OS33B-1263
A High Resolution Climate Record of the Western Tropical Atlantic During Marine Isotope Stage 5
We are constructing a high resolution climate record during the last major interglacial period (MIS 5; 80-130 ky bp) in the western tropical Atlantic. Mg/Ca sea surface temperature (SST) and δ18Ocalcite values were determined from analysis of the surface dwelling foraminifer, G. ruber, in piston core MD03-2620 from the anoxic Cariaco Basin on the Northern Venezuelan shelf. A centennial resolution record of δ18Ocalcite shows millennial scale variability consistent with the NGRIP δ18O record, including Greenland Interstadial events 19-25. On this timescale in the Cariaco Basin, we've found temperature excursions of a more gradual nature leading the relatively rapid excursions in oxygen isotopes. Substage 5e and Termination II are presented in multi-decadal resolution, allowing us to examine centennial patterns of climate change during the major interglacial period. Termination II shows a large Younger Dryas-like reversal and we find SSTs during MIS 5e reaching 29°C, approximately 1.5-2°C higher than peak Holocene values. These temperatures reach western Atlantic warm pool values, suggesting limited upwelling in the Cariaco Basin and a northward migration of the inter-tropical convergence zone for the duration of this extreme warm event.
OS33B-1264
High-Resolution Tropical Atlantic Climate Variability During Interstadial-12 From the Cariaco Basin
Few records from the tropical Atlantic have the ability to resolve climate fluctuations at subdecadal-scale resolution for just the last one thousand years, much less for times further into the past. The high-deposition rate sediments of the Cariaco Basin make it one of the premier archives of high-resolution tropical Atlantic variability. Here we present data with subdecadal-scale resolution from piston core sediments deposited during the latter part of Interstadial-12 (IS-12). Core MD03-2622 was sampled at consecutive one-millimeter intervals resulting in an average temporal resolution of approximately 2 years per sample based on correlations between sediment reflectivity and the Greenland Ice Core 2 stable oxygen isotope record. The foraminifer population is distinctly different from the modern assemblage. The tropical upwelling foraminifer Globigerina bulloides still dominates the total assemblage, but the relative abundance of other major taxa differ significantly from the modern. Orbulina universa is the second-most common species in the modern assemblage, but is nearly non-existent during the end of IS-12, and the average size of individual specimens is much smaller than those from today. Instead, Neogloboquadrina dutertrei (and P-D intergrade) is (are) the second most abundant taxa. In some ways, the Cariaco Basin foraminiferal assemblage during IS-12 resembles that of the modern Santa Barbara Basin assemblage, including the presence of Globigerina quinqueloba. Spectral analysis of the IS-12 G. bulloides abundance data reveals peaks at 8-10 yrs, 12.5-13.0 yrs, 16 yrs, 21 yrs, 32 yrs, and 140-500 yrs. Not only do we observe periodicities similar to those found under modern conditions during late-IS-12 (the decadal- and centennial-scale modes), we also see a pronounced interdecadal mode that is completely absent in the modern record. Additionally, the relative amplitudes of the 8-10 yrs and 12.5-13.0 yrs peaks are reduced compared to power in the interdecadal band. This apparent change in the behavior of the trade wind/ITCZ system is likely related to changes in the cross-equatorial sea surface temperature gradient.
OS33B-1265
The View from Below: Benthic Foraminifera as Indicators of Deep Water Change in Cariaco Basin
The Cariaco Basin off northern Venezuela is the largest anoxic basin in the world today after the Black Sea. High productivity from seasonal upwelling and circulation that is restricted by the relatively shallow (145 m) modern sill depth combine to produce the present state of anoxia below a depth of about 300 m, which in turn contributes to the preservation of finely laminated sediments on the basin floor. During the Last Glacial Maximum (LGM), and intermittently throughout the late Quaternary, anoxic conditions were interrupted and sediments show clear evidence of bioturbation and chemical signatures for the presence of oxygen. This implies a switch in the balance between oxygen supply and demand, the oceanographic and climatic controls for which are not fully understood. Benthic foraminifera preserved during intervals of deep basin oxygenation provide insights into the deep water conditions at those times. We report here benthic census data from the LGM section in ODP Site 1002. At first glance, oxygen in the deep waters during a time of lowered glacial sea level would seem counterintuitive since Cariaco Basin would have been largely isolated from the open Caribbean Sea and even more restricted in its circulation by inlet sills that shoaled to no more than 30 m depth. The paradigm that has emerged to date argues for reduced surface productivity and decreased oxygen consumption from organic remineralization as the key to the oxic conditions. However, benthic foraminifera suggest that increased salinity may play a role in driving basin ventilation at this time. Benthic foraminiferal diversity is greatest at the beginning and end of the LGM, with members of the genera Pyrgo, Quinqueloculina, and Triloculina conspicuously in low abundance. These miliolid genera, which are high-Mg calcite, dominate the middle of the LGM, together comprising as much as 95 percent of the benthic assemblage. Today, this sort of association is common in high salinity environments such as the Persian Gulf. Tritium measurements have implicated the input of warm, salty shelf waters as an important ventilation source for the modern basin. This mechanism may have helped maintain deep oxygen levels during the LGM when the basin was more restricted and the climate more arid than today.
