Thomas C. Johnson

Sedimentation in Lake Malawi (East Africa) during the past 10,000 years: a continuous paleoclimatic record from the southern tropics

A suite of piston cores recovered from Lake Malawi (9–14°S, 34–35°E), east Africa in 1986 has been analyzed for major and minor elements, organic C and N, calcium carbonate and diatoms. An internally consistent stratigraphy was constructed from calcium carbonate abundance and variations in the two most abundant diatom genera, Stephanodiscus and Melosira, with age control obtained primarily from 14C dating of the carbonate. Differences with time in Fe abundance in a transect of cores from different water depths have been interpreted to reflect changes in chemocline depth. The depth to the chemocline was on the order of 100 m shallower than present prior to 3500 yr B.P., indicating less seasonality. Carbonate production and preservation appears to be related to climatically induced changes in both salinity and chemical distributions in the water column. The carbonate, which precipitates from surface waters, is most abundant during the interval from about 10,000 to 6000 yr B.P. This micrite most likely represents periods of low lake level when salinity increased and carbonate precipitation was enhanced. Sedimentary evidence suggests that lake levels were 100–150 m lower than present during this period. This record is different from climatic trends in northern intertropical Africa, but appears to also be related to changes in insolation and monsoon circulation. This is the northernmost basin in Africa reported to exhibit a “southern hemisphere” response to the early Holocene northern hemisphere summer insolation maximum. The climatic hingeline north of Lake Malawi (∼ 9°S) implied by our results is significantly south of that indicated by general-circulation model simulations, however. The cores show evidence for periods of abrupt climate change during the interval of generally arid climate.

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.

Fluctuations in deep western North Atlantic circulation on the Blake Outer Ridge during the last deglaciation

Variations in the mean grain size of the detrital silt fraction from four piston cores along a depth transect on the Blake Outer Ridge were used to examine paleocirculation patterns and intensity between 2650 and 3818 m for the late Pleistocene and Holocene. Prior to 12 kyr, circulation was focused on the upper part of the ridge above 2700 m and may have corresponded to enhanced levels of intermediate water production in the North Atlantic. Deepwater circulation during deglaciation underwent a two-step decrease to minimum speeds from 15.5 to 13.5 kyr, followed by a three-step increase to maximum speeds at 10 kyr. Minima in circulation at 13.5 and 12 kyr can be directly associated with oxygen isotopic evidence for advection of reduced salinity surface water to the North Atlantic. Additional circulation minima at 15 and 10.5 kyr may also be the result of reduced salinity from high-latitude meltwater discharge. Evidence for a drop to moderate circulation intensity during the Holocene is supported by isolated geochemical records but could also be explained by decoupling of ocean ventilation rates from bottom circulation or a change in sediment provenance at the end of deglaciation.

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.

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.

Surface and Benthic Nepheloid Layers in the Western Arm of Lake Superior, 1983

Abstract Suspended sediment concentrations and thermal structure of the water column were measured in the western arm of Lake Superior on 19 occasions in the ice-free season (May to October) of 1983. A surface turbidity plume was well developed in the spring, subsided in early summer, and was briefly reinforced by a storm in early July. A benthic nepheloid layer developed and persisted throughout the period of thermal stratification. This layer was supplemented by sediment introduced by density flows and local resuspension. Although the benthic nepheloid layer was well developed and an easily mappable feature, its contribution to the annual sediment budget of the western arm was far less than that from spring runoff.

Surface sediment response to deepwater circulation on the Blake Outer Ridge, western North Atlantic: paleoceanographic implications

Abstract Two transects of box core, hydrographic and photographic stations were made along the Blake Outer Ridge on the southeastern U.S. continental rise to study how circulation patterns affect surface sediment properties. Circulation is strongest at 4200 m on the flanks of the ridge and is reflected in the suspended particulate distribution. A second turbidity maximum at 3600 m suggests that there may be a second axis of circulation at this depth. The intense circulation on the flanks of the ridge has resulted in winnowing of the sediment revealed by coarsening and flattening of the grain size distribution in the detrital silt fraction, and concentration of carbonate by the formation of a foraminiferal lag deposit. The mean grain size of the detrital silt fraction on the crest of the ridge decreases southeastwards with increasing distance from upstream sediment sources. Shifts in the position and strength of the Western Boundary Undercurrent affect the distance that relatively coarse silt is transported. Downcore changes in grain size can therefore be used as paleoceanographic proxy for changes in deepwater circulation.

Deltaic sedimentation in a rift valley lake: New seismic reflection data from Lake Malawi (Nyasa), East Africa

Digital seismic reflection data acquired over the subaqueous parts of five rift-lake deltas in Lake Malawi (Nyasa), Africa, reveal a striking variability in delta morphology. Gross variations appear to be controlled by tectonic setting. Border-fault margin and accommodationzone margin deltas have steeply sloping depositional surfaces, acoustic signatures dominated by chaotic seismic facies, and an overall aggradational character. Shoaling margin deltas that are opposite the main border faults and axial-margin deltas that parallel the rift axis have slopes generally

High resolution seismic profiles from Lake Malawi, Africa

Abstract Over 5000 km of high resolution seismic (HRS) profiles have been obtained from Lake Malawi. These can be classified as Type I, II or III, depending on whether the lake floor reflector is weak, intermediate or strong, respectively. Type I profiles typically have thick, acoustically transparent sediment and are most widespread in the offshore basins of southern and central Lake Malawi. Sediments recovered from regions of Type I profiles are diatom oozes and frequently laminated. Type II profiles usually show irregular lake floor morphology and non - parallel sub-bottom reflectors, also of intermediate strength. The sediments are diatomaceous silts and frequently are associated with debris flows or distal turbidites. Type III profiles show little sub-bottom stratigraphy because of the strength of the surface reflector. These are either sands and gravels associated with erosional surfaces in water depths less than about 100m, deltaic sands in shallow water or sandy turbidites in deep water. Type II and III profiles are most common in the northern half of the lake where rainfall is relatively high and where major border faults are located in close proximity to the lake.

Enhanced Atmospheric Circulation over North America During the Early Holocene: Evidence from Lake Superior

Profiles of grain sizes in five cores recovered from Lake Superior show that grain size increased with burial depth to the postglacial-glacial contact. The results reflect a substantial reduction in bottom-current velocity and the corresponding wind velocity from 9500 to 6500 years ago.