O Grande Vale do Rifte da África Oriental (Rift Valey)
é uma das maravilhas geológicas do mundo, um lugar onde as forças
tectônicas da Terra estão atualmente tentando criar novas placas,
separando das antigas. Um rifte pode ser visto como uma fenda na
superfície da Terra que se alarga com o tempo, ou mais tecnicamente,
como uma bacia alongada, delimitada por falhas normais que se afastam
acentuadamente em direções opostas.
O processo é tão bem exibido na África Oriental
(Etiópia-Quênia-Uganda-Tanzânia) que os geólogos atribuíram um nome a
nova placa. A Placa de Núbia compõe a maior parte da África, enquanto a placa menor que está se afastando foi denominada Placa da Somália(Figura 1). Essas duas placas estão se afastando umas das outras e também da Placa Arábica ao norte.
Figura 1:
Modelo digital de elevação mostrando os limites das placas tectônicas. O
mapa base é uma imagem de topografia de radar do ônibus espacial da
NASA.
A
placa mais antiga e melhor definida ocorre na região de Afar, na Etiópia. O
ponto em que essas três placas se encontram, na mesma região, forma o que é
conhecido como Junção Tripla, bem
ilustrada na Figura 2, onde duas das placas visivelmente são ocupadas pelo Mar
Vermelho (Red Sea) e pelo Golfo de Aden (Gulf of Aden) e a terceira é direcionada
para sul.
O rifte da África Oriental não se limita ao “Chifre da África”. Mais a
sul da junção também há muita atividade, incluindo o ramo oeste,
passando pelo Quênia, Tanzânia e pelo Rifte do Lago Albert (Albertine Rift) que contem a região dos “Grandes Lagos”, da África Oriental (Figura 2).
Figura 2
: Nomes de segmentos de fenda para o Sistema de Riftes da África
Oriental. O mapa base é uma imagem de topografia de radar do ônibus
espacial da NASA.
Já parte do ramo leste foi denominado Rifte do Quênia ou Gregory Rift (depois do geólogo que o mapeou no início dos anos 1900). Essas duas divisões juntas formam os Riftes da África Oriental, ambos são frequentemente agrupados com o Vale do Rifte da Etiópia para formar o Sistema de Riftes da África Oriental.
O sistema completo se estende por milhares de km somente na África e
por mais mil se incluirmos o Mar Vermelho e o Golfo de Áden como
extensões. Além disso, existem várias outras estruturas bem definidas,
mas menores, chamadas Grabens, que têm caráter de rifte e estão
claramente associados geologicamente aos riftes principais. Alguns
receberam nomes que refletem isso, como o Rifte de Nyanza, no oeste do Quênia, perto do lago Victoria.
Assim, pode-se assumir um único rifte em algum lugar da África
Oriental com uma série de outros distintos, todos relacionados e
produzindo a geologia e topografia distintas da África Oriental.
Como se formaram?
A teoria geológica mais atual sustenta que protuberâncias são
iniciadas pelo fluxo de calor elevado do manto (estritamente a
astenosfera), conhecida por pluma, aquecendo a crosta sobreposta e
fazendo com que ela se expanda e se frature, em uma série de falhas
normais, formando a estrutura clássica Horst e Graben dos vales de rifte (Figura 3). Essas protuberâncias podem ser facilmente vistas como planaltos elevados em qualquer mapa topográfico da área (Figura 1).
Figura 3: Formação Horst e Graben
em comparação com o terreno de rift real (canto superior direito) e
topografia (canto inferior direito). Observe como a largura ocupada
pelas áreas trapezoidais submetidas à formação normal de falha e horst e
graben aumenta de cima para baixo no painel esquerdo. As fendas são
consideradas características exteriores (as placas continentais estão se
separando) e, com frequência, exibem esse tipo de estrutura.
