Chemical and strontium isotopic compositions of interstitial waters from sediments of ODP Site 135-841

The depth variations in the major chemical components dissolved in interstitial waters from the Tonga margin (ODP Site 841) are much more pronounced than those usually observed in deep-sea sediments. The extensive alteration of volcanic Miocene sediments to secondary minerals such as analcime, clays, and thaumasite forms a CaCl2-rich brine. The brine results from a high exchange of Ca to Na, K, and Mg and an increase in Cl concentrations due to removal of H2O from the fluid during the authigenesis of hydrous minerals. The formation of thaumasite could have partly controlled the concentration of dissolved SO4, HCO3, and Ca in the Miocene sediments. The strontium isotopic signature of the interstitial water suggests that alteration of the volcanic Miocene sediments occurred a long time after sedimentation. A transient diffusion model indicates that molecular diffusion was not prevented by lithologic barriers and that the formation of secondary minerals in the Miocene sediment occurred over a short period of time (e.g.,

Lithium and strontium isotopic compositions of sediments and pore waters at ODP Sites 152-918 and 152-919

The distribution of Li isotopes in pore waters to a depth of 1157 m below seafloor is presented for ODP Sites 918 and 919 in the Irminger Basin, offshore Greenland. Lithium isotope data are accompanied by strontium isotope ratios to decipher diagenetic reactions in the sediments which are characterized by the pervasive presence of volcanic material, as well as by very high accumulation rates in the upper section. The lowering of the 87Sr/86Sr ratio below contemporaneous seawater values indicates several zones of volcanic material alteration. The Li isotope profiles are complex suggesting a variety of exchange reactions with the solid phases. These include cation exchange with NH4+ and mobilization from sediments at depth, in addition to the alteration of volcanic matter. Lithium isotopes are, therefore, a sensitive indicator of sediment-water interaction. d6Li values of pore waters at these two sites vary between -42 and -25?. At shallow depths (<100 mbsf), rapid decreases in the Li concentration, accompanied by a shift to heavier isotopic compositions, indicate uptake of Li into alteration products. A positive anomaly of d6Li observed at both sites is coincident with the NH4+ maximum produced by organic matter decomposition and may be related to ion exchange of Li from the sediments by NH4+. In the lower sediment column at Site 918, dissolved Li increases with depth and is characterized by enrichment of 6Li. The Li isotopic compositions of both the waters and the solid phase suggest that the enrichment of Li in deep interstitial waters is a result of release from pelagic sediments. The significance of sediment diagenesis and adsorption as sinks of oceanic Li is evaluated. The maximum diffusive flux into the sediment due to volcanic matter alteration can be no more than 5% of the combined inputs from rivers and submarine hydrothermal solutions. Adsorption on to sediments can only account for 5-10% of the total inputs from rivers and submarine hot springs.

Strontium isotope ratios of DSDP samples

The isotopic ratio of strontium-87 to strontium-86 shows no detectable variation in present-day ocean water but changes slowly over millions of years. The strontium contained in carbonate shells of marine organisms records the ratio of strontium-87 to strontium-86 of the oceans at the time that the shells form. Sedimentary rocks composed of accumulated fossil carbonate shells can be dated and correlated with the use of high precision measurements of the ratio of strontium-87 to strontium-86 with a resolution that is similar to that of other techniques used in age correlation. This method may prove valuable for many geological, paleontological, paleooceanographic, and geochemical problems.

Strontium isotopic ratios of evaporites of the Tyrrhenian Sea

Strontium isotopic ratios of gypsums recovered from upper Miocene (Messinian) evaporites at ODP Leg 107 Holes 652A, 653B, and 654A (Tyrrhenian Sea) are lower than expected. The values for the Messinian balatino-like gypsum, single gypsum crystals, and anhydrites range from 0.70861 to 0.70886 and are approximately 25 * 10**-5 less than would be expected for evaporites precipitated from Messinian seawater (0.70891-0.70902). Pre-evaporitic planktonic foraminifers from Hole 654A show variable degrees of dolomitization and 87Sr/86Sr values that irregularly decrease upward from normal marine values approximately 81m below the lowest evaporite occurrence. This suggests diagenetic alteration by advecting interstitial water with a low 87Sr/86Sr ratio or that the lower Sr isotopic ratios for the Messinian evaporites could have resulted from a greater influence of fresh water on the Sr isotopic composition of the desiccating Tyrrhenian Sea. Fluctuations of the 87Sr/86Sr-ratio for evaporites in the sedimentary cycles recognized for Holes 653B and 654A, the generally low Sr isotopic ratio of river water entering the Mediterranean Sea, and the presence of dwarf marine microfossils suggest that the 87Sr/86Sr ratio of the evaporites responded to hydrologic variations in a very restricted basin with variable rates of marine and fresh water input. The strontium isotopic ratios of the Messinian anhydrites from the proposed lacustrine sequence at Hole 652A fall in the same range as the marine evaporites from Holes 654A and 653B. This suggests a common or similar origin of the brines at the three locations. The complex depositional and hydrologic conditions in the Mediterranean during the Messinian salinity crisis preclude the use of Sr isotopic values from the evaporites for stratigraphic correlation and dating. They are, however, very useful in the interpretation of the depositional history of the basin. General calculations assuming a closed system suggest that the 87Sr/86Sr ratio of Messinian seawater (-0.7090) could be reduced to that of the evaporites (-0.7087) by mixing with fresh water (e.g., Nile River) in times of 10**4 to 10**5 yr.

