Late Oligocene-Miocene proto-Antarctic Circumpolar Current dynamics off the Wilkes Land margin, East Antarctica
Late Oligocene-Miocene proto-Antarctic Circumpolar Current dynamics off the Wilkes Land margin, East Antarctica
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DOI：
10.1016/j.gloplacha.2020.103221
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发表时间：
2020-08
影响因子：
3.9
通讯作者：
D. Evangelinos;C. Escutia;J. Etourneau;F. Hoem;P. Bijl;W. Boterblom;T. Flierdt;L. Valero;J. Flores;F. Rodríguez-Tovar;F. Jiménez-Espejo;Ariadna Salabarnada;Adrián López‐Quirós
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作者：
D. Evangelinos;C. Escutia;J. Etourneau;F. Hoem;P. Bijl;W. Boterblom;T. Flierdt;L. Valero;J. Flores;F. Rodríguez-Tovar;F. Jiménez-Espejo;Ariadna Salabarnada;Adrián López‐Quirós
目前，南大洋在全球气候系统和现代南极冰盖动态中发挥着重要作用。然而，人们对过去南大洋的形态知之甚少。这些信息仍然很重要，因为它可能为气候系统和过去在比现在更温暖的气候下的冰盖行为提供重要的见解。在这里，我们研究南大洋动力学在渐新世和中新世重建大气CO2浓度时，预计在本世纪。我们重建了东南极威尔克斯地深海平原渐新世晚期至中新世早期（~24.2-23 Ma）古海洋学条件的快照。为此，我们结合了联合收割机海洋沉积学，地球化学（X射线荧光，TEX 86，），孢粉学和同位素（εNd）记录从海洋沉积物回收深海钻探项目（DSDP）站点269。总的来说，我们发现，沉积物，交付到该网站的重力流和半远洋沉积在冰川间冰期周期，持续改造的原绕极深水（CDW）具有不同的强度，导致气候控制的锋面系统迁移。就在24马，陆源输入的主要是细粒沉积物沉积在弱原CDW强度和通风不良的底部条件下占主导地位。相比之下，24 Ma标志着增强的原始CDW流速的幕式事件的开始，与粗粒沉积物和更好的通风底部条件。特别是，在~ 24.2 Ma和23.6 Ma之间的沉积物中P-囊和低钙（Ca）的优势表明存在与高营养条件相关的活跃的开阔洋上升流。这得到了TEX 86衍生的海表温度（SST）数据的支持，表明海洋条件凉爽。从~ 23.6到23.2 Ma，我们的记录显示，钙在沉积物中的富集与增加钙质微体化石保存，大量的G-孢囊和增加TEX 86-SST。这意味着随着极锋向南迁移，温暖的水团到达南极边缘。再加上放射性Nd同位素数据表明现代一样的CDW值，我们的记录表明一个突出的proto-CDW在我们的研究地点向极扩展和减少AABW形成在最新的渐新世（即~23.2 Ma前）。我们的研究结果支持了一个根本不同的南大洋的概念，一个较弱的原ACC比目前在渐新世晚期和最早的中新世。
At present, the Southern Ocean plays an important role in the global climate system and in modern Antarctic ice sheet dynamics. Past Southern Ocean configurations are however poorly understood. This information is yet important as it may provide important insights into the climate system and past ice-sheet behavior under warmer than present day climates. Here we study Southern Ocean dynamics during the Oligocene and Miocene when reconstructed atmospheric CO2concentrations were similar to those expected during this century. We reconstruct snapshots of late Oligocene to earliest Miocene (~24.2–23 Ma) paleoceanographic conditions in the East Antarctic Wilkes Land abyssal plain. For this, we combine marine sedimentological, geochemical (X-ray fluorescence, TEX86,), palynological and isotopic (εNd) records from ocean sediments recovered at Deep Sea Drilling Project (DSDP) Site 269. Overall, we find that sediments, delivered to the site by gravity flows and hemipelagic settling during glacial-interglacial cycles, were persistently reworked by a proto-Circumpolar Deep Water (CDW) with varying strengths that result from climatically controlled frontal system migrations. Just prior to 24 Ma, terrigenous input of predominantly fine-grained sediments deposited under weak proto-CDW intensities and poorly ventilated bottom conditions dominates. In comparison, 24 Ma marks the start of episodic events of enhanced proto-CDW current velocities, associated with coarse-grained deposits and better-ventilated bottom conditions. In particular, the dominance of P-cyst and low Calcium (Ca) in the sediments between ~ 24.2 Ma and 23.6 Ma indicate the presence of an active open ocean upwelling associated with high nutrient conditions. This is supported by TEX86-derived sea surface temperature (SST) data pointing to cool ocean conditions. From ~ 23.6 to 23.2 Ma, our records reveal an enrichment of Ca in the sediments related to increased calcareous microfossil preservation, high amounts of G-cysts and increasing TEX86-SSTs. This implies warmer water masses reaching the Antarctic margin as the polar front migrated southward. Together with the radiogenic Nd isotope data indicating modern-like CDW values, our records suggest a prominent poleward expansion of proto-CDW over our study site and reduced AABW formation during the latest Oligocene (i.e. ~23.2 Ma ago). Our findings support the notion of a fundamentally different Southern Ocean, with a weaker proto-ACC than present during the late Oligocene and the earliest Miocene.