O
processo de separação associado à formação de fendas é frequentemente precedido
por enormes erupções vulcânicas que fluem por grandes áreas e são geralmente
preservadas/expostas nos flancos das fendas. Essas erupções são consideradas
por alguns geólogos como “basaltos de inundação” – a lava é rompida
ao longo de fraturas (e não em vulcões individuais) e atravessa a terra em
lençóis como a água durante uma enchente.
Tais erupções podem cobrir grandes áreas de terra e desenvolver
espessuras enormes (as Armadilhas Deccan da Índia e as Armadilhas
Siberianas são exemplos).
Se a separação da crosta continuar, se formará
uma “zona esticada” de crosta diluída, consistindo de uma mistura de
rochas basálticas e continentais que eventualmente caem abaixo do nível
do mar, como aconteceu no Mar Vermelho e no Golfo de Aden. Separações
adicionais levam à formação de crosta oceânica e ao nascimento de uma
nova bacia oceânica.
O rifte da África Oriental
Se
o processo de rifteamento descrito ocorrer em um cenário continental, teremos
uma situação semelhante à que está ocorrendo agora no Quênia, onde o rifte da
África Oriental está se formando. Nesse caso, é chamado de “rifte
continental” (por razões óbvias) e fornece uma visão do que pode ter sido
o desenvolvimento inicial do Vale do
Rifte da Etiópia.
Embora
os ramos oriental e ocidental tenham sido desenvolvidos pelo mesmo processo,
eles têm características muito diferentes.
O Ramo Oriental é caracterizado por
maior atividade vulcânica, enquanto o Ramo Ocidental é caracterizado por bacias
muito mais profundas que contêm grandes lagos e muitos sedimentos (incluindo os
Lagos Tanganyika, o segundo lago mais profundo do mundo e o Malawi).
Recentemente,
erupções de basalto e formação de fendas ativas foram observadas no Vale do Rifte da Etiópia, o que nos
permite observar diretamente a formação inicial de bacias oceânicas em terra.
Esta é uma das razões pelas quais o Sistema
de Riftes da África Oriental é tão interessante para os cientistas.
A
maioria das fendas em outras partes do mundo progrediu a tal ponto que agora
estão debaixo d’água ou foram preenchidas com sedimentos e, portanto, são
difíceis de estudar diretamente.
O Sistema
de Riftes da África Oriental, no entanto, é um excelente laboratório de
campo para estudar um sistema de rifteamento moderno e em desenvolvimento
ativo.
Esta
região também é importante para entender as raízes da evolução humana. Muitos
achados fósseis de hominídeos ocorrem dentro da fenda, e atualmente se pensa
que sua evolução possa ter desempenhado um papel integral na formação de nosso
desenvolvimento.
A estrutura e a evolução do rifte podem ter tornado a África Oriental
mais sensível às mudanças climáticas, que levam a muitas alternâncias
entre períodos chuvosos e áridos. Essa pressão ambiental poderia ter
sido o impulso necessário para que nossos ancestrais se tornassem
bípedes e mais inteligentes à medida que tentavam se adaptar a esses
climas instáveis.
Os
últimos dois milhões de anos da variabilidade climática da África
oriental estão atualmente muito restritos, apesar do interesse em
compreender seu papel assumido na evolução humana inicial1,2,3,4. Raros
registros paleoclimáticos do nordeste da África sugerem condições
progressivamente mais secas2,5 ou um hidroclima estável6. Por outro
lado, registros do Lago Malawi no sudeste tropical da África revelam uma
tendência de um clima progressivamente mais úmido nos últimos 1,3
milhões de anos7,8.
As forças climáticas que controlavam essas mudanças
hidrológicas passadas também são objeto de debate. Alguns estudos
sugerem uma insolação local dominante forçando mudanças
hidrológicas9,10,11, enquanto outros inferem uma potencial influência
das mudanças de temperatura da superfície do mar no Oceano
Índico8,12,13.
