Palaeogeography, Palaeoclimatology, Palaeoecology 245 (2007) 535 – 550 www.elsevier.com/locate/palaeo
High-resolution paleosalinity reconstruction from Laguna de la Leche, north coastal Cuba, using Sr, O, and C isotopes Matthew C. Peros a,⁎, Eduard G. Reinhardt b , Henry P. Schwarcz b , Anthony M. Davis a b
a Department of Geography, University of Toronto, 100 St. George Street, Toronto, Ontario, Canada M5S 2E1 School of Geography and Earth Sciences, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4M1
Received 12 December 2005; received in revised form 11 September 2006; accepted 14 September 2006
Abstract Isotopes of Sr, O, and C were studied from a 227-cm long sediment core in order to develop a high-resolution paleosalinity record to investigate the paleohydrology of Laguna de la Leche, north coastal Cuba, during the Middle to Late Holocene. Palynological, plant macrofossil, foraminiferal, ostracode, gastropod, and charophyte data from predominantly euryhaline taxa, coupled with a radiocarbon-based chronology, indicate that the wetland evolved through four phases: (1) an oligohaline (0.5–5‰) lake existed from ∼ 6200 to ∼ 4800 cal yr B.P.; (2) water level in the lake increased and the system freshened from ∼ 4800 to ∼ 4200 cal yr B.P.; (3) a mesohaline (5–18‰) lagoon replaced the lake ∼ 4200 cal yr B.P.; and (4) mangroves enclosed the lagoon beginning ∼ 1700 cal yr B.P., forming a mesohaline lake. Isotopic ratios were measured on specimens of the euryhaline foraminifer Ammonia beccarii, although several measurements were also made on other calcareous microfossils in order to identify potential taphonomic and/or vital effects. The 87Sr/86Sr results show that the average salinity of Laguna de la Leche was ∼1.7‰ during the early lake phase and ∼8‰ during the lagoon phase — a change driven by relative sea level rise. The δ18O results do not record the salinity increase seen in the 87Sr/86Sr data, but instead indicate high evaporation from the lake surface; this in turn suggests that the value of ∼ 1.7‰ may slightly underestimate the average salinity of the wetland, at least for the period prior to ∼ 4800 cal yr B.P. Variability in δ13C was controlled by plant productivity, episodic marine incursions, and vegetation community change. There is some evidence for seasonal effect and the lateral transport of microfossils prior to burial. The isotopic data independently confirm the hydrological interpretations made using the euryhaline indicators and have permitted the reconstruction of a significantly higher resolution paleosalinity record than would be possible using the paleoecological data. However, our results show that Sr isotopes, while often cited as a powerful paleosalinity tool, must be used in conjunction with other indicators when investigating paleosalinity trends; relying solely on any single isotopic or ecological indicator can lead to inaccurate results, especially in semienclosed and closed hydrological systems. © 2006 Elsevier B.V. All rights reserved. Keywords: Cuba; Holocene; Isotopes; Salinity; Micropaleontology; Coastal environment
⁎ Corresponding author. Present address: Laboratory for Paleoclimatology and Climatology, Department of Geography, University of Ottawa, 60 University Street, Ottawa, Ontario, K1N 6N5. Fax: +1 613 562 5145. E-mail address:
[email protected] (M.C. Peros). 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.09.006
1. Introduction Salinity is a critical control on the type and distribution of flora and fauna in marginal marine systems (Hogarth, 1999; Mitsch and Gosselink, 2000; Silvestri
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et al., 2005). The salinity of coastal wetlands is controlled by a number of factors; tides, for example, are important diurnal-scale processes (Kostaschuk, 2002), whereas over centennial to millennial timescales, transgressions and regressions associated with glacial eustasy add and remove marine water from coastal areas (Peltier, 2002). In addition to these natural processes, anthropogenically-induced salinity changes are increasingly widespread, especially through the diversion of rivers for irrigation (Ghasemi et al., 1995). Understanding the processes that influence the salinity of coastal wetlands has thus become a focal point for paleoenvironmental research in littoral zones. A range of proxies is available for reconstructing paleosalinity, including paleontological (Teeter, 1995; Dix et al., 1999) and elemental geochemical techniques (Ingram et al., 1998; Dwyer and Cronin, 2001). However, it is the use of isotopes of O, C, and Sr, measured on biogenic carbonates, that has emerged as perhaps the most powerful tool for paleosalinity reconstructions in marginal marine settings, because: (1) unlike many biological indicators, isotopes have the potential to establish high resolution paleosalinity records; (2) the combined use of O, C, and Sr can provide insights into the causes of salinity changes — an issue of considerable importance in coastal settings where both evaporation and marine incursions occur; and (3) O, C, and Sr isotopes have the potential to assess the degree that taphonomic processes, such as bioturbation and lateral transport, have influenced the fossil record (Clayton and Degens, 1959; Keith et al., 1964; Mook and Koene, 1975; Strain and Tan, 1979; Schmitz et al., 1991; Koch et al., 1992; Ingram and Sloan, 1992; Ingram and Depaolo, 1993; Bryant et al., 1995; Schmitz et al., 1997; Reinhardt et al., 1998, 2001, 2003). In this paper, we apply O, C, and Sr isotopes, measured on a variety of biogenic carbonates, to study centennial- to millennial-scale salinity and hydrological variability in the largest lake in Cuba, Laguna de la Leche. Palynological, micropaleontological, and radiocarbon data indicate that Laguna de la Leche has a long (∼6200 cal yr) and complicated hydrological history (Peros, 2005); O, C, and Sr isotopes were studied because of their potential to deconvolve the effects of relative sea level (RSL) change, climatic variability, and other salinity-influencing processes on this regionally important wetland. In addition, since the paleoecological record from Laguna de la Leche is dominated by euryhaline taxa, such as the foraminifers Ammonia beccarii and Triloculina oblonga, and plants like Chenopodiaceae and Rhizophora mangle, isotopes also offer the opportunity to generate a paleosalinity record with higher resolution than could be derived using other
