Sequential variation of carbonate cycles in South Atlantic DSDP locations

We report well-dated Late Cretaceous and Early Tertiary precessional climatic cycles, recorded by rhythmic carbonate maxima and minima in South Atlantic deep sea sites. Spectral analyses of digitized sediment color, a suitable carbonate proxy, show prominent regularities in the spacing marl-carbonate beds. Magnetostratigraphic dating over a number of magnetic chrons constrains the duration of the cycles, which can be detected over at least 20 Myr of sedimentation at 7 coring locations. Their mean absolute period of 23.5 +/- 4.4kyr agrees closely with the predicted late Cretaceous precessional period of 20.8 kyr. Because they can be matched to a physical forcing mechanism with a known repeat time, the cycles offer a new high-resolution tool to measure rates of climate change before and after the Cretaceous-Tertiary (K/T) boundary. From counts of carbonate cycles, we derive the position of the K/T boundary within C29R at 350 kyr after the base of the reversal. The constancy of cycle thickness (linearly related to sedimentation rate) and amplitude up to the “boundary clay” does not give evidence for climate instability preceding the boundary. Orbital chronometry records a step-function decrease in sediment accumulation rate at the Cretaceous-Tertiary boundary that is consistent with a geologically instantaneous event.

本研究报道了南大西洋深海钻探站位中由节律性碳酸盐含量峰与谷所记录的、定年精确的晚白垩世及早第三纪岁差气候周期（precessional climatic cycles）。针对作为合适碳酸盐替代指标（carbonate proxy）的数字化沉积物颜色开展频谱分析（spectral analyses），结果显示泥灰岩-碳酸盐岩层的间距存在显著规律性。基于多个磁极性带（magnetic chrons）的磁地层定年（Magnetostratigraphic dating），约束了这些周期的持续时长，该周期在7个钻探站位的至少20 Myr沉积记录中均可被识别。这些周期的平均绝对周期为23.5±4.4 kyr，与晚白垩世岁差周期的预测值20.8 kyr高度吻合。由于这些周期可与具有已知重复周期的物理强迫机制相匹配，因此它们为测定白垩纪-第三纪（K/T）边界（Cretaceous-Tertiary (K/T) boundary）前后的气候变化速率提供了一种新型高分辨率工具。通过对碳酸盐周期的计数，我们确定了K/T边界位于C29R磁极性带中，距离该反向极性带底界350 kyr的位置。直至“界线黏土”（boundary clay）层，周期厚度（与沉积速率呈线性相关）与振幅均保持恒定，未发现该边界前存在气候不稳定的证据。轨道计时法（orbital chronometry）记录到，在白垩纪-第三纪边界处沉积堆积速率发生阶跃式下降，这与地质瞬时事件（geologically instantaneous event）的特征相一致。