# Orbital and sea-level controls on long-term sediment deposition in the Amazon Fan

Gabriel R. Moizinho, Thiago P. Santos, Germain Bayon, Martin Roddaz, Natalia Vazquez Riveiros, Marina Rabineau, Daniel Aslanian, Victor Carreira, Elton L. Dantas, Roberto V. Santos

► To cite this version:

Gabriel R. Moizinho, Thiago P. Santos, Germain Bayon, Martin Roddaz, Natalia Vazquez Riveiros, et al.. Orbital and sea-level controls on long-term sediment deposition in the Amazon Fan. Global and Planetary Change, 2025, 255, 105068 (13p.). 10.1016/j.gloplacha.2025.105068. hal-05375134

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Research Article

# Orbital and sea-level controls on long-term sediment deposition in the Amazon Fan

<figure><img src="image_3.png" /></figure>

Gabriel R. Moizinho<sup>a,b,c,*</sup>, Thiago P. Santos<sup>d</sup>, Germain Bayon<sup>a</sup>, Martin Roddaz<sup>b</sup>, Natalia Vázquez Riveiros<sup>a</sup>, Marina Rabineau<sup>a</sup>, Daniel Aslanian<sup>a</sup>, Victor Carreira<sup>e</sup>, Elton L. Dantas<sup>c</sup>, Roberto V. Santos<sup>c</sup>

ARTICLE INFO

Editor: Dr. Fabienne Marret-Davies

Keywords:

Amazon Fan
Cyclostratigraphy
Orbital control
Neogene
Andes
Climate

A B S T R A C T

The Amazon Fan, one of the largest deep-sea fans on Earth, is a vital archive of the Amazon Basin's long-term climate and tectonic evolution. However, the factors driving sediment deposition patterns during the Neogene remain poorly understood. Here, we present an astronomical age model for a 4800 m-long sedimentary record (BP-3 well), spanning the past ~24 million years, a key interval encompassing the Amazon River's evolution into a transcontinental system. Gamma-ray (GR) data reveal strong Milankovitch cyclicity, highlighting the fan's sensitivity to orbital climate variability and enabling correlations with global mean sea level (GMSL). Our analysis shows persistently low sedimentation rates (~5–13 cm/kyr) during the late Miocene, despite Andean uplift and increased precipitation, likely due to sediment trapping by mega-wetlands in Western Amazonia. A marked increase in sedimentation (~50 cm/kyr) during the Early Pliocene (5.1–4.8 ± 1.78 Ma) reflects the collapse of these barriers, facilitating sediment transport to the Atlantic despite high GMSL. The GR record also captures the transition from obliquity- to eccentricity-dominated glacial-interglacial cycles during the mid-Pleistocene Transition (~1.3 Ma). These findings align with GMSL and benthic $δ^{18}$O data, supporting a minimal tuning approach and emphasizing the Amazon Fan's sensitivity to global climate changes. Our revised chronology provides a robust framework for understanding the Neogene evolution of the Amazon River and its links to Andean tectonics and climate variability.

# 1. Introduction

Continental margins host unique sediment records of past fluvial discharge to the oceans, sea-level fluctuations, oceanic circulation, geodynamic and climate change (Clift et al., 2008). Submarine fans are particularly significant among these deposits, representing continental margins' largest deep-water sedimentary systems that can be used to reconstruct the interplay of terrestrial climate, regional tectonics, and sea-level variations over geological timescales (Flood and Piper, 1995). The Amazon Fan, situated at the mouth of the Amazon River, ranks among the world's largest deep-sea fan systems, spanning an area of approximately 375,000 km² (Fig. 1; Figueiredo et al., 2009). The Amazon Fan developed in the mid-to-late Miocene, driven by Andean

orogeny and dynamic topography, which reorganized river systems across the Amazon Basin to flow eastward toward the Atlantic Ocean (Hoorn et al., 2017; Shephard et al., 2010). Since its inception, the Amazon Fan has served as the primary depocenter for sediments along the Brazilian Equatorial Margin (Figueiredo et al., 2009; Gorini et al., 2014; Hoorn et al., 2017), accumulating tremendous amounts of terrigenous material transported from the Amazon Basin. This scenario places the Amazon Fan as an exceptional repository for reconstructing northern South America's geodynamic and climatic evolution during the Neogene.

Previous efforts in this direction were conducted on sediment records drilled during the Ocean Drilling Program (ODP) Leg 155 Expedition (Fig. 1b), leading to various investigations exploring terrestrial organic

<footnote><sup>a</sup>Geo-Ocean, Univ. Brest, CNRS, Ifremer, UMR6538, F-29280 Plouzané, France</footnote>

<footnote><sup>b</sup>Géosciences-Environnement Toulouse, Université de Toulouse, Toulouse, France</footnote>

<footnote><sup>c</sup>Instituto de Geociências, Universidade de Brasília (UnB), Brasília, DF, Brazil</footnote>

<footnote><sup>d</sup>School of Arts, Sciences and Humanities, University of São Paulo, São Paulo, Brazil</footnote>

<footnote><sup>e</sup>Programa de Pós-Graduação em Dinâmica dos Oceanos e da Terra, Universidade Federal, Fluminense, Niterói, Brazil</footnote>

<footnote>* Corresponding author.
E-mail address: gabriel.ribeiro.moizinho@ifremer.fr (G.R. Moizinho).</footnote>
carbon accumulation in the fan, the interplay between surface and deep ocean circulation, regional moisture availability and freshwater discharge, and the evolution of the Amazon rainforest (Burns and Maslin, 1999; Maslin et al., 2000). The ODP Leg 154, while located in a more distal setting on the Ceará Rise, has been instrumental in documenting the onset of the Amazon River as a transcontinental system and the far-reaching impact of riverine discharge on the western Atlantic over million-year timescales (Stewart et al., 2016; Van Soelen et al., 2017). Recent high-resolution reconstructions of the last glacial-interglacial cycle have further emphasized the interactions between the Amazon climate and high-latitude regions, highlighting their potential impact on vegetation via changes in the Atlantic Meridional Overturning Circulation (Akabane et al., 2024 and references therein).

Seismic analyses indicate that cyclic sedimentation in the Amazon Fan comprises successions of pelagic interglacial calcareous oozes, channel-levee systems, and regionally extensive mass-transport deposits

(Manley and Flood, 1988; Maslin et al., 1998). During the past 11,000 years (and likely during older interglacials), the Amazon Fan was temporarily inactive as Amazon River sediments were mostly trapped on the inner continental shelf due to high sea level and surface water circulation (Maslin et al., 2005). In contrast, fan growth was most pronounced during periods of low sea level, when the Amazon River extended across the exposed continental shelf and discharged terrige-nous sediments directly into the Amazon Fan complex through the canyon and distributary channels (Figueiredo et al., 2007).

