East African weathering dynamics controlled by vegetation-climate feedbacks

Ivory, Sarah J.; McGlue, Michael M.; Ellis, Geoffrey S.; Boehlke, A.; Lézine, Anne Marie; Vincens, Annie; Cohen, Andrew S.

doi:10.1130/g38938.1

Cited by 17 publications

(9 citation statements)

References 25 publications

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“…Previous studies have shown that OC and SR accumulation are controlled by the complex interplay of primary productivity, dilution and preservation of OM, which are further influenced by a number of factors such as lake level fluctuations, changes in hinterland and basin slope angles and inorganic sediment and terrestrial OM input 3,6,7,[10][11][12][50][51][52][53][54] . Despite recent progress in understanding these processes (e.g., Bohacs et al 55 , Ellis et al 6 ), SR accumulation in rift-lake systems is particularly complex and therefore remains hard to generalize 3 .…”

Section: Discussionmentioning

confidence: 99%

“…Organic carbon (OC) accumulation and development of petroleum source rocks (SR) are complex processes involving a number of interrelated factors, the relative importance of which are not fully understood 1 , 2 . Furthermore, many of the factors can be scale dependent 3 what makes direct comparison with studies dealing with surface sediments, representing shorter time spans or different settings convoluted 4 – 7 . Lacustrine rift basins are especially challenging in this regard due to localized and evolving topography through the active rifting phase, which typically cause profound changes in factors controlling organic matter (OM) production and preservation 8 .…”

Section: Introductionmentioning

confidence: 99%

“…Significant insights into OC accumulation in lacustrine rift basin have been derived particularly from the modern East African lakes 6 , 7 , 9 – 11 . These studies have revealed the importance of factors such as width-depth ratio on the lake’s mixing potential, primary productivity and anoxia 10 .…”

Section: Introductionmentioning

confidence: 99%

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Factors controlling accumulation of organic carbon in a rift-lake, Oligocene Vietnam

Rizzi

Hovikoski

Schovsbo

et al. 2020

Sci Rep

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Understanding of the processes of petroleum source rock (SR) accumulation in lacustrine rift basins and the behavior of lake systems as long-term carbon sinks is fragmentary. Investigation of an 800 m thick (500 m core and ~ 300 m outcrop), deep-lacustrine, Oligocene section in Vietnam, provides a rare insight into the controls and deposition of organic carbon (OC) and SR formation in continental rift basins. A multidisciplinary dataset, combining elemental data, inorganic and organic geochemistry with sedimentology, shows that the richest alginite-prone, sapropelic SR developed during periods of relative tectonic quiescence characterized by moderate primary productivity in a mainly dysoxic lacustrine basin. Increased rift activity and further development of graben morphology intensified water column stratification and anoxia, which hindered nutrient recycling. Sapropelic organic matter (OM) continued to accumulate, but with increasing amorphous OM content and decreasing total OC values. Periods of increased seasonality were characterized by thermocline weakening, enhanced mixing of water columns, increased primary productivity and diatom blooming. The results suggest that a change from dysoxia towards anoxia or extreme primary productivity does not necessarily enhance OC burial and SR quality. External nutrient input from a phosphate-rich hinterland is sufficient for sapropel formation, whereas the main limiting factor is methanogenesis.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Factors controlling accumulation of organic carbon in a rift-lake, Oligocene Vietnam

Rizzi

Hovikoski

Schovsbo

et al. 2020

Sci Rep

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“…However, the evolution of Al/K ratios and other weathering proxies in sedimentary records is generally considered to reflect past fluctuations in alteration processes coupled with changing hydroclimate and vegetation patterns on continents (e.g. Schneider et al, 1997;Bayon et al, 2012;Ivory et al, 2017). At the Congo margin, the utility of Al/K for reconstructing past chemical weathering has been demonstrated using Hf isotopes, which represent another powerful indicator for silicate weathering intensity (Bayon et al, 2009(Bayon et al, , 2012.…”

Section: Past Weathering and Erosion Changes: Al/k Bit-index And Inimentioning

confidence: 99%

The roles of climate and human land-use in the late Holocene rainforest crisis of Central Africa

