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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 of Geology, University of’Delawure, Newark, DE 19716, USA

h Deprtment c.fGeology, Utah Stclte University, Logpn, 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 set-

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).

0377-839X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSD/O377-X398(95)00009-7

188 R.E. Murtin et 01. /Munne Micropalecjnt~/i~~~ 26 (1995) 1X7-206

tests in carbonate environments persist for relatively

r CALIFORNIA

long periods of time (up to hundreds or thousands of

I years, and perhaps longer; Martin, 1993; see also Kid-

Fig. I. Location of Choya Bay (adapted from Ftirsich and Flessa,

1987).

Based on experimental analyses of modern reef- dwelling foraminifera from Discovery Bay, Jamaica,

Martin and Liddell (1991) and Kotler et al. (1991,

1992) concluded that, once produced, foraminiferal

A) LOCATION OF CORE SITES 1-9

well and Behrensmeyer, 1993). Despite intensive, deep

( 2 1 m) bioturbation in such environments (Walter

and Burton, 1990)) the high shell content of the sedi-

ment apparently slows dissolution (Aller, 1982; Kid-

well, 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, pre-

sumably, foraminiferal preservation. Extensive Holo-

cene 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

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~~~~ 26 (1995) 1X7-206 18’)

been documented by other workers (Flessa, 1987; Ftir-

sich 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 (Fur-

sich 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). Hurri- canes normally occur between late May and early

November, although they are most common in Septem- ber 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 well- consolidated 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; Mel- dahl, 1987) repeatedly move fine-grained sediment

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; Can-

field and Raiswell, 1991).

Activities of CDFs 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

is - 125,000 years old ( -oxygen isotope stage Se;

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). Dis- tances (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 26 (1995) 187-206

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 encounter-

ing 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] . sed-

iment was scooped from the core barrel at 5 cm inter-

vals, 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 pre-

drilled holes-spaced every 5 cm-that had been

sealed with both electrical and duct tape wrapped com-

pletely 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 60 ml syringe. Total alkalinity

(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 demon- strated 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).

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: O- 13.1%) in sediment collected to depths of 40 cm during

July ( 1991) reconnaissance, preserved in buffered for-

malin, and stained with Sudan Black B (Walker et al., 1974). Moreover, the two largest populations of fora-

minifera ( 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 sam-

ple splitter. Foraminifera were concentrated from 10 gram sediment samples (determined by trial-and- error) 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), San-

dusky ( 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 Uni- versity of Delaware mainframe computer using Mini- tab 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 spec-

imens 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 sedi-

ment 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 sediment-

water 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 cor-

rected 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).

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.

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 atmos- pheric reservoirs (Flessa et al., 1993).

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 typi-

cally 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 for-

mation 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 distribution and abundance

4. I. Alkulinity and total carbonate weight percent

Alkalinity of Choya Bay porewaters is typically elevated somewhat above that of normal seawater ( N 2-

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 9

Legend l CORE 1

0 CORE 2

* CORE 3

3 CORE 4 -I_-

40 & CORE 5 -w-

a CORE 6 ---

45 q CORE.7 ,.

CORE 8 .,. . . . . ,I .,. ,,

50

55,

60,

W JULY 1992 ALKALINITY (MEQ/L)

55

1

L

Legend - CORE 1 m CORE 2

* CORE 3

0 CORE 4 ---

* CORE 5 --_

. CORE 6 --_ * CO,RE 7

* CORE 8 “I... ,.,..,,.,.....,...

Fig. 3. Downcore alkalinity (milliequivalents/liter) for each site in (A) January, 1992, and (13) July, 1992 (note scale change from Fig. 3A).

