Data Summary

Figures 2-5 summarize the AMS¹⁴C data collected on this leg. Only∆¹⁴C mea-surements with a quality flag value of 2 or 6 are included in each figure. Figure 2 shows the∆¹⁴C values with 2σ error bars plotted as a function of pressure. The data density in

this figure is representative for these legs: approximately 3 times as many samples were collected in the thermocline as in deep and bottom waters. The mid-depth∆¹⁴C minimum at approximately 2500 meters is clearly evident. Figure 3 shows the∆¹⁴C values plotted against silicate.The straight line shown in the figure is the least squares regression rela-tionship derived by Broecker et al. (1995) based on the GEOSECS global data set.

According to their analysis, this line (∆¹⁴C = -70 - Si) represents the relationship between naturally occurring radiocarbon and silicate for most of the ocean. They interpret devia-tions in∆¹⁴C above this line to be due to input of bomb-produced radiocarbon. Clearly, this relationship is not ideal for the P6 data set. The data points having silicate values greater than or equal to 60µmol/kg almost certainly have no bomb-radiocarbon compo-nent and should therefore lie on, rather than below, the line as seen in Figure 3. For these data the slope of the line needs to be steeper or/and the intercept needs to be lower. Also strongly diverging from the trend are one group of points having silicate concentrations between 105 - 125µmol/kg. These data are from the near-bottom water at stations between approximately 160°W and 180°W; that is, some form of northward flowing circumpolar bottom water. For silicate values greater than approximately 40µmol/kg the shape of the

∆¹⁴C vs. Si trend is better described as a backward “J” which is rotated counter-clockwise.

Figure 2:AMS∆¹⁴C results for P6 stations shown with 2σ error bars.Only those measurements having a quality control flag value of 2 are plotted.

Pressure (dB)

Delta C-14 (0/00)

0 2000 4000 6000

-200-1000100

Essentially all of the deep and bottom waters in the South Pacific examined during WOCE shown this same shape. The shape is, of course due to the fact that the∆¹⁴C and Si

extreme fall at different depths and have similar, but different ratios on either side of the extrema. If one follows Broecker’s argument, but modifies it so that only data which have no tritium and are above the shallower of the Si and∆¹⁴C extrema, one should still be able to get an estimate of the pre-bomb radiocarbon. To that end (and erring on the safe side), a least squares fit of the data from samples between 1 and 2 km depth (n=141; R²=.91) gives an intercept of -63±3 and an intercept value of -1.34±.03 both of which are significantly different than the -70, -1 which Broecker calculated for the GEOSECS global data set.

Figure 4 is an objectively contoured section (LeTraon, 1990) of the∆¹⁴C distribu-tion for the upper 1.5 kilometer of the water column The most prominent features of this section are the general upward tilt of the isopleths eastward of 120°W and the sharp upturn of the isopleths in the 250-750 meter depth range just off South America. This second trait is probably due to upwelling and is reflected in the low surface concentrations on the east end of the section. At this point, the upward slope of the deeper isopleths is interpreted as a shallow indication of the deeper southward flow at the eastern side of the section.

Figure 5 shows the entire section contoured. Both Figure 4 and Figure 5 were gridded using the method of LeTraon (1990), however, the horizontal correlation length scale used for Figure 5 was 2.5 times that used for Figure 4 to compensate for the sparser sampling in the deep and bottom water (i.e., the full water column section was significantly smoothed relative to the first) The∆¹⁴C minimum centered around 2500m depth is definitely not continuous across the section. The break in the minimum at 260° (100°W) is reflected in Figure 3:∆¹⁴C as a function of silicate for P6 AMS samples. The straight line shows the relationship proposed by Broecker, et al., 1995 (∆¹⁴C = -70 - Si with radiocarbon in ‰ and silicate inµ^mol/kg).

Silicate (umol/kg)

Delta C-14 (0/00)

0 20 40 60 80 100 120

-200-1000100

other tracers and is indicative of northward flow along the east side of the ridge.. .

Figure 4:∆¹⁴C concentration in the upper kilometer of TUNES leg 3; WOCE line P6) along 155°^W.

Gridding done using the method of Letraon (1990); all samples measured using the AMS technique (Key, 1996a,b; Key, et al., 1996). For most of the section the maximum concentration is found below the surface.

Figure 5:∆¹⁴C contour of WOCE section P6 at approximately 32°S. Longitudes are east of Greenich.

Objective gridding done using the LeTraon (1990) method with a relative long horizontal correlation length scale. The samples in the near bottom water mass centered on 190° and having relatively low∆^{14C are}

those which fall above the general deep water trend in Figure 3.

Contour Lines = Delta C-14 (0/00)

Contour Interval = 20Longitude

Depth (m)

160 180 200 220 240 260 280

-1500-1000-5000

-180 -200

-180 -180 -180 -180 -180

-160

160 180 200 220 240 260 280

-6000-5000-4000-3000-2000-10000

The minimum layer has 2 cores centered on 210° and 280°. Both are interpreted as southward flow of “old” water from farther north. The core which is against South Amer-ica was completely missed by GEOSECS due to lack of coverage during that program. At this point it is not clear whether the eastern core represents a return flow from the North Pacific or simply return flow of waters which have remained in the South Pacific.