Gulf since the
construction of Hoover Dam on the Colorado River in
the 1930s and subsequent development of irrigation
projects downriver (Maluf, 1983; Ftirsich and Flessa,
1987). Sediment at Choya Bay consists of fine to coarse
sand, and is presently derived locally from granitic
headlands and outcrops of semi-consolidated
to wellconsolidated sandstones and coquinas (Fiirsich and
Flessa, 1987; Zhang, 1994). Sedimentation
rates at
Choya Bay are therefore relatively low ( - 0.038 cm/
yr; Flessa et al., 1993).
Without high sedimentation
rates, conveyor belt
deposit feeders (CDFs; primarily callianassid shrimp
and polychaetes; Fursich and Flessa, 1987, 1991; Meldahl, 1987) repeatedly move fine-grained sediment
26 (1995) 1X7-206
18’)
downwards and then redeposit it at the sediment-water
interface while tending to concentrate coarse mollusc
debris in a relatively distinct subsurface shell layer
(Fig. 2B). CDFs also pump SO:- -rich seawater into
sediment, thereby causing the buildup-to
a certain
extent-of
alkalinity
by SOi- -reducing
bacteria,
which use SOi- as an electron acceptor in the oxidation
of organic matter (Goldhaber and Kaplan, 1980; Brett
and Baird, 1986). CDFs tend to counteract this effect,
however, by producing carbonic and sulfuric acids
through the oxidation of organic matter and sulfides
(HS -), respectively (Walter and Burton, 1990; Canfield and Raiswell, 1991).
Activities ofCDFs are most intense on the inner and
southern flat and decrease toward the outer flat and to
the north (Fiirsich and Flessa, 1987, and unpubl. observations). On the outer flat, sediment mixing is relatively shallow, and is accomplished by breaking waves
and vagile benthos (e.g., sand dollars). The depth to
the shell layer tends to shallow outward across the fat
and to the north from > 60 cm on the southern flat to
- IO cm in some places over a Pleistocene coquina that
isotope stage Se;
is - 125,000 years old ( -oxygen
Aberhan and Fursich, 1987), and that crops out over
the northern margin of the flat. CDF burrow densities
(estimated visually) also tend to decrease outward and
to the north, especially when sediment thickness is
< - 20-25 cm (unpubl. observations).
3. Methods
3.1. Coring
procedures
A total of 9 sites (Fig. 2A) are discussed for each of
three field seasons (summer: July 21-28, 199 I, and
July 26-August I, 1992; winter: January 3-9, 1992).
These sites were chosen based on extensive reconnaissance coring during July ( 199 I ) and reoccupied
in
January and July, 1992. Three sites each were occupied
on southern (sites l-3), middle (sites 4-6)) and northern (sites 7-9) transects, respectively; in this way, the
inner (sites 1, 4, 7), middle (sites 2, 5, 8), and outer
(sites 3, 6, 9) flat was also sampled (Fig. 2A). Distances (measured from permanent stations above high
tide) to inner flat sites varied from 50 to 200 m (typically 100 m), while distances to middle flat stations
were -700 m. Distances to outermost sites (3, 6, 9)
190
R.E. Martin et d. /Marine Micropaleontology
were deliberately varied by us (according to tide) in
order to sample the transition from outermost flat to
shallow subtidal (Fig. 2A).
Cores were taken using an apparatus modified from
Meldahl ( 1987). Core tubes made of schedule 40 (4”
diameter, 0.25” wall thickness) PVC were twisted into
sediment using a metal handle inserted through holes
drilled into the top of the core barrel. The handle was
then removed, the holes plugged with rubber stoppers,
and the top of the core capped with a plastic bag over
which was placed a PVC cap, which was secured with
a radiator hose clamp. The core was then excavated
from the sediment with shovels, and, upon encountering the base of the core tube, the bottom quickly sealed
by the same procedure as for the top. Upon return to
the laboratory [ Centro Intercultural de Estudios de
Desiertos y Oceanos (CEDO), Puerto Pefiasco] . sediment was scooped from the core barrel at 5 cm intervals, and air dried for shipment to Delaware.
3.2. Alkalinity and total carbonate (shell) weight
percent
Pore waters were retrieved from cores in the field
immediately after core excavation by insertion of the
plastic tip of 60 ml syringes into the core through predrilled holes-spaced
every 5 cm-that
had been
sealed with both electrical and duct tape wrapped completely around the core barrel. Usually 5-10 ml of pore
water was obtained in this way and emptied into 60 ml
centrifuge tubes with screw cap tops. Upon immediate
return to the laboratory, each water sample was filtered
separaely through 0.45 pm nylon filters using pressure
from
a
alkalinity
60
ml
syringe.
Total
(HCO; + CO: + other dissolved species such as
H,BO;
[borate]) was then calculated after titration
with dilute (0.1 N) HCl using apH meter, as demonstrated to REM by Dr. Chas. Culberson (pers. commun., 1991). Contributions
of dissolved borate and
other ions to seawater alkalinity are typically quite
small
so that carbonate
alkalinity
(HCO, +
CO:- ) = total alkalinity (Broecker and Peng, 1982).
Total carbonate weight percent (TCARB = total
shell content) was determined by the method of Schink
et al. ( 1978).
26 (1995) 187-206
3.3. Enumeration
offoraminifera
We used total (live+dead)
foraminifera in our
study. Zhang (1994, table 8) found very low numbers
of living foraminifera (typically <0.5% of the total
[live + dead] assemblage; mean: 1.7 + 3.7%; range: O13.1%) in sediment collected to depths of 40 cm during
July ( 1991) reconnaissance, preserved in buffered formalin, and stained with Sudan Black B (Walker et al.,
1974). Moreover, the two largest populations of foraminifera ( 11.I%, 13.1%) were found in samples with
low total numbers of tests (19 and 145 specimens,
respectively).