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 Jan- uary. By contrast, BuccelIa mansfieldi (Cushman) 1930, Elphidium clavatum Cushman 1930, and Elphi-

dium spp., characterized middle-to-outer flat sediments, as did the suborders Miliolina, Rotaliina, and

Textulariina, although the last taxon was relatively uncommon. Elphidium cf. E. crispum was most char- acteristic 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


\

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 advec-

tion of “impulse” tracers such as microtektites, vol- canic ash, or the radioactive tracer 137Cs by 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 disap- peared 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 sum-

mers 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

194 R.E. Mm-tin et ~1. /Murk Mi~ro~~uleonrolr~y 26 (1995) 187-206

4 JANUARY 1992 TOTAL CARBONATE WEIGHT (%)

0 10 20 30 40 50 . 40

* CORE 1

. CORE 3

0 CORE 4 -_1_-

* CORE 5 -_-

---

6) JULY 1992 TOTAL CARBONATE WEIGHT (8)

* CORE 1

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). Interest- ingly, 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 correlation with TCARB (Spearman’s p = 0.396; 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 coef-

ficients. 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 accounted for -5565% of variance, the second for

- 16-22%, and the third for - l&12%. The first factor

A)

JULY

19

91

FORAMS/GRAM SEDIMENT

0

25

Legend

* CORE 1

-___

CORE 2

* CORE 3 I 0

CORE 4

---

l CORE 5

--_

.

CO

RE

6

---

7 C

O&

r3

7

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. . . . . .

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. . . . . . . . . . . . ...‘.>

W

JANUARY

1992

Cl

JULY 1992

0 5

10

15

20

25

30

35

40

45

50

ii

c I' 0 I -.

FORAMS/GRAM

SEDIMENT

50

1

Legend

. CORE 1

., CORE 2

CORE 3

-*.*

', CORE ?

CORE 5

--_

CORE 6

---

) CORE 7

CORE 8

-

FORAMS,'GRAM SEDIMENT

0 25 Legend

L

60

= CORE 1

a CORE 2

- CORE 3

1 CORE 4

---

*

CO

RE

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---

a CORE 6

m--

* fZfJ!E 7

* CORE 8

.,.,....,..

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,.

*

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Fig.

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R.E. Martin et al. /Marine Micropaleontology 26 (1995) 187-206

I

Fig. 7. Sampling times (July, 1991, 1992; January, 1992) and sea-

sonal 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 Buc-

cella mansfieldi, Elphidium (E. clauatum, E. cf. E.

crispum, and Elphidium spp.) , discorbids + rosalinids,

the suborders Rotaliina and Miliolina, and foraminif-

era1 number, and tended to characterizeouter and north-

ern 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, sed- imentological, 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 typ-

ically 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

the Fall-Winter reproductive pulse, southern (sites l-

3)) middle (sites 4-6)) and northern (sites 7-9) tran-

sect 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 foraminif- era1 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 foram-

iniferal reproductive pulses had begun (Fig. 7)) north-

ern 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).

CAN 2 I *

R.E. Martin et al. /Marine Micropaleontology 26 (1995) 187-206

A) JANUARY 1992

B) JANUARY 1992

1 1

1 I 1

1

1 1

1 3 OUTEF;

197

~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.

198 R.E. Martin et al. /Murk Micropuleontology 26 (1995) 187-206

A) JULY 1992

CAN 2 5

NORTH 1 I

-f 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

CAN21

5 t

0 .

I

-1 .

I 3

3

1 OUTER

B) JULY 1991

199

Fig. 11. Plots of canonical variables I and 2 for July, 1991, for (A) southern ( 1 ), middle (2). and northern (3) transect sites and ( B) inner

( I 1, middle (21, and outer (3) flat sites. See Tables 1 and 2 for correlations between original variables and canonical variables.

