Martin, R.E., M.S. Harris * and W.D. Liddell. 1995. Taphonomy and Time

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 ofGeology, 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 tings, where much of the fossil record occurs (see Martin, 1993, for review). Differential preservation of For more than half a century, microfossils-espe- foraminiferal assemblages likely varies according to cially foraminifera-have been widely used as strati- depositional setting (Martin, 1993; see also Kidwell graphic and paleoenvironmental indicators. Despite and Bosence, 199 1; Powell, 1992). The frequency and countless studies of foraminiferal distribution and amount of shell (micro- and macrofossil) input to the diversity in modern sediments (see Murray, 1991, for surface mixed layer and rates of SO:- reduction (alka- review), and their wide usage in stratigraphic, paleoen- linity buildup), sedimentation, and bioturbation all vironmental, paleoceanographic, and paleoclimatic play a role in the modification of the surficial mixed studies; surprisingly little attention has been paid to the layer into a time-averaged fossil assemblage and its formation and preservation of foraminiferal assem- incorporation into the historical layer below (Martin, blages, especially in continental shelf and slope set- 1993).

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 Kid- CALIFORNIA I r well 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; 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, presumably, 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 Fig. I. Location of Choya Bay (adapted from Ftirsich and Flessa, 1987). Gulf of California; Figs. 1 and 2A), offer a variety of easily accessible environments in which to study the Based on experimental analyses of modern reef- subtle interplay of reproduction (shell input), shell dwelling foraminifera from Discovery Bay, Jamaica, content, bioturbation and pore water chemistry during Martin and Liddell (1991) and Kotler et al. (1991, the formation of foraminiferal assemblages. Choya Bay 1992) concluded that, once produced, foraminiferal was also chosen because its environments had already

