Age models for ODP Leg 172 sites

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.

Grain size and magnetic fabric analyses

The combined use of grain size and magnetic fabric analyses provides the ability to discriminate among depositional environments in deep-sea terrigenous sediments. We analyzed samples from three different depositional settings: turbidites, pelagic or hemipelagic interlayers, and sediment drifts. Results indicate that sediment samples from these different environments can be distinguished from each other on the basis of their median grain size, sorting, as well as the intensity and shape of magnetic fabric as determined from an examination of the anisotropy of magnetic susceptibility. We use these discriminators to interpret downcore samples from the Bermuda Rise sediment drift. We find that the finer grains of the Bermuda Rise (relative to the Blake Outer Ridge) do not result from lower depositional energy (current speed) and so may reflect a difference in the nature of sediment being delivered to the site (i.e., distance from source) between the two locations.

High-resolution paleomagnetic investigation in the Iceland Basin

Based on discrete samples, we report new high-resolution records of the ~185 kyr Iceland Basin (IB) geomagnetic excursion from Ocean Drilling Project (ODP) Site 1063 on the Bermuda Rise (sedimentation rate 32 cm/kyr) and from ODP Site 983 in the far North Atlantic (sedimentation rate 18 cm/kyr). Two records from Holes 1063A and 1063B are very consistent, and provide the highest resolution of the detailed field behaviour during the IB excursion obtained so far. Inclination records from Holes 983B and 983C in the far North Atlantic are also very consistent, whereas declination anomalies deviate more notably. The pseudo-Thellier (PT) technique was applied along with more conventional palaeointensity proxies (NRM/ARM and NRM/kappa) to recover relative palaeointensity (RPI) estimates from Hole 1063A and Hole 983B. As expected, these proxies indicate that the field intensity generally dropped at both sites during the IB excursion, but also that the history of RPI from the two sites is different. VGPs from Site 1063 indicate that the field at this location experienced some stop-and-go behaviour between patches of intense vertical flux over North America and the tip of South America, areas which coincide fairly well with patches of preferred transitional VGP clustering from reversals and zones of high seismic velocity in the lower mantle. Changes in RPI at this location were generally gradual, possibly due to the proximity of these flux patches, and the first period of VGP-clustering over North America was accompanied by a conspicuous increase in RPI. VGPs from Site 983 track along a different path, and the associated RPI changes are very abrupt and completely synchronous with the onset and termination of the excursion. The differing VGP paths from Sites 1063 and 983 indicate that the global field structure during the IB excursion was not dominated by a single dipole.