200 R.E. Martin et ui. /Marine Micropaleontology 26 (1995) 187-206

Table I Total canonical structure (total sample correlations between original

variables and canonical variables [ CV 1 I and 2) for southern, mid-

dle, and northern transect sites. Correlations rounded to second dec-

imal place

July, 1991

CVI cv2

January, 1992

CVl cv2

July, 1992

CVI cv2

BEC’*

BOL

BUC

DR

ECLV

ECSP

ESPP

HANZ

MIL

ROT

TEX

UNK

FNO

ALK

TCARB

- 0.25 0.29

0.18 0.34

0.05 0.30

0.49 0.01

- 0.3 I 0.42

0.27

0,s I 0.13

0.34 -0.15

0.14 0.29

0.15 0.42

0.12 -0.19

0.27 0.27

0.17 0.41

- 0.42

0.05

- 0.04

- 0.07

-0.31

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

- 0.12 -0.17

0.03 0.07

0.29 - 0.28

0.37 -0.26

0.11 - 0.40

0.41 0.04

0.33 -0.39

0.66 - 0.27

0.46 -0.16

0.62 -0.37

0.08 0.15

0.30 0.09

0.64 -0.36

0.30 0.03

0.18 - 0.07

’ * BEC = Ammo& heccurii; BOL = BolitCnn spp.; BUC = Buccello

manxfeldi; DR = Discorbids + rosalinids; ECLV = Elphidium ch-

tuturn; 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 (all); UNK = unknown;

FNO = Foraminiferal number (number of tests/gram sediment);

ALK = Alkalinity (meq/l) ; TCARB = Total carbonate weight %)

Foraminiferal number, suborders Rotaliina and Miliol-

ina, 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)) foraminif- era1 number, Buccella mansfieldi, discor-

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 resem- ble 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-

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 Rota-

liina, and foraminiferal number; CV 2, though, exhib-

ited 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 11 B) .

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 (2cr years 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

July, 1991 January, 1992 July, 1992

CVl cv2 CVl cv2 CVI cv2

BEC -0.12 0.56 -0.46 0.33 -0.34 -0.07

BOL -0.16 0.38 -0.05 -0.18 0.10 -0.17

BUC 0.67 - 0.58 0.91 -0.12 0.73 ~ 0.05

DR 0.19 0.00 0.17 - 0.44 0.61 0.15

ECLV 0.78 -0.13 0.62 - 0.2 I 0.39 0.33

ECSP 0.31 - 0.24 0.58 - 0.05 ESPP 0.19 0.35 0.06 - 0.37 -0.11 0.28

HANZ 0.10 0.50 -0.17 0.70 - 0.09 - 0.50

MIL 0.05 - 0.02 0.47 -0.18 0.65 - 0.04

ROT 0.62 0.02 0.63 - 0.26 0.33 -0.04 TEX -0.13 - 0.73 0.57 - 0.01 0.22 0.20

UNK 0.41 - 0.07 0.47 -0.12 0.59 0.42

FNO 0.54 - 0.01 0.62 - 0.25 0.53 - 0.05

ALK -0.30 -0.60 -0.14 0.57

TCARB 0.28 0.16 0.12 -0.47

Table 3

R. E. Martin et al. /Marine Micropaleontology 26 (I 99.5) 187-206 201

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 age Calendar age ( f 2~)

Site 8 2775 + 60 2026(1841-2278)

(O-IS cm sediment depth, above shell layer; NSF-Arizona AMS Facility Number AA1 1801) site Y 215O+SS 1309(1167-1508)

(20-25 cm sediment depth, within shell layer; NSF-Arizona AMS Facility Number AA I 1800)

5. Discussion

5.1. Foraminiferal distribution and abundance

The tidal hats of Choya Bay consist of an intergra-

dational 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 inner-

middle-outer flat zonation. Ammonia beccarii charac-

terizes 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, forami-

niferal 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; Gold- stein 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, per-

haps, 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 foraminif- era1 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 26 (1995) 187-206

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 plat-

form 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., foraminif-

era1 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 differ-

ences 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 pro- nounced) 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 grades, and temporal resolution

Studies of deep-seadissolution hold important implications for shallow shelf regimes. Broecker et al.