paleoecological data. This paper provides the first isotopic data for the Holocene of Cuba. It contributes to a growing database of paleoenvironmental information focused on understanding the hydrology of coastal environments — an urgent issue given the stresses that coastal systems are currently experiencing from the effects of agriculture, urbanization, and industry. Because of resort development and its associated infrastructure, north central Cuba is experiencing a surge in tourism-related development that will likely affect the water quality of local wetlands. This work thus provides critical long-term environmental baseline data against which ongoing hydrological changes in the region can be monitored and assessed. 2. Oxygen, carbon, and strontium isotopic interpretation Isotopic ratios of oxygen (18O and 16O, or δ18O), carbon (13C and 12C, or δ13C), and strontium (87Sr/86Sr) are influenced by numerous environmental factors. The purpose of this section is to outline how these isotopic ratios, as measured on biogenic carbonates, relate to salinity and other processes that are important in marginal marine systems. 2.1. Oxygen The δ18O of biogenic carbonate is determined by two main factors: the water temperature at which the carbonate precipitates and the δ18O of the ambient water at the time of carbonate formation (Urey et al., 1951; Craig, 1961, 1965; Leng and Marshall, 2004). In the case of temperature, the fractionation of lighter (16O) and heavier (18O) isotopes during carbonate precipitation is controlled by the temperature of the ambient water; the δ 18 O value of the carbonate becomes increasingly negative as water temperature increases. However, in the lowland tropical Americas, seasonal- to millennial-scale water temperature variability is small when compared to that of precipitation variability, so temperature-dependent fractionation is unlikely to have been important in this region, at least during the Holocene (Curtis et al., 1998). The factors that control the δ18O of the ambient water, on the other hand, are more significant for carbonate precipitation in the Neotropics. For example, because of its lower vapour pressure, water with the lighter isotope of oxygen, 16O, is evaporated more rapidly than water containing the heavier isotope, 18O (Craig et al., 1963; Craig and Gordon, 1965). Thus, an increase in the ratio of evaporation to precipitation (E/P), or a relatively drier
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climate, will cause 16O-enriched water to be selectively evaporated. In a closed-basin system, this removal of 16 O will leave the water enriched in 18O, resulting in a positive shift in its δ18O value. Conversely, a decrease in E/P, or a relatively wetter climate, will add isotopically lighter rainwater (enriched in 16O) to the basin, resulting in a negative shift in δ18O (Covich and Stuiver, 1974). In semi-open and open systems the situation is more complex; the δ18O of the basin water will also be influenced by the δ18O of groundwater, river water, and/or seawater. In the case of marginal marine settings, the mixing of coastal lake or lagoon water with seawater may have a greater role in affecting carbonate δ18O than changes in E/P. Since the δ18O value of seawater (∼ 0‰ VSMOW) is usually much higher than the local precipitation (Clark and Fritz, 1997), the incorporation of isotopically heavy seawater can significantly alter the δ18O of coastal lake or lagoon water — although in some arid regions river water can also be highly enriched in 18 O due to evaporation (Reinhardt et al., 2001). These conditions therefore suggest that, on the basis of oxygen isotopes alone, it may be difficult to determine whether an increase in the δ18O of coastal lake or lagoon water resulted from seawater mixing, say due to storm activity or relative sea level rise, an increase in E/P, or some other factor. 2.2. Carbon The main factors that control the δ13C value of lacustrine carbonates are the δ13C of the water feeding the lake, the exchange of lake water with atmospheric CO2, and biological productivity (i.e. biomass) (Siegenthaler and Eicher, 1986). The δ13C of the water is especially important in marginal-marine environments; groundwater and river-waters generally have lower δ13C values (−10 to −15‰ VPDB) than seawater, which has a δ13C value close to 0‰ (VPDB) (Reinhardt et al., 2003; Leng and Marshall, 2004). Thus, seawater incursions should result in positive shifts in the δ13C of the water of the coastal wetland (Strain and Tan, 1979; Tan, 1989). The exchange of lake water with atmospheric CO2 tends to be important in lakes and lagoons with long water residence times (i.e., closed, deep basins), as this enables isotopically heavier CO2 to be incorporated from the atmosphere, which can enrich δ13C by as much as 2‰ (VPDB) (Leng, 2003). The effect of biological productivity on δ13C is based on the principle that aquatic plants assimilate 12C in preference to 13C during photosynthesis (McConnaughay et al., 1997). Thus, during periods of enhanced plant productivity, the carbon pool in the water becomes depleted in 12C, which leads to a positive shift in δ13C in
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the water, and consequently in the carbonate that forms within it (Siegenthaler and Eicher, 1986). 2.3. Strontium The utility of 87Sr/86Sr as a paleosalinity proxy derives from the fact that the isotopes 87Sr and 86Sr have similar atomic weights, so that 87Sr/86Sr – unlike isotopes of the lighter elements (e.g., 18O and 16O, 13C and 12 C) – does not measurably fractionate during natural chemical reactions or physical transformations, such as evaporation (Faure and Powell, 1972; Hart et al., 2004). The Sr isotopic composition of seawater is uniform worldwide owing to the long residence time of Sr in the oceans (∼ 2.5 Ma) compared to its short mixing time (∼ 1.5 ka), and has remained essentially unchanged during the Holocene (Hodell et al., 1990). The Sr isotopic value of freshwater, on the other hand, is controlled by the 87Sr/86Sr values of the rocks and sediments the water flows over or through. The 87Sr/86Sr values of the rocks and sediments are in turn determined largely by their ages; 87Sr is the daughter of a radioactive isotope (87Rb), hence the older the rock, the higher its 87Sr/86Sr value is likely to be (although due to the extremely long half-life of 87Rb [∼ 4.7 × 1010 yr], the accumulation of 87 Sr via radioactive