Paul I. Abell

Lake level and paleoenvironmental history of Lake Tanganyika, Africa, as inferred from late Holocene and modern stromatolites

Fossil and living stromatolites are abundant around the margins of Lake Tanganyika, Africa, and provide a wealth of paleolimnologic and paleoclimatic information for the late Holocene. Six lines of evidence show that stromatolites and cements are precipitating in the lake today: (1) carbonate saturation state calculations, (2) documentation of living stromatolites and their depth distribution, (3) new stable isotope data showing the lake’s present mixing state and ancient evaporation and inflow balance, (4) new radiocarbon data and a reevaluation of apparent 14 C ages derived from Lake Tanganyika carbonates, (5) the presence of modern Mg-calcite cements derived from lake waters, and (6) the presence of modern, biologically mediated Mg-calcite precipitates in the lake. Lake Tanganyika’s lake levels have been remarkably stable over the past 2800 yr, fluctuating around the marginally open to marginally closed level through most of this time period. Lake lowstands and high δ 18 O values from the ninth century B.C. to the early fifth century A.D. indicate that the lake basin was comparatively dry during this time. However, the period prior to the most recent opening of Lake Kivu into the Lake Tanganyika basin (ca. A.D. 550) was not marked by major lake lowstands, nor was this opening accompanied by a dramatic lakelevel rise. The Kivu opening was roughly coincident with a significant shift toward isotopically lighter (δ 18 O and δ 13 C) lake water, which persists today. The lake remained close to its outlet level between the sixth and thirteenth centuries A.D. Lake levels rose between the fourteenth and sixteenth centuries. At some time between the late sixteenth and early nineteenth centuries, lake level fell to perhaps its lowest level in the past 2800 yr. By the early nineteenth century, lake level had begun to rise to the overflow level, apparently the result of a regional increase in precipitation/evaporation ratios. Weak δ 18 O/δ 13 C covariance for late Holocene carbonates suggests that the surface elevation of the lake has remained near the outlet level, with only occasional periods of closure. However, there is no simple relationship between solute input from Lake Kivu, isotope input from Lake Kivu, and lake levels in Lake Tanganyika. Lake Kivu waters are the primary source of major ions in Lake Tanganyika, but are much less important in controlling the δ 18 O and the lake level of Lake Tanganyika. Because the Ruzizi River’s discharge into Lake Tanganyika is largely derived from sources other than Lake Kivu, the overflow events in the two lakes have been uncoupled during the late Holocene.

PLIO-PLEISTOCENE LACUSTRINE STROMATOLITES FROM LAKE TURKANA, KENYA: MORPHOLOGY, STRATIGRAPHY AND STABLE ISOTOPES

Abell, P.I., Awramik, S.M., Osborne, R.H. and Tomellini, S., 1982. Plio-Pleistocene lacustrine stromatolites from Lake Turkana, Kenya: morphology, stratigraphy and stable isotopes. Sediment. Geol., 32: 1--26. A sequence of fossil stromatolites from Lake Turkana in Kenya was examined for 51SO and 513C content. These stromatolites, ranging in age from Holocene (~10,000 yrs B.P.) to Middle Pliocene (-3 m.y.) showed a variety of growth forms from oncolitic, columnar layered to bulbous heads. The stromatolites used in our study contain filamentous blue-green algae of one morphological type and rare coccoids; thus the stromatolites are considered biogenic. The stable isotope ratios for oxygen and carbon indicate changing climatic conditions, ranging from a cool, wet climate prior to ca. 1.9 m.y. to much drier, warmer conditions around 1.4 m.y., followed in turn by a somewhat cooler and wetter climate at the end of the Pleistocene.

Holocene palaeoclimates in northwestern Sudan: stable isotope studies on molluscs

Abstract Gastropod shells and bulk sedimentary carbonate deposits found in palaeolake sediments in the presently hyperarid regions of NW Sudan provide proxy materials for the evaluation of the vastly different and wetter climatic conditions that prevailed for several thousand years in that region at the beginning of the Holocene. Oxygen and carbon stable isotope ratio measurements on these shells and carbonates suggest that the African Summer Monsoon provided extensive rainfall up to 800 km further north than at present, creating substantial lakes and refilling the deep aquifers of the region with isotopically depleted water up to 21°N. Variations in stable isotope ratios, as one proceeds upwards through the sediments, indicate that the wettest phase occured about 9000 years B.P., and that a considerably drier period began after about 5600 years B.P., after which the record is obliterated by decreasing rainfall and subsequent deflation of the sediments. During the wet phase, large quantities of isotopically depleted (light) moisture — corroborating the convective origin of the rainfall — were brought to the Eastern Sahara by intensified monsoonal rains. Variations in the oxygen isotope ratios during the growth of individual shells demonstrate that considerable seasonality existed in yearly rainfall.

