Control of oceanic circulation on sediment distribution in the Southwestern Atlantic margin (23 o S to 55 o S)

. In this study, we interpret the role played by ocean circulation in sediment distribution on the Southwestern Atlantic margin using radiogenic Nd and Pb isotopes. The latitudinal trends for Pb and Nd isotopes reflect the different current systems acting on the margin. The utilization of the sediment fingerprinting method allowed us to associate the isotopic signatures to the main oceanographic features in the area. We recognized differences between Nd and Pb sources to the Argentinean shelf 20 (carried by the flow of Subantarctic Shelf Water) and slopes (transported by deeper flows). Sediments from Antarctica extend up to the Uruguayan margin, carried by the Upper- and Lower Circumpolar Deep Water. Our data confirm that, for shelf and intermediate areas (the upper 1,200 meters), the transfer of sediments from the Argentinean margin to the North of 35 o S is limited by the Subtropical Shelf Front and the basin-wide recirculated Antarctic Intermediate Water. On the southern Brazilian inner and middle shelf, it is possible to recognize the northward influence of the Río de la Plata 25 sediments carried by the Plata Plume Water. Another flow responsible for sediment transport and deposition on the outer shelf and slope is the southward flow of the Brazil Current. Finally, we propose that the Brazil-Malvinas Confluence and the Santos Bifurcation act as boundaries of geochemical provinces in the area. A conceptual model of sediment sources and transport is provided for the Southwestern Atlantic margin.

in the distribution of sediments. The results are interpreted with the aid of the output of a state-of-the-art circulation model to understand the role of oceanographic boundaries in the distribution of sediments along the area.
There is a general trend of narrowing the margin towards the North (Urien and Ewing, 1974;Zembruscki, 1979;Parker et al., 1996;Violante et al., 2017a). The shelf width varies from 850 km to the south to 70 km in its northernmost limit; the shelfbreak depth ranges from 80 m, in southern Brazil, to 200 meters, in Uruguay (Zembruscki, 1979;Muñoz et al., 2010;Lantzsch 95 et al., 2014). The shelf morphology is relatively flat, but sequences of scarps and terraces are recognized along the continental shelf at varying water depths (Corrêa, 1996;Parker et al., 1996;Baptista and Conti, 2009).

