Sven Oliver Franz

Astronomical age models for Pleistocene drift sediments from the western North Atlantic (ODP Sites 1055-1063)

Ten ODP sites drilled in a depth transect (2164–4775 m water depth) during Leg 172 recovered high-deposition rate (>20 cm/kyr) sedimentary sections from sediment drifts in the western North Atlantic. For each site an age model covering the past 0.8–0.9 Ma has been developed. The time scales have a resolution of 10–20 kyr and are derived by tuning variations of estimated carbonate content to the orbital parameters precession and obliquity. Based on the similarity in the signature of proxy records and the spectral character of the time series, the sites are divided into two groups: precession cycles are better developed in carbonate records from a group of shallow sites (2164–2975 m water depth, Sites 1055–1058) while the deeper sites (2995–4775 m water depth, Sites 1060–1063) are characterized by higher spectral density in the obliquity band. The resulting time scales show excellent coherence with other dated carbonate and isotope records from low latitudes. Besides the typical Milankovitch cyclicity significant variance of the resulting carbonate time series is concentrated at millennial-scale changes with periods of about 12, 6, 4, 2.5, and 1.5 kyr. Comparisons of carbonate records from the Blake Bahama Outer Ridge and the Bermuda Rise reveal a remarkable similarity in the time and frequency domain indicating a basin-wide uniform sedimentation pattern during the last 0.9 Ma.

Depositional changes along the Blake–Bahama Outer Ridge deep water transect during marine isotope stages 8 to 10 – links to the Deep Western Boundary Current

ODP sites 1055–1062 recover a bathymetric transect from 1800 to 4800 water depth in the subtropical NW-Atlantic (Carolina Slope, Blake–Bahama Outer Ridge). This sediment drift region is known for high deposition rates (>40 cm/kyr) and offers the excellent opportunity to investigate the history of water mass circulation and chemistry as well as depositional changes during the Quaternary. A late Pleistocene time interval from 250 to 350 kyr (marine isotope stages 8–10) was investigated with centennial- to millennial-scale time resolution. Stable isotope records of benthic foraminifera provide new detailed insights on variations in climate and deep water ventilation. The δ13C records indicate well ventilated North Atlantic water masses between 2200 and 3000 m water depth during the time interval from stage 9 to interstadial 8.5. During the glacial stages 8.4 and 10.2, however, the decrease in δ13C reflects an extension of nutrient-rich Antarctic Bottom Water (AABW) up to 2200 m water depth. This is paralleled by a shoaling of the lysocline as indicated by the carbonate records. A comparison between carbonate dissolution proxies points out that the carbonate dissolution at the hemipelagic sites was not only influenced by the different carbonate ion concentration of the water masses (AABW contra North Atlantic Deep Water) but also by the organic carbon flux to the sea floor especially at the shallower sites, whereby the decay of organic matter enhanced carbonate dissolution in the sediments. Additionally the varying depth position and strength of the Deep Western Boundary Current (DWBC) plays an important role on the depositional system along the Blake–Bahama Outer Ridge. Comparisons between current intensities as inferred from grain size analyses, sand, and carbonate contents suggest that high intensities of the DWBC during cold stages caused an erosion of the fine carbonate and an enrichment of the sand fraction at shallower depth. During warm stages the main core of DWBC moved to greater depth and allowed the settling of finer material at shallower depth. Synchronously this led to an increased supply of carbonate and extremely high carbonate concentrations below 3500 m water depth.