(Table 1) Gypsum, pyrite and glauconite at DSDP Hole 36-329

Authigenic gypsum, pyrite, and glauconite are disseminated throughout an unusually long (346 m) Miocene section of mixed biogenic carbonate and diatomaceous ooze drilled on the Falkland Plateau at DSDP Site 329 (water depth, 1519 m). The present organic carbon content of the sediment is low, ranging between 0.1 and 0.7%. Gypsum occurs as euhedral single or twinned crystals of selenite up to 5 mm in diameter, sometimes in the form of gypsum rosettes. These crystals are intact and unabraded, comprising up to 4% of the washed sample. The authigenic nature of the gypsum is demonstrated by the presence of diatoms and radiolarians embedded within the gypsum crystals. The gypsum co-occurs with pyrite and glauconite in these samples. The pyrite occurs as framboids, foraminiferal infillings, rods, and granular sheetlike masses composed of pyrite octahedra. The glauconite occurs as foraminiferal infillings and as free grains. The gypsum and pyrite were identified by energy-dispersive X-ray analysis and scanning electron micrographs. Some of the gypsum has grown on pyrite, indicating that it precipitated after the pyrite, perhaps in response to a change in pH conditions. The formation of the mineral suite can be explained by current models of in situ sulfide and sulfate precipitation coincident with diagenesis and oxidation of much of the original organic carbon.

Determination of millennial-scale climate variability based on alkenones in the western Atlantic Ocean

High-resolution records of alkenone-derived sea surface temperatures and elemental Ti/Ca ratios from a sediment core retrieved off northeastern Brazil (4° S) reveal short-term climate variability throughout the past 63,000 a. Large pulses of terrigenous sediment discharge, caused by increased precipitation in the Brazilian hinterland, coincide with Heinrich events and the Younger Dryas period. Terrigenous input maxima related to Heinrich events H6-H2 are characterized by rapid cooling of surface water ranging between 0.5 and 2° C. This signature is consistent with a climate model experiment where a reduction of the Atlantic meridional overturning circulation (AMOC) and related North Atlantic cooling causes intensification of NE trade winds and a southward movement of the Intertropical Convergence Zone, resulting in enhanced precipitation off northeastern Brazil. During deglaciation the surface temperature evolution at the core site predominantly followed the Antarctic warming trend, including a cooling, prior to the Younger Dryas period. An abrupt temperature rise preceding the onset of the Bølling/Allerød transition agrees with model experiments suggesting a Southern Hemisphere origin for the abrupt resumption of the AMOC during deglaciation caused by Southern Ocean warming and associated with northward flow anomalies of the South Atlantic western boundary current.