(Table 3) Stable isotope record of Cretaceous foraminifera from DSDP Hole 44-392A

Mid-Cretaceous (Barremian-Turonian) plankton preserved in deep-sea marl, organic-rich shale, and pelagic carbonate hold an important record of how the marine biosphere responded to short- and long-term changes in the ocean-climate system. Oceanic anoxic events (OAEs) were short-lived episodes of organic carbon burial that are distinguished by their widespread distribution as discrete beds of black shale and/or pronounced carbon isotopic excursions. OAE1a in the early Aptian (~120.5 Ma) and OAE2 at the Cenomanian/Turonian boundary (~93.5 Ma) were global in their distribution and associated with heightened marine productivity. OAE1b spans the Aptian/Albian boundary (~113-109 Ma) and represents a protracted interval of dysoxia with multiple discrete black shales across parts of Tethys (including Mexico), while OAE1d developed across eastern and western Tethys and in other locales during the latest Albian (~99.5 Ma). Mineralized plankton experienced accelerated rates of speciation and extinction at or near the major Cretaceous OAEs, and strontium isotopic evidence suggests a possible link to times of rapid oceanic plateau formation and/or increased rates of ridge crest volcanism. Elevated levels of trace metals in OAE1a and OAE2 strata suggest that marine productivity may have been facilitated by increased availability of dissolved iron. The association of plankton turnover and carbon isotopic excursions with each of the major OAEs, despite the variable geographic distribution of black shale accumulation, points to widespread changes in the ocean-climate system. Ocean crust production and hydrothermal activity increased in the late Aptian. Faster spreading rates [and/or increased ridge length] drove a long-term (Albian-early Turonian) rise in sea level and CO2-induced global warming. Changes in ocean circulation, water column stratification, and nutrient partitioning lead to a reorganization of plankton community structure and widespread carbonate (chalk) deposition during the Late Cretaceous. We conclude that there were important linkages between submarine volcanism, plankton evolution, and the cycling of carbon through the marine biosphere.

Age models and stable isotope ratios of SO12 and SO35 sediment cores

Based on organic carbon accumulation rates, nine time slices of oceanic export paleoproductivity (Pnew) are presented which depict the variability of Pnew on a global scale through the last 30,000 years and document that the basic distribution patterns did not change through glacial and interglacial times. However, the glacial ocean shows an increased contrast of high- versus low-productivity zones. d13C values of near-surface-dwelling planktonic foraminifera Globigerinoides ruber suggest that the same contrast applies to the glacial nutrient inventories of the ambient surface waters, with a significant glacial transfer of PO4 from low- to high-productivity zones. In this way, glacial Pnew increased by a global average of about 2-4 Gt C/yr and led, via an enhanced CaCO3 dissolution and alkalinity in the deep ocean, to a significant extraction of CO2 from the surface water and the atrnosphere.

Carbonate aggregate mineralogy and stable isotopes at DSDP Site 87-583

Crystalline aggregates composed of calcium carbonate were recovered in the uppermost 50 m of Nankai Trough sediments during DSDP Leg 87A. These aggregates decomposed with time to masses of sandy calcite as determined by X-ray diffraction analysis. Petrographic and scanning electron microscopy revealed textures suggestive of a precursor phrase prior to calcite, and this precursor has been tentatively identified as the mineral ikaite, CaCO3*6H2O. Stable isotope data suggest a large component of terrigenous organic matter as the carbon source, consistent with the appearance of these aggregates in highly reducing pyritic sediments containing abundant plant remains. We propose that these nodules formed in euxinic basins on the upper part of the Trough slope under normal seafloor conditions of pressure and temperature. Calculated temperatures of formation of this phase are not unusually low. The specimens from Site 583 are the first reported occurrences of ikaite in active margin sediments.

