Geochemistry of modern carbonaceous sediments overlain by a water mass showing photic zone anoxia in the saline meromictic Lake Kai-ike, southwest Japan: I. Early diagenesis of organic carbon, nitrogen, and phosphorus.
Geochemistry of modern carbonaceous sediments overlain by a water mass showing photic zone anoxia in the saline meromictic Lake Kai-ike, southwest Japan: I. Early diagenesis of organic carbon, nitrogen, and phosphorus.
复制标题
DOI：
10.1016/j.palaeo.2009.10.024
复制
发表时间：
2010-08
期刊：
影响因子：
--
通讯作者：
K. Yamaguchi;K. Oguri;N. Ogawa;S. Sakai;S. Hirano;H. Kitazato;N. Ohkouchi
中科院分区：
文献类型：
--
作者：
K. Yamaguchi;K. Oguri;N. Ogawa;S. Sakai;S. Hirano;H. Kitazato;N. Ohkouchi
利用日本西南部咸化分生湖Kai-ike沉积物岩心（KAI4）研究了缺氧水体覆盖的碳质沉积物中有机质分解的早期成岩作用。开湖是过去缺氧海洋环境的当前模型，其特点是光区缺氧和“细菌板”的发展，这在水中造成了永久的密度分层。近40年来，随着沉积物深度的增加，有机碳（~ 55%）、氮（~60%）和活性磷（ΔP; ~85%）含量迅速下降，而碳同位素（δ13Corg）稳定同位素组成增加1‰（正偏移），这是由于低δ13Corg组分在早期成岩作用中优先损失所致。摩尔的Corg/ΔP和N/ΔP比值从非常接近Redfield比值的初始值开始随深度增加。在有机质降解的早期成岩作用中，P的分解速率远快于Corg和N。相反，摩尔Corg/N比值对各种生物地球化学反应相当不敏感，证实了其作为有机质来源指标的实用性。虽然沉积OM中氮的稳定同位素组成（δ15N）是OM来源的有用指标，但KAI4岩心与上述参数的相关性尚不清楚。我们强调，古碳质沉积物或沉积岩中存在的OM很可能在其最初埋藏后迅速（<40年）分解，并且沉积时的原始碳、氮和磷含量可能大大高于保存（测量）的含量。碳质沉积物和沉积岩的δ 13corg和δ15N值在成岩作用早期也发生了小程度的正偏移（~1‰）。为了更好地利用沉积物或沉积岩的地球化学（即长、氮、磷）重建氧化还原层状海洋和湖泊的古环境条件，必须考虑早期成岩作用中OM的快速分解对相关生物地球化学指标的影响。该研究对白垩纪海洋缺氧事件和早期地球缺氧环境的地球化学重建具有重要意义。
The early diagenetic effects of organic matter (OM) decomposition in carbonaceous sediments overlain by an anoxic water body were investigated using a sediment core (KAI4) from the saline meromictic Lake Kai-ike, in southwest Japan. Lake Kai-ike, a current model for past anoxic ocean environments, is characterized by development of photic zone anoxia and a “bacterial plate,” which create permanent density stratification in the water. Over the last ~40years, rapid decreases in organic carbon (Corg; ~55%), nitrogen (~60%), and reactive phosphorus (ΔP; ~85%) contents accompanied by ~1‰ increase (positive shift) in the stable isotope compositions of Corg(δ13Corg), which is attributed to a preferential loss of low-δ13Corgcomponents during early diagenesis, are reflected with increasing depth of the sediments. The molar Corg/ΔP and N/ΔP ratios increase with depth, from initial values that are very close to those of the Redfield ratio. During the early diagenesis of OM degradation, the decomposition rate of P is much faster than that of Corgand N. In contrast, the molar Corg/N ratios are rather insensitive to various biogeochemical reactions, confirming their utility as a proxy for source OM. Although the stable isotope compositions of nitrogen (δ15N) in sedimentary OM are useful indicators of OM sources, correlations to the above-mentioned parameters for the KAI4 core remain unclear. We emphasize that the OM present in ancient carbonaceous sediments or sedimentary rocks was most likely subject to very rapid (<40years) decomposition after its initial burial, and that the original Corg, N, and P contents at the time of deposition could have been substantially higher than the preserved (measured) contents. The same cautions may be applied to the δ13Corgand δ15N values of carbonaceous sediments and sedimentary rocks, which also undergo a small degree of positive shift (~1‰) during early diagenesis. To better reconstruct the paleoenvironmental conditions for redox-stratified oceans and lakes using the geochemistries of sediments or sedimentary rocks (i.e., Corg, N, and P), one must take into account how rapid OM decomposition during early diagenesis affects the related biogeochemical proxies. This study has implications for the geochemical reconstructions of past anoxic environments, such as those during the Cretaceous OAEs (oceanic anoxic events) and the early Earth.