Elemental and isotope compositions of lipids, kerogens and asphaltenes from samples of DSDP Hole 41-368

The thermal effects of three (one major and two minor) Miocene diabase intrusions on Cretaceous black shales from DSDP site 41-368 have been analyzed. A concentration gradient was observed, especially for the hydrocarbons, decreasing towards the major intrusion and between the three sills. The thermally-altered samples in the proximity of and between the sills contained elemental sulfur and an excess of thermally-derived pristane over phytane. whereas, the unaltered sediments contained no elemental sulfur, and more phytane than pristane. A maximum yield of the extractable hydrocarbons was observed at a depth of 7 m below the major sill. Two classes of molecular markers were present in this bitumen suite. The first was sesqui-, di- and triterpenoids and steranes. which could be correlated with both terrigenous and autochthonous sources. They were geologically mature and showed no significant changes due to the thermal stress. The second class was found in the altered samples, which contained only polynuclear aromatic hydrocarbons with low alkyl substitution and sulfur and oxygen heterocyclic aromatic compounds. These compounds were derived from pyrolytic reactions during the thermal event. Kerogen was isolated from all of these samples, but only traces of humic substances were present. The H/C, N/C, d13C, d34S and dD all exhibit the expected effects of thermal stress. The kerogen becomes more aromatized and richer in 13C, 34S and D in the proximity of and between the sills. Maturation trends were also measured by the vitrinite reflectance and electron spin resonance, where the thermal stress could be correlated with an elevated country rock temperature and an increased degree of aromaticity. The effects of in situ thermal stress on the organic-rich shales resulted in the generation and expulsion of petroliferous material from the vicinity of the sills.

Stable isotope ratios of foraminifera from sediment cores off northeastern Brazil

The western South Atlantic boundary currents represent a sensitive system within the global thermohaline circulation (THC). We investigated the impact of deglacial THC changes on the western tropical Atlantic studied in six high resolution sediment cores from the upper continental slope of Brazil. The stratigraphy of the cores is mainly based on 14C AMS dating of monospecific foraminiferal samples. Changes in the upper layer tropical ocean during the deglaciation are inferred from stable oxygen isotope measurements on planktic and benthic foraminifera. Variations in the delta18O residuals are assumed to be mainly temperature related. During the Oldest and Younger Dryas cooling periods, two major deglacial THC disturbances are reported from North Atlantic sediment cores. Concomitant to the repeated THC slowdown, we observe an upper layer warming in the tropical ocean. A reduced northward heat export from the tropical areas during these periods (weak North Brazil Current) is additionally reflected by low meridional gradients in the stable oxygen records. This generally agrees with results from coupled ocean atmosphere models.

Isotope ratios of osmiun, carbon and oxygen of Miocene sediments and benthic foraminifera

Seawater 187Os/188Os ratios for the Middle Miocene were reconstructed by measuring the 187Os/188Os ratios of metalliferous carbonates from the Pacific (DSDP 598) and Atlantic (DSDP 521) oceans. Atlantic and Pacific 187Os/188Os measurements are nearly indistinguishable and are consistent with previously published Os isotope records from Pacific cores. The Atlantic data reported here provide the first direct evidence that the long-term sedimentary 187Os/188Os record reflects whole-ocean changes in the Os isotopic composition of seawater. The Pacific and the Atlantic Os measurements confirm a long-term 0.01/Myr increase in marine 187Os/188Os ratios that began no later than 16 Ma. The beginning of the Os isotopic increase coincided with a decrease in the rate of increase of marine 87Sr/86Sr ratios at 16 Ma. A large increase of 1? in benthic foraminiferal delta18O values, interpreted to reflect global cooling and ice sheet growth, began approximately 1 million years later at 14.8 Ma, and the long-term shift toward lower bulk carbonate delta13C values began more than 2 Myr later around 13.6 Ma. The post-16 Ma increase in marine 187Os/188Os ratios was most likely forced by weathering of radiogenic materials, either old sediments or sialic crust with a sedimentary protolith. We consider two possible Miocene-specific geologic events that can account for both this increase in marine 187Os/188Os ratios and also nearly constant 87Sr/86Sr ratios: (1) the first glacial erosion of sediment-covered cratons in the Northern Hemisphere; (2) the exhumation of the Australian passive margin-New Guinea arc system. The latter event offers a mechanism, via enhanced availability of soluble Ca and Mg silicates in the arc terrane, for the maintenance of assumed low CO2 levels after 15 Ma. The temporal resolution (three samples/Myr) of the 187Os/188Os record from Site 598, for which a stable isotope stratigraphy was also constructed, is significantly higher than that of previously published records. These high resolution data suggest oscillations with amplitudes of 0.01 to 0.02 and periods of around 1 Myr. Although variations in the 187Os/188Os record of this magnitude can be easily resolved analytically, this higher frequency signal must be verified at other sites before it can be safely interpreted as global in extent. However, the short-term 187Os/188Os variations may correlate inversely with short-term benthic foraminiferal delta18O and bulk carbonate delta13C variations that reflect glacioeustatic events.

