OS43C-01 INVITED
The Cariaco Basin Ocean Time Series Program: Linking Modern and Ancient Processes
The varved sediments accumulating in the Cariaco Basin are widely recognized as containing an incomparable archive of past tropical climate change. These sediments are used to study climate on multiple time scales, and to better determine the relationship and phasing between high latitude and low latitude climate change. The Cariaco Basin Ocean Time Series Program was started as a joint US-Venezuelan initiative in 1995 in an effort to link modern processes with this sedimentary record of climate change. The primary scientific objective of the time series is to better understand the interrelationships amongst climate, hydrography, primary production and sediment fluxes, and how signals of changes in these processes are preserved in seafloor sediments. Specifically, our monthly monitoring of water column properties and the continuous collection of sediment flux observations since the inception of the time series provide an ideal opportunity for refining and calibrating climate proxies, for use both in Cariaco Basin and elsewhere. In this talk we will review the accomplishments of the Cariaco Basin Ocean Time Series and the opportunities that exist for collaborations.
OS43C-02
The CARIACO Time Series: Carbon fluxes in a tropical, continental upwelling margin
Continental margins play an important role in the global carbon cycle, accounting for about 10-15% of the carbon produced in oceanic systems, and 40% of carbon deposition on the global ocean floor. In an effort to understand some of the processes operating along a wind-driven coastal upwelling margin, the CARIACO time-series project (CArbon Retention In A Colored Ocean) has carried out monthly oceanographic cruises to the Cariaco Basin on the continental shelf off Eastern Venezuela to collect hydrographic, biogeochemical and bio-optical observations since November 1995. Subtropical Underwater (SUW) provides new nutrients to the Basin, which support annual primary production of ~500 mgCm-2. These estimates are comparable to Monterrey Bay and higher than other upwelling sites such as off West Africa and the Oregon shelf. However, there is a disconnect between vertical, sinking Particulate Organic Carbon fluxes and primary production in the Basin, possibly due to lateral advection of particles. The POC budget between the surface and 275m indicates that about ~2.8x1011 mol C yr-1 are either remineralized or available for export as POC out of the Basin. Most of the carbon fixed by primary producers in the upper 20-50 m of the water column is exported out of the basin. Remineralization can contribute between 1-10% to the DIC values between the surface and 275 m per day. Rivers play a minor role in nutrient input. The data obtained from the CARIACO time-series helps assess the carbon budget and the seasonality of hydrographic variables in the South-Eastern Caribbean. Ultimately, these data are critical to understand the paleoclimate record of changes in the Atlantic Ocean stored in the laminated sediments at the bottom of the Cariaco Basin.
OS43C-03
A Nested Model of the Cariaco Basin: Study of the Hydrography and Interactions with the Open Ocean
The circulation of the Cariaco Basin (Venezuela) is modeled using the Regional Ocean Model System (ROMS) nested in the global 1/12 degree Hybrid Coordinate Ocean Model (HYCOM). The objective of this work is to obtain a better understanding of the Cariaco Basin circulation by studying the processes that link the basin with the Caribbean Sea. To our knowledge, this is the first attempt to model the circulation in the Cariaco Basin with a nested high resolution hydrodynamical model. In particular, we examined the interaction of the Cariaco Basin with the large-scale, open-ocean processes, as the westward Caribbean Current and the eastward subsurface counter-current flowing along the South America Caribbean coast. These two current systems connect the Cariaco Basin with the Caribbean Sea waters, and therefore are directly related to the ventilation of the basin. By studying the kinematics and dynamics of the Cariaco Basin we anticipate gaining a better understanding on how the past conditions affected the basin characteristics and hence the geological records obtained from the basin sediments. We will report on several years of observations from the continuous monitoring of currents within the basin, plus analyses of year-long model runs that provide a basin-wide, three-dimensional context for the circulation.
