ARTICLE IN PRESS Deep-Sea Research II 51 (2004) 927–943 www.elsevier.com/locate/dsr2 Processes of coastal upwelling and carbon flux in the Cariaco Basin Frank Muller-Kargera,Ã, RamonVarela b, Robert Thunellc, Yrene Astorb, Haiying Zhanga, Remy Luerssena, Chuanmin Hua aCollege of Marine Science, University of South Florida, St. Petersburg, FL 33701, USA bFundacion La Salle de Ciencias Naturales, Estacion de Investigaciones Marinas de Margarita, Apartado 144 Porlamar, Isla de Margarita, Venezuela cDepartment of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA Received 18 September 2002; accepted 6 October 2003 Available online 15 September 2004 Abstract Monthly hydrographic, phytoplankton biomass and primary production, bio-optical observations, and settling particulate organic carbon flux observations were collected at 10.51N, 64.671W within the Cariaco Basin, off Venezuela, for a period exceeding seven years starting in November 1995. These data were combined with a time series of Sea- viewing Wide-Field-of-view Sensor (SeaWiFS), advanced very high-resolution radiometer (AVHRR), and European Remote Sensing Satellite/QuikScat data to examine the spatial extent of a cold coastal upwelling plume and a phytoplankton bloom associated with it. The seasonal upwelling cycle was directly linked to the intensity of the Trade Winds, with sea-surface temperature (SST) changes lagging the wind by 1–2 weeks. The seasonal cycle of most properties was punctuated by transient phenomena, some of which caused subsurface ventilation and also high primary production events. Integrated primary production ranged from 650, 574, and 593 g C mÀ2 yrÀ1 in1996, 1997, and2001, respectively, to 372, 484, and 448 g C mÀ2 yrÀ1, respectively, in 1998, 1999, and 2000. The Rutgers vertical generalized productionmodel (VGPM) was modified to reflect anincreaseinAssimilationNumber (PB opt) with SST at the Cariaco time series station, because the original VGPM formulation suggested inhibition of primary production at SST4211C. Trap observations showed that between 9 and 10 g C mÀ2 yrÀ1 were delivered to the bottom at the Cariaco time series station, i.e. 1.33% of surface primary productivity. Annual particulate organic carbon flux to the bottom over the area of the Cariaco Basin(waters 4100 m), estimated using SeaWiFS and AVHRR variable inputs and the updated VGPM, ranged from 6.77 Â 1010 to 7.61 Â 1010 g C. These are likely underestimates due to lack of bathymetric corrections to flux. r 2004 Elsevier Ltd. All rights reserved. ÃCorresponding author. Tel.: +1-727-553-3335; fax: +1-727-553-1103. E-mail address: [email protected] (F. Muller-Karger). 0967-0645/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2003.10.010 ARTICLE IN PRESS 928 F. Muller-Karger et al. / Deep-Sea Research II 51 (2004) 927–943 1. Introduction field et al., 1963; Richards, 1975; Hastings and Emerson, 1988; Zhang and Millero, 1993; Scran- Over geological time scales, geochemical imbal- tonet al., 2001a ; Scranton et al., 2001b), develop- ances at the Earth’s surface are closely tied to ment of residence-time and box models (Scranton carbon sequestration by marine organisms. How- et al., 1984; Ward et al., 1987), and in the study of ever, whether this mechanism helps mitigate the metallic sulfides (Baconet al., 1980 ), among many increase of anthropogenic greenhouse gases over other organic matter remineralization and redox- years to decades represents one of the most sensitive process studies. The Cariaco Basin is pressing questions in oceanography (Longhurst, indeed the largest anoxic basin of truly oceanic 1991; de Haas et al., 2002). As we enter the new character and represents an important comple- millennium, only a handful of multi-year time ment to the Black Sea for chemical oceanographic series of insitu observationsare available to study studies. the connection between surface primary produc- More recently, the scientific community has tion and vertical carbon flux in the ocean. Clearly, come to appreciate the Cariaco Basinas an it is not practical to conduct such detailed in situ exceptional archive of past climate change (Over- observations in more than a few carefully selected peck et al., 1989; Hughenet al., 1996a, b, 1998, locations. Space-based observations therefore of- 2000; Haug et al., 1998; Black et al., 1999; fer promise inthe effort to quantifyglobal ocean Petersonet al., 1991, 2000 ; Werne et al., 2000). It productivity (Falkowski, 2002; Geider et al., 2001; is increasingly evident that the tropics play an Behrenfeld et al., 2001), but animportantaddi- important role in global climate, influencing the tional step is to assess the applicability of satellite hydrologic balance between Atlantic and Pacific data to assessing particulate