(Table 4) Mass accumulation rates of selected components in red, green, and black lithologies at DSDP Hole 75-530A

Deep-water sedimentary sequences of mid-Cretaceous age, rich in organic carbon, have been recovered at many DSDP sites in the Atlantic Ocean. Most of these sequences have a marked cyclicity in amount of organic carbon resulting in interbedded multicolored shale, marlstone, and (or) limestone that have cycle periods of 20,000 to 100,000 years and average 40,000 to 50,000 years. These cycles may be related to some climatic control on influx of terrigenous organic matter and sediment, rates of upwelling and sea-surface production of organic matter, and preservation of organic matter related to deeper-water dissolved oxygen concentration. These variations in supply of organic matter had pronounced effects on the potential of the sediment for subsequent diagenetic changes and geochemical partitioning in adjacent beds. Many trace elements are enriched in organic-carbon-rich lithologies relative to interbedded organic-carbon-poor lithologies. Elements that are most commonly enriched are Cr, Ni, V, Cu, Zn, and Mo. The association of high traceelement concentrations with organic matter may be the result of concentration of these elements by organisms or by chemical sorption and precipitation processes under anoxic conditions. Detailed trace-element profiles from organiccarbon- rich strata at Site 530 suggest that there may be differential mobility of trace elements, with diffusion of some elements over distances of at least tens of meters. The sequence of trace-element mobility, from highest to lowest, is approximately Ba, Mn, Pb, Ni, Co, Cr, Cu, Zn, V, Cd, and Mo. Slowly deposited, oxidized clays directly overlying some black shale sequences are enriched in some metals, particularly Fe, Mn, Zn, and Cu, relative to normal pelagic clays, and this enrichment may be the result of upward migration of metals in pore waters during compaction or diffusion from the underlying black shale. Most depositional models that have been used to explain the accumulation of the organic-carbon-rich strata imply that reducing conditions in the sediments (and therefore the increased degree of preservation of organic matter) were the result of anoxic or near-anoxic conditions in oceanic bottom waters, or in a midwater oxygen-minimum zone. Evidence from several DSDP sites in the Atlantic, however, indicate that some of these middle Cretaceous "black shale" beds may be the result of variations in rate of supply of organic matter that produced anoxia or near-anoxia within midwater oxygen-minimum zones and possibly, under extreme conditions, throughout much of the bottomwater mass. Although bottom-water anoxia may have occurred during periods of organic-carbon-rich strata, it was not necessarily the only cause for accumulation of these strata. The main reason for the accumulation of organic-carbonrich strata was an increase in the relative amount of organic debris being deposited. Some of this organic debris was derived from continental-margin areas of increased production, accumulation, and preservation of organic matter from marine, terrestrial, or mixed sources and transported to slope and basinal sites by turbidity currents.大西洋海域的诸多深海钻探计划（Deep Sea Drilling Project, DSDP）站位均已获取富含有机碳的白垩纪中期深水沉积序列。此类序列中的有机碳含量多呈现显著旋回性，形成互层状的多彩页岩、泥灰岩以及（或）灰岩，旋回周期介于2万年至10万年之间，平均周期为4万至5万年。这些旋回可能与以下气候调控因素相关：陆源有机质与沉积物的输入通量、上升流速率及海面有机质生产效率，以及与深水溶解氧浓度相关的有机质保存条件。有机质供给的上述变化，对沉积物后续成岩作用潜力及相邻地层的地球化学分异产生了显著影响。 相较于互层中的贫有机碳岩性，富有机碳岩相中富集了多种微量元素。通常富集的元素包括铬（Cr）、镍（Ni）、钒（V）、铜（Cu）、锌（Zn）及钼（Mo）。高微量元素浓度与有机质的伴生关系，可能源于生物对这些元素的富集作用，或是缺氧条件下的化学吸附与沉淀过程。530站位富有机碳地层的详细微量元素剖面显示，微量元素可能存在差异迁移性，部分元素的扩散距离至少可达数十米。微量元素迁移性从高到低的序列大致为：钡（Ba）、锰（Mn）、铅（Pb）、镍（Ni）、钴（Co）、铬（Cr）、铜（Cu）、锌（Zn）、钒（V）、镉（Cd）及钼（Mo）。直接覆于部分黑色页岩序列之上的缓沉积氧化黏土，相较于正常远洋黏土，富集了部分金属元素，尤其是铁（Fe）、锰（Mn）、锌（Zn）及铜（Cu），这种富集可能源于压实作用下孔隙水中金属的向上迁移，或是来自下伏黑色页岩的扩散作用。 以往用于解释富有机碳地层堆积的多数沉积模式均认为，沉积物中的还原环境（即有机质保存程度提升）是由海底水体缺氧或近乎缺氧，或是中层水含氧最低带所导致。然而，大西洋海域诸多DSDP站位的证据表明，部分白垩纪中期的"黑色页岩"地层，可能是由于有机质供给速率的变化在中层水含氧最低带内引发缺氧或近乎缺氧环境所致；极端情况下，甚至可能波及大部分底水团。尽管在富有机碳地层形成时期可能出现底水缺氧，但这并非此类地层堆积的唯一成因。富有机碳地层堆积的主要原因是沉积有机碎屑的相对占比提升。其中部分有机碎屑来自有机质生产、堆积及保存程度提升的大陆边缘区域，其来源涵盖海洋、陆相或混合源，并通过浊流搬运至斜坡与盆地站位。