Age models and foraminifer isotopes of four ODP Leg 202 sites

We present benthic isotope stratigraphies for Sites 1236, 1237, 1239, and 1241 that span the late Miocene-Pliocene time interval from 6 to 2.4 Ma. Orbitally tuned timescales were generated for Sites 1237 and 1241 by correlating the high-frequency variations in gamma ray attenuation density, percent sand of the carbonate fraction, and benthic d13C to variations in Earth's orbital parameters. The astronomical timescales for Sites 1237 and 1241 are in agreement with the one from Atlantic Site 925/926 (Ocean Drilling Program Leg 154). The comparison of benthic d18O and d13C records from the east Pacific sites and Atlantic Site 925/926 revealed a surprising clarity of the "41-k.y. signal" in d13C records and a remarkably good correlation between their d13C records. This suggests that the late Miocene-Pliocene amplitudes of obliquity-related d13C cycles reflect a magnitude of global response often larger than that provided by obliquity-related d18O cycles. At Site 1237, the orbitally derived ages of Pliocene magnetic reversal boundaries between the base of Réunion and the top of Thvera confirm astronomical datings of the generally accepted ATNTS2004 timescale, except for the top of Kaena and the base of Sidufjall. Our astronomical age for the top of Kaena is about one obliquity cycle older. The base of Sidufjall appears to be about one precession cycle younger. The age models of Sites 1236 and 1239 were established by correlating their benthic d18O and d13C records directly to the orbitally tuned isotope record of Site 1241.

本研究构建了1236、1237、1239及1241站位的底栖同位素地层学（benthic isotope stratigraphies）序列，时间覆盖晚中新世-上新世（6 Ma至2.4 Ma）时段。针对1237和1241站位，研究团队通过将伽马射线衰减密度（gamma ray attenuation density）、碳酸盐组分砂粒占比以及底栖碳同位素（benthic δ¹³C）的高频变化与地球轨道参数（Earth's orbital parameters）变化进行对比，构建了轨道调谐年代标尺。1237和1241站位的天文年代标尺与大西洋925/926站位（大洋钻探计划第154航次）的天文年代标尺一致。通过对比东太平洋各站位与大西洋925/926站位的底栖氧同位素（benthic δ¹⁸O）与底栖碳同位素记录，研究发现碳同位素记录中清晰呈现出“41千年信号”，且各站位的碳同位素记录之间具有极佳的相关性，该结果颇具意外性。这表明晚中新世-上新世时期，与斜度（obliquity）相关的碳同位素循环振幅所反映的全球响应幅度，通常大于斜度相关的氧同位素循环所反映的幅度。在1237站位，留尼旺（Réunion）极性亚时底界与瑟夫拉（Thvera）极性亚时顶界之间的上新世地磁反转界线的轨道推算年龄，与公认的ATNTS2004天文年代标尺的标定结果相符，仅卡埃纳（Kaena）极性亚时顶界和西杜菲约尔（Sidufjall）极性亚时底界除外。本研究得到的卡埃纳极性亚时顶界天文年龄比ATNTS2004标尺结果约老一个斜度周期，而西杜菲约尔极性亚时底界的天文年龄则比原标尺结果约年轻一个岁差（precession）周期。1236和1239站位的年龄模型，则是通过将其底栖氧同位素与碳同位素记录直接调谐至1241站位的轨道调谐同位素记录而建立的。