Biogenic silica deposition in Lake Malawi, East Africa over the past 150,000years

Johnson, Thomas C.; Brown, E. T.; Shi, Jim

doi:10.1016/j.palaeo.2010.01.024

Cited by 51 publications

(32 citation statements)

References 23 publications

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“…Two constraints are currently useful for defining this limit: (1) sediment cores in Northern Lake Malawi, 115 km South-South-East of Rungwe (Barker et al 2007; Barry et al 2002;Johnson et al , 2010Williams et al 1993; see also "Stratigraphy" section), contains RP ash, whereas (2) a core in Lake Rukwa, drilled 120 km North-West of Rungwe (Thevenon et al 2002), does not contain any tephra layers that could be correlated to RP (Fontijn et al 2010b). Furthermore, the isopach and isopleth maps (Fig.…”

Section: Total Grain Size Distributionmentioning

confidence: 99%

The ~4-ka Rungwe Pumice (South-Western Tanzania): a wind-still Plinian eruption

et al. 2011

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The ~4-ka trachytic Rungwe Pumice (RP) deposit from Rungwe Volcano in South-Western Tanzania is the first Plinian-style deposit from an African volcano to be closely documented focusing on its physical characterization. The RP is a mostly massive fall deposit with an inversely graded base. Empirical models suggest a maximum eruption column height HT of 30.5–35 km with an associated peak mass discharge rate of 2.8–4.8×108 kg/s.Analytical calculations result in HT values of 33±4 km(inversion of TEPHRA2 model on grain size data) corresponding to mass discharge ranging from 2.3 to 6.0×10^8 kg/s. Lake-core data allow extrapolation of the deposit thinning trend far beyond onland exposures. Empirical fitting of thickness data yields volume estimates between 3.2 and 5.8 km3 (corresponding to an erupted mass of 1.1–2.0×10^12 kg), whereas analytical derivation yields an erupted mass of 1.1×10^12 kg (inversion of TEPHRA2 model). Modelling and dispersal maps are consistent with nearly no-wind conditions during the eruption. The plume corner is estimated to have been ca. 11–12 km from the vent. After an opening phase with gradually increasing intensity, a high discharge rate was maintained throughout the eruption, without fountain collapse as is evidenced by a lack of pyroclastic density current deposits

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Section: Total Grain Size Distributionmentioning

confidence: 99%

The ~4-ka Rungwe Pumice (South-Western Tanzania): a wind-still Plinian eruption

et al. 2011

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“…XRF Core scanning elemental ratios such as Si/Ti have been used to estimate the amount of BSi in several studies (e.g. Johnson et al, 2011;Liu et al, 2013). However, in the case of our record, Si sources are multiple: basalts, potassic ash and biogenic.…”

Section: Proxy Interpretationmentioning

confidence: 98%

“…Often the presence of Ca is related to within "lake" formation/ deposition of organic CaCO 3 (e.g. Johnson et al, 2011;Liu et al, 2013). However, in a geological setting dominated of basaltic lava flows, carbonate deposition is unlikely.…”

Section: Proxy Interpretationmentioning

confidence: 99%

The Last Termination in the South Indian Ocean: A unique terrestrial record from Kerguelen Islands (49°S) situated within the Southern Hemisphere westerly belt

Putten

Verbruggen

Björck

et al. 2015

Quaternary Science Reviews

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International audienceThe awareness of the significance of the Southern Ocean in the Earth's climate system has become increasingly obvious. The deglacial atmospheric CO2 rise during warming periods in Antarctica has been attributed to CO2 ventilation from the deep ocean caused by enhanced upwelling around the Antarctic Divergence. It has been hypothesized that, more intense Southern Hemisphere westerly winds aligned with the Antarctic Circumpolar Current due to a southward shift of the wind belt from its Last Glacial Maximum equator-ward position, are the main drivers. Reconstructions of past changes in atmospheric circulation in the Southern Hemisphere are still scarce and the overall picture is patchy with sometimes contradictory results. For obvious reasons, most terrestrial records originate from southern South America and New Zealand. Here we present a terrestrial record from the Indian sector of the Southern Ocean, from Kerguelen Islands located at 49°S. A peat record is investigated using a multi-proxy approach (pollen and plant macrofossils, magnetic susceptibility, XRF analyses, biogenic silica content, Rock-Eval6 analysis and humification degree). Peat accumulation starts at about 16,000 cal yr BP with relatively warm and dry conditions. The most prominent change in our proxy data occurs at 13,600 cal yr BP, when peat ponds were established on the peat surface, resulting in lacustrine-type deposits, as a result of very high humidity, and with proxies implying very windy conditions. Within chronological uncertainties, this onset coincides with the onset of the so-called Oceanic Cold Reversal, based on the deuterium excess data in the EPICA Dome C ice core record. Kerguelen Islands are located in the moisture source area of Dome C and a change in atmospheric circulation at that time could explain both records. Around 12,900 cal yr BP, at the end of the Antarctic Cold Reversal, pond/lake sediments give way to more peaty deposits, with proxies suggesting slightly drier, less windy and probably warmer conditions. Kerguelen Islands became less influenced by the Southern Hemisphere westerly winds and these conditions were amplified during the early Holocene climate optimum as found in Antarctic ice core records

