Chemical and isotopic compositions of deep-sea carbonate sediments and interstitial waters from DSDP Sites 30-288 and 30-289

Interstitial waters and sediments from DSDP sites 288 and 289 contain information on the chemistry and diagenesis of carbonate in deep-sea sediments and on the role of volcanic matter alteration processes. Sr/Ca ratios are species dependent in unaltered foraminifera from site 289 and atom ratios (0.0012-0.0016) exceed those predicted by distribution coefficent data (~0.0004). During diagenesis Sr/Ca ratios of carbonates decrease and reach the theoretical distribution at a depth which is identical to the depth of Sr isotopic equilibration, where 87Sr/86Sr ratios of interstitial waters and carbonates converge. Mg/Ca ratios in the carbonates do not increase with depth as found in some other DSDP sites, possibly because of diagenetic re-equilibration with interstitial waters showing decreasing Mg(2+)/Ca(2+) ratios with depth due to Ca input and Mg removal by alteration of volcanic matter. Interstitial 18O/16O ratios increase with depth at site 289 to d18O = 0.67? (SMOW), reflecting carbonate recrystallization at elevated temperatures (>/= 20°C), the first recorded evidence of this effect in interstitial waters. Interstitial Sr2+ concentrations reach high levels, up to 1 mM, chiefly because of carbonate recrystallization. However, 87Sr/86Sr ratios decrease from 0.7092 to less than 0.7078, lower than for contemporaneous sea water, showing that there is a volcanic input of strontium at depth. This volcanic component is recorded in the Sr isotopic composition of recrystallized calcites. Isotopic compositions of the unrecrystallized calcites suggests that the rate of increase of the 87Sr/86Sr ratio of sea water with time has been faster since 3 my ago than in the preceding 13 my.

Stable and strontium isotope ratios and age determination of ODP Site 130-803

Stratigraphic information from strontium, oxygen, and carbon isotopic ratios has been integrated with diatom and planktonic foraminifer datums to refine the Oligocene to early Miocene chemostratigraphy of Site 803. The Sr isotope results are based on analyses of mixed species of planktonic foraminifer and bulk carbonate samples. 87Sr/86Sr ratios of bulk carbonate samples are, in most cases, less radiogenic than contemporaneous seawater. Estimated sediment ages based on planktonic foraminifer 87Sr/86Sr ratios, using the Sr-isotope-age relation determined by Hess and others in 1989, are in moderately good agreement with the biostratigraphic ages. Chronological resolution is significantly enhanced with the correlation of oxygen and carbon isotope records to those of the standard Oligocene section tied to the Geomagnetic Polarity Time Scale at Site 522. Ages revised by this method and other published ages of planktonic foraminifer datums are used to revise the Oligocene stratigraphy of Site 77 to correlate the stable isotope records of Sites 77 and 803. Comparison of the Cibicidoides stable isotope records of Sites 77 and 574 with paleodepths below 2500 m in the central equatorial Pacific, and Site 803 at about 2000-m paleodepth in the Ontong Java Plateau reveals inversions in the vertical d18O gradient at several times during the Oligocene and in the early Miocene. The shallower water site had significantly-higher d18O values than the deeper water sites after the earliest Oligocene 18O enrichment and before 34.5 Ma, in the late Oligocene from 27.5 to at least 25 Ma, and in the early Miocene from 22.5 to 20.5 Ma. It is not possible to ascertain if the d18O inversion persisted during the Oligocene/Miocene transition because the deeper sites have hiatuses spanning this interval. We interpret this pattern to reflect that waters at about 2000 m depth were cold and may have formed from mixing with colder waters originating in northern or southern high-latitude regions. The deeper water appear to have been warmer and may have been a mixture with warm saline waters from mid- or low-latitude regions. No apparent vertical d13C gradient is present during the Oligocene, suggesting that the age difference of these water masses was small.