The interpretation of sedimentary sequences in the Amazon Fan is complicated by the interplay between global mean sea level (GMSL) variability and autocyclic processes like channel avulsion or mass transport deposits. Simultaneously, GMSL carries a strong imprint of orbital signals driven by Milankovitch forcing (Miller et al., 2005). Under progressively colder climate conditions since the mid-Oligocene, the growth of massive ice sheets has been highly sensitive to the

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (a) Map of South America illustrating the modern drainage configuration of the Amazon Basin and the sediment transport pathways of the Amazon source-to-sink system. The purple area indicates the approximate extent of the Miocene mega-wetlands in Western Amazonia (b) The offshore sedimentary basins of the Brazilian Equatorial Margin are indicated as follows: GB: Guyana Basin, FAB: Foz do Amazonas Basin, PMB: Pará-Maranhão Basin. Arrows indicate the positions of the Northern Brazilian Current (NBC) and Northern Equatorial Counter Current (NECC) (c) Upper Paleogene-Neogene stratigraphic chart of the FAB central shelf, simplified from Cruz (2019). TU: Tucunaré, PI: Pirarucu, ORA: Orange, AMA: Amapá, MA: Marajó, and TA: Travosas Formations. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
interaction of precession, obliquity, and eccentricity, directly influ-
encing GMSL and the formation of third-order eustatic sequences
worldwide (Boulila et al., 2011; Laskar et al., 2004; Pálike et al., 2006;
Westerhold et al., 2020). Astronomical forcing also plays a significant
role in controlling long-term tropical hydroclimate variability and the
positioning of precipitation belts, as seasonal rainfall and temperature
gradients are significantly modulated by insolation, which can largely be
predicted by precession and obliquity (Clement et al., 2004; Laskar
et al., 2004). The well-established periodicity of precession, obliquity,
and eccentricity for the last 40 million years provides an opportunity to
use their sedimentary imprints as geochronometers, enabling the
refinement of geological timescales and the development of high-
resolution astronomically tuned chronologies (Hinnov, 2013). Such ef-
forts have advanced our understanding of the evolution of the Neogene
and the broader Cenozoic climate (Westerhold et al., 2020).

Surprisingly, cyclostratigraphy and astronomical tuning have yet to
be thoroughly applied to the Neogene Amazon Fan record. Although an
astronomical age model was developed for the Cretaceous Limoeiro
Formation, it pertains to a period predating the Amazon River’s estab-
lishment (Boulila et al., 2020). Cyclic sediment deposition, likely
influenced by obliquity, has been proposed as a driver of floodplain fill
packages in the Miocene wetlands of western Amazonia (Hoorn et al.,
2022b). However, this hypothesis did not explicitly test the presence or
absence of astronomical variability. This study recasts well-log data
from the 1-BP-3-APS exploration well drilled in the upper Amazon Fan.
Using the boundaries established by previous nannofossil biostrati-
graphic analyses (Figueiredo et al., 2009; Hoorn et al., 2017), we
construct an astronomical timescale with uncertainties derived from
gamma-ray (GR) well-log data. Our study uncovers previously unrec-
ognized details of Amazon Fan deposition, offering the first explicit
comparison with GMSL for the past ~24 million years. Despite the
relatively consistent correlation between GMSL and long-term GR pat-
terns, we propose that the exceptional sedimentation rate from the Plio-
Pleistocene cannot be attributed solely to the decline in GMSL during
this period.

## 2. Regional setting

The Foz do Amazonas Basin (FAB), the westernmost segment of the Brazilian Equatorial Margin, is bordered by the Guiana Basin to the west and the Pará-Maranhão Basin to the east (Fig. 1b). Covering an area of approximately 360,000 km², the FAB hosts sedimentary deposits up to 9000 m thick. Between the late Paleocene and late Miocene (ca. 61.7–8.0 Ma), carbonate platforms were the dominant depositional environment in the basin, resulting in the accumulation of around 4000 m of mixed carbonate-siliciclastic shelf sediments (Marajó and Amapá Formations) and deep-water calcilutites and mudstones (Fig. 1c; Figueiredo et al., 2007). During the Langhian stage of the mid-Miocene (~15.2 Ma), a globally warm climate led to high sea levels, marked in the FAB by a maximum flooding surface on the Amapá Platform. This transgressive event caused the Marajó Formation coastal systems to retreat and promoted carbonate platform development on the outer shelf (Figueiredo et al., 2007). However, subsequent sea-level decline during the Tortonian (~9.5–8.0 Ma), coupled with an increase in siliciclastic sediment supply, heightened water turbidity and drove shoreline progradation. These processes led to the exposure and erosion of carbonate platforms, effectively terminating carbonate deposition in the FAB (Figueiredo et al., 2009; Gorini et al., 2014). Cruz et al. (2019) further revealed a south-to-north sedimentary filling pattern in the FAB, with siliciclastic deposition beginning in the southeast around ~8 Ma and reaching the northwest by ~3.7 Ma. This progressive sedimentation preserved carbonate deposition in the northwest FAB for extended periods, reflecting spatial variability in sediment delivery and deposition dynamics.

The Neogene sedimentary history of the Amazon Fan reflects the
interplay of Andean orogeny, dynamic topography, climate variability,

and sea-level changes (Shephard et al., 2010; Hoorn et al., 2017; Albert
et al., 2018). During the mid-Miocene (~16–11.5 Ma), Andean surface
uplift induced flexural subsidence, resulting in the formation of the
Pebas system, a long-lived mega-wetland composed of swamps, lakes,
and fluvial systems influenced by marine incursions (Jaramillo et al.,
2017; Leandro et al., 2022). At that time, the Purus Arch, a prominent
structural high in Central Amazonia, may have acted as a critical barrier
preventing the western drainage from connecting to the eastern Amazon
Basin until the late Miocene (Figueiredo et al., 2009). However, other
authors have suggested that the Purus Arch only breached during the
Pleistocene, following the deposition of the Iça Formation (Nogueira
et al., 2013; Horbe et al., 2013; Rossetti et al., 2015). While the signif-
icance of the Purus Arch remains debated, alternative scenarios suggest
that the Amazon fluvial system may have bypassed this barrier during
the Neogene (Hoorn et al., 2017). Eventually, the interplay of tectonics
and long-term climate change led to the demise of these mega-wetlands,
the bypass of the Purus Arch, and the connection of eastern and western
Amazonia, facilitating the development of the modern transcontinental
Amazon River.

Finally, the formation of the Amazon Fan, commonly linked to the
onset of the transcontinental Amazon River, remains a topic of ongoing
debate. The Amazon Fan initially formed on the continental slope,
becoming the primary depocenter for siliciclastic input from the late
Miocene onward. Early estimates, based on extrapolated Quaternary
sedimentation rates from core samples, placed the onset of the trans-
continental Amazon River between 16.5 and 8.0 Ma (Damuth and
Kumar, 1975). Subsequent studies correlating well data, seismic pro-
files, and isotopic data suggested a later age of 11.8–10.0 Ma (Figueiredo
et al., 2007, 2009). More recently, Hoorn et al. (2017), using nannofossil
zonation calibrated to GTS 2012, proposed that Andean-derived sedi-
ments were first exported to the Amazon Fan between ~9.4 and 9.0 Ma.