(1991) developed an age (mixing) model for deep- sea 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 dis-

solution: 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 dis-

solves completely before grain type B begins to dis-

solve, 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 con-

trast, 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 pres-

ervation mechanism at Choya Bay is the unexpectedly

great age of the tests. Our studies suggest that Holocene shallow-water microfossil assemblages may be time- averaged 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 Kowa- lewski, 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


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 rela-

tively 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 “pip-

ing”) is not usually a serious problem for the biostra-

tigrapher. In most cases, microfossil-based

biostratigraphic zonations are sufficiently precise that

reworked specimens are typically recognized by their anomalous stratigraphic occurrence and state of pres-

ervation; 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 Bel-

perio ( 1994), for example, found that specimens of

Marginopora wrtebralis Blainville 1846 were

reworked from underlying Late Pleistocene rocks ( - 125,000 years age based on amino acid racemiza-

tion) 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.

5.4. Implications for time-averaging qf offshore microfossil assemblages

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 (Mar-

tin, 1993). For modern shelves, Flessa ( 1993) esti-

mated 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 assem-

blages 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-ontrol-

ling 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 alkalinity (i.e., environments which most closely

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 indi-

cator of test age; rather, it is an index of time of exposure at the sediment-water interface. Despite intensive dis-

solution 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 acknowl-

edge 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 dedi- cated 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

contacted via INTERNET; his address is

[email protected].

References

Aberhan, M. and Ftirsich, F.T., 1987. Paleoecology and paleoenvi-

ronments of the Pleistocene deposits of Bahia la Choya (Gulf of California, Sonora, Mexico). In: F.T. Fhrsich and K.W. Flessa

(Editors), Ecology, Taphonomy, and Paleoecology of Recent

And Pleistocene Molluscan Faunas of Bahia la Choya, Northern

Gulf of California. Zitteliana, 18: 135-163.

Alexandersson, T., 1972. Micritization of carbonate particles: Proc-

ess of precipitation and dissolution in modem shallow-water

sediments. Geol. Inst., Univ. Uppsala, Bull., N. Ser., 3: 201-236.

Aller, R.C., 1982. Carbonate dissolution in nearshore terrigenous

muds: the role of physical and biological reworking. J. Geol., 90:

79-9s.

Brazier, M.D., 1980. Microfossils. Allen and Unwin, London, 193

PP. Bremer, M.L. and Lohmann, G.P., 1982. Evidence for primary con-

trol of the distribution of certain Atlantic Ocean benthic fora-

minifera by degree of carbonate saturation. Deep-Sea Res., 29:

987-998.

Brett, C.E. and Baird, G.C., 1986. Comparative taphonomy: A key

to paleoenvironmental interpretation based on fossil preserva-

tion. Palaios, 1: 207-227.

Broecker, W.S. and Peng, T.-H., 1982. Tracers in the Sea. Eldigio

Press ( Lamont-Doherty Geol. Ohs.), Palisades, N.Y., 690 pp.

Broecker, W.S., Klas, M., Clark, E., Bonani, G.. Ivy, S. and Wolfli.

W., 1991. The influence of CaCO, dissolution on core top radi-

ocarbon ages for deep-sea sediments. Paleoceanography, 6: S93-

608.

Canfield, D.E. and Raiswell, R., 1991. Carbonate precipitation and

dissolution: Its relevance to fossil preservation. In: P.A. Allison

and D.E.G. Briggs (Editors), Taphonomy: Releasing the Data

Locked in the Fossil Record. Plenum, New York, pp. 4114.53.

Christensen,E.R. andGoetz, R.H., 1987. Historical fluxes of particle-

bound pollutants from deconvolved sedimentary records. Envi-

ron. Sci. Technol., 21: 1088-1096.

Corliss, B.H., 1985. Microhabitats of benthic foraminifera within

deep-sea sediments. Nature, 314: 435438.

Corliss, B.H. and Emerson, S., 1990. Distribution of rose bengal

stained deep-sea benthic foraminifera from the Nova Scotian

continental margin and Gulf of Maine. Deep-Sea Res., 37: 38 I-

400.

Corliss, B.H. and Honjo, S., 198 1. Dissolution of deep-sea benthonic

foraminifera. Micropaleontology, 27: 356-378.

