High-resolution cyclostratigraphy of the Cenomanian-Turonian interval: Paleoceanographic implications, a comparison between deep sea drilling project site 386 and 144

The purpose of this study was to determine the extent to which oceanic anoxic events (OAE's) are recorded in deep-water deposits of the former western Tethyan Sea, by investigating the Cenomanian-Turonian time interval characterized by the worldwide OAE 2 event. The study improved our knowledge of the possible controlling mechanisms that triggered this event at these sites, and furthered our understanding of this global phenomenon. This was examined by high-resolution, multi-proxy analyses of sediments at DSDP Sites 386 and 144, including sedimentology, scanning electron microscopy, stable isotopes, bulk and clay mineralogy, major and trace element geochemistry, biomarkers, and paleontological data. ^ The results provide a better stratigraphic resolution for the Cenomanian-Turonian, which allowed for more precise determination of chronologic boundaries, sedimentation rates at DSDP Site 386, and a more accurate calculation of the frequency of the cycles recorded in the sequence, which fall predominantly within the precession (∼23 kyr) and short eccentricity (∼100 kyr) ranges. The combined proxies allow assessment of the correlation of δ13Corg, and major and trace elements with the predominance of cyanobacteria. These organisms were the main producers of the organic matter during the dysoxic and euxinic conditions of OAE 2 at DSDP Site 386. A huge amount of microcrystalline quartz of eolian origin is also associated with OAE 2. The geochemical proxies further provide evidence that OAE 2 was linked to increased volcanism outside the deep water of the proto-Atlantic Ocean. The clays in the Turonian sediments are terrigenous and derived predominantly from eolian transport. Comparing DSDP Site 386 and 144 with stratotype sections, the δ13C org and TOC data indicate that OAE 2 seems diachronous throughout the proto-Atlantic Ocean. ^ This study concludes that the development of anoxic conditions in the deep water of the Atlantic during the latest Cenomanian-Turonian resulted from a combination of factors related to local oceanic setting and mitigated by global tectonism and climate. The data provide a more comprehensive view of the interacting factors that led to sustained high productivity of the cyanobacteria and photosynthetic protists that produced organic-carbon-rich deposits in the world's oceans. ^

Life cycle of Deccan trap magma chambers: A crystal scale elemental and strontium isotopic investigation

The Deccan Trap basalts are the remnants of a massive series of lava flows that erupted at the K/T boundary and covered 1-2 million km2 of west-central India. This eruptive event is of global interest because of its possible link to the major mass extinction event, and there is much debate about the duration of this massive volcanic event. In contrast to isotopic or paleomagnetic dating methods, I explore an alternative approach to determine the lifecycle of the magma chambers that supplied the lavas, and extend the concept to obtain a tighter constraint on Deccan’s duration. My method relies on extracting time information from elemental and isotopic diffusion across zone boundaries in individual crystals. I determined elemental and Sr-isotopic variations across abnormally large (2-5 cm) plagioclase crystals from the Thalghat and Kashele “Giant Plagioclase Basalts” from the lowermost Jawhar and Igatpuri Formations respectively in the thickest Western Ghats section near Mumbai. I also obtained bulk rock major, trace and rare earth element chemistry of each lava flow from the two formations. Thalghat flows contain only 12% zoned crystals, with 87 Sr/86Sr ratios of 0.7096 in the core and 0.7106 in the rim, separated by a sharp boundary. In contrast, all Kashele crystals have a wider range of 87Sr/86Sr values, with multiple zones. Geochemical modeling of the data suggests that the two types of crystals grew in distinct magmatic environments. Modeling intracrystalline diffusive equilibration between the core and rim of Thalghat crystals led me to obtain a crystal growth rate of 2.03x10-10 cm/s and a residence time of 780 years for the crystals in the magma chamber(s). Employing some assumptions based on field and geochronologic evidence, I extrapolated this residence time to the entire Western Ghats and obtained an estimate of 25,000–35,000 years for the duration of Western Ghats volcanism. This gave an eruptive rate of 30–40 km3/yr, which is much higher than any presently erupting volcano. This result will remain speculative until a similarly detailed analytical-modeling study is performed for the rest of the Western Ghats formations.

