John D. Halfman

Paleoclimate of the past 4000 years at Lake Turkana, Kenya, based on the isotopic composition of authigenic calcite

Abstract Authigenic calcite was analyzed for δ 13 C and δ 18 O in a 12 m long core from Lake Turkana that spans the last 4200 yr. The δ 18 O record is interpreted to reflect lake level history, and indicates high stands prior to 4000 yr B.P., from 1800 to 2000 yr B.P., and within the last century. This coincides better with the history of lake level change reported for other lakes in africa than did the previous lake level curve for Lake Turkana based on 14 C dates of exposed shoreline deposits. The δ 13 C curve roughly parallels the δ 18 O curve, and may reflect the influence of the Omo River plume. Both of the isotope records are strongly cyclic, with periodicites common to both at 80, 120, 150 and 225 yr. Other, somewhat longer periodicites are nearly equal in the two records, and there is close agreement with previously published cycles based on carbonate and lamination thickness analyses in the same Turkana core.

New AMS dates, stratigraphic correlations and decadal climatic cycles for the past 4 ka at Lake Turkana, Kenya

Abstract New AMS radiocarbon dates and stratigraphic correlations of 7 piston cores recovered from Lake Turkana, East Africa, show that the frequency of cyclic variability is greater than previously reported for a single piston core. Almost 60 new AMS dates of various carbonate fractions indicate that large (150 μm) ostracode carapaces yield the most accurate chronology for the sediment record due to the input of “old” fine-grained carbonate. Stratigraphic correlations among the cores by carbonate abundance and magnetic susceptibility profiles provide supporting evidence for the ostracode-based chronology. This chronology refines earlier estimates of sediment accumulation that impacts the recurrence interval of lamination deposition from one lamination every two years to one lamination per year in two north basin cores. The record of carbonate content within the north basin of the lake reflect its dilution with terrigenous sediments input from the Omo River but direct paleoclimatic interpretations are complicated by lateral migrations of the delta channels with changing lake levels. Time-series analysis of both individual cores and a combined carbonate profile show significant spectral peaks at 1000, 76, 32, 22, 18.6 and 11 yr. The latter three suggest an important link between the sediments accumulating in Lake Turkana and global cycles of climatic variability.

Climatic and tectonic effects on sedimentation in a rift-valley lake: Evidence from high-resolution seismic profiles, Lake Turkana, Kenya

Approximately 900 km of 28-kHz echo sounding and 770 km of 1-kHz seismic-reflection profiles were obtained from Lake Turkana in northern Kenya between March and December 1984. The profiles penetrate as much as 60 m below the lake floor, representing perhaps the last few tens of thousands of years. They show that the predominant pattern of sedimentation is one of simple and rapid basin infilling. The profiles suggest an abundance of gas (probably methane) in the Holocene sediments, particularly near the major rivers and in the deepest basins where sedimentation rates are relatively high. High-angle growth faults, characteristic of extensional regimes, are common and indicate that faulting is active today, particularly in the central basin of the lake. A major erosional unconformity indicates that lake level was about 60 m lower than present lake level, probably just prior to 10,000 yr B.P., when unusually arid conditions prevailed throughout much of east Africa.

Seismic stratigraphy of Waterton Lake, a sediment-starved glaciated basin in the Rocky Mountains of Alberta, Canada and Montana, USA

Abstract Upper and Middle Waterton lakes fill a glacially scoured bedrock basin in a large (614 km2) watershed in the eastern Front Ranges of the Rocky Mountains of southern Alberta, Canada and northern Montana, U.S.A. The stratigraphic infill of the lake has been imaged with 123 km of single-channel FM sonar (`chirp) reflection profiles. Offshore sonar data are combined with more than 2.5 km of multi-channel, land-based seismic reflection profiles collected from a large fan-delta. Three seismic stratigraphic successions (SSS I to III) are identified in Waterton Lake resting on a prominent basal reflector (bedrock) that reaches a maximum depth of about 250 m below lake level. High-standing rock steps (reigels) divide the lake into sub-basins that can be mapped using lake floor reflection coefficients. A lowermost transparent to poorly stratified seismic succession (SSS I, up to 30 m thick) is present locally between bedrock highs and has high seismic velocities (1750–2100 m/s) typical of compact till or outwash. A second stratigraphic succession (SSS II, up to 50 m thick), occurs throughout the lake basin and is characterised by continuous, closely spaced reflectors typical of repetitively bedded and rhythmically laminated silts and clays most likely deposited by underflows from fan-deltas; paleo-depositional surfaces identify likely source areas during deglaciation. Intervals of acoustically transparent seismic facies, up to 5 m thick, are present within SSS II. At the northern end of Upper Waterton Lake, SSS II has a hummocky surface underlain by collapse structures and chaotic facies recording the melt of buried ice. Sediment collapse may have triggered downslope mass flows and may account for massive facies in SSS II. A thin Holocene succession (SSS III,