Auxiliary material and basic geochemical and benthic foraminiferal stable isotope data for the interval bracketing Eocene Thermal Maximum 2 (ETM2) at DSDP Sites 401 and 550 in the northeastern Atlantic Ocean

Ever since its discovery, Eocene Thermal Maximum 2 (ETM2; ~53.7 Ma) has been considered as one of the "little brothers" of the Paleocene-Eocene Thermal Maximum (PETM; ~56 Ma) as it displays similar characteristics including abrupt warming, ocean acidification, and biotic shifts. One of the remaining key questions is what effect these lesser climate perturbations had on ocean circulation and ventilation and, ultimately, biotic disruptions. Here we characterize ETM2 sections of the NE Atlantic (Deep Sea Drilling Project Sites 401 and 550) using multispecies benthic foraminiferal stable isotopes, grain size analysis, XRF core scanning, and carbonate content. The magnitude of the carbon isotope excursion (0.85-1.10 per mil) and bottom water warming (2-2.5°C) during ETM2 seems slightly smaller than in South Atlantic records. The comparison of the lateral d13C gradient between the North and South Atlantic reveals that a transient circulation switch took place during ETM2, a similar pattern as observed for the PETM. New grain size and published faunal data support this hypothesis by indicating a reduction in deepwater current velocity. Following ETM2, we record a distinct intensification of bottom water currents influencing Atlantic carbonate accumulation and biotic communities, while a dramatic and persistent clay reduction hints at a weakening of the regional hydrological cycle. Our findings highlight the similarities and differences between the PETM and ETM2. Moreover, the heterogeneity of hyperthermal expression emphasizes the need to specifically characterize each hyperthermal event and its background conditions to minimalize artifacts in global climate and carbonate burial models for the early Paleogene.

Stable carbon and oxygen isotope compositions of benthic foraminifera N. truempyi samples and coarse fraction analyses from ODP Site 198-1209

We have compiled the first stratigraphically continuous high-resolution benthic foraminiferal stable isotope record for the Paleocene from a single site utilizing cores recovered at Pacific ODP Site 1209. The long-term trend in the benthic isotope record suggests a close coupling of volcanic CO2 input and deep sea warming. Over the short-term the record is characterized by slow excursions with a pronounced periodic beat related to the short (100-kyr) and long (405-kyr) eccentricity cycle. The phase relationship between the benthic isotope record and eccentricity is similar to patterns documented for the Oligocene and Miocene confirming the role of orbital forcing as the pace maker for paleoclimatic variability on Milankovitch time scales. In addition, the record documents an unusual transient warming of 2°C coeval with a 0.6 per mil carbon isotope excursion and a decrease in carbonate content at 61.75 Ma. This event, which bears some resemblance to Eocene hyperthermals, marks the onset of a long-term decline in d13C. The timing indicates it might be related to the initiation of volcanism along Greenland margin.

Stable isotope composition of carbonates in the Tyrrhenian Sea

The Messinian evaporitic succession recovered at ODP Sites 652, 653, and 654 in the Tyrrhenian Sea was generated under various environmental conditions which ranged from brackish to hypersaline, as deduced from the sedimentary facies and stable isotope compositions of the carbonate and sulfate deposits. Water in the basins had to be shallow to undergo such rapid and large geochemical variations. The marine influence was omnipresent in the basin at least during the deposition of sulfate evaporites; seawater or marine brines might have been supplied either by direct input into evaporitic lagoons as at Sites 653 and 654, or by subterraneous infiltration in marginal areas as at Site 652. Episodes of severe dilution by continental waters occurred frequently throughout Messinian times in the more basinal areas at Sites 653 and 654, while a fresh water body was standing permanently at Site 652. The high heat flow present at Site 652 was responsible for a major late authigenesis of iron-rich dolomites, which was initiated during the subsidence of the basin and ended before Pliocene.