Mid-Pleistocene Mg/Ca, temperature and stable isotopes from Globigerinoides ruber white and Globorotalia inflata of sediment core GeoB3801-6 from the subtropical South Atlantic

The glacial marine isotope stage 14 (MIS 14) appears in many climate records as an unusually warm glacial. During this period an almost monospecific, up to 1.5 m thick, laminated layer of the giant diatom Ethmodiscus rex has been deposited below the South Atlantic Subtropical Gyre. This oligotrophic region is today less favorable for diatom growth with sediments typically consisting of calcareous nannofossil oozes. We have reconstructed temperatures and the stable oxygen isotopic compositions of sea surface and thermocline water (d18Ow) from planktonic foraminiferal (Globigerinoides ruber and Globorotalia inflata) Mg/Ca and stable oxygen isotopes to test whether perturbations in surface ocean conditions contributed to the deposition of the diatom layer at ~530 kyr B.P. Temperatures and d18Ow values reconstructed from this diatom ooze interval are highly variable, with maxima similar to interglacial values. Since the area of the Ethmodiscus oozes resembles the region where Agulhas rings are present, we interpret these hydrographic changes to reflect the varying influence of warm and saline water of Indian Ocean origin that entered the Subtropical Gyre trapped in Agulhas rings. The formation of the Ethmodiscus oozes is associated with a period of maximum Agulhas leakage and a maximum frequency of Agulhas ring formation caused by a termination-type position of the Subtropical Front during the unusual warm MIS 14. The input of silica through the Agulhas rings enabled the shift in primary production from calcareous nannoplankton to diatoms, leading to the deposition of the massive diatom oozes.

Chemical and isotopic compositions of samples from the Haakon Mosby mud volcano and nearby

Authigenic carbonates in the caldera of an Arctic (72°N) submarine mud volcano with active methane-bearing fluid discharge are formed at the bottom surface during anaerobic microbial methane oxidation. The microbial community consists of specific methane-producing bacteria, which act as methanotrophic ones in conditions of excess methane, and sulfate reducers developing on hydrogen, which is an intermediate product of microbial CH4 oxidation. Isotopically light carbon (aver. d13C = -28.9 per mil) of CO2 produced during CH4 oxidation is the main carbonate carbon source. Heavy oxygen isotope ratio (aver. d18O = 5 per mil) in carbonates is inherited from seawater sulfate. Rapid sulfate reduction (up to 12 mg S/dm**3/day) results in total exhausting of sulfate ion in the upper sediment layer (10 cm). Because of this carbonates can only be formed in surface sediments near the water-bottom interface. Salinity as well as CO3/Ca and Mg/Ca ratios correspond to the field of non-magnesian calcium carbonate precipitation. Calcite is the dominant carbonate mineral in the methane seep caldera, where it occurs in the paragenetic association with barite. Radiocarbon age of carbonates is about 10 Ka.

(Table 1) Comparison between bulk ICP-OES measurements and LA-ICP-MS analyses for Mg/Ca and Sr/Ca ratios of Globigerinoides sacculifer from ODP Hole 165-1000A

