Equatorial Pacific Surface Research Articles

Local versus remote wind forcing of the equatorial Pacific surface temperature in July 2003

After the moderate El Niño of 2002 the tropical Pacific seemed poised to enter a La Niña phase. Wind anomalies during May and June 2003, however, led to a sudden termination of this event. Counter to earlier predictions, forecasts from July 2003 correctly predicted normal conditions for the remainder of the year. In this paper the causes of this abrupt turn of events are analyzed. It is shown that wind anomalies in the eastern tropical Pacific played a major role in terminating the incipient cold ENSO phase. Although westerly wind bursts in the western Pacific explained most of the ocean subsurface behavior to the west of 110°W, their impact on SST was confined to the central equatorial Pacific.

Mineralogical and geochemical compositions of the eolian dust from the northeast equatorial Pacific and their implications on paleolocation of the Intertropical Convergence Zone

Eolian components of a 328‐cm‐long piston core collected from the northeast equatorial Pacific at 16°12′N and 125°59′W were investigated for mineral and geochemical compositions in order to constrain the sources of dust and determine the latitudinal position of the Intertropical Convergence Zone (ITCZ) recorded in the core. The eolian components below 250 cm are characterized by smectite‐ and phillipsite‐rich mineral composition, depleted rare earth elements (REEs), and high Eu/Sm ratios, indicative of volcanic‐rich composition. These characteristics are found in equatorial and south Pacific surface sediments, of which eolian particles are supplied from Central and South America. In contrast, eolian components above 250 cm are characterized by quartz‐ and illite‐rich mineralogy, and more shale‐like REE and trace element compositions, which are common in surface sediments of the central Pacific north of the ITCZ, where eolian particles are sourced from the Asia and North America. The observed changes are attributed to the shifting of its eolian sources from the Central and South America to the China and North America across the hemispheric dust barrier of the ITCZ. This result suggests that smectite‐illite transition, a phenomenon that smectite amount increases over illite at a depth, can be used as a tracking tool for the paleolocation of the ITCZ in the northeast and central Pacific. Backtrack path construction of Pacific plate indicates paleolocation of the ITCZ north of 12°N (±2°) prior to late Miocene.

Influence of equatorial diatom processes on Si deposition and atmospheric CO2 cycles at glacial/interglacial timescales

The causes of the glacial cycle remain unknown, although the primary driver is changes in atmospheric CO2, likely controlled by the biological pump and biogeochemical cycles. The two most important regions of the ocean for exchange of CO2 with the atmosphere are the equatorial Pacific and the Southern Ocean (SO), the former a net source and the latter a net sink under present conditions. The equatorial Pacific has been shown to be a Si(OH)4‐limited ecosystem, a consequence of the low source Si(OH)4 concentrations in upwelled water that has its origin in the SO. This teleconnection for nutrients between the two regions suggests an oscillatory relationship that may influence or control glacial cycles. Opal mass accumulation rate (MAR) data and δ15N measurements in equatorial cores are interpreted with predictions from a one‐dimensional Si(OH)4‐limited ecosystem model (CoSINE) for the equatorial Pacific. The results suggest that equatorial Pacific surface CO2 processes are in opposite phase to that of the global atmosphere, providing a negative feedback to the glacial cycle. This negative feedback is implemented through the effect of the SO on the equatorial Si(OH)4 supply. An alternative hypothesis, that the whole ocean becomes Si(OH)4 poor during cooling periods, is suggested by low opal MAR in cores from both equatorial and Antarctic regions, perhaps as a result of low river input. terminations in this scenario would result from blooms of coccolithophorids triggered by low Si(OH)4 concentrations.

The influence of the Galápagos Islands on tropical temperatures, currents and the generation of tropical instability waves

The Galápagos Islands provide a topographic barrier for the Southern Equatorial Current (SEC) and the Equatorial Undercurrent (EUC). An island wake effect can be diagnosed from the difference of an ocean general circulation model simulation which includes the Galápagos Islands and one which ignores their presence. Cold thermocline water upwells on the western side of the islands, and only during boreal winter season these cold waters can linger around the Islands at a depth of about 80 m and affect the far eastern equatorial Pacific surface waters. This effect is partly offset by the westward transport of cold surface waters by the SEC which creates a wake on the western side of the Islands. It is furthermore shown that changes in horizontal current shear, induced by the presence of the Galápagos Islands modify the generation of tropical instability waves and lead to a basin scale SST anomaly pattern.

