Contributed by Robert M. Key, Princeton University
Prior to WOCE the only global-scale survey of the 14C distribution was GEOSECS. When completed, the U.S. WOCE effort in the Pacific will include approximately 10 times as many samples. About one-third of the stations where 14C was collected were sampled over the entire water column and the remainder over the upper 1 to 1.5 km. Measurements have been completed to date for four Pacific lines; the remaining Pacific samples will be measured during 1997-1998.
GEOSECS data are especially sparse in the eastern Pacific. To prepare the figures for this paper, the Pacific GEOSECS data from approximately the dateline eastward were considered to represent an average eastern Pacific section. This average GEOSECS section is then compared to the WOCE section along 135°W (P17). Property maps on density (or depth) surfaces for the Pacific clearly indicate that the primary trend of the property isolines is east to west rather than north to south, so the errors of this comparison should be reasonably small.
The upper ocean 14C distribution has been dominated by the influx of radiocarbon generated by atmospheric nuclear weapons tests during the 1950s and 1960s (Figure 1). During the early 1970s, the maximum 14C values were almost always found in surface samples. At that time, air-sea gas exchange, forced by the large gradient between surface waters and the atmosphere, was the primary factor controlling the upper ocean radiocarbon concentration. Twenty years later (during WOCE) the atmospheric concentration has dropped to approximately 25 percent of the 1965 maximum, and mixing and advection in the upper ocean have redistributed the surface signal into the interior. In the eastern Pacific WOCE sections, maximum concentrations now are found frequently below the surface at depths as great as 250 m.
Figure 2a compares the surface 14C values (shallowest sample less than 100 m) from the eastern Pacific GEOSECS stations and WOCE P17. The GEOSECS data are connected by line segments while a robust linear smoothing function was used to fit the WOCE data.
The most obvious changes in surface concentration in Figure 2a are the mid-latitude decrease and the low latitude equatorial increase. The mid-latitude change is greater in the North Pacific than in the South Pacific. GEOSECS 14C values as high as 205 ppt were measured around 30°N. The highest North Pacific surface value measured on P17 was 122 ppt at 25°N. During GEOSECS, the northern hemisphere mid-latitude surface values were higher than at similar latitudes in the southern hemisphere, reflecting the fact that most of the atmospheric bomb testing was performed in the north. This hemispheric difference is not apparent in the P17 WOCE data.
Southern Ocean surface values decreased between GEOSECS and WOCE. It is possible that natural variations in the circumpolar circulation regime or differences in sampling location are responsible for the decrease. A more plausible explanation is that radiocarbon lost from the Southern Ocean surface waters has been flushed into the subsurface South Pacific.
The equatorial eastern Pacific had a surface 14C concentration of about 50 ppt during GEOSECS. The concentration in this area increased to approximately 80 ppt by the time of the WOCE occupation. During both surveys, the low latitude surface minimum appears to be centered slightly south of the equator (Figure 2a). Both the equatorial 14C increase and the displacement of the minimum south of the equator are consistent with the circulation scenario proposed by Toggweiler et al. (1991). They argued that the low 14C equatorial surface waters originated as ~15°C water that had upwelled off Peru, and the upwelled waters were, in turn, derived from the 11°-14°C thermostad water of the Equatorial Undercurrent. At the time of GEOSECS, the undercurrent waters had not yet been contaminated by the bomb radiocarbon signal; however, by the time of the WOCE survey, the undercurrent waters did have a bomb radiocarbon signal.
