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). 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 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 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 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 con- centrations 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