The Response of Tropical Climate to the Equatorial Emergence of Spiciness Anomalies*

Abstract The ocean–atmosphere response to the surfacing of temperature anomalies from the oceanic thermocline is a key process in climate variability with decadal time scales. Using a coupled general circulation model, it is shown how density-compensating temperature and salinity (spiciness) anomalies emerging in the upwelling region of the equatorial Pacific modulate tropical climate. Upon reaching the surface in the central equatorial Pacific, warm and salty spiciness anomalies increase sea surface temperature and salinity, and vent their heat anomaly to the atmosphere, primarily by the latent heat flux. The associated surface buoyancy flux increases vertical mixing, and thereby dampens surface temperature anomalies. The moisture added to the atmosphere increases precipitation in the western Pacific and intertropical convergence zone, and strengthens the trade winds east, and weakens them west of the date line. Central equatorial Pacific surface temperatures are slightly warmed by the resulting deepened...

On Equatorial Pacific Surface Wind Changes around 1977: NCEP–NCAR Reanalysis versus COADS Observations*

Abstract This note compares equatorial Pacific surface wind changes around 1977 in the NCEP–NCAR reanalysis and the Comprehensive Ocean–Atmosphere Data Set (COADS) observations. Significant discrepancies are found in wind changes over the equatorial central and eastern Pacific. In the NCEP–NCAR reanalysis, the easterlies weakened over the eastern equatorial Pacific, while the southerlies strengthened over the north equatorial central Pacific. As a result, the low-level convergence and precipitation decreased over the equatorial central Pacific. These wind and precipitation anomalies are opposite to those derived from the COADS observations. Independent observations of ocean heat content are used to validate the changes in equatorial zonal wind, and it is found that the zonal slope of the thermocline in an ocean model forced by the COADS wind is more consistent with ocean observations than forced by the reanalysis wind. The equatorial wind biases are also identified in the NCEP–NCAR reanalysis climatology,...

Decadal Timescale Shift in the 14C Record of a Central Equatorial Pacific Coral

Coral skeletal radiocarbon records reflect seawater ∆14C and are useful for reconstructing the history of water mass movement and ventilation in the tropical oceans. Here, we reconstructed the inter-annual variability in central equatorial Pacific surface water ∆14C from 1922–1956 using near-monthly 14C measurements in a Porites sp. coral skeleton (FI5A) from the windward side of Fanning Island (3°54′32′′N, 159°18'88′′W). The most pronounced feature in this record is a large, positive shift in the ∆14C between 1947 and 1956 that coincides with the switch of the Pacific Decadal Oscillation (PDO) from a positive to a negative phase in the mid-1940s. Although the absolute ∆14C values from 1950–1955 in FI5A differ from the ∆14C values of another coral core collected from the opposite side of the island, both records show a large, positive shift in their ∆14C records at that time. The relative increase in the ∆14C of each record is consistent with the premise that a common mechanism is controlling the ∆14C records within each coral record. Overall, the Fanning ∆14C data support the notion that a significant amount of subtropical seawater is arriving at the Equator, but does not allow us to determine the mechanism for its transport.

A model of the iron cycle in the ocean

The IRONEX II in situ fertilization experiment showed decisively that availability of iron limits the growth of phytoplankton in equatorial Pacific surface waters. High‐nutrient low‐chlorophyll (HNLC) surface waters, potentially iron limited, are also found in the North Pacific and the Southern Ocean. A model of seawater iron geochemistry has been incorporated into a global ocean circulation and carbon cycle model and tuned to match the observed ocean Fe distribution. The model reproduces the observed HNLC areas in the equatorial Pacific and Southern Ocean but predicts nutrient depletion in the North Pacific (a region of high dust fluxes from Asia). The model is based on the idea that the concentrations of organic iron‐binding ligands in seawater control the subsurface Fe concentration and predicts that a majority (70–80%) of the global carbon export production can be sustained by upwelling of dissolved iron in seawater rather than by atmospheric deposition. The ocean iron cycle increases the efficiency of iron utilization in the biological pump, buffers new production against interannual fluctuations in iron deposition, and contributes several new intrinsic timescales to the ocean carbon cycle: several hundred years for the ocean iron concentration and perhaps several thousand for the concentration of ligand in the deep sea. Ocean recycling of Fe appears to play a major role in determining the strength of the biological pump in the ocean and the pCO2 of the atmosphere.

