Geochemistry of modern sediments from San Quintín coastal lagoon, Baja California: Implication for provenance

 

Composición geoquímica de sedimentos modernos de la laguna costera de San Quintín, Baja California: implicaciones para la proveniencia

 

Luis Walter Daesslé^{1, 2, *}, Gabriel Rendón–Márquez³, Víctor F. Camacho–Ibar¹, Efraín A. Gutiérrez–Galindo¹, Evgueny Shumilin⁴, and Eduardo Ortiz–Campos¹

 

¹ Universidad Autónoma de Baja California (UABC), Instituto de Investigaciones Oceanológicas , Carretera Tijuana–Ensenada Km. 107, 22830 Ensenada, Baja California, Mexico. ^*walter@uabc.mx

² Friedrich Alexander Universität Erlangen–Nürnberg FAU, Institut für Geologie und Mineralogie, Lehrstuhl für Angewandte Geologie, Schloβgarten 5, 91054 Erlangen, Germany.

³ Centro de Investigación Científica y Educación Superior de Ensenada (CICESE), Departamento de Geología, Carretera Tijuana–Ensenada Km. 107, 22830 Ensenada, Baja California, Mexico.

⁴ Instituto Politécnico Nacional–Centro Interdisciplinario de Ciencias Marinas (IPN–CICIMAR), Departamento de Oceanología, Av. IPN. S/N, Col. Playa Palo de Santa Rita, Apdo. Postal 592, 23096 La Paz, Baja California Sur, Mexico.

 

Manuscript received: June 16, 2008
Corrected manuscript received: Octuber 24, 2008
Manuscript acepted: Octuber 30, 2008

 

ABSTRACT

A detailed regional grid of 97 surficial sediment samples is studied for the San Quintín coastal lagoon, which is a shallow embayment located adjacent to a "regionally–rare" intraplate–type basaltic terrain known as San Quintín volcanic field. The influence that this unique lithology and other potential sources have on the recent sediment geochemistry is discussed on the basis of geochemical, petrographic and sedimentological results. The sandy silts and silts in the lagoon are enriched in ferromagnesian minerals such as pyroxenes and hornblende, which form up to 6 and 22%, respectively, of the total mineral count in the sand fraction. These relatively immature feldspathic sediments are characterized by the presence of abundant angular plagioclase (25–60%) and absence of lithics. The La–Sc–Th and Cr–Sc–Th discrimination diagrams suggest that mafic ferromagnesian minerals have a significant effect on the geochemical variance of the sediments. The Cr/Th (median=28) and Co/Th (median=59) ratios are similar to those reported for sands derived from basic rocks. A mafic provenance is probably responsible for the statistical association of Fe, Hf, U, Th, Sc, Cr, Ca, Na and the rare earth elements. An association of Fe, organic carbon and total P with the trace elements Sb, Cr, Br, As, Na, Sc and Co indicates that their distribution is mainly controlled by the presence of Fe–rich minerals, such as hornblende, and organic matter throughout Bahía San Quintín and the northernmost Bahía Falsa, beneath aquaculture racks. Low enrichment factors (<1) for Cr, Sb, As and P indicate that anthropogenic contaminant sources derived from agrochemicals are not significant.

Key words: sediment, geochemistry, volcaniclastic, heavy minerals, phosphorus, coastal lagoon, San Quintín, Mexico.

 

