Mud depocentres on the continental shelf: a neglected sink for anthropogenic contaminants from the coastal zone
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In this study, published and unpublished data from the Santos Estuarine Complex and Bay and the adjacent continental shelf (São Paulo State, Brazil) were gathered in order to evaluate the entrapment of anthropogenic chemical contaminants (hydrocarbons, heavy metals) in a mid-shelf mud depocentre. Results show that these contaminants, produced by industrial activities in the adjacent coastal zone and released into the bay waters, are distributed far over the shelf since they are found in the mid-shelf mudbelt in locally significant concentrations. Two main aspects are highlighted by this study. The first underlines the fact that the material stored in the mudbelt is not related to a specific fluvial source discharging to the shelf. Instead, the contaminants, used as tracers, stem from multiple injection sources along the heavily used coastline of the Santos industrial zone. The second finding suggests that the anthropogenic compounds are not only accumulating in the surface sediments of fine-grained shelf depocentres. Rather, these substances are also already found several centimetres below the modern seabed. They can, thus, be easily reinjected into the water column by storms, benthic activity, and human disturbances such as seabed dredging and bottom trawling.
KeywordsMudbelt Continental shelf Polycyclic aromatic hydrocarbons Metals Shelf sedimentation Source to sink
The continental shelf is a complex system in which seabed morphology, subbottom stratigraphy, sedimentary surface processes, benthic ecology, and human-related modifications interact in a complex environment where wind-driven waves, tides, ocean currents, material input from the continent, and human activity find an imprint through time and space (Nittrouer et al. 2007; Shi et al. 2012). Among numerous reasons which make continental shelves relevant to human societies, one might highlight that shelves (a) are the most proximal and sensitive marine environments in the land-to-sea system; (b) contain significant scarce resources of highest economic and ecological value; (c) influence the impact of hazardous events on the coastal zone and its infrastructure; and (d) represent a potential sink for many kinds of anthropogenic substances, produced either offshore, in the coastal zone, or further inland.
The majority of modern clastic shelves are characterized by the presence of confined, elongated mud deposits which extend at a specific bathymetrical zone and which show individual three-dimensional geometries and sedimentary characteristics. The formation patterns result from the regional sea-level history and tectonic setting, the regional climate regime, type and intensity of river sediment input, and the regional to local shelf hydrodynamics, among other controlling factors (e.g. McCave 1972a; Edwards 2002; Gonsalez et al. 2004; McKee et al. 2004; Palinkas et al. 2006; Dubrulle et al. 2007; Sommerfield and Wheatcroft 2007; Lantzsch et al. 2009; Rose and Kuehl 2010; Oberle et al. 2014a).
The potential of these mudbelts as sinks for anthropogenic substances was reported by several studies for more than 20 years (e.g. Palanques et al. 1990, 2008; Lin and Chen 1996; Gonsalez et al. 2007; Hartwell 2008; Liu et al. 2011). The pathways of the transfer from the continental source to the proximal-marine sink are usually, but not necessarily, related to the existence of a dominant fluvial source (Lin et al. 2002; Palanques et al. 2008). Also, the anthropogenic contamination can be either of organic (Lee et al. 2006; Hartwell 2008) or of inorganic origin (Lin and Chen 1996; Rúa et al. 2014). In this sense, sediment depocentres located at sites just off the coast show high concentrations of both organic and inorganic pollutants which are often affected by remobilization through human activities, e.g. bottom trawling fishing and dredging (Riemann and Hoffmann 1991; Warnken et al. 2003, Deepthi et al. 2014).
The south-eastern Brazilian shelf is adjacent to the most industrialized coastal area of Latin America, the Santos Estuarine Complex and Bay (SECB). The SECB houses one of the most important harbours in the Southern Hemisphere and an industrial complex with significant oil refining, petrochemical, and steel-producing industries. The area has a one-century history of organic and inorganic contamination which included intense water and air pollution, disposal of untreated sewage in the estuarine channels and mangrove fringes, and utilization of heavy metal-based antifouling ship paints. In fact, this situation started to change not earlier than in the early 1980s, when the government started a stricter control of environmental protection governance (Luiz-Silva et al. 2008; Martins et al. 2010, 2011).
