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Middle and Late Holocene sediments have not been extensively sampled in Lake Tanganyika, and much remains unknown about the response of the Rift Valley’s largest lake to major environmental shifts during the Holocene, including the termination of the African Humid Period (AHP). Here, we present an integrated study (sedimentology, mineralogy, and geochemistry) of a radiocarbon-dated sediment core from the Kavala Island Ridge (KIR) that reveals paleoenvironmental variability in Lake Tanganyika since the Middle Holocene with decadal to centennial resolution. Massive blue-gray sandy silts represent sediments deposited during the terminal AHP (~5880–4640 cal yr BP), with detrital particle size, carbon concentrations, light stable isotopes, and mineralogy suggesting an influx of river-borne soil organic matter and weathered clay minerals to the lake at that time. Enhanced by the AHP’s warm and wet conditions, chemical weathering and erosion of Lake Tanganyika’s watershed appears to have promoted considerable nutrient recharge to the lake system. Following a relatively gradual termination of the AHP over the period from ~4640 cal yr BP to ~3680 cal yr BP, laminated and organic carbon-rich sediments began accumulating on the KIR. δ¹⁵N_bulk, C/N, and hydrogen index data suggest high relative primary production from a mix of algae and cyanobacteria, most likely in response to nutrient availability in the water column under a cooler and seasonally dry climate from ~3680 to 1100 cal yr BP. Sediments deposited during the Common Era show considerable variability in magnetic susceptibility, total organic carbon content, carbon isotopes, and C/N, consistent with dynamic hydroclimate conditions that affected the depositional patterns, including substantial changes around the Medieval Climate Anomaly and Little Ice Age. Data from this study highlight the importance of sedimentary records to constrain boundary conditions in hydroclimate and nutrient flux that can inform long-term ecosystem response in Lake Tanganyika. 

Abstract We examine major reorganizations of the terrestrial ecosystem around Mono Lake, California during the last deglacial period from 16,000–9,000 cal yr BP using pollen, microcharcoal, and coprophilous fungal spores (Sporormiella) from a deep-water sediment core. The pollen results record the assemblage, decline, and replacement of a mixed wooded community of Sierran and Great Basin taxa with Alkali Sink and Sagebrush Steppe biomes around Mono Lake. In particular, the enigmatic presence ofSequoiadendron-type pollen and its extirpation during the early Holocene hint at substantial biogeographic reorganizations on the Sierran-Great Basin ecotone during deglaciation. Rapid regional hydroclimate changes produced structural alterations in pine–juniper woodlands facilitated by increases in wildfires at 14,800 cal yr BP, 13,900 cal yr BP, and 12,800 cal yr BP. The rapid canopy changes altered the availability of herbaceous understory plants, likely putting pressure on megafauna populations, which declined in a stepwise fashion at 15,000 cal yr BP and 12,700 cal yr BP before final extirpation from Mono Basin at 11,500 cal yr BP. However, wooded vegetation communities overall remained resistant to abrupt hydroclimate changes during the late Pleistocene; instead, they gradually declined and were replaced by Alkali Sink communities in the lowlands as temperature increased into the Early Holocene, and Mono Lake regressed. 

Jackson Lake supplies valuable cultural and provisioning ecosystem services to the Upper Snake River watershed in Wyoming and Idaho (western USA). Construction of Jackson Lake Dam in the early 20th century raised lake level by ∼12 m, generating an important water resource supporting agriculture and ranching, as well as tourism associated with Grand Teton National Park. Outlet engineering drastically altered Jackson Lake’s surface area, morphology, and relationship with the inflowing Snake River, yet the consequences for nutrient dynamics and algae in the lake are unknown. Here, we report the results of a retrospective environmental assessment completed for Jackson Lake using a paleolimnological approach. Paleoecological (diatoms) and geochemical datasets were developed on a well-dated sediment core and compared with available hydroclimate data from the region, to assess patterns of limnological change. The core spans the termination of the Little Ice Age and extends to the present day (∼1654–2019 CE). Diatom assemblages prior to dam installation are characterized by high relative abundances of plankton that thrive under low nutrient availability, most likely resulting from prolonged seasonal ice cover and perhaps a single, short episode of deep convective mixing. Following dam construction, diatom assemblages shifted to planktic species that favor more nutrient-rich waters. Elemental abundances of sedimentary nitrogen and phosphorous support the interpretation that dam installation resulted in a more mesotrophic state in Jackson Lake after ∼1916 CE. The data are consistent with enhanced nutrient loading associated with dam emplacement, which inundated deltaic wetlands and nearshore vegetation, and perhaps increased water residence times. The results of the study highlight the sensitivity of algal composition and productivity to changes in nutrient status that accompany outlet engineering of natural lakes by humans and have implications for water resource management. 

Dam installation on a deep hydrologically open lake provides the experimental framework necessary to study the influence of outlet engineering and changing base levels on limnogeological processes. Here, high-resolution seismic reflection profiles, sediment cores, and historical water level elevation datasets were employed to assess the recent depositional history of Jackson Lake, a dammed glacial lake located adjacent to the Teton fault in western Wyoming (USA). Prograding clinoforms imaged in the shallow stratigraphy indicate a recent lake-wide episode of delta abandonment. Submerged ∼11–12 m below the lake surface, these Gilbert-type paleo-deltas represent extensive submerged coarse-grained deposits along the axial and lateral margins of Jackson Lake that resulted from shoreline transgression following dam construction in the early 20th century. Other paleo-lake margin environments, including delta plain, shoreline, and glacial (drumlins, moraines) landforms were likewise inundated following dam installation, and now form prominent features on the lake floor. In deepwater, a detailed chronology was established using 137 Cs, 210 Pb, and reservoir-corrected 14 C for a sediment core that spans ∼1654–2019 Common Era (CE). Dam emplacement (1908–1916 CE) correlates with a nearly five-fold acceleration in accumulation rates and a depositional shift towards carbonaceous sediments. Interbedded organic-rich black diatomaceous oozes and tan silts track changes in reservoir water level elevation, which oscillated in response to regional climate and downstream water needs between 1908 and 2019 CE. Chemostratigraphic patterns of carbon, phosphorus, and sulfur are consistent with a change in nutrient status and productivity, controlled initially by transgression-driven flooding of supralittoral soils and vegetation, and subsequently with water level changes. A thin gravity flow deposit punctuates the deepwater strata and provides a benchmark for turbidite characterization driven by hydroclimate change. Because the Teton fault is a major seismic hazard, end-member characterization of turbidites is a critical first step for accurate discrimination of mass transport deposits controlled by earthquakes in more ancient Jackson Lake strata. Results from this study illustrate the influence of dam installation on sublacustrine geomorphology and sedimentation, which has implications for lake management and ecosystem services. Further, this study demonstrates that Jackson Lake contains an expanded, untapped sedimentary archive recording environmental changes in the American West.