Benthic foraminifera and stable isotope composition in Paleocene-Eocene sediments

In the late Paleocene to early Eocene, deep sea benthic foraminifera suffered their only global extinction of the last 75 million years and diversity decreased worldwide by 30-50% in a few thousand years. At Maud Rise (Weddell Sea, Antarctica; Sites 689 and 690, palaeodepths 1100 m and 1900 m) and Walvis Ridge (Southeastern Atlantic, Sites 525 and 527, palaeodepths 1600 m and 3400 m) post-extinction faunas were low-diversity and high-dominance, but the dominant species differed by geographical location. At Maud Rise, post-extinction faunas were dominated by small, biserial and triserial species, while the large, thick-walled, long-lived deep sea species Nuttallides truempyi was absent. At Walvis Ridge, by contrast, they were dominated by long-lived species such as N. truempyi, with common to abundant small abyssaminid species. The faunal dominance patterns at the two locations thus suggest different post-extinction seafloor environments: increased flux of organic matter and possibly decreased oxygen levels at Maud Rise, decreased flux at Walvis Ridge. The species-richness remained very low for about 50 000 years, then gradually increased. The extinction was synchronous with a large, negative, short-term excursion of carbon and oxygen isotopes in planktonic and benthic foraminifera and bulk carbonate. The isotope excursions reached peak negative values in a few thousand years and values returned to pre-excursion levels in about 50 000 years. The carbon isotope excursion was about -2 per mil for benthic foraminifera at Walvis Ridge and Maud Rise, and about -4 per mil for planktonic foraminifera at Maud Rise. At the latter sites vertical gradients thus decreased, possibly at least partially as a result of upwelling. The oxygen isotope excursion was about -1.5 per mil for benthic foraminifera at Walvis Ridge and Maud Rise, -1 per mil for planktonic foraminifera at Maud Rise. The rapid oxygen isotope excursion at a time when polar ice-sheets were absent or insignificant can be explained by an increase in temperature by 4-6°C of high latitude surface waters and deep waters world wide. The deep ocean temperature increase could have been caused by warming of surface waters at high latitudes and continued formation of the deep waters at these locations, or by a switch from dominant formation of deep waters at high latitudes to formation at lower latitudes. Benthic foraminiferal post-extinction biogeographical patterns favour the latter explanation. The short-term carbon isotope excursion occurred in deep and surface waters, and in soil concretions and mammal teeth in the continental record. It is associated with increased CaC03-dissolution over a wide depth range in the oceans, suggesting that a rapid transfer of isotopically light carbon from lithosphere or biosphere into the ocean-atmosphere system may have been involved. The rapidity of the initiation of the excursion (a few thousand years) and its short duration (50 000 years) suggest that such a transfer was probably not caused by changes in the ratio of organic carbon to carbonate deposition or erosion. Transfer of carbon from the terrestrial biosphere was probably not the cause, because it would require a much larger biosphere destruction than at the end of the Cretaceous, in conflict with the fossil record. It is difficult to explain the large shift by rapid emission into the atmosphere of volcanogenic CO2, although huge subaerial plateau basalt eruptions occurred at the time in the northern Atlantic. Probably a complex combination of processes and feedback was involved, including volcanogenic emission of CO2, changing circulation patterns, changing productivity in the oceans and possibly on land, and changes in the relative size of the oceanic and atmospheric carbon reservoirs.

Microfossil, stable isotope, and facies of a near-continuous core from the Gippsland Basin

Fossil, facies, and isotope analyses of an early high-paleolatitude (55°S) section suggests a highly unstable East Antarctic Ice Sheet from 32 to 27 Myr. The waxing and waning of this ice sheet from 140% to 40% of its present volume caused sea level changes of ±25 m (ranging from -30 to +50 m) related to periodic glacial (100,000 to 200,000 years) and shorter interglacial events. The near-field Gippsland sea level (GSL) curve shares many similarities to the far-field New Jersey sea level (NJSL) estimates. However, there are possible resolution errors due to biochronology, taphonomy, and paleodepth estimates and the relative lack of lowstand deposits (in NJSL) that prevent detailed correlations with GSL. Nevertheless, the lateral variations in sea level between the GSL section and NJSL record that suggest ocean siphoning and antisiphoning may have propagated synchronous yet variable sea levels.

