A 30 Ma history of the Amazon River inferred from terrigenous sediments and organic matter on the Ceará Rise

Abstract

The history of the Amazon River is a much-discussed subject, and the timing of the development of a transcontinental system in particular is a matter of some controversy, with estimations varying between the Early Miocene and the Pliocene or even the Pleistocene. To shed further light on this, we studied the sediment provenance of an Oligocene to Late Pleistocene marine sedimentary section from the Ceará Rise (ODP Site 925), a topographic high in the central Atlantic Ocean, using major element concentrations and Nd isotopic composition in 85 samples. In addition, the carbon isotopic composition of bulk organic matter and changes in the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) were used to identify periods of increased river outflow. On the basis of these results, we suggest that the history of the development of the Amazon River is characterized by specific steps. During the late Oligocene/Early Miocene (30-18.3 Ma), the terrigenous mass accumulation rates (TARs) were high, and sediment and GDGT compositions suggest that a large river system existed, which at times received weathering products from a younger and probably Andean sediment source. A shift to a younger Andean sediment provenance after 8.7 Ma indicates that the Amazon River became permanently connected with the Andes. Between 18.3 and 4.5 Ma, TARs were generally low, and GDGTs were derived for the most part from in situ production in marine waters. Around 4.5 Ma, the river expanded, probably due to ongoing tectonic activity, and uplift in the Andes increased Andean rock erosion. This led to a strong increase in terrigenous sediment deposition and enhanced organic matter preservation on the Ceará Rise, and the delivery of terrestrial (both soil and riverine) branched GDGTs to the Ceará Rise.

Could coastal plants in western Amazonia be relicts of past marine incursions?

“…Our hypothesis is that these candidate plant species growing in western Amazonia are, in a sense, living fossils that can help us to understand the interactions between landscape changes and biodiversity. The current distribution of candidate species and their patterns of richness underscore the interplay between biota and landscape processes, including Andean mountain building and marine incursions (Eakin, Lithgow‐Bertelloni, & Dávila, ; Hoorn, Wesselingh, ter Steege, et al., ; Hoorn et al., ; Shepard, Müller, Liu, & Gurnis, ; van Soelen et al., ).…”

Expedition 387 Scientific Prospectus: Amazon Margin

“…The significance of these points for our recovered record includes the following: (1) prior to the middle Miocene or later, terrigenous sediments on the Amazon margin were derived from small rivers sourced only in the easternmost region of tropical South America (Figure F3A), therefore pollen and other terrigenous proxies recovered offshore from pre-middle Miocene sediments will only record the environments of this eastern region; (2) it is possible that westward headward erosion (of the Gurupá and Purus Arches) progressively broadened the proto-Amazon drainage basin and source region through the course of the Cenozoic; (3) with breaching of the final divide (possibly the Iquitos Arch) somewhere between 10 and ~2 Ma, Andean sediments reached the Atlantic and transcontinental drainage was achieved. This is one of many plausible yet greatly disparate paleogeographic histories Latrubesse et al, 2007Latrubesse et al, , 2010van Soelen et al, 2017), none of which are sufficiently constrained by the sparse available data. Identifying the timing of these events will be elucidated by our recovery of complete sequences of Late Miocene and younger age, more precise determination of ages, and multiproxy records of provenance.…”

“…ODP Leg 154 (Curry, Shackleton, Richter, et al, 1995) drilled five sites in a depth transect from the top (3041 m water depth) to the deep flank (4356 m water depth) of the Ceará Rise, a carbonate sediment-draped aseismic ridge located at ~5°N, ~300 km beyond the offshore edge of the Amazon Fan. Leg 154 recovered sediments from late Paleocene to Holocene age, and these sediments have proven to be of enduring interest in paleoceanography (e.g., Stewart et al, 2016;Wilkens et al, 2017) and paleoclimatology (e.g., Rühlemann et al, 2001;Dobson et al, 2001;Harris and Mix, 2002;van Soelen et al, 2017). The Ceará Rise presently receives a minor input of terrigenous sediment, dominantly from an Amazon source.…”

River Network Rearrangements in Amazonia Shake Biogeography and Civil Security

“…30 ka ago (36,37). This scenario is refuted by the geological record of the submarine fan at the mouth of the Amazon (14,15), which shows that sediments of Andean origin have been depositing at the site for at least 9 Ma. Therefore, another explanation for the geologically young radiocarbon datings within present-day terra firme is needed.…”

“…For animals, large Amazonian rivers have long been regarded as potential dispersal barriers that promote vicariant speciation (10)(11)(12). Rivers can be expected to be more important as dispersal barriers in central Amazonia than in western Amazonia both because river channels and floodplains are wider downstream (13) and because the central Amazonian river network supposedly stabilized already in the Pliocene (14)(15)(16), giving more time and opportunities for vicariant speciation than in western Amazonia, where rivers are still dynamic due to ongoing tectonic activity (5)(6)(7)(8).…”