Upon arrival at Delaware, sediment from 5 cm core
intervals was subsampled for foraminifera using a sample splitter. Foraminifera were concentrated from 10
gram sediment samples (determined
by trial-anderror) via flotation techniques using heavy liquids
(Ccl,; Brasier, 1980). Sediment residue was checked
frequently after flotation for separation of tests from
sediment. Cushman (1930), Walton ( 1955), Sandusky ( 1969)) and Phleger ( 1960) were the primary
sources for species identification. Abundances of the
predominant
species are available from the senior
author upon request.
Herein, we assume that species-specific differences
in size or morphology produce little bias in counting
(Martin and Liddell, 1988, 1989). Although this is
obviously untrue of reef-dwelling foraminifera (Martin
and Liddell, 1988, 1989)) it appears to be a relatively
safe assumption based on our studies of Choya Bay
foraminifera.
3.4. Statistical analysis
Cluster, factor, and canonical discriminant analyses
were performed on combined data sets of downcore
foraminiferal abundance, TCARB, and alkalinity for
each sampling season on the University of Delaware
mainframe computer using SAS Version 6.0 (Cary,
NC). Other statistical analyses
(Mann Whitney
U= MWU and Spearman’s p) were run on the University of Delaware mainframe computer using Minitab Version 7.2 (Duxbury Press, Boston, MA).
3.5. Radiocarbon
dates
Accelerator Mass Spectrometer (AMS) j4C analyses were performed at the NSF-University
of Arizona
R.E. Martin et al. /Marine
Micropaleontology
26 (1995) 187-206
191
(Tucson) facility on combined samples of N 100 specimens total ( 2 1 mg CaCO, required for analysis) of
primarily Buccellu mansfieldi (Cushman)
1930, and,
secondarily, Elphidium cf. E. crispum (LinnC) 1788
(for site 9; Fig. 2A). Tests appeared pristine (lack of
obvious pits. perforations, borings, etc.) at the light
microscope level, and came from samples collected at
northern flat sites 8 (O-15 cm depth; shell-poor sediment a&e shell layer) and 9 (20-25 cm depth; within
shell layer; Fig. 2B) during July, 1992. Relative to sites
further south on the flat, these locations have relatively
high total shell (CaCO,) content near the sedimentwater interface and low rates of bioturbation.
All radiocarbon
dates discussed
herein were
obtained via the protocol described in Flessa et al.
( 1993; see also references therein). Radiocarbon dates
reported from the NSF-University
of Arizona laboratory were “conventional”
dates; i.e., by convention,
dates were normalized to 6°C = - 25%0 (assuming a
613C = O%O), and reported with respect to the Libby
half-life of 5568 years as years before 19.50. We corrected for the reservoir effect by the method of Flessa
et al. ( 1993), which was based on a specimen of Chione
(Chione) californiensis (Broderip) 1935 collected at
Choya Bay in 1949. Use of a bivalve date to correct
foraminiferal dates should make no difference in the
correction since the amount of “old” carbon stored in
the oceans will appear the same to both foraminifera
and bivalves (K.W. Flessa, pers. commun., 1994).
Conventional dates were converted to calendar years
using the calibration software of Stuiver and Reimer
(1993).
For the sake of comparison, we give ages in both
conventional and calendar years. The lg error (68%
probability of the true age falling within the range) for
conventional dates represents counting error only (no
correction for reservoir effect; Flessa et al., 1993). The
2a error (95.4% probability of the true age falling
within the range) for calendar year dates includes the
effects of error in counting and in modeling fluctuations
in the specific activity of carbon in oceanic and atmospheric reservoirs (Flessa et al., 1993).
2.5 meq/l; Chas. Culberson, pers. commun., 199 1).
Values were relatively uniform downcore, typically
ranging from 5 to 10 meq/l in both January (1992)
and July ( 1992; Figs. 3 and 4A), and did not differ
significantly between seasons (MWU) Values tended
to be low on the inner flat, where bioturbation was most
intense, then rose slightly on the middle flat, where
burrow densities decreased, before declining somewhat
on the outer flat, where wave agitation and shallow
bioturbation are extensive. During July, 1992, when air
(water) temperatures were quite warm and the activities of SOi--reducing
bacteria presumably enhanced,
values were somewhat higher (up to - 12 meq/l) at
site 6 (Figs. 3 and 4A; outer flat, middle transect), and
substantially higher ( - 20-50 meq/l) at site 8, on the
northern flat; both sites were characterized by a relatively thin sediment veneer (Fig. 2B), and greatly
decreased burrow densities, at the time.
TCARB typically ranged from 0 to 20% in surficial
sediment above the shell layer in both January and July
( 1992) but increased to > - 50-60% in the shell layer
(Figs. 4B and 5). The top of the shell layer was typically indicated by an abrupt increase in TCARB,
although sometimes the contact between the shell layer
and the overlying shell-poor mixed layer was gradational (Figs. 2B and 5). Downcore TCARB did not
change significantly between seasons (MWU; cf. Fig.
5), although it was substantially higher than summer
levels during January at site 9 on the outer portion of
the northern transect (Fig. 2A). As TCARB tended to
increase both across the flat (Fig. 4B) and into the
subtidal zone (Zhang, 1994), the January peak at site
9 was probably related fortuitously to the site location,
which varied for outer flat cores (Fig 2A; see also
“Coring Procedures”),
rather than a pulse of shell
input related to reproduction and die-off or to the formation of shell lags by storms. Although there was no
significant correlation between TCARB and alkalinity
(Spearman’s p) during either January or July ( 1992))
when these variables were measured, TCARB (like
alkalinity) increased to the north, especially in January
(Fig. 4B).
