Gerhard Daut

Climate change on the Tibetan Plateau in response to shifting atmospheric circulation since the LGM

The Tibetan Plateau (TP) is primarily influenced by the northern hemispheric middle latitude Westerlies and the Indian summer monsoon (ISM). The extent, long-distance effects and potential long-term changes of these two atmospheric circulations are not yet fully understood. Here, we analyse modern airborne pollen in a transition zone of seasonally alternating dominance of the Westerlies and the ISM to develop a pollen discrimination index (PDI) that allows us to distinguish between the intensities of the two circulation systems. This index is applied to interpret a continuous lacustrine sedimentary record from Lake Nam Co covering the past 24 cal kyr BP to investigate long-term variations in the atmospheric circulation systems. Climatic variations on the central TP widely correspond to those of the North Atlantic (NA) realm, but are controlled through different mechanisms resulting from the changing climatic conditions since the Last Glacial Maximum (LGM). During the LGM, until 16.5 cal kyr BP, the TP was dominated by the Westerlies. After 16.5 cal kyr BP, the climatic conditions were mainly controlled by the ISM. From 11.6 to 9 cal kyr BP, the TP was exposed to enhanced solar radiation at the low latitudes, resulting in greater water availability.

Comparisons between the chemical compositions of lake water, inflowing river water, and lake sediment in Nam Co, central Tibetan Plateau, China and their controlling mechanisms

ABSTRACT The chemical composition of lake water and inflowing river water was investigated in Nam Co (lake), Tibetan Plateau, in September 2005. Lake water samples (n = 76) were collected at different depths along a south-north transect of the eastern part of the lake while water samples were collected from 69 rivers flowing into the lake; a sediment core was collected at 64 m midway along the water survey transect to investigate salt precipitation. Na+ and Ca2+ were the dominant cations, accounting for 76.2% and 60.6% of the lake and river water cations, respectively while was the dominant anion accounting for 70.8% and 93.4% of lake and river anions, respectively. CaCO3 precipitation from the water column decreased Ca2+ in the lake water, with the relative proportion of other ions increasing significantly. Evaporation-crystallization processes largely control Nam Co lake-water chemistry, while rock weathering is the dominant processes influencing the chemistry of river water; carbonate and silicate weathering are the major sources of ions in these rivers.

A 700‐year record on the effects of climate and human impact on the southern cape coast inferred from lake sediments of eilandvlei, wilderness embayment, south africa

Abstract The southern Cape coast, outh frica, is sensitive to climate fluctuations as it is influenced by different atmospheric and oceanic circulation systems. Palaeoecological evidence of Holocene climate variations in this region is presently limited. Here, we present a lake sediment record spanning approximately the last 670 years from ilandvlei, a brackish coastal lake situated mid‐way between Cape Town and Port lizabeth. The results from geochemical and sedimentological analyses point to an increase in minerogenic sediment input from the catchment starting around ad 1400. Changes in the seasonal distribution of rainfall during the Little Ice Age may have altered river discharge and increased erosion rates and fluvial sediment transport in pre‐colonial times. A rising mean lake level, possibly associated with an altered water balance or relative sea‐level rise, may offer an explanation for the deposition of finer sediments. After ad 1450, reduced burial flux of elements associated with autochthonous sediment formation may have resulted from ecological changes in ilandvlei. Enhanced sedimentation rates, increasing carbon, nitrogen, phosphorous and biogenic silica concentrations, as well as high concentrations of proxies for allochthonous sediment input (e.g. aluminium, titanium, zirconium) point to increasing sediment and nutrient flux into ilandvlei from the late nineteenth century onwards. The most likely factor involved in these recent changes is land‐use change and other forms of human impact.

Reconstruction of the Depositional History of the Former Coastal Lagoon of Vilamoura (Algarve, Portugal): A Sedimentological, Microfaunal and Geophysical Approach

Abstract The late Holocene evolution of the former coastal lagoon of Vilamoura was reconstructed according to sediment cores and geophysical profiles. According to sedimentological analyses of the cores, five palaeoenvironmental stages were defined. (1) The pretransgression stage is represented by an erosive surface formed during incision of the river into the basement because of a lower sea level. This palaeosurface was retraced by refraction seismic profiles, showing that the marine transgression took place on a wide plain with several incised channels. (2) The development of an estuary started by transgression into the river valley corresponding to the postglacial sea level rise. Radiocarbon dating indicates a sea level not lower than −4 m at a minimum age of 4716 ± 72 Cal BP. After the transgressive maximum, infilling of the estuary started, beginning with (3) subtidal infilling related to the formation of a sandy barrier followed by (4) supratidal infilling with further accretion of the barrier, changing the previous open bay into a coastal lagoon. (5) Finally, the lagoon was fluvially filled with terrestrial sediments, changing the marine to a fluvial milieu with floodplain deposition. Analysis of benthic foraminiferal and ostracod assemblages revealed additional information about the environmental conditions during evolution of the estuary, which led to a further subdivision of the marine facies into stages with mainly estuarine, lagoonal, or marine influence. The end of the marine stage was dated at 2895 ± 48 Cal BP, indicating a pre-Roman onset of human-induced soil erosion.

