\n Deep-sea fan sediments carry physical and chemical signatures that reflect the conditions under which their native landscapes evolved. Fans are long-lived, strongly net-depositional heaps of terrigenous debris, in some cases directly connected to a major river catchment. Here we use recent findings from deep-sea fans around the world to reason that modern fans might be our most continuous record of Cenozoic landscape change over large geographic scales. We opt for the use of multiple signatures that indicate major shifts in composition, because this approach avoids the problems inherent to measuring sediment flux alone. We emphasize the importance of looking across grain size, in particular at the clay fraction, the fastest carrier of sedimentary signals. Highlighted cases show the range of environmental signals preserved in deep-sea fans, observed over millennial and longer time scales, and how these signals help us understand the climate–surface interactions important to the carbon cycle. Revisiting legacy core with new techniques, in addition to future drilling campaigns, can provide the observational constraints needed to fill recognized gaps in climate models and landscape–erosion projections.

Deep-sea fans: tapping into Earth's changing landscapes

Integrated, multi-method provenance studies of siliciclastic sedimentary deposits are increasingly used to reconstruct the history of source-to-sink transport, paleogeography, and tectonics. Invariably, analysis of large-scale depositional systems must confront issues regarding how to best sample the system and adequately cope with the details of sediment mixing. Potential biases including variations in grain size, sediment flux, and mineral concentration may cause provenance tracking tools to misrepresent the contributions of source-areas that contribute to large drainage networks. We have acquired U-Pb detrital zircon data from modern sand and whole-rock geochemistry from mud sampled from the Sacramento-San Joaquin drainage of central California to elucidate conditions that can skew provenance trends along the course of a major river system. This drainage network is fed by headwaters that tap the Mesozoic pluton-dominated southern Sierra Nevada, the Paleozoic-Mesozoic wallrock and volcanic-dominated northern Sierra Nevada, the ultramafic-dominated eastern Klamath Mountains, and the intermediate to mafic Cascades volcanic arc. Analysis of the results indicates that detrital zircon provenance trends effectively record source variations for the southern, granite-dominated portion of the drainage network where contrasts in lithology and inferred zircon fertility are relatively minor. In these circumstances, mixture modeling of U-Pb detrital zircon data calibrated with a measure of zircon fertility approximates relative sediment flux contributed by individual drainages. Alternatively, in the northern parts of the system, source regions underlain by ultramafic and/or volcanic rocks are poorly represented, or entirely missing, in downstream detrital zircon records. In some cases, mud geochemistry data more faithfully represent sediment provenance trends. Sampling performed at the confluence of the Sacramento, American, Mokelumne, and San Joaquin Rivers within the Sacramento-San Joaquin Delta region yields a detrital zircon age distribution that is indistinguishable from that of an independently established database of Sierra Nevada batholith crystallization ages. However, when the combined river flows along a recently established passage to the Pacific through the San Francisco Bay region, dredged sediment is found to be significantly contaminated by locally eroded material from the Franciscan Complex and other rocks that crop out within the Coast Ranges. Large variation of Zr concentrations measured throughout the Bay area document that significant hydrodynamic fractionation impacts sediment delivery through this segment of the system. The more Sierra Nevada-like detrital zircon age distribution yielded by a piston-core sample from the continental slope may be explained by either early-stage unroofing of the Coast Ranges or more efficient sand delivery from the delta to the Pacific by a free-flowing river driven by a low stand in sea level.

Dilution and propagation of provenance trends in sand and mud: Geochemistry and detrital zircon geochronology of modern sediment from central California (U.S.A.)

Poaceae (the grass family) originated in the Cretaceous, but first dominate the palynological records of the Amazon drainage basin (ADB) in the Neogene (23 to 2.5 million years ago (Ma)). However, the ecological role of grasses in the landscape during this time remains to be resolved. In this paper, we summarise the global significance of grasses and the relevance of the fossil record, and evaluate the history of the grasses in the ADB. We present a 3-stage model of the changing role of grasses, which we based on a revision of Neogene depositional environments, the palynological record, and modern grass distribution in the Neotropics. Our model comprises the following hypotheses: (H1) assumes that from c. 23 to 9 Ma western Amazonia was dominated by a megawetland (the ‘Pebas system’) that harboured large amounts of (aquatic?) grasses. In (H2) we propose that from c. 9 Ma Andean uplift prompted megafans (extremely large alluvial fans) that extended from the Andes into the lowlands. Meanwhile, the ‘Pebas’ megawetland gradually transformed into a fluvial system. In this scenario, grasses would have had a competitive advantage and were able to colonise the newly formed megafan and fluvial landscapes. Finally, in (H3) we suggest that landscape dynamics and climatic change intensified from c. 3.5 Ma, allowing for a renewed expansion of the grasses. In addition, both the fossil and molecular records suggest that from c. 5 Ma grasses were firmly established in the tropical alpine vegetation (paramo), the tropical lowland floodplains (varzeas), and savannas (cerrado). Although further study will have to confirm the precise nature of the ADB grass history, we anticipate that abiotic processes during the Neogene and Quaternary left a strong imprint in the grass phytogeography of northern South America.

