Quaternary Climate Change Research Articles

Recent, climate-driven river incision rate fluctuations in the Mercantour crystalline massif, southern French Alps

We present a new geomorphological analysis of the Tinée River tributaries in the southern French Alps based on numerical inverse and forward modelling of their longitudinal profiles. We model their relative uplift history with respect to the main channel, hence the incision rate history of this channel. Inverse models show that all tributaries have consistent incision rate histories with alternating high and low values. A comparison with global temperature curves shows that these variations correlate with quaternary climate changes. We suggest that during warm periods, a wave of regressive erosion propagates in the Tinée River, while its tributaries deeply incise their substratum to catch up with the falling base-level. We also show that the post 140 ka history of this landscape evolution is dominated by fluvial incision. We then perform forward models of river incision and simulate the incision of the Tinée River system over a time span of 600 ka. This model allows us to extract time and space incision rate variations of the Tinée River. With a background of a few mm.yr−1, incision rate can increase up to more than 1 cm yr−1 during short periods of time due to climatic oscillations. This result is compatible with published cosmogenic nuclide based dating, which evidenced incision rates from 0.2 to 24 mm yr−1. The part of the channel located between 12 and 20 km downstream from the source has undergone several periods of rapid incision rates, which could explain the steep hillslopes and the triggering of a landslide ∼10 ka ago.

Alternative glacial-interglacial refugia demographic hypotheses tested on Cephalocereus columna-trajani (Cactaceae) in the intertropical Mexican drylands.

Historic demography changes of plant species adapted to New World arid environments could be consistent with either the Glacial Refugium Hypothesis (GRH), which posits that populations contracted to refuges during the cold-dry glacial and expanded in warm-humid interglacial periods, or with the Interglacial Refugium Hypothesis (IRH), which suggests that populations contracted during interglacials and expanded in glacial times. These contrasting hypotheses are developed in the present study for the giant columnar cactus Cephalocereus columna-trajani in the intertropical Mexican drylands where the effects of Late Quaternary climatic changes on phylogeography of cacti remain largely unknown. In order to determine if the historic demography and phylogeographic structure of the species are consistent with either hypothesis, sequences of the chloroplast regions psbA-trnH and trnT-trnL from 110 individuals from 10 populations comprising the full distribution range of this species were analysed. Standard estimators of genetic diversity and structure were calculated. The historic demography was analysed using a Bayesian approach and the palaeodistribution was derived from ecological niche modelling to determine if, in the arid environments of south-central Mexico, glacial-interglacial cycles drove the genetic divergence and diversification of this species. Results reveal low but statistically significant population differentiation (FST = 0.124, P < 0.001), although very clear geographic clusters are not formed. Genetic diversity, haplotype network and Approximate Bayesian Computation (ABC) demographic analyses suggest a population expansion estimated to have taken place in the Last Interglacial (123.04 kya, 95% CI 115.3–130.03). The species palaeodistribution is consistent with the ABC analyses and indicates that the potential area of palaedistribution and climatic suitability were larger during the Last Interglacial and Holocene than in the Last Glacial Maximum. Overall, these results suggest that C. columna-trajani experienced an expansion following the warm conditions of interglacials, in accordance with the GRH.

Global variation in woodpecker species richness shaped by tree availability

AbstractAimSpecies richness patterns are generally thought to be determined by abiotic variables at broad spatial scales, with biotic factors being only important at fine spatial scales. However, many organism groups depend intimately on other organisms, raising questions about this generalization. As an example, woodpeckers (Picidae) are closely associated with trees and woody habitats because of multiple morphological and ecological specializations. In this study, we test whether this strong biotic association causes woodpecker diversity to be closely linked to tree availability at a global scale.LocationGlobal.MethodsWe used spatial and non‐spatial regressions to test for relationships between broad‐scale woodpecker species richness and predictor variables describing current and deep‐time availability of trees, current climate, Quaternary climate change, human impact, topographical heterogeneity and biogeographical region. We further used structural equation models to test for direct and indirect effects of predictor variables.ResultsThere was a strong positive relationship between woodpecker species richness and current tree cover and annual precipitation, respectively. Precipitation also showed a strong indirect effect on woodpecker richness via the effects on tree availability. Deep‐time tree availability, Quaternary climate change, human influence and other abiotic factors showed weaker direct effects. Human influence had a negative effect on tree availability, and hence a negative indirect effect on woodpecker species richness.Main conclusionsGlobal species richness of woodpeckers is primarily shaped by current tree cover and precipitation, reflecting a strong biotic association between woodpeckers and trees. Human influence can have a negative effect on woodpecker diversity when humans reduce tree availability. Hence, woodpeckers exemplify how broad‐scale diversity patterns are predominantly shaped by a biotic factor, and how climate and human influence can have indirect effects on animal biodiversity via the effects on tree availability and forest cover.

