DEPTH IN M 
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REGIONAL STATION NUMBER 
FIGURE 2.—Mean grain size of samples collected during years 1 (circle), 2 (square), and 3 (triangle). Error bars represent the 
standard deviation among samples collected in a given year. Stations are listed in order of increasing water depth. 
anomalously high value of 430 ppm (fig. 5) was ex- 
cluded from calculation of the mean. The scatter in 
the data probably indicates that Ba is not distrib- 
uted homogeneously over the sampling area. This 
heterogeneity is probably caused by the intermit- 
tent discharge of drilling fluids into a current field 
that continuously changes direction of flow 
throughout the tidal cycle. 
In agreement with the expected transport of drill- 
ing fluids to the west by the mean current flow, Ba 
concentrations were higher to the west than to the 
east. At station 18 (2 km to the west of the drill 
rig), maximum increase in Ba concentrations was 
about a factor of 2 as a result of drilling. The con- 
centrations of Ba decreased to predrilling levels, at 
the time of cruises 8 and 9 and then increased again. 
At the time of cruise 12, Ba concentrations were 36 
percent higher than predrilling levels and the dif- 
ference between mean values of cruises 1 and 12 is 
statistically significant at the 99-percent level of 
confidence (a=0.01). In contrast, at station 17 (2 km 
to the east of the drill rig), the maximum increase 
was only about 1.3 times the background Ba con- 
centration. A decrease in concentration was also 
seen at station 17 for cruises 8 and 9, then followed 
by an apparent increase. However, the concentra- 
tion measured on cruise-12 samples is not 
significantly different from.the cruise-1 concentra- 
tion at the 95-percent level of confidence. 
The concentration of Cr in these same samples 
(fig. 5) did not increase as a result of drilling even 
at the drill site. Similarly, we observed no changes 
in the concentrations of other metals in the bulk 
sediments during this period of monitoring. 
12 
In block 312 at station 5, the location of the site- 
specific survey, increases in Ba were observable 
following the initiation of drilling on December 8, 
1981 (fig. 6). The greatest increase (factor of 4.7 
above background) was observed at the drill site 
(station 5-1). At this location, there is a large stand- 
ard deviation among three individual replicates, and 
a considerable amount of scatter exists in the data 
following cruise 5. These trends are similar to those 
observed at station 16, also adjacent to a drill rig, 
and may reflect the heterogeneity of drilling mud 
distribution in the sediments near the rig. At sta- 
tions more than 0.5 km from the drill site, slightly 
higher increases were observed to the west than to 
the east, which is consistent with the preferential 
direction of transport. At all but station 5-10, the 
maximum concentration was observed during 
cruise 5, which was conducted immediately after 
drilling was completed. At most stations, the con- 
centrations decreased rapidly by cruises 6 and 7 and 
remained essentially constant through cruise 12. 
The concentration of Cr (fig. 6) or of other metals 
in bulk sediments at block 312 did not increase as 
a result of drilling. 
TRACE METALS IN THE FINE FRACTION OF SEDIMENT 
Within the sediment fraction finer than 60 pm 
(appendix table 4B), the Ba concentrations in- 
creased dramatically at stations near the drill rig 
at block 410 (fig. 7). At station 16, adjacent to the 
rig, the average Ba concentrations reached 8,000 
to 10,000 ppm between the third and seventh 
cruises. Lower concentrations were measured in 