4. Results
4.2. Foruminiferul
4. I. Alkulinity and total carbonate weight percent
Alkalinity of Choya Bay porewaters is typically elevated somewhat above that of normal seawater ( N 2-
distribution and abundance
Despite the superficial homogeneity of the tidal flat
environments at Choya Bay, foraminifera exhibited a
distinctive zonation across the flat that persisted down-
A)
JANUARY
1992
ALKALINITY
(MEQ/L)
6
7
8
W
JULY
1992
ALKALINITY
(MEQ/L)
9
Legend
- CORE 1
m CORE 2
*
Legend
l
CORE
1
CORE 5
--_
. CORE
--_ 6
* CO,RE 7
*
0 CORE 2
* CORE 3
40
3 -I_CORE 4
& -wCORE 5
45
a --CORE 6
q CORE.7
,.
CORE 3
0 --CORE 4
*
L
CORE 8
“I...,.,..,,.,.....,...
55
1
CORE
.,.
.... ,I8.,.
,,
50
55,
60,
Fig. 3. Downcore alkalinity
(milliequivalents/liter)
for each site in (A) January,
core. Only rarely did tests exhibit evidence of residual
protoplasm (either in surface or downcore samples)
that might indicate that the specimen was alive at the
time of collection (cf. Martin and Steinker, 1973;
Langer et al., 1989). Ammonia beccarii (LinnC) 1758
was most abundant on the inner flat, especially in January. By contrast, BuccelIa mansfieldi (Cushman)
1930, Elphidium clavatum Cushman 1930, and Elphimiddle-to-outer
flat sedidium spp., characterized
ments, as did the suborders Miliolina, Rotaliina, and
1992, and (13) July, 1992 (note scale change
from Fig. 3A).
Textulariina,
although the last taxon was relatively
uncommon. Elphidium cf. E. crispum was most characteristic of the outer northern flat (site 9) during July,
1992.
The effect of CDFs was evident in downcore profiles
of foraminiferal abundance (Fig. 6). A Fall-Winter
reproductive pulse, which was presumably caused by
overturn of the northern Gulf water column and associated phytoplankton blooms (Fig. 7)) was especially
noticeable in January at southern flat stations 2 and 3
R.E. Martin et al. /Marine
Micropaleontology
26 (1995) 187-206
193
A)
AVERAGE
ALKALINITY
AVERAGE TOTAL CARBONATE WEIGHT PERCENT
\
Fig. 4. (A) Map of average downcore alkalinity (average of alkalinity for each horizon at each site * I standard deviation) for January and
July, 1992. (B) Map of average downcore total carbonate weight percent (average of total carbonate weight percent for each horizon at each
site 5 I standard deviation) for January and July, 1992.
as a bulge in abundance at 5-10 cm depth that was
apparently being moved downward by CDFs (Fig.
6B). The bulge was reminiscent of downward advection of “impulse”
tracers such as microtektites, volcanic ash, or the radioactive tracer 137Csby bioturbators
(e.g., Guinasso and Schink, 1975; Christensen and
Goetz, 1987). The reproductive pulse was, however,
not evident at inner flat site 1, where CDFs were very
abundant, or at stations further north (except perhaps
for site 6)) where the depth to the shell layer ( = thickness of the overlying mixed layer) thinned markedly
(Fig. 2B) and CDFs were probably more efficient in
homogenizing
sediment. By the summer, when air
(water) temperature had increased and CDFs had
become more active (and dissolution presumably more
intense),
reproductive
pulses (bulges)
had disappeared and foraminiferal profiles were more irregular
downcore (Fig. 6). Thus, most foraminiferal tests at
Choya Bay persist for only a few months before they
dissolve (Fig. 7). Nevertheless, some tests persisted in
shallow sediments of northern (sites 8, 9) and outer
(site 6) flat stations and in the subsurface shell layer
(cf. Figs. 5 and 6).
The extent of dissolution varied between the summers of 1991 and 1992. Foraminiferal
numbers for
July, 1992, core samples were significantly less than
for July
( 1991)
and January
( 1992; MWU;
p < 0.0006). By contrast, foraminiferal numbers for
July, 199 1, and January were not significantly different
(MWU).
Like alkalinity and TCARB, foraminiferal number
increased northward and outward across the flat during
the summer, as the presumed reproductive
pulse
decayed (Fig. 8). Although foraminiferal
number
exhibited no significant correlations with TCARB for
January and July or alkalinity (for January), it did
exhibit a moderate (Sprinthall, 1982, p. 192) negative
correlation with alkalinity for July, 1992 (Spearman’s
R.E. Mm-tin et ~1. /Murk
194
4
JANUARY
Mi~ro~~uleonrolr~y
6)
1992
10
20
30
40
50
.
187-206
JULY 1992
TOTAL CARBONATE WEIGHT (8)
TOTAL CARBONATE WEIGHT (%)
0
26 (1995)
40
* CORE 1
* CORE 1
. CORE 3
0 CORE
-_1_- 4
* -_CORE 5
0 ---CORE 4
' --_
CORE 5
x --_
CORE 6
T CORE 7
---
Fig. 5. Total carbonate weight percent ( = TCARB = total shell content) downcore at each site for (A) January, 1992; and (B) July, 1992.