carbon flux to the Oceans, the heat balance between low and high bottom of the ocean(e.g., Deuser et al., 1988, latitudes, and consequently general ocean circula- 1990). Here we address this questionby studying tion( Enfield and Mayer, 1997; Giannini et al., time series observations collected within the 2000, 2001; Hoerling et al., 2002). Corals provide Cariaco Basin. only a short record of tropical climate, making it difficult to assess how past changes observed in ice 1.1. Geographic setting and significance of the cores at high latitudes relate to changes in the Cariaco Basin tropics. Surficial Cariaco Basin sediments instead contain a clear record of the last 15,000 years and The Cariaco Basin( Fig. 1) is a 1400 m deep have been used to reconstruct changes in the rate depression on the continental shelf off Venezuela. of formation of North Atlantic Deep Water and in It is openly connected to the surface Atlantic the surface circulationof the Atlantic Ocean Oceanabove a shallow ( 100 m) sill. There (Hughenet al., 1998, 2000 ). The most recent are two channels breaching this sill, one in oceandrillingprogram (ODP) cores collected in the northeast of 135 m depth (La Tortuga) and the Cariaco Basinextendthis record back over a narrower one in the northwest of 146 m 600,000 years, and suggest that the modern (Centinela)(Richards, 1975; Lidz et al., 1969). deposition patterns are observed during past The sill restricts water motion and lateral exchange interglacials, and that glacial periods show in- of material with the adjacent ocean. We may view creased ventilation and diminished organic carbon the Cariaco Basinconceptually as a natural accumulationinthe sediment( Haug et al., 1998). sediment trap within a continental shelf. Because The quiescent anoxic bottom conditions in the turnover of basin waters is slow (Deuser, 1973), Cariaco Basinandassociated absenceof bioturba- decomposition of the sinking material leads to tionallow the accumulationof sedimentvarves at permanent anoxia below about 275 m. the bottom of the basininthe present-day This scenario has served biogeochemists for oceanographic regime. These varves contain the over 40 years inthe constructionof stoichiometric climate record. Each varve is a couplet consisting models of organic matter remineralization (Red- of a light and a dark layer. The light laminae are ARTICLE IN PRESS F. Muller-Karger et al. / Deep-Sea Research II 51 (2004) 927–943 929 Fig. 1. The Cariaco Basin, location and representative AVHRR and SeaWiFS images. Top left: location of the Cariaco Basin in the CaribbeanSea; top right: Cariaco Basinbathymetry (m) andlocationof the CARIACO time series station(circle at 10 1300N, 641400W); bottom left: monthly AVHRR SST composite (March 2001); bottom right: monthly SeaWiFS Chl-a composite (March 2001). Black areas in the images represent land. Gray areas in the SeaWiFS image off the mouth of the Orinoco River represent highly turbid coastal waters. rich in plankton remains and are thought to be ited in the sediment, and about the effect of deposited during the upwelling period, or approxi- changes in hydrography, wind, phytoplankton mately betweenDecember andJuneevery year. In community composition, primary productivity, contrast, the dark laminae consist of detrital and terrestrial input, have not been tested. material or terrestrial minerals, presumably deliv- Indeed, very little is known about carbon fluxes ered to the basin during the rainy season between inthe tropics, andthe Cariaco Basinprovides a August and September. Assumptions underlying convenient and unique setting in which to study this simple conceptual model, and specifically the connection between near-surface ocean dy- assumptions about the origin of materials depos- namics, variability in primary production, and the ARTICLE IN PRESS 930 F. Muller-Karger et al. / Deep-Sea Research II 51 (2004) 927–943 settling of particulate organic carbon to depth wind by 1–2 weeks. This corrected the spurious within a tropical environment. results on the phasing between wind and upwelling found based on the anemometer data and pub- lished earlier by Muller-Karger et al. (2001). 2. Methods Sea-surface temperature (SST) was estimated from imagery collected by the AVHRR sensors on We base this study onthe CArbonRetentionIn the NOAA 11, 12, and 14 satellites. The satellite A Colored Ocean(CARIACO) time series, in- SSTs were derived using the split-window techni- itiated inNovember 1995 ( Muller-Karger et al., ques (Walton, 1988; Strong and McClain, 1984; 2000, 2001; Astor et al., 2003; Thunell et al., 1999, McClainet al., 1983 ). While non-linear algorithms 2000; Scranton et al., 2001a, 2001b; Taylor et al., developed for AVHRR data could be used, there is 2001). The time