Geochemistry of FeMn oxyhydroxides of IODP Site 303-U1302 and 303-1303

Regional weathering intensity must have changed dramatically at high latitudes during the Quaternary as a consequence of repeated continental glaciation. Investigation of these glacial/interglacial changes at high temporal resolution is possible with the recent development of Pb isotopes in FeMn oxyhydroxide phases as a proxy for region-specific weathering intensity, where increases in the radiogenic component are thought to correspond to increased continental weathering fluxes. Here we present a Pb isotope record sourced from the FeMn oxyhydroxide fraction in marine sediments from IODP Sites U1302/3 on Orphan Knoll (~3500 mbsl, NW Atlantic), spanning the last 37 ka. Located at the eastern edge of the Laurentide Ice Sheet (LIS), Site U1302/3 is well-placed to monitor changes in weathering intensity associated with LIS glacial history. Overall, the data show a close correspondence to local surface water d18O, with least radiogenic values during times of heavy d18O (glacial maximum) and most radiogenic values during times of light d18O (Holocene). This supports the prediction that weathering intensity in glaciated regions of the North Atlantic correlates with the exposure age of glacial debris. Superimposed on these background trends are extreme radiogenic excursions (e.g. variation in 206Pb/204Pb from ~19.2-21.0) contemporaneous with Heinrich events and the Younger Dryas. These data are substantially more radiogenic than existing records from the NW Atlantic, and most likely represent episodes of exceptionally high inputs of pre-formed FeMn oxyhydroxides during drainage of the LIS. Due to its extreme isotope composition, at least in the NW Atlantic region, Pb would appear to be a good proxy for the fluxes of weathered continental material and perhaps, by inference, nutrients to the surface ocean

Age and strontium isotope composition for Cretaceous in DSDP Hole 74-525

Firm stratigraphic correlations are needed to evaluate the global significance of unconformity bounded units (sequences). We correlate the well-developed uppermost Campanian and Maestrichtian sequences of the New Jersey Coastal Plain to the geomagnetic polarity time scale (GPTS) by integrating Sr-isotopic stratigraphy and biostratigraphy. To do this, we developed a Maestrichtian (ca. 73-65 Ma) Sr-isotopic reference section at Deep Sea Drilling Project Hole 525A in the southeastern Atlantic Ocean. Maestrichtian strata can then be dated by measuring their 87Sr/86Sr composition, calibrating to the GPTS of S. C. Cande and D. V. Kent (1993, personal commun.), and using the equation Age (Ma) = 37326.894-52639.89 (87Sr/86Sr). Sr-stratigraphic resolution for the Maestrichtian is estimated as +-1.2 to +-2 m.y. At least two unconformity-bounded units comprise the uppermost Campanian to Maestrichtian strata in New Jersey. The lower one, the Marshalltown sequence, is assigned to calcareous nannofossil Zones CC20/21 (~NC19) and CC22b (~NC20). It ranges in age from ~74.1 to 69.9 Ma based on Sr-isotope age estimates. The overlying Navesink sequence is assigned to calcareous nannoplankton Zones CC25-26 (~NC21-23); it ranges in age from 69.3 to 65 Ma based on Sr-isotope age estimates. The upper part of this sequence, the Tinton Formation, has no calcareous planktonic control; Sr-isotopes provide an age estimate of 66 +- 1.2 Ma (latest Maestrichtian). Sequence boundaries at the base and the top of the Marshalltown sequence match boundaries elsewhere in the Atlantic Coastal Plain (Owens and Gohn, 1985) and the inferred global sea-level record of Haq et al. (1987); they support eustatic changes as the mechanism controlling depositional history of this sequence. However, the latest Maestrichtian record in New Jersey does not agree with Haq et al. (1987); we attribute this to correlation and time-scale differences near the Cretaceous/Paleogene boundary. High sedimentation rates in the latest Maestrichtian of New Jersey (Shrewsbury Member of the Red Bank Formation and the Tinton Formation) suggest tectonic uplift and/or rapid progradation during deposition of the highstand systems tract.