We constructed a high-resolution Mg/Ca record on the planktonic foraminifer Globigerinoides sacculifer in order to explore the change in sea surface temperature (SST) due to the shoaling of the Isthmus of Panama as well as the impact of secondary factors like diagenesis and large salinity fluctuations. The study covers the latest Miocene and the early Pliocene (5.6-3.9 Ma) and was combined with d18O to isolate changes in sea surface salinity (SSS). Before 4.5 Ma, SSTMg/Ca and SSS show moderate fluctuations, indicating a free exchange of surface ocean water masses between the Pacific and the Atlantic. The increase in d18O after 4.5 Ma represents increasing salinities in the Caribbean due to the progressive closure of the Panamanian Gateway. The increase in Mg/Ca toward values of maximum 7 mmol/mol suggests that secondary influences have played a significant role. Evidence of crystalline overgrowths on the foraminiferal tests in correlation with aragonite, Sr/Ca, and productivity cyclicities indicates a diagenetic overprint on the foraminiferal tests. Laser ablation inductively coupled plasma-mass spectrometry analyses, however, do not show significantly increased Mg/Ca ratios in the crystalline overgrowths, and neither do calculations based on pore water data conclusively result in significantly elevated Mg/Ca ratios in the crystalline overgrowths. Alternatively, the elevated Mg/Ca ratios might have been caused by salinity as the d18O record of Site 1000 has been interpreted to represent large fluctuations in SSS, and cultivating experiments have shown an increase in Mg/Ca with increasing salinity. We conclude that the Mg/Ca record <4.5 Ma can only reliably be considered for paleoceanographical purposes when the minimum values, not showing any evidence of secondary influences, are used, resulting in a warming of central Caribbean surface water masses after 4.5 Ma of ~2°C.

(Table 1, 2) Foraminifera biometric data, and boron isotopic composition of Globigerinoides sacculifer of sediments from the Ontong-Java Plateau

Sediment samples from the Ontong-Java Plateau in the Pacific and the 90° east ridge in the Indian Ocean were used to investigate whether shell size and early diagenesis affect d11B of the symbiont-bearing planktonic foraminifer Globigerinoides sacculifer. In pristine shells from both study locations we found a systematic increase of d11B and Mg/Ca with shell size. Shells in the sieve size class 515-865 µm revealed d11B values +2.1 to +2.3 per mil higher than shells in the 250-380 µm class. This pattern is most likely due to differences in symbiont photosynthetic activity and its integrated effect on the pH of the foraminiferal microenvironment. We therefore suggest smaller individuals must live at approximately 50-100 m water depth where ambient light levels are lower. Using the empirical calibration curve for d11B in G. sacculifer, only shells larger than 425 µm reflect surface seawater pH. Partial dissolution of shells derived from deeper sediment cores was determined by shell weight analyses and investigation of the shell surface microstructure by scanning electron microscopy. The d11B in partially dissolved shells is up to 2 per mil lower relative to pristine shells of the same size class. In agreement with a relatively higher weight loss in smaller shells, samples from the Ontong-Java Plateau show a more pronounced dissolution effect than larger shells. On the basis of the primary size effect and potential postdepositional dissolution effects, we recommend the use of shells that are visually pristine and, in the case of G. sacculifer, larger than 500 ?m for paleoreconstructions.

Mg/Ca and stable oxygen isotope ratios of Uvigerina spp. from the Southern Ocean

The sensitivity to temperature of Mg/Ca ratios in the shallow-infaunal benthic foraminifera Uvigerina spp. has been assessed. Core-top calibrations over ~1-20 °C show a range in sensitivity of 0.065-0.084 mmol/mol/°C but few data are available spanning the temperature range anticipated in deep-sea records over glacial-interglacial cycles. In contrast to epibenthic foraminiferal species, carbonate ion saturation appears not to affect Mg/Ca significantly. A method based on estimating the ratio of the temperature sensitivity of foraminiferal Mg/Ca to that of d18Ocalcite shows that sensitivity for Mg/Ca at the high end of the observed core-top range (~0.1 mmol/mol/°C) is required for consistency with LGM-Holocene differences in each property as constrained by independent proxy data. This is supported by a Mg/Ca record for Uvigerina spp. generated for the Southern Ocean over the past 440,000 years from Ocean Drilling Program Site 1123 (Chatham Rise, New Zealand). The record shows variability that correlates with climate oscillations. The LGM deep ocean temperature derived from the Mg/Ca record is -1.1 ± 0.3 °C. Transformation to temperature allows estimates to be made of changes in bottom water temperature and seawater d18O and comparison made with literature records. Analysis reveals a ~2.5-kyr lead in the record of temperature over calcite d18O and a longer lead over seawater d18O. This is a reflection of larger phase offsets at eccentricity periods; phase offsets at tilt and precession are within error zero.

