Radiocarbon‐based carbon source quantification of anomalous isotopic foraminifera in last glacial sediments in the western North Pacific
Radiocarbon‐based carbon source quantification of anomalous isotopic foraminifera in last glacial sediments in the western North Pacific
复制标题
DOI：
10.1029/2006gc001558
复制
发表时间：
2008-04
期刊：
影响因子：
3.7
通讯作者：
M. Uchida;K. Ohkushi;K. Kimoto;F. Inagaki;Toyoho Ishimura;U. Tsunogai;Taqumi Tuzino;Y. Shibata
中科院分区：
文献类型：
--
作者：
M. Uchida;K. Ohkushi;K. Kimoto;F. Inagaki;Toyoho Ishimura;U. Tsunogai;Taqumi Tuzino;Y. Shibata
先前的一项研究解释了最后一次冰川沉积物(17500至25400卡年B.P.)浮游和底栖有孔虫的13C极度耗尽的漂移。从日本岛下半岛和北海道附近海域取回的岩心照片，作为甲烷水合物解离产生的甲烷周期性释放的证据。为了更好地了解~(13)C耗尽漂移的形成过程，我们进行了高分辨率的天然放射性碳测量和生物地球化学分析。在浮游有孔虫和底栖有孔虫中，我们分别发现了从−10.2‰到−1.6‰和从−6.8‰到−1.6‰的高度耗尽的13C漂移。这些层位的有孔虫测试大多是棕色的，很可能是沉积后蚀变的结果，反映了测试表面自生碳酸盐的形成。镁方解石含量高和异常有孔虫的酸浸试验也支持这些蚀变。为了评估改变的有孔虫测试中的碳源，我们使用耦合质量平衡同位素模型(14C/C和13C/12C)定量了不含14C的甲烷来源的碳源对耗尽13C漂移的有孔虫中自生碳酸盐形成的相对贡献。浮游和底栖生物的~(13)C贫乏有孔虫的~(13)C年龄比附近地层的正常测试结果大约早600~2000年。来自甲烷氧化过程的自生碳酸盐对浮游有孔虫的相对贡献率达到∼22wt%，对底栖有孔虫的相对贡献率达到∼15wt%。根据质量平衡模型计算的甲烷的δ13C值，浮游有孔虫在−29‰到−68‰之间，底栖有孔虫在−40‰到−108‰之间，与全球甲烷水合物储藏库中热生和非生物成因甲烷的δ13C值一致。这些数据一致地表明，包括δ13C异常在内的层位中发生了与甲烷有关的剧烈环境变化。这项研究为解释与气候有关的甲烷水合物不稳定的地质记录提供了重要信息。
A previous study interpreted extremely 13C‐depleted excursions of planktonic and benthic foraminifera in last glacial sediments (17,500 to 25,400 cal years B.P.) of the core retrieved from off Shimokita Peninsula and off Hokkaido, Japan, as evidence for periodic releases of methane, arising from the dissociation of methane hydrate. To better understand the formation process of the 13C‐depleted excursions, we conducted high‐resolution natural radiocarbon measurements and biogeochemical analyses. We found highly depleted 13C excursions ranging from −10.2‰ to −1.6‰ and −6.8‰ to −1.6‰ in planktonic and benthic foraminifera, respectively. Most of the foraminiferal tests in these horizons were brown, most likely as a result of postdepositional alteration, reflecting the formation of authigenic carbonate on the surface of tests. These alterations were also supported by high levels of Mg‐calcite and the acid‐leaching test for anomalous foraminifera. To evaluate the carbon sources in the altered foraminifera tests, we quantified the relative contributions of 14C‐free methane‐derived carbon sources to the formation of authigenic carbonates in foraminifera with depleted 13C excursions using a coupled mass balance isotopic model (14C/C and 13C/12C). The radiocarbon ages of both planktonic and benthic 13C‐depleted foraminifera were approximately 600 to 2000 years older than those of normal tests from nearby horizons. The relative contributions of authigenic carbonates derived from the methane oxidizing process reached to ∼22 wt% for planktonic foraminifera and ∼15 wt% for benthic foraminifera. The δ13C values of methane calculated from the mass balance model were between −29‰ and −68‰ for planktonic foraminifera and between −40‰ and −108‰ for benthic foraminifera, consistent with δ13C values reported for thermogenic and abiogenic methane in global methane hydrate reservoirs. These data consistently suggest that methane‐related drastic environmental change occurred in the horizons that included δ13C anomalies. This study provides important information for interpreting geological records of the methane hydrate instability associated with climate.