ALBERT

Description: 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.

Description: We studied magnesium:calcium (Mg/Ca) ratios in shells of the deep-sea ostracode genus Krithe from a short interval in the middle Pliocene between 3.29 and 2.97 Ma using deep-sea drilling sites in the North and South Atlantic in order to estimate bottom water temperatures (BWT) during a period of climatic warmth. Results from DSDP and ODP Sites 552A, 610A, 607, 658A, 659A, 661A and 704 for the period Ma reveal both depth and latitudinal gradients of mean Mg/Ca values. Shallower sites (552A, 610A and 607) have higher mean Mg/Ca ratios (10.3, 9.7, 10.1 mmol/mol) than deeper sites (661A, 6.3 mmol/mol), and high latitude North Atlantic sites (552A, 610 and 607) have higher Mg/Ca ratios than low latitude (658A: 9.8 mmol/mol, 659A: 7.7 mmol/mol, 661A: 6.3 mmol/mol) and Southern Ocean (704: 8.0 mmol/mol) sites. Converting Mg/Ca ratios into estimated temperatures using the calibration of Dwyer et al. (1995) [Dwyer, G.S., Cronin, T.M., Baker, P.A., Raymo, M.E., Buzas, J.S., Corrège, T., 1995. North Atlantic deepwater temperature change during late Pliocene and late Quaternary climatic cycles. Science 270, 1347–1351] suggests that mean middle Pliocene bottom water temperatures at the study sites in the deep Atlantic were about the same as modern temperatures. However, brief pulses of elevated BWT occurred several times between 3.29 and 2.97 Ma in both the North and South Atlantic Ocean suggesting short-term changes in deep ocean circulation.

Description: The strontium isotope ratios of authigenic carbonates from Indian Ocean sea-floor basalts have been used to determine the timing of carbonate mineral precipitation and fluid flow. The samples include calcites from 57.2 Ma crust from Ocean Drilling Project (ODP) Site 715, and calcites, aragonites, and siderites from 63.7 Ma crust from ODP Site 707. At Site 715, calcite precipitation may have begun at any time after the basalts cooled, and it continued until approximately 31 Ma, or 26 m.y. after basalt eruption. At Site 707, aragonite and siderite did not begin to precipitate until about 36 Ma, almost 30 m.y. after basalt eruption, and continued to precipitate until at least 30 and 28 Ma, respectively. Calcite precipitation began at approximately 32 Ma and continued until 22 Ma. These ages suggest that vein mineral deposition and low-temperature fluid circulation in the ocean crust may continue for much longer periods of time than previously observed.

Description: This report presents the results of a study of the stable isotopic and chemical composition of secondary carbonate minerals precipitated within basalts at Ocean Drilling Program Sites 707 and 715. At Site 715, the secondary carbonates are all composed of calcite and display a narrow range of carbon and oxygen stable isotope ratios, with values ranging from -2.75 per mil to 1.95 per mil PDB and -0.27 per mil to 2.86 per mil PDB, respectively. Strontium, iron, and manganese values of the samples are generally low. The geochemistry of Site 715 samples indicates that they precipitated from seawater-domi- nated fluids, at low temperatures, as is typical of secondary carbonates from most Deep Sea Drilling Project sites. In contrast, at Site 707, aragonite, siderite, and manganese-rich calcite occur as secondary carbonates in addition to calcite. The carbon isotopes of the Site 707 carbonates of all rock types are depleted in 13C. Values range from -2.79 per mil to -16.43 per mil PDB. Oxygen isotope values do not show a wide variation, ranging from -1.78 per mil to 1.17 per mil. The strontium contents of the samples range from 5200 to 8100 ppm for aragonites, and from 145 to 862 ppm for calcites. Iron and manganese contents are high in calcites and siderites and low in aragonites. Site 707 carbonates precipitated at low temperatures in a fairly closed system, in which basalt-seawater interaction has greatly influenced the chemistry of the pore fluids. The reactions occurring within the system before and in conjunction with secondary carbonate precipita- tion include oxidation of isotopically light methane, derived from fluids circulating within the basalts, and reduction of substantial amounts of iron and manganese oxides from the basalts.

Description: Several celestite nodules were recovered on DSDP Leg 90 from four drilling sites on the Lord Howe Rise, southwest Pacific Ocean. The sediments at these sites are predominantly very pure calcareous nannofossil oozes and chalks. As a result of a higher-than average accumulation rate, they undergo relatively rapid burial diagenesis, which causes the expulsion of Sr from the biogenic calcite, to the interstitial waters. Another result of the high accumulation rate is the occurrence of microbial sulfate reduction in the interstitial waters. The downcore Sr increase is proportionately greater than the sulfate decrease, and celestite precipitates below about 100 m subbottom at each of these sites. The celestite contains high concentrations of many substituent cations: 4.3-7.8 mole% BaSO4, 1.7-6.4 mole% CaSO4, 1100-2600 ppm Al, and 400-750 ppm K. Ion activity products of Sr and sulfate at each site were calculated from the Pitzer equations and the measured concentrations of porewater constituents, and are in close agreement with the celestite solubility product corrected for in situ temperatures and pressures. Strontium concentrations of the porewaters are nearly at equilibrium with respect to celestite, and are controlled by the extent of microbial sulfate reduction. Celestite solubility increases with increasing water depth, in excellent agreement with values of the standard state partial molal volume change.

Description: About 200 pore-water samples were recovered and analyzed from eight sites of DSDP Leg 90. At all sites on the carbonate- rich Lord Howe Rise, Ca2+ concentrations increase and Mg2+ concentrations decrease with increasing sub-bottom depth. The value of dCa2+/dMg2+ averages -0.45 mol/mol at these sites, an unusually small negative value in comparison with sites on basaltic crust. This supports the argument that the crust of the Lord Howe Rise is siliceous. Li+ concentrations increase and K+ concentrations decrease with depth. Both of these constituents are affected by reactions within the sediment column. Microbial sulfate reduction occurs to a small extent at all sites. In the upper 200 m of Site 594 on the south side of the Chatham Rise, sulfate reduction and alkalinity production are more pronounced. Carbonate re-crystallization is indicated by large increases in Sr2+ concentrations with depth at all sites. The exact value of the Sr2+ maximum at each site is determined by equilibrium with respect to celestite (SrSO4). The greater the degree of microbial sulfate reduction, the higher is the pore water Sr2+ concentration. The diffusive flux of Sr2 out of the pore waters is consistent with an initial rate of carbonate re-crystallization of 10%/m.y. This re-crystallization must alter the isotopic and trace elemental compositions of some carbonate components.