decay is negligible in Holocene-age biogenic carbonates [Faure and Powell, 1972]). The Sr concentration of freshwater is likewise determined by the concentration of Sr within the local lithology, the resistance of the rocks to weathering, and climate. Limestones, for example, typically have higher Sr concentrations (∼ 1000 ppm), but lower 87Sr/86Sr values (0.706 – 0.709), than silicates (Palmer and Edmond, 1992). The 87Sr/86Sr ratio of the water in a coastal wetland reflects the degree that seawater and freshwater are mixed, and this ratio will therefore have a value between that of seawater and the freshwater feeding the system. Thus, assuming that the Sr isotopic and concentration values of the end-members feeding the system are known, the salinity of the water in an open coastal wetland can be inferred based on its Sr isotopic ratio (Ingram and Sloan, 1992). Critically for paleoenvironmental studies, 87Sr/86Sr is incorporated into carbonates without any biological fractionation, meaning that the 87 Sr/86Sr ratios of biogenic carbonates are reliable indicators of the 87Sr/86Sr of the waters in which they formed — so long as diagenesis can be ruled out (Reinhardt et al., 1999). Despite this, since the concentration of Sr in seawater (∼ 8 ppm) is much higher than in freshwater (b 1 ppm), a significant amount of dilution is needed to alter the 87Sr/86Sr of seawater; thus
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the Sr isotopic method is best used in lower-salinity systems (i.e., b20‰) (Reinhardt et al., 1998). Furthermore, in closed basins, salinity can vary with changes in E/P, but salinity changes caused by E/P variability will not alter the Sr isotopic ratio of the water, because 87 Sr/86Sr does not measurably fractionate. Therefore, when relatively closed systems are studied, it is necessary to examine O and C isotopes in conjunction with Sr isotopes in order to interpret salinity variability (Reinhardt et al., 1998). 2.4. Biological and taphonomic processes In addition to climatic and sea level changes, stable isotopic values are also influenced by biological and taphonomic processes. A potentially important biological process is vital effect — a biologically induced fractionation that results in a departure of the δ18O and δ13C values of the shell of the organism from that of the water in which it formed (Woodruff et al., 1980). Vital effect appears to be caused by the incorporation of isotopically light metabolic CO2 into the shell of the organism, which leaves biogenic carbonates generally depleted in the heavy isotope (i.e., 18O, 13C) relative to equilibrium conditions (Grossman, 1987). Another biological process that influences stable isotopic composition relates to the life-cycle of the organism. For example, the lifespan of individuals within a given benthic foraminiferal species can range from weeks to several years (Murray, 1991). Thus, for long-lived specimens, an isotopic measurement made using the entire test will produce a result indicative of the average environmental conditions the organism experienced over multiple seasons. However, organisms that complete their life-cycle within one season will have isotopic compositions indicative of a much shorter period of time, which in areas of high seasonality can be quite different from the annual average (Wefer and Berger, 1980). Seasonal effect can be mitigated by combining several tests for a single isotopic determination or by analyzing longlived specimens. Taphonomic processes can also influence the isotopic record at a given location. One of the most common processes is bioturbation — the vertical mixing of sediment via biological activity which results in stratigraphic ‘time-averaging’ (Martin, 1999). Lateral transport can mix particles that formed under a variety of environ-
mental conditions, especially when they are small and relatively buoyant (e.g., foraminifera) and are located in areas where transport is likely, such as in an estuary or where intense storms are common (Martin, 1999). Intuitively, carbonates that form under the same environmental conditions should have the same isotopic values (assuming that vital effect is negligible); thus, it may be possible to assess the degree that taphonomic processes influenced the fossil record by measuring the stable isotopic compositions of several different taxa from the same stratigraphic level (Reinhardt et al., 2003). Significant differences in the isotopic values of contemporaneous taxa may be the result of horizontal movement or vertical mixing prior to or following burial. Conversely, uniform isotopic values among several taxa are not necessarily indicative of a lack of movement. In the case of lateral transport, for example, microfossils formed at the same time could have originated from different areas that possessed relatively homogenous environmental conditions. 3. Site background Laguna de la Leche is a large (∼ 67 km2), shallow (≤3 m) lake located on the north coast of central Cuba (Fig. 1). Topographic maps indicate that the surface of Laguna de la Leche has an elevation of ∼ 50 cm above mean sea level. No rivers presently enter Laguna de la Leche, although a freshwater canal (Canal de Júcaro) connects the lake to the town of Morón to the south. Laguna de la Leche is fringed by mangroves, especially Rhizophora mangle (red mangrove), although stands of Avicennia germinans (black mangrove), Conocarpus erectus (buttonwood mangrove), and Typha domingensis (southern cattail), lie to the northwest and east. For most of the 20th century, Laguna de la Leche was connected to the hyperhaline Bahía de Buena Vista and Bahía de Perros by two 15–20 m wide canals to the northwest (Canal de Chicola) and east (Canal Manatí). In an effort to convert Laguna de la Leche to a freshwater lake, these canals were blocked in 1986 to prevent seawater incursion. The Laguna de la Leche region has a winter dry period from November to March, a summer dry period in June and July, and two rainy seasons from April to May and August to October (Borhidi, 1996). Mean annual mean daily temperature and precipitation are
Fig. 1. Map of the study region with coring site (LL1) indicated. Only the 25 m contours are shown; virtually the entire region is below 5 m above mean sea level. The maximum elevations of the diapirs are: Punta Alegre, 138 m; Turiguano, 105 m; Cunagua, 315 m. The capital letters (A–F) correspond to the water samples listed in Table 2.