Figure 2b repeats the lines from Figure 2a for comparison to an ocean global circulation model (OGCM). The OGCM results (from January 1973 and January 1993) use the coarse resolution OGCM of Toggweiler and Samuels (1995 a, b); the values plotted are from 135°W and 25 m. The model results for 1973 are quite close to the GEOSECS data between 10°S and 10°N but differ substantially elsewhere. For latitudes between 10° and 20°, the model results (blue dashed line) are low by as much as 25 ppt. At higher latitudes, the model surface values are much higher than the measurements. The model predicts a maximum 14C of 327 ppt around 40°N, which is approximately 120 ppt higher than measured. Differences greater than 100 ppt also exist in the southern hemisphere. The 1993 model results (red dashed line) are reasonably close to the WOCE P17 measurements (red solid line). Between 45°S and 25°N, the model results are approximately 25 ppt too low. Poleward of these two latitudes, the model results diverge from the measurements until the model is approximately 50 ppt too high at both high latitude extremes. The model difference between 1973 and 1993 is much larger than the GEOSECS to WOCE difference. This implies that the model is removing 14C from the surface ocean (presumably downward) much faster than is actually occurring.
Figure 3 summarizes subsurface changes between GEOSECS and WOCE for the eastern Pacific. This figure was prepared by individually gridding the eastern Pacific GEOSECS data and the P17 data, then contouring the difference between the two gridded sections, presented as 14C (ppt).
The near surface waters seen in Figure 3a repeat the trend illustrated in Figure 2a-an increase around the equator and a decrease elsewhere. The decrease is larger in the northern gyre than in the southern, but the 0 ppt isoline is at approximately the same depth. The equatorial near surface increase extends down to approximately 150 m. At depths of 150-250 m, the waters just south of the equator show an increase in concentration while those just north have decreased generally. A second zone of slightly increased concentration is located in the 300-500 m range at the equator.
The most remarkable feature in Figure 3a is the overall asymmetry about the equator. At the depth of mode and intermediate waters, 14C values in the southern subtropical gyre increased by as much as 60 ppt. The equivalent northern gyre waters showed a maximum increase of only 40 ppt, and the areal extent is significantly smaller than in the south. The difference is because these density layers in the south communicate freely with the circumpolar circulation regime. At the time of GEOSECS, very little (if any) of the bomb signal had penetrated the intermediate waters of the southern gyre. By the early 1990s the bomb signal had penetrated northward to at least 10°S. The intermediate waters in the North Pacific subtropical gyre are not ventilated so efficiently. The flow pathway of the intermediate and mode waters from the circumpolar region into the subtropical gyre cannot be determined from the data available at this point.
The overall shape of the contour pattern from the numerical model shown in Figure 3b is similar to Figure 3a; however, there are important differences. In the upper 200 m, the model results are approximately symmetrical about the equator, and the loss of radiocarbon is at least a factor of two larger than the measurements. These very large negative values near the surface are a result of the model overestimation of the 1973 surface concentration. Below 200 m the model results are generally within 20 ppt of the data, with the model showing larger differences. In the northern hemisphere, both the data and the model show a region of increased 14C. In the model this region is somewhat deeper, covers a larger latitude span, and appears to be more dissociated from the surface.
The changes in the southern hemisphere are more pronounced. Both sections show a triangular shaped cell of positive change (orange). In both cases the southern end of the triangle extends from approximately 200 to 1000 m. The bottom of the cell shoals equatorward in both cases; however, in the model this region clearly extends across the equator while the region stops around 10°S in the data. The maximum model change for this cell is approximately +60 ppt. Finally, the model section (Figure 3b) has a second cell of positive values between 60°S and 40°S, which is totally absent in the data. In the data section ( Figure 3a), the high southern latitude change between 300 and 900 m decreases rather uniformly going southward from 40°S to values of 14C~40 ppt at the southern boundary. In the model, the southern boundary value is approximately 0 ppt except at the surface.
One can strongly infer the connection between the circumpolar circulation and the gyre ventilation by examining the WOCE data in density space rather than in depth space. Figure 4 shows 14C contours for samples collected in the upper 1200 km of WOCE P17 that had a potential density () between 23.5 and 27.4 kg liter-1. At the north end of the section, only the 100 ppt and 50 ppt 14C isolines intersect the ocean surface. At the southern end, contours at least as low as -50 ppt outcrop. The fact that the 0 ppt and -50 ppt contours are essentially horizontal from the southern outcrop to approximately 25°S implies that these levels can be ventilated primarily by advection. The mean trend of the deeper contours (-150 ppt to 0 ppt) is upward to the north. As in Figure 3a, there is an asymmetry about the equator. There is a distinct peak (representing a minimum in 14C caused by upwelling and advection processes around the equator) in the 50 ppt and 0 ppt contours, which are centered around 8°-10°N. This relative peak is present, but less pronounced, in the deeper contours (-50 ppt and -100 ppt) where it is shifted slightly further north.
