Early Cenozoic Atlantic and Pacific benthic foraminiferal isotopes from 12 sites

Oxygen and carbon isotope records are important tools used to reconstruct past ocean and climate conditions, with those of benthic foraminifera providing information on the deep oceans. Reconstructions are complicated by interspecies isotopic offsets that result from microhabitat preferences (carbonate precipitation in isotopically distinct environments) and vital effects (species-specific metabolic variation in isotopic fractionation). We provide correction factors for early Cenozoic benthic foraminifera commonly used for isotopic measurements (Cibicidoides spp., Nuttallides truempyi, Oridorsalis spp., Stensioina beccariiformis, Hanzawaia ammophila, and Bulimina spp.), showing that most yield reliable isotopic proxies of environmental change. The statistical methods and larger data sets used in this study provide more robust correction factors than do previous studies. Interspecies isotopic offsets appear to have changed through the Cenozoic, either (1) as a result of evolutionary changes or (2) as an artifact of different statistical methods and data set sizes used to determine the offsets in different studies. Regardless of the reason, the assumption that isotopic offsets have remained constant through the Cenozoic has introduced an 1-2°C uncertainty into deep sea paleotemperature calculations. In addition, we compare multiple species isotopic data from a western North Atlantic section that includes the Paleocene-Eocene thermal maximum to determine the most reliable isotopic indicator for this event. We propose that Oridorsalis spp. was the most reliable deepwater isotopic recorder at this location because it was best able to withstand the harsh water conditions that existed at this time; it may be the best recorder at other locations and for other extreme events also.

Geochemical data derived from an absolutely dated shell-based chronology for the Irish Sea

The identification in various proxy records of periods of rapid (decadal scale) climate change over recent millennia, together with the possibility that feedback mechanisms may amplify climate system responses to increasing atmospheric CO2, highlights the importance of a detailed understanding, at high spatial and temporal resolutions, of forcings and feedbacks within the system. Such an understanding has hitherto been limited because the temperate marine environment has lacked an absolute timescale of the kind provided by tree-rings for the terrestrial environment and by corals for the tropical marine environment. Here we present the first annually resolved, multi-centennial (489-year), absolutely dated, shell-based marine master chronology. The chronology has been constructed by detrending and averaging annual growth increment widths in the shells of multiple specimens of the very long-lived bivalve mollusc Arctica islandica, collected from sites to the south and west of the Isle of Man in the Irish Sea. The strength of the common environmental signal expressed in the chronology is fully comparable with equivalent statistics for tree-ring chronologies. Analysis of the 14C signal in the shells shows no trend in the marine radiocarbon reservoir correction (DR), although it may be more variable before ~1750. The d13C signal shows a very significant (R**2 = 0.456, p < 0.0001) trend due to the 13C Suess effect.

Nannofossil and planktic foraminiferal events of ODP Site 166-1006 sediments

Detailed biostratigraphy in Site 1006 based on planktonic foraminifers and nannofossils shows large-scale sedimentation rate variability in the Florida Strait west of the Great Bahama Bank. A 'floating' cyclostratigraphy based mainly on resistivity logs and magnetic susceptibility data has been fixed to the biostratigraphy in the absence of magnetostratigraphy. The strongest orbital cycle present is the precessional beat, which is present in the borehole logs throughout the record. Counting the cycles resulted in an accurate time scale and thus a sedimentation rate time series. Spectral analysis of the sedimentation rate time series shows that the short-term cycle of eccentricity (~125 k.y.) and the long term cycle of eccentricity (~400 k.y.) are pervasive throughout the Miocene record, together with the long-term ~2-m.y. eccentricity cycle. The Great Bahama Bank produced pulses of shallow carbonate input once every precessional (sea level) cycle during the Miocene and perhaps two pulses per cycle in the early Pliocene. The amount of sediment exported in these pulses appears to be controlled by eccentricity modulation of the precessional amplitude and therefore the amplitude of the sea-level rise. Finally, an increase in sedimentation rate just after the Miocene/Pliocene boundary is attributed to a change in the location and strength of sediment drift currents in the Florida Strait due to reorganization of the currents following the closure of the Panama Isthmus.