RESUMEN

Se estudia en detalle una malla regional de 97 muestras de sedimento superficial de la laguna costera de San Quintín. Dicha laguna adyace un terreno basáltico de tipo intraplaca con composición regionalmente atípica, denominado Campo Volcánico de San Quintín. Con base en resultados geoquímicos, petrográficos y sedimentológicos se discute la influencia que tiene la litología característica del campo volcánico, así como otras fuentes potenciales, en la composición del sedimento en la laguna. Los limos arenosos de la laguna están enriquecidos en minerales ferromagnesianos como piroxenos y hornblenda, los cuales forman hasta un 6 y 22%, respectivamente, del conteo de minerales en la fracción de arenas. Estos sedimentos feldespáticos inmaduros se caracterizan por la presencia de abundante plagioclasa angular (25–60%) y la ausencia de líticos. Los diagramas de discriminación de La–Sc–Th y Cr–Sc–Th señalan que los minerales máficos ferromagnesianos tienen un efecto significativo en la variablilidad geoquímica de los sedimentos. Las razones Cr/Th (mediana= 28) y Co/Th (mediana= 59) son similares a aquéllas reportadas para arenas derivadas de rocas básicas. Una proveniencia máfica es probablemente responsable de la asociación entre Fe, Hf, U, Th, Sc, Cr, Ca, Na y los elementos de las Tierras Raras. La asociación entre Fe, carbono orgánico y P total con los elementos traza Sb, Cr, Br, As, Na, Sc y Co, señala que la distribución de estos elementos está controlada dominantemente por la presencia de minerales de Fe, como la horblenda, y por la materia orgánica a lo largo de Bahía San Quintín y el norte de Bahía Falsa, debajo de los sitios de acuacultura. Los bajos factores de enriquecimiento (<1) para Cr, Sb, As y P indican que la contaminación antropogénica por el aporte de agroquímicos no es significativa.

Palabras clave: sedimento, geoquímica, volcaniclástico, metales pesados, laguna costera, San Quintín, México.

 

INTRODUCTION

The San Quintín coastal lagoon (SQCL) is a shallow embayment located adjacent to a "regionally–rare" intraplate–type basaltic terrain known as San Quintín volcanic field (SQVF). The SQVF is a group of cinder cones that were active from Pleistocene to Holocene, and probably until historic times (Woodford, 1928; Figure 1). These rocks have an uncommon composition compared with the lithology of the Baja California peninsula. They are composed of alkaline intraplate–type basalts and contain upper mantle peridotite and lower crustal granulite xenoliths (Basu and Murthy, 1977; Rogers et al., 1985; Saunders et al., 1987; Luhr et al., 1995). Stable minerals identified in the basalts are olivine, spinel inclusions, plagioclase, clinopyroxene, titanomagnetite and ilmenite (Luhr et al., 1995). Although the volcanic rocks show varying degrees of erosion and form most of the inner shoreline of SQCL, no evidence for significant weathering of the basalts was found by Gorsline and Stewart (1962). These authors observed uniformity in the mineral composition throughout the bay, which, in addition to the absence of rock fragments and coarse material, led them to conclude that the SQVF only contributed relatively small amounts of sediment to the lagoon.

Recent interest on the biogeochemistry of SQCL has motivated research on the role that the sediment may play as a sink and/or a source of dissolved chemicals in the system. Little is known about the regional heterogeneity of sediment composition and its likely relationship with the biogeochemistry of SQCL. Sediments in the SQCL play an important role in the non–conservative fluxes of dissolved inorganic phosphorus (DIP), and therefore in the primary productivity of this system. Bed sediments act as a net source of phosphorus to the water column because of the organic matter re–mineralization (Camacho–Ibar et al., 2003; Ibarra–Obando et al., 2004). However, the chemical and mineralogical composition of surficial sediments seems to allow them to act as sinks of DIP through sorption during resuspension (Ortiz–Hernández et al., 2004). The finding of uncommonly high Fe and Ti concentrations in the sediments from the SQCL and their heterogenous regional distribution in the SQCL, as well as the lack of correlation of these metals with the mud (silt+clay) grain size fraction, led Navarro et al. (2006) to conclude that lithics and/or heavy minerals from the SQVF may be important hosts of these metals in the sediments. Gutiérrez–Galindo et al. (2007) suggested that the spatial distributions of Cr and Ni in 39 samples studied from SQCL in 1992 could also be the result of SQVF influence, but that Cd, Cu and Zn could be influenced by upwelling, because of their association with organic matter. No evidence has yet been found for contamination by anthropogenic sources despite the intensive agriculture in the adjacent San Quintín valley and oyster aquaculture in the lagoon. Even though oyster aquaculture can induce changes in shallow coastal ecosystems, including oxygen depletion, alteration of sedimentary geochemical processes, and increased sedimentation beneath culture racks (see Newell et al., 2002; Forrest and Creese, 2006 and references therein), the effect of oyster aquaculture on sediment geochemistry in the SQCL has not been evaluated. Thus characterizing in detail the composition and sedimentology of surface sediments in the SQCL is important not only from the geochemical point of view, but also from an environmental perspective.