Moreover, apparently no major fluvial sources exist along this coast which could supply freshwater and terrigenous sediments to the shelf, since the drainage systems are rather limited in size due to the presence of a pronounced mountain chain along the coast (Serra do Mar). The maximum river flow which drains the area does not exceed 1000 m3 s−1 and is centred about 200 km to the south. Therefore, the distribution of the various anthropogenic substances to the adjacent shelf was for decades considered as being widely limited (Kowsmann et al. 1977). The possibility that any export of anthropogenic products to the shelf would find an imprint in the mid-shelf mud depocentres has, consequently, always been neglected. However, such an injection of contaminants to the ocean habitats at considerable levels would probably have severe ecological consequences.
The aim of this study is to characterize the continental shelf mud depocentres off the SECB as potential sinks for anthropogenic organic (particularly polycyclic aromatic hydrocarbons/PAHs and aliphatic hydrocarbons) and inorganic (heavy metals) components. The SECB continental shelf seafloor is located in close adjacency to the most industrialized area of South America and, thus, deserves a more detailed analysis on its role as a remote fate location of anthropogenic substances.
The shelf off the SECB (Fig. 1) corresponds to the central part of the São Paulo Bight, an arc-shaped sector of the Brazilian inner continental margin, which extends from 22°S to 28°S (Zembruscki 1979). The area in front of the SECB is characterized by a flat topography (1:1300) and very wide extent (230 km). The shelf-break occurs at about 160–180 mbsl (metres below modern sea level).
Due to the lack of a main fluvial input, the surface sediment distribution pattern is controlled by hydrodynamics and the local bathymetry (Mahiques et al. 2004). The inner shelf is covered by fine to very fine sands, extending down to the 50-m isobath. This sandy facies results from both wind-driven wave action and a northward-directed wind-driven shelf current, the latter being more effective during austral winter months.
The outer shelf is characterized by a wide sheet of medium- to coarse-grained relict sands being result of the permanent floor-polishing effect of the meandering Brazil Current (Mahiques et al. 2002, 2004). In this outermost shelf zone, modern sedimentation rates are negligible and surface sediments date back to early Holocene ages (Mahiques et al. 2011).
The inner and mid-shelf hydrographical structure is determined by the seasonal displacement of three water masses with strong seasonal variation (Castro et al. 1987). Between November and March, the South Atlantic Central Water (SACW; T = 14.0 °C, S = 35.5) moves on the bottom towards the coast, leading to an oceanward displacement of the less dense superficial Coastal Water (CW; T = 22.0 °C, S < 35.0) and keeping the outer-shelf Tropical Water (TW; T = 25.0 °C, S = 37.1) relatively distant from the coastline. This period corresponds to the rainy season in SE Brazil and, thus, to an increased terrigenous flux to the coastal waters. This superficial displacement of the CW is the most important factor in the transport of terrigenous organic matter towards the deeper areas of the shelf. From March to November, the retreat of the SACW leads to a greater influence of the TW on shelf processes (Mahiques et al. 1999).
Materials and methods
This paper gathers partially published (Figueira et al. 2006; Martins et al. 2007, 2008, 2010) and unpublished data from surface sediments and sediment cores (Fig. 1) collected in the Santos Estuarine Complex and Bay, as well as on the adjacent shelf, including a mid-shelf mudbelt depocentre.
For organic component analysis, eight sediment samples were collected in the Santos Bay while 21 samples were taken on the adjoining continental shelf at the sites shown in Fig. 1, using a box corer during the austral winter of 2005. The sediment surface layer (0–2 cm) was stored in pre-cleaned aluminium foil at −15 °C. Three of the box cores, taken in muddy areas, were continuously sampled in one-cm intervals. The sediments were freeze-dried, homogenized in a mortar, and stored in glass bottles until laboratory analysis.
