5:15 PM - 7:15 PM
[MIS14-P32] Paleoenvironmental analyses of Lake Baikal sediments since the last interglacial period based on the reestablished age model
Keywords:Marine Isotope Stage 5d, Glacial erosion
Lake Baikal in southeast Siberia is the most sensitive area on Earth which has been responded to changes in solar insolation. Numerous studies have been conducted so far using the bottom sediment in Lake Baikal (e.g. Prokopenko et al., 2006). Of these, the biogenic silica (BioSi) and the fossil pollen records clearly showed variations on the glacial–interglacial (tens of thousands of years) scales and millennial scales during interglacial periods. Meanwhile, such biological proxies were nearly constant with low levels during glacial periods, and therefore the use of the data is difficult to evaluate environmental changes in that time. In this study, we have conducted continuous measurements on inorganic geochemical proxies highly independent of biological origin, such as grain size of mineral fraction (silicate phase), uranium (U) as well as BioSi contents in Lake Baikal sediment (BDP-99-1). In this poster presentation, we report the results of paleoenvironment analyses of the 140 kyrs records after reestablishing the sediment chronology.
In 1999, BDP-99-1 (0–113.3 m) and BDP-99-2 (109–251.9 m) were drilled from Posolskaya Bank which is located at boundary between Southern and Central Basins of the Lake Baikal (BDP99-Members, 2005). The age–depth relationship of BDP99 cores was determined by comparing Lake Baikal diatom content to marine oxygen isotope variations. Of these, the upper 22.3 m (13.6 kyrs) for BDP-99-1 was analyzed in this study. At first, the sediment samples for analyses were sliced every 2 cm and freeze-dried. Then, for grain size and BioSi analyses, 50 mg sample was pretreated with 10% H2O2 and 1 M HCl to remove organic matter and carbonates, respectively. BioSi was eluted with 2 M Na2CO3 and the residue, mineral fraction (silicate phase), were extracted. The BioSi content and grain sizes were measured by ICP-AES at Gifu University and laser light scattering instruments at Tono Geoscience Center (JAEA), respectively. For U and thorium (Th) contents, the 50 mg sample was treated with 3 mL HNO3 (61%), 2 mL H2O2 (30%), and 4 mL HF (46%) to digest completely (Murakami et al., 2010). ICP-MS at H-RISE and JAEA were used to quantify U and Th. 14C ages on soil organic carbon was measured using an accelerator mass spectrometry at JAEA and the acquired dates were converted to calibrated dates using the OxCal program (Ramsey, 2009). End-member modeling analysis of grain-size distribution was used for determination of grain size composition (Paterson and Heslop, 2015).
The composite chronology was constructed using 14C dates in the upper 721.5 cm-sediment and BioSi stratigraphy in 125–3306 cm depth. At first, reliable twelve 14C dates were selected from the fourteen dates and then were used to determine the chronology of the upper 721.5 cm-sediment. A 14C reservoir effect was calculated by the upper five 14C dates (24–125 cm). This is likely a result of old particulate matter from the watershed being delivered to lake (Prokopenko et al., 2007). The resulting topmost sediment age was estimated as 743 years ago. We subtracted 743 years from twelve 14C ages and then obtained the calibrated dates. On the other hand, the chronology for 125–3306 cm depth was determined by comparison of BioSi stratigraphy of previous study (BDP99-Members, 2005) and BioSi content of this study. The results indicate that the depth of the base MIS 5c (104 ka) was revised from 1833 cm to 1673 cm and the base MIS 5e (130 ka) from 2024 cm to 2069 cm. The average sedimentation rate calculated by the revised chronology was as follows: MIS 1–5.6 cm/ka, MIS 2–13.3 cm/ka, MIS 3–27.1 cm/ka, MIS 4–23.2 cm/ka, and MIS 5–12.1 cm/ka.
In BDP-99-1 core, both BioSi content and U/Th ratio showed high values during interglacial periods (MIS 1 and 5) and low values during glacial periods (MIS 2 and 4). Previous studies have reported that BioSi content becomes a paleotemperature proxy and U/Th ratio reflects moisture conditions for Lake Baikal sediment (Prokopenko et al., 2006; Murakami et al., 2012). The average U/ Th ratio is 0.72 ± 0.20 in MIS 1, while it is 0.58 ± 0.09 in MIS 5e. This suggests that MIS 1 had a wetter climate than that of MIS 5e. Meanwhile, six end members (EM1: 0.26 μm, EM2: 4.5 μm, EM3: 7.7 μm, EM4: 11.6 μm, EM5: 15.2 μm, EM6: 133 μm) explained 99.7% for all data in our BDP-99-1 core samples. A high contribution of EM5 was found in MIS 1 and 5e, while that of EM6 was in MIS 2, 3, and 4. The results suggest that EM5 reflect silt-sized suspended matters due to enhanced river input during interglacial periods, while EM6 is due to ice rafted detritus originating from Khamar-Daban Mountain Range on the southeastern coast of the lake.
