OS33A-0569 1340h
Sedimentological Signatures of Transient Depositional Events in the Cariaco Basin, Venezuela
The varved sediments that have accumulated in the Cariaco Basin throughout the Holocene provide a detailed archive of the region's climatic history, and act as a historical record for the occurrence of phenomena such as earthquakes and coastal flooding. In this study we compare the sedimentological characteristics of lithogenic material collected from the water column during transient depositional events to those of normal hemipelagic sedimentation in the basin. Specifically, we have examined the clay mineralogy and grain size distribution of detrital material delivered to the basin by the July 9, 1997 earthquake near Cumana, Venezuela and the coastal flooding of Venezuela in late 1999. The sample material used in our study was collected as part of an ongoing sediment trap time series in the Cariaco Basin. The sedimentological signatures associated with these two events are distinctive from the typical lithogenic input to the basin. Preliminary data for biweekly samples collected from 1997-1999 shows a tri-modal particle size distribution, with peaks at 3, 22, and 80 m. However, material collected from the deep basin immediately following the 1997 earthquake is characterized by a particle diameter distribution at 6 and 22 m with a smaller than normal peak at 80 m; this variance suggests an alternate source of material was delivered to the basin via a turbidity flow induced by the earthquake. Supporting this theory, the clay mineralogy of the same sediment trap samples shows a higher than average ratio of kaolinite to quartz for sediments delivered to the basin following both the earthquake and flooding. We hope to extend the use of these sedimentological methods to identify past transient depositional events in Cariaco Basin cores.
OS33A-0570 1340h
The Link between Climate and Terrigenous Deposition in the Cariaco Basin, Venezuela
We are investigating seasonal variations in terrigenous input to the Cariaco Basin, Venezuela, by studying the major and trace element chemistry in sediment trap material gathered over two years (1996-1998) as part of the CARIACO program. Our main goal is to compare changes in the flux and composition of the terrigenous matter with meteorological and hydrological records (e.g., precipitation and SST) in order to discern the modern climate forcing of terrigenous deposition in the Basin. Previous studies have used chemical proxies of the terrigenous component to infer changes in the paleoposition of the Intertropical Convergence Zone (ITCZ). Our study aims to discriminate the sources of this material and understand how modern shifting of the ITCZ controls changes in these sources' relative contributions. Understanding this link in the modern is crucial to help calibrate the paleorecords and interpret past climate changes in this tropical setting. Preliminary results from two traps (275m and 440m) show that the detrital component has its highest concentration during the wet season. However, terrigenous flux does not show seasonality, which suggests that parameters other than river run off are affecting the terrigenous deposition. Some chemical tracers of the detrital fraction clearly document fluctuations between a rainy and a dry season signal, suggesting seasonal changes in the relative contribution of at least two different sources. Multivariate statistical analysis suggests mixing between two or three compositional end-members. We are also studying shelf sediments (Unare Platform). These sediments record a dilution pattern as more proximal samples show the highest concentrations of Ti, Al, Fe, and other detrital tracers while the more distal samples show the lowest concentrations (biogenic dilution). Thus, for example, variations in Ti concentration in marine cores may reflect dilution, not real changes in the terrigenous flux. Considering the reported composition of potential sources and the results we have at this point, changes in the contribution of local rivers, Saharan aerosol, and the Orinoco River may contribute to the observed seasonality. On-going work by our team is assessing the importance of these potential sources.