Abrupt change in shell content downcore in January indicates top of shell-rich layer discussed in text, although contacts are sometimes gradational
(cf. Fig. 2).
p = - 0.487;
p < 0.01) ,when alkalinity was relatively
high (especially at site 8; Figs. 3 and 4A). Interestingly, July ( 1992) foraminiferal number exhibited a
moderatepositive
correlation with Hanzawaia strattoni
( Applin) 1925 (Spearman’s
p = 0.657; p < 0.01))
which also exhibited a moderate negative correlation
with July ( 1992) alkalinity (Spearman’s p = - 0.666;
p < 0.01) . This species also displayed a positive corwith
TCARB
(Spearman’s
p = 0.396;
relation
p < 0.01) and tended to increase to the north, although
it was most abundant on the inner flat, where alkalinity
tended to be low.
4.3. Multivariate
statistical analyses
Cluster analysis revealed no meaningful
groups
despite repeated attempts with different clustering coefficients. Ftirsich and Flessa (1987) also found relatively indistinct groupings in cluster analyses of middle
and outer flat macroinvertebrates.
Principal factor analysis produced somewhat better
results. Depending upon the sampling season, three
factors accounted for - 85 to 98% of the variance. For
each of the three sampling seasons, the first factor
of variance, the second for
accounted for -5565%
- 16-22%, and the third for - l&12%. The first factor
25
FORAMS/GRAM
JULY
2
1
I
CORE
-___
CORE
7
7
+
CORE
......8
. .
CORE
9 ...‘.>
...........
CO&r3
6
5
4
. --CORE
l
0 --CORE
CORE
--_
* CORE 3
*
Legend
SEDIMENT
1991
0
c I'
ii
50
45
40
35 -.
30
25
20
15
10
5
0 I
W
CORE
) CORE
CORE
--_
CORE
---
3
8
7
6
5
?
CORE
-*.*
', CORE
1
2
., CORE
CORE
Legend
.
-
SEDIMENT
FORAMS/GRAM
50
1992
JANUARY
1
60
0
Cl
JULY
1992
L
Legend
CORE
---
5
Fig. 6. Foraminiferal
The July
curves in
* .,.,....,..
CORE 8.,,,
,.
* .,.l.~:,.o,...l,.,
CORE
9
CORE 6
m-* fZfJ!E7
a
*
1 --CORE 4
- CORE 3
a CORE 2
= CORE 1
25
FORAMS,'GRAM SEDIMENT
1992, and (C) July, 1992. Bell-shaped
number (number of foraminifera/gram
sediment) downcore at each site for (A) July, 199 I, (B ) January.
January,
1992, at southem
flat stations appear to represent downward
movement of presumed Fall-Winter reproductive pulse by CDFs (see text for further discussion).
( 199 1) Season
was used primarily for reconnaissance for future sampling; hence, sample intervals are not always spaced every 5 cm.
0
A)
R.E. Martin et al. /Marine
Micropaleontology
I
Fig. 7. Sampling times (July, 1991, 1992; January, 1992) and seasonal reproduction of foraminifera ( = P) in relation to overturn of
nutrient-rich thermocline in the Gulf of California (based on depth
to thermocline; Robinson, 1973), associated phytoplankton blooms,
and intensity of bioturbation and SOi- reduction.
exhibited moderate to high positive loadings of Buccella mansfieldi, Elphidium (E. clauatum, E. cf. E.
crispum, and Elphidium spp.) , discorbids + rosalinids,
the suborders Rotaliina and Miliolina, and foraminifera1 number, and tended to characterizeouter and northern flat stations. Factor two was characterized by
moderate positive loadings of Ammonia beccarii and
low-to-moderate
loadings of Buccella mansfieldi and
Elphidium spp.; this factor appeared to contrast inner
and southern flat stations versus outer and northern flat
stations. No clear pattern emerged from factor 3.
Canonical discriminant
analysis was much more
effective in revealing the intricacies of biological, sedimentological,
and geochemical processes that occur
on the flat through the year (Figs. 9-l l), and tended
to confirm observations based on raw data and factor
analysis. The first two canonical correlations were typically significant at p < 0.025 (oneway ANOVA; SAS
User’s Guide: Basic Statistics);
canonical variable
(CV) 1 normally accounted for - 75-80%, and CV 2
for 15-20%, of variance. During January, following
26 (1995) 187-206
the Fall-Winter reproductive pulse, southern (sites l3)) middle (sites 4-6)) and northern (sites 7-9) transect assemblages were highly gradational (Fig. 9A),
and correlations of the original variables with canonical
variables were mostly low ( r = 0.2-0.4; Sprinthall,
1982; Table 1) ; CV 1 primarily represented an inverse
relationship between TCARB and Ammonia beccarii,
whereas CV 2 represented an inverse relationship
between alkalinity and Hanzawaia strattoni. Inner
(sites 1, 4, 7) and middle flat (sites 2, 5, 8) stations
resembled each other more strongly than outer flat
(sites 3, 6, 9) assemblages (Fig. 9B; Table 2); in this
case, CV 1 represented primarily Buccella mansfieldi,
and, secondarily, Elphidium clauatum, the suborders
Miliolina, Rotaliina, and Textulariina, and foraminifera1 number; Ammonia beccarii exhibits an inverse
relationship to these groupings. Canonical variable 2
again represented an inverse relation between alkalinity
and Hanzawaia strattoni.
By July ( 1992)) however, well after decay of foraminiferal reproductive pulses had begun (Fig. 7)) northern transect and outer flat sites, although quite variable,
were relatively distinct from remaining sites (Fig. 10).
AVERAGE FORAM NUMBER
1
Fig. 8. Map of average downcore foraminiferal number (average of
counts for each horizon sampled at each site f 1 standard deviation)
for July (1991) and January and July (1992).
