OS32A-01 10:20h
Temporal Variability in the Nutrient Chemistry of the Cariaco Basin
Nutrient data have been collected monthly at the CARIACO time series site in the Cariaco Basin since 1995, providing a uniquely detailed picture of the cycling of NO3-, NO2-, NH4+, PO4 and SiO2 in this permanently anoxic system underlying a major coastal upwelling zone. Our data indicate that nutrients for phytoplankton growth are primarily supplied by upwelling of subsurface water on a seasonal basis. In addition coastal runoff seems to supply important amounts of silica and ammonium to surface waters on a few occasions. We saw no indication of local nitrogen fixation in the Cariaco surface waters, as nutrients appear in Redfield ratio amounts at depths (35-55 m) immediately below the euphotic layer throughout the 8.5 year study period. In the suboxic zone, our data are not sufficiently detailed to resolve all important features. However, they do support an important influence of intrusions on the chemistry of the redox gradient. For example, we occasionally, but do not always, observe partial phosphate removal in a zone above the first appearance of sulfide, which is associated with intermittent intrusions of oxygenated water. When there is no evidence of an intrusion, the phosphate removal is not seen. In the suboxic zone, there appear to be thin layers where ammonium and nitrite coexist, potentially permitting anaerobic ammonium oxidation (anammox) to take place. We also observe decreases in alkalinity near the interface which appear to be associated with sulfide oxidation. In the deep waters, concentrations of ammonium, phosphate and silica continue to increase at a rate consistent with prior studies. However rates of sulfide increase are lower than predicted, probably due to microbial and/or chemical sulfide removal associated with recent oxygen intrusions.
OS32A-02 10:35h
Seasonal and Interannual fCO2 Variability in a Tropical Coastal Upwelling System
Monthly seawater pH and alkalinity measurements were collected between January 1996 and December 2000 at 10° 30' N and 64° 40' W as part of the CARIACO oceanographic time series (CArbon Retention In A Colored Ocean Program). The objective was to study temporal variability in Total CO2 (TCO2) concentrations and the CO2 fugacity (fCO2) at a tropical coastal wind-driven upwelling site. The difference between atmospheric and surface ocean CO2 concentrations (fCO2) ranged from -64.3 to +62.3 atm. Physical and biochemical factors, specifically upwelling, temperature, primary production, and TCO2 concentrations, interact to control temporal variation fCO2. Air-sea CO2 fluxes showed interannual variability. They were typically depressed (0 to +10 mmol C m-2 d-1) in the first few months of the year during the upwelling season and rose during June-November (+10 to 20 mmol C m-2 d-1). While fluxes were not related to changes in salinity, very low positive flux values were seen during the period of very heavy rains and land-derived runoff which depressed surface salinities during the second half of 1999. During the period examined, two events of negative air-sea CO2 flux were observed, namely in March 1997 (\$sim\$ -10 mmol C m-2 d-1) and in March 1998 (\$sim\$ 50 mmol C m-2 d-1), when extremely strong but short upwelling events occurred. Therefore, the Cariaco Basin generally acts as a source of CO2 to the atmosphere in spite of annual primary productivity in excess of between 300 and 600 gC m-2 y-1.
OS32A-03 10:50h
Biogeochemical Fluxes of Carbon and Nitrogen in Surface Waters of the Cariaco Basin: A Combined Rate Measurement and Isotopic Approach
As a tropical ocean margin influenced by seasonal upwelling, the plankton productivity of the Cariaco Basin varies greatly between some of the largest rates measured in ocean waters, to near oligotrophic conditions, over the course of a year. The fluxes of carbon and nitrogen associated with these biological variations also change significantly and will be analyzed in the context of a consistent time series of physical, chemical and biological data collected monthly over the last several years. New (nitrate) productivity increases in response to the upwelling of nutrients in winter and also to a secondary (although somewhat smaller) upwelling period in summer. There appears to be significant regenerated nutrients (such as ammonium and dissolved organic matter) to fuel production at other times of the year as well. As an additional dimension in the analysis of carbon and nitrogen flow, the isotopic signatures of the zooplankton collected in net hauls were examined over a two-year period. The C-13/C-12 ratio of the zooplankton pool was significantly heavier in the winter upwelling season, but showed little response to summer upwelling. The N-15/N-14 ratio, on the other hand, increased in both upwelling periods and also exhibited a two-year trend towards higher values. These variations are potentially a rich source of information on the dynamics of the epipelagic environment and likely reflect in part, the lower pCO2 levels and larger phytoplankton cells in the winter upwelling season, as well as the intrusion of nitrate from outside of the basin during both the winter and summer upwelling periods.
OS32A-04 INVITED 11:05h
The Nitrogen Isotope Dynamics of the Cariaco Basin
We describe the controls on the N-15/N-14 of water column nitrate and ammonium in the Cariaco Basin and their implications for sinking and sedimentary N. Water column denitrification occurring in the basin has only a very small isotopic imprint on nitrate in the basin because nitrate consumption is nearly complete in the actively denitrifying water near the oxic/anoxic interface (275 m). Being free of a large denitrification signal, the delta-N-15 of shallow thermocline nitrate is 3.5 permil, significantly lower than the mean deep ocean nitrate delta-N-15 of 5 permil. This may be due to the nitrification of newly fixed N, whether it occurs within the basin or in open Atlantic waters that flow into the Cariaco over the sill. The N-15/N-14 of the sinking flux in the deepest trap (1250 m) is similar to that of thermocline nitrate, as expected given the complete consumption of nitrate in the surface layer. Moreover, the N-15/N-14 of the seafloor sediment is similar to that of the sinking flux, as is common in environments of high export production, low bottom water oxygen, and good organic matter preservation. Thus, surface sediment in the modern Cariaco Basin records the N-15/N-14 of the thermocline nitrate, which, in turn, may record the input of newly fixed N to the upper ocean, be it local or more regional in origin. We consider briefly whether these isotopic relationships may have changed in the past and how such changes would affect the downcore record of sediment N-15/N-14 in the Cariaco Basin.