Cushman, J.A., 1930. The foraminifera of the Atlantic Ocean. Part

7. Nonionidae, Camerinidae, Peneroplidae, and Alveolinellidae.

US. Nat. Mus. Bull., 104: l-79.

Davies, D.J., Powell, E.N. and Stanton, R.J., 1989. Taphonomic

signature as a function of environmental process: Shells and shell

beds in a hurricane-influenced inlet on the Texas coast. Palaeo-

geogr., Palaeoclimatol., Palaeoecol., 72: 3 17-356.

Denne, R.A. and Sen Gupta. S.K., 1989. Effects of taphonomy and

habitat on the record of benthic foraminifera in modem sedi-

ments Palaios, 4: 414-423.

Dubois, L.G. and Prell, W.L., 1988. Effects of carbonate dissolution

on the radiocarbon age structure of sediment mixed layers. Deep-

Sea Res., 3.5: 1875-1885.

Flessa, K.W. (Editor), 1987. Paleoecology and Taphonomy of

Recent to Pleistocene Intertidal Deposits, Gulf of California.

Paleontol. Sot. Spec. Publ., 2, 237 pp.

Flessa, K.W., 1993. Time-averaging and temporal resolution in

Recent marine shelly faunas. In: SM. Kidwell and A.K. Beh-

rensmeyer (Editors), Taphonomic Approaches to Time Reso-

lution in Fossil Assemblages. Paleontol. Sot. Short Courses

Paleontol., 6: 9-33.

Flessa, K.W. and Kowalewski, M., 1994. Shell survival and time-

averaging in nearshore and shelf environments: Estimates from

the radiocarbon literature. Lethaia, 27, in press,

Flessa, K.W., Cutler, A.H. and Meldahl. K.H., 1993. Time and

taphonomy: quantitative estimates of time-averaging and strati-


graphic disorder in a shallow marine habitat. Paleobiology, 19: ing foraminifera: Implications for the reservation of biogenic 26&286. carbonate. Palaios, 7: 244-276.

Fiirsich. F.T. and Flessa, K.W., 1987. Tapbonomy of tidal flat mol-

IUSCS in the northern Gulf of California: Paleoenvironmental

analysis despite the perils of preservation. Palaios, 2: 543-559.

Fiirsich. F.T. and Flessa. K.W., 1991. Ecology, taphonomy, and

paleoecology of Recent and Pleistocene molluscan faunas of

Bahia la Choya. northern Gulf of California. Zitteliana, 18: l-

180.

Goldhaber. M.B. and Kaplan, I.R., 1980. Mechanisms of sulfur

incorporation and isotope fractionation during early diagenesis

of sediments in the Gulf of California. Mar. Chem., 9: 95-143.

Goldstein, S.T. and Harben, E.B., 1993. Taphofacies implications of

infaunal foraminiferal assemblages in a Georgia salt marsh,

Sapelo Island. Micropaleontology, 39: 5342.

Green, M.A., Aller, R.C. and Aller, J.Y., 1993. Carbonatedissolution

and temporal abundances of foraminifera in Long Island Sound

sediments. Limnol. Oceanogr.. 38: 331-345.

Guinasso. N.L. and Schink, D.R., 1975. Quantitative estimates of

biological mixing rates in abyssal sediments. J. Geophys. Res..

80: 3032-3043.

Langer, M., Hottinger, L. and Huber, B.. 1989. Functional morphol-

ogy in low-diverse foraminiferaI assemblages from tidal flats of

the North Sea. Senckenberg. Marit., 20: 81-99.

Loubere, P., 1989. Bioturbation and sedimentation rate control 01

bentbic microfossil taxon abundances in surface sediments: A

theoretical approach to the analysis of species microhabitatr.

Mar. Micropaleontol., 14: 317-325.

Loubere, P. and Gary, A.. 1990. Taphonomic process and species

microhabitats in the living to fossil assemblage transition 01

deeper water benthic foraminifera. Palaios, 5: 375-38 1.

Loubere, P., Gary, A. and Lagoe, M., 1993. Generation of the benthic

foraminiferal assemblage: Theory and preliminary data. Mar.