Fossil diatoms and the mid to late Holocene paleolimnology of Lake Turkana, Kenya: a reconnaissance study

A 12 m sediment core recovered from the south basin of Lake Turkana, northwestern Kenya, reveals four major diatom assemblages that span approximately 5450 to 1070 years BP based on AMS radiocarbon analyses. The oldest assemblage, Zone D (5450 to 4850 yr BP), is dominated by Melosira nyassensis and Stephanodiscus spp. and is interpreted to reflect higher lake levels, fresher water and more variable seasonal mixing of the water column than the modern lake. Melosira dominates the assemblage in Zone C (4850 to 3900 yr BP) with some Surirella engleri and Stephanodiscus. This assemblage indicates a continuation of relatively high lake levels and seasonal mixing of a stratified lake. The brief peak of Surirella, interpreted as benthic, suggests an episode of slightly lower lake level. Thalassiosira rudolfi and Surirella predominate since the beginning of Zone B (3900 to 1900 yr BP), reflecting a decrease in lake level and increase in water column salinity. Increasing dominance of Surirella in Zone A (1900 to 1070 yr BP) may suggest that the lake continued to decrease in depth. Salinity probably rose to levels comparable with the modern lake. These results are consistent with paleoclimatic interpretations based on carbonate abundance, lamination thickness, oxygen isotope and bulk geochemistry profiles from this core and cores recovered from the north basin. It extends the known paleolimnology beyond 4000 yr BP of the earlier research to 5450 yr BP and into the early to mid Holocene pluvial phase in northern intertropical east Africa.

Effects of bottom currents and fish on sedimentation in a deep-water, lacustrine environment

Sands and muds accumulating under the influence of apparently strong bottom currents in a deep-water environment of Lake Superior were studied in detail using 3.5-kHz echo sounding; side-scan SONAR; piston, gravity, and box coring; and lake-floor photography. The study area includes a well-defined slope and basin environment. Bottom currents maintain a scoured trough, 20 m deep and 2 km wide, at the base of the slope and modify the shapes of large ringlike depressions that are common in many regions of Lake Superior. Side-scan SONAR records reveal lineations subparallel to the direction of bottom-current flow that are interpreted in different areas to be sand ribbons, slight depressions, or depositional furrows. The sediments range from silty sand in the scoured trough to sandy clays farther offshore. Radiocarbon dating and 210 Pb dating show the sediments to be postglacial rather than relic glacial-lacustrine sediments. Grain size distributions are affected by bottom currents and a nearby source of sand. Some sedimentary structures, including plane laminations and interbedded sands and muds, are due to bottom currents. Fish create dish laminations, however, that often obscure the effects of the bottom currents. Fish and fish-scour depressions are common in lake-floor photographs, whereas evidence for bottom currents is not. Biological activity therefore appears to erase traces of intermittent currents.

Water Column Characteristics from Modern CTD Data, Lake Malawi, Africa

Conductivity, temperature, and dissolved oxygen profiles from Lake Malawi (Lake Nyasa), East Africa, provide a modern snapshot of water column characteristics during January, 1992. Temperature and dissolved oxygen concentration of the water column generally decrease with depth from over 28° C and over 80% saturation at the surface to just below 22.7° C and anoxic conditions deeper than 300 m. Specific conductance (κ25) of the water column typically increases with depth from 258 μS cm−1 at the surface to 270 μS cm−1 at 690 m with approximately 50% of the increase between 160 and 240 m. The increase in salinity complements a decrease in temperature across the chemocline to restrict vertical mixing during the rainy, warm season, and salinity may be the only barrier to vertical mixing across the chemocline during the windy, cool season. However, fluctuating bottom-water temperatures, complied over the past 50 years from the deep-basin of the lake, suggest that cooler surface or near surface water is occasionally introduced into the monimolimnion. Conductivity profiles offshore of the Ruhuhu River show a mid-depth (30 - 65 m) layer of relatively fresh water (minimum of 213 μS cm−1) that is interpreted as an interflow injection of river discharge. The discharge introduces surface or near surface waters to deeper depths in the lake.