OS33A-0571 1340h
Sources of autochtonous and allochtonous particulate material to the Cariaco Basin, Venezuela
The temporal variability of physical and biogeochemical oceanographic parameters of the Cariaco Basin, located along the southern margin of the Caribbean Sea, were studied using data collected during two cruises to the eastern half of the Basin in September 2003 and March 2004. Specific objectives were to examine the hydrography of the upwelling plume and the spatial distribution of particles in the area, to help determine the relative importance of particles produced in situ vs. those delivered laterally from terrigenous origins. Particle abundance was examined using continuous vertical profiles of c-beam attenuation and chlorophyll fluorescence. During the dry season (March 2004), Subtropical Underwater (SUW) occupied the entire water column over the northern Cariaco sill. As the SUW entered the basin, this water mass sank about 30-50 m, spreading southward and upwelling along the southeastern corner of the basin. The upwelling plume brought nutrients, dissolved organic carbon (DOC) and colored dissolved organic material (CDOM) to the surface. The particles generated by primary production near the surface spread to the northwest. The vertical flux of this organic material sustains a microbial population with marked vertical structure in biomass as observed in beam attenuation and CDOM fluorescence profiles. During the rainy season (September 2003) the in situ production of organic material is low. However, there is an influx of particles of terrestrial origin, delivered to the basin by local rivers. Though most of the sediment settles within the first 2-3 km from the mouth of the Unare and Neveri Rivers, smaller suspended particles are carried farther within low salinity plumes that reach the CARIACO time series station (10o 30' N; 64o 40' W).
OS33A-0572 1340h
The importance of small but local vs. large rivers in the region on the Cariaco Basin, Venezuela
Two cruises were conducted to sample the eastern half of the Cariaco Basin, located along the southern margin of the Caribbean Sea, during September 2003 and March 2004. The objectives of these cruises were to examine the seasonal influence of the Caribbean Sea and of rivers in the region on the hydrography of the Cariaco Basin. Specifically, the cruises sought to clarify whether strong seasonal salinity variation observed in surface waters at the CARIACO time series station (10.5oN, 64.66oW) is due to influence of the Orinoco and Amazon River or due to dispersal of plumes from local rivers entering the basin along its southern margin. During the rainy season (September 2003), surface salinities inside the basin were generally higher relative to Caribbean Sea waters outside the basin. Salinity patterns indicated that the Orinoco and Amazon don't influence the basin directly. However, there is a strong correlation between DOC concentration and CDOM absorption in the Cariaco basin for March 2004 (r2 = 0.846), regardless of the location sampled. The influence of local rivers was evident during the rainy season. Low salinity plumes lined the southern margin of the Cariaco Basin relative to the north around the sill. The primary rivers that affect the basin are the Unare and Neveri. While their sediment input is limited and affects mostly the shallow platform, their plumes are carried northwestward toward the CARIACO time series station near the surface. Near the Manzanares river, off the city of Cumana, and near Cubagua Island, located south of Margarita Island, large particle concentrations were observed at depth (70-150 m), pointing to a possible deep sediment pool year-round. However, more observations are necessary to determine the nature and origin of these particles. The high resolution climate record contained in Cariaco Basin sediments can only be properly interpreted by considering the significant influence of small local rivers relative to the small to minimal influence of large rivers of the region.
OS33A-0573 1340h
Toward Resolution of Imbalance between Microbiological Energy Demand and Supply to Cariaco's Redoxcline: Horizontal Productivity Gradients
The Cariaco Basin's redoxcline or transition zone between oxic and anoxic waters typically resides between 250 and 450 m and hosts anomalously high microbiological production. At Station CARIACO, chemoautotrophic bacterial production in the redoxcline on average represents 70% (median) of local primary production in surface waters (upper 100 m). Presumably chemoautotrophs are fueled by inorganic end products (reduced S species, NH$_{4}^{ +}$, CH$_{4}$, H$_{2}$) supplied by remineralization of residual surface production in the Basin's interior. However, less than 10% of local surface production (in C units) sinks to the redoxcline. Simple 1-D mass balance calculations based on measurements from our time-series station demonstrate that $<$10% of chemoautotrophs' demands for reductant (energy) can be met by local vertical flux of biogenic debris. Both temporal and spatial variability in chemical and biological processes are likely to be important in creating this perceived imbalance. We hypothesize that enriched, non-local sources of reductant are advected in bottom waters to support higher-than-expected chemoautotrophic production at Station CARIACO. Four years of SeaWiFS ocean color satellite imagery clearly demonstrate that mean annual primary production is significantly higher east of Station Cariaco and that seasonal blooms propagate westward. Using SeaWiFS images and published algorithms, we will estimate local and basin-wide production and organic sedimentation based on bathymetry and flux models. From accepted diagenetic models and biological stoichiometries, we will estimate potential reductant production and requisite transit velocities required to balance observed biological demand.
