Calcium carbonate, isotopes and Rock-Eval pyrolysis at DSDP Holes 77-535 and 77-540

At DSDP Sites 534 (Central Atlantic) and 535 and 540 (Gulf of Mexico), and in the Vocontian Basin (France), Lower Cretaceous deposits show a very pronounced alternation of limestone and marl. This rhythm characterizes the pelagic background sedimentation and is independent of detritic intercalations related to contour and turbidity currents.
Bed-scale cycles, estimated to be 6000-26,000 yr. long, comprise major and minor units. Their biological and mineralogic components, burrowing, heavy isotopes C and O, and some geochemical indicators, vary in close correlation with CaCO3 content. Vertical changes of frequency and asymmetry of the cycles are connected with fluctuations of the sedimentation rate. Plots of cycle thickness ("cyclograms") permit detailed correlations of the three areas and improve the stratigraphic subdivision of Neocomian deposits at the DSDP sites.
Small-scale alternations, only observed in DSDP cores, comprise centimetric to millimetric banding and millimetric to micrometric lamination, here interpreted as varvelike alternations between laminae that are rich in calcareous plankton and others rich in clay. The laminations are estimated to correspond to cycles approximately 1,3, and 13 yr. in duration.
The cyclic patterns appear to be governed by an interplay of continental and oceanic processes. Oceanic controls express themselves in variations of the biogenic carbonate flux, which depends on variations of such elements as temperature, oxygenation, salinity, and nutrient content. Continental controls modulate the influxes of terrigenous material, organic matter, and nutrients derived from cyclic erosion on land.
Among the possible causes of cyclic sedimentation, episodic carbonate dissolution has been ruled out in favor of climatic fluctuations with a large range of periods. Such fluctuations are consistent with the great geographic extension shown by alternation controls and with the continuous spectrum of scales that characterizes limestone-marl cycles. The climatic variations induced by the Earth's orbital parameters (Milankovitch cycles) could be connected to bed-interbed alternations.

在深海钻探计划（Deep Sea Drilling Project, DSDP）的534站位（大西洋中部）、535与540站位（墨西哥湾），以及法国沃康松盆地（Vocontian Basin）中，下白垩统地层均发育特征显著的石灰岩与泥灰岩互层序列。这种沉积节律代表了远洋背景沉积作用的典型特征，且与等深流、浊流相关的碎屑夹层无关。
层序尺度旋回的估算时长为6000~26000年，可划分为一级与次级单元。其生物、矿物组成，生物潜穴，碳、氧重同位素以及部分地球化学指标，均与碳酸钙（Calcium Carbonate, CaCO₃）含量呈现显著的相关性变化。旋回的出现频率与垂向不对称性的变化，与沉积速率的波动密切相关。通过旋回厚度绘制的旋回曲线图（cyclograms），可实现三个研究区域的高精度地层对比，并完善了DSDP站位处凡兰吟阶（Neocomian）沉积的地层划分方案。
仅在DSDP岩心中观测到的小型互层序列，表现为厘米级至毫米级条带与毫米至微米级纹层，本文将其解释为钙质浮游生物富集纹层与黏土富集纹层之间的纹泥状互层。经估算，这些纹层对应的旋回时长分别约为1年、3年与13年。
上述旋回模式似乎受大陆与海洋过程的共同调控。海洋控制因素体现为生物碳酸盐通量的变化，而该通量又取决于温度、溶氧度、盐度以及营养盐含量等要素的波动；大陆控制因素则调节了陆源物质、有机质以及由陆地周期性侵蚀产生的营养盐的输入量。
在可能的旋回沉积成因中，间歇性碳酸盐溶解已被排除，而大范围周期的气候波动成为更合理的解释。这类气候波动与互层序列所体现的广泛地理分布，以及石灰岩-泥灰岩旋回所具有的连续尺度谱特征均相吻合。由地球轨道参数驱动的气候波动（米兰科维奇旋回，Milankovitch cycles），或与层序-夹层互层现象存在关联。