Clearly, significant changes have occurred in the distribution of radiocarbon in the Pacific since GEOSECS. The next step in the data synthesis process is to quantify these changes and to estimate the total inventory of bomb radiocarbon. Preliminary investigations indicate that current techniques used to differentiate bomb radiocarbon from natural radiocarbon will need modification.
In addition to illustrating the invasion of bomb 14C into the Pacific Ocean thermocline, the new data clearly demonstrate that significant improvements to existing numerical models are needed. Not only must the models be able to reproduce the structure evident in the detailed WOCE data set, they also must be able to describe the changes that have occurred in the bomb radiocarbon distribution over the past two decades. These new demands are significantly more stringent than required by the GEOSECS data set alone.
Broecker, W. S., S. Sutherland, W. Smethie, T.-H. Peng, and G. Ostlund. 1995. Oceanic radiocarbon: Separation of the natural and bomb components. Global Biogeochem. Cycles , 9: 263-288.
Key, R. M., P. D. Quay, G. A. Jones, A. P. McNichol, K. F. von Reden, and R. J. Schneider. 1996. WOCE AMS radiocarbon I: Pacific Ocean results; P6, P16 & P17. Radiocarbon, 38(2): in press.
Stuiver, M., G. Ostlund, R. M. Key, and P. J. Reimer. 1996. Large volume WOCE radiocarbon sampling in the Pacific Ocean. Radiocarbon , 38(2): in press.
Toggweiler, J. R., K. Dixon, and K. Bryan. 1989. Simulations of radiocarbon in a coarse-resolution world ocean model I. Steady state pre-bomb distributions. J. Geophys. Res., 94(C6): 8217-8242.
Toggweiler, J. R., K. Dixon, and W. S. Broecker. 1991. The Peru upwelling and the ventilation of the South Pacific thermocline. J. Geophys. Res., 96(C11): 20,467-20,497.
Toggweiler, J. R., and B. Samuels. 1995a. Effect of sea ice on the salinity of Antarctic bottom waters. J. Phys. Oceanogr., 25: 1980-1997.
Toggweiler, J. R., and B. Samuels. 1995b. Effect of Drake Passage on the global thermohaline circulation. Deep-Sea Res., 42: 477-500.
|Figure 1||Atmospheric time history of 14C in the northern (N) and southern (S) hemispheres showing the large change due to the atmospheric testing of nuclear weapons (Broecker et al., 1995).|
|Figure 2||a) Surface values from eastern Pacific GEOSECS section compared to WOCE section P17 data collected along 135°W. The values in the temperate zones of both hemispheres have decreased while the values in the tropical and equatorial latitudes have increased. b) Lines from a) plus results from recent OGCM for 1973 and 1993. WOCE data are from Key et al. (1996).|
|Figure 3||a) Change in the 14C (WOCE P17 [black asterisks] - eastern Pacific GEOSECS [gray Xs]) between 1973 and 1992 is indicated by color. b) Similar to a), but results are from Toggweiler's numerical model. WOCE data are from Key et al. (1996) and Stuiver et al. (1996).|
|Figure 4||14C contours in potential density anomaly space () for WOCE P17. A few values were clipped by the upper plot boundary (23.5 ¾ ¾ 27.4) in the near surface tropical waters, but no additional contour lines would have been drawn had the points not been omitted. The northernmost station was within 10 kilometers of Alaska. The isopycnal surfaces having 14C values of 0 ppt or less do not outcrop at the north (at least at the time of year the samples were collected - May-June 1993).|