Asymmetric response of forest and grassy biomes to climate variability across the African Humid Period : influenced by anthropogenic disturbance?

A comprehensive understanding of the relationship between land cover, climate change and disturbance dynamics is needed to inform scenarios of vegetation change on the African continent. Although significant advances have been made, large uncertainties exist in projections of future biodiversity and ecosystem change for the world's largest tropical landmass. To better illustrate the effects of climate–disturbance–ecosystem interactions on continental‐scale vegetation change, we apply a novel statistical multivariate envelope approach to subfossil pollen data and climate model outputs (TraCE‐21ka). We target paleoenvironmental records across continental Africa, from the African Humid Period (AHP: ca 14 700–5500 yr BP) – an interval of spatially and temporally variable hydroclimatic conditions – until recent times, to improve our understanding of overarching vegetation trends and to compare changes between forest and grassy biomes (savanna and grassland). Our results suggest that although climate variability was the dominant driver of change, forest and grassy biomes responded asymmetrically: 1) the climatic envelope of grassy biomes expanded, or persisted in increasingly diverse climatic conditions, during the second half of the AHP whilst that of forest did not; 2) forest retreat occurred much more slowly during the mid to late Holocene compared to the early AHP forest expansion; and 3) as forest and grassy biomes diverged during the second half of the AHP, their ecological relationship (envelope overlap) fundamentally changed. Based on these asymmetries and associated changes in human land use, we propose and discuss three hypotheses about the influence of anthropogenic disturbance on continental‐scale vegetation change

Late Holocene linkages between decade–century scale climate variability and productivity at Lake Tanganyika, Africa

Microlaminated sediment cores from the Kalya slope region of Lake Tanganyika provide a near-annually resolved paleoclimate record between ~~2,840 and 1,420 cal. yr B.P. demonstrating strong linkages between climate variability and lacustrine productivity. Laminae couplets comprise dark, terrigenous-dominated half couplets, interpreted as low density underflows deposited from riverine sources during the rainy season, alternating with light, planktonic diatomaceous ooze, with little terrigenous component, interpreted as windy/dry season deposits. Laminated portions of the studied cores consist of conspicuous dark and light colored bundles of laminae couplets. Light and dark bundles alternate at decadal time scales. Within dark bundles, both light and dark half couplets are significantly thinner than within light bundles, implying slower sediment accumulation rates during both seasons over those intervals. Time series analyses of laminae thickness patterns demonstrate significant periodicities at interannual¿centennial time scales. Longer time scale periodicities (multidecadal to centennial scale) of light and dark half couplet thicknesses are coherent and in some cases are similar to solar cycle periods on these time scales. Although laminae thickness cycles do not strongly covary with the actual ¿14C record for this same time period, two large ¿14C anomalies are associated with substantial decreases in both light and dark laminae thickness. In contrast to the multidecadal¿ centennial time scale, significant annual to decadal periodicities, which are broadly consistent with ENSO/PDO forcing and their impact on East African climate, are not coherent between light and dark half couplets. The coherency of light¿dark couplets at decadal¿centennial time scales, but not at shorter time scales, is consistent with a model of a long-term relationship between precipitation (recorded in wet season dark laminae thickness) and productivity (light laminae thickness), which is not manifest at shorter time scales. We hypothesize that this coupling results from long-term recharging of internal nutrient loading during wet periods (higher erosion of soil P) and reduced loading during drought intervals. The relationship is not expressed on short time scales during which the dominant control on productivity is wind-driven, dry season upwelling, which is uncorrelated with wet-season precipitation. Our record greatly extends the temporal record of this quasi-periodic behavior throughout the late Holocene and provides the first evidence linking decade- to century-scale episodes of enhanced productivity to enhanced precipitation levels and nutrient recharge in a productive tropical lake

Late Quaternary lacustrine ostracods (Ostracoda, Crustacea) and charophytes (Charophyta, Charales) from the Puna Plateau, Argentina

Relatively little is known about the recent palaeontological records of the high-altitude closed basin lakes of the Central Andes, but a great need exists to remedy this knowledge gap if microfossils are to be used to infer climatic and ecological transitions from lake sediment cores. Here, eight species of non-marine ostracods and two calcareous algae are recorded from modern sediments and late Quaternary strata from Laguna de los Pozuelos, Jujuy Province, northwestern Argentina. Four species, Limnocythere alexanderi, Limnocythere foresteri, Limnocythere lysandrosi and Limnocythere ruipunctifinalis, are described as new species. Limnocythere titicaca Lerner-Seggev, 1973 has only been recorded in Lake Titicaca prior to this study and this is the first time the species is recognized outside of Bolivia. The cypridoideans Ilyocypris ramirezi Cusminsky & Whatley, 1996, Eucypris virgata Cusminsky & Whatley, 1996 and Chlamydotheca pseudobrasiliensis Martens & Behen 1994 are known species that are rare in the stratigraphic sequence in core LP06-6A. Also, the gyrogonites of two well-known species of Charophyta, Chara filiformis Hertzsch and Chara vulgaris Linnaeus, are occasionally present in the sediment from Laguna de los Pozuelos. The results provide a new vehicle for clarifying the Quaternary palaeohydrological history of the Pozuelos Basin, which is a RAMSAR wetland that is likely to be sensitive to global environmental change.Fil: Palacios Fest, Manuel R.. Terra Nostra Earth Sciences Research,; Estados UnidosFil: Cusminsky, Gabriela Catalina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; ArgentinaFil: McGlue, Michael M.. University of Kentucky; Estados Unido