OS33A-0574 1340h
Characterization of phosphorus in sinking particles in Cariaco Basin, Venezuela
Phosphorus (P) is an essential nutrient utilized by all organisms for biological productivity, yet little is known about its cycling within the marine realm. In this study, we used a five-step sequential sediment extraction method (SEDEX) to examine the composition of P in sinking particles obtained from one of the world's largest anoxic basins, Cariaco Basin, Venezuela. This method, which is usually applied to sediments, chemically separates particulate P into 5 phases: loosely bound or exchangeable (P$_{Ex}$), iron-bound (P$_{Fe}$), authigenic (P$_{Aut}$), detrital (P$_{Det}$), and organic (P$_{Org}$). Samples were collected from November 2000 to April 2002 using four automated sediment traps moored at depths of 275, 455, 930 and 1250 m, with the 275 m trap located just above the oxic/anoxic interface. Our results indicate that the composition of particulate P changes dramatically as the material sinks through the water column. The 275 m trap was comprised of a mixture of P$_{Ex}$ (57 %), P$_{Org}$ (23 %) and P$_{Fe}$ (15 %), whereas the deepest trap was dominated by P$_{Org}$ (55 %) and P$_{Ex}$ (26 %). Total P fluxes decreased by a factor of four between 275 and 1250 m (85.7 versus 18.5 \mu mol m$^{-2}$ d$^{-1}$), with much of this decrease due to the $>$ 75 % loss of P$_{Ex}$ (49.2 to 4.9 \mu mol m$^{-2}$ d$^{-1}$) and P$_{Fe}$ (12.6 to 1.8 \mu mol m$^{-2}$ d$^{-1}$). P$_{Org}$ decreased by only 50 % between these two depths (19.9 versus 10.2 \mu mol m$^{-2}$ d$^{-1}$). Our results are consistent with the expectation that P$_{Ex}$ is the most labile fraction of P and P$_{Fe}$ is rapidly reduced in the anoxic bottom waters of Cariaco Basin. P$_{Ex}$ correlates well with organic C (C$_{Org}$) only in samples from the shallowest trap (r$^{2}$ = 0.6), while in the deeper traps there is a strong correlation with terrigenous material (r$^{2}$ = 0.6). This implies a change in the major source, and hence lability of this material as it sinks to the sea floor. P$_{Org}$ was very well correlated to C$_{Org}$ at all depths (r$^{2}$ $>$ 0.9), implying little to no preferential remineralization below 275 m. Additional strong relationships between Porg and opal and CaCO$_{3}$ became apparent when upwelling versus non-upwelling time periods were separated, implying different mechanisms of Porg production throughout the year. Our results imply that water column P transformations are complex and need to be considered when studying the cycling of P in marine systems.