Aqui mostramos que o hidroclima no sudeste da África
(20–25 ° S) é controlado pela interação entre o forçamento de insolação
de baixa latitude (precessão e excentricidade) e mudanças no volume de
gelo em altas latitudes.
Os nossos resultados baseiam-se numa
reconstrução múltipla de alterações hidrológicas na bacia hidrográfica
do rio Limpopo, combinada com uma reconstrução da temperatura da
superfície do mar no sudoeste do Oceano Índico nos últimos 2.14 milhões
de anos. Encontramos uma aridificação a longo prazo na bacia
hidrográfica do Limpopo, entre cerca de 1 e 0,6 milhões de anos atrás,
em oposição à evolução hidroclimática sugerida pelos registos do Lago
Malawi.
Nossos resultados, juntamente com a evidência de umedecimento no
Lago Malawi, implicam que a correia de chuva se contrai em direção ao
Equador em resposta ao aumento do volume de gelo em altas latitudes.
Ao
reduzir a extensão das florestas e zonas úmidas em ecossistemas
terrestres, as mudanças observadas no hidroclima do sudeste da África -
tanto em termos de estado de longo prazo quanto de marcada variabilidade
precessional - poderiam ter tido um papel na evolução dos primeiros
hominídeos, particularmente em a extinção do Paranthropus robustus.
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T.C.
is supported by CNRS-INSU. Funding from LEFE-IMAGO CNRS INSU project
SeaSalt is acknowledged. T.C. was partly supported by the ‘Laboratoire
d’Excellence’ LabexMER (ANR-10-LABX-19) and co-funded by a grant from
the French government under the program ‘Investissements d’Avenir’, and
by a grant from the Regional Council of Brittany (SAD programme). J.A.C.
acknowledges funding from the ERC project ‘STEEPClim’. E.S. and L.D.
acknowledge funding through the DFG Research Center/Cluster of
Excellence ‘The Ocean in the Earth System’ at MARUM – Center for
Environmental Sciences. A.S. acknowledges funding through the LaScArBx, a
programme supported by the Agence Nationale de la Recherche
(ANR-10-LABX-52). C.G.-C. was supported by CREST (grant number
JPMJCR12A3; P.I. SLS) funded by the Japan Science and Technology (JST).
Core MD96-2048 was collected during the MOZAPHARE cruise of the RV
Marion Dufresne, supported by the French agencies Ministère de
l’Education Nationale de la Recherche et de la Technologie, Centre
National de la Recherche Scientifique (CNRS) and Institut Paul Emile
Victor (IPEV).
Reviewer information
Nature thanks C. O’Brien, M. Petraglia, K. Uno
and the other anonymous reviewer(s) for their contribution to the peer
review of this work.
Author information
Affiliations
EPOC, UMR 5805, CNRS, University of Bordeaux, Pessac, France
Thibaut Caley
, Thomas Extier
, Bruno Malaizé
, Linda Rossignol
, Frédérique Eynaud
, Philippe Martinez
, Karine Charlier
, Mélanie Wary
, Pierre-Yves Gourves
, Isabelle Billy
& Jacques Giraudeau
Laboratoire
des Sciences du Climat et de l’Environnement, LSCE/IPSL, CEA-CNRS-UVSQ,
Université Paris-Saclay, Gif-sur-Yvette, France
Thomas Extier
& Didier M. Roche
GFZ
– German Research Center for Geosciences, Section 5.1 Geomorphology,
Organic Surface Geochemistry Laboratory, Potsdam, Germany
James A. Collins
Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
James A. Collins
MARUM – Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany
Enno Schefuß
& Lydie Dupont
PACEA, UMR 5199, CNRS, University of Bordeaux, Pessac, France
Antoine Souron
Department of Geography, Durham University, Durham, UK
Erin L. McClymont
Department of Biogeochemistry (JAMSTEC), Yokosuka, Japan
Francisco J. Jimenez-Espejo
Research and Development Center for Global Change, (JAMSTEC), Yokohama, Japan
Carmen García-Comas
Ecology Group, University of Vic – Central University of Catalonia, Barcelona, Spain
Carmen García-Comas
Vrije Universiteit Amsterdam, Faculty of Science, Cluster Earth and Climate, Amsterdam, The Netherlands
Didier M. Roche
Unité Géosciences Marines, Institut Français de Recherche pour l’Exploitation de la Mer (IFREMER), Plouzané, France
Stephan J. Jorry
Contributions
T.C.