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25.8 °C and 1150 mm respectively, and the seasonal average mean daily temperature normally varies by only 5 °C (Nuevo Atlas Nacional de Cuba, 1989). Precipitation is often less than 50 mm/month during the dry periods and over 180 mm/month during the rainy seasons (Nuevo Atlas Nacional de Cuba, 1989; Borhidi, 1996). North-central Cuba lies within the North American plate and is underlain by the tectonically stable Bahamas Platform (Lewis and Draper, 1990; Iturralde-Vinent, 1994). This area of Cuba can be divided into a series of narrow linear belts that parallel the coast of the island. Each one of these belts is characterized by a diagnostic stratigraphy (Khudoley, 1967; Iturralde-Vinent, 1994). The Laguna de la Leche region lies within the Cayo Coco belt, which consists of Pleistocene limestones underlain by sedimentary rocks that include Early to Middle Eocene limestones and marls; Early Cretaceous limestones and dolomites; and Late Jurassic or Early Cre-
taceous interbedded dolomites and anhydrites (Pardo, 1975). The region has generally low relief (b 5 m), with the exception of three salt diapirs (Meyerhoff and Hatten, 1968). To the northeast of Laguna de la Leche, the Turiguano diapir outcrops with a maximum elevation of 109 m above sea level, and consists of Late Jurassic or Early Cretaceous limestone and dolomite blocks within a gypsum matrix, surrounded by Eocene and Miocene limestones (Iturralde-Vinent and Roque Marrero, 1982) (Fig. 2). The Turiguano diapir appears to have pierced the surface during the Pliocene (Meyerhoff and Hatten, 1968). 4. Methodology The core (LL1) examined in this paper was lifted with a Livingstone piston-sampler from the centre of Laguna de la Leche in 3 m of water (Fig. 1). The core was
Fig. 2. Bedrock geology of the region of Laguna de la Leche and the Turiguano Diapir (Meyerhoff and Hatten, 1968; Iturralde-Vinent and Roque Marrero, 1982). The letters G and H correspond to the locations of the near freshwater end-members used in the paleosalinity reconstruction. The letters I and J indicate the locations of water samples with salinities of 4.2‰ and 4.7‰ respectively.
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wrapped in plastic and transported to Toronto where it was stored at ∼ 4 °C. The core was studied for pollen (Faegri and Iversen, 1989), calcareous microfossils (foraminifera, ostracodes, gastropods, and charophytes) (Scott et al., 2001), plant macrofossils, and bulk sediment composition by loss on ignition (Dean, 1974). Pollen concentration (grains/cm3) was estimated by spiking each sample with a known quantity of exotic spores; pollen influx values (grains/cm2/yr) were then determined by multiplying the pollen concentration by the sediment accumulation rate (cm/yr) for each level where pollen was counted (Faegri and Iversen, 1989; Peros et al., in press). Core chronology was established by submitting five samples of bulk sediment (several grams each) to Seoul National University for Accelerator Mass Spectrometry dating. Bulk sediment was submitted because no suitable organic matter of terrestrial origin was discovered in the core. In Seoul, all carbonates were removed from the samples by treatment with dilute HCl before AMS analysis was undertaken (Kim, J.C., personal communication, 2003). Isotopic analyses were undertaken on thirty-seven, 1 cm thick samples throughout the length of the core. From each sample, four or five tests of the foraminifer Ammonia beccarii were picked with a fine brush for δ18O and δ13C determinations. In order to assess potential vital effects and/or taphonomic processes, a single test of Elphidium spp., a miliolid, several articulated ostracodes, a gastropod, and a charophyte were also picked for δ18O and δ13C analyses from the top, centre, and base of the core. Only microfossils lacking evidence of recrystallization were selected for analysis in order to rule-out diagenetic activities. Each microfossil was sonicated in distilled water for approximately 5 s and then cleaned with 0.5 N HCl to remove any encrusting materials. The δ18O and δ13C determinations were performed on a VG Autocarb system attached to an Optima mass spectrometer at McMaster University, Hamilton, Ontario. Precision was ± 0.07‰ for δ18O and ± 0.10‰ for δ13C (Reinhardt et al., 2003). Isotopic values are expressed in conventional delta (δ) notation as the per mil (‰) deviation from Vienna PeeDee Belemnite. Strontium isotopic analysis was undertaken after the majority of the δ18O and δ13C results had been determined. Approximately 400 specimens of Ammonia beccarii were picked from six foraminiferal samples that produced widely ranging δ18O and δ13C results. The foraminifera were sonicated in distilled water and then cleaned with 0.5 HCl to remove any encrustations. Sr was extracted by ion exchange following standard procedures outlined in Patterson et al. (1995). Sr isotopic
