Geochemistry of Eocene sediments from ODP Site 171-1052 on the Black Nose, Atlantic Ocean

We use an X-ray fluorescence (XRF) Core Scanner to obtain records of elemental concentrations in sediment cores from Ocean Drilling Program (ODP) Leg 171B, Site 1052 (Blake Nose, Atlantic margin of northern Florida).This record spans the Middle to Late Eocene, as indicated by bio- and magnetostratigraphy, and displays cyclicity that can be attributed to the orbital forcing of a combination of climate, ocean circulation, or productivity. We use the XRF counts of iron and calcium as a proxy of the relative contribution from calcium carbonate and terrestrial material to construct a new composite depth record. This new composite depth record provides the basis to extend the astronomically calibrated geological time scale into the Middle Eocene and results in revised estimates for the age and duration of magnetochrons C16 through C18. In addition, we find an apparent change in the dominance of orbitally driven changes in obliquity and climatic precession at around 36.7 Ma on our new time scale. Long term amplitude modulation patterns of eccentricity and obliquity in the data do not seem to match the current astronomical model any more, suggesting the possibility of new constraints on astronomical calculations.

Roxana Páez, Manuel Puig. Del pop a la extrañeza. Buenos Aires, Ed. Almagesto, Colección Perfiles, 1995, 171 páginas

Fil: Sancholuz, Carolina. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.

Isotopic record across the Aptian/Albian boundary of ODP Hole 171-1049C

Geochemical analyses of extraordinarily well preserved late Aptian-early Albian foraminifera from Blake Nose (Ocean Drilling Program Site 1049) reveal rapid shifts of d18O, d13C, and 87Sr/88Sr in the subtropical North Atlantic that may be linked to a major planktic foraminifer extinction event across the Aptian/Albian boundary. The abruptness of the observed geochemical shifts and their coincidence with a sharp lithologic contact is explained as an artifact of a previously undetected hiatus of 0.8-1.4 million years at the boundary contact, but the values before and after the hiatus indicate that major oceanographic changes occurred at this time. 87Sr/88Sr increase by ~0.000200, d13C values decrease by 1.5 per mil to 2.2 per mil, and d18O values decrease by ~1.0 per mil (planktics) to 0.5 per mil (benthics) across the hiatus. Further, both 87Sr/88Sr ratios and d18O values during the Albian are anomalously high. The 87Sr/88Sr values deviate from known patterns to such a degree that an explanation requires either the presence of inter-basin differences in seawater 87Sr/88Sr during the Albian or revision of the seawater curve. For d18O, planktic values in some Aptian samples likely reflect a diagenetic overprint, but preservation is excellent in the rest of the section. In well preserved material, benthic foraminiferal values are largely between 0.5 and 0.0 per mil and planktic samples are largely between 0.0 per mil to -1.0 per mil, with a brief excursion to -2.0 per mil during OAE 1b. Using standard assumptions for Cretaceous isotopic paleotemperature calculations, the d18O values suggest bottom water temperatures (at ~1000 -1500 m) of 8-10°C and surface temperatures of 10-14°C, which are 4-6°C and 10-16°C cooler, respectively, than present-day conditions at the same latitude. The cool subtropical sea surface temperature estimates are especially problematic because other paleoclimate proxy data for the mid-Cretaceous and climate model predictions suggest that subtropical sea surface temperatures should have been the same as or warmer than at present. Because of their exquisite preservation, whole scale alteration of the analyzed foraminifera is an untenable explanation. Our proposed solution is a high evaporative fractionation factor in the early Albian North Atlantic that resulted in surface waters with higher d18O values at elevated salinities than commonly cited in Cretaceous studies. A high fractionation factor is consistent with high rates of vapor export and a vigorous hydrological cycle and, like the Sr isotopes, implies limited connectivity among the individual basins of the Early Cretaceous proto-Atlantic ocean.

Iron X-ray fluorescence (XRF) scannings of ODP Holes 165-1001A and 171-1050C

Abstract: Ocean Drilling Program Sites 1001A (Caribbean Sea) and 1050C (western North Atlantic) display obliquity and precession cycles throughout polarity zone C27 of the late Danian stage (earliest Cenozoic time). Sliding-window spectra analysis and direct cycle counting on downhole logs and high-resolution Fe variations at both sites yield the equivalent of 35-36 obliquity cycles. This cycle-tuned duration for polarity chron C27 of 1.45 Ma (applying a modern mean obliquity period of 40.4 ka) is consistent with trends from astronomical tuning of early Danian polarity chron C29 and 40Ar/39Ar age calibration of the Campanian-Maastrichtian magnetic polarity time scale. The cycle-tuned Danian stage (sensu Berggren et al. 1995, in SEPM Special Publications, 54, 129-212) spans 3.65 Ma (65.5-61.85 Ma). Spreading rates on a reference South Atlantic synthetic profile display progressive slowing during the Maastrichtian to Danian stages, then remained relatively constant through late Palaeocene and early Eocene time.