## 3. Methodology

### 3.1. 1-BP-3-APS well-log data

The 1-BP-3-APS (hereafter BP-3) is an industry well drilled in the upper Amazon Fan (3° 2′ 58.660" N, 47° 44′ 45.801" W; 754 m below sea level – mbl – Figs. 1b and 2a) reaching a max depth of 5655 mbl. This well has been referenced under various names in prior studies, including Well 2 (Figueiredo et al., 2009; Hoorn et al., 2017; Lammerstma et al., 2018), Well 4 (Gorini et al., 2014), and Well P1 (Reis et al., 2016). The GR well-log data was acquired by Schlumberger and provided by the Brazilian Petroleum and Gas Agency (ANP). GR data have a vertical resolution of 15–20 cm, and values are presented in API (American Petroleum Institute) units. This study focuses on the interval between 1450 and 4800 mbl. The lithology of this interval includes alternating limestone and mudstone from 4800 to 4080 mbl, transitioning to siltstone and fine sandstone until 3620 mbl, and intercalated mudstone and siltstone down to 1450 mbl. Due to drilling-related artifacts, the GR well-log is discontinuous; therefore, we divided it into three internally consistent intervals (Fig. 2a): Section 3 (S3, 4425–4800 mbl), Section 2 (S2, 3680–4415 mbl), and Section 1 (S1, 1480–3670 mbl). It should be noted that these 10 m non-measurement segments are not caused by stratigraphic discontinuities. The break between S1 and S2 coincides with a total mud loss at 3679 m during casing and cementing operations. This caused a baseline shift in the GR record by reducing the attenuation typically provided by drilling mud. Similarly, the break between S2 and S3 corresponds to a sequence of severe mud losses during casing operations. These events altered the absolute magnitude of the GR baseline but did not affect the relative variability of the signal, which remains consistent with lithological changes. For this reason, S1, S2, and S3 are treated as independent GR segments that can be interpreted stratigraphically within each interval but are not directly comparable in absolute values across boundaries.
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Multi-taper spectral analysis of gamma-ray (GR) data from the BP3 well for sections S3, S2, and S1. (a) GR data from the BP3 well plotted alongside lithological information and Nannofossil zonation from previous studies (Figueiredo et al., 2009; Hoorn et al., 2017). (b), (c), and (d) Multi-taper spectral analysis of GR data in the time domain, tuned to the biostratigraphic age model. Yellow bars highlight spectral bands corresponding to 405-kyr long eccentricity (E), ~100-kyr short eccentricity (e1 and e2), 41-kyr obliquity (o), and ~21-kyr precession (p1 and p2) cycles. (e), (f), and (g) Multi-taper spectral analysis of GR data in the stratigraphic domain, with yellow bars indicating filtered wavelengths associated with orbital signals. (h) The spectral amplitude of the orbital solution (Laskar et al., 2004) for the past ~24 Myr. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

## 3.2. Prior biochronology of BP-3 well and a novel astronomical time scale

An age model for BP-3 was initially developed using calcareous nannofossil biostratigraphy on ditch-cutting samples (Varol Research, 2004; Figueiredo et al., 2009) and was subsequently updated to the GTS2012 timescale (Hoorn et al., 2017). According to this prior biostratigraphic evaluation, the 1450 to 4800 mbsl interval encompasses the Late Oligocene (NP25) to Pleistocene (NN19) period. Unlike the continental shelf or deeper fan regions, sedimentation on the upper fan has remained relatively uninterrupted during past sea-level fluctuations. However, stratigraphic breaks have been reported immediately above 4145, 3338, and 2654 mbsl. The first hiatus above 4145 m (Section S2) corresponds at least to the complete absence of biozone NN8. The hiatuses at 3338 and 2654 mbsl are associated with the partial absence of the lower parts of biozones NN16 and NN19, spanning ~1.0

and 0.73 Ma, respectively (Varol Research, 2004; Hoorn et al., 2017). It should be recognized that chronological inferences within a given biozone based on sparse biostratigraphic data remain problematic, as the limited temporal resolution imposes serious constraints on any refined internal interpretation. In addition, short-term intervals of poor preservation, whether caused by carbonate dissolution, dilution from the sudden influx of siliciclastic material, or other ecological factors, may further bias such analyses. Later in the manuscript (Section 3.3), based on the suite of statistical analyses conducted here, we challenge these earlier interpretations and, within the bounds of our uncertainty evaluation, argue instead for a largely complete chronostratigraphic succession in Section S1.

The nannofossil biostratigraphy published by Figueiredo et al. (2009) was accompanied by a portion of the GR data employed in the current study. However, to date, the potential of these data for
constructing a coherent astronomical time scale has not been explored. We investigated the eventual expression of orbital forcing using statistical cyclostratigraphic methods, applying these techniques to the GR data from the BP-3 well. GR data reflect the combined radiogenic emission of potassium, thorium, and uranium, typically present in minerals such as feldspar, mica, clays, and organic-rich particles. Consequently, higher GR values are likely associated with clay- or organic-rich sediments, while lower GR values generally correspond to coarser carbonate and sandstone components (Cantalejo and Pickering, 2015). The balance between these sediment fractions within a basin is strongly influenced by the surrounding climate, which enhances the GR signal-to-noise ratio and enables more efficient detection of astronomical signals from stratigraphic noise (Li et al., 2019; Weedon, 2003). Limitations on this assumption rely on the non-climatic factors influencing the radiogenic signatures of the mentioned elements, possibly linked to high concentrations of heavy minerals, such as zircons (Simfek et al., 2012).

Our approach begins with an independent verification of astronomical periodicities before orbital tuning. We leveraged nannofossil biostratigraphic published by Figueiredo et al. (2009) and subsequently updated by Hoorn et al. (2017) as a prior age-depth control. This biostratigraphic age model was applied across the three segments where the GR data were collected, placing the intervals S3, S2, and S1 between the late Oligocene and late Pleistocene. At present, biostratigraphy remains the only viable means of anchoring this well to an absolute timescale. The use of biostratigraphic constraints to guide subsequent cyclostratigraphic refinement is a well-established strategy in astrochronological studies (e.g., Groeneveld et al., 2021; Koutsodendris et al., 2021; Raffi and Backman, 2022).

After applying the nannofossil age-depth model to each GR section, we performed a linear interpolation based on their respective mean temporal resolution and detrended the data using a LOWESS regression with a 35 % window. This process was designed to remove very long, non-astronomical frequencies mixed in the data. The significant frequencies preserved in each GR section were verified using power spectral analysis through the prolate multi-taper spectral estimator (MTM) (Thomson, 1982). The analysis was performed with $2\pi$ tapers and a robust red noise null hypothesis model supported by a theoretical first-order autoregressive process fitted to the data and reported regarding the 90 %, 95 %, and 99 % spectral confidence levels (Fig. 2).