Geochemistry and foraminiferal stable isotoe record of sediment core SCS90-36

Changes in the Southeast Asia monsoon winds and surface circulation patterns since the last glaciation are inferred using multiple paleoceanographic indicators including planktic foraminifer faunal abundances, fauna and alkenones sea-surface temperature (SST) estimates, oxygen and carbon isotopes of planktic and benthic foraminifers, and sedimentary fluxes of carbonates and organic carbon obtained from deep-sea core SCS90-36 from the South China Sea (SCS) (17°59.70'N, 111°29.64'E at water depth 2050 m). All these paleoceanographic evidences indicate marked changes in the SCS ocean system over the last glacial toward the Holocene. Planktic foraminiferal faunal SST estimates show stable warm-season SST of 28.6°C, close to the modern value, throughout the glacial-interglacial cycle. In contrast, cold-season SST increases gradually from 23.6°C in the last glacial to a mean value of 26.4°C in the Holocene with a fluctuation of about 3°C during 13-16 ka. SST estimates by UK'37 method reveal less variability and are in average 1-3°C lower than the fauna-derived winter-season SST. These patterns reveal that the seasonality of the SST is not only higher by about 3-4°C in the glacial, but also a function of the winter season SST. Sedimentation rates decrease from the last glacial-deglacial stage to the Holocene due to a reduction in supply of terrigenous components, which led to an increase of carbonate contents. Total organic carbon (TOC) contents of primarily marine sources decrease from the last glacial-deglacial to the Holocene. The last deglaciation is also characterized by high surface productivity as indicated by increased ketones abundances and high mass accumulation rates (MAR) of the TOC and carbonates. The gradient of planktic foraminifer ocygen and carbon isotopes of between surface dwellers and deep dwellers increases significantly toward Termination I and Holocene, and is indiscernibly small in the carbon isotope gradient of between 14 and 24 ka, revealing a deep-mixing condition in surface layers prior to 10 ka. The glacial-interglacial fluctuation of the carbon isotope value of a benthic foraminifer is 0.61%. which is significantly larger than a global mean value. The large carbon isotope fluctuation indicates an amplification of marginal-sea effects which is most likely resulted from an increase in surface productivity in the northern SCS during the last glacial-deglacial stage. The multiple proxies consistently indicate that the last glacial-deglacial stage winter monsoon in the Southeast Asia was probably strengthened in the northern SCS, leading to a development of deep-mixing surface layer conditions and a more efficient nutrient cycling which supports more marine organic carbon production.

Geochemistry and stable isotope record of carbonate oozes and chalks of ODP Hole 130-807B

Light greenish gray and pale purple color bands are common in the ooze and chalk of the Ontong Java Plateau. Analyses of Pleistocene and Pliocene ooze samples that contain abundant bands indicate that the purple bands are colored by finely disseminated iron sulfide, whereas the green bands are colored by finely disseminated Fe- and Al-bearing silicates (probably clays). No local contrasts in the total organic carbon contents, carbon and oxygen isotopic compositions, and grain sizes were found. Band abundances, counted from core photographs of all Leg 130 holes, can be correlated from hole to hole on the basis of age rather than depth. The temporal distribution of these color bands is also comparable with that of the green bands described from the Lord Howe Rise, which were previously interpreted as products of altered volcanic glass. This may indicate that the green and purple bands on the Ontong Java Plateau originate from the early alteration of volcanic ash. The crosscutting relationships between the green and purple bands and original structures in the host sediment indicate that the bands have been locally altered by redox conditions in the sediments after the bands were formed.

(Table 1) Calcite, siderite, and stable isotopes d13C and d18O at DSDP Hole 93-603B

Diagenesis of the fine-grained, feldspathic sandstones in the Lower Cretaceous submarine fan complex cored in DSDP Hole 603B can be considered to have occurred in three stages: (1) replacement of matrix and framework grains by pyrite, siderite, phillipsite (?), and particularly by ferroan calcite; (2) dissolution of ferroan calcite and feldspars to produce secondary macroporosity; and (3) development of sparse feldspar and quartz overgrowths, and authigenic modification of remnant matrix. Only ferroan calcite is a volumetrically important diagenetic mineral phase (up to 50 vol.%). Matrix in thin sandstone turbidite deposits has been extensively replaced by ferroan calcite. Carbon stable isotope data suggest that organic diagenesis had only a minor influence on calcite precipitation. Oxygen stable isotope data indicate that the minimum average calcite precipitation temperature was 40° C. Preliminary calculations show that steadystate diffusion of Ca+ + from the dissolution of nannoplankton skeletal material in the interbedded pelagic marls to the associated sandstones is a feasible transport mechanism. A thick sandstone unit from 1234-1263 m sub-bottom is extensively replaced by calcite near the upper and lower contacts. Farther into the sand body away from the contacts, the sandstone has good secondary porosity resulting from the dissolution of ferroan calcite that partially replaced matrix and framework grains. The central portion of the thick sand appears to be a channel with high-energy clean sand. We believe that the channel provided a conduit for focused flow of diagenetic compactional fluids responsible for dissolution. Focused flow may be the result of the earlier lithification of the pelagic limestones and thin-bedded sandstones which, then formed vertical permeability barriers. Calcite dissolution has occurred and may still be occurring at temperatures less than 65°C.