EU/Th AND 14C isotope dating of lake sediments from sacred lake and lake Nkunga, Kenya

In the tropical regions, lake and swamp sediment core chronologies have traditionally been established solely by radiocarbon dating. In several instances, however, the radiocarbon sampling resolution has been coarse, entailing extrapolations over time periods where there may have been considerable change in sedimentation rates related, for example, to significant, albeit abrupt, palaeoclimatic and palaeoenvironmental change. Moreover, some cores may age-wise exceed the radiocarbon dating limit of ca.40,000 yr, thus entailing tenuous extrapolations of radiocarbon dates obtained in the younger sections of the core in order to obtain a whole corechronology. In this paper, the chronology of lake sediment cores retrieved from Sacred Lake and Lake Nkunga on the north-eastern flank of Mount Kenya is established using a combination of highresolution radiocarbon dating and experimental U/Th dating to circumvent the drawbacks mentionedabove. The derived chronosequences, which show that these sediment records span almost the whole of the late Quaternary period, demonstrate the efficacy and synergism of these dating techniques

The Holocene isotopic record of aquatic cellulose from Lake Äntu Sinijärv, Estonia: Influence of changing climate and organic-matter sources

The well characterized oxygen-isotopic fractionation during cellulose biosynthesis has been utilised by numerous studies of stable isotopes in fine-grained aquatic cellulose. We measured the δ13Ccelluloseand δ18Ocellulosevalues of bulk cellulose and moss fragments from an ∼11.4ka-long core obtained from a shallow, productive, spring-fed, hardwater lake, Äntu Sinijärv, Estonia (59˚3.8′N; 26˚14.5′E; 94.6 m a.s.l.; maximum depth 7.3 m), in order to reconstruct regional Holocene climate and lake-basin evolution. Isotopically, the modern waterbody is a well-behaved, open, hydrological system with negligible evaporative effects. Cellulose-isotope records were compared with down-core measurements of loss-on-ignition (LOI), carbonate and mineral contents, total organic carbon (TOC), total nitrogen (TN), C/N ratio, δ13CTOC, biomarker indices (Palgand Paq), published palaeoecological data and a δ18Ocarbonaterecord from the same palaeolake. Green microalgae, freshwater macroalgae (Chara) and aquatic bryophytes were important sources of sedimentary cellulose during different phases in the environmental history of the lake. Although a strong palaeoclimatic imprint can be detected in the δ18Ocelluloserecord from Äntu Sinijärv, notably the Preboreal oscillation, the 8.2ka event and an unnamed cold oscillation ∼3.25ka BP, the isotopic signal of these events may have been amplified by increases in18O-depleted spring snowmelt. In contrast, δ13Ccellulosewas tightly coupled to the Holocene evolution of terrestrial ecosystems and soils by significant inputs of biogenic carbon from the catchment and sublacustrine springs. During the early Holocene, ∼11 – 9ka BP, the δ18Ocelluloseand δ18Ocarbonaterecords diverge markedly, which can be attributed to “no-analogue” seasonal, climatic, hydrological and isotopic conditions resulting from orbital forcing and residual ice-sheet impacts

What have we learnt from palaeoclimate simulations?

There has been a gradual evolution in the way that palaeoclimate modelling and palaeoenvironmental data are used together to understand how the Earth System works, from an initial and largely descriptive phase through explicit hypothesis testing to diagnosis of underlying mechanisms. Analyses of past climate states are now regarded as integral to the evaluation of climate models, and have become part of the toolkit used to assess the likely realism of future projections. Palaeoclimate assessment has demonstrated that changes in large-scale features of climate that are governed by the energy and water balance show consistent responses to changes in forcing in different climate states, and these consistent responses are reproduced by climate models. However, state-of-the-art models are still largely unable to reproduce observed changes in climate at a regional scale reliably. While palaeoclimate analyses of state-of-the-art climate models suggest an urgent need for model improvement, much work is also needed on extending and improving palaeoclimate reconstructions and quantifying and reducing both numerical and interpretative uncertainties

Pollen, biomarker and stable isotope evidence of late Quaternary environmental change at Lake McKenzie, southeast Queensland