An endemic rat species complex is evidence of moderate environmental changes in the terrestrial biodiversity centre of China through the late Quaternary

The underlying mechanisms that allow the Hengduan Mountains (HDM), the terrestrial biodiversity centre of China, to harbour high levels of species diversity remain poorly understood. Here, we sought to explore the biogeographic history of the endemic rat, Niviventer andersoni species complex (NASC), and to understand the long-term persistence of high species diversity in this region. In contrast to previous studies that have proposed regional refuges in eastern or southern of the HDM and emphasized the influence of climatic oscillations on local vertebrates, we found that HDM as a whole acted as refuge for the NASC and that the historical range shifts of NASC mainly occurred in the marginal regions. Demographic analyses revealed slight recent population decline in Yunnan and south-eastern Tibet, whereas of the populations in Sichuan and of the entire NASC were stable. This pattern differs greatly from classic paradigms of temperate or alpine and holarctic species. Interestingly, the mean elevation, area and climate of potential habitats of clade a (N. excelsior), an alpine inhabitant, showed larger variations than did those of clade b (N. andersoni), a middle-high altitude inhabitant. These species represent the evolutionary history of montane small mammals in regions that were less affected by the Quaternary climatic changes.

The demographic response of a deciduous shrub (the Indigofera bungeana complex, Fabaceae) to the Pleistocene climate changes in East Asia

East Asia harbors the highest level of floristic diversity among the world’s temperate regions. Despite the increase in phylogeographic studies of temperate plants in East Asia, far less attention has been paid to widely distributed deciduous shrubs that widespread across several floral regions. We sequenced two chloroplast DNA (cpDNA) fragments (ndhJ-trnF and trnD-trnT) and one nuclear DNA (Pgk1) of 472 individuals from 51 populations of such a group, the Indigofera bungeana complex. We used population genetic data as well as ecological niche modelling to examine the evolutionary history and glacial refugia during the Last Glacial Maximum (LGM) of this group. We recovered 133 cpDNA and 68 nuclear haplotypes. The star-phylogeny of the recovered cpDNA and nuclear haplotypes and demographic analyses suggested distinct range expansion of I. bungeana complex have occurred during the early and middle Pleistocene. The climate change of the LGM might have affected little on the distribution of this complex based on the niche modelling. However, these climate changes and geographic isolation probably resulted in fixtures of the private haplotypes and genetic differentiations between regions. Our results suggested that this arid-tolerant species complex may have different responses to the Quaternary climate changes with those climate-sensitive species.

Chasing ghosts: allopolyploid origin of Oxyria sinensis (Polygonaceae) from its only diploid congener and an unknown ancestor.