Sedimentary cover 105
The Southwestern Atlantic margin is dominated by a terrigenous, siliciclastic sedimentary cover, with extensive sand sheets (Lonardi and Ewing, 1971;Frenz et al., 2003;Figueiredo and Madureira, 2004). The Argentinean and Uruguayan shelves are capped mainly by a 5 to 15 m thick post-Late Glacial transgressive sandy sheet (with decreasing thickness towards the south) composed of dominant medium to fine sands (sometimes muddy), with varying amounts of shells (more abundant in the Uruguayan shelf) and gravels (more abundant in the Patagonian shelf). 110 Sandy and shelly sediments are mainly relicts of coastal and inner shelf environments that evolved during Pleistocene transgressive-regressive events (Kowsmann and Costa, 1979;Urien et al., 1980;Lantzsch et al., 2014). Therefore, they are considered relict and palimpsest, whereas gravelly-dominated sediments on the southern Argentinean shelf result from glacifluvial origin. More recent works emphasize the existence of mud depocenters as potential fates of modern sediments on the southern Brazilian shelf (Nagai et al., 2014a;de Mahiques et al., 2017;Lourenço et al., 2017;de Mahiques et al., 2020). 115 In the slope and rise, there is a prevalence of very fine sands and silty sands, resulting from exclusively submarine processes occurring across-(gravitational) and along-(contouritic) slope, together with pelagic sedimentation (Violante et al., 2010;Bozzano et al., 2011;Franco-Fraguas et al., 2016;Schattner et al., 2020). However, coarse sands and gravels occur at or near the head of submarine canyons and in contouritic channels and moats (Lonardi and Ewing, 1971;Bozzano et al., 2011;Reis et al., 2016;Franco-Fraguas et al., 2017). Razik et al. (2015) indicate increasing grain size towards coarse sands due to 120 sediment remobilization and redistribution due to upwelling and downwelling resulting from eddies and vertical water movement generated by the slope off southern Brazil meandering Brazil Current.
At the BMC, centered at 37-39°S (Maamaatuaiahutapu et al., 1992), the southward-flowing Brazil Current (BC) encounters 130 the northward-flowing Malvinas Current (MC) (Schmid and Garzoli, 2009), transporting and mixing water masses with contrasting thermohaline characteristics. The BC is a baroclinic boundary current that concentrates its main flow in the upper 500 m water-depth upstream of 28 o S, carrying the Tropical Water (TW) at the surface (Emilsson, 1961;Palma et al., 2008), and the South Atlantic Central Water (SACW) at pycnoclinic levels (Emilsson, 1961;Signorini, 1978). Near the BMC, a significant fraction of the BC transport is below 500m. The MC is a strong barotropic boundary current that advects the 135 Subantarctic Water (SAW) near the surface (Spadone and Provost, 2009), and the Antarctic Intermediate Water (AAIW) at intermediate levels (Tomczak and Godfrey, 1994a, b).
At the BMC, water masses are transported eastwards as part of the southern limb of the basin-wide Anticyclonic Atlantic Subtropical Gyre (Boebel et al., 1997;Boebel et al., 1999a;Schmid et al., 2000;Núñez-Riboni et al., 2005;Legeais et al., 2013). At the intermediate levels of the westward flow of the gyre, the water reaches the South American margin near 28 o S, 140 where it splits into two branches, forming the Santos Bifurcation (Boebel et al., 1999a;Legeais et al., 2013). From the bifurcation, one-quarter of the transport at 40 o W flows northward along the continental slope (mainly between the 800 and 1,200 m isobaths), forming the Intermediate Western Boundary Current (IWBC) (Fernandes et al., 2009;Biló et al., 2014).
About three-quarters feed the BC, flowing south until its separation from the coast at the BMC (Schmid et al., 2000;Piola and Matano, 2019). This configuration leads to an overall southward flow on the outer shelf and the outer to middle slope, from 145 28 o S up to the BMC.
Concerning deep circulation, the North Atlantic Deep Water (NADW) (Sverdrup et al., 1942), transported from the northern hemisphere high latitudes by the Deep Western Boundary Current, occupies the region between the 2,000 and 3,000 m isobaths.
The NADW flows between two northward-flowing branches of the Circumpolar Water (i.e., Upper and Lower Circumpolar Deep Water). The abyssal circulation (> 3,500 m) is dominated by the Antarctic Bottom Water (AABW), which is partially 150 trapped in the Argentine Basin (Tarakanov and Morozov, 2015).
Over the shelf, the extension of the BMC, known as the Subtropical Shelf Front (STSF), separates Subtropical Shelf Waters (STSW, formed by the mixture of the TW and SACW) and Subantarctic Shelf Waters (SASW) (Piola et al., 2000). This narrow and sharp front extends between 32°S at 50 m of water column depth and 36°S over the shelf break, and its position appears stable throughout the year (Piola et al., 2000;Berden et al., 2020). The main branch of the STSF is mixed with waters 155 transported by the BC and exported offshore along with the BMC. A secondary branch is diluted with the PPW and TW and returns along the shelf (Berden et al., 2020).
At the surface, the low-salinity RdlP plume flows northward along the inner Uruguayan continental shelf during the austral winter. In the summer and during El Niño events, the plume remains off the RdlP mouth and extends along the entire upper continental margin (Piola et al., 2000;Piola et al., 2005;. 160