Pliocene-Pleistocene stable oxygen isotope record of Cibicidoides wuellerstorfi from the North Atlantic

We generated benthic isotope records from Ocean Drilling Program (ODP) site 981 on the Feni drift (2173 m water depth) and from ODP site 983 on the Gardar drift (1983 m water depth) to examine the interaction between North Atlantic Deep Water (NADW) and Glacial North Atlantic Intermediate Water (GNAIW) formation from 2.0 to 1.4 Ma. We find NADW at both sites during interglacial periods, and a mix of NADW and Southern Ocean water at the Feini drift during most glacial periods. Prior to 1.7 Ma we find no evidence ofr GNAIW at the Gardar drift site. Instead, glacial Gardar drift delta13C values are as low or lower than values for all other sites in the North Atlantic and reflect continued glacial overflow from the Nordic seas. After 1.7 Ma Gardar drift delta13C values increase and suggest that there was GNAIW at the Gardar drift site during some glacial intervals. Overall, we find that NADW and GNAIW production changed around 1.7 Ma in concert with changes in sea surface temperature and salinity and in the Earth's obliquity cycle.

Stable isotope record of sediment core MD98-2177

We present a reconstruction of El Niño Southern Oscillation (ENSO) variability spanning the Medieval Climate Anomaly (MCA, A.D. 800-1300) and the Little Ice Age (LIA, A.D. 1500-1850). Changes in ENSO are estimated by comparing the spread and symmetry of d18O values of individual specimens of the thermocline-dwelling planktonic foraminifer Pulleniatina obliquiloculata extracted from discrete time horizons of a sediment core collected in the Sulawesi Sea, at the edge of the western tropical Pacific warm pool. The spread of individual d18O values is interpreted to be a measure of the strength of both phases of ENSO while the symmetry of the d18O distributions is used to evaluate the relative strength/frequency of El Niño and La Niña events. In contrast to previous studies, we use robust and resistant statistics to quantify the spread and symmetry of the d18O distributions; an approach motivated by the relatively small sample size and the presence of outliers. Furthermore, we use a pseudo-proxy approach to investigate the effects of the different paleo-environmental factors on the statistics of the d18O distributions, which could bias the paleo-ENSO reconstruction. We find no systematic difference in the magnitude/strength of ENSO during the Northern Hemisphere MCA or LIA. However, our results suggest that ENSO during the MCA was skewed toward stronger/more frequent La Niña than El Niño, an observation consistent with the medieval megadroughts documented from sites in western North America.

Stable carbon isotope record of Bahama Bank sediments

The carbon-isotopic composition (d13C) of bulk carbonates, obtained from a transect of sites drilled through platform and periplatform sediments of Holocene to Early Miocene age, has been compared to ascertain whether changes in the d13C can be correlated between sediments of equivalent ages and whether such changes can be related to global changes in the d13C of the dissolved inorganic carbon in the oceans over this time period. Five of the sites were drilled during Leg 166 of the Ocean Drilling Project (1003-1007) in a transect ranging from five km to 25 km away from the platform margin and penetrating sediments of Holocene to Oligocene age that are contained in 17 depositional sequences (A-Q). Two shallow-water sites, Clino and Unda were situated on a extension of the same transect on Great Bahama Bank in a water depth of 10-15 m. With the exception of Unda and Clino, the d13C of the carbonates ranges from +5 per mil in the younger sequences to +1 per mil in the Early Miocene. In each of the sites, the d13C is strongly positively correlated with the percentage of aragonite. As a consequence, the d13C of sequences A through F is strongly correlated, reflecting the decreasing amount of aragonite with increasing depth. In the two platform sites, the d13C is significantly lower in the younger portions of the cores as a result of the influences of meteoric diagenesis during repeated exposure during the Pleistocene. Although the d13C of the individual sequences can be correlated in most instances between the ODP holes, the changes are not related to global changes in the d13C of the oceans which in contrast to the d13C of the platform sediments become isotopically lower towards the present day. Instead variations in the d13C appear to be related to varying mixtures of d13C-rich banktop sediments and pelagic material.