Reconstructing the origin of a polyploid species is particularly challenging when an ancestor has become extinct. Under such circumstances, the extinct donor of a genome found in the polyploid may be treated as a 'ghost' species in that its prior existence is recognized through the presence of its genome in the polyploid. In this study, we aimed to determine the polyploid origin of Oxyria sinensis (2n=40) for which only one congeneric species is known, that is diploid O.digyna (2n=14). Genomic insitu hybridization (GISH), transcriptome, phylogenetic and demographic analyses, and ecological niche modelling were conducted for this purpose. GISH revealed that O.sinensis comprised 14 chromosomes from O.digyna and 26 chromosomes from an unknown ancestor. Transcriptome analysis indicated that following divergence from O.digyna, involving genome duplication around 12 million years ago (Ma), a second genome duplication occurred approximately 6Ma to give rise to O.sinensis. Oxyria sinensis was shown to contain homologous gene sequences divergent from those present in O.digyna in addition to a set that clustered with those in O.digyna. Coalescent simulations indicated that O.sinensis expanded its distribution approximately 6-7Ma, possibly following the second polyploidization event, whereas O.digyna expanded its range much later. It was also indicated that the distributions of both species contracted and re-expanded during the Pleistocene climatic oscillations. Ecological niche modelling similarly suggested that both species experienced changes in their distributional ranges in response to Quaternary climatic changes. The extinction of the unknown 'ghost' tetraploid species implicated in the origin of O.sinensis could have resulted from superior adaptation of O.sinensis to repeated climatic changes in the region where it now occurs.

The distribution, diversity, and conservation status of Cycas in China.

As ancient gymnosperm and woody plants, cycads have survived through dramatic tectonic activities, climate fluctuation, and environmental variations making them of great significance in studying the origin and evolution of flora biodiversity. However, they are among the most threatened plant groups in the world. The principal aim of this review is to outline the distribution, diversity, and conservation status of Cycas in China and provide suggestions for conservation practices. In this review, we describe the taxonomy, distribution, and conservation status of Cycas in China. By comparing Chinese Cycas species with its relatives worldwide, we then discuss the current genetic diversity, genetic differentiation of Cycas, and try to disentangle the potential effects of Quaternary climate changes and topographical events on Cycas. We review conservation practices from both researchers and practitioners for these rare and endangered species. High genetic diversity at the species level and strong genetic differentiation within Cycas have been observed. Most Cycas species in southwest China have experienced population retreats in contrast to the coastal Cycas's expansion during the Quaternary glaciation. Additionally, human activities and habitat fragmentation have pushed these endangered taxa to the brink of extinction. Although numerous efforts have been made to mitigate threats to Cycas survival, implementation and compliance monitoring in protection zones are currently inadequate. We outline six proposals to strengthen conservation measures for Cycas in China and anticipate that these measures will provide guidelines for further research on population genetics as well as conservation biology of not only cycads but also other endangered species worldwide.

Late Quaternary alluvial fan terraces: Langshan, Inner Mongolia, China

Alluvial fan terraces of the Langshan Range in the northwestern Hetao Basin of northern China records uplift on range-bounding faults and late Quaternary climate change. Alluvial fan profiles are segmented in the central part of the range front, which lack strath terraces that developed in the southwest (SW) and northeast (NE) parts. The stream channels in the central part have higher channel gradients and lower ratios of the valley floor width to valley height (V). Four episodes of alluvial fan terraces were upstream from the range-bounding fault, which formation ages were estimated as ~116, 64, 35, and 10ka BP according to optically stimulated luminescence (OSL) dating combined with previous 14C and OSL ages. The correlation between time of the terraces formation and climatic fluctuations recorded in a Guliya ice core reveals that alluvial fan aggradation was concentrated during the cold stages and that incision occurred during the succeeding cold-to-warm transitions. A majority of alluvial fan terraces developed in accelerated uplift since ~64ka, which indicates that tectonic uplift provided the driving force for continuous downcutting after climatic change triggered it. Geomorphic indices (channel gradient and V) distribution reconstructs the model of discrepant uplift in the Langshan Range with the fastest uplift rate in the central part, which is controlled by range-bounding faults.