In 1999, BDP-99-1 (0–113.3 m) and BDP-99-2 (109–251.9 m) were drilled from Posolskaya Bank which is located at boundary between Southern and Central Basins of the Lake Baikal (BDP99-Members, 2005). The age–depth relationship of BDP99 cores was determined by comparing Lake Baikal diatom content to marine oxygen isotope variations. Of these, the upper 22.3 m (13.6 kyrs) for BDP-99-1 was analyzed in this study. At first, the sediment samples for analyses were sliced every 2 cm and freeze-dried. Then, for grain size and BioSi analyses, 50 mg sample was pretreated with 10% H2O2 and 1 M HCl to remove organic matter and carbonates, respectively. BioSi was eluted with 2 M Na2CO3 and the residue, mineral fraction (silicate phase), were extracted. The BioSi content and grain sizes were measured by ICP-AES at Gifu University and laser light scattering instruments at Tono Geoscience Center (JAEA), respectively. For U and thorium (Th) contents, the 50 mg sample was treated with 3 mL HNO3 (61%), 2 mL H2O2 (30%), and 4 mL HF (46%) to digest completely (Murakami et al., 2010). ICP-MS at H-RISE and JAEA were used to quantify U and Th. 14C ages on soil organic carbon was measured using an accelerator mass spectrometry at JAEA and the acquired dates were converted to calibrated dates using the OxCal program (Ramsey, 2009). End-member modeling analysis of grain-size distribution was used for determination of grain size composition (Paterson and Heslop, 2015).
The composite chronology was constructed using 14C dates in the upper 721.5 cm-sediment and BioSi stratigraphy in 125–3306 cm depth. At first, reliable twelve 14C dates were selected from the fourteen dates and then were used to determine the chronology of the upper 721.5 cm-sediment. A 14C reservoir effect was calculated by the upper five 14C dates (24–125 cm). This is likely a result of old particulate matter from the watershed being delivered to lake (Prokopenko et al., 2007). The resulting topmost sediment age was estimated as 743 years ago. We subtracted 743 years from twelve 14C ages and then obtained the calibrated dates. On the other hand, the chronology for 125–3306 cm depth was determined by comparison of BioSi stratigraphy of previous study (BDP99-Members, 2005) and BioSi content of this study. The results indicate that the depth of the base MIS 5c (104 ka) was revised from 1833 cm to 1673 cm and the base MIS 5e (130 ka) from 2024 cm to 2069 cm. The average sedimentation rate calculated by the revised chronology was as follows: MIS 1–5.6 cm/ka, MIS 2–13.3 cm/ka, MIS 3–27.1 cm/ka, MIS 4–23.2 cm/ka, and MIS 5–12.1 cm/ka.
In BDP-99-1 core, both BioSi content and U/Th ratio showed high values during interglacial periods (MIS 1 and 5) and low values during glacial periods (MIS 2 and 4). Previous studies have reported that BioSi content becomes a paleotemperature proxy and U/Th ratio reflects moisture conditions for Lake Baikal sediment (Prokopenko et al., 2006; Murakami et al., 2012). The average U/ Th ratio is 0.72 ± 0.20 in MIS 1, while it is 0.58 ± 0.09 in MIS 5e. This suggests that MIS 1 had a wetter climate than that of MIS 5e. Meanwhile, six end members (EM1: 0.26 μm, EM2: 4.5 μm, EM3: 7.7 μm, EM4: 11.6 μm, EM5: 15.2 μm, EM6: 133 μm) explained 99.7% for all data in our BDP-99-1 core samples. A high contribution of EM5 was found in MIS 1 and 5e, while that of EM6 was in MIS 2, 3, and 4. The results suggest that EM5 reflect silt-sized suspended matters due to enhanced river input during interglacial periods, while EM6 is due to ice rafted detritus originating from Khamar-Daban Mountain Range on the southeastern coast of the lake.