Strontium and Oxygen isotope compositions from DSDP Holes 54-424A, 54-424B and 70-509B

Strontium and O isotope compositions of green clay minerals from sediment cores of three boreholes drilled into (sites 424A and 509B) and close to a hydrothermal mound (site 424B) near the Galapagos Spreading Center (DSDP Legs 54 and 70) were determined. The green clays consist mostly of a transition from Fesmectite (nontronite) to glauconite. 87Sr/86Sr ratios were measured on clay size-fractions after gentle acid leaching and on the recovered leachates from different samples. The 87Sr/86Sr ratios of the clay residues from both the 424A and B sites are well below the modern seawater value, which points consistently to precipitation from hydrothermal fluids that contained variable amounts of seawater, even away from mound. However, most of the clay residues from mound site 509B have 87Sr/86Sr ratios significantly above the seawater value, suggesting the occurrence of a detrital component together with the new authigenic particles. The clay minerals of the hydrothermal mound are mixed with detrital components, and that of the sample taken outside but near the mound as a reference for the surrounding oceanic environment, yields a hydrothermal signature. Crystallization temperatures of the clays range from 32 to 63 °C assuming a d18O value of +2.2 per mil for the mineralizing fluids. Hydrothermal fluids generated in the underlying oceanic crust, mixed in varied proportions with ambient seawater and migrated into beds of the mound in a sequence of recurrent processes that ultimately resulted in the formation of the observed clay minerals. No significant temperature differences were detected for crystallization of the K-rich glauconite and K-depleted nontronite. The 87Sr/86Sr ratios of the Sr leached off the clay particles are near the value of modern seawater, inferring a progressive replacement of the hydrothermal fluids by seawater in the pore space of the mound sediments.

Stable oxygen isotope ratios of pore waters from ODP Leg 166 sites

This study investigates the d18O of pore waters from Sites 1003 through 1007, drilled along the western margin of the Great Bahama Bank during Leg 166 of the Ocean Drilling Program. These pore waters generally show a positive correlation between d18O and the concentration of chloride. The exception to this trend is Site 1006, where the pore waters exhibit nonlinear behavior with respect to chloride. The correlation between the concentration of Cl- and d18O at most of the sites appears to be a coincidence because although the increase in Cl- is a result of diffusion from an underlying source, the increases in d18O result from the recrystallization of metastable carbonates in the presence of a geothermal gradient. The difference in behavior in the d18O of the pore water at Site 1006 is probably a result of the relative reduced rate of carbonate recrystallization at this site. The d18O of the pore waters in the upper portion of the cores shows a pattern similar to the concentration of chloride in that there is an interval of 30-50 m in which neither the d18O nor the concentration of Cl- changes. This interval is consistent with either an interval of very rapid deposition of sediment or the advection of fluid through the platform. Both the d18O and the concentration of Cl- increase toward the platform, suggesting an input of saline and isotopically heavy water from the platform surface.

Late Albian foraminiferal carbon and oxygen isotope data and biostratigraphic datums from ODP holes 171-1050C and 171-1052E

A late Albian-early Cenomanian record (~103.3 to 99.0 Ma), including organic-rich deposits and a d13C increase associated with oceanic anoxic event 1d (OAE 1d), is described from Ocean Drilling Program sites 1050 and 1052 in the subtropical Atlantic. Foraminifera are well preserved at these sites. Paleotemperatures estimated from benthic d18O values average ~14°C for middle bathyal Site 1050 and ~17°C for upper bathyal Site 1052, whereas surface temperatures are estimated to have ranged from 26°C to 31°C at both sites. Among planktonic foraminifera, there is a steady balance of speciation and extinction with no discrete time of major faunal turnover. OAE 1d is recognized on the basis of a 1.2 per mill d13C increase (~100.0-99.6 Ma), which is similar in age and magnitude to d13C excursions documented in the North Atlantic and western Tethys. Organic-rich "black shales" are present throughout the studied interval at both sites. However, deposition of individual black shale beds was not synchronous between sites, and most of the black shale was deposited before the OAE 1d d13C increase. A similar pattern is observed at the other sites where OAE 1d has been recognized indicating that the site(s) of excess organic carbon burial that could have caused the d13C increase has (have) yet to be found. Our findings add weight to the view that OAEs should be chemostratigraphically (d13C) rather than lithostratigraphically defined.

Coral oxygen isotope and Sr/Ca data from the Northern Gulf of Aqaba (Red Sea)