Subsequently, we searched for the respective orbital cycle found in the first step, but in the stratigraphic domain, choosing the one that best fit in terms of number of cycles. A Gaussian bandpass filter was used to isolate the most appropriate wavelength in each section. We followed a minimum tuning approach, aligning each minimum in our filtered data with a corresponding minimum in the target astronomical signal for the chronological interval yielded by the biostratigraphic age model (Hinnov, 2013; Laskar et al., 2004). For intervals S3 and S2, the target astronomical frequency was the $g_2 - g_5$ 405-kyr cycle, also known as the 'metronome'. The 405-kyr cycle is considered the most stable astronomical cycle (Laskar, 2020). For sections S3 and S2, we filtered frequencies corresponding to 1/0.01866 and 1/0.04269 cycle/m (wavelengths of ~53.59 m and ~23.42 m, respectively) (Figs. 2f and g). For interval S1, the target astronomical frequency was the $g_4 - g_5$ ~ 95-kyr short eccentricity cycle. Among the various eccentricity cycles within this band, the ~95-kyr cycle exhibits the highest power spectrum during the Plio-Pleistocene interval (Laskar et al., 2004). For Section S1, we filtered the frequency at 1/0.02097 cycles/m (wavelength of ~47.68 m). In Figs. 2e-g, we indicated additional frequencies with respect to the distribution related to the frequency ratio method (20:5:2:1) (Laskar et al., 2004); however, we reinforce that they were not used for tuning purposes since some of them do not overcome the spectral confidence levels.

Uncertainty evaluation remains a topic of ongoing debate in cyclostratigraphy, with no standardized methodology for quantifying it (Sinnesael et al., 2019). Age uncertainty can arise from various factors,

including the non-linear response of the climate system, which can distort climate signals, the effects of sampling and statistical methods, resolution limitations, and the inherently incomplete nature of the stratigraphic record (Meyers, 2019). Insights from stochastic sedimentation modeling suggest that the average deviation in total duration when tuning to the 405-kyr and ~ 100-kyr cycles is approximately 14.5 % and 21.5 %, respectively (Wang et al., 2024). We directly incorporated the average deviation of 14.5 % for interval S3. For intervals S2 and S1, in addition to the average deviation for the 405-kyr and ~ 100-kyr cycles, we accounted for an additional uncertainty of approximately 1.2 Ma and 0.19 Ma due to the potential absence of three 405-kyr and two ~95-kyr cycles, respectively (Figs. 3 and 4). Ultimately, our age model did not diverge significantly from previous efforts based on biostratigraphic data (Figueiredo et al., 2009; Hoorn et al., 2017) (Supplementary Fig. 1). Still, it substantially improves chronostratigraphic control while addressing more realistic uncertainties not previously assumed (Supplementary Fig. 1). Supplementary Tables 1–3 present the age models for intervals S1, S2, and S3, including their uncertainty estimates and the Gaussian band-pass filters used for astronomical tuning.

### 3.3. A bayesian age model approach for the Plio-Pleistocene interval

An additional experiment was conducted in the upper part of section S1 (upper ~3000 mbls). In this interval, we examined the capabilities of Integrated Prediction Error Filter Analysis (INPEFA), a feature within Cyclogolog® software (Djin Nio et al., 2005). INPEFA calculates prediction error by creating new data points based on prior samples, with each point representing the numerical error between predicted and observed values at corresponding depths. A 10 m moving window was applied to the data. Assuming the record exhibits cyclic patterns due to climatic variations, INPEFA leverages these patterns to assess continuity and discontinuity in the dataset by quantifying spectral changes. The approach generates new sample points where each value represents either a positive or negative prediction error, indicating overestimation or underestimation of the GR values. Overestimation (negative error) suggests a “sandy” and more regressive segment, while underestimation (positive error) suggests a “muddy” and more transgressive trend. We interpret that significant changes in the depositional system occur at inflection points between predominantly negative and positive trends (Djin Nio et al., 2005). In this study, we worked exclusively on the mid-term INPEFA output.

Then, we attempt a probabilistic tuning between the results of the INPEFA analysis and the reference deep-ocean benthic oxygen isotope ($δ^{18}$O) LR04 stack (Lisiecki and Raymo, 2005) to generate an alternative age model for the upper part of section S3. This experiment employed Bayesian Inference Gaussian Process regression and Multiproxy Alignment for Continuous Stacks (BIGMACS) (Lee et al., 2023). BIGMACS is a newly developed software package designed to probabilistically align ocean sediment core data and construct multiproxy age models. It formally incorporates multiple sources of age uncertainty to produce probabilistic alignments that are more informative and less subjective than traditional tie-point approaches (Lee et al., 2023). Median age models and their uncertainties are derived from the distribution of Markov chain Monte Carlo samples. Lastly, the correlation between mid-term INPEFA and GMSL was assessed using Pyleoclim, a Python 3.9 application (Khider et al., 2022). Both datasets were interpolated to their mean resolution and standardized using Pyleoclim tools. We applied time-series correlation with the default isospectral method, supported by 5000 Monte Carlo simulations. As mentioned in Section 3.2 and later discussed in Section 5.3, the exceptional results obtained through the combined application of INPEFA and BIGMACS indicate a nearly complete stratigraphic succession for the upper ~3000 mbls. This evidence leads us to question the existence of the hiatus associated with the partial absence of biozone NN19 at 2654 mbls. A similar reasoning may be applied to the reported hiatus for biozone NN16 at 3338 mbls,
<figure><img src="image_6.png" /><figcaption>Fig. 3. Astronomical tuning for interval S3 of BP3 well. (a) Representation of the 405-kyr long eccentricity cycle (Laskar et al., 2004). (b) LOWESS-detrended gamma ray (GR) data from BP3 well (light grey) together with the Gaussian bandpass filter applied to GR data (blue), with cutoff frequencies of 0.0149–0.0224 cycles/m, in the stratigraphic domain. Thin black lines indicate the alignment of each eccentricity minimum in filtered GR data. (c) Target 405-kyr cycle (black) compared with filtered GR data (black) in the time domain. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. Astronomical tuning for interval S2 of BP3 well. (a) Representation of the 405-kyr long eccentricity cycle (Laskar et al., 2004). (b) LOWESS-detrended gamma ray (GR) data from BP3 well (light grey) together with the Gaussian bandpass filter applied to GR data (blue), with cutoff frequencies of 0.03869–0.04669 cycles/m, in the stratigraphic domain. Thin black lines indicate the alignment of each eccentricity minimum within the filtered GR data. (c) The target 405-kyr cycle (black) compared with the filtered GR data (black) in the time domain. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
given its comparable nature.

## 4. Results

The GR values of the BP-3 well range from 40 to 130 API, with the maximum values occurring near the top of interval S2 (~4000–3600 mbsl; Fig. 2a). An abrupt change in mean values marks the transition between intervals S2 and S3 (~3600 mbsl), where GR decreases from approximately 120 to 80 API. In interval S1, a noticeable shift to higher frequency variability is observed around ~2450 mbsl, accompanied by a gradually decreasing trend toward the top of the well (Fig. 2a).

