New insights of the influence of ocean circulation on the sedimentary distribution in the Southwestern Atlantic margin (23oS to 55oS) based on Nd and Pb isotopic fingerprinting

In this work, we provide an extensive inventory of Pb and Nd radiogenic isotopes in surface sediments from the Southwestern Atlantic margin, aiming to interpret the role played by ocean circulation in sediment distribution. There are 20 latitudinal trends for Pb and Nd isotopes, reflecting the different current systems acting on the margin. The utilization of sediment fingerprinting allowed us to associate the isotopic signatures to the main oceanographic forcings in the area. We recognized differences between the Nd and Pb sources for the sediments to the Argentinean shelf, carried by the Subantarctic Shelf Water, and slope, transported by deeper flows. Sediments from Antarctica extend up to the Uruguayan margin, carried by the Upperand Lower Circumpolar Deep Water. Our data confirm that, for shelf and intermediate (up to 1,200 m water 25 depth) areas, the transfer of sediments from the Argentinean margin to the North of 35oS is limited by the Subtropical Shelf Front and the recirculated Antarctic Intermediate Water. On the southern Brazilian margin, it is possible to recognize the northward influence of the Río de la Plata sediments carried by the Plata Plume Water. This influence is limited by the southward flow of waters transported by the Brazil Current. Finally, we propose that the Subtropical Shelf Front and the Santos Bifurcation act as boundaries of geochemical provinces in the area. 30 Finally, a qualitative model of sediment sources and transport is provided for the Southwestern Atlantic margin.


Morphology
The Southwestern Atlantic margin is a typical segmented volcanic-rifted margin, where several transverse basins are recognized (Bassetto et al., 2000;Moulin et al., 2010;Soto et al., 2011). Its origin and evolution are intrinsically related to 75 the opening of the South Atlantic (Nürnberg and Müller, 1991), whose rifting processes first occurred during the Triassic (Lovecchio et al., 2020) but effectively took place in the Jurassic and Cretaceous.
There is a general trend of narrowing the margin's width 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 shelf-break depth ranges from 80 m, in southern Brazil, to 200 meters, in Uruguay (Zembruscki, 1979;Muñoz et al., 2010;80 Lantzsch et al., 2014). As a rule, the shelf morphology is relatively flat, but sequences of scarps and terraces are recognized along the whole continental shelf at varying water depths (Corrêa, 1996;Parker et al., 1996;Baptista and Conti, 2009). The sequence of terraces and adjacent scarps is associated with sea-level stabilization intervals after the Last Glacial Maximum (Corrêa, 1996;Violante et al., 2014). Relicts of complex barrier islands, sandbanks, ancient beaches, and shorelines, developed during the post-Last Glacial transgression, are also found along the shelf (Urien and Ewing, 1974;Urien et al., 1980a;Parker 85 et al., 2008;Violante et al., 2014;Cooper et al., 2018).
This fan-shaped feature extends from the outer shelf to the base of the slope, representing the deposition of more than 10 km of terrigenous sediments (Miller et al., 2015a;Alberoni et al., 2019). Some studies associate this feature with the input of sediments from the Río de la Plata under conditions of lowstand (Corrêa et al., 2014;Razik et al., 2015). 100 The base of the slope can extend up to 4500 m (Violante et al., 2017b), but is significantly shallower in the North, where an intense halokinesis promoted the development of a unique geomorphological feature, the São Paulo Plateau (Kumar and Gambôa, 1979;Mohriak et al., 2008;de Almeida and Kowsmann, 2016). https://doi.org/10.5194/os-2021-44 Preprint. Discussion started: 1 June 2021 c Author(s) 2021. CC BY 4.0 License.

