Links between surface sediment composition, morphometry and hydrodynamics in a large shallow coastal lagoon

Journal Pre-proof Links between surface sediment composition, morphometry and hydrodynamics in a large shallow coastal lagoon

H.M. Vieira, Jair Weschenfelder, Elisa Helena Fernandes, Heline Alves Oliveira, Osmar Olinto Möller, Felipe García-Rodríguez PII:

S0037-0738(20)30003-8

DOI:

https://doi.org/10.1016/j.sedgeo.2020.105591

Reference:

SEDGEO 105591

To appear in:

Sedimentary Geology

Received date:

19 October 2019

Revised date:

6 January 2020

Accepted date:

7 January 2020

Please cite this article as: H.M. Vieira, J. Weschenfelder, E.H. Fernandes, et al., Links between surface sediment composition, morphometry and hydrodynamics in a large shallow coastal lagoon, Sedimentary Geology(2020), https://doi.org/10.1016/ j.sedgeo.2020.105591

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© 2020 Published by Elsevier.

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Links between surface sediment composition, morphometry and

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hydrodynamics in a large shallow coastal lagoon

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H.M. Vieira1†, Jair Weschenfelder2, Elisa Helena Fernandes1, Heline Alves Oliveira1

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Osmar Olinto Möller1, Felipe García-Rodríguez1,3

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† Deceased on September 2nd 1997

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1 Laboratório de Oceanografia Costeira e Estuarina (LOCOSTE), Instituto de

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Oceanografia – Universidade Federal do Rio Grande (FURG), Rio Grande, Brazil

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2 Centro de Estudos de Geologia Costeira e Oceânica (CECO), Instituto de Geociências,

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Universidade Federal do Rio Grande do Sul/UFRGS, Porto Alegre, RS, Brazil

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3 Centro Universitario Regional del Este, Sede CURE-Rocha, Ruta 9 s/n , Rocha

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(27000), Uruguay

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Corresponding author: [email protected]

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Abstract

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This study addresses the surface sediment composition and distribution in Mirim

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Lagoon, a large coastal shallow transboundary system located on the border of South

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Brazil and Uruguay, which is 3749 km2 and maximum depth is around 6 m, in relation

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to the hydrodynamic conditions evolved from predominant winds. Surface water

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currents in the marginal area displayed maximum speed values of 0.25 to 0.3 m s-1

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flowing parallel to the coastline either under NE of SW wind predominance. The

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marginal zone, above the 6 m bathymetric contour comprise sandy sediments, indicating

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that under such a combination of bathymetric and hydrodynamic conditions, the

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resuspension of fine sediments is dominant. Lower surface current velocity observed in

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the central zone of the lagoon located below the 6 m isobath (i.e., 0.05 m s-1), together

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with an increase in the maximum width of the lagoon, promoted conditions for

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deposition of the clay sediment fraction. Therefore, the combined use of

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sedimentological and hydrodynamic data represents a useful tool to infer patterns of

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deposition and resuspension in coastal systems. Given the large size of the Mirim

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Lagoon and associated difficulties in sampling and monitoring the system, the

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information presented here is important for environmental management, and particularly

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for planning future paleolimnological research and set the coring stations on appropriate

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coordinates within the central region, where relatively calm conditions and dominance

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of clay sediments are observed.

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Keywords: deposition, hydrodynamics, morphometry, resuspension,

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Introduction

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The coastal zone contains shallow transitional aquatic environments that are highly

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sensitive to external forcing. Such transitional environments are characterized by

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restricted water circulation where natural events such as sea-level oscillations, abnormal

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river floods, storms and anthropic activities lead to modifications of a number of key

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physical and chemical process, but also morphological and sedimentological features.

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Furthermore, the interaction between marine and terrestrial processes induce high

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variability of both physical and sedimentological properties (Spagnoli and Andresini,

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2018), which are mainly modulated by water circulation evolved from wind intensity

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and direction (Costi et al., 2018; Oliveira et al., 2019). In this sense, water circulation of

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lenthic systems is important in explaining the composition and distribution of the

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sediment bed (Ulmann et al., 2003).