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

Continental drainage systems archive complex records of rock uplift, source area relief, precipitation, glaciation, and carbon cyclicity driven largely by tectonics and climate. Significant progress has been made in linking such external environmental forcings to the geomorphic expression of landscapes and the stratigraphic record of depositional basins in coastal and offshore areas. However, there are large uncertainties in the degree to which sediment dispersal processes can modify signals between the erosional sources and the depositional sinks. We investigate a Holocene sediment transfer zone with contrasting fluvial and eolian sediment transport mechanisms to understand how river and wind processes impact the propagation of environmental signals in continental-scale drainage systems. To quantify these processes, we employ sediment fingerprinting methods for unconsolidated sand samples (detrital zircon U-Pb geochronology), incorporate sediment mixing models, and correlate the findings with the regional geologic and geomorphic framework. Three contrasting source regions deliver sediment to the Andean foreland: volcanic rocks of the Frontal Cordillera, sedimentary rocks of the Precordillera, and metamorphic basement of the Sierras Pampeanas. Although all samples of Holocene eolian dunes accurately record sediment input from three fluvial source regions, spatial variations in U-Pb results are consistent with north-directed paleowinds, whereby river sediments from Frontal Cordillera sources are transported northward and progressively mixed with river sediments from Precordillera and Sierras Pampeanas sources. In contrast, samples of modern rivers show progressive southward (downstream) mixing along a large axial fluvial system. Sediment mixing induced by eolian transport and reworking of various sources is likely a critical, climate-modulated process in the propagation of environmental signals, potentially involving the aliasing of tectonic signals, local storage and recycling of synorogenic river sediment, and cyclical patterns of sediment starvation and delivery to distal zones of accumulation.

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Fluvial and eolian sediment mixing during changing climate conditions recorded in Holocene Andean foreland deposits from Argentina (31-33° S)