Bayon

Schefuß

Dupont

et al. 2019

Earth and Planetary Science Letters

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There is increasing evidence that abrupt vegetation shifts and large-scale erosive phases occurred in Central Africa during the third millennium before present. Debate exists as to whether these events were caused by climate change and/or intensifying human activities related to the Bantu expansion. In this study, we report on a multi-proxy investigation of a sediment core (KZR-23) recovered from the Congo submarine canyon. Our aim was to reconstruct climate, erosion and vegetation patterns in the Congo Basin for the last 10,000 yrs, with a particular emphasis on the late Holocene period. Samples of modern riverine suspended particulates were also analyzed to characterize sediment source geochemical signatures from across the Congo watershed. We find that a sudden increase of bulk sediment aluminium-to-potassium (Al/K) ratios and initial radiocarbon ages of bulk organic matter occurred after 2,200 yrs ago, coincident with a pollen-inferred vegetation change suggesting forest retreat and development of savannas. Although hydrogen isotope compositions of plant waxes (δD wax) do not reveal a substantial hydroclimate shift during this period, neodymium isotopes and rare earth elements in detrital fractions indicate provenance changes for the sediment exported from the Congo Basin at that time, hence suggesting a reorganization of spatial rainfall patterns across Central Africa during this event. Taken together, these findings provide evidence for changing landscapes in Central Africa from about 2,200 yrs ago, associated with synchronous events of vegetation changes and enhanced erosion of preaged and highly weathered soils. These events coincided remarkably well with the arrival of Iron Age communities into the rainforest, as inferred from comparison to regional archaeological syntheses. While the human impact on the environment remains difficult to quantify at the scale of the vast Congo Basin, we tentatively propose that strengthening of El Niño-Southern Oscillation (ENSO) variability at that time played a key role in triggering the observed environmental changes, and possibly acted as a driver for the eastward migration of Bantu-speaking peoples across Central Africa.

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“…The bulk geochemistry of sediments in the EARS reflects their provenance and weathering, as well as their depositional, chemical and diagenetic histories, which result from a complex interplay between geological and climatological controls (WoldeGabriel et al ., ; Owen et al ., ). Weathering, for example, leads to loss of labile elements (Nesbitt & Young, ), but the weathering intensity might reflect either climate (Ivory et al ., ) or relative uplift and subsidence rates, which control exposure time to weathering.…”

Section: Introductionmentioning

confidence: 99%

Spatial and temporal geochemical variability in lacustrine sedimentation in the East African Rift System: Evidence from the Kenya Rift and regional analyses

2018

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Many previous studies on lacustrine basins in the East African Rift System have directed their attention to climatic controls on contemporary sedimentation or climate change as part of palaeoenvironmental reconstruction. In contrast, this research focuses on the impact of tectonism and volcanism on rift deposition and develops models that help to explain their roles and relative importance. The study focuses on the spatial and temporal variability in bulk sediment geochemistry from a diverse range of modern and ancient rift sediments through an analysis of 519 samples and 50 major and trace elements. The basins examined variously include, or have contained, wetlands and/or shallow to deep, fresh to hypersaline lakes. Substantial spatial variability is documented for Holocene to modern deposits in lakes Turkana, Baringo, Bogoria, Magadi and Malawi. Mio‐Pleistocene sediments in the Central Kenya Rift and Quaternary deposits of the southern Kenya Rift illustrate temporal variability. Tectonic and volcanic controls on geochemical variability are explained in terms of: (i) primary controlling factors (faulting, subsidence, uplift, volcanism, magma evolution and antecedent lithologies and landscapes); (ii) secondary controls (bedrock types, rift shoulder and axis elevations, accommodation space, meteoric and hydrothermal fluids and mantle CO2); and (iii) response factors (catchment area size, orographic rains, rain shadows, vegetation densities, erosion and weathering rates, and spring/runoff ratios). The models developed have, in turn, important implications for palaeoenvironmental interpretation in other depositional basins.

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East African weathering dynamics controlled by vegetation-climate feedbacks

Cited by 17 publications

References 25 publications

Factors controlling accumulation of organic carbon in a rift-lake, Oligocene Vietnam

Factors controlling accumulation of organic carbon in a rift-lake, Oligocene Vietnam

The roles of climate and human land-use in the late Holocene rainforest crisis of Central Africa

Spatial and temporal geochemical variability in lacustrine sedimentation in the East African Rift System: Evidence from the Kenya Rift and regional analyses

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