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Knowledge of the surface sediment compositon and distribution of aquatic systems is

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important prior to the implementation of human activities but also for conservation and

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envirommental management purposes. The formation of sedimentary facies in large

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coastal shallow lagoons depends on the supply of suspended solids from river networks,

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which is then either deposited or resuspended within the lenthic domain (Afri et al.,

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1994; Petti et al., 2018). Thus, surface sediment distribution is a consequence of

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morphodynamic processes involving water circulation and bottom morphometry (Toldo,

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1991; Arfi et al., 1994; Naya et al., 2004; Gardner et al, 2018). The study of surface

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sediments is particularly interesting in large coastal shallow systems, because the

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interaction between aquatic and sedimentary processes is highly dynamic, given the

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large contact surface area succeptible to resuspension and deposition (Gardner and

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Journal Pre-proof Doyle, 2018). The east coast of South America holds the world’s largest coastal shallow

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lagoonal system of the world, the Patos-Mirim System, with an area of 13,749 km2,

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where the Patos Lagoon comprises 10,000 km2 and Mirim Lagoon 3749 km2 (Toldo,

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1994; Friedrich et al., 2006). As the system is connected to the Atlantic Ocean through a

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single microtidal inlet, the dynamics of the both lagoons is driven by freshwater

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discharge contributions and wind effects. Thus, this system represents an excellent

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study case for assessing trends in the distribution and composition of surface sediments

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in relation to dominant winds and wind-driven hydrodynamics. Given that several

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studies on the sedminent bed composition have been published for Patos Lagoon

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(Toldo, 1991; 1994; Toldo et al., 2000; Calliari et al., 2009), it is important to study the

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surface sediment of Mirim Lagoon for providing a more precise sedimentological

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understanding of the Patos-Mirim System as a whole (Fig. 1).

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Vieira (1995) presented a thorough review of the geological setting of the watershed,

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formation of the lagoon, Quaternary evolution, and surface sediment properties and

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origin. More recent studies foccused on understanding the hydrodynamic response of

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the Mirim Lagoon to the wind effect and water circulation. Hirata et al. (2010) observed

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that the long-term water level oscillations in the lagoon were strongly linked to the El

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Nino-South Oscillation (ENSO). Oliveira et al. (2015) characterized the hydrological

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regime of the main water inputs to the Mirim lagoon and constructed a stage-discharge

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rating curve. Costi et al. (2018) used a combination of 2D numerical simulations, gauge

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station data, and Synthetic Aperture RADAR imaging to evaluate the influence of

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incident winds and main tributaries discharge on the system water level and the

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establishment of barotropic gradients between the Mirim lagoon and São Gonçalo

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channel. Oliveira et al. (2019) presented a novel integrated approach between the Patos

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and Mirim lagoons and the adjacent coastal region, thus generating a realistic prediction

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of the Mirim Lagoon hydrodynamic response. The authors suggested that the lagoon

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can be divided in three regions where currents have a distinct behaviour.

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In this paper, we have re-visited the grain size distribution/composition database from

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Vieira (1995), and related them to the dominant hydrodynamic conditions to infer the

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processes modulating sediment deposition and resuspension in this large shallow coastal

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system.

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Study area

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The Patos and Mirim lagoons are connected through the 76 km long São Gonçalo

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Channel, forming the so-called Patos-Mirim System (Fig. 1). This transboundary

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lagoonal system is located in the coastal plain of the State of Rio Grande do Sul (RS,

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Brazil) and eastern Uruguay (Fig. 1). This system integrates the Pelotas basin, which is

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one of several sedimentary basins occurring along the Brazilian and Uruguayan

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continental margin. The onset of the Pelotas basin occurred in the early Cretaceous and

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it evolved as a passive marginal basin (Dias et al., 1994), with itlandward limit being

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with Precambrian and Paleozoic highlands, and coastal limits at Santa Marta Cape

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(Santa Catarina State, Brazil) and La Coronilla (Rocha State, Uruguay) (Fig. 1). Both

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allogenic and autogenic forcing factors influenced the depositional sequence framework

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of the coastal zone (Bortolin et al., 2018), where sea-level changes play the allogenic

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control responsible for the regional geologic evolution (Tomazelli and Villwock, 2000;

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Bortolin et al., 2019).

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Journal Pre-proof The rivers of the Mirim Lagoon watershed (i.e., Cebolatti, Taquari, Jaguarão and

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Piratini) drain landward bedrock highlands of the Precambrian basement (Rio Grande

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do Sul and Uruguay Shield), whose eroded sediments are fluvially transported and

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deposited off the coastal zone sedimentary system. A fringe of coarser sediment (mainly

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gravel, coarse sand, and mud) of the alluvial fan system (up to 50 km wide) makes the

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transition between the highlands (up to 500 m altitude) and the low-lying coastal plain.