Aligned with the nannofossil biostratigraphic age model, intervals S3, S2, and S1 are constrained to 24.8–19.6 Ma, 19.2–5.09 Ma, and 5.03–0.43 Ma, respectively. The MTM power spectral analysis indicates that intervals S3 and S2 are predominantly characterized by lower frequency variability (>1/100 cycles/ka) (Fig. 2c and d). Within this frequency band, the 405-kyr cycle was identified, exceeding the 90 % and 99 % confidence levels for intervals S3 and S2, respectively. Lower frequency variability is also observed in interval S1, with well-defined 405-kyr-long eccentricity and ~100 and ~130-kyr short eccentricity cycles (99 % confidence level) (Fig. 2b). Additionally, higher-frequency variability within obliquity and precession bands is thoroughly represented in section S1, while in sections S3 and S2, their representation is not constant (Figs. 2c and d). It is crucial to highlight that no orbital tuning was conducted at this stage, underscoring the significant influence of orbitally induced climate modulation on this dataset.

In the stratigraphic domain, the equivalent long eccentricity cycles for intervals S3 and S2, and the short eccentricity cycle for interval S1, were isolated using Gaussian bandpass filtering. For intervals S3 and S2, the long eccentricity cycle corresponds to wavelengths of approximately 53.5 and 23.4 cycles/m, respectively. In interval S1, the short eccentricity cycle is associated with a frequency of about 47.68 cycles/m (Fig. 2e-g). The minimal tuning approach aligned each minimum in the filtered wavelengths with the corresponding orbital cycle within the periods indicated by the biostratigraphic data (Figs. 3–5). For intervals S3 and S2, the orbital solution suggests the presence of seven and 35 minima in the 405-kyr band, while seven and 31 sedimentary cycles are

likely associated with the 405-kyr band, respectively (Figs. 4 and 5). We interpret that no missing beats occur in interval S3, while approximately 1.2 Ma may be absent in interval S2. For interval S1, the orbital solution indicates the existence of 49 minima in the ~95-kyr eccentricity band, while 46 sedimentary cycles are correlated. The caveats listed in Section 3.2, plus the potential existence of hiatuses, as outlined in Figueiredo et al. (2009) and Hoorn et al. (2017), result in final astronomical age models with root mean square error constrained between 24.9 and 22.0 ± 0.003 Ma, 17.6–5.1 ± 2.0 Ma and 4.81–0.42 ± 0.65 Ma for sections S3, S2 and S1, respectively (Figs. 3–5; Supplementary Fig. 1).

## 5. Discussion

### 5.1. Orbital Pacing and Global mean Sea Level in the Amazon Fan

The Neogene history of sediment deposition in the Amazon Fan is highly complex, driven by the interaction between Andean orogeny, dynamic topography, climate variability, and sea-level fluctuations (Hoorn et al., 1995). The interplay of these processes is believed to have facilitated the formation of diverse environments, ranging from shallow marine conditions to extensive fluvio-lacustrine mega-lakes in the western Amazon during the early- to mid-Miocene (Boonstra et al., 2015; Jaramillo et al., 2017; Leandro et al., 2022). Over time, these geological and climatic factors contributed to the development of the transcontinental Amazon River and played a crucial role in shaping the remarkable biodiversity of the Amazon Basin (Sawakuchi et al., 2022).

A numerical model developed to investigate the formation of the Amazon River, incorporating tectonic activities, climate variations, and global mean sea level (GMSL) changes, indicates that sea level played a critical role in controlling intermittent marine incursions in the western Amazon (Sacek et al., 2023). Additionally, in coastal regions, periods of sea level drop were generally associated with increased sedimentation rates (Sacek et al., 2023). Furthermore, the application of detrital zircon geochronology indicates that sea-level fluctuations played a critical role in controlling sediment sources in the Amazon Fan in the Pleistocene (Mason et al., 2019). During warm periods, sea-level highstands promoted a greater influx of craton-derived sediments. In contrast, cold

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Astronomical tuning for interval S1 of the BP3 well. (a) Representation of the ~95-kyr short eccentricity cycle (Laskar et al., 2004). (b) LOWESS-detrended gamma ray (GR) data from the BP3 well (light grey) alongside the Gaussian bandpass filter applied to the GR data (blue), with cutoff frequencies of 0.01697–0.02497 cycles/m, in the stratigraphic domain. Thin black lines indicate the alignment of each eccentricity minimum within the filtered GR data. (c) The target ~95-kyr cycle (black) compared with the filtered GR data (black) in the time domain. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
intervals were marked by shelf incision, base-level lowering, and enhanced connectivity between terrestrial, shelf, and deep-sea environments. These conditions facilitated the downslope transport of Andean-sourced zircons (Mason et al., 2019). Gamma ray (GR) data from sedimentary records spanning the continental shelf to the upper slope further reflect the impact of sea-level oscillations (Li et al., 2018; Tortarolo, 2023). Transgressive phases and sea-level highstands typically coincide with increased deposition of fine-grained pelagic sediments, terrigenous clay minerals, and organic matter—resulting in elevated GR values (Wagreich and Koukal, 2020). In contrast, regressive phases and sea-level lowstands are associated with the preferential accumulation of coarser siliciclastic and carbonate sediments, reflected by lower GR values.

Fig. 6 compares the GR data for the BP-3 well, aligned with the developed astronomical timescale, to the GMSL of Miller et al. (2020). A general agreement is observed between the long-term trends in GR and GMSL, both during periods above and below present-day sea level. Notable correlations are evident in intervals S3 and S1 (Figs. 6a and c), while in interval S2, the relationship is stronger during the near ice-free conditions of the Miocene Climatic Optimum. In the mid-Miocene, the GMSL declines in two distinct steps, although this pattern is not precisely mirrored in the GR data. However, during the late Miocene, both proxies exhibit a similar upward trend toward the Pliocene (Fig. 6b). The Plio-Pleistocene is characterized by a gradual GR increase in the BP-3 well from ~4.7 to 2.6 Ma, aligning with periods of high sea level during the Pliocene Climatic Optimum. Subsequently, both proxies display a gradual decline throughout the Pleistocene. The mechanism unifying GR and GMSL is likely the dynamic interplay between the growth and decay of continental-scale glaciations, which began in Antarctica during the early Oligocene and was later intensified by Northern Hemisphere glaciation (Miller et al., 2020). Eustatic sea-level fluctuations driven by

ice volume changes ultimately influence depositional dynamics and the nature of sediments in deltaic, upper slope, and deep-sea fan environments, all affecting the GR signal.