Micropaleontol., 20: 165-18 1.

Kidwell. S.M., 1986a. Models for fossil concentrations: Paleobiol-

ogic implications. Paleobiology. 12: 6-24.

Kidwell. SM., 1986b. Taphonomic feedback in Miocene assem-

blages: Testing the role of dead hardparts in benthic communities.

Palaios, 3: 239-255.

Kidwell. S.M.. 1989. Stratigraphic condensation of marine trans-

gressive records: origin of major shell deposits in the Miocene

of Maryland. J. Geol.. 97: l-24.

Maluf, L.Y., 1983. Physical Oceanography. In: T.J. Case and M.L.

Cody (Editors), Island Biogeography in the Sea of Cortez. Univ.

California Press, Berkeley, pp. 2611.5.

Martin. R.E., 1993. Time and taphonomy: Actualistic evidence fat

time-averaging of benthic foraminiferal assemblages. In: S.M.

Kidwell and A.K. Behrensmeyer (Editors), Taphonomic

Approaches to Time Resolution in Fossil Assemblages. Paleon-

tol. Sot. Short Courses Paleontol.. 6: 34-56.

Martin, R.E. and Liddell. W.D., 1988. Foraminiferal biofacies on a

north coast fringing reef ( I-75 m). Discovery Bay, Jamaica.

Palaios, 3: 298-3 14.

Kidwell, S.M.. I99 I. The stratigraphy of shell concentrations. In:

P.A. Allison and D.E.G. Briggs (Editors), Taphonomy: Releas-

ing the Data Locked in the Fossil Record. Plenum, New York,

pp. 21 I-290.

Kidwell, SM.. 1993a. Patterns of time-averaging in the shallow

marine fossil record. In: S.M. Kidwell and A.K. Behrensmeyer

(Editors), Taphonomic Approaches to Time Resolution in Fossil

Assemblages. Paleontol. Sot. Short Courses Paleontol., 6: 27%

300.

Martin, R.E. and Liddell, W.D., 1989. Relation of counting methods

to taphonomic gradients and biofacies zonation of foraminiferal

sediment assemblages. Mar. Micropaleontol., IS: 67-89.

Martin, R.E. and Liddell, W.D., 1991. Taphonomy of foraminifera

in modern carbonate environments: Implications for the forma:

tion of foraminiferal assemblages. In: S.K. Donovan (Editors),

Fossilization: The Processes of Taphonomy. Belhaven Prcrs.

London, pp. I70- 194.

Martin, R.E. and Steinker, D.C., 1973. Evaluation of techniques for

recognition of living foraminifera. Compass (Sigma Gamma

Epsilon), SO: 26-30.

Kidwell, S.M., 199% Taphonomic expressions of sedimentary hia-

tuses: Field observations on bioclastic concentrations and

sequence anatomy in low, moderate, and high subsidence set-

tings. Geol. Rundsch., 82: 189-22.

Kidwell. S.M. and Behrensmeyer, A.K.. 1993. Summary: Estimates

of Time-Averaging. In: S.M. Kidwell and A.K. Behrensmeyer

(Editors), Taphonomic Approaches to Time Resolution in Fossil

Assemblages. Paleontol. Sot. Short Courses Paleontol., 6: 301-

302.

Kidwell, S.M. and Bosence, D.W.J., 1991. Taphonomy and time-

averaging of marine shelly faunas. In: P.A. Allison and D.E.G.

Briggs (Editors), Taphonomy: Releasing the DataLocked in the

Fossil Record. Plenum, New York, pp. 115-209.

Kotler. E.. Martin, R.E. and Liddell, W.D., 1991. Abrasion-resistance

of modern reef-dwelling foraminifera from Discovery Bay,

Jamaica-implications for test preservation. In: R. Bain

(Editor), Proc. 5th Symp. Geology Bahamas. pp. 125-138.

