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This paper presents the first result of a study of two long-term natural archives in the southwestern part of the Western Siberian Plain. Bchron age models based on 17 AMS dates allowed us to determine a chronology of changes in properties of lake and peat deposits. Significant changes in the Holocene were identified according to geochemical indices, accumulation rates, and compositions of the plant macrofossils of sediment cores in Lake Kyrtyma and Oshukovskoe peat bog. Sediments with a low content of organic matter and maximum weathering accumulated in the Late Glacial, below the border of the Holocene. The transition to the Holocene manifested as a black organic gyttja with ostracods and marked warming ∼12–11.2 ka BP. At about 11.2–10.1 ka BP it was replaced by the Preboreal oscillation. Subsequent changes in lake sedimentation were associated with general warming and an unstable hydrological regime. An increase in humidification ∼8.2 ka BP was reflected by lake level rise and contributed to swamping of the watersheds. The aridest Holocene phase at ∼7.1–5.5 ka BP was reliably recorded by geochemical data, but poorly reflected in the stratigraphy of both profiles. Subsequent cooling and the resulting decrease in evaporation began at ∼5.5–4.9 ka BP. A pronounced cardinal shift in sedimentation conditions due to a gradual increase in precipitation was recorded at ∼4.9–2.8 ka BP, though short-term fluctuations towards dryness were also marked at ∼3.5–3.3 ka BP. The most significant increase in humidification and cooling began at 2.8 ka BP. Geochemical indices in the lacustrine sediments are climate sensitive and allowing us to obtain enough reliable data associated with changes in sedimentation environments. The stratigraphy of peat reflected the nuances of climatic changes less clearly and primarily demonstrated the stages of bog self-development; however, sphagnum peat provided a better-resolution record. [ABSTRACT FROM AUTHOR]

The aim of this paper is to reconstruct an 11,000-year history of depositional environmental change in southeastern Brazil, based upon the integration of particulate organic matter and stable isotope (C and N) data from a 136-cm sediment core from Lake Canto Grande. These proxies are used to explore the evolution of terrestrial and marine influence on the lake. Isotopic (δ 13 C: −27.87‰ to −31.9‰; δ 15 N: −0.07‰–4.9‰) and elemental (total organic carbon - TOC: 0.58%–37.19%; total nitrogen - TN: 0.08%–1.73%; C/N: 0.3 to 54.7) values recorded in Lake Canto Grande suggest that the sedimentary organic matter was derived from mostly C 3 land plants and freshwater phytoplankton. Particulate organic matter and cluster analyses distinguished four associations characterized by the predominance of amorphous organic matter, followed by phytoclasts and palynomorphs. These results indicate two different phases of lake evolution. The first phase (136 - 65 cm; ∼10,943 cal yr. B.P. to ∼8529 cal yr. B.P.) is recorded by sand layers interbedded with mud, which contain amorphous organic matter (AOM, 45–59%) and phytoclasts (opaques - OP: 6–18%; non-opaques – NOP: 17–23%) which indicate a floodplain area. The second phase (65–0 cm; ∼8529 cal yr. B.P. to ∼662 cal yr. B.P.) comprises mud, AOM (68–86%) and palynomorphs (PAL, 8–16%) related to lake establishment comparable to modern conditions. Thus, characterizing particulate organic matter, in combination with stable isotopes, proved to be invaluable proxies for lacustrine paleoenvironmental change through the Holocene. [ABSTRACT FROM AUTHOR]

Abstract: This paper applies meta-analysis techniques to a database of 401 14C-dated Holocene fluvial units in New Zealand. We use the database to produce a probability-based reconstruction of Holocene river behaviour at a national and regional scale. Records of river activity in northern (North Island) and southern (South Island) New Zealand are compared with independent hydro-climate proxy records that reflect regional, tropical and polar influences on Southern Hemisphere climate. During the Holocene, 12 multi-centennial length episodes of river activity and flooding were identified in the North Island, and in the South Island record 11 periods exceed the mean relative probability of activity. These records show that episodes of river activity have exhibited a predominantly out-of-phase relationship, suggesting the relationship between ENSO and SAM, and the relative dominance of the two modes, may be influencing Holocene river activity in New Zealand. The emerging pattern in the South Island Holocene fluvial record is one of increased river activity in response to enhanced westerly atmospheric circulation associated with a predominance of trough regime synoptic type (negative SAM-like circulation). In the North Island episodes of river activity are driven by increased meridional atmospheric circulation associated with blocking regime synoptic conditions (La Niña-like and positive SAM-like circulation). Analysis of floodplain sedimentation rates shows a rapid increase after ∼500 cal yr BP following the arrival of humans and the beginning of widespread deforestation. Regional climate complexity in New Zealand presents opportunities for palaeoclimate reconstruction, with the New Zealand fluvial 14C-database ideally placed to fill geographical gaps in the long-term hydrological record. [Copyright &y& Elsevier]