Mg/Ca and Sr/Ca in calcites and estimated distribution coefficients in marine sediments (Table 2)

Recrystallization processes in marine sediments can alter the extent to which biogenic calcite composition serves as a proxy of oceanic chemical and isotopic history. Models of calcite recrystallization developed to date have resulted in significant insights into these processes, but are not completely adequate to describe the conditions of recrystallization. Marine sediments frequently have concentration gradients in interstitial dissolved calcium, magnesium, and strontium which have probably evolved during sediment accumulation. Realistic, albeit simplified, models of the temporal evolution of interstitial water profiles of Ca, Mg, and Sr were used with several patterns of recrystallization rate variation to predict the composition of recrystallized inorganic calcite. Comparison of predictions with measured Mg/Ca and Sr/Ca ratios in severely altered calcite samples from several Deep Sea Drilling Project sites demonstrates that models incorporating temporal variation in interstitial water composition more successfully predict observed calcite compositions than do models which rely solely on present-day interstitial water chemistry. Temporal changes in interstitial composition are particularly important in interpreting Mg/Ca ratios in conjunction with Sr/Ca ratios. Estimates of Mg distribution coefficients from previous observations in marine sediments, much lower than those in laboratory studies of inorganic calcite, are confirmed by these results. Evaluation of the effects of diagenetic alteration of biogenic calcium carbonate sediment must be a site-specific process, taking into account accumulation history, present interstitial chemistry and its variation in the past, and sample depths and ages.

87Sr/86Sr ratios, Sr and Rb concentrations and stable oxygen isotope values for some basalts from ODP Hole 504B (Table 1)

Seventeen whole-rock samples, generally taken at 25-50 m intervals from 5 to 560 m sub-basement in Hole 504B, drilled in 6.2 m.y. old crust, were analysed for 87Sr/86Sr ratios, Sr and Rb concentrations, and 18O/16O ratios. Sr isotope ratios for 8 samples from the upper 260 m of the hole range from 0.70287 to 0.70377, with a mean of 0.70320. In the 330-560 m interval, 5 samples have a restricted range of 0.70255-0.70279, with a mean of 0.70266, the average value for fresh mid-ocean ridge basalts (MORB). In the 260-330 m interval, approximately intermediate Sr isotopic ratios are found. Delta18O values (?) range from 6.4 to 7.8 in the upper 260 m, 6.2-6.4 in the 270-320 m interval, and 5.8-6.2 in the 320-560 m interval. The values in the upper 260 m are typical for basalts which have undergone low-temperature seawater alteration, whereas the values for the 320-560 m interval correspond to MORB which have experienced essentially no oxygen isotopic alteration. The higher 87Sr/86Sr and 18O/16O ratios in the upper part of the hole can be interpreted as the result of a greater overall water/rock ratio in the upper part of the Hole 504B crust than in the lower part. Interaction of basalt with seawater (87Sr/86Sr = 0.7091) increased basalt 87Sr/86Sr ratios and produced smectitic alteration products which raised whole-rock delta18O values. Seawater circulation in the lower basalts may have been partly restricted by the greater number of relatively impermeable massive lava flows below about 230 m sub-basement. These flows may have helped to seal off lower basalts from through-flowing seawater.

Dinoflagellate cyst assemblages, oxygen isotope records of foraminifera, and alkenone and Mg/Ca-based SST estimates for the time interval 3.40-3.18 Ma (Late Pliocene) from five North Atlantic and Caribbean sediment cores

The early Late Pliocene (3.6 to ~3.0 million years ago) is the last extended interval in Earth's history when atmospheric CO2 concentrations were comparable to today's and global climate was warmer. Yet a severe global glaciation during marine isotope stage (MIS) M2 interrupted this phase of global warmth ~3.30 million years ago, and is seen as a premature attempt of the climate system to establish an ice-age world. Our geochemical and palynological records from five marine sediment cores along a Caribbean to eastern North Atlantic transect show that increased Pacific-to-Atlantic flow via the Central American Seaway weakened the North Atlantic Current (NAC) and attendant northward heat transport prior to MIS M2. The consequent cooling of the northern high latitude oceans permitted expansion of the Greenland ice sheet during MIS M2, despite near-modern atmospheric CO2 concentrations. Before and after MIS M2, heat transport via the NAC was crucial in maintaining warm climates comparable to those predicted for the end of this century.