R.E. Martin
et al. /Marine
26 (1995) 187-206
Micropaleontology
197
A) JANUARY 1992
CAN 2
I *
B) JANUARY 1992
1
1
1
I
1
1
1
1
3 OUTEF;
1
~3
.
. . ..~~~~~~~......~.......~.*..--~.~................*~..........*...........*....~~~..............~~~.........~....
-3
-2
~1
0
...~.~~.*..
I
2
4
5
6
’ CAN
1
Fig. 9. Plots of canonical variables 1 and 2 for January, 1992, for (A) southern (l), middle (2). and northern (3) transect sites and (B) inner
( I ), middle (2). and outer ( 3) flat sites. See Tables 1 and 2 for correlations between original variables and canonical variables.
R.E. Martin et al. /Murk
198
Micropuleontology 26 (1995) 187-206
A) JULY 1992
NORTH
I
1
-f
5
CAN 2
5
6 CAN
1
B) JULY 1992
7
’
3
OUTER
3
-1
*
---.--~-------*----------*-.........*......~~......................~.*.........~.~......~.~~......~~~~~.._____~~.~.~~...___~~~~
~1
-2
-1
0
1
1
3
4
5
6
7
8 CAN
1
Fig. 10. Plots of canonical variables I and 2 for July, 1992, for (A) southern (l), middle (2), and northern (3) transect sites and (B) inner
( 1). middle (2). and outer (3) flat sites. See Tables 1 and 2 for correlations between original variables and canonical variables.
RX. Martin et al. /Marine Micropaleontology 26 (I 995) 187-206
CAN
2
A) JULY 1991
B) JULY 1991
CAN21
5t
0
199
.
I
3
I
3
-1
.
1
OUTER
Fig. 11.Plots of canonical variables I and 2 for July, 1991, for (A) southern ( 1 ), middle (2). and northern (3) transect sites and
( I 1, middle (21, and outer (3) flat sites. See Tables 1 and 2 for correlations between original variables and canonical variables.
( B) inner
200
R.E. Martin et ui. /Marine
Micropaleontology
Table I
Total canonical structure (total sample correlations between original
variables and canonical variables [ CV 1 I and 2) for southern, middle, and northern transect sites. Correlations rounded to second decimal place
July, 1991
CVI
BEC’*
BOL
BUC
DR
ECLV
ECSP
ESPP
HANZ
MIL
ROT
TEX
UNK
FNO
ALK
TCARB
- 0.25
0.18
0.05
0.49
- 0.3 I
0.27
0,s I
0.34
0.14
0.15
0.12
0.27
0.17
January,
1992
July, 1992
CVl
cv2
CVI
0.29
0.34
0.30
0.01
0.42
- 0.42
0.05
- 0.04
- 0.07
-0.31
0.13
-0.15
0.29
0.42
-0.19
0.27
0.41
0.06
0.35
0.08
-0.12
- 0.22
- 0.22
- 0.08
- 0.29
0.54
- 0.29
0.19
0.12
0.08
0.23
- 0.08
0.10
-0.60
0.20
0.12
0.12
0.17
0.15
0.52
0.27
cv2
0.12
- 0.03
0.29
0.37
0.11
0.41
0.33
0.66
0.46
0.62
0.08
0.30
0.64
0.30
0.18
cv2
-0.17
0.07
- 0.28
-0.26
- 0.40
0.04
-0.39
- 0.27
-0.16
-0.37
0.15
0.09
-0.36
0.03
- 0.07
’ *BEC = Ammo& heccurii; BOL = BolitCnn spp.; BUC = Buccello
manxfeldi; DR = Discorbids + rosalinids; ECLV = Elphidium chtuturn; ECSP = Elphidium cf. E. crispum (counted in Elphidium spp.
for July, 199 I ); ESPP= Elphidium spp. (mainly E. urticulatum and
intergradational
forms) ;
HANZ= Hunzawain
strattoni;
MIL= suborder Miliolina (all) ; ROT = suborder Rotaliina (all) ;
TEX = suborder
Textulariina
UNK = unknown;
(all);
FNO = Foraminiferal
number (number of tests/gram
sediment);
ALK = Alkalinity (meq/l) ; TCARB = Total carbonate weight %)
Foraminiferal number, suborders Rotaliina and Miliolina, Hanzawaia strattoni, and Elphidium cf. E. crispum
exhibited moderate positive correlations (r = 0.4-0.7;
Sprinthall, 1982) with CV 1 in canonical discriminant
analysis of southern, middle, and northern transect
sites, whereas correlations between original variables
and CV 2 were relatively low (Table 1) . In analysis of
inner, middle, and outer flat sites (Table 2)) foraminifera1
Buccella
discornumber,
mansfieldi,
bids +rosalinids,
Elphidium cf. E. crispum, and
suborder Miliolina exhibited moderate positive correlations with CV 1, and alkalinity displayed moderate
positive, and TCARB and Hanzawaia strattoni moderate negative, correlations with CV 2.
The behavior of sites for July, 1991, tended to resemble those for January, 1992. As for January, 1992,
southern, middle, and northern flat sites for July, 1991,
were highly gradational (Fig. 11A; cf. Fig. 9A). Cor-
26 (1995) 187-206
relations between Elphidium clavatum and Hanzawaia
and CVl for July ( 1991) were essentially identical to
those of January, but, unlike January, correlations
between discorbids + rosalinids and Elphidium spp.
and CVl were higher than for other variables; CV2 was
not statistically significant (Table 1) The behavior of
inner, middle, and outer flat sites also differed from that
for January (Table 2): CV 1 represented Buccella
mans$eldi and Elphidium clavatum, the suborder Rotaliina, and foraminiferal number; CV 2, though, exhibited a stronger inverse relationship between Ammonia
beccarii and Hanzawaia strattoni, on the one hand, and
Buccella mansfieldi and the suborder Textulariina on
the other. Plots of canonical discriminant functions for
July, 1991, and January inner, middle, and outer flat
sites differ accordingly (cf. Figs. 9B and 11B) .