Kotler, E., Martin, R.E. and Liddell, W.D., 1992. Experimental anal-

ysis of abrasion and dissolution resistance of modem reef-dwell-

McCave, 1.N.. 1988. Biological pumping upwards of the coarse frac-

tion of deep-sea sediments. J. Sediment. Petrol.. 58: 148-1.58.

Meldahl, K.H., 1987. Sedimentologic and taphonomic implications

of biogenic stratification. Palaios, 2: 350-358

Meldahl, K.H., 1990. Sampling, species abundance, and the strati-

graphic signature of mass extinction: A test using Holocene tidal

flat molluscs. Geology, 18: 890-893.

Murray, J.W.. 1989. Syndepositional dissolution of calcareous for-

aminifera in modern shallow-water sediments. Mar. Micropa-

leontol.. IS: 117-121.

Murray, J.W.. 1991. Ecology and Palaeoecology of Benthic Fora-

minifera. John Wiley. New York, 397 pp.

Murray-Wallace, C.V. and Belperio. A.P.. 1994. Identification of

remanie fossils using amino acid racemisation. Alcheringa. IX: 219-227.

Peterson. C.H., 1991. Intertidal zonation of marine invertebrates in

sand and mud. Am. Sci., 79: 236-249.

Phleger, F.B.. 1960. Ecology and Distribution of Recent Foramini-

fera. Johns Hopkins Press, Baltimore, 297 pp.

206 R. E. Murtin et ul. /Marine Micropaleontology 26 (1995) 187-206

Powell, E.N., 1992. A model for death assemblage formation: Can

sediment shelliness be explained? J. Mar. Res., 50: 229-265.

Powell, E.N.: Cummins, H., Stanton, R.J. and Staff, G., 1984. Esti-

mation of the size of molluscan larval settlement using the death

assemblage Est. Coastal Shelf Sci., 18: 367-384.

Pride, C.J.. Dean, W.E. and Thunell, R.C., 1994. Sedimentation in

the Gulf of California: Fluxes and accumulation rates of biogenic

sediments and trace elements. Geol. Sot. Am. Abstr. Progr., 26:

A23.

Robinson, M.K., 1973. Atlas of monthly mean sea surface and sub-

surface temperatures in the Gulf of California. San Diego Sot.

Nat. Hist. Mem.. 5, 97 pp. Roden, G.I., 1964. Oceanographic aspects of Gulf of California.

AAPG Mem., 3: 30-S8.

Sandusky, CL., 1969. Sedimentology of Ester0 Marua, Sonora,

Mexico. MS. Thesis, Dep. Geol., Univ. Arizona, Tucson, 84 pp.

(Unpubl.)

Schink, J.C., Stockwell, J.H. and Ellis, R.A., 1978. An improved

device for gasometric determination of carbonate in sediment. J.

Sediment. Petrol., 49: 651-653.

Smith, R.K., 1987. Fossilization potential in modem shallow-water

benthic foraminiferal assemblages. J. Foraminiferal Res., 17:

117-122.

Sprinthall, R.C., 1982. Basic Statistical Analysis. Addison-Wesley,

Menlo Park, 459 pp.

Stuiver, M. and Reimer, P.J., 1993. Extended 14C data base and

revised CALEB 3.0 14C age calibration program. Radiocarbon,

35: 215-230.

Walker, D.A., Linton, A.E. and Schafer, C.T., 1974. Sudan Black 8:

A superior stain to rose bengal for distinguishing living from

non-living foraminifera. J. Foraminiferal Res., 4: 205-2 IS.

Walter, L.M. and Button, E.A., 1990. Dissolution of recent platform

carbonate sediments in marine pore fluids. Am. J. Sci., 290: 601-

643.

Walton, W.R., 1955, Ecology of living benthonic foraminifera,

Todos Santos Bay, California. J. Paleontol., 29: 952-1018.

Zhang, L., 1994. Sedimentology and foraminiferal taphonomy in

siliciclastic environments: The northern Gulf of California, Mex-

ico. M.S. Thesis, Dep. Geol., Utah State Univ., Logan, 107 pp.

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