OS33A-0575 1340h
Seasonal to interannual depth-dependent changes in phosphorus flux in Cariaco Basin, Venezuela
One of the major removal pathways of phosphorus (P) from the water column is through the formation, sinking, and burial of particles formed during marine biological production. Yet the flux of P containing particles to the seafloor remains one of the least studied components of the P cycle. In this study, particulate inorganic P (PIP) and particulate organic P (POP) fluxes were measured in a series of samples collected from sediment traps ranging in depth from 275 to 1255 m from November 1995 - November 2002 in Cariaco Basin, Venezuela. PIP concentrations averaged 40- 60 % (depending on depth) of the total particulate P (TPP) measured in the traps. PIP fluxes decreased by 75 % between the surface and deep waters, from a median of 28.6 to 6.3 \mu mol m$^{-2}$ d$^{-1}$, whereas POP fluxes decreased by only 50 %, from 17.2 to 8.5 \mu mol m$^{-2}$ d$^{-1}$. TPP, PIP and POP all vary seasonally and higher fluxes follow higher production during the upwelling season from late January to April. The relationship between particulate organic C (POC) and POP is relatively constant (POC:POP = 283) throughout the entire water column over the entire period (r$^{2}$ = 0.58). However, there is a much tighter relationship between POP and POC in upwelling (January through April, r$^{2}$ = 0.85) versus non upwelling (May through December, r$^{2}$ = 0.40) seasons. Furthermore, upwelling, and hence higher production appears to be associated with higher POC:POP ratios (327 versus 258 in non upwelling periods). Higher than Redfield POC:POP ratios may indicate that preferential release of P containing organic matter is occurring, but if true, it is restricted to the upper 250 m of the water column above the shallowest sediment trap. An alternative explanation may be that the composition of plankton in the Cariaco Basin does not conform to the Redfield-ratio. Plankton tow samples collected over the upper 200 m with a $>$ 200 um mesh had POC:POP ratios of 294 +/- 38. However, there is no other evidence that the euphotic zone of Cariaco Basin is dominated by unique flora. The lower POC:POP ratios associated with non upwelling, low production events imply either 1) a different consortium of species is dominant during these periods, and/or 2) that a large portion of the POP pool is derived from other, nonbiologically derived particles.
OS33A-0576 1340h
Chemical Composition of Sinking Particulate Organic Matter in the Water Column and Sediments of the Cariaco Basin
Sinking particles have been collected at bi-weekly intervals from three different water depths in the eastern region of the Cariaco Basin since late 1995 as part of the CARIACO time series study. A subset of samples collected from 1997 to 1999 have been analyzed by alkaline CuO oxidation to characterize the magnitude and composition of the organic matter exported from the euphotic zone over several upwelling-stratification cycles. As part of this study we quantified CuO reaction products from a variety of common biochemicals, including amino acids, lipids (fatty acids and dicarboxylic acids) and lignin. The compositions of the sinking particles in the Cariaco Basin were dominated by amino acid and lipid-derived products, which in past studies have been shown to be predominantly derived from marine organic matter sources such as phytoplankton and zooplankton. The seasonal fluxes of these products correlated well with both organic carbon and opal fluxes, further reinforcing the interpretation that marine organic matter is the major source of these compounds. In contrast, lignin-derived products, which are characteristic of terrigenous organic matter, showed seasonal fluxes that correlated best with lithogenic fluxes. We will investigate the attenuation in the export fluxes of all these compounds as they travel through the water column of the Cariaco Basin in order to provide insight into organic matter degradation processes. Additionally, we will compare the magnitude of the annually integrated water column export fluxes of these compounds to their accumulation fluxes in underlying sediments in order to assess the burial efficiency of different organic matter components. Finally, we will reconstruct the burial fluxes of these compounds over a 6,000 year time period by analyzing samples from sediment cores from the basin in order to provide a historical record of organic matter preservation at this location.
OS33A-0577 1340h
Major Sterol Fluxes in Sinking Particles and Surface Sediments in the Cariaco Basin.
Sterols in sediments are used to trace past ecosystem dynamics in the upper ocean. It is therefore important to know what factors control the creation of sterol fluxes, degradation of sterols in the water column and eventual burial in the sediments. To this end we examined the major sterols fluxes in sediment traps during 1996-1997 (at depths of 275, 455 and 975m) and surface sediments in the Cariaco Basin. Sterol flux data in the sediment traps were compared with hydrographic data collected as part of the CARIACO Project. Diatom sterols 24-methylcholesta-5,22-dien-3b-ol (brassicasterol), 24-methylcholesta-5,24(28)-dien-3b-ol (24methlyene-cholesterol) fluxes were greatest during upwelling. 24methlyene-cholesterol was well correlated with biogenic opal flux (r$^{2}$ = 0.88) suggesting that 24methlyene-cholesterol is an excellent biomarker for diatom production. 4a,23,24-trimethyl-5a(H)-cholest-22-en-3b-ol (dinosterol) exhibited a post upwelling maximum indicating that fluxes of dinoflagellate-derived materials were dominant during stratified conditions. Sterols were degraded with depth but the relative composition of the major sterols remained fairly constant. There is a sharp decrease in the magnitude of total sterol fluxes between the sediment traps (955m; 143 ug m$^{-2}$ d$^{-1}$) and the surface sediments (core depth 460m; 11.7 ug m$^{-2}$ $^{-1}$) indicating that a large portion of the flux is lost at the sediment water interface. It is at this transition in which the relative compositions of the sterols are also altered. Dinosterol, which is a minor component of the sediment trap fluxes, is 3 to 4 times greater than that of cholesterol in the sediment. While the ratio of dinosterol to cholesterol changed significantly, the ratio between the two diatom sterol fluxes, brassicasterol and 24methlyene-cholesterol, and cholesterol remained within the range of values observed in the sediment traps.