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The sedimentary deposits around the Mirim Lagoon (Dillenburg et al., 2017) alternate

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mainly fine sands of coastal barriers and lagoon muds of the various lagoon-barrier

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depositional systems deposited there during the Quaternary (Fig. 1; bottom right panel).

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The coastal plain was subaerially exposed and deeply fluvially incised during numerous

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sea-level rise and fall events of the Quaternary period (Weschenfelder et al., 2008,

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2010, 2014, 2016), and more clearly after the last interglacial maximum of 120 ky BP

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(Cooper et al., 2018; Bortolin et al., 2018, 2019). Fluvial dissection of the coastal relief

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during forced regression events led to the formation of a series of adjacent incised

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valleys, accommodating base-level fall and long lowstand periods (Weschenfelder and

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Corrêa, 2018), which were flooded during subsequent transgressive events

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(Weschenfelder et al., 2014; Santos-Fischer et al., 2016, 2018; Bortolin et al., 2019;

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Scottá et al., 2019). The regional low-relief coastal plain is one of the widest in the

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world, in which its width reaches up to 80 km. Sandy barriers and several lagoons with

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“restinga” vegetation characterize this coastal plain. The shelf exhibits a smooth

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morphology, very gentle slope (~1.4 m/km) and it is 125 km wide. It shows several

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marine terraces inherited as a morphological testimony of periods of sea-level

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stabilization during the Holocene transgression (Corrêa, 1996). The regional coastal

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plain sedimentary deposits alternate coastal-barrier sands and lagoonal muds by the

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juxtaposition of four barrier/lagoon depositional systems designated I to IV (oldest to

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youngest) by Villwock et al. (1986). They have been correlated with distinct Pleistocene

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and Holocene highstands (Tomazelli and Villwock, 2000), corresponding to high-

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frequency depositional sequences (Fisher and McGowen, 1967; Villwock and

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Tomazelli, 1995; Rosa et al., 2011, 2017), which in turn should confirm the correlation

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between sea-level variation and climate changes occurring during the Quaternary

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period.

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The Patos and Mirim system is warm-temperate and covers around 14,000 km2,

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extending up to 500 km in the broad NE-SW direction and averaging 40 km wide and 6

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m depth. Local waves up to 1.6 m high influence the bottom and margins of the lagoon

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(Toldo et al., 2000). The bottom sediments comprise a fringe of marginal sands (~ 60%

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in area) and internal muds, where water depths around 5 and 6 m separate both areas.

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The Mirim Lagoon covers 3749 km2 and has a maximum depth of around 6 m. The

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Mirim Lagoon´s main flow is into the Patos Lagoon (Oliveira et al., 2015), as there is a

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dam preventing brackish waters entering from the Patos Lagoon (Vieira and Rangle,

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1988; Gouvêa et al., 2010). Besides its importance for the quality of life of approximate

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1 million people who live within its catchment basin, the Mirim Lagoon plays an

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important role as a reservouir of freshwater (Oliveira et al., 2015). It is essential for the

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maintenance of humidity in Taim's wetlands, recognized as a Biosphere Reserve by

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UNESCO and feeding and breeding grounds for migrant birds. Furthermore, rice culture

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is the main economic activity in its catchment area, using lagoon waters for irrigation

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(Olivera et al., 2019).

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The hydrodynamics of the lagoon are modulated by the local wind acting on time scales

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of 2-12 days to establish water slopes of up to 3 m between extremities and promote

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water level increase (decrease) at the north during winds from the SW (NE) (Oliveira et

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al., 2015). Current velocities are stronger and recirculation cells are more evident during

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SW winds, with maximum current velocities at the surface of the Mirim Lagoon

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occurring around São Gonçalo Channel (Fig. 1) during periods of high discharge

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(Oliveira et al., 2019).