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“…In lacustrine sediment cores, BSi content can range from the detection limit (0.01 wt %) to > 70 wt % SiO 2 (Frings et al, 2014a). The downcore variations in BSi through time vary from as little as 2 wt % SiO 2 (Adams and Finkelstein, 2010;Ampel et al, 2008) to a high of 10-40 wt % SiO 2 (Johnson et al, 2011;Prokopenko et al, 2006;Van der Putten et al, 2015) and depends on several interacting factors such as mineral matter or organic matter accumulation, diatom productivity and preservation/dissolution processes. Hence, these processes control the relative effect that tephra constituents (< 3 wt % SiO 2 , Fig.…”

Section: Implications For Palaeoecological Studiesmentioning

confidence: 99%

“…Hence, the magnitude of BSi accumulation in soils is a key component in the biological buffering capacity of the Si cycle in an ecosystem. Palaeoecologists use BSi as a proxy for diatom productivity, and apply this to infer changes in, for example, nutrient availability (Conley et al, 1993;Heathcote et al, 2014), hydrology (Andresen et al, 2004), atmospheric circulation (Harper et al, 1986;Johnson et al, 2011;Verschuren et al, 2002) and temperature (Adams and Finkelstein, 2010;Prokopenko et al, 2006).…”

Section: Introductionmentioning

confidence: 99%

The contribution of tephra constituents during biogenic silica determination: implications for soil and palaeoecological studies

et al. 2015

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Abstract. Biogenic silica (BSi) is used as a proxy by soil scientists to identify biological effects on the Si cycle and by palaeoecologists to study environmental changes. Alkaline extractions are typically used to measure BSi in both terrestrial and aquatic environments. The dissolution properties of volcanic glass in tephra deposits and their nanocrystalline weathering products are hypothesized to overlap those of BSi; however, data to support this behaviour are lacking. The potential that Si-bearing fractions dissolve in alkaline media (Si Alk ) that do not necessarily correspond to BSi brings the applicability of BSi as a proxy into question. Here, analysis of 15 samples reported as tephra-containing allows us to reject the hypothesis that tephra constituents produce an identical dissolution signal to that of BSi during alkaline extraction. We found that dissolution of volcanic glass shards is incomplete during alkaline dissolution. Simultaneous measurement of Al and Si used here during alkaline dissolution provides an important parameter to enable us to separate glass shard dissolution from dissolution of BSi and other Si-bearing fractions. The contribution from volcanic glass shards (between 0.2 and 4 wt % SiO 2 ), the main constituent of distal tephra, during alkaline dissolution can be substantial depending on the total Si Alk . Hence, soils and lake sediments with low BSi concentrations are highly sensitive to the additional dissolution from tephra constituents and its weathering products. We advise evaluation of the potential for volcanic or other non-biogenic contributions for all types of studies using BSi as an environmental proxy.

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Biogenic silica deposition in Lake Malawi, East Africa over the past 150,000years

Cited by 51 publications

References 23 publications

The ~4-ka Rungwe Pumice (South-Western Tanzania): a wind-still Plinian eruption

The ~4-ka Rungwe Pumice (South-Western Tanzania): a wind-still Plinian eruption

The Last Termination in the South Indian Ocean: A unique terrestrial record from Kerguelen Islands (49°S) situated within the Southern Hemisphere westerly belt

The contribution of tephra constituents during biogenic silica determination: implications for soil and palaeoecological studies

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