designed the study. T.C., T.E., M.W. and P.-Y.G. performed the Mg/Ca
measurements. L.R. analysed the foraminifera assemblages, and T.C. and
F.E. analysed the results and performed the transfer function. T.C.,
T.E., B.M. and K.C. performed the δ18O analyses on
foraminifera. T.C., J.G., P.M. and I.B. performed the XRF measurements
and F.J.J.-E. and C.G.-C. conducted the statistical analyses on XRF.
J.A.C. and E.S. performed plant-wax δD and δ13C analyses.
L.D. performed the pollen analysis. A.S. and T.C. produced the synthesis
on the ecology and environments of South African hominins and conducted
the comparisons to the marine record. T.C. and D.M.R. performed and
analysed the iLOVECLIM model results. T.C. analysed the results and all
authors participated in the interpretation. T.C. wrote the manuscript
with contributions from all authors.
Calcium
and iron both have complex and multiple origins in marine sediments.
Iron can be related to redox variations, detrital and fluvial input,
among others, and calcium can be related to the biogenic fraction
(foraminifera or nannofossils) and detrital input. To properly interpret
the ln(Fe/Ca) ratio at our study location, we applied principal
components analysis120. a,
PC1 describes 66% of the total variance for the entire site MD96-2048.
The negative loadings for PC1 are calcium and strontium, and all other
elements (aluminium, silicon, potassium, titanium, iron and zirconium)
have positive loadings. Calcium and strontium are associated with
biogenic carbonate and are mainly related to presence of foraminifera.
Element matrix correlation shows a strong positive linear correlation (R > 0.70)
between iron and typically detrital elements, such as aluminium,
silicon, titanium and potassium. Calcium shows negative correlation with
iron (R = −0.5). b, ln(Fe/Ca) shows a strong correlation with PC1 (R = 0.94) and a strong relationship with Limpopo runoff proxies (Extended Data Fig. 3).
Iron and titanium elements are related to terrigenous and siliciclastic
components (heavy minerals and oxides) and the variation in carbonate
content (calcium) is mainly due to dilution by terrigenous sediment.
ln(Fe/Ca) is therefore a proxy of Limpopo runoff, consistent with
previous studies in riverine basins throughout the African continent10,121,122,123,124.
To confirm a weak influence of sea-level changes on the Fe/Ca record,
we compared our ln(Fe/Ca) record with a previous reconstruction of the
deep-water δ18O component for relative sea level125, (b,
bottom). Both records are plotted against the LR04 chronology. Visual
inspection and statistical testing do not support a dominant effect of
sea-level changes on the ln(Fe/Ca) record (R = 0.05). PC3, which
describes 11% of the total variance for the entire site MD96-2048, is
closely related to sea-level changes. The negative loadings for PC3 are
mainly strontium and, to a lesser degree, potassium and titanium, and
the main positive loadings are zirconium and, to a lesser degree,
silicon.
a, b, Seasonal δD composition of precipitation (a) and amount of precipitation at Pretoria station126 (b), in comparison to the results of the iLOVECLIM model at the corresponding latitude and longitude48,49. All data are centred on their annual average. Depleted δD values are indicative of increasing amounts of rainfall127. c,
Results of the transient simulation with the isotope-enabled numerical
climate model iLOVECLIM for the δD composition of precipitation and
precipitation in the Limpopo catchment (about −27.5° S to −22° S and
30° E to 36° E), for the past 150 kyr (Methods)51.