Abstract: Sediment deposition in floodplains is an essential part of the sediment dynamics of a catchment. These sediment dynamics can vary largely on a Holocene time scale under the influence of driving forces such as land use and climatic variations. In this paper we review the Holocene floodplain sediment deposition history of rivers in West and Central Europe, with special attention to the influence of climate and land use variability. During the early Holocene (Preboreal to Atlantic Period), most floodplains were rather stable with limited floodplain aggradation. After this initial phase of relative stability, sedimentation rates increase during the Middle and Late Holocene, with often the most important deposition phase during the last 1000 to 2000 a. The start of the increase in sedimentation varies spatially, with an earlier start in the west (France, Germany) then in the east (Poland, Belarus, Ukraine), while there are also local differences between regions which are settled early (e.g. loess regions) compared to later settled regions. The sedimentation history of most floodplains can be linked to the local land use history, while influence of climatic variations on floodplain aggradation is often reported to be of minor importance. Processes related to bedload are often reported to be more influenced by climatic events. Relationships between the driving forces and the sediment history are mainly based on synchronicity, and there is seldom proof for a direct causal relationship. Future research should focus on a system-based approach, integrating the floodplain deposition in the entire sediment dynamics which includes also erosion, colluvial deposition and export. Main research questions that need further attention include integration of system-based concepts, like landscape connectivity and buffering effects, and also the assessment of the individual contribution of land use changes and climatologic variations on the sediment dynamics. Detailed sediment budget studies combined with modeling studies can make an important contribution to the understanding of floodplain sediment dynamics. [Copyright &y& Elsevier]

Abstract: The reconstruction of paleoenvironmental changes from diatoms in lake sediments is based on the usual assumption that, in each studied sample, the dominant species reflect the environment that prevailed during the time of deposition. If the environment changed significantly during the period of deposition (several years) one can expect a mixture of species having different or contradictory ecological affinities. In this paper we present analyses of diatoms in surface sediment samples collected in the Lake Ossa area (3°50′N, 9°36E) and fossil diatoms from a mid-late Holocene core retrieved in the deepest part of the lake. The Lake Ossa area alternates between a short dry season centered at around the northern winter and a long rainy season during the rest of the year entailing significant changes in water level and pH. Based on multivariate analyses, we will show here that mean annual water depth is the most significant variable explaining the distribution of diatoms in the entire Lake. However, seasonal changes of water level are poorly recorded by diatom assemblages, except in some flat areas on the borders of the lakes where a mixing between species with different affinities to water depth is likely due to seasonal changes in water level. Inferred water depth based on a quantitative transfer function reflected essentially secular to millennial changes in the studied core. The relationship between pH and diatoms is not statistically significant but seasonal to multi-annual pH variations mainly observed in the central parts of the lake are reflected by a mixing of acidophilous and alkaliphilous species. Hierarchical ascending cluster analysis (HAC) considered as the most efficient mean of describing diatom mixing shows that seasonal to multi-annual changes in pH are recorded both in the modern and fossil assemblages. According to the degree of mixing between diatoms with different pH affinities we conclude that short-term pH variability was weaker than today between 5200 and 2700calyrBP, stronger between 2700 and 2000calyrBP, weaker again between 2000 and 600calyrBP and similar to present from 400calyrBP onwards. Short-term changes were thus superimposed on secular to millennium trends recorded by modifications in the abundance of alkaliphilous diatoms. All these changes are interpreted as variations in precipitation according to a previous model showing that pH is strongly controlled by acidic meteoric water. Inferred water depth slightly changed over the last 5500years showing weak variations of precipitation minus evaporation balance at secular to millennial time scales. These results will be used to refine previously published paleoclimatic interpretations, which explained changes in precipitation and P–E balance by modifications in the vertical structure of the atmosphere and subsequently by changes in cloud cover, convective or stratiform. [Copyright &y& Elsevier]