Strontium and neodymium concentrations and isotope compositions in fossil fish teeth and pore waters at ODP Sites 130-807 and 125-786

We analyzed Nd and Sr isotopic compositions of Neogene fossil fish teeth from two sites in the Pacific in order to determine the effect of cleaning protocols and burial diagenesis on the preservation of seawater isotopic values. Sr is incorporated into the teeth at the time of growth; thus Sr isotopes are potentially valuable for chemostratigraphy. Nd isotopes are potential conservative tracers of paleocirculation; however, Nd is incorporated post-mortem, and may record diagenetic pore waters rather than seawater. We evaluated samples from two sites (Site 807A, Ontong Java Plateau and Site 786A, Izu-Bonin Arc) that were exposed to similar bottom waters, but have distinct lithologies and pore water chemistries. The Sr isotopic values of the fish teeth appear to accurately reflect contemporaneous seawater at both sites. The excellent correlation between the Nd isotopic values of teeth from the two sites suggests that the Nd is incorporated while the teeth are in chemical equilibrium with seawater, and that the signal is preserved over geologic timescales and subsequent burial. These data also corroborate paleoseawater Nd isotopic compositions derived from Pacific ferromanganese crusts that were recovered from similar water depths (Ling et al., 1997; doi:10.1016/S0012-821X(96)00224-5). This corroboration strongly suggests that both materials preserve seawater Nd isotope values. Variations in Pacific deepwater e-Nd values are consistent with predictions for the shoaling of the Isthmus of Panama and the subsequent initiation of nonradiogenic North Atlantic Deep Water that entered the Pacific via the Antarctic Circumpolar Current.

Sedimentology and Lead and Strontium isotope ratios of Miocene sediments from the Kerguelen Plateau

Characterization of sediment from Ocean Drilling Program Site 745, representing the East Kerguelen Ridge sediment drift, addresses important issues surrounding the timing of Miocene to present East Antarctic ice sheet stability and oceanic environmental change. Our results show three periods of greatly enhanced accumulation of Antarctic-derived sediment, at 6.4-5.9 Ma, 4.9-4.4 Ma and 1.1-0.8 Ma, potentially indicative of warmer, less stable ice sheets at these times. Conversely, the accumulation of Antarctic-derived material is comparatively less during the middle of the Pliocene warm epoch (4.8-3.2 Ma). The deep flow forming the Kerguelen drift was stronger during the latest Miocene and earliest Pliocene and has decreased in intensity continuously since then.

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.

Strontium isotopic composition of interstitial waters from DSDP sites 25-245 and 38-336

Measurements of 87Sr/86Sr ratios of interstitial waters from leg 25, site 245 and leg 38, site 336 of the Deep Sea Drilling Project show that the enrichment of Sr[2+] with depth is caused both by the alteration of volcanic material and by the introduction of strontium derived from calcium carbonate. 87Sr/86 Sr ratios range from 0.70913 to 0.70794 at site 245 and from 0.70916 to 0.70694 at site 336. The low ratios compared with contemporaneous seawater reflect the release of Sr from a volcanic source having, according to material-balance calculations, a 87Sr/86 Sr ratio of about 0.7034 at site 336. At this site the source appears to be volcanic ash and not basaltic basement which acts as a sink for Sr[2+] during in situ low-temperature weathering. The volcanic contribution to the strontium enrichment in the basal interstitial waters varies from <10% at site 245 to >50% at site 336. The remaining Sr[2+] is derived from Sr-rich biogenic carbonate during diagenetic recrystallization to form Sr-poor calcite.

