Paleo-environment and flooding of the Limpopo River-plain, Mozambique, between c. AD 1200–2000 (2023)

Paleochannel sedimentary sequences can provide abundant information on regional environmental changes. A typical paleochannel (paleo-oxbow lake type) section of the Yellow River was identified within the Zoige Basin on the NE Tibetan Plateau. A multi-index approach was used to accurately identify sediments of different genetic types, such as riverbed deposits of the Yellow River, paleo-oxbow lake deposits, and overbank flood deposits (OFD) in the section. Based on optically stimulated luminescence (OSL) and AMS ¹⁴C dates, we examined the environmental evolution recorded by the section. The results show that: (1) The section is a record of environmental change since 4.17 ± 0.49 ka. During 4.17 ± 0.49 to 3.24 ± 0.26 ka, the ancient Yellow River occupied the channel. At 3.24 ± 0.26 ka, the paleochannel experienced a neck cutoff, and the fluvial environment began to change into the oxbow lake environment. After 2.45 ± 0.11 ka, the paleo-oxbow lake gradually disappeared. Subalpine meadow soil has developed at this site since 1.31 ± 0.05 ka. (2) Paleoenvironmental proxies indicate that the Zoige Basin was warmer and wetter before ~3.00 ka, and became drier and colder after ~3.00 ka, which may be mainly related to the weakening of the East Asian summer monsoon (EASM) and the strengthening of the Westerlies. (3) Two episodes of extreme overbank flooding occurred at 2.96 ± 0.24 to 2.87 ± 0.27 ka and 1.84 ± 0.20 to 1.70 ± 0.16 ka, correlated with climate shift period from the mid-Holocene climatic optimum to the late Holocene and the Dark Age Cold Period (DACP), respectively. Due to the relatively cold and dry climate in these periods, glaciers generally advanced on the Tibetan Plateau, and the contribution of snow and ice meltwater weakened. Therefore, the strong rainfall caused by the abnormal atmospheric circulation may be the main cause of these extreme overbank flooding.

Transitioning lagoon-barrier systems and coastal transgressive dunes offer the valuable opportunity to correlate their formation and interactions with seasonal dry and wet climate conditions, stepped relative sea-level rise of ∼3.5 m, and sediment supply at various scales. This manuscript examines the Holocene sea level changes and recent climate conditions and controls on a 150 m-high coastal barrier system in Mozambique, southeastern Africa. The methodological approach was based on the use of 7 kyr B.P. relative sea-level curve; 37 years of wind records from 1979 to 2016; 52 years of rainfall records from 1960 to 2012; and 56 years of average recorded temperatures from 1960 to 2016. Local dunefield migrations were monitored and the sand transport rate was measured from 2016 to 2017. The combined effects of the relative sea-level rise and sediment supply indicate the formation of the lagoon-barrier system in southern Mozambique. While the recent dry and wet climate conditions suggest that they might be a controlling factor on the generation of transgressive dunefields that migrate landward. This migration is reflected on the sand transport rate of 1.4 kg m⁻¹ s⁻¹, which is controlled by winds from the SSW, the S and the SSE quadrants. The active parabolic dunes monitored, indicated a SE-NW migration rate of 22.5 m yr⁻¹, which rapidly buries lakes and lagoons systems. The formation of different geomorphological features on the transgressive paleodunes and modern dunes reflect their exposure to prevailing S, SSE, SE, E and N winds, the annual rainfall of 1600 mm yr⁻¹, and the absence of rain for 7 months yr⁻¹. The stepped Holocene sea-level changes combined with high sediment supply and persistent acting of seasonal dry and wet climatic conditions led on the sedimentation and definition of transgressive dunefields morphology in southern Mozambique coast.