A paleoenvironmental reconstruction of Laguna Babícora, Chihuahua, Mexico based on ostracode paleoecology and trace element shell chemistry

Paleoecology of Laguna Babícora, Chihuahua, Mexico was reconstructed using ostracode faunal assemblages and shell chemistry. The paleolimnological record is used to show the magnitude of paleoclimatic changes in the area from 25,000 years to the present. Faunal assemblages consist of four species of the genus Limnocythere: L. sappaensis, L. ceriotuberosa, L. bradburyi and L. platyforma, all associated with Candona caudata, Candona patzcuaro and Cypridopsis vidua. A paleosalinity index developed from these assemblages indicates that the lake's salinity fluctuated frequently from oligo- to meso-haline conditions during the last bsim 25,000 years. This pattern and low salinity range are in good agreement with modern TDS (here used as an indicator of salinity) values recorded from 26 wells and one spring from the area (258–975 mg l–1). To estimate paleotemperature we examined the trace element content (Mg/Ca ratios) from individual valves of L. ceriotuberosa and L. platyforma, the two species most commonly recorded in Laguna Babícora. Shell Mg/Ca ratios of 204 specimens of these two species were used to estimate water temperature (Mg/Ca) by means of experimental standard coefficients. Our data show that paleowater temperature ranged from 5.6–21.3 °C (with 2sgr values ranging from 0.2–4.8 °C), which suggest a close correlation with atmospheric temperatures around the lake. These results are in good agreement with a modern mean winter temperature (3.5 °C) and mean summer temperature (20 °C) recorded in the area between 1970 and 1980

Ostracode biofacies and shell chemistry reveal quaternary aquatic transitions in the Pozuelos basin (Argentina)

Here we present the first use of calcareous microfossils to examine the late Quaternary paleoecology of the endorheic Pozuelos Basin (Argentina). Modern deposition in the basin centers on Laguna de los Pozuelos (LP),a shallow playa-lake that is fed by axial rivers and groundwater and dominantly accumulates siliciclastic sediments.Today, the distribution of limnocytherid and cypridoidean ostracodes across southern LP is strongly influenced bydistance to the Rıo Cincel delta, whereas the northern end of the playa-lake is characterized by a paucity of ostracodes due to frequent sub-aerial exposure. Ten ostracode biofacies define a sediment core retrieved from LP,which reveal progressive changes in aquatic environments that varied in salinity, depth, and proximity to deltas over the late Pleistocene. Closed lakes occupied the basin from ~ 37.6?30.7 ka, ~ 28.0?25.0 ka, and ~ 23.0?16.6 ka,whereas saline wetlands occurred when these lakes contracted. Extant LP has no analog in the late Pleistocene record; it formed after ~ 7.2 ka, following a hiatus that removed the Pleistocene?Holocene transition.Paleoecological evidence indicates that the core site was influenced by deltaic inflows from the eastern basin margin until ~ 24.3 ka, an area where today dry alluvial fans are found. Reorganization of the watershed by normalfaulting, most likely at ~ 18.0 ka, appears to have reduced the influence of these deltaic inflows. Extensional neotectonics, perhaps induced by incorporation of the Pozuelos Basin into the Andean hinterland, is a mechanism that along with tropical climate change is potentially important to water balance and ecology in high-altitude convergent orogenic basins.Fil: McGlue, Michael M.. University of Kentucky; Estados UnidosFil: Palacios Fest, Manuel R.. Terra Nostra Earth Sciences Research; Estados UnidosFil: Cusminsky, Gabriela Catalina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; ArgentinaFil: Camacho, Maria. Universidad Nacional de Jujuy; ArgentinaFil: Ivory, Sarah J.. University of Kentucky; Estados Unidos. University Brown; Estados UnidosFil: Kowler, Andrew L.. University of California; Estados UnidosFil: Chakraborty, Suvankar. University of Utah; Estados Unido

Paleolimnological investigations of anthropogenic environmental change in Lake Tanganyika, I: an introduction to the project

We investigated paleolimnological records from a series of river deltas around the northeastern rim of Lake Tanganyika, East Africa (Tanzania and Burundi) in order to understand the history of anthropogenic activity in the lake's catchment over the last several centuries, and to determine the impact of these activities on the biodiversity of littoral and sublittoral lake communities. Sediment pollution caused by increased rates of soil erosion in deforested watersheds has caused significant changes in aquatic communities along much of the lake's shoreline. We analyzed the effects of sediment discharge on biodiversity around six deltas or delta complexes on the east coast of Lake Tanganyika: the Lubulungu River delta, Kabesi River delta, Nyasanga/Kahama River deltas, and Mwamgongo River delta in Tanzania; and the Nyamuseni River delta and Karonge/Kirasa River deltas in Burundi. Collectively, these deltas and their associated rivers were chosen to represent a spectrum of drainage-basin sizes and disturbance levels. By comparing deltas that are similar in watershed attributes (other than disturbance levels), our goal was to explore a series of historical "experiments" at the watershed scale, with which we could more clearly evaluate hypotheses of land use or other effects on nearshore ecosystems. Here we discuss these deltas, their geologic and physiographic characteristics, and the field procedures used for coring and sampling the deltas, and various indicators of anthropogenic impact