Browsing by Subject "Younger Dryas"
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Item A Multidisciplinary Investigation of the Intermediate Depths of the Atlantic Ocean: AAIW delta^13C Variability During the Younger Dryas and Lithoherms in the Straits of Florida(2012-02-14) Brookshire, BrianA transect of cores ranging from 798 m to 1585 m water depth in the South Atlantic Ocean document the relative intermediate water mass nutrient geometry and stable isotopic variability of AAIW during the Younger Dryas cooling event. The data reveal concurrent delta^13 C and delta^18 O excursions of 0.59 ppt and 0.37 ppt within the core of Antarctic Intermediate Water (AAIW) centered at 11,381 calendar years before present based on radiometric age control. A portion of the delta^1 3C variability (0.22 ppt) can be explained by a shift in thermodynamic equilibrium concurrent with a drop in temperature of 1.8?C at the locus of AAIW formation. The remaining 0.37 ppt increase in delta^13 C most likely resulted from increased wind velocities, and a greater coupling between the ocean and the atmosphere at the locus of AAIW formation (increased efficiency of the thermodynamic process). Deepwater coral mounds are aggregates of corals, other organisms, their skeletal remains, and sediments that occur on the seafloor of the world?s oceans. In the Straits of Florida, these features have been referred to as lithoherms. We use digital, side-scan sonar data collected from the submarine NR-1 from an 10.9 km^2 area at ~650 m water depth to characterize quantitatively aspects of the morphology of 216. Their lengths, widths, heights, areas, orientations and concentration on the seafloor have been determined. Analysis indicates that the outlines of relatively small to medium sized lithoherms can be effectively described with a piriform function. This shape is less applicable to the largest lithoherms because they are aggregates of smaller lithoherms. Nearly all of the lithoherms studied have axes parallel to the northward flowing Florida Current, and the heads of 80 percent of these features face into the current. The shape and orientation of the lithoherms, and evidence of megaripples and scouring in the sonar data suggest that these features are formed by a unidirectional current. Following an extensive investigation of over 200 lithoherms via side-scan sonar imagery and direct observation, we have developed a qualitative model for the formation of the lithoherm type of deep-water coral mounds in the Straits of Florida. Lithoherm formation can be characterized by four main stages of development: nucleating, juvenile, mature singular, and fused. Fused lithoherms can form via transverse and/or longitudinal accretion, however, transverse accretion at the head of the mound is likely the most efficient mechanism. A comparison of lithoherm spatial relationship to local bathymetry agrees with previous observations of deep-water coral mound formations along the levied margins of density flow scour channels.Item Effect of Atlantic Meridional Overturning Circulation Changes on Tropical Coupled Ocean-Atmosphere System(2010-01-14) Wan, XiuquanThe objective of this study is to investigate the effect of Atlantic meridional overturning circulation (AMOC) changes on tropical coupled ocean-atmosphere system via oceanic and atmospheric processes. A suite of numerical simulations have been conducted and the results show that both oceanic and atmospheric circulation changes induced by AMOC changes can have a profound impact on tropical sea surface temperature (SST) and sea surface salinity (SSS) conditions, but their dominance varies in different parts of the tropical oceans. The oceanic process has a dominant control on SST and SSS response to AMOC changes in the South Tropical Atlantic, while the atmospheric teleconnection is mainly responsible for SST and SSS changes over the North Tropical Atlantic and Pacific Oceans during the period of reduced AMOC. The finding has significant implication for the interpretation of the paleotemperature reconstructions over the southern Caribbean and the western Tropical Atlantic regions during the Younger Dryas. It suggests that the strong spatial inhomogeneity of the SST change revealed by the proxy records in these regions may be attributed to the competing oceanic and atmospheric processes that dominate the SST response. Similar mechanisms may also explain the reconstructed paleo-salinity change in the tropical Atlantic, which shows a basin-wide increase in SSS during the Younger Dryas, according to recent paleo climate studies. Finally, we show that atmospheric teleconnection induced by the surface cooling of the North Atlantic and the North Pacific in response to a weakened AMOC, is a leading physical mechanism that dictates the behavior of El Nino/Southern Oscillation (ENSO) response to AMOC changes. However, depending on its origin, the atmospheric teleconnection can affect ENSO variability in different ways. The atmospheric process associated with the North Atlantic cooling tends to enhance El Nino occurrence with a deepened mean thermocline depth in the eastern Pacific, whereas the atmospheric process associated with the North Pacific cooling tends to produce more La Nina events with a reduced mean thermocline depth in the eastern Pacific. Preliminary analysis suggests that the change in ENSO characteristics is associated with the change in internal atmospheric variability caused by the surface cooling in the North Atlantic and North Pacific. Complex nature of the underlying dynamics concerning the effect of the AMOC on ENSO calls for further investigation into this problem.