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Gayana (Concepción)

On-line version ISSN 0717-6538

Gayana (Concepc.) vol.70  suppl.1 Concepción Oct. 2006

http://dx.doi.org/10.4067/S0717-65382006000300002 

Suplemento Gayana 70: 1-5, 2006

 


Seasonal variability in the hydrological and chemical structure of the suboxic waters at the cariaco time-series station



Variabilidad estacional en la estructura hidrologica y química de las aguas subxicas en la estación serie de tiempo cariaco


Yrene M. Astor1; Mary Scranton2, Laurencia Guzmán1, Robert Thunell3 Frank Muller-Karger4, Gordon Taylor2, Kent Fanning4 & Ramón Varela1


1. La Salle Foundation of Natural Sciences, Margarita Marine Research Station, Margarita Island, Venezuela, yastor@edimar.org

2. Marine Sciences Research Center, Stony Brook University, Stony Brook, NY, USA.

3. University of South Carolina, Department of Geological Science, Columbia, SC, USA.

4. University of South Florida, College of Marine Science, St. Petersburg, FL, USA.


ABSTRACT

For more than ten years, the CARIACO Time-Series Program has studied seasonal patterns and interannual variability at the Cariaco Basin. Monthly core physical and biogeochemical observations have been collected since November 1995 at the CARIACO Time-Series station (10o 30' N, 64o 40' W). A region with suboxic conditions (oxygen and sulfide concentrations <1 µM) is found at intermediate waters at the station. The characteristics of this region show time and space variability, depending on the dynamics of the water. In general, ammonium, nitrate and nitrite disappeared within the suboxic region, although there is a steep ammonium concentration increase with depth after the oxic-anoxic interface suggesting its consumption at this boundary. Our data shows that intrusions are a major factor in affecting the chemical characteristics of the water column.

Keywords: Oxic-anoxic interface, ventilation, Cariaco basin, intrusions


RESUMEN

Por más de diez años, el Programa Serie de Tiempo CARIACO ha estudiado los patrones estacionales y la variabilidad interanual en la Fosa de Cariaco. Observaciones de los principales parámetros físicos y biogeoquímicos se han obtenido mensualmente desde noviembre de 1995 en la estación Serie de Tiempo CARIACO ubicada a 10o 30' N, 64o 40' W. Una región con condiciones subóxicas (concentraciones de oxígeno y sulfuro <1 µM) se encuentran presentes en las aguas intermedias de la fosa. Las características de esta región muestran una variabilidad temporal y espacial que depende de la dinámica de las aguas. En general, los niveles de amonio, nitrito y nitrato son prácticamente indetectables en esta región, aunque se observa un incremento marcado del amonio después de la interfaz lo que sugiere que está siendo consumido en esta zona. Nuestros datos muestran que las intrusiones son un factor importante en alterar las condiciones químicas de la columna de agua.

Palabras Claves: Interfaz óxico-anóxica, ventilación, Fosa de Cariaco, intrusiones


INTRODUCTION

The Cariaco Basin is a permanently anoxic marine depression located on the Venezuelan continental margin. It is divided into two sub-basins (eastern and western), each of about 1400 m depth, separated by a 900 m deep sill. The Cariaco Basin itself opens to the Caribbean Sea to the north, across a sill approximately 200 km long and which reaches some 140 m depth in two submarine channels. The CARIACO oceanographic Time-Series program has collected a set of core physical and biogeochemical observations over the past ten years, including hydrography, biological productivity, vertical particle flux, and currents at a single location (10° 30' N and 64° 40' W) in the eastern basin. Hydrochemical measurements at 20 m depths are collected routinely on a monthly basis, and several spatial distribution surveys have been conducted in the basin. This summary covers observations collected between January 1996 and December 2005.

Waters in the Cariaco Basin are exchanged annually with Caribbean waters across the northern sill. Both wind-driven upwelling and a strong density gradient control and restrict vertical mixing in the surface layer. Seasonal upwelling leads to high production during the first half of the year. The downward flux of organic matter production leads to sub- and anoxic conditions below 200-250 m. Five separate oceanographic regimes can be defined annually in shallow waters (<200 m) at the basin (Astor et al. 2005). Specifically, this include: beginning of upwelling (December or January), upwelling peak (March or April), transition period (June), secondary upwelling (July or August), and relaxation (September and October). The water column can be divided into three layers: 1) a well mixed superficial layer (1-100 m) with high oxygen content and productivity that undergoes strong seasonal and interannual changes (Muller-Karger et al. 2001), 2) an intermediate layer (200 - 350 m) where suboxic conditions exist and a strong redox gradient is present, and 3) a deep layer (>350 m) with no dissolved oxygen and where methane and hydrogen sulfide (H2S) are present.


RESULTS AND DISCUSSION

The depth of the oxic/anoxic interface in the eastern basin changes with time and space depending on the dynamic conditions of the water (Astor et al. 2003; Ho et al. 2004). For example, the depth of the surface at which oxygen equals 10 µM can be up to 60 m deeper near the margins of the eastern sub-basin than at the center during the upwelling period (Fig. 1). During the period of relaxation or non-upwelling, the topography of this iso-surface does not vary by more than ~20 m across the eastern Cariaco basin.

The suboxic zone has an active microbial loop and near the interface layer, bacteria accumulate contributing to the particulate organic matter observed at this surface. The beam attenuation coefficient of red light (transmissometer data) shows a number of peaks throughout the upper portions of the water column, with the deepest peak marking the presence of bacteria associated with the top of the sulfidic zone (Scranton & Taylor, personal communication), where hydrogen sulfide concentrations start to increase above 1 µM. Since hydrogen sulfide has not been measured on a regular basis at the time series station, the light scattering data has been used to estimate the depth of the oxic-anoxic interface layer. The depth of this interface has fluctuated between about 250 and 350 m over the ten year study period.


