Middle Miocene stable carbon and oxygen isotope record of ODP Holes 130-805B and 130-806B

Bulk carbon isotope records are an effective chemostratigraphic tool for the middle Miocene because of the large and systematic variation in first-order d13C signals. Bulk d13C measurements support the presence of a hiatus at 305 mbsf in Hole 805B (latest middle Miocene), provisionally located while on board ship using biostratigraphic and magnetostratigraphic events. Records at Holes 805B and 806B show the middle Miocene Monterey carbon isotope excursion although the record at Hole 806B is apparently more stratigraphically continuous. Detailed analysis of multispecies foraminiferal carbon isotope records during the middle Miocene ("Monterey excursion") segment at Hole 806B support the assertion that this carbon isotope excursion comprises mainly between-reservoir effects. The benthic d18O data increase after 15.3 Ma, which we suggest corresponds to the mid-Miocene cooling step/ice volume increase of other authors. Planktonic foraminiferal d18O evidence exists for steepening of the thermocline at 17.4 Ma. A second-order d13C excursion superimposed at 13.8 Ma on the first-order Monterey excursion is associated with a second-order negative d18O excursion.

Stable Oxygen and carbon isotope record of benthic foraminifera of ODP Leg 121 holes

Oxygen and carbon isotopic records have been developed for the Cenozoic carbonate oozes of Sites 752, 754, 756, and 757 based on the analysis of monospecific benthic foraminifers. The intent of this report is to provide a basic isotopic stratigraphy for use in other paleoceanographic studies. The oxygen isotope record displays the enrichments associated with cooling or ice volume buildup at the Eocene/Oligocene boundary, in the middle Miocene, and in the upper Pliocene. The carbon isotopic record contains the Chron 16 enrichment in the lower Miocene and the Chron 6 depletion in the uppermost Miocene.

Part of the global DOC versus AOU (dissolved organic carbon/apparent oxygen utilization) data compilation, Azores-I

Temperate, transitional and subtropical waters of the remote Azores Front region east of Azores (24-40°N, 22-32°W) were sampled during three cruises conducted under increasing stratification conditions (April 1999, May 1997 and August 1998). Despite the temporal increase of surface temperature (by 5 °C) and stratification (by 2.1 1/min**2), as well as the thermocline shoaling (by ~15 m), dissolved organic carbon (DOC) and nitrogen (DON) in the surface layer were not significantly different for the early spring, late spring and summer periods, with average concentrations of 69±2 µM-C and 5.2±0.4 µM-N, respectively. The surface excess of semi-labile DOC, compared with the baseline DOC concentration in the deep ocean (47±2 µM-C), represents 33% of the bulk DOC concentration and as much as 85% of the TOC (=POC+DOC) excess. When compared with the winter baseline (56±2 µM-C), the seasonal surface DOC excess is 20% of the bulk DOC concentration and 87% of the seasonal TOC excess. These results confirm the major role played by DOC in the carbon cycle of surface waters of the Azores Front region. The total amount of bioreactive DOC transported from the temperate to the subtropical North Atlantic by the Ekman flux between March and December represents only ~15% of the average annual primary production, and ~15% and ~30% of the measured sinking POC flux+vertical DOC eddy diffusion during early spring and summer, respectively. Vertical eddy diffusion is 35% and 2% of the spring and summer sinking POC flux, respectively. On the other hand, DOC only contributes 13% to the local oxidation of organic matter in subsurface waters (between the pycnocline and 500 m) of the study region.

Organic carbon content, stable isotopes and planktonic foraminifera in sediments of tghe norther Arabian Sea oxygen minimum zone

The northern Arabian Sea is one of the few regions in the open ocean where thermocline water is severely depleted in oxygen. The intensity of this oxygen minimum zone (OMZ) has been reconstructed over the past 225,000 years using proxies for surface water productivity, water column denitrification, winter mixing, and the aragonite compensation depth (ACD). Changes in OMZ intensity occurred on orbital and suborbital timescales. Lowest O2 levels correlate with productivity maxima and shallow winter mixing. Precession-related productivity maxima lag early summer insolation maxima by ~6 kyr, which we attribute to a prolonged summer monsoon season related to higher insolation at the end of the summer. Periods with a weakened or even non-existent OMZ are characterized by low productivity conditions and deep winter mixing attributed to strong and cold winter monsoonal winds. The timing of deep winter mixing events corresponds with that of periods of climatic cooling in the North Atlantic region.