The complete analytical protocol for PAH analysis is described in UNEP (1992). Around 20 g of sediment was extracted over 8 h by Soxhlet using 80 mL of a mixture of (1:1) dichloromethane (DCM) and n-hexane. Surrogates (naphthalene-d8, acenaphthene-d10, phenanthrene-d10, chrysene-d12, and perylene-d12) were added prior to sample extraction. The DCM/n-hexane extract was purified by column chromatography using 5 % deactivated alumina (1.8 g) and silica (3.2 g). The elution was done with 10 mL n-hexane (fraction 1—aliphatic hydrocarbons) and 15 mL (3:7) DCM/n-hexane mixture (fraction 2—PAHs). An aliquot of 1 μL from each extract was injected for gas chromatographic analysis.
The PAH analyses were performed with an Agilent GC model 6890 coupled to a Agilent Mass Spectrometer Detector (model 5973) and an Ultra-2 capillary fused silica column coated with 5 % diphenyl/dimethylsiloxane (30 m, 0.32 mm ID and 0.25 μm film thickness). The oven temperature was programmed to 40 °C holding for 2 min, to 40–60 °C with a temperature increase of 20 °C min−1, then to 250 °C with 5 min−1, and finally to 300 °C with 6 min−1, where this temperature was always held for 5 min. Helium was used as the carrier gas, and the data acquisition was done in single-ion monitoring mode (SIM). The aliphatic hydrocarbons analyses were performed with an Agilent GC (model 6890) equipped with a flame ionization detector. The fused silica column and the temperature programme used to identify aliphatic hydrocarbons were identical to the procedure used for the quantification of PAHs.
Procedural blanks contained a few minor contaminant peaks, but these did not interfere with the analyses of target compounds. Detection limits (DL) for each PAH were approximately 0.50 ng g−1 dry weight. A surrogate and spiked-recovery experiment was conducted simultaneously with the extraction of the samples and the recovery ranged from 50 to 120 % (Martins et al. 2007, 2011). Measured concentrations of target PAHs and n-alkanes in the IAEA-417 reference material were within 90 to 115 % of the certified values provided by the International Atomic Energy Agency (IAEA). Also, regular analyses of reference material (NIST 1941b) from the National Institute of Standards and Technology, USA, and annual participation in the inter-comparison exercises promoted by the Marine Environment Laboratory of International Atomic Energy Agency (MEL-IAEA) have shown satisfactory quality control.
Elemental analyses (Al, Sc, Cu, Pb, and Zn) were performed using the ICP-OES technique with a Varian model VISTA-MPX. The analysis followed the procedures described in Method 3050b of the SW-846 series (USEPA 2008). Approximately 1 g of dry sediment was digested with 10 mL of 1:1 HNO3 at 95 °C for 15 min. After cooling, another 5 mL of concentrated HNO3 was added, and the solution was heated for 30 min. This second procedure was repeated until the digestion of the sample was completed. 2 mL of water and 3 mL of 30 % H2O2 were added as the sample was heated until the elimination of the organic matter was completed.
After this step, 10 mL of concentrated HCl was added, and the solution was heated for 15 min. Finally, the solution was filtered through a Whatman 41 filter, and 10 mL of concentrated HCl was added to the digestate. Finally, the solution was filtered again with a Whatman 41 filter and the filtrate was collected in a 100-mL flask. The volume was filled up to 100 mL, and the solution was analysed in a Varian ICP-OES, model VISTA-MPX. The measurement precision for all elements was at least 5 %. Method accuracy was obtained by analysing certified standards of contaminated soils (SS-1 and SS-2 EnviroMAT).