OS43C-04 INVITED
The History and Dynamics of Anoxia in Cariaco Basin
The Cariaco Basin is the largest modern anoxic marine basin in the world after the Black Sea. Although anoxia involves complex interactions between biological, chemical, and physical processes, the presence or absence of anoxic conditions in the marine environment ultimately reflects a balance between oxygen consumption and supply. A history of this balance in Cariaco Basin, and clues to the climatic and oceanographic controls, is preserved in the underlying sediment record. Much of the late Quaternary sequence in Cariaco Basin is laminated, indicating deposition under anoxic conditions that preclude biological mixing. However, the existence of significant bioturbated intervals indicate past oscillations between oxic and anoxic conditions in the deep basin. To a first-order, glacioeustatic sea level changes appear to play a key role in whether or not anoxic conditions in Cariaco Basin develop, with the effects of sea level on sill depth controlling the availability of nutrients and hence surface productivity. During glacial lowstands, reduced input of nutrients to the basin results in low productivity and oxygenated conditions, while interglacial times of high sea level are linked to strong upwelling, an increased supply of sinking organic detritus and sea floor anoxia. Superimposed on this pattern are millennial-scale oscillations in oxygen levels, best revealed by high-resolution scanning XRF analyses of redox-sensitive elements such as Mo and Cd. During the last glacial, intervals of anoxic deposition coincide one for one with warm interstadials as recorded in Greenland ice cores and require a different explanation. At such times, high productivity may be stimulated by riverine delivery of nutrients rather than by upwelling.
OS43C-05
Tropical Climate Variability During Marine Isotope Stage 3: Results from the Cariaco Basin
The last glacial period is characterized by a number of well-defined, millennial scale stadial and interstadial events that have been documented in climate records from low to high latitudes. We combine detailed oxygen isotope and Mg/Ca records for the planktonic foraminifera G. ruber from Cariaco Basin core MD03-2621 in order to assess the magnitude of sea surface temperature, and to reconstruct seawater δ18O variations and sea surface salinity changes associated with 10 stadial/interstadial oscillations during MIS3. The lowest oxygen isotope values \{~0.25‰\} occur during interstadials \{IS\} with the highest values reaching 0.85‰ during stadial periods. The δ18O of G. bulloides is also measured in MD03-2621 and covaries well with G. ruber δ18O, though, the G. bulloides δ18O values are slightly lighter than G. ruber during stadials events. The variability in the oxygen isotopic composition for G. ruber \{white and pink\} and G. bulloides along with the δ13C record of G. bulloides correlates very well with sediment color reflectance data, which is considered a proxy for marine productivity. The Mg/Ca results for G. ruber across several of these stadial/interstadial oscillations tend to co vary with the δ18O data. Mean Mg/Ca values range from a minimum of ~3.0 mmol/mol during stadial periods just prior to and immediately after IS-7 and IS-12, to a maximum of ~4.9 mmol/mol during IS-12. Interestingly IS 12 exhibits a double peak in both the δ18O and Mg/Ca records with the trace element record leading by about 400 years. The Mg/Ca and δ18O records for G. ruber begin to diverge during the latter part of IS 13 and shortly after interstadials 9 and 6 suggesting warmer, saltier conditions during these stadial periods. We are comparing this record with a Greenland and Antarctic ice cores to further evaluate of the role the tropics play in millennial scale climate change.
OS43C-06
Selective Preservation and the Interpretation of Pollen Data from Hypoxic Marine Conditions: Evidence from Cariaco Basin Core MD03-2620
Oxic/anoxic transitions in marine cores raise questions about the interpretation of pollen data because pollen tend to be grouped with other calcareous organisms which disappear with the onset of oxic conditions. However, pollen grains are extremely resistant in diverse mediums, and their sensitivity to oxic environments has been asserted, but not proven. We present a robust calculation of pollen grains from hypoxic conditions and an oxic/anoxic transition zone in core MD02-2620 from the Cariaco Basin and correlate the pollen data with organic content (grey scale), Ti and Ca concentration, and sedimentation rate. We also consider the effect of upwelling and the contribution of riverine pollen. The oxic/anoxic transition is determined from lithological data. Our statistical analysis shows that the minimum of pollen concentration does not coincide with the maximum of Ti input and the minimum of Ca concentration in the sediment, but follows it with a 300-400 year lag. The composition of the pollen assemblages indicates low diversity and an abrupt decrease of arboreal elements, mostly from the lowland vegetation group, but also an increase in non-arboreal pollen and spores. These observations are inconsistent with the assumption that the pollen spectrum is controlled primarily by selective preservation due to onset of oxic conditions. In fact, the only manifestation of selective preservation is the disappearance of the smallest grains in any group, regardless of ecology. Comparison of our results with pollen data from other marine cores allows us to conclude that unusual pollen assemblages provide information about climate change despite some selective preservation.