Sediment dynamics and hydrologic events affecting small lacustrine systems on the southern-central Tibetan Plateau – the example of TT Lake

A sedimentological, geochemical, micropaleontological, and palynological study of a lacustrine sediment record from the small TT Lake (southern-central Tibetan Plateau) shows that the background sedimentation was frequently interrupted by event-related deposits. These event-related deposits are interpreted as the result of hydrologic events that are triggered by above-average precipitation events. In total, 11 events were recorded in the TT Lake sequence. Two types can be differentiated: fluvial runoff events caused by precipitation that carried sediment in suspension into the lake and a sediment mass transport caused by torrential precipitation. The hydrologic events appear to be decoupled from long-term climate and environmental variations, but there is evidence that anthropogenic impact, in terms of pastoralism, might have favored the runoff events. The multi-proxy approach proved to be valuable and allowed for a detailed study of sedimentary processes within the lake and its watershed in order to assess their triggering processes and dynamics. The findings show the complexity of these sedimentary processes and their controlling factors, and the study aims to improve their understanding. This study is the first effort to investigate event-related deposits and sedimentary processes on the Tibetan Plateau and its triggering processes and dynamics by utilizing lacustrine sediment records.

High‐resolution paleomagnetic and sedimentological investigations on the Tibetan Plateau for the past 16 ka cal B.P.—The Tangra Yumco record

The spatial distribution of paleomagnetic secular variation (PSV) records on the Tibetan Plateau and adjacent areas covering the Holocene and Late Glacial is sparse. In order to reconstruct PSV in this area, a piston core covering the past 17.5 ka cal B.P. retrieved from Lake Tangra Yumco, southern‐central Tibetan Plateau, was analyzed. In the laminated sediments, several event layers are intercalated. Those were identified by sedimentological analysis and excluded for age‐depth modeling and interpretation. Paleomagnetic measurements on u‐channels reveal two contrasting core sections. The lower section (dated to 17.5–15.9 ka cal B.P.) is very coarse grained (up to 220 µm) and characterized by low intensities (0.8 mA m−1) and high maximum angular deviation values (mean 25°), making it unsuitable for PSV reconstruction. In contrast, the upper unit (dated to <15.9 ka cal B.P.) yields ideal properties with a well‐defined magnetization carried by low‐coercivity minerals in the pseudo single domain state making those younger sediments a proper record for PSV studies. The robustness of the PSV reconstruction for the past 3000 years is highlighted by a comparable inclination and declination pattern of three short sediment cores (2 m) from Tangra Yumco. On a regional scale, the obtained inclination signal for the past 15.9 ka cal B.P. is in good agreement with the Lake Baikal record (3000 km to the North), PSV stack for East Asia, as well as with predictions of geomagnetic field models. This study is a step forward in constructing a PSV reference curve for central Asia. -- Keywords : paleomagnetic secular variations (PSV) ; magnetostratigraphy ; Tibetan Plateau ; lake sediments ; Tangra Yumco ; inclination.

Towards an integration of process measurements, archive analysis and modelling in geomorphology — the Kärkevagge experimental site, Abisko area, northern Sweden

The analysis of Holocene geomorphic process activity demands long–term data sets, which are available for the Kärkevagge catchment due to 50 years of intensive geomorphologic field studies. This data set is used in combination with additional field measurements, remote sensing and digital elevation model (DEM) analysis to provide input data for modelling Holocene valley development. On the basis of this information, geomorphic process units (GPUs) are defined by means of GIS modelling. These units represent areas of homogeneous process composition that transfer sediments. Since the data base enables the quantification of single processes, the interaction of processes within the units can also be quantified. Applying this concept permits calculation of recent sediment transfer rates and hence leads to a better understanding of actual geomorphic landscape development activity. To extrapolate these data in time and space the process–related sediments in the valley are analysed for depth and total volume, primarily using geophysical methods. In this fashion the validity of measured process rates is evaluated for the Holocene time scale. Results from this analysis are exemplified in a cross–profile showing some of the principal sediment units in the valley. For example, the measured modern rates on a slush torrent debris fan seem to represent the Holocene mean rate. This approach should also be suitable for revealing Holocene geomorphic landscape development in terms of climate change.

