Search

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]