A 13,600-year diatom oxygen isotope record from the South Carpathians (Romania): Reflection of winter conditions and possible links with North Atlantic circulation changes

摘要

This study provides a continuous lateglacial and Holocene record of diatom silica oxygen isotope changes (δ~(18)Odiat) in a subalpine lake sediment sequence obtained from the Retezat Mts (Taul dintre Brazi, 1740 m a.s.l.). This through-flow, shallow, high-altitude lake with a surface area of only 0.4 ha has short water residence time and is predominantly fed by snowmelt and rainwater. Its δ~(18)Odiat record principally reflects the oxygen isotope composition of the winter and spring precipitation, as diatom blooms occur mainly in the spring and early summer. Hence, changes in δ~(18)Odiat are interpreted as seasonal scale changes: in the amount of winter precipitation. Low oxygen isotope values (27-28.5‰) occurred during the lateglacial until 12,300 cal BP, followed by a sharp increase thereafter. In the Holocene δ~(18)0DiAt values ranged from 29 to 31‰ until 3200 cal BP, followed by generally lower values during the late Holocene (27-30‰). Short-term decreases in the isotopic values were found between 10,140-9570, 9000-8500, 7800-7300, 6300-5800, 5500-5000 and at 8015, 4400, 4000 cal BP. After 3200 cal BP a decreasing trend was visible with the lowest values between 3100—2500 and after 2100 cal BP The general trend in the record suggests that contribution of winter precipitation was generally lower between 11,680 and 3200 cal BP, followed by increased contribution during the last millennia. The late Holocene decrease in δ~(18)Odiat shows good agreement with the speleothem δ~(18)O, lake level and testate amoebae records from the Carpathian Mountains that also display gradual 818O decrease and lake level/mire water table level rise after 3200 cal BP. Strong positive correlation with North Atlantic circulation and solar activity proxies, such as the Austrian and Hungarian speleothem records, furthermore suggested that short-term increases in the isotopic ratios in the early and mid Holocene are likely connectable to high solar activity phases and high frequency of positive North Atlantic Oscillation indexes that may have resulted in decreased winter precipitation in this region.