Zircons, isotope ratios and element analyses from tonalite and oxide gabbro of the mid-ocean ridge from ODP Hole 176-735B

A limiting factor in the accuracy and precision of U/Pb zircon dates is accurate correction for initial disequilibrium in the 238U and 235U decay chains. The longest-lived-and therefore most abundant-intermediate daughter product in the 235U isotopic decay chain is 231Pa (T1/2 = 32.71 ka), and the partitioning behavior of Pa in zircon is not well constrained. Here we report high-precision thermal ionization mass spectrometry (TIMS) U-Pb zircon data from two samples from Ocean Drilling Program (ODP) Hole 735B, which show evidence for incorporation of excess 231Pa during zircon crystallization. The most precise analyses from the two samples have consistent Th-corrected 206Pb/238U dates with weighted means of 11.9325 ± 0.0039 Ma (n = 9) and 11.920 ± 0.011 Ma (n = 4), but distinctly older 207Pb/235U dates that vary from 12.330 ± 0.048 Ma to 12.140 ± 0.044 Ma and 12.03 ± 0.24 to 12.40 ± 0.27 Ma, respectively. If the excess 207Pb is due to variable initial excess 231Pa, calculated initial (231Pa)/(235U) activity ratios for the two samples range from 5.6 ± 1.0 to 9.6 ± 1.1 and 3.5 ± 5.2 to 11.4 ± 5.8. The data from the more precisely dated sample yields estimated DPazircon/DUzircon from 2.2-3.8 and 5.6-9.6, assuming (231Pa)/(235U) of the melt equal to the global average of recently erupted mid-ocean ridge basaltic glasses or secular equilibrium, respectively. High precision ID-TIMS analyses from nine additional samples from Hole 735B and nearby Hole 1105A suggest similar partitioning. The lower range of DPazircon/DUzircon is consistent with ion microprobe measurements of 231Pa in zircons from Holocene and Pleistocene rhyolitic eruptions (Schmitt (2007; doi:10.2138/am.2007.2449) and Schmitt (2011; doi:10.1146/annurev-earth-040610-133330)). The data suggest that 231Pa is preferentially incorporated during zircon crystallization over a range of magmatic compositions, and excess initial 231Pa may be more common in zircons than acknowledged. The degree of initial disequilibrium in the 235U decay chain suggested by the data from this study, and other recent high precision datasets, leads to resolvable discordance in high precision dates of Cenozoic to Mesozoic zircons. Minor discordance in zircons of this age may therefore reflect initial excess 231Pa and does not require either inheritance or Pb loss.

Rb, Sr, Sm and Nd concentrations and isotopic composition of basalts from DSDP Leg 74 (Table 1)