Late Miocene obduction and microplate migration around the southern Banda Sea and the closure of the Indonesian Seaway

Miocene shallowing and closure of the Indonesian Seaway between the Indian Ocean and the Pacific is related to plate-tectonic developments at the southern margins of the Banda Sea. Ultramafites, mainly lherzolite, closely associated with quartzo-pelitic metamorphic rocks, on the northern coast of Timor and on smaller islands in the southern Outer Banda Arc and also on southwest Seram in the northern Banda Arc, are fragments of Middle Miocene oceanic lithosphere obducted in the Late Miocene on sole rock of Australian continental origin. Initially cool sole rock was dynamo-thermally metamorphosed by heating from above by very hot, overriding oceanic lithosphere. Temperature equilibration between the sole and the Kaibobo lherzolitic complex (southwest Seram) took place at about 740°C and 4–5 kbar. Anatectic granitic magma formed in the sole and intruded in the ultramafite. 40Ar 39Ar determinations on muscovite and biotite from the sole and anatectic granite indicate that the system cooled through 400°C 6.0 Ma ago and through 320°C 0.5 Ma later. P-T-t modelling of the obduction/post-obduction exhumation curve, which is based on thermobarometry of the Kaibobo ultramafic complex and its sole, suggests that obduction started about 9.5 Ma, emplacement was completed about 8 Ma ago and that fast vertical movements continued until about 7 Ma. The obduction of the Kaibobo lherzolitic complex actually took place in an area northeast of the current position of Tanimbar, where Seram (as a micro-continental Australian fragment) lay 8 Ma ago, as it migrated northward with the Australian Plate. The reconstructed obduction site of the southwest Seram ultramafites and those between northern Timor and Tanimbar determine an 850 km-long, ENE-trending zone along the southern margin of the Banda Sea. As the obduction of the lherzolitic complex on northern Timor also took place 8 Ma ago and cooling to 300°C occurred 5.5 Ma ago, a similar time setting to Kaibobo is inferred. It is argued that oceanic lithosphere was being formed in the Early Miocene (about 6 Ma prior to the start of obduction) along this zone in the southern Banda Sea. The lherzolitic composition and petrographical and geochemical details suggest that the obducted ultramafites are remnants of weakly depleted, lherzolitic lithosphere, formed close to a passive margin by processes of very slow spreading. This notion and published palaeomagnetic data relating to the Neogene positions of Timor and the Australian continent as well as the types and ages of magmatic rocks in the Banda volcanic arc lead to a model for the Middle and Late Miocene geotectonic developments in the southern Banda Sea between the Eurasian and the Australian plates. The proposed model features interarc spreading in a short-lived, interarc Timor Plate (16–9.5 Ma) just north of the line Timor-Tanimbar, lengthening of the east Sunda volcanic arc by the creation of the volcanic Banda Arc, obduction-emplacement and exhumatian of ultramafic complexes, and migration of the Seram microcontinent. The model is in good agreement, in space and in time, with the 9.9–7.5 Ma history of shallowing and eventual closure of the Indonesian Seaway, as inferred in the literature from developments in biogeographic patterns and vertical thermal structure evolution of Miocene equatorial Pacific surface waters.

A timescale for dissolved organic carbon production in equatorial Pacific surface waters

Concentrations of dissolved organic carbon (DOC) in the equatorial Pacific Ocean are high (roughly 80 μM) in sunlit surface waters and low (∼40 μM) in subsurface waters. Concentrations in recently upwelled waters in the central equatorial region and in the coastal upwelling zone near Peru are intermediate between the surface water and deep water values (∼65 μM). The data appear to be consistent with the existence of a short‐lived “semilabile” DOC component, generated in surface waters, superimposed on a long‐lived “refractory” DOC concentration of ∼40 μM. We assume that the oligotrophic 80 μM end‐member is close to a biochemical steady state where production is balanced by consumption, and the lower concentrations in recently upwelled water represent a transient approach to steady state. The steady state assumption for oligotrophic surface waters can be used to couple the kinetic rate constants of DOC production and consumption, so that the only tunable parameter in the model is the e‐folding timescale for the approach to steady state. We tested and assessed the implications of this model by embedding surface ocean production and concentration‐dependent consumption of DOC into the three‐dimensional flow field of a high resolution equatorial circulation model. The distribution of DOC near the equator can be reproduced using a grow‐in timescale of 30–120 days, consistent with inferred values from field data and with direct field measurements. Implied gross DOC production rates are roughly half of measured particulate primary production rates [Barber et al., 1996]; this proportion applied globally would imply a global rate of DOC production of 50–100 Gton C yr−1, an order of magnitude higher than previous estimates [Siegenthaler and Sarmiento, 1993]. The DOC export production rate predicted by the model from the equatorial region (5°N to 5°S and 90°W to the dateline) is 0.2–0.4 Gton C yr−1, which is equivalent to 20–40% of the non‐El Niño 15NO3− uptake based new production estimate from the equatorial region [McCarthy et al., 1996; Murray et al., 1997].

