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Late oligocene-miocene proto-antarctic circumpolar current dynamics off the Wilkes Land margin, East Antarctica
Global and Planetary Change ( IF 3.9 ) Pub Date : 2020-08-01 , DOI: 10.1016/j.gloplacha.2020.103221
Dimitris Evangelinos , Carlota Escutia , Johan Etourneau , Frida Hoem , Peter Bijl , Wilrieke Boterblom , Tina van de Flierdt , Luis Valero , José-Abel Flores , Francisco J. Rodriguez-Tovar , Francisco J. Jimenez-Espejo , Ariadna Salabarnada , Adrián López-Quirós

Abstract 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 CO2 concentrations 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 (eNd) 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.

中文翻译:

南极东部威尔克斯陆缘晚渐新世-中新世原南极环极洋流动力学

摘要 目前,南大洋在全球气候系统和现代南极冰盖动力学中发挥着重要作用。然而,人们对过去的南大洋构造知之甚少。这些信息仍然很重要,因为它可以提供对气候系统和过去在比现在温暖的气候下的冰盖行为的重要见解。在这里,我们研究了渐新世和中新世期间重建的大气 CO2 浓度与本世纪预期的相似时的南大洋动力学。我们重建了南极东部威尔克斯地深海平原晚渐新世到最早的中新世(~24.2-23 Ma)古海洋条件的快照。为此,我们结合了海洋沉积学、地球化学(X 射线荧光,TEX86,),深海钻探项目 (DSDP) 站点 269 回收的海洋沉积物的孢粉学和同位素 (eNd) 记录。总体而言,我们发现在冰期-间冰期循环期间通过重力流和半远洋沉降输送到该站点的沉积物被持续地重新加工由于气候控制的锋面系统迁移,具有不同强度的原环极深水 (CDW)。就在 24 Ma 之前,在弱原始 CDW 强度和底部通风不良条件下沉积的主要是细粒沉积物的陆源输入占主导地位。相比之下,24 Ma 标志着原始 CDW 电流速度增强的偶发事件的开始,与粗粒沉积物和更好的通风底部条件相关。特别是沉积物中 P-cyst 和低钙 (Ca) 的优势介于 ~ 24 之间。2 Ma 和 23.6 Ma 表明存在与高营养条件相关的活跃的公海上升流。这得到了 TEX86 派生的海面温度 (SST) 数据的支持,这些数据指向凉爽的海洋条件。从~23.6 到 23.2 Ma,我们的记录显示沉积物中 Ca 的富集与钙质微化石保存增加、大量 G 囊肿和 TEX86-SST 增加有关。这意味着随着极地锋向南迁移,更温暖的水团到达南极边缘。连同表明现代 CDW 值的放射成因 Nd 同位素数据,我们的记录表明,原始 CDW 在我们的研究地点显着向极地扩张,并且在最新的渐新世(即约 23.2 Ma 前)期间 AABW 形成减少。我们的发现支持了一个根本不同的南大洋的概念,
更新日期:2020-08-01
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