Understanding the Link between the Ocean Surface and the Sinking Flux of Particulate Carbon in the Cariaco Basin

Since November 1995, the CARIACO Ocean Time Series program has been studying the relationship between surface primary production (carbon fixation rates by photosynthesis of planktonic algae), regional hydrography, physical forcing variables (such as the wind), and the settling flux of particulate organic carbon in the Cariaco Basin. This tectonic depression, located on the continental shelf of Venezuela (Map), shows marked seasonal and interannual variation in hydrography and primary production induced in part by the regular migration of the Intertropical Convergence Zone (ITCZ).

The Cariaco Basin hydrography is affected by North-Atlantic gyre-scale processes, including dispersal of Subtropical Underwater and western boundary current variability, cross-equatorial flow of water masses (Wust, 1964; Muller-Karger et al., 1989), wind-driven upwelling compounded by geostrophic circulation (Richards, 1975; Muller-Karger and Aparicio-Castro, 1994), ventilation forced by Caribbean Sea eddies (Astor et al., 2003), and river discharge (Yarincik et al., 2000; Lorenzoni, et al. 2009). Due to its restricted circulation and high primary production, the basin is anoxic below ~250 m (Muller-Karger et al., 2001; 2010). In the late 1990’s and early 2000’s, CARIACO observations measured annual primary production rates of more than 500 gC/m²y, of which over 15-20% was generated by events lasting one month or less. Since 2004 there has been a decrease in primary production rates (annual averages of less than 400 gC/m²y). Still, the annual primary production rates in the Cariaco Basin are comparable to rates estimated using time series observations for Monterey Bay (460 gC/m² y; Chavez, 1996), and higher than rates estimated for Georges Bank, the New York Shelf, and the Oregon Shelf (380, 300, and 190 gC/m² y, respectively; Walsh, 1988). Primary production and vertical particulate organic matter fluxes in the Cariaco Basin are higher than those observed at the oligotrophic BATS and HOT locations (Karl et al., 2001; Steinberg et al., 2001; Thunell et al., 2007). The Basin also experiences sedimentological events caused by earthquakes (Thunell et al., 1999; Lorenzoni et al., 2012) and coastal flooding (Percy et al., 2008; Lorenzoni et al., 2009). All of these phenomena can influence the sediments.

Due to its high rates of sedimentation (30 to >100 cm/ky; Peterson et al., 2000) and excellent sediment preservation, the varved sediments of the Cariaco Basin offer the unique opportunity to study high resolution paleoclimate and better understand the role of the tropics in global climate change ( Black et al., 1999; Peterson et al., 2000; Haug et al., 2001; Black et al., 2004; Hughen et al., 2004). An ocean time series in the Cariaco Basin is particularly important as it helps elucidate the connection between forcing mechanisms, processes in the water column and how they are translated into the sedimentary record (Montes et al., 2012). Sediment traps maintained by the CARIACO program show that over 5% of autochtonous material reaches 275 m depth, and that nearly 2% reaches 1,400 m. The significance of this flux is that it represents a sink for carbon and that it helps explain the record of ancient climate stored at the bottom of the Cariaco Basin (Thunell et al., 2000).

Acknowledgements: This ocean time series work was supported by the National Science Foundation (NSF), the National Aeronautics and Space Administration (NASA), and Venezuela’s Fondo Nacional de Ciencia, Tecnología e Innovación (FONACIT). For more information please see this Acknowledgements link

RECENT SCIENTIFIC HIGHLIGHTS

• The source of siliciclastic detritus to the Cariaco Basin is largely from the local rivers draining the coastal region to the south of the Cariaco station. The sediment trap samples provide a faithful instantaneous picture of the settling flux of lithogenous elements that can be used for comparison with average basinfloor burial rates over several years (Calvert et al., 2015).
• TChla, absorption and accessory pigments vary seasonally in the Cariaco Basin in response to changes in the phytoplankton community composition. The POC:TChla ratio at CARIACO is also variable and dependent on bulk carbon (not necessarily related to phytoplankton) and the functional groups present at any given time, underscoring the fact that using a fixed ratio of POC:Chla in biogeochemical models can lead to large uncertainties in carbon budgets from coastal zones (Lorenzoni et al., 2015).
• Biometric characteristics of Orbulina universa (d’Orbigny) were used to differentiate two morphotypes present in sediment trap samples collected from the sediment trap array deployed at the CARIACO station. Data from this research provided field evidence that thin and thick morphotypes of O. universa may experience different environmental conditions during the formation of their final chamber and, therefore, should not be combined in geochemical analyses for reconstructing past surface ocean conditions (Marshall et al., 2015).
• Analysis of phosphorus (P) within suspended particulate samples collected at the CARIACO station revealed secondary peaks in total suspended particulate P (TSPP) in the redoxcline that were similar in magnitude to those measured in the upper 100 m. However, the composition of TSPP was significantly different between the surface and the redoxcline. Abrupt increases and transformations in the TSPP pool within the redoxcline result from both abiotic reactions associated with a manganese and iron redox shuttle and biotic reactions associated with a large and diverse chemoautotrophic prokaryotic community. These results suggest that the biogeochemical cycling of P may be altered within oxygen depleted water columns in unexpected ways (McParland et al., 2015).
• A species distributions model was used to evaluate whether the dominant phytoplankton species present at the CARIACO station can adapt to changing environmental conditions or whether they have fixed environmental preferences. The results suggested that most of the dominant species were able to adapt their realized niches to track average increases in water temperature and irradiance, but the majority of species exhibited a fixed niche for nitrate. Community ecosystem models should no longer assume that phytoplankton cannot adapt (Irwin et al., 2015).
• Decreasing pH and increasing pCO2 trends due to uptake of anthropogenic carbon are already observable in the ocean. At CARIACO, change of surface pCO2 was one of the highest measured among 9 ship-based, biogeochemical time-series distributed globally; this increase in pCO2 can be attributed to warming of surface waters linked to a reduction in upwelling which in turn leads to lower biological productivity (Bates et al., 2014; Tanhua et al., 2015).