OS33A-0578 1340h
Holocene and Glacial Variability of the Hydrologic Cycle in the Tropical Atlantic: Evidence from the Cariaco Basin
Climate variability in the Central American tropics is largely forced by changes in the intensity of the Hadley circulation and the strength and position of the Intertropical Convergence Zone (ITCZ). The two most direct consequences of seasonal ITCZ motion for Cariaco Basin and its sediments are the pronounced alternation between the upwelling that occurs in the winter-spring and the increased river runoff that takes place in the summer-fall. This seasonal contrast, coupled with the lack of bioturbation during anoxic phases of deposition in the basin, has produced a high deposition rate (ca. 30 to $>$150 cm/kyr). Sediment sequence in most portions consisting of mm-scale, annually-paired plankton-rich biogenic laminae and terrigenous detrital-rich laminae. The built-in seasonal clock that results from ITCZ movement has allowed development of annually-resolved chronologies over intervals where laminae are distinct enough to reliably count yielding nearly unparalleled temporal resolution for a marine sequence. In a series of recent papers, we have utilized measurements of sedimentary Ti and Fe contents as proxies for the total terrigenous fraction of Cariaco Basin sediments. Downcore variations in Ti and Fe have been adopted as indices of past regional hydrologic change, with higher values interpreted to reflect increased rainfall, river discharge, and terrigenous sediment delivery to the basin. Bulk Ti and Fe and other major element patterns in sediment recovered from IMAGES core MD03-2621 and Ocean Drilling Program (ODP) Hole 1002C were measured at a 50 to 500 um spacing, which reflects a temporal spacing of up to 10 measurements per varve, using a new profiling x-ray fluorescence scanner located at GFZ Potsdam. Here, we present a new data set spanning the Holocene and parts of the last glacial period.
OS33A-0579 1340h
Tropical Sea Surface Temperature Variability During Marine Isotope Stage 3: Mg/Ca results from the Cariaco Basin, Venezuela
Magnesium/calcium \{Mg/Ca\} ratios were measured in the surface dwelling planktonic foraminiferal species \(Globigerinoides ruber\) \{white variety\} from a high deposition rate sediment core collected in the Cariaco Basin, located on the northern continental shelf of Venezuela, in order to assess the magnitude of sea surface temperature \{SST\} changes associated with stadial/interstadial oscillations during Marine Isotope Stage 3. Specifically, we have generated a high resolution SST record for interstadials 5-8 \{$\sim$35 to 45 ka\}. The Mg/Ca ratios of \(G. ruber\) \{white\} document large surface temperature oscillations that appear to co-vary with the Greenland ice-core air temperature records. The lowest values were obtained from the end of interstadial 6 \{$\sim3$mmol/mol\} and the highest values were obtained during interstadial 7 \{$\sim5$mmol/mol\}. This represents a temperature change of $\sim$$5.5\deg$C. We have also measured the Mg/Ca ratio in two other species of planktonic foraminifera, \(G. ruber\) \{pink variety\} and \(Globigerina bulloides\). The trace metal data is compared with foraminiferal oxygen isotope measurements made on the same suite of samples to assess the internal consistency of these temperature proxies.