Limits of detection and quantification as well as recovery percentage of the metal standards used in this study
Limit of detection (mg kg−1)
Limit of quantification (mg kg−1)
Reference value (mg kg−1)
Mean of five measurements (mg kg−1)
Reference value (mg kg−1)
Mean of five measurements (mg kg−1)
Results and initial interpretation
Synthesis of the concentrations of polycyclic aromatic hydrocarbons (PAHs), aliphatic hydrocarbons, and metals in the studied environments
ΣPAH (ng gN.A.1)
Σ Aliphatic (µg gN.A.1)
Pb (mg kgN.A.1)
Cu (mg kgN.A.1)
Fe (mg kgN.A.1)
Mn (mg kgN.A.1)
Zn (mg kgN.A.1)
ΣPAH (ng gN.A.1)
Σ Aliphatic (µg gN.A.1)
Pb (mg kgN.A.1)
Cu (mg kgN.A.1)
Fe (mg kgN.A.1)
Mn (mg kgN.A.1)
Zn (mg kgN.A.1)
Down-core concentration profiles of polycyclic aromatic hydrocarbons (PAHs) and aliphatic hydrocarbons in the three box cores discussed
Σ PAHs (ng g−1)
Σ Aliphatics (µg g−1)
Σ PAHs (ng g−1)
Σ Aliphatics (µg g−1)
Σ PAHs (ng g−1)
Σ Aliphatics (µg g−1)
Trace metals in surface and subsurface sediments of the SECB and adjacent continental shelf
Inside the Santos Estuarine Complex, the values of lead (Pb) concentrations varied from 9 mg kg−1 (pre-industrialized, non-contaminated sediments) to 59 mg kg−1 (top core samples from the harbour). Concerning zinc (Zn), the values ranged from 54 to 128 mg kg−1 (same samples as for Pb).
Surface sediments of Santos Bay showed a variation in the same order of magnitude as for Pb (N.D., i.e. below the limit of detection to 38 mg kg−1), but at least one significantly higher value for Zn (range from 9 to 274 mg kg−1). As for the organic compounds, the spatial variance of heavy metals in the sediments of Santos Bay is much higher than in the other two environments.
Elemental copper occurs in small concentrations in the three environments (maximum of 48 mg kg−1 in the cores from the Estuarine Complex, 38 mg kg−1 in the Santos Bay samples, and 10 mg kg−1 in the shelf sediments).
It is noticeable that the metal concentrations in samples from the Estuarine Complex and Santos Bay exceeded the Interim Sediment Quality Guidelines (ISQG) and even the Probable Effect Levels (PEL) for Cu, Pb, and especially Zn (available at http://st-ts.ccme.ca/en/index.html).
Organic compounds in surface and subsurface sediments of the SECB and the adjacent continental shelf
Inside the Santos Estuarine Complex, the ΣPAH concentrations in the core sediments varied greatly, ranging from 3.3 ng g−1 in basal, i.e. pre-industrial layers to 7987.1 ng g−1 at highly contaminated places.
In the Santos Bay, the ΣPAH concentration of surface sediments varied from N.D. to 319.1 ng g−1, with a mean concentration of 158.5 ± 103.5 ng g−1 (n = 6). The highest concentrations (319.1 ng g−1) were found in the proximity to a sewage outfall, as well as close to the main entrance of Santos Harbour (227.5 ng g−1).
The ΣPAH concentrations in surface sediments on the adjoining continental shelf varied from N.D. to 147.3 ng g−1, with a mean concentration 83.07 (n = 4). PAH concentrations were detected only at Stations 7, 9, 12, and 21. Stations 7 and 9 (66.4 and 147.3 ng g−1, respectively) are those located close to an area at the exit of Santos Bay, which is used as disposal area of the sediments dredged in the SECB. Sites 12 and 21 (112.1 and 6.65 ng g−1, respectively) are located in a mid-shelf mudbelt at the 100-m isobath (Fig. 6).
In the estuarine sediments, the concentrations of the total aliphatic hydrocarbons (ΣAliph) exhibit clear differences among the sediment cores. Two of the cores show very high ΣAliph concentrations (maxima of 464.2 and 667.8 µg g−1) while two others have maxima of 21.0 and 32.2 µg g−1, giving the spatial deposition of these organic compounds along the estuarine system a local character.