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Materials and methods

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Grain size data of 142 surface sediment samples of the Mirim Lagoon (Fig. 2a) obtained

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from Vieira (1995) were used to plot the sediment distribution and infer sedimentary

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facies (Fig. 2c). Samples were treated with hydrogen peroxide and hydrchloric acid to

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remove organic matter and carbonate respectively. The samples were then rinsed and

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dried in the oven. Samples were sieved with a rotup using sive intervals of 0.25ɸ, and

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the silt-clay fraction was assessed by pipetting (Krumbein and Pettijohn, 1938). Surface

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sediments were classified according to Wentworth (1922), Shepard (1954) textural

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classification and Folk and Ward (1957) statistical parameters. Data were expressed as

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percentage of sand, silt and clay, as well as the statistical parameters of arithmetic mean

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(Mz) (Fig. 3). The bathymetric map of Mirim Lagoon (Fig. 2b) is slightly modified

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from the original presented by Vieira (1995), at a scale of 1:260,000, by adding a depth

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gradient in shade of gray scale to improve the graphical quality and visual perception of

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the bottom morphometry.

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Journal Pre-proof Because of the large area covered by the Patos-Mirim system, the high variability in

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physical forcing and the limited field data available, numerical modelling is a

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recommended tool, as it allows high spatio-temporal resolution. From model results, it

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is possible to accomplish a detailed study of the system, and infer its response to

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physical forcing. Hydrodynamic information was generated by the numerical model

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TELEMAC-3D (www.opentelemac.org), developed by the Laboratoire National

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d´Hydraulique et Environment, of Companhie Electricité de France (©EDF). This

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model has been extensively applied to the Patos Lagoon (Fernandes et al., 2001, 2002,

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2004, 2005; Marques et al. 2009, 2010a ,2010b), and more recently to the Mirim

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Lagoon by Costi et al. (2018, 2019) and Oliveira et al. (2019).

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The TELEMAC-3D hydrodynamic model solves the Navier-Stokes equations

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considering the local variation in the fluid free surface, ignoring the density variations

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in the mass conservation equation and applying the Boussinesq approximation to solve

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momentum equations (Hervouet, 2007). The resulting partial differential equations,

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combined with initial and boundary conditions, were discretized in the model by a finite

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element, triangular, non-structured mesh (see Supplementary material). This approach

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makes it possible to set spatially varying mesh resolutions, with increased resolution for

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regions with large velocity gradients and/or complex bottom topography. The correct

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representation of these features increase model efficiency in predicting the

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environmental behaviour and also provides a more stable numerical solution.

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Oliveira et al. (2019) describes in detail the TELEMAC-3D model calibration,

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validation and application to the Patos-Mirim System for a three-dimensional 1-year

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long simulation, and their hydrodynamic results will be used in this study.

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Results

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Sediment distribution and morphometry

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The Mirim Lagoon is a large shallow system, whose deepest section has in the same

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orientation as the maximum-length axis (Fig. 2b). The maximum depth in such areas

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reaches 6 m and the sediment composition is dominated by the fine fraction (i.e., 30-

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40% silt and 20-30% clay, Fig. 3). Three main regions can be identified. The extreme

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northeastern region, being the largest section and displaying the maximum width of the

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lagoon, shows the highest relative percentages of fine sediment (Fig. 3), which accounts

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together for almost 100% of the relative sediment composition. The central region,

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where the transversal axis is narrower than that of the northeastern section, has a

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sediment composition consist of 30-40% silt, 20-30% clay, and the remaining fraction

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contains at most 30% of sand (Fig. 3). Finally, the extreme southwestern region, which

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is similar in both sediment composition and dimensions to those of central region.

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According to the above mentioned grain size distributions, the main sedimentary facies

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consist of a peripheric region, which encompasses the whole lagoon littoral contour

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zone down to the 3-4 m isobath (Fig. 2). Such a sedimentary facies corresponds

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basically to sand and to a lesser extent silty-sand and sandy-silt. Below the 4-5 m

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isobath (Fig. 2), there is a shift in the sedimentary facies classification, where the facies

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are dominated by clayey-silt in both the northern and southern extremes. In the central

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region below the 4-5 m isobath, sedimentary facies consists of silty-sand and sandy-silt

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(Fig. 2).

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Current velocity and direction

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Fig. 4 shows the calculated surface current velocity vectors at different regions of the

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Mirim Lagoon under the maximum SW and NE winds observed during a one-year long

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simulation period (Oliveira et al., 2019). Results indicate that during such maximum

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events the water flow displays a clear pattern in relation to the wind direction (Olivera

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et al. 2019), being transported towards the Patos Lagoon during SW winds, and towards

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Uruguay during NE winds (Fig. 4). Recirculation cells are evident throughout the

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lagoon mainly during NE wind, which is actually the predominant wind direction over

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the region. In addition, the relative distribution of current direction and intensity over

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one year is shown in the directional roses, which were derived from three transects

