Abstract The organic matter (OM) of surface sediments (0–2 cm) from the Santos Basin was characterized by employing elemental (C, N, P), isotopic (δ 13C and δ 15N), biochemical (total carbohydrates, proteins, and lipids), and pigment (chlorophyll-a and phaeopigments) proxies, as part of the Santos Project – The Santos Basin Regional Environmental Characterization (PCR-BS) – coordinated by PETROBRAS. Samples were collected in a regional scale throughout the entire basin, including 11 isobaths from eight cross-margin transects ranging from 25 to 2,400 m water depths, and 12 additional samples obtained from the São Paulo Plateau. The aims of this study were two-fold, namely, to identify the major oceanographic and depositional processes driving OM transport and accumulation and to provide information on OM origin and availability to benthic secondary producers. The multiple employed proxies and the use of machine learning allowed to categorize distinct OM accumulation provinces displaying two major regional features: shallow sediments (< 50-m isobaths) are OM-poor (TOC < 2.0 mg g -1) but the available amount is readily hydrolysable, and mid-shelf deposits (75-m and 100-m isobaths) throughout the entire basin present muddy and OM-rich sediments (TOC ranging from 7.0 to 14.0 mg g-1) seemingly available to heterotrophs. Similarly, relatively OM-rich deposits are found at depths ranging from 700 to 1,300-m isobaths throughout the basin, but their composition and nutritional value (as suggested by biochemical indicators) are not related to mid-shelf deposits. The general overview of OM accumulation and composition provided herein validated the selected organic geochemical proxies for benthic ecology studies at the shelf and deep ocean sectors of the studied region. (OM 0–2 02 0 2 (0– cm C, C (C N P, P , P) 15N, 15N 15N) total carbohydrates proteins lipids, lipids lipids) chlorophylla chlorophyll phaeopigments PCRBS PCR BS (PCR-BS PETROBRAS 1 crossmargin cross margin 2400 400 2,40 Plateau twofold, twofold fold, fold two-fold namely producers features ( 50m 50 OMpoor poor TOC 20 2. -1 hydrolysable midshelf mid 75m 75 100m 100 OMrich rich 70 7 7. 140 14 14. g1 g-1 heterotrophs Similarly 1,300m 1300m 1,300 300 indicators region 0– (0 240 40 2,4 5 - 10 g- 300m 1300 1,30 30 24 4 2, 130 1,3 3 13 1,

Abstract This study aims model the distribution of meiofauna indicators in relation to environmental variables from the Santos Basin continental margin, SE Brazil, using machine learning techniques, to provide baseline information and foster future monitoring programs. A total of 100 sampling stations were distributed in eight transects and 11 isobaths (25 to 2,400 m) perpendicular to the coast. In each station, three replicates were sampled for meiofauna and 38 environmental parameters. A total of 28 meiofauna taxa were found, with a mean richness varying from 3 to 15 taxa per station. Meiofauna mean density varied between 55 and 2,001 ind. 10 cm-2. Density of meiofauna and its most frequent taxa (Nematoda, Copepoda, Kinorhyncha, and Polychaeta), and taxa richness were used as descriptors for the models. Meiofauna and nematode density showed the highest training and testing accuracies, with R² values above 0.74. Based on the distribution of meiofauna descriptors and their responses to environmental conditions, we suggest a mosaic of six benthic zones. The La Plata Plume zone and the Cabo Frio Upwelling zone are two of the most diverse and productive zones in the continental shelf, wich are separated by the less productive Central Continental Shelf zone. A fourth zone, with very low meiofauna densities, corresponds to the carbonated sediments of the shelf-break. The Upper and Mid-Slope is a narrow zone along the entire basin, with intermediate densities and small amounts of high-quality organic carbon. The largest, impoverished zone, the Lower Slope and Plateau comprises the deepest areas and the São Paulo Plateau. The study showed that, although some zones can be recognized by most meiofauna descriptors, others are better characterized by specific ones, implying that meiofauna indicators should be monitored concomitantly. We recommend the optimization of sampling design based on our model to reduce costs and increase our understanding of the system. margin Brazil techniques programs 1 25 (2 2400 2 400 2,40 m coast station parameters found 5 2001 001 2,00 ind cm2. cm2 cm 2. cm-2 Nematoda, Nematoda (Nematoda Copepoda Kinorhyncha Polychaeta, Polychaeta , Polychaeta) models accuracies R 074 0 74 0.74 conditions shelf shelfbreak. shelfbreak break. break shelf-break MidSlope Mid basin highquality high quality carbon largest ones concomitantly system ( 240 40 2,4 200 00 2,0 cm- 07 7 0.7 24 4 2, 20 0.