Basement intersected in DSDP holes 525A, 528 and 527 on the Walvis Ridge consists of submarine basalt flows and pillows with minor intercalated sediments. These holes are situated on the crest and mid and lower northwest flank of a NNW-SSE-trending ridge block which would have closely paralleled the paleo mid-ocean ridge (Rabinowitz and LaBrecque, 1979 doi:10.1029/JB084iB11p05973, Moore et al. (1983 doi:10.1130/0016-7606(1983)94<907:TWRTDS>2.0.CO;2). The basalts were erupted approximately 70 m.y. ago, an age equivalent to that of immediately adjacent oceanic crust in the Angola Basin and coraistent with formation at the paleo mid-ocean ridge (Moore et al., 1983). The basalt types vary from aphyric quartz tholeiites on the ridge crest to highly plagioclase phyric olivine tholeiites on the ridge flank. These show systematic differences in incompatible trace element and isotopic composition. Many element and isotope ratio pairs form systematic trends with the ridge crest basalts at one end and the highly phyric ridge flank basalts at the other. The low 143Nd/144Nd (0.51238), 206Pb/204Pb (17.54), 207Pb/204Pb (15.47), 208Pb/204Pb (38.14) and high 87Sr/86Sr (0.70512) ratios of the ridge crest basalts suggest derivation from an old Nd/Sm-, Rb/Sr- and Pb/U-enriched mantle source. This isotopic signature is similar to that of alkaline basalts on Tristan da Cunha but offset to significantly lower Nd and Pb isotopic ratios. The isotopic ratio trends may be extrapolated beyond the ridge flank basalts with higher 143Nd/144Nd (0.51270), 206Pb/204Pb (18.32), 207Pb/204Pb (15.52), 208Pb/204Pb (38.77) and lower 87Sr/86Sr (0.70417) ratios in the direction of increasingly Nd/Sm-, Rb/Sr- and Pb/U-depleted source compositions. These isotopic correlations are equally consistent with mixing of depleted and enriched end member melts or partial melting of an inhomogeneous, variably enriched mantle source. However, observed Zr-Ba-Nb-Y interelement relationships are inconsistent with any simple two-component model of magma mixing, as might result from the rise of a lower mantle plume through the upper mantle. Incompatible element and Pb isotopic systematics also preclude extensive involvement of depleted (N-type) MORB material or its mantle sources. In our preferred petrogenetic model the Walvis Ridge basalts were derived by partial melting of mantle similar to an enriched (E-type) MORB source which had become heterogeneous on a small scale due to the introduction of small-volume melts and metasomatic fluids.

(Table 1) Sr, Nd, and Pb isotopic composition of ODP Leg 129 basalts

Basaltic rocks recovered from three drill sites in the western Pacific during Ocean Drilling Program Leg 129 have fairly distinct Sr, Nd, and Pb isotopic compositions. The Cretaceous alkali olivine dolerites from Site 800 in the northern part of Pigafetta Basin have fairly low 87Sr/86Sri (0.70292-0.70320) and 143Nd/144Ndi (0.51277-0.51281) and high present-day Pb isotopic ratios (206Pb/204Pb = 20.53-21.45; 207Pb/204Pb = 15.70-15.77; 208Pb/204Pb = 40.02-40.68). The Middle Jurassic tholeiites from Site 801 in the southern part of the basin have low 87Sr/86Sri (0.70237-0.70248), high 143Nd/144Ndi (0.51298-0.51322), and moderate present-day Pb isotopic ratios (206Pb/204Pb = 18.20-19.12; 207Pb/204Pb = 15.47-15.60; 208Pb/204Pb = 37.56-38.18); isotopic compositions of the alkali olivine basalts overlying the tholeiites fall between those of the tholeiites and Site 800 dolerites. The Cretaceous tholeiites from Site 802 in the East Mariana Basin have high 87Sr/86Sri (0.70360-0.70372), fairly low 143Nd/144Ndi (0.51277-0.51280), and fairly low and homogeneous present-day Pb isotopic ratios (206Pb/204Pb = 18.37-18.39; 207Pb/204Pb = 15.49-15.51; 208Pb/204Pb = 38.34-38.39). Isotopic compositions of Site 801 tholeiites are indistinguishable from those of modern mid-ocean ridge basalts, consistent with the proposal that these tholeiites are a part of the oldest Pacific crust. The diverse isotopic compositions of the younger basalts appear to be the result of Jurassic Pacific plate migration over the geologically anomalous south-central Pacific region, wherein they acquired their distinct isotopic compositions. The anomalous region was volcanically more active during the Cretaceous than at present.