Stable isotope record of planktonic foraminifera from sediment core MD96-2085

A high-resolution (~4-5cm/kyr) giant piston core record (MD962085) retrieved during an IMAGES II-NAUSICAA cruise from the continental slope of the southeast Atlantic Ocean reveals striking variations in planktonic foraminifer faunal abundances and sea-surface temperatures (SST) during the past 600 000 yr. The location and high-quality sedimentary record of the core provide a good opportunity to assess the variability of the Benguela Current system and associated important features of the ocean-climate system in the southeast Atlantic. The planktonic foraminifer faunal abundances of the core are dominated by three assemblages: (1) Neogloboquadrina pachyderma (right coiling) + Neogloboquadrina dutertrei, (2) Globigerina bulloides, and (3) Globorotalia inflata. The assemblage of N. pachyderma (right coiling) + N. dutertrei shows distinctive abundance changes which are nearly in-phase with glacial-interglacial variations. The high abundances of this assemblage are associated with major glacial conditions, possibly representing low SST/high nutrient level conditions in the southwestern Africa margin. In contrast, the G. bulloides and G. inflata assemblages show greater high-frequency abundance change patterns, which are not parallel to the glacial-interglacial changes. These patterns may indicate rapid oceanic frontal movements from the south, and a rapid change in the intensity of the Benguela upwelling system from the east. A single episode of maximum abundances of a polar water species N. pachyderma (left coiling) occurred in the beginning of stage 9 (~340-330 kyr). The event of the maximum occurrence of this species shown in this record may indicate instability in the Benguela coastal upwelling, or the Antarctic polar front zone position. A winter season SST estimate using transfer function techniques for this record shows primarily glacial-interglacial variations. The SST is maximal during the transitions from the major glacial to interglacial stages (Terminations I, II, IV, V), and is associated with the abundance maxima of a warm water species indicator Globigerinoides ruber. Cross-spectral analyses of the SST record and the SPECMAP stack reveal statistically significant concentrations of variance and coherencies in three major orbital frequency bands. The SST precedes changes in the global ice volume in all orbital frequency bands, indicating a dominant southern Hemispheric climate effect over the Benguela Current region in the southeast Atlantic.

Radiocarbon, isotope, biogenic, and 230Th measurements for sediment core RC10-196

The subarctic North Pacific Ocean holds a large CO2 reservoir that is currently isolated from the atmosphere by a low-salinity layer. It has recently been hypothesized that the reorganization of these high-CO2 waters may have played a crucial role in the degassing of carbon dioxide to the atmosphere during the last deglaciation. This reorganization would leave some imprint on paleo-productivity records. Here we present 230Th-normalized biogenic fluxes from an intermediate depth sediment core in the Northwest Pacific (RC10-196, 54.7°N, 177.1°E, 1007 m) and place them within the context of a synthesis of previously-published biogenic flux data from 49 deep-sea cores north of 20°N, ranging from 420 to 3968 m water depth. The 230Th-normalized opal, carbonate, and organic carbon fluxes from RC10-196 peak approximately 13,000 calendar years BP during the Bølling/Allerød (B/A) period. Our data synthesis suggests that biogenic fluxes were in general lowest during the last glacial period, increased somewhat in the Northwest Pacific during Heinrich Event 1, and reached a maximum across the entire North Pacific during the B/A period. We evaluate several mechanisms as possible drivers of deglacial change in biogenic fluxes in the North Pacific, including changes in preservation, sediment focusing, sea ice extent, iron inputs, stratification, and circulation shifts initiated in the North Atlantic and North Pacific. Our analysis suggests that while micronutrient sources likely contributed to some of the observed changes, the heterogeneity in timing of glaciogenic retreat and sea level make these mechanisms unlikely causes of region-wide contemporaneous peaks in export production. We argue that paleo-observations are most consistent with ventilation increases in both the North Pacific (during H1) and North Atlantic (during B/A) being the primary drivers of increases in biogenic flux during the deglaciation, as respectively they were likely to bring nutrients to the surface via increased vertical mixing and shoaling of the global thermocline.