A sediment sequence from a coastal, hydrologically isolated lake in southern Mozambique was analysed for leaf wax δD, n-alkane abundance (ACL) and bulk organic geochemistry (δ¹³C, TOC, %N), providing a record of past rainfall variability and savanna dynamics over the last 1500 years. The δD_wax rainfall reconstruction reveals a stable hydroclimate between 500–700 CE, while ACL and δ¹³C together with previous pollen data suggest savanna vegetation was characterized by a relatively dense woody cover. Highly variable hydroclimate conditions are inferred by δD_wax between 800–1350 CE, with repeated centennial scale intervals of extreme dry and wet conditions overlapping the Medieval Climate Anomaly (MCA; 950–1250 CE). Savanna tree cover stayed relatively intact over this phase. After ca 1250 CE, a progressive change towards drier conditions was initiated, leading up to maximum aridity during the AD 1700s, a period associated with the Little Ice age (LIA; 1500–1850 CE). Tree cover was now replaced by a more grass-dominated savanna. The clear antiphase rainfall patterns between Nhaucati and equatorial East African proxy records gives support to the notion that Indian Ocean sea surface temperature (SST) gradients act as modulator of southern African climate on a multi-decadal time scale, possibly forced by long-term El Niño Southern Oscillation (ENSO) variability. We suggest that strong ENSO variability and greater occurrence of La Niña events triggered the generally wet and unstable MCA in southern Africa. From around 1250 CE, a shift towards a predominance of El Niño induced drier conditions in south-east Africa during the LIA. Our study of vegetation and hydroclimate proxies in parallel suggests that savanna tree and shrub cover was relatively resilient to the abrupt shifts in hydroclimate over the MCA, but more sensitive to the long-term progressive drying over the LIA.

The Three Gorges Dam has significantly interrupted fluvial continuity and modified the mass transfer regime along river continuums. Flow regulation following regular dam operations drives dramatic hydrological regime shifts, which facilitates sediment dispersal in the water-level fluctuation zone over episodic inundation periods. How flow regulation modulates sediment redistribution, however, remains unclear. In this study, we depict absolute particle size composition of suspended sediment and sink sediment in the water-level fluctuation zone, and these are interpreted in the context of flow regulation controls on sediment sorting. Multiple sampling strategies were applied at different spatial and temporal scales, to overcome limitations of labour and cost input in a large-scale field study and to collect representative samples. The results revealed a longitudinal fining trend and seasonal variability in particle size composition for suspended sediment. Sink sediment collected from the water-level fluctuation zone during a single summer flood event displayed a similar longitudinal fining trend, reflecting preferential settling of coarser fractions in the backwater reaches where flow velocity declines sharply. Surface sediment demonstrated a laterally coarsening trend with increasing elevations along a slope profile. Flooding duration, frequency and timing represent key factors in determining the elevation-dependent variations in the magnitude of sedimentation and its source inputs. Relatively longer flooding duration and frequent intermediate summer floods with high suspended sediment flux are responsible for high sedimentation rates in the lower portions with distal upstream source inputs, while low sedimentation rates in the upper portions are principally associated with water impoundment and sediment produced from local bank erosion. Vertical particle size variability was observed along a sedimentary core profile, which most likely reflects seasonal differences in source supply with contrasting particle size characteristics. We conclude that absolute particle size differentiation explains flow regulation controls on sediment sorting in the water-level fluctuation zone of the Three Gorges Reservoir.

Understanding environmental hazards presented by river flooding has been enhanced by paleoflood analysis, which uses sedimentary records to document floods beyond historical records. Bottomland overbank deposits (e.g., natural levees, floodbasins, meander scars, low terraces) have the potential as continuous paleoflood archives of flood frequency and magnitude, but they have been under-utilized because of uncertainty about their ability to derive flood magnitude estimates. The purpose of this paper is to provide a case study that illuminates tremendous potential of bottomland overbank sediments as reliable proxies of both flood frequency and magnitude. Methods involve correlation of particle-size measurements of the coarse tail of overbank deposits (>0.25mm sand) from three separate sites with historical flood discharge records for the upper Little Tennessee River in the Blue Ridge Mountains of the southeastern United States. Results show that essentially all floods larger than a 20% probability event can be detected by the coarse tail of particle-size distributions, especially if the temporal resolution of sampling is annual or sub-annual. Coarser temporal resolution (1.0 to 2.5year sample intervals) provides an adequate record of large floods, but is unable to discriminate individual floods separated by only one to three years. Measurements of >0.25mm sand that are normalized against a smoothed trend line through the down-column data produce highly significant correlations (R² values of 0.50 to 0.60 with p-values of 0.004 to <0.001) between sand peak values and flood peak discharges, indicating that flood magnitude can be reliably estimated. In summary, bottomland overbank deposits can provide excellent continuous records of paleofloods when the following conditions are met: 1) Stable depositional sites should be chosen; 2) Analysis should concentrate on the coarse tails of particle-size distributions; 3) Sampling of sediment intervals should achieve annual or better resolution; 4) Time-series data of particle-size should be detrended to minimize variation from dynamic aspects of fluvial sedimentation that are not related to flood magnitude; and 5) Multiple sites should be chosen to allow for replication of findings.