Oxygen isotope ratios in modern African gastropod shells: a data base for paleoclimatology

Abstract The oxygen isotope ratios of ∼ 80 modern gastropod shells, collected throughout Africa, show a fair correlation with rainfall patterns as signalled by permanent vegetative cover, but some regions show substantial anomalies. Gastropods from the East African Rift Valley lakes showed oxygen isotope ratios 3–6%o more positive, and those from the northeastern regions of South Africa were both more positive and more variable than expected. Temperature effects on precipitation and evaporative isotope enrichment of rainfall and water bodies are the principal causes of these discrepancies. The possibilities for the use of isotope ratios of fossil shells for paleoclimatology are discussed.

OXYGEN AND CARBON ISOTOPE RATIOS IN GASTROPOD SHELLS AS INDICATORS OF PALEOENVIRONMENTS IN THE AFAR REGION OF ETHIOPIA

Abstract The oxygen and carbon isotope ratios in the shells of ancient freshwater gastropods, coupled with geological, geographical and hydrological data from their area and time of origin, can yield information concerning general climatic conditions prevailing at the time the gastropod lived. In addition, sequential oxygen isotope analyses along the growth spiral of the shell provides indications of seasonal variability in the life span of the organisms. Such data were collected for some freshwater gastropods collected from two lakes and two springs in the Afar region of Ethiopia, spanning late Pleistocene to modern age. Episodes of high rainfall in the Ethiopian highlands were clearly evident in the depleted δ18O values in some shells, whiel arid spells produced enriched levels in others. Hot springs introduced an added complication to the isotope record. This study, on a region of particularly complex hydrology and highly variable environmental history, indicates the utility of stable isotope analyses as proxies for past climates, but also emphasizes the problems involved.

The Pleistocene/Holocene transition in South Africa: evidence for the Younger Dryas event

Abstract Evidence for the Younger Dryas (YD) “cold episode” event and other complexities in the warming up of world climates at the end of the Pleistocene is rather sparse outside the North Atlantic region. There are indications from a number of studies, however, that it was indeed a worldwide event. Seeking evidence to define the Pleistocene/Holocene transition in southern Africa, the oxygen isotope ratios in shell fragments of the giant land snail, Achatina sp. (as derived from dated strata in the archaeological excavations of the Bushman Rock Shelter in the Transvaal region of South Africa), were measured. A sequence of colder episodes, including what we believe to be the YD event, is indicated, and there is evidence as for an earlier (Older Dryas?) event as well. We also measured the oxygen isotope ratios of modern land snail shells from South Africa and showed that they exhibit a good correlation with latitude, enabling us to make some deductions as to the general climatic conditions during the Pleistocene/Holocene transition compared to today.

Seasonality in the early Holocene climate of Northwest Sudan: interpretation of Etheria elliptica shell isotopic data

Abstract The oxygen isotope ratios in the incremental growth layers in the shells of Etheria elliptica constitute a proxy record of rainfall patterns, and thus a record of seasonality. Analyses of shells of early Holocene age (6800 14 C years BP or 5600 BC) collected from the lower reaches of Wadi Howar, near the confluence of that now-extinct river with the Nile, show an annual pattern of two rainy seasons in the present-day hyperarid southeastern Sahara, similar to that which prevails today in much of East Africa.

The shells of Etheria elliptica as recorders of seasonality at Lake Victoria

Abstract Reconstruction of past environments is an essential part of evaluating the ecology of organisms including early man and assessing the pressures for evolutionary change. In this study we have demonstrated that Etheria elliptica , the freshwater oyster of equatorial Africa, accretes layers of shell having a lunar month periodicity. Separation and oxygen isotopic analyses of the individual monthly layers affords a qualitative picture of seasonal change. Fourteen monthly samples of E. elliptica , collected near Kisumu on the Winam Gulf of Lake Victoria, gave oxygen isotope ratios in the penultimate layer of their shells in excellent agreement with the individual layers of a shell whose growth covered the same period. The variation in isotope ratios correlates clearly with the rainfall pattern of that region of East Africa.

HBr catalyzed photoisomerization of allyl halides

Abstract The gas phase bond isomerizations of allyl fluoride, allyl chloride, and allyl bromide to the corresponding 1-halo-1-propenes have been studied. HBr and ultraviolet light catalyze the reaction very effectively in the temperature range 150 to 250 °C and at pressures of about 5 to 50 Torr. The equilibrium constants for the reactions of allyl halide to give 1-halo-1-propene have been determined as functions of temperature for the three halides, and activation energies and enthalpy differences have been calculated. New determinations of the cis/trans ratios of the 1-halo-1-propenes have been made and found to be essentially independent of the halogen in the halopropene.

Paleoenvironments in the Chalbi Basin of Kenya

Abstract Stable oxygen and carbon isotope ratios in the shells of the freshwater gastropod, Melanoides tuberculata , collected at five early Holocene sites in the Chalbi Basin of north-central Kenya, have been measured. These ratios indicate that at 10,000–11,000 yr.ago, paleo Lake Chalbi was a freshwater lake of considerable stability, despite its large extent and shallow depth.