In view of the geochemical heterogeneity of modern sediments in the SQCL and the influence of metal–rich volcaniclastic sources, the aim of the present work is to identify the sediment sources to the SQCL using a much wider range of geochemical (including Ca, Na, Fe, Sc, Cr, Co, Br, Ba, Th, U, and rare earth elements), petrographic and sedimentological variables, and a higher sampling resolution than before. The information on surficial sediments (upper 3 cm) of the SQCL floor is used to assess the influence of erosion and/or weathering of metal–rich particles from the SQVF and other sources on the modern sediment geochemistry. In addition, the presence of potential contaminants (P, As, Cr and Sb) associated with input from agrochemicals is assessed. The information in this work is also intended as an important tool for ongoing biogeochemical and biological research projects in the area.

 

REGIONAL SETTING

The SQCL is a shallow coastal lagoon located 350 km south of the Mexico – US Pacific border. Its geomorphology was defined mainly by the eruption of cinder cones belonging to the SQVF and a large dune and beach tombolo separating the lagoon from the Pacific Ocean (Figure 1). The SQCL has an average depth of 2 m, with extensive tidal flats, but with depths that reach 9 m along the tidal channels and the region adjacent to the mouth connecting it with the ocean (Figure 1). It is divided in two sections: Bahía Falsa (BF) and Bahía San Quintín (BSQ). Most of the rocks surrounding the SQCL originated from strombolian volcanic eruptions forming pyroclastic and lava deposits covering an area of 50 – 5000 m² (Luhr et al., 1995). The coastal plain is composed of beach sands and fluvial gravel. East of the SQCL (~40 km) crop out the plutons belonging to the Peninsular Range batholith, and andesites and rhyolite tuffs from the Alisitos Formation (Gastil et al., 1975).

The climate in the region is dry with a mean annual rainfall of 150 mm and a mean evaporation of 1400 mm. There is little freshwater and sediment supply from land, as the San Simón watercourse, the main stream draining into the bay, is dry most of the time. Land inputs through this stream seem to occur only during winters of wet years, for example, under El Niño conditions. Tides are mixed semidiurnal with a range of 2.5 m during spring tides. Maximum tidal current velocities occur in the channels, with values up to 160 cm s^–1 in the mouth (Flores–Vidal, 2006). Hydrodynamic conditions are different in the two bays, as indicated for example by the difference in water exchange time. During summer, water exchange time is approximately one week in BF and three weeks in BSQ (Camacho–Ibar et al., 2003). One of the main biological features related to sediment transport and accumulation in the bay is the ubiquitous presence of seagrass (Zostera marina L. and Ruppia maritima L.) in the intertidal and subtidal areas (Ward et al., 2003). San Quintín has ~5,021 inhabitants. The most important economic activity in the SQCL is oyster aquaculture, which by regulation is concentrated in BF, where it covers ~300 ha. On land, agriculture is the most important activity.

