Oxygen isotope composition of diatoms from sediments of Lake Kotokel (Buryatia)

Abstract

This is a summary of new oxygen isotope data for diatoms from Lake Kotokel sediments, with implications for responses of the lake system and its environment to global change over the past 46 kyr. Fossil diatoms in all samples are free from visible contamination signatures and contain no more than 2.5% Al 2 O 3 , which ensures reliable reconstructions. The δ 18 O values in diatoms vary between +23.7 and +31.2‰ over the record. The results mainly represent diatom assemblages of summer blooming periods, except for the time span between 36 and 32 kyr, when the isotopic signal rather records a shift from summer to spring blooming conditions. Possible water temperature changes only partly explain the changes in the isotopic record. The observed isotopic patterns are produced mainly by isotope changes in lake water in response to variations in air temperature, hydrology, and atmospheric circulation in the region. During Marine Isotope Stage (MIS) 2 (Last Glacial maximum), high δ 18 O diatom resulted from rapid evaporation and low fluvial inputs. The high δ 18 O values of about +29 to +30‰ during the first half of MIS 1 (Holocene interglacial) suggest an increased share of summer rainfalls associated with southern/southeastern air transport. The δ 18 O decrease to +24‰ during the second half of MIS 1 is due to the overall hemispheric cooling and increased moisture supply to the area by the Atlantic transport. The record of Lake Kotokel sediments provides an example of complex interplay among several climatic controls of δ 18 O diatom in the Late Pleistocene and the Holocene.

Moisture origin and stable isotope characteristics of precipitation in southeast Siberia

“…The oxygen isotope record of diatoms preserved in the Lake Kotokel bottom sediment (Kostrova et al, 2013, 2014, 2016) suggests that the south‐easterly moisture transport played a more significant role in the Baikal region during the Early Holocene with higher‐than‐present summer insolation and intensified monsoon circulation. Oppositely, the westerly moisture transport was weaker in the Eurasian midlatitudes during the Early Holocene and became stronger (similar to today) during the Late Holocene (Tarasov, Bezrukova, Müller, Kostrova, & White, 2017).…”

“…Since the 1990s, the region has increasingly become a centre of palaeoenvironmental studies using sedimentary archives from Lake Baikal (Kravchinsky, 2017; Kuz'min, Khursevich, Prokopenko, Fedenya, & Karabanov, 2009) and numerous small lakes (Bezrukova, Tarasov, Solovieva, Krivonogov, & Riedel, 2010; Danilenko, Solotchin, & Solotchina, 2015; Leonova et al, 2018; Mackay, Bezrukova, et al, 2013; Sklyarov et al, 2010). In particular, oxygen isotope records from diatoms (Kalmychkov, Kuz'min, Pokrovskii, & Kostrova, 2007; Kostrova et al, 2013, 2014, 2016; Mackay et al, 2011, Mackay, Swann, et al, 2013; Morley, Leng, Mackay, & Sloane, 2005; Swann et al, 2018) and ostracods (Tarasov et al, 2019) have been analysed and demonstrate the relationship between isotope signals preserved in sedimentary archives and ancient atmospheric precipitation patterns. All these studies require the knowledge of the stable isotope composition of recent regional precipitation.…”

Hydrological (in)stability in Southern Siberia during the Younger Dryas and early Holocene

“…Changes in ẟ 18 O diatom values from Kotokel are thought to be due to variations in the ẟ 18 O lakewater in response to changes in air temperature, hydrology and atmospheric circulation (Kostrova et al, 2016).…”

“…Variations in the proportion of summer precipitation from different source regions are implicated in changes to the ẟ 18 O diatom throughout the Holocene, with an increased share of precipitation from southern sources during the Early Holocene, with a shift to more Atlantic sourced moisture during the Mid-Late Holocene (Kostrova et al, 2016). Additionally, as a large but shallow lake, evaporation may also be a feature, although during the studied time period, this will be less influential than during its older history, when the lake was a closed basin, and thus, more strongly influenced by evaporation (Leng and Barker, 2006).…”

“…ẟ 18 O is influenced by changes in external environmental and climatic factors, including the quantity or source of precipitation, atmospheric temperature variability, and evaporation regimes. ẟ 18 O diatom reconstructions are available for key southern Siberian sites, including Lake Baikal (Mackay et al, 2013b;Mackay et al, 2011) and Lake Kotokel, located to the east of the Baikal central basin (Kostrova et al, 2013b(Kostrova et al, , 2014(Kostrova et al, , 2016. In Lake Baikal, ẟ 18 O diatom variability has been linked to factors including the relative proportion of southern versus northern rivers feeding the lake, due to its enormous catchment (Mackay et al, 2011), and at Lake Kotokel to the interplay of evaporation and the ẟ 18 O of precipitation, which is linked to air temperature and atmospheric circulation (Kostrova et al, 2013b(Kostrova et al, , 2014(Kostrova et al, , 2016.…”