A) LOCATION OF CORE SITES 1-9 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- downwards and then redeposit it at the sediment-water sich and Flessa, 1987) and were the subject of ongoing interface while tending to concentrate coarse mollusc taphonomic research (Ftirsich and Flessa, 1987, 1991; debris in a relatively distinct subsurface shell layer Meldahl, 1987, 1990; Flessa, 1993; Flessa et al., 1993; (Fig. 2B). CDFs also pump SO:- -rich seawater into Flessa and Kowalewski, 1994). 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 2. Oceanographic and geologic setting of organic matter (Goldhaber and Kaplan, 1980; Brett and Baird, 1986). CDFs tend to counteract this effect, Choya Bay lies at the northern extreme of the Gulf however, by producing carbonic and sulfuric acids of California adjacent to the Sonoran Desert. Nearby through the oxidation of organic matter and sulfides Puerto Peiiasco receives an annual average rainfall of (HS -), respectively (Walter and Burton, 1990; Can- 74 mm (evaporation exceeds rainfall; Maluf, 1983), field and Raiswell, 1991). and air (water) temperatures range from 11.6”C Activities ofCDFs are most intense on the inner and ( 13.8”C) in January to 30°C (29.4”C) in August (Fur- southern flat and decrease toward the outer flat and to sich and Flessa, 1987); offshore surface salinities in the north (Fiirsich and Flessa, 1987, and unpubl. obser- the northern Gulf range from -35.5 to 37.5%0, vations). On the outer flat, sediment mixing is rela- although they may range higher in restricted areas tively shallow, and is accomplished by breaking waves (Maluf, 1983). Tides are semidiurnal and spring tides and vagile benthos (e.g., sand dollars). The depth to range up to - 8 m (Fursich and Flessa, 1987). Hurri- the shell layer tends to shallow outward across the fat canes normally occur between late May and early and to the north from > 60 cm on the southern flat to November, although they are most common in Septem- - IO cm in some places over a Pleistocene coquina that ber and October (Roden, 1964). There is seasonal is - 125,000 years old ( -oxygen isotope stage Se; overturn of the nutrient-rich thermocline in the northern Aberhan and Fursich, 1987), and that crops out over Gulf (as indicated by depth to the thermocline; Roden, the northern margin of the flat. CDF burrow densities 1964; Robinson, 1973)) which causes seasonal pulses (estimated visually) also tend to decrease outward and of phytoplankton reproduction (Maluf, 1983; Pride et to the north, especially when sediment thickness is al., 1994). < - 20-25 cm (unpubl. observations). The tidal flat at Choya Bay is a potentially useful modern analog for studying the formation of shell concentrations on ancient shallow, sediment-starved 3. Methods shelves: sedimentation is held constant while hardpart input varies seasonally (cf. Kidwell, 1986a). There has 3.1. Coring procedures been little sediment input to the northern Gulf since the construction of Hoover Dam on the Colorado River in A total of 9 sites (Fig. 2A) are discussed for each of the 1930s and subsequent development of irrigation three field seasons (summer: July 21-28, 199 I, and projects downriver (Maluf, 1983; Ftirsich and Flessa, July 26-August I, 1992; winter: January 3-9, 1992). 1987). Sediment at Choya Bay consists of fine to coarse These sites were chosen based on extensive reconnais- sand, and is presently derived locally from granitic sance coring during July ( 199 I ) and reoccupied in headlands and outcrops of semi-consolidated to well- January and July, 1992. Three sites each were occupied consolidated sandstones and coquinas (Fiirsich and on southern (sites l-3), middle (sites 4-6)) and north- Flessa, 1987; Zhang, 1994). Sedimentation rates at ern (sites 7-9) transects, respectively; in this way, the Choya Bay are therefore relatively low ( - 0.038 cm/ inner (sites 1, 4, 7), middle (sites 2, 5, 8), and outer yr; Flessa et al., 1993). (sites 3, 6, 9) flat was also sampled (Fig. 2A). Dis- Without high sedimentation rates, conveyor belt tances (measured from permanent stations above high deposit feeders (CDFs; primarily callianassid shrimp tide) to inner flat sites varied from 50 to 200 m (typi- and polychaetes; Fursich and Flessa, 1987, 1991; Mel- cally 100 m), while distances to middle flat stations dahl, 1987) repeatedly move fine-grained sediment 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 3.3. Enumeration offoraminifera order to sample the transition from outermost flat to shallow subtidal (Fig. 2A). We used total (live+dead) foraminifera in our Cores were taken using an apparatus modified from study. Zhang (1994, table 8) found very low numbers Meldahl ( 1987). Core tubes made of schedule 40 (4” of living foraminifera (typically <0.5% of the total diameter, 0.25” wall thickness) PVC were twisted into [live + dead] assemblage; mean: 1.7 + 3.7%; range: O- sediment using a metal handle inserted through holes 13.1%) in sediment collected to depths of 40 cm during drilled into the top of the core barrel. The handle was July ( 1991) reconnaissance, preserved in buffered for- then removed, the holes plugged with rubber stoppers, malin, and stained with Sudan Black B (Walker et al., and the top of the core capped with a plastic bag over 1974). Moreover, the two largest populations of fora- which was placed a PVC cap, which was secured with minifera ( 11. I%, 13.1%) were found in samples with a radiator hose clamp. The core was then excavated low total numbers of tests (19 and 145 specimens, respectively). from the sediment with shovels, and, upon encounter- Upon arrival at Delaware, sediment from 5 cm core ing the base of the core tube, the bottom quickly sealed intervals was subsampled for foraminifera using a sam- by the same procedure as for the top. Upon return to ple splitter. Foraminifera were concentrated from 10 the laboratory [ Centro Intercultural de Estudios de gram sediment samples (determined by trial-and- Desiertos y Oceanos (CEDO), Puerto Pefiasco] . sed- error) via flotation techniques using heavy liquids iment was scooped from the core barrel at 5 cm inter- (Ccl,; Brasier, 1980). Sediment residue was checked vals, and air dried for shipment to Delaware. frequently after flotation for separation of tests from sediment. Cushman (1930), Walton ( 1955), San- dusky ( 1969)) and Phleger ( 1960) were the primary 3.2. Alkalinity and total carbonate (shell) weight