The waxing and waning of ice sheets since the late Oligocene exhibit strong orbital pacing, with GMSL responding to 21-kyr precession, 41-kyr obliquity, ~100-kyr, and 405-kyr eccentricity cycles, along with amplitude modulation of obliquity and eccentricity at ~1.2- and ~2.4-Myr rhythms, respectively (Boulila et al., 2011; Miller et al., 2020; Pälike et al., 2006). A wavelet transform analysis (Torrence and Compo, 1998) of the GR data reveals that deposition in the Amazon Fan also preserves a wide range of Milankovitch frequencies (Fig. 6). During the sea-level fall associated with the Mi1 event at the Oligo-Miocene transition (~23 Ma), spectral power in the 405-kyr band is prominent in both GR and GMSL records. Benthic $\delta^{18}$O and $\delta^{13}$C records from the western equatorial Atlantic (ODP Site 926 – Ceará Rise) indicate an increase in the 405-kyr cycle amplitude across the Mi1 event, suggesting a pivotal shift in the global climate system driven by positive feedback mechanisms on the global carbon cycle (Zachos, 2001). In the early Miocene, the GMSL lowstand is further marked by strong spectral power in the 21-kyr precession and 41-kyr obliquity bands in the BP-3 data (Fig. 6c). Additionally, a longer temporal variability at Myr-scale (~1.2/2.4-Myr cycle) is evident in interval S2 (early to mid-Miocene), agreeing with GMSL (Fig. 6b). Continental margin deposition may act as a low-pass filter, amplifying Myr-scale variability and aliasing higher-frequency signals, particularly under low sedimentation rates (<6 cm/kyr) (Miller et al., 2020). Like the early Miocene, higher frequencies in the 41-kyr and ~100-kyr bands are also observed in the GR data, corresponding with the series of GMSL falls (Mi2–Mi4) marking the end of the Miocene Climatic Optimum (Fig. 6b).

The GMSL record exhibits a weak expression of the 405-kyr cycle during the Plio-Pleistocene interval, and the underlying cause remains

<figure><img src="image_9.png" /><figcaption>Fig. 6. Estimated sedimentation rates derived from the astronomical age model, and comparison of gamma-ray (GR) data with global mean sea level (GMSL) (Miller et al., 2020). Panels (a-c) from top to bottom: sedimentation rate (cm/kyr; shaded brown area); GR data (thin black line) with a 35 % LOWESS regression (thick black line); GMSL (thin blue line) with a 35 % LOWESS regression (thick blue line); and wavelet transform (Torrence and Compo, 1998) of standardized GR data. Dashed white lines denote the positions of obliquity (1.2-Myr) and eccentricity (2.4-Myr) amplitude modulations, long eccentricity (405-kyr), short eccentricity (~100-kyr), obliquity (41-kyr), and precession (21-kyr). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
uncertain (Miller et al., 2020). Conversely, the 405-kyr cycle is clearly preserved in the GR data (Fig. 6a). Coupled simulations of global ice volume over the past 5 million years suggest that the 405-kyr modulation is present in Plio-Pleistocene ice volume variability but becomes obscured due to the dominant influence of Northern Hemisphere ice sheets linked to the 41- and ~100-kyr cycles since 2.9 Ma (De Boer et al., 2014). Cosmogenic radionuclide data from the southern Andes provide direct evidence of a 405-kyr cyclic control over erosion rates (Fisher et al., 2023). This may reflect a direct influence of the long eccentricity cycle on moisture transport from the Atlantic or, more likely, an indirect effect through amplitude modulation of the 21-kyr precession. Such mechanisms could explain the consistent expression of the 405-kyr cycle in the GR data, as tropical hydroclimate is notably sensitive to seasonal insolation changes, particularly within the precession band (e.g., Cheng et al., 2013; Marzocchi et al., 2015). The nonlinear interaction of orbitally driven climatic factors, such as continental precipitation, with GMSL variability may account for the observed discrepancies between the GR data and GMSL at this point.

Furthermore, geochemical and environmental magnetic proxy records on the northwest Australian shelf show strong modulation of the monsoon dynamics by the 405-kyr cycle between 2.9 and 1.6 Ma, providing direct support for the preservation of this cycle in certain Quaternary sedimentary records (Zhao et al., 2024). Thus, the persistence of the 405-kyr cycle within the last ~4.8 Myr section of BP-3 is consistent with other records. The last ~1.3 Myr period also displays an enhanced expression of the ~100-kyr cycle, which will be examined in detail in the subsequent section (Section 5.3; Fig. 6a). Detecting multiple astronomical signals in the BP-3 GR spectrograms ultimately validates our minimal tuning approach and suggests no inherent bias toward the selected target frequency for each interval (Fig. 6).

## 5.2. The imprint of orbital forcing on sedimentation rates at the Amazon Fan

The novel astronomical age model for the BP-3 well also enables a reassessment of Neogene sedimentation rates in the Amazon Fan. Figueiredo et al. (2009) proposed three stages of development that led to the exceptionally high sedimentation rates observed in the most recent period: 1) before approximately 6.8 Ma, when sedimentation rates were relatively low (<5 cm/kyr); 2) between ~6.8 and 2.4 Ma when sediment discharge increased sixfold despite a general rise in GMSL; and finally, 3) from 2.4 Ma to the present, a period characterized by a notable increase in sedimentation rates with a mean value of 122 cm/kyr, but punctually peaking at 1116 cm/kyr (Figueiredo et al., 2009). Our revised estimate for the sedimentation rate during the Plio-Pleistocene interval is more moderate, as the regular placement of tie-points by cyclostratigraphy avoids abrupt time compressions. In our model, the substantial increase in sediment discharge occurred between 5.1 and 4.8 ± 1.78 Ma (Fig. 6a and b and Supplementary fig. 1), slightly later than Figueiredo et al. (2009) suggested, but still consistent when considering the full uncertainty range. Paleogeographic reconstructions based on 2D and 3D seismic data (Rouby et al., 2022) show that the BP-3 site remained within the main depocenter of the Brazilian Equatorial Margin throughout the 23-0 Ma interval. Thus, the marked increase in sedimentation rates after ~5 Ma is best interpreted as reflecting an overall increase in sediment supply to the Amazon Fan system, consistent with the regional signal documented by Figueiredo et al. (2009), rather than a lateral migration of fan lobes. This increased sedimentation rate is also consistent with observations of a Messinian worldwide tectonic revolution, as summarized by Leroux et al. (2018).

Notably, despite the evident influence of GMSL on the GR signal, no comparable effect on sedimentation rates is observed. In the S3 section, the calculated sedimentation rate changes are minimal (<1 cm/kyr). The transition from S3 to S2 is marked by a two-fold decrease in sedimentation rates, and although S2 exhibits more significant fluctuations (~2 cm/kyr), these variations are unlikely to be attributed to the ~30 m

GMSL drop from the late Oligocene to the late Miocene (Figs. 6b and c). The early Pliocene in section S1 is a critical period for this evaluation. When approaching the Pliocene Climatic Optimum, the sea level was approximately 20 m above the present, and, despite this, the Amazon Fan experienced significant sediment discharge of ~50 cm/kyr. This suggests that the sea-level highstand of the early Pliocene did not impede substantial sediment transport to the Atlantic Ocean.

