Stable isotope and Mg/Ca record of Globigerinoides ruger morphotypes

Tests of the planktonic foraminifer Globigerinoides ruber (white; d'Orbigny) have become a standard tool for reconstructing past oceanic environments. Paleoceanographers often utilize the Mg/Ca ratios of the foraminiferal tests for reconstructing low-latitude ocean glacial-interglacial changes in sea surface temperatures (SST). We report herein a comparison of Mg/Ca measurements on sample pairs (n = 20) of two G. ruber (white) morphotypes (G. ruber sensu stricto (s.s.) and G. ruber sensu lato (s.l.)) from surface and downcore samples of the western Pacific and Indian Oceans. G. ruber s.s. refers to specimens with spherical chambers sitting symmetrically over previous sutures with a wide, high arched aperture, whereas G. ruber s.l. refers to a more compact test with a diminutive final chamber and small aperture. The G. ruber s.s. specimens generally show significantly higher Mg/Ca ratios compared to G. ruber s.l. Our results from the Mg/Ca ratio analysis suggest that G. ruber s.l. specimens precipitated their shells in slightly colder surface waters than G. ruber s.s. specimens. This conclusion is supported by the differences in delta18O and delta13C values between the two morphotypes. Although it is still unclear if these two morphotypes represent phenotypic variants or sibling species, our findings seem to support the hypothesis of depth and/or seasonal allopatry within a single morphospecies.

Rb-Sr isotope systematics and Sr/Ca-Ba/Ca ratios at DSDP Hole 89-462A

Four samples of Nauru Basin basalts (Cores 94 to 109 of Hole 462A, sub-bottom depth 1077-1209 m) have 87Sr/86Sr ratios in the range 0.7037 to 0.7038, which is distinctly higher than the ratios of N-type MORB. The Rb contents of the samples are depleted in comparison with those of MORB and ocean-island basalts. These chemical and isotopic characteristics are identical to those of the basalts previously drilled during Leg 61 (Cores 75 to 90 of Hole 462A), and are explained in terms of inhomogeneity of the source region in the mantle or later alteration effects. Sr/Ca-Ba/Ca systematics of 15 samples from Cores 462A-94 to 462A-109 and 14 samples from Cores 462A-75 to 462A-90 suggest that the Nauru Basin basalts are derived from a mantle peridotite by 20 to 30% partial melting with subsequent Plagioclase crystallization.

X-ray fluorescence (XRF) Ca intensity of sediment cores MD04-2760 and MD04-2788

Accelerator mass spectrometry (AMS) radiocarbon dating of ostracod and gastropod shells from the southwestern Black Sea cores combined with tephrochronology provides the basis for studying reservoir age changes in the lateglacial Black Sea. The comparison of our data with records from the northwestern Black Sea shows that an apparent reservoir age of ~1450 14C yr found in the glacial is characteristic of a homogenized water column. This apparent reservoir age is most likely due to the hardwater effect. Though data indicate that a reservoir age of ~1450 14C yr may have persisted until the Bølling-Allerød warm period, a comparison with the GISP2 ice-core record suggests a gradual reduction of the reservoir age to ~1000 14C yr, which might have been caused by dilution effects of inflowing meltwater. During the Bølling-Allerød warm period, soil development and increased vegetation cover in the catchment area of the Black Sea could have hampered erosion of carbonate bedrock, and hence diminished contamination by "old" carbon brought to the Black Sea basin by rivers. A further reduction of the reservoir age most probably occurred contemporary to the precipitation of inorganic carbonates triggered by increased phytoplankton activity, and was confined to the upper water column. Intensified deep water formation subsequently enhanced the mixing/convection and renewal of intermediate water. During the Younger Dryas, the age of the upper water column was close to 0 yr, while the intermediate water was ~900 14C yr older. The first inflow of saline Mediterranean water, at ~8300 14C yr BP, shifted the surface water age towards the recent value of ~400 14C yr.