Surface sediments from Santos Bay show values which are coherent with the less contaminated sediments from the estuarine cores, with a ΣAliph average value of 8.4 ± 10.5 µg g−1, ranging from 0.33 to 22.97 µg g−1 (n = 8). Worth noting is the high dispersion of the ΣAliph values which, in fact, follows the grain size variations in the area (Fukumoto et al. 2004).
Surface sediments from the adjacent shelf show a pattern of low ΣAliph values, ranging from 0.7 to 6.4 µg g−1 (average 1.68 ± 1.42 µg g−1), with the highest values located in the mid-shelf mudbelt.
Organic compounds in box cores
A second core also collected from the mid-shelf mudbelt showed detectable concentrations of PAHs and aliphatic hydrocarbons throughout its whole sedimentary succession.
A third core (#9) was collected at the disposal site of dredged materials of the Santos Channel and, thus, presents values as high as 1,574.0 ng g−1 of ΣPAH and 27.0 µg g−1 of ΣAliph. In this core, the concentrations have drastically increased between 9 and 7 cm and gradually decreased to lower values since then.
Given the absence of bioturbation as indicated by a millimetre-scale laminated texture found in the sediment cores of the mid-shelf mudbelt off Santos, as also verified by macroscopic analysis, two observations are noticeable: the deposition of the organic compounds in the mid-shelf mudbelt reaches locally significant levels of contamination on the one hand, and did not accumulate at constant rates through time and not in equal spatial distribution on the other. This observation might be related to one of the following two processes: (a) Lateral variations in local sedimentation rates or in local shelf hydrodynamics led to a differentiated distribution pattern of particulate contaminant-absorbing matter. Shelf dynamics and local sources of sediment lead to the generation of a full mosaic of sediment types and (b) Production and injection of anthropogenic compounds have changed through time (Martins et al. 2007). Given the comparably low values measured in the uppermost centimetres, this second scenario might be related to the stricter policy for pollution control since the 1980s (Luiz-Silva et al. 2008).
While the contamination of sediments in estuaries and bays is demonstrated for many systems (Cearreta et al. 2013; Gao et al. 2013), the fact that these pollutants are also stored at significant concentrations in open-shelf mudbelts is yet little documented (e.g. McKee et al. 2004).
The laterally offshore spreading of the riverine suspension tongue distributing the contaminants from the coastal zone towards the inner and middle continental shelf might be driven by one of the following hydrodynamic processes.
a) Seasonally enhanced fluvial run-off as a result of heavy precipitation events often leads to peaks in suspension load (e.g. Summerfield and Nittrouer 1999; Geyer et al. 2004). This plume would then spread out allowing the particles to rain down in the open-shelf waters, as stated by Mahiques et al. (1999) for the studied area;
b) Bottom waters enriched in sediment suspension tend to be offshore advected by gravity, thus flowing down the comparably steep shoreface to inner-shelf gradient towards the mid-shelf zone (e.g. McCave 1972; Traykovski et al. 2000). The high bottom sediment concentration might be the result of an early freshwater-plume/suspension-plume separation after being supplied into the ocean (e.g. Hill et al. 2007; Geyer et al. 2004; McKee et al. 2004), or of intense remobilization of fine-grained inner-shelf seabed sediments by storm events; (c) An alternative mechanism for offshore transport of suspended matter is the wind-driven normal-to-coast circulation of coastal waters (Vitorino et al. 2002; Geyer et al. 2004). Such a local downwelling scenario enhances the gravity-driven offshore motion of the bottom layer (see b) and injects additional fines to the bottom turbidity layer.
With increasing distance to the coast, the river- and wind-driven surface influences become less effective and the seabed shows a gentler gradient. Thus, the inner boundary of the mid-shelf mud depocentres is commonly controlled by the vertical limit of the storm weather wave base combined with the decrease in the efficiency of gravity forcing (e.g. McCave 1972; Jouanneau et al. 2002).
