A multidisciplinary investigation of the intermediate depths of the Atlantic Ocean: AAIW delta-carbon-13 variability during the Younger Dryas and lithoherms in the Straits of Florida.

摘要

A 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 delta13C and delta18 O excursions of 0.59 ‰ and 0.37 ‰ 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 13C variability (0.22 ‰) 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 ‰ increase in delta 13C 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 km2 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% 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.