Pliocene-Pleistocene stack of globally distributed benthic stable oxygen isotope records

We present a 5.3-Myr stack (the ''LR04'' stack) of benthic d18O records from 57 globally distributed sites aligned by an automated graphic correlation algorithm. This is the first benthic delta18O stack composed of more than three records to extend beyond 850 ka, and we use its improved signal quality to identify 24 new marine isotope stages in the early Pliocene. We also present a new LR04 age model for the Pliocene-Pleistocene derived from tuning the delta18O stack to a simple ice model based on 21 June insolation at 65 N. Stacked sedimentation rates provide additional age model constraints to prevent overtuning. Despite a conservative tuning strategy, the LR04 benthic stack exhibits significant coherency with insolation in the obliquity band throughout the entire 5.3 Myr and in the precession band for more than half of the record. The LR04 stack contains significantly more variance in benthic delta18O than previously published stacks of the late Pleistocene as the result of higher resolution records, a better alignment technique, and a greater percentage of records from the Atlantic. Finally, the relative phases of the stack's 41- and 23-kyr components suggest that the precession component of delta18O from 2.7-1.6 Ma is primarily a deep-water temperature signal and that the phase of d18O precession response changed suddenly at 1.6 Ma.

Sulfur geochemistry at ODP Site 112-680 and 112-686

We have measured the concentrations of (1) pore-water sulfide and (2) solid-phase pyrite, iron monosulfide (=acid volatile sulfide), elemental sulfur, and extractable and nonextractable organic ("kerogen") sulfur in sediments from Ocean Drilling Program (ODP) Sites 680 and 686. Pore-water sulfide defines classic "bell-shaped" profiles. Maximum concentrations of 6 to 12 mM occur where sulfate is exhausted, or is most depleted, at depths between 15 and 50 mbsf. Sulfide resulting from bacterial sulfate reduction reacts in three ways: (1) some is reoxidized to elemental sulfur in surface sediments; (2) some reacts with detrital iron minerals to form iron monosulfide and pyrite, primarily in the top meter or two of the sediment; and (3) some reacts with, and is incorporated into, kerogen. Incorporation of reduced sulfur into kerogen occurs over the top 15 m of the sediment at both Sites 680 and 686, after the main phase of pyrite formation. Up to 45% of the total sedimentary sulfur is organically bound, and concentrations of 12 wt% sulfur are reached in the kerogen. These values are like those measured in lithologically similar, but more deeply buried, sediments from the Monterey Formation.