4.4. Foraminiferal radiocarbon dates
Despite extensive dissolution of foraminifera
at
Choya Bay, test ages were surprisingly old (Table 3) :
1309 calendar years (2a range: 1167-1508) for site 9
(20-25 cm depth; tests from within shell layer) and
2026 calendar years (2cr range: 184 l-2278) for site 8
(O-15 cm depth; tests from shell-poor sediment above
shell layer).
Table 2
Total canonical structure (total sample correlations between original
variables and canonical variables [CV I 1 and 2) for inner, middle,
and outer flat sites. Correlations rounded to second decimal place
See Table 1 for abbreviations
BEC
BOL
BUC
DR
ECLV
ECSP
ESPP
HANZ
MIL
ROT
TEX
UNK
FNO
ALK
TCARB
July, 1991
January,
1992
July, 1992
CVl
cv2
CVl
cv2
CVI
cv2
-0.12
-0.16
0.67
0.19
0.78
0.56
0.38
- 0.58
0.00
-0.13
-0.46
-0.05
0.91
0.17
0.62
0.33
-0.18
-0.12
- 0.44
- 0.2 I
-0.34
0.10
0.73
0.61
0.39
-0.07
-0.17
~ 0.05
0.15
0.33
0.19
0.10
0.05
0.62
-0.13
0.41
0.54
0.35
0.50
0.02
0.02
0.73
0.07
0.01
0.31
0.06
-0.17
0.47
0.63
0.57
0.47
0.62
-0.30
0.28
- 0.24
- 0.37
0.70
-0.18
- 0.26
- 0.01
-0.12
- 0.25
-0.60
0.16
0.58
-0.11
- 0.09
0.65
0.33
0.22
0.59
0.53
-0.14
0.12
- 0.05
0.28
- 0.50
- 0.04
-0.04
0.20
0.42
- 0.05
0.57
-0.47
-
R. E. Martin et al. /Marine Micropaleontology 26 (I 99.5) 187-206
201
Table 3
Radiocarbon dates for foraminifera (mainly Buccella mansjieldi + some Elphidium cf. E. crispurn) from northern tidal flat stations of Choya
Bay. Ranges represent 2a (see text for further discussion)
Conventional
Site 8
(O-IS cm sediment depth, above shell layer; NSF-Arizona
site Y
(20-25
cm sediment depth, within shell layer; NSF-Arizona
distribution
Calendar age ( f 2~)
2775 + 60
2026(1841-2278)
215O+SS
1309(1167-1508)
AMS Facility Number AA1 1801)
AMS Facility Number AA I 1800)
5. Discussion
5.1. Foraminiferal
age
and abundance
The tidal hats of Choya Bay consist of an intergradational intertidal zonation that strongly reflects the
subtle interactions of organisms and sediment (see
Peterson, 199 1, for genera1 review; see Fiirsich and
Flessa, 1987, 1991, for Choya Bay). CDFs are most
likely abundant on the inner flat because of relatively
low wave energy and the availability
of abundant
organic matter (food; Peterson, 1991) ; CDFs also
cause extensive dissolution in this environment (e.g.,
low alkalinity). On the middle flat, the abundance of
CDFs declines and alkalinity rises somewhat, but as
the outer flat is approached, wave energy and shallow
infaunal and epifaunal burrowing tend to increase,
which lowers alkalinity to inner flat values (Figs. 3 and
4A).
Foraminifera abundance tends to follow the innermiddle-outer flat zonation. Ammonia beccarii characterizes the inner flat, where the rigors of temperature
and salinity are no doubt highest (Murray, 1991),
whereas Elphidium clauatum, Buccella mansfieldi,
Elphidium cf. E. crispum, and the suborders Rotaliina,
Miliolina, and Textulariina characterize the outer flat.
The occurrence of certain species in both the middle
and outer flat (as opposed to the inner flat) may reflect
not only more optima1 environments, but perhaps also
transportation of outer flat species onto the middle flat
(Zhang, 1994). Despite intensive burrowing, foraminiferal numbers tend to be relatively high near the tops
of cores and decrease downward (Fig. 6), suggesting
that only relatively small populations, at best, live at
greater depths in the sediment (cf. Corliss, 1985;
Langer et al., 1989; Corliss and Emerson, 1990; Goldstein and Harben, 1993).
The zonation at Choya Bay bears the strong overprint
of antecent topography. To the north, as the Pleistocene
platform shallows, both the depth to the shell layer and
the thickness of the overlying shell-poor mixed layer
decrease, just as they tend to do toward the outer flat
(Fig. 2B). The same foraminiferal species that increase
in abundance in sediment from inner to outer flat also
tend to increase to the north, most likely because of
increased habitat availability on rocky outcrops, less
extreme temperature and salinity fluctuations, and, perhaps, changes in pore water chemistry.