Nd isotope data for planktonic foraminifera from within and around sapropels S1 and S5 from ODP Site 160-967

The development of widespread anoxic conditions in the deep oceans is evidenced by the accumulation and preservation of organic-carbon-rich sediments, but its precise cause remains controversial. The two most popular hypotheses involve (1) circulation-induced increased stratification resulting in reduced oxygenation of deep waters or (2) enhanced productivity in the surface ocean, increasing the raining down of organic matter and overwhelming the oxic remineralization potential of the deep ocean. In the periodic development of deep-water anoxia in the Pliocene-Pleistocene Mediterranean Sea, increased riverine runoff has been implicated both as a source for nutrients that fuel enhanced photic-zone productivity and a source of a less dense freshwater cap leading to reduced circulation, basin-wide stagnation, and deep-water oxygen starvation. Monsoon-driven increases in Nile River discharge and increased regional precipitation due to enhanced westerly activity-two mechanisms that represent fundamentally different climatic driving forces-have both been suggested as causes of the altered freshwater balance. Here we present data that confirm a distinctive neodymium (Nd) isotope signature for the Nile River relative to the Eastern Mediterranean-providing a new tracer of enhanced Nile outflow into the Mediterranean in the past. We further present Nd isotope data for planktonic foraminifera that suggest a clear increase in Nile discharge during the central intense period of two recent anoxic events. Our data also suggest, however, that other regional freshwater sources were more important at the beginning and end of the anoxic events. Taken at face value, the data appear to imply a temporal link between peaks in Nile discharge and enhanced westerly activity.

(Table 1) Oxygen and carbon isotope ratios of benthic foraminifera of DSDP Holes 12-119 and 48-401

We present here oxygen and carbon isotopic records of Eocene to Oligocene benthic foraminifera from two Bay of Biscay Deep Sea Drilling Project (DSDP) sites (119 and 401). d18O of benthic foraminifera increases 1.9 per mil from a middle Eocene minimum (Zones P10-P11) to an earliest Oligocene maximum (Zone NP21). Approximately 1.4 per mil of the increase in benthic foraminiferal d18O occurs during the late Eocene to earliest Oligocene (Zones P15/16-NP21). Previous results from other North Atlantic DSDP sites (400A and 398) have significantly lower d18O values of benthic foraminifera, some by as much as 2 per mil (Vergnaud-Grazzini et al., 1978; 1989, doi:10.2973/dsdp.proc.48.119.1979; Vergnaud-Grazzini, 1979, doi:10.2973/dsdp.proc.47-2.117.1979 ). We believe that these differences result from diagenetic alteration of the sediments in the deeper-buried Sites 400A and 398.

In-situ Iron isotope measurements and mineral compositions from ODP Hole 206-1256D and IODP Hole 335-U1256D