Figure 1. Spatial distribution of the 10 µM O2 surface at the eastern basin of the Cariaco Basin during non-upwelling (a) and upwelling (b) periods.

The oxic-anoxic interface layer within the Cariaco Basin changes in thickness and depth through time (Ho et al. 2004). We define this layer by the depths where both oxygen and sulfide concentrations are less than 1 µM and, if oxygen is detectable, where there is a lack of a visible vertical gradient. A transition region (Fig. 2) is observed between the disappearance of dissolved oxygen and the first appearance of sulfide. The thickness of this region varies from ~0 to 100 m through time. Lateral transport and ventilation events at the depth of the oxic-anoxic interface are observed occasionally (Astor et al. 2003, Scranton et al. 2001), suggesting that these events may have an effect in the depth and separation of these two boundaries.


Figure 2. Selected profiles from the Cariaco Time-Series Station show oxygen-CTD (continuous line), beam attenuation coefficient (broken line), sulfide (cross) and discrete oxygen (closed diamonds) concentrations.


The vertical broken line and the continuous line mark the 1 and 5 µM concentration respectively. The horizontal continuous line shows the depths at which dissolved oxygen disappears and the first appearance of hydrogen sulfide, the horizontal broken line marks the last depth at which sulfide is less than 1 µM.

Nutrients have a unique vertical distribution within the Cariaco Basin differing greatly from that typically found in the open ocean. Maximum nitrate values (~10 - 12 µM), resulting from oxidative regeneration (nitritification) of organic matter, are found at an almost constant depth of 160 m (Thunell et al. 2004, Scranton et al. 2006). This portion of the water column, located between the photic (>0.1 % light level) and the anoxic zone, may expand in depth depending on water dynamics. Nitrate concentrations drop at the base of the oxic zone (<5 µM O2) without an associated decrease in phosphates. This linear decrease of NO3- with depth is probably due to heterotrophic denitrification and is associated with a production of alkalinity. Although nitrite concentrations are uniformly very low throughout the water column (<0.5 µM), occasionally two nitrite maxima (>0.5 µM) are observed. The first peak occurs consistently within the photic layer, and probably arises from phytoplankton excretion or nitrification. The second one occurs below 200 m and may be the result of heterotrophic denitrification and/or chemo-denitrification reactions in which nitrate reacts with NH4+ to form N2, and where NO2- acts in the process as the electron acceptor (Fig. 3).

Ammonium concentrations frequently are undetectable or with values less than 0.5 µM in the first 200 m of the water column. However, there is a steep concentration increase defining a sharp gradient at the oxic-anoxic interface. This occurs just below the 26.42 isopycnal surface (Fig. 3) and suggests that there is an upward flux of ammonia that is being consumed at the interface. This region is usually characterized by the disappearance of nitrates and the appearance of the secondary maximum of NO2- suggesting a possible relation with anaerobic ammonium oxidation.

Phosphate concentrations increase with depth with an intervening minimum at intermediate waters (Fig. 3). This minimum is probably due to the scavenging of phosphates during the formation of ferrous and manganese oxides which may sink into the reducing zone where they re-dissolve (Scranton et al. 2006).


Figure 3. Selected profiles from Cariaco Time-Series Station showing nitrate (closed circle), ammonia (closed triangle), phosphate (open triangle) and nitrite (closed diamond) concentrations.


Hydrogen sulfide concentrations were determined every six months at the CARIACO Time Series Station. Hydrogen sulfide is absent above the oxic-anoxic interface, but increases monotonically below this depth. Over the course of this study, the depth of the appearance of sulfide has varied from 210 to 330 m. Deep concentrations of H2S also have shown temporal variability with values ranging from 30.9 to 76.2 µM at 1310 m over a period of ten years. A decrease in the sulfide concentrations from 70.2 to 50.4 µM was observed after the earthquake of July 1997, and another drop in the H2S concentrations of 20 µM was seen by May 1998 after a series of strong ventilation events occurred in the basin. Sulfide values remained below 40 µM between May 1999 and January 2002, they increased through time to 69.75 µM by May 2004. In May 2001, irregularities in the H2S profile may indicate possible lateral isopycnal transport.

Although ventilation events in the Cariaco Basin seem to be random and sporadic phenomena, they have an important effect on the chemical and biological characteristics of the water column, especially near the oxic-anoxic interface and the layers above and below it. During the ten year study period, such events were observed during the years of greater upwelling intensity, specifically in 1997 (5 occasions), 1998 (3), 2001 (2), 2002 (2) and 2003 (2). This activity seems to be connected to an increase in eddy activity outside the basin, although not always seems to be the case.

The CARIACO Time Series Program has elucidated the major seasonal patterns that characterize the oceanography of the basin, and has documented interannual variability, as well as episodic and local events. Changes in Cariaco Basin have been linked directly to seismic activity (Thunell et al. 1999), eddies (Astor et al. 2003), and water intrusions (Scranton et al. 2001; Astor et al. 2003). The CARIACO Time Series is assembling the information needed to understand how the different physical and biochemical process interact to control the cycling of carbon and nutrients in this basin, and how this variability is connected to the southern Caribbean region and the Atlantic Ocean. This will help understand how an upwelling margin works, and also how climate signals are recorded in the sediment records from this unique basin.


ACKNOWLEDGEMENTS

The National Science Foundation (NSF) and Fondo Nacional de Ciencia y Tecnología (FONACIT) supported this work. We are thankful to the personnel of Fundación La Salle de Ciencias Naturales, Estación de Investigaciones Marinas de Margarita (EDIMAR) for their professional support and Dr. Pablo Mandazen Soto (Hermano Ginés) for his confidence in our activities.


REFERENCES

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