OS33B-1266
Organic-Carbon Burial and Oxygen Depletion Along the Margin of Western North America During the Past 60,000 Years: Teleconnections With Greenland Ice and the Cariaco Basin
During Oxygen Isotope Stage 3 (OIS3; 60-24 cal ka),), the Bolling/Allerod warm interval (B/A; 15-13 cal ka), and the Holocene, bottom-water oxygen levels were much lower, and organic carbon (Corg) burial was much higher on all margins of the Californias (Alta and Baja) than during the last glacial interval (LGI; 24-15 cal ka) and the Younger Dryas cold interval (YD; 13-11.6 cal ka). Burial of Corg during the LGI-B/A-Holocene transition in the Cariaco Basin on the margin of northern Venezuela is remarkably similar to that on the California margin. Correlation between Dansgaard-Oeschger (D-O) cycles in Greenland ice with gray-scale measurements in varved sediments in the Cariaco Basin, and cycles of Corg burial on the Baja California margin are well established. Synchronous climate-driven changes as recorded in the sediments on the margins of the Californias, sediments from the Cariaco Basin, and in the GISP-2 Greenland ice core support the hypothesis that changes in atmospheric dynamics played a major role in abrupt climate change during the last 60 ky.
OS33B-1267
Coupling of Sinking Biogenic Particulate Fluxes and Primary Production in the Euphotic Zone of the Cariaco Basin, Venezuela
Only 1% of the organic matter produced in the upper ocean by photosynthesis reaches depths below 1500 m due to dissolution and microbial degradation. Recent work shows that the vertical flux of particulate organic carbon (POC) is strongly correlated with the settling flux of minerals like calcium carbonate, opal and lithogenic material. These act as ballast and also provide physical protection against degradation of POC. Results from the CARIACO (Carbon Retention in a Colored Ocean) time series program support this hypothesis. For over ten years, CARIACO has been studying the connections between primary production (PP) and the biogeochemical features of sinking particles in the Cariaco Basin, Venezuela, with moored sediment traps that collect settling matter at five depths between 125 and 1300 m on a bi-weekly basis. The geomorphology of the basin restricts deep water ventilation, leading to anoxia below 250 m. Although the Cariaco Basin exhibits strong seasonal production cycles related to wind-driven upwelling, the flux of biogenic matter at all depths below the oxic-anoxic interface is not significantly correlated to primary production. In order to understand the flux of particles in the upper 100 m of the water column, deployments of drifting sediment traps in the Cariaco Basin were carried out from March to July 2007, collecting settling material at 50 and 100 m. The hypothesis is that the flux of sinking material through the euphotic zone may be less affected by decomposition and dissolution than material reaching the deep moored traps. Initial results show significant differences in POC, PON and carbonate flux rates between 50 and 100 m. They also exhibit significant differences in the flux rates of these components between upwelling vs. relaxation periods, suggesting potential connections among seasonal changes in surface chlorophyll a concentrations, plankton community structure, and the vertical export of biogenic materials. We describe results from this research, including comparisons between shallow and deep sediment trap data.