Black curves show the results after filtering with a low-pass filter.
The δD composition of precipitation and precipitation amount in the
Limpopo catchment are negatively correlated (R = −0.63, [Math Processing Error]
)
for the past 150 kyr. Maxima of precipitation are phased with maxima in
austral summer insolation at 30° S and lead to more-depleted δDprecipitation (amount effect).
a, Comparison between the ln(Fe/Ca) XRF signal and austral summer local insolation at 30° S31. b, Comparison between the ln(Fe/Ca) XRF signal and the brGDGT concentration in the sediment15. brGDGTs are commonly found in soil and can be attributed to Limpopo River runoff15. c, Comparison between the ln(Fe/Ca) XRF signal and the C31n-alkane δ13C record16. An increased amount of Limpopo River discharge is associated with more C4 plant input and an increase in austral summer insolation at 30° S. d, Comparison between inverted ln(Fe/Ca) XRF signal and the accumulation rate (AR) of CaCO3
as a measure of biogenic carbonate. The ln(Fe/Ca) XRF record is not
primarily controlled by dilution due to biological productivity (R = 0.1). A previous study of the past 0.8 Myr of core MD96-2048 interpreted shifts towards more-depleted δ13Cwax as potentially reflecting more-humid conditions16. However, the anti-correlation between δ13Cwax and δDwax values (Extended Data Fig. 4) in our study indicates that enriched δ13Cwax values are associated with more-humid conditions. Because C4 plants in the Limpopo catchment are dominant in the interior (Fig. 1), we propose that more-enriched δ13Cwax values indicate a higher relative contribution from sources located farther upstream (more C4 plants) during times of high runoff, compared to only downstream sources (more C3
plants) during times of low discharge. In addition, humid conditions
would have favoured the extension of sedge-rich vegetation (Cyperaceae,
of which 20–60% are C4 plants in this region128)
in riverine swamps and floodplains along the river course, explaining
the detected increase in Cyperaceae pollen at times of increased fluvial
discharge (Fig. 2).
Studies of sediments from the adjacent Zambezi catchment similarly
suggest the extension of swampy sedge-rich vegetation—including C4-Cyperaceae—when river discharge was high, and infer that more C4 plant waxes are exported to the ocean when the flooding of floodplains occurs during rainfall maxima10,129.
a, Correlation between the record of δ13C C31n-alkanes and the record of δD C31n-alkanes, with or without vegetation and ice-volume correction (vc-ivf) over the past 2.14 Myr (n = 19 samples). An anti-correlation exists between the δ13C and the δD signals of the C31n-alkanes. The C31n-alkane is used because it is the most abundant homologue in the samples. b, Raw δ13Cwax, δDwax data and δDwax adjusted for ice-volume and vegetation changes from core MD96-2048. Mean analytical uncertainties are indicated. Top, δ13Cwax of the C31 homologue (data from a previous study16 in light green, and data from this study in dark green). Middle, δDwax of the C31 homologue. Bottom, δDwax of the C31 homologue adjusted for ice-volume changes (ivf) using a seawater δ18O curve125
and converting to δD assuming an increase of 7.2‰ at the Last Glacial
Maximum. We use 7.2‰ because measurements of sediment pore water δ18O and δD suggest that the glacial ocean δD increase has a mean value of 7.2‰130. We also adjusted the δDwax record for vegetation changes (vc) using published fractionation factors (−123‰ ± 31‰ for C3 trees, −139‰ ± 27‰ for C4 grasses131) and the δ13Cwax signal following a previously published procedure132. End-member δ13Cwax values used for C3 and C4 vegetation were −36‰ and −21.5‰, respectively133. The error ranges for the vegetation fractionation factors are very large131.