 

METHODS

Sediment samples were collected in 2004 on a regional grid with an in–house designed PVC–polyethylene corer designed to retrieve only the upper ~3 cm of the lagoon floor sediments, in order to study the most recent sediments only. Homogenized sub–samples were used for petrographic, particle size distribution and chemical analyses. For petrography, 16 samples from different sites covering the entire SQCL were selected (Figure 1, Table 1). They were wet–sieved, oven dried and impregnated with epoxic resin. Thin slides were cut and stained to identify quartz, K–feldspar and plagioclase, as well as heavy minerals. Three hundred points were counted using a spacing of ~0.1 mm. Particle size distribution was determined by means of a HORIBA LA910 laser/tungsten analyzer, and the % sand (> 62.5 µm), silt (4–62.5 µm) and clay (< 4 µm) reported (Table 2, Appendix). Prior to geochemical analyses, the sediments were finely ground with an agate pestle and mortar. Major, trace and rare earth elements (REE) in the samples were analyzed by means of instrumental neutron activation analysis (INAA), along with duplicates, a laboratory in–house reference material, as well as USGS MAG–1 marine reference sediment. Only those elements determined with analytical bias and precision better than ±15% are discussed. These elements include seven REE, and Na, Ca, Ba, Sc, Cr, Fe, Co, As, Sb, Th, U, Br and Hf. Organic carbon (C_org) was determined after eliminating carbonates with 0.5 M HCl overnight and rinsing with deionized water. The C_org analyses were carried out with a LECO CHNS 932 elemental analyzer. Total P was determined, after ignition at 550°C and leaching with 1M HCl, with a Varian Cary 50 spectrophotometer (Aspila et al. 1976).

 

RESULTS

Sediment distribution

Sediments in the SQCL are mainly composed of green and grayish green sandy silts and clay, with clay size particles exceeding 20% mainly in the heads of BF and BSQ, and a localized spot in southern BSQ (Figures 2a and b). Consistently, muds (silt+clay) are found mainly in shallow waters in the heads of BSQ and BF, as well as in central BSQ, where depths are < 2 m. Sands are dominant near the mouth of the bay toward the ocean along the deeper (<9 m) tidal channels and adjacent to the dune and beach sand bar. Coarse sands are absent. Near the discharge site of the San Simón watercourse, silty sands are dominant and no evidence is seen for preferential coarse sediment deposition associated with stream inflow. This arroyo is flooded only occasionally during extreme rainfall, and its riverbed is currently covered by sand and shrubs. Field observations of the extensive sand deposits along the arroyo, marshland along the coastline and the absence of a defined delta structure (cf. Gorsline and Stewart 1962), suggest that any occasional sediment input from this source may have been already mixed by the dominant and continuous coastal hydrodynamics.

Petrography

The minerals counted are shown in Table 1. The modal analysis of the sandy fraction shows a high content of plagioclase and quartz (both >75% of total count), and a low K–feldspar content. Lithic fragments are practically absent, probably because coarse sand is also absent. According to the mineralogic classification of Okada (1971), the sands are classified as feldspathic sediment, with plagiocalse dominant relative to K–felspar, and can be classified as having an uplifted basement source (Figure 3; Dickinson et al., 1983). The high abundance of hornblende, monocrystalline quartz and lesser amounts of K–feldspar suggests the importance of the granitic regional basement. Heavy mineral content in the determined samples is as much as 30% of the total mineral count, and includes hornblende, pyroxene, mica (biotite > muscovite) and opaque minerals (magnetite > titanomagnetite). Hornblende and pyroxene are present as euhedral crystals (Figure 4) without evidence for extensive weathering and/or transport, suggestive of a local source. Thus, two predominant mineral sources can be defined for the SQCL: (1) of local volcanic origin belonging to the SQVF, and (2) from the erosion of the batholitic basement. Scanning electronic microscopy (SEM) confirmed the presence of the minerals identified with the petrographic microscope. Owing to its unstable nature, olivine was seldom identified and only as small crystals. Figure 4 shows a pyroxene crystal from the SQCL with a Si, Al, Ca, Mg and Fe general composition, as identified by SEM. This chemical composition, that corresponds to diopside, is only possible from an ultramafic xenolith source from San Quintín, as described by Basu (1975). No chromite or spinel was identified in the samples.