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depicted on the maps (Fig. 4). According to the annual mean current velocity/direction

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frequency distribution, it was possible to divide the lagoon into three main

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hydrodynamic areas (Fig. 4): the extreme north, where northeastern current velocities

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reaching 0.35-0,40 m s-1 are predominant, flowing towards the Patos Lagoon; the

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central area, where current velocities between 0.05-0.1 m s-1 are predominant, and

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although the mean flux is towards the Patos Lagoon, a broad direction spectrum was

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observed; and the extreme south, where the number of recirculation cells is more

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significant, resulting in high current direction variability, mainly ranging from 0.1–0.15

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m s-1. Although the wind action promotes water level slopes between the lagoon

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extremities (Olivera et al., 2015; Costi et al., 2018; Olivera et al., 2019), and creates and

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shapes recirculation cells, it does not change the orientation of the flow direction

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towards the Patos Lagoon, even under northern wind conditions (Costi et al., 2018;

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Oliveira et al., 2019, Vieira da Silva et al., 2019) (Fig. 4). In this context, the flow

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orientation can be only reversed during extremely low water level inside the Mirim

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Lagoon, but even under such conditions, the Patos Lagoon salt wedge does not intrude

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Journal Pre-proof into the Mirim Lagoon due to the presence of the São Gonçalo Channel dam (Gouvea et

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al., 2010) (Fig. 1). Historical data and associated modelling results indicate that the

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Mirim Lagoon maximum surface current velocity is attained at the São Gonçalo

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Channel, usually ranging from 0.2 to 0.5 m s-1 under high water discharge conditions

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(Fig. 4). Under the dominance of strong SW winds, the highest surface current velocity

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within the system can achieve values of 0.6 m s-1 (Comissão da Lagoa Mirim - CLM,

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1970). Such wind conditions and current velocity, combined with the occurrence of

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shallow water areas, promote the occurrence of sand spits and the relatively flat bottom

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topography (Oliveira et al., 2015).

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Discussion

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The Patos-Mirim system is a highly turbid environment fed by several turbid large

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rivers, which total supply of fine suspended sediments is 5.1 x 106 ton yr-1 to the Patos

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Lagoon (Jung et al., submitted) from which the Mirim Lagoon accounts for 1.8 x 106

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ton yr -1 (Jung et al., submitted). The final fate of the fine suspended material is the

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lagoon system in the first place, and the inner continental shelf secondarily (Calliari et

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al., 2009, Marques et al., 2009; Vinzon et al., 2009). In the case of the Patos Lagoon

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there is evidence from the morphodynamics, composition and distribution of the surface

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sediment bed (Toldo, 1991, 1994; Toldo et al., 2000; Calliari et al., 2009; Lisboa et al.,

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2015) but in the case of the Mirim Lagoon, there is comparatively an important lack of

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information.

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The surface sediment composition and distribution of Mirim Lagoon consists of

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an outer littoral zone dominated by sandy sediments. However, the composition and

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origin of such sandy facies within the west and east littoral, is different. The sand-

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during the Pleistocene and consists, therefore, of well sorted sand with very reduced silt

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content (Vieira, 1995). The sand on the west littoral is supplied by large turbid rivers

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(i.e., Jaguarão, Taquarí and Cebollatí Rivers) and inundated areas (Oliveira et al., 2015:

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Costi et al., 2018) and therefore, such sandy facies is poorly selected as it contains silt

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and clay fractions (Vieira, 1995). The perimeter boundary of this external sandy facies

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can be set on the 5 to 6 m isobath (see comparison between morphometry and

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sedimentary facies, Fig. 2) and it is distributed parallel to the maximum-length axis of

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the system. The surface current velocity at the marginal areas displays medium/low

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values (i.e., 0.25 m s-1) at both margins of the lagoon, flowing parallel to the coast either

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under NE or SW wind dominance.

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The combination of such morphometric and hydrodynamic conditions within the

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west margin is likely to promote fine sediment resuspension rather than deposition. On

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the east margin, however, this superficial facies exhibits the lowest relative abundance

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of fine sediment particles (i.e., clayey) as the dominant fraction is well sorted sand.

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Thus, in this region currents are likely to resuspend sand. In this sense, the littoral

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surface sediment composition is modulated by the relationship between depth and

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current velocity/direction, mainly parallel to the maximum length axis, under either NE

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or SW wind directions (Fig. 4). Similar environmental conditions for resuspension have

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been observed in this shallow system (Toldo, 1994; Cózar et al., 2005) and in other

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regions of the world (Naya et al., 2004).