OS32A-05 11:20h
The Sedimentary N Isotope Record From Cariaco Basin - Local and Global Implications
In an earlier study, Haug et al. (1998) showed that the 15N/14N of bulk sedimentary N from Cariaco Basin is lower during interglacials, when organic carbon concentrations are higher. These results were interpreted to reflect increased nitrogen fixation responding to increased denitrification during interglacials. However, it could not be discerned whether this feedback was operating strictly on a local basis (within the Cariaco) or whether either the denitrification-driven N loss or the N2-fixation-driven response was occurring on a regional or global scale. Higher resolution measurements of Terminations I and V show a clear time lag of about 4 kyr between the increase in sedimentary N concentration (coincident with the onset of sediment lamination at Termination I) and the decrease in 15N/14N. These data argue against a diagenetic change as the cause for the 15N/14N change. Since the Cariaco is a relatively small basin, this points towards a regional to global cause for at least part of the 15N/14N drop. Moreover, the new data from the last deglaciation show 15N/14N maxima for the period of the Bolling/Allerod and early Holocene, interrupted by the Younger Dryas, and a subsequent decrease until the mid Holocene. Based on studies of the modern Cariaco water column, a local mechanism for these changes can be imagined and will be presented. However, similar deglacial 15N/14N changes in records from Indian and Pacific sediment records suggest that, since early deglaciation, whole ocean changes in nitrate 15N/14N have been recorded in the Cariaco Basin and are responsible for many of the above deglacial changes. Thus, despite the clear structure of the sediment 15N/14N changes, distinguishing local from regional and global signals in the Cariaco Basin sediment record remains a challenge.
OS32A-06 11:35h
Climate and Hydrologic Conditions Over Northernmost South America as Recorded in Cariaco Basin Sediments
The Cariaco Basin has proven its worth as a remarkable repository of paleoenvironmental information in the tropical Atlantic Ocean. Laminated sediments that have accumulated rapidly in the deep basin under largely anoxic conditions preserve an "ice core-like" record of marine and terrestrial variability that can be linked to widely-distributed sites showing evidence for abrupt climate change. In Cariaco Basin, a strong modern seasonality exists because of the annual migration of the Intertropical Convergence Zone (ITCZ). On time-scales ranging from interdecadal to millennial back through marine isotope stage (MIS) 4, a coherent climatologic response is observed in sedimentary parameters that can be attributed to a pattern of ITCZ migration that mimics the seasonal cycle. During periods of cool North Atlantic sea surface temperature (SST), decreased terrigenous delivery from local rivers provides evidence of a southward shift in the mean latitudinal position of the ITCZ and its belt of convective rainfall, while a northward shift in ITCZ position can be inferred for periods of warm North Atlantic SST from increased terrigenous input to the basin. Data from MIS 5, however, at Ocean Drilling Program Site 1002 suggest an opposite pattern, with increased terrigenous delivery having occurred during the cooler portions of the last interglacial, including MIS 5b, 5d and during a clear "Younger Dryas-like" event that characterizes Termination II. To the extent that mean ITCZ position is sensitive to orbitally-induced changes in tropical insolation, we speculate that the higher amplitude of the precessionally-forced insolation signal during MIS 5 may have led to periodic migration of the Atlantic ITCZ to a position well north of Cariaco Basin. This would have left the basin more sensitive to southward excursions of the ITCZ, as opposed to today where it sits near the northern limit of the range of ITCZ motion. If true, this highlights the important role that orbital precession plays in the tropics, especially in driving the hydrological cycle.
OS32A-07 11:50h
Seasonal to Climate-Scale Variability in the Vertical and Burial Fluxes of Terrigenous Organic Matter in the Cariaco Basin
Biweekly sediment trap samples have been collected continuously since 1995 at multiple depths in the water column as part of the Cariaco Basin ocean time series. A selected set of samples of sinking particulate matter from 1997 to 1999 have been analyzed by alkaline CuO oxidation to measure the fluxes of lignin phenol products. Since lignin is uniquely derived from terrestrial vascular plants, we are using lignin phenols to trace the inputs of terrigenous organic matter at this site. The results of this study show two seasonal peaks in the flux of lignin phenols of over 0.2 mg/m2/d relative to background fluxes of about 0.05 mg/m2/d. The timing on one of the peaks of that coincides with the initiation of the main upwelling period in November-December of each year. A second peak in the flux of lignin phenols was also found during the initiation of the secondary upwelling event between May and June. The seasonal and inter-annual differences in lignin, and by inference terrigenous organic matter, fluxes will be discussed in the context of temporal changes in wind, surface salinity, freshwater runoff and upwelling-controlled biologic productivity. The compositions of the lignin phenols are being investigated to search for seasonal differences that may help characterize the provenance terrigenous organic matter and help understand the processes responsible for the measured fluxes. Lignin compositions and burial fluxes from a gravity core representing 6,000 years of sedimentation are being analyzed to investigate the longer time scale record of terrigenous organic matter inputs to Cariaco Basin.
OS32A-08 12:05h
Isotope Studies on Cariaco Basin Methane
We report the first measurements of d2H-CH4, d13C-CH4, and 14C-CH4 on water column and sediment samples from the Cariaco Basin. Previous studies on Cariaco Basin methane geochemistry consider release of diagenetic methane from sediments as the source of water column methane. Our observations indicate that a sediment diagenetic source is present, but that the water column methane inventory is dominated by an unexpected and previously unknown fossil methane source.