 

Sediment geochemistry and statistical factor analyses

The raw geochemical results are given in the Appendix and summarized in Table 2. Of all the elements studied, Cr has a unique regional distribution, in that it is relatively enriched adjacent to the entire coast surrounding BF (above average lagoon concentrations), and also in the eastern SQCL, adjacent to the San Simón discharge site (Figure 5). This distribution partially resembles that of Fe, especially near the entrance of the San Simón watercourse (Figure 5b). The regional distribution of P shows enrichment in the head of BF and central BSQ (Figure 5c). Although its distribution is similar to that of Fe, high concentrations are also found in sites where silts are dominant (Figure 2a).

The chondrite normalized REE patterns show a distribution that is similar to that of rocks from the SQVF (Luhr et al., 1995). They are enriched in light REE (LREE) and depleted in heavy REE (HREE) (average La_n/Lu_n~4). However, some samples show a slight HREE enrichment in relation to the medium REE (MREE). This enrichment is better assessed by using chondrite normalized Tb_n/Lu_n ratios. The Tb_n/Lu_n ratios in the sediments average 1.7 (0.7–4.3), and are similar to those of the SQVF rocks (Luhr et al., 1995), with Tb_n/Lu_n = 1.9 (1.7–2.1). The Tb_n/Lu_n ratios in the SQVF and SQCL are only slightly higher than those in the upper continental crust (UCC; Rudnick and Gao, 2004), and in the North American shale composite (NASC; Gromet et al., 1984), which are 1.5 and 1.2, respectively. In order to closer assess any similarities between the REE patterns of the sediments with those of the SQVF, the concentrations of the seven reported REE were normalized to the average REE composition of Kenton volcano (Luhr et al., 1995). Three types of SQVF–normalized REE distributions were empirically identified on the basis of their Tb_n/Lu_n ratios, indicating three different groups in the sediments (Figure 6).

Varimax rotated factor analysis was used to describe the main sediment geochemical components in SQCL and to better explain the sedimentary and/or hydrodynamic factors controlling sediment composition. In addition to the elements determined with INAA, the factor analysis includes results for abundance of sand, silt, clay, C_org, and P. Only those samples for which all the mentioned variables could be determined, were used for statistical analyses (n=77; Table 3). Three factors explain 58% of the total geochemical variance in the lagoon. Factor 1 (accounting for 31% of the total variance) groups Fe, Ca, Cr, Na, Hf, Sc, Th, U and the REE. Positive factor scores (>0.3) for this factor are found in most samples from BSQ (except northern BSQ) and in some from western BF (Figure 7a). The second factor groups those sediments with high silt, clay, C_org and P content. Positive Factor 2 scores are found in samples from northern and central BSQ, and northern BF (Figure 7b). The third factor groups Fe, C_org and P with As, Br, Ca, Co, Cr, Na and Sc. Unlike the first two factors, Factor 3 scores are positive in most of BSQ and only in a few samples in northernmost BF, where the aquaculture racks are located (Figure 7c).

 