Consistent role of Quaternary climate change in shaping current plant functional diversity patterns across European plant orders

Current and historical environmental conditions are known to determine jointly contemporary species distributions and richness patterns. However, whether historical dynamics in species distributions and richness translate to functional diversity patterns remains, for the most part, unknown. The geographic patterns of plant functional space size (richness) and packing (dispersion) for six widely distributed orders of European angiosperms were estimated using atlas distribution data and trait information. Then the relative importance of late-Quaternary glacial-interglacial climate change and contemporary environmental factors (climate, productivity, and topography) as determinants of functional diversity of evaluated orders was assesed. Functional diversity patterns of all evaluated orders exhibited prominent glacial-interglacial climate change imprints, complementing the influence of contemporary environmental conditions. The importance of Quaternary glacial-interglacial climate change factors was comparable to that of contemporary environmental factors across evaluated orders. Therefore, high long-term paleoclimate variability has imposed consistent supplementary constraints on functional diversity of multiple plant groups, a legacy that may permeate to ecosystem functioning and resilience. These findings suggest that strong near-future anthropogenic climate change may elicit long-term functional disequilibria in plant functional diversity.

Radiocarbon Dating of Leaf Waxes in the Loess-Paleosol Sequence Kurtak, Central Siberia

AbstractLoess-paleosol sequences (LPS) are valuable archives for Quaternary climate and environmental changes. So far, LPS are generally dated using luminescence, with ~10% uncertainties, or radiocarbon (14C) analyses in the rare cases that charcoal or macrofossils are available. For this study, we determined 14C ages of leaf waxes (long-chain n-alkanes) extracted from the LPS Kurtak in central Siberia. 14C ages range from 16.7 to 22.9 ka cal BP for the last glacial loess and from 24.5 to 35.3 ka cal BP for the paleosol correlated with marine isotope stage (MIS) 3. Overall, this is in good agreement with independent age control based on stratigraphy, infrared stimulated luminescence (IRSL) dating, and 14C dating on charcoal and macrofossils. However, strong cryoturbation and solifluction seem to have affected the MIS 3 paleosol early during MIS 2. Our results corroborate the stratigraphic integrity of leaf waxes, and highlight their potential for dating LPS back to ~35–40 ka BP. Compared to compound-specific 14C analyses, which are very time-consuming and require specialized instrumentation (gas chromatograph with fraction collector), 14C dating of leaf waxes as a whole compound class is relatively quick and straightforward and warrants further investigation as a chronological tool.

Mineralogical, geochemical, and magnetic signatures of surface sediments from the Canadian Beaufort Shelf and Amundsen Gulf (Canadian Arctic)

AbstractMineralogical, geochemical, magnetic, and siliciclastic grain‐size signatures of 34 surface sediment samples from the Mackenzie‐Beaufort Sea Slope and Amundsen Gulf were studied in order to better constrain the redox status, detrital particle provenance, and sediment dynamics in the western Canadian Arctic. Redox‐sensitive elements (Mn, Fe, V, Cr, Zn) indicate that modern sedimentary deposition within the Mackenzie‐Beaufort Sea Slope and Amundsen Gulf took place under oxic bottom‐water conditions, with more turbulent mixing conditions and thus a well‐oxygenated water column prevailing within the Amundsen Gulf. The analytical data obtained, combined with multivariate statistical (notably, principal component and fuzzy c‐means clustering analyses) and spatial analyses, allowed the division of the study area into four provinces with distinct sedimentary compositions: (1) the Mackenzie Trough‐Canadian Beaufort Shelf with high phyllosilicate‐Fe oxide‐magnetite and Al‐K‐Ti‐Fe‐Cr‐V‐Zn‐P contents; (2) Southwestern Banks Island, characterized by high dolomite‐K‐feldspar and Ca‐Mg‐LOI contents; (3) the Central Amundsen Gulf, a transitional zone typified by intermediate phyllosilicate‐magnetite‐K‐feldspar‐dolomite and Al‐K‐Ti‐Fe‐Mn‐V‐Zn‐Sr‐Ca‐Mg‐LOI contents; and (4) mud volcanoes on the Canadian Beaufort Shelf distinguished by poorly sorted coarse‐silt with high quartz‐plagioclase‐authigenic carbonate and Si‐Zr contents, as well as high magnetic susceptibility. Our results also confirm that the present‐day sedimentary dynamics on the Canadian Beaufort Shelf is mainly controlled by sediment supply from the Mackenzie River. Overall, these insights provide a basis for future studies using mineralogical, geochemical, and magnetic signatures of Canadian Arctic sediments in order to reconstruct past variations in sediment inputs and transport pathways related to late Quaternary climate and oceanographic changes.