In-situ Fe isotope measurements have been carried out to estimate the impact of the hydrothermal metamorphic overprint on the Fe isotopic composition of Fe-Ti-oxides and Fe-sulfides of the different lithologies of the drilled rocks from IODP Hole 1256D (eastern equatorial Pacific; 15 Ma crust formed at the East Pacific Rise). Most igneous rocks normally have a very restricted range in their 56Fe/54Fe ratio. In contrast, Fe isotope compositions of hot fluids (> 300 °C) from mid-ocean-ridge spreading centers define a narrow range that is shifted to lower delta 56Fe values by 0.2 per mil - 0.5 per mil as compared to igneous rocks. Therefore, it is expected that mineral phases that contain large amounts of Fe are especially affected by the interaction with a fluid that fractionates Fe isotopes during exsolution/precipitation of those minerals. We have used a femtosecond UV-Laser ablation system to determine mineral 56Fe/54Fe ratios of selected samples with a precision of < 0.1 per mil (2 sigma level) at micrometer-scale. We have found significant variations of the delta 56Fe (IRMM-014) values in the minerals between different samples as well as within samples and mineral grains. The overall observed scale of delta 56Fe (magnetite) in 1256D rocks ranges from - 0.12 to + 0.64 per mil, and of delta 56Fe (ilmenite) from - 0.77 to + 0.01 per mil. Pyrite in the lowermost sheeted dike section is clearly distinguishable from the other investigated lithological units, having positive delta 56Fe values between + 0.29 and + 0.56 per mil, whereas pyrite in the other samples has generally negative delta 56Fe values from - 1.10 to - 0.59 permil. One key observation is that the temperature dependent inter-mineral fractionations of Fe isotopes between magnetite and ilmenite are systematically shifted towards higher values when compared to theoretically expected values, while synthesized, well equilibrated magnetite-ilmenite pairs are compatible with the theoretical predictions. Theoretical considerations including beta-factors of different aqueous Fe-chlorides and Rayleigh-type fractionations in the presence of a hydrous, chlorine-bearing fluid can explain this observation. The disagreement between observed and theoretical equilibrium fractionation, the fact that magnetite, in contrast to ilmenite shows a slight downhole trend in the delta 56Fe values, and the observation of small scale heterogeneities within single mineral grains imply that a general re-equilibration of the magnetite-ilmenite pairs is overprinted by kinetic fractionation effects, caused by the interaction of magnetite/ilmenite with hydrothermal fluids penetrating the upper oceanic crust during cooling, or incomplete re-equilibration at low temperatures. Furthermore, the observation of significant small-scale variations in the 56Fe/54Fe ratios of single minerals in this study highlights the importance of high spatial-resolution-analyses of stable isotope ratios for further investigations.

Stable carbon and oxygen isotope records of Cibicidoides wuellerstorfi in DSDP and ODP sites in the Atlantic and Pacific Ocean

Carbon isotopic records from benthic foraminifera are used to map patterns of deep ocean circulation between 3 and 2 million years ago, the interval when significant northern hemisphere glaciation began. The delta18O and delta13C data from four Atlantic sites (552, 607, 610, and 704) and one Pacific site (677) show that global cooling over this interval was associated with increased suppression of North Atlantic Deep Water (NADW) formation. However, the relative strength of NADW production was always greater than is observed during late Pleistocene glaciations when extreme decreases in NADW are observed in the deep North Atlantic. Our data indicate that an increase in the equator-to-pole temperature gradient associated with the onset of northern hemisphere glaciation did not intensify deepwater production in the North Atlantic but rather the opposite occurred. This is not unexpected as it is the "warm high-salinity" characteristic, rather than the "low temperature", of thermocline waters that is critical to the deepwater formation process in this region today.

Nitrogen isotope, total organic carbon and element concentration data for piston core M77/2-024-5 from the Peruvian continental margin

Piston core M77/2-024-5 was retrieved during the M77/2 cruise of Research Vessel Meteor in December 2008. Total organic carbon concentrations were determined using a Carlo Erba Element Analyzer (NA1500). Prior to analysis carbon bound to carbonate minerals was removed by leaching the sediment with 1 M HCl. Bulk nitrogen isotope ratios were determined using a Carlo Erba Element Analyzer (NA1500) coupled to a DeltaPlusXL isotope ratio mass spectrometer. Major and trace metals were analyzed after microwave-assisted (CEM MARS-5) acid digestion (HCl, HNO3 and HF) by inductively coupled plasma optical emission spectrometry (aluminum, titanium and iron) (Teledyne Leeman Prodigy) and inductively coupled plasma mass spectrometry (molybdenum and uranium) (THERMO X-Series 2).

