460
Holmesetal.
a)
5
6
28
26
-
U
0
24 - fC/)
8
C/)
22
7
20
- 0
0-!2
Z 6
L{)
co
5
5
4~
4
0
- c
3.8 I -
m
U
2 .~ c
m
OJ
I -
1 0
I
I
I
I
0
0
50
100
150
Age (kyr)
Fig. 10. Bulk sedimentary 8"N values, organic carbon content (GeoB 1016-3 from MUller et al. 1994 and GeoB
1008-3 from Schneider et al. 1997) and sea surface temperature (SST) based on alkenone analyses (from Schneider
et al. \995) in (a) GeoB 1008-3 and (b, right) GeoB 10\6-3. Stratigraphy was established from 8 1 '0 ofGlobigerina
ruber (pink) (Schneider et al. \995). Oxygen isotope chrons are shown at the top.
isotopic composition of bulk sediments (Altabet et
al. 1995; Ganeshram et al. 1995). In the Arabian
Sea, 8 1S N variations are driven by summer monsoons, which lead to intensified upwelling, which
in tum instigates elevated denitrification and IsNI
1 4 N ratios. Likewise, decreases in denitrification
resulted in lower giacial8 1s N values in sediments
from the eastern tropical North Pacific. Presentday oxygen concentrations in the Angola Basin are
above 0.2 ml 1. 1 (the threshold above which
denitrification does not occur; Skennan and McRae
1957; Packard et al. 1983), except over some shelf
areas (SNEC II 1986; Chapman and Shannon
1987). Furthennore, it is unlikely that denitrification
played an important role in the Angola Basin duringthe past 300 kyr, since OC content is inversely
correlated with 8 IS N. High productivity rates can
deplete oxygen concentrations, leading to
denitrification. Thus, denitrification (and elevated
8 1 5N) would be expected to be associated with high