Changes in pH in the eastern equatorial Pacific across stage 5–6 boundary based on boron isotopes in foraminifera

Estimates of paleo‐pH for the eastern equatorial Pacific Ocean across the oxygen isotopic stage 5–6 boundary have been made based on the boron isotopic composition of planktonic (Orbulina universa) and benthic (mixed species) foraminifera from core V19‐28. The estimated deep ocean pH during the penultimate glacial period was about 0.3 ± 0.1 pH units higher compared to the modern deep ocean. This is consistent with previously estimated deep ocean pH changes across the stage l–2 boundary in the western equatorial Pacific and tropical Atlantic, thus arguing against the possibility that the benthic foraminifera analyzed to estimate deep ocean pH changes have been significantly affected by anomalous local environment and/or diagenesis. The estimated changes in the deep ocean carbonate chemistry require a decoupling (of several kilometers) between the saturation horizon and the lysocline during the glacial periods. Though such a decoupling could be achieved by enhanced respiration CO2 driven calcite dissolution in sediments during glacial periods, it lacks support from the calcite sedimentary records. The boron isotopic compositions of planktonic foraminifera, on the other hand, indicate no significant pH change in the eastern equatorial Pacific surface ocean during the glacial‐interglacial transition. This is inconsistent with an expected higher surface ocean pH during the glacial period due to lower atmospheric pCO2 and is also in contrast with the previously estimated boron isotope based glacial‐interglacial pH change of 0.2 ± 0.1 pH units in the western equatorial Pacific and tropical Atlantic. The lack of change in eastern equatorial Pacific surface ocean pH between glacial‐interglacial periods could be attributed to less nutrient utilization efficiency and/or enhanced calcite production during glacial periods. Such a decrease in nutrient utilization efficiency and/or increase in calcite production would lead to a greater disequilibrium between the pCO2 of eastern equatorial Pacific surface ocean and that of the atmosphere, making this part of the ocean a greater source of CO2 to the atmosphere during glacial periods compared to today.

Late Paleocene to Eocene paleoceanography of the equatorial Pacific Ocean: Stable isotopes recorded at Ocean Drilling Program Site 865, Allison Guyot

An expanded and largely complete upper Paleocene to upper Eocene section was recovered from the pelagic cap overlying Allison Guyot, Mid‐Pacific Mountains at Ocean Drilling Program (ODP) Site 865 (18°26′N, 179°33′W; paleodepth 1300–1500 m). Reconstructions show that the site was within a few degrees of the equator during the Paleogene. Because no other Paleogene sections have been recovered in the Pacific Ocean at such a low latitude, Site 865 provides a unique record of equatorial Pacific paleoceanography. Detailed stable isotopic investigations were conducted on three planktonic foraminiferal taxa (species of Acarinina, Morozovella, and Subbotina). We studied benthic foraminiferal isotopes at much lower resolution on species of Cibicidoides and Lenticulina, Nuttallides truempyi and Gavelinella beccariiformis, because of their exceptional rarity. The δ18O and δ13C stratigraphies from Site 865 are generally similar to those derived from other Paleocene and Eocene sections. The planktonic foraminiferal records at Site 865, however, include significantly less short‐term, single‐sample variability than those from higher‐latitude sites, indicating that this tropical, oligotrophic location had a comparatively stable water column structure with a deep mixed layer and less seasonal variability. Low‐amplitude (0.1–0.8‰) oscillations on timescales of 250,000 to 300,000 years correlate between the δ13C records of all planktonic taxa and may represent fluctuations in the mixing intensity of surface waters. Peak sea surface temperatures of 24°–25°C occurred in the earliest Eocene, followed by a rapid cooling of 3–6°C in the late early Eocene. Temperatures remained cool and stable through the middle Eocene. In the late Eocene, surface water temperatures decreased further. Vertical temperature gradients decreased dramatically in the late Paleocene and were relatively constant through much of the Eocene but increased markedly in the late Eocene. Intermediate waters warmed through the late Paleocene, reaching a maximum temperature of 10°C in the early Eocene. Cooling in the middle and late Eocene paralleled that of surface waters, with latest Eocene temperatures below 5°C. Extinction patterns of benthic foraminifera in the latest Paleocene were similar to those observed at other Pacific sites and were coeval with a short‐term, very rapid negative excursion in δ13C values in planktonic and benthic taxa as at other sites. During this excursion, benthic foraminiferal δ18O values decreased markedly, indicating warming of 4 to 6°C for tropical intermediate waters, while planktonic taxa show slight warming (1°C) followed by 2°C of cooling. Convergence of δ18O values of planktonic and benthic foraminifera suggests that thermal gradients in the water column in this tropical location collapsed during the excursion. These data are consistent with the hypothesis that equatorial Pacific surface waters were a potential source of warm, higher salinity waters which filled portions of the deep ocean in the latest Paleocene. Oxygen isotopic data indicate that equator to high southern latitude sea surface thermal gradients decreased to as little as 4°C at the peak of the excursion, suggesting some fundamental change in global heat transport.