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The sedimentary facies lying below the 6 m isobath, is mostly dominated by

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clayey-silty sediments, except for the constricted narrowest part of the central section

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(as depicted in Fig. 4), where the maximum current velocity (under SW winds) is

Journal Pre-proof observed (i.e., 0.35 to 0.4 m s -1). Hence, under such conditions of wind dominance and

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high surface current velocities, resuspension is expected to be dominant, as inferred

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from the dominance of sandy sediments. Therefore, the most abundant sediment

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fraction is fine sand, and the associated inferred sedimentary facies is silty-sand and

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sandy-silt (see Figs. 2 and 3). A similar distribution pattern has been inferred for the

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Patos Lagoon by Toldo (1994), who identified a littoral zone dominated by sandy

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sediments, and an internal central zone dominated by fine sediments. The surface

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current velocity sharply decreases towards the thickened section of the central zone

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(Fig. 4), as the lagoon width increases from about 20 km to 40 km. Because of such an

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increase in lagoon width, together with the decreased current velocity, calm water

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conditions are promoted in this region, and thus, the finest sediment fraction is

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deposited below the 6 m isobath. Likewise, the extreme south region (as shown in Fig,

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4) also displays relatively low surface water velocity, and for the same reason as

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explained above, the finest sediment fraction is deposited below the 6 m isobath and the

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associated inferred sedimentary facies is silty-sand and sandy-silt.

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the river discharge presents an intra-seasonal to annual timescales variability

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(Costi et al., 2018; Vieira da Silva et al., 2019), and the associated mean residence time

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is 180 days, although the northern (350 days) and southern (100 days) section exhibit

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remarkably different residence times (Vieira da Silva et al., 2019). Such a residence

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time is important in regulating the source-to-sink process is very involved in the

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formation of the recent sedimentary bed containing a high-resolution record of the

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contemporary human impacts from river dams, agriculture, milk industry, mining and

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forestry activities. Therefore, this sedimentary facies, distributed in the central region of

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the lagoon below the 6 m isobath appears to represent an excellent source of

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paleolimnological information.

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Concluding remarks

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The sediment bed composition of Mirim Lagoon is a consequence of the water

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circulation regime evolved from predominant wind exposure, which generates a littoral

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resuspension zone dominated by sand, and a central deposition zone dominated by silt.

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The inferred sedimentary facies from both resuspension and deposition zones are set

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parallel to the maximum length axis of the lagoon and they are distributed in relation to

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the bathymetric features.

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Acknowledgements

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This paper is dedicated to the memory of H.M. Vieira who devoted her

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professional life to sedimentological research in Rio Grande, Brazil. Thanks to FINEP

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for

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01.11.0141.01) and REHMANSA (grant 01.12.0064.00), and to CNPq for the research

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grants 551436/2011-5 (EHF), 308274/2011-3 (EHF), and 302231/2010–2 (OOM).

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Jasper Knight and an anonymous reviewer provided helpful comments that improved

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this paper.

sponsoring

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Figure captions

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Fig. 1. Upper left panel: regional extension of the Patos-Mirim system. Central panel:

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The Mirim Lagoon catchment area and main large inflowing rivers. Lower left panel:

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general geological setting according to Dillemburg et al. (2017).

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Fig. 2. Surface sediment sampling stations, morphometry and surface sedimentary

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facies of Mirim Lagoon.

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Fig. 3. Surface sediment composition expressed as Mz Wenworth size classes sand

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percentage of sand, silt and clay. The associated sedimentary facies are shown in Fig. 2.

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Fig. 4. Calculated surface current velocity vectors at different Mirim Lagoon regions

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for the maximum southwestern wind (left panels) and maximum northeastern wind

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(right panels) observed during a one year simulation period. Center panels: monthly

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frequency distribution of surface currents for extreme north (transect T1), central

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(transect T2), and extreme south (transect T3) regions.Modified from Oliveira et al.

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(2019).

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Journal Pre-proof Highlights Large shallow coastal turbid system driven by wind. Current direction and speed modulate resuspension and deposition processes. Below/above the 6 m isobath, sediment deposition/resuspension were inferred. Deposition/resuspension zone dominated by clay/sand.

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Hydrodynamics + morphometry = sediment composition/distribution

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Figure 1

Figure 2

Figure 3

Figure 4