Geochemical Analyses
The samples were organized in five distinct sectors in this study, corresponding to the Santos, Pelotas, and Punta del Este All chemical procedures were performed in class 10,000 cleanroom equipped with laminar flow hoods class 100. All reagents were purified before use. Water was distilled and then purified on a Milli-Q System (®Millipore Corporation) ('ultrapure' water -"Type 1"). The acids were purified in sub boiling distillers (DST-1000, ®Savillex) and sub boiling stills (®Savillex) at low temperatures.
All of the samples were previously decarbonated with HCl. Sediment powder (70 mg) was dissolved with HF, HNO3, and 180 HCl acids. Dissolution was done on a MARS-5 microwave oven. Both Pb and Nd were purified by the ion-exchange technique. The first stage of ion-exchange chromatography involves separating Pb from the other matrix elements using columns packed with anion exchange AG1-X8, 200-400 mesh (Biorad) resin. After Pb collection, the remaining solution is dried out, and the residue is retaken to separate the Rare Earth Elements using RE resin (EIChroM Industries Inc.) from the bulk solution. Nd was then separated using Ln resin (EIChroM Industries Inc.). 185 Pb isotopic compositions were measured on a Finnigan MAT 262 Mass Spectrometer. Samples were loaded on Re filaments with H3PO4 and silica gel. Every single analysis consisted of 60 ratio measurements. The Pb ratios were corrected for mass fractionation of 0.13%/amu based on repeated analysis of the NBS-981 standard ( 206 Pb/ 204 Pb = 16.893 ± 0.003; 207 Pb/ 204 Pb = 15.432 ± 0.004, and 208 Pb/ 204 Pb = 36.512 ± 0.014; n = 11), which yielded mass discrimination and fractionation corrections of 1.0024 ( 206 Pb/ 204 Pb), 1.0038 ( 207 Pb/ 204 Pb) and 1.0051 ( 208 Pb/ 204 Pb). The combination of these uncertainties and within-run 190 uncertainties are typically 0.15%-0.48% for 206 Pb/ 204 Pb, 0.13%-1.07% for 207 Pb/ 204 Pb and 0.10%-0.45% for 208 Pb/ 204 Pb, all at the 2σ (95%) confidence level. The total Pb blank contribution, <1 ng, is negligible.
The Nd analyses, here reported as ɛNd, values, were prepared by standard methods by the analytical procedures described by Sato et al. (1995) and Magdaleno et al. (2017) To recognize the distinct isotopic domains over the study area, we applied the Geochemical Fingerprinting procedure, similar to the approaches proposed by Walling (2013), Miller et al. (2015), and Palazon and Navas (2017). First, a Kruskal-Wallis non-parametric analysis of variance was applied for each variable, followed by a Mann-Whitney pairwise post-hoc test to identify which variables presented statistically significant differences. Finally, a Discriminant Analysis with standardized 215 values was used to determine the correct classification for the previously assigned groups.

The LLC2160 Circulation Model
To support the geochemical data distribution interpretation, we analyzed the output of the LLC2160 simulation, a global 1/24º forward run of the Massachusetts Institute of Technology General Circulation Model (MITGCM) that was spun up from We used annual-mean fields of LLC2160 simulation to identify two key features: the Santos Bifurcation (SB) and the 225 Subtropical Shelf Front (STSF). The SB is recognized as the region on the continental slope where the flow within the AAIW depth range (550-1400 m) is negligible. Specifically, we search on different isobaths ranging from 500 m to 1500 m for the region where the AAIW flow is weaker than 0.01 m/s. We emphasize that the SB is not a stagnation point where the flow is zero but a shadow zone that spans nearly 100 km, wherein the intermediate flow is feeble (see the schematic SB in Figure 1).
In our discussion below, we present the mean position and the latitudinal extension of the SB as a function depth. 230 To identify the mean position of the STSF, we searched for the local maximum of the potential temperature gradient, which is a very distinct feature on the northern Argentina/southern Brazil shelf. We compute the potential temperature gradients at 40 m to avoid contamination by RdlP water (e.g., Piola et al., 2008). When applied to the LLC2160 output using seasonal averages, our method yielded frontal locations consistent with those identified by applying the isothermal criteria at 40 m proposed by Piola et al. (2008). In the yearly fields, the front follows approximately the 14 ºC isotherm. 235

Results
The results of isotopic analyses are presented in the Supplementary Material and summarized in the box plots shown in Figure   2. We also present the latitudinal variation of each isotope (Figure 3). Nd values show a northward trend to less radiogenic values, varying from -0.1 (Argentina) to -17.1 (Santos Basin) ( Figure   3a). The latitudinal variation of the Pb isotopes is less clear but still visible for 207 Pb/ 204 Pb and 208 Pb/ 204 Pb (Figures 3c and 3d From the Kruskal-Wallis analysis, we observe that except for 206 Pb/ 204 Pb, the variables show significant differences among the 255 compartments, thus allowing us to proceed with the Discriminant Analysis. Furthermore, the Mann-Whitney analysis allowed us to recognize the pairwise differences among the other variables (Table 1). Finally, it is to be noted that sediments from Argentina showed statistically significant differences with all of the variables analyzed, suggesting that they are distinct from those located towards the North. On the other hand, sediments from the Rio de la Plata are statistically similar to those from the Pelotas sector for all of the variables. 260  (Figure 4). It is possible to recognize that samples from Argentina are Graphical representations of the LLC2160 output are presented for both the Santos Bifurcation ( Figure 5) and Subtropical Shelf Front (Figure 6). The Santos Bifurcation is identified as the maximum horizontal velocity divergence region at the AAIW 275 level, identified in Figure 5A close to 26 o S. The visualization based on the horizontal fields is more complicated but still visible as the sector with velocities close to 0 m/s ( Figure 5B). The Subtropical Shelf Front ( Figure 6A) is identified as a maximum subsurface temperature gradient ( Figure 6B). Vertically it is well marked below the 30 m isobath ( Figure 6C).