Phytoplankton photosynthesis parameters along 140°W in the equatorial Pacific

The role of trace metal limitation in the maintenance of the high nutrient and low chlorophyll condition in eastern equatorial Pacific surface waters remains controversial. On the EqPac Survey II cruise, photosynthesis versus irradiance ( P-I) and phytoplankton spectral absorption were measured along 140°W from 12°N to 12°S in the eastern tropical Pacific to determine the spatial variability of P- I parameters in relation to the light, macronutrient, and trace metal regime. The latitudinal patterns of the maximum quantum yield of photosynthesis ( φ m) and carotenoid-corrected spectral absorption covaried with the concentration of N03 at 60 m, a proxy for the macronutrient supply rate. At the equator, the maximum quantum yield of phytoplankton photosynthesis was less than half that expected for nutrient-replete phytoplankton. This indicates that phytoplankton photosynthesis was nutrient limited even though N03 concentrations at the equator exceeded 6 μmol kg −1. When φ m was corrected for absorption by photoprotective carotenoids, maximum values were found at 12°N and values declined linearly with latitude to 12°S. This pattern is coincident with the putative atmospheric iron flux and the distribution of dissolved iron. These observations support the hypothesis that the photosynthesis of equatorial phytoplankton is regulated by iron availability. Phytoplankton photosynthetic performance and in situ chlorophyll-normalized primary production along 140W appeared to be regulated by the interaction of macronutrient and trace metal supply rates on phytoplankton photoacclimation processes.

Quantitative estimation of surface ocean productivity and bottom water oxygen concentration using benthic foraminifera

An electronic supplement of this material may be obtained on adiskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN toAGU's FTP account using ANONYMOUS as the usemame andGUEST as the password. Go to the right directory by typing CDAPEND. Type LS to see what files are available. Type GET and thename of the file to get it. Finally, type EXIT to leave the system.)(Paper 94PA01624, Quantitative estimation of surface oceanproductivity and bottom water concentration using benthicforaminifera, by P. Loubere). Diskette may be ordered from AmericanGeophysical Union, 2000 Florida Avenue, N.W., Washington, DC20009; $15.00. Payment must accompany order. Quantitative estimation of surface ocean productivity and bottom water oxygen concentration with benthic foraminifera was attempted using 70 samples from equatorial and North Pacific surface sediments. These samples come from a well defined depth range in the ocean, between 2200 and 3200 m, so that depth related factors do not interfere with the estimation. Samples were selected so that foraminifera were well preserved in the sediments and temperature and salinity were nearly uniform (T = 1.5° C; S = 34.6‰). The sample set was also assembled so as to minimize the correlation often seen between surface ocean productivity and bottom water oxygen values (r² = 0.23 for prediction purposes in this case). This procedure reduced the chances of spurious results due to correlations between the environmental variables. The samples encompass a range of productivities from about 25 to >300 gC m−2 yr−1, and a bottom water oxygen range from 1.8 to 3.5 ml/L. Benthic foraminiferal assemblages were quantified using the >62 µm fraction of the sediments and 46 taxon categories. MANOVA multivariate regression was used to project the faunal matrix onto the two environmental dimensions using published values for productivity and bottom water oxygen to calibrate this operation. The success of this regression was measured with the multivariate r² which was 0.98 for the productivity dimension and 0.96 for the oxygen dimension. These high coefficients indicate that both environmental variables are strongly imbedded in the faunal data matrix. Analysis of the beta regression coefficients shows that the environmental signals are carried by groups of taxa which are consistent with previous work characterizing benthic foraminiferal responses to productivity and bottom water oxygen. The results of this study suggest that benthic foraminiferal assemblages can be used for quantitative reconstruction of surface ocean productivity and bottom water oxygen concentrations if suitable surface sediment calibration data sets are developed and appropriate means for detecting no‐analog samples are found.