Demographic stability and high historical connectivity explain the diversity of a savanna tree species in the Quaternary.

Cyclic glaciations were frequent throughout the Quaternary and this affected species distribution and population differentiation worldwide. The present study reconstructed the demographic history and dispersal routes of Eugenia dysenterica lineages and investigated the effects of Quaternary climate change on its spatial pattern of genetic diversity. A total of 333 individuals were sampled from 23 populations and analysed by sequencing four regions of the chloroplast DNA and the internal transcribed spacer of the nuclear DNA. The analyses were performed using a multi-model inference approach based on ecological niche modelling and statistical phylogeography. Coalescent simulation showed that population stability through time is the most likely scenario. The palaeodistribution dynamics predicted by the ecological niche models revealed that the species was potentially distributed across a large area, extending over Central-Western Brazil through the last glaciation. The lineages of E. dysenterica dispersed from Central Brazil towards populations at the northern, western and south-eastern regions. A historical refugium through time may have favoured lineage dispersal and the maintenance of genetic diversity. The results suggest that the central region of the Cerrado biome is probably the centre of distribution of E. dysenterica and that the spatial pattern of its genetic diversity may be the outcome of population stability throughout the Quaternary. The lower genetic diversity in populations in the south-eastern Cerrado biome is probably due to local climatic instability during the Quaternary.

One million years of glaciation and denudation history in west Greenland

The influence of major Quaternary climatic changes on growth and decay of the Greenland Ice Sheet, and associated erosional impact on the landscapes, is virtually unknown beyond the last deglaciation. Here we quantify exposure and denudation histories in west Greenland by applying a novel Markov-Chain Monte Carlo modelling approach to all available paired cosmogenic 10Be-26Al bedrock data from Greenland. We find that long-term denudation rates in west Greenland range from >50 m Myr−1 in low-lying areas to ∼2 m Myr−1 at high elevations, hereby quantifying systematic variations in denudation rate among different glacial landforms caused by variations in ice thickness across the landscape. We furthermore show that the present day ice-free areas only were ice covered ca. 45% of the past 1 million years, and even less at high-elevation sites, implying that the Greenland Ice Sheet for much of the time was of similar size or even smaller than today.

Quaternary climate change drives allo-peripatric speciation and refugial divergence in the Dysosma versipellis-pleiantha complex from different forest types in China

Subtropical China harbours the world’s most diverse temperate flora, but little is known about the roles of geographical and eco-climatic factors underlying the region’s exceptionally high levels of species diversity and endemism. Here we address this key question by investigating the spatio-temporal and ecological processes of divergence within the Dysosma versipellis-pleiantha species complex, endemic to subtropical China. Our cpDNA phylogeny showed that this monophyletic group of understory herbs is derived from a Late Pliocene ancestor of the Qinghai-Tibetan Plateau (QTP)/Southwest China. Genetic and ENM data in conjunction with niche differentiation analyses support that the early divergence of D. versipellis and D. pleiantha proceeded through allo-peripatric speciation, possibly triggered by Early Pleistocene climate change, while subsequent climate-induced cycles of range contractions/expansions enhanced the eco-geographical isolation of both taxa. Furthermore, modelling of population-genetic data indicated that major lineage divergences within D. versipellis likely resulted from long-term allopatric population isolation in multiple localized refugia over the last glacial/interglacial periods, and which in turn fostered endemic species formation (D. difformis, D. majoensis) from within D. versipellis in Southwest China. These findings point to an overriding role of Quaternary climate change in triggering essentially allopatric (incipient) speciation in this group of forest-restricted plant species in subtropical China.