Natural remanent magnetization (NRM) from ODP Site 1263 covering magnetochron C20r and C21n and high-resolution bulk carbon isotope (d13C) records from Sites 702 and 1263

To explore cause and consequences of past climate change, very accurate age models such as those provided by the astronomical timescale (ATS) are needed. Beyond 40 million years the accuracy of the ATS critically depends on the correctness of orbital models and radioisotopic dating techniques. Discrepancies in the age dating of sedimentary successions and the lack of suitable records spanning the middle Eocene have prevented development of a continuous astronomically calibrated geological timescale for the entire Cenozoic Era. We now solve this problem by constructing an independent astrochronological stratigraphy based on Earth's stable 405 kyr eccentricity cycle between 41 and 48 million years ago (Ma) with new data from deep-sea sedimentary sequences in the South Atlantic Ocean. This new link completes the Paleogene astronomical timescale and confirms the intercalibration of radioisotopic and astronomical dating methods back through the Paleocene-Eocene Thermal Maximum (PETM, 55.930 Ma) and the Cretaceous-Paleogene boundary (66.022 Ma). Coupling of the Paleogene 405 kyr cyclostratigraphic frameworks across the middle Eocene further paves the way for extending the ATS into the Mesozoic.

Stable carbon and oxygen isotope compositions of bulk rock samples analyzed from the Zumaia section

Astronomical tuning of sedimentary records to precise orbital solutions has led to unprecedented resolution in the geological time scale. However, the construction of a consistent astronomical time scale for the Paleocene is controversial due to uncertainties in the recognition of the exact number of 405-kyr eccentricity cycles and accurate correlation between key records. Here, we present a new Danian integrated stratigraphic framework using the land-based Zumaia and Sopelana hemipelagic sections from the Basque Basin and deep-sea records drilled during Ocean Drilling Program (ODP) Legs 198 (Shatsky Rise, North Pacific) and 208 (Walvis Ridge, South Atlantic) that solves previous discrepancies. The new coherent stratigraphy utilises composite images from ODP cores, a new whole-rock d13C isotope record at Zumaia and new magnetostratigraphic data from Sopelana. We consistently observe 11 405-kyr eccentricity cycles in all studied Danian successions. We achieve a robust correlation of bioevents and stable isotope events between all studied sections at the ~100-kyr short-eccentricity level, a prerequisite for paleoclimatic interpretations. Comparison with and subsequent tuning of the records to the latest orbital solution La2011 provides astronomically calibrated ages of 66.022 ± 0.040 Ma and 61.607 ± 0.040 Ma for the Cretaceous-Paleogene (K-Pg) and Danian-Selandian 105 (D-S) boundaries respectively. Low sedimentation rates appear common in all records in the mid-Danian interval, including conspicuous condensed intervals in the oceanic records that in the past have hampered the proper identification of cycles. The comprehensive interbasinal approach applied here reveals pitfalls in time scale construction, filtering techniques in particular, and indicates that some caution and scrutiny has to be applied when building orbital chronologies. Finally, the Zumaia section, already hosting the Selandian Global Boundary Stratotype Section and Point (GSSP), could serve as the global Danian unit stratotype in the future.

Trace metals in shells of mussels and clams from deep-sea hydrothermal vent fields of the Mid-Atlantic Ridge and East Pacific Rise

Bioaccumulation of trace metals in carbonate shells of mussels and clams was investigated at seven hydrothermal vent fields of the Mid-Atlantic Ridge (Menez Gwen, Snake Pit, Rainbow, and Broken Spur) and the Eastern Pacific (9°N and 21°N at the East Pacific Rise and the southern trough of Guaymas Basin, Gulf of California). Mineralogical analysis showed that carbonate skeletons of mytilid mussel Bathymodiolus sp. and vesicomyid clam Calyptogena m. are composed mainly of calcite and aragonite, respectively. The first data were obtained for contents of a variety of chemical elements in bivalve carbonate shells from various hydrothermal vent sites. Analyses of chemical compositions (including Fe, Mn, Zn, Cu, Cd, Pb, Ag, Ni, Cr, Co, As, Se, Sb, and Hg) of 35 shell samples and 14 water samples from mollusk biotopes revealed influences of environmental conditions and some biological parameters on bioaccumulation of metals. Bivalve shells from hydrothermal fields with black smokers are enriched in Fe and Mn by factor of 20-30 relative to the same species from the Menez Gwen low-temperature vent site. It was shown that essential elements (Fe, Mn, Ni, and Cu) more actively accumulated during early ontogeny of the shells. High enrichment factors of most metals (n x 100 - n x 10000) indicate efficient accumulation function of bivalve carbonate shells. Passive metal accumulation owing to adsorption on shell surfaces was estimated to be no higher than 50% of total amount and varied from 14% for Fe to 46% for Mn.