Suspended Sediments in Lake Malawi, Africa: A Reconnaissance Study

Abstract In order to investigate the source and distributional pathways of suspended sediment in a tropical rift-valley lake, water-column profiles of light transmission (inversely related to water turbidity) were coupled with filtered water samples to determine total suspended sediment (TSS) concentrations at 28 stations throughout Lake Malawi (Lake Nyasa), East Africa, during January, 1992. TSS concentrations of the water column are typically between 0.1 and 0.5xa0mg/L, lower than those reported for the lower Laurentian Great Lakes. Elevated levels of turbidity are detected both at stations near the shoreline and offshore at four depths: at or near the surface, at the lake floor, at the chemocline, and in a mid-depth layer that is unique to the stations offshore of the Ruhuhu Delta. The latter had the most turbid water detected on this cruise and is interpreted as an interflow injection of fluvial material related to a 2-day rainstorm. Estimates suggest that this event contributed a significant portion of the annual sediment load to the delta. Other important sources of suspended sediment in Lake Malawi probably include biological productivity at the surface, chemical or biological processes at the chemocline, and resuspension events produced by wind-induced waves and currents and by seiche activity.

Acoustic architecture of glaciolacustrine sediments deformed during zonal stagnation of the Laurentide Ice Sheet; Mazinaw Lake, Ontario, Canada

Abstract In North America, the last (Laurentide) Ice Sheet retreated from much of the Canadian Shield by ‘zonal stagnation’. Masses of dead ice, severed from the main ice sheet by emerging bedrock highs, downwasted in situ within valleys and lake basins and were commonly buried by sediment. Consequently, the flat sediment floors of many valleys and lakes are now pitted by steep-sided, enclosed depressions (kettle basins) that record the melt of stagnant ice blocks and collapse of sediment. At Mazinaw Lake in eastern Ontario, Canada, high-resolution seismic reflection, magnetic and bathymetric surveys, integrated with onland outcrop and hammer seismic investigations, were conducted to identify the types of structural disturbance associated with the formation of kettle basins in glaciolacustrine sediments. Basins formed as a result of ice blocks being trapped within a regionally extensive proglacial lake (Glacial Lake Iroquois ∼12,500 to 11,400 years BP) that flooded eastern Ontario during deglaciation. Kettles occur within a thick (>30 m) succession of parallel, high-frequency acoustic facies consisting of rhythmically laminated (varved?) Iroquois silty-clays. Iroquois strata underlying and surrounding kettle basins show large-scale normal faults, fractures, rotational failures and incoherent chaotically bedded sediment formed by slumping and collapse. Mazinaw Lake lies along part of the Ottawa Graben and while neotectonic earthquake activity cannot be entirely dismissed, deformation is most likely to have occurred as a result of the rapid melt of buried ice blocks. Seismic data do not fully penetrate the entire basin sediment fill but the structure and topography of bedrock can be inferred from magnetometer data. The location and shape of buried ice masses was closely controlled by the graben-like form of the underlying bedrock surface.

Authigenic low-Mg calcite in Lake Turkana, Kenya

Abstract Lake Turkana is an important modern analog to ancient rift environments in east Africa and elsewhere. Carbonate is second in abundance after the detrital silicate fraction of the sediments in the lake. It consists of ostracod tests and a silt-sized micrite. Scanning electron microscopy and X-ray diffraction analysis revealed the micrite to be euhedral crystals of low-Mg calcite about 10 microns in length. Carbon isotope values of the micrite indicate that it forms in the upper water column rather than in the sediments after burial. The oxygen isotope analyses indicate that the micrite could be at isotopic equilibrium with the modern lake waters at temperatures of 36°C. This suggests that the micrite forms either in the surface waters of the open lake or in isolated shallow bays, and is then transported to the deep lake.