sources for species identification. Abundances of the percent predominant species are available from the senior author upon request. Herein, we assume that species-specific differences Pore waters were retrieved from cores in the field in size or morphology produce little bias in counting immediately after core excavation by insertion of the (Martin and Liddell, 1988, 1989). Although this is plastic tip of 60 ml syringes into the core through pre- obviously untrue of reef-dwelling foraminifera (Martin drilled holes-spaced every 5 cm-that had been and Liddell, 1988, 1989)) it appears to be a relatively sealed with both electrical and duct tape wrapped com- safe assumption based on our studies of Choya Bay pletely around the core barrel. Usually 5-10 ml of pore foraminifera. water was obtained in this way and emptied into 60 ml centrifuge tubes with screw cap tops. Upon immediate 3.4. Statistical analysis return to the laboratory, each water sample was filtered separaely through 0.45 pm nylon filters using pressure Cluster, factor, and canonical discriminant analyses from a 60 ml syringe. Total alkalinity were performed on combined data sets of downcore (HCO; + CO: + other dissolved species such as foraminiferal abundance, TCARB, and alkalinity for H,BO; [borate]) was then calculated after titration each sampling season on the University of Delaware with dilute (0.1 N) HCl using apH meter, as demon- mainframe computer using SAS Version 6.0 (Cary, strated to REM by Dr. Chas. Culberson (pers. com- NC). Other statistical analyses (Mann Whitney mun., 1991). Contributions of dissolved borate and U= MWU and Spearman’s p) were run on the Uni- versity of Delaware mainframe computer using Mini- other ions to seawater alkalinity are typically quite tab Version 7.2 (Duxbury Press, Boston, MA). small so that carbonate alkalinity (HCO, + CO:- ) = total alkalinity (Broecker and Peng, 1982). 3.5. Radiocarbon dates Total carbonate weight percent (TCARB = total shell content) was determined by the method of Schink Accelerator Mass Spectrometer (AMS) j4C analy- et al. ( 1978). ses 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- 2.5 meq/l; Chas. Culberson, pers. commun., 199 1). imens total ( 2 1 mg CaCO, required for analysis) of Values were relatively uniform downcore, typically primarily Buccellu mansfieldi (Cushman) 1930, and, ranging from 5 to 10 meq/l in both January (1992) secondarily, Elphidium cf. E. crispum (LinnC) 1788 and July ( 1992; Figs. 3 and 4A), and did not differ (for site 9; Fig. 2A). Tests appeared pristine (lack of significantly between seasons (MWU) Values tended obvious pits. perforations, borings, etc.) at the light to be low on the inner flat, where bioturbation was most microscope level, and came from samples collected at intense, then rose slightly on the middle flat, where northern flat sites 8 (O-15 cm depth; shell-poor sedi- burrow densities decreased, before declining somewhat ment a&e shell layer) and 9 (20-25 cm depth; within on the outer flat, where wave agitation and shallow shell layer; Fig. 2B) during July, 1992. Relative to sites bioturbation are extensive. During July, 1992, when air further south on the flat, these locations have relatively (water) temperatures were quite warm and the activi- high total shell (CaCO,) content near the sediment- ties of SOi--reducing bacteria presumably enhanced, water interface and low rates of bioturbation. values were somewhat higher (up to - 12 meq/l) at All radiocarbon dates discussed herein were site 6 (Figs. 3 and 4A; outer flat, middle transect), and obtained via the protocol described in Flessa et al. substantially higher ( - 20-50 meq/l) at site 8, on the ( 1993; see also references therein). Radiocarbon dates northern flat; both sites were characterized by a rela- reported from the NSF-University of Arizona labora- tively thin sediment veneer (Fig. 2B), and greatly tory were “conventional” dates; i.e., by convention, decreased burrow densities, at the time. dates were normalized to 6°C = - 25%0 (assuming a TCARB typically ranged from 0 to 20% in surficial 613C = O%O), and reported with respect to the Libby sediment above the shell layer in both January and July half-life of 5568 years as years before 19.50. We cor- ( 1992) but increased to > - 50-60% in the shell layer rected for the reservoir effect by the method of Flessa (Figs. 4B and 5). The top of the shell layer was typi- et al. ( 1993), which was based on a specimen of Chione cally indicated by an abrupt increase in TCARB, (Chione) californiensis (Broderip) 1935 collected at although sometimes the contact between the shell layer Choya Bay in 1949. Use of a bivalve date to correct and the overlying shell-poor mixed layer was grada- foraminiferal dates should make no difference in the tional (Figs. 2B and 5). Downcore TCARB did not correction since the amount of “old” carbon stored in change significantly between seasons (MWU; cf. Fig. the oceans will appear the same to both foraminifera 5), although it was substantially higher than summer and bivalves (K.W. Flessa, pers. commun., 1994). levels during January at site 9 on the outer portion of Conventional dates were converted to calendar years the northern transect (Fig. 2A). As TCARB tended to using the calibration software of Stuiver and Reimer increase both across the flat (Fig. 4B) and into the (1993). subtidal zone (Zhang, 1994), the January peak at site For the sake of comparison, we give ages in both 9 was probably related fortuitously to the site location, conventional and calendar years. The lg error (68% which varied for outer flat cores (Fig 2A; see also probability of the true age falling within the range) for “Coring Procedures”), rather than a pulse of shell conventional dates represents counting error only (no input related to reproduction and die-off or to the for- correction for reservoir effect; Flessa et al., 1993). The mation of shell lags by storms. Although there was no 2a error (95.4% probability of the true age falling significant correlation between TCARB and alkalinity within the range) for calendar year dates includes the (Spearman’s p) during either January or July ( 1992)) effects of error in counting and in modeling fluctuations when these variables were measured, TCARB (like in the specific activity of carbon in oceanic and atmos- alkalinity) increased to the north, especially in January pheric reservoirs (Flessa et al., 1993). (Fig. 4B).

