德国Chiemsee湖磁性细菌干湖泥的磁学性质

THE MAGNETIC PROPERTIES OF THE BACTERIAL SEDIMENTS IN LAKE CHIEMSEE,GERMANY

对经历过实验室磁性细菌培养的Chiemsee湖的4块干湖泥样品进行了磁学参数、粒度和XRD测量,结果表明样品中的主要磁性矿物以磁铁矿为主,并含有少量的硫铁矿。培养深度1～2cm处的样品在磁化率、百分比频率磁化率、无磁滞剩磁、饱和磁化强度和矫顽力等磁学特征上表现出和其余样品差别较大,分析认为这种差异可能是由于该层位的样品受到较多的磁性细菌产生的磁小体影响所致。样品的低温饱和剩磁测量表明样品中除了单畴(SD)磁铁矿之外,还存在多畴(MD)磁铁矿颗粒,利用粒度频率分布曲线对沉积物的来源进行了分析,表明Chiemsee湖的沉积物可能主要是沉积在湖泊周围的一些风积物在地表水流作用下,与一些粗粒级沙粒一并进入湖泊所形成,样品中的粗粒多畴磁铁矿可能是这些沉积物所携带的,与后期的磁性细菌关系不大。因此,沉积物中的磁学信号除了磁性细菌产生的磁小体的贡献以外,其他物源沉积物的贡献显然也是不可忽略的重要因素。

In this paper,we measured the magnetic parameters,grain size and XRD for four dry mud samples,which were taken from Lake Chiemsee, Bavaria of Germany. The Lake Chiemsee （47°54＇N, 12°30＇E ; 520m above sea level） located in the piedmont of Alps,covers about 80kin2. After collected by piston cores, the samples had experienced the magnetotactic bacteria culture in laboratory, then were air-dried and divided into four parts （various depths） which were named Bio-tesl, Bio-lcs2, Bio-lcs3, Bio-lcs4. The results of the experiments show that the main magnetic minerals in the samples are magnetite, and a few pyrite simultaneously exist. Compared with other samples, the sample cultured at 1 - 2cm depth displays some different properties in magnetic susceptibility （Z） , percentage frequency magnetic susceptibility （ Zfd% , anhysteretic remanent magnetization （ ARM ） , saturation magnetization （M） and eoercivity（ H o） , likely indicating higher concentration of magnetosomes produced by more magnetotactie bacteria in such certain depth. The results of low-temperature saturation remanence show clear Verwey transition, suggesting presence of multi-domain （MD） magnetite in the samples. That coarse size is out range of reported magnetosomes. That was also detected by particle size measurements showing a few peaks for the distribution curves. Of which,two main peaks are all larger than that of single domain SD/MD boundary. That likely indicates that lake Chiemsee sediments may be a mixture sources, together with aeolian deposit down to the lake. Some coarser sands and aeolian deposit around the lake would be possibly washed down to the lake by surface water flow. The multi-domain magnetite measured in the samples could be within these coarser sediments.