Os isotope chemostratigraphy applied to organic‐rich marine sediments from the Eocene‐Oligocene transition on the West African margin (ODP Site 959)

Abstract

[1] The known temporal relationship between the benthic foraminiferal d18 O record and the marine Os isotope record is used to reinterpret the absolute chronology and paleoceanographic context of an episode of organic carbon burial on the West African margin Ocean Drilling Program (ODP) Site 959. Although organic-rich sediments require significant corrections for in situ decay of 187 Re to 187 Os, these results demonstrate the utility of the marine Os isotope record for chemostratigraphic correlation of organic-rich sediments that are devoid of age diagnostic microfossils with pelagic carbonate sequences. Revision of the ODP Site 959 chronology shifts the age assignment of an interval of biosiliceous, organic-rich sediment deposition from the Oligocene to the late Eocene and earliest Oligocene, likely culminating with the first major glaciation of the Oligocene (Oi1). We speculate that enhanced organic carbon burial over much of the West African margin may have contributed to drawdown of atmospheric carbon dioxide before and during the Oi1 event and suggest that Os isotope chemostratigraphy provides a valuable tool for further exploring this possibility.Citation: Ravizza, G., and F. Paquay (2008), Os isotope chemostratigraphy applied to organic-rich marine sediments from the Eocene-Oligocene transition on the West African margin (ODP Site 959), Paleoceanography, 23, PA2204,

Major shifts in calcareous phytoplankton assemblages through the Eocene‐Oligocene transition of Tanzania and their implications for low‐latitude primary production

“…However, the calcareous nannofossil record presented here strongly indicates an increased availability of nutrients in the low-latitude surface ocean, adding to the growing number of studies that document an increase in surface ocean productivity directly coincident with the EOT [Anderson and Delaney, 2005;Diester-Haass et al, 1996;Diester-Haass and Zachos, 2003;Latimer and Filippelli, 2002;Ravizza and Paquay, 2008;Salamy and Zachos, 1999]. This discrepancy between surface ocean productivity records and the extent of organic carbon burial across the EOT needs to be addressed and may indicate a widespread decoupling of productivity and carbon burial due to the increased ventilation of the deep ocean.…”

“…It is feasible that increased mixing in the Southern Ocean, coincident with Antarctic cooling and glaciation, enhanced the formation of SAMW which effectively connected southern high-latitude cooling with the tropical/subtropical oceans. New records are required to ascertain the extent of low-latitude organic carbon burial associated with these productivity increases [Ravizza and Paquay, 2008], which appears to be largely absent from the Southern Ocean [Rea and Lyle, 2005], and so test recent assertions that organic carbon burial did not play a significant role in the positive d 13 C excursion and deepening of the CCD [Merico et al, 2008].…”

“…There is however a lack of comparable paleoproductivity records from the low latitudes [but see Diester-Haass and Zachos, 2003;Nilsen et al, 2003;Ravizza and Paquay, 2008] where quantifying changes in primary carbonate and silica fluxes at many deep-sea sites is hampered by the dramatic deepening of the carbonate compensation depth (CCD) and the associated shift from siliceous to calcareous sediments through the EOT [Nilsen et al, 2003].…”

Absence of geochemical evidence for an impact event at the Bølling–Allerød/Younger Dryas transition

“…(54). Third, the Os and Pt concentrations are above average eroding crustal values (48), typical of organic-rich sediments from other continental margin settings (65,66). Fourth, the range in the Os/Ir ratios (1.6-12.8) indicates a minimal terrigenous influence (Os/Ir Ϸ 1) supporting authigenic enrichment of Os relative to Ir.…”

Re–Os and Mo isotope systematics of black shales from the Middle Proterozoic Velkerri and Wollogorang Formations, McArthur Basin, northern Australia

“…Finally, our new Re-Os ages for the Velkerri Formation demonstrate the utility of Re-Os geochronology of black shales deposited from both suboxic and euxinic bottom water conditions. Initial Os isotope compositions (hereafter I Os ) from ReOs black shale isochron regressions may record the Os isotope composition of the contemporaneous seawater at the time of sediment deposition (Ravizza and Turekian, 1992;Ravizza and Paquay, 2008). Large uncertainties are associated with the I Os from the Re-Os isochron regressions for the Velkerri Formation (0.29 ± 0.18 and 0.06 ± 0.22 for the upper and lower intervals, respectively) because only samples with radiogenic present-day 187 Os/ 188 Os isotope ratios (between $9 and 14) were analyzed.…”