Detrital zircons reveal sea-level and hydroclimate controls on Amazon River to deep-sea fan sediment transfer

Abstract

We use new U-Pb detrital zircon (DZ) geochronology from the Pleistocene Amazon submarine fan (n = 1352 grains), integrated with onshore DZ age data, to propose a sedimentary model for sea level-modulated and hydroclimate-modulated sediment transfer in Earth's largest source-to-sink system. DZ ages from the modern Amazon River sediment display a progressive downstream dilution by older cratonic zircons, leading to the expectation of a submarine fan with high proportions of craton-derived sediment. Our new DZ age data from the submarine fan and mixture modeling suggest that higher proportions of sediment were supplied from the distant central Andes to the Amazon fan during the last two glacioeustatic lowstands, and thus the observed DZ age spectra of the modern lower Amazon River indicate a relative increase in craton-derived sediment during the Holocene. We interpret that during interglacials, when sea level was high and the submarine fan inactive, the lower Amazon River did not efficiently transfer sand-sized sediment to the margin and thus became enriched in craton-derived sediment. During sea-level lowstands, increased gradients and incision in the lower Amazon River due to base-level lowering resulted in enhanced connectivity and transfer of Andes-sourced zircons to the deep sea. These results are also consistent with interpreted patterns of Andean-Amazon hydroclimate anti-phasing (enhanced precipitation in the central Andes and increased aridity in the northern Amazon Basin) during the Last Glacial Maximum. Our results suggest that sand-sized sediment in the Amazon submarine fan records multi-millennial patterns of sea level and South American hydroclimate.

Quantifying the three‐dimensional stratigraphic expression of cyclic steps by integrating seafloor and deep‐water outcrop observations

“…Submarine canyon–channel‐fan systems are important for transferring sediment and other materials (for example, organic carbon, microplastics and other pollutants) from terrestrial to deep marine environments (Normark et al ., 1993; Hessler & Fildani, 2019; Kane et al ., 2020; Pohl et al ., 2020). Their sedimentary deposits store a wealth of information about the Earth’s history (for example, climate and tectonic events, Clift, 2006; Castelltort et al ., 2017; Daniels et al ., 2018; Fildani et al ., 2018; Mason et al ., 2019), provide a long‐term record of turbidity current events required for seafloor hazard assessments (e.g. Goldfinger, 2011; Clare et al ., 2015, 2016; Jobe et al ., 2018) and host significant natural resources (for example, hydrocarbons, Pettingill & Weimer, 2002).…”

The onset of grasses in the Amazon drainage basin, evidence from the fossil record

“…Harris and Mix (2002) showed that up to 8 Ma (except for the interval 12 to 10 Ma) precipitation rates were relatively high, followed by a shift to drier conditions from 8 Ma. The transition from Neogene to Quaternary coincided with the rise of Poaceae pollen, as observed in the Amazon submarine fan record (Hoorn et al 2017), and also with a general increase in terrigenous Andean sediment (Lammertsma et al 2018;Mason et al, 2019). The further decline in the ratio of oxide minerals from 4.5 Ma, indicative of aridification, is paralleled by a rise in Poaceae and the introduction of high Andean taxa.…”

Denuding a Craton: Thermochronology Record of Phanerozoic Unroofing From the Pilbara Craton, Australia

“…The sediments show a predominance of Mesoproterozoic detrital U/Pb zircon ages, with relatively few Archean grains (e.g., Cawood & Nemchin, 2000), suggesting that despite its close proximity, the extensive Yilgarn Craton was not an important provenance for basin fill. Furthermore, recent detrital records of river systems and coastal deposits adjacent to shields do not contain the expected significant zircon populations given their close location to exposed cratonic areas (e.g., Western Australia Quaternary coastal deposits, Sircombe & Freeman, 1999; Mississippi River, Blum et al, 2017; Amazon River, Mason et al, 2019). One explanation for this lack of detrital contribution from cratons is that their peneplanation was complete before the Quaternary and that denudation rates have been minimal since that time (Sircombe & Freeman, 1999).…”