OS43C-07 INVITED
Modern Climate Forcing of Terrigenous Deposition in the Tropics (Cariaco Basin, Venezuela)
One approach to deciphering tropical Quaternary paleoclimate records has been to study the composition of terrigenous material in order to infer past changes in hydrologic and atmospheric conditions. We have analyzed the inorganic geochemistry (major and trace elements) of modern sediment traps and shelf sediments in the Cariaco Basin, Venezuela, in order to characterize seasonal variation in deposition of the terrigenous component and to document linkages within the ocean-atmosphere-climate system. Our results show that variation in the chemistry of terrigenous input to the Basin is a sensitive monitor of the annual meridional migration of the Intertropical Convergence Zone (ITCZ). Additionally, terrigenous relative abundances are decoupled from terrigenous absolute fluxes, showing that abundances are not directly related to terrigenous delivery. Changes in mixing between local fluvial sediments, fluvial and/or eolian mafic material, and Saharan dust explains the observed seasonality. The modern seasonal variations are significantly smaller than those observed in the glacial-interglacial paleorecord, indicating that climate sensitivity at the two time scales is very different. Significant contrast in the behavior of Ti/Al between the longer glacial-interglacial time scale and the modern seasonality also suggests that long-term terrigenous deposition is not linked simply to 100-kyr shifting of the ITCZ. The modern data can be reconciled with the glacial-interglacial record by invoking generally drier conditions during glacial periods, combined with intermittent wet conditions caused by precessional forcing. Results from three additional years of time-series data as well as from the continental margin surrounding the basin, will also be discussed.
OS43C-08
Importance of Bottom Nephelolid Layers on the Transport and Delivery of Ssediment to the Eastern Cariaco Basin, Venezuela
Due to their high topographic relief and susceptibility to erosion, mountainous coastal rivers may potentially deliver high sediment and organic matter loads to continental margins. Three small mountainous rivers (Manzanares, Neveri and Unare) that empty onto the shelf of the eastern Cariaco Basin (Venezuela) were examined during the rainy season in September 2003 and 2006 under the framework of the CARIACO (CArbon Retention In A Colored Ocean) Time-Series Project. Optical transmissometer measurements were coupled with particulate organic matter (POM) samples collected in the water column and near the bottom. Suspended sediments entering the basin sink rapidly to the bottom close to the mouth of the rivers (< 10 km). Over the shelf, river sediments dispersed via bottom nepheloid layers (BNL) up to 50 km from the source, reaching the 100m isobath. BNL were observed near the mouths of the three rivers, varying in thickness from 2 to 20 m, with the thickest and most extended BNL observed off the Unare River. BNL particle concentrations were 2-60% higher than water column values. Most of the BNL were lower in particulate organic carbon (POC), nitrogen (PON) and phosphate (POP) (~85 mg C m-3, ~9.6 mg N m-3, ~ 0.8 mg P m-3, respectively), compared to measurements in the overlying water column (~100 mg C m-3, ~10.7 mg N m-3, ~ 0.92 mg P m-3, respectively), suggesting that BNL are not a primary mechanism for delivering terrigenous POM to the deeper anoxic waters of the Cariaco Basin during the rainy season. Nonetheless, terrigenous sediments originating in the Cordillera de la Costa (Coastal Mountain Range) of Venezuela are found in the deep sediments of Cariaco Basin, and thus may serve as important components of mineral ballast that acts to transport POC from surface waters to depth. BNL also represent a potential, transitory source of iron to the suboxic zone of the Cariaco Basin.