Radiocarbon ages, carbon and oxygen isotopes and Mg/Ca and Sr/Ca ratios of Globigerinoides in sediment core MD99-2203, Cape Hatteras

This study presents high-resolution foraminiferal-based sea surface temperature, sea surface salinity and upper water column stratification reconstructions off Cape Hatteras, a region sensitive to atmospheric and thermohaline circulation changes associated with the Gulf Stream. We focus on the last 10,000 years (10 ka) to study the surface hydrology changes under our current climate conditions and discuss the centennial to millennial time scale variability. We observed opposite evolutions between the conditions off Cape Hatteras and those south of Iceland, known today for the North Atlantic Oscillation pattern. We interpret the temperature and salinity changes in both regions as co-variation of activities of the subtropical and subpolar gyres. Around 8.3 ka and 5.2-3.5 ka, positive salinity anomalies are reconstructed off Cape Hatteras. We demonstrate, for the 5.2-3.5 ka period, that the salinity increase was caused by the cessation of the low salinity surface flow coming from the north. A northward displacement of the Gulf Stream, blocking the southbound low-salinity flow, concomitant to a reduced Meridional Overturning Circulation is the most likely scenario. Finally, wavelet transform analysis revealed a 1000-year period pacing the d18O signal over the early Holocene. This 1000-year frequency band is significantly coherent with the 1000-year frequency band of Total Solar Irradiance (TSI) between 9.5 ka and 7 ka and both signals are in phase over the rest of the studied period.

Stable oxygen isotope and Mg/Ca ratios of planktonic foraminifera from the Holocene and the Last Glacial Maximum

We measured oxygen isotopes and Mg/Ca ratios in the surface-dwelling planktonic foraminifer Globigerinoides ruber (white s.s.) and the thermocline dweller Pulleniatina obliquiloculata to investigate upper ocean spatial variability in the Indo-Pacific Warm Pool (IPWP). We focused on three critical time intervals: the Last Glacial Maximum (LGM; 18-21.5 ka), the early Holocene (8-9 ka), and the late Holocene (0-2 ka). Our records from 24 stations in the South China Sea, Timor Sea, Indonesian seas, and western Pacific indicate overall dry and cool conditions in the IPWP during the LGM with a low thermal gradient between surface and thermocline waters. During the early Holocene, sea surface temperatures increased by ~3°C over the entire region, indicating intensification of the IPWP. However, in the eastern Indian Ocean (Timor Sea), the thermocline gradually shoaled from the LGM to early Holocene, reflecting intensification of the subsurface Indonesian Throughflow (ITF). Increased surface salinity in the South China Sea during the Holocene appears related to northward displacement of the monsoonal rain belt over the Asian continent together with enhanced influx of saltier Pacific surface water through the Luzon Strait and freshwater export through the Java Sea. Opening of the freshwater portal through the Java Sea in the early Holocene led to a change in the vertical structure of the ITF from surface- to thermocline-dominated flow and to substantial freshening of Timor Sea thermocline waters.

Organic carbon concentrations and Cd/Ca ratios of sediment core RC13-229

A comparison of cadmium/calcium (Cd/Ca) records of benthic foraminifera from a deep Cape Basin and a deep eastern equatorial Pacific core suggests that over the past 400,000 years, the nutrient concentration of Circumpolar Deep Water (CPDW) has always been lower than that of the deep Pacific. The data further suggest that at the 100,000- and 23,000-year orbital periods, the contribution of North Atlantic Deep Water to CPDW is at a maximum during periods of ice growth and at a minimum during periods of ice decay. These results are not in agreement with results based on carbon isotope records of benthic foraminifera, which suggest intervals of CPDW nutrient enrichment relative to the deep Pacific and an approximately in-phase relationship between CPDW nutrient concentration and ice volume. Resolution of the apparent conflict between delta13C and Cd/Ca data may provide important constraints on past deep-ocean circulation and nutrient variability.