The heat budget of the equatorial western Pacific surface mixed layer

The turbulent heat flux through the equatorial western Pacific surface mixed layer is estimated, using conductivity‐temperature‐depth and Doppler profiler data obtained in January 1986 and the assumption that a useful approximation to the heat flux can be obtained with the Pacanowski‐Philander eddy diffusivity. Fluxes appeared to be an order of magnitude less along the equator than in the central Pacific because the only layer of very low Richardson number (and hence large eddy diffusivity) lay in the isothermal but salt‐stratified “barrier layer.” The top of the thermocline, just below the barrier layer, coincided with the core of the (subsurface) South Equatorial Current; hence current shears, eddy diffusivities, and heat fluxes were small in this region. Away from the equator, heat flux estimates were of the order of 3 W m−2. Use of the recent Peters et al. diffusivities reduces these estimates still further. In combination with previous analyses of heat advection in the equatorial western Pacific heat pool, our results indicate that the net surface heat flux entering the region is near zero. There must therefore be systematic errors in several recent estimates of this net heat flux in the equatorial western Pacific, which range between 20 and 80 W m−2.

Plutonium concentrations in fish and seawater from Kwajalein Atoll.

A follow-up study has been made to assess the concentrations of 239+240Pu and 137Cs in the marine environment of Kwajalein Atoll. Fish collected from the atoll in 1972 had body burdens of plutonium that were substantially higher than concentrations in similar species from locations contaminated only with global fallout. Our recent results, however, indicated that Kwajalein lagoon seawater contained levels of plutonium more similar to global fallout levels found in north equatorial Pacific surface waters. No satisfactory explanation for the reported plutonium levels in fish from Kwajalein collected in 1972 could be deduced from the available data. The highest plutonium concentrations reported for the 1972 reef species of fish could expose man, through ingestion of marine foods, to a dose rate as high as 25% of the proposed EPA guideline for annual total transuranic dose rate to bone (3 mrad/yr over 70 yr). Our present results show the dose rate from the marine food pathway is nearer to 0.005% of the recommended EPA value and is consistent with the view that Kwajalein Atoll contains plutonium concentrations that are expected from global fallout. The magnitude of the plutonium levels reported in fish collected from Kwajalein lagoon during 1972 was excessively high, and these results appear to be inconsistent with other environmental data from the lagoon. Our results also show that concentration factors for plutonium in fish muscle and bone tissues appear to be independent of species, trophic level and location, which leads us to believe that there is a great deal of validity in the concept of a concentration factor for estimating concentrations of plutonium in fish.

Radium and thorium isotopes in the surface waters of the East Pacific and coastal Southern California

A fiber extraction technique is used to concentrate Ra and Th isotopes from 1000 liters or more of seawater. Natural 226Ra and 234Th are used as yield tracers. In the equatorial Pacific the 228Ra activity of surface water varies from 20 to 1 dpm/1000 kg and generally decreases away from continental shelf areas. Across the Peru Current System, this decrease is modeled as one-dimensional diffusion and indicates the possibility of two flow regimes with distinct characteristic mixing lengths and apparent eddy diffusivities of 10 5 and 10 7 cm 2/s. The perturbing effects of advection and equatorial upwelling west of the Galapagos Islands are noted. Off the coast of Southern California a vertical 228Ra distribution gives an apparent diffusivity of 1.6 cm 2/s for the upper thermocline. 226Ra concentrations near the coast appear to be higher than the open ocean values at comparable depths, which may reflect supply of this isotope from continental shelf sediments and/or upwelling. The insoluble daughter/soluble parent activity ratios 228Th/ 228Ra and 234Th/ 238U in the equatorial Pacific surface water display latitudinal trends which may be correlated with productivity variations. Near the coast of California these ratios reflect the differing oceanographic conditions north and south of Pt. Conception indicating a mean chemical removal time constant on the order of 4 months for Th and other highly reactive elements within the Southern California Bight. The 232Th content of seawater sampled is less than 0.1 μg/1000 1; most of the published values for seawater 232Th could well be too high. A comparison of the two methods of determining 228Ra (via 228Ac and via 228Th) made on 64 seawater samples shows that the time delay required by the 228Th method is more than compensated by its better analytical simplicity and precision.