Seafloor hydrothermal activity in sediment core MV0502-15JC

A large diameter piston core containing 8.35 m of metalliferous sediment has been recovered from a small abyssal valley in the remote Southwest Pacific Basin (31°42.194'S, 143°30.331'W; 5082 m water depth), providing unique insight into hydrothermal activity and eolian sedimentation there since the early Oligocene. A combination of fish-teeth Sr-isotope stratigraphy and INAA geochemical data reveals an exponentially decreasing hydrothermal flux 31 Ma to the present. Although hydrothermal sedimentation related to seafloor spreading explains this trend, a complex history of late Eocene/early Oligocene ridge jumps, propagating rifts and plate tectonic reorganization of South Pacific seafloor could have also played a role. A possible hiatus in deposition, as recorded by changes in core composition just below 2 m depth, is beyond the resolution of the fish teeth Sr isotope dating method employed here; however, the timing of this interval may be coincident with extinction of the Pacific-Farallon Ridge at ~20 Ma. A low flux eolian component accumulating at this site shows an increase relative to the hydrothermal component above 2 m depth, consistent with dust-generating continental sources far to the west (Australia/New Zealand). This is the first long-term paleoceanographic record obtained from within the South Pacific "bare zone" (Rea et al., 2006), an anomalous region where Pacific seafloor has largely escaped sediment accumulation since the Late Cretaceous.

Whole rock and Cr-rich spinel geochemistry of ODP Legs 152 and 163 and DSDP Hole 38-338 samples

Coring during Ocean Drilling Program and Deep Sea Drilling Project Legs 163, 152, 104, 81, and 38 recovered sequences of altered basalt from North Atlantic seaward-dipping reflector sequences (SDRS) erupted during the initial rifting of Greenland from northern Europe and likely associated with excessive mantle temperatures caused by an impacting mantle plume head. Cr-rich spinel is found abundantly as inclusions and groundmass crystals within the olivine-rich lavas of Hole 917A (Leg 152) cored into the Southeast Greenland SDRS, but only rarely as inclusions within plagioclase in the lavas of the Vøring Plateau SDRS, and it is absent from other cored SDRS lavas from the Rockall Plateau and Southeast Greenland. Eruptive melt compositions determined from inferred, thermodynamically-defined, spinel-melt exchange equilibria indicate that the most primitive melts represented by Hole 917A basalts have Mg/(Mg + Fe2+) at least as high as 0.70 and approach near-primary mantle melt compositions. In contrast, Cr-rich spinels from Hole 338 (Leg 38) lavas on the Vøring Plateau SDRS give evidence for melt with Mg/(Mg + Fe2+) only as high as 0.64. This study underlines that primitive melts similar to those from Hole 917A comprise only a small fraction of the eruptive North Atlantic SDRS melts, and that most SDRS basalts were, in fact, too evolved to have precipitated Cr-rich spinel, with true melt Mg/(Mg + Fe2+) likely below 0.60. The evolved nature of the SDRS basalts implies large amounts of fractionation at the base of the crust or deep within it, consistent with seismic results that indicate an abnormally thick Layer 3 underlying the SDRS.

Miocene stable isotope stratigraphy of ODP Hole 120-747A

We correlated Miocene d18O increases at Ocean Drilling Program Site 747 with d18O increases previously identified at North Atlantic Deep Sea Drilling Project Sites 563 and 608. The d18O increases have been directly tied to the Geomagnetic Polarity Time Scale (GPTS) at Site 563 and 608, and thus our correlations at Site 747 provide a second-order correlation to the GPTS. Comparison of the oxygen isotope record at Site 747 with records at Sites 563 and 608 indicates that three as-yet-undescribed global Miocene d18O increases may be recognized and used to define stable isotope zones. The d18O maxima associated with the bases of Zones Mila, Milb, and Mi7 have magnetochronologic age estimates of 21.8, 18.3, and 8.5 Ma, respectively. The correlation of a d18O maximum at 70 mbsf at Site 747 to the base of Miocene isotope Zone Mi3 (13.6 Ma) provides a revised interpretation of four middle Miocene normal polarity intervals observed between 77 and 63 mbsf at Hole 747A. Oxygen isotope stratigraphy indicates that the reversed polarity interval at 70 mbsf, initially interpreted as Chronozone C5AAr, should be C5ABr. Instead of a concatenated Chronozone C5AD-C5AC with distinct Chronozones C5AB, C5AA, and C5A (as in the preliminary interpretation), d18O stratigraphy suggests that these normal polarity intervals are Chronozones C5AD, C5AC, and C5AB, whereas Chronozones C5AA-C5A are concatenated. This interpretation is supported by the d13C correlations. The upper Miocene magnetostratigraphic record at Hole 747A is ambiguous. Two upper Miocene d18O events at Site 747 can be correlated to the oxygen isotope records at Site 563 and 608 using the magnetostratigraphy derived at Hole 747B. Our chronostratigraphic revisions highlight the importance of stable isotope stratigraphy in attaining an integrated stratigraphic framework for the Miocene.