4. Results 4.2. Foruminiferul distribution and abundance

4. I. Alkulinity and total carbonate weight percent Despite the superficial homogeneity of the tidal flat Alkalinity of Choya Bay porewaters is typically ele- environments at Choya Bay, foraminifera exhibited a vated somewhat above that of normal seawater ( N 2- distinctive zonation across the flat that persisted down- A) JANUARY 1992 W JULY 1992 ALKALINITY (MEQ/L) ALKALINITY (MEQ/L) 6 7 8 9

Legend - CORE 1 m CORE 2 Legend * CORE 3 0 ---CORE 4

l CORE 1 * --_CORE 5 0 CORE 2 . --_CORE 6 * CO,RE 7 * CORE 3 * CORE 8 “I...,.,..,,.,.....,... 3 -I_-CORE 4 L 40 & CORE 5 -w- 55 a CORE 6 --- 1 45 q CORE.7,. .,.CORE... . ,I8.,. ,, 50

55,

60,

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 Textulariina, although the last taxon was relatively protoplasm (either in surface or downcore samples) uncommon. Elphidium cf. E. crispum was most char- that might indicate that the specimen was alive at the acteristic of the outer northern flat (site 9) during July, time of collection (cf. Martin and Steinker, 1973; 1992. Langer et al., 1989). Ammonia beccarii (LinnC) 1758 The effect of CDFs was evident in downcore profiles was most abundant on the inner flat, especially in Jan- of foraminiferal abundance (Fig. 6). A Fall-Winter uary. By contrast, BuccelIa mansfieldi (Cushman) reproductive pulse, which was presumably caused by 1930, Elphidium clavatum Cushman 1930, and Elphi- overturn of the northern Gulf water column and asso- dium spp., characterized middle-to-outer flat sedi- ciated phytoplankton blooms (Fig. 7)) was especially ments, as did the suborders Miliolina, Rotaliina, and 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 dissolve (Fig. 7). Nevertheless, some tests persisted in apparently being moved downward by CDFs (Fig. shallow sediments of northern (sites 8, 9) and outer 6B). The bulge was reminiscent of downward advec- (site 6) flat stations and in the subsurface shell layer tion of “impulse” tracers such as microtektites, vol- (cf. Figs. 5 and 6). canic ash, or the radioactive tracer 137Csby bioturbators The extent of dissolution varied between the sum- (e.g., Guinasso and Schink, 1975; Christensen and mers of 1991 and 1992. Foraminiferal numbers for Goetz, 1987). The reproductive pulse was, however, July, 1992, core samples were significantly less than not evident at inner flat site 1, where CDFs were very for July ( 1991) and January ( 1992; MWU; abundant, or at stations further north (except perhaps p < 0.0006). By contrast, foraminiferal numbers for for site 6)) where the depth to the shell layer ( = thick- July, 199 1, and January were not significantly different ness of the overlying mixed layer) thinned markedly (MWU). (Fig. 2B) and CDFs were probably more efficient in Like alkalinity and TCARB, foraminiferal number homogenizing sediment. By the summer, when air increased northward and outward across the flat during (water) temperature had increased and CDFs had the summer, as the presumed reproductive pulse become more active (and dissolution presumably more decayed (Fig. 8). Although foraminiferal number intense), reproductive pulses (bulges) had disap- exhibited no significant correlations with TCARB for peared and foraminiferal profiles were more irregular January and July or alkalinity (for January), it did downcore (Fig. 6). Thus, most foraminiferal tests at exhibit a moderate (Sprinthall, 1982, p. 192) negative Choya Bay persist for only a few months before they 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 6) JULY 1992 TOTAL CARBONATE WEIGHT (%) TOTAL CARBONATE WEIGHT (8) 0 10 20 30 40 50. 40

* CORE 1 * CORE 1

. CORE 3

0 -_1_-CORE 4 0 ----CORE 4 * -_-CORE 5 ' --_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 4.3. Multivariate statistical analyses high (especially at site 8; Figs. 3 and 4A). Interest- ingly, July ( 1992) foraminiferal number exhibited a Cluster analysis revealed no meaningful groups moderatepositive correlation with Hanzawaia strattoni despite repeated attempts with different clustering coef- ( Applin) 1925 (Spearman’s p = 0.657; p < 0.01)) ficients. Ftirsich and Flessa (1987) also found rela- which also exhibited a moderate negative correlation tively indistinct groupings in cluster analyses of middle with July ( 1992) alkalinity (Spearman’s p = - 0.666; and outer flat macroinvertebrates. p < 0.01) . This species also displayed a positive cor- Principal factor analysis produced somewhat better relation with TCARB (Spearman’s p = 0.396; results. Depending upon the sampling season, three p < 0.01) and tended to increase to the north, although factors accounted for - 85 to 98% of the variance. For it was most abundant on the inner flat, where alkalinity each of the three sampling seasons, the first factor tended to be low. accounted for -5565% of variance, the second for - 16-22%, and the third for - l&12%. The first factor A) JULY 1991 W JANUARY 1992 Cl JULY 1992 FORAMS/GRAM SEDIMENT FORAMS/GRAM SEDIMENT FORAMS,'GRAM SEDIMENT 0 25 0 50 1 0 25 0 I

30 Legend Legend 35 -. . CORE 1 Legend * -___CORE 1 ., CORE 2 = CORE 1 CORE 2 40 -*.*CORE 3 a CORE 2 * CORE 3 ', CORE ? - CORE 3 0 ---CORE 4 45 --_CORE 5 1 ---CORE 4 l CORE 5 --_ CORE 5 ---CORE 6 * --- . ---CORE 6 50 ) CORE 7 a m--CORE 6 7 CO&r3 7 CORE 8 * fZfJ!E7 + CORE...... 8 ii * .,.,....,..CORE 8.,,, ,. . .CORE...... 9 ...‘.> * .,.l.~:,.o,...l,.,CORE 9 - L c I' 60

1992, and (C) July, 1992. Bell-shaped curves in Fig. 6. Foraminiferal number (number of foraminifera/gramI 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). The July ( 199 1) Seasonwas used primarily for reconnaissance for future sampling; hence, sample intervals are not always spaced every 5 cm. R.E. Martin et al. /Marine Micropaleontology 26 (1995) 187-206

the Fall-Winter reproductive pulse, southern (sites l- I 3)) 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 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 foraminiferal reproductive pulses had begun (Fig. 7)) northern transect and outer flat sites, although quite variable, Fig. 7. Sampling times (July, 1991, 1992; January, 1992) and seasonal reproduction of foraminifera ( = P) in relation to overturn of were relatively distinct from remaining sites (Fig. 10). nutrient-rich thermocline in the Gulf of California (based on depth to thermocline; Robinson, 1973), associated phytoplankton blooms, AVERAGE FORAM NUMBER and intensity of bioturbation and SOi- reduction. 1 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 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, 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 typically significant at p < 0.025 (oneway ANOVA; SAS

User’s Guide: Basic Statistics); canonical variable Fig. 8. Map of average downcore foraminiferal number (average of (CV) 1 normally accounted for - 75-80%, and CV 2 counts for each horizon sampled at each site f 1 standard deviation) for 15-20%, of variance. During January, following for July (1991) and January and July (1992). R.E. Martin et al. /Marine Micropaleontology 26 (1995) 187-206 197

CAN 2 I * A) JANUARY 1992

B) JANUARY 1992

1 1

1 I 1 1 1 1

1 3 OUTEF;

~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

NORTH 1 I

-f 5 6 CAN 1

CAN 2 5 B) JULY 1992

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) 187-206 199

CAN 2 A) JULY 1991

CAN21 B) JULY 1991 5 t

0 .