OS33A-0580 1340h
Nitrogen Isotopic Evidence for Climate-Induced Variations in Nitrogen Cycling in the Cariaco Basin Upwelling System from the Younger Dryas to Present
A high-resolution nitrogen isotopic study in the sediments of the Cariaco Basin, Venezuela, indicates that nitrogen cycling in the Cariaco Basin has varied dramatically over the past 12 14C kyr as a result of climate-induced variations in wind-driven upwelling intensity. The d15N of sedimentary organic matter (SOM) varies from as low as 1% during the Younger Dryas and later Holocene to values between 4 and 5.5% during the early Holocene (10 and 7 14C kybp). In the modern Cariaco Basin, strong correspondence between the d15N of thermocline nitrate, particulate organic nitrogen filtered from 275 and 1250 m water depth, and core-top SOM implies that the basin sediments accurately record the isotopic composition of thermocline nitrate (Thunnell et al., 2004). Thermocline nitrate in the Cariaco basin is 15N-depleted relative to deep-ocean nitrate, indicating the importance of newly fixed N to the planktonic ecosystem. d15N of SOM around 1% during the Younger Dryas and the mid-late Holocene indicate substantial changes in the nitrogen cycling and the overwhelming importance newly fixed N as a source for sedimentary N. At times of high productivity, low N:P ratios in surface waters and the proximity to a land source to supply needed Fe are factors capable of promoting enhanced N-fixation. Alternatively , either direct ammonium assimilation or the recently identified process of anaerobic ammonium oxidation could be responsible for the 15N-depleted nitrogen isotopic signature in the sediments during periods characterized by high productivity such as the Younger Dryas. Temporal variations in the biogeochemical cycling of nitrogen from the Younger Dryas to present will be discussed in the context of contemporary nitrogen cycling as identified by the CARIACO time-series, to previous sedimentary 15N-studies spanning glacial-interglacial periods in the Cariaco Basin (Haug et al., 1998), and to innovative studies of nitrogen cycling in other modern marine anoxic basins.
OS33A-0581 1340h
Soledad Basin, Baja California: a Twin to Cariaco Basin for Monitoring the Eastern Tropical Pacific Today and the Past?
Soledad Basin, a semi-enclosed basin on the Pacific margin of southern Baja California at 25oN, is ideally located to document past variations of ocean/atmosphere interactions responding to the Pacific Decadal Oscillation (PDO) and the El Nino-Southern Oscillation (ENSO). This presentation focuses on the hydrography and geochemistry of the basin in the context of a potential monitoring program that could reach the scale of current activities in Cariaco Basin. Soledad Basin (sometimes referred to as Magdalena Basin or San Lazaro Basin) has been studied intermittently since the 1970's although detailed studies to exploit its paleoceanographic potential have started only recently. A very flat bottom with a maximum depth of 540 m was mapped with SeaBeam. A comparison of hydrographic profiles collected inside and outside the basin indicates a sill depth of 290 m. Bioturbation is currently inhibited within the basin primarily because of low oxygen concentration in adjacent source waters, rather than oxygen consumption within the basin as is the case for Cariaco and Santa Barbara Basins. Radiocarbon dating of planktonic foraminifera indicates a very high sedimentation rates of ~108 cm/kyr up through the end of the Bolling/Allerod 13 kyr ago (van Geen et al., Paleoceanography, v. 8, no. 4, 2003). A non-bioturbated section, characterized by sub-cm dark brown to black, coarse, mm- to cm-scale laminations rather than by mm-scale fine laminations, extends almost continuously from the top of a piston core to ~9 m depth, an interval dated at 10.0 ka. In addition, thin white mm-scale laminae composed almost entirely of coccoliths packed in faecal pellets extend to a depth of ~11 m (11.3 ka). A selection of promising results based on diffuse spectral reflectance records obtained at 1-cm resolution, planktonic Mg/Ca data, and the acccumulation of authigenic Mo will be presented.