Chemical and isotopic compositions of authigenic carbonates from bottom sediments of Northwest Pacific marginal seas

The paper presents data on authigenic carbonate distribution in Holocene - Upper Pleistocene deposits of the Okhotsk, Japan, East China, Philippine and South China Seas. Description of carbonate samples, their chemical and isotope compositions are given. Chemical analysis of the samples indicates that almost all authigenic carbonates are composed of calcite or magnesian calcite; and only in one case, of siderite. Oxygen isotopic composition (d18O) ranges from +37.7 to +26.1 per mil (SMOW); it is, probably, connected with different temperatures of carbonate formation. A distinct geographic regularity is traced. Decrease in d18O values is observed from the cold Okhotsk Sea to the warm South China Sea. A very wide range of carbon isotopic composition (d13C from -42 to +3.8 per mil) indicates different sources of carbonic acid required for formation of these carbonates. As a basis for carbon isotopic composition we can distinguish three sources of carbonic acid in the studied sediments: microbiological methane oxidation, organic matter destruction during sediment diagenesis, and dissolved organogenic limestone. Thus, formation of authigenic carbonates in sediments from the marginal seas of the Northwest Pacific results from: 1) sediment diagenesis, 2) methane oxidation in zones of gas anomalies, 3) their precipitation from the supersaturated by carbonates sea shoal waters of tropical sea lagoons.

Mineralogical and stable isotope analyses of carbonates from the eastern Black Sea

Authigenic carbonates associated with cold seeps provide valuable archives of changes in the long-term seepage activity. To investigate the role of shallow-buried hydrates on the seepage strength and fluid composition we analysed methane-derived carbonate precipitates from a high-flux hydrocarbon seepage area ("Batumi seep area") located on the south-eastern Black Sea slope in ca. 850 m. In a novel approach, we combined computerized X-ray tomography (CT) with mineralogical and isotope geochemical methods to get additional insights into the three-dimensional internal structure of the carbonate build-ups. X-ray diffractometry revealed the presence of two different authigenic carbonate phases, i.e. pure aragonitic rims associated with vital microbial mats and high-Mg calcite cementing the hemipelagic sediment. As indicated by the CT images, the initial sediment has been strongly deformed, first plastic then brittle, leading to brecciation of the progressively cemented sediment. The aragonitic rims on the other hand, represent a presumably recent carbonate growth phase since they cover the already deformed sediment. The stable oxygen isotope signature indicates that the high-Mg calcite cement incorporated pore water mixed with substantial hydrate water amounts. This points at a dominant role of high gas/fluid flux from decomposing gas hydrates leading to the deformation and cementation of the overlying sediment. In contrast, the aragonitic rims do not show an influence of 18O-enriched hydrate water. The differences in d18O between the presumably recent aragonite precipitates and the older high-Mg cements suggest that periods of hydrate dissociation and vigorous fluid discharge alternated with times of hydrate stability and moderate fluid flow. These results indicate that shallow-buried gas hydrates are prone to episodic decomposition with associated vigorous fluid flow. This might have a profound impact on the seafloor morphology resulting e.g. in the formation of carbonate pavements and pockmark-like structures but might also affect the local carbon cycle.

Bulk stable carbon and oxygen isotopes from ODP Site 1258, and Fe intensities from ODP Site 1262, and minimal tuning age model options for ODP Site 1258 and 1262

Timing is crucial to understanding the causes and consequences of events in Earth history. The calibration of geological time relies heavily on the accuracy of radioisotopic and astronomical dating. Uncertainties in the computations of Earth's orbital parameters and in radioisotopic dating have hampered the construction of a reliable astronomically calibrated time scale beyond 40 Ma. Attempts to construct a robust astronomically tuned time scale for the early Paleogene by integrating radioisotopic and astronomical dating are only partially consistent. Here, using the new La2010 and La2011 orbital solutions, we present the first accurate astronomically calibrated time scale for the early Paleogene (47-65 Ma) uniquely based on astronomical tuning and thus independent of the radioisotopic determination of the Fish Canyon standard. Comparison with geological data confirms the stability of the new La2011 solution back to ~54 Ma. Subsequent anchoring of floating chronologies to the La2011 solution using the very long eccentricity nodes provides an absolute age of 55.530 {plus minus} 0.05 Ma for the onset of the Paleocene/Eocene Thermal Maximum (PETM), 54.850 {plus minus} 0.05 Ma for the early Eocene ash -17, and 65.250 {plus minus} 0.06 Ma for the K/Pg boundary. The new astrochronology presented here indicates that the intercalibration and synchronization of U/Pb and 40Ar/39Ar radiometric geochronology is much more challenging than previously thought.