Here we present a palaecological reconstruction covering the last 1700yr from Lydenburg fen, located in the north-eastern grassland biome, Mpumalange, South Africa. A 300cm peat sequence was analysed for biogenic (grass phytoliths, diatoms) and geochemical proxies (δ¹³C, δ¹⁵N, carbon/nitrogen content) to infer past grassland dynamics and hydro-climatic changes. The Lydenburg record reports a C₄ dominated grassland throughout the studied period, with more or less pronounced fluxes between C₄-Chloridoideae and C₄-Panicoideae grass subfamilies. The record reflects moderate to dry conditions from AD 400 to 1000; more mesic conditions until around AD 1250; followed by a significantly drier period between c. AD 1250 and c. AD 1350, when Chloridoideae grasses expand at the expense of Panicoideae grasses. During this phase, the δ¹³C-record reports more enriched values indicating higher influx of C₄ grasses. Furthermore, lithological evidence indicates highly erosive conditions, with significant gravel input from the surrounding hills. After AD 1350, proxy indications suggest a shift towards more mesic conditions. During this increasingly mesic but also unstable period, farming communities using specialized agricultural practices (e.g. the people in Bokoni) expanded their settlements into new regions (Delius et al., 2008). This expansion was also coupled to population growth, suggesting these communities applied techniques that enabled improved food production under environmentally challenging conditions. Over the last century, Lydenburg δ¹³C-values indicate increased input of C₃ taxa. The phytolith record shows that this increase is not coupled to an increase in Pooideae (C₃) grasses, suggesting that the C₃ input may be related to woody encroachment.

Global and Planetary Change, Volume 177, 2019, pp. 225-238

Organic Geochemistry, Volume 107, 2017, pp. 21-32

Organic molecular proxies provide a record of the hydrogen isotopic composition of ambient precipitation and paleoenvironmental temperature. In terrestrial settings, two classes of organic biomarkers are commonly utilized to reconstruct these past environmental conditions: straight-chained normal alkanes (n-alkanes) and branched glycerol dialkyl glycerol tetraethers (brGDGTs). We measured the δD_n-alkane and the ratio of cyclisation and methylation indexes of branched tetraethers (CBT/MBT) from surface soils along a ∼1000km transect along the southeast margin of the Tibetan Plateau to assess how these proxies record changes in climate parameters that are related to elevation, despite variations in ecosystem. Abundance-weighted δD_nC27-31 data record the δD of precipitation across the orogen and indicate minimal change in apparent fractionation (ε_{n-alkane/water}=−133‰) with elevation. n-Alkane δD mirrors the pattern of Rayleigh distillation of an air mass undergoing rainout as moisture moves from the Bay of Bengal into the SE margin of the Tibetan plateau. Soil tetraether temperatures provide a reliable measure of modern temperature with an MBT/CBT calculated temperature lapse rate (5.8°C/km) close to the moist adiabat. These data reflect “ground-level” conditions that are independent of the shape of the orogen or the source of moisture. In combination with a Rayleigh distillation model that incorporates temperature and precipitation isotope data, paired tetraether temperature and leaf wax isotope data produce a reliable record of changes in environmental conditions associated with modern elevation change across the largest orogen. However, some tetraether calibrations may impart systematic temperature bias, most notably in cold or dry climates. These calibration uncertainties can result in underestimates of paleoelevations by up to 1.5–2km. Paired isotopic and temperature paleoelevation reconstructions can minimize errors associated with an individual proxy method, however we recommend that application of tetraether temperature paleoelevation reconstruction take potential calibration biases into account in paleoelevation estimates.