This persistently low sedimentation rate in the Amazon Fan until approximately 5 Ma may be attributed to the presence of mega-wetland systems in the western Amazon Basin (Boonstra et al., 2015; Hoorn et al., 2022a). This system, known as the Pebas system, comprised a complex mosaic of swamps, lakes, and fluvial networks influenced by episodic marine incursions. Acting as a sediment trap, the Pebas system likely retained the majority of Andean and cratonic sediments within the Amazon Basin. While the Pebas System is considered to have existed no later than ~13 Ma (Hoorn et al., 2022a; Jaramillo et al., 2017), other findings suggest that a connection between western Amazonia and the Caribbean region may have persisted longer (Aguilera et al., 2013; Espinosa et al., 2021). Minor marine influences have been identified in the early Pliocene (~4.7, 4.2-4.1, and 3.8 Ma) (Leandro et al., 2022), potentially contributing to the observed low sedimentation rates during this time. If such marine incursions impacted sediment discharge to the Amazon Fan, they might explain why sedimentation rates remained low until the early Pliocene. However, it should be noted that this scenario of prolonged marine incursions in western Amazonia is not supported in all models (Sacek et al., 2023). Applying our astronomical age model in conjunction with previously published neodymium (Nd) isotopic data from the same well (Hoorn et al., 2017), we observe the influx of Andean-derived sediment as early as 12.2 ± 1.3 Ma. This transition is marked by an abrupt shift in εNd values from highly negative (−18.8 ± 1.7, *n* = 9) to more radiogenic signatures (−10.7 ± 1.8, *n* = 15), reflecting the increased contribution of juvenile material sourced from Andean volcano-sedimentary Cenozoic deposits. This finding indicates that while sediment traps such as the Pebas System significantly delayed sediment export, they did not entirely hinder the onset of transcontinental drainage and sediment delivery to the deep sea during the Late Miocene.

If GMSL decline was not the primary driver of the massive sediment discharge starting in the early Pliocene, another mechanism must have been at play. Geochemical proxies indicate that the northern Altiplano experienced rapid uplift between 9.1 and 5.4 Ma, likely reaching its modern elevation during this period (Kar et al., 2016). This surface uplift facilitated the organization and intensification of the South American Low-Level Jet, leading to substantial (>400 %) increases in precipitation across the central Altiplano. This increase was due to convective activity triggered by latent heat release as air masses converged on the plateau (Poulsen et al., 2010). Fossil flora evidence suggests that precipitation in the central Altiplano was nearly three times higher than current levels during the mid-Miocene, with these wet conditions persisting into the early Pliocene (Martínez et al., 2020). Additionally, pollen records from equatorial South America indicate continuously wet conditions during the early Pliocene, supporting the idea of a southward displacement of the Intertropical Convergence Zone, which would have transported moisture from the Atlantic across Amazonia (Grimmer et al., 2018). This increase in humidity from the mid-Miocene onward likely intensified the denudation rates of Andean rocks due to enhanced precipitation (Sacek et al., 2023). In addition, the Pliocene uplift of the Fitzcarrald Arch (Espurt et al., 2007), a positive topographic area that covers 400,000 km² of the Peruvian-Brazilian retroarc basins (Regard et al., 2009), may have been an additional trigger in favoring erosion of the Amazonian retroarc foreland basin sediments and in reducing the accommodation space in the Amazon retroarc foreland basins. Therefore, we propose that the resulting sediments were initially retained within wetland systems in western Amazonia, acting as efficient sediment traps. With the disappearance of these barriers during the early Pliocene, sediments produced by these processes found an unrestricted pathway to the
Atlantic Ocean. This sediment flush was so substantial that it exceeded the influence of high GMSL during the early Pliocene.

To summarize, the spectral analysis of BP-3 GR data reveals a comprehensive archive of orbital-induced climate variability influencing sedimentation at the Amazon Fan and, consequently, the stacking of sedimentary units from Amazon River-derived material. The developed astronomical timescale enables a detailed comparison between Amazon Fan dynamics and GMSL over the past ~24 million years, facilitating the identification of the factors that led to an exceptional sedimentation rate in this region.

### 5.3. The Quaternary Ice Ages and the mid-Pleistocene transition

The middle portion of section S1, corresponding to the last ~3.5 Myr (upper ~3000 mbls), is marked by a shift to a high-frequency variability in GR data. This shift is unrelated to the astronomical tuning, as observed in the stratigraphic domain (Figs. 1 and 2). Globally, the late Pliocene to early Pleistocene transition is characterized by a fundamental shift in Earth's climate, as inferred from benthic foraminifera $\delta^{18}$O marine records (Lisiecki and Raymo, 2005; Westerhold et al., 2020). During this period, the oxygen isotopic composition of the global oceans became progressively heavier, indicating an expansion in global ice volume with a concomitant sea-level decrease and a reduction in global temperature (Shackleton, 1987). The amplitude of benthic $\delta^{18}$O variability also gradually increased, leading to the series of ice ages that punctuated the Quaternary period. These changes significantly impacted GMSL, with amplitude variability reaching approximately 120 m during more recent glacial intervals (Miller et al., 2020).

To further quantify the linkages between global climate change and Amazon Fan dynamics, we applied INPEFA over the upper ~3000 m. The inflection points of the INPEFA curves indicate significant shifts in depositional regimes, likely influenced by GMSL and climate change (Djin Nio et al., 2005). INPEFA outputs successfully identified the depositional changes associated with GMSL in the Espírito Santo basin (western South Atlantic) from the late Paleocene to the early Miocene (Santos et al., 2023). When aligned to the developed astronomical model

for section S1 and compared with the GMSL, we found a striking visual association, mainly from the Pleistocene onset at 2.58 Ma (Fig. 7a). The sequence of peaks in the mid-term INPEFA nearly corresponds to elevated GMSL (and lower benthic foraminifera $\delta^{18}$O) constrained during the brief Pleistocene interglacials.

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Wavelet transform of the standardized mid-term Integrated Prediction Error Filter (INPEFA) applied to BP3 gamma ray (GR) data in the time domain for the upper part of the S3 section (upper panel) and the benthic foraminifera oxygen isotope record ($\delta^{18}$O) from the LR04 reference stack (lower panel) (Lisiecki and Raymo, 2005). The white rectangle highlights the interval associated with the mid-Pleistocene Transition (MPT). Dashed white lines indicate the spectral bands corresponding to 41-kyr and ~100-kyr orbital cycles.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Comparison of gamma ray (GR) data and the mid-term Integrated Prediction Error Filter Analysis (INPEFA) results for the upper ~3600 mbls of the BP3 well. (a) from top to bottom: reference benthic oxygen isotope ($\delta^{18}$O) records from CENOGRID (light blue line) (Westerhold et al., 2020) and LR04 (pink line) (Lisiecki and Raymo, 2005); GR data (thin black line) with a 35 % LOWESS regression (thick black line); GMSL (blue line) (Miller et al., 2020) and mid-term INPEFA signal summed with the 35 % LOWESS regression used for GR data. (b) same as (a), but the mid-term INPEFA signal is plotted using the probabilistic age model developed with BIGMACS (Lee et al., 2023). Vertical grey bars highlight interglacial periods, and numbers indicate selected Marine Isotope Stages between ~0.5–2.58 Ma. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

<figure><img src="image_12.png" /></figure>
Interestingly, the distribution of spectral content in mid-term INPEFA exhibits strong power, mainly at the obliquity band during the early to mid-Pleistocene (Fig. 8). From ~1.3–1.2 Ma, there is a clear increment in the ~100-kyr frequency that prevailed until the uppermost part of the record. These features are consistent with the glacial-interglacial variability observed in benthic foraminiferal $\delta^{18}$O records, which shifts from obliquity- to eccentricity-dominated signal at ~1.3–1.2 Ma (Fig. 8). This transition is known as the mid-Pleistocene Transition and is characterized by an increased resilience of northern hemisphere ice caps, making ice ages spent longer (Berends et al., 2021; Clark and Pollard, 1998) (Fig. 7a). Why Earth's climate responded so strongly to a weak forcing in eccentricity at that time remains fiercely debated in the literature (e.g., Barker et al., 2022). However, identifying these Pleistocene climate shifts in our GR series tuned to ~100-kyr eccentricity demonstrates that the same climate variability was recorded in the Amazon Fan sedimentary system.

