Abstract
The effect of sea level change on nutrient supply to the anoxic Cariaco
demonstrates the fundamental importance of nitrogen (N2) fixation
and phosphate to oceanic production. As N2 fixation produces
biomass of low delta15N and has been reported to be an important
component of the nitrogen cycle in the modern Cariaco Basin, we propose that
it contributes to the light interglacial delta15N (~2-3)
values observed in the Ocean Drilling Program (ODP) site 1002 sediment record.
During the glacials the sediments are bioturbated (oxic conditions) with low
total organic carbon (TOC) contents and sedimentary delta15N values
of ~5, suggesting that nitrogen (N2) fixation contributed
little to the N nutrition of Cariaco surface waters. The most plausible
explanation for the inferred glacial/interglacial changes in N2
fixation in the Cariaco is that they have occurred in response to variations
in the N/P ratio of the nutrient supply, driven by changes in denitrification.
Reprinted by permission of the American Geophysical Union.
Abstract
Continuous drill cores from the Caribbean Sea of up to 1 km thick
mixed sediments and carbonates were recovered by Ocean Drilling
Program Leg 165. The sediments range in porosity from 10% to 80% and
in carbonate content from 20% to 95%. Unconfined P-wave velocities
from this material were measured by ODP on freshly retrieved water
saturated material, and are here compared to similar data from ODP Leg
130 (Ontong Java Plateau) where more than 95% pure carbonates were
recovered. To fit low porosity as well as high porosity data, Wyllie's
equation can be modified by replacing the porosity by the ratio
between porosity and critical porosity. Irrespective of clay content
the same modified Wyllie type linear relationship was found between
the P-wave travel time and porosity. For samples from the Caribbean a
critical porosity of 73% was found. A critical porosity of 68% was
found for samples from the Ontong Java Plateau.