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 ( 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 relations between Elphidium clavatum and Hanzawaia Total canonical structure (total sample correlations between original and CVl for July ( 1991) were essentially identical to variables and canonical variables [ CV 1 I and 2) for southern, middle, and northern transect sites. Correlations rounded to second dec- those of January, but, unlike January, correlations imal place between discorbids + rosalinids and Elphidium spp. and CVl were higher than for other variables; CV2 was July, 1991 January, 1992 July, 1992 not statistically significant (Table 1) The behavior of inner, middle, and outer flat sites also differed from that CVI cv2 CVl cv2 CVI cv2 for January (Table 2): CV 1 represented Buccella BEC’* - 0.25 0.29 - 0.42 - 0.29 0.12 -0.17 mans$eldi and Elphidium clavatum, the suborder Rota- BOL 0.18 0.34 0.05 0.19 - 0.03 0.07 liina, and foraminiferal number; CV 2, though, exhib- BUC 0.05 0.30 - 0.04 0.12 0.29 - 0.28 ited a stronger inverse relationship between Ammonia DR 0.49 0.01 - 0.07 0.08 0.37 -0.26 beccarii and Hanzawaia strattoni, on the one hand, and ECLV - 0.3 I 0.42 -0.31 0.23 0.11 - 0.40 ECSP 0.27 - 0.08 0.41 0.04 Buccella mansfieldi and the suborder Textulariina on ESPP 0,s I 0.13 0.06 0.10 0.33 -0.39 the other. Plots of canonical discriminant functions for HANZ 0.34 -0.15 0.35 -0.60 0.66 - 0.27 July, 1991, and January inner, middle, and outer flat MIL 0.14 0.29 0.08 0.20 0.46 -0.16 sites differ accordingly (cf. Figs. 9B and 11B) . ROT 0.15 0.42 -0.12 0.12 0.62 -0.37 TEX 0.12 -0.19 - 0.22 0.12 0.08 0.15 4.4. Foraminiferal radiocarbon dates UNK 0.27 0.27 - 0.22 0.17 0.30 0.09 FNO 0.17 0.41 - 0.08 0.15 0.64 -0.36 ALK - 0.29 0.52 0.30 0.03 Despite extensive dissolution of foraminifera at TCARB 0.54 0.27 0.18 - 0.07 Choya Bay, test ages were surprisingly old (Table 3) : 1309 calendar years (2a range: 1167-1508) for site 9 ’ *BEC = Ammo& heccurii; BOL = BolitCnn spp.; BUC = Buccello (20-25 cm depth; tests from within shell layer) and manxfeldi; DR = Discorbids + rosalinids; ECLV = Elphidium ch- tuturn; ECSP = Elphidium cf. E. crispum (counted in Elphidium spp. 2026 calendar years (2cr range: 184 l-2278) for site 8 for July, 199 I ); ESPP= Elphidium spp. (mainly E. urticulatum and (O-15 cm depth; tests from shell-poor sediment above intergradational forms) ; HANZ= Hunzawain strattoni; shell layer). MIL= suborder Miliolina (all) ; ROT = suborder Rotaliina (all) ; TEX = suborder Textulariina (all); UNK = unknown; Table 2 FNO = Foraminiferal number (number of tests/gram sediment); Total canonical structure (total sample correlations between original ALK = Alkalinity (meq/l) ; TCARB = Total carbonate weight %) variables and canonical variables [CV I 1 and 2) for inner, middle, and outer flat sites. Correlations rounded to second decimal place Foraminiferal number, suborders Rotaliina and Miliol- See Table 1 for abbreviations ina, Hanzawaia strattoni, and Elphidium cf. E. crispum July, 1991 January, 1992 July, 1992 exhibited moderate positive correlations (r = 0.4-0.7; Sprinthall, 1982) with CV 1 in canonical discriminant CVl cv2 CVl cv2 CVI cv2 analysis of southern, middle, and northern transect sites, whereas correlations between original variables 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 and CV 2 were relatively low (Table 1) . In analysis of BUC 0.67 - 0.58 0.91 -0.12 0.73 ~ 0.05 inner, middle, and outer flat sites (Table 2)) foraminif- DR 0.19 0.00 0.17 - 0.44 0.61 0.15 era1 number, Buccella mansfieldi, discor- ECLV 0.78 -0.13 0.62 - 0.2 I 0.39 0.33 bids +rosalinids, Elphidium cf. E. crispum, and ECSP 0.31 - 0.24 0.58 - 0.05 suborder Miliolina exhibited moderate positive corre- ESPP 0.19 0.35 0.06 - 0.37 -0.11 0.28 lations with CV 1, and alkalinity displayed moderate 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 positive, and TCARB and Hanzawaia strattoni mod- ROT 0.62 0.02 0.63 - 0.26 0.33 -0.04 erate negative, correlations with CV 2. TEX -0.13 - 0.73 0.57 - 0.01 0.22 0.20 The behavior of sites for July, 1991, tended to resem- UNK 0.41 - 0.07 0.47 -0.12 0.59 0.42 ble those for January, 1992. As for January, 1992, FNO 0.54 - 0.01 0.62 - 0.25 0.53 - 0.05 southern, middle, and northern flat sites for July, 1991, ALK -0.30 -0.60 -0.14 0.57 TCARB 0.28 0.16 0.12 -0.47 were highly gradational (Fig. 11A; cf. Fig. 9A). Cor- 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 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 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 5.1. Foraminiferal distribution and abundance the thickness of the overlying shell-poor mixed layer decrease, just as they tend to do toward the outer flat The tidal hats of Choya Bay consist of an intergra- (Fig. 2B). The same foraminiferal species that increase dational intertidal zonation that strongly reflects the in abundance in sediment from inner to outer flat also subtle interactions of organisms and sediment (see tend to increase to the north, most likely because of Peterson, 199 1, for genera1 review; see Fiirsich and increased habitat availability on rocky outcrops, less Flessa, 1987, 