Isotopic paleolimnology of Lake Kinneret

The carbonate, organic C, and N contents and their respective d13C were measured in four cores from Lake Kinneret, Israel. Using these data and the isotopic difference between coexisting carbonate and organic C, Dd13C, we attempted to reconstruct the lake’s productivity and regional climatic conditions during the late Holocene. The sedimentary record of the last 120 yr (for which climatological data are available) demonstrates that during periods of intense productivity, organic C content and its dCorg are high, whereas the Dd13C values are low. During wet periods, characterized by intense input of particulates and nutrients from external sources, the CaCO3 content, its dCcar and dOcar, and the organic C : N ratios are low. Based on these correlations, the longer sedimentary record of the last ca. 3,300 yr was divided into five stages, which differ in productivity and/or climatic characteristics. This longer sedimentary record indicates that the primary production in Lake Kinneret has varied in the past over a range similar to that observed presently, but the rate of these changes was much slower. The rainiest period recorded in the sediments was from about 200 B.C. to A.D. 200, i.e. during the Roman period, when the region was heavily populated. The recent decrease in d13C of atmospheric CO2 that accompanies the global anthropogenic CO2 rise seems to be recorded by a commensurate decrease in dCcar of the sediments. The concomitant increase in Dd13C expresses an increase in the isotopic fractionation of the lake’s primary producers, which may also be related to the atmospheric CO2 rise. During the last decade it has been well established that the C isotopic fractionation of marine and freshwater phytoplankton is controlled by their rate of growth, the ambient concentration of dissolved CO2 (CO2(aq)), and the phytoplankton size and shape (e.g, Degens 1969; Goericke et al. 1994; Laws et al. 1995; Rau et al. 1996). Thus, the fractionation of C isotopes is becoming an important tool for deciphering paleoenvironmental changes in aquatic systems (e.g., Hollander et al. 1993; Jasper et al. 1994) Because of their smaller size and higher sedimentation rates, lakes are more sensitive than oceans to climatic and environmental changes. Their sedimentary records therefore allow finer time resolution and direct comparison with known historic and climatic records on nearby land. Lake Kinneret (358319E, 328539N) is a warm monomictic lake characterized by an annual winter–spring bloom of Per1 To whom correspondence should be addressed. Present address: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 54-1320, 77 Massachusetts Ave., Cambridge, Massachusetts 02139 ([email protected]). Acknowledgments This research was supported by the Belfer Foundation for Energy Research, by grant DISUM-BMBF 0033-01421 from the Israeli Ministry of Science and the German Ministry of Science and Technology, and by the Moshe Shilo Minerva Center for Marine Biogeochemistry. We thank E. Halicz and A. Ayalon for their help with the isotopic analyses and A. Kaufman and Y. Kolodny for their help in radioactive dating of the sediments. We thank U. Pollinger and T. Zohary from the Yigal Alon Kinneret Limnological Laboratory (KLL) for their algal biomass data, and the boat crew of KLL for their help in obtaining the cores. We thank reviewers J. M. Hayes and M. R. Talbot for their productive remarks and efforts to improve this manuscript. idinium gatunense. This dinoflagellate is present all year round but during the bloom represents .95% of the phytoplankton biomass. The lake surface, at 211 m below mean sea level, is about 167 km2 in area, and its maximum depth is 42 m (average depth, 24 m). Most of the lake’s watershed is located on the north side and is drained by the Jordan River, which is the main nutrient supplier to the lake. The moderate size of the lake and its relatively large watershed (2,730 km2) makes it very sensitive to both climatic and anthropogenic changes in the region. Significant climatic changes in the region occurred during the upper Pleistocene to late Holocene and during historical times (e.g., Neev and Emery 1995; Bar-Matthews et al. 1999). During the last century, many environmental changes have resulted from intensive anthropogenic activity in the lake watershed. The effects of these activities on the biogeochemical system are still unclear, despite the many studies that have been carried out in the lake (e.g., Berman et al. 1992; Zohary et al. 1994; Berman-Frank et al. 1998). Because the lake is the main source of freshwater in Israel, it is of great importance to study and understand the effects of global and local changes on this ecosystem. Detailed monitoring of the lake and its ecosystem has been conducted since 1969, and meteorological data and measurements of the lake surface levels have been recorded since 1880. The longer record of environmental change preserved in the lake sediments provides an additional perspective that may help us to understand and better manage the recent changes in the lake. Previous paleolimnological studies on Lake Kinneret were performed in the early 1980s as part of an interdisciplinary study (Stiller et al. 1984). These studies were based upon three cores, about 5 m long, collected in 1979 by Thompson and coworkers (1985) at stations U, D, and F (see Fig. 1). 69 Isotopic paleolimnology of Lake Kinneret Fig. 1. Map of Lake Kinneret, Israel. The short cores (1, 4, and 5) were obtained at station A (43 m depth), and the long core (KINU8) was obtained in 1979 at station U. The analyses included paleomagnetic profiles and radioactive dating using 14C (Thompson et al. 1985), pollen records (Baruch 1986), distribution of diatoms and nonsilicate algae (Ehrlich 1985; Pollingher et al. 1986), and isotopic composition of carbonate (Stiller and Kaufman 1985). The present study provides a high-resolution analysis of the sedimentary carbonates and organic matter in three short cores, representing the last century. Contemporaneous limnological and climatological data from the area allow a better understanding of the sediment records and the effects of recent anthropogenic activities in and around the lake. The organic C and N contents and the isotopic composition of organic C were also analyzed on one of the longer 1979 cores (core KINU8 from station U). Combining these data with previous paleolimnological findings and with the information gained from the study of the short cores allowed an improved estimate of the lake’s paleoproductivity and of the region’s paleoclimate over the last 3,300 yr.