A lower to middle Eocene astrochronology for the Mentelle Basin (Australia) and its implications for the geologic time scale
A lower to middle Eocene astrochronology for the Mentelle Basin (Australia) and its implications for the geologic time scale
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DOI：
10.1016/j.epsl.2019.115865
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发表时间：
2020-01
影响因子：
5.3
通讯作者：
M. Vahlenkamp;D. De Vleeschouwer;S. Batenburg;K. Edgar;E. Hanson;Mathieu Martinez;H. Pälike;K. MacLe
中科院分区：
文献类型：
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作者：
M. Vahlenkamp;D. De Vleeschouwer;S. Batenburg;K. Edgar;E. Hanson;Mathieu Martinez;H. Pälike;K. MacLe
新生代的地质时间尺度在过去几十年中得到了显着改善，这得益于综合地层学，将高分辨率天文年代学、生物地层学和磁性地层学与高精度放射性同位素日期相结合。然而，始新世中期仍然是一个薄弱环节。所谓的“始新世时间尺度差距”反映了缺乏合适的研究剖面，这些剖面具有明显的天文强迫碳酸盐含量变化，主要是因为在始新世温室期间大部分海洋缺乏碳酸盐。国际海洋发现计划 (IODP) 369 号探险队对 Mentelle 盆地（澳大利亚西南近海 U1514 地点）始新世富含碳酸盐的沉积序列进行了取芯。该序列由超微化石白垩组成，并表现出有节奏的粘土含量变化。在这里，我们展示了 IODP 站点 U1514 允许使用 3 厘米分辨率的 X 射线荧光 (XRF) 核心扫描提取天文信号并构建始新世天文年代学。 XRF 得出的钙和铁含量 (Ca/Fe) 之间的比率可追踪岩性变异性，并作为我们 U1514 天文年代学的基础。我们展示了长达 1600 万年（40-56 Ma）的始新世沉积几乎连续的历史，其变化随偏心率和倾斜度而变化。我们用低分辨率散装碳和氧同位素补充高分辨率 XRF 数据，记录从古新世-始新世热最大值（PETM – 约 56 Ma）到始新世中期（约 40 Ma）的长期冷却趋势。我们的早期始新世天文年表证实了基于深海遗址和意大利陆地部分的现有年表。对于始新世中期，U1514 的沉积学记录提供了单点地球化学主干，从而为在轨道分辨率下完全整合新生代地质时间尺度迈出了进一步的一步。
The geologic time scale for the Cenozoic Era has been notably improved over the last decades by virtue of integrated stratigraphy, combining high-resolution astrochronologies, biostratigraphy and magnetostratigraphy with high-precision radioisotopic dates. However, the middle Eocene remains a weak link. The so-called “Eocene time scale gap” reflects the scarcity of suitable study sections with clear astronomically-forced variations in carbonate content, primarily because large parts of the oceans were starved of carbonate during the Eocene greenhouse. International Ocean Discovery Program (IODP) Expedition 369 cored a carbonate-rich sedimentary sequence of Eocene age in the Mentelle Basin (Site U1514, offshore southwest Australia). The sequence consists of nannofossil chalk and exhibits rhythmic clay content variability. Here, we show that IODP Site U1514 allows for the extraction of an astronomical signal and the construction of an Eocene astrochronology, using 3-cm resolution X-Ray fluorescence (XRF) core scans. The XRF-derived ratio between calcium and iron content (Ca/Fe) tracks the lithologic variability and serves as the basis for our U1514 astrochronology. We present a 16 million-year-long (40-56 Ma) nearly continuous history of Eocene sedimentation with variations paced by eccentricity and obliquity. We supplement the high-resolution XRF data with low-resolution bulk carbon and oxygen isotopes, recording the long-term cooling trend from the Paleocene-Eocene Thermal Maximum (PETM – ca. 56 Ma) into the middle Eocene (ca. 40 Ma). Our early Eocene astrochronology corroborates existing chronologies based on deep-sea sites and Italian land sections. For the middle Eocene, the sedimentological record at U1514 provides a single-site geochemical backbone and thus offers a further step towards a fully integrated Cenozoic geologic time scale at orbital resolution.