5.2. Foraminiferal
reproduction and preservation
Foraminiferal reproduction at Choya Bay appears to
occur in discrete (ca. a few weeks) seasonal pulses,
which are then followed by periods of homogenization
and dissolution of several months duration (Fig. 7) ;
i.e., small populations of living foraminifera are not the
result of rapid sedimentation (Walton, 1955; Phleger,
1960, pp. 189-212). Green et al. ( 1993) also calculated a mean residence time for foraminiferal tests in
Long Island Sound sediments of 86-t 13 days, and
Powell et al. (1984) estimated half-lives of 100 days
for the smallest (0.8-3.1 mm) juveniles of molluscan
death assemblages. The significantly lower foraminifera1 numbers in July ( 1992) than in July ( 1991) may
reflect the unpredictability
of the exact timing of seasonal reproduction as well as our sampling at a somewhat later time in July ( 1992) than in July ( 199 I ),
thereby allowing slightly more time for dissolution of
assemblages.
The same factors that determine the distribution of
living foraminifera at Choya Bay also strongly influence their preservation. Foraminiferal test dissolution
at Choya Bay is much more pervasive than at Discovery
Bay (Martin, 1993; see also Alexandersson,
1972;
Smith, 1987; Murray, 1989). Despite the lack of sig-
202
R.E. Martin et al. /Marine
Micropaleontology
nificant correlations between foraminiferal abundance
and alkalinity and TCARB, foraminifera persist for
longer periods of time at northern flat stations (Fig. 8).
Although the shell-poor mixed layer overlying the subsurface shell layer is probably stirred more rapidly by
CDFs to the north because of the shallowing of the
Pleistocene platform (and accompanying thinning of
the overlying mixed layer), the shallowness of the platform tends to inhibit bioturbation, allows buildup of
alkalinity (as high as 50 meq/l; Fig. 4A), and keeps
shell material relatively close to the surface (Figs. 2B
and 4B), all of which slow dissolution; i.e., foraminifera1 preservation in relatively shell-rich siliciclastic
sediments at Choya Bay most closely approximates
carbonate regimes, as predicted by Martin and Liddell
( 199 1) and Kotler et al. ( 199 1, 1992; see also Aller,
1982; Kidwell, 1989). In effect, antecent topography
(Pleistocene outcrop) serves as a kind of ’ ‘taphonomic
feedback” (Kidwell, 1986b) on the development of
microfossil assemblages.
Tests from Choya Bay are quite small ( < 250 pm),
and are characterized by a high surface/volume
ratio
and presumably high chemical reactivity. Indeed, the
relatively pristine test surfaces (at the light microscope
level) also implies that most tests dissolve rapidly
(Figs. 6-8); i.e., test microstructure, mineralogy, etc.,
typically make little difference in the taphonomic
behavior of foraminifera at this locale. Nevertheless,
the inverse relationship between Hanzawaia strattoni
and alkalinity suggests that tests of this species may
survive dissolution at Choya Bay because of relatively
thick walls or microstructure.
Moreover, the differences in correlations between original variables and
canonical variables between July ( 199 1) and July
( 1992) suggest that some foraminiferal taxa may decay
at slightly different rates. Similar (but more pronounced) behavior has been documented for other
foraminiferal species: Corliss and Honjo (1981) and
Bremer and Lohmann ( 1982)) for example, found that
deep-sea species of foraminifera that live below the
CCD are more resistant to dissolution than those, such
as Amphistegina, that characterize reef sediments.
5.3. Dissolution models, taphonomic
temporal resolution
grades, and
Studies of deep-seadissolution
hold important implications for shallow shelf regimes. Broecker et al.
26 (1995) 187-206
(1991) developed an age (mixing) model for deepsea sediments based on the assumption that dissolution
within the zone of bioturbation should be proportional
to the residence (replacement)
time of grains within
the mixed layer. They distinguished two forms of dissolution: homogeneous and sequential. In homogeneous dissolution, each grain loses a constant fraction of
its mass per unit time (irrespective of grain type),
which shifts the mass distribution of assemblages in
the mixed layer toward younger grains in core top
assemblages because the replacement time of grains in
the mixed layer by new grains from the pelagic rain is
reduced. In sequential dissolution, grain type A dissolves completely before grain type B begins to dissolve, and so on; in this case, core top ages presumably
increase with the extent of dissolution (see Martin,
1993, for review).
With respect to calcareous microfossils, sequential
dissolution was predicted to predominate in shelfal carbonate environments (such as Discovery Bay) ; by contrast, homogeneous
dissolution
was predicted to
dominate in siliciclastic regimes, such as Choya Bay
(Martin, 1993). Obviously,
dissolution
is neither
purely homogeneous or sequential at Choya Bay. At
Choya Bay, tests that survive dissolution probably do
so because they are rapidly advected downward by
CDFS into the shell layer and preserved there until,
much later, they are reworked upward by biological
activity (e.g., McCave, 1988) or storms (K.H. Meldahl
and A. Olivera, pers. commun., 1994).
What is most surprising about the foraminiferal preservation mechanism at Choya Bay is the unexpectedly
great age of the tests. Our studies suggest that Holocene
shallow-water microfossil assemblages may be timeaveraged over as much as hundreds to thousands of
years. Our results are corroborated by Flessa (1993)
and Flessa et al. (1993; see also Flessa and Kowalewski, 1994) for bivalves (Chione spp.) from Choya
Bay. The age of foraminiferal assemblages analyzed
by us falls within the range of ages for disarticulated
Chione spp. collected by Flessa et al. ( 1993) from the
sediment-water interface of the inner flat of Choya Bay
[ 2 0 ( “post-bomb” ; i.e., A.D. 1950 or younger) to
3569 calendar years], although foraminiferal ages tend
to fall near the higher end of the age range for Chione
spp. (Flessaet al., 1993, table 1). Flessaet al.‘s ( 1993,
table 2) shells from the inner flat exhibited a broad
range of taphonomic grades (surface condition), but
R.E. Martin
et al. /Marine
Micropaleontology
taphonomic grade was not an infallible indicator of
shell age (time since death); old specimens ( - 1900
years) were sometimes relatively pristine, whereas relatively young shells ( - several hundred years) were
sometimes more highly degraded. Flessa ( 1993) and
Flessa et al. ( 1993) suggested that the condition of a
shell’s surface is primarily indicative of the residence
time of the shell at the sediment-water
interface and
not its age (see also Kidwell, 1991, 1993a,b). Even if
a shell is rapidly buried by downward advection by
burrowing organisms (such as at Choya Bay), rather
than by rapid sediment influx, it may still remain relatively pristine because it has been removed from the
Taphonomically
Active Zone (TAZ; Davies et al.,
1989) near the surface.