These mud depocentres represent a major sink for fine-grained continent-derived materials and form in a water-depth interval of calm hydrodynamic conditions (e.g. McCave 1972; Walsh and Nittrouer 2009). Since most of these deposits develop since several millennia (e.g. Hanebuth et al. 2015), the contaminants should be assumed as being retained for long times. These mud depocentres serve as cradle for benthic life on lower and higher tropic levels of the food chain, including fish larvae (e.g. Thrush and Dayton 2002). It should, thus, be expected that these organisms accumulate these contaminants in significant amounts. When they incorporated them into their fabric, they do not only recycle these substances, but also inject them back into the water column or into the food chain, respectively. In addition and in some regions of high relevance, the disturbance of these mud deposits by recently changing regional climatic conditions (for instance, increase in high-energy storm events; e.g. Pezza and Simmonds 2005; McTaggart-Cowan et al. 2006; George and Hill 2008) or by chronic human seabed activity (material dredging, bottom trawling fishery; e.g. Pilskaln et al. 1998; Torres et al. 2009) probably leads to a progressive release of these contaminants into the water column at severe levels.
Studies have further shown that significant amounts of suspended muds which were originally introduced to the shelf system by the regional rivers are not stored on the shelf itself but are transported towards the open, i.e. deep ocean (e.g. Nittrouer et al. 2007; Oberle et al. 2014b). Accepting these general bypassing and seasonal remobilization processes of the particulate suspended matter as being relevant, a long-distance offshore distribution of these contaminants and its ecological impact is probably much severe than yet expected.
Despite the absence of direct river input to the continental shelf, an export process of anthropogenic materials exists in the study area being responsible for the offshore transport of contaminants in measureable and partly significant amounts, which finally deposit in mudbelts located on the mid-shelf off SE Brazil. This coastal material distribution process might be linked to seasonal flood-like run-off after heavy rain events, to gravity-driven advection of suspension-rich bottom waters or to wind-driven vertical, normal-to-coast circulation of coastal waters. The only potential source of these anthropogenic materials is the metropolitan zone of Santos Harbour, the largest port in South America.
This study shows that industry-related contamination in coastal regions is not solely restricted to the immediate coastal zone, comprising river mouth and estuarine environments. Rather, these contaminants are widely distributed over the adjacent continental shelf and might be available for recycling processes for an unestimated long time.
The observed elevated concentrations of anthropogenic substances in deeper, i.e., older stratigraphic layers of the mudbelt deposit indicate that either input or accumulation was not constant trough time. Bioturbation or physical vertical material mixing can be excluded as being responsible for a downward transport in the sediment column because the sediments are finely laminated and the values in deeper strata are, also, sometimes even higher than those measured at the sediment surface. Changes in the hydrodynamic system or in the production of the contaminants might be actual scenarios to explain the heterogeneous distribution pattern found in the mud depocentre. To prove these potential relations (local internal control; industrial production history; climate variability) will be a future task.
This article is related to the project entitled “A Influência do Complexo Estuarino da Baixada Santista sobre o Ecossistema da Plataforma Adjacente—ECOSAN” sponsored by FAPESP (Grant No. 03/09932-1). FAPESP also supported grant 10/06147-5 for the acquisition of R.V. Alpha Crucis which allowed the obtainance of chirp subbottom echo sounder data. Acknowledgement is due to K. Lindhorst, University of Kiel, who kindly processed the chirp profile shown in Fig. 2. The authors would also like to express their gratitude to the crews of the B.Pq. Velliger II and the R.V. Prof. W. Besnard for the collection of samples. Michel Mahiques acknowledges the Brazilian National Research Council (CNPq) for Research Grant No. 301106/2010-0. This publication is related to the INQUA International Focus Group “Rapid environmental changes and human impact on continental shelves”.
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