Characteristics and indications of hydrogen and oxygen isotopes distribution in lake ice body.
Stable isotopes have been used to identify the characteristics of precipitation, evaporation, basin hydrology, and residence times. However, lakes in the cold regions are usually covered by ice for 5-6 months. To get a better understanding of stable isotopes characteristics and indications in lake ice bodies, ice and water were sampled during the icebound season in both the ice and water bodies in Dali Lake, and deuterium, oxygen-18 total nitrogen (TN), and the major ions were analyzed. The results showed that deuterium and oxygen-18 compositions (δD-δ¹⁸O) compositions in the ice body were greater than in the water body beneath, scattered on a straight line, and deviating downward from the global meteoric water line in the top right. The ice profile showed that the δD-δ¹⁸O compositions increased from the ice surface downward and decreased near to the bottom. In contrast, the TN and the major ions in the ice decreased from the ice surface downward and increased near to the bottom, meaning that the concentrations of δ¹⁸O had a negative correlation with the concentrations of TN and major ions. These indicated that stable isotopes can be used for tracing the nutriment and ion transport processes in the ice body.
Characteristics of palaeosalinity and palaeoredox records in sediment from Dali Lake: Climate change in North China from 0 to 2100 cal BP
Abstract To understand the salinity, redox characteristics, and climatology of Dali Lake during the late Holocene, we analysed trace element records from a high-resolution sediment core (DL-1), including Al, Mn, Zn, Mo, Pb, Fe, Sr, and Ba. The core was dated to 2100 cal BP based on accelerator mass spectrometry (AMS) 14C dating combined with the constant rate of supply (CRS) model. We identified five palaeoclimatic stages based on the down-core trace elemental characteristics. From ca. 2100 to 1650 cal BP (Stage Ⅰ), Mn-MAR (mass accumulation rates) and Fe-MAR showed no correlations with Mo-MAR, suggesting that anaerobic conditions limited the adsorption of Mo onto Mn–Fe oxides. From ca. 1650 to 1050 cal BP (Stage Ⅱ), we identified a weakened aerobic environment, coinciding with the Dark Ages Cold Period due to a decrease in the organic matter (OM), nutrients, Fe-MAR, and Zn-MAR, which inhibited Mo deposition. Salinity (Sr/Ba) did not influence the trace elemental fluxes, OM, or nutrients throughout Stages I and II. We identified aerobic conditions during Stage Ⅲ (between ca. 1050 and 650 cal BP), coinciding with the Medieval Warm Period, with observed lower salinity that facilitated Al-MAR, Mn-MAR, and Pb-MAR deposition. During the Little Ice Age (between ca. 650 and 30 cal BP; Stage Ⅳ), Mn-MAR, Zn-MAR, and Fe-MAR showed no correlation with Mo-MAR, implying anaerobic conditions. We also identified high salinity and OM concentrations that facilitated trace element deposition. Over the past 95 years (Stage V), high salinity levels have produced the deposition of Al-MAR, Mn-MAR, Zn-MAR, and Fe-MAR. However, the redox conditions in the lake sediment are difficult to interpret due to anthropogenic influences on trace elemental fluxes. We identified a salinity periodicity (Sr/Ba) of 100–150 years and a redox periodicity (Mo-MAR) of 100–260 years using the wavelet power spectrum. Our results indicate a gradual weakening of the East Asian summer monsoon, which may be controlled by insolation in the Dali Lake basin.
Accumulation of mantle gases in a permanently stratified volcanic lake (Lac Pavin, France)
Abstract Lac Pavin is a volcanic crater lake in the Massif Central (France), characterized by a permanent vertical density stratification resulting from a strong and persistent chemocline between about 60 and 70 m depth. The deep water below the chemocline forms the monimolimnion, in which most dissolved ions as well as helium, carbon dioxide, and methane are strongly enriched. The 3He/4He isotope ratio of the excess helium is (9.09 ± 0.01) · 10−6, or (6.57 ± 0.01) Ra. These findings clearly indicate a flux of mantle-derived magmatic gases into the monimolimnion. In order to derive the fluxes of magmatic volatiles into Lac Pavin, it is essential to understand the hydrologic characteristics of the lake. Previously published two-box models have assumed groundwater input at the lake bottom, a short residence time in the monimolimnion, and biogenic origin of the CO2. We propose an alternative model with a flux of magmatic gases, but not of water, into the monimolimnion, and a weak diffusive coupling between the monimolimnion and the overlying mixolimnion which leads to a long deep-water residence time (≈ 70 yr). We reassess the carbon budget of the lake and conclude that the major part of the accumulated CO2 in the monimolimnion is of magmatic origin. From the model-derived water exchange rates, we calculated a mantle 4He flux of (6 ± 2) · 1011 atoms m−2 s−1. This value lies near the lower end of the range found in comparable volcanic lakes. The flux of magmatic CO2 is estimated as (1.2 ± 0.4) · 10−7 mol m−2 s−1, which is also comparatively low. The monimolimnion appears to be in steady state with respect to these fluxes, therefore no further, potentially hazardous, accumulation of CO2 takes place.