Discussion
The integration of both isotopic signatures (Nd,207 Pb/ 204 Pb,and 208 Pb/ 204 Pb) and hydrographic (water masses) and hydrodynamic (currents) information is presented in figures 7 to 9, respectively. This information is essential to infer both 295 sediment sources and the role played by ocean circulation in the distribution of sediments in the study area.

Sediment sources
Recognizing the role of circulation on the deposition of sediments requires an association of the sedimentary provinces with potential source areas. Indeed, radiogenic isotopes are considered good sediment source fingerprints (Owens et al., 2016). Two seminal papers, by Goldstein et al. (1984) and Bayon et al. (2015), used Nd isotopes and other proxies from the world´s rivers and provided the basis for comprehending distribution detrital Nd in the world´s oceans. Beny et al. (2020) provided 320 the summary of Nd, Pb, and Sr signatures in the South Atlantic, looking for the potential sources and circulation in the area.
More recently, a work by Höppner et al. (2021) provided new data on the isotopic signatures of the river sediments that feed the RdlP basin. Worth noting that the RdlP-Paraná-Uruguay riverine system drains several types of terranes, such as pre-Cambrian rocks of the Brazilian shield, Paleozoic sediments, and tholeiitic basalts from the Paraná Basin, and Cenozoic Andean rocks. 325 Table 2 provides a list of Nd and Pb isotopic signatures of potential continental materials (rocks and sediments) for the study area. It is possible to recognize a trend of decreasing values of Nd towards the north, as already observed in our samples.
Concerning Pb isotopes, the small number of data hampers the recognition of a trend. Isotopic distinctions and similarities among the sectors are recognized from the interpretation of the results of the Discriminant Analysis ( Figure 4). Sediments from Argentina and part of the Punta del Este Basin present isotopic signatures similar to the 335 values obtained for Patagonia (Gaiero et al., 2007;Bayon et al., 2015;Khondoker et al., 2018). The deepest samples of the dataset, located in the Punta del Este basin, at water depths between 3579 and 4066 meters, present Nd values of -5.33 and -4.26, respectively. These values are consistent with those from the Antarctic Peninsula and West Antarctica (Roy et al., 2007).
They can indicate a provenance of sediments via the flow of the Upper-and Lower-Circumpolar Deep-water masses (UCDW and LCDW, respectively) (Beny et al., 2020) or even from ice-rafted debris (Bozzano et al., 2021). Another sample, located 340 at 2378 meters, presents an Nd value of -4.80 but is presently under the influence of the NADW southward flow (Figure 7).
The distinct character of these samples also resides in the fact that they are lower 207 Pb/ 204 Pb and higher 208 Pb/ 204 Pb radiogenic than those located in shallower areas (Figures 8 and 9). The remainder of the Punta del Este basin samples, situated on the shelf, might represent a mixture of Patagonian and Río de la Plata sediments. The samples from Santos Basin present lower radiogenic Nd and higher radiogenic Pb values, thus indicating a Pre-Cambrian 355 source, as Mantovanelli et al. (2018) stated. Nevertheless, the values obtained for Pb isotopes differ significantly from those reported by the literature for the Precambrian metasediments and granites of the southeastern Brazilian coast (Ragatky et al., 2000;Moraes et al., 2004;Mendes et al., 2011). A possible explanation for this discrepancy is that the input of sediments from the adjacent coast is hampered by the Serra do Mar mountain chain, limiting the development of expressive drainage basins in the area (Riccomini et al., 2010;Cogné et al., 2011). In this sense, we cannot rule out the possibility that a significant part of 360 sediments that presently cover the shelf and upper slope of the Santos basin is originated further north and transported by the Brazil Current and derived shelf dynamics (Castro and Miranda, 1998;Silveira et al., 2017).

