High resolution cyclostratigraphy of the early Eocene – new insights into the origin of the Cenozoic cooling trend
High resolution cyclostratigraphy of the early Eocene – new insights into the origin of the Cenozoic cooling trend
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DOI：
10.5194/cp-5-309-2009
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发表时间：
2009-07
影响因子：
4.3
通讯作者：
T. Westerhold;U. Röhl
中科院分区：
文献类型：
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作者：
T. Westerhold;U. Röhl
抽象的。在这里，我们展示了基于从热带西大西洋（德梅拉拉海隆，ODP Leg 207，站点 1258）检索到的新记录的 X 射线荧光 (XRF) 岩心扫描数据的高分辨率旋回地层学。 ODP 站点 1258 的始新世沉积物涵盖磁时线 C20 至 C24，并显示出发育良好的旋回。该记录包括重新评估地磁极性时间尺度（GPTS）早始新世部分的缺失间隔，还为重建早始新世气候最佳期（EECO）期间的高分辨率气候变率提供了关键方面。详细的光谱分析表明，早期始新世沉积旋回的特征是进动频率受短偏心率（100 kyr）和长偏心率（405 kyr）调制，通常具有较小的倾角分量。对进动和偏心率周期的计数会导致磁同步器 C21r 至 C24n 持续时间的修正估计。我们的旋回地层框架还证实，始新世绿河组（美国怀俄明州）的地质年代学仍然存在疑问，主要是由于“第六凝灰岩”与 GPTS 的相关性不确定。就在长期新生代冷却趋势开始时，ODP 1258 号站点的主要偏心率调制进动周期被强倾角周期打断，在磁同步器 C22r 中部持续约 800 kyr。这个低纬度地点的这些独特的倾角周期表明（1）从50.1到49.4 Ma的全球气候变化的高纬度驱动机制，以及（2）似乎与大气二氧化碳浓度显着下降到低于工业化前水平（PAL）2到3倍之间的临界阈值一致。这里新发现的低偏心率轨道结构与大约 800 kyr 期间的高倾角振幅相结合，以及临界 pCO2 阈值的跨越可能导致早在大约 50 Ma 前南极洲第一片短暂冰盖的形成。
Abstract. Here we present a high-resolution cyclostratigraphy based on X-ray fluorescence (XRF) core scanning data from a new record retrieved from the tropical western Atlantic (Demerara Rise, ODP Leg 207, Site 1258). The Eocene sediments from ODP Site 1258 cover magnetochrons C20 to C24 and show well developed cycles. This record includes the missing interval for reevaluating the early Eocene part of the Geomagnetic Polarity Time Scale (GPTS), also providing key aspects for reconstructing high-resolution climate variability during the Early Eocene Climatic Optimum (EECO). Detailed spectral analysis demonstrates that early Eocene sedimentary cycles are characterized by precession frequencies modulated by short (100 kyr) and long (405 kyr) eccentricity with a generally minor obliquity component. Counting of both the precession and eccentricity cycles results in revised estimates for the duration of magnetochrons C21r through C24n. Our cyclostratigraphic framework also corroborates that the geochronology of the Eocene Green River Formation (Wyoming, USA) is still questionable mainly due to the uncertain correlation of the "Sixth tuff" to the GPTS. Right at the onset of the long-term Cenozoic cooling trend the dominant eccentricity-modulated precession cycles of ODP Site 1258 are interrupted by strong obliquity cycles for a period of ~800 kyr in the middle of magnetochron C22r. These distinct obliquity cycles at this low latitude site point to (1) a high-latitude driving mechanism on global climate variability from 50.1 to 49.4 Ma, and (2) seem to coincide with a significant drop in atmospheric CO2 concentration below a critical threshold between 2- and 3-times the pre-industrial level (PAL). The here newly identified orbital configuration of low eccentricity in combination with high obliquity amplitudes during this ~800-kyr period and the crossing of a critical pCO2 threshold may have led to the formation of the first ephemeral ice sheet on Antarctica as early as ~50 Ma ago.