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analysis was performed on a VG 354 multicollector mass spectrometer at McMaster University. Replicate analysis of the NBS 987 Sr standard yielded a 87Sr/86Sr ratio of 0.71028 ± 2 × 10− 5 and all 87Sr/86Sr values reported here have been normalized to 0.71025. Internal precision (standard deviation of the mean) for all analyses was less than 2 × 10− 5 (Reinhardt et al., 2003). In order to construct the Sr mixing curve, surface water samples were collected from the marine and freshwater end-members that feed Laguna de la Leche. For the marine end-member, water samples were collected from both sides of a causeway connecting the mainland to the offshore coral reef (Fig. 1). Since no natural rivers currently enter Laguna de la Leche, samples from the southern end of the lake were used as ‘near’ freshwater end-members. The source of the freshwater that enters Laguna de la Leche likely derives from a combination of overland runoff as well as water temporarily stored in shallow aquifers within the surrounding carbonates, and also, possibly, the Turiguano diapir. Thus, the near freshwater end-member is a reflection of the collective mixing of the groundwater entering the system whose Sr isotopic composition is determined by the dissolution of the rock formations in the drainage basin. All water samples were measured for conductivity using a Hack meter; conductivity values were then converted to units of salinity (http://www.fivecreeks.org/ monitor/sal.html). Strontium concentration values were assayed by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP AES) on a Perkin Elmer Model Optima 3000DV unit in the Analest Laboratory, Department of Chemistry, University of Toronto. A strontium isotopic measurement was made on one of the freshwater end-member samples, whereas the marine end-member was assumed to have the same 87Sr/86Sr value as seawater (Hodell et al., 1990). When multiple measurements were made, mean values of each parameter were calculated. The following mixing equation was used to express the relationship among the Sr concentration, Sr isotopic ratio, and salinity of sea- and freshwater end-members (Bryant et al., 1995): Rmix ¼
½Rsw Csw S þ Rfw Cfw ð1−SÞ ½ðCsw SÞ þ Cfw ð1−SÞ
Where R is the ratio of 87Sr/86Sr, C is Sr concentration, and S is a salinity factor calculated as a fraction of the salinities of the marine and freshwater end-members. The subscripts sw and fw refer to seawater and freshwater respectively.
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5. Results and discussion 5.1. Lithology, chronology, and biostratigraphy Core LL1 is 227 cm long (Fig. 3). It consists of three lithologic units: greenish-grey sand from 217–227 cm; grey mud from 97–217 cm; and shell-rich marl from 97 cm to the top of the core. Two 1 cm thick gastropod layers are present in the grey mud unit at 116–117 cm and 123–124 cm. The gastropods in these layers are whole specimens and have an average maximum diameter of ∼ 3 mm. Silicate abundance is highest in the basal mud unit (∼ 70%), whereas carbonates dominate the remainder of the core. Five 14C dates on core LL1 provide age control. A 1600 cal yr offset is apparent when the age-depth model is extrapolated to 0 cm, suggesting that the dates have been influenced by a reservoir effect (i.e., age offset caused by the incorporation of 14C-deficient carbon)
(MacDonald et al., 1987). Several lines of evidence support a systematic age-offset for the AMS dates. First, numerous macrofossils of several aquatic plants (e.g., Najas) have been found throughout the core (Peros et al., in press). These plants normally assimilate bicarbonate from the water in which they grow (Olsson and Kaup, 2001), and because bulk organic matter was submitted for dating, it is likely that detritus from these plants was measured for the 14C analysis. Second, four 14 C-dated gastropods from a core taken from Dune Pass Bay Pond, Bahamas, show an almost identical age offset, which has been attributed to a reservoir effect in that geologically similar area (Dix et al., 1999). Third, the limestone rocks surrounding Laguna de la Leche would provide a significant source of 14C-deficient carbon. In order to account for the apparent reservoir effect at Laguna de la Leche, we subtracted 1600 cal yr from each calibrated 14C date. When this correction is made, changes in the paleoecological record from Laguna de la Leche
Fig. 3. Stratigraphy, LOI, and radiocarbon data from core LL1, Laguna de la Leche. The crosses are uncorrected, calibrated radiocarbon dates in yr B. P. (the vertical shafts refer to the thickness of each sample used for dating). The black boxes represent the same samples corrected for a reservoir error of 1600 cal yr. The open circles are radiocarbon data from Dune Pass Bay Pond, Bahamas (Dix et al., 1999). In all cases the vertical bars are equivalent to the 2σ error-range. Note how the best-fit lines for the Cuban and Bahamian data converge at 1600 cal yr B.P. Figure reproduced from Peros et al. (in press).
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Fig. 4. Summary diagram of pollen, plant macrofossil, foraminiferal, ostracode, gastropod, and charophyte results with O, C, and Sr isotopic data. The δ18O and δ13C data points associated with the lines were measured on tests of A. beccarii. The 87Sr/86Sr value of modern seawater (0.709172) is indicated by the grey bar on the right of the diagram (Hodell et al., 1990). The subscripts are: M (miliolid); E (Elphidium); G (gastropod); O (ostracode); C (charophyte). The zonation is based on stratigraphically-constrained cluster analysis of the microfossil data. The terms ‘oligohaline’ and ‘mesohaline’ refer to salinity ranges of 0.5–5‰ and 5–18‰ respectively.