They derive from the compilation of a global dataset from individual
plants, which is not comparable to an ecosystem fractionation in a
specific catchment (such in the Limpopo) that will fractionate with a
much smaller uncertainty. However, as we do not know the exact
fractionation factor in the Limpopo catchment and regard the
uncertainties from the global compilation as unrealistic for a specific
ecosystem we refrained from propagating this uncertainty into the
vegetation corrections. The vegetation and ice-volume-adjusted δDwax record is very similar to the unadjusted record, highlighting the fact that the adjustments have a minor effect.
a, Spectral power for ln(Fe/Ca) by wavelet analysis realized with a previously published MatLab package134.
The thick contour designates the 5% significance level against red
noise. Dashed black lines indicate the variability at the precession,
obliquity and eccentricity periods. b, Spectral analysis of ln(Fe/Ca) with REDFIT135.
The red line shows the false-alarm level at the 95% confidence
interval. Spectral peaks exceeding the false-alarm level can be
considered significant135. c,
Blackman–Tukey cross correlation between ln(Fe/Ca) XRF and
eccentricity–tilt–precession (ETP) realized with the Analyseries
software37
for the past 2.14 Myr. ETP is constructed by normalizing and stacking
eccentricity, tilt (obliquity) and negative precession to evaluate
coherence and phase (timing) relative to orbital extremes136.
The red curve shows the spectral power for ln(Fe/Ca) record. The black
curve shows the spectral power for ETP. The coherency, which varies
between 0 and 1, is represented by the grey curve and gives the interval
within which the spectrum is significant. In our case, the non-zero
coherency is higher than 0.55 and is significant at the 95% confidence
interval (grey line). There are significant spectral peaks for
eccentricity and precession but not for obliquity. The ln(Fe/Ca) XRF
record and ETP are in phase at the 400-kyr period, the eccentricity
leads by 16 kyr the ln(Fe/Ca) record at the 100-kyr period and the
ln(Fe/Ca) record is in anti-phase with negative precession (in-phase
with positive precession) at the 19- and 23-kyr periods. The three
statistical analyses are consistent and indicate significant variability
at the 400-, 100-, 23- and 19-kyr periods and insignificant variability
at the 41-kyr period. d, Comparison between the precessional component of the ln(Fe/Ca) record (Gaussian filter frequency 1/23,000; bandwidth: 5 ×10−6) obtained with the Analyseries software37
and the precession index. Maxima of the ln(Fe/Ca) precession component
are in phase with precession index maxima. The precession cycles in the
ln(Fe/Ca) record appear particularly strong between about 0.9 and
0.6 Ma. e–g, The same statistical analyses as in a–c, respectively, but for the PC1 SST record. In e,
dashed white lines indicate the variability at the precession,
obliquity and eccentricity periods. The three statistical analyses
indicate significant variability at the 100- and 41-kyr periods but not
significant power for the 400-kyr and 23-kyr (precession) periods.
a, Reconstruction of SST using two different methods: Mg/Ca reconstruction based on previous15 and new data (Mg/Ca ratios were converted into temperature values by applying a previously established equation40)
and foraminifera transfer function reconstruction using the modern
analogue technique. Error bars represent the error on the calibrations40 (Extended Data Fig. 7). b, Empirical orthogonal function analysis47
of the two SST records for the past 2.14 Myr. PC1 contains 74% of the
total variance for the past 2.14 Myr. Correlation between SST proxies
and PC1 for the past 2.14 Myr is R = 0.71.
a, Location of the modern database, composed of 367 core tops from the south Indian Ocean45 with present-day SST from the World Ocean Atlas (WOA) 200929. b, Test for the modern database45
yielding to a precision of 0.8 °C for the annual SST reconstructions.
Modern hydrological parameters were obtained from the WOA (1998)
database using a previously developed tool (http://www.geo.uni-bremen.de/geomod/Sonst/Staff/csn/woasample.html).
Extended Data Table 1 Fossil finds, their location and associated ages
Extended Data Table 2 δ13C
enamel of hominin and contemporaneous herbivores and associated
statistical parameters for different sites in the Limpopo catchment