Strontium chemistry of anhydrite from DSDP/ODP Hole 504B

A unique record of the chemical evolution of seawater during hydrothermal recharge into oceanic crust is preserved by anhydrite from the volcanic sequences and sheeted dike complex in ODP Hole 504B. Chemical and isotopic analyses 87Sr/86Sr, delta18O, delta34S of anhydrite constrain the changing composition of fluids due to reaction with basalt. There is a general trend of decreasing 87Sr/86Sr of anhydrite, corresponding to the minor incorporation of basaltic strontium with depth in the volcanic rocks. 87Sr/86Sr ratios decrease rapidly with depth in the dikes to values identical to host basalt (0.7029). Sr/Ca ratios (<0.1 mmol/mol) suggest that recharge fluids have very low Sr concentrations and fluids evolve by first precipitating Sr-bearing phases before extensive exchange of Sr with the host basalt. There is a background trend of decreasing sulfate delta18O with depth from +12-13? in the lower volcanics to +7? in the lower sheeted dikes recording an increase in recharge fluid temperature from c. 150° to c. 250°C, and confirming the presence of sulfate in hydrothermal fluids at elevated temperatures. From the amount of anhydrite recovered from Hole 504B and the amount of seawater sulfur that has been reduced to sulfide, a minimum seawater recharge flux can be calculated. This value is 4-25 times lower than estimates of high-temperature fluid fluxes based on either thermal constraints or global chemical budgets and suggests that there is significant deficit of seawater-derived sulfur in the oceanic crust. Only a minor proportion of the seawater that percolates into the crust near the axis is heated to high temperatures and exits as black smoker-type fluids. A significant proportion of the axial heat loss must be advected at 200-250°C by sulfate-bearing hydrothermal solutions that egress diffusely from the crust. These fluids penetrate into the dikes and exchange both heat and chemical tracers without the extensive clogging of porosity by anhydrite precipitation, which would halt hydrothermal circulation for any reasonable fluid flux. The heating of the major proportion of hydrothermal fluids to only moderate temperatures (c. 250°C) reconciles estimates of hydrothermal fluxes derived from thermal models and global geochemical budgets. The flux of hydrothermal sulfate would be of a magnitude similar to the riverine input, and oxygen-isotopic exchange at 200-250°C between dissolved sulfate and recharge fluids during hydrothermal circulation provides a mechanism to continuously buffer seawater sulfate oxygen to the light isotopic composition observed.

Strontium isotopes in pore waters of east equatorial Pacific sediments

Pore-water samples from the equatorial sedimentary bulge area show reversals in depth profiles of 87Sr/86Sr ratios at the sediment/basement interface. Results of this work support inferences made from previous pore-water data (from DSDP drilling in the area) that large-scale horizontal advection of seawater has occurred through the basement underlying the thick sedimentary sequence in this region. The area of apparent advection includes the eastern part of the equatorial high-productivity zone and part of the Guatemala Basin. We attempted to find links between the observed near-basement reversals in pore-water chemistry and sedimentary thickness, age, and topography of the area. Most of the sites that show horizontal advection have disturbed basement topography or outcrops within 10 to 20 km, suggesting that the cooling effects of outcrops may extend for at least 20 km horizontally. Heat-flow data from the area were compared to determine whether sites showing near-bottom chemistry reversals were consistent with areas of low conductive heat flow. This was generally true for the area of the sedimentary bulge and Guatemala Basin. Not enough pore-water data from the Nazca Plate were available to establish any reliable systematics. Because the high-productivity area is well-sealed from hydrothermal circulation, the missing heat must be lost by horizontal advective heat transport. From profiles of strontium isotopes and other elements that show departure from seawater values with increasing depth in the sediments, but return to seawater values near the basement, it appears that water flows relatively freely through much of the oceanic crust, even when sealed by considerable sedimentary cover.

Isotopic composition of strontium in planktonic foraminiferas from sediments of the DSDP Holes 90-588 and 94-607

Measurements of 87Sr/86Sr on samples of planktonic foraminifers were used to reconstruct changes in the Sr isotopic composition of seawater for the past 8 Ma. The late Neogene was marked by a general, but not regular, increase in 87S/86Sr with two breaks in slope at 5.5 and 2.5 Ma. These times mark the beginning of two periods of steep increase in 87Sr/86Sr values, relative to preceding periods characterized by essentially constant values. During the last 2.5 Ma, 87Sr/86Sr values increased at an average rate of 0.000054/Ma. This steep increase suggests that the modem ocean is not in Sr isotopic equilibrium relative to its major input fluxes. A non-equilibrium model for the modern Sr budget suggests that the residence time of Sr is ~2.5 Ma, which is significantly less than previously accepted estimates of 4-5 Ma. Modelling results suggest that the increase in 87Sr/86Sr over the past 8 Ma could have resulted from a 25% increase in the riverine flux of Sr or an increase in the average 87Sr/86Sr of this flux by 0.0006. The dominant cause of increasing 87Sr/86Sr values of seawater during the late Neogene is believed to be increased rates of uplift and chemical weathering of mountainous regions. Calculations suggest that uplift and weathering of the Himalayan-Tibetan region alone can account for the majority of the observed 87Sr/86Sr increase since the early Late Miocene. Exhumation of Precambrian shield areas by continental ice-sheets may have contributed secondarily to accelerated mechanical and chemical weathering of old crustal silicates with high 87Sr/86Sr values. In fact, the upturn in 87Sr/86Sr at 2.5 Ma coincides with increased glacial activity in the Northern Hemisphere. A variety of geochemical (87Sr/86Sr, Ge/Si, d13C, CCD, etc.) and sedimentologic data (accumulation rates) from the marine sedimentary record are compatible with a progressive increase in the chemical weathering rate of continents and dissolved riverine fluxes during the late Cenozoic. We hypothesize that chemical weathering of the continents and dissolved riverine fluxes to the oceans reached a maximum during the late Pleistocene because of repeated glaciations, increased continental exposure by lowered sea level, and increased continental relief resulting from high rates of tectonism.