DISCUSSION

Weathering and provenance

Sediments from the SQCL have a relatively low abundance of C_org (0.07–2.1%). The absence of lithics in most of the sediments studied suggests that, if any volcanic rock fragments were present, these were rapidly weathered during more humid past conditions, and weathering products were dispersed out of the basin by the active tidal currents and/or are currently buried. This would explain the low abundance of clay–sized sediments in the SQCL, except for a few samples (Figure 2b). Gorsline and Stewart (1962) reported unusual high abundances of hornblende, exceeding 50% of the total heavy mineral counts in the SQCL sediments. These authors however, did not report the presence of pyroxenes identified in the present work as a dominant (as much as 6%) heavy mineral component (Table 1). Thus, it is possible that clinopyroxenes eroded from the SQVF (more likely from the ultramafic xenoliths), remained in the SQCL and were distributed by tidal currents. The angular appearance of these minerals (as well as that of plagioclase) is suggestive of a nearby source and low degree of weathering (Figure 4). Luhr et al. (1995) identified clinopyroxenes in several rock samples from the SQVF. Clinopyroxenes (>5%) are found in mounts Kenton, Basu, Woodford and Mazo, mainly as microphenocrysts. Phenocrysts are present in Mount Mazo (at the end of the dune and beach sand tombolo; Figure 1), but are rare in other cones surrounding the lagoon. One phenocrystic pyroxene from mount Mazo is reported to have exceptionally high SiO₂, Cr₂O₃, NiO, and MgO concentrations, whereas clinopyroxene megacrysts analyzed from the Woodford complex are characterized by very low Cr₂O₃ and NiO concentrations (<0.02%; Luhr et al., 1995). Oxide mineral microphenocrysts such as Al–Fe–Cr–Mg spinel, spinel inclusions in olivines, groundmass spinel and groundmass ilmenite form >5% of mounts Basu, Kenton and Woodford (Luhr et al., 1995). According to these authors, spinel inclusions may have 24% Cr₂O₃. Chromian spinels are described as being resistant to weathering and abrasion (Pooley, 2004), whereas pyroxenes are only moderately resistant to weathering. Thus, the presence of chromian spinels together with clynopyroxene phenocrysts and microphenocrysts may play an important role in determining the distribution of Fe and Cr in the SQCL (and probably also of V, Co and Ni, as indicated by the partition coefficients of these metals in clinopyroxenes; Rollinson, 1993). However, in the SQCL only a few olivine–bearing, and no spinel–bearing, sediment samples were identified.

Selected element ratios are used to compare the composition of sediments from the SQCL with that published for the SQVF, the Peninsular Ranges batholith, UCC, NASC, post–Archean Australian shale (PAAS) and other sediments eroded from basic and felsic rocks (Table 4; Nagarajan et al., 2007). The La/Sc ratios in sediments from the SQCL (0.1–3.1) are similar to those in the SQVF, and sediments from basaltic rocks. However they are on the lower range of the Peninsular Ranges batholith, UCC, NASC, PAAS and sediment from felsic rocks. This suggests the presence of minerals from a mafic source in sediments in the SQCL, but with an important contribution from felsic rocks (Dokuz and Tanyolu, 2006). High Cr/Th and Co/Cr ratios also suggest a mafic source (Table 4), when compared to the felsic UCC, NASC and PAAS ratios. Sediments with a mafic geochemical signature are ubiquitous in the SQCL, except perhaps those samples adjacent to the dune and beach sand bar (around samples 15, 16 and 94), as well as the inner coast of BF (around sample 107), which also have unusually high REE concentrations (∑REE >100 µg g–1), and are probably identified as part of the elements associated with heavy minerals in statistical Factor 1. High Cr/Th and Co/Th ratios were also found in the Magdalena–Almejas lagoon (Baja California Sur), adjacent to the Magdalena and Margarita Islands (Table 4), as well as near Cedros Island (Baja California). Enrichments in both areas were probably caused by the weathering and erosion of the ophiolitic rocks in these islands (Daesslé et al., 2000; Rodríguez–Meza, 2005).