Quaternary Science Reviews, Volume 103, 2014, pp. 1-18

Journal of African Earth Sciences, Volume 109, 2015, pp. 1-10

In this study, data from 41 wells were used to quantify the evolution of the sedimentary budget in the Southern Mozambique passive margin basin, with a high temporal resolution for the Cenozoic period. We found that the drainage areas, which supplied sediments to the Southern Mozambique Basin, were eroded in two episodes. The first, of Mid–Late Cretaceous in age, is concordant with both thermochronological datation and sedimentary fluxes estimated by other studies in the Namibian and South African and Northern Mozambique margins. This erosion episode ended when the African surface, as defined by Burke and Gunnel (2008), had become flat and low-lying over most of the South African Plateau by ∼65Ma. Carbonate sediment deposition became more important in the shallow waters of the Mozambique basin after that time. The second erosion episode began at ∼23Ma and is likely due to an uplift event of the North-eastern part of the South African Plateau. It seems that the Limpopo catchment and the whole area sourcing the studied basin have inherited their present relief from two epeirogenic uplift pulses of Late Cretaceous and Miocene ages.

Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 580, 2021, Article 110607

Kisima Ngeda (KN), a spring on the northern margin of saline Lake Eyasi, Tanzania, sustains an Acacia-Hyphaene palm woodland and Typha swamps, while the surrounding vegetation is semi-desert. To study the vegetation changes associated with this spring, which represents a plausible modern analog for the fossil springs documented in the nearby paleoanthropological and archaeological sites of Olduvai Gorge, we analyzed the pollen content of a 43cm-long sediment core that documents vegetation changes since the last ~1200years (from cal yrs. C.E. 841 to 2011). Our results show that (1) Hyphaene palms, which require meso-halophytic soil conditions were most abundant in the area of the coring site until cal yrs. C.E. ~1150 when the groundwater supplying the KN spring was likely lower than at present, allowing intrusions of saline lake waters. (2) From cal yrs. C.E. ~1200, a peat began to develop, the palm woodland was replaced by a Mimosaceae woodland, and the increased presence of Typha pollen indicates the presence of more wetlands. (3) From cal yrs. C.E. 1600, the groundwater level of the KN spring increased and reached its highest level in the last 1200years. (4) Peaks of wetland expansion, which reflect increased groundwater flow and level in response to amplified rainfall in the recharge area (Mt Oldeani, Ngorongoro Highlands), occurred at cal yrs. C.E. ~1200–1400 and ~1650–2011. These outflows of groundwater at Kisima Ngeda were linked to the intensity and frequency of positive Indian Ocean Dipole (IOD) events, which trigger heavy rains in eastern Africa. We conclude that the Kisima Ngeda hydrological system, which has been active for more than 1200years, responds rapidly to regional climate change driven by changes in the sea surface temperatures (SSTs) of the Indian Ocean. Yet, it is also capable of remaining active during dry intervals as inferred from the Kisima Ngeda record prior to cal yrs. C.E. 1200. Our results support the hypothesis that this type of system helped to maintain Plio-Pleistocene hominin populations and activities in the arid lowlands of the rift on a multi-decennial scale.

Global and Planetary Change, Volume 128, 2015, pp. 83-89

We have reconstructed decadally-resolved continuous sea surface temperature and seawater δ¹⁸O (hence salinity) records over the last 1300yr from alkenone and planktonic foraminiferal oxygen isotope ratio analyses of the East Sea/Japan Sea marine sediments to investigate East Asia monsoon variability. Comparisons of the records with other paleoclimate records indicate a possible connection between changes in the mid-latitude East Asia monsoon and Pacific Decadal Oscillation (PDO) over this period. The results show that during the Medieval Climate Anomaly (MCA) when the PDO index was negative, East Asia was characterized by surface warming with a strengthened summer monsoon. Summer monsoon-related precipitation increased and pluvials possibly dominated in the region at that time. Onset of Asia monsoon failure and severe drought occurred at the end of the MCA and extended to the Little Ice Age (LIA) when the PDO became positive.