Sedimentary cover
The Southwestern Atlantic margin is dominated by a terrigenous, siliciclastic sedimentary cover, with extensive sand sheets 105 (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 (in general 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).
Sandy and shelly sediments are mainly relicts of coastal and inner shelf environments evolved during Pleistocene transgressive-110 regressive events (Kowsmann and Costa, 1979;Urien et al., 1980b;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;Lourenço et al., 2017;. In the slope and rise, there is a prevalence of very fine sands and silty sands, resulting from exclusively submarine processes 115 occurring across-(gravitational) and along-(contouritic) slope, together with pelagic sedimentation 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). On the slope off southern Brazil, Razik et al. (2015) indicate increasing grain size towards coarse sands due to sediment remobilization and redistribution due to upwelling and downwelling resulting from 120 eddies and vertical water movement generated by the meandering Brazil Current.
Andean-sourced sediments, defined by Teruggi (1954) as "volcanic-pyroclastic Pampean-Patagonian association," dominate 125 most of the Argentinean margin. They are transferred to the sea from the mountain and peri-mountain regions to the plains and coast by eolian, fluvial, and glacial activity along with intricate multicyclic processes. Wave erosion of old continental sequences outcropping at the coast is the main factor as a sediment supplier to the shelf as it was also during lower-than-today sea-level positions (Gaiero et al., 2003;Violante and Parker, 2004;Isla and Cortizo, 2014;Violante et al., 2014). Another sediment source for the local concentration of gravels in the slope and rise are the icebergs that supplied large amounts of ice-130 rafted debris (containing rock fragments from Antarctica) during glacial times and concentrated in some places by bottom flows (Krinsley and Biscaye, 1973;Muñoz et al., 2012;Bozzano et al., 2021). Part of this ice-rafted debris reached the present Uruguayan margin (Franco-Fraguas et al., 2014).
The Plata basin-sourced sediments are mainly supplied by the Río de la Plata along the Uruguayan coast, reaching the southern Brazilian shelf (Mathias et al., 2014). A mixture of different sources is observed in these sediments (Manassero et al., 2008), 135 reflecting distinct sources. These sources include the metamorphic rocks from the Brazilian shield, basalts from the upper https://doi.org/10.5194/os-2021-44 Preprint. Discussion started: 1 June 2021 c Author(s) 2021. CC BY 4.0 License.
Paraná basin, and sedimentites/volcanic rocks supplied by the Pilcomayo and Bermejo rivers basins from the northern Argentine/Bolivian Andes and the Chaco plains.
Sediments from the Brazilian craton are originated from sources located to the north of the area of study (Anjos et al., 2007;Cruz et al., 2018) and transported by the Brazil Current (Viana et al., 1998;de Mahiques et al., 2004). The presence of a 140 1,000-km long coastal mountain range (Serra do Mar), extending from 28 o S to 23 o S, hampers the direct input of sediments to the adjacent margin (Cogné et al., 2011;.

Ocean Circulation
The Southwestern Atlantic margin is characterized by complex hydrography (Matano et al., 2010). It presents two main oceanographic boundaries, the Subtropical Shelf Front (STSF), as the shelf extension of the Brazil -Malvinas Convergence 145 (BMC) (Piola et al., 2000;Severov et al., 2012), and the less-studied Santos Bifurcation (SB) (Boebel et al., 1997;Boebel et al., 1999a). It is also influenced by the Rio de la Plata (RdlP), which discharges freshwater from the second-largest hydrographic basin in South America, with an average value of 22,000 m 3 s -1 (Framiñan and Brown, 1996). This regional circulation system experiences seasonal latitudinal shifts in response to wind regimes (Schmid et al., 2000;Piola and Matano, 2001;. 150 At the BMC, centered at 37-39°S (Maamaatuaiahutapu et al., 1992), the southward-flowing Brazil Current (BC) encounters the northward-flowing Malvinas Current (MC) (Schmid and Garzoli, 2009), displacing water masses with contrasting thermohaline characteristics. The BC is a baroclinic boundary current that concentrates its main flow up to 500 m water-depth, carrying the Tropical Water (TW) at the surface (Emilsson, 1961;Palma et al., 2008), and the South Atlantic Central Water (SACW) at pycnocline levels (Emilsson, 1961;Signorini, 1978). The MC is a strong barotropic boundary current that advects 155 the Subantarctic Water (SAW) at 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 displaced eastwards as 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, where it splits into two branches, 160 inducing the formation of 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 margin (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 flow to the south, following the BC, until its recirculation in the BMC (Schmid et al., 2000;Piola and Matano, 2019). This configuration leads to an overall southward flow on the outer shelf and outer to middle slope, from 28 o S up to the BMC. 165 Concerning deep circulation, the southward flow of the North Atlantic Deep Water (NADW) (Sverdrup et al., 1942), This large-scale oceanographic process is closely related to shelf circulation (Matano et al., 2010). 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 distribution appears stable throughout the year (Piola et al., 2000;Berden et al., 2020). At the STSF, the main branch is mixed with waters transported by the BC and 175 exported offshoreward 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 Nino events, the plume remains off the RdlP mouth and extends along the entire upper continental margin (Piola et al., 2000;Piola et al., 2005;. 180