Discrimination diagrams (Figures 8 and 9) are further used to assess the geochemical affinity of the sediments with the two most likely geochemical end–members in the region: the SQVF and the Peninsular Ranges batholith, which are located ~40 km east of the SQCL. Although a mixing and integration of the geochemical signatures by the hydrodynamic forces in the lagoon is highly likely, the distinctive composition of the end–members is thought to be reflected to some extent in the sediments. The La–Sc–Th diagram includes the data of the average composition of the Eastern and Western batholith. Sediments from the SQCL have La–Sc–Th compositions that increase in La from a batholith–type composition toward and beyond a SQVF composition (Figure 8), suggestive of an additional sediment component enriched in La, probably heavy minerals. Figure 9 shows the proportions of Cr–Sc–Th, in order to assess the potential sources of Cr in the sediment. In the diagram, the sediments show a distribution similar to the trend seen from East to West in the Peninsular batholith rocks (Silver and Chappell, 1987), reaching proportions of Sc comparable to those in the SQVF, but still not the same Th depletion and Cr enrichment as in these rocks. The regional distribution of bulk Cr concentrations strongly suggest that Cr–bearing minerals such as diopside from the xenoliths (with Cr above the average of 30 µg g^–1) have preferentially been deposited along the entire coast of BF and throughout BSQ. Mantle–derived ultramafic xenoliths are abundant in the SQVF (Basu, 1975). They are characterized by chromium–diopside rich lherzolite. These rocks are easily friable and may provide the anomalous sources of Sc and Cr in theSQCL. However, a significant enrichment of Cr (along with Fe) along the shallow eastern coast of BSQ (adjacent to the San Simón drainage area), is indicative of peculiar conditions that favor the deposition of these metals there. Since Cr concentrations in the Western batholith (67 µg g^–1) are almost three times those from the Eastern batholith (24 µg g^–1) (~150 km east), a felsic source for Cr at that specific site (probably as hornblende) could be partially responsible for this enrichment. However, as no opaque minerals (including chromite) were identified in this area, a diagenetic signal may be responsible at least in part for the enrichment in Fe and Cr there, probably as pyrite.

 

While most of the samples show a similar REE pattern, which is almost identical to that of SQVF rocks, the slight differences in Tb_n/Lu_n (normalized to REE concentrations in SQVF rocks) allow for the identification of two additional factors controlling their distribution (Figure 6). Slight enrichments of HREE along most of the western coast of BF, the southern coast of BSQ and some sites adjacent to the inner coast of BSQ, may indicate the presence (although in small amounts) of minerals such as orthopyroxenes, olivine or other (most likely mafic) minerals enriched in HREE (Rollinson, 1993). This distribution of samples with Tb_n/Lu_n <0.7 closely resembles that of the factor scores belonging to statistical Factor 1 (Figure 7a), which associates the REE with elements such as Sc, Cr, Hf, U, Th, Ca, Na and Fe.

Grain size effect and element enrichments

Commonly, fine grained particles are a dominant factor controlling metal distribution in aquatic sediments (Lakhan et al., 2003). This suggests the mechanistic link between the sedimentation of organic matter and clastic fines, either through sorption and/or comparable sedimentation conditions. In the SQCL, the abundance of clay and silt are statistically associated only to C_org and P, signaling sites of low hydrodynamic energy in the heads of BF and BSQ, as well as in a shallow mud–bank in central BSQ (Figure 2a and b). However, unlike other marine sediments where small grain size is an important factor controlling metal enrichment (Covelli and Fantolan, 1997 and references therein), in SQLC the grain–size effect plays only a secondary role in explaining the compositional variance of the sediments. This is suggested from the bivariate scatter plot of Fe against silt+clay (Figure 10a), where no correlation is seen between these variables. Results by Navarro et al. (2006) indicate that neither does Al correlate with grain size in the SQCL. This unusual no–correlation between grain size and metal concentrations may be explained by the abundance of Fe–rich hornblende in the fine sand–size fraction throughout SQCL. Thus, normalization against grain size is not an appropriate tool to determine element enrichments in the SQCL. However, Fe appears to control part of the enrichment of Cr and P in most of the samples, as suggested by the positive correlation between these elements in the SQCL (Figures 10b and 10c).

The association of Fe in Factor 1, with elements such as Hf, Th, Cr, Sc and the REE indicates that, in parts of BF and BSQ, these elements are contained in heavy minerals (Figure 7a). However, the association of Fe in factor 3 (Figure 7c) with several other trace elements (As, Br, Co, Cr, Sb and Sc), C_org and P in BSQ and the northernmost BF (underneath aquaculture sites), may be caused by combined sources in addition to the heavy minerals. Likely additional sediment components that may be responsible for the element association seen in factor 3 are: a) sorption of anions to Fe oxyhydroxides or other Fe minerals; b) formation of authigenic pyrite associated to sediment anoxia in the shallow mud flat adjacent to the mouth of the San Simón watercourse and underneath the aquaculture racks in BF; c) organic matter; and/or d) input of fertilizers containing P, As and Sb (He and Yang, 1999; Otero et al., 2005), or arsenical pesticides (Robinson and Ayuso, 2004) that might be carried from nearby agriculture fields during flooding of the San Simón.

