Strontium isotopic-paleontological method as a high-resolution paleosalinity tool for lagoonal environments

A combined strontium isotopic (*'^Sr/^*Sr) and paleontológica! metliod is newly applied to a modem lagoon in Egypt's Nile River delta to test its applicability as a paleosalinity proxy. Analyses of 22 surficial samples collected throughout the lagoon include 81 Sr isotopic analyses of mollusks, foraminifera, ostracods, barnacles, bryozoans, serpulid worm tubes, pore water, and gypsum crystals. Two salinity groups are distinguished in each sample: a lower salinity group (~1 ppt) mixed with a higher salinity group (-3-10 ppt) that, respectively, are interpreted as the modem biocoenosis and an older relict fauna. The relict fauna denotes higher salinity conditions in the lagoon prior to closure of the Aswan High Dam (1964), and the modern fauna records freshening of the lagoon. Recent decreased salinity is a response to regulated Nile River tlow and increased discharge into Manzala of fresh water via canals and drains. Quantifícation of this short-term salinity change holds promise for study of modern lagoons in other world settings, and may provide paleoclimatic information for older lagoon sequences in the Nile River delta and the geologic record. INTRODUCTION Lagoon environments, typically quasi-closed coastal settings that receive water from lluvial, grotind-water, and marine sources, have highly variable salinities that may range from hyposaline to hypcrsaline (1 to >35 ppt). The paleontological record in ancient lagoon deposits may be ambiguous with regard to salinity because many invertebrate taxa have wide tolerances to environmental factors (Dodd and Stanton, 1990, and references therein). To derive sea-level and paleoclimatic information from lagoon deposits in the geologic record, an accurate paleosalinity proxy is required. To develop such a proxy, our study of Man/ala lagoon, in Egypt's northeastern Nile River delta, evaluates a combined strontium isotopic and paleontological methodology lor the measurement of high-resolution salinity changes in lagoon environments (Fig. 1). Strontium isotopes ("^Sr/**'\Sr) have been tiscd as a paleosalinity proxy in some marginal mai-inc environments (Schmit/. ct al., 1991, 1997; Ingram and Sloan, 1992; Ingram and DePaolo, 1993; Bryant et al., 1995; Reinhardt et al., 1998b). Sr isotopes are an ideal paleosalinity proxy in these .settings for several reasons. (1) Biogenic carbonates incorporate Sr isotopes in their crystal lattice during precipitation with no vital effect, recording the ^''Sr/'^^Sr ratio of the waters in which they grew (Reinhardt et al., 1998a). (2) Typically, the Sr isotopic ratios of river waters aie either higher or lower than the worldwide marine value (0.70917) and there is a simple mixing relationship between the two (Palmer and Edmond, 1989; Modell et al., 1990; Ingram and DePaolo, 1993; Andcrsson et al., 1994; Bryant et al., 199.5). (3) Where any diagencsis can be ruled out, deviation from this marine Sr isotopic value is the result of frcsh-waler dilution. The method, however, has limitations because concentration r)f Sr in marine waters (~8 ppm) is much higher than in fresh water (<1 ppm; Palmer and Edmond, 1989). Thus, a significant amount of dilution by fresh water is needed to alter the ^•'Sr/^^Sr ratio of marine watere, and the method is best suited for determining lower salinity gradients, which can be found in lagoonal environments. Despite numerous advantages, Sr isotopes have not been used extensively as a paleosalinity proxy in paleontological studies, and to date, the method has not been tested extensively in lagoonal environments where die method has perhaps the most potential. Since 1964, date of closure of the Aswan High Dam, diere has been a significant increase of fresh water discharged from canals and drains into Manzala lagoon (Shaheen and Yousef, 1978; Randazzo et al., 1998, and references therein). The purpose of this study is to determine the salinity range in surficial deposits of this lagoon so as to independently detect this freshening and, if possible, quantify the salinity shift in the paleontological and Sr isotopic records (Fig. 1 ). If the method can detect and measure the anthropogenic shift induced by increased waterway discharge into the lagoon during the past 33 years, it then offers potential as a proxy for detecting short-term changes in Nile River flow in the late Quatemaiy and earlier geological record. METHODOLOGY We analyzed 81 specimens in 22 samples, including 21 invertebrate species (Fig. 1 ; Table DR 1 ' ). Identification of various invertebrate taxa follows previously established taxonomy (Bemasconi and Stanley, 1994, and references therein). Particular attention was paid to different taxa and sedimentary components (pore waters and gypsum ci7stals) analyzed in two specific samples to determine the potenfial salinity variation and overall faunal diversity at two salinity extremes along sample transects: MZl 5, near fresh-water canal discharge; and MZ79, near the coast and proximal to salt water sources (Fig. 1). On the basis of this information from these two samples (MZ15 and 79) and the ecological constraints of the taxa (i.e. marine, euryhaline, freshwater; Bernasconi and Stanley, 1994) indicator species were selected for the areal analysis of Manzala lagoon. Shell material for Sr isotopic analysis was selected based on taphonomic character and. 'Data Repository item 9893, Table DR I, '*'Sr/'**Sr results, is available on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301. E-mail: editing(rígeosociety.org. brancfi ¡^^-^ ^ «> •vrfi '^ /:;ínanlyíK>¿¿Ti\ ' • canaik ^^s 37 Mediterranean Sea