The mechanism of microfossil preservation at Choya
Bay grades into the upward reworking (“leaking”)
of
much older tests into younger sediments (“remanie”;
Murray-Wallace and Belperio, 1994; Kidwell, 1993a).
Reworking of substantially
older microfossils
into
younger sediments (or vice versa by downward “piping”) is not usually a serious problem for the biostratigrapher.
In
most
microfossil-based
cases,
biostratigraphic zonations are sufficiently precise that
reworked specimens are typically recognized by their
anomalous stratigraphic occurrence and state of preservation; such specimens were noted only ~:ev rarely
in our samples, especially at site 7 (inner northern flat),
and they were not used in 14C analyses.
Time-averaging of microfossil assemblages is much
more insidious, however. Murray-Wallace
and Belperio ( 1994), for example, found that specimens of
wrtebralis
Blainville
1846 were
Marginopora
reworked from underlying
Late Pleistocene
rocks
( - 125,000 years age based on amino acid racemization) into modern tidal flat sediments, and that the
surfaces of reworked Marginopora
exhibited little
taphonomic alteration. Thus, substantial numbers of
significantly older shells may be mixed into younger
assemblages (depending on shell content of the sediment and intensity of bioturbation)
and the time scales
of accumulation affected accordingly, with little or no
observable change in the character of the microfossil
assemblages themselves.
26 (1995) 187-206
5.4. Implications for time-averaging
microfossil assemblages
203
qf offshore
The extent of mixing on short temporal scales-and
the exact limits of stratigraphic resolution inherent to
each taphonomic environment
(“taphofacies”)-no
doubt vary across the continental shelf and slope (Martin, 1993). For modern shelves, Flessa ( 1993) estimated time-averaging in the nearshore zone of - 1000
years (more-or-less in agreement with our results) and
of up to 10,000 years for shelves exclusive of the nearshore zone. Whether or not shelfal microfossil assemblages formed offshore show similar degrees of
time-averaging
remains to be determined and will
require extensive study of the sedimentary dynamics
and shell input to each taphofacies (cf. Denne and Sen
Gupta, 1989; Loubere. 1989; Loubere and Gary, 1990;
Loubere et al., 1993). Indeed, Dubois and Prell ( 1988)
concluded that although sediment may have the same
radiocarbon age, the proportions of the components
producing that age may not be the same if the particles
have different preservational histories, and that in order
to use 14C dates in stratigraphy, the processes c-ontrolling hardpart input and loss must be er,aluated. The
similarity in age, and, apparently, mechanism of preservation of foraminiferal and bivalve assemblages at
Choya Bay suggests that stratigraphic and taphonomic
criteria derived for macrofossils (e.g., Kidwell, 199 I,
1993a, 1993b) may be useful in assessing the formation
and degree of time-averaging
of microfossil assemblages. The study of shallow-water assemblages
is only
a first step in deciphering the complex, and often subtle,
processes that form microfossil assemblages and their
relative time scales of accumulation.
6. Conclusion
Holocene tidal flat environments at Choya Bay arc
surprisingly complex in terms of the subtle interplay
between shell input, bioturbation, pore water chemistry, and shell preservation. At Choya Bay, foraminifera
persist longest at northern flat stations because of
decreased bioturbation and elevated shell content and
which most closely
alkalinity
(i.e., environments
approximate carbonate regimes). These three factors
are, in turn, a function of shallowing of a Pleistocene
rocky platform around the northern margin of Choya
204
R.E. Martin et al. /Marine Micropaleontology 26 (1995) 187-206
Bay (antecent topography and taphonomic feedback).
The taphonomic grade of tests is not a reliable indicator of test age; rather, it is an index of time of exposure
at the sediment-water
interface. Despite intensive dissolution of foraminifera, tests are surprisingly old (up
to - 2000 years based on AMS 14C dates). Moreover,
these tests are relatively pristine at the light microscope
level. We hypothesize that some tests survive dissolution by rapid downward piping by conveyor belt
deposit feeders into a subsurface shell layer, and are
preserved there until they are reworked upward by biological activity or storms. Thus, the dynamics of shell
input and preservation must be accounted for in assessing time-averaging of microfossil assemblages.
Acknowledgements
Our studies
at Choya Bay have been funded by NSF
Grant Number EAR-9017864. We gratefully acknowledge the support of the NSF-University
of Arizona
AMS Facility. Thanks to Karl Flessa and Jim Pizzuto
for advice on radiocarbon dating, and to Barun Sen
Gupta and Tim Patterson for constructive reviews.
Many thanks also to geology undergraduates Maryanne
Johnson, Dave Lawrence, Darren Rasmussen, and
Dave Sterling of Utah State University for their dedicated assistance in the field and laboratory, without
which our studies could not have been completed. Barb
Broge drafted the figures.
For further information the senior author can also be
via
INTERNET;
his
address
is
contacted
[email protected].
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