Materials and Methods
In this study, the samples were organized in five distinct sectors, with names corresponding to the Santos, Pelotas, and Punta 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' 195 water -"Type 1"). The acids were purified in sub boiling distillers (DST-1000, ®Savillex) and sub boiling stills (®Savillex) at low temperatures. resin. After Pb collection, the remaining solution is dried out, and the residue retaken for separation of the rare earth elements using RE resin (EIChroM Industries Inc.) from the bulk solution. Nd was then separated using Ln resin (EIChroM Industries Inc.).
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 205 fractionation of 0.13%/amu based on repeated analysis of the NBS-981 standard (Pb/Pb = 16.893 ± 0.003; Pb/Pb = 15.432 ± 0.004, and Pb/Pb = 36.512 ± 0.014; n = 11), which yielded mass discrimination and fractionation corrections of 1.0024 (Pb/Pb), 1.0038 (Pb/Pb) and 1.0051 (Pb/Pb). The combination of these uncertainties and within-run uncertainties are typically 0.15%-0.48% for Pb/Pb, 0.13%-1.07% for Pb/Pb and 0.10%-0.45% for Pb/Pb, all at the 2σ (95%) confidence level. The total Pb blank contribution, <1 ng, is negligible. 210 The Nd analyses, here reported as ɛNd, were prepared by standard methods by the analytical procedures described by Sato et al. (1995) and Magdaleno et al. (2017), involving the removal of calcium carbonate, HF-HNO3 dissolution plus HCl cation exchange using a Teflon Powder column to separate REE. No visible solid residues were observed after dissolution. Samples with incomplete dissolution were discarded.
Nd determinations were performed on a Thermo Neptune Plus ICP-MS. Nd isotopic ratios (Nd/Nd) were normalized to the 215 value of 146 Nd/Nd = 0.7219 (DePaolo, 1981) and Nd/Nd = 0.512103 of the JNDi-1 standard (laboratory average of the last 12 months). Usually, a single analysis consisted of 60 measurements of Nd. The Nd/Nd mean average of the JNDi standard during the analyses was 0.512095 ± 0.000007 (n = 3) and 0.512096 ± 0.000005 between July and November of 2013 (n = 56).
To recognize the distinct isotopic domains over the study area, we applied a procedure similar to the one proposed by Walling (2013), Miller et al. (2015b), and Palazon and Navas (2017). First, a Kruskal-Wallis non-parametric analysis of variance was 225 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 values was used to determine the correct classification for the previously assigned groups.
To support the interpretation of the geochemical data distribution, we analyzed the output of the LLC2160 simulation, a global We used annual-mean fields of LLC2160 simulation to identify two key features: the Santos Bifurcation (SB) and the 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). 240 In our discussion below, we present the mean position and the latitudinal extension of the SB as a function depth.
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 245 Piola et al. (2008). In the yearly fields, the front follows approximately the 14 ºC isotherm.

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).   From the Kruskal-Wallis analysis, we observe that except for Pb/Pb, the variables show significant differences among the compartments, thus proceeding 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 275 Pelotas sector for all of the variables.

Discussion
Recognizing the role of circulation on the deposition of sediments requires associating the sedimentary provinces with potential source areas of sediments. Indeed, radiogenic isotopes are considered good sediment source fingerprints (Owens et al., 2016). 310 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 the comprehension of distribution detrital Nd in the world´s oceans. In the South Atlantic case, an important outcome provided by Beny et al. (2020) is the summary of Nd, Pb, and Sr signatures provided for the potential sources and circulation in the area. i. Neoarchean and Proterozoic metasediments from the coastal region of southeastern Brazil (Ragatky et al., 2000); 330 j.
Proterozoic granites from the coastal region of southeastern Brazil (Mendes et al., 2011).  The samples from Santos Basin present lower radiogenic Nd and higher radiogenic Pb values, thus indicating a Pre-Cambrian 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., 365 2000;Moraes et al., 2004;Mendes et al., 2011). A possible explanation for this discrepancy lies in the fact that the input of sediments from the adjacent coast is hampered by the presence of the Serra do Mar mountain chain, which limits the development of expressive drainage basins in the area. In this sense, we cannot discard the possibility that a significant part of 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). 370 Another point of view can be obtained from the classed plot according to latitude and water depth (Figures 8 and 9).    Boebel et al., 1999a;Boebel et al., 1999b;Piola et al., 2008). The LLC2160 output allows us to present, together with εNd, the bathymetrical variations of those features. As observed, there are clear distinctions in the signature corresponding to both fronts. In the case of the STSF, our findings confirm the conclusions previously stated by Franco-Fraguas et al. (2014). The STSF presents only minor seasonal variations, 390 and its control is probably related to the interaction between the RdlP plume and the subsurface water masses distribution.
During austral summer, the strong stratification  inhibits RdlP sedimentation southward of the STSF.
During austral winter, the northeastward RdlP plume promotes the offshore displacement of subtropical waters , enabling the deposition of fine sediments on the shelf north of the STSF. On the upper and middle slope, the southward displacement of the Brazil Current, as well as of the recirculated AAIW, likely limits RdlP sedimentation (Schmid et al., 2000). 395 There is

405
This integrated analysis allows us to interpret that there is no transport of sediments from the Argentinean sector to the southern Brazilian margin. On 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 are also limiting distinct geochemical provinces on the 410 southwestern Atlantic margin.

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. 430 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, 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 recognized as the primary influencer of the sediments off southern Brazil up to the 27 o S parallel. The 435 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.
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. 440 We propose that the main oceanographic boundaries of the southwestern South Atlantic margin, i.e., the Subtropical Shelf Front and the Santos Bifurcation, act as limits of distinct geochemical provinces. Thus, the boundary represented by the STSF extends towards deeper areas along with the Brazil -Malvinas Confluence.

Data availability
All of the data used in this paper is presented as Supplementary Material 445

Acknowledgments
The authors acknowledge the crew of R.V. Alpha Crucis and participants of research cruises Mudbelts I and II and Talude I and II for helping during the sampling surveys. This work is a contribution to the Grupo de Investigación en Ciencia y Tecnología Marina (CINCYTEMA) as well as to the MOU between the Oceanographic Institute of University of São Paulo (Brazil)