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correlate with other major environmental changes in the region, providing independent support for the revised chronology (Peros et al., in press). Past vegetation dynamics were deduced from fossil pollen and plant macrofossils (Fig. 4). There is little pollen preserved in the sand at the base of the core. From 101– 210 cm, the pollen is dominated by Typha domingensis (southern cattail), Asteraceae (Aster family), and Chenopodiaceae (Goosefoot family), with smaller quantities of Batis maritima (saltwort), Adiantum spp. (maidenhair fern), and Acrostichum aureum (mangrove fern). The overlying unit, from 101 cm to the top of the core, is dominated by the pollen of the mangroves Rhizophora mangle (red mangrove), Avicennia germinans (black mangrove), and Conocarpus erectus (buttonwood mangrove), as well as Cyperaceae (Sedge family), Poaceae (Grass family), and upland taxa such as Pinus caribaea (Caribbean pine). Macrofossils of the aquatic plants Najas spp. (water-nymph), Ruppia maritima (widgeongrass), and Potamogeton spp. (pond-weed) are present; Najas occurs from 65 cm to the base of the core, whereas Potamogeton and R. maritima are abundant only in the grey mud unit. A wide range of calcareous microfossils were studied. The microfossil assemblage from 126–215 cm consists solely of the foraminifer Ammonia beccarii, with smaller quantities of gastropods (all belonging to the fresh/ brackish-water family Hydrobiidae; possibly consisting of Heleobops spp. and Littoridinops spp.) and ostracodes (including the euryhaline taxa Cyprideis edentate and Perissocytheridea cribrosa and the freshwater taxa Physocypria spp. and Cypridopsis vidua). From 102– 123 cm, the gastropods and ostracodes increase in abundance and oogonia of the charophyte Chara canescens f. hitsuta or C. fibrosa appear. The ostracodes, gastropods, and charophyte oogonia abruptly decline in abundance at 95 cm and are replaced by the foraminifers Elphidium spp. and Triloculina spp. 5.2. δ18O and δ13C — A. beccarii The isotopic data are graphed in Fig. 4 and are listed in Table 1. The δ18O measurements on A. beccarii show two broad patterns: a subtle shift toward more negative values from the base (∼ 0‰) to the top of the core (∼− 1.2‰) and quasi-periodic fluctuations of ∼ 1‰ every 10–20 cm throughout the core. Exceptions to these patterns are the low δ18O value at the base of the core (− 1.24‰); larger (∼ 3‰), higher-frequency fluctuations at 116–126 cm; and a shift toward more positive values (− 1.84‰ to ∼ − 0.8‰) from 19 cm to the top of the core. The decrease in δ18O from 1.41‰ (126 cm) to − 0.82‰
Table 1 Isotopic data from core LL1, Laguna de la Leche Depth in core (cm)
δ13C (‰-VPDB)
δ18O (‰-VPDB)
Group
87
2 2 2 4 9 12 17 19 30 45 50 50 68 68 68 75 82 91 95 102 108 115 116 116 116 119 123 126 132 135 138 147 155 158 165 171 176 177 186 190 195 195 195 211 215 219
− 5.27 − 3.55 − 7.79 − 5.40 − 5.52 − 4.64 − 5.23 − 5.00 − 4.91 − 4.60 − 4.55 − 4.53 − 5.13 − 5.85 − 6.94 − 4.09 − 4.10 − 4.53 − 4.00 − 4.39 − 1.04 − 5.05 − 3.33 − 0.37 − 0.17 0.91 − 0.60 − 1.79 − 3.19 − 2.38 − 1.09 − 5.31 − 4.35 − 1.93 − 2.20 − 2.88 − 2.76 − 5.63 − 3.25 − 3.42 − 7.38 − 6.21 − 2.50 − 4.06 − 2.27 − 2.20
− 0.33 0.38 − 1.72 − 1.24 − 1.15 − 0.44 − 1.04 − 1.64 − 1.07 − 0.96 − 1.84 0.62 − 0.71 − 0.03 − 0.52 − 0.05 − 0.82 0.19 − 0.85 − 0.23 − 0.44 − 0.41 − 2.29 − 0.33 − 1.12 0.33 − 0.82 1.41 − 0.53 0.13 0.05 − 1.13 − 0.25 − 0.14 − 0.46 0.36 0.58 0.50 − 0.03 0.39 − 0.17 1.16 − 0.48 0.70 0.16 − 1.24
A M O A A A A A A A A E A O1 O2 A A A A A A A A O C A A A A A A A A A A A A A A A A O G A A A
0.70898
Sr/86Sr
0.70895
0.70850 0.70852
0.70851
0.70849
The groups are: (A) Ammonia beccarii; (E) Elphidium spp.; (M) miliolid; (O) Ostracode; (G) Gastropod; and (C) Charophyte.
(123 cm) is coincident with an abrupt increase in ostracode and gastropod abundance. The charophyte peak is synchronous with the isotopically lightest foraminiferal sample (−2.29‰) at 116 cm. The δ13C measurements on A. beccarii record three broad patterns: fluctuations of ∼ 4‰ from the base of the
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Fig. 5. Strontium mixing curve for the Laguna de la Leche region (black line). The black circles represent the positions of the near-freshwater and marine end-members. A strontium mixing curve for the Peace River, Florida, is shown in grey for comparison; the end-members for this curve are also indicated by filled circles (Bryant et al., 1995). The dotted lines represent two 87Sr/86Sr values measured on the foraminifera and their corresponding salinity values.
core to 102 cm; an overall shift toward more positive values for the same section (102–219 cm); and a shift toward greater negative values (− 4.00‰ to − 5.27‰) from 2–95 cm, with much less variation than in the lower section of the core. The oxygen and carbon isotopic results from the lake phase are similar to δ18O and δ13C data measured on A. beccarii from a 130 cm-long core from Dune Pass Bay Pond, Bahamas, which show sympathetic decreases in both δ18O and δ13C with isotopic values of comparable magnitude to the Laguna de la Leche data (Dix et al., 1999). 5.3.
