Ba/Sr ratio in marine barite from five DSDP and ODP holes

The Paleocene-Eocene Thermal Maximum (PETM), ca. 55 Ma, was a period of extreme global warming caused by rapid emission of greenhouse gases. It is unknown what ended this episode of greenhouse warming, but high oceanic export productivity over thousands of years (as indicated by high accumulation rates of barium, Ba) may have been a factor in ending this warm period by carbon sequestration. However, Ba has a short oceanic residence time (~10 k.y.), so a prolonged global increase in Ba accumulation rates requires an increase in input of Ba to the ocean, increasing barite saturation. We use a novel proxy for barite saturation (Sr/Ba in marine barite) to demonstrate that the seawater saturation state with respect to barite did not change across the PETM. The observations of increased barite burial, no change in saturation, and the short residence time can be reconciled if Ba burial decreased at continental margin and shelf sites due to widespread occurrence of suboxic conditions, leading to Ba release into the water column, combined with increased biological export production at some pelagic sites, resulting in Ba sink reorganization.

(Table 2) Age controls used to derive a time scale for DSDP Site 73-522

The stratotype section for the base of the Miocene is at a reversed (below) to normal (above) magnetic transition that is claimed to represent magnetic chron C6Cn.2n (o). Deep Sea Drilling Project (DSDP) Site 522 is the only location we are aware of that unambiguously records the three normal events of C6Cn. We have quantitatively determined the range of the short-lived nannofossil Sphenolithus delphix and the lower limit of S. disbelemnos in DSDP Holes 522 and 522A in order to calibrate their precise relationship to the magnetostratigraphy and to confirm the completeness of the record at this site. Astronomical tuning of Ocean Drilling Program (ODP) Sites 926, 928, and 929 shows that S. disbelemnos appears at 22.67 Ma and that the entire range of S. delphix is from about 22.98 Ma to 23.24 Ma. Using these ages, linear interpolation in DSDP Site 522 suggests that the age of C6Cn.2n (o) and of the Oligocene-Miocene boundary is 22.92+/-0.04 Ma. Our value, conservatively expressed as 22.9+/-0.1 Ma, is 0.9 m.y. younger than the currently accepted age of the Oligocene-Miocene boundary and of C6Cn.2n (o), which was assigned an age of 23.8 Ma, based on an estimate of 23.8+/-1 Ma for the Oligocene-Miocene boundary. The bulk-sediment carbon isotope data from DSDP Site 522 is correlated to the record from benthic foraminifera at ODP Site 929 to refine the calibration of magnetic reversals from C6Cn.1n (o) to C7n.2n (o) at DSDP Site 522 on the astronomical time scale.

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.