Oligocene-Miocene strontium isotopes of ODP Hole 120-747A

This study tests and improves on previously published early and middle Miocene 87Sr/86Sr marine correlations, presents Sr isotopic age correlations for this interval using the new timescale of Cande and Kent [1992 doi:10.1029/92JB01202], and evaluates Sr isotopic changes against an inferred glacioeustatic proxy. We generated a latest Oligocene to early late Miocene 87Sr/86Sr isotope record from Ocean Drilling Program (ODP) Hole 747A; this site provides an excellent magnetostratigraphic record during most of this interval for independent age estimates, very good foraminiferal preservation, and excellent core recovery. Comparisons of new 87Sr/86Sr data from Hole 747A with previously published data from Deep Sea Drilling Project (DSDP) Sites 608 [Miller et al., 1991 doi:10.1029/90PA01941] and 588 [Hodell et al., 1991 doi:10.1130/0091-7613(1991)019<0024:VITSIC>2.3.CO;2] yield the following results: (1) confirmation and refinement of the early Miocene Sr isotope changes, (2) improved definition of the timing of the changes in slope of 87Sr/86Sr near 15.4 Ma and 22.8 Ma, (3) improved Sr isotopic age resolution for the middle Miocene with resolution as good as +/- 0.7 m.y., and (4) identification of an inflection in the Sr isotope record at 28.0 Ma based on the combined records from DSDP Site 522 [Miller et al., 1988 doi:10.1029/PA003i002p00223] and ODP Hole 747A. We have been unable to determine the cause of middle Miocene offset between Site 588 and Hole 747A data, although we believe it may be attributed to problems in the age assignments for Hole 588A for the interval ~14-11 Ma and Site 747 for the interval 11-8 Ma. Because Hole 747A results provide a better chronology than Site 588 for most of the Miocene and a better middle Miocene Sr isotope record than Site 608, we propose that Hole 747A serves as the best reference section for Miocene 87Sr/86Sr variations from ca. 23 to 11 Ma. Using 87Sr/86Sr data from Sites 522, 608, and 747A, we relate late Eocene to early Miocene inflections in the 87Sr/86Sr isotope record to oxygen isotope increases and decreases inferred to represent glacioeustatic events. The decreases (deglaciations) observed in the ?18O record apparently lead the 87Sr/86Sr inflections by 1 to 1.5 m.y.

Isotopic composition and Strontium/Calcium ratios of foraminifera of ODP Holes 113-689B and 113-690C

Oxygen and carbon isotopic ratios were measured from Maestrichtian benthic and planktonic foraminifer species and bulk carbonate samples from ODP Sites 689 and 690, drilled on the Maud Rise during Leg 113. Careful scanning electron microscope observations reveal that test calcite in some intervals was diagenetically altered, although Sr/Ca and isotopic ratios of these tests do not appear to have been modified significantly. Foraminifer d18O values at both sites document a cooling trend during early Maestrichtian time, a rapid drop in water temperatures at the time of the first appearance of Abathomphalus mayaroensis in the high southern latitude regions (about 69.9 Ma), and lower water temperatures during late Maestrichtian time. d13C values record a depletion in 13C in the latest early Maestrichtian time beginning at about 72.2 Ma, just prior to the sharp late Maestrichtian increase in d18O values. These trends are similar to those previously reported for well-preserved benthic foraminifer species from Seymour Island, in the Antarctic Peninsula. Paleotemperature estimates are also comparable to those at Seymour Island and suggest temperate climatic conditions in Antarctica and that bottom waters in the southern South Atlantic region were of Antarctic origin. Benthic and planktonic foraminifer 613C values fluctuate sympathetically and are higher in upper Maestrichtian sediments than in the lower Maestrichtian sequence.