Age models and stabile isotope ratios from SO35 cores

Stable isotope measurements on the planktonic foraminifer Globigerinoides ruber (white) have been carried out on a number of selected deep-seas sediment cores from the South Lau and Norlh Fiji Basins. The d18O-curves show good correlation with the inter-ocean oraphic correlation composite d18O-record of the standard reference section (Prell et al. 1986), which, in combination with the chronostratigraphic classifications of Herterich & Sarnthein (1984, modified) and Imbrie et al. 1984), allows a detailed dating of the sedimentary sequences. The deepest layers in core no. 119 (southern Lau Basin) could be assigned to Isotope Stage 24. Measurements made on bulk carbonate in two cores show a much higher glacial-interglacial amplitude, allowing the general identification of the conventional oxygen isotope stages. The d13C-values of the benthic foraminifer Cibicidoides wuellerstorfi show progressively lighter values northwards reflecting an increasing contribution of the isotopically lighter CO2 from the remineralisation of organic matter during the general northward movement of the deep water masses. Cyclicities in the sedimentation rates were observed in core nos. 117 and 119 (both southern Lau Basin) where the interglacials exhibit higher levels than the glacials. Calculated new or export paleoproductivity show that the glacials had higher productivity in the euphotic zone. From the oxygen isotope stratigraphy, the five ash layers in core nos. 117 and 119 could be dated as about 530 ka B.P. in Stage 14, 695 ka B.P. in Stage 18, 775 ka B.P. in Stage 21, 790 ka B.P. and 825 ka B.P. in Stage 22. Carbonate dissolution occurred during stages 5, 8 and 10 to 12.

Age determination and stable isotope record of foraminifera of sediment core SO82_5-2

Surface and deepwater paleoclimate records in Irminger Sea core SO82-5 (59°N, 31°W) and Icelandic Sea core PS2644 (68°N, 22°W) exhibit large fluctuations in thermohaline circulation (THC) from 60 to 18 calendar kyr B.P., with a dominant periodicity of 1460 years from 46 to 22 calendar kyr B.P., matching the Dansgaard-Oeschger (D-O) cycles in the Greenland Ice Sheet Project 2 (GISP2) temperature record [Grootes and Stuiver, 1997, doi:10.1029/97JC00880]. During interstadials, summer sea surface temperatures (SSTsu) in the Irminger Sea averaged to 8°C, and sea surface salinities (SSS) averaged to ~36.5, recording a strong Irminger Current and Atlantic THC. During stadials, SSTsu dropped to 2°-4°C, in phase with SSS drops by ~1-2. They reveal major meltwater injections along with the East Greenland Current, which turned off the North Atlantic deepwater convection and hence the heat advection to the north, in harmony with various ocean circulation and ice models. On the basis of the IRD composition, icebergs came from Iceland, east Greenland, and perhaps Svalbard and other northern ice sheets. However, the southward drifting icebergs were initially jammed in the Denmark Strait, reaching the Irminger Sea only with a lag of 155-195 years. We also conclude that the abrupt stadial terminations, the D-O warming events, were tied to iceberg melt via abundant seasonal sea ice and brine water formation in the meltwater-covered northwestern North Atlantic. In the 1/1460-year frequency band, benthic ?18O brine water spikes led the temperature maxima above Greenland and in the Irminger Sea by as little as 95 years. Thus abundant brine formation, which was induced by seasonal freezing of large parts of the northwestern Atlantic, may have finally entrained a current of warm surface water from the subtropics and thereby triggered the sudden reactivation of the THC. In summary, the internal dynamics of the east Greenland ice sheet may have formed the ultimate pacemaker of D-O cycles.

Stable carbon and oxygen isotope ratios of bulk sediment from the Weddell Sea

Carbon isotope measurements were made on bulk sediments from the well preserved calcareous sequences recovered at ODP Sites 689 and 690 on the Maud Rise, Weddell Sea, Antarctica. The very positive delta13C values that characterize the late Paleocene and the rapid trend toward lighter values in the early Eocene established in other sites are clearly recorded here and may be of value for long-distance stratigraphic correlation. However, values in the late Eocene are significantly more positive than have been reported from other areas. The general pattern of the records from Sites 689 and 690 is sufficiently unlike those previously reported from lower latitudes that we suggest that carbon isotope data should be used only with considerable caution for correlating sequences from such high latitudes with lower latitude records.