87
Pliocene carbonates (Compton, 1997) – rocks of roughly similar age and composition to those of the Laguna de la Leche region – and both areas have a similar climatic regime, so weathering rates are comparable. The Floridian data appear to provide independent validation for the Cuban Sr mixing curve. Using the Cuban curve, the 87Sr/86Sr value of 0.70850, which characterizes the lower unit of the core, represents a salinity of 1.7‰, whereas 0.70895 and 0.70898 reflect salinities of 7.1‰ and 8.1‰ respectively. Table 3 shows the upper and lower limits for each salinity value when
Sr/86Sr
Of the six samples analyzed, four are from the lower unit of the core and have virtually identical 87Sr/86Sr values (∼0.70850); the remaining two samples are from the upper unit and have higher Sr isotopic ratios of 0.70895 and 0.70898, which approach the average global marine value of 0.709172 (Table 1; Fig. 4) (Hodell et al., 1990). The Sr mixing curve is plotted in Fig. 5; the data used to construct it are listed in Table 2. The mixing curve is very similar to data for the Peace River, Florida, plotted on the same graph, whose waters have a 87Sr/86Sr ratio of ∼ 0.7083 and a Sr concentration of 0.75 (Bryant et al., 1995). The similarity between the Cuban and Floridian mixing curves may be due to the fact that the bedrock of the Peace River drainage basin consists of Miocene and
Table 2 Salinity, Sr concentration, and 87Sr/86Sr values for water samples from north-central Cuba
Marine end-members
Sample Salinity Sr (ppm) ID (‰)
87
A B C D E F
– – – – – – 0.709172 a 0.70865 ± 0.00002 – 0.70865 ± 0.00002
Mean Freshwater end-members G H Mean a
54.9 56.7 55.5 47.6 53.0 55.3 53.9 1.7
12.6 12.2 14.4 13.3 13.1 14.4 13.3 0.99
1.7 1.7
0.97 0.98
From Hodell et al. (1990); error not reported.
Sr/86Sr
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Table 3 Calculated salinities from the 87Sr/86Sr values are shown in bold, with corresponding 2σ errors above and below the calculated salinities Depth in core (cm)
87
2
0.70900 0.70898 0.70896 0.70897 0.70895 0.70893 0.70852 0.70850 0.70848 0.70854 0.70852 0.70850 0.70853 0.70851 0.70849 0.70851 0.70849 0.70847
68
108
116
123
195
Sr/86Sr ± 0.00002
Salinity (‰) 9.02 8.14 7.39 7.75 7.06 6.47 1.77 1.67 1.59 1.87 1.77 1.68 1.82 1.72 1.63 1.72 1.63 1.55
the ±2 × 10− 5 analytical uncertainty for the Sr isotope measurements is considered. Because of the hyperbolic nature of the curve, higher salinity values – as well as the upper error ranges for each salinity measurement – necessarily have larger errors. Thus, the error associated with the 1.7‰ result is negligible (±∼0.1‰), while the errors associated with the 7.1‰ and 8.1‰ results are roughly ±0.8‰. Additional, minor errors may result from measurements associated with the salinity and Sr concentration data and the use of samples collected from a single season. Nevertheless, the 87Sr/86Sr record from Laguna de la Leche records a clear trend of increasing salinity, from ∼1.7 ± 0.1‰ to ∼8.0 ± 0.8‰, consistent with the wide range of palynological and benthic invertebrate data generated from the core. 5.4. Taphonomic, vital, or seasonal effects? The non-A. beccarii δ18O and δ13C results are mostly inconsistent with those of A. beccarii at the same levels (Fig. 4). At 2 cm, the ostracode has lower δ18O and δ13C values, whereas the miliolid's values are higher, and both the ostracode and miliolid have a larger deviation in δ13C compared to δ18O. The Elphidium spp. sample at 50 cm has a δ13C value close to, but slightly more positive than, A. beccarii; the δ18O value, however, is significantly more (2.45‰) positive than A. beccarii. The two ostracode samples at 68 cm have δ13C values more negative, whereas the δ18O values are more positive. At 116 cm, the δ18O and δ13C of the ostracode and charophyte samples are both more positive than the A. beccarii
values by 1.96‰ and 1.17‰ (for δ18O) and 2.97‰ and 3.17‰ (for δ13C), respectively. Near the base of the core, at 195 cm, the ostracode δ18O and δ13C are more positive by 1.33‰ and 1.18‰ respectively; the gastropod δ13C exhibits a large, positive deviation (4.89‰), unlike the δ18O value, which is only 0.31‰ more negative. When compared to the A. beccarii data, the δ13C measurements on the non-A. beccarii samples tend to deviate more than the δ18O measurements, with the exception of the Elphidium spp. specimen. Also, the foraminifera (the Elphidium spp. and miliolid tests) have more positive values than A. beccarii, unlike the other taxa which have both positive and negative values. Deconvolving the reasons behind the widely ranging non-A. beccarii results is difficult, although tentative interpretations can be drawn. Seasonal effect probably explains much of the variability among contemporaneous non-A. beccarii specimens. During the early lake phase of the core, the A. beccarii results suggest that bioturbation was negligible, since bioturbation would likely blur the high-frequency, high-magnitude fluctuations seen in the δ13C data in this section of the core (Martin, 1999). Given the small size of the foraminifera, lateral reworking is also a distinct possibility, especially during hurricane events. If this was the case, it suggests a higher spatial variability in past salinities than the A. beccarii data themselves imply. Furthermore, despite recent evidence that suggests biological fractionation may be relatively unimportant among shallow-water biogenic carbonates (Reinhardt et al., 2003; Lécuyer et al., 2004), vital effect cannot be completely ruled out. 6. Discussion The O, C, and Sr isotopic results, in conjunction with geochemical, palynological and other micropaleontological data, permit the following paleohydrological reconstruction of Laguna de la Leche. From ∼ 6800 to ∼ 4800 cal yr B.P., Laguna de la Leche was a relatively closed, shallow, oligohaline lake. High δ18O values at this time were caused by intense evaporation, similar to the Florida Everglades where freshwater can have a δ18O value as high as 3‰ — in comparison to marine water with values of 0–1‰ (Alvarez Zarikian et al., 2001). In addition, the closed nature of the basin may also explain the low salinity of ∼ 1.7‰ inferred from the Sr isotopes for this time; intense evaporation may have caused the salinity of the pond to have been slightly higher, but this would not be recorded with the Sr isotopes because they are unaffected by evaporation. Fluctuations in δ13C were driven by periods of intensified plant productivity and episodic marine incursions. Fig. 6 shows a strong
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Fig. 6. Comparison of δ13C and pollen influx results. There is not an exact correspondence between peaks because the foraminiferal and pollen samples were taken at different depths in the core.