1991, for Choya Bay). CDFs are most extreme temperature and salinity fluctuations, and, per- likely abundant on the inner flat because of relatively haps, changes in pore water chemistry. low wave energy and the availability of abundant organic matter (food; Peterson, 1991) ; CDFs also 5.2. Foraminiferal reproduction and preservation cause extensive dissolution in this environment (e.g., low alkalinity). On the middle flat, the abundance of Foraminiferal reproduction at Choya Bay appears to CDFs declines and alkalinity rises somewhat, but as occur in discrete (ca. a few weeks) seasonal pulses, the outer flat is approached, wave energy and shallow which are then followed by periods of homogenization infaunal and epifaunal burrowing tend to increase, and dissolution of several months duration (Fig. 7) ; which lowers alkalinity to inner flat values (Figs. 3 and i.e., small populations of living foraminifera are not the 4A). result of rapid sedimentation (Walton, 1955; Phleger, Foraminifera abundance tends to follow the inner- 1960, pp. 189-212). Green et al. ( 1993) also calcu- middle-outer flat zonation. Ammonia beccarii charac- lated a mean residence time for foraminiferal tests in terizes the inner flat, where the rigors of temperature Long Island Sound sediments of 86-t 13 days, and and salinity are no doubt highest (Murray, 1991), Powell et al. (1984) estimated half-lives of 100 days whereas Elphidium clauatum, Buccella mansfieldi, for the smallest (0.8-3.1 mm) juveniles of molluscan Elphidium cf. E. crispum, and the suborders Rotaliina, death assemblages. The significantly lower foraminif- Miliolina, and Textulariina characterize the outer flat. era1 numbers in July ( 1992) than in July ( 1991) may The occurrence of certain species in both the middle reflect the unpredictability of the exact timing of sea- and outer flat (as opposed to the inner flat) may reflect sonal reproduction as well as our sampling at a some- not only more optima1 environments, but perhaps also what later time in July ( 1992) than in July ( 199 I ), transportation of outer flat species onto the middle flat thereby allowing slightly more time for dissolution of (Zhang, 1994). Despite intensive burrowing, forami- assemblages. niferal numbers tend to be relatively high near the tops The same factors that determine the distribution of of cores and decrease downward (Fig. 6), suggesting living foraminifera at Choya Bay also strongly influ- that only relatively small populations, at best, live at ence their preservation. Foraminiferal test dissolution greater depths in the sediment (cf. Corliss, 1985; at Choya Bay is much more pervasive than at Discovery Langer et al., 1989; Corliss and Emerson, 1990; Gold- Bay (Martin, 1993; see also Alexandersson, 1972; stein and Harben, 1993). 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 (1991) developed an age (mixing) model for deep- and alkalinity and TCARB, foraminifera persist for sea sediments based on the assumption that dissolution longer periods of time at northern flat stations (Fig. 8). within the zone of bioturbation should be proportional Although the shell-poor mixed layer overlying the sub- to the residence (replacement) time of grains within surface shell layer is probably stirred more rapidly by the mixed layer. They distinguished two forms of dis- CDFs to the north because of the shallowing of the solution: homogeneous and sequential. In homogene- Pleistocene platform (and accompanying thinning of ous dissolution, each grain loses a constant fraction of the overlying mixed layer), the shallowness of the plat- its mass per unit time (irrespective of grain type), form tends to inhibit bioturbation, allows buildup of which shifts the mass distribution of assemblages in alkalinity (as high as 50 meq/l; Fig. 4A), and keeps the mixed layer toward younger grains in core top shell material relatively close to the surface (Figs. 2B assemblages because the replacement time of grains in and 4B), all of which slow dissolution; i.e., foraminif- the mixed layer by new grains from the pelagic rain is era1 preservation in relatively shell-rich siliciclastic reduced. In sequential dissolution, grain type A dis- sediments at Choya Bay most closely approximates solves completely before grain type B begins to dis- carbonate regimes, as predicted by Martin and Liddell solve, and so on; in this case, core top ages presumably ( 199 1) and Kotler et al. ( 199 1, 1992; see also Aller, increase with the extent of dissolution (see Martin, 1982; Kidwell, 1989). In effect, antecent topography 1993, for review). (Pleistocene outcrop) serves as a kind of ’ ‘taphonomic With respect to calcareous microfossils, sequential feedback” (Kidwell, 1986b) on the development of dissolution was predicted to predominate in shelfal car- microfossil assemblages. bonate environments (such as Discovery Bay) ; by con- Tests from Choya Bay are quite small ( < 250 pm), trast, homogeneous dissolution was predicted to and are characterized by a high surface/volume ratio dominate in siliciclastic regimes, such as Choya Bay and presumably high chemical reactivity. Indeed, the (Martin, 1993). Obviously, dissolution is neither