日本大沼湖沉积物碳氮比值、有机碳同位素特征及其近400年的古气候环境意义

CHARACTERISTICS OF ORGANIC STABLE CARBON ISOTOPE AND C/N RATIO OF SEDIMENTS IN LAKE ONUMA,JAPAN AND THEIR ENVIRONMENTAL IMPLICATIONS FOR THE LAST 400 YEARS

通过对日本大沼湖沉积物样品总有机碳含量(TOC)及其稳定同位素、总氮含量(TN)和C/N值的测定,结合沉积岩芯火山灰和^(14)C年代,分析了大沼湖沉积物中有机质的来源,探讨了沉积物有机碳同位素的主要影响因素。结果表明:大沼湖沉积物中有机质主要来源于自生藻类,受陆源输入的影响较小;沉积物δ^(13)C值指示了夏季温度的变化。过去400年来,大沼湖地区存在1660~1730年和1780~1860年两个明显的冷期,与邻近地区冰芯、树轮和湖泊沉积物记录的冷期基本一致,分别对应于Maunder和Dalton太阳黑子极小期。

Lacustrine sediments have high-resolution regional environmental information on lake and its surrounding catchments.Therefore they are of great use for reconstructing past climate fluctuations,understanding lakecatchment processes.Lake Onuma,located in Hokkaido,is a dammed lake formed by volcanic eruption of Mt.Komagatake in 1640,enables its deposits record the climatic and environmental change sensitively.A 392cm long sediment core （ON12C） was retrieved from the northeastern basin of Lake Onuma in 2012 at a water depth of 11.7m （42°00′9.3″N,140°41′31.7″E） using a piston core sampler.The sediment core was sliced at 2.5cm intervals.The data of total organic carbon （TOC） content and its stable isotope （δ13C）,total nitrogen content and the total organic carbon and nitrogen （C/N） ratio of sediments were measured to reconstruct the environmental change of Lake Onuma,Japan.The chronological model is based on the volcanic tephra ages and the AMS14C age of plant remnants.In the Late Holocene,four major plinian eruptions have occurred in Mt.Komagatake since 1640 and been recorded in the stratigraphy,namely Ko-d in 1640A.D.,Ko-c2 in 1694A.D.,Ko-c1 in 1856 A.D.and Ko-a in 1929A.D.,respectively.Combining with the data of AMS 14C,the Ko-d,Ko-c1,and Ko-a were observed in the core sediments.Therefore,we will discuss the environmental change in Lake Onuma from 1640A.D.based on the 120 samples in the upper 301cm sediments,the average resolution is about 1.0～6.5 years.The origin of organic matter was established using C/N ratio and δ13C.Regression of TOC verses TN in samples from the core ON12C showed a strong linear relationship,with a negative intercept near zero.This shows that nitrogen is present in predominantly organic form and that inorganic nitrogen is negligible.This analytical approach yields C/N ratios that faithfully represent organic matter origins.δ13C of atmospheric CO2 has been declining in the past 200 years as an effect of the accelerating release of CO2 with low 13C content by fossil fuel burning to the atmosphere and deforestation,so the δ13C of sediments in Lake Onuma for the Suess effect is corrected.The C/N ratios in the sediments of Lake Onuma were 8.2～ 13.1,below or slightly above 10 and δ13C of bulk organic matter ranged from-29.1‰o to-26.0‰o,similar to that of aquatic-derived lacustrine organic matter,demonstrating that the organic matter preserved in the sediments is mainly autochthonous,the contribution of allochthonous organic matter to the lake sediment was negligible for most of the record.Through comparisons for δ13C values of organic matter in sediment core at Lake Onuma,summer temperature variations after 1874A.D.from meteorological records at Hakodate and the CO2 concentrations of atmosphere,results show that the paleotemperature trend correlates well with meteorological records from Hakodate.There are two notable cold intervals at Lake Onuma over the past 400 years,namely 1660～ 1730A.D.and 1780 ～ 1860A.D.,these cold periods at Lake Onuma correspond well with those recorded from ice core,tree ring and lake sediment data from adjacent regions in China that have been observed in the conventional Little Ice Age （LIA）,indicating that δ13C of Lake Onuma is an effective proxy of paleotemperature.The trend in paleotemperature variations at Lake Onuma is also consistent with a recorded shift in solar activity,suggesting that solar activity may dominate temperature variations on decadal scales at the Lake Onuma.The sun plays a significant role in the Earth＇s climate variability in the LIA.