The last interglacial period (about 125,000 years ago) is thought to have been at least as warm as the present climate (Kukla et al., 2002, doi:10.1006/qres.2001.2316). Owing to changes in the Earth's orbit around the Sun, it is thought that insolation in the Northern Hemisphere varied more strongly than today on seasonal timescales (Berger, 1987, doi:10.1175/1520-0469(1978)035<2362:LTVODI>2.0.CO;2), which would have led to corresponding changes in the seasonal temperature cycle (Montoya et al., 2000, doi:10.1175/1520-0442(2000)013<1057:CSFKBW>2.0.CO;2). Here we present seasonally resolved proxy records using corals from the northernmost Red Sea, which record climate during the last interglacial period, the late Holocene epoch and the present. We find an increased seasonality in the temperature recorded in the last interglacial coral. Today, climate in the northern Red Sea is sensitive to the North Atlantic Oscillation (Felis et al., 2000 doi:10.1029/1999PA000477; Rimbu et al., 2001, doi:10.1029/2001GL013083), a climate oscillation that strongly influences winter temperatures and precipitation in the North Atlantic region. From our coral records and simulations with a coupled atmosphere-ocean circulation model, we conclude that a tendency towards the high-index state of the North Atlantic Oscillation during the last interglacial period, which is consistent with European proxy records (Zagwijn, 1996, doi:10.1016/0277-3791(96)00011-X; Aalbersberg and Litt, 1998, doi:10.1002/(SICI)1099-1417(1998090)13:5<367::AID-JQS400>3.0.CO;2-I; Klotz et al., 2003, doi:10.1016/S0921-8181(02)00222-9), contributed to the larger amplitude of the seasonal cycle in the Middle East.

Stable carbon and oxygen isotope record of Cretaceous planktonic foraminifera from DSDP (Table 1)

The Cretaceous Heterohelix moremani (Cushman) was the only biserial planktonic foraminiferal species from its first appearance in the late Albian up to the Cenomanian/Turonian boundary. Within that time, it increased gradually in abundance relative to other planktonic foraminifera in five Circum-North Atlantic sections. It is generally rare in upper Albian sediments, common in most of the Cenomanian and very abundant in sediments representing the latest Cenomanian Oceanic Anoxic Event. Short-term variations on the overall abundance trend correlate with positive excursions in the bulk carbonate delta13C record. Maximum rain rates of H. moremani during OAE2 show that this species was an opportunist that did well in extreme conditions, but its overall distribution indicates that it is not necessarily a marker for very high palaeoproductivity environments. Stable oxygen and carbon isotope measurements on foraminiferal species indicate that H. moremani was a surface water dweller at least in part of its geographic range, but incorporated 13C out of equilibrium with ambient seawater. It is depleted in delta13C relative to other planktonic foraminifera, which is attributed to vital effects related to its opportunistic character.

Sable oxygen isotope and Mg/Ca ratios of planktonic foraminifera from DSDP Hole 31-292

This study presents new evidence of when and how the Western Pacific Warm Pool (WPWP) was established in its present form. We analyzed planktic foraminifera, oxygen isotopes, and Mg/Ca ratios in upper Miocene through Pleistocene sediments collected at Deep Sea Drilling Program (DSDP) Site 292. These data were then compared with those reported from Ocean Drilling Program (ODP) Site 806. Both drilling sites are located in the western Pacific Ocean. DSDP Site 292 is located in the northern margin of the modern WPWP and ODP Site 806 near the center of the WPWP. Three stages of development in surface-water conditions are identified in the region using planktic foraminferal data. During the initial stage, from 8.5 to 4.4 Ma, Site 806 was overlain by warm surface water but Site 292 was not, as indicated by the differences in faunal compositions and sea-surface temperature (SST) between the two sites. In addition, the vertical thermal gradient at Site 292 was weak during this period, as indicated by the small differences in the delta18O values between Globigerinoides sacculifer and Pulleniatina spp. During stage two, from 4.4 to 3.6 Ma, the SST at Site 292 rapidly increased to 27 °C, but the vertical thermal gradient had not yet be strengthened, as shown by Mg/Ca ratios and the presence of both mixed-layer dwellers and thermocline dwellers. Finally, a warm mixed layer with a high SST ca. 28 °C and a strong vertical thermal gradient were established at Site 292 by 3.6 Ma. This event is marked by the dominance of mixed-layer dwellers, a high and stable SST, and a larger differences in the delta18O values between G. sacculifer and Pulleniatina spp. Thus, evidence of surface-water evolution in the western Pacific suggests that Site 292 came under the influence of the WPWP at 3.6 Ma. The northward expansion of the WPWP from 4.4 to 3.6 Ma and the establishment of the modern WPWP by 3.6 Ma appear to be closely related to the closure of the Indonesian and Central American seaways.

(Table 3) Oxygen isotope ratios of benthic foraminifera from DSDP Hole 80-548

Presently, the intermediate depths of the North Atlantic Ocean are occupied by a great lens of warm, saline water whose source is the Mediterranean Sea. This water flows both westward and northward, finally entering the Norwegian Sea where it may contribute to the formation of North Atlantic Deep Water. The Late Neogene history of Mediterranean Outflow in the Atlantic can be monitored at DSDP-IPOD Site 548 on the continental slope Southwest of Ireland using benthic Foraminifera oxygen isotope values. Isotopic data from 154 samples indicate that Mediterranean water was absent from the mid-depth North Atlantic from 3.4 to 3.2 Ma ago. However, at about 2.9 Ma ago the isotopic values at Site 548 diverge from those recorded from the deep North Atlantic and they can be interpreted to indicate the appearance of a new water mass, possibly Mediterranean water, in the North Atlantic water column. This appearance may be related to climatic changes that occurred around the Mediterranean Basin at about 2.9 Ma ago. The analysis of 189 samples for grain-size distributions shows that a significant increase in the silt-size fraction occurs at the same level that isotopic analysis indicates a change in bottom waters at Site 548. The grainsize data support the hypothesis that mid-depth water-mass changes occurred at about 2.9 Ma ago.