correlation between pollen influx and δ13C; higher pollen influx values, suggesting greater biomass (Peros et al., in press), may have been triggered by lowmagnitude water level fluctuations (perhaps climatically induced) that affected plants with shallow water preferences, such as T. domingensis and B. maritima (Tobe et al., 1998). Storm surges or other short-term increases in RSL may have led to marine incursions; during such events synchronous increases in both δ13C and δ18O would be expected, such as occurring at 119 cm and 138 cm. The gradual, overall increase in δ13C values may be due to marine waters entering and remaining in the closed basin from these incursions, resulting in a steady accumulation of dissolved inorganic carbon (DIC)-rich water over time. The period from ∼4800 to ∼ 4200 cal yr B.P. is associated with deeper and possibly fresher water, as evidenced by increases in the gastropods, ostracodes, and charophytes, and the lowest δ18O value measured. This δ18O value may have been caused by an influx of isotopically-depleted rain and/or hurricane water (Lawrence and Gedzelman, 1996; Clark and Fritz, 1997). It is unclear whether indicators of freshening are due to a regional climatic shift of a local-scale change in drainage, which delivered increased runoff to the lake during this period. Since rainwater does not contain strontium, stable 87Sr/86Sr values at this time are not inconsistent with an increase in precipitation.
From ∼4200 to ∼ 1700 cal yr B.P, relatively open and deeper conditions existed and Laguna de la Leche experienced frequent, and perhaps continuous, connection with the Bahía de Perros. This hydrological change was caused by an increase in RSL which contributed saline marine water to the basin. With the open conditions, the water in Laguna de la Leche had a lower residence time compared to the earlier, oligohaline lake phase; since evaporation therefore had much less of an effect on the water, average δ18O values decreased, despite enhanced marine influence. A severe 100–200 yr drought, centered at 4200 cal yr B.P. and documented at low and midlatitude sites worldwide (Booth et al., 2005), may have also contributed to the salinity increase around 4200 cal yr B.P. The period from ∼ 1700 cal yr B.P to the present has been characterized by extensive mangrove development around the perimeter of Laguna de la Leche, which led to an impoundment of the basin. The slight increase in average δ18O values at this time suggests greater evaporation, although salinity may not have been affected. The salinity of ∼ 8‰ inferred for the sample at 2 cm indicates that this sediment was deposited prior to the closing of the canals in 1986, when lake salinity was higher. Since then, average salinity decreased from ∼8‰ to its present value of 2–4.5‰ due to recent human modifications of the canals that connect Laguna de la Leche to the Bahía de Perros.
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The decrease in δ13C from ∼ 4200 cal yr B.P to the present may reflect changes in local and regional vegetation. Many salt marsh plants, such as members of the Poaceae (e.g., Spartina alterniflora, Distichlis spicata) use the C4 photosynthetic pathway and have δ13C values of − 9 to − 17‰; others, such as Salicornia spp., use the Crassulacean Acid Metabolism (CAM) pathway and have δ13C values that range from − 12 to − 20‰ (Ingram et al., 1998; Mitsch and Gosselink, 2000; Leng, 2003). Mangroves, on the other hand, are C3 plants and have δ13C values of − 24 to − 28‰ (Hogarth, 1999). While the palynological and plant macrofossil results indicate that much of the local vegetation during the pond phase consisted of C3 vegetation (e.g., T. domingensis), and the low-taxonomic resolution of the pollen data prevents the identification of many taxa to the species level, we argue that the change from an oligohaline pond to a mangrovesurrounded lake may have decreased the δ13C value of the freshwater end-member feeding the system, resulting in an overall negative shift in δ13C. Ingram et al. (1998) documented a similar situation from Petaluma marsh, northern California, where despite increasingly saline conditions in the marsh, δ13C values decreased over time as the CAM plant Salicornia spp. increased in importance. 7. Conclusions Oxygen, carbon, and strontium isotopic data have yielded information on the Middle to Late Holocene paleohydrology and paleosalinity of Laguna de la Leche, north coastal Cuba. Specifically, they allowed for a determination of the degree of connectivity that Laguna de le Leche has had with the sea and how this has changed with time, the causes of these changes, and the effects of these changes on the salinity and water depth of the wetland. Our results underscore the importance of using Sr as well as O and C isotopes in paleosalinity reconstructions, since O and C isotopes can be affected by a range of factors unrelated to salinity, such as vegetation and climate. Furthermore, the Sr results provided reliable, quantitative data on past salinity variation. A study of addition microfossil specimens and taxa will allow for a better understanding of the taphonomic processes that have affected the fossil record of the wetland. Acknowledgments We thank Jorge Calvera for acquiring permits to work at Laguna de la Leche and assisting us in the field. David Smith helped with coring. Beverly Goodman and Laura Kobayashi assisted with foraminiferal preparation
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