Holocene lake level fluctuations and environmental changes at Taro Co, southwestern Tibet, based on ostracod-inferred water depth reconstruction

Lake level change is an important indicator of changes in the hydrological status of lakes triggered by the influence of atmospheric circulation systems. Previous studies show that the interaction of the Indian monsoon and the Westerlies, and their influence on environmental changes in the Tibetan Plateau region, remain unclear. Here, we reconstruct the Holocene lake level history of a lake, Taro Co, in southwestern Tibet, based on the relationship between ostracod assemblages and water depth from 34 surface sediment samples spanning the depth range 0 to 125 m. Measurement and analysis of multi-proxy data from a 310 cm sediment core from Taro Co lead to the following conclusions: (1) From 10,200 to 9400 cal. a BP, the lake received large inflows of freshwater (mainly glacial melt water) resulting in a rapid rise in lake level. Following a cold and dry interval and low lake level from 9400 to 8800 cal. a BP, the lake maintained a high level between 8800 and 7500 cal. a BP. This was followed by a less warm and wet period until 6400 cal. a BP. From 6400 to 1200 cal. a BP, the lake probably overflowed after reaching a high level, and after 1200 cal. a BP, the lake level decreased and fluctuated around the modern depth of 60 m at the coring site. (2) The region experienced warm and wet conditions from 10,200 to 6400 cal. a BP, with a cold event at around 9000 cal. a BP, which probably resulted from the influence of the Indian monsoon. However, the inferred wet conditions between 6400 and 1200 cal. a BP are not observed in other records from the monsoon region; they are consistent with numerous records from the regions under the influence of the Westerlies, which probably indicate an interval of Westerlies-domination in the Taro Co region.

ENSO and monsoon variability during the past 1.5 kyr as reflected in sediments from Lake Kalimpaa, Central Sulawesi (Indonesia):

The climate of Sulawesi is driven by the monsoon system as well as the El Niño-Southern Oscillation (ENSO). Until now, mechanisms and long-term variations of these complex interacting climate processes have been poorly understood. This paper uses a sediment record from Lake Kalimpaa to investigate long-term rainfall trends of the past ~1500 years. Granulometric and geochemical data provide indications for an increasingly wetter climate (higher rainfall intensities and/or mean rainfall) on centennial to millennial time scales from approximately ad 560 to the 20th century. Highest rainfall intensities probably occurred at the end of the ‘Little Ice Age’ (LIA). The trend towards wetter conditions during this time could also be detected in other palaeoclimatic studies from the region. A plausible explanation for these observations is the southward displacement of the Intertropical Convergence Zone (ITCZ) associated with changes in monsoon dynamics. However, comparison of the results with other proxy and model data indicates that the long-term rainfall variability in Central Sulawesi is also determined by variations in ENSO. During the 20th century, the climate signal in the Kalimpaa record is superimposed by human impact. Moreover, the data suggest that two periods of disturbance occurred within the lake catchment between about ad 1090–1190 and ad 1450–1620. Comparison with fire frequency derived from macro-charcoal analysis indicates that these events were caused by forest fires which likely took place during periods of drought. Broadly simultaneous drought periods have been detected in records from East Java suggesting a regional drought occurrence affecting at least East Java and Sulawesi.

The Warming of Large Lakes on the Tibetan Plateau: Evidence From a Lake Model Simulation of Nam Co, China, During 1979–2012

Lakes are considered as indicators of climate change on the Tibetan Plateau (TP). In the present study, we use the General Lake Model to simulate water temperature changes in Nam Co, the second largest lake on the central TP, for the period 1979-2012. The calibration and validation results demonstrate that this model is well suited for thermal simulation of Nam Co. The simulation results indicate that Nam Co has responded to the recent warming climate. The average summer surface water temperature fluctuated yearly, but its trend is positive at a rate of 0.52 +/- 0.25 degrees C per decade. At the same time, the onset of summer stratification advanced by 4.20 +/- 2.02d per decade, and the duration increased at a rate of 6.00 +/- 3.54d per decade. To explore the roles of air temperature and longwave radiation in lake warming, three sensitivity experiments are conducted by removing long-term trends from time series of air temperature and longwave radiation in the forcing data. These experiments prove that both increased air temperature and downward longwave radiation are two driving factors responsible for the warming of Nam Co.