Mitsugu Nishimura

Paleomagnetic record from Academician Ridge, Lake Baikal: a reversal excursion at the base of marine oxygen isotope stage 6

Abstract Paleomagnetic and rock-magnetic studies on a hydraulic piston core (Ver98-1, St.6) from Academician Ridge, Lake Baikal showed the occurrence of a reversal excursion at 670–696 cm depth, which is at the base of marine oxygen isotope stage 6. A correlation of X-ray CT values, as a proxy of relative density, to the marine oxygen isotope record provides an age of 177–183 ka for this reversal excursion. It can be correlated with other excursion records from Lake Baikal, found in Core 287-K2 from Academician Ridge [King et al., Russ. Geol. Geophys. 34 (1993) 148–162] and in core BDP93-1 drilled on the Buguldeika saddle [BDP-93, Quat. Int. 37 (1997) 3–17]. We correlate the Lake Baikal reversal excursion with a well documented excursion in the Brunhes Chron, the Iceland Basin event (186–189 ka) from ODP Sites 983 and 984 in the North Atlantic [Channell, J. Geophys. Res. 104 (1999) 22937–22951]. Also the relative paleointensity record agrees well with that from ODP Site 983 [Channell, J. Geophys. Res. 104 (1999) 22937–22951]. The Lake Baikal excursion and the Iceland Basin event correspond to the minimum of relative intensity at 188 ka in Sint-800 [Guyodo and Valet, Nature 399 (1999) 249–252]. We argue that it is distinct from the Jamaica/Pringle Falls excursion, estimated at 205–215 ka [Langereis et al., Geophys. J. Int. 129 (1997) 75–94]. This is supported by the recalibration of the age of another excursion found in Core St.16 in Lake Baikal [Sakai et al., Bull. Nagoya Univ. Furukawa Mus. 13 (1997) 11–22] with an age of ∼223 ka, which is close to the age of the Jamaica/Pringle Falls excursion, as suggested earlier [King et al., Russ. Geol. Geophys. 34 (1993) 148–162]. The VGP path of the reversal excursion (177–183 ka) consists of a southward swing through the North Atlantic, followed by a loop through Africa and the Indian Ocean. The path morphology is similar to that of the Iceland Basin event from the North Atlantic [Channell, J. Geophys. Res. 104 (1999) 22937–22951].

First investigation of ultraoligotrophic alpine Lake Puma Yumco in the pre-Himalayas, China

Lake Puma Yumco is a typical alpine lake (altitude 5030 m) located in the pre-Himalayas of Tibet, China, and this study was the first limnological investigation ever conducted on it. Lake Puma Yumco (28°34″N, 90°24″E) has the following morphometric properties: maximum length 31 km, maximum width 14 km, mean width 9 km, shoreline 90 km, surface area 280 km2, and shoreline development 1.5. Transparency was approximately 10 m, even in the thawing season. The extinction coefficient of the lake water was calculated as 0.15 m−1. Annual maximum transparency was estimated from the depth of the Chara zone to be 30 m. Dissolved oxygen was 7 mg O2 l−1 and showed saturated values, and salinity was 360 mg l−1. The chemical type of the lake water was Mg-Ca-HCO3-SO4, and it was slightly alkaline in character. Total nitrogenous nutrients (sum of ammonia, nitrite, nitrate, and urea nitrogen), phosphate, and silicate were extremely low at 1, 0.02, and 9 µM, respectively. Dissolved organic carbon, nitrogen, and phosphorus concentrations were 160, 11, and 0.08 µM and the molar ratio was calculated as 2100 : 140 : 1. Chlorophyll a concentration was 0.2 mg m−3. Phytoplankton and zooplankton were dominated by Aphanocapsa sp. and Diaptomidae. Both nitrogen and phosphorus appear to be the limiting parameters for phytoplankton growth. Organic carbon and nitrogen contents in lake sediments were low and the sediments contained a large amount of CaCO3. The grain size of sediment was that of silt-sand in most cases. The present results indicate that the pre-Himalayan alpine freshwater Lake Puma Yumco is an ultraoligotrophic lake.