Bromine is an element typically enriched in organic matter formed in saline waters, and shows a strong correlation with organic carbon in sediment (Cosgrove, 1970). This explains the association of Br with C_org and P in factor 3. However, considering the relatively low concentration of P, As and Sb (Appendix) in the sediments directly adjacent to the mouth of the San Simón watercourse (samples 1–5 and 109; Figure 1), it is unlikely that these elements were derived from agrochemical pollution. Furthermore, they have low element enrichment factors (EF ≤ 1) when compared to NASC. The EF (calculated as EF = [element_sample/Fe_sample] / [element_NASC/Fe_NASC]) in these samples are 0–0.8 for P, 0.02–0.3 for Cr, 0.01–0.3 for As and 0–0.6 for Sb. Samples throughout SQCL have a mean EF of 0.7, 0.3, 0.1 and 0.1 for P, Cr, As and Sb, respectively. These results indicate that anthropogenic sources for these elements, if any, are minor in the SQCL, and that Fe mineralogy (as heavy minerals and probably diagenetic sulphides) and organic matter are the dominant sediment components controlling the geochemistry of trace elements and P.

 

CONCLUSIONS

The sediment in the San Quintín coastal lagoon is immature with high feldspar contents relative to quartz and can be classified as having an uplifted basement source. The abundant ferromagnesian minerals have a significant effect on the geochemical variance of SQCL sediments. The presence of ferromagnesian minerals other than hornblende is mainly attributed to their erosion from the San Quintín volcanic field, and their dispersion throughout SQCL by the action of tidal currents, especially near the coastline. Furthermore, elemental ratios (La/Sc, Cr/Th and Co/Th) also suggest a mafic provenance. Discrimination diagrams (La–Sc–Th and Cr–Sc–Th), that include local igneous endmembers and other sediments of known sources for comparison, indicate a dominant felsic source (Peninsular Ranges batholith diorites), with a superimposed mafic (SQVF and its ultramafic xenoliths) secondary heavy mineral source. Factor analyses and REE patterns (especially the normalized MREE/HREE ratios) also suggest a ubiquitous influence of ferromagnesian minerals in the SQCL. No correlation exists between the abundance of mud and the concentration of Fe or other metals, and thus grain size cannot be used as a normalizer for assessing contaminant sources in this coastal lagoon. However, the association of Sb, Cr, Br, As, Na, Sc and Co with Fe, C_org and P indicates that, although secondary, the distribution of these trace elements and P is mainly controlled by the presence of Fe–rich minerals such as hornblende (and probably also Fe–oxides and diagenetic Fe–sulphides), and organic matter throughout BSQ and northernmost BF below the aquaculture racks. No evidence for contamination by P, As, Cr and Sb from the use of agrochemicals was found.

 

ACKNOWLEDGMENTS

We are grateful to V. Guerrero, G. Paniagua and V.A. Macías for their invaluable boating skills while sampling in shallow San Quintín waters. Thanks to E. Navarro for his field and laboratory assistance, A. Siqueiros for helping out with phosphorus analyses, and D. Saposhnikov for INAA analyses. We acknowledge the reviewers of this manuscript for helping us to improve it, especially J. Madhavaraju and A.C. Edwards. Thanks to Prof. H.J Tobschall at the University Erlangen–Nürnberg for his unconditional support and friendship. This work benefited from funding by the Mexican Research and Technology Council CONACYT grant 40144–F to V.F. Camacho–Ibar and a Georg Forster fellowship to L.W. Daesslé from the Alexander von Humboldt Foundation in Germany.

 

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