relatively pristine test surfaces (at the light microscope purely homogeneous or sequential at Choya Bay. At level) also implies that most tests dissolve rapidly Choya Bay, tests that survive dissolution probably do (Figs. 6-8); i.e., test microstructure, mineralogy, etc., so because they are rapidly advected downward by typically make little difference in the taphonomic CDFS into the shell layer and preserved there until, behavior of foraminifera at this locale. Nevertheless, much later, they are reworked upward by biological the inverse relationship between Hanzawaia strattoni activity (e.g., McCave, 1988) or storms (K.H. Meldahl and alkalinity suggests that tests of this species may and A. Olivera, pers. commun., 1994). survive dissolution at Choya Bay because of relatively What is most surprising about the foraminiferal pres- thick walls or microstructure. Moreover, the differ- ervation mechanism at Choya Bay is the unexpectedly ences in correlations between original variables and great age of the tests. Our studies suggest that Holocene canonical variables between July ( 199 1) and July shallow-water microfossil assemblages may be time- ( 1992) suggest that some foraminiferal taxa may decay averaged over as much as hundreds to thousands of at slightly different rates. Similar (but more pro- years. Our results are corroborated by Flessa (1993) nounced) behavior has been documented for other and Flessa et al. (1993; see also Flessa and Kowa- foraminiferal species: Corliss and Honjo (1981) and lewski, 1994) for bivalves (Chione spp.) from Choya Bremer and Lohmann ( 1982)) for example, found that Bay. The age of foraminiferal assemblages analyzed deep-sea species of foraminifera that live below the by us falls within the range of ages for disarticulated CCD are more resistant to dissolution than those, such Chione spp. collected by Flessa et al. ( 1993) from the as Amphistegina, that characterize reef sediments. sediment-water interface of the inner flat of Choya Bay [ 2 0 ( “post-bomb” ; i.e., A.D. 1950 or younger) to 5.3. Dissolution models, taphonomic grades, and 3569 calendar years], although foraminiferal ages tend temporal resolution to fall near the higher end of the age range for Chione spp. (Flessaet al., 1993, table 1). Flessaet al.‘s ( 1993, Studies of deep-seadissolution hold important impli- table 2) shells from the inner flat exhibited a broad cations for shallow shelf regimes. Broecker et al. range of taphonomic grades (surface condition), but R.E. Martin et al. /Marine Micropaleontology 26 (1995) 187-206 203 taphonomic grade was not an infallible indicator of 5.4. Implications for time-averaging qf offshore shell age (time since death); old specimens ( - 1900 microfossil assemblages years) were sometimes relatively pristine, whereas relatively young shells ( - several hundred years) were The extent of mixing on short temporal scales-and sometimes more highly degraded. Flessa ( 1993) and the exact limits of stratigraphic resolution inherent to Flessa et al. ( 1993) suggested that the condition of a each taphonomic environment (“taphofacies”)-no shell’s surface is primarily indicative of the residence doubt vary across the continental shelf and slope (Mar- time of the shell at the sediment-water interface and tin, 1993). For modern shelves, Flessa ( 1993) esti- not its age (see also Kidwell, 1991, 1993a,b). Even if mated time-averaging in the nearshore zone of - 1000 a shell is rapidly buried by downward advection by years (more-or-less in agreement with our results) and burrowing organisms (such as at Choya Bay), rather of up to 10,000 years for shelves exclusive of the near- than by rapid sediment influx, it may still remain rela- shore zone. Whether or not shelfal microfossil assem- tively pristine because it has been removed from the blages formed offshore show similar degrees of Taphonomically Active Zone (TAZ; Davies et al., time-averaging remains to be determined and will 1989) near the surface. require extensive study of the sedimentary dynamics The mechanism of microfossil preservation at Choya and shell input to each taphofacies (cf. Denne and Sen Bay grades into the upward reworking (“leaking”) of Gupta, 1989; Loubere. 1989; Loubere and Gary, 1990; much older tests into younger sediments (“remanie”; Loubere et al., 1993). Indeed, Dubois and Prell ( 1988) Murray-Wallace and Belperio, 1994; Kidwell, 1993a). concluded that although sediment may have the same Reworking of substantially older microfossils into radiocarbon age, the proportions of the components younger sediments (or vice versa by downward “pip- producing that age may not be the same if the particles ing”) is not usually a serious problem for the biostra- have different preservational histories, and that in order tigrapher. In most cases, microfossil-based to use 14C dates in stratigraphy, the processes c-ontrol- biostratigraphic zonations are sufficiently precise that ling hardpart input and loss must be er,aluated. The reworked specimens are typically recognized by their similarity in age, and, apparently, mechanism of pres- anomalous stratigraphic occurrence and state of preservation of foraminiferal and bivalve assemblages at ervation; such specimens were noted only ~:ev rarely Choya Bay suggests that stratigraphic and taphonomic in our samples, especially at site 7 (inner northern flat), criteria derived for macrofossils (e.g., Kidwell, 199 I, and they were not used in 14C analyses. 