Understanding ancient tectonic settings through detrital zircon analysis

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. INTRODUCTION The dynamics of fluvial to deep-sea sediment transfer in response to climate and sea-level fluctuations over multi-millennial time scales is complex and nonlinear (Blum and Törnqvist, 2000; Clift, 2006; Blum and Hattier-Womack, 2009). Early research on major fluvial systems focused on the role of base level (Vail et al., 1977) in the context of climate change (Fisk, 1944), while subsequent research modified conceptual models in recognition of the influence of upstream climate-modulated sediment supply and hydrology (Saucier, 1996). Other studies have considered the controls on the timing of submarine fan activity (Covault and Graham, 2010; Maslin, 2009), sediment caliber and volumes delivered to submarine fans (Sweet and Blum, 2016), as well as sediment exhumation and transit rates and the role of river avulsions in sediment transfer to the deep sea (Blum et al., 2018). Provenance techniques applied to major river–to–submarine fan systems are a valuable tool to improve models for the controls on source-to-sink sediment routing system behavior over multi-millennial time scales (Fildani et al., 2018). The Amazon River and deep-sea fan represent a globally important end-member system for understanding land-to-sea sediment dynamics. Submarine fans have been shown to archive histories of output fluxes and onshore environmental change (Normark and Reid, 2003; Fildani et al., 2016; Hessler et al., 2018), and have been used to invert up-system climate forcings in glaciated continent-scale rivers (Clift et al., 2008; Fildani et al., 2016, 2018; Mason et al., 2017; Li et al., 2019). The Amazon catchment straddles equatorial latitudes and has distinct sediment and detrital zircon (DZ) source terranes and physiography (Fig. 1), and the modern fluvial system is characterized by published DZ U-Pb data (Fig. 1B) (Campbell and Allen, 2008; Mapes, 2009; Pepper et al., 2016). We present 1352 new U-Pb DZ ages from 10 samples from cores recovered in the deep-sea Amazon Fan (Sites 936, 945, 946) during Ocean Drilling Program (ODP) Leg 155 (Flood et al., 1995). We integrate our data and results with published U-Pb DZ data from the modern onshore Amazon fluvial system and perform mixture modeling of DZ age spectra to elucidate effects of changing sea level and hydroclimate on temporal patterns of sediment provenance over multi-millennial time scales (marine isotope stages 6–2; MIS 6–MIS 2) within Earth’s largest fluvial to deep-sea sediment routing system. THE AMAZON SOURCETO-SINK SYSTEM The Amazon River represents Earth’s largest freshwater discharge to the ocean (15%–20% of global total) and largest total sediment load (~1200 Mt/yr), and is the largest fluvial system in terms of drainage basin area (~7 × 106 km2) (Milliman and Farnsworth, 2011). Amazon Basin hydroclimate controls water and sediment discharge to the sea and is influenced by the South American monsoon system and the locations of the Intertropical Convergence Zone and South Atlantic Convergence Zone (Novello et al., 2017). Hydroclimate in the central and northern Amazon Basin was overall more arid during the Last Glacial Maximum (LGM) (Häggi et al., 2017), while precipitation increased for the central Andes and the southeastern Amazon craton (Cheng et al., 2013; Baker and Fritz, 2015). Amazon floodplain incision during the LGM significantly altered the eleCITATION: Mason, C.C., et al., 2019, Detrital zircons reveal sea-level and hydroclimate controls on Amazon River to deep-sea fan sediment transfer: Geology, v. 47, p. 1–5, https:// doi .org /10 .1130 /G45852.1 Manuscript received 12 November 2018 Revised manuscript received 2 March 2019 Manuscript accepted 31 March 2019 https://doi.org/10.1130/G45852.1 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online XX Month 2019 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G45852.1/4678806/g45852.pdf by Univ West Georgia user on 17 April 2019 2 www.gsapubs.org | Volume 47 | Number 6 | GEOLOGY | Geological Society of America vation and slope of the river bed in the lower Amazon River (i.e., from ~0.7 to ~7 cm/km) (Mertes and Dunne, 2007). The degree of upstream propagation of incision is uncertain, but may have influenced bed elevations as far as 1700 km from the modern coast (Mertes and Dunne, 2007). The Amazon submarine fan is a late Miocene through Pleistocene accumulation of mainly terrigenous siliciclastic sediment as much as 4–5 km thick (Figueiredo et al., 2009). During highstands, coarse-grained sediment is deposited and stored in floodplains in the lower Amazon River valley and on the shallow continental shelf. When sea levels fall by >40 m relative to Holocene sea level, river water and sediment is transferred across the continental shelf and through the Amazon canyon to the submarine fan (Damuth et al., 1995). The modern onshore Amazon system is partially constrained by existing detrital geochronology (Figs. 1A and 1B) (Campbell and Allen, 2008; Mapes, 2009; Pepper et al., 2016). DZ age data from the Solimões (upper Amazon) and lower Amazon Rivers (Fig. 1A) display a progressive dilution of young zircon age modes (<1.3 Ga) by older craton age modes (Fig. 1B). Given the observed pattern of downstream dilution by cratonic zircon sources, we might expect a submarine fan enriched in a cratonic DZ signature. METHODS AND RESULTS We collected 10 samples of fineto mediumgrained sand from turbidite beds in cores recovered during ODP Leg 155 (Flood et al., 1995), Sites 936, 945, and 946, in the lower Amazon Fan (Fig. 1A for locations; Data File DR1 in the GSA Data Repository1). All sample preparation, analyses, and data reduction were conducted at the UTChron facility at the University of Texas at Austin (USA), where we used standard techniques of mineral separation, and applied laser ablation–inductively coupled plasma–mass spectrometry U-Pb dating of zircon grains (methods of Thomson et al. [2017] and references therein). Resultant DZ ages are presented within their composite stratigraphic context (Fig. 2A) as kernel density estimates (KDEs; Figs. 2B and 2C) and plotted using a multidimensional scaling (MDS) map (Fig. 2D). Full isotopic measurements are reported in Data File DR2. The MDS map (Fig. 2D) indicates that the Amazon Fan is a mixture of Andean and cratonic sources, and is most closely related to the central Andes, lower Amazon, and craton sources in MDS space. Sample KDEs from the Amazon Fan (Fig. 2B) show little evidence for systematic stratigraphic trends, but display variable proportions of Phanerozoic and Pan-African–Brasiliano age modes (Fig. 2B). KDE cross-correlation coefficients for all Amazon Fan samples (see Table DR2 in the Data Repository) have a mean and standard deviation (1σ) of 0.45 ± 0.08, which is similar to or slightly less than coefficients for synthetic subsamples drawn from the same parent sample (Saylor and Sundell, 2016). Collectively, Amazon Fan DZ samples (Fig. 2C) have age modes nearly identical to those of published samples from the Amazon River (Mapes, 2009; Campbell and Allen, 2008) (Fig. 2C). However, (1) fan samples are qualitatively more homogenous than modern samples from the length of Amazon River, and (2) Amazon Fan DZ age spectra contain higher proportions of Phanerozoic through Sunsás orogeny–aged zircons (76% U-Pb ages <1.3 Ga) than samples from the lower Amazon (64% <1.3 Ga) (Figs. 1B and 2D; Table DR1), while the modern lower Amazon contains higher proportions of >1.3 Ga zircons derived from the craton (Fig. 1B). To further explore this observed variability in provenance signature between Holocene river and Pleistocene submarine fan samples, we applied a top-down unmixing algorithm (sensu Sharman and Johnstone, 2017; after Mason et al., 2017; Fildani et al., 2018) to quantify the relative contributions of zircons from tributary river components (parents) present in composite (daughter) mixtures, or the lower Amazon River and Amazon Fan. We selected published DZ samples (Campbell and Allen, 2008; Mapes, 2009; Pepper et al., 2016) based on their geographic locations within the Amazon drainage basin, with the goal of characterizing broad swaths of DZ terranes using modern river sands. Results can be viewe

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Russel W. Mapes

Papers

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