Radiocarbon ages of different fractions of peat on coastal lowland of Bohai Bay: marine influence?

Peat in boreholes is the most important ^(14)C dating material used for constructing age framework. 20 bulk peat samples were collected from five boreholes, the ^(14)C ages of two fractions(organic sediment fraction and peat fraction) of the bulk peat samples were investigated by AMS-dating and which fraction is better to help construct an age framework for the boreholes were compared and discussed. The results indicated that the peat fraction give a good dating results sequence in the boreholes, compared with the corresponding organic sediment fraction. And the dating results of organic sediment fraction show 161-6 702 years older than corresponding peat fraction, which was caused by marine influence. Then, we suggest an experience formula as y =0.99 x-466.5 by the correlation analysis for correcting the marine influenced organic sediment ages within the conventional ages between 4 000 to 9 000 yrs BP, and more study should be carried out for the AMS ^(14)C dating of the bulk organic sediments.

钻孔岩心黏土混浊水电导率、黄铁矿、pH相关性分析及其在古沉积环境复原的应用：以渤海湾西岸平原DC01孔为例

为探讨复原海岸平原沉积环境和精确划分海陆相地层的有效方法,对渤海湾西岸海河以南平原深达30 m的DC01孔岩心以约20cm间距取样,测试黏土混浊水电导率(EC)、黄铁矿质量分数(w(FeS_2))和pH值,并开展相关分析研究。结果显示,EC与w(FeS_2)正相关(相关系数r为0.47),而EC与pH以及w(FeS_2)与pH均负相关(r分别为-0.43和-0.52)。根据EC值的大小将DC01孔自下至上分成5个带(Ⅰ—Ⅴ),进一步分别对5个带的EC、w(FeS_2)及pH做相关分析,结果显示,Ⅱ带相关性最为突出:EC与w(FeS_2)的相关系数为0.83,极强正相关;EC与pH的相关系数为-0.77,强负相关;w(FeS_2)与pH的相关系数为-0.45,呈中等程度负相关。根据Ⅱ带的EC和w(FeS_2)极强正相关、且EC和w(FeS_2)均明显偏高以及多发育黑色泥炭和腐殖质黏土等特征,推断Ⅱ带为受海水影响的盐沼环境;结合AMS14 C测年结果,推测8 260~7 470cal.B.P.期间,海水曾经影响到DC01孔的位置。另外,Ⅳ带的EC值也偏高,但w(FeS_2)却较低,其EC与w(FeS_2)的相关系数为-0.03,不呈正相关关系,其原因是因为接近地表的上部沉积物因淋溶作用,在Ⅳ带形成淀积层导致EC异常偏高;最上层的Ⅴ带为淋溶层,这样就导致钻孔上部(深6.7m以上)Ⅳ带和Ⅴ带EC与w(FeS_2)的相关性出现异常。因此在讨论沉积物上部沉积环境时,将EC与w(FeS_2)综合分析可以更精确地划分钻孔岩心的沉积环境。