Organic geochemistry and sedimentology of lower to mid-Cretaceous at DSDP Leg 77 Holes

Analyses of 40 carbonate core samples - 27 from Site 535, 12 from Site 540, and 1 from Site 538A - have confirmed many of the findings of the Shipboard Scientific Party. The samples, all but one Early to mid-Cretaceous in age (Berriasian to Cenomanian), reflect sequences of cyclically anoxic and oxic depositional environments. They are moderately to very dark colored, dominantly planar-parallel, laminated lime mudstones. Most show the effects of intense mechanical compaction. Visual kerogen characteristics and conventional Rock-Eval parameters indicate that these deep basinal carbonates contain varying mixtures of thermally immature kerogen derived from both marine and terrigenous precursors. However, variations in kerogen chemistry are evident upon analysis of the pyrolysis mass spectral data in conjunction with the other geochemical analyses. Particularly diagnostic is the reduction index, Rl, a measure of H2S produced during pyrolysis. Total organic carbon, TOC, ranges from 0.6 to 6.6%, with an overall average of 2.4%. Average TOCs for these fine-grained mudstones are: late Eocene 2.5% (1 sample), Cenomanian 2.2% (6), Albian 2.0% (10), Aptian 1.3% (1), Barremian-Hauterivian 2.8% (11), late Valanginian 4.8% (3), Berriasian-early Valanginian 1.6% (7). Most of the carbonates have source-potential ratings of fair to very good of predominantly oil-prone to mixed kerogen, with only a few gas-prone samples. The ratings correlate well with the inferred depositional environments, i.e., whether oxic or anoxic. Several new organic-geochemical parameters, especially Rl, based on pyrolysis mass spectrometry of powdered whole-rock samples, support this view. Tar from fractures in laminated to bioturbated limestones of Unit IV (late Valanginian) at 535-58-4, 19-20 cm (530 m sub-bottom) appears to be mature, biodegraded, and of migrated rather than on site indigenous origin.

Results of biogeochemical studies in the Russian Arctic seas: isotope and radioisotope data

Comprehensive biogeochemical studies including determination of isotopic composition of organic carbon in both suspended matter and surface layer (0-1 cm) bottom sediments (more than 260 determinations of d13C-Corg) were carried out for five Arctic shelf seas: White, Barents, Kara, East Siberian, and Chukchi Seas. The aim of this study is to elucidate causes that change isotopic composition of particulate organic carbon at the water-sediment boundary. It is shown that isotopic composition of organic carbon in sediments from seas with high river run-off (White, Kara, and East Siberian Seas) does not inherit isotopic composition of organic carbon in particles precipitating from the water column, but is enriched in 13C. Seas with low river run-off (Barents and Chukchi Seas) show insignificant difference between d13C-Corg values in both suspended load and sediments because of low content of isotopically light allochthonous organic matter in suspended matter. Biogeochemical studies with radioisotope tracers (14CO2, 35S, and 14CH4) revealed existence of specific microbial filter formed from heterotrophic and autotrophic organisms at the water-sediment boundary. This filter prevents mass influx of products of organic matter decomposition into the water column, as well as reduces influx of OM contained in suspended matter from water into sediments.

Isotopic survey of diagenetic carbonates, at DSDP Leg 63 Holes

Authigenic carbonates, principally calcium-rich dolomites, with extremely variable isotopic compositions were recovered in organic-rich marine sediments during Leg 63 drilling off southern California and Baja California. These carbonates occur as thin layers in fine-grained, diatomaceous sediments and siliceous rocks, mostly deposited during the Neogene. A combination of textural, geochemical, and isotopic evidence indicates these dolomites formed as cements and precipitates in shallow subsurface zones of high alkalinity spawned by abundant CO2 and methane production during progressive microbial decay of organic matter. Depths and approximate temperatures of formation estimated from oxygen isotopes are 87 to 658 meters and 10°C to 50°C, respectively. Within any sedimentary section, dolomites may form simultaneously at several depths or at different times within the same interval. Highly variable carbon isotopes (-30 to +16 per mil) reflect the isotopic reservoir in which the carbonates formed. Oxidation of organic matter through microbial reduction of sulfate at shallow depths favors light-carbon carbonates such as those at Sites 468 and 471; heavy-carbon carbonates at Site 467 most likely formed below this zone where HC**12O3**- is preferentially removed by reduction of CO2 to methane during methanogenesis. An important controlling factor is the sedimentation rate, which dictates both the preservation of organic matter on the sea floor and depth distribution of subsurface zones of organic-matter decay.