OS33B-1268
Description of an Unrecognized Secondary Upwelling Peak in the Southeastern Caribbean
Strong trade wind-driven upwelling during winter and early spring has been extensively described in the Southern Caribbean. The two main upwelling areas are: the Southeastern region (Peninsulas of Paria and Araya) and the South-central Caribbean region (Peninsulas of Paraguaná and La Guajira). However, the existence of a secondary short-lived upwelling event around June-August, that occurs when the Inter Tropical Convergence Zone migrates northward and the coastal winds are relatively weak, has been rarely acknowledged in the literature. Ten years of records from the CARIACO time series, located in the southeastern Caribbean, clearly show the secondary upwelling peak. This is also supported by thirteen years of historical records of in situ measurements of sea temperature and water densities of the area. The use of old tide gauge data to study past upwelling cycles is supported by a high correlation (R=0.85, p<0.001) between contemporary monthly means of satellite sea surface temperature (SST) and sea level records obtained from a local tide gauge. Three decades of monthly sea level anomalies which date back to 1948 clearly show the upwelling annual cycle, including the secondary summer peak. Thirteen years of SST weekly means from the Southern Caribbean show that the secondary peak takes place simultaneously in the two upwelling areas, and that it occurs in the same focuses as the main upwelling. The secondary upwelling extends the presence of cool waters (< 25.5° C) in the southern Caribbean for 7 months or more. The results of this research confirm that the secondary upwelling is a regular feature of the whole southern Caribbean upwelling system. On going research will focus on the relation between the secondary upwelling to local and remote forcing
OS33B-1269
Fish Effects on Ocean Current Observations in the Cariaco Basin
Multiple years of moored current meter observations from the Cariaco Basin imply the persistent diurnal movement of fish species known to populate the basin. In agreement with short-term observations from 1979, the more recent observations with acoustic Doppler current profilers provide evidence of the multi-decadal presence and behavior of these species. An unwanted corollary however, is a bias in both the vertical and horizontal components of velocity due to the fish movements. Removal of this fish-bias results in large data loss (approximately 72%); however, an interpolated, non-biased data set is developed with depth-averaged horizontal velocities comparable to the observations, demonstrating successful elimination of the bias. Further comparisons show that the interpolated data results in minimal variance density loss at low frequencies and a reduction of variance density at high frequencies such that the interpolated data fits Kolmogorov's 5/3 law for turbulence at subinertial frequencies. A net downward velocity of 0.18cm s-1 is a reflection of a biogenic particle flux and some residual fish contamination. The mean settling speed of particles in the Cariaco Basin is calculated, via Stokes Law, to be smaller than 0.04 cm s-1. Velocity observations from acoustic current meters at depths greater than 400m are impacted by the water clarity, therefore alternate methods should be used to make velocity measurements at depth.
OS33B-1270
Stable isotope probing of chemoautotrophic biomarkers in the Cariaco Basin
In the redoxcline (250-450 m) of the Cariaco Basin, particulate carbon, nitrogen, and biomass (prokaryote, flagellate, viral) concentrations can be > 2-fold the concentrations found in the upper water column. Chemoautotrophic production is sufficient to meet the carbon demands in the redoxcline, yet local electron donor and acceptor sources are insufficient to account for the energy demands of this chemoautotrophy. Parallel experiments using thiosulfate amendments produced a depth-dependent 4 to 33-fold increase in the 14C fixation rate. We are using stable isotope probing on redoxcline waters incubated with 13C labeled bicarbonate and 15N-ammonium with and without thiosulfate amendments to establish the phylogeny and identify the lipid biomarkers associated with the active chemoautotrophic microbes. In incubations with only H13CO3- (and 15N- ammonium) addition (nr5), fatty acid 16:2 had the highest percent of label uptake (~25%). 16:1, 18:u and 14:1 all had >20% 13C incorporation. Other fatty acids with moderate (5-15%) 13C uptake include 12:0, 14:0 16:0, 18:1w7. Very low uptake was observed in i-15:0, a15:0; 15:0, 18:1w9, and 18:0 were all present but did not incorporate detectable 13C. In incubations with thiosulfate added with the H13CO3- (and 15N- ammonium; nr7), the total amount of fatty acids was at least 4x greater. There was also both a shift in the distribution of fatty acids incorporating the label and an increase in the total amount of label incorporation. Fatty acids with >30% 13C include 12:0, 14:1, 14:0, 2 isomers of 16:1, 18:u, and 18:w7. 16:0 also incorporated 13C (~28%). As in nr5, 15:0, a15:0; 15:0, 18:1w9 and 18:0 were all present but showed no 13C enrichment. The labeled fatty acids are consistent with production by sulfur oxidizers. 13C labeled carbon was incorporated into a suite of fatty acids known to be produced by sulfur oxidizers (e.g. Thioploca); production of these lipids also appears to be stimulated with the addition of thiosulfate. On-going work will analyze the phylogenetic affiliations of 13C and 15N labeled DNA, and future experiments will include other potential electron donors and acceptors.