Stable isotope ratios in shells of marine organisms

Oxygen and carbon isotope analyses have been carried out on calcareous skeletons of important recent groups of organisms. Annual temperature ranges and distinct developmental stages can be reconstructed from single shells with the aid of the micro-sampling technique made possible by modern mass-spectrometers. This is in contrast to the results of earlier studies which used bulk sampIes. The skeletons analysed are from Bermuda, the Philippines, the Persian Gulf and the continental margin off Peru. In these environments, seasonal salinity ranges and thus annual variations in the isotopic composition of the water are small. In addition, environmental parameters are weIl documented in these areas. The recognition of seasonal isotopic variations is dependant on the type of calcification. Shells built up by carbonate deposition at the margin, such as molluscs, are suitable for isotopic studies. Analysis is more difficult where chambers are added at the margin of the shell but where older chambers are simultaneously covered by a thin veneer of carbonate e. g. in rotaliid foraminifera. Organisms such as calcareous algae or echinoderms that thicken existing calcareous parts as weIl as growing in length and breadth are the most difficult to analyse. All organisms analysed show temperature related oxygen-isotope fractionation. The most recent groups fractionate oxygen isotopes in accordance with established d18O temperature relationships (Tab. 18, Fig. 42). These groups are deep-sea foraminifera, planktonic foraminifera, serpulids, brachiopods, bryozoa, almost all molluscs, sea urchins, and fish (otoliths). A second group of organisms including the calcareous algae Padina, Acetabularia, and Penicillus, as weIl as barnacles, cause enrichment of the heavy isotope 18O. Finally, the calcareous algae Amphiroa, Cymopolia and Halimeda, the larger foraminifera, corals, starfish, and holothurians cause enrichment of the lighter isotope 16O. Organisms causing non-equilibrium fractionation also record seasonal temperature variations within their skeletons which are reflected in stable-oxygen-isotope patterns. With the exception of the green algae Halimeda and Penicillus, all organisms analysed show lower d13C values than calculated equilibrium values (Tab. 18, Fig. 42). Especially enriched with the lighter isotope 12C are animals such as hermatypic corals and larger foraminifera which exist in symbiosis with other organisms, but also ahermatypic corals, starfish, and holothurians. With increasing age of the organisms, seven different d13C trends were observed within the skeletons. 1) No d13C variations are observed in deep-sea foraminifera presumably due to relatively stable environmental conditions. 2) Lower d13C values occur in miliolid larger foraminifera and are possibly related to increased growth with increasing age of the foraminifera. 3) Higher values are found in planktonic foraminifera and rotaliid larger foraminifera and can be explained by a slowing down of growth with increasing age. 4) A sudden change to lower d13C values at a distinct shell size occurs in molluscs and is possibly caused by the first reproductive event. 5) A low-high-Iow cycle in calcareous algae is possibly caused by variations in the stage of calcification or growth. 6) A positive correlation between d18O and d13C values is found in some hermatypic corals, all ahermatypic corals, in the septa of Nautilus and in the otoliths of fish. In hermatypic corals from tropical areas, this correlation is the result of the inverse relationship between temperature and light caused by summer cloud cover; in other groups it is inferred to be due to metabolic processes. 7) A negative correlation between d18O and d13C values found in hermatypic corals from the subtropics is explained by the sympathetic relationship between temperature and light in these latitudes. These trends show that the carbon isotope fractionation is controlled by the biology of the respective carbonate producing organisms. Thus, the carbon isotope distribution can provide information on the symbiont-host relationship, on metabolic processes and calcification and growth stages during ontogenesis of calcareous marine organisms.

Age model and stable isotope record of DSDP holes 48-401 and 86-577

Early Paleogene warm climates may have been linked to different modes and sources of deepwater formation. Warm polar temperatures of the Paleocene and Eocene may have resulted from either increased atmospheric trace gases or increased heat transport through deep and intermediate waters. The possibility of increasing ocean heat transport through the production of warm saline deep waters (WSDW) in the Tethyan region has generated considerable interest. In addition, General Circulation Model results indicate that deepwater source regions may be highly sensitive to changing basin configurations. To decipher deepwater changes, we examined detailed benthic foraminiferal faunal and isotopic records of the late Paleocene through the early Eocene (~60 to 50 Ma) from two critical regions: the North Atlantic (Bay of Biscay Site 401) and the Pacific (Shatsky Rise Site 577). These records are compared with published data from the Southern Ocean (Maud Rise Site 690, Islas Orcadas Rise Site 702). During the late Paleocene, similar benthic foraminiferal delta18O values were recorded at all four sites. This indicates uniform deepwater temperatures, consistent with a single source of deep water. The highest delta13C values were recorded in the Southern Ocean and were 0.5 per mil more positive than those of the Pacific. We infer that the Southern Ocean was proximal to a source of nutrient-depleted deep water during the late Paleocene. Upper Paleocene Reflector Ab was cut on the western Bermuda Rise by cyclonically circulating bottom water, also suggesting a vigorous source of bottom water in the Southern Ocean. A dramatic negative excursion in both carbon and oxygen isotopes occurred in the latest Paleocene in the Southern Ocean. This is a short-term (<100 kyr), globally synchronous event which also is apparent in both the Atlantic and Pacific records as a carbon isotopic excursion of approximately 1 per mil. Faunal analyses from the North Atlantic and Pacific sites indicate that the largest benthic foraminiferal faunal turnover of the Cenozoic was synchronous with the isotopic excursion, lending support to the hypothesis that the extinctions were caused by a change in deepwater circulation. We speculate that the Southern Ocean deepwater source was reduced or eliminated at the time of the excursion. During the early Eocene, Southern Ocean delta13C values remained enriched relative to the North Atlantic and Pacific. However, the Southern Ocean was also enriched in delta18O relative to these basins. We interpret that these patterns indicate that although the Southern Ocean was proximal to a source of cool, nutrient-depleted water, the intermediate to upper deep water sites of the North Atlantic and Pacific were ventilated by a different source that probably originated in low latitudes, i.e., WSDW.