层状钙华及其地球化学指标的古气候/环境意义

层状钙华作为高分辨率古气候环境重建记录已被国内外许多学者所关注.钙华的微层厚度、氧碳同位素组成和微量元素的含量都呈现出随季节变化的周期性特点.钙华年层的形成与方解石沉积速率或者微生物生长活性的季节变化有关;氧同位素组成的变化主要受温度和沉积水体的δ18O控制,但蒸发作用(尤其是在干旱区)也有重要影响;钙华的稳定碳同位素...

A REVIEW OF PALAEOCLIMATIC IMPLICATIONS FROM LAMINATED TUFA AND TRAVERTINE

It is wildly accepted that tufa and travertine with laminae can be used for high-resolution reconstruction of Quaternary paleoclimate.Alternating laminae of dense micrite and porous calcites,combined with stable isotope(δ¹⁸ O and δ¹³ C )and trace elements from recent tufa show seasonal changes which reflect the climatic information.These alternating laminae have been interpreted to represent seasonally controlled differences in the rate of calcification and/or biotic activity of microbes.Variability in δ¹⁸ O is driven mainly by changes in water temperature and the δ¹⁸ O value of the recharging water,while evaporation,especially in arid climates,also needs to be accounted.Variability in δ¹³ C reflects principally the relative contribution of isotopically different carbon origins to dissolved inorganic carbon(DIC).CO₂ degassing as well as aquatic photosynthesis can take up ¹² C-enriched CO₂ in water and thus leave isotopically heavier DIC,which can also be given rise to by precipitation of calcite along the stream or river flow path.Meanwhile,the contents of trace element(e.g.Mg,Ba,Sr)in tufa are supposed to be related to temperature,their concentration in water and rainfall.However,whether aggrading neomorphism and reprecipitation change isotope record of tufa,and reliable chronologies need further research.In a word,as these deposits accumulate rapidly,laminated tufas and travertines offer an excellent opportunity to study seasonality in Quaternary palaeoclimate,and may become a promising record in future research.