1993a, 1993b) may be useful in assessing the formation Time-averaging of microfossil assemblages is much and degree of time-averaging of microfossil assem- more insidious, however. Murray-Wallace and Bel- blages. The study of shallow-water assemblages is only perio ( 1994), for example, found that specimens of a first step in deciphering the complex, and often subtle, Marginopora wrtebralis Blainville 1846 were processes that form microfossil assemblages and their reworked from underlying Late Pleistocene rocks relative time scales of accumulation. ( - 125,000 years age based on amino acid racemiza- tion) into modern tidal flat sediments, and that the surfaces of reworked Marginopora exhibited little 6. Conclusion taphonomic alteration. Thus, substantial numbers of significantly older shells may be mixed into younger Holocene tidal flat environments at Choya Bay arc assemblages (depending on shell content of the sedi- surprisingly complex in terms of the subtle interplay ment and intensity of bioturbation) and the time scales between shell input, bioturbation, pore water chemis- of accumulation affected accordingly, with little or no try, and shell preservation. At Choya Bay, foraminifera observable change in the character of the microfossil persist longest at northern flat stations because of assemblages themselves. 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). Brazier, M.D., 1980. Microfossils. Allen and Unwin, London, 193 The taphonomic grade of tests is not a reliable indi- PP. Bremer, M.L. and Lohmann, G.P., 1982. Evidence for primary con- cator of test age; rather, it is an index of time of exposure trol of the distribution of certain Atlantic Ocean benthic fora- at the sediment-water interface. Despite intensive dis- minifera by degree of carbonate saturation. Deep-Sea Res., 29: solution of foraminifera, tests are surprisingly old (up 987-998. to - 2000 years based on AMS 14C dates). Moreover, Brett, C.E. and Baird, G.C., 1986. Comparative taphonomy: A key these tests are relatively pristine at the light microscope to paleoenvironmental interpretation based on fossil preserva- level. We hypothesize that some tests survive dissolution. Palaios, 1: 207-227. Broecker, W.S. and Peng, T.-H., 1982. Tracers in the Sea. Eldigio tion by rapid downward piping by conveyor belt Press ( Lamont-Doherty Geol. Ohs.), Palisades, N.Y., 690 pp. deposit feeders into a subsurface shell layer, and are Broecker, W.S., Klas, M., Clark, E., Bonani, G.. Ivy, S. and Wolfli. preserved there until they are reworked upward by bio- W., 1991. The influence of CaCO, dissolution on core top radi- logical activity or storms. Thus, the dynamics of shell ocarbon ages for deep-sea sediments. Paleoceanography, 6: S93- input and preservation must be accounted for in assess- 608. ing time-averaging of microfossil assemblages. 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. Acknowledgements Christensen,E.R. andGoetz, R.H., 1987. Historical fluxes of particle- bound pollutants from deconvolved sedimentary records. Envi- ron. Sci. Technol., 21: 1088-1096. Our studies at Choya Bay have been funded by NSF Corliss, B.H., 1985. Microhabitats of benthic foraminifera within Grant Number EAR-9017864. We gratefully acknowl- deep-sea sediments. Nature, 314: 435438. edge the support of the NSF-University of Arizona Corliss, B.H. and Emerson, S., 1990. Distribution of rose bengal AMS Facility. Thanks to Karl Flessa and Jim Pizzuto stained deep-sea benthic foraminifera from the Nova Scotian continental margin and Gulf of Maine. Deep-Sea Res., 37: 38 I- for advice on radiocarbon dating, and to Barun Sen 400. Gupta and Tim Patterson for constructive reviews. Corliss, B.H. and Honjo, S., 198 1. Dissolution of deep-sea benthonic Many thanks also to geology undergraduates Maryanne foraminifera. Micropaleontology, 27: 356-378. Johnson, Dave Lawrence, Darren Rasmussen, and Cushman, J.A., 1930. The foraminifera of the Atlantic Ocean. Part Dave Sterling of Utah State University for their dedi- 7. Nonionidae, Camerinidae, Peneroplidae, and Alveolinellidae. cated assistance in the field and laboratory, without US. Nat. Mus. Bull., 104: l-79. Davies, D.J., Powell, E.N. and Stanton, R.J., 1989. Taphonomic which our studies could not have been completed. Barb signature as a function of environmental process: Shells and shell Broge drafted the figures. beds in a hurricane-influenced inlet on the Texas coast. Palaeo- For further information the senior author can also be geogr., Palaeoclimatol., Palaeoecol., 72: 3 17-356. contacted via INTERNET; his address is Denne, R.A. and Sen Gupta. S.K., 1989. Effects of taphonomy and [email protected]. habitat on the record of benthic foraminifera in modem sediments 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- References Sea Res., 3.5: 1875-1885. Flessa, K.W. (Editor), 1987. 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