The second investigation of Lake Puma Yum Co located in the Southern Tibetan Plateau, China

Limnological features of Lake Puma Yum Co, a typical alpine lake located at the altitude of 5030 m in the Tibetan Plateau, China, are described based on the findings of the second expedition in September 2004. The lake surface is about 280km2 and maximal depth is 65 m. Total inflow just after the rainy season was estimated to be about 860 000m3 day−1, and the lake water was drained from a newly excavated channel at a rate of 960 000m3 day−1. We may have to expect undesirable lowering of the water level by this new drainage, especially in the dry season. Thermocline developed from 20 to 30 m depth, and the euphotic zone reached the 50 m depth. Dissolved oxygen in surface water was supersaturated as in productive lakes, although there were no large point and nonpoint sources of nutrient in the catchment. Vertical distribution of phytoplankton biomass and primary production suggested the presence of photoinhibition. What should be noted about the flora and fauna is that a Chara zone and a shell zone were distributed at about 30 or 40 m of depth.

Further discussion about the features of Lake Puma Yum Co, South Tibet, China

Further discussion about the limnological features of Lake Puma Yum Co, South Tibet, China, is provided based on the results of several investigations. By using depth data from all over the lake, the whole submarine topography has been compiled. Horizontal analysis of the waters physicochemical features indicates that compared with the relatively uniform water features at other lake areas, apparent spatial heterogeneity exists in the water of the subaquatic alluvial fan induced by the Jiaqu River, the biggest inflow. Vertical analysis of water characteristics using two-factor analysis of variance with no re-experiment indicates that temperature, dissolved oxygen, and pH of the water vary with water depth rhythmically, whereas other parameters demonstrate no evident vertical variation, which shows that chemical stratification is not obvious. But this does not exclude slightly higher concentrations of Ca2+ induced by lower pH at the bottom of deep lake water. The hydrochemistry difference between inflow water and lake water reveals the loss of Ca2+ in lake water, which indicates calcite deposition may be an important characteristic of lake sediment.

A New 14C Data Set of the PY608W-PC Sediment Core from Lake Pumoyum Co (Southeastern Tibetan Plateau) over the Last 19 kyr

A new continuous sediment core (PY608W-PC; 3.8 m length) for reconstruction of climatic and environmental changes in the southeastern Tibetan Plateau was taken from the eastern part of Lake Pumoyum Co in August 2006. Sediment layers of the lower part of PY608W-PC (380-300 cm depth) were composed mainly of relatively large plant residues (up to ~3 cm in length) with an admixture of fine sand and sandy silt. The large plant residues disappeared at ~300-290 cm depth in core PY608W-PC and were replaced by silt-silty clay. The large plant residues from the lower part of PY608W-PC could be aquatic, because the plant residues were extremely enriched in 13C (up to -3.0‰, -5.6 ± 2.3‰ on average). On the other hand, the plant residue concentrates (PRC fractions) from the upper part of the core (290-0 cm in depth) could be terrestrial C3 plants (?13C = -21.8 ± 1.7‰ on average). Radiocarbon dating was performed on the large plant residues and PRC fractions from the PY608W-PC sediment core, which represented the chronology from ~19,000 cal BP to present.

14C Dating of Holocene Soils from an Island in Lake Pumoyum Co (Southeastern Tibetan Plateau)

Soil samples from an 85-cm-long continuous section (PY608ES) were collected from an island in Lake Pumoyum Co (southeastern Tibetan Plateau, ~5020 m asl) in August 2006. To estimate past environmental conditions of Lake Pumoyum Co during the Holocene, we analyzed radiocarbon ages, stable carbon isotope compositions, and total organic carbon/total nitrogen (TOC/TN) atomic ratios of the soil samples. The 14C measurements were performed with the Tandetron accelerator mass spectrometry system at the Center for Chronological Research, Nagoya University. The 14C concentration in the surface layer (101 pMC; 5-10 cm soil depth) was nearly modern. A 14C chronology of the sequence indicated that continuous soil development began on the island in Lake Pumoyum Co at ~5800 cal BP (at 63 cm soil depth, the top of a gravel layer). These results may reflect a decrease in the lake level in the middle Holocene. The age of the obvious lithologic boundary (~5800 cal BP) corresponds to the end of Holocene climate optimum.