Unravelling links between climate change and vegetation response during the Quaternary is important if the climate–environment interactions of modern systems are to be fully understood. Using a sediment core from Lake McKenzie, Fraser Island, we reconstruct changes in the lake ecosystem and surrounding vegetation over the last ca. 36.9 cal kyr. Evidence is drawn from multiple sources, including pollen, micro-charcoal, biomarker and stable isotope (C and N) analyses, and is used to gain a better understanding of the nature and timing of past ecological changes that have occurred at the site. The glacial period of the record, from ca. 36.9 to 18.3 cal kyr BP, is characterised by an increased abundance of plants of the aquatic and littoral zone, indicating lower lake water levels. High abundance of biomarkers and microfossils of the colonial green alga Botryococcus occurred at this time and included large variation in individual botryococcene d13C values. A slowing or ceasing of sediment accumulation occurred during the time period from ca. 18.3 to 14.0 cal kyr BP. By around 14.0 cal kyr BP fire activity in the area was reduced, as was abundance of littoral plants and terrestrial herbs, suggesting wetter conditions from that time. The Lake McKenzie pollen record conforms to existing records from Fraser Island by containing evidence of a period of reduced effective precipitation that commenced in the mid-Holocene

Late Holocene sedimentology and palaeoenvironment of Kiluli Swamp, Mount Kenya

Kiluli Swamp is an extensive valley swamp near the lower limit of the montane forest on the eastern slopes of Mount Kenya, East Africa. The swamp is fed by a small spring on the northeastern margin, and the water table lies a few centimetres below the surface. The swamp's sediments modify water chemistry: the Na-Mg-HCO3 water-type at the input changes to a Ca- Mg-HCO3 water-type in the central parts of the swamp. A short sediment core (2.12m) was retrieved from the central part of the swamp using a modified Livingstone piston corer. The sediments were mainly composed of silty organic mud, silty clay and coarse silt. Three radiocarbon dates were obtained. A suite of sedimentological analyses was carried out in order to reconstruct the palaeoenvironmental history of the area, and these included: mineral magnetic characteristics (susceptibility, IRM etc.); total organic carbon (TOC); total nitrogen (TN), and stable carbon isotopes. The sediment record stretches from about 4,000 yr BP (before present) to present. Indications are that the valley was initially dry and the catchment vegetation was characterised by dominant C4-type grassland. The initiation of true swamp conditions occurred at ca.470 yr BP immediately following a phase of deep ponding and high diatom productivity within the swamp between 600 and 470 yr BP. A high incidence of charcoal from 470 to 0 yr BP probably marks the period of persistent anthropogenic activities within the catchment. There is a change in vegetation type from a predominantly C4-type to predominantly C3-type at about 130 yr BP that is attributed to crop cultivation within the swamp rather than due to climate change, since the arid phase which marks this zone would have, under natural conditions, abetted the continued dominance of C4 plants which are more drought-resistant than C3 plants. The changes observed are broadly synchronous with other palaeoenvironmental records from Mount Kenya and the surrounding region. African Journal of Science and Technology Vol.4(2) 2003: 12-2

Holocene environmental change in the West African Sahel: sedimentological and mineral-magnetic analyses of lake sediments from Jikariya Lake, northeastern Nigeria

Previous studies have demonstrated long-term changes in effective moisture in sub-Saharan Africa. Here, we reconstruct Holocene environments using a similar to 7 m lake-sediment sequence recovered from the northeastern Nigerian Sahel and attempt to distinguish basin-specific changes from regional climatic variations. The sequence was analysed for sedimentological properties, mineral magnetism and pollen, and dated by (137)Cs, (210)Pb excess and (14)C. Extremely arid conditions of the terminal Pleistocene ended ca. 11 500 cal. BP (calendar years) when climate ameliorated and a lake developed until the occurrence of an arid event leading to lake desiccation at similar to 11 200 cal. BP. Following this, climate ameliorated and a water body re-emerged. Very wet conditions predominated 11 200-5600 cal. BP, followed by drought between 5600 and 5500 cal. BP and a return to moderate humidity from 5500 to 4000 cal. BP. After 4000 cal. BP, a marked deterioration occurred, culminating in lake desiccation at ca. 800 cal. BP. After this time the climate remained generally dry and the re-emerging lake was shallow. Comparison of these results with other well-dated sequences in the region demonstrates the importance of basin-specific influences on the palaeolimnological records in addition to regional climatic controls. Disentangling these different controls, as well as the reconstruction of Holocene climate, therefore requires a multiple-basin approach. Copyright (C) 2008 John Wiley & Sons, Ltd.Geosciences, MultidisciplinaryPaleontologySCI(E)6ARTICLE5449-4602