Do 120,000 years of plant–pollinator interactions predict floral phenotype divergence in Calceolaria polyrhiza? A reconstruction using species distribution models

Quaternary climatic changes impacted species’ demography and distribution worldwide. Although response to climate change could have been modulated by mutualistic interactions with other species, studies exploring the dynamics of these interactions and their role facilitating species persistence during past climatic variations are scarce. In this work, we attempt to explore the spatial dynamic of Calceolaria polyrhiza and its oil-collecting bee pollinators during the last 120,000 years, identifying stable areas of persistence and statistically determining whether the distribution of pollinator-related floral ecotypes is associated with these shared areas of persistence. To do this, we used 395 presence records of the interacting species and constructed species palaeodistribution models. Additionally, we gathered phenotypic measures of the plant and used decision tree and multiple regression analyses to link the plant phenotypic divergence with the distribution of stable areas. Our species distribution models suggest that past climatic changes affected the interaction between C. polyrhiza and both bee species in time and space. While the interaction between the plant and C. caeruleus predominated in the Andean-Patagonian forest and was relatively stable in space and time, that was not the case for the pollinator C. cineraria in the Patagonian steppe. This, along with our analyses of spatial phenotypic divergence, indicates that current floral phenotypes are the result of two historical different pollination regimes.

Late Quaternary climatic control of Lake Baikal (Russia) turbidite systems: Implications for turbidite systems worldwide

Lake Baikal (Russia) contains a variety of turbidite systems in different tectonic and depositional settings that provide tests for the role of Quaternary climatic change on turbidite system growth. During Pleistocene glacial climates, all types of systems exhibit increased sediment supply (high sedimentation rates, high net sand percent, thick sand turbidites) and progradation. During Holocene interglacial climate, all systems exhibit reduced sediment supply and retreat. Seismic profiles from the large Selenga Fan and small Tompuda Fan show (1) maximum fan growth during the late Pleistocene glacial melt time, where lobes and large channels reached the distal outer fan, and (2) fan retreat during the transition to the fully developed Holocene interglacial climate. For example, the Selenga Fan surface lobes backstepped ∼55 km from the distal outer fan to the distal inner fan, and the large outer fan surface channel (∼750 m wide, ∼20 m levee relief) evolved to a smaller surface channel (∼450 m wide, ∼13 m levee relief) that extended only to the end of the inner fan. These results show that Quaternary climate controls the growth of the Lake Baikal turbidite systems in a setting where there are no significant water-level changes, which often are cited as the main control on turbidite system growth. The Lake Baikal and other marine turbidite systems suggest that climatic control of sediment supply, unrelated to sea-level lowering and tectonic effects, may have been a much more important control for turbidite systems than previously believed, not only during the Pleistocene, but perhaps also for ancient systems.

High gene flow in the American badger overrides habitat preferences and limits broadscale genetic structure.

Habitat associations are a function of habitat preferences and dispersal capabilities, both of which can influence how species responded to Quaternary climatic changes and contemporary habitat heterogeneity. Predicting resultant genetic structure is not always straightforward, especially in species where high dispersal potential and habitat preferences yield opposing predictions. The American badger has high dispersal capabilities that predict widespread panmixia, but avoids closed-canopy forests and clay soils, which could restrict gene flow and create ecologically based population genetic structure. We used mitochondrial sequence and microsatellite data sets to characterize how these opposing forces contribute to genetic structure in badgers at a continent-wide scale. Our data revealed an overall lack of ecologically based population genetic structure, suggesting that high dispersal capabilities were sufficiently realized to overcome most habitat-based genetic structure. At a broadscale, badger gene flow is limited only by geographic distance (isolation by distance) and large water barriers (Lake Michigan and the Mississippi River). The absence of genetic structure in a species with strong avoidance of unsuitable habitats advances our understanding of when and how genetic structure emerges in widespread, highly mobile species.

