Stable Isotopes Of Modern Carbonate From Genggahai Lake,Northeastern Qinghai-Tibetan Plateau: Implications For Environmental Changes

Carbon isotopes(?¹³Ccarb)and oxygen isotopes??18Ocarb?of carbonates from lake sediments have long been recognized as proxies for climatic and environmental changes.However,due to the complicated factors and multiple processes both within lake and in lake catchments,potentiallyleading to changes in ?¹³Ccarb and ?18Ocarb,the climatic and environmental implications of ?¹³Ccarb and ?18Ocarb are often interpreted from a tentative and empirical perspective.A thorough understanding of changes in ?¹³C and ?18O of modern lacustrine carbonates and their responses to within-lake and watershed processes are crucial to further elucidate environmental implications of ?¹³Ccarb and ?18Ocarb.Genggahai Lake is located in the central Gonghe Basin on the northeastern Qinghai-Tibetan Plateau.It is a shallow lake,presently occupied by dense aquatic vegetation.The lake is favorable for formation of geogenic and biogenic carbonates.The sedimentary environment of the lake is stable and the hydrological cycle of the lake system is relatively simple.Furthermore,there is less human activity around the lake.Therefore,Genggahai Lake is an ideal site to study changes in ?¹³Ccarb and ?18Ocarb and those potential affecting factors.From May 2012 to September 2015,we had monitered the chemical and physical properties of lake water?e.g.temperature,dissolved oxygen concentrations,pH values?,analyzed the ion concertration,dissolved inorganic carbon?DIC?concertration and hydrogen and oxygen isotopes of the lake water and the carbon isotopes of DIC(?¹³CDIC)precipitated from lake water and the water bodies within the lake catchment.Moreover,the carbon and oxygen isotopic compositions of modern carbonates,e.g.encrustations on stems of living Chara spp.,mollusc shells,and the carbonate component of trapped sediments were measured.In addition,the carbon isotopes of organic matter(?¹³Corg)from submerged plants were measures.In combination with meteoric data from Gonghe station,water chemistry characteristics and controlling factors within the catchment of Genggahai Lake and the relationship between the isotope composition of different carbonate components and ?¹³Corg of different submerged plants and the modern water environment and regional climatic regimes were analyzed.Furthermore,the climatic implication of ?¹³Ccarb and ?18Ocarb and ?¹³Corg were discussed.The main conclusions are drawn as follows:1.The main cations in spring and river of Genggahai watershed are Ca2+ and Na+,and their dominant source is the weathering of rocks;the main anion is HCO3-and its dominant sources are weathering of rocks and CO2 derived from soil respiration in the catchment.In lake water,the main cation is Na+ and the main anion is Cl-.The ion contents are mainly affected by inflowing spring water and evaporation enrichment within lake.Moreover,the low contents of Ca2+ and HCO3-in lake water are related to photosynthesis of aquatic plants in the lake.2.Changes in carbon isotopes of lake water DIC(?¹³CDIC-L)are attributed to variations in carbon isotopes of inflowing water DIC(?¹³CDIC-I),responding to carbon-flow path-ways in the catchment.The intense photosynthesis and hence calcification lead to the positive ?¹³CDIC-L and the negative ?¹³Cencrust.Carbon isotopic compositions of mollusc shells(?¹³Cshell)are primarily affected by the ?¹³CDIC-L.Among these different carbonate species,the carbonate component of trapped sediments is the most enriched in 13 C,whereas mollusc shells are the most depleted in 13 C.The carbon isotopic differences between different carbonate species suggests an existence of isotopic disequilibrium during formation of the carbonates,which may result from “microenvironmental metabolic effect”,and/or “vital effect” of mollusc.3.Variations in ?18OL of Genggahai Lake are mainly controlled by the ?18O of inflowing spring??18OI?which are driven by moisture history,although evaporation process indeed causes isotopic enrichment of lake water.“Kinetic effect” induced by rapid carbonate precipitation,due to intense photosynthesis of Charophyte in the lake would lead to negative shifts in the oxygen isotopes of stem encrustations of Chara plants??18Oencrust?during intervals with high-temperatures.Oxygen isotopes of mollusc shells??18Oshell?are primarily affected by the ?18OI on yearly timescale.Among these carbonate species,mollusc shells are the most enriched in 18 O,whereas stem encrustations are the most depleted in 18 O,which may have resulted from either “kinetic effect” and/or “mineral effect” pertinent to different carbonate minerals.4.The degree of isotopic disequilibrium fractionatioin between mollusc shells and lake water is relatively low,thus changes in ?¹³Cshell and ?18Oshell largely represent variations in ?¹³CDIC-L and ?18OL,respectively.Therefore,changes in ?¹³Cshell and ?18Oshell have the potential for indicating changes in carbon–flow pathways within the catchment and ?18O of precipitation and hence atmospheric circulation patterns over the study area,respectively.However,we have observed extreme isotopic disequilibrium between stem encrustations and lake water.Besides the influence of isotopic compositions of lake water,the variations in ?¹³Cencrust and ?18Oencrust may have been modulated by in-lake processes,such as “microenvironmental metabolic effect” and “kinetic effect”.5.The high level of photosynthesis efficiency of Chara spp.would result in lower ?¹³Corg values?-18.6--14.0‰?of Charophyte,compared to vascular submerged plants?Potamogeton pectinatus and Myriophyllum spicatum??-14.9--9.2‰?.The carbon isotopes of vascular plants could represent changes in ?¹³CDIC-L.In addition,the isotopic enrichment factor between carbon isotopes of Potamogeton pectinatus and ?¹³CDIC-L??Potamogeton-DIC?(?Potamogeton-DIC = ?¹³CPotamogeton-?¹³CDIC-L)would be indicative of former HCO3-concerntrations that is related to changes in lake nutrient status.