Investigation of Mantle Dynamics from Platinum Group Elements and Rhenium-Osmium Isotope Systematics of Mantle Xenoliths from Oahu, Hawaii

Intraplate volcanism that has created the Hawaiian-Emperor seamount chain is generally thought to be formed by a deep-seated mantle plume. While the idea of a Hawaiian plume has not met with substantial opposition, whether or not the Hawaiian plume shows any geochemical signal of receiving materials from the Earth’s Outer Core and how the plume may or may not be reacting with the overriding lithosphere remain debatable issues. In an effort to understand how the Hawaiian plume works I report on the first in-situ sulfides and bulk rock Platinum Group Element (PGE) concentrations, together with Os isotope ratios on well-characterized garnet pyroxenite xenoliths from the island of Oahu in Hawaii. The sulfides are Fe-Ni Monosulfide Solid Solution and show fractionated PGE patterns. Based on the major elements, Platinum Group Elements and experimental data I interpret the Hawaiian sulfides as an immiscible melt that separated from a melt similar to the Honolulu Volcanics (HV) alkali lavas at a pressure-temperature condition of 1530 ± 100OC and 3.1±0.6 GPa., i.e. near the base or slightly below the Pacific lithosphere. The 187Os/188Os ratios of the bulk rock vary from subchondritic to suprachondritic (0.123-0.164); and the 187Os/188Os ratio strongly correlates with major element, High Field Strength Element (HFSE), Rare Earth Element (REE) and PGE abundances. These correlations strongly suggest that PGE concentrations and Os isotope ratios reflect primary mantle processes. I interpret these correlations as the result of melt-mantle reaction at the base of the lithosphere: I suggest that the parental melt that crystallized the pyroxenites selectively picked up radiogenic Os from the grain boundary sulfides, while percolating through the Pacific lithosphere. Thus the sampled pyroxenites essentially represent crystallized melts from different stages of this melt-mantle reaction process at the base of the lithosphere. I further show that the relatively low Pt/Re ratios of the Hawaiian sulfides and the bulk rock pyroxenites suggest that, upon ageing, such pyroxenites plus their sulfides cannot generate the coupled 186Os- 187Os isotope enrichments observed in Hawaiian lavas. Therefore, recycling of mantle sulfides of pyroxenitic parentage is unlikely to explain the enriched Pt-Re-Os isotope systematics of plume-derived lavas.

Paleoenvironments and geochemical signals from the late Barremian to the middle Aptian in a Tethyan marginal basin, northeast Spain: Implications for carbon sequestration in restricted basins

The hallmark of oceanic anoxic event 1a (OAE1a) (early Aptian ~125 Ma) corresponds to worldwide deposition of black shales with total organic carbon (TOC) content > 2% and a δ13C positive excursion up to ~5‰. OAE1a has been related to large igneous province volcanism and dissociation of methane hydrates during the Lower Cretaceous. However, the occurrence of atypical, coeval and diachronous organic-rich deposits associated with OAE1a, which are also characterized by positive spikes of the δ 13C in epicontinental to restricted marine environments of the Tethys Ocean, indicates localized responses decoupled from complex global forcing factors. ^ The present research is a high-resolution, multiproxy approach to assess the paleoenvironmental conditions that led to enhanced carbon sequestration from the late Barremian to the middle Aptian in a restricted, Tethyan marginal basin prior to and during OAE1a. I studied the lower 240 m of the El Pui section, Organyà Basin, Spanish Pyrenees. The basin developed as the result of extensional tectonism linked to the opening of the Atlantic Ocean. At the field scale the section consists of a sequence of alternating beds of cm – m-scale, medium-gray to grayish-black limestones and marlstones with TOC up to ~4%. ^ The results indicate that the lowest 85 m of the section, from latest Barremian -earliest Aptian, characterize a deepening phase of the basin concomitant with sustained riverine flux and intensified primary productivity. These changes induced a shift in the sedimentation pattern and decreased the oxygen levels in the water column through organic matter respiration and limited ventilation of the basin. ^ The upper 155 m comprising the earliest – late-early Aptian document the occurrence of OAE1a and its associated geochemical signatures (TOC up to 3% and a positive shift in δ13C of ~5‰). However, a low enrichment of redox-sensitive trace elements indicates that the basin did not achieve anoxic conditions. The results also suggest that a shallower-phase of the basin, coeval with platform progradation, may have increased ventilation of the basin at the same time that heightened sedimentation rates and additional input of organic matter from terrestrial sources increased the burial and preservation rate of TOC in the sediment.^