Isotope fingerprinting and ocean circulation
The geochemical fingerprinting approach confirmed the suitability of using Nd and Pb isotopes (except 206 Pb/ 204 Pb) as reliable proxies for the discrimination among the distinct sectors of the Southwestern Atlantic margin. Indeed, the first axis provided more than 99% of explained variance (Figure 4). The distribution of the samples and, together with the recognition of the 370 potential sources (Table 2), allows tracing a correlation among isotopic signatures, sediment sources, and ocean circulation.  Boebel et al., 1999a;Boebel et al., 1999b;Piola et al., 2008). The LLC2160 output, together with the isotopic values, allows us to present the bathymetrical variations of those 375 features. As observed, there are clear distinctions in the signature corresponding to both fronts. The STSF presents only minor seasonal variations, and its control is probably related to the interaction between the RdlP plume and the subsurface water This integrated analysis suggests no transport of sediments from the Argentinean sector to the southern Brazilian margin. On 385 the other hand, based on the same analysis, we can confirm that sediments from the Rio de la Plata reach, at least partially, the Santos sector, i.e., to the north of 28 o S. Concerning the SB, there is a clear distinction in isotopic signatures below the 500 m isobath, less radiogenic Nd prevailing to the north of the bifurcation. We thus argue that both STSF and SB also separate distinct geochemical provinces on the Southwestern Atlantic margin.
The Argentinean and part of the Uruguayan upper margins are covered by Andean-Patagonian sediments, redistributed by the 390 shelf circulation and Malvinas Currents. The STSF and BMC block the transport of these sediments to the north. This finding corroborates Hernández- Molina et al. (2016) and Franco-Fraguas et al. (2016), who defined the northernmost limit of a megacontouritic feature on the Uruguayan slope. Sediments located more profound than the 2,000 m isobath present an Antarctic signature, transported either by the bottom circulation (UCDW and LCDW) or ice-rafted debris.
Sediments from the Río de la Plata estuary advance along the inner shelf towards southern Brazil and a mixture of Pelotas and 395 Santos signatures are observed between 28 o S and 30 o S. This mixture is visible in the scatter plot presented in Figure 4, in which sediments of Pelotas Basin constitute a mixture of distinct populations, i.e., Santos Basin and Río de la Plata. It is essential to highlight that, on interannual time scales, the influence of El Niño-Southern Oscillation (ENSO) in the precipitation regime determines changes in the freshwater outflow of the RdlP. Cold and warm episodes of ENSO cause drought and abundant rainfall in Uruguay, southern Brazil, and north-eastern Argentina (Pisciottano et al., 1994;Cazes-Boezio et al., 2003). 400 In addition, changes in the wind patterns during the warm phase of ENSO determine the influence of the PPW towards higher latitudes. In conjunction with the Coriolis force, this phenomenon determines that the PPW follows a NE direction close to the shelf break, explaining the distribution up to 28 o S and, in anomalous years, 25 o S (Piola et al., 2005).
Finally, sediments located northward of 27 o S originate from the Precambrian rocks that dominate the coastal domains off SE and E Brazil, being mainly transported by the intense flow of the BC on the outer shelf and upper slope. Limited input comes 405 from the small rivers that drain the mountainous areas of the Serra do Mar, as previously reported by Lourenço et al. (2017) and de  410

Conclusions
In this paper, we use Nd and Pb radiogenic isotopes to recognize the role of ocean circulation in the sediment distribution of the Southwestern Atlantic margin.
Andean and continental Patagonian sediments are the primary source for the deposits of the Argentinean and Uruguayan shelves, while the lower slope is more influenced by more distant sources, such as the Antarctic Peninsula. Nevertheless, 415 sediments on the shelf and upper slope are carried by the flows of the SASW and AAIW, while the UCDW and LCDW transport sediments from the lower slope.
The Río de la Plata is the primary influencer of the sediments off southern Brazil up to the 27 o S parallel. The sediments are transported northwards by the PPW, which is transported by a wind-driven current. A mixture of sediments from the PPW and the north is transported towards the slope between 34 o S and 28 o S. 420 Finally, Pre-Cambrian terrains are the primary sources of the sediments deposited further north. They are originated from rivers located northward of the area of study and, on a smaller scale, by the small drainages that face the ocean in the Serra do Mar region.
We propose that the main oceanographic boundaries of the southwestern South Atlantic margin, i.e., the Subtropical Shelf Front and the Santos Bifurcation, separate distinct geochemical provinces. 425