Phylogeography and historical demography of the orchid bee Euglossa iopoecila: signs of vicariant events associated to Quaternary climatic changes

The aim of this study was to investigate whether Pleistocene climatic instability influenced the phylogeographic structure and historical demography of an endemic Atlantic Forest (AF) orchid bee, Euglossa iopoecila Dressler, which shows two main patterns of integument colors over of its geographical distribution. We based our analysis on the concatenated sequence of four mtDNA segments belonging to genes 16S (357 bp), Cytb (651 bp) and COI (1206 bp), totaling 2234 bp. Samples of E. iopoecila populations were collected in 14 AF remnants along its geographic distribution. Median-Joining haplotype networks, SAMOVA and BAPS results indicated three lineages (southern, central and northern clusters) for E. iopoecila, with two important phylogeographic ruptures. We found higher genetic diversity among samples collected in the central region of the AF, which coincides with predicted areas of climatic stability, according to recent AF stability–extinction model. The demographic analysis suggests that only the southern cluster had undergone recent population expansion, which probably started after the last glacial maximum (LGM). Our data suggest that the differentiation observed in the three mitochondrial lineages of E. iopoecila is the result of past disconnections and multiple extinction/recolonization events involving climate fluctuations. In terms of conservation, we would emphasize the importance of considering: (1) the region of the central clade as the location of the highest genetic diversity of mtDNA of E. iopoecila populations; (2) the philopatric behavior of females that tends to restrict mtDNA gene flow in particular, with direct implications for the conservation of the total genetic diversity in euglossine populations.

Climatic and tectonic controls on strath terraces along the upper Weihe River in central China

The Weihe River in central China is the largest tributary of the Yellow River and contains a well-developed strath terrace system. A new chronology for the past 1.11 Ma for a spectacular flight of strath terraces along the upper Weihe River near Longxi is defined based on field investigations of loess–paleosol sequences and magnetostratigraphy. All the strath terraces are strikingly similar, having several meters of paleosols that have developed directly on top of fluvial deposits located on the terrace treads. This suggests that the abandonment of each strath terrace by river incision occurred during the transition from glacial to interglacial climates. The average fluvial incision rates during 1.11–0.71 Ma and since 0.13 Ma are 0.35 and 0.32 m/ka, respectively. These incision rates are considerably higher than the average incision rate of 0.16 m/km for the intervening period between 0.71 and 0.13 Ma. Over all our results suggest that cyclic Quaternary climate change has been the main driving factor for strath terrace formation with enhanced episodic uplift.

Interplay of Structural, Climatic, and Volcanic Controls On Late Quaternary Lacustrine–Deltaic Sedimentation Patterns In the Western Branch of the East African Rift System, Rukwa Rift Basin, Tanzania

Abstract This paper presents the first detailed, outcrop-based sedimentologic investigation of the Pleistocene–Holocene upper Lake Beds succession in the Rukwa Rift Basin, located in the Western Branch of the East African Rift System, southwestern Tanzania. The goal of this investigation is to examine the sedimentary facies and reconstruct the depositional environments of this important archive of Quaternary climate and environmental change. Eleven diagnostic facies associations comprising 24 facies were identified and provide the basis for recognition of three key deposystems: 1) alluvial-to-fluvial channel system; 2) lake delta system; and 3) profundal lacustrine system. Analysis of paleocurrent indicators and sandstone provenance indicate widely dispersed source regions and drainage patterns that were strongly influenced by major border-fault systems and episodic volcanism. Six stratigraphic sequences (A–F), ranging from ∼ 2 to 17 m thick, were identified based on stratal stacking patterns and the development of sequence-bounding unconformities and lacustrine flooding surfaces. Sedimentation processes, facies architecture, and stratigraphic packaging record a complex interplay between Quaternary climate fluctuations and intense episodic volcanism in the nearby Rungwe Volcanic Province, set against large-scale tectonic controls associated with synchronous development of the East African Rift System. Sequence stratigraphic analysis reveals that the Rukwa Rift Basin episodically shifted between a balanced-fill lake basin and an overfilled lake basin. Deep water, basin-wide lake expansion occurred at different times during the late Quaternary. The final depositional sequence preserved in the basin, a fluvial–underfilled lake basin, initiated ∼ 7.9 ka and has persisted to the present day. High-frequency climate change played the key role in sequence development in the upper Lake Beds. However, voluminous, rift-related volcanism and erosion of abundant labile volcanic materials from the Rungwe Volcanic Province, as well as syntectonic evolution of the rift, led to high sedimentation rates and transformation of flash floods and debris flows in the hinterlands (rift margin) to hyperpycnal flows towards the basin depocenter.