To further explore the apparent coupling between mid-term INPEFA and both GMSL and benthic foraminiferal $\delta^{18}$O records, we applied a probabilistic tuning approach for the upper part of the S1 section, aligning our data directly with the benthic foraminifera LR04 reference (Lisiecki and Raymo, 2005) using the routine of BIGMACS software (Lee et al., 2023). In our case, BIGMACS facilitated a probabilistic stratigraphic alignment of benthic $\delta^{18}$O with mid-term INPEFA. The resulting age model was slightly compressed (~0.42–3.40 Ma) relative to that generated by minimal tuning with the ~100-kyr cycle (~0.42–3.50 Ma) but with an order of magnitude lower uncertainty, ranging ±0.01–0.025 Ma (Supplementary Fig. 2). Interestingly, the association between mid-term INPEFA and GMSL was notably robust, with a clear imprint of Marine Isotope Stages (MIS) in the variability of the well-log data (Fig. 7b). This is not easily found, but when available, it supports building accurate high-resolution age models (e.g., Dearing Crampton-Flood et al., 2020; Zhang et al., 2019).

In the late Pliocene (~2.6–3.5 Ma), visual alignment between INPEFA and GMSL remains consistent. However, the isospectral correlation test indicates a lack of statistical significance in this portion ($R = 0.09, p = 0.39$), likely due to the software's limited capacity to match finer-scale variability between GMSL and mid-term INPEFA. A marked shift occurs at 2.6 Ma, where INPEFA cyclicity aligns closely with individual MIS, with peaks (lows) in INPEFA matching interglacial (glacial) periods characterized by higher (lower) sea levels and lower (higher) benthic $\delta^{18}$O values (Fig. 7b). Exceptions to this alignment are solely observed during MIS 101, 85, and 71. We argue that the identification of nearly all MIS between ~2.5 and 0.5 Ma provides strong evidence that the upper ~3000 mbls of BP-3 constitutes an essentially complete stratigraphic section, rendering the hiatus inferred from the partial absence of biozone NN19 more unlikely (Hoorn et al., 2017). A similar interpretation may be extended to the lower hiatus associated with the partial absence of biozone NN16. However, this inference should be approached carefully, as this lower interval was not examined with the same set of analytical tools. Ultimately, this fourth to fifth-order stratigraphic analysis reveals that the BP-3 site holds a continuous sediment record spanning the last ~3.0 Myr, making it a compelling target for scientific coring to investigate Plio-Pleistocene variability in the Amazon River system. The statistically significant correlation ($R = 0.42, p < 0.05$) between mid-term INPEFA and GMSL underscores the Amazon Fan's high sensitivity to the onset of early Pleistocene glaciations. This sensitivity initially structured sedimentary architecture in response to the 41-kyr glacial-interglacial cycles and later to the 100-kyr cycles (Figs. 7 and 8).

# 6. Conclusions

This study underscores the interplay between orbital forcing, sea-level changes, and regional geodynamic events in controlling sediment deposition in the Amazon Fan. Our reassessment of the GR data for the BP-3 well reveals a strong imprint of Milankovitch cycles over the last 24

million years, emphasizing the fan's sensitivity to orbital climate variability. Our new astronomically tuned age model refined prior chronologies, enabling robust correlations between sedimentary cycles and GMSL. We show that low sedimentation rates prevailed during the Miocene, at a time when the combination of active Andean tectonics, uplift of the Fitzcarrald Arch, and enhanced regional precipitation in the Amazon Basin was most likely associated with high erosion rates. This discrepancy is best interpreted as reflecting efficient sediment trapping by mega-lake systems in western Amazonia. The subsequent drastic increase in sediment discharge to the Atlantic starting from the Early Pliocene indicates a major drainage reorganization across the Amazon Basin at that time, most likely caused by the breach of the Purus Arch, leading to enhanced sediment export to the Atlantic Ocean. Finally, in the Plio-Pleistocene, the GR record captures a major sensitivity of the fan to the GMSL changes imposed by the glacial-interglacial variability seen through the expression of isotopic stages. In this sense, the glaciation of the northern hemisphere has become a major force in fan sedimentary architecture. The shift from obliquity- to eccentricity-dominated glacial-interglacial cycles characterizing the mid-Pleistocene Transition (~1.3 Ma) further highlights the Amazon Fan's responsiveness to global climatic shifts. These findings provide a new framework for future high-resolution proxy investigations of the detailed evolution of the Amazon Basin during the Neogene and its link to global climate.

## CRediT authorship contribution statement

**Gabriel R. Moizinho:** Writing – original draft, Visualization, Investigation, Data curation, Conceptualization. **Thiago P. Santos:** Writing – original draft, Investigation, Funding acquisition, Formal analysis, Conceptualization. **Germain Bayon:** Writing – review & editing, Supervision, Project administration. **Martin Roddaz:** Writing – review & editing, Supervision, Resources. **Natalia Vázquez Riveiros:** Writing – review & editing, Supervision, Project administration. **Marina Rabineau:** Writing – review & editing, Validation, Supervision. **Daniel Aslanian:** Writing – review & editing, Validation, Supervision. **Victor Carreira:** Software, Methodology. **Elton L. Dantas:** Writing – review & editing. **Roberto V. Santos:** Writing – review & editing, Supervision, Resources, Project administration.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

The authors confirm that all data necessary for supporting the scientific findings of this paper have been provided.

## Acknowledgments

This research had financial support from the Coordination for the Improvement of Higher Personnel – Brazil (CAPES) – Finance Code 001, the Agence Nationale de Recherche ECLIP-SED project (ANR-20-01CE-0003), and the CAPES-COFECUB program through their funding of actions 32/2022 and Te 1003/23 (project numbers 8881.712022/2022-1 and 49558SM). TPS also thanks the Programa de Apoio a Novos Docentes (process 22.1.09345.01.2) of the University of São Paulo. We acknowledge the Brazilian Oil and Gas Agency (ANP) for giving access to the BP-3 well data (SAA 48610.204116/2022-88). We are very grateful for Dr. Fabienne Marret-Davis for her editorial handling and an anonymous reviewer for their constructive comments and review.
# Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.gloplacha.2025.105068.

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