Cyclic sediment deposition by orbital forcing in the Miocene wetland of western Amazonia? New insights from a multidisciplinary approach

Authors: Carina Hoorn*, Tyler Kukla, Giovanni Bogotá-Angel, Els van Soelen, Catalina González-Arango, Frank P. Wesselingh, Hubert Vonhof, Pedro Val, Gaspar Morcote-Rios, Martin Roddaz, Elton Luiz Dantas, Roberto Ventura Santos, Jaap S. Sinninghe Damsté, Jung-Hyun Kim, Robert J. Morley*

Supplementary data

In the Miocene, a large wetland system extended from the Andean foothills into western Amazonia. This system has no modern analogue and the driving mechanisms are not yet fully understood. In this study we investigate the presumed orbital cyclicity that controlled sediment deposition, while also assessing sediment source and biomes in the Miocene wetland. We do this by integrating lithological, palynological, malacological and geochemical data from the Los Chorros site (Amazon River, Colombia), and by placing our data in a sequence stratigraphic framework. In this sequence biostratigraphic evaluation, the Los Chorros succession is visualised to be composed of a series of flood-fill packages, with a rapid initial flood, marine-influenced conditions at the time of maximum flood, followed by a longer regressive infill phase.
Marine, mangrove, and lacustrine indicators suggest that the outcrops at Los Chorros represent predominant marine-influenced lacustrine conditions during periods of sea level highstand. The sequence biostratigraphic evaluation further points to eight 41 kyr obliquity-driven depositional cycles, with rapid phases of transgression. Mangrove elements would have colonised within the timeframe of each sea level rise. Based on this relative age constraint and comparison to regional records, deposition likely took place prior to the 13.8 Myr global sea level fall, and most likely during the period just after 14.5 Ma, towards the end of the Middle Miocene Climatic Optimum (MMCO). Palynological evidence further suggests that to the west, surface elevation ranged from ~1000 up to ~3500 m and hosted protoparamo vegetation, the oldest yet reported and in agreement with predictions from molecular studies. In contrast, contemporaneous sites to the northeast of the wetland consisted of fluvial and cratonic formations, as shown by their Nd and Sr isotopic sediment signature. In summary, our data lead to an improved understanding of how geological and astronomical mechanisms controlled the floral and faunal distribution and controlled sediment deposition in western Amazonia during the middle Miocene. As Miocene conditions strongly contrast with modern western Amazonia, our data provide an important context for the deep time history and evolution of the modern western Amazon rainforest.

作者：Carina Hoorn*、Tyler Kukla、Giovanni Bogotá-Angel、Els van Soelen、Catalina González-Arango、Frank P. Wesselingh、Hubert Vonhof、Pedro Val、Gaspar Morcote-Rios、Martin Roddaz、Elton Luiz Dantas、Roberto Ventura Santos、Jaap S. Sinninghe Damsté、Jung-Hyun Kim、Robert J. Morley*

补充数据

中新世时期，大型湿地系统从安第斯山麓延伸至亚马孙西部区域。该湿地系统无现代同类生态系统可类比，其形成驱动机制尚未完全阐明。本研究旨在探究调控沉积物沉积的轨道旋回（orbital cyclicity）周期，同时评估该中新世湿地的沉积物源区与生物群区。研究整合了哥伦比亚亚马孙河洛斯乔罗斯（Los Chorros）遗址的岩性、孢粉学、软体动物学及地球化学数据，并将数据置于层序地层学（sequence stratigraphic）框架下开展分析。通过本次层序生物地层学（sequence biostratigraphic）评估，洛斯乔罗斯沉积序列被揭示由一系列洪水充填旋回构成：初始阶段为快速洪水充填过程，最大洪水期处于受海洋影响的环境，随后进入时长更长的退积充填阶段。

海洋、红树林及湖相沉积指标显示，洛斯乔罗斯露头在海平面高位期主要处于受海洋影响的湖相环境。层序生物地层学评估进一步揭示了8个由41千年斜率旋回驱动的沉积周期，伴随快速海侵阶段。红树林类群应在每次海平面上升期完成定植。结合该相对年代约束与区域记录对比，本次沉积作用应发生于13.8百万年全球海平面下降事件之前，最可能处于14.5 Ma（中新世中期气候适宜期（Middle Miocene Climatic Optimum, MMCO））末期前后。孢粉学证据进一步表明，湿地西侧的地表海拔介于约1000米至3500米之间，发育原帕拉莫（protoparamo）植被——这是目前已报道的最古老原帕拉莫植被记录，与分子生物学研究的预测结果一致。与之形成对比的是，该湿地东北侧的同期遗址以河流与克拉通地层为主，这一点可由其Nd、Sr同位素沉积物指纹特征得到证实。

综上，本研究数据增进了我们对中新世中期地质与天文机制如何调控亚马孙西部动植物分布与沉积物沉积过程的理解。由于中新世环境与现代亚马孙西部环境差异显著，本研究数据为现代亚马孙西部雨林的深时演化历史提供了重要背景参考。